CNL-23-012, Revision to the Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02)

Revision to the Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02)

ML23101A179
Person / Time
Site:	Sequoyah
Issue date:	04/11/2023
From:	Hulvey K Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation, Document Control Desk
Shared Package
ML23101A178	List: CNL-23-012, Revision to the Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02)
References
CNL-23-012, EPID L-2020-LLA-0004
Download: ML23101A179 (1)

Text

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4 1101 Market Street, Chattanooga, Tennessee 37402 CNL-23-012

$SULO, 2023 10 CFR 50.90 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328

Subject:

Revision to the Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02)(EPID L-2020-LLA-0004)

References:

1. TVA letter to NRC, CNL-19-066, "Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02) (EPID L-2020-LLA-0004)," dated January14, 2020 (ML20016A396 and ML20016A397)

 NRC email to TVA, SQN Hydrologic Analysis LAR Acceptance Review,

dated February 13, 2020 (ML20049A032)

 TVA letter to NRC, CNL-20-026, Supplement to Application to Revise

Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis

Report Regarding Changes to Hydrologic Analysis (TS-19-02), dated

February 18, 2020 (ML20049H184)

 NRC email to TVA, Acceptance of License Amendment Request to Modify

the UFSAR based on a new Hydrologic Analysis (EPID L-2020-LLA-0004),

dated February 20, 2020 (ML20054B553)

 NRC letter to G. Williams (TVA), Request for Additional Information

Regarding the Hydrologic Analysis LAR (EPID L-2020-LLA-0004) dated

April 14, 2020 (ML20106F104)

 TVA Letter to NRC, CNL-20-032, Application to Revise Sequoyah Nuclear

Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding

Changes to Hydrologic Analysis - Response to Request for Additional

Information (TS-19-02) (EPID L-2020-LLA-0004), dated May 14, 2020 (ML20135H067)

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4 U.S. Nuclear Regulatory Commission CNL-23-012 Page 2

$SULO, 2023

7. NRC e-mail to TVA, Request for Additional Information Regarding Hydrologic UFSAR Update (EPID L-2020-LLA-0004), dated July 1, 2020 (ML20189A211)

8. TVA Letter to NRC, CNL-20-057, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Partial Response to Additional Request for Additional Information (TS-19-02) (EPID L-2020-LLA-0004), dated August 12, 2020 (ML20225A170)

9. NRC Electronic Mail to TVA, Sequoyah Nuclear Plant, Units 1 and 2 -

Request for Additional Information Regarding Hydrologic UFSAR Update (EPID L-2020-LLA-0004), dated September 14, 2020 (ML20261H417)

10. NRC Letter to TVA, Summary of October 13, 2020, Closed Meeting With Tennessee Valley Authority to Discuss Responses to Requests for Additional Information Regarding Updated Final Safety Analysis Report Hydrologic Analysis License Amendment Request (EPID L-2020-LLA-0004), dated October 27, 2020 (ML20293A080)

11. TVA Letter to NRC, CNL-20-082, Partial Response to Request for Additional Information Regarding Application to Revise Sequoyah Nuclear Plants Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (TS-19-02) (EPID L-2020-LLA-0004), dated November 10, 2020 (ML20328A093)

12. TVA Letter to NRC, CNL-21-024, Partial Response to Additional Request for Additional Information (QUESTION 2) Regarding Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (TS-19-02)

(EPID L-2020-LLA-0004), dated June 15, 2021 (ML21203A335) 13.TVA Letter to NRC, CNL-21-072, Partial Response to Additional Request for Additional Information (QUESTION 2), Regarding Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Correction of Submitted File Types Contained on Digital Versatile Disc (TS-19-02)

(EPID L-2020-LLA-0004), dated August 13, 2021 (ML21239A115)

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4 U.S. Nuclear Regulatory Commission CNL-23-012 Page 3

$SULO, 2023

14. TVA Letter to NRC, CNL-21-095, Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Software Dedication Report 16 Update to Revision 4 (TS-19-02) (EPID L-2020-LLA-0004), dated December 21, 2021 (ML22013A278)

15. TVA letter to NRC, CNL-22-053, Response to Request for Additional Information Regarding Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Second Partial Response to Additional Request for Additional Information (TS 19 02) (EPID L-2020 LLA-0004), dated August 22, 2022 (ML21239A115)

In Reference 1, Tennessee Valley Authority (TVA) submitted a request for an amendment to Renewed Facility Operating License Nos. DPR-77 and DPR-79 for Sequoyah Nuclear Plant (SQN), Units 1 and 2, respectively. The submitted license amendment request (LAR) proposed to revise the SQN Units 1 and 2, Updated Final Safety Analysis Report (UFSAR) to reflect the results from the new hydrologic analysis. TVA determined that the proposed changes to the SQN UFSAR require prior Nuclear Regulatory Commission (NRC) approval.

In References 3, 6, and 8, TVA provided supplemental information to the Reference 1 LAR.

Following receipt of the Reference 9 NRC Request for Additional Information (RAI), TVA determined that there was a potentially unconservative unverified assumption in the calculations supporting the Apalachia Dam failure analysis and entered this issue into the TVA corrective action program. TVA communicated this information to the NRC during a closed meeting on October 13, 2020 (Reference 10). This unverified assumption impacted some of the Reference 9 requested calculation files. As a result of this impact, TVA provided in Reference 11, a partial response to the Reference 9 request. Reference 11 included the calculation files which were considered to not be impacted by the unverified assumption issue related to the Apalachia Dam failure analysis. Reference 14 provided an update to the Software Dedication Report 16-01 to Revision 4 and transmitted in Reference 11. Reference 15 responded to the remaining Reference 9 RAIs not previously answered in Reference 11.

As stated above, TVA identified issues with dam stability assumptions utilized in hydrologic analysis supporting the results and conclusions in Reference 1. These issues, documented in the TVA corrective action program, involved (1) an assumption of dam stability at the Apalachia Dam for a headwater and tailwater elevation not supported by an analysis and (2) an assumption of dam stability of several embankment supported spillways at floodwater flows in excess of the original design assumptions. Resolution of these dam stability issues has resulted in a required revision to all eight Enclosures provided in Reference 1.

Revised Enclosures 1, 2, 3, and 4 contain security-related information that TVA is requesting be withheld from public disclosure in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 2.390. Revised Enclosures 5, 6, 7, and 8 contain the public versions of the SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4 U.S. Nuclear Regulatory Commission CNL-23-012 Page 4

$SULO  2023 information provided in revised Enclosures 1, 2, 3, and 4, with the security-related information withheld from the public. An affidavit supporting TVAs request for withholding is included as .

Enclosures 1 and 5 of this letter provide a description and evaluation of the proposed technical changes to SQN Units 1 and 2 UFSAR Section 2.4, Hydrologic Engineering, and Appendix 2.4A, Flood Protection Plan. Changes to Enclosure 1 submitted in Reference 1 are indicated with additions in underlined text and deletions with lined-through text. Updated Enclosures 2 and 6 provide the current SQN Units 1 and 2 UFSAR text marked up to show the proposed changes. Updated Enclosures 3 and 7 provide the proposed Tables and Figures for SQN Units 1 and 2 UFSAR Section 2.4. A description of the SQN UFSAR Table and Figure changes is included in the revised Enclosures 1 and 5 of this letter. The updated Enclosures 4 and 8 provide the retyped SQN Units 1 and 2 UFSAR incorporating the proposed changes.

Lastly, with exception of the changes provided in the enclosures to this submittal, the previous RAIs and responses referenced above, remain applicable to the Reference 1 LAR and this supplement.

TVA determined that there are no significant hazard considerations associated with the proposed changes and that the change qualifies for a categorical exclusion from environmental review pursuant to the provisions of 10 CFR 51.22(c)(9). Additionally, in accordance with 10 CFR 50.91(b)(1), TVA is sending a copy of this letter and the enclosures to the Tennessee Department of Environment and Conservation.

TVA requests approval of the proposed license amendment within one year from the date of this submittal with implementation within 60 days of issuance of the amendment.

There are no new commitments associated with this submittal. Please address any questions regarding this submittal to Stuart L. Rymer, Senior Manager, Fleet Licensing at slrymer@tva.gov.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this

th day of $SULO 2023.

Respectfully, Digitally signed by Edmondson, Carla Date: 2023.04.11 11:39:11 -04'00' Kimberly D. Hulvey Director, Nuclear Regulatory Affairs Enclosures cc: See Page 5 SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4 U.S. Nuclear Regulatory Commission CNL-23-012 Page 5

$SULO, 2023

Enclosures:

1. Evaluation of the Proposed Changes (Mark-Ups of R0 Submittal) (Security-Related)

2. Proposed SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A (Mark-Ups)

(Security-Related)

3. Proposed SQN Units 1 and 2 UFSAR Section 2.4 Tables and Figures (Mark-Ups)

(Security-Related)

4. Proposed SQN Units 1 and 2 UFSAR 2.4 (Final Typed) (For Information Only)

(Security-Related)

5. Evaluation of the Proposed Changes (Public)

6. Proposed SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A (Mark-Ups)

(Public)

7. Proposed SQN Units 1 and 2 UFSAR Section 2.4 Tables and Figures (Final Typed)

(Public)

8. Proposed SQN Units 1 and 2 UFSAR 2.4 (Final Typed) (For Information Only) (Public)

9. Affidavit Pursuant to 10 CFR 2.390 (Public) cc (with Enclosures):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Sequoyah Nuclear Plant, Units 1 and 2 NRC Project Manager - Sequoyah Nuclear Plant Director, Division of Radiological Health - Tennessee State Department of Environment and Conservation SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 This letter is decontrolled when separated from Enclosures 1, 2, 3, and 4

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 ENCLOSURE 1, R1 Evaluation of the Proposed Changes (Mark-Ups of R0 Submittal)

(Security-Related)

CNL-23-012 SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 Enclosure 2, R1 Proposed SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A (Mark-Ups) (Security-Related)

CNL-23-012 SECURITY RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390

SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 Enclosure 3, R1 Proposed SQN Units 1 and 2 UFSAR Section 2.4 Tables and Figures (Mark-Ups) (Security-Related)

The following table provides a list of the current SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A Tables by Table Number and Current Title. The table provides the Proposed Title for the respective Table Number. Table Numbers indicated n/a in the Current Title column represent a new Table being proposed in Section 2.4. The proposed UFSAR Tables are provided on pages E3-8 to E3-35 of this Enclosure. A description of the UFSAR Table changes is provided near the end of Section 2.1, Proposed Changes, of of this letter.

CNL-23-012 SECURITY RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 Enclosure 4, R1 Proposed SQN Units 1 and 2 UFSAR Section 2.4 (Final Typed) (Security-Related)

CNL-23-012 SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390

ENCLOSURE 5, R1 Evaluation of the Proposed Changes (Public)

CNL-23-012

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5HYLVHGEvaluation of Proposed Change

Subject:

Application to Revise Sequoyah Nuclear Plant (SQN) Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (TS-19-02)

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Justification: 7KHH[LVWLQJGHVLJQEDVLVPRGHOXWLOL]HV62&+VRIWZDUH7KHXSGDWHG

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E. HEC-RAS Model Geometry Changes Due to Overbank Volume Error in TVA HEC-RAS Model 7KH+(&5$6PRGHOJHRPHWU\IRUWKHUHVHUYRLUDERYH'RXJODV'DPRQWKH)UHQFK%URDG DQG1ROLFKXFN\5LYHUVIRUWKHUHVHUYRLUDQGDERYH&KHURNHH'DPVRQWKH+ROVWRQ5LYHUDQG WKHGRZQVWUHDPSRUWLRQRIWKH6RXWK)RUN+ROVWRQ5LYHUKDVEHHQXSGDWHGWRPRUH DFFXUDWHO\FKDUDFWHUL]HRYHUEDQNVWRUDJHWKURXJKWKHDGGLWLRQDOFURVVVHFWLRQVDQGVWRUDJH DUHDVFRQQHFWHGE\ODWHUDOVWUXFWXUHV2YHUEDQNVWRUDJHYROXPHLQWKHRWKHUPRGHOHG UHVHUYRLUVKDYHEHHQDGMXVWHGE\XVLQJWKHDYHUDJHUHDFKOHQJWKLQWKHRYHUEDQNVWRUDJH YROXPHFDOFXODWLRQ

Existing Design Basis: 7KH641H[LVWLQJGHVLJQEDVLVLVWKH62&+0RGHO7KHIROORZLQJ GLVFXVVLRQGHVFULEHVXSGDWHVWRWKH+(&5$6PRGHOJHRPHWU\IURPWKDWSUHYLRXVO\

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Figure 4 - Off Channel Storage

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Table 1 - TVA and HEC-RAS Overbank Storage Evaluation for Major Reservoirs

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Figure 5 - TVA and HEC-RAS Overbank Storage Calculation Updated Design Basis: 7RFRUUHFWWKHYROXPHVWRUDJHHUURUVGLVFXVVHGDERYHWKHUH

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Figure 6 - General Approach to Resolve Cherokee and Douglas Reservoir Storage ErroU

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Figure 7 - Off Channel Storage Calculation - Mathematical Fix 7RFRQILUPWKDWWKH+(&5$6PRGHORIWKH7HQQHVVHH5LYHUDFFXUDWHO\DFFRXQWHGIRUZDWHU

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% Difference = [(Outflows + Storage) - Inflows] / [InflowsInitial Storage]

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Table 2 - Volume Check Comparison 79$DQG+(&

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Updated Design Basis:,QWKHXSGDWHGDQDO\VLVWKHWZR\HDUZLQGVSHHGLVWKH

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G. Seismically Induced Dam Failure Flooding Analysis Changes to Align with NRC JLD-ISG-2013-01

1. Seismic Flood Event Combinations and Multiple Dam Failure Considerations 6HLVPLFDOO\LQGXFHGGDPIDLOXUHIORRGLQJDQDO\VHVDUHXSGDWHGWRFRQVLGHUWKHPRUH VHYHUHRIWZRFRPELQDWLRQV  VHLVPLFKD]DUGZLWKDQDQQXDOH[FHHGDQFHSUREDELOLW\RI

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Existing Design Basis: 7KHH[LVWLQJGHVLJQEDVLVIRUVHLVPLFDOO\LQGXFHGGDPIDLOXUHV

H[DPLQHGWZRVSHFLILFDOWHUQDWLYHGDPIDLOXUHFRPELQDWLRQVFRQVLVWHQWZLWK5*

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Table 3 - Seismically Induced Dam Failure Combinations

- Existing Design Basis OBE Failures with one-half PMF SQN Flood Elevation (ft)

(Plant Grade is 705 ft)

 [CEII]

 [CEII]

 [CEII]



[CEII]

SSE Failures with 25-yr Flood SQN Flood Elevation (ft)

 [CEII]

 [CEII]

Updated Design Basis: ,QWKHXSGDWHGDQDO\VLVWKHEDVLVIRUDQDO\]LQJVHLVPLFGDP

IDLOXUHVLQFRPELQDWLRQZLWKIORRGVLVUHYLVHGWREHFRQVLVWHQWZLWK15&-/',6*

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Table 4 - Seismically Induced Dam Failure Combinations

- Updated Design Basis Simulation One-half of 10-4 Seismic Ground Motion SQN Failures with 500 Yr Flood Elevation

 [CEII] [CEII]

 [CEII @@ [CEII]

[CEII] 

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Simulation 10-4 Seismic Ground Motion Failure SQN With 25 Yr Flood Elevation

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3. Dam Stability

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4. Seismically Induced Dam Failure Flooding SOCH Model Changed to HEC-RAS0RGHO 7KHK\GURORJLFDOXQVWHDG\IORZPRGHOXVHGLQWKHVLPXODWLRQRIVHLVPLFDOO\LQGXFHGGDP

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Storm Type  General Tropical Duration (in hours) Duration (in hours)

Watershed PMP Event Watershed Sub-basins in PMP Area Area 1 6 12 24 48 72 1 6 12 24 48 72 (sq.mi.)

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Storm Type  General Tropical Duration (in hours) Duration (in hours)

Watershed PMP Event Watershed Basins in PMP Area Area 1 6 12 24 48 72 1 6 12 24 48 72 (sq.mi.)

Above Cherokee-Douglas-Fontana-Chatuge-Nottely-1-6, 9-15, 19-22, 38, 39, 42 10,174.59 1.12 3.84 7.42 9.19 13.75 14.44 0.99 4.78 7.22 9.05 10.93 12.48 Blue Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-22, 38, 39, 42 9,003.10 1.22 4.19 7.93 9.78 14.33 15.04 1.06 5.03 7.50 9.42 11.55 13.16 Douglas-Fontana-Chatuge-Nottely-Blue Ridge Cherokee to Boone and Above Douglas-Fontana-1-6, 12-15, 19-22, 38-40, 42 8,900.49 1.27 4.39 8.33 10.16 14.84 15.49 1.11 5.21 7.77 9.76 12.11 13.87 Hiwassee-Blue Ridge Above Blue Ridge-Hiwassee-Fontana-Douglas-38-40, 42, 19-22, 1-6, 9-15 10,739.66 1.11 3.85 7.44 9.23 13.81 14.48 0.99 4.76 7.24 9.08 10.99 12.59 Cherokee Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-22, 38-40, 42 9,568.16 1.19 4.10 7.86 9.67 14.33 15.00 1.04 4.98 7.48 9.38 11.49 13.16 Douglas-Fontana-Hiwassee-Blue Ridge Above Cherokee - Douglas - Fontana 1-6, 9-15, 19-22 9,539.61 1.15 3.91 7.47 9.27 13.73 14.46 1.01 4.82 7.24 9.06 10.96 12.45 Cherokee to Boone and Above Douglas-Tellico-1-6, 12-15, 19-24, 38, 39, 42 9,390.29 1.23 4.22 8.09 9.91 14.55 15.23 1.08 5.12 7.68 9.62 11.80 13.49 Chatuge-Nottely-Blue Ridge Above Cherokee-Douglas-Tellico-Chatuge-Nottely-1-6, 9-15, 19-24, 38, 39, 42 11,229.46 1.09 3.79 7.32 9.15 13.64 14.33 0.98 4.71 7.18 9.04 10.85 12.40 Blue Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24, 38, 39, 42 10,057.96 1.16 3.95 7.66 9.46 14.08 14.77 1.02 4.90 7.40 9.26 11.23 12.83 Douglas-Tellico-Chatuge-Nottely-Blue Ridge Cherokee to Boone and Above Douglas-Tellico-1-6, 12-15, 19-24, 38-40, 42 9,955.35 1.20 4.11 7.99 9.77 14.52 15.16 1.05 5.06 7.63 9.56 11.69 13.43 Hiwassee-Blue Ridge Above Cherokee-Douglas-Tellico-Hiwassee-Blue 1-6, 9-15, 19-24, 38-40, 42 11,794.52 1.08 3.80 7.32 9.16 13.68 14.35 0.97 4.68 7.18 9.06 10.88 12.47 Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24, 38-40, 42 10,623.03 1.14 3.96 7.66 9.48 14.13 14.80 1.01 4.88 7.40 9.29 11.27 12.92 Douglas-Tellico-Hiwassee-Blue Ridge Cherokee to Boone and Above Douglas-Tellico 1-6, 12-15, 19-24 8,755.30 1.28 4.34 8.20 10.06 14.59 15.30 1.10 5.19 7.72 9.69 11.90 13.53 Above Cherokee-Douglas-Tellico 1-6, 9-15, 19-24 10,594.47 1.11 3.79 7.32 9.13 13.59 14.32 0.98 4.74 7.19 9.01 10.81 12.30 Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24 9,422.97 1.20 4.06 7.75 9.59 14.10 14.84 1.04 4.96 7.43 9.31 11.31 12.85 Douglas-Tellico Cherokee to Boone and Above Douglas 1-6, 12, 13, 14&15 6,129.54 1.37 4.60 8.25 10.18 14.23 15.10 1.16 5.31 7.72 9.64 11.65 12.92 Above Cherokee-Douglas 1-6, 9-15 7,968.71 1.21 4.02 7.40 9.27 13.35 14.20 1.04 4.88 7.21 8.97 10.68 11.88 Cherokee to South Holston-Watauga and Above 1-6, 11-13, 14&15 6,797.21 1.30 4.33 7.80 9.74 13.78 14.67 1.10 5.08 7.43 9.25 11.09 12.29 Douglas Cherokee to Boone 12, 13, 14&15 1,586.29 1.93 5.78 8.30 11.62 14.23 15.05 1.27 5.20 7.31 8.93 9.07 9.07 Cherokee to Ft. Patrick Henry 13, 14&15 1,523.52 1.96 5.87 8.40 11.71 14.30 15.11 1.29 5.26 7.39 9.03 9.17 9.17 Above Cherokee 9-15 3,425.46 1.52 4.99 7.87 10.26 13.58 14.49 1.20 5.15 7.28 9.03 10.25 10.90 Cherokee to South Holston-Watauga 11-13, 14&15 2,253.96 1.69 5.19 7.76 10.71 13.52 14.35 1.19 5.00 6.99 8.57 9.11 9.33 Above Chatuge-Nottely 38-39 403.37 3.74 13.93 17.71 18.80 22.23 22.48 4.04 11.79 14.40 21.81 25.97 29.33 Above Chatuge 38 189.08 4.83 17.13 21.03 23.18 26.61 27.71 4.98 14.83 17.76 27.11 30.97 35.14 Above Douglas-Fontana-Chatuge-Nottely-Blue Ridge 1-6, 19-22, 38, 39, 42 6,749.13 1.48 5.34 9.73 11.65 16.32 16.82 1.31 5.88 8.58 10.93 14.12 16.26 Above Blue Ridge-Hiwassee-Fontana-Douglas 1-6, 19-22, 38-40, 42 7,314.20 1.44 5.23 9.63 11.52 16.30 16.77 1.28 5.82 8.54 10.85 13.99 16.17 Above Douglas - Fontana 1-6, 19-22 6,114.15 1.52 5.46 9.83 11.79 16.34 16.87 1.34 5.94 8.62 11.02 14.23 16.32 Above Douglas-Tellico-Chatuge-Nottely-Blue Ridge 1-6, 19-24, 38, 39, 42 7,804.00 1.41 5.02 9.32 11.19 15.92 16.43 1.24 5.71 8.41 10.64 13.53 15.61

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Storm Type General Tropical Duration (in hours) Duration (in hours)

Watershed PMP Event Watershed Basins in PMP Area Area 1 6 12 24 48 72 1 6 12 24 48 72 (sq.mi.)

Above Douglas-Tellico-Hiwassee-Blue Ridge 1-6, 19-24, 38-40, 42 8,369.06 1.37 4.89 9.20 11.03 15.87 16.35 1.22 5.64 8.36 10.54 13.38 15.49 Above Douglas-Tellico 1-6, 19-24 7,169.01 1.45 5.15 9.46 11.37 15.98 16.53 1.27 5.78 8.47 10.74 13.69 15.73 Above Douglas 1-6 4,543.25 1.57 5.66 9.69 11.64 15.71 16.32 1.39 6.01 8.59 10.89 13.80 15.55 Fort Loudoun-Fontana to Cherokee 1-8, 16-22 7,694.28 1.42 4.88 9.05 10.94 15.44 15.98 1.22 5.64 8.31 10.47 13.01 14.75 Fort Loudoun to Boone 1-8, 12-18 7,709.68 1.32 4.27 7.88 9.79 13.85 14.66 1.10 5.18 7.63 9.48 11.16 12.26 Ft. Loudoun to Cherokee-Douglas 7-8, 16-18 1,580.14 2.36 8.41 12.20 13.78 17.29 17.73 1.78 7.30 10.31 12.87 14.34 14.97 Fort Loudoun to Cherokee 1-8, 16-18 6,123.39 1.49 5.02 9.03 10.94 15.05 15.68 1.26 5.76 8.37 10.45 12.70 14.13 Above Ft. Loudoun 1-18 9,548.85 1.15 3.68 7.04 8.89 12.98 13.79 0.98 4.73 7.10 8.82 10.24 11.31 Fort Loudoun to South Holston-Watauga 1-8, 11-18 8,377.35 1.25 4.01 7.47 9.39 13.43 14.27 1.04 4.96 7.35 9.13 10.67 11.72 Above Blue Ridge-Hiwassee-Fontana 38-40, 42, 19-22 2,770.95 2.30 9.33 14.48 16.61 21.52 21.83 2.36 8.16 11.30 15.40 21.10 24.19 Above Fontana 19-22 1,570.89 2.77 11.23 16.28 18.05 22.84 23.13 2.86 9.24 12.67 17.80 23.95 27.37 Ft. Patrick Henry to Boone 12 62.77 4.05 8.06 9.97 16.07 18.18 18.51 1.48 5.95 8.19 9.02 9.10 9.10 Above Ft. Patrick Henry 9-12 1,901.94 1.79 6.34 9.37 11.83 14.91 15.43 1.47 5.80 8.08 10.19 12.50 13.69 Ft. Patrick Henry to South Holston-Watauga 11-12 730.44 2.47 7.01 9.30 12.75 15.23 15.66 1.35 5.41 7.52 9.58 11.08 11.87 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge to 1-8, 12-24, 38, 39, 42 10,970.42 1.17 3.93 7.63 9.51 13.99 14.66 1.02 4.91 7.47 9.38 11.22 12.70 Boone Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge 1-24, 38, 39, 42 12,809.60 1.05 3.66 7.02 8.96 13.24 13.93 0.94 4.55 7.04 8.93 10.52 11.90 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge to 1-8, 11-24, 38, 39, 42 11,638.10 1.12 3.80 7.34 9.26 13.64 14.34 0.98 4.74 7.26 9.14 10.86 12.28 South-Holston-Watauga Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Boone-1-8, 12-18, 23, 24, 38-40, 42 9,964.60 1.20 3.90 7.58 9.43 13.81 14.53 1.02 4.98 7.50 9.34 11.03 12.36 Fontana Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Boone 1-8, 12-24, 38-40, 42 11,535.49 1.15 3.93 7.62 9.51 14.02 14.67 1.01 4.87 7.45 9.38 11.24 12.77 Fort Loudoun-Tellico-Hiwassee-Blue Ridge to 7, 8, 16-18, 23-24, 38-39, 40, 42 3,835.06 1.98 7.09 11.62 13.57 17.88 18.35 1.71 7.01 9.94 12.64 15.72 17.40 Cherokee-Douglas-Fontana Fort Loudoun-Tellico-Hiwassee-Blue Ridge to 7, 8, 16-24, 38, 39, 40, 42 5,405.95 1.79 6.29 11.19 13.33 18.03 18.48 1.50 6.64 9.59 12.28 15.74 17.85 Cherokee-Douglas Above Fort Loudoun-Tellico-Hiwassee-Blue Ridge 1-24, 38-40, 42 13,374.66 1.03 3.65 7.00 8.95 13.26 13.93 0.94 4.51 7.02 8.93 10.53 11.95 Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South 1-8, 11-18, 23, 24, 38-40, 42 10,632.27 1.14 3.77 7.29 9.17 13.47 14.21 0.99 4.80 7.28 9.09 10.67 11.94 Holston-Watauga-Fontana Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South 1-8, 11-24, 38-40, 42 12,203.16 1.10 3.80 7.33 9.26 13.67 14.35 0.97 4.71 7.24 9.15 10.88 12.34 Holston-Watauga Ft. Loudoun-Tellico to Boone-Fontana 1-8, 12-18, 23, 24 8,764.54 1.27 4.08 7.71 9.61 13.75 14.55 1.06 5.08 7.56 9.40 11.05 12.22 Ft. Loudoun-Tellico to Boone 1-8, 12-24 10,335.44 1.19 3.93 7.65 9.51 13.96 14.67 1.02 4.95 7.49 9.37 11.19 12.62 Ft. Loudoun-Tellico to Cherokee-Douglas-Fontana 7-8, 16-18, 23-24 2,635.00 2.14 7.59 11.70 13.47 17.38 17.85 1.79 7.18 10.06 12.71 14.97 16.11 Ft. Loudoun-Tellico to Cherokee-Douglas 7-8, 16-24 4,205.90 1.96 6.98 11.71 13.75 18.23 18.70 1.67 6.94 9.87 12.81 16.17 18.07 Fort Loudoun-Tellico to Cherokee-Fontana 1-8, 16-18, 23-24 7,178.25 1.43 4.80 8.81 10.71 14.92 15.54 1.21 5.65 8.28 10.32 12.49 13.96 Fort Loudoun-Tellico to Cherokee 1-8, 16-24 8,749.15 1.34 4.58 8.69 10.54 15.11 15.66 1.16 5.47 8.15 10.22 12.56 14.29 Ft. Loudoun-Tellico to Fontana 1-18, 23, 24 10,603.72 1.11 3.60 6.95 8.82 12.93 13.73 0.96 4.66 7.06 8.81 10.21 11.33 Above Ft. Loudoun - Tellico 1-24 12,174.61 1.07 3.66 7.05 8.96 13.22 13.94 0.95 4.59 7.06 8.92 10.50 11.83

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Storm Type  General Tropical Duration (in hours) Duration (in hours)

Watershed PMP Event Watershed Basins in PMP Area Area 1 6 12 24 48 72 1 6 12 24 48 72 (sq.mi.)

Fort Loudoun-Tellico to South Holston-Watauga-1-8, 11-18, 23, 24 9,432.22 1.20 3.83 7.30 9.20 13.34 14.15 1.01 4.87 7.29 9.06 10.57 11.68 Fontana Fort Loudoun-Tellico to South Holston-Watauga 1-8, 11-24 11,003.11 1.14 3.80 7.35 9.26 13.61 14.34 0.99 4.78 7.27 9.13 10.83 12.19 Guntersville to Chickamauga 46, 47A, 47B, 48-50 3,671.27 2.37 7.52 12.22 14.20 18.68 18.95 1.72 7.45 10.57 12.92 15.82 18.65 Above Guntersville 1-50 24,452.06 0.79 2.92 5.43 8.04 11.34 11.83 0.76 3.62 6.12 8.26 9.05 9.71 7-8, 16-18, 23-25, 27, 33-37, 41, Guntersville to Blue Ridge-Hiwassee-Fontana-43, 44A, 44B, 45, 46, 47A, 47B, 10,799.61 1.50 4.17 8.17 10.68 14.51 14.88 1.12 5.52 8.38 10.43 11.33 13.19 Douglas-Cherokee-Norris 48-50 41, 43, 44A, 44B, 45, 46, 47A, Guntersville to Watts Bar-Hiwassee-Blue Ridge 5,958.48 1.97 5.81 10.43 12.64 16.96 17.21 1.41 6.70 9.73 11.89 13.57 16.40 47B, 48-50 Above Blue Ridge - Hiwassee 38-40, 42 1,200.05 2.89 11.53 16.30 17.71 22.22 22.48 3.09 9.25 12.50 17.36 23.37 26.66 Hiwassee to Chatuge-Nottely 40 565.07 3.69 14.03 18.04 19.48 23.34 23.69 3.97 11.64 14.27 21.65 26.03 29.62 Above Hiwassee 38-40 968.44 3.02 11.92 16.53 17.78 22.14 22.40 3.23 9.55 12.81 17.98 23.82 27.16 Above Melton Hill 26-27 3,344.67 1.97 6.17 9.86 11.47 15.04 15.48 1.47 6.39 9.05 11.06 11.64 11.91 Melton Hill to Norris 27 431.87 3.51 12.23 15.56 16.85 19.70 19.97 2.70 9.65 12.55 15.66 17.21 18.39 Above Nickajack 1-47B 21,852.90 0.80 2.99 5.53 8.02 11.46 11.99 0.78 3.66 6.19 8.33 9.17 9.95 Nickajack to Blue Ridge-Hiwassee-Fontana-Douglas- 7-8, 16-18, 23-25, 27, 33-37, 41, 8,200.45 1.64 4.72 8.86 11.13 15.01 15.43 1.23 5.95 8.82 10.86 11.95 14.05 Cherokee-Norris 43, 44A, 44B, 45, 46, 47A, 47B Above Blue Ridge-Hiwassee-Fontana-Douglas-38-40, 42, 19-22, 1-6, 9-15, 26 13,652.45 1.02 3.52 6.73 8.71 12.82 13.46 0.91 4.38 6.84 8.72 10.13 11.36 Cherokee-Norris Above Norris-Cherokee-Douglas-Fontana 1-6, 9-15, 19-22, 26 12,452.40 1.06 3.51 6.75 8.69 12.74 13.43 0.92 4.45 6.87 8.69 10.06 11.19 Above Norris - Cherokee-Douglas 1-6, 9-15, 26 10,881.51 1.10 3.43 6.62 8.52 12.41 13.17 0.92 4.51 6.85 8.55 9.71 10.59 Above Norris - Cherokee 9-15, 26 6,338.26 1.39 4.13 7.38 9.29 12.93 13.82 1.07 5.04 7.34 9.01 9.79 10.21 Above Norris 26 2,912.79 1.98 6.32 9.87 11.46 14.88 15.34 1.49 6.36 8.96 10.97 11.42 11.59 Above Nottely 39 214.30 3.87 13.78 17.00 18.02 20.71 21.55 3.77 11.22 14.01 20.48 24.45 27.63 Ocoee #1 to Blue Ridge 43 362.64 3.75 13.79 17.43 18.44 21.72 21.96 3.85 11.20 13.74 20.41 24.74 27.79 Above Ocoee #1 42-43 594.25 3.52 13.37 17.28 18.66 22.41 22.74 3.67 10.77 13.56 19.84 24.60 27.91 Above South Holston-Watauga-Douglas 1-6, 9, 10 5,714.75 1.42 5.00 8.93 10.81 14.94 15.58 1.25 5.65 8.17 10.27 12.83 14.50 Above South Holston-Watauga-Douglas-Fontana 1-6, 9, 10, 19-22 7,285.64 1.38 4.93 9.06 10.91 15.46 16.01 1.23 5.59 8.20 10.39 13.19 15.15 Above South Holston-Watauga 9-10 1,171.50 2.19 7.95 11.19 13.20 16.34 16.75 1.81 6.71 9.38 12.16 15.37 17.01 Above South Holston 9 703.25 2.47 7.53 9.93 12.69 15.15 15.55 1.53 6.13 8.54 10.89 12.55 13.43 Tellico-Hiwassee-Blue Ridge to Fontana 23, 24, 38, 39, 40, 42 2,254.92 2.28 9.06 13.73 15.70 20.12 20.51 2.33 8.02 11.05 14.56 19.27 21.78 Tellico-Hiwassee-Blue Ridge 19-24, 38, 39, 40, 42 3,825.81 2.06 7.96 13.05 15.20 20.06 20.43 1.93 7.41 10.45 13.92 18.77 21.51 Tellico to Fontana 23-24 1,054.86 2.74 10.40 14.56 16.17 19.95 20.43 2.52 8.44 11.62 15.49 19.54 21.76 Above Tellico 19-24 2,625.76 2.28 9.04 13.93 15.98 20.63 21.03 2.26 8.01 11.10 15.01 19.98 22.68 Above Tims Ford 59 533.31 3.44 12.67 16.20 17.50 20.90 21.22 3.36 9.86 12.74 16.58 21.07 24.57 Above Watts Bar-Chatuge-Nottely-Blue Ridge 1-38, 39, 42 17,928.52 0.85 3.17 5.94 8.23 11.87 12.48 0.83 3.92 6.43 8.47 9.45 10.42 Above Watts Bar-Hiwassee-Blue Ridge 1-40, 42 18,493.58 0.83 3.16 5.88 8.20 11.86 12.45 0.82 3.86 6.39 8.45 9.44 10.42 Watts Bar-Hiwassee-Blue Ridge to Norris-Cherokee-7,8,16-18, 23-25, 27, 33-40, 42 6,041.18 1.75 5.54 9.97 12.07 16.24 16.69 1.38 6.40 9.29 11.52 13.64 15.46 Douglas-Fontana Watts Bar-Hiwassee-Blue Ridge to Norris-Cherokee-7, 8, 16-25, 27, 33-40, 42 7,612.08 1.62 5.26 9.76 11.87 16.35 16.76 1.32 6.13 9.02 11.31 13.67 15.78 Douglas

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Storm Type General Tropical Duration (in hours) Duration (in hours)

Watershed PMP Event Watershed Basins in PMP Area Area 1 6 12 24 48 72 1 6 12 24 48 72 (sq.mi.)

Watts Bar to Fort Loudoun 19-27, 33-37 7,744.68 1.59 4.84 8.99 11.06 15.16 15.60 1.24 5.83 8.59 10.72 12.36 13.96 Watts Bar to Fort Loudoun-Tellico 25-27, 33-37 5,118.92 1.78 5.13 9.08 10.94 14.63 15.09 1.31 6.15 8.87 10.81 11.69 12.67 Above Watts Bar 1-37 17,293.53 0.88 3.19 6.00 8.27 11.89 12.52 0.84 3.98 6.48 8.48 9.47 10.42 Watts Bar to Norris-Cherokee-Douglas-Fontana 7-8, 16-18, 23-25, 27, 33-37 4,841.13 1.86 5.75 10.04 12.06 15.99 16.47 1.43 6.57 9.45 11.63 13.41 14.95 Watts Bar to Norris-Cherokee-Douglas 7-8, 16-25, 27, 33-37 6,412.02 1.74 5.55 10.04 12.17 16.46 16.90 1.38 6.34 9.22 11.58 13.90 15.87 Watts Bar to Norris-Cherokee 1-8, 16-25, 27, 33-37 10,955.27 1.25 4.05 7.90 9.90 14.22 14.73 1.06 5.15 7.83 9.82 11.55 13.22 Watts Bar to Norris-Fort Loudoun 19-25, 27, 33-37 4,831.88 1.92 6.36 11.10 13.25 17.66 18.05 1.55 6.81 9.78 12.45 15.56 17.94 Watts Bar to Norris-Fort Loudoun-Tellico 25, 27, 33-37 2,206.13 2.38 7.66 11.59 13.21 16.93 17.22 1.96 7.44 10.41 12.75 15.57 17.54 Watts Bar to Norris 1-25, 27-37 14,380.73 1.01 3.49 6.66 8.78 12.74 13.40 0.91 4.39 6.91 8.84 10.12 11.38 Wheeler to Chickamauga 46, 47A, 47B, 48-65 8,811.96 1.85 5.09 9.70 12.23 16.63 16.82 1.27 6.19 9.23 11.36 11.98 15.05 Above Wheeler 1-65 29,592.76 0.78 2.76 5.18 7.99 11.04 11.47 0.73 3.51 5.94 8.05 8.80 9.26 7-8, 16-18, 23-25, 27, 33-43, Wheeler to Norris-Cherokee-Douglas-Fontana 17,140.36 1.09 3.74 7.09 9.97 13.71 13.96 0.98 4.63 7.52 9.84 10.62 11.65 44A, 44B, 45, 46, 47A, 47B, 48-65 Wheeler to Guntersville 51-58, 60-65 4,607.39 2.26 6.96 11.98 14.09 18.90 19.18 1.57 7.23 10.36 12.63 14.93 17.81 7-8, 16-18, 23-25, 27, 33-37, 41, Wheeler to Tims Ford-Blue Ridge-Hiwassee-Fontana-43, 44A, 44B, 45, 46, 47A, 47B, 15,406.99 1.22 3.87 7.41 10.25 13.96 14.24 1.02 4.89 7.78 10.03 10.75 11.99 Douglas-Cherokee-Norris 48-50, 51-58, 60-65 41, 43, 44A, 44B, 45, 46, 47A, Wheeler to Watts Bar-Hiwassee-Blue Ridge 11,099.17 1.61 4.45 8.70 11.39 15.50 15.70 1.15 5.69 8.66 10.79 11.51 13.83 47B, 48-65 Above Watauga-Douglas 1-6, 10 5,011.50 1.51 5.38 9.50 11.44 15.60 16.20 1.33 5.92 8.51 10.75 13.64 15.46 Above Watauga 10 468.25 3.26 12.26 15.59 16.84 19.89 20.25 3.03 10.00 12.94 19.58 23.20 26.32

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Table B Projects with Potential to Impact TVA Nuclear Sites Table 1 Projects with potential to impact TVA nuclear sites Apalachia Hiwassee Blue Ridge Melton Hill Boone Nickajack Chatuge Norris Cherokee Nottely Chickamauga South Holston Douglas Tellico Fontana Tims Ford Ft. Loudoun Watauga Ft. Patrick Henry Watts Bar Guntersville Wheeler 7RLGHQWLI\WKHPRVWVHYHUHUHDVRQDEO\SRVVLEOHK\SRWKHWLFDOIORRGHYHQWWRVDWLVI\WKH

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Table B-2a - Candidate Watersheds Watershed PMP Event Watershed Sub-basins in PMP Area Area (sq.mi.)

Apalachia to Chatuge-Nottely 40-41 614.9 Above Apalachia 38-41 1,018.3 Apalachia to Hiwassee 41 49.8 Above Boone and Douglas 1-6, 9-11 6,382.4 Above Boone 9-11 1,839.2 Above Boone-Douglas-Fontana 1-6, 9-11, 19-22 7,953.3 Boone to South Holston-Watauga 11 667.7 Above Blue Ridge 42 231.6 Chickamauga-Tellico to Norris-Fort Loudoun-Fontana 23-25, 27, 33-43, 44A, 44B, 45 6,748.3 Chickamauga-Tellico to Norris-Fort Loudoun 19-25, 27, 33-43, 44A, 44B,45 8,319.1 Chickamauga-Tellico to Watts Bar-Fontana 23, 24, 38-43, 44A, 44B, 45 4,542.1 Chickamauga and Tellico to Watts Bar 19-24, 38-43, 44A, 44B, 45 6,113.0 Above Chickamauga 1-45 20,780.8 Chickamauga to Norris-Cherokee-Douglas-Chatuge- 7-8, 16-25, 27, 33-37, 40, 41, 43, 9,264.3 Nottely-Blue Ridge 44A, 44B, 45 Chickamauga to Norris-Cherokee-Douglas-Fontana- 7-8, 16-18, 23-25, 27, 33-37, 40, 7,693.4 Chatuge-Nottely-Blue Ridge 41, 43, 44A, 44B, 45 Chickamauga to Norris-Cherokee-Douglas-Fontana- 7-8, 16-18, 23-25, 27, 33-37, 41, 7,128.3 Hiwassee-Blue Ridge 43, 44A, 44B, 45 7-8, 16-18, 23-25, 27, 33-43, Chickamauga to Norris-Cherokee-Douglas-Fontana 8,328.4 44A, 44B, 45 Chickamauga to Norris-Cherokee-Douglas-Hiwassee- 7-8, 16-25, 27, 33-37, 41, 43, 8,699.2 Blue Ridge 44A, 44B, 45 7-8, 16-25, 27, 33-43, 44A, 44B, Chickamauga to Norris-Cherokee-Douglas 9,899.3 45 Chickamauga to Norris-Fort Loudoun-Chatuge-Nottely- 19-25, 27, 33-37, 40, 41, 43, 44A, 7,684.2 Blue Ridge 44B, 45 Chickamauga to Norris-Fort Loudoun-Hiwassee-Blue 19-25, 27, 33-37, 41, 43, 44A, 7,119.1 Ridge 44B, 45 Chickamauga to Norris-Fort Loudoun-Tellico-Chatuge- 25, 27, 33-37, 40, 41, 43, 44A, 5,058.4 Nottely-Blue Ridge 44B, 45 Chickamauga to Norris-Fort Loudoun-Tellico- 25, 27, 33-37, 41, 43, 44A, 44B, 4,493.3 Hiwassee-Blue Ridge 45 Chickamauga to Norris-Fort Loudoun-Tellico 25, 27, 33-43, 44A, 44B, 45 5,693.4 Chickamauga to Watts Bar-Chatuge-Nottely-Blue 40, 41, 43, 44A, 44B, 45 2,852.3 Ridge Chickamauga to Watts Bar-Hiwassee-Blue Ridge 41, 43, 44A, 44B, 45 2,287.2 

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Table B-2b - Candidate Watersheds Watershed PMP Event Watershed Basins in PMP Area Area (sq.mi.)

Chickamauga to Watts Bar 38-43, 44A, 44B, 45 3,487.3 Cherokee to Boone and Above Douglas-Fontana-1-6, 12-15, 19-22, 38, 39, 42 8,335.4 Chatuge-Nottely-Blue Ridge Above Cherokee-Douglas-Fontana-Chatuge-Nottely-1-6, 9-15, 19-22, 38, 39, 42 10,174.6 Blue Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-22, 38, 39, 42 9,003.1 Douglas-Fontana-Chatuge-Nottely-Blue Ridge Cherokee to Boone and Above Douglas-Fontana-1-6, 12-15, 19-22, 38-40, 42 8,900.5 Hiwassee-Blue Ridge Above Blue Ridge-Hiwassee-Fontana-Douglas-38-40, 42, 19-22, 1-6, 9-15 10,739.7 Cherokee Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-22, 38-40, 42 9,568.2 Douglas-Fontana-Hiwassee-Blue Ridge Above Cherokee - Douglas - Fontana 1-6, 9-15, 19-22 9,539.6 Cherokee to Boone and Above Douglas-Tellico-1-6, 12-15, 19-24, 38, 39, 42 9,390.3 Chatuge-Nottely-Blue Ridge Above Cherokee-Douglas-Tellico-Chatuge-Nottely-1-6, 9-15, 19-24, 38, 39, 42 11,229.5 Blue Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24, 38, 39, 42 10,058.0 Douglas-Tellico-Chatuge-Nottely-Blue Ridge Cherokee to Boone and Above Douglas-Tellico-1-6, 12-15, 19-24, 38-40, 42 9,955.4 Hiwassee-Blue Ridge Above Cherokee-Douglas-Tellico-Hiwassee-Blue 1-6, 9-15, 19-24, 38-40, 42 11,794.5 Ridge Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24, 38-40, 42 10,623.0 Douglas-Tellico-Hiwassee-Blue Ridge Cherokee to Boone and Above Douglas-Tellico 1-6, 12-15, 19-24 8,755.3 Above Cherokee-Douglas-Tellico 1-6, 9-15, 19-24 10,594.5 Cherokee to South Holston-Watauga and Above 1-6, 11-15, 19-24 9,423.0 Douglas-Tellico Cherokee to Boone and Above Douglas 1-6, 12, 13, 14&15 6,129.5 Above Cherokee-Douglas 1-6, 9-15 7,968.7 Cherokee to South Holston-Watauga and Above 1-6, 11-13, 14&15 6,797.2 Douglas Cherokee to Boone 12, 13, 14&15 1,586.3 Cherokee to Ft. Patrick Henry 13, 14&15 1,523.5 Above Cherokee 9-15 3,425.5 Cherokee to South Holston-Watauga 11-13, 14&15 2,254.0 

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Table B-2c - Candidate Watersheds Watershed PMP Event Watershed Basins in PMP Area Area (sq.mi.)

Above Chatuge-Nottely 38-39 403.4 Above Chatuge 38 189.1 Above Douglas-Fontana-Chatuge-Nottely-Blue Ridge 1-6, 19-22, 38, 39, 42 6,749.1 Above Blue Ridge-Hiwassee-Fontana-Douglas 1-6, 19-22, 38-40, 42 7,314.2 Above Douglas - Fontana 1-6, 19-22 6,114.1 Above Douglas-Tellico-Chatuge-Nottely-Blue Ridge 1-6, 19-24, 38, 39, 42 7,804.0 Above Douglas-Tellico-Hiwassee-Blue Ridge 1-6, 19-24, 38-40, 42 8,369.1 Above Douglas-Tellico 1-6, 19-24 7,169.0 Above Douglas 1-6 4,543.3 Fort Loudoun-Fontana to Cherokee 1-8, 16-22 7,694.3 Fort Loudoun to Boone 1-8, 12-18 7,709.7 Ft. Loudoun to Cherokee-Douglas 7-8, 16-18 1,580.1 Fort Loudoun to Cherokee 1-8, 16-18 6,123.4 Above Ft. Loudoun 1-18 9,548.9 Fort Loudoun to South Holston-Watauga 1-8, 11-18 8,377.4 Above Blue Ridge-Hiwassee-Fontana 38-40, 42, 19-22 2,770.9 Above Fontana 19-22 1,570.9 Ft. Patrick Henry to Boone 12 62.8 Above Ft. Patrick Henry 9-12 1,901.9 Ft. Patrick Henry to South Holston-Watauga 11-12 730.4 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge to 1-8, 12-24, 38, 39, 42 10,970.4 Boone Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge 1-24, 38, 39, 42 12,809.6 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge to 1-8, 11-24, 38, 39, 42 11,638.1 South-Holston-Watauga Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Boone-1-8, 12-18, 23, 24, 38-40, 42 9,964.6 Fontana Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Boone 1-8, 12-24, 38-40, 42 11,535.5 Fort Loudoun-Tellico-Hiwassee-Blue Ridge to 7, 8, 16-18, 23-24, 38-39, 40, 42 3,835.1 Cherokee-Douglas-Fontana Fort Loudoun-Tellico-Hiwassee-Blue Ridge to 7, 8, 16-24, 38, 39, 40, 42 5,405.9 Cherokee-Douglas Above Fort Loudoun-Tellico-Hiwassee-Blue Ridge 1-24, 38-40, 42 13,374.7 Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South 1-8, 11-18, 23, 24, 38-40, 42 10,632.3 Holston-Watauga-Fontana Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South 1-8, 11-24, 38-40, 42 12,203.2 Holston-Watauga 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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Table B-2d - Candidate Watersheds Watershed PMP Event Watershed Basins in PMP Area Area (sq.mi.)

Ft. Loudoun-Tellico to Boone-Fontana 1-8, 12-18, 23, 24 8,764.5 Ft. Loudoun-Tellico to Boone 1-8, 12-24 10,335.4 Ft. Loudoun-Tellico to Cherokee-Douglas-Fontana 7-8, 16-18, 23-24 2,635.0 Ft. Loudoun-Tellico to Cherokee-Douglas 7-8, 16-24 4,205.9 Fort Loudoun-Tellico to Cherokee-Fontana 1-8, 16-18, 23-24 7,178.3 Fort Loudoun-Tellico to Cherokee 1-8, 16-24 8,749.1 Ft. Loudoun-Tellico to Fontana 1-18, 23, 24 10,603.7 Above Ft. Loudoun - Tellico 1-24 12,174.6 Fort Loudoun-Tellico to South Holston-Watauga-1-8, 11-18, 23, 24 9,432.2 Fontana Fort Loudoun-Tellico to South Holston-Watauga 1-8, 11-24 11,003.1 Guntersville to Chickamauga 46, 47A, 47B, 48-50 3,671.3 Above Guntersville 1-50 24,452.1 7-8, 16-18, 23-25, 27, 33-37, 41, Guntersville to Blue Ridge-Hiwassee-Fontana-43, 44A, 44B, 45, 46, 47A, 47B, 10,799.6 Douglas-Cherokee-Norris 48-50 41, 43, 44A, 44B, 45, 46, 47A, Guntersville to Watts Bar-Hiwassee-Blue Ridge 5,958.5 47B, 48-50 Above Blue Ridge - Hiwassee 38-40, 42 1,200.1 Hiwassee to Chatuge-Nottely 40 565.1 Above Hiwassee 38-40 968.4 Above Melton Hill 26-27 3,344.7 Melton Hill to Norris 27 431.9 Above Nickajack 1-47B 21,852.9 Nickajack to Blue Ridge-Hiwassee-Fontana-Douglas- 7-8, 16-18, 23-25, 27, 33-37, 41, 8,200.4 Cherokee-Norris 43, 44A, 44B, 45, 46, 47A, 47B Above Blue Ridge-Hiwassee-Fontana-Douglas-38-40, 42, 19-22, 1-6, 9-15, 26 13,652.5 Cherokee-Norris Above Norris-Cherokee-Douglas-Fontana 1-6, 9-15, 19-22, 26 12,452.4 Above Norris - Cherokee-Douglas 1-6, 9-15, 26 10,881.5 Above Norris - Cherokee 9-15, 26 6,338.3 Above Norris 26 2,912.8 Above Nottely 39 214.3 Ocoee #1 to Blue Ridge 43 362.6 Above Ocoee #1 42-43 594.3 Above South Holston-Watauga-Douglas 1-6, 9, 10 5,714.8 Above South Holston-Watauga-Douglas-Fontana 1-6, 9, 10, 19-22 7,285.6 Above South Holston-Watauga 9-10 1,171.5 

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Table B-2e - Candidate Watersheds Watershed PMP Event Watershed Basins in PMP Area Area (sq.mi.)

Above South Holston 9 703.3 Tellico-Hiwassee-Blue Ridge to Fontana 23, 24, 38, 39, 40, 42 2,254.9 Tellico-Hiwassee-Blue Ridge 19-24, 38, 39, 40, 42 3,825.8 Tellico to Fontana 23-24 1,054.9 Above Tellico 19-24 2,625.8 Above Tims Ford 59 533.3 Above Watts Bar-Chatuge-Nottely-Blue Ridge 1-38, 39, 42 17,928.5 Above Watts Bar-Hiwassee-Blue Ridge 1-40, 42 18,493.6 Watts Bar-Hiwassee-Blue Ridge to Norris-Cherokee-7,8,16-18, 23-25, 27, 33-40, 42 6,041.2 Douglas-Fontana Watts Bar-Hiwassee-Blue Ridge to Norris-Cherokee-7, 8, 16-25, 27, 33-40, 42 7,612.1 Douglas Watts Bar to Fort Loudoun 19-27, 33-37 7,744.7 Watts Bar to Fort Loudoun-Tellico 25-27, 33-37 5,118.9 Above Watts Bar 1-37 17,293.5 Watts Bar to Norris-Cherokee-Douglas-Fontana 7-8, 16-18, 23-25, 27, 33-37 4,841.1 Watts Bar to Norris-Cherokee-Douglas 7-8, 16-25, 27, 33-37 6,412.0 Watts Bar to Norris-Cherokee 1-8, 16-25, 27, 33-37 10,955.3 Watts Bar to Norris-Fort Loudoun 19-25, 27, 33-37 4,831.9 Watts Bar to Norris-Fort Loudoun-Tellico 25, 27, 33-37 2,206.1 Watts Bar to Norris 1-25, 27-37 14,380.7 Wheeler to Chickamauga 46, 47A, 47B, 48-65 8,812.0 Above Wheeler 1-65 29,592.8 7-8, 16-18, 23-25, 27, 33-43, Wheeler to Norris-Cherokee-Douglas-Fontana 17,140.4 44A, 44B, 45, 46, 47A, 47B, 48-65 Wheeler to Guntersville 51-58, 60-65 4,607.4 7-8, 16-18, 23-25, 27, 33-37, 41, Wheeler to Tims Ford-Blue Ridge-Hiwassee-Fontana-43, 44A, 44B, 45, 46, 47A, 47B, 15,407.0 Douglas-Cherokee-Norris 48-50, 51-58, 60-65 41, 43, 44A, 44B, 45, 46, 47A, Wheeler to Watts Bar-Hiwassee-Blue Ridge 11,099.2 47B, 48-65 Above Watauga-Douglas 1-6, 10 5,011.5 Above Watauga 10 468.2 

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Vsx AsxDtx where: Vsx = rainfall volume over secondary basin x (in sq.mi.*inches)

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Table B Fort Patrick Henry Example Adjustment Sub-basin #

9 10 11 12 13 14&15 Volume Sub-basin Area (sq.mi.) 703.25 468.25 667.67 62.77 668.89 854.63 (in sq mi.-in.)

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Table B 72-hour Duration Rainfall for CHH_105 Scenario

1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW

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Table B 72-hour Duration Rainfall for CHH_105 Scenario 1RWH³<HOORZ'KLJKOLJKWHGFROXPQVUHSUHVHQWWKHVXEEDVLQVLQWKH3ULPDU\$UHDRI,QWHUHVW 32, 7KHRWKHUFRORUKLJKOLJKWHGJURXSLQJVUHSUHVHQWVXEEDVLQVLQHDFK6HFRQGDU\$UHDVRI,QWHUHVW 62, VWDUWLQJEHORZWKH

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Enclosure 6, R1 Proposed SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A (Mark-Ups) (Public)

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Exterior accesses are also provided to each of the class IE electrical systems manholes and handholes at elevations varying from 700 ft MSL to 724 feet ft MSL, depending upon the location of each structure.

The relationship of the plant site to the surrounding area can be seen in Figures 2.1.2-1 and 2.4.1-1. It can be seen from these figures that significant natural drainage features of the site have not been altered. Local surface runoff drains into the Tennessee River.

2.4.1.2 Hydrosphere The Sequoyah Nuclear Plant (SQN) site comprises approximately 525 acres on a peninsula on the western shore of Chickamauga Lake at Tennessee River Mile (TRM) 484.5. As shown by Figure 2.4.1-1, the site is on high ground with the Tennessee River being the only potential source of flooding. SQN is located in the Middle Tennessee Chickamauga watershed, U.S. Geological Survey (USGS) hydrologic unit code 06020001, one of 32 watersheds in the Region 06 - Tennessee River Watershed (Figure 2.4.1-2).

The Tennessee River above SQN site drains 20,650 square miles. The drainage area at Chickamauga Dam, 13.5 miles downstream, is 20,790 square miles. Three major tributaries, Hiwassee, Little Tennessee, and French Broad Rivers, rise to the east in the rugged Southern Appalachian Highlands. They flow northwestward through the Appalachian Divide which is essentially defined by the North Carolina-Tennessee border to join the Tennessee River which flows southwestward. The Tennessee River and its Clinch and Holston River tributaries flow southwest through the Valley and ridge physiographic province which, while not as rugged as the Southern Highlands, features a number of mountains including the Clinch and Powell Mountain chains. The drainage pattern is shown on Figure 2.1.1-1. About 20 percent of the watershed rises above elevation 3000 ft with a maximum elevation of 6,684 ft at Mt. Mitchell, North Carolina. The watershed is about 70 percent forested with much of the mountainous area being 100 percent forested.

The climate of the watershed is humid temperate. Mean annual precipitation for the Tennessee Valley is shown by Figure 2.4.1 2. Above Chickamauga Dam, annual rainfall averages 51 inches and varies from a low of 40 inches at sheltered locations in the mountains to high spots of 85 inches on the southern and eastern divide. Rainfall occurs relatively evenly throughout the year. See Section 2.3 for a discussion of rainfall. As shown in Table 2.3.2-19, the lowest site monthly average is 2.9 inches in October and the highest site monthly average is 6.8 inches in March, with January a close second with an average of 6.0 inches.

Major flood producing storms are of two three general types; the cool season, winter type, and the warm season, hurricane general, tropical, and local types. Most floods on the Tennessee River near at SQN, however, have been produced byJHQHUDOwinter-type storms in the main flood-season months of January through early AprilMay.

Watershed snowfall is relatively light, averaging only about 14 inches annually above the plant.

Snowfall above the 3000 ft elevation averages 22 inches annually. The maximum highest average annual snowfall of in the basin is 63 inches occurs at Mt. Mitchell, the highest point east of the Mississippi River. The overall snowfall average above the 3,000 foot elevation, however, is only 22 inches annually. Individual snowfalls are normally light, with an average of 13 snowfalls per year.

Snowmelt is not a factor in maximum flood determinations.

The Tennessee River, particularly above Chattanooga, Tennessee, is one of the most highly-regulated rivers in the United States. The TVA reservoir system is operated for flood control, navigation, and power generation with flood control a prime purpose with particular emphasis on protection for Chattanooga, 20 miles downstream from SQN.

Chickamauga Dam, 13.5 miles downstream, affects water surface elevations at SQN. Normal (Vummer) full pool elevation is 683.0682.5 feetft. At this elevation the reservoir is 58.9 miles long on the Tennessee River and 32 miles long on the Hiwassee River, covering an area of 35,40036,050 acres, with a volume of 628,000622,500 acre-feet. The reservoir has an average width of nearly 1 mile, ranging from 700 feet ft to 1.7 miles. At SQNthe SQN site, the reservoir is about 3,000 feet ft wide with depths ranging between 12 feet ft and 50 feet ft at normal pool elevation.

S2-4.doc 2.4-2 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

The Tennessee River above Chattanooga, Tennessee, is one of the best regulated rivers in the United States. A prime purpose of the TVA water control system is flood control with particular emphasis on protection for Chattanooga, 20 miles downstream from SQN.

There are 20 17 major reservoirs in the TVA system upstream from the plant, 13 of which have substantial reserved flood detention capacity during the main flood season. Table 2.4.1 1 lists pertinent data for TVA's major dams prior to modifications made by the Dam Safety Program (see Table 2.4.1 5).dams (South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, Blue Ridge, Hiwassee, Chatuge, Nottely, Apalachia and Watts Bar) in the TVA system upstream from SQN, 14 of which (those previously identified excluding Fort Patrick Henry, Melton Hill, and Apalachia) provide about 3.9 million acre-ft of reserved flood-detention (March 15) capacity during the main flood season. Table 2.4.1-2 lists pertinent data for TVA's dams and reservoirs. Figure 2.4.1-3 presents a simplified flow diagram for the Tennessee River system. Table 2.4.1-3 provides the relative distances in river miles of upstream dams to the SQN site.

Details for TVA dam outlet works are provided in Table 2.4.1-4. In addition, there are six four major dams owned by Brookfield Renewable Energy Partners (Calderwood, Chilhowee, Santeetlah, and Cheoah Dams) and two major dams owned by Duke Energy (Nantahala and Mission Dams)the Aluminum Company of America (ALCOA). The These ALCOA reservoirs often contribute to flood reduction but were ignored in this analysis because they do not have dependable reserved flood detention capacity. The locations of these dams and the minor dams, Nolichucky and Walters (Waterville Lake), are shown on Figure 2.1.1 1. Table 2.4.1 22.4.1-5 lists pertinent data for the major and minor ALCOAnon-TVA owned dams and reservoirsWalters Dam. The locations of these dams are shown on Figure 2.1.1-1.

The flood detention capacity reserved in the TVA system varies seasonally, with the greatest amounts during the flood season. Figure 2.4.1 3, containing 14 sheets, shows tributary and main river reservoir seasonal operating guides for those reservoirs having major influence on SQN flood flows. Table 2.4.1 3 shows the flood control reservations at the multiple purpose projects above SQN at the beginning and end of the winter flood season and in the summer. Assured system detention capacity above the plant varies from 5.6 inches on January 1 to 4.5 inches on March 15, decreasing to 1.0 inch during the summer and fall. Actual detention capacity may exceed these amounts, depending upon inflows and power demands.

Flood control above SQN is provided largely by 11 12 tributary reservoirs (Table 2.4.1-6). Tellico Dam is counted as a tributary reservoir because it is located on the Little Tennessee River, although, because of canal connection with Fort Loudoun Dam, it also functions as a main river dam. On March 15, near the end of the flood season, these provide a minimum of 4,436,0003,604,500 acre-feet of detention capacity, equivalent to 4.65.8 inches on the 14,708476 square-mile area they control. This is 90 92 percent of the total available above Chickamauga Reservoir. The two main river reservoirs, Fort Loudoun and Watts Bar, provide 490,0321,000 acre-feet, equivalent to 1.51.6 inches of detention capacity on the remaining area above the plant Chickamauga Reservoir.

The flood detention capacity reserved in the TVA system varies seasonally, with the greatest amounts during the January through March flood season. Figure 2.4.1-4 (16 sheets) shows the reservoir seasonal operating guides for reservoirs above the plant site. Table 2.4.1-6 shows the flood control reservations at the multiple-purpose projects above SQN at the beginning and end of the winter flood season and in the summer. Total assured system detention capacity above the Chickamauga Reservoir varies from approximately 5.0 inches on January 1 to approximately 4.0 inches on March 15 and decreasing to approximately 1.5 inches during the summer and fall. Actual detention capacity may exceed these amounts, depending upon inflows and power demands.

Chickamauga Dam, the elevation of which affects flood elevations at the plant, has a drainage area of 20,790 square miles, 3,480 square miles more than Watts Bar Dam. There are five major tributary dams (Chatuge, Nottely, Hiwassee, Apalachia, and Blue Ridge in the 3,480-square-mile intervening watershed, of which four have substantial reserved capacity (Apalachia excluded). On March 15, near the end of the flood season, these provide a minimum of 329,800 acre-ft equivalent to 5.2 inches on the 1,200-square-mile controlled area. Chickamauga Dam contains 258,300 acre-ft of detention capacity at median guide elevation 678 ft on March 15 equivalent to 2.1 inches on the remaining 2,280 square miles. Figure 2.4.1-4 (Sheet 1) shows the seasonal operating guide for Chickamauga.

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Elevation-storage relationships for the reservoirs above the site and Chickamauga Dam, downstream, are shown in Figure 2.4.1-5 (17 sheets).

Daily flow volumes at the plant, for all practical purposes, are represented by discharges from Chickamauga Dam with drainage area of 20,790 square miles, only 140 square miles more than at the plant. Momentary flows at the nuclear plant may vary considerably from daily averages, depending upon turbine operations at Watts Bar Dam upstream and Chickamauga Dam downstream. There may be periods of several hours when there are no releases from either or both Watts Bar and Chickamauga Dams. Rapid turbine shutdown at Chickamauga may sometimes cause periods of up streamreverse flow in Chickamauga Reservoir.

Based upon discharge records since closure of Chickamauga Dam in 1940, the average daily streamflow at the plant is 32,600 cfs. The maximum daily discharge was 223,200 cfs on May 8, 1984.

Except for two special operations on March 30 and 31, 1968, when discharge was zero to control milfoil, the minimum daily discharge was 700 cfs on November 1, 1953. Flow data for water years 1951-1972 indicate an average rate of about 27,600 cfs during the summer months (May-October) and about 38,500 cfs during the winter months (November-April). Flow durations based upon Chickamauga Dam discharge records for the period 1951-1972 are tabulated below.

Average Daily Percent of Time Discharge, cfs Equaled or Exceeded 5,000 99.6 10,000 97.7 15,000 93.3 20,000 84.0 25,000 69.3 30,000 46.8 35,000 31.7 Channel velocities at SQN average about 0.6 fps under normal winter conditions. Because of lower flows and higher reservoir elevations in the summer months, channel velocities average about 0.3 fps.

As listed on Table 2.4.1 42.4.1-1, there are 23 surface water users within the 98.6-mile reach of the Tennessee River between Dayton, TN and Stevenson, AL. These include fifteen industrial water supplies and eight public water supplies.

The industrial users exclusive of SQN withdraw about 497 500 million gallons per day from the Tennessee River. Most of this water is returned to the river after use with varying degrees of contamination.

The public surface water supply intake (Savannah Valley Utility District), originally located across Chickamauga Reservoir from the plant site at TRM 483.6, has been removed. Savannah Valley Utility District has been converted to a ground water supply. The nearest public downstream intake is the

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Groundwater resources in the immediate SQN site are described in Section 2.4.13Subsection 2.4.13.2.

2.4.1.3 TVA Dam Safety Program Most of the dams upstream from SQN were designed and built before the hydrometerological approach to spillway design had gained its current level of acceptance. Spillway design capacity was generally less than would be provided today. The original FSAR analyses were based on the existing dam system before dam safety modifications were made and included failure of some upstream dams from overtopping.

In 1982, TVA officially began a safety review of its dams. The TVA Dam Safety Program was S2-4.doc 2.4-4 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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The turbine, control, and auxiliary buildingTurbine, Control, and Auxiliary Buildings will be allowed to flood. All equipment required to maintain the plant safely during the flood, and for 100 days after the beginning of the flood, is either designed to operate submerged, is located above the DBF plus wind wave runup (external), DBF plus surge (internal) or is otherwise protected.

Wind wave run up during the PMF at the diesel generator building reaches elevation [CEII] which is

[CEII]

feet below the operating floor. Consequently, wind wave run up will not impair the safety function of systems in the diesel generator building.

The accesses and penetrations below this elevation in the diesel generator building are designed and constructed to minimize leakage into the buildings. Redundant sump pumps are provided within the building to remove minor leakage. Protective measures are taken to ensure that all safety related systems and equipment in the Emergency Raw Cooling Water (ERCW) pump station will remain functional when subjected to the maximum flood level.

Class IE electrical cables, located below the Probable Maximum Flood (PMF) plus wind wave activity and required in a flood, are designed for submerged operation.

2.4.2.3 Effects of Local Intense Precipitation Maximum water levels at buildings expected to result from the local plant PMP were determined using a transient flow (unsteady flow) model with hydraulically connected storage areas. Much of the plant site is flat, particularly at the switchyards, and a single flow path is not well defined. A transient model with interconnected storage areas very roughly approximates a two-dimensional model using one-dimensional methods by providing multiple simultaneous outlet paths for the exterior areas adjacent to plant buildings.

Structures housing safety-related facilities, systems, and equipment are protected from flooding during a local PMF by the slope of the plant yard. The yard is graded so that the surface runoff will be carried to Chickamauga Reservoir without exceeding the elevation of the external accesses given in Paragraph 2.4.1.1 except those at the intake pumping station whose pumps can operate submerged.

PMP for the plant drainage system and roofs of safety-related structures is defined in Topical Report TVA-NPG-AWA16-A, TVA Overall Basis Probable Maximum Precipitation and Local Intense Precipitation Analysis, CDQ0000002016000041, Revision 1 [3]. The probable maximum storm used to test the adequacy of the local drainage system would produce a maximum one-hour depth of 13.8 inches. Three different temporal distributions were applied to the model, with peak intensity shifted between early, middle, and late occurrence. No precipitation losses were applied. Runoff was made equal to rainfall.

The separate watershed subareas and flowpaths are shown on Figure 2.4.3-19.

The western plant site was evaluated as six interconnected storage areas with four primary weir-flow outlets and one connected transient flow stream-course model. Runoff from the western plant site will flow: Northwest to a channel along the main plant tracks then across the main access highway (Area 7); to the West through a parking lot (Areas 6A, 6C, and 6E connected to transient flow model);

Southwest through the vehicle barrier system directly to Chickamauga Lake (Area 6E); or South through the vehicle barrier system to the Yard Drainage and other Ponds (Area 6C). The maximum water surface elevations in Areas 6A and 6BS are below critical floor elevation 706 ft [54].

The eastern plant site was evaluated as three interconnected storage areas with three weir-flow outlets and two connected transient flow stream-course models. Runoff from the eastern plant site will flow: North around the West and East ends of the Multipurpose Building to the intake canal (Area 5 connected to two transient flow models); South to the Condenser Circulating Water Discharge Channel (Areas 4 and 6D); or Southwest into the western plant site (Area 6D into 6C). The maximum water surface elevations in Areas 4, 5, and 6D are below critical flood elevation 706 ft [54].

Underground drains were assumed clogged throughout the storm. For fence sections, the Mannings S2-4.doc 2.4-8 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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PMP values are calculated using a Basin PMP Evaluation Tool provided in Topical Report TVA-NPG-AWA16-A and described in Topical Report Sections 5.6 and 5.7. The PMP Evaluation Tool applies moisture transposition, in-place maximization and orographic transposition adjustment factors to an analyzed storm depth-area-duration value for the area size and duration of interest to yield an adjusted rainfall value. The analyzed storms adjusted rainfall value is then compared with the adjusted rainfall values of each storm in Topical Report TVA-NPG-AWA16-A storm database that is transpositionable to the target grid point. The maximum adjusted rainfall value determined in this comparison is the unique PMP for that grid point location. This process is repeated for each grid point on the gridded network to determine the final point PMP values at each grid point location for a given storm duration. These point PMP depths represent a worst-case estimated rainfall for the historically largest observed storm events transposable to a given grid point for a defined area of interest and duration.

The areal application of point PMP values is determined using an event nesting and residual rainfall methodology [16]. Nesting is the occurrence of PMP events over small areas or sub-basins during a larger area PMP event. Nesting methodology ensures that the total average PMP rainfall depth of a larger sub-basin area which includes the average PMP rainfall depth of a smaller sub-basin area of interest is consistent with the average PMP rainfall depth developed from the Topical Report for the larger sub-basin area.

The nesting methodology [16] creates potential controlling storm events using a multi-step process.

First, gridded point PMP values over a primary area defined as the area of interest for a rainfall duration are determined using Topical Report TVA-NPG-AWA16-A and the PMP Evaluation Tool. A primary area may be a single sub-basin that is the watershed for a single dam project or may be multiple sub-basins above a dam project. The average PMP rainfall depth over the primary area for each rainfall duration is calculated by applying geographic information system (GIS) functionalities to create a PMP depth surface over the primary area. Next, residual rainfall in sub-basin areas above Wheeler Dam and outside the primary area is determined. As shown in Figure 2.4.3-5a, the drainage area above Wheeler Dam consists of 61 sub-basins. The Wheeler Dam project is the downstream boundary for the watershed potentially affecting TVA nuclear sites. Within the large multi-basin Wheeler dam drainage area, there are multiple possible sub-basin areas (secondary areas of interest) encompassing or nesting the primary area of interest. Each possible sub-basin area has an average PMP rainfall depth unique to that specific sub-basin area and rainfall duration. The nesting methodology ensures that the average PMP rainfall depth over a selected secondary sub-basin area (including the nested primary area of interest) is consistent with the average PMP rainfall depth developed from Topical Report TVA-NPG-AWA16-A by reducing the average PMP rainfall depth in the secondary sub-basin area outside of the primary area of interest. The nesting approach begins with the sub-basin areas at the upper boundary of the total basin drainage area encompassing the primary area and extends incrementally to downstream dam projects until the lower drainage boundary at Wheeler dam is reached. The final secondary area of interest is the total watershed above Wheeler dam. Since there are multiple combinations of larger sub-basin areas encompassing the primary area of interest, there are multiple potentially controlling PMP storm events for the SQN site.

Controlling PMP depths for 21,400 square mile and 7,980 square mile areas are tabulated below.

These storms would occur in March. Depths for other months would be less.

Depth, Inches 72 Hour Main Storm Sq. Miles Antecedent Storm 6 Hour 24 Hour 72 Hour 21,400 6.7 5.03 11.18 16.78 7,980 8.1 7.02 14.04 20.36 Two possible isohyetal patterns producing the total area depths are presented in Report No. 41. The one critical to this study is the "downstream pattern" shown in Figure 2.4.3 1. The isohyetal pattern for the 7,980 square mile storm is shown in Figure 2.4.3 2. The pattern is not orographically fixed and can be moved parallel to the long axis northeast and southwest along the Valley.

A 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> storm three days antecedent to the main storm was assumed to occur in all PMP situations with storm depths equivalent to 40 percent of the main storm.All PMP storms are nine-day events. A S2-4.doc 2.4-10 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

three-day antecedent storm was postulated to occur three days prior to the three-day PMP storm in all PMF determinations. Variations of the temporal rainfall distribution within the 3-day PMP storm were considered in the controlling storm determination. As recommended in Hydrometeorological Report No. 41, the antecedent rainfall of 40 percent of the main storm depth was conservatively applied with the spatial distribution of the antecedent coincident with that of the main storm PMP.

Potential storm amounts differing by seasons were analyzed in sufficient number to make certain that the March storms would be controlling. Enough centerings were investigated to assure that a most critical position was used.

Storms producing PMP above upstream tributary dams, whose failure has the potential to create maximum flood levels, were evaluated in the original FSAR analysis. Dam safety modifications at upstream tributary dams have eliminated these potential failures and subsequent plant site flood levels.

A standard time distribution pattern was adopted for all storms based upon major observed storms transposable to the Tennessee Valley and in conformance with the usual practice of Federal agencies.

The adopted distribution is shown on Figure 2.4.3 3.

The critical probable maximum storm was determined to be a total basin storm with downstream orographically fixed pattern (Figure 2.4.3 1) which would follow an antecedent storm commencing on March 15. Translation of the PMP from Report No. 41 to the basin results in an antecedent storm producing an average precipitation of 6.4 inches in three days, followed by a three day dry period, and then by the main storm producing an average precipitation of 16.5 inches in three days. Figure 2.4.3 4 is an isohyetal map of the maximum three day PMP. Basin rainfall depths are given in Table 2.4.3 1.

To evaluate the local plant drainage system for a PMP event, Topical Report TVA-NPG-AWA16-AHydrometeorological Report No. 56 was used to calculate a 1-hr storm totaling 16.2113.81 inches.

Three different temporal distributions were applied to the model with peak intensity of 2.81 39 inches/5-min = 33.72 in/hr shifted between early, middle, and late occurrence. Depths for each 5-minute increment of the controlling late peak distribution were 0.5868, 0.6678, 0.8775, 0.9782, 0.9783, 0.991.17, 1.081.26, 1.161.36, 1.321.55, 2.392.81, 1.742.04, and 1.491.75. Rainfall on the plant building roofs was assumed to discharge to the ground surface.

To determine the controlling storm event for each primary area of interest, multiple possible storm events are postulated for each primary area of interest based on the selection of multiple secondary areas of interest [16]. These potentially controlling storm events are then either simulated using the stream course model described in Section 2.4.3.3 to determine the PMF elevation at the SQN site or screened out as not a potentially controlling simulations. The model simulation resulting in the highest the PMF elevation at the SQN site is the controlling PMP storm event at the SQN site.

The controlling PMP for the SQN site resulted from consideration of drainage area above the Blue Ridge Dam project as the primary PMP watershed nested with multiple secondary area PMPs over the Hiwassee-Blue Ride, Fontana-Hiwassee-Blue Ridge, Tellico-Hiwassee-Blue Ridge, Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas, and Fort Loudoun-Tellico-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above Chickamauga, Nickajack, Guntersville, and Wheeler dam projects [55]. The PMP rainfall depths over the secondary watershed areas outside the primary watershed area were reduced to maintain the average PMP rainfall depth over the secondary watershed area. Figure 2.4.3-5a and Figure 2.4.3-1 provide a graphic representation of the controlling primary and secondary area nesting sequence and the controlling PMP rainfall depth spatial distribution over the watershed. Table 2.4.3-1 provides a graphic representation of the controlling storm rainfall depths for duration of the main and antecedent storms.

2.4.3.2 Precipitation Losses Precipitation losses in the probable maximum storm are estimated with multivariable relationships used in the day to day operation of the TVA system. These relationships, developed from a study of S2-4.doc 2.4-11 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

storm and flood records, relate the amount of precipitation excess (and hence the precipitation loss) to the week of the year, an antecedent precipitation index (API), and geographic location. The relationships are such that the loss subtraction from rainfall to compute precipitation excess is greatest at the start of the storm and decreases to no subtraction when the storm rainfall totals from 7 to 16 inches. Precipitation losses become zero in the late part of extreme storms.

For this probable maximum flood analysis, median moisture conditions as determined from past records were used to determine the API at the start of the storm sequence. The antecedent storm is so large, however, that the precipitation excess computed for the later main storm is not sensitive to variations in adopted initial moisture conditions. The precipitation loss in the critical probable maximum storm totals 4.13 inches, 2.30 inches in the antecedent storm amounting to 36 percent of the 3 day 6.44 inch rainfall, and 1.83 inches in the main storm amounting to 11 percent of the 3 day, 16.46 inch rainfall. Table 2.4.3 1 displays the API, rain, and precipitation excess for each of the 45 subwatersheds of the hydrologic model for the SQN probable maximum flood.

No precipitation loss was applied in the probable maximum storm on the local area used to test the adequacy of the site drainage system and roofs of safety related structures. Runoff was made equal to rainfall.A multi-variable relationship, used in the day-to-day operation of the TVA reservoir system, has been applied to determine precipitation excess directly. The relationships were developed from observed storm and flood data. They relate precipitation excess to the rainfall, week of the year, geographic location, and antecedent precipitation index (API). In their application, precipitation excess becomes an increasing fraction of rainfall as the storm progresses in time and becomes equal to rainfall in the later part of extreme storms. An API determined from an 18-year period of historical rainfall records (1997-2015) was used at the start of the antecedent storm. The precipitation excess computed for the main storm is not sensitive to variations in adopted initial moisture conditions because of the large antecedent storm.

Basin rainfall and precipitation excess for the PMP storm event controlling the PMF elevation at the SQN site are provided in Table 2.4.3-1. The average precipitation loss for the 20,790 sq mi watershed above Chickamauga Dam is 2.68 inches for the three-day antecedent storm and 2.46 inches for the three-day main storm. The losses are approximately 56 percent of antecedent rainfall and 20 percent of the PMP, respectively. The precipitation loss of 2.68 inches in the antecedent storm compares favorably with that of historical flood events shown in Table 2.4.3-3.

2.4.3.3 Runoff Model The runoff model used to determine Tennessee River flood hydrographs at SQN is divided into 45 61 unit areas and includes the total watershed above Wheeler Dam. Unit hydrographs are used to compute flows from these areas. The watershed unit areas above Chickamauga Dam are shown in Figure 2.4.3-5. The unit area flows are combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream tributary reservoirs, which in turn are routed through the reservoirs, using standard routing techniques. Resulting outflows are combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures, including unsteady flow routing. Figure 2.4.3 5 shows unit areas of the watershed upstream from SQN.

The runoff model used in this updated FSAR differs from that used previously because of refinements made in some elements of the model during PMF studies for other nuclear plants and those made from information gained from the 1973 flood, the largest that has occurred during present reservoir conditions.

Changes are identified when appropriate in the text. They include both additional and revised unit hydrographs and additional and revised unsteady flow stream course models.

Unit hydrographs were developed for each unit area for which discharge records were available from maximum flood hydrographs either recorded at stream gauging gaging stations or estimated from reservoir headwater elevation, inflow, and discharge data using the procedures described by Newton and Vineyard [23]. For non-gaged unit areas unit graphs were developed from relationships of unit hydrographs from similar watersheds relating the unit hydrograph peak flow to the drainage area size, time to peak in terms of watershed slope and length, and the shape to the unit hydrograph peak S2-4.doc 2.4-12 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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2,100 foot long canal.

The unsteady flow routing model for the 72.4 mile long Watts Bar Reservoir was divided into thirty four 2.13 mile reaches. The model was verified at two gauged points within the reservoir using 1963 flood data.

The unsteady flow mathematical model for the total 58.9 mile long Chickamauga Reservoir was divided into twenty eight 2.1 mile reaches providing twenty nine equally spaced grid points. The grid point at mile 483.62 is nearest to the plant, mile 484.5. The unsteady flow model was verified at four gauged points within Chickamauga Reservoir using 1973 flood data. This differs from the previous submission in that the 1973 flood was added for verification, replacing the 1963 flood. The 1973 flood occurred during preparation of the FSAR and therefore, was not available for verification. The 1973 flood is the largest which has occurred since closure of South Holston Dam in 1950. Comparisons between observed and computed stages in Chickamauga Reservoir are shown in Figure 2.4.3 7.

It is impossible to verify the models with actual data approaching the magnitude of the probable maximum flood. The best remaining alternative was to compare the model elevations in a state of steady flow with elevations computed by the standard step method. This was done for steady flows ranging up to 1,500,000 cfs. An example shown by the rating curve of Figure 2.4.3 8 shows the good agreement.

The watershed runoff model was verified by using it to reproduce the March 1963 and March 1973 floods; the largest recorded since closure of South Holston Dam. This differs from the previous submission in that the 1973 flood was added for verification, replacing the 1957 flood. Observed volumes of precipitation excess were used in verification. Comparisons between observed and computed outflows from Watts Bar and Chickamauga Dams for the 1973 and 1963 floods are shown in Figures 2.4.3 9 and 2.4.3 10, respectively.

From a study of the basic units of the predicting system and its response to alterations in various basic elements, it is concluded that the model serves adequately and conservatively to determine maximum flood levels.

The USACE Hydrologic Engineering Center River Analysis System software (HEC-RAS) [24] performs one-dimensional steady and unsteady flow calculations. The HEC-RAS models are used in flood routing calculations for reservoirs in the Tennessee River System upstream of Wilson Dam to predict flood elevations and discharges for floods of varying magnitudes. Model inputs include previously calibrated geometry, unsteady flow rules, and inflows. Model calibration ensures accurate replication of observed river discharges and elevations for known historic events. Once calibrated, the model can be used to reliably predict flood elevations and discharges for events of varying magnitudes.

The TVA total watershed HEC-RAS model extends along the Tennessee River from Wilson Dam upstream to its source at the confluence of the Holston and French Broad Rivers, along the Elk River from its mouth at the Tennessee River to Tims Ford Dam, along the Hiwassee from its mouth at the Tennessee River to Chatuge Dam, along the Nottely River from its mouth at the Hiwassee River to Nottely Dam, along the Ocoee River from its mouth at the Hiwassee River to Blue Ridge Dam, along the Clinch River from its mouth at the Tennessee River to a gage at RM 159.8, along the Powell River from its mouth at the Clinch River to a gage at RM 65.4, along the Little Tennessee River from its mouth at the Tennessee River to a gage at RM 92.9, along the Tuckasegee River from its mouth at the Little Tennessee River to a gage at RM 12.6, along the Holston River from its mouth at the Tennessee River to its source at the confluence of the South Fork Holston River and the North Fork Holston River, along the South Fork Holston River from its mouth at the Holston River to South Holston Dam, along the Watauga River from its mouth at the South Fork Holston River to Watauga Dam, along the French Broad River from its mouth at the Tennessee River to a gage at RM 77.5, along the Nolichucky River from its mouth at the French Broad River to a gage at RM 10.3, along Cove Creek from its mouth at the Clinch River to RM 12.2, along Big Creek from its mouth at the Clinch River to RM 11.8, and along North Chickamauga Creek from its mouth at the Tennessee River to RM 12.82. The model also incorporates the Dallas Bay / Lick Branch rim leak and the Fort Loudoun canal by modeling these reaches. Figure 2.4.3-4 (2 sheets) shows the extent of the model, as well as the location of dams.

S2-4.doc 2.4-14 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Using the inflows, multiple flood-routing simulations were performed for a PMP storm event over various dam project primary areas above Chickamauga dam combined with PMP events on larger secondary sub-basin areas above Wheeler Dam including the primary area PMP storm event. Storm-specific decisions, such as if dam failure assumption is necessary, were required to perform the simulations and are documented for each storm simulation. In general, a specific storm simulation was performed and the headwater/tailwater/discharge results were reviewed at each dam. If it was determined that a dam should be failed in the simulation, the model was modified to allow dam failure at a specified headwater and the model was re-run. The new results were analyzed and new failures simulated in an iterative fashion until the composite flood-wave was routed downstream to the SQN site and Chickamauga dam. Checking tools were used to verify the headwater/tailwater/discharge relationship predicted by HEC-RAS at each dam agreed with approved dam rating curves (DRC). DRCs are provided in Figure 2.4.3-7 (17 sheets). Volume checks were performed as well to ensure that volume was preserved in the model simulation. Unsteady flow rules have been developed for the main Tennessee River and its tributaries and have been incorporated into the verified HEC-RAS unsteady flow model. Elevation and discharge hydrographs for the PMP storm event producing the highest water elevation at the SQN site are presented in Figure 2.4.3-18. Hydrographs for dams in the controlling PMF simulation are provided in Figure 2.4.3-25 (27 sheets). A summary of the results at the dams for the PMF is provided in Table 2.4.3-4 (2 sheets). 2.4.3.3.2 Model Setup The TVA total watershed HEC-RAS model extends along the Tennessee River from Wilson Dam upstream to its source at the confluence of the Holston and French Broad Rivers, along the Elk River from its mouth at the Tennessee River to Tims Ford Dam, along the Hiwassee from its mouth at the Tennessee River to Chatuge Dam, along the Nottely River from its mouth at the Hiwassee River to Nottely Dam, along the Ocoee River from its mouth at the Hiwassee River to Blue Ridge Dam, along the Clinch River from its mouth at the Tennessee River to a gage at RM 159.8, along the Powell River from its mouth at the Clinch River to a gage at RM 65.4, along the Little Tennessee River from its mouth at the Tennessee River to a gage at RM 92.9, along the Tuckasegee River from its mouth at the Little Tennessee River to a gage at RM 12.6, along the Holston River from its mouth at the Tennessee River to its source at the confluence of the South Fork Holston River and the North Fork Holston River, along the South Fork Holston River from its mouth at the Holston River to South Holston Dam, along the Watauga River from its mouth at the South Fork Holston River to Watauga Dam, along the French Broad River from its mouth at the Tennessee River to a gage at RM 77.5, and along the Nolichucky River from its mouth at the French Broad River to a gage at RM 10.3, along Cove Creek from its mouth at the Clinch River to RM 12.2, along Big Creek from its mouth at the Clinch River to RM 11.8, and along North Chickamauga Creek from its mouth at the Tennessee River to RM 12.82. The model also incorporates the Dallas Bay / Lick Branch rim leak and the Fort Loudoun canal by modeling these reaches. Figure 2.4.3-4 shows the extent of the model. HEC-RAS models developed for the individual reservoirs had to be connected into a composite model in order to perform a continuous simulation of the Tennessee River system from TVAs uppermost tributary reservoirs downstream to Wilson Dam. The calibrated geometry for each reservoir was imported into the composite geometry file within HEC-RAS. HEC-RAS Inline Structures were added to model the dams and utilized data presented in DRC calculations [31] and tributary unsteady flow rules [32]. When an Inline Structure is used to model a dam, a headwater cross-section is located upstream and the tailwater section downstream of the dam. Reach lengths are modified to account for adjustments at the dam river station. HEC-RAS Lateral Structures are used at Apalachia, Chatuge, Douglas, Nottely, Ocoee No. 2, Ocoee No.3, and South Holston, Tellico and Watts Bar Dams, to model saddle dams and turbine discharges. Additionally, lateral structures are used on the Holston River, South Fork Holston River, French Broad River, and Nolichucky River to connect storage areas to the rivers. After compiling the separate river geometry files into a composite model, the overall geometry file requires additional modifications before it is adequate for use. These modifications include the addition of junctions and inline structures, copying or interpolating additional cross-sections to allow for the application of inflows, addition of junctions or to enhance model stability, and the addition of pilot channels. If a cross-section is copied or interpolated, the reach lengths associated with the new section are adjusted. S2-4.doc 2.4-16 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The reservoir operating guides applied during the model simulations mimic, to the extent possible, operating policies and are within the current reservoir operating flexibility. In addition to spillway discharge, turbine and sluice discharges were used to release water from the tributary reservoirs. Turbine discharges were also used at the main river reservoirs up to the point where the head differentials are too small and/or the powerhouse would flood. All discharge outlets (spillway gates, sluice gates, and valves) for projects in the reservoir system will remain operable without failure up to the point the operating deck is flooded for the passage of water when and as needed during the flood. A high confidence that all gates/outlets will be operable is provided by periodic inspections by TVA plant personnel, the intermediate and five-year dam safety engineering inspections consistent with Federal Guidelines for Dam Safety, and the significant capability of the emergency response teams to direct and manage resources to address issues potentially impacting gate/outlet functionality. The unsteady flow rules incorporate the seasonal Flood Operational Guides [34] adjusted to weekly values [60], as they provide operating ranges of reservoir levels for the 32 modeled reservoirs upstream of Wilson Dam. The rules reflect the flexibility provided in the guides to respond to unusual or extreme circumstances, such as the PMF event, through the use of primary guide and recovery curves. If the maximum discharge of the primary guide or recovery curve is exceeded, the discharges are from the DRCs [31]. The DRCs account for flow over other components such as non-overflow sections, navigation locks, tops of open spillway gates, tops of spillway piers, saddle dams, and rim leaks. Therefore, the DRCs and the flood operational guides define the dam discharge as a function of headwater elevation, tailwater elevation, and outlet configuration. If, during the event, the headwater elevation does not exceed the elevation of the operating deck, discharges are determined in accordance with the flood operational guides during the flood recession. In the event the operating deck is inundated, the dam rating curves determine the discharge during flood recession. There are configuration parameters in each set of rules that are simulation specific. Model configuration parameters including failure elevation, gate position, operational allowances, armoring embankments, failure timing, and seismic triggers are initially set with input from the modeler. The HEC-RAS model is set-up to run all modeled rivers and reservoirs as a contiguous system to be run continuously. The model cannot be started and stopped in the middle of a simulation; however, some scenarios will require iterative simulations to determine necessary configuration parameters. Inflows were distributed for use in the composite HEC-RAS model of the Tennessee River System upstream of Wilson Dam. Inflow hydrographs presented in the inflow calculation [33] were used as an input to the composite HEC-RAS model. The hydrographs provide inflow data for individual basins in the Tennessee River System. 2.4.3.3.3 Main Stem Geometry The validated geometry for each reservoir was calibrated for use in the unsteady flow model. This validated geometry consists of Fort Loudoun, Tellico, Melton Hill, Watts Bar, Chickamauga, Nickajack, Guntersville, and Wheeler Reservoirs. Wilson Reservoir geometry, although a part of the main stem, was provided by TVA River Management and verified in the same manner as the tributary geometry. Cross-section data was obtained from the geometry verification calculations [36-43] and used to develop the HEC-RAS geometry. Cross-section data obtained from the geometry verification calculations were generally spaced about two miles apart on the main stem. Generally, constricted channel locations were selected for cross-section locations. These smaller, constricted sections do not accurately represent the reach storage available (the storage capacity between cross sections) in an unsteady flow model. Therefore, a mathematical augmentation of selected cross sections with off-channel ineffective flow areas was performed, so the constricted geometry could accurately account for the additional reach storage available. To account for total reach storage, the reach storage contained between the constricted cross-sections was compared to the total reservoir volume information, if available. Overbank volumes were computed using the average overbank reach length rather than as separate left and right overbank reach volumes to coincide with the internal HEC-RAS computations. If reservoir storage information was not available, such as at higher elevations of steep reaches, GIS obtained volumes were used for comparison to the model reach storage capacities. The reach storage between cross-sections was evaluated at incremental elevations. Reach storage was adjusted until the desired total cumulative storage was reached. Where additional reach storage was required, an additional ineffective flow area was added. A check of reach volume for the entire S2-4.doc 2.4-17 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 reservoir is also performed to verify that model volume is representative of the published actual reservoir volume [44]. 2.4.3.3.4 Tributary Geometry The tributary geometry has been developed for use in the HEC-RAS models. The tributary geometry was developed in one of three manners [35]:

1. TVA River Management developed the geometry. The geometry was verified in accordance with 10 CFR 50 Appendix B Quality Assurance requirements for use in safety related applications. The tributary geometry developed by River Management included the following:

Apalachia Reservoir, Ocoee River, Toccoa River, Blue Ridge Reservoir, Boone Reservoir, Watauga River, Wilbur Reservoir, South Fork Holston River, Little Tennessee River, Fort Patrick Henry Reservoir, Hiwassee River and Reservoir, Nottely River, and the Elk River.

2. If no geometry previously existed, the geometry was generated and verified for nuclear application. The tributaries that required geometry generation and verification are: Fontana Reservoir, Tuckasegee River, Norris Reservoir, Powell River, Big Creek, and Cove Creek.

3. The TVA River Management developed geometry for the Holston River and the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dams as re-generated and verified to more accurately determine the reservoir storage at these locations.

Verification of River Management Developed Geometry The verification of the tributary geometry previously developed by TVA River Management included verification of the location and orientation of each section, the Mannings n values, the cross-section shape with respect to historic channel geometry, the underwater portion of the section, and storage volume between sections. The location of each cross-section provided by River Management and its orientation were examined. Adjustments were made to the cross-sections and additional cross-sections were added if required to better represent the river. The River Management provided cross-sections were compared to geographic information system (GIS) generated cross-sections above the water surface and historical channel geometry below the water surface elevation. The revised cross-sections were plotted with historic channel geometry cross-sections and the width at the water surface of the new cross-section was compared to and verified against the historic cross-sections. The composite GIS/historic channel geometry cross-sections were then compared to those developed by River Management. When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if necessary. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as to confirm reach lengths. Augmented ineffective flow areas were updated after overbank volumes were computed using average overbank reach lengths. Generation and Verification of New Geometries Development of the HEC-RAS geometry for Fontana Reservoir, Tuckasegee River, Norris Reservoir, Powell River, Big Creek, and Cove Creek were developed by extracting cross-sections from a GIS TIN and comparing the cross-sections to historic cross-sections. Available stream centerline and elevation data were compiled in GIS. USGS topographic maps were examined to identify desired cross-section locations. Once the cross-section locations were established, generic Mannings n values were added in the HEC-RAS geometry. The revised cross-sections were plotted with historic channel geometry cross-sections. The width at the water surface of the new cross-section was compared to and verified against the historic cross-sections. S2-4.doc 2.4-18 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if the data were not representative of the cross-section. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as confirm reach lengths. Once the cross-sections were developed and/or verified, a reach storage augmentation procedure was performed so the model storage accurately reflects the actual reach storage capacities. For more information on the reach storage augmentation procedure for tributaries other than Holston River, and the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dams, see Section 2.4.3.3.3. Re-Generation and Verification of Holston, French Broad, Nolichucky Geometries Re-Generation and verification of the geometries of the Holston River and the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dam began with the tributary geometry previously developed by TVA River Management. To account for total reach storage in the Holston River, the downstream portion of the South Fork Holston River, the French Broad River and the Nolichucky River, additional cross-sections and storage areas, connected by lateral structures, were added as necessary to match published reservoir or GIS based reach storage. Cross-sections were added to achieve approximate spacing of 1000 ft and to achieve accurate approximation of volume in the reach such as small embayments or just upstream and downstream of an embayment. Storage areas were used where there were larger embayments not easily captured with cross-sections. New cross-sections were generated by extracting a cross-section from HEC-GeoRAS and then re-projecting the cross-section in ArcGIS and adjusting the reach lengths. HEC-RAS was then used to create interpolated sections at the new identified locations. The underwater portions of the new cross-sections were developed using bathymetric data where available or interpolated cross-section data. Because of the large number of new cross-sections and the sinuosity of the main river channel, it was necessary to block off parts of the overbank area for several cross-sections to avoid overestimating overbank reach volume. To represent large embayments, storage areas were inserted with an elevation/storage curve that equates to the additional storage needed for the entire reach. The storage area was connected with a lateral structure. The weir crest of the lateral structure was defined by the bathymetry of the bordering overbank extensions of the adjacent cross-sections. When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if the data were not representative of the cross-section. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as confirm reach lengths. Once the cross-sections and storage areas were developed, storage volumes were adjusted iteratively beginning at the lowest elevation and working toward the highest elevation to retain the integrity of the overall volume of the system. 2.4.3.3.5 Calibration Model calibration is performed to adjust model parameters so that the model will accurately predict the outcome of a known historic event. In the case of the HEC-RAS models, the model results must accurately replicate observed elevations and discharges for known historic flood events. A calibrated model is therefore considered reliable at predicting the outcome of events of other magnitudes. 2.4.3.3.5.1 Main Stem River The main river model uses the USACE HEC-RAS software. The main river model extends from Wilson Dam upstream to Norris, Cherokee, Douglas, and Chilhowee Dams, and the Charleston Gage at River Mile (RM) 18.9 on the Hiwassee River. The nine reservoirs upstream of Wilson Dam (Wilson, Wheeler, Guntersville, Nickajack, Chickamauga, Watts Bar, Tellico, Fort Loudoun, and Melton Hill) were individually calibrated for use to reliably predict flood elevations and discharges for events of S2-4.doc 2.4-19 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 varying magnitudes. The reservoirs that impact PMF elevations at the SQN site are: Chickamauga, Watts Bar, Tellico, Fort Loudoun and Melton Hill. Initial unsteady-flow runs are conducted to replicate the historic flood events. Initial unsteady-flow runs are conducted for each individual reservoirs model. The initial runs used channel roughness (Mannings n) values from the previous calibrated models in an attempt to replicate the historic flood events. Following the initial runs, roughness values for each of the model segments were evaluated and adjusted as needed. The model was rerun and the results were again compared to the observed elevations at the gage stations. The process was repeated in an iterative fashion until good agreement was reached between the HEC-RAS computed elevations and the observed gage elevations. Adjustments to the roughness values in the HEC-RAS models were kept within a reasonable range for the ground coverage in the vicinity of the cross section. In general, the computed peak elevations are within one foot, but not below, the observed gage elevations. In some cases, the computed elevations are more than 1 foot above the observed gage elevations; however this was necessary to avoid impacts to the computed peak elevations at other gage locations. A schematic of the model for Watts Bar Reservoir is shown in Figure 2.4.3-11. The calibration results of the March 1973 flood is shown in Figure 2.4.3-12 (2 Sheets) and the calibration results of the May 2003 flood is shown in Figure 2.4.3-13 (2 Sheets). A schematic of the unsteady flow model for Chickamauga Reservoir is shown in Figure 2.4.3-14. The calibration results of the March 1973 flood is shown in Figure 2.4.3-15 (3 Sheets) and the calibration results of the May 2003 flood is shown in Figure 2.4.3-16 (3 Sheets). The configuration for the Fort Loudoun-Tellico complex is shown by the schematic in Figure 2.4.3-8. The Fort Loudoun-Tellico complex was verified by two different methods as follows:

1. Using the available data for the March 1973 flood on Fort Loudoun Reservoir and for the French Broad and Holston rivers. The verification of the 1973 flood is shown in Figure 2.4.3-9 (4 Sheets).

Because there were limited data to verify against on the French Broad and Holston Rivers, the steady state HEC-RAS model was used to replicate the Federal Emergency Management Agency (FEMA) published 100- and 500-year profiles. Tellico Dam was not closed until 1979, thus was not in place during the 1973 flood for verification.

2. Using available data for the May 2003 flood for the Fort Loudoun-Tellico complex. The verification of the May 2003 flood is shown in Figure 2.4.3-10 (5 Sheets). The Tellico Reservoir steady state HEC-RAS model was also used to replicate the FEMA published 100- and 500-year profiles.

In addition to roughness adjustments, the calibration sequence is used to verify that an adequate time step and appropriate mixed flow parameters are selected. To verify the time step, a series of simulations were conducted using PMF flows and varying time steps. The results indicated that a time step of five minutes provides for a stable simulation and the results are comparable with shorter time steps. Above five minutes, there is more variation in the results. The mixed flow regime option is used in the HEC-RAS models because the topographic relief, dam failures, and high flows evaluated for the PMF could produce supercritical flow or hydraulic jumps. Higher values of the mixed flow regime parameters produce more accurate results, but if too high can cause model instability. A comparison of water surface errors between simulations with varying parameters is used to verify appropriate values for the parameters are selected. Once each reservoirs model was adequately calibrated, they were combined into a composite model of the entire main stem for use in a continuous run simulation. This calibration process provided model results that satisfactorily reproduced the two historic floods (1973 and 2003). The HEC-RAS unsteady flow model accurately replicated observed gage elevations and discharges for two large historic flood events. Therefore, the HEC-RAS unsteady flow model of main stem reservoirs upstream of Wilson Dam can be used to reliably predict flood elevations and discharges for events of other magnitudes and is adequate for use in predicting flood elevations and discharges for the PMF. S2-4.doc 2.4-20 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.3.3.5.2 Tributary Calibration Tributaries were calibrated using a combination of steady-state and unsteady simulations. Steady-state calibration was to Federal Emergency Management Agency (FEMA) 100-year and 500-year flood profiles or, if not available, project manuals. Unsteady calibration, at a minimum, utilized the worst two historical storms experienced on each tributary, as tabulated below: Tributary Calibration - Largest Recorded Storms Hiwassee River between Hiwassee and March 1994 and April 1998 Apalachia Dams Ocoee River and Toccoa River from Ocoee 1 to April 1998, May 2003, and September 2004 Blue Ridge Dam Boone Reservoir March 2002 and November 2003 Wilbur Reservoir March 2002 and November 2003 Cherokee Reservoir, Holston and South Fork March 2002 and February 2003 Holston Rivers French Broad River and Nolichucky River May 2003 and September 2004 Little Tennessee River and Tuckasegee River May 2003 and September 2004 Fort Patrick Henry Reservoir March 2002 and November 2003 Hiwassee River and Nottely River May 2003 and December 2004 Hiwassee River below Apalachia Dam and May 2003 and September 2004 Ocoee River below Ocoee #1 Dam Clinch River above Norris Dam March 2002 and February 2003 Elk River, Subbasin 1 March 2002 and February 2004 Elk River, Subbasins 2 and 3 February 2004 and January 2006 Elk River, Subbasins 4 and 5 March 1973 and December 2004 Initial tributary geometry segments were obtained from the HEC-RAS Tributary Geometry Development calculation [35]. The required local inflows and associated distribution for unsteady flow modeling were determined from the HEC-RAS Model Calibration and Model Set-up calculations [30, 45]. In most cases, tributary segments were calibrated to FEMA 100-year and the 500-year flood profiles. In some cases, flood profiles were available in published flood insurance studies: in others the profiles were reproduced by running HEC-RAS or HEC-2 files from various TVA studies (e.g., reservoir sedimentation studies, floodplain models, and FEMA flood studies). Some tributary segments only had one FEMA profile available. Some did not have any profiles, in those cases other steady-state profile data were used such as those provided in project manuals. Roughness (Mannings n) values were adjusted iteratively until the steady-state computed profiles were in good agreement with the FEMA or project manual profiles. Following the steady-state calibration procedure, unsteady calibration simulations were performed on the tributary models, similar to the main stem calibration process. Observed historic flood event data were obtained from various available sources such as unit hydrograph calculations or gage data. Results of the unsteady flow simulations were compared to the observed elevation and discharge hydrographs. If the computed results were in good agreement with the observed hydrographs, the calibration was considered complete. In some cases, Mannings n values required further adjustment after comparison of unsteady-flow results. In those cases, the steady-state profiles were rerun to verify agreement with FEMA profiles. This calibration process provided model results that, through the combination of reach storage, unit hydrograph runoff, and inflow distribution, satisfactorily reproduced historic floods and available steady state profiles (FEMA or project manual flood profiles) for the tributary reaches. The HEC-RAS S2-4.doc 2.4-21 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The separate watershed subareas and flowpaths are shown on Figure 2.4.3 13a. The western plant site was evaluated as six interconnected storage areas with four primary weir flow outlets and one connected transient flow stream course model. Runoff from the western plant site will flow: Northwest to a channel along the main plant tracks and then across the main access highway (Area 7); to the West through a parking lot (Areas 6A, 6C, and 6E connected to transient flow model); southwest through the vehicle barrier system directly to Chickamauga Lake (Area 6E); or South through the vehicle barrier system to the Yard Drainage and other Ponds (Area 6C). The maximum water surface elevations in Areas 6A and 6BS are below critical floor elevation 706. The eastern plant site was evaluated as three interconnected storage areas with three weir flow outlets and two connected transient flow stream course models. Runoff from the eastern plant site will flow: North around the West and East ends of the multipurpose building to the intake channel (Area 5connect to two transient flow models); South to the Condenser Circulating Water Discharge Channel (Areas 4 and 6d); or Southwest into the western plant site (Area 6D into 6C). The maximum water surface elevations in Areas 4, 5, and 6D are below critical floor elevation 706. Underground drains were assumed clogged throughout the storm. For fence sections, the Mannings n value was doubled to account for increased resistance to flow and the potential for debris blockage. The only stream adjacent to SQN is the Tennessee River. There are no streams within the site. The 1 percent chance floodplain of the Tennessee River at the site is delineated on Figure 2.4.3 14. Details of the analyses used in the computation of the 1 percent chance flood flow and water elevation are described in a study made by TVA for the Federal Insurance Administration (FIA) and published in February 1979 [5]. The only structures located in the 1 percent chance floodplain are transmission towers, the intake pumping station skimmer wall, and the ERCW pump station deck. The ERCW pumps are located on the pump station deck at elevation 720.5, well above the 1 percent chance flood level. These structures are shown on Figure 2.4.3 14. The structures that are located in the floodplain will not alter flood flows or elevations. The 20,650 square mile drainage area is not altered and the reduction in flow area at the site is infinitesimal and at the fringe of the flooded area. The site will be well maintained and any debris generated from it will be minimal and will present no problem to downstream facilities. The controlling PMF elevation at the SQN was determined to be [CEII] ft, produced by the controlling PMP for the SQN site resulted from consideration of the drainage area above the Blue Ridge Dam project as the primary PMP watershed nested with multiple secondary area PMPs over the Hiwassee-Blue Ridge, Fontana-Hiwassee-Blue Ridge, Tellico-Hiwassee-Blue Ridge, Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas, and Fort Loudoun-Tellico-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above Chickamauga, Nickajack, Guntersville, and Wheeler dam projects [55]. The PMF elevation hydrograph is shown on Figure 2.4.3-18. Elevations were computed concurrently with discharges using the unsteady flow reservoir model described in Subsection 2.4.3.3. 2.4.3.6 Coincident Wind-Wave Activity Some wind waves are likely when the probable maximum floodPMF crests at SQN. The controlling flood would be near its crest for a day beginning about 2 1/22 days after cessation of the probable maximum storm. The day of occurrence would most likely be in the month of March or possibly the first week in AprilMay. For the SQN site, the two-year extreme wind for the season in which the PMF could occur was adopted to associate with the PMF crest as specified in Regulatory Guide 1.59. The wind data used to determine the two-year wind is taken from Automated Surface Observation System (ASOS) Surface 1-minute data from the National Climatic Data Center for five surrounding airport data stations (Knoxville, Chattanooga and Tri-Cities, Tennessee, Huntsville, Alabama and Asheville, North Carolina) [56]. . Data from January 1, 2000 through June 30, 2014 was used in the analysis. The 2-minute average wind speed, reported for each 1-minute interval, is used in the wind speed determination as recommended by the ASOS Users Guide as more representative of wind speeds that influence wave S2-4.doc 2.4-25 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 formation. Since a 20-minute sustained wind is sufficient to cause wind wave activity, the 2-minute average winds were analyzed to find the peak 20-minute average wind speed for each year at the SQN site. Wind direction was considered by calculating the X and Y velocity components based on direction and solving for the resultant wind velocity with respect to the critical fetch direction for every 20-minute window. From these 20-minute average resultant wind velocities, the maximum 20-minute resultant wind velocity was found for the SQN site for each year. A two-year wind is defined as the wind speed that has a 50 percent chance of being exceeded in any given year. To determine the two-year overland wind, the calculated peak 20-minute average resultant velocities for each year for each airport reporting site was statistically analyzed to generate a curve of best fit for wind speed versus probability of exceedence using a Pearson Type III transformation [58]. The two-year wind for each airport reporting site was taken from the curve at the point of a 50 percent probability of exceedence. Using the inverse of the cube of the distance from the SQN site to the airport reporting station, the effective weight the two-year wind speed of each reporting station relative to the SQN site was determined. The sum of the weighted wind speeds from each of the five airport reporting station is taken as the two-year overland wind speed for the SQN site. The overland two-year wind speed for general structures at the SQN site is 23.65 miles per hour [56]. To account for the increase in wind speed over water, these two-year overland wind speeds are converted to over water wind speeds with regard to the effective fetch length [59]. A conservatively high velocity of 45 miles per hour over water was adopted to associate with the probable maximum flood crest. A 45 mile per hour overwater velocity exceeds maximum March one hour velocities observed in severe March windstorms of record in a homogeneous region as reported by the Corps of Engineers [6]. That a 45 mile per hour overwater wind is conservatively high, is supported also by an analysis of March day maximum winds of record collected at Knoxville and Chattanooga, Tennessee. The records analyzed varied from 30 years at Chattanooga to 26 years at Knoxville, providing samples ranging from 930 to 806 March days. The recorded fastest mile wind on each March day was used rather than hourly data because this information is readily available in National Weather Service publications. Relationships to convert fastest mile winds to winds of other durations were developed from Knoxville and Chattanooga wind data contained in USWB Form 1001 and the maximum storm information contained in Technical Bulletin No. 2 [6]. From the wind frequency analysis it was determined that the 45 mile per hour overwater wind for the critical minimum duration of 20 minutes had an 0.1 percent chance of occurrence on any given March day. The probability that this wind might occur on the specific day that the probable maximum flood would crest is extremely remote. Even assuming that the flood was to crest once during the 40 year plant life, the probability of the wind occurring on that particular day is in the order of 1 x 10 6. TVA estimates that the probability of the flood and wind occurring in a given year on the same day to be in the order of 1 x 10 11 to 1 x 10 13. Computation of wind waves at the SQN site was made using the procedures of the Corps of Engineers [7][59]. Using the two-year over water wind speed and critical fetch direction, the significant wave height (Hs) and wave period (Ts) are determined with Exhibits 3 and 4 [59]. The maximum wave height (Hmax), wave length (Ls) and wave steepness are determined by the following equations [59]: Hmax = 1.67 x Hs Wave Length = 5.12 x Ts2 Wave steepness = Hs / Ls Relative wave runup (R/H) is determined using Exhibit 8 [59]. Wave runup (R), wind setup (S) and S2-4.doc 2.4-26 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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3. Broken Waves The dynamic effect of broken waves on the walls of safety related structures was investigated using a method proposed by the U.S. Army Coastal Engineering Research Center [7]. This method of design yielded concrete and reinforcing stresses within allowable limits.

All safety related structures are designed to withstand the static and dynamic effects of the water and waves as stated in Section 2.4.2.2. 2.4.4 Potential Dam Failures, (Seismically and Otherwise Induced) The guidance described in Appendix A of Regulatory Guide 1.59 supplemented by the NRC guidance provided in Report JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure, Interim Staff Guidance, were followed when evaluating potential flood levels from seismically induced dam failures. The plant site and upstream reservoirs are located in the Southern Appalachian Tectonic Province and, therefore, subject to moderate earthquake forces with possible attendant failure. Upstream dams whose failure has the potential to cause flood problems at the plant were investigated to determine if failure from seismic events would endanger plant safety. Details of the dam failure analysis are discussed in Subsection 2.4.4.1, Dam Failure Permutations. There are 20 major dams above SQN. These were examined individually and in groups to determine if failure might result from a seismic event and if such failure or failures occurring concurrently with storm runoff would create critical flood levels at the plant. Two situations were examined: (1) a one half Safe Shutdown Earthquake (SSE) as defined in Subsection 2.5.2, imposed concurrently with one half the probable maximum flood and (2) a Safe Shutdown Earthquake (SSE) as defined in Subsection 2.5.2, imposed concurrently with a 25 year flood. Neither of these conditions would create levels greater than the hydrologic probable maximum flood at SQN, described previously in 2.4.3. Details of the dam failure analysis are discussed in Section 2.4.4.2, Dam Failure Permutations. Failure of Chickamauga Dam, downstream, can affect cooling water supplies at the plant. Consequently for conservatism, an arbitrary failure was imposed. This resulting condition would not be critical to plant operation, as discussed in Section 2.4.11.6. 2.4.4.1 Reservoir Description Characteristics of dams that influence river conditions at SQN are contained in Tables 2.4.1 1 and 2.4.1 2. Their location with respect to the plant is shown on Figure 2.1.1 1. Seismic safety criteria were not incorporated in the design of dams upstream from SQN, except Tellico and Norris. Those projects having a potential to influence plant flooding levels were examined, as described in Section 2.4.4.2. Elevation storage relationships and seasonally varying storage allocations in the major projects are shown on the 14 sheets of Figure 2.4.1 3. 2.4.4.2 Dam Failure Permutations The plant site and upstream reservoirs are located in the Southern Appalachian Tectonic Province and, therefore, subject to moderate earthquake forces with possible attendant failure. All upstream dams, whose failure has the potential to cause flood problems at the plant, were investigated to determine if failure from seismic or hydrologic events would endanger plant safety. Potential failures from both seismic and hydrologic events and the resulting consequences are discussed in this section. It should be clearly understood that these studies have been made solely to ensure the safety of SQN against failure by floods caused from excessive rainfall or by the assumed failure of dams due to seismic forces. To assure that safe shutdown of SQN is not impaired by flood waters, TVA has in these studies added conservative assumptions to conservative assumptions to be able to show that the plant can be safely controlled even in the event that all these unlikely events occur in just the S2-4.doc 2.4-28 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.4.1 Dam Failure Permutations NRC JLD-ISG-13-01, Section 1.4.3, Seismic Failure, states that a dam should be assumed to fail due to seismic hazard if it cannot withstand the more severe of the following combinations: x 10-4 annual exceedance seismic hazard combined with a 25-year flood x One-half of the 10-4 ground motion, combined with a 500-year flood The seismic hazard for key TVA dams whose failure could potentially result in flooding of the SQN site is defined by a probabilistic seismic hazard analysis (PSHA) performed by TVAs Dam Safety organization. A site specific PSHA and time histories for each dam were developed for 17 major dams upstream of the SQN site on the Tennessee River and its tributaries whose failure could potentially result in site flooding. Analyzed dam locations with respect to the SQN site are shown in Figure 2.4.1-3. [CEII] The TVA Dam Safety PSHA utilized NUREG-2115, Central and Eastern United States Seismic Source Characterization for Nuclear Facilities (2012), [52] as seismic source characterization (SSC) for the analyzed dams along with (OHFWULF3RZHU5HVHDUFK,QVWLWXWH EPRI s 2004/2006 ground motion prediction models [53]. The uniform hazard response spectra corresponding to the appropriate structural frequency range of 1 Hz for embankment dam structures and 10 Hz for concrete dam structures was used to determine the controlling earthquake for each dam location. Three sets of spectrally matched time histories are developed for the 1 Hz and 10 Hz structural frequencies. Each set consists of three statistically independent time history records. Site response analyses were completed for the dams not founded on hard rock and are considered best estimate analyses with the shear wave velocities developed from direct measurement of the soils and rock at each site. NUREG/ CR-6728, Table 4-5, was used to define vertical to horizontal ratios. After the site specific seismic hazard for each of the 22 dams were established, the concrete and/or earth embankment structures (except Apalachia) were evaluated as described below using TVA Dam Safety procedure, RO-SPP-27.1, Design and Evaluation of New and Existing Dams. Concrete Dam Structures The method of analysis of concrete structures is the two or three dimensional finite element method (FEM), which closely models the actual geometry of the dam as well as interaction with the foundation. After the structural model was developed, a dynamic analysis of the concrete dam structure is performed by response spectrum modal analysis or time history analysis. The purpose of the dynamic analysis is to assess the post-earthquake damaged state of the dam and to determine if the dam can continue to resist the applied static loads in a damaged state. The dynamic analysis includes the dynamic effects of the reservoir water mass. The dam/foundation interface is assumed to crack whenever tensile stress normal to the dam/foundation interface is indicated. After the seismic event damaged state of the concrete dam structure has been determined, the post-earthquake stability of the dam is assessed. Forces applied to the dam include hydrostatic forces due the maximum normal reservoir level, dead weight, silt pressure, earth backfill pressure, nappe forces (spillway), and uplift pressure due to degraded drains and base cracking. Cohesion at the rock-concrete interfaces is conservatively neglected, unless sufficient data is available to justify the use of cohesion. The post-earthquake stability of the concrete structure is confirmed, if the sliding factor of safety is 1.3 or greater and the overturning resultant is within the base of the concrete structure. S2-4.doc 2.4-30 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The flow capacity of flood control outlet spillways with concrete weirs on earthen foundations and either concrete lined or unlined flumes is limited to the original design flow for the spillway. If the design flow is exceeded, the spillway is assumed to fail completely to the most shallow erosion resistant layer. The standard method of computing stability is used. The maximum base compressive stress, average base shear stress, the factor of safety against overturning, and the shear strength required for a shear friction factor of safety of 1 are determined. To find the shear strength required to provide a safety factor of 1, a coefficient of friction of 0.65 is assigned at the elevation of the base under consideration. As stated in Section 2.4.1.2, all of the original stability analyses and postulated dam failure assumptions in the 1998 reanalyses were conservatively assumed to occur in the same manner and in combination with the same postulated rainfall events. The analyses for earthquake are based on the static analysis method as given by Hinds [10] with increased hydrodynamic pressures determined by the method developed by Bustamante and Flores [11]. These analyses include applying masonry inertia forces and increased water pressure to the structure resulting from the acceleration of the structure horizontally in the upstream direction and simultaneously in a downward direction. The masonry inertia forces are determined by a dynamic analysis of the structure which takes into account amplification of the accelerations above the foundation rock. No reduction of hydrostatic or hydrodynamic forces due to the decrease of the unit weight of water from the downward acceleration of the reservoir bottom is included in this analysis. Waves created at the free surface of the reservoir by an earthquake are considered of no importance. Based upon studies by Chopra [12] and Zienkiewicz [13], it is our judgment that before waves of any significant height have time to develop, the earthquake will be over. The duration of earthquake used in this analysis is in the range of 20 to 30 seconds. Although accumulated silt on the reservoir bottom would dampen vertically traveling waves, the effect of silt on structures is not considered. There is only a small amount of silt now present, and the accumulation rate is slow, as measured by TVA for many years [14].Embankment Dam Structures The seismic analysis for earth and rock-fill embankment structures begins with defining the geometry and foundation of the embankment to screen for liquefiable materials. Soil densities, shear strengths, and resistance to liquefaction are evaluated by consideration of laboratory and field test data and comparison with industry source data and past experiences. If materials within the dam have a factor of safety less than 1.4 for liquefaction triggering, post-earthquake analysis is performed using appropriate shear strengths assigned to the potentially liquefiable materials based on standard industry methods. Non-liquefiable materials are evaluated for strain softening and assigned appropriate drained or undrained shear strengths depending on material properties and phreatic surfaces. The post-earthquake analysis is then computed using static equilibrium slope stability analysis utilizing the normal summer pool elevation and shear strengths typically represented as Mohr-Coulomb failure envelope or nonlinear relationships between shear strength and normal stress on the failure surface. Circular, wedge-type and irregular failure surfaces are evaluated. If the search routine develops factor of safety values less than 1.1, then the embankment structure is considered potentially unstable. If liquefiable materials are not present in the embankment and/or foundation, then a static equilibrium analysis is performed using a pseudo-static analysis technique by applying the ground motion as a horizontal force on the critical slip plane from the steady state seepage conditions in the direction of potential failure. The shear strengths are applied from the drained and undrained parameters with the initial effective normal consolidation pressures at normal pool. If the factor of safety is greater than 1.1, the embankment is deemed stable. If the factor of safety is less than 1.1, then a simplified deformation analysis is performed utilizing Newmark Analysis, or other method deemed appropriate. If the deformations by the simplified method are two feet or less and less than one-half the thickness of the filter, the dam is considered stable. Otherwise additional more sophisticated deformation analysis S2-4.doc 2.4-31 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.4.42.4.4.3 Water Level at Plant Site Maximum water level at the plant from different postulated combinations of seismic dam failures coincident with floods would be elevation 707.9, excluding wind wave effects. It would result from the one half SSE failure of Fontana, Hiwassee, Apalachia, and Blue Ridge Dams coincident with one half the probable maximum flood. March wind with one percent exceedance probability over the 1.4 mile effective fetch from the critical north northwest direction is 26 miles per hour over land. This would cause reservoir waves to reach elevation 709.6. Runup could reach elevation 710.4 on a smooth 4:1 slope, elevation 712.8 on a vertical wall in shallow (4.9 feet) water, and elevation 710.4 on a vertical wall in deep water.The unsteady flow analyses [57] of the four postulated combinations of seismic dam failures coincident with floods analyzed yields a maximum elevation of [CEII] ft at SQN excluding wind wave effects as shown in Figure 2.4.4-17 (Sheet 3). The maximum elevation would result from [CEII] due to one-half the ground motion of a 10 -4 AFE seismic event ground motion centered at [CEII] in combination with a 500-year flood. Coincident wind wave activity for the PMF is described in Section 2.4.3.6. Wind waves were not computed for the seismic events, but superimposed wind wave activity from guide specified two-year wind speed would result in water surface elevations several feet below the calculated PMF elevation [CEII] ft. described in section 2.4.3. For the design basis flood level, see Section 2.4.14.1. 2.4.5 Probable Maximum Surge and Seiche Flooding (HISTORICAL INFORMATION) Chickamauga Lake level during non-flood conditions could be no higher than elevation 685.44, top of gates, and is not likely to would not exceed elevation 682.5 ft, normal summer maximum pool level, for any significant time. No conceivable hurricane or cyclonic type windsmeteorological conditions could produce the over 20 feet of wave height required toa seiche nor reservoir operations a surge which would reach plant grade elevation 705705.0 ft, some 22.5 ft above normal maximum pool level. 2.4.6 Probable Maximum Tsunami Flooding (HISTORICAL INFORMATION) Because of its inland location, SQN is not endangered by tsunami flooding. 2.4.7 Ice Flooding and Landslides (HISTORICAL INFORMATION) Because of the location in a temperate climate, significant amounts of ice do not form on the Tennessee Valley rivers and lakes. SQN is in no danger from ice flooding.lakes and rivers in the plant vicinity and ice jams are not a source of major flooding. Flood waves from landslides into upstream reservoirs pose no danger because of the absence of major elevation relief in nearby upstream reservoirs and because the prevailing thin soils offer small slide volume potential compared to the available detention space in reservoirs. 2.4.8 Cooling Water Canals and Reservoirs (HISTORICAL INFORMATION) 2.4.8.1 Canals The intake channel, as shown in Figure 2.1.2-1, referenced in paragraph 2.4.1.1, is designed for a flow of 2,250 cfs. At minimum pool (elevation 675), as shown in Figure 2.4.8-1, this flow is maintained at a velocity of 2.7 fps. The protection of the intake channel slopes from wind-wave activity is afforded by the placement of riprap, shown in Figure 2.4.8-1, in accordance with TVA Design Standards, from elevation 665 to elevation 690. The riprap is designed for a wind velocity of 45 mph. 2.4.8.2 Reservoirs (HISTORICAL INFORMATION) Chickamauga Reservoir provides the cooling water for SQN. This reservoir and the extensive TVA system of upstream reservoirs, which regulate inflows, are described in Table 2.4.1 12.4.1-2. The S2-4.doc 2.4-48 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 be considered would be evaluated before implementation. 2.4.11.5 Plant Requirements 2.4.11.5.1 Two-Unit Operation The safety related water supply systems requiring river water are: the essential raw cooling water (ERCW) (Subsection 9.2.2), and that portion of the high-pressure fire-protection system (HPFP) (Subsection 2.4A.4.12.4.14.4.1) supplying emergency feedwater to the steam generators. The fire/flood mode pumps are submersible pumps located in the condenser circulating water (CCW) intake pumping station. The CCW intake pumping station sump is at elevation [CEII] ft. The entrances to the suction pipes for the fire/flood mode pumps are at elevation [CEII] ft [CEII] which is about [CEII] feet, respectively, below the maximum normal water elevation of [CEII] ft3.0 and the normal minimum elevation of [CEII] ft for the reservoir. Abnormal reservoir level is elevation [CEII] ft with a technical specification limit of elevation [CEII] ft. For flow requirements of the HPFP during engineering safety feature operation (Reference 22). The ERCW pump sump in this independent station is at elevation [CEII] ft, which is [CEII] ft below maximum normal water elevation, [CEII] ft below minimum normal water elevation, and [CEII] ft below the [CEII] ft minimum possible elevation of the river. Since the ERCW pumping station has direct communication with the river for all water levels and is above probable maximum flood, the ERCW system for two-unit plant operation always operates in an open cooling cycle. 2.4.11.6 Heat Sink Dependability Requirements The ultimate heat sink, its design bases and its operation, under all normal and credible accident conditions is described in detail in Subsection 9.2.5. As discussed in Subsection 9.2.5, the sink was modified by a new essential raw cooling water (ERCW) pumping station before unit 2 began operation. The design basis and operation of the ERCW system, both with the original ERCW intake station and with the new ERCW intake station, is presented in Subsection 9.2.2. As described in these sections, the new ERCW station is designed to guarantee a continued adequate supply of essential cooling water for all plant design basis conditions. This position is further assured since additional river water may be provided from TVA's upstream multiple-purpose reservoirs, as previously discussed during Low Flow in Rivers and Streams. 2.4.11.6.1 Loss of Downstream Dam The loss of downstream dam will not result in any adverse effects on the availability of water to the ERCW system or these portions of the original HPFP supplying emergency feedwater to the steam generator. Loss of downstream dam reduces ERCW flow about 7% to the component cooling and containment spray heat exchangers. ERCW flow does not decrease below that assumed in the analysis (analyzed as [CEII] ft) until more than two hours after the peak containment temperature and pressure occurs. (See Section 6.2.1.3.4.) 2.4.11.6.2 Adequacy of Minimum Flow The cooling requirements for plant safety-related features are provided by the ERCW system. The required ERCW flow rates under the most demanding modes of operation (including loss of downstream dam) as given in Subsection 9.2.2 are contained in TVA calculations and flow diagrams. Two other safety-related functions may require water from the ultimate heat sink; these are fire protection water (refer to Subparagraph 2.4.11.6.3) and emergency steam generator feedwater (refer to Subsection 10.4.7). These two functions have smaller flow requirements than the ERCW systems. Consequently, the relative abundance of the river flow, even under the worst conditions, assures the availability of an adequate water supply for all safety-related plant cooling water requirements. River operationsTVA River Management methodology for maintaining UHS temperatures are discussed in Monitoring and Moderating Sequoyah Ultimate Heat Sink, Reference 21. S2-4.doc 2.4-51 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.11.6.3 Fire-Protection Water Refer to the Fire Protection Report discussed in Section 9.5.1. 2.4.12 Environmental Acceptance of Effluents The ability of surface waters near SQN, located on the right bank near Tennessee River Mile (TRM) 484.5, to dilute and disperse radioactive liquid effluents accidentally released from the plant is discussed herein. Routine radioactive liquid releases are discussed in Section 11.2. The Tennessee River is the sole surface water pathway between SQN and surface water users along the river. Liquid effluent from SQN flows into the river from a diffuser pond through a system of diffuser pipes located at TRM 483.65. An accidental, radioactive liquid effluent release from SQN would enter the Tennessee River after it reached the diffuser pond and entered the diffuser pipes. The contents of the diffuser pond enter the diffuser pipes and mix with the river flow upon discharge. The diffusers are designed to provide rapid mixing of the discharged effluent with the river flow. The flow through the diffusers is driven by the elevation head difference between the diffuser pond and the river [1] (McCold 1979). Descriptions of the diffusers and SQN operating modes are given in Paragraph 10.4.5.2. Flow is discharged into the diffuser pond via the blowdown line, ERCW System (Subsection 9.2.2) and CCW System (Subsection 10.4.5). A layout of SQN is given in Figures 2.1.2-1 and 2.1.2-2. Two pipes comprise the diffuser system and are set alongside each other on the river bottom. They extend from the right bank of the river into the main channel. The main channel begins near the right bank of the river and is approximately 900 feet wide at SQN [1] (McCold, 1979). Each diffuser pipe has a 350-foot section through which flow is discharged into the river. The downstream diffuser leg discharges across a section 0 to 350 feet from the right bank of the main channel. The upstream diffuser leg starts at the end of the downstream diffuser leg and discharges across a section 350 to 700 feet from the right bank of the main channel. The two diffusers therefore provide mixing across nearly the entire main channel width. The river flow near SQN is governed by hydro power operations of Watts Bar Dam upstream (TRM 529.9) and Chickamauga Dam downstream (TRM 471.0). The backwater of Chickamauga Dam extends to Watts Bar Dam. Peaking hydro power operations of the dams cause short periods of zero (i.e., stagnant) and reverse (i.e., upstream) flow near the plant. Effluent released from the diffusers during these zero and reverse flow periods will not concentrate near the plant or affect any water intake upstream. The maximum flow-reversal during 1978-1981 were not long enough to cause discharge from the diffusers to extend upstream to the SQN intake [2] (El-Ashry, 1983), which is the nearest intake and located at the right bank near TRM 484.7. Moreover, the warm buoyant discharge from the diffusers will tend toward the water surface as it mixes the river flow and away from the cooler, denser water found near the intake opening below the skimmer wall. The intake opening extends the first 10 feet above the riverbed elevation of about [CEII] feetft mean sea level (MSL). The minimum flow depth at the intake is approximately [CEII] feet [3] (Ungate and Howerton, 1979). There are no other surface water users between the diffusers and this intake. Subsection 2.4.13 discusses groundwater movement at SQN. Effluent released through the diffusers will have no impact on SQN groundwater sources along the banks of the river. Paragraph 2.2.3.8 discusses the effect on plant safety features from flammable or toxic materials released in the river near SQN. The predominant transport and effect of a diffuser release is along the main channel and in the downstream direction. The nearest downstream surface water intake is located along the left bank at TRM 473.0 (Table 2.4.1 42.4.1-1). A mathematical analysis is used to estimate the downstream transport and dilution of a contaminant released in the Tennessee River during an accidental spill at SQN. Only the main channel flow area without the adjacent overbank regions is considered in the analysis. The mathematical analysis of a potential spill scenario can involve: (1) a slug release, which can be modeled as an instantaneous release; (2) a continuous release, which can be modeled as a steady-state release; (3) a bank release, which can be modeled as a vertical line source; and (4) a diffuser release, which can be modeled either as a vertical line or plane source, depending on the width of the diffuser with respect to S2-4.doc 2.4-52 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 the channel width. The following assumptions are used in the mathematical analyses to compute the minimum dilution expected downstream from SQN and, in particular, at the nearest water intake.

1. Mixing calculations are based on unstratified steady flow in the reservoir. River flow, Q, is assumed to be 27,474 cubic feet per second (cfs), which is equalledequaled or exceeded in the reservoir approximately 50 percent of the time (Paragraph 2.4.1.2). Because various combinations of the upstream and downstream hydro power dam operations can create upstream flows past SQN, a minimum flow is not well defined. Larger (smaller) flows will decrease (increase) the travel time to the nearest intake but cause less than an order of magnitude change in the calculated dilution.

2. Because the SQN diffusers and the nearest downstream water intake are on opposite banks of the river, and the diffusers extend across most of the main channel width, an analysis using a diffuser release (rather than a bank release) is selected to yield a lesser (i.e., more conservative) dilution at the intake. Thus, the accidental spill is modeled as a vertical plane source across the width of the main channel.

3. The contaminant concentration profile from a slug release is assumed to be Gaussian (i.e.,

normal) in the longitudinal direction.

4. The contaminant is conservative, i.e., it does not degrade through radioactive decay, chemical or biological processes, nor is it removed from the reservoir by adsorption to sediments or by volatilization.

5. The transport of the contaminant is described using the motion of the river flow, i.e., the contaminant is neutrally buoyant and does not rise or sink due to gravity.

The main channel and dynamic, flow-dependent processes of the reservoir reach between SQN and the first downstream water intake are modeled as a channel of constant rectangular cross section with the following constant geometric, hydraulic and dispersion characteristics. Longitudinal distance, x = 10.6 miles Average water surface elevation = 678.5 feet MSL (Figure 2.4.1 3 (1)) Average width, W = 1175 feet Average depth, H = [CEII] Average velocity, U (= Q/(W H)) = 0.468 feet per second (fps) Average travel time (for approximate peak contaminant), t (= x/U) = 1.4 days Manning coefficient n (surface roughness) = 0.03 Longitudinal dispersion parameter, alpha = 200 where: alpha = Ex / (H u) Ex = constant longitudinal dispersion coefficient (square feet per second) u = shear velocity (fps) = gRS g = acceleration due to gravity = 32.174 ft/s2 R = hydraulic radius (ft) S2-4.doc 2.4-53 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 S = slope of the energy line (ft/ft) The average width and depth were estimated from measurements of 9 cross sections in the reach [4] (TVA) [5] (TVA). For wide channels (i.e., large width-to-depth ratio), the hydraulic radius can be approximated as the average depth. The value of alpha = 200 is on the conservative (i.e., low) side [6] (Fischer, et al., 1979). The value of the Manning coefficient n is representative for natural rivers [7] (Chow, 1959). The equation used to describe the maximum downstream activity (or concentration), C, at a point of interest due to an instantaneous plane source release of volume V is [8] (Guide 1.113): C V

     =

Co W H 4S E x t (2.4.12-1) where: Co = initial activity (or concentration) in the plant of the released contaminant S = 3.14156 Any consistent set of units can be used on each side of Equation 2.4.12-1 (e.g., C and C o in mCi/ml; V in cf; W and H in ft; Ex in ft2/s; t in s). The term, C/Co, is the relative (i.e., dimensionless) activity (or concentration) and its reciprocal is the dimensionless dilution factor. Equation 2.4.12-1 simplifies to C/C o = 8.3E-10

• V (V expressed in cubic feet (cf)) when the parameters are substituted and the Manning equation [7] (Chow, 1959) is used in the definition of the shear velocity, u. In the substitution, u = 0.028 ft/s and E x = 282.1 ft2/s.

The equation used to describe the maximum downstream concentration at a point of interest due to a continuous plane source release rate, Qs, where Qs << Q, is [8] (Guide 1.113): (2.4.12-2) C Qs

                 =

Co Q Any consistent set of units can be used on each side of Equation 2.4.12-2 (e.g., C and C o in mCi/ml; Qs and Q in cfs). Equation 2.4.12-2 simplifies to C/Co = 3.64E-05

• Qs (Qs expressed in cfs) for Q = 27,474 cfs.

Examples of quantities and concentrations of potential contaminant releases and the use of Equations 2.4.12-1 and 2.4.12-2 follow. Because Co is defined as the in-plant activity (or concentration) and not that of the diffuser release, an estimate of the dilution of liquid waste occurring in the diffuser pond and diffuser pipes is not needed. This is because the flow available for dilution in the plant (e.g., CCW and ERCW) is taken from and returned to the river. Only effluent extraneous to the river flow requires consideration in the analyses to calculate the dilution. More information on the possible means which liquid waste from the plant enters the diffuser pond is contained in Subsection 10.4.5. The largest outdoor tanks whose contents flow into the diffuser pond are the two condensate storage tanks (Paragraph 11.2.3.1), which each have an overflow capacity of 398,000 gallons. Liquid waste that reaches the diffuser pond enters the Tennessee River through the diffuser system. The diffuser pond is approximately 2000 feet long and 500 feet wide with a depth that, although it depends on the Chickamauga Reservoir elevation, averages about 10 feet [9] (McIntosh, et al., 1982). The design S2-4.doc 2.4-54 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 flow residence time of the pond is approximately one hour (i.e., diffuser design flow is 2,480 cfs at maximum plant capacity [3] [Ungate and Howerton, 1979]). For example, assume an instantaneous plane source release into the Tennessee River of the contents of one condensate storage drain tank. Assume the full 398,000 gallon (53,210 cf) volume contains Iodine-131 (I-131) at an activity of 1.5E-06 mCi/gm (Table 10.4.1-1). From Equation 2.4.12-1, the activity, C, at the first downstream water intake would be 6.6E-11 mCi/gm>@, which is within the acceptable limit [10] (CFR) for soluble I-131. For a continuous plane source release, assume the contents of the 398,000 gallon (53,210 cf) floor drain tank leak out steadily over a 24-hour period. The effective release rate is 0.6 cfs at an activity of 1.5E-06 mCi/gm. The expected activity at the first downstream water intake would be 3.4E-11 mCi/gm >@ using Equation 2.4.12-2 and is within the acceptable limit [10] (CFR) for soluble I-131. REFERENCES (for Section 2.4.12 only) [1] McCold, L. N. (March 1979), "Model Study and Analysis of Sequoyah Nuclear Plant Submerged Multiport Diffuser," TVA, Division of Water Resources, Water System Development Branch, Norris, TN, Report No. WR28-1-45-103. [2] El-Ashry, Mohammed T., Director of Environmental Quality, TVA, February 1983 letter to Paul Davis, Manager, Permit Section, Tennessee Division of Water Quality Control, SEQUOYAH NUCLEAR PLANT---NPDES PERMIT NO. T0026450. [3] Ungate, C. D., and Howerton, K. A. (April 1978; revised March 1979), "Effect of Sequoyah Nuclear Plant Discharges on Chickamauga Lake Water Temperatures," TVA, Division of Water Management, Water Systems Development Branch, Norris, TN, Report No. WR28-1-45-101. [4] TVA, Chickamauga Reservoir Sediment Investigations, Cross Sections, 1940-1961, Division of Water Control Planning, Hydraulic Data Branch. [5] TVA, Measured Cross Sections of Chickamauga Reservoir, 1972, Flood Protection Branch. [6] Fischer, H. B., List, E. J., Koh, R.C.Y., Imberger, J., Brooks, N. H. (1979), Mixing in Inland and Costal Waters, Academic Press, New York. [7] Chow, V. T. (1959) Open-Channel Hydraulics, McGraw-Hill, New York. [8] United States Nuclear Regulatory Commission, Office of Standards Development, Regulatory Guide 1.113 (April 1977), "Estimating Aquatic Dispersion of Effluents from Accidental and Routine Reactor Releases for the Purpose of Implementing Appendix I," Revision 1. [9] McIntosh, D. A., Johnson, B. E. and Speaks, E. B. (October 1982), "A Field Verification of Sequoyah Nuclear Plant Diffuser Performance Model: One-Unit Operation," TVA, Office of Natural Resources, Division of Air and Water Resources, Water Systems Development Branch, Norris, TN, Report No. WR28-1-45-110. [10] 10 CFR Part 20, Appendix B, Table II, Column 2. [11] TVA SQN Calculation SQN-SQS2-0242, SQN Site Iodine-131 Release Concentration in Tennessee River. S2-4.doc 2.4-55 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.13 Groundwater (HISTORICAL INFORMATION) 2.4.13.1 Description and Onsite Use The peninsula on which SQN is located is underlain by the Conasauga Shale, a poor water-bearing formation. About 2,000 feet northwest of the plant site, the trace of the Kingston Fault separates this outcrop area of the Conasauga Shale from a wide belt of Knox Dolomite. The Knox is the major water bearing formation of eastern Tennessee. Groundwater in the Conasauga Shale occurs in small openings along fractures and bedding planes; these rapidly decrease in size with depth, and few openings exist below a depth of 300 feet. Groundwater in the Knox Dolomite occurs in solutionally enlarged openings formed along fractures and bedding planes and also in locally thick cherty clay overburden. There is no groundwater use at SQN. 2.4.13.2 Sources The source of groundwater at SQN is recharged by local, onsite precipitation. Discharge occurs by movement mainly along strike of bedrock, to the northeast and southwest, into Chickamauga Lake. Rises in the level of Chickamauga Lake result in corresponding rises in the water table and recharge along the periphery of the lake, extending inland for short distances. Lateral extent of this effect varies with local slope of the water table, but probably nowhere exceeds 500 feet. Lowering levels of Chickamauga Lake results in corresponding declines in the water table along the lake periphery, and short-term increase in groundwater discharge. When SQN was initially evaluated in the early 1970s, it was in a rural area, and only a few houses within a two-mile radius of the plant site were supplied by individual wells in the Knox Dolomite (see Table 2.4.13-1, Figure 2.4.13-1). Because the average domestic use probably does not exceed 500 gallons per day per house, groundwater withdrawal within a two-mile radius of the plant site was less than 50,000 gallons per day. Such a small volume withdrawal over the area would have essentially no effect on areal groundwater levels and gradients. Although development of the area has increased, public supplies are available and overall groundwater use is not expected to increase. Public and industrial groundwater supplies within a 20 mile radius of the site in 1985 are listed in Table 2.4.13-2. The area groundwater gradient is towards Chickamauga Lake, under water table conditions, and at a gradient of less than 120 feet per mile. The water table system is shallow, the surface of which conforms in general to the topography of the land surface. Depth to water ranges from less than 10 feet in topographically low areas to more than 75 feet in higher areas underlain by Knox Dolomite. Figure 2.4.13-2 is a generalized water-table map of SQN, based on water level data from five onsite observation wells, and in private wells adjacent to the site in April 1973, and also based on surface resistivity measurements of depth to water table made in 1972. Because permeability across strike in the Conasauga Shale is extremely low, and nearly all water movement is in a southwest-northeast direction, along strike, the Conasauga-Knox Dolomite Contact is a hydraulic barrier, across which only a very small volume of water could migrate in the event large groundwater withdrawals were made from the adjacent Knox. Although some water can cross this boundary, the permeability normal to strike of the Conasauga is too low to allow development of an areally extensive cone of depression. Groundwater recharge occurs to the Conasauga Shale at the plant site. Recharge water moves no more than 3,000 feet before being discharged to Chickamauga Lake. 2.4.13.3 Accident Effects Design features in SQN further protect groundwater from contamination. Category I structures in the SQN facility are designed to assure that all system components perform their designed function, including maintenance of integrity during earthquake. S2-4.doc 2.4-56 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Buildings in which radioactive liquids could be released due to the equipment failure, overflow, or spillage are designed to retain such liquids even if subject to an earthquake equivalent to the safe shutdown earthquake. Outdoor tanks that contain radioactive liquids are designed so that if they overflow, the overflow liquid is redirected to the building where the liquid is collected in the radwaste system. Two outdoor tanks that contain low concentrations of radioactivity at times overflow to yard drains which discharge into the diffuser pond. Overflow liquid is discharged near the discharge diffuser. The capacity for dispersion and dilution of contaminants by the groundwater system of the Conasauga Shale is low. Dispersion would occur slowly because water movement is limited to small openings along fractures and bedding planes in the shale. Clay minerals of the Conasauga Shale do, however, have a relatively high exchange capacity, and some of the radioactive ions would be absorbed by these minerals. Any ions moving through the groundwater system eventually would be discharged to Chickamauga Lake. The Conasauga Shale is heterogeneous and anisotropic vertically and horizontally. Water-bearing characteristics change abruptly within short distances. Standard aquifer analyses cannot be applied, and meaningful values for permeability, time of travel, or dilution factors cannot be obtained. Bedrock porosity is estimated to be less than 3 percent based on examination of results of exploratory core drilling. It is known from experience elsewhere in this region that water movement in the Conasauga Shale occurs almost entirely parallel to strike. Subsurface movement of a liquid radwaste release at the plant site would be about 1,000 feet to the northeast or about 2,000 feet to the southwest before discharge to Chickamauga Lake. Time of travel can only be estimated as being a few weeks for first arrival, a few months for peak concentration arrival, and perhaps two or more years for total discharge. The computed mean time of travel of groundwater from SQN to Chickamauga Lake is 303 days. No radwaste discharge would reach a groundwater user. At the nearest point, the reservation boundary lies 2,200 feet northwest of the plant site, across strike. Groundwater movement will not occur from the plant site in this direction across this distance. During initial licensing, the radionuclide concentrations were determined for both groundwater and surface water movement to the nearest potable water intake (Savannah Valley Utility District, which is no longer in service) and found to be of no concern (see Safety Evaluation Report, March 1979, Section 2.4.4 Groundwater). 2.4.13.4 Monitoring or Safeguard Requirements SQN is on a peninsula of low-permeability rock; the groundwater system of the site is essentially hydraulically isolated and potential hazard to groundwater users of the area is minimal. The environmental radiological monitoring program is addressed in Section 11.6. Monitor wells 1, 2, 3, and 4 were sampled and analyzed for radioactivity during the period from 1976 through 1978. Well 5 was not monitored because of insufficient flow. An additional well (Well 6) was drilled in late 1978 downgradient from the plant and a pump sampler installed. Wells 1, 2, 4, and 5 are each 150 feet deep, Well 6 is 250 feet deep, and Wells L6 and L7 are 75-80 feet deep. All of the wells are cased in the residuum and open bore in the Conasauga Shale. 2.4.13.5 Conclusions SQN was designed to provide protection of groundwater resources by preventing the escape of the leaks of radionuclides. Site soils and underlying geology provide further protection in that they retard the movement of water and attenuate any contaminants that would be released. All groundwater movement is toward Chickamauga Lake. The Knox Dolomite is essentially hydraulically separated from the Conasauga Shale; therefore, offsite pumping, including future development, should have little effect upon the groundwater table in the Conasauga Shale at the plant. S2-4.doc 2.4-57 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Even though the potential for accidental contamination of the groundwater system is extremely low, the radiological monitoring program will provide ample lead times to mitigate any offsite contamination. As a consequence of the geohydrologic conditions that remain unchanged from evaluations conducted in the 1970s, the information in Chapter 2.4.13 Groundwater is historical and should not be subject to updating revisions. 2.4.14 Technical Requirements and Emergency OperationFlooding Protection Requirements Emergency flood protection plans, designed to minimize impact of floods above plant grade on safety related facilities, are described in Appendix 2.4A. Procedures for predicting rainfall floods, arrangements to warn of upstream dam failure floods, and lead times available and types of action to be taken to meet related safety requirements for both sources of flooding are described therein. The Technical Requirements Manual specify the action to be taken to minimize the consequences of floods.The plant grade elevation at SQN can be exceeded by large rainfall and seismically-induced dam failure floods. Assurance that SQN can be safely shut down and maintained in these extreme flood conditions (Subsection 2.4.2.2 and this Section 2.4.14) is provided by the discussions given in Sections 3.4, 3.8.1, and 3.8.4. 2.4A.12.4.14.1 Introduction This appendix Subsection describes the methods by which the Sequoyah Nuclear PlantSQN will be madeis capable of tolerating floods above plant grade without jeopardizing public safety. Since flooding of this magnitude, as explained in section 2.4, is most unlikely, extreme steps are considered acceptable including actions that create or allow extensive economic damage to the plant, as illustrated in Sections 2.4.2 and 2.4.4 is most unlikely, extreme steps are considered acceptable, including actions that create or allow extensive economic consequence to the plant. The actions described herein will be implemented for floods ranging from slightly below plant grade, to allow for wave runup, to the Design Basis Flood (DBF). The plant Flood Protection Plan (Technical Requirements Manual (TRM) Requirement 8.7.2 specifies the flood warning conditions and subsequent actions. 2.4A.1.12.4.14.1.1 Design Basis Flood The DBF is the calculated upper limit flood that includes the probable maximum flood (PMF) plus the wave runup caused by an 45 mile per hour overwater wind; this is discussed in subsection 2.4.3.6. The table below gives representative levels of the DBF at different plant locations. Design Bases Flood (DBF) Levels [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] The lower flood elevations listed above are actual DBF elevations and are not normally used for the purpose of design but are typically used in plant procedures including procedures which direct plant actions in response to postulated DBF. For purposes of designing the flood protection for systems, structures, and components, the following higher elevations should be used thus ensuring additional margin has been included in the development of design analysis. S2-4.doc 2.4-58 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Design Analysis Flood Levels [CEII] [CEII] [CEII] [CEII] [CEII] [CEII] See FSAR References 2.4A.10 127 and 2.4A.10 228. In addition to level considerations, plant flood preparations will cope with the "fastest rising" flood which is the calculated flood including seismically induced floods that can exceed plant grade with the shortest prediction noticewarning time. Reservoir levels for large floods in the Tennessee Valley can be predicted well in advance. A minimum of 27 hours, divided into two stages, is provided for safe plant shutdown by use of this prediction capability. Stage I, a minimum of 10 hours long, will commence upon a prediction that flood-producing conditions might develop. Stage II, a minimum of 17 hours long, will commence on a confirmed estimate that conditions will provide a flood above plant grade. This two-stage scheme is designed to prevent excessive economic loss in case a potential flood does not fully develop. Refer to Subsection 2.4.14.4. 2.4A.1.22.4.14.1.2 Combinations of Events Because floods above plant grade, earthquakes, tornadoes, or design basis accidents, including a loss-of-coolant accident (LOCA), are individually very unlikely, a combination of a flood plus any of these events or the occurrence of one of these during the flood recovery time or of the flood during the recovery time after one of these events is considered incredible. Surges from seismic failure of upstream dams, however, can exceed plant grade, but to lower DBF levels, when imposed coincident with wind and certain floods. A minimum 27 hours of warning is assured so that ample time is available to prepare the plant for flooding.However, as an exception, certain reduced levels of floods are considered together with seismic events. Refer to Section 2.4.14.10 and 2.4.4. 2.4A.1.32.4.14.1.3 Post Flood Period Because of the improbability of a flood above plant grade, no detailed procedures will be established for return of the plant to normal operation unless and until a flood actually occurs. If flood mode operation (subsection 2.4A.2Subsection 2.4.14.2) should ever become necessary, it will be possible to maintain this mode of operation for a sufficient period of time (100 days) so that appropriate recovery steps can be formulated and taken. The actual flood waters are expected to recede below plant grade within 1 to 6 7 days. 2.4A.1.42.4.14.1.4 Localized Floods Localized plant site flooding due to the probable maximum storm (subsection 2.4.3Subsection 2.4.2.3) will not enter vital structures or endanger the plant. Plant shutdown will be forced by water ponding on the switchyard and around buildings, but this shutdown will not differ from a loss of offsite power situation as described in Chapter 15. The other steps described in this appendix subsection are not applicable to this case. 2.4A.22.4.14.2 Plant Operation During Floods Above Grade "Flood mode" operation is defined as the set of conditions described below by means of which the plant will be safely maintained during the time when flood waters exceed plant grade (elevation 705705.0 ft) and during the subsequent period until recovery (subsection 2.4A.7Subsection 2.4.14.7) is accomplished. S2-4.doc 2.4-59 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4A.2.12.4.14.2.1 Flooding of Structures Only theThe Reactor Building, the Diesel Generator Building (DGB), and the Essential Raw Cooling (ERCW) Water Intake Station will be maintained essentially water tight during the flood mode. Walls and penetrations are designed to withstand all static and dynamic forces imposed by the DBF. The Reactor Buildings protect SSCs contained within that are required for Flood Mode operations. All penetrations below the Design Analysis Flood DBF level including wind wave runup of elevation [CEII] have been sealed with seals, which are tested to withstand hydrostatic forces generated by the Design Basis FloodDBF. Analysis demonstrates the acceptability of minor leakage through the seals into the annulus. The lowest floor of the DGB is at elevation 722 722.0 ft with its doors on the uphill side facing away from the main body of flood water. This elevation is lower higher than the previous DBF elevation including wind wave runup of [CEII] . The 1998 reanalysis determined the still water elevation to be [CEII] with wind wave runup at the DGB to elevation [CEII] Therefore, flood levels do not exceed floor elevation of 722722.0 ft. The entrances into safety-related areas and all mechanical and electrical penetrations into safety-related areas are sealed either prior to or during flood mode to prevent major leakage into the building for water up to the PMFDBF, including wind wave runup. Due to the 1998 reanalysis this only applies to below grade features. Redundant sump pumps are provided within the building to remove minor leakage. The Essential Raw Cooling Water (ERCW) intake station is designed to remain fully functional for floods up to the PMFDBF, including wind wave runup. The deck elevation (elevation 720720.0 ft) is below the PMF DBF plus wind wave runup, but the deck it is protected from flooding by the outside walls. The traveling screen wells extend above the deck elevation up to the design basis DBF surge level. The wall penetration for water drainage from the deck in nonflood conditions is below the DBF DBF elevation, but it is designed for sealing in event of a flood. All other exterior penetrations of the station below the PMF DBF are permanently sealed. Redundant sump pumps are provided on the deck and in the interior rooms to remove rainfall on the deck and water seepage. All other structures, including the service, turbine, auxiliary, and control buildings, will be allowed to flood as the water exceeds their grade level entrances. All equipment, including power cables, that is located in these structures and required for operation in the flood mode is either above the DBF or designed for submerged operation, or otherwise protected. 2.4A.2.22.4.14.2.2 Fuel Cooling Spent Fuel Pit Fuel in the spent fuel pit will be cooled by the normal Spent Fuel Pit Cooling (SFPC) System. The pumps are located on a platform at elevation 721 721.0 ft which is above the DBF surge level of [CEII] The pumps are protected by a watertight enclosure equipped with redundant sump pumps to remove minor leakage. During the flood mode of operation, heat will be removed from the heat exchangers by ERCW instead of component cooling water. As a backup to spent fuel cooling, water from the Fire Protection (FP) System can be dumped into the spent fuel pool, and steam removed by the area ventilation system. Reactors Residual core heat will be removed from the fuel in the reactors by natural circulation in the Reactor Coolant (RC) system. Heat removal from the steam generators will be accomplished by adding river water from the FP System (subsection 9.5.1) and relieving steam to the atmosphere through the power relief valves. Primary system pressure will be maintained at less than 500 lb/in2g by operation of the pressurizer relief valves and heaters. This low pressure will lessen leakage from the system. Secondary side pressure will be maintained at or below 90 psig by operation of the steam line relief valves. S2-4.doc 2.4-60 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 An analysis has been performed to ensure that the limiting atmospheric relief capacity would be sufficient to remove steam generated by decay heat. At times beyond approximately 10 hours following shutdown of the plant two relief valves have sufficient capacity to remove the steam generated by decay heat. Since a minimum of 27 hours flood warning is available it is concluded that the plant could be safely shutdown and decay heat removed by operation of only two relief valves. Reference 28.FSAR 2.4A.10 1. The main steam power operated relief valves will be adjusted to maintain the steam pressure at or below 90 psig. If this control system malfunctions, then the controls in the main control room can be utilized to operate the valves in an open-closed manner. Also, a manual loading station and the relief valve handwheel provide additional backup control for each relief valve. The secondary side steam pressure can be maintained for an indefinite time by the means outlined above. The cooling water flow paths conform to the single failure criteria as defined in FSAR Section 3.1.1. In particular, all active components of the secondary side feedwater supply and ERCW supply are redundant and can therefore tolerate a single failure in the short or long term. A passive failure, consistent with the 50 gpm loss rate specified in FSAR Section 3.1.1, can be tolerated for an indefinite period without interrupting the required performance in either supply. If one or both reactors are open to the containment atmosphere as during the refueling operations, then the decay heat of any fuel in the open unit(s) and spent fuel pit will be removed in the following manner. The refueling cavity will be filled with borated water (approximately 2000 ppm boron concentration) from the refueling water storage tank. The SFPC System pump will take suction from the spent fuel pit and will discharge to the SFPC System heat exchangers. The SFPC System heat exchanger output flow will be directed by a piping connection to the Residual Heat Removal (RHR) System heat exchanger bypass line. The tie-in locations in the SFPC System and the RHR System are shown in Figures 9.1.3-1 and 5.5.7-1, respectively. This connection will be made using prefabricated, in- position piping which is normally disconnected. During flood mode preparations, the piping will be connected using prefabricated spool pieces. Prior to flooding, valve number 78-513 (refer to Figure 9.1.3-1) and valves FCV 74-33, and 74-35 (refer to Figure 5.5.7-1) will be closed; valves HCV 74-36, 74-37, FCV 74-16, 74-28, 63-93, and 63-94 (refer to Figure 5.5.7-1 and 6.3.1-1) will be opened or verified open. This arrangement will permit flow through the RHR heat exchangers and the four normal cold leg injection paths to the reactor vessel. The water will then flow downward through the annulus, upward through the core (thus cooling the fuel), then exit the vessel directly into the refueling cavity. This results in a water level differential between the spent fuel pit and the refueling cavity with sufficient water head to assure the required return flow through the 20-inch diameter fuel transfer tube thereby completing the path to the spent fuel pit. Except for a portion of the RHR System piping, the only RHR System components utilized below flood elevation are the RHR System heat exchangers. Inundation of these passive components will not degrade their performance for flood mode operation. After alignment, all valves in this cooling circuit located below the maximum flood elevation will be disconnected from their power source to assure that they remain in a safe position. The modified cooling circuit for open reactor cooling will be assured of two operable SFPC System pumps (a third pump is available as a backup) as well as two SFPC System heat exchangers. Also, the large RHR System heat exchangers are supplied with essential raw cooling water during the open reactor mode of fuel cooling; these heat exchangers provide an additional heat sink not available for normal spent fuel cooling. Fuel coolant temperature calculations, assuming conservative heat loads and the most limiting, single active failure in the SFPC System, indicate that the coolant temperatures are acceptable. The temperatures can be maintained at a value appreciably less than the fuel pit temperature calculated for the nonflood spent fuel cooling case when assuming the loss of one equipment train. S2-4.doc 2.4-61 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 As further assurance, the open reactor cooling circuit was aligned and tested, during pre-operational testing, to confirm flow adequacy. Normal operation of the RHR System and SFPC System heat exchangers will confirm the heat removal capabilities of the heat exchangers. High spent fuel pit temperature will cause an annunciation in the MCR, thus indicating equipment malfunction. Additionally, that portion of the cooling system above flood water will be frequently inspected to confirm continued proper operation. For either mode of reactor cooling, leakage from the Reactor Coolant System will be collected, to the extent possible, in the reactor coolant drain tank; nonrecoverable leakage will be made up from supplies of clean water stored in the four cold leg accumulators, the pressurizer relief tank, the cask decontamination tank, and the demineralized water tank. If these sources prove insufficient, the FP System can be connected to the Auxiliary Charging System (subsection 9.3.5) as a backup. Whatever the source, makeup water will be filtered, demineralized, tested, and borated, as necessary, to the normal refueling concentration, and pumped by the Auxiliary Charging System into the reactor (see Figures 2.4A 22.4.14-1 and 2.4A 32.4.14-2). Power Electric power will be supplied by the onsite diesel generators starting at the beginning of Stage II or when offsite power is lost, whichever occurs first (subsection 2.4A.5.3). 2.4.14.2.3 Cooling of Plant Loads Plant cooling requirements, with the exception of the FP System which must supply feedwater to the steam generators, will be met by the ERCW System (refer to subsection Section 9.2.2). 2.4.14.2.4 Power Electric power will be supplied by the onsite diesel generators starting at the beginning of Stage II or when offsite power is lost, whichever occurs first (Subsection 2.4.14.5.3). 2.4.14.2.5 Plant Water Supply The plant water supply is thoroughly discussed in subsection Section 9.2.2. The following is a summary description of the water supply provided for use during flooded plant conditions. The ERCW station is designed to remain fully functional for all floods up to and including the DBF. The CCW intake forebay will provide a water supply for the fire/flood mode pumps. If the flood approaches DBF proportions, there is a remote possibility that Chickamauga Dam will fail. Such an event would leave the Sequoyah SQNPlant CCW intake forebay isolated from the river as flood water recedes below EL 665 ft. Should this event occur, the CCW forebay has the capacity of retained water to supply two steam generators in each unit and provide spent fuel pit with evaporation makeup flow until CCW forebay inventory makeup is established. The ERCW station is designed to be operable for all plant conditions and includes provisions for makeup to the forebay. See Reference 28.FSAR 2.4A.10 1. 2.4A.32.4.14.3 Warning PlanScheme See Subsection 2.4.14.8 (Warning Plan). Plant grade elevation 705 can be exceeded by both rainfall floods and seismic caused dam failure floods. A warning plan is needed to assure plant safety from these floods. 2.4A.3.1 Rainfall Floods Protection of the Sequoyah Plant from the low probability rainfall floods that might exceed plant grade depends on a flood warning issued by TVA's River Operations as described in Section 2.4A.8. With TVA's extensive climate monitoring and flood predicting systems and flood control facilities, floods in the Sequoyah area can be reliably predicted well in advance. The Sequoyah Nuclear Plant flood warning plan will provide a minimum preparation time of 27 hours including a 3 hour margin for operation in the flood mode. Four additional, preceding hours will provide time to gather data and S2-4.doc 2.4-62 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 produce the warning. The warning plan will be divided into two stages the first a minimum of 10 hours long and the second of 17 hours so that unnecessary economic penalty can be avoided while adequate time is ensured for preparing for operation in the flood mode. The first stage, Stage I, of shutdown will begin when there is sufficient rainfall on the ground in the upstream watershed to yield a projected plant site water level of 697 in the winter months (October 1 through April 15) and 703 in the summer (April 16 through September 30). This assures that the additional time required is available when shutdown is initiated. The water level of 703 (two feet below plant grade) will allow margin so that waves due to high winds cannot disrupt the flood mode preparation. Stage I will allow preparation steps causing some damage to be sustained but will withhold major economic damage until the Stage II warning assures a forthcoming flood above grade. The plant preparation status will be held at Stage I until either Stage II begins or TVA's River Operations determines that flood waters will not exceed elevation 703 at the plant. The Stage II warning will be issued only when enough rain has fallen to predict that elevation 703 is likely to be exceeded. 2.4A.3.2 Seismic Dam Failure Floods Protection of the Sequoyah plant from flood waves generated by seismically caused dam failures which exceed plant grade depends on TVAs River Operation organization to identify when a critical combination of dam failures and floods exist. [CEII] [CEII] 2.4A.42.4.14.4 Preparation for Flood Mode An abnormal operating procedure is available to support operation of the plant. At the time the initial flood warning is issued, the plant may be operating in any normal mode. This means that either or both units may be at power or either unit may be in any stage of refueling. 2.4A.4.12.4.14.4.1 Reactors Initially Operating at Power If both reactors are operating at power, Stage I and then, if necessary, Stage II procedures will be initiated. Stage I procedures will consist of a controlled reactor shutdown and other easily revokable revocable steps such as moving supplies necessary to the flood protection plan above the DBF level and making temporary connections and load adjustments on the onsite power supply. Stage II procedures will be the less easily revokable revocable and more damaging steps necessary to have the plant in the flood mode when the flood exceeds plant grade. The fire/flood mode pumps may supply auxiliary feedwater for reactor cooling (Reference 3) [29]. Other essential plant cooling loads will be transferred from the component cooling water to the ERCW System (subsection 9.2.2). Radioactive Waste System (Chapter 11) and CVCS tanks, which are susceptible to flotation will be secured by either filling the tanks during flood preparations or by opening the tanks to allow floodwaters to enter, tanks which are adequately anchored to prevent floatation are exempt from these requirements. Some power and communication lines running beneath the DBF and not designed for submerged operation will require disconnection. Batteries beneath the DBF will be disconnected. 2.4A.4.22.4.14.4.2 Reactor Initially Refueling If time permits, fuel will beis removed from the unit(s) undergoing refueling and placed in the spent fuel pit; otherwise fuel cooling will be accomplished as described in subsection 2.4A.2.2Subsection 2.4.14.2.2. If the refueling canal is not already flooded, the mode of cooling described in subsection 2.4A.2.2Subsection 2.4.14.2.2 requires that the canal be flooded with borated water from the refueling water storage tank. If the flood warning occurs after the reactor vessel head has been removed or at a time when it could be removed before the flood exceeds plant grade, the flood mode reactor cooling water will flow directly from the vessel into the refueling cavity. If the warning time available does not permit this, then the upper head injection piping will be disconnected above the vessel head to allow the discharge of water through the four upper head injection standpipes. Additionally, it is required S2-4.doc 2.4-63 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 that the prefabricated piping be installed to connect the RHR and SFPC Systems, and that ERCW be directed to the secondary side of the RHR System and SFPC System heat exchangers. 2.4A.4.32.4.14.4.3 Plant Preparation Time All The steps needed to prepare the plant for flood mode operation can be accomplished within 24 hours of receipt of the initial warning that a flood above plant grade is possible. An additional 3 hours are available for contingency margin before wave runup from the rising flood might enter the buildings. Site grading and building design prevent any flooding before the end of the 27 hour preflood period. 2.4A.52.4.14.5 Equipment Both normal plant components and specialized flood-oriented supplements will be utilized in coping with floods. All such equipment required in the flood mode is either located above the DBF or is within a non-flooded structure or is designed for submerged operation or otherwise protected. Systems and components needed only in the preflood period are protected only during that period. 2.4A.5.12.4.14.5.1 Equipment Qualification To ensure capable performance in this highly unlikely but rigorous, limiting design case, only high quality components will be utilized. Active components are redundant or their functions diversely supplied. Since no rapidly changing events are associated with the flood, repairability offers reinforcement for both active and passive components during the long period of flood mode operation. Equipment potentially requiring maintenance will be accessible throughout its use, including components in the Diesel Generator Building. 2.4A.5.22.4.14.5.2 Temporary Modification and Setup Normal plant components used in flood mode operation and in preparation for flood mode operation may require modification from their normal plant operating configuration. Such modification, since it is for a limiting design condition and since extensive economic damage is acceptable, will be permitted to damage existing facilities for their normal plant functions. However, most alterations will be only temporary and nondestructive in nature. For example, the switchover of plant cooling loads from the component cooling water to the ERCW System will be done through valves and a prefabricated spool piece, causing little system disturbance or damage. Equipment especially provided for the flood design case includes both permanently installed components and more portable apparatus that will be emplaced and connected into other systems during the preflood period. Detailed procedures to be used under flood mode operation have been developed and are incorporated in the plant's Abnormal Operating InstructionsProcedures. 2.4A.5.32.4.14.5.3 Electric Power Because there is a possibility that high winds may destroy powerlines and disconnect the plant from offsite power at any time during the preflood transition period, only onsite power will be used once Stage II of the preparation period begins. While most equipment requiring alternating current electric power is a part of the permanent emergency onsite power system, other components will be temporarily connected, when the time comes, by prefabricated jumper cables. All loads that are normally supplied by onsite power but are not required for the flood will be switched out of the system during the preflood period. Those loads used during the preflood period but not during flood mode operation will be disconnected when they are no longer needed. During the preparation period, all power cables running beneath the DBF level, except those especially designed for submerged operation, will be disconnected from the onsite power system. Similarly, direct current electric power will be disconnected from unused loads and potentially flooded lines. Charging will be maintained for each battery by the onsite alternating current power system as long as it is required. S2-4.doc 2.4-64 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Batteries that are beneath the DBF will be disconnected during the preflood period when they are no longer needed. 2.4A.5.42.4.14.5.4 Instrument Control, Communication and Ventilation Systems All instrument, control, and communication lines that will be required for operation in the flood mode are either above the DBF or within a non-flooded structure or are designed for submerged operation or otherwise protected. Unneeded cables that run below the DBF will be disconnected to prevent short circuits. Redundant means of communications are provided between the central control area (the main and auxiliary control rooms) and all other vital areas that might require operator attention, such as the Diesel Generator Building. Instrumentation is provided to monitor all vital plant parameters such as the reactor coolant temperature and pressure and steam generator pressure and level. Control of the pressurizer heaters and relief valves and steam generator feedwater flow and atmospheric relief valves will ensure continued natural circulation core cooling during the flood mode. All other important plant functions will be either monitored and controlled from the main control area or, in some cases where time margins permit, from other points in the plant that are in close communication with the main control area. Ventilation, when necessary, and limited heating or air-conditioning will be maintained for all points throughout the plant where operators might be required to go or where required by equipment heat loads. 2.4A.62.4.14.6 Supplies All equipment and most supplies required for the flood are on hand in the plant at all times. Some supplies will require replenishment before the end of the period in which the plant is in the flood mode. In such cases supplies on hand will be sufficient to last through the short time (subsection 2.4A.1.3Subsection 2.4.14.1.3) that flood waters will be above plant grade and until replenishment can be supplied. For instance, there is sufficient diesel generator fuel available at the plant to last for 3 or 4 weeks; this will allow sufficient margin for the flood to recede and for transportation routes to be reestablished. 2.4A.72.4.14.7 Plant Recovery The plant is designed to continue safely in the flood mode for 100 days even though the water is not expected to remain above plant grade for more than 1 to 6 7 days. After recession of the flood, damage will be assessed and detailed recovery plans developed. Arrangements will then be made for reestablishment of offsite power and removal of spent fuel. The 100-day period provides more than adequate time for the development of procedures for any maintenance, inspection, or installation of replacements for the recovery of the plant or for a continuation of flood mode operations in excess of 100 days. A decision based on economics will be made on whether or not to regain the plant for power production. In either case, detailed plans will be formulated after the flood, when damage can be accurately assessed. 2.4A.82.4.14.8 Basis For Flood Protection Plan In Rainfall FloodsWarning Plan SummaryPlant grade elevation 705.0 ft can be exceeded by both rainfall floods and seismic-caused dam failure floods. A warning plan is needed to assure plant safety from these floods. The warning plan is divided into two stages: Stage I, a minimum of 10 hours long and Stage II, a minimum of 17 hours so that unnecessary economic consequences can be avoided, while adequate time is allowed for preparing for operation in the flood mode. Stage I allows preparation steps causing minimal economic consequences to be sustained but will postpone major economic damage until the Stage II warning forecasts a likely forthcoming flood above elevation 705.0 ft. S2-4.doc 2.4-65 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.14.8.1 Rainfall Floods Protection of the SQN from the low probability rainfall floods that might exceed plant grade depends on a flood warning issued by TVA's River Management. With TVA's extensive climate monitoring and flood forecasting systems and flood control facilities, floods in the SQN area can be reliably predicted well in advance. The SQN flood warning plan will provide a minimum preparation time of 27 hours to prepare for operation in the flood mode. Four additional, preceding hours will provide time to gather data and produce the warning. To be certain of 27 hours for pre-flood preparation, flood warnings with the prospect of reaching elevation 705.0 ft must be issued early when lower levels of rainfall on the ground are forecast to reach plant grade in 27 hours. Consequently, some of the warnings may later prove to have been unnecessary. For this reason pre-flood preparations are divided into two stages. Stage I steps requiring 10 hours are easily revocable and cause minimum economic consequences. The estimated probability is small that a Stage I warning will be issued during the life of the plant. Added rain on the ground, stream-flow data and other available storm related information obtained during Stage I activity will determine if the more serious steps of Stage II need to be taken with the assurance that at least 17 hours will be available before elevation 705.0 ft is reached. The probability of a Stage II warning during the life of the plant is very small. The plant preparation status will be held at Stage I until either Stage II begins or TVA River Management determines that flood waters will not exceed elevation 705.0 ft at the plant. The Stage II warning will be issued only when enough rain has fallen to predict that elevation 705.0 ft is likely to be exceeded. 2.4.14.8.2 Seismically-Induced Dam Failure Floods One postulated combination of seismically induced dam failures and coincident storm conditions was shown to result in a flood which could exceed elevation 705.0 ft at the plant. SQNs notification of these floods utilizes TVA River Management forecast system to identify when a critical combination exists. Stage I shutdown is initiated upon notification that a critical dam failure has occurred or loss of communication prevents determining if a potential flood exceeding plant grade is expected to occur. Stage I shutdown continues until it has been determined positively that flooding exceeding plant grade will not occur. If communications do not document this certainty, shutdown procedures continue into Stage II activity. Stage II shutdown continues to completion or until it has been determined positively that flooding exceeding plant grade will not occur. 2.4.14.9 Basis For Flood Protection Plan In Rainfall Floods 2.4.14.9.1 Overview Large Tennessee River floods can exceed plant grade elevation 705 705.0 ft at Sequoyah Nuclear PlantSQN. Plant safety in such an event requires shutdown procedures, which may takecan be implemented within 247 hours to implement. TVA flood forecast procedures will provide at least 27 hours of warning before river levels reach elevation 703705.0 ft. Stage II activities required to be completed prior to flood waters reach 705.0 ft are not impacted by wind wave effects.Use of elevation 703, 2 feet below plant grade, provides enough freeboard to prevent waves from 45 mile per hour, overwater winds from endangering plant safety during the final hours of shutdown activity. For conservatism the fetches calculated for the PMF (Figures 2.4.3 15 and 2.4.3 16) were used to calculate maximum wind wave additive to the reservoir surface at elevation 703 feet msl. The maximum wind additive to the reservoir surface would be 2.8 feet and would not endanger plant safety during the final hours of shutdown. This is due Due to the long shallow approach and the waves breaking at the perimeter road (elevation 705 feet msl705.0 ft)., After the waves break there is not sufficient depth or distance between the perimeter road and the safety-related facilities for new waves to be generated. Forecast will be based upon rainfall already reported to be on the ground and supplemented by available stream flow data and other storm related information which can aid in predicting downstream flooding elevations. S2-4.doc 2.4-66 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Different target river level criteria are needed for winter use and for summer use to allow for seasonally varied reservoir levels and rainfall potential. To be certain of 27 hours for preflood preparation, warnings of floods with the prospect of reaching elevation 703 705.0 ft must be issued early; consequently, some of the warnings may later prove to have been unnecessary. For this reason preflood preparations are divided into two stages. Stage I steps, requiring 10 hours, would be easily revokable revocable and cause minimum economic damage. The estimated probability is less than 0.0026 that a Stage I warning will be issued during the 40 year life of the plant. Additional rain and streamflow information obtained during Stage I activity will determine if the more damaging steps of Stage II need to be taken with the assurance that at least 17 hours will be available before elevation 703 705.0 ft is reached. The estimated probability is less than 0.0010 that shutdown will need to continue into Stage II during plant life. Flood forecasting and warnings, to assure adequate warning time for safe plant shutdown during floods, will be conducted by River Operations of River System OperationsTVA River Management. 2.4.14.9.2 TVA Forecast System (HISTORICAL INFORMATION) TVA has in constant use an extensive, effective system to forecast inflow and control elevation as needed in the Tennessee River Basin. This permits efficient operation of the reservoir system and provides warning of when water levels will exceed critical elevations at selected, sensitive locations, which includes SQN. The TVA River Forecast Center (RFC) normal operation produces daily forecasts twice daily using the most recent data observations. During major flood events, the RFC may issue forecasts as frequently as every 6 hours. Elements of the forecast system include the following: 1 More than 200 rain gages measure rainfall, with an average density of about 200 sq mi per rain gage. The gages are Geostationary Operational Environmental Satellites (GOES) Data Collection Platform (DCP) satellite telemetered gages. The gages are relayed to TVA directly and also to the National Weather Service (NWS), which combines this gage data with radar data to return TVA a gridded rainfall observation dataset every hour. TVA backs up this feed with an in-house gage interpolation gridding. The rainfall gages transmit 15-minute rainfall accumulations during normal operations.

2. Stage data from ~100 gages are collected and converted to flow using rating curves. The gages are GOES Data Collection Platform satellite telemetered gages. The satellite gages transmit 15-minute stage data during normal operations.

3. Real-time headwater elevation, tailwater elevation, and discharge data are received from 31 TVA hydro projects and hourly data are received from minor non-power projects and non-TVA hydro plants, such as those owned by Brookfield Smoky Mountain Hydro and from the USACE for areas upstream of Barkley Dam.

4 Gridded weather forecasts, including a quantitative precipitation forecast (QPF), are received up to four times twice daily and at other times when changes are expected. QPF forecasts are sourced from the NWS Weather Prediction Center (WPC) for the most likely, 95% maximum and 95% minimum forecasts. QPFs are also sourced from the European Centre for Medium-Range Weather Forecasts (ECMWF), North American Ensemble Forecast System (NAEFS), and the High-Resolution Rapid Refresh (HRRR) System. Gridded temperature, dewpoint, pressure, wind, cloud cover, relative humidity, and shortwave radiation forecasts are also received from the NWS.

5. Computer programs are used two to four times daily to translate rainfall into streamflow based on current runoff conditions and which permit a forecast of flows and elevations based upon S2-4.doc 2.4-67 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 both observed and predicted rainfall. The hydrologic model has 140 sub-basins, each of which can be hand-adjusted at each forecast period to assimilate any discrepancies between the model and reality. A network of severs and computers are utilized and are designed to provide backup for each other. One computer is used primarily for data collection, with the other used for executing forecasting programs for reservoir operations. The time interval between receiving input data and producing a forecast is less than 4 hours. Forecasts normally cover at least a three-day period.

6. At each forecast period, a series of RiverWare reservoir routing models are executed in order to meet the operating policy and objectives described in the TVA Reservoir Operations Policy. These models are well-parameterized and easy for experienced operators to use for making fast tradeoff decisions. The output from the RiverWare routing model runs is used as input into a HEC-RAS model that is used to predict river level elevations at locations along the Tennessee River.

As effective as the forecast system already is, it is constantly being improved as new technology provides better methods to interrogate the watershed during floods and as the watershed mathematical model and computer system are improved. TVA has transitioned to a system that is built of components, which are widely used throughout the industry. This makes them easier to maintain and allows for easy enhancements as the industry advances. The TVA forecast center is manned 24 hours a day, 7 days per week, 365 days per year.TVA has in constant use an extensive, effective system to forecast flow and elevation as needed in the Tennessee River Basin. This permits efficient operation of the reservoir system and provides warning of when water levels will exceed critical elevations at selected, sensitive locations. Elements of the present (2001) forecast system above Sequoyah Nuclear Plant include the following:

1. One hundred sixty (160) rain gages measure rainfall, with an average density of 165 square miles per rain gage. Of these gages 112 are owned by TVA, 35 are owned by the National Weather Service (NWS), 7 are owned by the United States Geological Service (USGS), 2 are owned by the United States Corps of Engineers (USACE), and 4 are owned by Alcoa. Most of these gages are tipping buckets collector type and the transmission of the data is either by satellite or telephone. At some of the gages located at hydroplants, the data is manually read.

Information normally is received daily from the gages at 6 a.m. and at least every 6 hours during flood periods. Close interval rainfall reports can be obtained from a majority of the gages if needed.

2. Streamflow data are received for 35 gages from 16 TVA gages amd 19 USGS gages. These gages trasmit their data either by satellite or telephone or both. Discharge data are received from 26 hydroplants. Of these plants, 25 also transmit headwater elevation data, and 13 transmit tailwater elevation data. Therefore, steamflow data are available from 61 locations.

Streamflow data are received daily at 8 a.m. and at least every 2 hours if needed during flood operations.

3. Weather forecasts including quantitative precipitation forecasts are received four times daily and at other times when changes are expected.

4. Computer programs which translate rainfall into streamflow based on current runoff conditions and which permit a forecast of flows and elevations based upon both observed and predicted rainfall. Two separate computers are utilized and are designed to provide backup for each other. One computer is used primarily for data collection, with the other used for executing forecasting programs for reservoir operations. The time interval between receiving input data and producing a forecast is less than 4 hours. Forecasts normally cover at least a 8 day period.

As effective as the forecast system already is, it is constantly being improved as new technology provides better methods to interrogate the watershed during floods and as the watershed mathematical model and computer system are improved. Also, in the future, improved quantitative S2-4.doc 2.4-68 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 precipitation forecasts may provide a more reliable early alert of impending major storm conditions and thus provide greater flood warning time. The TVA forecast center is manned 24 hours a day. Normal operation produces two forecasts daily, one by 12 noon based on data collected at 6 a.m. Central time, and the second by 4 a.m. based on data collected at midnight Central Time. When serious flood situations demand, forecasts are produced every 4 hours. 2.4.14.9.3 Basic Analysis The forecast procedure to assure safe shutdown of SQN for flooding is based upon an analysis of hypothetical storms up to PMP magnitude. The storms enveloped potentially critical areal and seasonal variations and time distributions of rainfall. To be certain that fastest rising flood conditions were included, the effects of varied time distribution of rainfall were tested by consideration of the maximum daily PMP in the first, middle, and the last day of the three-day main storm as well as various PMP nesting combinations using the methodology described in Subsection 2.4.3.1. Multiple project PMF, warning time specific and seismic simulations potentially controlling for warning time were reviewed and formally documented in calculations. The PMP nesting combinations producing the maximum flood elevation at the SQN site do not produce the quickest warning time PMP nesting combination because the maximum flood elevation PMPs include dam failures upstream of SQN occurring after the flood elevation reached plant grade of 705.0 ft. The warning system is based on those storm situations which resulted in the shortest time interval between watershed rainfall selected to initiate a River Operations Stage I warning and elevation 705.0 ft at SQN, thus assuring that this elevation could be predicted at least 27 hours in advance. The procedures used to compute flood flows and elevations for those flood conditions which establish controlling elements of the forecast system are described in Section 2.4.3.To develop a forecast procedure to assure safe shutdown of Sequoyah Nuclear Plant for flooding, 17 hypothetical PMP storms, including their antecedent storms, were analyzed. They enveloped potentially critical seasonal variations and time distributions of rainfall. To be certain that fastest rising flood conditions were included, the effects of varied time distribution of rainfall were tested by alternatively placing the maximum daily PMP on the first, the middle, and the last day of the 3 day main storm. In each day the maximum 6 hour depth was placed during the second interval except when the maximum daily rain was placed on the last day. Then the maximum 6 hour amount was placed in the last 6 hours. The procedures used to compute flood flows and elevations are described in subsections 2.4.3.1, 2.4.3.2, and 2.4.3.3. Some flood events were analyzed using earlier versions of the watershed model described in subsection 2.4.3.3. Those events which established important elements of the warning system or those where the present model might produce significant differences in warning times have been reevaluated. Events reevaluated have been noted either in tables or figures where appropriate. The warning system is based on those storm situations which resulted in the shortest time interval between watershed rainfall and elevation 703, thus assuring that this elevation could be predicted at least 27 hours in advance. 2.4.14.9.4 Hydrologic Basis for Warning System A minimum of 27 hours has been allowed for preparation of the plant for operation in the flood mode. An additional 4 hours for communication and forecasting computations are provided to allow TVA River Forecasting Center to translate rain on the ground to river elevations at the plant. Hence the warning plan must provide 31 hours from arrival of rain on the ground until critical elevation 703 705.0 ft could be reached. The 27 hours allowed for shutdown at the plant are utilized for a minimum of 10 hours of Stage I preparation and an additional 17 hours for Stage II preparation that is not concurrent with River Management Stage I activity. This 27 hour allocation includes a 3 hour margin. Although river elevation 703, 2 feet below plant grade to allow for wind waves, is critical during final stages of plant shutdown for flooding, lower forecast target levels are used in most situations to assure that the 27 hours preflood transition interval will always be available. The target river levels differ with season. When rain on the ground on the watershed above Chickamauga Dam reaches 3.0 inches in 72 hours S2-4.doc 2.4-69 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 or less, the River Operations Forecast Center begins predicting flooding levels at the SQN site. Communication is maintained between the Forecast Center and Plant Operations. A Stage I warning, which starts shutdown procedures, will be initiated as soon as 5.6 inches of rainfall on the ground in 72 hours or less is reached and elevation 705.0 ft is predicted at the SQN site. A Stage I warning is also initiated when the failure of a critical dam has occurred. Stage II shutdown will be initiated and carried to completion if rain on the ground reaches 7.6 inches in 72 hours or less and when target river elevation 705.0 ft at SQN has been forecast in 17 hours or less. Regardless of rain on the ground, a Stage II shutdown warning is given when plant grade is projected to be reached in 17 hours or less. During the October 1 through April 15 "winter" season, Stage I shutdown procedures will be started as soon as target river elevation 697 has been forecast. Shutdown will be carried to completion if and when target river elevation 703 has been forecast. Corresponding target river elevation for the April 16 through September 30 "summer" season is 703. The one target river elevation in the summer season permits waiting to initiate shutdown procedures until enough rain is on the ground to forecast reaching critical elevation 703; shutdown would then be initiated and carried to completion. Inasmuch as the hydrologic procedures and target river elevations have been designed to provide adequate shutdown time in the fastest rising flood, longer times will be available in other floods. In such cases there will may be a waiting period after the Stage I 10-hour shutdown activity during which activities shall be in abeyance until it is predicted from recorded rainfall that Stage II shutdown should be implemented or it is determined from weather conditions that plant operation can be resumedweather conditions determine if normal plant operation can be resumed, or if Stage II shutdown should be implemented. Resumption of plant operation following Stage I shutdown activities will be allowable only after flood levels and weather conditions, as determined by TVA River Management, have returned to a condition in which 27 hours of warning will again be available. River Scheduling of River Operations prepares at least an 8 day water level forecast seven days per week for Tennessee River locations. During prospective flooding conditions forecasts can be prepared 4 times a day so that warnings for Sequoyah will assure that 27 hours always will be available to shut down the plant and prepare it for flooding. 2.4.14.9.5 Hydrologic Basis for Target StagesWarning Times and Triggers The analysis of multiple PMF simulations performed in Section 2.4.3 show that the simulations with the highest water surface elevation at SQN result from postulated upstream and cascading dam failures starting on the [CEII] Since PMF simulations with the highest water surface elevation may not be the fastest rising PMF simulation to reach plant grade, additional simulations were required to determine alternate storms that reach plant grade in the least amount of time. Preliminary analysis of PMP storms with potentially the shortest warning time indicated sensitivity to spatial, temporal, and seasonal occurrences. This sensitivity required iterative analysis of various temporal distributions (front, medial and back loaded rainfall), variable spatial distribution nesting sequences and multiple week/season considerations. The analysis for SQN shows that two PMF events produced the rainfall lower boundary decision limits. (1) The Ocoee #1 below Blue Ridge primary nesting with the 72-hour back-loaded temporal distribution during week 21 produced the rainfall lower boundary decision limits of 4.36 hours for modeling and communications time as well as 32.55 hours for total warning time. (2) The Chatuge/Nottely primary nesting with a 48-hour front-loaded temporal distribution during week 19 produced the rainfall lower boundary decision limit of 27.35 hours for the Stage I call to plant shutdown period and 17.36 hours for the Stage II call to plant shutdown period. These values are greater than or equal to the specified minimum design basis warning times. S2-4.doc 2.4-70 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The seismic event resulting in consequential flooding at SQN has a calculated warning time that includes a maximum 1.5-hours for review and communications [64], and 27-hours of flood mode preparation time. The results of the analysis of these simulations were reviewed against the required minimum warning time durations of 4 hours for modeling and communication, 10 hours for Stage I activities and 17 hours for Stage II activities. Based on this review, the minimum rain of the ground values used by the River Forecast Center to (1) begin analysis of the potential flooding impact at the SQN site, (2) initiate the Stage I warning trigger review, and (3) initiate the Stage II warning trigger review were selected. The rain on the ground values represent the initial rainfall expected in the initial stages of the 72 hour main storm with the rainfall in the balance of the main storm still to come. A rain on the ground value of 3.0 inches to initiate Forecasting Center analysis provided a minimum of 4 hours for analysis and communication as well as at least 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. A rain on the ground value of 5.6 inches, gave a minimum of 27 hours for Stage I and Stage II activities and more than 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. A rain on the ground value of 7.6 inches provided at least 17 hours for Stage II activities and more than 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. To ensure that flood level forecast at the SQN site are not subject to sudden changes from dam failures, a Stage I warning is initiated for the failure of any critical dam upstream of SQN. Using the rain on the ground trigger values combined with River Forecast Center projections of flooding at the SQN site, the minimum durations required for flood mode operation activities are provided. Figure 2.4.14-3 (Sheet 1 and 2) show the controlling simulations for PMP rainfall distribution, the target forecast flood warning time and the rain on the ground thresholds above Chickamauga Dam which assure adequate warning times for SQN. The fastest rising PMF simulation with the limiting initial notification time (>4 hours) and the limiting overall warning time (>31 hours) is shown in Figure 2.4.14-3 (Sheet 1). Figure 2.4.14-3 (Sheet 2) shows the rainfall distribution for the fastest rising PMP storm controlling the limiting Stage I rain on the ground notification (>27 hours) and the limiting Stage II warning time (>17 hours). The PMP storms have been preceded three days earlier by a three-day storm having 40 percent of PMP storm rainfall applied with the spatial distribution coincident with that of the main storm PMP. Figure 2.4A. 4, in four parts, shows how target forecast flood elevations at the Sequoyah plant have been determined to assure adequate warning times. The floods shown are the fastest rising floods at the site which are produced by the 21,400 square mile PMP with downstream centering described in subsection 2.4.3.1. The storms are the main PMP amounts and have been preceded 3 days earlier by a 3 day storm having 40 percent of the main storm rainfall. This has caused soil moisture to be high and reservoirs to be well above seasonal levels when the main storm begins. Figure 2.4A. 4 (A, B, and C) shows the winter PMP which could produce the fastest rising flood which would cross plant grade and variations caused by changed time distribution. The fastest rising flood occurs during a PMP when the 6 hour increments increase throughout the storm with the maximum 6 hours occurring in the last period. Figure 2.4A 4 (B) shows the essential elements of this storm which provides the basis for the warning scheme. In this flood 9.2 inches of rain would have fallen 31 hours (27 + 4) prior to the flood crossing elevation 703 and would produce elevation 697 at the plant. Hence, any time rain on the ground results in a predicted plant stage of 697 a Stage I shutdown warning will be issued. Examination of Figure 2.4A. 4 (A and C) shows that following this procedure in these noncritical floods would result in a lapsed time of 42 and 44 hours between when 9.2 inches had fallen and the flood would cross critical elevation 703. An additional 2.2 inches of rain must fall promptly for a total of 11.4 inches of rain to cause the flood to cross critical elevation 703. In the fastest rising flood, Figure 2.4A. 4 (B), this rain would have fallen in the next 5 hours. A Stage II warning would be issued within the next 4 hours. Thus, the Stage II warning would be issued 5 hours after issuance of a Stage I warning and 22 hours before the flood would cross critical flood elevation 703. In the slower rising floods, Figure 2.4A. 4 (A and C), the time between issuance of a Stage I warning and when the 11.4 inches of rain required to put the flood to elevation 703 would have occurred is 6 and 10 hours respectively. This would result in issuance of a Stage II warning not less than 4 hours later or 32 and 30 hours respectively before the flood would reach elevation 703. S2-4.doc 2.4-71 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The summer flood shown by Figure 2.4A. 4 (D), with the maximum 1 day rain on the last day provides controlling conditions when reservoirs are at summer levels. At a time 31 hours (27 + 4) before the flood reaches elevation 703, 11 inches of rain would have fallen. This 11 inches of rain, under these runoff conditions, would produce critical elevation 703, so this level becomes both the Stage I and Stage II target. The above criteria all relate to forecasts which use rain on the ground. The rain on the ground criteria are selected to be small enough such that protective actions are taken in time to provide the required durations for flood protection activities should the remaining rainfall in the PMP main storm defined by Topical Report TVA-NPG-AWA16-A follow. In actual practice quantitativeQuantitative rain forecasts, which are already a part of daily operations, would be used by the River Operations Forecast Center in flooding projections to provide advance alertsStage I or Stage II warnings that flood mode configurations are needed for shutdown may be imminent. Only rain on the ground, however, is included in the procedure for firm warning use. Because the above analyses have used fastest possible rising floods at the plant, all other floods will allow longer warning times than required for all physical plant shutdown activity. In summary, the predicted target levelsforecast rain on the ground which will assure adequate shutdown times are: Forecast Rain on the Ground Above Chickamauga Dam Initiation of Monitoring Stage I shutdown Stage II shutdown 3.0 inches 5.6 inches 7.6 inches in 72 hours or less in 72 hours or less in 72 hours or less Forecast Flood Elevations at Sequoyah For For Season Stage I Shutdown Stage II Shutdown Winter (October 1 April 15) 697 703 Summer (April 16 September 30) 703 703 2.4.14.9.6 Communications Reliability (HISTORICAL INFORMATION) Communication between projects in the TVA power system is via a (a) TVA owned microwave network, (b) Fiber-Optic System, and (c) by commercial telephone. In emergencies, additional communication links are provided by Transmission Power Supply radio network. The four These networks provide a high level of dependability against emergencies. Additionally, River Management has available satellite telephone communications with the TVA hydro projects upstream of Chattanooga. The hydrologic network for the watershed above Sequoyah that would be available in flood emergencies if commecial telephone communications is lost include 138 rainfall gages (24 at power installations and 114 satellite and file transfer gages) and 47 streamflow gages (26 at hydroplants, 20 satellite gages, and 1 file transfer gage). River Scheduling River Management is linked to the TVA power system by all four communication networks. The data from the satellite gages are received via a data collection platform-satellite computer system located in the River Schedulings River Management office. These are so distributed over the watershed that reasonable flood forecasting can be done from this data while the balance of data is being secured from the remaining hydrologic network stations. The preferred, complete coverage of the watershed, employ 160 rainfall and 61 streamflow locations above the Sequoyah plant. Involved in the communications link to these locations are routine radio, radio satellite, and commercial telephone system networks. In an emergency, available radio communications would be called upon to assist. S2-4.doc 2.4-72 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The various networks proved to be capable in the large floods of 1957, 1963, 1973, 1984, 1994, and 1998 of providing the rain and streamflow data needed for reliable forecasts. 2.4A.92.4.14.10 Basis for Flood Protection Plan in Seismic-Caused Dam Failures Plant grade would be exceeded by one of the four candidate seismic failure combinations evaluated, thus requiring emergency measures. Table 2.4.4-1, shows the maximum elevations at SQN for the candidate combinations. [CEII]

                                                                                    Figure 2.4.14-3 (Sheet 3) shows the time between the seismic event and the resulting flood wave crossing plant grade elevation 705.0 ft is 28.5 hours.Floods resulting from combined seismic and flood events can exceed plant grade, thus requiring emergency measures. The 1998 reanalysis showed that only two combinations of seismic dam failures coincident with a flood would result in floods above plant grade:

(1) [CEII]

           , (2)          [CEII]                                                                  . As shown in Table 2.4.4 1 all other potentially critical candidates would create flood levels below plant grade elevation 705.

Dam failure during non-flood periods would not present a problem at the plant. The reanalysis showed that failure in a non flood period and at summer flood guide levels in the most critical dam failure combination ( [CEII] ) would produce a maximum elevation of [CEII] All other combinations in non flood periods would produce elevations much lower.was not evaluated, but would be bounded by the four critical failure combinations. The time from seismic occurrence to arrival of failure surge at the plant is adequate to permit safe plant shutdown in readiness for flooding. Table 2.4A 2 lists the time between the postulated seismic event and when the flood wave would exceed plant grade elevation 705 and elevation 703. Use of elevation 703 provides a margin for possible wind wave effects. The warning plan for safe plant shutdown is based on the fact that a combination of critically centered large earthquake and rain produced flood conditions must coincide before the flood wave from seismically caused dam failures will cross approach plant grade. In flood situations, an extreme earthquake must be precisely located [CEII] before a flood threat to the site would exist. The combination producing the shortest time interval between seismic event and plant grade crossing is a [CEII] [CEII] The time between the seismic event and the resulting flood wave crossing plant grade elevation 705 is 40 hours. The time to elevation 703, which allows a margin for wind wave considerations, is 35 hours. The event producing the next shortest time interval to elevation 703 involves the [CEII] resulting in a time interval of 63 hours. The warning system utilizes TVA's flood forecast system to identify when flood conditions will be such that seismic failure of critical dams could cause a flood wave to exceed elevation 703 at the plant site. In addition to the critical combinations, failure of a single major upstream dam will lead to an early warning. A Stage I warning is declared once a failure of [CEII] [CEII] has been confirmed. Two levels of warning will be provided: (1) an early warning will be issued to SQN whenever a dam S2-4.doc 2.4-73 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 failure has occurred or is imminent for any single critical dam; or it appears from rain and flood forecasts that a critical situation may develop and (2) a flood warning or alert to begin preparation for plant shutdown when a critical situation exists that will result in the flood level to exceeding plant grade. A Stage I flood warning is declared once failure of critical dams has been confirmed and flood conditions are such that the flood surge will exceed plant grade. It shall be issued at least 27 hours before the flood level exceeds elevation 703 at the site. A Stage II flood warning will be issued at least 17 hours before the flood level exceeds elevation 703 at the site. Communication will be established and maintained during these two levels of warning to assure the 27 hour flood preparation period. Any prolonged interruption of communication or failure to confirm that a critical case has not occurred will result in the initiation of flood preparation at the plant site. The flood preparation shall continue until completion, unless communication is re established and the site is notified that a critical case has not occurred.If loss of or damage to an upstream dam is suspected based on monitoring by TVA River Management, efforts will be made by TVA to determine whether dam failure has occurred. If the critical case has occurred or it cannot be determined that it has not occurred, Stage I shutdown will be initiated. Once initiated, the flood preparation procedures will be carried to completion, unless it is determined that the critical case has not occurred. Communication between projects in the TVA power system is discussed in Subsection 2.4.14.9.6). Communications between the plant, dams, power system control center, and River Operations at Knoxville, Tennessee, are provided by microwave networks, fiber optic network, radio networks, and commercial telephone service. 2.4.14.11 Special Condition Allowance The flood protection plan is based upon the minimum time available for the worst case. This worst case provides adequate preparation time including contingency margin for normal and anticipated plant conditions including anticipated maintenance operations. It is conceivable, however, that a plant condition might develop for which maintenance operations would make a longer warning time desirable. In such a situation the Plant Manager determines the desirable warning time and contacts TVA River Management to determine if the desired warning time is available. If weather and reservoir conditions are such that the desired time can be provided, special warning procedures will be developed, if necessary, to ensure the time is available. This special case continues until the Plant Manager (or designee) notifies TVA River Management that maintenance has been completed. If threatening storm conditions are forecast which might shorten the available time for special maintenance, the Plant Manager is notified by River Management and steps taken to assure that the plant is placed in a safe shutdown mode. 2.4.15 References

1. U.S. Weather Bureau, "Probable Maximum and TVA Precipitation Over The Tennessee River Basin Above Chattanooga," Hydrometeorological Report No. 41, 1965Not used.

2. U.S. Weather Bureau, "Probable Maximum and TVA Precipitation for Tennessee River Basins Up To 3,000 Square Miles in Area and Duration to 72 Hours," Hydrometeorological Report No.

45, 1969Not used.

3. Tennessee Valley Authority, Submittal of Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, Revision 1, (EPID L-2016-TOP-0011), May 21, 2019.Garrison, J.

M., Granju, J. P., and Price, J. T., "Unsteady Flow Simulation in Rivers and Reservoirs," Journal of the Hydraulics Division, ASCE, Vol. 95, No. HY5, Proceedings Paper 6771, September 1969, pp. 15559 1576.

4. PSAR, Phipps Bend Nuclear Plant, Docket Nos. 50 553, 50 554.Not used.

5. Tennessee Valley Authority, "Flood Insurance Study, Hamilton County, Tennessee, (Unincorporated Areas)," Division of Water Resources, February 1979Not used.

6. U.S. Army Engineering, Corps of Engineers, Omaha, Nebraska, "Severe Windstorms of Record," Technical Bulletin No. 2, Civil Works Investigations Project CW 178 Freeboard Criteria for Dams and Levees, January 1960Not used.

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

7. U.S. Army Corps of Engineers, "Computation of Freeboard Allowances for Waves in Reservoirs," Engineering Engineer Technical Letter No. 1110 2 8, August 1966Not used.

8. U.S. Army Coastal Engineering Research Center, "Shore Protection, Planning, and Design," 3rd Edition, 1966Not used.

9. Reference removed per Amendment 6.

10. Hinds, Julian, Creager, William P., and Justin, Joel D., "Engineering For Dams," Vol. II, Concrete Dams, John Wiley and Sons, Inc., 1944Not used.

11. Bustamante, Jurge I., Flores, Arando, "Water Pressure in Dams Subject to Earthquakes,"

Journal of the Engineering Mechanics Division, ASCE Proceedings, October 1966Not used.

12. Chopra, Anil K., "Hydrodynamic Pressures on Dams During Earthquakes," Journal of the Engineering Mechanics Division, ASCE Proceedings, December 1967Not used.

13. Zienkiewicz, O. C., "Hydrodynamic Pressures Due to Earthquakes," Water Power, Vol. 16, September 1964, pp. 382 388Not used.

14. Tennessee Valley Authority, "Sedimentation in TVA Reservoirs," TVA Report No. 0 6693, Division of Water Control Planning, February 1968Not used.

15. Reference removed per Amendment 6.

16. Tennessee Valley Authority, Hydrology Project White Paper, Areal Application of PMP Event Data for the Tennessee River Model above Wheeler Dam (B41170420001)Cristofano, E. A.,

     "Method of Computing Erosion Rate for Failure of Earthfill Dams," Engineering and Research Center, Bureau of Reclamation, Denver 1966.

17. "The Breaching of the Oros Earth Dam in the State of Ceara, North East Brazil," Water and Water Engineering, August 1960Not used.

18. NRC letter to TVA dated December 8, 1989, "Chickamauga Reservoir Sediment Deposition and Erosion Sequoyah Nuclear Plant, Units 1 and 2Not used."

19. Programmatic Environmental Impact Statement, TVA Reservoir Operations Study, Record of Decision, May 2004.

20. Tennessee Valley Authority, RvM-SOP-10.05.06, Nuclear Notifications and Flood Warning Procedure, Revision 0005, dated 11-15-2021Updated Predictions of Chickamauga Reservoir Recession Resulting from Postulated Failure of the South Embankment at Chickamauga Dam; TVA River System Operations and Environment, Revised June 2004 (B85 070509 001).

21. Monitoring and Moderating Sequoyah Ultimate Heat Sink, June 2004, River System Operations and Environment, River Operations, River Scheduling (B85 070509 001).

22. Tennessee Valley Authority, SQN Calculation MDQ0026970001A, High Pressure Fire Protection Supply to the Steam Generators for Flood Mode Operation.

23. Newton, Donald W., and Vineyard, J. W., "Computer-Determined Unit Hydrographs From Floods," Journal of the Hydraulics Division, ASCE, Volume 93, No. HY5, September 1967.

24. U.S. Army Corps of Engineers, Hydrologic Engineering Center, River Analysis System, HEC-RAS computer software, version 3.1.3.

25. Federal Emergency Management Agency (FEMA), "Federal Guidelines for Dam Safety:

Earthquake Analysis and Design of Dams, FEMA 65, May 2005. S2-4.doc 2.4-75 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

26. Price, J. T. and Garrison, J. M., Flood Waves From Hydrologic and Seismic Dam Failures,"

paper presented at the 1973 ASCE National Water Resources Engineering Meeting, Washington, D. C. 2.4A.10 References 127. Tennessee Valley Authority, SQN-DC-V-1.1, Design of Reinforced Concrete Structures Design Criteria. 228. Tennessee Valley Authority, SQN-DC-V-12.1, Flood Protection Provisions Design Criteria. 329. Tennessee Valley Authority, SQN-DC-V-43.0, High Pressure Fire Protection Water Supply System.

30. Tennessee Valley Authority, Calculation CDQ0000002014000018, HEC-RAS Tributary Model Calibration, Revision 2.

31. Tennessee Valley Authority, Calculation CDQ0000002014000016, Tributary Dam Rating Curves, Revision 3 and Calculation CDQ0000002013000007, Main Stem Initial Dam Rating Curves, Revision 4.

32. Tennessee Valley Authority, Calculation CDQ0000002014000019, HEC-RAS Tributary Model Unsteady Flow Rules, Revision 3.

33. Tennessee Valley Authority, Calculation CDQ000020080053, PMF Inflow Determination, Revision 1.

34. Tennessee Valley Authority, Calculation CDQ000020080050, Flood Operational Guide, Revision 3.

35. Tennessee Valley Authority, Calculation CDQ0000002014000017, HEC-RAS Tributary Geometry Development, Revision 1.

36. Tennessee Valley Authority, Calculation CDQ000020080024, SOCH Geometry Verification - Ft.

Loudoun Reservoir, French Broad River, and Holston River, Revision 2.

37. Tennessee Valley Authority, Calculation CDQ000020080025, SOCH Geometry Verification -

Tellico Reservoir and Tellico/Ft. Loudoun Canal, Revision 2.

38. Tennessee Valley Authority, Calculation CDQ000020080026, SOCH Geometry Verification -

Watts Bar Reservoir, Revision 2.

39. Tennessee Valley Authority, Calculation CDQ000020080029, SOCH Geometry Verification -

Melton Hill Reservoir, Revision 2.

40. Tennessee Valley Authority, Calculation CDQ000020080030. SOCH Geometry Verification -

Chickamauga Reservoir, Revision 2.

41. Tennessee Valley Authority, Calculation CDQ000020080031, SOCH Geometry Verification -

Nickajack Reservoir, North Chickamauga Creek, Lick Branch (Dallas Bay), Revision 2.

42. Tennessee Valley Authority, Calculation CDQ000020080032, SOCH Geometry Verification -

Guntersville Reservoir, Revision 3.

43. Tennessee Valley Authority, Calculation CDQ000020080033, SOCH Geometry Verification -

Wheeler Reservoir, Revision 0.

44. Tennessee Valley Authority, Calculation CDQ000020080051, Reservoir Storage Tables, Revision 2.

45. Tennessee Valley Authority, Calculation CDQ0000002014000021, HEC-RAS Model Setup, Revision 5.

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

46. River Operations Procedure RO-SPP-27.1, "RO-Design and Evaluation of New and Existing Dams," Revision 2.

47. Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Subcommittee, Interagency Advisory Committee on Water Data, Office of Water Data Coordination, Geological Survey, U.S. Department of the Interior, Revised September 1981 with March 1982 Editorial Corrections.

48. Moore, James N. and Ray C. Riley, Comparison of Temporal Rainfall Distributions for Near Probable Maximum Precipitation Storm Events for Dam Design, National Water Management Center, Natural Resources Conservation Service, (NRCS), Little Rock, Arkansas.

49. Hovey, Peter and Thomas DeFiore, Using Modern Computing Tools to Fit the Pearson Type III Distribution to Aviation Loads Data, Report # DOT/FAA/AR-03/62, Office of Aviation Research, Federal Aviation Administration, U.S. Department of Transportation, Washington, D.C.,

September 2003.

50. Bonnin, Geoffrey M., Deborah Martin, Bingzhang Lin, Tye Parzybok, Michael Yekta, David Riley, NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 2 Version 3.0:

Delaware, District of Columbia, Illinois, Indiana, Kentucky, Maryland, New Jersey, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland, 2004, revised 2006.

51. Hershfield, David M., Technical Paper No. 40 Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years, Department of Commerce, Cooperative Studies Section, Hydrologic Services Division for Engineering Division, Soil Conservation Service, U.S. Department of Agriculture; Washington, D.C. May 1961, Repaginated and Reprinted January 1963.

52. NRC, NUREG-2115, Central and Eastern United States Seismic Source Characterization for Nuclear Facilities, January 2012.

53. EPRI, 2004/2006 Ground-Motion Models; CEUS Ground Motion Project Final Report 1009684, 2004; Program on Technology Innovation: Truncation of the lognormal distribution and value of the standard deviation for ground motion models in the Central and Eastern United States, Final Report 1014381, 2006.

54. Tennessee Valley Authority, Calculation CDQ0000002013000057, Sequoyah Local Intense Precipitation Evaluation-SAR Design Basis (Case 1) and Flood Hazard Re-evaluation Report (Case 2), Revision 4.

55. Tennessee Valley Authority, Calculation CDQ0000002017000059, Chickamauga Dam (CHH)

Probable Maximum Flood Analysis, Revision 1.

56. Tennessee Valley Authority, Calculation CDQ0000002014000033, Wind Waves for Combined Effect Floods, Revision 4.

57. Tennessee Valley Authority, Calculation CDQ0000002014000024, Seismic Dam Failure Combined with Rainfall Event Simulations, Revision 6.

58. US Army Corps of Engineers Hydrologic Engineering Center, HEC-DSSVue HEC Data Storage System Visual Utility Engine Users Manual, Version 2.0, July 2009.

59. Department of the Army, Office of the Chief of Engineers, Engineering Technical Letter No.

1110-2-8, August 1, 1966.

60. Tennessee Valley Authority, Calculation CDQ0000002016000043, Controlling Storm Type and Week Analysis, Revision 1.

S2-4.doc 2.4-77 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

61. Tennessee Valley Authority, Calculation, CDQ0000002017000081, Loss of Chickamauga Reservoir Drain Down Analysis, Revision 0.

62. United States Army Corps of Engineers (USACE), Coastal Engineering Manual-Part VI Chapter 5, Fundamentals of Design, EM 1110-2-1100, updated 2015.

63. Von Thun, J Lawrence and David R. Gillette, Guidance on Breach Parameters, unpublished internal document, U.S. Bureau of Reclamation, March 13, 1990.

64. Tennessee Valley Authority, Calculation, CDQ0000002017000076, Watts Bar (WBN) and Sequoyah (SQN) Nuclear Plant Warning Time Analyses, Revision 1..

65.
Froehlich, D. C., Peak Outflow from Breach Embankment Dam Journal of Water Resources Planning and Management, Volume 121, Issue 1, January 1995 S2-4.doc 2.4-78 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

Enclosure 7, R1 Proposed SQN Units 1 and 2 UFSAR Section 2.4 Tables and Figures (Public) The following table provides a list of the current SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A Tables by Table Number and Current Title. The table then provides the Proposed Title for the respective Table Number. Table Numbers indicated n/a in the Current Title column represent a new Table being proposed in Section 2.4. The proposed UFSAR Tables are provided on pages E7-8 to E7-35 of this Enclosure. A description of the UFSAR Table changes is provided near the end of Section 2.1, Proposed Changes, of of this letter. CNL-23-012

Enclosure 7, R1 PROPOSED SQN UNITS 1 and 2 UFSAR SECTION 2.4 TABLES TABLE # CURRENT TITLE PROPOSED TITLE 2.4.1-1 Facts about Major TVA Dams and Reservoirs Public and Industrial Surface Water Supplies Withdrawn from the 98.6 Mile Reach of the Tennessee River Between Dayton, Tennessee, and Meade Corporation, Stevenson, Alabama 2.4.1-2 Facts about Non-TVA Dam and Reservoir Projects Facts about TVA Dams and Reservoirs (2 Sheets) 2.4.1-3 Flood Detention Capacity - TVA Projects above Sequoyah TVA Dams - River Mile Distances to SQNP (2 Sheets) Nuclear Plant 2.4.1-4 Public and Industrial Surface Water Supplies Withdrawn Facts about TVA Dams Above Chickamauga from the 98.6 Mile Reach of the Tennessee River Between Dayton, Tennessee, and Meade Corporation, Stevenson, Alabama 2.4.1-5 Dam Safety Modification Status (Hydrologic) (2 Sheets) Facts About Non-TVA Dams and Reservoirs 2.4.1-6 n/a Flood Detention Capacity - TVA Projects Above Sequoyah Nuclear Plant 2.4.2-1 n/a Peak Streamflow of the Tennessee River at Chattanooga TN (USGS Station 03568000) 1867 - 2007 (5 Sheets) 2.4.3-1 Probable Maximum Storm Rainfall and Precipitation Probable Maximum Storm Precipitation and Precipitation Excess (2 Sheets) Excess (2 Sheets) 2.4.3-2 Unit Hydrograph Data Table Deleted 2.4.3-3 n/a Historical Flood Events 2.4.3-4 n/a Summary of Results at the Dams for the Controlling Event (2 Sheets) 2.4.4-1 Floods from Postulated Seismic Failure of Upstream Floods from Postulated Seismic Failure of Upstream Dams Dams 2.4.13-1 Well and Spring Inventory Within 2-Mile Radius of Well and Spring Inventory Within 2-Mile Radius of Sequoyah Nuclear Plant Site (3 Sheets) Sequoyah Nuclear Plant Site (3 Sheets) 2.4.13-2 Groundwater Supplies Within 20-Mile Radius of the Plant Groundwater Supplies Within 20-Mile Radius of the Plant Site (4 Sheets) Site (4 Sheets) 2.4A-2 Critical Cases - Seismic Caused Dam Failures Table Deleted CNL-23-012 Page E7-2

Enclosure 7, R1 The following table provides a list of the current SQN Units 1 and 2 UFSAR Section 2.4 and Appendix 2.4A Figures by Figure Number and Current Title. The table then provides the Proposed Title for the respective Figure Number. Figure Numbers indicated n/a in the Current Title column represent a new Figure being proposed in Section 2.4. The proposed Figures are provided on pages E7-36 to E7-183 of this Enclosure. A description of the UFSAR Figure changes is provided near the end of Section 2.1, Proposed Changes, of Enclosure 5 of this letter. CNL-23-012 Page E7-3

Enclosure 7, R1 PROPOSED SQN UNITS 1 and 2 UFSAR SECTION 2.4 FIGURES FIGURE# CURRENT TITLE PROPOSED TITLE 2.4.1-1 Topographic Map, Plant Vicinity Topographic Map, Plant Vicinity 2.4.1-2 Tennessee River Basin Mean Annual Precipitation, 30-yr USGS Hydrologic Units within the Tennessee River Period, 1935 - 1964 Watershed 2.4.1-3 Multiple - Purpose Reservoir Operations (Various Dam TVA Water Control System Projects) (14 sheets) 2.4.1-4 n/a Seasonal Operating Curves (Various Dams) (16 Sheets) 2.4.1-5 n/a Reservoir Elevation - Storage Relationship (Various Dams) (17 Sheets) 2.4.2-1 Flood Distribution Diagram, Chattanooga, TN Tennessee River Mile 464.2 - Distribution of Floods at Chattanooga, Tennessee 2.4.3-1 Probable Maximum March Isohyets (21,400-sq mi Sub-Basin Rainfall Depths for Controlling Event, Primary downstream), 1st 6 hours (in.) Water Shed Above Blue Ridge Dam 2.4.3-2 Probable Maximum March Isohyets (7980 sq mi), 1st 6 Figure Deleted hours (in.) 2.4.3-3 Rainfall-Time Distribution, Adopted Standard Mass Curve Rainfall Time Distribution, Typical Mass Curve 2.4.3-4 72-Hr March Probable Maximum Storm Depths (IN) Extent of HEC-RAS Modeling (2 sheets) 2.4.3-5 Hydrologic Model Unit Areas Drainage Areas above Chickamauga Dam 2.4.3-5a n/a Sub-Basins Above Wheeler Dam and Nesting Sequence for Controlling Event 2.4.3-6 6-Hour Unit Hydrographs (11 sheets) Unit Hydrographs, (Areas above Various Dams) (8 sheets) 2.4.3-7 1973 Flood - Chickamauga Reservoir Unsteady Flow Discharge Rating Curves (Various Dams) (17 sheets) Model Verification 2.4.3-8 Steady State Model Verification, Watts Bar Dam Tailwater Fort Loudoun & Tellico HEC-RAS Schematic Rating Curve 2.4.3-9 Hydrologic Model Verification-1973 Flood Unsteady Flow Model, Fort Loudoun Reservoir, March 1973 Flood (4 sheets) 2.4.3-10 Hydrologic Model Verification -1963 Flood Unsteady Flow Model, Fort Loudoun-Tellico Reservoir, May 2003 Flood (5 sheets) 2.4.3-11 Sequoyah Nuclear Plant Probable Maximum Flood Watts Bar HEC-RAS Unsteady Flow Model Schematic Discharge 2.4.3-12 Sequoyah Nuclear Plant Probable Maximum Flood Unsteady Flow Model, Watts Bar Reservoir March 1973 Elevation Flood (2 Sheets) CNL-23-012 Page E7-4

Enclosure 7, R1 FIGURE# CURRENT TITLE PROPOSED TITLE 2.4.3-13 n/a Unsteady Flow Model, Watts Bar Reservoir, May 2003 Flood (2 Sheets) 2.4.3-13a General Grading for Site Drainage Number Not Used 2.4.3-14 ERCW Pump Station Location Chickamauga HEC-RAS Unsteady Flow Model Schematic 2.4.3-15 Sequoyah Nuclear Plant, NNW Wind Wave Fetch Unsteady Flow Model, Chickamauga Reservoir, March 1973 Flood (3 Sheets) 2.4.3-16 Sequoyah Nuclear Plant, NE Wind Wave Fetch Unsteady Flow Model, Chickamauga Reservoir, May 2003 Flood (3 Sheets) 2.4.3-17 (Topography Surrounding Diesel Generator Building and Figure Deleted Cooling Towers) 2.4.3-18 n/a PMF Rainfall Hyetograph and Elevation and Discharge Hydrographs at Sequoyah Nuclear Plant 2.4.3-19 n/a General Grading for Site Drainage 2.4.3-20 n/a Sequoyah Nuclear Plant, Wind Wave Fetch (Various) Building Critical Fetch Length (5 Sheets) 2.4.3-22 n/a (Various Dam), Total Failure Section Plots (2 Sheets) 2.4.3-23 n/a West Saddle Dike, Cross Section #3 2.4.3-24 n/a West Saddle Dam, Discharge Rating Curves 2.4.3-25 n/a PMF Rainfall Hyetograph and Elevation and Discharge Hydrographs at (Various) Dams (27 Sheets) 2.4.4-1 Watts Bar Dam, Powerhouse & Spillway, Results of Figure Deleted Analysis for Operating Basis Earthquake 2.4.4-2 Embankment Watts Bar Dam, Results of Analysis for 1/2 Figure Deleted SSE 2.4.4-3 Spillway Gate Positions for 25 Year Flood - 1/2 Probable Figure Deleted Maximum Flood 2.4.4-4 Powerhouse and Spillway, Fort Loudoun Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-5 Embankment, Fort Loudoun Dam, Results of Analysis for Figure Deleted 1/2 SSE 2.4.4-6 Nonoverflow and Spillway, Tellico Dam, Figure Deleted Results of Analysis Operating Basis Earthquake 2.4.4-7 Embankment - Tellico Dam, Results of Analysis for 1/2 Figure Deleted SSE CNL-23-012 Page E7-5

Enclosure 7, R1 FIGURE# CURRENT TITLE PROPOSED TITLE 2.4.4-8 Spillway and Nonoverflow, Norris Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-9 Norris Dam, Analysis for 1/2 SSE and 1/2 Maximum Figure Deleted Possible Flood 2.4.4-10 Spillway and Nonoverflow, Cherokee Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-11 Embankment, Cherokee Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-12 Cherokee Dam, Assumed Condition of Dam After Failure, Figure Deleted 1/2 SSE and 1/2 Maximum Possible Flood 2.4.4-13 Spillway and Overflow, Douglas Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-14 Saddle Dam No. 1, Douglas Dam, Figure Deleted Results of Analysis for 1/2 SSE 2.4.4-15 Douglas Dam, Assumed Condition of Dam After Failure, Figure Deleted 1/2 SSE and 1/2 Maximum Possible Flood 2.4.4-16 Fontana Dam - Assumed Condition of Dam After 1/2 SEE Figure Deleted and 1/2 Maximum Possible Flood 2.4.4-17 Fontana Project - Concrete Strengthening of Blks 33, 34, Single Seismic Dam Failure Curves (Various Dams) and 35 (2 sheets) Multiple Seismic Dam Failure (Various Dams) (2 sheets) 2.4.4-18 1/2 SSE with Epicenter Within Area Shown Figure Deleted 2.4.4-21 Seismic Flood Analysis - Fontana, Hiwassee, Apalachia Figure Deleted and Blue Ridge, OBE Failure in 1/2 PMF - Sequoyah Plant 2.4.4-24 Spillway, Fort Loudoun Dam, Results of Analysis for SSE Figure Deleted 2.4.4-25 Embankment, Fort Loudoun Dam, Results of Analysis for Figure Deleted SSE 2.4.4-26 Fort Loudoun Dam, Assumed Condition of Dam After Figure Deleted Failure, SSE Combined With 25 Year Flood 2.4.4-27 Tellico Dam, Tellico Project Assumed Condition of Dam Figure Deleted After Failure - SSE Combined with 25 Year Flood CNL-23-012 Page E7-6

Enclosure 7, R1 FIGURE# CURRENT TITLE PROPOSED TITLE 2.4.4-28 SSE Plus 25 Year Flood, Judged Condition of Dam After Figure Deleted Failure, Norris Dam 2.4.4-29 SSE With Epicenter in North Knoxville Vicinity Figure Deleted 2.4.4-30 Seismic Flood Analysis - Norris, Cherokee and Douglas Figure Deleted SSE with 25-year Flood, Sequoyah Plant 2.4.4-31 SSE With Epicenter in West Knoxville Vicinity Figure Deleted 2.4.4-37 General Plan, Elevation and Sections, Watts Bar Project Figure Deleted 2.4.4-38 Topographic Map, General Area - Watts Bar Dam Figure Deleted 2.4.4-39 West Saddle Dam, Location, Plan and Section - Watts Figure Deleted Bar Project 2.4.4-40 n/a Inflows above Watts Bar Dam for 1/2 of 10-4 Seismic Ground Motion Failures with 500-Year Flood (2 Sheets) 2.4.8-1 Grading Plan, Intake Channel, Dwg No. 10N213 R4 Grading Plan Intake Channel 2.4A-2 Flow Diagram-Flood Protection Provisions with New Flow Diagram-Flood Protection Provisions with New to ERCW Intake Station in Operation - Natural Convection ERCW Intake Station in Operation - Natural Convection 2.4.14-1 Cooling Cooling 2.4A-3 Flow Diagram - Flood Protection Provisions with New Flow Diagram - Flood Protection Provisions with New to ERCW Intake Station in Operation - Open Reactor ERCW Intake Station in Operation - Open Reactor 2.4.14-2 Cooling Cooling 2.4A-4 Sequoyah Nuclear Plant Flood Protection Plan, Basis for Sequoyah Nuclear Plant Rainfall Flood Warning Time to Safe Shutdown for Plant Flooding Basis for Safe Shutdown for Plant Flooding 3 Sheets) 2.4.14-3 CNL-23-012 Page E7-7

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2.4.1-2 Facts About TVA Dams and Reservoirs (Page 1 of 2)

                      &(,,

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2.4.1-3 TVA Dams - River Mile Distances to SQNP (Page 1 of 2) Distance from River Structure/River Mouth River Mile(a) SQNP (mi.) 7HQQHVVHH 5LYHU

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     &OLQFK 5LYHU                                                            

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    +ROVWRQ5LYHU                                                                   
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 )UHQFK%URDG5LYHU                                                                 
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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2.4.1-4 Facts about TVA Dams Above Chickamauga

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2 4 1-5 Facts About Non-TVA Dams and Reservoirs

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2.4.2-1 Peak Streamflow of the Tennessee River at Chattanooga TN (USGS Station 03568000) 1867 - 2007 (Page 1 of 5) Water Year(a) Date Discharge (cfs)

                                          
                                          
                                          
                                         
                                          
                                          
                                          
                                          
                                         
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          
                                          

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 Table 2.4.2-1 Peak Streamflow of the Tennessee River at Chattanooga, TN (USGS Station 03568000) 1867 - 2007 (Page 4 of 5) (a) Water Year Date Discharge (cfs)

                                        
                                        
                                       
                                        
                                       
                                         
                                        
                                        
                                        
                                        
                                         
                                        
                                        
                                        
                                        
                                         
                                        
                                        
                                         
                                         
                                         
                                        
                                         
                                        
                                        
                                       
                                       
                                        
                                        
                                         
                                        

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        )UHQFK%URDG5LYHUDW$VKHYLOOH                                    
        )UHQFK%URDG5LYHU1HZSRUWWR$VKHYLOOH                            
        3LJHRQ5LYHUDW1HZSRUW                                            
        1ROLFKXFN\5LYHUDW(PEUHHYLOOH                                    

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        'RXJODV'DPORFDO                                                  
        /LWWOH3LJHRQ5LYHUDW6HYLHUYLOOH                                 
        )UHQFK%URDG5LYHUORFDO                                           
        6RXWK+ROVWRQ'DP                                                   
       :DWDXJD'DP                                                        
       %RRQHORFDO                                                         
       )RUW3DWULFN+HQU\                                                 
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       /LWWOH5LYHUDWPRXWK                                              
       )RUW/RXGRXQORFDO                                                 
       /LWWOH7HQQHVVHH5LYHUDW1HHGPRUH                                 
       1DQWDKDOD                                                         
       7XFNDVHJHH5LYHUDW%U\VRQ&LW\                                    
       )RQWDQDORFDO                                                      
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          &KLOKRZHH'DP                                                      
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       :DWWV%DUORFDODERYH&OLQFK5LYHU                                  
       &OLQFK5LYHUDW1RUULV'DP                                          
       0HOWRQ+LOOORFDO                                                   
       &OLQFK5LYHUORFDODERYHPLOH                                    

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      3RSODU&UHHNDWPRXWK                                                
      (PRU\5LYHUDWPRXWK                                                 
      &OLQFK5LYHUORFDOPRXWKWRPLOH                                 
      :DWWV%DUORFDOEHORZ&OLQFK5LYHU                                   
      &KDWXJH'DP                                                        
      1RWWHO\'DP                                                        
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      $SDODFKLDORFDO                                                     
      %OXH5LGJH'DP                                                    

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      &KLFNDPDXJDORFDO                                                    

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VXEPHUVLEOH 2 35 q13'23" 85 q06'12" 75 720 685 .5 Submersible pump 3 35 q13'30" 85 q06'47" 116 745 -- .5 Submersible pump 4 35 q13'58" 85 q05'45" 42 700 696 3.0 5 35 q14'15" 85 q06'25" -- 680 -- .5 1/4-hp pump 6 35 q14'34" 85 q06'46" 85 720 -- 15 Submersible pump 7 35 q14'35" 85 q06'52" 65 720 670 2.5 3/4-hp pump 8 35 q14'36" 85 q06'57" 73 735 687 .5 1/3-hp pump 9 35 q15'06" 85 q06'32" 27 780 761 5.0 Bucket 10 35 q14'46" 85 q06'16" 110 720 -- .5 Submersible 11 35 q14'55" 85 q06'15" -- 725 -- - 12 35 q14'53" 85 q06'13" 77 800 -- .5 13 35 q14'52" 85 q06'13" -- 800 -- - Summer home 14 35 q14'50" 85 q06'12" -- 800 -- - Summer home 15 35 q14'45" 85 q06'14" 50 720 680 .5 16 35 q14'44" 85 q06'18" 275 795 525 .5 1-hp submersible pump 17 35 q14'45" 85 q06'22" -- 740 -- .5 1-hp pump 18 35 q14'21" 85 q05'30" -- 695 -- - 19 35 q14'26" 85 q05'27" 200 695 -- .5 1-hp pump 20 35 q14'34" 85 q05'29" 150 695 -- .5 1/2-hp pump 21 35 q14'31" 85 q05'29" -- 695 -- .5 22 35 q14'29" 85 q05'29" 110 690 -- .5 1-hp pump 23 35 q14'23" 85 q05'32" 85 700 -- .75 1-hp jet pump 24 35 q14'22" 85 q05'40" -- 695 -- .5 Serves 2 families; 1-hp pump 25 35 q14'24" 85 q05'46" 52 710 680 .5 3/4-hp pump 26 35 q14'28" 85 q05'45" 130 740 620 .5 27 35 q14'26" 85 q05'41" 90 740 710 .5 28 35 q14'32" 85 q05'44" 141 740 650 .5 29 35 q14'34" 85 q05'44" -- 735 -- - Summer home 30 35 q14'38" 85 q05'41" 58 700 670 .5 1/3-hp pump 31 35 q14'41" 85 q05'41" -- 720 -- .5 32 35 q14'45" 85 q05'46" -- 715 -- - 33 35 q14'43" 85 q05'47" -- 720 -- - 34 35 q14'41" 85 q05'48" -- 695 -- - Summer home 35 35 q14'39" 85 q05'50" 48 695 650 .5 1-hp pump 36 35 q14'39" 85 q05'53" 60 700 -- .5 Submersible pump 37 35 q14'40" 85 q05'58" -- 695 653 .5 1-hp pump 38 35 q14'41" 85 q05'56" 50 695 655 .5 3/4-hp pump 39 35 q14'35" 85 q05'54" -- 700 -- - Summer home 40 35 q14'36" 85 q05'57" -- 700 -- - 41 35 q14'37" 85 q06'01" -- 715 -- - Summer home 42 35 q14'33" 85 q05'02" 223 720 530 .5 Note: The information in this table is historic and not subject to updating revisions. 6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 641 7$%/( 6KHHW  &RQWLQXHG WELL AND SPRINGINVENTORY WITHIN 2-MILE RADIUSOF SEQUOYAH NUCLEAR PLANT SITE (1972 Survey Only) Estimated Map Elevation, Feet Well Ident. Location Well Depth Water Dia., No. Latitude Longitude Feet Ground Surface Feet Remarks 43 35 q14'46" 85 q05'54" 65 695 655 .5 3/4-hp pump 44 35 q14'47" 85 q05'54" 95 705 655 .5 45 35 q14'48" 85 q05'53" -- 700 -- - Summer home 46 35 q14'50" 85 q05'53" 257 695 665 .5 1-hp submersible pump 47 35 q14'52" 85 q05'48" -- 710 -- - Summer home 48 35 q15'04" 85 q05'56" -- 725 -- - Summer home 49 35 q15'06" 85 q06'02" -- 720 -- - Summer home 50 35 q15'06" 85 q06'05" 90 705 625 .5 Submersible pump 51 35 q14'58" 85 q06'06" -- 695 -- - Summer home 52 35 q15'01" 85 q06'02" 65 720 680 .5 3/4-hp pump 53 35 q14'47" 85 q05'57" 46 700 670 .5 2 families;1-hp pump 54 35 q14'42" 85 q06'01" 48 695 675 .5 1/2-hp pump 55 35 q14'41" 85 q06'02" -- 695 -- - Summer home 56 35 q14'40" 85 q06'03" -- 695 -- - Summer home 57 35 q14'37" 85 q06'08" 155 690 670 .5 1-hp pump 58 35 q14'34" 85 q06'09" -- 695 -- - 59 35 q14'23" 85 q05'53" -- 760 -- .5 Submersible pump 60 35 q14'49" 85 q05'58" -- 705 -- - 61 35 q13'01" 85 q04'41" -- 720 -- - Summer home 62 35 q13'18" 85 q04'24" -- 845 -- .5 1-hp pump 63 35 q13'19" 85 q04'23" 206 845 645 .5 1/2-hp pump 64 35 q13'33" 85 q04'19" 50 720 680 .5 1-hp pump 65 35 q13'49" 85 q04'14" 100 720 640 .5 Servies clubhouse, 15 houses 66 35 q13'57" 85 q03'55" 175 741 -- .6 1-hp pump 67 35 q13'53" 85 q03'49" 100 738 690 .5 1-hp submersible pump 68 35 q13'50" 85 q03'52" 133 720 675 .5 1/2-hp pump 69 35 q13'48" 85 q03'43" 85 736 -- .5 1-hp pump 70 35 q13'43" 85 q03'38" 80 780 -- .5 1-hp pump 71 35 q13'37" 85 q03'36" 130 800 715 .5 1-hp pump 72 35 q13'38" 85 q03'43" -- 800 -- - Well not used 73 35 q13'16" 85 q03'30" 227 880 680 .5 Submersible pump 74 35 q13'09" 85 q03'41" 397 900 820 .5 2-hp pump 75 35 q12'47" 85 q03'58" 190 860 800 .5 Serves 2 families; submersible 76 35 q13'03" 85 q04'17" -- 720 -- - Summer home 77 35 q13'05" 85 q04'10" 90 740 670 .5 1/2-hp pump 78 35 q12'50" 85 q04'13" 85 760 -- .5 1-hp pump 79 35 q12'45" 85 q03'59" 190 880 -- .5 Serves 2 families; 1-hp pump 80 35 q12'26" 85 q04'07" 290 860 -- .5 Serves 5 families; submersible Note: The information in this table is historic and not subject to updating revisions. 6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++E/'81'(5&)5 641 7$%/(  6KHHW  &RQWLQXHG WELL AND SPRINGINVENTORY WITHIN 2-MILE RADIUSOF SEQUOYAH NUCLEAR PLANT SITE (1972 Survey Only) Estimated Map Well Elevation, Feet Well Ident. Location Depth, Water Dia , No. Latitude Longitude Feet Ground Surface Feet Remarks 81 35 q12'20" 85 q04'33" 265 940 -- .5 Submersible pump 82 35 q12'15" 85 q04'34" 250 965 735 .5 1-hp submersible pump 83 35 q12'24" 85 q04'35" 305 965 665 .5 Submersible pump 84 35 q12'22" 85 q05'05" 135 740 690 .5 1-hp pump 85 35 q12'21" 85 q05'08" 120 740 -- .5 Serves 2 families; 3/4-hp jet pump

            q            q                                                    KS                l submersible SXPS 87           35 q12'23"          85 q05'09"            --        740              --             .55    1-hp 1 h pump 88           35 q12'16"          85 q05'12"           55         740              720            2.5    Bucket 89           35 q12'07"          85 q05'09"          251         775              700            .5     Serves 2 families; 3/4-hp pump 90           35 q11'54"          85 q04'56"          170         980              --             .5     1/2-hp pump 91           35 q12'19"          85 q05'20"          125         740              705            .5     Submersible pump 92           35 q12'22"          85 q05'33"           --         725              --             -      Summer home 93           35 q12'22"          85 q05'35"            --        700              --             -      1-hp pump 94           35 q12'22"          85 q05'36"           --         705              --             -      Summer home 95           35 q12'20"          85 q05'44"           --         700              --             -      Summer home 96           35 q12'04"          85 q05'56"          160         700              --             .5     Serves 5 families; 1-hp pump 97           35 q12'04"          85 q05'59            65         700              --             .5     House and cottage; 1-hp pump Note: The information in this table is historic and not subject to updating revisions .

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 The relationship of the plant site to the surrounding area can be seen in Figures 2.1.2-1 and 2.4.1-1. It can be seen from these figures that significant natural drainage features of the site have not been altered. Local surface runoff drains into the Tennessee River. 2.4.1.2 Hydrosphere The SQN site comprises approximately 525 acres on a peninsula on the western shore of Chickamauga Lake at Tennessee River Mile (TRM) 484.5. As shown by Figure 2.4.1-1, the site is on high ground with the Tennessee River being the only potential source of flooding. SQN is located in the Middle Tennessee Chickamauga watershed, U.S. Geological Survey (USGS) hydrologic unit code 06020001, one of 32 watersheds in the Region 06 - Tennessee River Watershed (Figure 2.4.1-2). The Tennessee River above SQN site drains 20,650 square miles. The drainage area at Chickamauga Dam, 13.5 miles downstream, is 20,790 square miles. Three major tributaries, Hiwassee, Little Tennessee, and French Broad Rivers, rise to the east in the rugged Southern Appalachian Highlands. They flow northwestward through the Appalachian Divide which is essentially defined by the North Carolina-Tennessee border to join the Tennessee River which flows southwestward. The Tennessee River and its Clinch and Holston River tributaries flow southwest through the Valley and ridge physiographic province which, while not as rugged as the Southern Highlands, features a number of mountains including the Clinch and Powell Mountain chains. The drainage pattern is shown on Figure 2.1.1-1. About 20 percent of the watershed rises above elevation 3,000 ft with a maximum elevation of 6,684 ft at Mt. Mitchell, North Carolina. The watershed is about 70 percent forested with much of the mountainous area being 100 percent forested. The climate of the watershed is humid temperate. Above Chickamauga Dam, annual rainfall averages 51 inches and varies from a low of 40 inches at sheltered locations in the mountains to high spots of 85 inches on the southern and eastern divide. Rainfall occurs relatively evenly throughout the year. See Section 2.3 for a discussion of rainfall. As shown in Table 2.3.2-19, the lowest site monthly average is 2.9 inches in October and the highest site monthly average is 6.8 inches in March, with January a close second with an average of 6.0 inches. Major flood producing storms are of three types; general, tropical, and local types. Most floods on the Tennessee River near SQN, however, have been produced by JHQHUDO storms in the main flood-season months of January through early May. Watershed snowfall is relatively light, averaging only about 14 inches annually above the plant. Snowfall above the 3,000 ft elevation averages 22 inches annually. The highest average annual snowfall in the basin is 63 inches at Mt. Mitchell, the highest point east of the Mississippi River. Individual snowfalls are normally light, with an average of 13 snowfalls per year. Snowmelt is not a factor in maximum flood determinations. The Tennessee River, particularly above Chattanooga, Tennessee, is one of the most highly-regulated rivers in the United States. The TVA reservoir system is operated for flood control, navigation, and power generation with flood control a prime purpose with particular emphasis on protection for Chattanooga, 20 miles downstream from SQN. Chickamauga Dam, 13.5 miles downstream, affects water surface elevations at SQN. Normal (Summer) full pool elevation is 682.5 ft. At this elevation the reservoir is 58.9 miles long on the Tennessee River and 32 miles long on the Hiwassee River, covering an area of 36,050 acres, with a volume of 622,500 acre-feet. The reservoir has an average width of nearly 1 mile, ranging from 700 ft to 1.7 miles. At the SQN site, the reservoir is about 3,000 ft wide with depths ranging between 12 ft and 50 ft at normal pool elevation. There are 17 PDMRUdams (South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, Blue Ridge, Hiwassee, Chatuge, Nottely, Apalachia and Watts Bar) in the TVA system upstream from SQN, 14 of which (those previously identified excluding Fort Patrick Henry, Melton Hill, and Apalachia) provide about 3.9 million acre-ft of reserved flood-detention (March 15) capacity during the main flood season. Table 2.4.1-2 lists pertinent data for 79$ VGDPVDQGUHVHUYRLUV S2-4.doc 2.4-2 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Figure 2.4.1-3 presents a simplified flow diagram for the Tennessee River system. Table 2.4.1-3 provides the relative distances in river miles of upstream dams to the SQN site. Details for TVA dam outlet works are provided in Table 2.4.1-4. In addition, there are four major dams owned by Brookfield Renewable Energy Partners (Calderwood, Chilhowee, Santeetlah, and Cheoah Dams) and two major dams owned by Duke Energy (Nantahala and Mission Dams). These reservoirs often contribute to flood reduction but they do not have dependable reserved flood detention capacity. Table 2.4.1-5 lists pertinent data for the non-TVA owned dams and reservoirs. The locations of these dams are shown on Figure 2.1.1-1. Flood control above SQN is provided largely by 12 tributary reservoirs (Table 2.4.1-6). Tellico Dam is counted as a tributary reservoir because it is located on the Little Tennessee River, although, because of canal connection with Fort Loudoun Dam, it also functions as a main river dam. On March 15, near the end of the flood season, these provide a minimum of 3,604,500 acre-feet of detention capacity, equivalent to 4.6 inches on the 14,708 square-mile area they control. This is 92 percent of the total available above Chickamauga Reservoir. The two main river reservoirs, Fort Loudoun and Watts Bar, provide 321,000 acre-feet, equivalent to 1.6 inches of detention capacity on the remaining area above the Chickamauga Reservoir. The flood detention capacity reserved in the TVA system varies seasonally, with the greatest amounts during the January through March flood season. Figure 2.4.1-4 (16 sheets) shows the reservoir seasonal operating guides for reservoirs above the plant site. Table 2.4.1-6 shows the flood control reservations at the multiple-purpose projects above SQN at the beginning and end of the winter flood season and in the summer. Total assured system detention capacity above the Chickamauga Reservoir varies from approximately 5.0 inches on January 1 to approximately 4.0 inches on March 15 and decreasing to approximately 1.5 inches during the summer and fall. Actual detention capacity may exceed these amounts, depending upon inflows and power demands. Chickamauga Dam, the elevation of which affects flood elevations at the plant, has a drainage area of 20,790 square miles, 3,480 square miles more than Watts Bar Dam. There are five major tributary dams (Chatuge, Nottely, Hiwassee, Apalachia, and Blue Ridge in the 3,480-square-mile intervening watershed, of which four have substantial reserved capacity (Apalachia excluded). On March 15, near the end of the flood season, these provide a minimum of 329,800 acre-ft equivalent to 5.2 inches on the 1,200-square-mile controlled area. Chickamauga Dam contains 258,300 acre-ft of detention capacity at median guide elevation 678 ft on March 15 equivalent to 2.1 inches on the remaining 2,280 square miles. Figure 2.4.1-4 (Sheet 1) shows the seasonal operating guide for Chickamauga. Elevation-storage relationships for the reservoirs above the site and Chickamauga Dam, downstream, are shown in Figure 2.4.1-5 (17 sheets). Daily flow volumes at the plant, for all practical purposes, are represented by discharges from Chickamauga Dam with drainage area of 20,790 square miles, only 140 square miles more than at the plant. Momentary flows at the nuclear plant may vary considerably from daily averages, depending upon turbine operations at Watts Bar Dam upstream and Chickamauga Dam downstream. There may be periods of several hours when there are no releases from either or both Watts Bar and Chickamauga Dams. Rapid turbine shutdown at Chickamauga may sometimes cause periods of reverse flow in Chickamauga Reservoir. Based upon discharge records since closure of Chickamauga Dam in 1940, the average daily streamflow at the plant is 32,600 cfs. The maximum daily discharge was 223,200 cfs on May 8, 1984. Except for two special operations on March 30 and 31, 1968, when discharge was zero to control milfoil, the minimum daily discharge was 700 cfs on November 1, 1953. Flow data for water years 1951-1972 indicate an average rate of about 27,600 cfs during the summer months (May-October) and about 38,500 cfs during the winter months (November-April). Flow durations based upon Chickamauga Dam discharge records for the period 1951-1972 are tabulated below. Average Daily Percent of Time Discharge, cfs Equaled or Exceeded 5,000 99.6 10,000 97.7 S2-4.doc 2.4-3 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 15,000 93.3 20,000 84.0 25,000 69.3 30,000 46.8 35,000 31.7 Channel velocities at SQN average about 0.6 fps under normal winter conditions. Because of lower flows and higher reservoir elevations in the summer months, channel velocities average about 0.3 fps. As listed on Table 2.4.1-1, there are 23 surface water users within the 98.6-mile reach of the Tennessee River between Dayton, TN and Stevenson, AL. These include fifteen industrial water supplies and eight public water supplies. The industrial users exclusive of SQN withdraw about 500 million gallons per day from the Tennessee River. Most of this water is returned to the river after use with varying degrees of contamination. The public surface water supply intake (Savannah Valley Utility District), originally located across Chickamauga Reservoir from the plant site at TRM 483.6, has been removed. Savannah Valley Utility District has been converted to a ground water supply. [:LWKKHOGE\6WDWXWH] Groundwater resources in the immediate SQN site are described in Subsection 2.4.13.2. 2.4.2 Floods 2.4.2.1 Flood History The nearest location with extensive formal flood records is 20 miles downstream at Chattanooga, Tennessee, where continuous records are available since 1874. Knowledge about significant floods extends back to 1826, based upon newspaper and historical reports. Flood flows and stages at Chattanooga have been altered by TVA's reservoir system beginning with the closure of Norris Dam in 1936 and reaching essentially the present level of control in 1952 with closure of Boone Dam, the last major dam with significant reserved flood detention capacity constructed above Chattanooga. Tellico Dam, with closure in 1979, provides additional reserved flood detention capacity; however, the percentage increase in total detention capacity above the SQN site is small. Thus, for practical purposes, flood records for the period 1952 to date can be considered representative of prevailing conditions. Table 2.4.2-1 provides annual peak flow data at Chattanooga. Figure 2.4.2-1 shows the known flood experience at Chattanooga in diagram form. The maximum known flood under natural conditions occurred in 1867. This flood was estimated to reach elevation 690.5 ft at SQN site with a discharge of about 450,000 cfs. The maximum flood elevation at the site under present-day regulation reached elevation 687.9 ft at the site on May 9, 1984. The following table lists the highest floods at SQN site under present-day regulation: Estimated Elevation Discharge Date at SQN (ft) at Chickamauga Dam (cfs) February 3, 1957 683.7 180,000 March 13, 1963 684.8 205,000 March 18, 1973 687.0 219,000 April 6, 1977 685.0 150,000 May 9, 1984 687.9 250,000 April 20, 1998 685.9 180,000 May 7, 2003 687.8 225,000 There are no records of flooding from seiches, dam failures, or ice jams. Historic information about icing is provided in Section 2.4.7. S2-4.doc 2.4-4 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.2.2 Flood Design Considerations TVA has planned the SQN project to conform with Regulatory Guide 1.59, including position 2, and supplemented by NRC guidance provided in Report JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure, Interim Staff Guidance, when evaluating potential flood levels from seismically induced dam failures.as described herein and in Section 2.4.4. The types of events evaluated to determine the worst potential flood included (1) Probable Maximum Precipitation (PMP) on the total watershed and critical sub-watersheds, including seasonal variations and potential consequent dam failures and (2) dam failures in postulated seismic events with guide specified concurrent flood conditions. Specific analysis of Tennessee River flood levels resulting from ocean front surges and tsunamis is not required because of the inland location of the plant. Snow melt and ice jam considerations are also unnecessary because of the temperate zone location of the plant. Flood waves from landslides into upstream reservoirs required no specific analysis, in part because of the absence of major elevation relief in nearby upstream reservoirs and because the prevailing thin soils offer small slide volume potential compared to the available detention space in reservoirs. Seiches pose no flood threats because of the size and configuration of the lake and the elevation difference between normal lake level and plant grade. The controlling PMF level would result from a primary PMP storm over the Blue Ridge Dam rainfall drainage area and secondary larger area PMP storms over multiple rainfall drainage areas above Wheeler Dam as described in Section 2.4.3. Wind waves based on an overland wind speed of 23.65 miles per hour were assumed to occur coincident with the flood peak. This would create maximum wind waves up to [CEII] ft high (trough to crest). All safety related facilities, systems, and equipment are housed in structures which provide protection from flooding for all flood conditions up to plant grade at elevation 705.0 ft. See Section 2.4.10 for more specific information. Other rainfall floods could exceed plant grade, elevation 705.0 ft, and require plant shutdown. Section 2.4.14 describes emergency protective measures to be taken in flood events exceeding plant grade. Seismic and concurrent flood events could cause dam failure flood waves exceeding plant grade elevation of 705.0 ft. Section 2.4.14 describes emergency protective measures to be taken in induced flood events exceeding plant grade. For the condition where flooding exceeds plant grade, as described in Sections 2.4.3 and 2.4.4, those safety-related facilities, systems, and equipment located in the containment structure are protected from flooding by the Shield Building structure with those accesses and penetrations below the DBF plus wind wave runup being designed and constructed as essentially watertight elements. Wind wave run-up during the DBF at the Diesel Generator Building reaches elevation [CEII] ft, which is [CEII] ft below the operating floor. Consequently, wind wave run up will not impair the safety function of systems in the Diesel Generator Building. The accesses and penetrations below the DBF plus wind wave run-up are designed and constructed to minimize leakage into the buildings. Redundant sump pumps are provided within the building to remove minor leakage. Protective measures are taken to ensure that all safety-related systems and equipment in the Essential Raw Cooling Water (ERCW) Intake Pumping Station and the Diesel Generator Building will remain functional when subjected to the DBF plus wind wave runup. Those Class 1E electrical system conduit banks located below the DBF plus wind wave run up flood level are designed to function submerged with either continuous cable runs or qualified, type tested splices. The Turbine, Control, and Auxiliary Buildings will be allowed to flood. All equipment required to S2-4.doc 2.4-5 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 maintain the plant safely during the flood, and for 100 days after the beginning of the flood, is either designed to operate submerged, is located above the DBF plus wind wave runup (external), DBF plus surge (internal) or is otherwise protected. 2.4.2.3 Effects of Local Intense Precipitation Maximum water levels at buildings expected to result from the local plant PMP were determined using a transient flow (unsteady flow) model with hydraulically connected storage areas. Much of the plant site is flat, particularly at the switchyards, and a single flow path is not well defined. A transient model with interconnected storage areas very roughly approximates a two-dimensional model using one-dimensional methods by providing multiple simultaneous outlet paths for the exterior areas adjacent to plant buildings. Structures housing safety-related facilities, systems, and equipment are protected from flooding during a local PMF by the slope of the plant yard. The yard is graded so that the surface runoff will be carried to Chickamauga Reservoir without exceeding the elevation of the external accesses given in Paragraph 2.4.1.1 except those at the intake pumping station whose pumps can operate submerged. PMP for the plant drainage system and roofs of safety-related structures is defined in Topical Report TVA-NPG-AWA16-A, TVA Overall Basis Probable Maximum Precipitation and Local Intense Precipitation Analysis, CDQ0000002016000041, Revision 1 [3]. The probable maximum storm used to test the adequacy of the local drainage system would produce a maximum one-hour depth of 13.8 inches. Three different temporal distributions were applied to the model, with peak intensity shifted between early, middle, and late occurrence. No precipitation losses were applied. Runoff was made equal to rainfall. The separate watershed subareas and flowpaths are shown on Figure 2.4.3-19. The western plant site was evaluated as six interconnected storage areas with four primary weir-flow outlets and one connected transient flow stream-course model. Runoff from the western plant site will flow: Northwest to a channel along the main plant tracks then across the main access highway (Area 7); to the West through a parking lot (Areas 6A, 6C, and 6E connected to transient flow model); Southwest through the vehicle barrier system directly to Chickamauga Lake (Area 6E); or South through the vehicle barrier system to the Yard Drainage and other Ponds (Area 6C). The maximum water surface elevations in Areas 6A and 6BS are below critical floor elevation 706 ft [54]. The eastern plant site was evaluated as three interconnected storage areas with three weir-flow outlets and two connected transient flow stream-course models. Runoff from the eastern plant site will flow: North around the West and East ends of the Multipurpose Building to the intake canal (Area 5 connected to two transient flow models); South to the Condenser Circulating Water Discharge Channel (Areas 4 and 6D); or Southwest into the western plant site (Area 6D into 6C). The maximum water surface elevations in Areas 4, 5, and 6D are below critical flood elevation 706 ft [54]. Underground drains were assumed clogged throughout the storm. Rainfall on plant building roofs was assumed to discharge to the ground surface. 2.4.3 Probable Maximum Flood (PMF) on Streams and Rivers The guidance of Appendix A of Regulatory Guide 1.59 was followed in determining the PMF. The PMF was determined from PMP for the watershed above the plant with consideration given to seasonal and areal variation in rainfall. The PMP is developed using Topical Report TVA -NPG-AWA16-A as described in Section 2.4.3.1. Topical Report TVA-NPG-AWA16-A in combination with the included Basin PMP Evaluation Tools provides PMP rainfall depth-area duration data for a gridded network over the Tennessee River and tributary drainage basins above Wheeler Dam. Multiple potential PMP storm event events, developed from the Topical Report TVA-NPG-AWA16-A and an event nesting methodology described in Reference 16, are analyzed using the run-off and stream course model described in Section 2.4.3.3 to determine the controlling PMP and the resulting PMF elevation at the SQN site. The controlling PMP for the SQN site resulted from consideration of drainage area above the Blue Ridge Dam project as the primary PMP watershed nested with multiple secondary area PMPs over the Hiwassee-Blue Ridge, Fontana-Hiwassee-Blue Ridge, Tellico-S2-4.doc 2.4-6 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Hiwassee-Blue Ridge, Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas, and Fort Loudoun-Tellico-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above Chickamauga, Nickajack, Guntersville, and Wheeler dam projects [55]. Based on TVA Dam Safety procedures, TVA has evaluated the stability of 16 critical dams above Chickamauga dam at PMF headwater/tailwater conditions. These dams are: Blue Ridge, Boone, Chatuge, Cherokee, Douglas, Fontana, Fort Loudoun, Fort Patrick Henry, Hiwassee, Melton Hill, Norris, Nottely, South Holston, Tellico, Watauga, and Watts Bar. [CEII] The hydrologic failure of low margin dams is postulated during the PMF. In the controlling PMP storm event, [CEII] is also postulated to occur. Other dams not evaluated for stability are postulated to fail at critical headwater conditions [55]. No other dam failures would occur. Maximum discharge at the plant would be 1,344,915 cfs for the controlling PMP storm. The resulting calculated PMF elevation at the plant would be [CEII] ft excluding wind wave effects [55]. 2.4.3.1 Probable Maximum Precipitation Probable maximum precipitation (PMP) for the Tennessee River watershed, in which SQN is located, is developed in Topical Report TVA-NPG-AWA16-A. Snowmelt is not a factor in generating maximum floods at the plant site. Topical Report TVA-NPG-AWA16-A defines the PMP depth-area-duration characteristics at each point of a gridded network over the TVA drainage basin for three different storm types: local, general and tropical. The resolution of the PMP grid is 0.025 x 0.025 decimal degrees with each grid cell having an approximate area of 2.5-square miles for a total of 12,966 grid points above Wheeler Dam. Gridded PMP values are calculated using a Basin PMP Evaluation Tool provided in Topical Report TVA-NPG-AWA16-A and described in Topical Report Sections 5.6 and 5.7. The PMP Evaluation Tool applies moisture transposition, in-place maximization and orographic transposition adjustment factors to an analyzed storm depth-area-duration value for the area size and duration of interest to yield an adjusted rainfall value. The analyzed storms adjusted rainfall value is then compared with the adjusted rainfall values of each storm in Topical Report TVA-NPG-AWA16-A storm database that is transpositionable to the target grid point. The maximum adjusted rainfall value determined in this comparison is the unique PMP for that grid point location. This process is repeated for each grid point on the gridded network to determine the final point PMP values at each grid point location for a given storm duration. These point PMP depths represent a worst-case estimated rainfall for the historically largest observed storm events transposable to a given grid point for a defined area of interest and duration. The areal application of point PMP values is determined using an event nesting and residual rainfall methodology [16]. Nesting is the occurrence of PMP events over small areas or sub-basins during a larger area PMP event. Nesting methodology ensures that the total average PMP rainfall depth of a larger sub-basin area which includes the average PMP rainfall depth of a smaller sub-basin area of interest is consistent with the average PMP rainfall depth developed from the Topical Report for the larger sub-basin area. The nesting methodology [16] creates potential controlling storm events using a multi-step process. First, gridded point PMP values over a primary area defined as the area of interest for a rainfall duration are determined using Topical Report TVA-NPG-AWA16-A and the PMP Evaluation Tool. A primary area may be a single sub-basin that is the watershed for a single dam project or may be multiple sub-basins above a dam project. The average PMP rainfall depth over the primary area for each rainfall duration is calculated by applying geographic information system (GIS) functionalities to create a PMP depth surface over the primary area. Next, residual rainfall in sub-basin areas above Wheeler Dam and outside the primary area is determined. As shown in Figure 2.4.3-5a, the drainage area above Wheeler Dam consists of 61 sub-basins. The Wheeler Dam project is the downstream boundary for the watershed potentially affecting TVA nuclear sites. Within the large multi-basin Wheeler dam drainage area, there are multiple possible sub-basin areas (secondary areas of interest) S2-4.doc 2.4-7 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 encompassing or nesting the primary area of interest. Each possible sub-basin area has an average PMP rainfall depth unique to that specific sub-basin area and rainfall duration. The nesting methodology ensures that the average PMP rainfall depth over a selected secondary sub-basin area (including the nested primary area of interest) is consistent with the average PMP rainfall depth developed from Topical Report TVA-NPG-AWA16-A by reducing the average PMP rainfall depth in the secondary sub-basin area outside of the primary area of interest. The nesting approach begins with the sub-basin areas at the upper boundary of the total basin drainage area encompassing the primary area and extends incrementally to downstream dam projects until the lower drainage boundary at Wheeler dam is reached. The final secondary area of interest is the total watershed above Wheeler dam. Since there are multiple combinations of larger sub-basin areas encompassing the primary area of interest, there are multiple potentially controlling PMP storm events for the SQN site. All PMP storms are nine-day events. A three-day antecedent storm was postulated to occur three days prior to the three-day PMP storm in all PMF determinations. Variations of the temporal rainfall distribution within the 3-day PMP storm were considered in the controlling storm determination. As recommended in Hydrometeorological Report No. 41, the antecedent rainfall of 40 percent of the main storm depth was conservatively applied with the spatial distribution of the antecedent coincident with that of the main storm PMP. To evaluate the local plant drainage system for a PMP event, Topical Report TVA-NPG-AWA16-A was used to calculate a 1-hr storm totaling 13.81 inches. Three different temporal distributions were applied to the model with peak intensity of 2.39 inches/5-min shifted between early, middle, and late occurrence. Depths for each 5-minute increment of the controlling late peak distribution were 0.58, 0.66, 0.75, 0.82, 0.83, 0.99, 1.08, 1.16, 1.32, 2.39, 1.74, and 1.49. Rainfall on the plant building roofs was assumed to discharge to the ground surface. To determine the controlling storm event for each primary area of interest, multiple possible storm events are postulated for each primary area of interest based on the selection of multiple secondary areas of interest [16]. These potentially controlling storm events are then either simulated using the stream course model described in Section 2.4.3.3 to determine the PMF elevation at the SQN site or screened out as not a potentially controlling simulations. The model simulation resulting in the highest the PMF elevation at the SQN site is the controlling PMP storm event at the SQN site. The controlling PMP for the SQN site resulted from consideration of drainage area above the Blue Ridge Dam project as the primary PMP watershed nested with multiple secondary area PMPs over the Hiwassee-Blue Ride, Fontana-Hiwassee-Blue Ridge, Tellico-Hiwassee-Blue Ridge, Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas, and Fort Loudoun-Tellico-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above Chickamauga, Nickajack, Guntersville, and Wheeler dam projects [55]. The PMP rainfall depths over the secondary watershed areas outside the primary watershed area were reduced to maintain the average PMP rainfall depth over the secondary watershed area. Figure 2.4.3-5a and Figure 2.4.3-1 provide a graphic representation of the controlling primary and secondary area nesting sequence and the controlling PMP rainfall depth spatial distribution over the watershed. Table 2.4.3-1 provides a graphic representation of the controlling storm rainfall depths for duration of the main and antecedent storms. 2.4.3.2 Precipitation Losses A multi-variable relationship, used in the day-to-day operation of the TVA reservoir system, has been applied to determine precipitation excess directly. The relationships were developed from observed storm and flood data. They relate precipitation excess to the rainfall, week of the year, geographic location, and antecedent precipitation index (API). In their application, precipitation excess becomes an increasing fraction of rainfall as the storm progresses in time and becomes equal to rainfall in the later part of extreme storms. An API determined from an 18-year period of historical rainfall records (1997-2015) was used at the start of the antecedent storm. The precipitation excess computed for the main storm is not sensitive to variations in adopted initial moisture conditions because of the large antecedent storm. Basin rainfall and precipitation excess for the PMP storm event controlling the PMF elevation at the SQN site are provided in Table 2.4.3-1. The average precipitation loss for the 20,790 sq mi watershed above Chickamauga Dam is 2.68 inches for the three-day antecedent storm and 2.46 inches for the S2-4.doc 2.4-8 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 three-day main storm. The losses are approximately 56 percent of antecedent rainfall and 20 percent of the PMP, respectively. The precipitation loss of 2.68 inches in the antecedent storm compares favorably with that of historical flood events shown in Table 2.4.3-3. 2.4.3.3 Runoff Model The runoff model used to determine Tennessee River flood hydrographs at SQN is divided into 61 unit areas and includes the total watershed above Wheeler Dam. Unit hydrographs are used to compute flows from these areas. The watershed unit areas above Chickamauga Dam are shown in Figure 2.4.3-5. The unit area flows are combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream tributary reservoirs, which in turn are routed through the reservoirs, using standard routing techniques. Resulting outflows are combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures, including unsteady flow routing. Unit hydrographs were developed for each unit area for which discharge records were available from maximum flood hydrographs either recorded at stream gaging stations or estimated from reservoir headwater elevation, inflow, and discharge data using the procedures described by Newton and Vineyard [23]. For non-gaged unit areas unit graphs were developed from relationships of unit hydrographs from similar watersheds relating the unit hydrograph peak flow to the drainage area size, time to peak in terms of watershed slope and length, and the shape to the unit hydrograph peak discharge in cubic feet per second (cfs) per square mile. Unit hydrograph plots for the unit areas above Chickamauga dam are provided in Figure 2.4.3-6 (8 Sheets). The USACE Hydrologic Engineering Center River Analysis System software (HEC-RAS) [24] performs one-dimensional steady and unsteady flow calculations. The HEC-RAS models are used in flood routing calculations for reservoirs in the Tennessee River System upstream of Wilson Dam to predict flood elevations and discharges for floods of varying magnitudes. Model inputs include previously calibrated geometry, unsteady flow rules, and inflows. Model calibration ensures accurate replication of observed river discharges and elevations for known historic events. Once calibrated, the model can be used to reliably predict flood elevations and discharges for events of varying magnitudes. The TVA total watershed HEC-RAS model extends along the Tennessee River from Wilson Dam upstream to its source at the confluence of the Holston and French Broad Rivers, along the Elk River from its mouth at the Tennessee River to Tims Ford Dam, along the Hiwassee from its mouth at the Tennessee River to Chatuge Dam, along the Nottely River from its mouth at the Hiwassee River to Nottely Dam, along the Ocoee River from its mouth at the Hiwassee River to Blue Ridge Dam, along the Clinch River from its mouth at the Tennessee River to a gage at RM 159.8, along the Powell River from its mouth at the Clinch River to a gage at RM 65.4, along the Little Tennessee River from its mouth at the Tennessee River to a gage at RM 92.9, along the Tuckasegee River from its mouth at the Little Tennessee River to a gage at RM 12.6, along the Holston River from its mouth at the Tennessee River to its source at the confluence of the South Fork Holston River and the North Fork Holston River, along the South Fork Holston River from its mouth at the Holston River to South Holston Dam, along the Watauga River from its mouth at the South Fork Holston River to Watauga Dam, along the French Broad River from its mouth at the Tennessee River to a gage at RM 77.5, along the Nolichucky River from its mouth at the French Broad River to a gage at RM 10.3, along Cove Creek from its mouth at the Clinch River to RM 12.2, along Big Creek from its mouth at the Clinch River to RM 11.8, and along North Chickamauga Creek from its mouth at the Tennessee River to RM 12.82. The model also incorporates the Dallas Bay / Lick Branch rim leak and the Fort Loudoun canal by modeling these reaches. Figure 2.4.3-4 (2 sheets) shows the extent of the model, as well as the location of dams. This TVA total watershed HEC-RAS model performs a continuous simulation of the Tennessee River system from the uppermost tributary reservoirs downstream through Wilson Dam. The composite model is used to perform flood simulations of potentially controlling PMP design storms. Discharge rating curves are provided in Figure 2.4.3-7 (17 Sheets) for the reservoirs in the watershed at and above the Chickamauga dam reservoir. The discharge rating curve for Chickamauga Dam is for the proposed final lock modification with five spillways blocked, conservatively bounding the original and ongoing lock modification in regard to impact to the SQN plant site. Seven Watts Bar Reservoir rim leaks were added as additional discharge components to the Watts Bar Dam discharge S2-4.doc 2.4-9 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 simulated in an iterative fashion until the composite flood-wave was routed downstream to the SQN site and Chickamauga dam. Checking tools were used to verify the headwater/tailwater/discharge relationship predicted by HEC-RAS at each dam agreed with approved dam rating curves (DRC). DRCs are provided in Figure 2.4.3-7 (17 sheets). Volume checks were performed as well to ensure that volume was preserved in the model simulation. Unsteady flow rules have been developed for the main Tennessee River and its tributaries and have been incorporated into the verified HEC-RAS unsteady flow model. Elevation and discharge hydrographs for the PMP storm event producing the highest water elevation at the SQN site are presented in Figure 2.4.3-18. Hydrographs for dams in the controlling PMF simulation are provided in Figure 2.4.3-25 (27 sheets). A summary of the results at the dams for the PMF is provided in Table 2.4.3-4 (2 sheets). 2.4.3.3.2 Model Setup The TVA total watershed HEC-RAS model extends along the Tennessee River from Wilson Dam upstream to its source at the confluence of the Holston and French Broad Rivers, along the Elk River from its mouth at the Tennessee River to Tims Ford Dam, along the Hiwassee from its mouth at the Tennessee River to Chatuge Dam, along the Nottely River from its mouth at the Hiwassee River to Nottely Dam, along the Ocoee River from its mouth at the Hiwassee River to Blue Ridge Dam, along the Clinch River from its mouth at the Tennessee River to a gage at RM 159.8, along the Powell River from its mouth at the Clinch River to a gage at RM 65.4, along the Little Tennessee River from its mouth at the Tennessee River to a gage at RM 92.9, along the Tuckasegee River from its mouth at the Little Tennessee River to a gage at RM 12.6, along the Holston River from its mouth at the Tennessee River to its source at the confluence of the South Fork Holston River and the North Fork Holston River, along the South Fork Holston River from its mouth at the Holston River to South Holston Dam, along the Watauga River from its mouth at the South Fork Holston River to Watauga Dam, along the French Broad River from its mouth at the Tennessee River to a gage at RM 77.5, and along the Nolichucky River from its mouth at the French Broad River to a gage at RM 10.3, along Cove Creek from its mouth at the Clinch River to RM 12.2, along Big Creek from its mouth at the Clinch River to RM 11.8, and along North Chickamauga Creek from its mouth at the Tennessee River to RM 12.82. The model also incorporates the Dallas Bay / Lick Branch rim leak and the Fort Loudoun canal by modeling these reaches. Figure 2.4.3-4 shows the extent of the model. HEC-RAS models developed for the individual reservoirs had to be connected into a composite model in order to perform a continuous simulation of the Tennessee River system from TVAs uppermost tributary reservoirs downstream to Wilson Dam. The calibrated geometry for each reservoir was imported into the composite geometry file within HEC-RAS. HEC-RAS Inline Structures were added to model the dams and utilized data presented in DRC calculations [31] and tributary unsteady flow rules [32]. When an Inline Structure is used to model a dam, a headwater cross-section is located upstream and the tailwater section downstream of the dam. Reach lengths are modified to account for adjustments at the dam river station. HEC-RAS Lateral Structures are used at Apalachia, Chatuge, Douglas, Nottely, Ocoee No. 2, Ocoee No.3, and South Holston, Tellico and Watts Bar Dams, to model saddle dams and turbine discharges. Additionally, lateral structures are used on the Holston River, South Fork Holston River, French Broad River, and Nolichucky River to connect storage areas to the rivers. After compiling the separate river geometry files into a composite model, the overall geometry file requires additional modifications before it is adequate for use. These modifications include the addition of junctions and inline structures, copying or interpolating additional cross-sections to allow for the application of inflows, addition of junctions or to enhance model stability, and the addition of pilot channels. If a cross-section is copied or interpolated, the reach lengths associated with the new section are adjusted. The reservoir operating guides applied during the model simulations mimic, to the extent possible, operating policies and are within the current reservoir operating flexibility. In addition to spillway discharge, turbine and sluice discharges were used to release water from the tributary reservoirs. Turbine discharges were also used at the main river reservoirs up to the point where the head differentials are too small and/or the powerhouse would flood. All discharge outlets (spillway gates, sluice gates, and valves) for projects in the reservoir system will remain operable without failure up to the point the operating deck is flooded for the passage of water when and as needed during the flood. A high confidence that all gates/outlets will be operable is provided by periodic inspections by TVA plant personnel, the intermediate and five-year dam safety engineering inspections consistent with S2-4.doc 2.4-11 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Federal Guidelines for Dam Safety, and the significant capability of the emergency response teams to direct and manage resources to address issues potentially impacting gate/outlet functionality. The unsteady flow rules incorporate the seasonal Flood Operational Guides [34] adjusted to weekly values [60], as they provide operating ranges of reservoir levels for the 32 modeled reservoirs upstream of Wilson Dam. The rules reflect the flexibility provided in the guides to respond to unusual or extreme circumstances, such as the PMF event, through the use of primary guide and recovery curves. If the maximum discharge of the primary guide or recovery curve is exceeded, the discharges are from the DRCs [31]. The DRCs account for flow over other components such as non-overflow sections, navigation locks, tops of open spillway gates, tops of spillway piers, saddle dams, and rim leaks. Therefore, the DRCs and the flood operational guides define the dam discharge as a function of headwater elevation, tailwater elevation, and outlet configuration. If, during the event, the headwater elevation does not exceed the elevation of the operating deck, discharges are determined in accordance with the flood operational guides during the flood recession. In the event the operating deck is inundated, the dam rating curves determine the discharge during flood recession. There are configuration parameters in each set of rules that are simulation specific. Model configuration parameters including failure elevation, gate position, operational allowances, armoring embankments, failure timing, and seismic triggers are initially set with input from the modeler. The HEC-RAS model is set-up to run all modeled rivers and reservoirs as a contiguous system to be run continuously. The model cannot be started and stopped in the middle of a simulation; however, some scenarios will require iterative simulations to determine necessary configuration parameters. Inflows were distributed for use in the composite HEC-RAS model of the Tennessee River System upstream of Wilson Dam. Inflow hydrographs presented in the inflow calculation [33] were used as an input to the composite HEC-RAS model. The hydrographs provide inflow data for individual basins in the Tennessee River System. 2.4.3.3.3 Main Stem Geometry The validated geometry for each reservoir was calibrated for use in the unsteady flow model. This validated geometry consists of Fort Loudoun, Tellico, Melton Hill, Watts Bar, Chickamauga, Nickajack, Guntersville, and Wheeler Reservoirs. Wilson Reservoir geometry, although a part of the main stem, was provided by TVA River Management and verified in the same manner as the tributary geometry. Cross-section data was obtained from the geometry verification calculations [36-43] and used to develop the HEC-RAS geometry. Cross-section data obtained from the geometry verification calculations were generally spaced about two miles apart on the main stem. Generally, constricted channel locations were selected for cross-section locations. These smaller, constricted sections do not accurately represent the reach storage available (the storage capacity between cross sections) in an unsteady flow model. Therefore, a mathematical augmentation of selected cross sections with off-channel ineffective flow areas was performed, so the constricted geometry could accurately account for the additional reach storage available. To account for total reach storage, the reach storage contained between the constricted cross-sections was compared to the total reservoir volume information, if available. Overbank volumes were computed using the average overbank reach length rather than as separate left and right overbank reach volumes to coincide with the internal HEC-RAS computations. If reservoir storage information was not available, such as at higher elevations of steep reaches, GIS obtained volumes were used for comparison to the model reach storage capacities. The reach storage between cross-sections was evaluated at incremental elevations. Reach storage was adjusted until the desired total cumulative storage was reached. Where additional reach storage was required, an additional ineffective flow area was added. A check of reach volume for the entire reservoir is also performed to verify that model volume is representative of the published actual reservoir volume [44]. 2.4.3.3.4 Tributary Geometry The tributary geometry has been developed for use in the HEC-RAS models. The tributary geometry was developed in one of three manners [35]:

1. TVA River Management developed the geometry. The geometry was verified in accordance S2-4.doc 2.4-12 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 with 10 CFR 50 Appendix B Quality Assurance requirements for use in safety related applications. The tributary geometry developed by River Management included the following: Apalachia Reservoir, Ocoee River, Toccoa River, Blue Ridge Reservoir, Boone Reservoir, Watauga River, Wilbur Reservoir, South Fork Holston River, Little Tennessee River, Fort Patrick Henry Reservoir, Hiwassee River and Reservoir, Nottely River, and the Elk River.

2. If no geometry previously existed, the geometry was generated and verified for nuclear application. The tributaries that required geometry generation and verification are: Fontana Reservoir, Tuckasegee River, Norris Reservoir, Powell River, Big Creek, and Cove Creek.

3. The TVA River Management developed geometry for the Holston River and the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dams as re-generated and verified to more accurately determine the reservoir storage at these locations.

Verification of River Management Developed Geometry The verification of the tributary geometry previously developed by TVA River Management included verification of the location and orientation of each section, the Mannings n values, the cross-section shape with respect to historic channel geometry, the underwater portion of the section, and storage volume between sections. The location of each cross-section provided by River Management and its orientation were examined. Adjustments were made to the cross-sections and additional cross-sections were added if required to better represent the river. The River Management provided cross-sections were compared to geographic information system (GIS) generated cross-sections above the water surface and historical channel geometry below the water surface elevation. The revised cross-sections were plotted with historic channel geometry cross-sections and the width at the water surface of the new cross-section was compared to and verified against the historic cross-sections. The composite GIS/historic channel geometry cross-sections were then compared to those developed by River Management. When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if necessary. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as to confirm reach lengths. Augmented ineffective flow areas were updated after overbank volumes were computed using average overbank reach lengths. Generation and Verification of New Geometries Development of the HEC-RAS geometry for Fontana Reservoir, Tuckasegee River, Norris Reservoir, Powell River, Big Creek, and Cove Creek were developed by extracting cross-sections from a GIS TIN and comparing the cross-sections to historic cross-sections. Available stream centerline and elevation data were compiled in GIS. USGS topographic maps were examined to identify desired cross-section locations. Once the cross-section locations were established, generic Mannings n values were added in the HEC-RAS geometry. The revised cross-sections were plotted with historic channel geometry cross-sections. The width at the water surface of the new cross-section was compared to and verified against the historic cross-sections. When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if the data were not representative of the cross-section. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as confirm reach lengths. Once the cross-sections were developed and/or verified, a reach storage augmentation procedure was performed so the model storage accurately reflects the actual reach storage capacities. For more information on the reach storage augmentation procedure for tributaries other than Holston River, and S2-4.doc 2.4-13 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dams, see Section 2.4.3.3.3. Re-Generation and Verification of Holston, French Broad, Nolichucky Geometries Re-Generation and verification of the geometries of the Holston River and the downstream portion of the South Fork Holston River above Cherokee Dam and the French Broad and Nolichucky Rivers above Douglas Dam began with the tributary geometry previously developed by TVA River Management. To account for total reach storage in the Holston River, the downstream portion of the South Fork Holston River, the French Broad River and the Nolichucky River, additional cross-sections and storage areas, connected by lateral structures, were added as necessary to match published reservoir or GIS based reach storage. Cross-sections were added to achieve approximate spacing of 1000 ft and to achieve accurate approximation of volume in the reach such as small embayments or just upstream and downstream of an embayment. Storage areas were used where there were larger embayments not easily captured with cross-sections. New cross-sections were generated by extracting a cross-section from HEC-GeoRAS and then re-projecting the cross-section in ArcGIS and adjusting the reach lengths. HEC-RAS was then used to create interpolated sections at the new identified locations. The underwater portions of the new cross-sections were developed using bathymetric data where available or interpolated cross-section data. Because of the large number of new cross-sections and the sinuosity of the main river channel, it was necessary to block off parts of the overbank area for several cross-sections to avoid overestimating overbank reach volume. To represent large embayments, storage areas were inserted with an elevation/storage curve that equates to the additional storage needed for the entire reach. The storage area was connected with a lateral structure. The weir crest of the lateral structure was defined by the bathymetry of the bordering overbank extensions of the adjacent cross-sections. When the shape of each cross-section had been verified, additional geometry data including Mannings n values, ineffective flow areas, and flow lengths were evaluated and adjustments or corrections were made if the data were not representative of the cross-section. Mannings n values were confirmed using aerial photographs. USGS topographic maps were used to identify and confirm ineffective flow areas, as well as confirm reach lengths. Once the cross-sections and storage areas were developed, storage volumes were adjusted iteratively beginning at the lowest elevation and working toward the highest elevation to retain the integrity of the overall volume of the system. 2.4.3.3.5 Calibration Model calibration is performed to adjust model parameters so that the model will accurately predict the outcome of a known historic event. In the case of the HEC-RAS models, the model results must accurately replicate observed elevations and discharges for known historic flood events. A calibrated model is therefore considered reliable at predicting the outcome of events of other magnitudes. 2.4.3.3.5.1 Main Stem River The main river model uses the USACE HEC-RAS software. The main river model extends from Wilson Dam upstream to Norris, Cherokee, Douglas, and Chilhowee Dams, and the Charleston Gage at River Mile (RM) 18.9 on the Hiwassee River. The nine reservoirs upstream of Wilson Dam (Wilson, Wheeler, Guntersville, Nickajack, Chickamauga, Watts Bar, Tellico, Fort Loudoun, and Melton Hill) were individually calibrated for use to reliably predict flood elevations and discharges for events of varying magnitudes. The reservoirs that impact PMF elevations at the SQN site are: Chickamauga, Watts Bar, Tellico, Fort Loudoun and Melton Hill. Initial unsteady-flow runs are conducted to replicate the historic flood events. Initial unsteady-flow runs are conducted for each individual reservoirs model. The initial runs used channel roughness (Mannings n) values from the previous calibrated models in an attempt to replicate the historic flood events. Following the initial runs, roughness values for each of the model segments were evaluated and adjusted as needed. The model was rerun and the results were again compared to the observed elevations at the gage stations. The process was repeated in an iterative fashion until good S2-4.doc 2.4-14 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 agreement was reached between the HEC-RAS computed elevations and the observed gage elevations. Adjustments to the roughness values in the HEC-RAS models were kept within a reasonable range for the ground coverage in the vicinity of the cross section. In general, the computed peak elevations are within one foot, but not below, the observed gage elevations. In some cases, the computed elevations are more than 1 foot above the observed gage elevations; however this was necessary to avoid impacts to the computed peak elevations at other gage locations. A schematic of the model for Watts Bar Reservoir is shown in Figure 2.4.3-11. The calibration results of the March 1973 flood is shown in Figure 2.4.3-12 (2 Sheets) and the calibration results of the May 2003 flood is shown in Figure 2.4.3-13 (2 Sheets). A schematic of the unsteady flow model for Chickamauga Reservoir is shown in Figure 2.4.3-14. The calibration results of the March 1973 flood is shown in Figure 2.4.3-15 (3 Sheets) and the calibration results of the May 2003 flood is shown in Figure 2.4.3-16 (3 Sheets). The configuration for the Fort Loudoun-Tellico complex is shown by the schematic in Figure 2.4.3-8. The Fort Loudoun-Tellico complex was verified by two different methods as follows:

1. Using the available data for the March 1973 flood on Fort Loudoun Reservoir and for the French Broad and Holston rivers. The verification of the 1973 flood is shown in Figure 2.4.3-9 (4 Sheets).

Because there were limited data to verify against on the French Broad and Holston Rivers, the steady state HEC-RAS model was used to replicate the Federal Emergency Management Agency (FEMA) published 100- and 500-year profiles. Tellico Dam was not closed until 1979, thus was not in place during the 1973 flood for verification.

2. Using available data for the May 2003 flood for the Fort Loudoun-Tellico complex. The verification of the May 2003 flood is shown in Figure 2.4.3-10 (5 Sheets). The Tellico Reservoir steady state HEC-RAS model was also used to replicate the FEMA published 100- and 500-year profiles.

In addition to roughness adjustments, the calibration sequence is used to verify that an adequate time step and appropriate mixed flow parameters are selected. To verify the time step, a series of simulations were conducted using PMF flows and varying time steps. The results indicated that a time step of five minutes provides for a stable simulation and the results are comparable with shorter time steps. Above five minutes, there is more variation in the results. The mixed flow regime option is used in the HEC-RAS models because the topographic relief, dam failures, and high flows evaluated for the PMF could produce supercritical flow or hydraulic jumps. Higher values of the mixed flow regime parameters produce more accurate results, but if too high can cause model instability. A comparison of water surface errors between simulations with varying parameters is used to verify appropriate values for the parameters are selected. Once each reservoirs model was adequately calibrated, they were combined into a composite model of the entire main stem for use in a continuous run simulation. This calibration process provided model results that satisfactorily reproduced the two historic floods (1973 and 2003). The HEC-RAS unsteady flow model accurately replicated observed gage elevations and discharges for two large historic flood events. Therefore, the HEC-RAS unsteady flow model of main stem reservoirs upstream of Wilson Dam can be used to reliably predict flood elevations and discharges for events of other magnitudes and is adequate for use in predicting flood elevations and discharges for the PMF. 2.4.3.3.5.2 Tributary Calibration Tributaries were calibrated using a combination of steady-state and unsteady simulations. Steady-state calibration was to Federal Emergency Management Agency (FEMA) 100-year and 500-year flood profiles or, if not available, project manuals. Unsteady calibration, at a minimum, utilized the S2-4.doc 2.4-15 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 worst two historical storms experienced on each tributary, as tabulated below: Tributary Calibration - Largest Recorded Storms Hiwassee River between Hiwassee and March 1994 and April 1998 Apalachia Dams Ocoee River and Toccoa River from Ocoee 1 to April 1998, May 2003, and September 2004 Blue Ridge Dam Boone Reservoir March 2002 and November 2003 Wilbur Reservoir March 2002 and November 2003 Cherokee Reservoir, Holston and South Fork March 2002 and February 2003 Holston Rivers French Broad River and Nolichucky River May 2003 and September 2004 Little Tennessee River and Tuckasegee River May 2003 and September 2004 Fort Patrick Henry Reservoir March 2002 and November 2003 Hiwassee River and Nottely River May 2003 and December 2004 Hiwassee River below Apalachia Dam and May 2003 and September 2004 Ocoee River below Ocoee #1 Dam Clinch River above Norris Dam March 2002 and February 2003 Elk River, Subbasin 1 March 2002 and February 2004 Elk River, Subbasins 2 and 3 February 2004 and January 2006 Elk River, Subbasins 4 and 5 March 1973 and December 2004 Initial tributary geometry segments were obtained from the HEC-RAS Tributary Geometry Development calculation [35]. The required local inflows and associated distribution for unsteady flow modeling were determined from the HEC-RAS Model Calibration and Model Set-up calculations [30, 45]. In most cases, tributary segments were calibrated to FEMA 100-year and the 500-year flood profiles. In some cases, flood profiles were available in published flood insurance studies: in others the profiles were reproduced by running HEC-RAS or HEC-2 files from various TVA studies (e.g., reservoir sedimentation studies, floodplain models, and FEMA flood studies). Some tributary segments only had one FEMA profile available. Some did not have any profiles, in those cases other steady-state profile data were used such as those provided in project manuals. Roughness (Mannings n) values were adjusted iteratively until the steady-state computed profiles were in good agreement with the FEMA or project manual profiles. Following the steady-state calibration procedure, unsteady calibration simulations were performed on the tributary models, similar to the main stem calibration process. Observed historic flood event data were obtained from various available sources such as unit hydrograph calculations or gage data. Results of the unsteady flow simulations were compared to the observed elevation and discharge hydrographs. If the computed results were in good agreement with the observed hydrographs, the calibration was considered complete. In some cases, Mannings n values required further adjustment after comparison of unsteady-flow results. In those cases, the steady-state profiles were rerun to verify agreement with FEMA profiles. This calibration process provided model results that, through the combination of reach storage, unit hydrograph runoff, and inflow distribution, satisfactorily reproduced historic floods and available steady state profiles (FEMA or project manual flood profiles) for the tributary reaches. The HEC-RAS unsteady flow model produced elevations and discharges for large historic flood events appropriate for the intended use of predicting elevations at SQN. Therefore, the HEC-RAS unsteady flow model of the tributaries of the Tennessee River System can be used with the model of the greater Tennessee River System to reliably predict flood elevations and discharges for events of other magnitudes and is adequate for use in predicting flood elevations and discharges for the PMF. S2-4.doc 2.4-16 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Spillway Gates During peak PMF conditions, the radial spillway gates of [CEII] are wide open with flow over the gates and under the gates. For this condition, both the static and dynamic load stresses in the main structural members of the [CEII] spillway gate are determined to be less than the yield stress and the stress in the trunnion pin is less than the allowable design stress. The open radial spillway gates [CEII] spillway gate analysis. Waterborne Objects Consideration has been given to the effect of waterborne objects striking the spillway gates and bents supporting the bridge across [CEII] at peak water level at the dam. [CEII] CEII CEII Lock Gates The lock gates at [CEII] were examined for possible failure with the conclusion that no potential for failure exists. The lock gate structural elements may experience localized yielding and may not function normally following the most severe headwater/tailwater conditions. 2.4.3.5 Water Level Determinations The controlling PMF elevation at the SQN was determined to be [CEII] ft, produced by the controlling PMP for the SQN site resulted from consideration of the drainage area above the Blue Ridge Dam project as the primary PMP watershed nested with multiple secondary area PMPs over the Hiwassee-Blue Ridge, Fontana-Hiwassee-Blue Ridge, Tellico-Hiwassee-Blue Ridge, Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas, and Fort Loudoun-Tellico-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above Chickamauga, Nickajack, Guntersville, and Wheeler dam projects [55]. The PMF elevation hydrograph is shown on Figure 2.4.3-18. Elevations were computed concurrently with discharges using the unsteady flow reservoir model described in Subsection 2.4.3.3. 2.4.3.6 Coincident Wind-Wave Activity Some wind waves are likely when the PMF crests at SQN. The controlling flood would be near its crest for a day beginning about 2 days after cessation of the probable maximum storm. The day of occurrence would most likely be in the month of May. For the SQN site, the two-year extreme wind for the season in which the PMF could occur was adopted to associate with the PMF crest as specified in Regulatory Guide 1.59. The wind data used to S2-4.doc 2.4-18 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 determine the two-year wind is taken from Automated Surface Observation System (ASOS) Surface 1-minute data from the National Climatic Data Center for five surrounding airport data stations (Knoxville, Chattanooga and Tri-Cities, Tennessee, Huntsville, Alabama and Asheville, North Carolina) [56]. Data from January 1, 2000 through June 30, 2014 was used in the analysis. The 2-minute average wind speed, reported for each 1-minute interval, is used in the wind speed determination as recommended by the ASOS Users Guide as more representative of wind speeds that influence wave formation. Since a 20-minute sustained wind is sufficient to cause wind wave activity, the 2-minute average winds were analyzed to find the peak 20-minute average wind speed for each year at the SQN site. Wind direction was considered by calculating the X and Y velocity components based on direction and solving for the resultant wind velocity with respect to the critical fetch direction for every 20-minute window. From these 20-minute average resultant wind velocities, the maximum 20-minute resultant wind velocity was found for the SQN site for each year. A two-year wind is defined as the wind speed that has a 50 percent chance of being exceeded in any given year. To determine the two-year overland wind, the calculated peak 20-minute average resultant velocities for each year for each airport reporting site was statistically analyzed to generate a curve of best fit for wind speed versus probability of exceedence using a Pearson Type III transformation [58]. The two-year wind for each airport reporting site was taken from the curve at the point of a 50 percent probability of exceedence. Using the inverse of the cube of the distance from the SQN site to the airport reporting station, the effective weight the two-year wind speed of each reporting station relative to the SQN site was determined. The sum of the weighted wind speeds from each of the five airport reporting station is taken as the two-year overland wind speed for the SQN site. The overland two-year wind speed for general structures at the SQN site is 23.65 miles per hour [56]. To account for the increase in wind speed over water, these two-year overland wind speeds are converted to over water wind speeds with regard to the effective fetch length [59]. Computation of wind waves at the SQN site was made using the procedures of the Corps of Engineers [59]. Using the two-year over water wind speed and critical fetch direction, the significant wave height (Hs) and wave period (Ts) are determined with Exhibits 3 and 4 [59]. The maximum wave height (Hmax), wave length (Ls) and wave steepness are determined by the following equations [59]: Hmax = 1.67 x Hs Wave Length = 5.12 x Ts2 Wave steepness = Hs / Ls Relative wave runup (R/H) is determined using Exhibit 8 [59]. Wave runup (R), wind setup (S) and total wave height are determined with the following equations [59]: Wave runup (R) = Hmax x Relative wave runup (R/H) Wind setup (S) = (U2 x Fs) / (1400 x D) Total Wave Height = R+S Where: U = Average wind velocity over fetch distance (mph) Fs = Setup Fetch = 2 x critical Fetch (mi) D = Average depth of water generally along the fetch line (ft) The critical fetch directions were from the north and northeast with effective fetches of 1.83 and 1.92 miles, for the ERCW Pumping Station and the Unit 1 Shield Building, respectively. The critical fetch for the Diesel Generator Building was 1.86 miles from the northeast direction. The critical wind wave fetches for the structures are shown in Figure 2.4.3-20, sheets 1 through 5. The maximum water level attained due to the PMF plus wind-wave activity is elevation [CEII] at the vertical face of the nuclear island structures (shield, auxiliary, and control building. The runup on the vertical face of the Unit 1 Shield Building is [CEI ] ft and reaches elevation [CEII] ft, including wind setup. Runup on the vertical face of the Unit 2 Shield Building is [CEII] ft and reaches [CEII] ft, including wind S2-4.doc 2.4-19 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 setup. The wind-wave runup plus wind setup coincident with the maximum flood level for the Diesel Generator Building is elevation [CEII] ft. The level inside structures that are allowed to flood is elevation [CEII] ft. The flood elevations used as design bases are given in Subsection 2.4.14.1.1. The static effect of wind waves was accounted for by taking the static water pressure from the maximum height of the runup. The dynamic effects of wind waves at the ERCW Pumping Station were investigated using the Sainflou-method [62]. Concrete and reinforcing stresses were found to be within allowable limits. 2.4.4 Potential Dam Failures, Seismically Induced The guidance described in Appendix A of Regulatory Guide 1.59 supplemented by the NRC guidance provided in Report JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure, Interim Staff Guidance, were followed when evaluating potential flood levels from seismically induced dam failures. The plant site and upstream reservoirs are located in the Southern Appalachian Tectonic Province and, therefore, subject to moderate earthquake forces with possible attendant failure. Upstream dams whose failure has the potential to cause flood problems at the plant were investigated to determine if failure from seismic events would endanger plant safety. Details of the dam failure analysis are discussed in Subsection 2.4.4.1, Dam Failure Permutations. 2.4.4.1 Dam Failure Permutations NRC JLD-ISG-13-01, Section 1.4.3, Seismic Failure, states that a dam should be assumed to fail due to seismic hazard if it cannot withstand the more severe of the following combinations: x 10-4 annual exceedance seismic hazard combined with a 25-year flood x One-half of the 10-4 ground motion, combined with a 500-year flood The seismic hazard for key TVA dams whose failure could potentially result in flooding of the SQN site is defined by a probabilistic seismic hazard analysis (PSHA) performed by TVAs Dam Safety organization. A site specific PSHA and time histories for each dam were developed for 17 major dams upstream of the SQN site on the Tennessee River and its tributaries whose failure could potentially result in site flooding. Analyzed dam locations with respect to the SQN site are shown in Figure 2.4.1-3. [CEII] The TVA Dam Safety PSHA utilized NUREG-2115, Central and Eastern United States Seismic Source Characterization for Nuclear Facilities (2012), [52] as seismic source characterization (SSC) for the analyzed dams along with Electric Power Research Institute (EPRI)s 2004/2006 ground motion prediction models [53]. The uniform hazard response spectra corresponding to the appropriate structural frequency range of 1 Hz for embankment dam structures and 10 Hz for concrete dam structures was used to determine the controlling earthquake for each dam location. Three sets of spectrally matched time histories are developed for the 1 Hz and 10 Hz structural frequencies. Each set consists of three statistically independent time history records. Site response analyses were completed for the dams not founded on hard rock and are considered best estimate analyses with the shear wave velocities developed from direct measurement of the soils and rock at each site. NUREG/ CR-6728, Table 4-5, was used to define vertical to horizontal ratios. S2-4.doc 2.4-20 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 After the site specific seismic hazard for each of the 22 dams were established, the concrete and/or earth embankment structures (except Apalachia) were evaluated as described below using TVA Dam Safety procedure, RO-SPP-27.1, Design and Evaluation of New and Existing Dams. Concrete Dam Structures The method of analysis of concrete structures is the two or three dimensional finite element method (FEM), which closely models the actual geometry of the dam as well as interaction with the foundation. After the structural model was developed, a dynamic analysis of the concrete dam structure is performed by response spectrum modal analysis or time history analysis. The purpose of the dynamic analysis is to assess the post-earthquake damaged state of the dam and to determine if the dam can continue to resist the applied static loads in a damaged state. The dynamic analysis includes the dynamic effects of the reservoir water mass. The dam/foundation interface is assumed to crack whenever tensile stress normal to the dam/foundation interface is indicated. After the seismic event damaged state of the concrete dam structure has been determined, the post-earthquake stability of the dam is assessed. Forces applied to the dam include hydrostatic forces due the maximum normal reservoir level, dead weight, silt pressure, earth backfill pressure, nappe forces (spillway), and uplift pressure due to degraded drains and base cracking. Cohesion at the rock-concrete interfaces is conservatively neglected, unless sufficient data is available to justify the use of cohesion. The post-earthquake stability of the concrete structure is confirmed, if the sliding factor of safety is 1.3 or greater and the overturning resultant is within the base of the concrete structure. The flow capacity of flood control outlet spillways with concrete weirs on earthen foundations and either concrete lined or unlined flumes is limited to the original design flow for the spillway. If the design flow is exceeded, the spillway is assumed to fail completely to the most shallow erosion resistant layer. Embankment Dam Structures The seismic analysis for earth and rock-fill embankment structures begins with defining the geometry and foundation of the embankment to screen for liquefiable materials. Soil densities, shear strengths, and resistance to liquefaction are evaluated by consideration of laboratory and field test data and comparison with industry source data and past experiences. If materials within the dam have a factor of safety less than 1.4 for liquefaction triggering, post-earthquake analysis is performed using appropriate shear strengths assigned to the potentially liquefiable materials based on standard industry methods. Non-liquefiable materials are evaluated for strain softening and assigned appropriate drained or undrained shear strengths depending on material properties and phreatic surfaces. The post-earthquake analysis is then computed using static equilibrium slope stability analysis utilizing the normal summer pool elevation and shear strengths typically represented as Mohr-Coulomb failure envelope or nonlinear relationships between shear strength and normal stress on the failure surface. Circular, wedge-type and irregular failure surfaces are evaluated. If the search routine develops factor of safety values less than 1.1, then the embankment structure is considered potentially unstable. If liquefiable materials are not present in the embankment and/or foundation, then a static equilibrium analysis is performed using a pseudo-static analysis technique by applying the ground motion as a horizontal force on the critical slip plane from the steady state seepage conditions in the direction of potential failure. The shear strengths are applied from the drained and undrained parameters with the initial effective normal consolidation pressures at normal pool. If the factor of safety is greater than 1.1, the embankment is deemed stable. If the factor of safety is less than 1.1, then a simplified deformation analysis is performed utilizing Newmark Analysis, or other method deemed appropriate. If the deformations by the simplified method are two feet or less and less than one-half the thickness of the filter, the dam is considered stable. Otherwise additional more sophisticated deformation analysis methods may be considered. S2-4.doc 2.4-21 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Flood Routing Multiple Seismic Dam Failures Multiple dam failures in the Tennessee River and tributary river systems could potentially occur in a single seismic event. The guidance in NRC JLD-ISG-2013-01 is applied to determine which combination of dams would potentially fail due to the seismic hazard. Deaggregation of site specific seismic hazard for the 17 dams upstream of SQN and 5 downstream dams was performed for a seismic event with an annual frequency of exceedance (AFE) of 10 -4 and a ground motion level equal to one-half the ground motion of the 10 -4 AFE event. The deaggregation was performed for two ground motion frequencies, 10 Hz for concrete structures and 1 Hz for embankment structures. The cutoff distance for each dam site used in the analysis defines the area containing at least 85 percent of the seismic hazard. Cutoff distances for an AFE 10 -4 event ranged from 50 to 100 kilometers for 10 Hz and 300 to 750 kilometers for 1 Hz. Cutoff distances for one-half the ground motion of a 10-4 AFE ranged from 50 to 125 kilometers for 10 Hz and 300 to 750 kilometers for 1 Hz. The results of the deaggregation provided potential combinations of seismic dam failures due to a single seismic event for the 10-4 AFE hazard and one-half the ground motion of the 10-4 AFE hazard. The large number of potential seismic dam failure combinations for the deaggregation analysis was further evaluated to determine the combination of seismic dam failures resulting in the greatest impact to the SQN site. Using the volume of water released from each single seismic dam failure, the cumulative water volume released for each potential seismic dam failure combination was determined. The seismic dam failure combinations having the largest water volume released were selected as the controlling seismic-flooding combinations. Ground Motion: 10-4 AFE seismic event: [CEII] Ground Motion: One-half the ground motion of the 10-4 AFE seismic event ground motion: [CEII] In addition to these multiple dam failures, a reservoir volume analysis was performed to determine if individual dam failures in a seismic event would exceed SQN plant grade. As a result of this analysis, two additional seismic-flooding combinations were selected as potentially controlling: Ground Motion: One-half the ground motion of the 10-4 AFE seismic event ground motion: [CEII] Ground Motion: One-half the ground motion of the 10-4 AFE seismic event ground motion: [CEII] S2-4.doc 2.4-22 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 [CEII] 25 Year and 500 Year Flood Inflows Flood inflow hydrographs were developed by using watershed gaged data to scale prototypical inflow hydrographs to meet estimated 25-year and 500-year volume targets. Guidance for development of probabilistic point rainfall estimates is published in Reference 50. Reference 50, Section 5, indicates point rainfall estimate data represents rainfall frequency at a point approximately 0.5-miles square and is not directly applicable for larger areas. Reference 51 states that point estimates may be applied to larger areas after adjustment through the use of Areal Reduction Factors (ARFs) for areas up to 400 sq mi. The watershed impacting the SQN Site is 17,310 sq mi above Watts Bar Dam. Because this area is significantly beyond the published limits for ARFs, the application for ARF adjusted point rainfall based on Reference 51 was judged not suitable. Therefore, an alternate methodology for production of scaled inflow hydrographs was developed to meet the requirements. This methodology uses historical gaged data across the watershed above the Watts Bar Dam aggregated into annual maximum series for 1- to 5-day durations to estimate 25-year and 500-year frequency stream flows. TVA has maintained Estimated Local Flow (ELF) data at gaged points in the Tennessee River watershed since 1903. These data represent inflows at the referenced gage point and are independent of river regulation. The daily data from 1903 through 2013 were compiled into X-day values representing the corresponding durational flows (in cfs per X days) for incremental daily durations of 1 to 5 days. The daily average for the X-days was centered on each date for the odd durations and even durations were calculated using the average based on the leading center day. The series data were checked for conflict between same X-day duration water years to identify and eliminate any overlapping events at the end of one water year and the beginning of the subsequent year. Conflicts were resolved by keeping the larger of the two series values and selecting the next highest non-overlapping annual value for the lower value water year. The X-day data sets were arranged by water year (October 1 - September 30) and the annual maximum values for each duration for each water year were identified. Following the guidance of Reference 47, an annual duration series (yearly X-day maximum) was developed for each X-day duration data set. A log-Pearson Type III distribution was applied to the resulting annual series following the methodologies described in References 47 and 49. Correction for data skew and elimination of low and high outliers were performed on the final distribution. A 10 percent significance level K value was used for the outlier check per guidance in Reference 47, Appendix 4. The resulting distributions provide both the 25- and the 500-year X-day durational average streamflows. Because the resulting streamflows represent average flows over the respective duration, the estimates were used as streamflow volumes (i.e. durational streamflow x respective duration). The durational volumes above the Watts Bar project watershed were then selected as the target values for adjustment of the prototype inflow hydrographs. The prototype inflow hydrographs are a representative storm event using published National Weather Service Atlas 14 data. A 25-year point rainfall at the centroid of the watershed above Chickamauga Dam was selected as the prototype rainfall for the watershed. A uniform rainfall areal distribution was applied over all sub-basins with a temporal distribution placing the peak rainfall according to a World Curve approach for a 24-hour event [48]. Rainfall was applied with losses using the NRCS curve number methodology with validated curve numbers for the season, and baseflows applied were June average monthly values. Runoff transformation was accomplished by manual spreadsheet convolution using validated sub-basin unit hydrographs (UHs). Resulting inflow hydrograph data were multiplied by scaling factors applied to all sub-basins to achieve the target volumes for the 25-yr and 500-yr events at each daily duration from 1 to 5 days. Adjustment ratios at the maxima were varied= iteratively to achieve an acceptable difference in volume between the targets and the final summed= hydrographs for the 1- through 4-day values. The 5-day volume target was included in order to= maintain an acceptable slope between the 4th and 5th day maxima which made the adjustment to= meet the 4-day volume more reasonable. However, the 5-day maximum ratio tended to be very high= since the applied rainfall was a 4-day event with losses. This 5-day ratio generated hydrograph= ordinates that were considered to be artifacts. However, the volumes met the target values and were= judged reasonable. Additionally, time steps more than 1 day after the 4-day peak ordinate applied a= recession constant of 10 percent per day to the ratio values to smooth the falling limb and minimize= ratio generated artifacts. The final hydrograph ordinates were summed and volumes calculated to S2-4.doc 2.4-23 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 confirm that the target volumes had been met or exceeded. The adjusted surface runoff values were limited to be no smaller than the constant baseflow. During postulated single and multiple project failure events, the [CEII] is conservatively included in the model flood inflows. [CEII] are located across the sub-basins with conveyances having differing sinuosity, length, slope, cross-sectional, and roughness characteristics. As a result, the postulated failure waves are expected to pass through a variety of supercritical, critical, and subcritical flow regimes as they traverse the respective reaches, starting at the failure location and ending at the respective model input points. The resulting translation reduces the peak flows and spreads the time base of the volume input. A simplified calculation approach was used to account for the [CEII] under these failure conditions. A time to peak of 20 minutes was assumed for the failure hydrographs. A Froehlich approach was used to postulate the individual failure hydrograph peak flows. The individual hydrographs were then combined into a composite triangular hydrograph based on distance of the [CEII] from the model, and the peaks were adjusted to preserve volume ensuring that the entire [CEII] was included in the failure flows (Figure 2.4.4-40). 2.4.4.2 Unsteady Flow Analysis of Potential Dam Failures The calibrated HEC-RAS runoff model described in Subsection 2.4.3.3 was used to perform an unsteady flow analysis of the four potentially critical seismic-flood event combinations involving dam failures above the plant [57]. Using the regulatory guidance provided in NRC JLD-ISG-13-01, Section 1.4.3, seismic events having one-half the ground motion of the 10 -4 AFE seismic event ground motion were combined with a 500-year flood. A seismic event having a ground motion of a10 -4 AFE seismic event was combined with a 25-year flood. Dam structures within the simulation model that have been determined to be stable post-seismic event using TVA Dam Safety dam stability criteria are credited in the model. [CEII] Initial reservoir elevations in the model are consistent with the flood operational guides shown in Figure 2.4.1-4, Sheets 1 through 16. Reservoir operating procedures used were those applicable to the season and flood inflows. One-half the ground motion of 10-4 AFE seismic event ground motion combined with a 500-year flood Simulation 1: [CEII] Centering-Individual Dam Failure [CEII] In the HEC-RAS simulation model, [CEII] S2-4.doc 2.4-24 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 (Sheet 3) elevation and discharge hydrographs. The maximum flow at SQN is 919,651 cubic feet per second. [CEII] 10-4 AFE seismic event ground motion combined with a 25-year flood: Simulation 4: [CEII] Centering-Multiple Dam Failure The post-seismic evaluation of dams affected by the deaggregation of the seismic hazard resulting from the ground motion of the 10-4 AFE event centered at [CEII] In the HEC-RAS simulation model, [CEII] The results of the HEC-RAS unsteady flow model simulation are shown in Figure 2.4.4-17 (Sheet 4) elevation and discharge hydrographs. The maximum flow at SQN is 820,117 cubic feet per second. [CEII] 2.4.4.3 Water Level at Plant Site The unsteady flow analyses [57] of the four postulated combinations of seismic dam failures coincident with floods analyzed yields a maximum elevation of [CEII] ft at SQN excluding wind wave effects as shown in Figure 2.4.4-17 (Sheet 3). The maximum elevation would result from [CEII] due to one-half the ground motion of a 10-4 AFE seismic event ground motion centered at [CEII] in combination with a 500-year flood. Coincident wind wave activity for the PMF is described in Section 2.4.3.6. Wind waves were not computed for the seismic events, but superimposed wind wave activity from guide specified two-year wind speed would result in water surface elevations several feet below the calculated PMF elevation [CEII] ft. described in section 2.4.3. For the design basis flood level, see Section 2.4.14.1. 2.4.5 Probable Maximum Surge and Seiche Flooding Chickamauga Lake level during non-flood conditions would not exceed elevation 682.5 ft, normal maximum pool level, for any significant time. No conceivable meteorological conditions could produce a seiche nor reservoir operations a surge which would reach plant grade elevation 705.0 ft, some 22.5 ft above normal maximum pool level. 2.4.6 Probable Maximum Tsunami Flooding Because of its inland location, SQN is not endangered by tsunami flooding. S2-4.doc 2.4-26 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.7 Ice and Landslides Because of the location in a temperate climate, significant amounts of ice do not form on lakes and rivers in the plant vicinity and ice jams are not a source of major flooding. Flood waves from landslides into upstream reservoirs pose no danger because of the absence of major elevation relief in nearby upstream reservoirs and because the prevailing thin soils offer small slide volume potential compared to the available detention space in reservoirs. 2.4.8 Cooling Water Canals and Reservoirs 2.4.8.1 Canals The intake channel, as shown in Figure 2.1.2-1, referenced in paragraph 2.4.1.1, is designed for a flow of 2,250 cfs. At minimum pool (elevation 675), as shown in Figure 2.4.8-1, this flow is maintained at a velocity of 2.7 fps. The protection of the intake channel slopes from wind-wave activity is afforded by the placement of riprap, shown in Figure 2.4.8-1, in accordance with TVA Design Standards, from elevation 665 to elevation 690. The riprap is designed for a wind velocity of 45 mph. 2.4.8.2 Reservoirs Chickamauga Reservoir provides the cooling water for SQN. This reservoir and the extensive TVA system of upstream reservoirs, which regulate inflows, are described in Table 2.4.1-2. The location in an area of ample runoff and the extensive reservoir system assures sufficient cooling waterflow for the plant. 2.4.9 Channel Diversions Channel diversion is not a potential problem for the plant. There are no channel diversions upstream of SQN that would cause diverting or rerouting of the source of plant cooling water, and none are anticipated in the future. The floodplain is such that large floods do not produce major channel meanders or cutoffs. Carbon 14 dating of material at the high terrace levels shows that the Tennessee River has essentially maintained its present alignment for over 35,000 years. The topography is such that only an unimaginable catastrophic event could result in flow diversion above the plant. 2.4.10 Flooding Protection Requirements Assurance that safety-related facilities are capable of surviving all possible flood conditions is provided by the discussions given in Sections 2.4.14, 3.4, 3.8.1, 3.8.2, and 3.8.4. The plant is designed to be shutdown and remain in a safe shutdown condition for any rainfall flood exceeding plant grade, up to the "design basis flood" discussed in Section 2.4.3 and for lower, seismic-caused floods discussed in Section 2.4.4. Any rainfall flood exceeding plant grade will be predicted at least 27 hours in advance by TVA's River Forecast Center. Warning of seismic failure of key upstream dams will be available at the plant approximately 27 hours before a resulting flood surge would reach plant grade. Hence, there is adequate time to prepare the plant for any flood. See Section 2.4.14 for a detailed presentation of the flood protection plan. 2.4.11 Low Water Considerations Because of its location on Chickamauga Reservoir, maintaining minimum water levels at SQN is not a problem. The high rainfall and runoff of the watershed and the regulation afforded by upstream dams assure minimum flows for plant cooling. S2-4.doc 2.4-27 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.11.1 Low Flow in Rivers and Streams The targeted minimum water level at SQN is elevation [CEII] ft and would occur in the winter flood season as a result of Chickamauga Reservoir operation. On rare occasions, the water level may be slightly lower (.1 or .2 tenths of a foot) for a brief period of time (hours) due to hydropower peaking operations at Chickamauga and Watts Bar Dams during the winter season. A minimum elevation of [CEII] ft must be maintained in order to provide the prescribed commercial navigation depth in Chickamauga Reservoir. The Preferred Alternative Reservoir Operating Policy was designed to provide increased recreation opportunities while avoiding or reducing adverse impacts on other operating objectives and resource areas. Under the Preferred Alternative, TVA will no longer target specific summer pool elevations at 10 tributary storage reservoirs. Instead, TVA tends to manage the flow of water through the system to meet operating objectives. TVA will use weekly average system flow requirements to limit the drawdown of 10 tributary reservoirs (Blue Ridge, Chatuge, Cherokee, Douglas, Fontana, Nottely, Hiawassee, Norris, South Holston, and Watauga) June 1 through Labor Day to increase recreation opportunities. For four main stem reservoirs (Chickamauga, Guntersville, Wheeler, and Pickwick), summer operating zones will be maintained through Labor Day. For Watts Bar Reservoir, the summer operating zone will be maintained through November 1. Weekly average system minimum flow requirements from June 1 through Labor Day, measured at Chickamauga Dam, are determined by the total volume of water in storage at the 10 tributary reservoirs compared to the seasonal total tributary system minimum operating guide (SMOG). If the volume of water in storage is above the SMOG, the weekly average system minimum flow requirement will be increased each week from 14,000 cfs the first week of June to 25,000 cfs the last week of July. Beginning August 1 and continuing through Labor Day, the weekly average flow requirement will be 29,000 cfs. If the volume of water in storage is below the SMOG curve, 13,000 cfs weekly average minimum flows will be released from Chickamauga Dam between June 1 and July 31, and 25,000 cfs weekly average minimum flows will be released from August 1 through Labor Day [19]. Within these weekly averages, TVA has the flexibility to schedule daily and hourly flows to best meet all operating objectives, including water supply for TVAs thermal power generating plants. Flows may be higher than these stated minimums if additional releases are required at tributary or main river reservoirs to maintain allocated flood storage space or during critical power situations to maintain the integrity and reliability of the TVA power supply system. In the assumed event of [CEII] [61]. The estimated minimum river flow requirement for the ERCW system is only 45 cfs. 2.4.11.2 Low Water Resulting From Surges, Seiches, or Tsunamis Because of Sequoyahs inland location on a relatively small, narrow lake, low water levels resulting from surges, seiches, or tsunamis are not a potential problem. 2.4.11.3 Historical Low Water From the beginning of stream gauge records at Chattanooga in 1874 until the closure of Chickamauga Dam in January 1940, the lowest daily flow in the Tennessee River at SQN was 3,200 cfs on September 7 and 13, 1925. The next lowest daily flow of 4,600 cfs occurred in 1881 and also in 1883. Since January 1942, low flows at the site have been regulated by TVA reservoirs, particularly by Watts Bar and Chickamauga Dams. Under normal operating conditions, there may be periods of several hours daily when there are no releases from either or both dams, but average daily flows at the site S2-4.doc 2.4-28 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 from 1976 to October 2022 have been less than 5,000 cfs about 1.6 percent of the time and have been less than 10,000 cfs about 12 percent of the time. On March 30 and 31, 1968, during special operations for the control of watermilfoil, there were no releases from either Watts Bar or Chickamauga Dams during the two-day period. Over the period 1986 - 2010, the number of zero flow days at Watts Bar and Chickamauga Dams have been 0 and 2, respectively. Since January 1940, water levels at the plant have been controlled by Chickamauga Dam. For the period (1940 - 2010), the minimum level at the dam was elevation [CEII] ft on January 21, 1942. TVA no longer routinely conducts pre-flood drawdowns below elevation [CEII] ft at Chickamauga Reservoir and the minimum elevation in the past 20 years (1987 - 2006) was elevation [CEII] ft at Chickamauga head water. 2.4.11.4 Future Control Future added controls which could alter low flow conditions at the plant are not anticipated because no sites that would have a significant influence remain to be developed. However, any control that might be considered would be evaluated before implementation. 2.4.11.5 Plant Requirements 2.4.11.5.1 Two-Unit Operation The safety related water supply systems requiring river water are: the essential raw cooling water (ERCW) (Subsection 9.2.2), and that portion of the high-pressure fire-protection system (HPFP) (Subsection 2.4.14.4.1) supplying emergency feedwater to the steam generators. The fire/flood mode pumps are submersible pumps located in the condenser circulating water (CCW) intake pumping station. The CCW intake pumping station sump is at elevation [CEII] ft. The entrances to the suction pipes for the fire/flood mode pumps are at elevation [CEII] ft which is about [CEII] feet, respectively, below the maximum normal water elevation of [CEII] ft and the normal minimum elevation of [CEII] ft for the reservoir. Abnormal reservoir level is elevation [CEII] ft with a technical specification limit of elevation [CEII] ft. For flow requirements of the HPFP during engineering safety feature operation (Reference 22). The ERCW pump sump in this independent station is at elevation [CEII] ft, which is [CEII] ft below maximum normal water elevation, [CEII] ft below minimum normal water elevation, and [CEII] ft below the [CEII] ft minimum possible elevation of the river. Since the ERCW pumping station has direct communication with the river for all water levels and is above probable maximum flood, the ERCW system for two-unit plant operation always operates in an open cooling cycle. 2.4.11.6 Heat Sink Dependability Requirements The ultimate heat sink, its design bases and its operation, under all normal and credible accident conditions is described in detail in Subsection 9.2.5. As discussed in Subsection 9.2.5, the sink was modified by a new ERCW pumping station before unit 2 began operation. The design basis and operation of the ERCW system, both with the original ERCW intake station and with the new ERCW intake station, is presented in Subsection 9.2.2. As described in these sections, the new ERCW station is designed to guarantee a continued adequate supply of essential cooling water for all plant design basis conditions. This position is further assured since additional river water may be provided from TVA's upstream multiple-purpose reservoirs, as previously discussed during Low Flow in Rivers and Streams. 2.4.11.6.1 Loss of Downstream Dam The loss of downstream dam will not result in any adverse effects on the availability of water to the ERCW system or these portions of the original HPFP supplying emergency feedwater to the steam generator. Loss of downstream dam reduces ERCW flow about 7% to the component cooling and containment spray heat exchangers. ERCW flow does not decrease below that assumed in the S2-4.doc 2.4-29 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 analysis (analyzed as [CEII] ft) until more than two hours after the peak containment temperature and pressure occurs. (See Section 6.2.1.3.4.) 2.4.11.6.2 Adequacy of Minimum Flow The cooling requirements for plant safety-related features are provided by the ERCW system. The required ERCW flow rates under the most demanding modes of operation (including loss of downstream dam) as given in Subsection 9.2.2 are contained in TVA calculations and flow diagrams. Two other safety-related functions may require water from the ultimate heat sink; these are fire protection water (refer to Subparagraph 2.4.11.6.3) and emergency steam generator feedwater (refer to Subsection 10.4.7). These two functions have smaller flow requirements than the ERCW systems. Consequently, the relative abundance of the river flow, even under the worst conditions, assures the availability of an adequate water supply for all safety-related plant cooling water requirements. TVA River Management methodology for maintaining UHS temperatures are discussed in Monitoring and Moderating Sequoyah Ultimate Heat Sink, Reference 21. 2.4.11.6.3 Fire-Protection Water Refer to the Fire Protection Report discussed in Section 9.5.1. 2.4.12 Environmental Acceptance of Effluents The ability of surface waters near SQN, located on the right bank near TRM 484.5, to dilute and disperse radioactive liquid effluents accidentally released from the plant is discussed herein. Routine radioactive liquid releases are discussed in Section 11.2. The Tennessee River is the sole surface water pathway between SQN and surface water users along the river. Liquid effluent from SQN flows into the river from a diffuser pond through a system of diffuser pipes located at TRM 483.65. An accidental, radioactive liquid effluent release from SQN would enter the Tennessee River after it reached the diffuser pond and entered the diffuser pipes. The contents of the diffuser pond enter the diffuser pipes and mix with the river flow upon discharge. The diffusers are designed to provide rapid mixing of the discharged effluent with the river flow. The flow through the diffusers is driven by the elevation head difference between the diffuser pond and the river [1] (McCold 1979). Descriptions of the diffusers and SQN operating modes are given in Paragraph 10.4.5.2. Flow is discharged into the diffuser pond via the blowdown line, ERCW System (Subsection 9.2.2) and CCW System (Subsection 10.4.5). A layout of SQN is given in Figures 2.1.2-1 and 2.1.2-2. Two pipes comprise the diffuser system and are set alongside each other on the river bottom. They extend from the right bank of the river into the main channel. The main channel begins near the right bank of the river and is approximately 900 feet wide at SQN [1] (McCold, 1979). Each diffuser pipe has a 350-foot section through which flow is discharged into the river. The downstream diffuser leg discharges across a section 0 to 350 feet from the right bank of the main channel. The upstream diffuser leg starts at the end of the downstream diffuser leg and discharges across a section 350 to 700 feet from the right bank of the main channel. The two diffusers therefore provide mixing across nearly the entire main channel width. The river flow near SQN is governed by hydro power operations of Watts Bar Dam upstream (TRM 529.9) and Chickamauga Dam downstream (TRM 471.0). The backwater of Chickamauga Dam extends to Watts Bar Dam. Peaking hydro power operations of the dams cause short periods of zero (i.e., stagnant) and reverse (i.e., upstream) flow near the plant. Effluent released from the diffusers during these zero and reverse flow periods will not concentrate near the plant or affect any water intake upstream. The maximum flow-reversal during 1978-1981 were not long enough to cause discharge from the diffusers to extend upstream to the SQN intake [2] (El-Ashry, 1983), which is the nearest intake and located at the right bank near TRM 484.7. Moreover, the warm buoyant discharge from the diffusers will tend toward the water surface as it mixes the river flow and away from the cooler, denser water found near the intake opening below the skimmer wall. The intake opening extends the first 10 feet above the riverbed elevation of about [CEII] ft MSL. The minimum flow depth at the intake is approximately [CEII] [3] (Ungate and Howerton, 1979). There are no other surface water users between the diffusers and this intake. S2-4.doc 2.4-30 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Subsection 2.4.13 discusses groundwater movement at SQN. Effluent released through the diffusers will have no impact on SQN groundwater sources along the banks of the river. Paragraph 2.2.3.8 discusses the effect on plant safety features from flammable or toxic materials released in the river near SQN. The predominant transport and effect of a diffuser release is along the main channel and in the downstream direction. The nearest downstream surface water intake is located along the left bank at TRM 473.0 (Table 2.4.1-1). A mathematical analysis is used to estimate the downstream transport and dilution of a contaminant released in the Tennessee River during an accidental spill at SQN. Only the main channel flow area without the adjacent overbank regions is considered in the analysis. The mathematical analysis of a potential spill scenario can involve: (1) a slug release, which can be modeled as an instantaneous release; (2) a continuous release, which can be modeled as a steady-state release; (3) a bank release, which can be modeled as a vertical line source; and (4) a diffuser release, which can be modeled either as a vertical line or plane source, depending on the width of the diffuser with respect to the channel width. The following assumptions are used in the mathematical analyses to compute the minimum dilution expected downstream from SQN and, in particular, at the nearest water intake.

1. Mixing calculations are based on unstratified steady flow in the reservoir. River flow, Q, is assumed to be 27,474 cubic feet per second (cfs), which is equaled or exceeded in the reservoir approximately 50 percent of the time (Paragraph 2.4.1.2). Because various combinations of the upstream and downstream hydro power dam operations can create upstream flows past SQN, a minimum flow is not well defined. Larger (smaller) flows will decrease (increase) the travel time to the nearest intake but cause less than an order of magnitude change in the calculated dilution.

2. Because the SQN diffusers and the nearest downstream water intake are on opposite banks of the river, and the diffusers extend across most of the main channel width, an analysis using a diffuser release (rather than a bank release) is selected to yield a lesser (i.e., more conservative) dilution at the intake. Thus, the accidental spill is modeled as a vertical plane source across the width of the main channel.

3. The contaminant concentration profile from a slug release is assumed to be Gaussian (i.e.,

normal) in the longitudinal direction.

4. The contaminant is conservative, i.e., it does not degrade through radioactive decay, chemical or biological processes, nor is it removed from the reservoir by adsorption to sediments or by volatilization.

5. The transport of the contaminant is described using the motion of the river flow, i.e., the contaminant is neutrally buoyant and does not rise or sink due to gravity.

The main channel and dynamic, flow-dependent processes of the reservoir reach between SQN and the first downstream water intake are modeled as a channel of constant rectangular cross section with the following constant geometric, hydraulic and dispersion characteristics. Longitudinal distance, x = 10.6 miles Average water surface elevation = 678.5 feet MSL (1) Average width, W = 1175 feet Average depth, H = [CEII] Average velocity, U (= Q/(W H)) = 0.468 feet per second (fps) Average travel time (for approximate peak contaminant), t (= x/U) = 1.4 days S2-4.doc 2.4-31 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Manning coefficient n (surface roughness) = 0.03 Longitudinal dispersion parameter, alpha = 200 where: alpha = Ex / (H u) Ex = constant longitudinal dispersion coefficient (square feet per second) u = shear velocity (fps) = gRS g = acceleration due to gravity = 32.174 ft/s2 R = hydraulic radius (ft) S = slope of the energy line (ft/ft) The average width and depth were estimated from measurements of 9 cross sections in the reach [4] (TVA) [5] (TVA). For wide channels (i.e., large width-to-depth ratio), the hydraulic radius can be approximated as the average depth. The value of alpha = 200 is on the conservative (i.e., low) side [6] (Fischer, et al., 1979). The value of the Manning coefficient n is representative for natural rivers [7] (Chow, 1959). The equation used to describe the maximum downstream activity (or concentration), C, at a point of interest due to an instantaneous plane source release of volume V is [8] (Guide 1.113): C V

     =

Co W H 4S E x t (2.4.12-1) where: Co = initial activity (or concentration) in the plant of the released contaminant S = 3.14156 Any consistent set of units can be used on each side of Equation 2.4.12-1 (e.g., C and C o in mCi/ml; V in cf; W and H in ft; Ex in ft2/s; t in s). The term, C/Co, is the relative (i.e., dimensionless) activity (or concentration) and its reciprocal is the dimensionless dilution factor. Equation 2.4.12-1 simplifies to C/C o = 8.3E-10

• V (V expressed in cubic feet (cf)) when the parameters are substituted and the Manning equation [7] (Chow, 1959) is used in the definition of the shear velocity, u. In the substitution, u = 0.028 ft/s and E x = 282.1 ft2/s.

The equation used to describe the maximum downstream concentration at a point of interest due to a continuous plane source release rate, Qs, where Qs << Q, is [8] (Guide 1.113): (2.4.12-2) C Qs

                 =

Co Q Any consistent set of units can be used on each side of Equation 2.4.12-2 (e.g., C and C o in mCi/ml; Qs and Q in cfs). S2-4.doc 2.4-32 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Equation 2.4.12-2 simplifies to C/Co = 3.64E-05

• Qs (Qs expressed in cfs) for Q = 27,474 cfs.

Examples of quantities and concentrations of potential contaminant releases and the use of Equations 2.4.12-1 and 2.4.12-2 follow. Because Co is defined as the in-plant activity (or concentration) and not that of the diffuser release, an estimate of the dilution of liquid waste occurring in the diffuser pond and diffuser pipes is not needed. This is because the flow available for dilution in the plant (e.g., CCW and ERCW) is taken from and returned to the river. Only effluent extraneous to the river flow requires consideration in the analyses to calculate the dilution. More information on the possible means which liquid waste from the plant enters the diffuser pond is contained in Subsection 10.4.5. The largest outdoor tanks whose contents flow into the diffuser pond are the two condensate storage tanks (Paragraph 11.2.3.1), which each have an overflow capacity of 398,000 gallons. Liquid waste that reaches the diffuser pond enters the Tennessee River through the diffuser system. The diffuser pond is approximately 2000 feet long and 500 feet wide with a depth that, although it depends on the Chickamauga Reservoir elevation, averages about 10 feet [9] (McIntosh, et al., 1982). The design flow residence time of the pond is approximately one hour (i.e., diffuser design flow is 2,480 cfs at maximum plant capacity [3] [Ungate and Howerton, 1979]). For example, assume an instantaneous plane source release into the Tennessee River of the contents of one condensate storage drain tank. Assume the full 398,000 gallon (53,210 cf) volume contains Iodine-131 (I-131) at an activity of 1.5E-06 mCi/gm (Table 10.4.1-1). From Equation 2.4.12-1, the activity, C, at the first downstream water intake would be 6.6E-11 mCi/gm>@, which is within the acceptable limit [10] (CFR) for soluble I-131. For a continuous plane source release, assume the contents of the 398,000 gallon (53,210 cf) floor drain tank leak out steadily over a 24-hour period. The effective release rate is 0.6 cfs at an activity of 1.5E-06 mCi/gm. The expected activity at the first downstream water intake would be 3.4E-11 mCi/gm >@ using Equation 2.4.12-2 and is within the acceptable limit [10] (CFR) for soluble I-131. REFERENCES (for Section 2.4.12 only) [1] McCold, L. N. (March 1979), "Model Study and Analysis of Sequoyah Nuclear Plant Submerged Multiport Diffuser," TVA, Division of Water Resources, Water System Development Branch, Norris, TN, Report No. WR28-1-45-103. [2] El-Ashry, Mohammed T., Director of Environmental Quality, TVA, February 1983 letter to Paul Davis, Manager, Permit Section, Tennessee Division of Water Quality Control, SEQUOYAH NUCLEAR PLANT---NPDES PERMIT NO. T0026450. [3] Ungate, C. D., and Howerton, K. A. (April 1978; revised March 1979), "Effect of Sequoyah Nuclear Plant Discharges on Chickamauga Lake Water Temperatures," TVA, Division of Water Management, Water Systems Development Branch, Norris, TN, Report No. WR28-1-45-101. [4] TVA, Chickamauga Reservoir Sediment Investigations, Cross Sections, 1940-1961, Division of Water Control Planning, Hydraulic Data Branch. [5] TVA, Measured Cross Sections of Chickamauga Reservoir, 1972, Flood Protection Branch. [6] Fischer, H. B., List, E. J., Koh, R.C.Y., Imberger, J., Brooks, N. H. (1979), Mixing in Inland and Costal Waters, Academic Press, New York. [7] Chow, V. T. (1959) Open-Channel Hydraulics, McGraw-Hill, New York. [8] United States Nuclear Regulatory Commission, Office of Standards Development, Regulatory Guide 1.113 (April 1977), "Estimating Aquatic Dispersion of Effluents from Accidental and Routine Reactor Releases for the Purpose of Implementing Appendix I," Revision 1. [9] McIntosh, D. A., Johnson, B. E. and Speaks, E. B. (October 1982), "A Field Verification of Sequoyah Nuclear Plant Diffuser Performance Model: One-Unit Operation," TVA, Office of S2-4.doc 2.4-33 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Natural Resources, Division of Air and Water Resources, Water Systems Development Branch, Norris, TN, Report No. WR28-1-45-110. [10] 10 CFR Part 20, Appendix B, Table II, Column 2. [11] TVA SQN Calculation SQN-SQS2-0242, SQN Site Iodine-131 Release Concentration in Tennessee River. S2-4.doc 2.4-34 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.13 Groundwater 2.4.13.1 Description and Onsite Use The peninsula on which SQN is located is underlain by the Conasauga Shale, a poor water-bearing formation. About 2,000 feet northwest of the plant site, the trace of the Kingston Fault separates this outcrop area of the Conasauga Shale from a wide belt of Knox Dolomite. The Knox is the major water bearing formation of eastern Tennessee. Groundwater in the Conasauga Shale occurs in small openings along fractures and bedding planes; these rapidly decrease in size with depth, and few openings exist below a depth of 300 feet. Groundwater in the Knox Dolomite occurs in solutionally enlarged openings formed along fractures and bedding planes and also in locally thick cherty clay overburden. There is no groundwater use at SQN. 2.4.13.2 Sources The source of groundwater at SQN is recharged by local, onsite precipitation. Discharge occurs by movement mainly along strike of bedrock, to the northeast and southwest, into Chickamauga Lake. Rises in the level of Chickamauga Lake result in corresponding rises in the water table and recharge along the periphery of the lake, extending inland for short distances. Lateral extent of this effect varies with local slope of the water table, but probably nowhere exceeds 500 feet. Lowering levels of Chickamauga Lake results in corresponding declines in the water table along the lake periphery, and short-term increase in groundwater discharge. When SQN was initially evaluated in the early 1970s, it was in a rural area, and only a few houses within a two-mile radius of the plant site were supplied by individual wells in the Knox Dolomite (see Table 2.4.13-1, Figure 2.4.13-1). Because the average domestic use probably does not exceed 500 gallons per day per house, groundwater withdrawal within a two-mile radius of the plant site was less than 50,000 gallons per day. Such a small volume withdrawal over the area would have essentially no effect on areal groundwater levels and gradients. Although development of the area has increased, public supplies are available and overall groundwater use is not expected to increase. Public and industrial groundwater supplies within a 20 mile radius of the site in 1985 are listed in Table 2.4.13-2. The area groundwater gradient is towards Chickamauga Lake, under water table conditions, and at a gradient of less than 120 feet per mile. The water table system is shallow, the surface of which conforms in general to the topography of the land surface. Depth to water ranges from less than 10 feet in topographically low areas to more than 75 feet in higher areas underlain by Knox Dolomite. Figure 2.4.13-2 is a generalized water-table map of SQN, based on water level data from five onsite observation wells, and in private wells adjacent to the site in April 1973, and also based on surface resistivity measurements of depth to water table made in 1972. Because permeability across strike in the Conasauga Shale is extremely low, and nearly all water movement is in a southwest-northeast direction, along strike, the Conasauga-Knox Dolomite Contact is a hydraulic barrier, across which only a very small volume of water could migrate in the event large groundwater withdrawals were made from the adjacent Knox. Although some water can cross this boundary, the permeability normal to strike of the Conasauga is too low to allow development of an areally extensive cone of depression. Groundwater recharge occurs to the Conasauga Shale at the plant site. Recharge water moves no more than 3,000 feet before being discharged to Chickamauga Lake. 2.4.13.3 Accident Effects Design features in SQN further protect groundwater from contamination. Category I structures in the SQN facility are designed to assure that all system components perform their designed function, including maintenance of integrity during earthquake. S2-4.doc 2.4-35 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Buildings in which radioactive liquids could be released due to the equipment failure, overflow, or spillage are designed to retain such liquids even if subject to an earthquake equivalent to the safe shutdown earthquake. Outdoor tanks that contain radioactive liquids are designed so that if they overflow, the overflow liquid is redirected to the building where the liquid is collected in the radwaste system. Two outdoor tanks that contain low concentrations of radioactivity at times overflow to yard drains which discharge into the diffuser pond. Overflow liquid is discharged near the discharge diffuser. The capacity for dispersion and dilution of contaminants by the groundwater system of the Conasauga Shale is low. Dispersion would occur slowly because water movement is limited to small openings along fractures and bedding planes in the shale. Clay minerals of the Conasauga Shale do, however, have a relatively high exchange capacity, and some of the radioactive ions would be absorbed by these minerals. Any ions moving through the groundwater system eventually would be discharged to Chickamauga Lake. The Conasauga Shale is heterogeneous and anisotropic vertically and horizontally. Water-bearing characteristics change abruptly within short distances. Standard aquifer analyses cannot be applied, and meaningful values for permeability, time of travel, or dilution factors cannot be obtained. Bedrock porosity is estimated to be less than 3 percent based on examination of results of exploratory core drilling. It is known from experience elsewhere in this region that water movement in the Conasauga Shale occurs almost entirely parallel to strike. Subsurface movement of a liquid radwaste release at the plant site would be about 1,000 feet to the northeast or about 2,000 feet to the southwest before discharge to Chickamauga Lake. Time of travel can only be estimated as being a few weeks for first arrival, a few months for peak concentration arrival, and perhaps two or more years for total discharge. The computed mean time of travel of groundwater from SQN to Chickamauga Lake is 303 days. No radwaste discharge would reach a groundwater user. At the nearest point, the reservation boundary lies 2,200 feet northwest of the plant site, across strike. Groundwater movement will not occur from the plant site in this direction across this distance. During initial licensing, the radionuclide concentrations were determined for both groundwater and surface water movement to the nearest potable water intake (Savannah Valley Utility District, which is no longer in service) and found to be of no concern (see Safety Evaluation Report, March 1979, Section 2.4.4 Groundwater). 2.4.13.4 Monitoring or Safeguard Requirements SQN is on a peninsula of low-permeability rock; the groundwater system of the site is essentially hydraulically isolated and potential hazard to groundwater users of the area is minimal. The environmental radiological monitoring program is addressed in Section 11.6. Monitor wells 1, 2, 3, and 4 were sampled and analyzed for radioactivity during the period from 1976 through 1978. Well 5 was not monitored because of insufficient flow. An additional well (Well 6) was drilled in late 1978 downgradient from the plant and a pump sampler installed. Wells 1, 2, 4, and 5 are each 150 feet deep, Well 6 is 250 feet deep, and Wells L6 and L7 are 75-80 feet deep. All of the wells are cased in the residuum and open bore in the Conasauga Shale. 2.4.13.5 Conclusions SQN was designed to provide protection of groundwater resources by preventing the escape of the leaks of radionuclides. Site soils and underlying geology provide further protection in that they retard the movement of water and attenuate any contaminants that would be released. All groundwater movement is toward Chickamauga Lake. The Knox Dolomite is essentially hydraulically separated from the Conasauga Shale; therefore, offsite pumping, including future development, should have little effect upon the groundwater table in the Conasauga Shale at the plant. S2-4.doc 2.4-36 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 In addition to level considerations, plant flood preparations will cope with the "fastest rising" flood which is the calculated flood including seismically induced floods that can exceed plant grade with the shortest warning time. Reservoir levels for large floods in the Tennessee Valley can be predicted well in advance. A minimum of 27 hours, divided into two stages, is provided for safe plant shutdown by use of this prediction capability. Stage I, a minimum of 10 hours long, will commence upon a prediction that flood-producing conditions might develop. Stage II, a minimum of 17 hours long, will commence on a confirmed estimate that conditions will provide a flood above plant grade. This two-stage scheme is designed to prevent excessive economic loss in case a potential flood does not fully develop. Refer to Subsection 2.4.14.4. 2.4.14.1.2 Combinations of Events Because floods above plant grade, earthquakes, tornadoes, or design basis accidents, including a loss-of-coolant accident (LOCA), are individually very unlikely, a combination of a flood plus any of these events or the occurrence of one of these during the flood recovery time or of the flood during the recovery time after one of these events is considered incredible. However, as an exception, certain reduced levels of floods are considered together with seismic events. Refer to Section 2.4.14.10 and 2.4.4. 2.4.14.1.3 Post Flood Period Because of the improbability of a flood above plant grade, no detailed procedures will be established for return of the plant to normal operation unless and until a flood actually occurs. If flood mode operation (Subsection 2.4.14.2) should ever become necessary, it will be possible to maintain this mode of operation for a sufficient period of time (100 days) so that appropriate recovery steps can be formulated and taken. The actual flood waters are expected to recede below plant grade within 1 to 7 days. 2.4.14.1.4 Localized Floods Localized plant site flooding due to the probable maximum storm (Subsection 2.4.2.3) will not enter vital structures or endanger the plant. Plant shutdown will be forced by water ponding on the switchyard and around buildings, but this shutdown will not differ from a loss of offsite power situation as described in Chapter 15. The other steps described in this subsection are not applicable to this case. 2.4.14.2 Plant Operation During Floods Above Grade "Flood mode" operation is defined as the set of conditions described below by means of which the plant will be safely maintained during the time when flood waters exceed plant grade (elevation 705.0 ft) and during the subsequent period until recovery (Subsection 2.4.14.7) is accomplished. 2.4.14.2.1 Flooding of Structures The Reactor Building, the Diesel Generator Building (DGB), and the Essential Raw Cooling (ERCW) Water Intake Station will be maintained essentially water tight during the flood mode. Walls and penetrations are designed to withstand all static and dynamic forces imposed by the DBF. The Reactor Buildings protect SSCs contained within that are required for Flood Mode operations. All penetrations below the DBF level including wind wave runup have been sealed with seals, which are tested to withstand hydrostatic forces generated by the DBF. Analysis demonstrates the acceptability of minor leakage through the seals into the annulus. The lowest floor of the DGB is at elevation 722.0 ft with its doors on the uphill side facing away from the main body of flood water. This elevation is higher than the DBF elevation including wind wave runup of 720.6 ft.. Therefore, flood levels do not exceed floor elevation of 722.0 ft. The entrances into S2-4.doc 2.4-38 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 safety-related areas and all mechanical and electrical penetrations into safety-related areas are sealed either prior to or during flood mode to prevent major leakage into the building for water up to the DBF, including wind wave runup. Redundant sump pumps are provided within the building to remove minor leakage. The ERCW intake station is designed to remain fully functional for floods up to the DBF, including wind wave runup. The deck elevation (elevation 720.0 ft) is below the DBF plus wind wave runup, but the deck is protected from flooding by the outside walls. The traveling screen wells extend above the deck elevation up to the DBF surge level. The wall penetration for water drainage from the deck in nonflood conditions is below the DBF elevation, but it is designed for sealing in event of a flood. All other exterior penetrations of the station below the DBF are permanently sealed. Redundant sump pumps are provided on the deck and in the interior rooms to remove rainfall on the deck and water seepage. All other structures, including the service, turbine, auxiliary, and control buildings, will be allowed to flood as the water exceeds their grade level entrances. All equipment, including power cables, that is located in these structures and required for operation in the flood mode is either above the DBF or designed for submerged operation, or otherwise protected. 2.4.14.2.2 Fuel Cooling Spent Fuel Pit Fuel in the spent fuel pit will be cooled by the normal Spent Fuel Pit Cooling (SFPC) System. The pumps are located on a platform at elevation 721.0 ft which is above the DBF surge level. The pumps are protected by a watertight enclosure equipped with redundant sump pumps to remove minor leakage. During the flood mode of operation, heat will be removed from the heat exchangers by ERCW instead of component cooling water. As a backup to spent fuel cooling, water from the Fire Protection (FP) System can be dumped into the spent fuel pool, and steam removed by the area ventilation system. Reactors Residual core heat will be removed from the fuel in the reactors by natural circulation in the Reactor Coolant (RC) system. Heat removal from the steam generators will be accomplished by adding river water from the FP System (subsection 9.5.1) and relieving steam to the atmosphere through the power relief valves. Primary system pressure will be maintained at less than 500 lb/in2g by operation of the pressurizer relief valves and heaters. This low pressure will lessen leakage from the system. Secondary side pressure will be maintained at or below 90 psig by operation of the steam line relief valves. An analysis has been performed to ensure that the limiting atmospheric relief capacity would be sufficient to remove steam generated by decay heat. At times beyond approximately 10 hours following shutdown of the plant two relief valves have sufficient capacity to remove the steam generated by decay heat. Since a minimum of 27 hours flood warning is available it is concluded that the plant could be safely shutdown and decay heat removed by operation of only two relief valves. Reference 28. The main steam power operated relief valves will be adjusted to maintain the steam pressure at or below 90 psig. If this control system malfunctions, then the controls in the main control room can be utilized to operate the valves in an open-closed manner. Also, a manual loading station and the relief valve handwheel provide additional backup control for each relief valve. The secondary side steam pressure can be maintained for an indefinite time by the means outlined above. The cooling water flow paths conform to the single failure criteria as defined in FSAR Section 3.1.1. In particular, all active components of the secondary side feedwater supply and ERCW supply are redundant and can therefore tolerate a single failure in the short or long term. A passive failure, consistent with the 50 gpm loss rate specified in FSAR Section 3.1.1, can be tolerated for an indefinite period without interrupting the required performance in either supply. S2-4.doc 2.4-39 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 If one or both reactors are open to the containment atmosphere as during the refueling operations, then the decay heat of any fuel in the open unit(s) and spent fuel pit will be removed in the following manner. The refueling cavity will be filled with borated water (approximately 2000 ppm boron concentration) from the refueling water storage tank. The SFPC System pump will take suction from the spent fuel pit and will discharge to the SFPC System heat exchangers. The SFPC System heat exchanger output flow will be directed by a piping connection to the Residual Heat Removal (RHR) System heat exchanger bypass line. The tie-in locations in the SFPC System and the RHR System are shown in Figures 9.1.3-1 and 5.5.7-1, respectively. This connection will be made using prefabricated, in- position piping which is normally disconnected. During flood mode preparations, the piping will be connected using prefabricated spool pieces. Prior to flooding, valve number 78-513 (refer to Figure 9.1.3-1) and valves FCV 74-33, and 74-35 (refer to Figure 5.5.7-1) will be closed; valves HCV 74-36, 74-37, FCV 74-16, 74-28, 63-93, and 63-94 (refer to Figure 5.5.7-1 and 6.3.1-1) will be opened or verified open. This arrangement will permit flow through the RHR heat exchangers and the four normal cold leg injection paths to the reactor vessel. The water will then flow downward through the annulus, upward through the core (thus cooling the fuel), then exit the vessel directly into the refueling cavity. This results in a water level differential between the spent fuel pit and the refueling cavity with sufficient water head to assure the required return flow through the 20-inch diameter fuel transfer tube thereby completing the path to the spent fuel pit. Except for a portion of the RHR System piping, the only RHR System components utilized below flood elevation are the RHR System heat exchangers. Inundation of these passive components will not degrade their performance for flood mode operation. After alignment, all valves in this cooling circuit located below the maximum flood elevation will be disconnected from their power source to assure that they remain in a safe position. The modified cooling circuit for open reactor cooling will be assured of two operable SFPC System pumps (a third pump is available as a backup) as well as two SFPC System heat exchangers. Also, the large RHR System heat exchangers are supplied with essential raw cooling water during the open reactor mode of fuel cooling; these heat exchangers provide an additional heat sink not available for normal spent fuel cooling. Fuel coolant temperature calculations, assuming conservative heat loads and the most limiting, single active failure in the SFPC System, indicate that the coolant temperatures are acceptable. The temperatures can be maintained at a value appreciably less than the fuel pit temperature calculated for the nonflood spent fuel cooling case when assuming the loss of one equipment train. As further assurance, the open reactor cooling circuit was aligned and tested, during pre-operational testing, to confirm flow adequacy. Normal operation of the RHR System and SFPC System heat exchangers will confirm the heat removal capabilities of the heat exchangers. High spent fuel pit temperature will cause an annunciation in the MCR, thus indicating equipment malfunction. Additionally, that portion of the cooling system above flood water will be frequently inspected to confirm continued proper operation. For either mode of reactor cooling, leakage from the Reactor Coolant System will be collected, to the extent possible, in the reactor coolant drain tank; nonrecoverable leakage will be made up from supplies of clean water stored in the four cold leg accumulators, the pressurizer relief tank, the cask decontamination tank, and the demineralized water tank. If these sources prove insufficient, the FP System can be connected to the Auxiliary Charging System (subsection 9.3.5) as a backup. Whatever the source, makeup water will be filtered, demineralized, tested, and borated, as necessary, to the normal refueling concentration, and pumped by the Auxiliary Charging System into the reactor (see Figures 2.4.14-1 and 2.4.14-2). 2.4.14.2.3 Cooling of Plant Loads Plant cooling requirements, with the exception of the FP System which must supply feedwater to the steam generators, will be met by the ERCW System (refer to Section 9.2.2). S2-4.doc 2.4-40 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.14.2.4 Power Electric power will be supplied by the onsite diesel generators starting at the beginning of Stage II or when offsite power is lost, whichever occurs first (Subsection 2.4.14.5.3). 2.4.14.2.5 Plant Water Supply The plant water supply is thoroughly discussed in Section 9.2.2. The following is a summary description of the water supply provided for use during flooded plant conditions. The ERCW station is designed to remain fully functional for all floods up to and including the DBF. The CCW intake forebay will provide a water supply for the fire/flood mode pumps. If the flood approaches DBF proportions, there is a possibility that Chickamauga Dam will fail. Such an event would leave the SQN CCW intake forebay isolated from the river as flood water recedes below EL 665 ft. Should this event occur, the CCW forebay has the capacity of retained water to supply two steam generators in each unit and provide spent fuel pit with evaporation makeup flow until CCW forebay inventory makeup is established. The ERCW station is designed to be operable for all plant conditions and includes provisions for makeup to the forebay. See Reference 28. 2.4.14.3 Warning Scheme See Subsection 2.4.14.8 (Warning Plan). 2.4.14.4 Preparation for Flood Mode An abnormal operating procedure is available to support operation of the plant. At the time the initial flood warning is issued, the plant may be operating in any normal mode. This means that either or both units may be at power or either unit may be in any stage of refueling. 2.4.14.4.1 Reactors Initially Operating at Power If both reactors are operating at power, Stage I and then, if necessary, Stage II procedures will be initiated. Stage I procedures will consist of a controlled reactor shutdown and other easily revocable steps such as moving supplies necessary to the flood protection plan above the DBF level and making temporary connections and load adjustments on the onsite power supply. Stage II procedures will be the less easily revocable and more damaging steps necessary to have the plant in the flood mode when the flood exceeds plant grade. The fire/flood mode pumps may supply auxiliary feedwater for reactor cooling [29]. Other essential plant cooling loads will be transferred from the component cooling water to the ERCW System (subsection 9.2.2). Radioactive Waste System (Chapter 11) and CVCS tanks, which are susceptible to flotation will be secured by either filling the tanks during flood preparations or by opening the tanks to allow floodwaters to enter, tanks which are adequately anchored to prevent floatation are exempt from these requirements. Some power and communication lines running beneath the DBF and not designed for submerged operation will require disconnection. Batteries beneath the DBF will be disconnected. 2.4.14.4.2 Reactor Initially Refueling If time permits, fuel is removed from the unit(s) undergoing refueling and placed in the spent fuel pit; otherwise fuel cooling will be accomplished as described in Subsection 2.4.14.2.2. If the refueling canal is not already flooded, the mode of cooling described in Subsection 2.4.14.2.2 requires that the canal be flooded with borated water from the refueling water storage tank. If the flood warning occurs after the reactor vessel head has been removed or at a time when it could be removed before the flood exceeds plant grade, the flood mode reactor cooling water will flow directly from the vessel into the refueling cavity. If the warning time available does not permit this, then the upper head injection piping will be disconnected above the vessel head to allow the discharge of water through the four upper head injection standpipes. Additionally, it is required that the prefabricated piping be installed to connect the RHR and SFPC Systems, and that ERCW be directed to the secondary side of the RHR System and SFPC System heat exchangers. S2-4.doc 2.4-41 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.14.4.3 Plant Preparation Time The steps needed to prepare the plant for flood mode operation can be accomplished within 24 hours of receipt of the initial warning that a flood above plant grade is possible. An additional 3 hours are available for contingency margin before wave runup from the rising flood might enter the buildings. Site grading and building design prevent any flooding before the end of the 27 hour preflood period. 2.4.14.5 Equipment Both normal plant components and specialized flood-oriented supplements will be utilized in coping with floods. All such equipment required in the flood mode is either located above the DBF or is within a non-flooded structure or is designed for submerged operation or otherwise protected. Systems and components needed only in the preflood period are protected only during that period. 2.4.14.5.1 Equipment Qualification To ensure capable performance in this highly unlikely but rigorous, limiting design case, only high quality components will be utilized. Active components are redundant or their functions diversely supplied. Since no rapidly changing events are associated with the flood, repairability offers reinforcement for both active and passive components during the long period of flood mode operation. Equipment potentially requiring maintenance will be accessible throughout its use, including components in the Diesel Generator Building. 2.4.14.5.2 Temporary Modification and Setup Normal plant components used in flood mode operation and in preparation for flood mode operation may require modification from their normal plant operating configuration. Such modification, since it is for a limiting design condition and since extensive economic damage is acceptable, will be permitted to damage existing facilities for their normal plant functions. However, most alterations will be only temporary and nondestructive in nature. For example, the switchover of plant cooling loads from the component cooling water to the ERCW System will be done through valves and a prefabricated spool piece, causing little system disturbance or damage. Equipment especially provided for the flood design case includes both permanently installed components and more portable apparatus that will be emplaced and connected into other systems during the preflood period. Detailed procedures to be used under flood mode operation have been developed and are incorporated in the plant's Abnormal Operating Procedures. 2.4.14.5.3 Electric Power Because there is a possibility that high winds may destroy powerlines and disconnect the plant from offsite power at any time during the preflood transition period, only onsite power will be used once Stage II of the preparation period begins. While most equipment requiring alternating current electric power is a part of the permanent emergency onsite power system, other components will be temporarily connected, when the time comes, by prefabricated jumper cables. All loads that are normally supplied by onsite power but are not required for the flood will be switched out of the system during the preflood period. Those loads used during the preflood period but not during flood mode operation will be disconnected when they are no longer needed. During the preparation period, all power cables running beneath the DBF level, except those especially designed for submerged operation, will be disconnected from the onsite power system. Similarly, direct current electric power will be disconnected from unused loads and potentially flooded lines. Charging will be maintained for each battery by the onsite alternating current power system as long as it is required. Batteries that are beneath the DBF will be disconnected during the preflood period when they are no longer needed. S2-4.doc 2.4-42 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.14.5.4 Instrument Control, Communication and Ventilation Systems All instrument, control, and communication lines that will be required for operation in the flood mode are either above the DBF or within a non-flooded structure or are designed for submerged operation or otherwise protected. Unneeded cables that run below the DBF will be disconnected to prevent short circuits. Redundant means of communications are provided between the central control area (the main and auxiliary control rooms) and all other vital areas that might require operator attention, such as the Diesel Generator Building. Instrumentation is provided to monitor all vital plant parameters such as the reactor coolant temperature and pressure and steam generator pressure and level. Control of the pressurizer heaters and relief valves and steam generator feedwater flow and atmospheric relief valves will ensure continued natural circulation core cooling during the flood mode. All other important plant functions will be either monitored and controlled from the main control area or, in some cases where time margins permit, from other points in the plant that are in close communication with the main control area. Ventilation, when necessary, and limited heating or air-conditioning will be maintained for all points throughout the plant where operators might be required to go or where required by equipment heat loads. 2.4.14.6 Supplies All equipment and most supplies required for the flood are on hand in the plant at all times. Some supplies will require replenishment before the end of the period in which the plant is in the flood mode. In such cases supplies on hand will be sufficient to last through the short time (Subsection 2.4.14.1.3) that flood waters will be above plant grade and until replenishment can be supplied. For instance, there is sufficient diesel generator fuel available at the plant to last for 3 or 4 weeks; this will allow sufficient margin for the flood to recede and for transportation routes to be reestablished. 2.4.14.7 Plant Recovery The plant is designed to continue safely in the flood mode for 100 days even though the water is not expected to remain above plant grade for more than 1 to 7 days. After recession of the flood, damage will be assessed and detailed recovery plans developed. Arrangements will then be made for reestablishment of offsite power and removal of spent fuel. The 100-day period provides more than adequate time for the development of procedures for any maintenance, inspection, or installation of replacements for the recovery of the plant or for a continuation of flood mode operations in excess of 100 days. A decision based on economics will be made on whether or not to regain the plant for power production. In either case, detailed plans will be formulated after the flood, when damage can be accurately assessed. 2.4.14.8 Warning Plan Plant grade elevation 705.0 ft can be exceeded by both rainfall floods and seismic-caused dam failure floods. A warning plan is needed to assure plant safety from these floods. The warning plan is divided into two stages: Stage I, a minimum of 10 hours long and Stage II, a minimum of 17 hours so that unnecessary economic consequences can be avoided, while adequate time is allowed for preparing for operation in the flood mode. Stage I allows preparation steps causing minimal economic consequences to be sustained but will postpone major economic damage until the Stage II warning forecasts a likely forthcoming flood above elevation 705.0 ft. 2.4.14.8.1 Rainfall Floods Protection of the SQN from the low probability rainfall floods that might exceed plant grade depends on a flood warning issued by TVA's River Management. With TVA's extensive climate monitoring and flood forecasting systems and flood control facilities, floods in the SQN area can be reliably predicted well in advance. The SQN flood warning plan will provide a minimum preparation time of 27 hours to S2-4.doc 2.4-43 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 prepare for operation in the flood mode. Four additional, preceding hours will provide time to gather data and produce the warning. To be certain of 27 hours for pre-flood preparation, flood warnings with the prospect of reaching elevation 705.0 ft must be issued early when lower levels of rainfall on the ground are forecast to reach plant grade in 27 hours. Consequently, some of the warnings may later prove to have been unnecessary. For this reason pre-flood preparations are divided into two stages. Stage I steps requiring 10 hours are easily revocable and cause minimum economic consequences. The estimated probability is small that a Stage I warning will be issued during the life of the plant. Added rain on the ground, stream-flow data and other available storm related information obtained during Stage I activity will determine if the more serious steps of Stage II need to be taken with the assurance that at least 17 hours will be available before elevation 705.0 ft is reached. The probability of a Stage II warning during the life of the plant is very small. The plant preparation status will be held at Stage I until either Stage II begins or TVA River Management determines that flood waters will not exceed elevation 705.0 ft at the plant. The Stage II warning will be issued only when enough rain has fallen to predict that elevation 705.0 ft is likely to be exceeded. 2.4.14.8.2 Seismically-Induced Dam Failure Floods One postulated combination of seismically induced dam failures and coincident storm conditions was shown to result in a flood which could exceed elevation 705.0 ft at the plant. SQNs notification of these floods utilizes TVA River Management forecast system to identify when a critical combination exists. Stage I shutdown is initiated upon notification that a critical dam failure has occurred or loss of communication prevents determining if a potential flood exceeding plant grade is expected to occur. Stage I shutdown continues until it has been determined positively that flooding exceeding plant grade will not occur. If communications do not document this certainty, shutdown procedures continue into Stage II activity. Stage II shutdown continues to completion or until it has been determined positively that flooding exceeding plant grade will not occur. 2.4.14.9 Basis For Flood Protection Plan In Rainfall Floods 2.4.14.9.1 Overview Large Tennessee River floods can exceed plant grade elevation 705.0 ft at SQN. Plant safety in such an event requires shutdown procedures, which can be implemented within 27 hours. TVA flood forecast procedures will provide at least 27 hours of warning before river levels reach elevation 705.0 ft. Stage II activities required to be completed prior to flood waters reach 705.0 ft are not impacted by wind wave effects. Due to the long shallow approach and the waves breaking at the perimeter road (elevation 705.0 ft), there is not sufficient depth or distance between the perimeter road and the safety-related facilities for new waves to be generated. Forecast will be based upon rainfall already reported to be on the ground and supplemented by available stream flow data and other storm related information which can aid in predicting downstream flooding elevations. To be certain of 27 hours for preflood preparation, warnings of floods with the prospect of reaching elevation 705.0 ft must be issued early; consequently, some of the warnings may later prove to have been unnecessary. For this reason preflood preparations are divided into two stages. Stage I steps, requiring 10 hours, would be easily revocable and cause minimum economic damage. Additional rain and streamflow information obtained during Stage I activity will determine if the more damaging steps of Stage II need to be taken with the assurance that at least 17 hours will be available before elevation 705.0 ft is reached. Flood forecasting and warnings, to assure adequate warning time for safe plant shutdown during floods, will be conducted by TVA River Management. S2-4.doc 2.4-44 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 2.4.14.9.2 TVA Forecast System TVA has in constant use an extensive, effective system to forecast inflow and control elevation as needed in the Tennessee River Basin. This permits efficient operation of the reservoir system and provides warning of when water levels will exceed critical elevations at selected, sensitive locations, which includes SQN. The TVA River Forecast Center (RFC) normal operation produces daily forecasts twice daily using the most recent data observations. During major flood events, the RFC may issue forecasts as frequently as every 6 hours. Elements of the forecast system include the following: 1 More than 200 rain gages measure rainfall, with an average density of about 200 sq mi per rain gage. The gages are Geostationary Operational Environmental Satellites (GOES) Data Collection Platform (DCP) satellite telemetered gages. The gages are relayed to TVA directly and also to the National Weather Service (NWS), which combines this gage data with radar data to return TVA a gridded rainfall observation dataset every hour. TVA backs up this feed with an in-house gage interpolation gridding. The rainfall gages transmit 15-minute rainfall accumulations during normal operations.

2. Stage data from ~100 gages are collected and converted to flow using rating curves. The gages are GOES Data Collection Platform satellite telemetered gages. The satellite gages transmit 15-minute stage data during normal operations.

3. Real-time headwater elevation, tailwater elevation, and discharge data are received from 31 TVA hydro projects and hourly data are received from minor non-power projects and non-TVA hydro plants, such as those owned by Brookfield Smoky Mountain Hydro and from the USACE for areas upstream of Barkley Dam.

4 Gridded weather forecasts, including a quantitative precipitation forecast (QPF), are received up to four times twice daily and at other times when changes are expected. QPF forecasts are sourced from the NWS Weather Prediction Center (WPC) for the most likely, 95% maximum and 95% minimum forecasts. QPFs are also sourced from the European Centre for Medium-Range Weather Forecasts (ECMWF), North American Ensemble Forecast System (NAEFS), and the High-Resolution Rapid Refresh (HRRR) System. Gridded temperature, dewpoint, pressure, wind, cloud cover, relative humidity, and shortwave radiation forecasts are also received from the NWS.

5. Computer programs are used two to four times daily to translate rainfall into streamflow based on current runoff conditions and which permit a forecast of flows and elevations based upon both observed and predicted rainfall. The hydrologic model has 140 sub-basins, each of which can be hand-adjusted at each forecast period to assimilate any discrepancies between the model and reality. A network of severs and computers are utilized and are designed to provide backup for each other. One computer is used primarily for data collection, with the other used for executing forecasting programs for reservoir operations. The time interval between receiving input data and producing a forecast is less than 4 hours. Forecasts normally cover at least a three-day period.

6. At each forecast period, a series of RiverWare reservoir routing models are executed in order to meet the operating policy and objectives described in the TVA Reservoir Operations Policy. These models are well-parameterized and easy for experienced operators to use for making fast tradeoff decisions. The output from the RiverWare routing model runs is used as input into a HEC-RAS model that is used to predict river level elevations at locations along the Tennessee River.

As effective as the forecast system already is, it is constantly being improved as new technology provides better methods to interrogate the watershed during floods and as the watershed mathematical model and computer system are improved. TVA has transitioned to a system that is S2-4.doc 2.4-45 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 built of components, which are widely used throughout the industry. This makes them easier to maintain and allows for easy enhancements as the industry advances. The TVA forecast center is manned 24 hours a day, 7 days per week, 365 days per year. 2.4.14.9.3 Basic Analysis The forecast procedure to assure safe shutdown of SQN for flooding is based upon an analysis of hypothetical storms up to PMP magnitude. The storms enveloped potentially critical areal and seasonal variations and time distributions of rainfall. To be certain that fastest rising flood conditions were included, the effects of varied time distribution of rainfall were tested by consideration of the maximum daily PMP in the first, middle, and the last day of the three-day main storm as well as various PMP nesting combinations using the methodology described in Subsection 2.4.3.1. Multiple project PMF, warning time specific and seismic simulations potentially controlling for warning time were reviewed and formally documented in calculations. The PMP nesting combinations producing the maximum flood elevation at the SQN site do not produce the quickest warning time PMP nesting combination because the maximum flood elevation PMPs include dam failures upstream of SQN occurring after the flood elevation reached plant grade of 705.0 ft. The warning system is based on those storm situations which resulted in the shortest time interval between watershed rainfall selected to initiate a River Operations Stage I warning and elevation 705.0 ft at SQN, thus assuring that this elevation could be predicted at least 27 hours in advance. The procedures used to compute flood flows and elevations for those flood conditions which establish controlling elements of the forecast system are described in Section 2.4.3. 2.4.14.9.4 Hydrologic Basis for Warning System A minimum of 27 hours has been allowed for preparation of the plant for operation in the flood mode. An additional 4 hours for communication and forecasting computations are provided to allow TVA River Forecasting Center to translate rain on the ground to river elevations at the plant. Hence the warning plan must provide 31 hours from arrival of rain on the ground until elevation 705.0 ft could be reached. The 27 hours allowed for shutdown at the plant are utilized for a minimum of 10 hours of Stage I preparation and an additional 17 hours for Stage II preparation that is not concurrent with River Management Stage I activity. When rain on the ground on the watershed above Chickamauga Dam reaches 3.0 inches in 72 hours or less, the River Operations Forecast Center begins predicting flooding levels at the SQN site. Communication is maintained between the Forecast Center and Plant Operations. A Stage I warning, which starts shutdown procedures, will be initiated as soon as 5.6 inches of rainfall on the ground in 72 hours or less is reached and elevation 705.0 ft is predicted at the SQN site. A Stage I warning is also initiated when the failure of a critical dam has occurred. Stage II shutdown will be initiated and carried to completion if rain on the ground reaches 7.6 inches in 72 hours or less and when target river elevation 705.0 ft at SQN has been forecast in 17 hours or less. Regardless of rain on the ground, a Stage II shutdown warning is given when plant grade is projected to be reached in 17 hours or less. Inasmuch as the hydrologic procedures and target river elevations have been designed to provide adequate shutdown time in the fastest rising flood, longer times will be available in other floods. In such cases there may be a waiting period after the Stage I 10-hour shutdown activity during which activities shall be in abeyance until weather conditions determine if normal plant operation can be resumed, or if Stage II shutdown should be implemented. Resumption of plant operation following Stage I shutdown activities will be allowable only after flood levels and weather conditions, as determined by TVA River Management, have returned to a condition in which 27 hours of warning will again be available. 2.4.14.9.5 Hydrologic Basis for Warning Times and Triggers The analysis of multiple PMF simulations performed in Section 2.4.3 show that the simulations with the highest water surface elevation at SQN result from postulated upstream and cascading dam failures S2-4.doc 2.4-46 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 starting on the [CEII] Since PMF simulations with the highest water surface elevation may not be the fastest rising PMF simulation to reach plant grade, additional simulations were required to determine alternate storms that reach plant grade in the least amount of time. Preliminary analysis of PMP storms with potentially the shortest warning time indicated sensitivity to spatial, temporal, and seasonal occurrences. This sensitivity required iterative analysis of various temporal distributions (front, medial and back loaded rainfall), variable spatial distribution nesting sequences and multiple week/season considerations. The analysis for SQN shows that two PMF events produced the rainfall lower boundary decision limits. (1) The Ocoee #1 below Blue Ridge primary nesting with the 72-hour back-loaded temporal distribution during week 21 produced the rainfall lower boundary decision limits of 4.36 hours for modeling and communications time as well as 32.55 hours for total warning time. (2) The Chatuge/Nottely primary nesting with a 48-hour front-loaded temporal distribution during week 19 produced the rainfall lower boundary decision limit of 27.35 hours for the Stage I call to plant shutdown period and 17.36 hours for the Stage II call to plant shutdown period. These values are greater than or equal to the specified minimum design basis warning times. The seismic event resulting in consequential flooding at SQN has a calculated warning time that includes a maximum 1.5-hours for review and communications [64], and 27-hours of flood mode preparation time. The results of the analysis of these simulations were reviewed against the required minimum warning time durations of 4 hours for modeling and communication, 10 hours for Stage I activities and 17 hours for Stage II activities. Based on this review, the minimum rain of the ground values used by the River Forecast Center to (1) begin analysis of the potential flooding impact at the SQN site, (2) initiate the Stage I warning trigger review, and (3) initiate the Stage II warning trigger review were selected. The rain on the ground values represent the initial rainfall expected in the initial stages of the 72 hour main storm with the rainfall in the balance of the main storm still to come. A rain on the ground value of 3.0 inches to initiate Forecasting Center analysis provided a minimum of 4 hours for analysis and communication as well as at least 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. A rain on the ground value of 5.6 inches, gave a minimum of 27 hours for Stage I and Stage II activities and more than 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. A rain on the ground value of 7.6 inches provided at least 17 hours for Stage II activities and more than 31 total hours to reach plant grade when considering the remaining rainfall in the main storm. To ensure that flood level forecast at the SQN site are not subject to sudden changes from dam failures, a Stage I warning is initiated for the failure of any critical dam upstream of SQN. Using the rain on the ground trigger values combined with River Forecast Center projections of flooding at the SQN site, the minimum durations required for flood mode operation activities are provided. Figure 2.4.14-3 (Sheet 1 and 2) show the controlling simulations for PMP rainfall distribution, the target forecast flood warning time and the rain on the ground thresholds above Chickamauga Dam which assure adequate warning times for SQN. The fastest rising PMF simulation with the limiting initial notification time (>4 hours) and the limiting overall warning time (>31 hours) is shown in Figure 2.4.14-3 (Sheet 1). Figure 2.4.14-3 (Sheet 2) shows the rainfall distribution for the fastest rising PMP storm controlling the limiting Stage I rain on the ground notification (>27 hours) and the limiting Stage II warning time (>17 hours). The PMP storms have been preceded three days earlier by a three-day storm having 40 percent of PMP storm rainfall applied with the spatial distribution coincident with that of the main storm PMP. The above criteria all relate to rain on the ground. The rain on the ground criteria are selected to be small enough such that protective actions are taken in time to provide the required durations for flood protection activities should the remaining rainfall in the PMP main storm defined by Topical Report TVA-NPG-AWA16-A follow. Quantitative rain forecasts, which are a part of daily operations, would be S2-4.doc 2.4-47 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

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2V?gltt?lV?lp?SPjPZT?vl?Pk? yntvqTPj?SPj?`t?tytnTRuTS?QPtTS?lk?jlk_ulr_kZ?Q}?DJ"??_zTp? A SlR?  ?

        @($I>2FL?>(5!G(&?28.:>6!H2:8? K2H01(5&?I8&(>? ?$.>? ?

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 Management, efforts will be made by TVA to determine whether dam failure has occurred. If the critical case has occurred or it cannot be determined that it has not occurred, Stage I shutdown will be initiated. Once initiated, the flood preparation procedures will be carried to completion, unless it is determined that the critical case has not occurred. Communication between projects in the TVA power system is discussed in Subsection 2.4.14.9.6. 2.4.14.11 Special Condition Allowance The flood protection plan is based upon the minimum time available for the worst case. This worst case provides adequate preparation time including contingency margin for normal and anticipated plant conditions including anticipated maintenance operations. It is conceivable, however, that a plant condition might develop for which maintenance operations would make a longer warning time desirable. In such a situation the Plant Manager determines the desirable warning time and contacts TVA River Management to determine if the desired warning time is available. If weather and reservoir conditions are such that the desired time can be provided, special warning procedures will be developed, if necessary, to ensure the time is available. This special case continues until the Plant Manager (or designee) notifies TVA River Management that maintenance has been completed. If threatening storm conditions are forecast which might shorten the available time for special maintenance, the Plant Manager is notified by River Management and steps taken to assure that the plant is placed in a safe shutdown mode. 2.4.15 References

1. Not used.

2. Not used.

3. Tennessee Valley Authority, Submittal of Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, Revision 1, (EPID L-2016-TOP-0011), May 21, 2019.

4. Not used.

5. Not used.

6. Not used.

7. Not used.

8. Not used.

9. Reference removed per Amendment 6.

10. Not used.

11. Not used.

12. Not used.

13. Not used.

14. Not used.

15. Reference removed per Amendment 6.

16. Tennessee Valley Authority, Hydrology Project White Paper, Areal Application of PMP Event Data for the Tennessee River Model above Wheeler Dam (B41170420001).

17. Not used.

S2-4.doc 2.4-49 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

18. Not used.

19. Programmatic Environmental Impact Statement, TVA Reservoir Operations Study, Record of Decision, May 2004.

20. Tennessee Valley Authority, RvM-SOP-10.05.06, Nuclear Notifications and Flood Warning Procedure, Revision 0005, dated 11-15-2021.

21. Monitoring and Moderating Sequoyah Ultimate Heat Sink, June 2004, River System Operations and Environment, River Operations, River Scheduling (B85 070509 001).

22. Tennessee Valley Authority, SQN Calculation MDQ0026970001A, High Pressure Fire Protection Supply to the Steam Generators for Flood Mode Operation.

23. Newton, Donald W., and Vineyard, J. W., "Computer-Determined Unit Hydrographs From Floods," Journal of the Hydraulics Division, ASCE, Volume 93, No. HY5, September 1967.

24. U.S. Army Corps of Engineers, Hydrologic Engineering Center, River Analysis System, HEC-RAS computer software, version 3.1.3.

25. Federal Emergency Management Agency (FEMA), "Federal Guidelines for Dam Safety:

Earthquake Analysis and Design of Dams, FEMA 65, May 2005.

26. Price, J. T. and Garrison, J. M., Flood Waves From Hydrologic and Seismic Dam Failures,"

paper presented at the 1973 ASCE National Water Resources Engineering Meeting, Washington, D. C.

27. Tennessee Valley Authority, SQN-DC-V-1.1, Design of Reinforced Concrete Structures Design Criteria.

28. Tennessee Valley Authority, SQN-DC-V-12.1, Flood Protection Provisions Design Criteria.

29. Tennessee Valley Authority, SQN-DC-V-43.0, High Pressure Fire Protection Water Supply System.

30. Tennessee Valley Authority, Calculation CDQ0000002014000018, HEC-RAS Tributary Model Calibration, Revision 2.

31. Tennessee Valley Authority, Calculation CDQ0000002014000016, Tributary Dam Rating Curves, Revision 3 and Calculation CDQ0000002013000007, Main Stem Initial Dam Rating Curves, Revision 4.

32. Tennessee Valley Authority, Calculation CDQ0000002014000019, HEC-RAS Tributary Model Unsteady Flow Rules, Revision 3.

33. Tennessee Valley Authority, Calculation CDQ000020080053, PMF Inflow Determination, Revision 1.

34. Tennessee Valley Authority, Calculation CDQ000020080050, Flood Operational Guide, Revision 3.

35. Tennessee Valley Authority, Calculation CDQ0000002014000017, HEC-RAS Tributary Geometry Development, Revision 1.

36. Tennessee Valley Authority, Calculation CDQ000020080024, SOCH Geometry Verification - Ft.

Loudoun Reservoir, French Broad River, and Holston River, Revision 2.

37. Tennessee Valley Authority, Calculation CDQ000020080025, SOCH Geometry Verification -

Tellico Reservoir and Tellico/Ft. Loudoun Canal, Revision 2. S2-4.doc 2.4-50 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

38. Tennessee Valley Authority, Calculation CDQ000020080026, SOCH Geometry Verification -

Watts Bar Reservoir, Revision 2.

39. Tennessee Valley Authority, Calculation CDQ000020080029, SOCH Geometry Verification -

Melton Hill Reservoir, Revision 2.

40. Tennessee Valley Authority, Calculation CDQ000020080030. SOCH Geometry Verification -

Chickamauga Reservoir, Revision 2.

41. Tennessee Valley Authority, Calculation CDQ000020080031, SOCH Geometry Verification -

Nickajack Reservoir, North Chickamauga Creek, Lick Branch (Dallas Bay), Revision 2.

42. Tennessee Valley Authority, Calculation CDQ000020080032, SOCH Geometry Verification -

Guntersville Reservoir, Revision 3.

43. Tennessee Valley Authority, Calculation CDQ000020080033, SOCH Geometry Verification -

Wheeler Reservoir, Revision 0.

44. Tennessee Valley Authority, Calculation CDQ000020080051, Reservoir Storage Tables, Revision 2.

45. Tennessee Valley Authority, Calculation CDQ0000002014000021, HEC-RAS Model Setup, Revision 5.

46. River Operations Procedure RO-SPP-27.1, "RO-Design and Evaluation of New and Existing Dams," Revision 2.

47. Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Subcommittee, Interagency Advisory Committee on Water Data, Office of Water Data Coordination, Geological Survey, U.S. Department of the Interior, Revised September 1981 with March 1982 Editorial Corrections.

48. Moore, James N. and Ray C. Riley, Comparison of Temporal Rainfall Distributions for Near Probable Maximum Precipitation Storm Events for Dam Design, National Water Management Center, Natural Resources Conservation Service, (NRCS), Little Rock, Arkansas.

49. Hovey, Peter and Thomas DeFiore, Using Modern Computing Tools to Fit the Pearson Type III Distribution to Aviation Loads Data, Report # DOT/FAA/AR-03/62, Office of Aviation Research, Federal Aviation Administration, U.S. Department of Transportation, Washington, D.C.,

September 2003.

50. Bonnin, Geoffrey M., Deborah Martin, Bingzhang Lin, Tye Parzybok, Michael Yekta, David Riley, NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 2 Version 3.0:

Delaware, District of Columbia, Illinois, Indiana, Kentucky, Maryland, New Jersey, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland, 2004, revised 2006.

51. Hershfield, David M., Technical Paper No. 40 Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years, Department of Commerce, Cooperative Studies Section, Hydrologic Services Division for Engineering Division, Soil Conservation Service, U.S. Department of Agriculture; Washington, D.C. May 1961, Repaginated and Reprinted January 1963.

52. NRC, NUREG-2115, Central and Eastern United States Seismic Source Characterization for Nuclear Facilities, January 2012.

53. EPRI, 2004/2006 Ground-Motion Models; CEUS Ground Motion Project Final Report 1009684, 2004; Program on Technology Innovation: Truncation of the lognormal distribution and value of S2-4.doc 2.4-51 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5 the standard deviation for ground motion models in the Central and Eastern United States, Final Report 1014381, 2006.  Tennessee Valley Authority, Calculation CDQ0000002013000057, Sequoyah Local Intense
Precipitation Evaluation-SAR Design Basis (Case 1) and Flood Hazard Re-evaluation Report
(Case 2), Revision 4.  Tennessee Valley Authority, Calculation CDQ0000002017000059, Chickamauga Dam (CHH)
Probable Maximum Flood Analysis, Revision 1.  Tennessee Valley Authority, Calculation CDQ0000002014000033, Wind Waves for Combined
Effect Floods, Revision 4.  Tennessee Valley Authority, Calculation CDQ0000002014000024, Seismic Dam Failure
Combined with Rainfall Event Simulations, Revision 6.  US Army Corps of Engineers Hydrologic Engineering Center, HEC-DSSVue HEC Data Storage
System Visual Utility Engine Users Manual, Version 2.0, July 2009.  Department of the Army, Office of the Chief of Engineers, Engineering Technical Letter No.
1110-2-8, August 1, 1966.  Tennessee Valley Authority, Calculation CDQ0000002016000043, Controlling Storm Type and
Week Analysis, Revision 1.  Tennessee Valley Authority, Calculation, CDQ0000002017000081, Loss of Chickamauga
Reservoir Drain Down Analysis, Revision 0.  United States Army Corps of Engineers (USACE), Coastal Engineering Manual-Part VI
Chapter 5, Fundamentals of Design, EM 1110-2-1100, updated 2015.  Von Thun, J Lawrence and David R. Gillette, Guidance on Breach Parameters, unpublished
internal document, U.S. Bureau of Reclamation, March 13, 1990.  Tennessee Valley Authority, Calculation, CDQ0000002017000076, Watts Bar (WBN) and
Sequoyah (SQN) Nuclear Plant Warning Time Analyses, Revision 1..  Froehlich, D. C., Peak Outflow from Breach Embankment Dam Journal of Water Resources
Planning and Management, Volume 121, Issue 1, January 1995 S2-4.doc 2.4-52 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

Enclosure 9 TVA Affidavit for Enclosures 1, 2, 3, 4 (2 pages) CNL-23-012

A FFID AV IT

1. My name is Kimberly D. Hulvey. I am Director, Nuclear Regulatory Affairs, for Tennessee Valley Authority (TVA), and as such I am authorized to execute this Affidavit. I am familiar with TVAs Revision to the Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, and have personal knowledge of the matters stated herein.

2. I am submitting this affidavit in accordance with 10 CFR §2.390(a)(3) and 10 CFR §9.17, and because certain information contained in this submittal has been determined by TVA to be of the type customarily held in strict confidence by the agency and is exempted from disclosure by statute under Tennessee Code Annotated 10-7-504(a)(21)(A)(i).

3. The information that should not be released to the public has been collected and organized into Enclosures 1, 2, 3, and 4 of this submittal.

4. Enclosures 1, 2, 3, and 4 of this submittal include information exempted from disclosure by statute per 10 CFR §2.390(a)(3).

a. This submittal contains information related to the location of municipal water intakes which have been provided to TVA by Tennessee state governmental agencies. These Tennessee governmental agencies have requested that TVA withhold this information from public disclosure based on their obligations under the Tennessee Code Annotated 10-7-504(a)(21)(A)(i). Tennessee Code Annotated 10-7-504(a)(21)(A)(i) requires the withholding of records that would allow a person to identify areas of structural or operational vulnerability of a utility service provider (including water storage or processing system, water distribution system, or any similar services) or that would permit unlawful disruption to, or CNL-23-012 E9-1 of 2

interference with, the services provided by a utility service provider. The location of municipal water intakes is encompassed within the information that must be withheld from public disclosure under the statute.

b. This submittal also contains information that may be non-public Critical Energy/Electric Infrastructure Information (CEII) as defined by the Federal Energy Regulatory Commission (FERC) in 18 CFR § 388.113. In accordance with NRC Yellow Announcement YA-18-0051 and the NRC guidance for Control of Sensitive Unclassified Non-Safeguards Information, information that may be CEII has been redacted and designated as such.

5. The information sought to be withheld (the information) is being submitted to the NRC in confidence.

6. The information is the sort of information regularly and customarily held in confidence by TVA based on the statutes requiring the withholding of such information; and is, in fact, so held.

7. The information has consistently been held in confidence by TVA, and no public disclosure of the information has been made by TVA.

8. All disclosures of the information to third parties by TVA, including any transmittals to the NRC, have been made pursuant to regulatory provisions or proprietary agreements which provide that the information is to be maintained in confidence.

9. The foregoing statements are true and correct to the best of my knowledge, information, and belief.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 11th day of April 2023. Digitally signed by Edmondson, Carla Date: 2023.04.11 09:41:15 -04'00' Kimberly D. Hulvey Director, Nuclear Regulatory Affairs CNL-23-012 E9-2 of 2}}