Text

Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (BFN-TS-550)
	ML25043A263
Person / Time
Site:	Browns Ferry
Issue date:	02/12/2025
From:	Hulvey K; Tennessee Valley Authority
To:	; Office of Nuclear Reactor Regulation, Document Control Desk
Shared Package
ML25043A262	List: CNL-24-006, Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (BFN-TS-550);
References
	CNL-24-006, BFN-TS-550
	Download: ML25043A263 (1)
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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 10 CFR 50.90 1101 Market Street, Chattanooga, Tennessee 37402 CNL-24-006 February 12, 2025 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Browns Ferry Nuclear Plant, Unit 1, 2 and 3 Renewed Facility Operating License Nos. DPR-33, DPR-52 and DPR-68 NRC Docket Nos. 50-259, 50-260 and 50-296

Subject:

Application to Revise Browns Ferry Nuclear Plant Units 1, 2, and 3 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (BFN-TS-550)

Reference:

1. NRC Letter to TVA, Verification Letter of the Approval Version of Tennessee Valley Authority Topical Report TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation, Analysis Calculation CDQ0000002016000041, Revision 1 (EPID L-2016-TOP- 0011), dated July 3, 2019 (ML19158A395)

2. TVA letter to NRC, CNL-23-012, Application to Revise Sequoyah Nuclear Plant (SQN) Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, dated April 11, 2023 (ML23101A179, ML23101A180)

In accordance with the provisions of Title 10 of the Code of Federal Regulations (10 CFR) 50.90, "Application for amendment of license, construction permit, or early site permit," Tennessee Valley Authority (TVA) is submitting a request for an amendment to Facility Operating License Nos. DPR-33, DPR-52, and DPR-68 for Browns Ferry Nuclear Plant (BFN), Units 1, 2, and 3, respectively. This license amendment request (LAR) revises the BFN Units 1, 2, and 3 Updated Final Safety Analysis Report (UFSAR) to reflect the results from a new hydrologic analysis. TVA has determined that the proposed changes to the BFN UFSAR require prior Nuclear Regulatory Commission (NRC) approval. There are no technical specifications changes associated with this request.

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-24-006 Page 2 February 12, 2025 On July 3, 2019, the NRC determined that Reference 1, Topical Report (TR) TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, is acceptable for referencing in licensing applications for nuclear power plants to the extent specified and under the limitations and conditions delineated in the accepted versions of the TR.

This LAR requests NRC approval of a change to the BFN Units 1, 2, and 3 UFSAR to adopt a revised hydrologic analysis for the BFN site that uses the Probable Maximum Precipitation (PMP) methodology contained in the approved TR.

In Reference 2, TVA submitted a similar hydrological analysis update LAR request for Sequoyah Nuclear Plant (SQN), Units 1 and 2. As noted in Enclosure 1, Attachment C of this LAR, the NRC requests for additional information (RAI) related to the SQN LAR (Reference 2) are relevant to the BFN LAR. Enclosure 1, Appendix C defines which TVA RAI responses are fully or partially applicable to BFN. For those RAI responses not applicable to BFN, RAI responses unique to BFN are provided.

Enclosures 1, 2, 3, and 4 contain security-related information that TVA is requesting be withheld from public disclosure in accordance with 10 CFR 2.390. Enclosure 5 contains the public version of the information provided in Enclosure 1 with the security-related information withheld from the public.

Enclosures 1 and 5 of this letter provide a description and evaluation of the proposed technical changes to BFN Units 1, 2, and 3 UFSAR Section 2.4, Hydrology, Water Quality, and Aquatic Biology, and Appendix 2.4A, Probable Maximum Flood (PMF). provides the current BFN Units 1, 2, and 3 UFSAR text marked up to show the proposed changes to UFSAR Section 2.4, Hydrology, Water Quality, and Aquatic Biology, and Appendix 2.4A, Probable Maximum Flood (PMF). Enclosure 3 provides the proposed Tables and Figures for BFN Units 1, 2, and 3 UFSAR Section 2.4, Appendix 2.4A. A description of the BFN UFSAR, Section 2.4, Appendix 2.4A Table and Figure changes is included in of this letter. Enclosure 4 provides the retyped BFN Units 1, 2, and 3 UFSAR incorporating the proposed changes. An affidavit supporting TVAs request for withholding is included as Enclosure 6.

TVA has determined that there are no significant hazard considerations associated with the proposed change 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 Alabama Department of Public Health.

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 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-24-006 Page 3 February 12, 2025 There are no new regulatory commitments contained in this submittal. TVA requests approval of the proposed license amendment one year from the date of this submittal, with the amendment being implemented within 90 days.

Please address any questions regarding this request to Amber Aboulfaida, Senior Manager, Fleet Licensing at avaboulfaida@tva.gov.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 12th day of February 2025.

Respectfully, Kimberly D. Hulvey General Manager, Nuclear Regulatory Affairs & Emergency Preparedness Enclosures

1. Evaluation of the Proposed Changes (Security Related)

2. Proposed BFN Units 1, 2 and 3 UFSAR Section 2.4, Appendix 2.4A (Mark-Ups)

(Security Related)

3. Proposed BFN Units 1, 2 and 3 UFSAR Section 2.4, Appendix 2.4A Tables and Figures (Final Typed) (Security Related)

4. Proposed BFN Units 1, 2 and 3 UFSAR Section 2.4, Appendix 2.4A (Final Typed) (For Information Only) (Security Related)

5. Evaluation of the Proposed Changes (Public)

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

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Brown Ferry Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant State Health Officer, Alabama Department of Public Health Digitally signed by Edmondson, Carla Date: 2025.02.12 06:38:39

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SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 1 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Evaluation of the Proposed Changes (Public)

Subject:

Application to Revise Browns Ferry Nuclear Plant Units 1, 2 and 3 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (BFN-TS-550)

CONTENTS 1.0

SUMMARY

DESCRIPTION........................................................................................ 2 1.1 Plant Site................................................................................................................ 2 1.2 Flood Design Considerations.................................................................................. 3 1.3 Probable Maximum Flood....................................................................................... 3 1.4 Water Levels at Plant Site....................................................................................... 3 1.5 Warning Plan.......................................................................................................... 4 2.0 DETAILED DESCRIPTION......................................................................................... 4 2.1 Background............................................................................................................. 4 2.2 Need for Proposed Change.................................................................................... 4 2.3 Proposed Changes................................................................................................. 5 2.4 Condition Intended to Resolve.............................................................................. 24

3.0 TECHNICAL EVALUATION

...................................................................................... 24 3.1 Evaluation............................................................................................................. 24 3.2 Uncertainties......................................................................................................... 41 3.3 Margins................................................................................................................. 43 3.4 Conclusions.......................................................................................................... 43

4.0 REGULATORY EVALUATION

................................................................................. 44 4.1 Applicable Regulatory Requirements and Criteria................................................. 44 4.2 Precedence........................................................................................................... 45 4.3 Significant Hazards Consideration........................................................................ 45 4.4 Conclusions.......................................................................................................... 47

5.0 ENVIRONMENTAL CONSIDERATION

.................................................................... 47

6.0 REFERENCES

......................................................................................................... 47 ATTACHMENTS A. Topical Report PMP Evaluation Tool and Application Using GIS B. Formulation of Candidate PMP Storms by Applying Nesting Methodology and Primary/

Secondary Areas of Interest (AOI)

C. Applicability of TVA Submittals Responding to NRC Questions Relative to the Sequoyah LAR TS-19-02

CNL-24-006 E5-Page 2 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 1.0

SUMMARY

DESCRIPTION Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.90, "Application for amendment of license, construction permit, or early site permit," Tennessee Valley Authority (TVA) is requesting a license amendment to the Browns Ferry Nuclear Plant (BFN,) Units 1, 2, and 3 Facility Operating Licenses (FOL). The proposed change will revise BFN Units 1, 2, and 3 Updated Final Safety Analysis Report (UFSAR) Section 2.4 (Hydrology, Water Quality, and Aquatic Biology), Appendix 2.4A, Probable Maximum Flood (PMF), related tables and figures to reflect the results from new hydrologic analysis. TVA determined that the proposed changes to the BFN UFSAR require prior Nuclear Regulatory Commission (NRC) approval. There are no technical specification changes associated with this request.

1.1 PLANT SITE The BFN site comprises approximately 880 acres on the north shore of the Wheeler Lake at Tennessee River Mile (TRM) 294 with plant grade elevation at 565.0 feet (ft.) mean sea level (MSL). The plant has been designed to have the capability for safe shutdown of all operating units to a Cold Shutdown condition in floods up to the computed maximum water level.

The Tennessee River at BFN drains a 27,130 square miles (sq. mi.) watershed area above the plant. Guntersville Dam, 55 miles upstream on the Tennessee River, has a drainage area of 24,450 sq. mi. Wheeler Dam, approximately 19 miles downstream on the Tennessee River, has a drainage area of 29,590 sq. mi. Major tributaries of the Tennessee River include the French Broad and Holston Rivers flowing into the Tennessee River above Knoxville, Tennessee (TN),

the Little Tennessee River flowing into the Tennessee River below Knoxville, TN and the Clinch River, entering the Tennessee River above Watts Bar Dam near Kingston, TN, the Hiwassee River flowing into the Tennessee River above Chattanooga, TN and the Elk River entering the Tennessee River downstream of BFN.

The climate of the watershed is humid and temperate. Above Wheeler Dam, annual rainfall averages 51 inches and varies from a low of 40 inches at sheltered locations within the mountains to high spots of 90 inches on the southern and eastern divide. Most floods at BFN have been produced by winter-type storms in the main flood-season months of March through June.

The Tennessee River is highly regulated, operated for flood control, navigation, and power generation. There are 20 major reservoirs (South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, Watts Bar, Blue Ridge, Apalachia, Hiwassee, Chatuge, Nottely, Chickamauga, Nickajack, and Guntersville) in the TVA system upstream of BFN. Twelve (12) of these dam reservoirs (Guntersville, Chickamauga, Watts Bar, Fort Loudoun, Fontana, Hiwassee, Norris, Tellico, Douglas, Cherokee, South Holston, and Watauga) provide substantial reserved flood-detention capacity during the main flood season. In addition, there are six major non-TVA dams that often contribute to flood reduction, but do not have dependable reserved flood detention capacity. The flood detention capacity reserved in the TVA system varies seasonally with the greatest amounts during the January through March flood season.

Daily flow volumes at the BFN site are generally represented by discharges from Wheeler Dam, an average daily stream flow of 51,000 cubic feet per second (cfs.) since January 2005.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 1.2 FLOOD DESIGN CONSIDERATIONS BFN structures that house safety-related facilities, systems, and equipment are protected from flooding during a local probable maximum precipitation (PMP) by the slope of the plant yard and the switchyard and west basin drainage channels, carrying precipitation runoff to the Cool Water Channels and the Wheeler Reservoir. The local intense precipitation (LIP) PMP for plant drainage is defined by Hydrometeorological Report (HMR) No. 56, using storm intensities for the maximum one-hour rainfall. Underground drains were assumed to be clogged, and runoff was assumed equal to rainfall. Computed maximum water surface elevations at safety-related structures are below critical floor elevation 565.0 ft.

The types of events evaluated to determine the worst potential river flooding at the BFN site include (1) PMP on the total watershed and critical sub-watersheds, including seasonal variations and potential consequential dam failures, and (2) dam failures in postulated seismic events with guide specified concurrent flood conditions.

The potential PMPs for determining the probable maximum flood (PMF) are defined by National Weather Service in HMRs No. 41 and No. 47. The PMP rainfall depth, duration, and spatial distribution were defined in the HMRs as three basic storms with four possible isohyetal patterns and seasonal variations. The critical PMP rainfall depth and location on the TVA watershed above Chattanooga is prescribed by the fixed 21,400-sq-mi-storm defined in HMR No. 41.

1.3 PROBABLE MAXIMUM FLOOD The controlling PMF storm event is the 21,400-square-mile storm producing an elevation of ft. at the BFN plant site. The design and licensing basis PMF elevation is maintained at ft.

((CEII))

The runoff model used to determine Tennessee River flood hydrographs at BFN was divided into 62-unit areas and included the total watershed above Wheeler Dam. Unit hydrographs were used to compute flows from the unit areas. The unit area flows were combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream reservoirs, which in turn were routed through the reservoirs using standard techniques. Resulting outflows were combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures including unsteady flow routing.

The main river dams upstream from Wheeler include earth embankments which could fail if overtopped. Maximum flood level determinations at BFN are based on the postulated failure of any earth embankment which is overtopped with time of failure based on results of the breach analysis of each embankment. The west saddle dam at Watts Bar, and earthen embankments at Nickajack, Guntersville, and Chickamauga dams were overtopped and are postulated to fail in the hydraulic model analysis.

Seismically induced dam failure flooding scenarios are not in the BFN current licensing basis.

1.4 WATER LEVELS AT PLANT SITE The above analyses determined that BFN plant grade elevation could be exceeded by large rainfall floods.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 3 of 49

((CEII))

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 1.5 WARNING PLAN As outlined in plant operating procedures, preparations to cope with a flood at grade level are initiated when Wheeler Reservoir level is at or above elevation 558.0 ft. and is expected to rise above plant grade elevation 565.0 ft. Entry into plant procedures for this flooding event includes the expectation to have all three BFN Units in Cold Shutdown within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months and to verify closed flood protection features such as watertight doors, floodgates, hatches and floor drain plugs. As delineated in BFN FSAR Figure 16, Section 2.4, Appendix 2.4A (the controlling flood hydrograph at the site), the timeframe from when the flood waters reach elevation 558.0 ft. to when plant grade elevation 565.0 ft. is reached, is approximately 5.5 days, providing sufficient time for plant flood mode preparations.

2.0 DETAILED DESCRIPTION

2.1 BACKGROUND

In May 2021, TVA identified issues with dam stability assumptions utilized in the hydrological analysis supporting the results and conclusions of the PMF analysis in the BFN UFSAR. These issues, documented in the TVA corrective action program, involved an assumption of dam stability of several embankment supported spillways at flood water flows in excess of the original design assumptions.

Utilizing (1) the updated PMP provided in NRC approved (Reference 1 and 2) Topical Report TVA-NPG-AWA16-A, (2) the US Army Corp of Engineers industry-standard Hydrologic Engineering Center River Analysis System (HEC-RAS) hydrological software in lieu of the TVA unique Simulated Open Channel Hydraulics (SOCH) software, (3) the updated model geometry and overbank reservoir storage, (4) the current dam stability conclusions and the original flow capacity of embankment supported spillways, TVA revised the BFN LIP and stream course hydrologic models used for predicting flooding impacts at the BFN site. In addition, TVA completed a seismically induced dam failure flooding analysis utilizing the current NRC guidance provided in Reference 3, NRC JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure.

2.2 NEED FOR PROPOSED CHANGE The update of the hydrologic analysis for BFN Units 1, 2, and 3 includes changes in the PMP used in the LIP and the rivers and streams flooding models, revision of the FSAR model geometry and/or reservoir overbank storage compatibility with the new HEC-RAS model, update of wind speed used in the wind wave analysis, update of dam stability assumptions, inclusion of a seismically induced dam failure flooding analysis to current NRC guidance, and inclusion of an update of the warning time plan contained in plant procedures that results from these changes.

As a result of the BFN hydrologic analysis update, the calculated PMF elevation at the BFN site decreased from a still water elevation of ft. For purposes of designing the SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 4 of 49

((CEII))

The calculated still water river PMF level is elevation ((CEII)) ft, excluding wind wave effects.

An additional 0.8 ft. of margin is provided for a design and licensing basis PMF at elevation

((CEII)) ft. A windstorm producing 45 mph sustained wind speeds is assumed to occur coincidently with the PMF, generating a 5 ft. wave, crest to trough. The wave runup on a vertical wall is about 5 ft. A maximum flood elevation of ((CEII)) ft. at the plant-site results from a combination of the PMF and wind wave runup on a vertical wall.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 5 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 flood protection for BFN Units 1, 2, and 3 systems, structures, and components (SSCs), the existing design and licensing basis PMF elevation is retained at ft. for stillwater conditions and ft. when wind wave effects are included. The existing BFN flooding protection measures are still effective as designed.

The proposed changes are necessary to address the updated dam stability and spillway flow capacity assumptions in the hydrologic simulation models.

2.3 PROPOSED CHANGE

S BFN UFSAR Section 2.4, Appendix 2.4A section is being changed to reflect the updated hydrologic analysis, including changes in the PMP used in the local intense precipitation (LIP) and the rivers and streams flooding models, application of HEC-RAS modeling software, revision of the geometry and/or reservoir overbank storage for compatibility with the HEC-RAS model, updated wind speed used in the wind wave analysis, inclusion of a seismically induced dam failure flooding analysis applying current NRC guidance, and inclusion of an update of the warning time plan contained in plant procedures resulting from these changes. to this submittal provides the existing BFN Units 1, 2 and 3 UFSAR text pages marked up to show the proposed changes. Enclosure 3 provides the proposed replacement BFN Units 1, 2 and 3 UFSAR Section 2.4, Appendix 2.4A Tables and Figures. Enclosure 4 provides the clean typed BFN Units 1, 2 and 3 UFSAR pages included in Enclosure 2 with the proposed changes incorporated. Enclosures 1, 2, 3, and 4 contain security related information that TVA is requesting be withheld from public disclosure in accordance with 10 CFR 2.390.

The following discusses the corresponding BFN UFSAR revisions based on the updated hydrology analysis. The Section number and headings correspond to the UFSAR Section in of this letter. (Note: in the updated UFSAR, numbers have been assigned to the existing primary headings to facilitate referencing. Also, in the updated UFSAR, content has been rearranged to be more consistent with the order of presentation of topics in NUREG-0800, Section 2.4.)

Section 2.4, Appendix 2.4A, 1.0 Introduction Technical changes are proposed for BFN Units 1, 2 and 3 UFSAR Section 2.4, Appendix 2.4A, Introduction.

These changes include the following:

Revised the PMP definition used in the local intense site flooding analysis as well as the rivers and streams flooding analysis.

Added a Definition for the term seismically induced dam failure flooding and denotes the use of NRC document JLD-ISG-2013-01 as a guide in evaluation of seismic dam failure flooding.

Removed the current HMR PMP reference.

((CEII

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 Section 2.4, Appendix 2.4A, 2.0 Summary of Results As described in BFN UFSAR Appendix 2.4A, Summary of Results, the design basis maximum possible flow in the Tennessee River at BFN of 1,2000,000 cfs. and the corresponding PMF elevation of ((CEIi ft. or ((CEIi ft. including wind wave effects are provided. Postulated hydrologic dam failures 'associated with saddle dam and embankment dam overtopping upstream of BFN are defined. In the updated analysis, the design basis maximum flow in the Tennessee River and the maximum PMF elevation at the BFN site is unchanged. Hydrologic dam failures in the controlling event for the BFN site are postulated for low margin and unanalyzed dams as well as overtopped saddle and embankment dams.

In the current BFN UFSAR Appendix 2.4A, Summary of Results, seismically induced dam failure flooding is not discussed and is not an event evaluated in the BFN design basis. In the updated analysis, the simulation results of three seismically induced dam failure events concurrent with flooding conditions are added to the BFN UFSAR. Stability failures of analyzed dams and the assumed failures of unanalyzed dams are provided. The controlling seismically induced dam failure flood elevation at the BFN site is ((CEIi ft., below plant grade elevation 565.0 ft. The BFN site is not flooded in the seismically induced dam failure flooding event.

Section 2.4, Appendix 2.4A. 3.0 The Watershed As described in BFN UFSAR Section 2.4, Appendix 2.4A, The Watershed, the major flood producing storms are of the cool-season, winter-type storms and the warm season, hurricane type storms. Most floods at BFN have resulted from storms occurring in the months of March through June. In the updated analysis, major flooding at the BFN site is attributed to general, tropical or local type produced in spring storms primarily during the months of March through June. This change reflects the application of TV A Topical Report TV A-NPG-AWA 16-A to define the watershed PMP (Reference 1 ).

Section 2.4. Appendix 2.4A. 4.0 Reservoir System As described in BFN UFSAR Section 2.4, Appendix 2.4A, Reservoir System, there are 22 major reservoirs upstream of the BFN site, 11 of which have substantial flood detention capacity on March 15. The tributary reservoir system above the BFN site provides 4,484,000 acre-feet of storage on March 15 or approximately 5.5 inches of rain on the tributary basins. The total flood detention capacity above Wheeler dam is 3. 7 inches mid-March. In the updated analysis, there are 22 major reservoirs upstream of the BFN site, 16 of which have significant reserved flood detention capacity. Using current operating guides, the tributary reservoirs provide 3,604,500 acre-feet of storage mid-March or approximately 6 inches of rain on the 14,708 sq. mi. tributary basins. The mid-March flood detention above Wheeler dam is approximately 2.8 inches of precipitation. This change was made to reflect updated TVA dam storage information.

Section 2.4. Appendix 2.4A 5.0 Local Intense Precipitation {LIP) -Site Drainage (Note: Heading relocated within Appendix 2.4A and retitled to be consistent with more current terminology.)

As described in BFN UFSAR Section 2.4, Appendix 2.4A, Local Intense Precipitation (LIP)-Site Drainage, the PMP for determining LIP flooding was defined by National Weather Service for TVA in HMR No. 56. The maximum 1-hour rainfall amount is 16.7 inches within a 6-hour storm producing 34.4 inches of total rainfall over a 1 sq. mi. area. The peak flood water elevation in the main plant area at Class 1 structures was determined to be 565.0 ft.

CNL-24-006 E5-Page 6 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 The updated BFN LIP PMP is defined by Topical Report TVA-NPG-AWA16-A as a maximum rainfall amount of 11.6 in. The peak flood water elevation in the main plant area at Class I structures was determined to be ((CEIi)) ft. Temporal distributions are unchanged.

Safety-related equipment is not impacted.

Section 2.4. Appendix 2.4A 6.0 Probable Maximum Flood (PMF) on Rivers and Streams (Note:

A new heading and introductory content are inserted in the updated UFSAR to group topics related to the PMF model and to provide results of simulations and wind waves under one heading.)

A brief general description of the development of the controlling PMF for the BFN site is added as an introduction to Section 6.0 of the updated BFN UFSAR. The Topical Report provides rainfall depth-area-duration data to define the PMP for BFN. Since multiple combinations of areas and depth are possible for a given rainfall duration, multiple storm events are analyzed using the run-off and stream course model to determine the controlling flood levels at the BFN site.

Section 2.4. Appendix 2.4A 6.1 Probable Maximum Precipitation (Note: Sub-section relocated in the updated BFN UFSAR for better grouping of related content)

As described in BFN UFSAR Section 2.4, Appendix 2.4A, Probable Maximum Precipitation, the PMP for the watershed above Wheeler is defined for TVA by the Hydrometeorological Branch of the National Weather Service (NWS) in HMRs No. 41 and 47. The PMP rainfall depth, duration, and spatial distributions for BFN were defined as two basic storms: (1) a 21,400 sq. mi. storm on the watershed above Chattanooga with the downstream centered orographically fixed isohyetal pattern defined in HMR No. 41 and (2) a 16,170 sq. mi. storm on the watershed above Wheeler Dam and below the major tributary dams with an isohyetal pattern defined in HMR No. 47. A third 7,980 sq. mi. storm isohyetal pattern defined in HMR No. 41 was eliminated from consideration based on rainfall depth on the watershed above Wheeler Dam.

PMP seasonality is defined in the HMR No. 41 by variable rainfall depths and durations for the period beginning in March and ending in September for each isohyetal pattern.

The updated BFN PMP is defined by Topical Report TVA-NPG-AWA16-A and is applicable to the assessment of river flooding effects. The updated PMP depth-area-duration characteristics at each point of a gridded network over the Tennessee River drainage basin for three storm types: local, general and tropical. The resolution of the PMP gridded network is 0.025 by 0.025 decimal degrees with each grid cell having an approximate area of 2.5 sq. mi. for a total of 12,966 grid points above Wheeler Dam. Gridded PMP values were calculated using the PMP Evaluation Tool, described in Topical Report TVA-NPG-AWA16-A. This tool applies rainfall data adjusted for moisture transposition, in-place maximization and orographic transposition adjustment factors to analyzed storm depth-area-duration curves for the PMP area size and duration of interest to yield an adjusted rainfall value at each grid point location. The analyzed storm's adjusted rainfall value at the target grid point location was then compared with the adjusted rainfall values of each storm in the Topical Report storm database that was transpositionable to the target grid point location. The maximum adjusted rainfall value determined in this comparison is the unique PMP depth for that grid point location for the analyzed area and duration. This process was repeated for each grid point on the gridded network within the PMP area of interest to determine the final PMP rainfall depths at all grid points. These point PMP depths represent a worst-case estimated rainfall for the historically SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 7 of 49

CNL-24-006 E5-Page 8 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 largest observed storm events transposable to a given grid point for a defined PMP area of interest and duration.

In the updated BFN PMP analysis, the areal application of the Topical Report point PMP values was determined using an event nesting and residual rainfall methodology. Nesting is the occurrence of PMP events over small areas or sub-basins during a larger area PMP event. The TVA nesting methodology 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 were determined using Topical Report 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 or between dam projects. The average PMP rainfall depth over the primary area for each rainfall duration was calculated by applying geographic information system (GIS) functionalities to create a PMP depth surface over the primary area and calculate the average durational rainfall for sub-basins in the primary area. Next, residual rainfalls in basin or sub-basin areas above Wheeler Dam and outside the primary area were determined.

As shown in updated BFN UFSAR Figure 4, the drainage area above Wheeler Dam consists of 61 sub-basins. The Wheeler Dam project is the furthest downstream river model boundary for the watershed that could potentially affect flooding at the TVA nuclear sites. Within the large multi-basin Wheeler Dam drainage area, there were multiple possible sub-basin areas (secondary areas of interest) encompassing (or nesting with) the primary area of interest for each storm. Each possible sub-basin area has an average PMP rainfall depth unique to that specific sub-basin area and rainfall duration as determined from the Topical Report. To ensure that the average PMP rainfall depth over a selected secondary sub-basin area (including the nested primary area of interest) was consistent with the average PMP rainfall depth developed from the Topical Report, the average PMP rainfall depth in the secondary sub-basin area outside of the primary area of interest was reduced (See Figure 1 below as an example of a Cherokee Total PMP composed of a PMP on the watershed above Cherokee Dam and a downstream secondary PMP on the basins between Fort Loudoun Dam and Cherokee Dam).

As a result, the average PMP rainfall durational depth over each basin or sub-basin area equates to the average PMP rainfall durational depth derived from the Topical Report. 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 by incremental project watershed 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. Because there were multiple combinations of larger sub-basin areas encompassing the primary area of interest, there were multiple potentially controlling PMP storm events for the BFN site.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

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CNL-24-006 E5-Page 10 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 As described in BFN UFSAR Section 2.4, Appendix 2.4A, Probable Maximum Precipitation and in the updated analysis, storms evaluated were nine-day events with a three-day antecedent storm postulated to occur three days prior to a three-day PMP storm in all PMF simulations.

Temporal rainfall distribution within the three-day PMP storm followed the guidance provided in HMR No. 41. As recommended in HMR No. 41, the antecedent rainfall of 40% of the main storm depth was applied. In the BFN UFSAR, Section 2.4, Appendix 2.4A, the antecedent rainfall was applied uniformly as 40% of the average depth of the main storm. In the updated analysis, the antecedent rainfall was applied as a spatial distribution replicating 40% of the spatial distribution of the main storm PMP although the Topical Report found no data supporting an event antecedent to the PMP. This change was made to align the antecedent storm with the spatial distribution of the PMP defined by Topical Report TVA-NPG-AWA16-A.

In the updated BFN PMF analysis, multiple PMPs with the potential to influence the flooding elevation at the BFN site were evaluated to determine the worst-case flooding impact to specific dam projects and to the BFN site. To determine the controlling PMP storm event for each primary area of interest (such as a dam project), multiple possible storm events were postulated for the primary area of interest based on the selection of multiple secondary areas of interest.

These potentially controlling PMP storm events were then either simulated using the stream course model described in updated BFN UFSAR, Section 2.4, Appendix 2.4A, to determine the PMF elevation at the BFN site or screened out as having no potential impact. The model simulation resulting in the highest PMF elevation for the BFN site was the controlling PMP storm event for the BFN site. The controlling PMP for the BFN site resulted from consideration of the drainage area above the Hiwassee and Blue Ridge dam projects as the primary PMP watershed nested with multiple secondary area PMPs over the Fontana-Hiwassee-Blue Ridge, Fort Loudon-Tellico-Hiwassee-Blue Ridge, Watts Bar-Hiwassee-Blue Ridge combined watersheds and nested with secondary area PMPs on the watersheds above the Chickamauga, Nickajack, Guntersville, and Wheeler dam projects. 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. The controlling PMP produced 11.47 inches of rain above Wheeler Dam in three days. Figure 2 provides a graphic representation of the controlling primary and secondary area nesting sequence. Figure 3 illustrates the controlling PMP rainfall depth spatial distribution over the watershed.

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SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 12 of 49 Section 2.4, Appendix 2.4A, 6.2 Rain Runoff Relationships As described in BFN UFSAR Section 2.4, Appendix 2.4A, Rain Runoff Relationships, a multi-variable relationship, used in the day-to-day river operations of the TVA system, has been applied to determine precipitation excess directly. The relationships were developed from observed data. They relate precipitation excess to rainfall, week of the year, geographic location, and antecedent precipitation index (API). For the current design basis, a median API, as determined from past records (1997-2007), was used at the start of the antecedent storm.

The updated analysis proposes updating the inputs for defining API using an 18-year period of historical rainfall records (1997-2015) at the start of the antecedent storm.

Section 2.4, Appendix 2.4A, Hydrograph Determination (Note: Unit hydrograph information has been updated and included in a new Section 6.3 Runoff and Stream Course Model in BFN UFSAR update)

Unit Hydrographs: As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Unit Hydrographs, the entire watershed above Wheeler Dam is divided into 62-unit areas. Unit hydrographs were developed for each unit area from flood hydrographs recorded at stream gaging stations or estimated from reservoir headwater elevation, inflow, and discharge data.

In the updated analysis, 2-unit areas (sub-basins 14 and 15) were combined into 1 hydrograph resulting in 61-unit areas above Wheeler Dam. For non-gaged unit areas, unit graphs were developed from relationships of unit hydrographs from similar watersheds relating 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 cfs. per sq. mi.

Reservoir Routing and Model Assumptions: (Note: Re-titled in BFN UFSAR update and included content of sub-section Reservoir Operations in current BFN UFSAR): As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Reservoir Routing, and design basis calculations (Reference 15), tributary reservoir routings were made using standard reservoir routing procedures and flat pool storage conditions. The tributary reservoirs of Tellico and Melton Hill dams as well as the main river reservoirs were routed using unsteady flow techniques in a three-segment model, beginning upstream on tributaries at the Cherokee, Douglas, Chilhowee, and Norris dams and ending downstream at Wheeler Dam. The equations of unsteady flow were solved in each segment of the mathematical model using TVA unique software. Upstream to downstream segments were manually connected through boundary data for continuity at the Watts Bar and Guntersville Dam boundaries to the downstream terminus at Wheeler Dam. Reservoir routings were started with all reservoirs at their respective median mid-March elevations.

In the updated analysis, the USACE Hydrologic Engineering Center River Analysis System software (HEC-RAS) performs one-dimensional steady and unsteady flow calculations. The TVA total watershed single composite HEC-RAS model is used in flood routing calculations for reservoirs in the Tennessee River System upstream of Wilson Dam through the uppermost tributary reservoirs to predict flood elevations and discharges for multiple PMP combinations.

Median pool levels for the appropriate week are used for the initial elevations at the beginning of the PMF event.

In the current design basis analysis (Reference 15) supporting the results provided in BFN UFSAR, Section 2.4, Appendix 2.4A, the discharge rating curve at Chickamauga Dam applies the original dam configuration before the construction of the new lock. In the updated SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 13 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 analysis, the discharge rating curve for Chickamauga Dam considers the bounding spillway configuration of either the original dam configuration or the proposed final modified lock configuration.

In the current design basis analysis (Reference 15) supporting the results provided in BFN UFSAR, Section 2.4, Appendix 2.4A, reservoir rim leaks at Watts Bar and Nickajack dams were neglected as non-significant. In the updated analysis, seven Watts Bar reservoir rim leaks and a single Nickajack Reservoir rim leak were postulated.

Model Geometry-Main Stem and Tributaries: (Note: New Section added in BFN UFSAR update): In the existing BFN UFSAR, Section 2.4, Appendix 2.4A, a section on Model Geometry-Main Stem and Tributaries did not exist. This subsection is added to fully describe the changes that took place in converting from the TVA unique SOCH model to a TVA total watershed HEC-RAS model performing a continuous simulation of the Tennessee River system from the uppermost tributary reservoirs downstream through Wilson Dam.

For the main stem geometry (simulation model geometry excluding the tributaries below interface points), cross-section data was obtained from the geometry verification calculations 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.

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.

The tributary geometry was developed as described below:

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 TVA 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. 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, French Broad River, Nolichucky River, and the downstream portion of the South Fork Holston River was regenerated and verified to more accurately determine the reservoir storage at these locations.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 14 of 49 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. Adjustments were made to the cross-sections and additional cross-sections were added if required to better represent the river. Mannings n values, ineffective flow areas, and flow lengths were evaluated, and adjustments or corrections were made if necessary. Augmented ineffective flow areas were updated after overbank storage volumes were computed using average overbank reach lengths. When the shape of each cross-section was verified, additional geometry data were evaluated, and adjustments or corrections were made if the data were not representative of the cross-section.

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 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 that were not easily captured with cross-sections.

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.

Calibration (Note: The existing Verification sub-title in the BFN UFSAR is updated to the more common terminology, 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.

Section 2.4, Appendix 2.4A, 6.4 Dam Stability Determination (Note: New Section added in BFN UFSAR update):

Earth Embankment Structures: A new subsection is added under the new section on Dam Stability Determination to provide a discussion on embankment dam sections global stability analysis, including the acceptance criteria based on TVAs Dam Safety procedures.

Concrete Structures: A new subsection is added under the new section on Dam Stability Determination to provide a discussion on concrete structure dam sections global stability analysis, including the acceptance criteria based on TVAs Dam Safety procedures.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 Controlling Storm: As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Critical Storm, storm arrangements including storm centerings, seasonal variability and potential dam failures were investigated to ensure selection of the controlling PMP for the BFN site. The critical storm was determined to be the 21,4000 sq. mi. downstream centered storm following an antecedent storm beginning on March 15. The antecedent storm would produce an average precipitation of 6.08 inches on the basins above Wheeler Dam, followed by a 3-day dry period and then by the main storm producing an average precipitation of 14.48 inches in 3-days.

In the updated analysis, storm arrangements including different primary and secondary watershed areas of interest combinations, seasonal variability, temporal distributions, and potential dam failures were reviewed to determine the critical, controlling storm for the BFN site. The controlling PMP storm was determined to be a primary PMP over the Hiwassee-Blue Ridge projects combined drainage basin concurrent with secondary PMPs over other downstream watersheds as described in the Probable Maximum Precipitation section above. The antecedent storm would produce an average precipitation of 4.59 inches on the basins above Wheeler Dam, followed by a 3-day dry period and then by the main storm producing an average precipitation of 11.4 7 inches in 3-days.

Precipitation Excess: As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Precipitation Excess, median moisture conditions as determined from an 11-year period of historical records ( 1992-2007) were used to determine the API at the start of the storm sequence.

In the updated analysis, the input for defining the API uses an 18-year period of historical rainfall records (1997-2015) at the start of the antecedent storm.

Dam Failures: As described in BFN UFSAR, Section 2.4, Appendix 2.4A, ((CEIi))

in the controlling event. All concrete dams were

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found to be stable.

In the updated analysis ((CEIi))

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HESCO Concertainersare not used to prevent overtopping of any embankments. No other dam failures were postulated to occur in the controlling PMF.

Section 2.4. Appendix 2.4A. 6.6 Probable Maximum Flood As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Probable Maximum Flood, the peak PMF discharge at the BFN site is 1, 193,671 cfs. and reaches a PMF elevation of ((CEIi ft. In the updated analysis, the peak PMF discharge at the BFN site is 1,013,315 cfs. ana reaches an SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 15 of 49 Section 2.4, Appendix 2.4A, 6.5 Conditions Creating Probable Maximum Flood

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 16 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 elevation of ft. The design basis flow of 1,200,000 cfs. and flood elevation at the BFN site ft. is retained.

Section 2.4, Appendix 2.4A, 6.7 Coincident Wind Waves (Note: Re-titled sub-section in the BFN UFSAR update)

As described in BFN UFSAR, Section 2.4, Appendix 2.4A, Wind Waves, a windstorm producing 45 mph sustained wind was assumed to occur concurrently with the controlling PMF, producing 5 ft. waves (crest to trough). The analysis used the 1% wave and produced wave runup above flood level of about 5 ft. on a vertical wall.

In the updated wind wave analysis, wind speed data from the National Climatic Data Center for five airport data stations surrounding the BFN site but primarily from Huntsville Airport were used. Two-minute average wind speed data reported for each one-minute interval from January 2000 to June 2014 were used in the analysis as recommended by the ASOS Users Guide. The overland two-year wind speed for general structures at the BFN site was 25.2 mph. Equations in the USACE procedures were used to calculate wind wave characteristics and specific USACE monographs were used in the wave runup and wave setup calculations. Wave runup and setup is estimated at 6.7 ft. on plant structures.

Section 2.4, Appendix 2.4A, 7.0 Floods Due to Seismically Induced Dam Failures (Note: this is a new sub-section in the BFN UFSAR update.)

In the existing BFN UFSAR, Section 2.4, Appendix 2.4A, a section on Seismically Induced Dam Failures does not exist and is not in the current BFN design basis. This subsection was added to fully describe analysis performed to determine the potential flooding impact to the BFN site associated with various scenarios of seismically induced dam failures. NRC Report JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure, Interim Staff Guidance, was used for guidance when evaluating potential flood levels from seismically induced dam failures.

Dam Failure Permutations: NRC guidance states a dam should be assumed to fail if it cannot withstand the more severe of the following combinations:

10-4 annual exceedance seismic hazard combined with a 25-year flood.

Half of the 10-4 ground motion, combined with a 500-year flood.

The seismic hazard for key TVA dams is defined by a site specific probabilistic seismic hazard analysis (PHSA). The PHSA utilized NUREG-2115 (Reference 8), Central and Eastern United States Seismic Source Characterization for Nuclear Facilities (2012), as seismic source characterization (SSC) along with EPRIs 2004/2006 ground motion prediction models (Reference 9). 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 the key dam locations.

Three sets of time histories, spectrally matched in accordance with NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, Section 3.7.1, criteria 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, Technical Basis for Revision of Regulator Guidance on Design Ground Motions: Hazard-and Risk-Consistent Ground Motion Spectra

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CNL-24-006 E5-Page 17 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Guidelines, Table 4-5, Recommended V/H Ratios for CEUS Rock Site Conditions, was used to define vertical to horizontal ratios.

After the site-specific seismic hazard for each of the key dams was established, the concrete and earth embankment structures were evaluated using TVA Dam Safety procedures.

Concrete Dam Structures: The current TVA Dam Safety seismic dam stability methods for concrete dams were used. 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 to 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. Flood control outlet spillway flow is limited to the original design flow capacity of the spillway; otherwise, failure is assumed.

Embankment Dam Structures: For the updated analysis, the TVA Dam Safety seismic dam stability methods for embankment dams were used. 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 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 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 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.

Multiple Seismic Dam Failures: Flood inflows utilized in combination with seismic events were examined for two alternatives based on the current guidance provided in NRC JLD-ISG-2013-01, Section 1.4.3: (1) seismic hazard with an annual exceedance probability of 104 combined with a 25-year flood and (2) seismic hazard defined by half of the 104 annual exceedance probability ground motion combined with a 500-year flood. The inflow hydrographs for the 25-year and 500-year floods were developed by using watershed gaged data to scale prototypical inflow hydrographs to meet estimated 25-and 500-year volume targets. The final hydrograph ordinates were summed, and volumes calculated to 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 concurrent failure of National Inventory of Dams (NID) identified projects outside the model is conservatively included in the model flood inflows. This change to include seismic failure of dams in the NID database was made to more closely align with the regulatory guidance provided in NRC JLD-ISG-2013-01, Section 3.2.

Unsteady Flow Analysis of Potential Dam Failures: The calibrated HEC-RAS runoff model described in Section 6.0 was used to perform an unsteady flow analysis of the three potentially critical seismic-flood event combinations involving dam failures above the plant.

Using the guidance provided in NRC JLD-ISG-2013-01, Section 1.4.3, seismic events having one-half the ground motion of the 104 annual frequency of exceedance (AFE) seismic event ground motion were combined with a 500-year flood. A seismic event having a ground motion of a 104 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 TV A Dam Safety dam stability criteria are credited in the model.

Dam structures without stability evaluations or dam structures determined to not be stable post-seismic event are assumed either to fail in the seismic event or to fail at a time in the event that is conservative to the simulation results.

Overtopped earthen dam structures upstream of BFN are assumed to fail after overtopping either instantaneously or as prescribed by the Von Thun and Gillette (Reference 10) or Froehlich methods (Reference 12). Overtopped earthen dam structures downstream of BFN are conservatively assumed to not fail when overtopped.

Water Level at Plant Site: The unsteady flow analyses of the four postulated combinations of seismic dam failures coincident with floods analyzed yield a maximum elevation of ((CEIi ft. at BFN excluding wind wave effects. The maximum elevation would result from multiple dam failures upstream of Chickamauga dam due to one-half the ground motion of a 104 AFE seismic event ground motion centered at Douglas dam in combination with a 500-year flood.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 18 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 n((CEII)) I BFN Elevation One-half of 104 Seismic Ground Motion Failures with 500 Yr. Flood n((CEII)) l I

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Section 2.4. Appendix 2.4A. 8.0 Low Water Considerations (Note: this is a new sub-section in the BFN UFSAR.)

In the updated BFN UFSAR, Section 2.4, Appendix 2.4A, a reference is inserted to provide a crosstie reference to the low water evaluations provided in Section 2.4.2.2.2, Stream Flow.

Section 2.4. Appendix 2.4A. 9.0 Flooding Protection Requirements (Note: this is a new sub-section in the BFN UFSAR.)

In the existing BFN UFSAR, Section 2.4, Appendix 2.4A, a section on Flooding Protection Requirements did not exist. Since plant grade elevation 565.0 ft. can be exceeded by large rainfall floods, this subsection is added to fully describe the basis by which BFN can tolerate flooding without jeopardizing public safety. Seismically induced dam failure floods will not exceed plant grade.

Design Basis Flood: The design basis flood (DBF) is the calculated upper limit flood that includes the probable maximum flood (PMF) plus the wave runup caused by an overwater wind and setup as discussed in Section 6.7. The table below gives representative levels of the DBF at different plant locations.

(([CEIi)) i mcruu i

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Includes Reactor Buildings, Diesel Generator Buildings, Radwaste Building, Turbine Building, and Standby Gas Treatment Buildings.

The flood elevations in the table 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 action in response to a postulated DBF. For purposes of designing the flood protection for systems, structures, and components, the higher design analysis flood (OAF) elevations in the table below should be used ensuring additional margin has been included in the development of design analysis.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 19 of 49

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In addition to flood level considerations, plant flood preparations cope with the "fastest rising" flood which is the calculated flood that can exceed plant grade with the shortest warning time. Reservoir levels for large rainfall floods in the Tennessee Valley can be predicted well in advance, permitting sufficient time to complete plant protective actions to place BFN in cold shutdown prior to flood waters exceeding plant grade.

Flooding of Structures: BFN structures housing equipment required for Cold Shutdown operation will be protected from flooding for a PMF of ((CEI I ft. ( or ((CEIi ft. for exterior walls exposed to wind wave effects). These structures are the Reactor l3uilding, the Diesel Generator Building, the Residual Heat Removal Service Water (RHRSW) Intake Pumping Station rooms, and the Radwaste Building. The Offgas Treatment Building and Chimney are protected from flooding up to 568.0 ft. The Turbine Building, Off Gas Stack, and the Standby Gas Treatment Building are permitted to flood.

Warning Plan: Per plant procedures, operating units at BFN initiate shut down to Cold Shutdown (Mode 4) per plant operating procedures when the Wheeler reservoir at the site is forecast by TVA River Management to rise above target elevation 563.5 ft. (below plant grade 565.0 ft.). The protection of BFN equipment required for Cold Shutdown from rainfall floods exceeding plant grade utilizes a flood warning issued by TVA's River Management.

TVA's climatic monitoring and flood forecasting systems and flood control facilities permit early identification of potentially critical flood producing conditions and reliable prediction of floods which may exceed plant grade well in advance of the event.

Basis for Flood Protection Plan in Rainfall Floods: Large Tennessee River floods can exceed plant grade elevation 565.0 ft. at BFN. Plant safety in such an event requires Cold Shutdown procedures which can be implemented within the allowable timeframe. The proposed BFN flood warning plan provides a minimum of 25 hours2.893519e-4 days 0.00694 hours 4.133598e-5 weeks 9.5125e-6 months to be in Cold Shutdown after initiation of the plan. Four additional preceding hours would be available to gather and analyze rainfall data and produce the warning to the site. To ensure flood operation preparation activities are not adversely impacted by wind generated waves during the final hours of shutdown activity, a flood level target of 563.5 ft. (below plant grade 565.0 ft.) is used to establish the minimum available period for completion of necessary activities and reaching Cold Shutdown.

Initiation of the site warning plan begins when the Wheeler Reservoir at BFN reaches an elevation of 558.0 ft. and Wheeler Reservoir flood levels are projected to reach 563.5 ft. in 25 hours2.893519e-4 days 0.00694 hours 4.133598e-5 weeks 9.5125e-6 months . Plant flood preparation status continues unless TV A's River Management determines that floodwaters will not exceed elevation 563.5 ft. at the plant.

The 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. Quantitative rain forecasts, which are a SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 20 of 49

((CEII))

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 21 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 part of daily operations, would be used by the River Management Forecast Center in flooding projections.

Basis of Analysis for Rainfall Floods: The forecast procedure to assure safe shutdown of BFN for flooding is based upon an analysis of hypothetical storms up to PMP magnitude defined in Tennessee Valley Authority, Topical Report (TR) TVA-NPG-AWA16-A. The selected storms envelope potentially critical areal and seasonal variations and time distributions of rainfall. The warning system is based on those storm situations which resulted in the shortest time interval between the time of the initial River Management warning at 558.0 ft. in the Wheeler Reservoir and time projected to exceed Wheeler Reservoir elevation 563.5 ft. The result of this analysis assures at least 25 hours2.893519e-4 days 0.00694 hours 4.133598e-5 weeks 9.5125e-6 months are provided for BFN to complete flood preparation activities and to reach Cold Shutdown of the operating units.

The HEC-RAS analysis of a minimal warning time primary nesting PMP on the Wheeler to Tims Ford-Guntersville Dam drainage basin with a 48-hour median-loaded temporal distribution during week 11 demonstrates that more than 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months are provided from the River Management warning of flood expected to exceed Wheeler Reservoir elevation 563.5.0 ft. Four hours are included for River Management modeling and communication time for more than 28 hours3.240741e-4 days 0.00778 hours 4.62963e-5 weeks 1.0654e-5 months of total warning time.

Basis for Flood Protection Plan in Seismic-Caused Dam Failures: Seismically induced dam failure floods do not exceed a Wheeler Reservoir elevation of ft. Due to the configuration of site grades and structures, there are no wind wave effects at safety-related structures. A site warning plan for plant safety is not required.

TVA Forecast System: The TVA River Forecast Center (RFC) normal operation produces forecasts twice daily using data observations collected over the prior 12-hour period. During major flood events, the RFC may issue forecasts as frequently as every 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months . The TVA RFC is constantly manned.

Forecast System Reliability: Communication between projects in the TVA power system is via Fiber-Optic System and commercial telephone. In emergencies, additional communication links are provided by TVAs Transmission Power Supply radio networks and satellite telephone communications, as required.

Section 2.4, Appendix 2.4A, Tables In support of the technical changes proposed for the BFN UFSAR, and to reflect the most current information for the hydrologic analysis for BFN, the tables associated with BFN UFSAR are proposed to be revised as follows:

UFSAR Table 1, (Sheets 1 and 2), Facts about Dams and Reservoirs is updated to reflect current information and sources.

UFSAR Table 2, Unit Hydrograph Data (Sheets 1 and 2) is replaced with Table 2, Flood Detention Capacity - TVA Projects Above BFN to reflect current information and sources.

UFSAR Table 3 (Sheets 1 and 2), Probable Maximum Flood - Rainfall and Precipitation Excess has been updated to Table 3 (Sheets 1, 2 and 3), Probable Maximum Storm Precipitation and Precipitation Excess. The data represented the rainfall and runoff data for

((CEII

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 22 of 49 design storms from HMR No.41 which has been replaced by the rainfall and runoff data from Topical Report TVA-NPG-AWA16-A.

UFSAR Table 4, Dam Failure Statistics, previously deleted in Amendment 25 has been reinstated as Table 4, Floods from Postulated Seismic Failure of Upstream Dams to provide the Wheeler Reservoir flooding level results of the seismically induced dam failure simulations in the updated Section 2.4, Appendix 2.4A of the UFSAR.

Section 2.4, Appendix 2.4A, Figures In support of the technical changes proposed for the BFN UFSAR, and to reflect the most current information for the hydrologic analysis for BFN, the figures associated with the BFN UFSAR are proposed to be revised as follows:

BFN UFSAR Figure 2, Seasonal Operating Curves, Cherokee is revised to Figure 2, Seasonal Operating Curves (Sheets 1 to 20) for key reservoirs upstream of Wheeler Dam which support the updated hydrologic analysis in the updated BFN UFSAR.

BFN UFSAR Figure 3, Seasonal Operating Curves, Guntersville is deleted since the Guntersville Operating Curve is included in Figure 2.

BFN UFSAR Figure 4, Browns Ferry Nuclear Plant, Hydrologic Model, Unit Areas is updated to represent the unit hydrograph areas applied in the hydrological model in the updated BFN UFSAR.

BFN UFSAR Figure 5 (Sheets 1 through 11), Unit Hydrographs, Areas (Various) have been revised to logically group the unit hydrographs into distinct reservoir/basin areas in support of the updated hydrologic analysis in the BFN UFSAR BFN UFSAR Figure 6, Hydrologic Model Verification - 1973 Flood has been revised to update the flood elevation calibration of the Wheeler Reservoir model to the March 1973 flood due to the revised geometry and overbank storage calculations in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 7, Hydrologic Model Verification - 1973 Flood has been revised to update the flood flow calibration of the Wheeler Reservoir model to the March 1973 flood due to the revised geometry and overbank storage calculations in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 8, Hydrologic Model Verification - 2004 Flood has been revised to update the flood elevation calibration of the Wheeler Reservoir model to the December 2004 flood due to the revised geometry and overbank storage calculations in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 9, Hydrologic Model Verification - 2004 Flood has been revised to update the flood flow calibration of the Wheeler Reservoir model to the December 2004 flood due to the revised geometry and overbank storage calculations in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 10, Hydrologic Model Verification - Steady-State Profiles has been revised to provide the extent of the hydrologic HEC-RAS model limits, Sheets 1 and 2.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 23 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Enclosure 5 Steady-state calibration is no longer performed in the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 11, Hydrologic Model Verification - Guntersville Tailwater Curve has been revised to Deleted since the tailwater verification is no longer performed in the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 12, Guntersville Hydro Plant, General Plan has been revised to Deleted. The figure is not referenced in the proposed BFN UFSAR text.

BFN UFSAR Figure 13 is updated and retitled Hiwassee Blue Ridge Primary Nesting Sequence to provide the basin nesting grouping for the PMP utilized in the BFN controlling PMF event.

BFN UFSAR Figure 14 is updated and retitled Rainfall Distribution in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 15 has revised to PMF Elevation and Discharge to provide the controlling PMF elevation and discharge at the BFN plant site for the updated hydrologic analysis.

BFN UFSAR Figure 16 is updated to Deleted. The 21,4000 sq. mi. downstream centered storm is no longer applicable to the updated BFN hydrologic analysis.

BFN UFSAR Figure 18 previously deleted in Amendment 25 is being reinstated as Figure 18, Single Seismic Dam Failure, Guntersville Dam, Half-10,000 Year Ground Motion with 500-Yr Flood Event in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 19 previously deleted in Amendment 25 is being reinstated as Figure 19, Multiple Seismic Dam Failure, Douglas Dam Centering, Half-10,000 Year Seismic Ground Motion Failures with 500-Year Flood in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 20 previously deleted in Amendment 25 is being reinstated as Figure 20, Multiple Seismic Dam Failure, Fort Loudoun Dam Centering, 10,000 Year Seismic Ground Motion Failures with 25-Year Flood in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 21 is updated and retitled to Sub-Basins to show the drainage sub-basins feeding the west and east channels and the cooling tower and the plant area basins in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 21A, titled West Area is added to show the west channel and the west channel drainage areas.

BFN UFSAR Figure 22 is updated and retitled to East Area to show the east channel and its drainage sub-basins in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 22A is updated and retitled to Plant Area to show the PMP drainage sub-basins in area surrounding the main plant structures in support of the updated hydrologic analysis in the BFN UFSAR.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 BFN UFSAR Figure 22B, Probable Maximum Precipitation Point Rainfall has been updated and retitled to Typical Mass Curve in support of the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 23, One Hour Unit Hydrograph for Unnamed Stream North-West of Plant has been retitled and updated to West Channel Unit Hydrographs for the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 24, Maximum Possible Flood (Probable Maximum Flood) for Unnamed Stream Northwest of Plant has been retitled and updated to West Channel Discharge for the updated hydrologic analysis in the BFN UFSAR.

BFN UFSAR Figure 27, is added and titled Cooling Tower Area Basins to show the sub-basins emptying into the Cool Water Channel that are used in the updated hydrologic analysis in the BFN UFSAR.

2.4 CONDITION INTENDED TO RESOLVE The proposed changes are necessary to address the errors in the flow capacity and stability assumptions of embankment supported spillways upstream of Wheeler Dam. The update of the hydrologic analysis for BFN Units 1, 2, and 3 includes changes in the PMP used in the LIP and the rivers and streams flooding models, revision of the geometry and/or reservoir overbank storage in the HEC-RAS model, update of dam stability criteria to TVA Dam Safety standards, update of wind speed used in the wind wave analysis, inclusion of a seismically induced dam failure flooding analysis to current NRC guidance in NRC JLD-ISG-2013-01, and the inclusion of a warning time plan resulting from these changes.

3.0 TECHNICAL EVALUATION

3.1 EVALUATION TVA proposes to update the BFN hydrologic analysis to address the dam stability issues associated with embankment supported spillways and Apalachia Dam. In this update, the existing HMR-based PMP is proposed to be replaced with a plant specific PMP using Topical Report TVA-NPG-AWA16-A. Utilization of the Topical Report PMP gridded rain data format requires changes in the process of defining the PMP rainfall depth for each basin in the TVA watershed impacting the rivers and streams rainfall flooding levels at the BFN site. TVA proposes to update this process of defining the average PMP rainfall depth on each basin.

As part of this proposed update, TVA proposes to replace the current BFN hydrologic TVA-unique SOCH modeling software with HEC-RAS, the industry standard hydrologic modeling software, and to update the existing model geometry and overbank storage methodology to be consistent with the HEC-RAS software methods for determining overbank storage.

TVA proposes to update the overland wind speed data used in the determination of wind wave effects at the BFN site to reflect more current data and inclusion of additional wind speed data reporting stations. In addition, TVA proposes to add a seismically induced dam flooding analysis for the BFN site to reflect the current NRC guidance provided in JLD-ISG-2013-01.

TVA also proposes to add a warning time analysis for the BFN site using Topical Report TVA-NPG-AWA16-A for the PMP. Because the BFN site flooding associated with seismically SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 24 of 49

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 induced dam failures does not reach plant grade, a warning time analysis is not required for this event.

As a result of the BFN hydrologic analysis update, the calculated PMF elevation, and the resulting DBF elevations, at the BFN site is decreased from a still water elevation of ((CEIi ft. to

((CEIi ft. For purposes of designing the flood protection for BFN Units 1, 2, and 3 SSCs, DAF elevations based on a still water elevation of ((CEIi ft. are established to maintain margin. The SSCs that are required for flood protection are not impacted by the decreased DBF elevation.

The existing flooding protection measures are still effective as designed.

Each of these proposed changes is evaluated in the following sections.

(Note: In Reference 13, TVA submitted a LAR (TS-19-02) to update the Sequoyah Nuclear Plant (SQN) UFSAR hydrologic analysis. TVA has submitted responses to NRC questions in regard to Reference 13, the SQN LAR. Attachment C provides a listing of those responses which are either totally or partially applicable to this BFN LAR. For those responses not applicable, responses to the NRC questions in regard to BFN are provided.)

A. Probable Maximum Precipitation (PMP) Basis Document and Process of Determining Controlling PMP Storms The design basis PMP used in the BFN site drainage analysis and the rivers and streams rainfall flooding hydrological analysis is updated to use Topical Report TVA-NPG-AWA16-A.

Due to the Topical Report gridded rainfall depth-area-duration data format, the method used to determine the rainfall pattern in PMP storm development has been updated. Temporary flood barriers at the Cherokee, Tellico, Fort Loudoun, and Watts Bar dams have been replaced with permanent modifications. Boundary conditions at the Chickamauga Dam were updated to the bounding configuration between the currently in-progress lock modification which blocks five of the original spillways and the original spillway.

Existing Design Basis: The existing PMP for the BFN site local intense precipitation (LIP) drainage analysis is defined by HMR No. 56, as described in BFN UFSAR Section 2.4, Appendix 2.4A, Local Drainage. The maximum 1-hour rainfall amount is 16.7 inches within a 6-hour storm producing 34.4 inches of total rainfall over an area of 1 sq. mi. Early, middle, and late temporal distributions were modeled. The results of the current site drainage analysis show the maximum water surface elevation in the vicinity of the radioactive waste, reactor, and diesel generator buildings will not exceed surface elevations of 565.0 ft.

The existing PMP defined in HMR No. 41 and No. 47 for the BFN rivers and streams rainfall flooding hydrologic analysis results from consideration of two basic design storms:

(1) a HMR No. 41 sequence of storms producing PMP depths on the 21,400 sq. mi.

watershed above Chattanooga, Tennessee with an upstream and downstream centering (BFN UFSAR Figure 13), and (2) a HMR No. 47 sequence of storms producing PMP depths on the 16,170 sq. mi. basin above Wheeler Dam and below the five major tributary dams (Norris, Cherokee, Douglas, Fontana, and Hiwassee)(BFN UFSAR Figure 14). A 7,980 sq.

mi. storm described in HMR No. 41 was eliminated from further analysis based on rainfall depths on the watershed above Wheeler Dam. The 21,400-sq-mi-storm isohyetal pattern (BFN UFSAR Figure 13) with a downstream centering, produces the critical rainfall pattern on the watershed for BFN.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 25 of 49

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 Estimates of PMP depths for the watershed above Chattanooga are fully defined in HMR No. 41. The PMP depth for a 72-Hour storm on the 21,400-square-mile watershed above Chattanooga is 14.48 inches, occurring in March.

Estimates of PMP depths for the 16, 170-square-mile watershed above Wheeler Dam but below the major tributaries are contained in HMR No. 47. The PMP depths for a 72-Hour storm on the 16, 170-square-mile watershed is 15.55 inches. The pattern and depths are for a storm centered within 35 miles of Nickajack Dam.

A 72-hour storm 3 days antecedent to the main storm was assumed to occur in all PMP situations outlined in HMR-41 with storm depths equivalent to 40 percent of the main storm outlined in Bulletin 41. Precipitation losses are determined using TVA's antecedent precipitation index (API), developed from observed storm and flood data over an 11-year period (1997-2007). The API defines excess precipitation based on rainfall depth, week of year, and geographic location.

As described in existing BFN UFSAR Section 2.4, Appendix 2.4A, Hydrograph Determination, unit hydrographs are used to compute flows from each sub-basin. The unit area flows are combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream tributary reservoirs, which are then 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 using unsteady flow routing.

Tributary reservoir routings were made using standard reservoir routing procedures and flat pool storage conditions. The tributary reservoirs, Tellico and Melton Hill, and the main river reservoirs were routed using unsteady flow techniques.

As described in BFN UFSAR Section 2.4, Appendix 2.4A, Reservoir Routing, unsteady flow routings were computer solved with a mathematical model based on the equations of unsteady flow. This model is described in a paper by Jack M. Garrison, Jean-Pierre Granju, and James T. Price entitled "Unsteady Flow Simulation in Rivers and Reservoirs,"

Journal of the Hydraulics Division, ASCE, Volume 95, No. HYS, September 1969 (Reference 14). Boundary conditions prescribed were inflow hydrographs at the upstream boundary, local inflows, and headwater discharge relationships at the downstream boundary based upon standard operating rules operating curves where geometry controlled, as appropriate.

Through the application of the run-off and stream course model, the controlling design storm PMP that produces the highest flooding elevation at the BFN site, is determined to be the 21,400-sq-mi-PMP downstream centered storm. The storm results in 14.48 inches of rain in the three-day storm preceded by a three-day antecedent rainfall of 6.08 inches ending three days prior to the main storm above Chattanooga. The controlling design storm results in a flood elevation of ((CEIi ft. (Reference 15) at the BFN site providing 0.8 ft. of margin to the design basis PmF elevation of ((CEIi ft.

Updated Design Basis: The proposed update of the PMP applied in the BFN site LIP drainage analysis and the rivers and streams rainfall flooding hydrologic analysis is defined by Topical Report TVA-NPG-AWA16-A.

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SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 The updated PMP for the BFN site local intense precipitation drainage analysis defines a cumulative 1-hour, 1-mi2 PMP rainfall depth of 11.6 inches with early, middle, and late temporal distributions modeled. The results of the updated site drainage analysis show the maximum water surface elevations in the vicinity of the Radioactive Waste, Reactor, and Diesel Generator Buildings are at or below elevation ((CEIi)) ft. at critical water intrusion locations.

The updated PMP, as defined by Topical Report TVA-NPG-AWA16-A, for the BFN rivers and streams rainfall flooding hydrologic analysis provides depth-area-duration characteristics for three storm types at each point of a gridded network over the Tennessee River drainage basin for three storm types: local, general, and tropical. The resolution of the PMP gridded network is 0.025 by 0.025 decimal degrees with each grid cell having an approximate area of 2.5 sq. mi. for a total of 12,966 grid point above Wheeler Dam. Gridded PMP values were calculated using the PMP Evaluation Tool, described in the Topical Report and in Attachment A to this enclosure. This tool applies moisture transposition, in-place maximization, and orographic transposition adjustment factors to analyzed storm depth-area-duration values for the PMP area size and duration of interest to yield an adjusted rainfall value at a single grid point location-The analyzed storm's adjusted rainfall value at the target grid point location and specified duration is then compared with the adjusted rainfall values of each storm in the Topical Report storm database that is transpositionable to the target grid point location. The maximum adjusted rainfall value determined in this comparison is the unique PMP depth for that grid point location at the specified duration. This process is repeated for each grid point on the gridded network within the PMP area of interest to determine the final point PMP rainfall depths. These point PMP depths at all specified durations represent a worst-case estimated rainfall for the historically largest observed storm events transposable to a given grid point for a defined PMP area of interest and duration.

The areal application of the point PMP values was determined using an event nesting and residual rainfall methodology. The TVA nesting methodology creates potential controlling storm events using a multi-step process, as described further in Attachment B to this enclosure. First, gridded point PMP values over a primary area, defined as the area of interest for a rainfall duration, were determined using the Topical Report 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 was calculated by applying geographic information system (GIS) functionalities to create a PMP depth surface over the primary area. Residual rainfall in basin or sub-basin areas above Wheeler Dam and outside the primary area was then determined.

The drainage area above Wheeler Dam consists of 61 sub-basins. The Wheeler Dam project is the furthest downstream river boundary for the watershed that could potentially affect flooding at the 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 for each storm. Each possible sub basin area has an average PMP rainfall depth unique to that specific sub-basin area and rainfall duration as determined from the Topical Report. To ensure that the average PMP rainfall depth over a selected secondary sub-basin area (including the nested primary area of interest) was consistent with the average PMP rainfall depth developed from the Topical Report, the average PMP rainfall depth in the secondary sub-basin area outside of the SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 27 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 primary area of interest was reduced. As a result, the average PMP rainfall depth over each basin or sub-basin equates to the average PMP rainfall depth derived from the Topical Report. 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. Because there were multiple combinations of larger sub-basin areas encompassing the primary area of interest, there were multiple potentially controlling PMP storm events for the BFN site.

PMP storms evaluated were nine-day events with a three-day antecedent storm postulated to occur three days prior to a three-day PMP storm in all PMF simulations. Variations of the temporal rainfall distribution within the three-day PMP storm were considered in the controlling storm determination.

Actual rainfall events were utilized to determine the temporal rainfall distributions for the design storms of various types, durations, and quantile loadings. Multiple meteorologically feasible rainfall distributions were used to aid in finding the controlling PMF events without unduly compounding the probability of occurrence. Meteorologically feasible rainfall distributions were obtained via the application of statistical analysis on actual rainfall events that were provided in the Topical Report and used in development of the PMP for the Tennessee River watershed. The statistical analysis utilized in the development of the temporal distributions that were applied in the determination of the controlling PMF events, was based on the process laid out in "Development and Utility of Huff Curves for Disaggregating Precipitation Amounts" by J.V. Bonta (Reference 7). This type of statistical analysis is consistent with the methodologies applied in Volumes 2 and 9 of the NOAA Atlas 14 Precipitation Frequency Analyses encompassing Tennessee, Virginia, North Carolina, and Alabama.

Actual rainfall events, as presented in the Storm Precipitation Analysis System (SPAS) data from the Topical Report for 31 general storm events, 19 local storm events, and 8 tropical storm events, were applied with the aforementioned methodology to develop meteorologically feasible rainfall temporal distributions. Median, first, and last quantile distributions were developed based at the 50, 10, and 90 percentiles, respectively, from the SPAS storm data to provide dimensionless storm data that were used in development of the associated temporal hourly rainfall rankings.

The antecedent rainfall was applied as a spatial distribution replicating 40% of the spatial distribution of the main storm PMP. Precipitation losses were determined using TVA's API, developed from observed storm and flood data. The API defines excess precipitation based on rainfall depth, week of year, and geographic location.

Unit hydrographs (UHs) were used to compute flows from each sub-basin. The unit area flows were combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream tributary reservoirs, which were then routed through the reservoirs using standard routing techniques. Resulting outflows were combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures using unsteady flow routing.

Unit Hydrographs (UHs) for the 61 TVA model sub-basins are non-synthetic, data based UHs. Historical large rainfall event data were used in the UH development and the UH data were validated against multiple gaged storm events. These UHs were applied in all SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 28 of 49

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 previous TV A simulations for the approved licensing bases. The UH data were used as previously validated, reviewed, and approved in the WBN 2012 Hydrology LAR and the 2014 LAR Supplement.

Application of effective rainfall follows the methodology detailed in the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) Technical Reference Manual, Chapter 6. Rainfall may be input in any time increment and the "discrete rainfall pulses" are summed to match the UH duration prior to convolution calculations. While the current gridded rainfall data set was created in one-hour increments, the rainfall was not shaped and was applied in 1-, 6-, 12-, 18-, 24-, 48-and 72-hour durations as uniform rainfall across the given duration (i.e., hourly rainfall increments for hours 2 through 6 are equal, hourly increments for hours 7 through 12 are equal, etc.). While these incremental depths may be separated when placed into the selected temporal distribution, as described in the HEC-HMS manual, the rainfall applied according to the UH duration will be summed, and the correct total effective rainfall volume was applied uniformly across the duration. These validated UH data have been used in previously approved submittals.

USACE HEC-RAS software was used to perform one-dimensional steady and unsteady flow calculations for each design storm in a continuous simulation of the Tennessee River system from the uppermost tributary reservoirs downstream through Wilson Dam. The unsteady flow model calibration was to known historic flood events. Flood operational guides were used to operate dams before spillway gates are fully opened. Median pool levels for the appropriate week were used in the runoff and stream course model at the beginning of the storm event. Operational allowances were implemented at Douglas and Norris Dams. Changes to the geometry of the HEC-RAS unsteady flow model are discussed in Section 3.1.B.

The updated model utilizes the bounding configuration between the proposed final configuration of an in-process major lock modification at the Chickamauga Dam and original Chickamauga Dam configuration. Temporary flood barriers credited in the current BFN UFSAR simulations were replaced with permanent modifications. The updated model includes seven Watts Bar Reservoir rim leak locations discharging downstream of Watts Bar Dam at Watts Creek and Yellow Creek as well as an additional rim leak TVA has identified northeast of the Nickajack Dam and back to the Tennessee River below the dam.

To determine the controlling PMP storm event for each primary area of interest (such as a dam project), multiple possible storm events were postulated for the primary area of interest based on the selection of multiple secondary areas of interest. These potentially controlling PMP storm events were then either simulated using the stream course model to determine the PMF elevation at the BFN site or screened out as having no potential impact. The model simulation resulting in the highest PMF elevation at the BFN site is the controlling PMP storm event at the BFN site. Project PMPs evaluated specifically for BFN impacts include the Chickamauga, Nickajack, Guntersville and Wheeler dam reservoirs.

The controlling PMP that results in the highest flood water elevation at the BFN site resulted from consideration of the drainage area above the Hiwassee and Blue Ridge Dam projects as the primary PMP watershed, nested with multiple secondary area PMPs over the Fontana-Hiwassee-Blue Ridge, Fort Loudon-Tellico-Hiwassee-Blue Ridge, Watts Bar Hiwassee-Blue Ridge projects combined watersheds, and nested with secondary area PMPs on the watersheds above the Chickamauga, Nickajack, Guntersville, and Wheeler Dam projects (Reference 16). The PMP rainfall depths over the secondary watershed SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 29 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 areas outside the primary watershed area were reduced to maintain the average PMP rainfall depth over the secondary watershed area.

This storm results in 11.47 inches of rainfall in a three-day main storm preceded by a three-day antecedent rainfall of 4.59 inches ending three days prior to the main storm above the Wheeler Dam. ((CEIi))

((CEIi))

No other dam failures would occur in the controlling PMF.

The resulting flood discharge at the BFN site was determined to be 1,013,315 cfs. and a peak still water elevation of ((CEIi ft. (References 16 and 17). For purposes of designing the flood protection for BFN Units 1, 2' and 3 SSCs, OAF elevations based on a still water elevation of ((CEIi ft. are established to maintain margin. The SSCs that are required for flood protection are not impacted by the decreased DBF elevation. The existing flooding protection measures are still effective as designed.

Justification: Topical Report TVA-NPG-AWA16-A updates the HMR PMP by including storms that occurred after the HM Rs were completed, by updating the HMR processes used to address orographic effects, and by applying reproducible procedures in the derivation of the PMP.

PMP rainfall depth-area-duration data in the Topical Report are provided in a gridded matrix across the total TV A watershed as compared to the design storm isohyetal pattern defined in HMR No. 41 and HMR No. 47. The updated method is comprehensive with many spatial rainfall patterns considered compared to the two design storms in the existing design basis.

The multiple PMP rainfall patterns challenge each of the potentially critical drainage basins above the BFN site in the determination of the storms having the most impact at the site. In addition, the rainfall volume from multiple PMPs covering the total drainage basin above Wheeler Dam were applied concurrently in each updated design storm compared to the PMP in the existing analysis covering only the isohyetal patterns described in HMR No. 41 and 47.

The NRC approved Topical Report TVA-NPG-AWA16-A for use in determining the controlling PMP for the BFN site in a safety evaluation dated March 18, 2019 (Reference 2).

The updated PMP for the BFN site reflects utilization of Topical Report TVA-NPG-AWA16-A.

The use of temporary flood barriers at upstream dams to prevent overtopping in the proposed model was eliminated, because permanent modifications have been completed to prevent an overtopping failure.

The proposed model configuration of Chickamauga Dam was updated to consider the bounding configuration between the final configuration of the in-process lock configuration and the original dam configuration.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 30 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 B. Changed Methodology for Determining PMF - SOCH Model Changed to HEC-RAS Model The TVA SOCH model has been replaced in its entirety by the industry standard USACE HEC-RAS and HEC-HMS software to model the Tennessee River Watershed to perform a continuous simulation of the Tennessee River system from the uppermost tributary reservoirs downstream through Wilson Dam.

Existing Design Basis: The main river reservoirs routings were made using the TVA unsteady flow SOCH mathematical model. The SOCH model for BFN consist of 3 model segments: Segment 1 begins on the Holston River tributary to Cherokee Dam and the French Broad tributary to Douglas Dam, joining to form the Tennessee River at TRM 652.22.

The Tennessee River is then joined with the Little Tennessee River tributary to Chilhowee Dam and the Clinch River tributary to Norris Dam. Segment 1 downstream boundary is Watts Bar Dam on the Tennessee River. Segment 2 begins at Watts Bar Dam on the Tennessee River and is joined with the Hiwassee River tributary to Charleston Gage at HRM 18.9 and North Chickamauga Creek below Chickamauga Dam. Segment 2 downstream boundary is the Guntersville Dam. Segment 3 begins at Nickajack Dam on the Tennessee River and terminates at Wheeler Dam. Discharges from Segment 1 were input to the upstream end of Segment 2. The discharges from Nickajack Dam computed in Segment 2 were input to upstream boundary of Segment 3. General input to the segment models includes the reservoir geometry, upstream boundary inflow hydrograph, local inflows, and the downstream boundary headwater discharge relationships based upon operating guides or rating curves when the structure geometry controls. Tributary inflows were combined with the appropriate time sequencing or channel routing procedures to compute inflows into the upstream tributary reservoirs based on sub-basin location and routing parameters. The reservoir inflows were then routed through the reservoirs using standard techniques and flat pool storage conditions to provide inflows at the SOCH input points. The discharge-rating curve for Chickamauga Dam is for the original lock configuration with the 18 spillway bays available.

The unsteady flow mathematical model for the 49.9-mile-long Fort Loudoun Reservoir was divided into 29 cross-sections with 28 reaches. The model was verified at gauged points within Fort Loudoun Reservoir, Holston River and French Broad River using 1973 (6 gaged locations) and 2003 (5 gaged locations) flood data. The unsteady flow model was extended upstream on the French Broad and Holston Rivers to Douglas and Cherokee dams, respectively. The French Broad River and Holston River unsteady flow models were verified at one gaged point each at miles 7.4 and 5.5, respectively, using 1973 flood data. Since Tellico Dam was constructed in 1979, Tellico Dam and the Little Tennessee River tributary upstream to Chilhowee Dam were included in the 2003 model. The Tellico Reservoir was verified at one gaged point at mile 0.3 using 2003 flood data.

The unsteady flow routing model for the 72.4-mile-long Watts Bar Reservoir was divided into forty reaches with forty-one cross-sections. A second model for the Clinch River up to Melton Hill Dam included 23 reaches. The model was verified at three gaged points within the reservoir using 1973 and 2003 flood data.

The unsteady flow mathematical model for the total 58.9-mile-long Chickamauga Reservoir was divided into 28 reaches, providing 29 equally-spaced cross-sections. The unsteady SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 31 of 49

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 32 of 49 flow model was verified at four gauged points using 1973 flood data and three gaged locations using 2003 flood data within the Chickamauga Reservoir.

The unsteady flow mathematical model for the total 46.3-mile-long Nickajack Reservoir was divided into 36 reaches, providing 37 cross-sections. The unsteady flow model was verified at nine gauged points within Nickajack Reservoir using 1973 flood data and three gauged locations for the 2003 flood data.

The unsteady flow mathematical model for the total 75.7-mile-long Guntersville Reservoir was divided into 36 reaches, providing 37 cross-sections. The unsteady flow model was verified at six gauged points within Guntersville Reservoir using 1973 flood data and four gauged locations using 2003 flood data.

The unsteady flow mathematical model for the total 74.1-mile-long Wheeler Reservoir was divided into 36 reaches, providing 37 equally-spaced cross-sections. The unsteady flow model was verified at four gauged points within Wheeler Reservoir using 1973 flood data and at three gauged locations using 2004 flood data.

Reservoir operating guides applied during the SOCH model simulations mimic, to the extent possible, operating policies and are within the current reservoir operating flexibility. Median initial reservoir elevations for the appropriate season were used at the start of the storm sequence.

Updated Design Basis: The proposed changes to BFN UFSAR Section 2.4, Appendix 2.4A, Reservoir Routing and Modeling Assumptions, update the discussion of PMF on streams and rivers to reflect the most current information available as inputs, and to use updated methodologies, such as the USACE HEC-RAS software, for elements of the hydrologic analysis for determining the PMF for streams and rivers for BFN. The runoff model setup, geometry, and calibration are the same as previously reviewed for the SQN license amendment request (LAR) (Reference 13). 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 UFSAR Section 2.4, Appendix 2.4A, Probable Maximum Precipitation.

Inputs to the simulations include calibrated HEC-RAS models (geometry files with updated ineffective flow areas and Mannings n values) of each reservoir, operational guides and initial median reservoir levels, initial dam rating curves, as well as inflow hydrographs. 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.

Initial dam rating curves have been developed for the main stem dams and tributary dams to be used as inputs to the HEC-RAS models. The initial dam rating curves were developed using an average tailwater to determine outflow from the dam based on data from steady-state profiles at varying flows. The initial dam rating curves for each dam provide total dam discharge as a function of headwater and tailwater elevations and are used to define the beginning conditions for the hydraulic analysis. Final dam rating curves are simulation specific and determined in the HEC-RAS model, which incorporates tailwater effects and relationships of upstream and downstream dams.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 33 of 49 A HEC-RAS model analysis of hypothetical storms on the Tennessee River Watershed was conducted using the methodology described above. The lock configuration used in the simulation modeling considered the bounding condition between the original dam configuration and the in-progress modification of Chickamauga Dam with five spillway bays blocked.

Justification: The existing design basis model utilizes SOCH software. The updated design basis model adapts the industry standard HEC-RAS modeling software to perform flooding simulations.

C. Overland Wind Speed Data Update Meteorological wind speed data was updated to include more current wind data and data sources from airports in Chattanooga, Knoxville, and Tri-Cities, Tennessee, in Asheville, North Carolina, and in Huntsville, Alabama. BFN site windspeeds are essentially reflective of the windspeeds at the nearby Huntsville Airport. As a result of this change, critical fetch and wind wave effects were updated for BFN critical structures.

Existing Design Basis: Some wind waves are likely when the PMF is cresting at BFN. The flood would be near its crest elevation for a day beginning approximately 6-1/2 days after cessation of the PMP storm.

A reasonably severe windstorm producing 45 mph sustained wind speeds could occur coincidentally with the PMF. A wind from the SE will produce the largest waves at the site.

A wind of this magnitude and from this direction can generate 5-foot waves (crest to trough).

The analysis of wave heights used the 1% wave of which about 5 per hour will occur.

Consequent wave runup above the flood level would be about 5 feet on a vertical wall.

Updated Design Basis: In the updated analysis, the two-year wind speed was the Automated Surface Observing System (ASOS) surface one-minute data from the National Climatic Data Center for five airport data stations. Two-minute average wind speed data, reported for each one-minute interval from January 2000 to June 2014, were used in the analysis as recommended by the ASOS Users Guide. Because a 20-minute sustained wind is sufficient to cause wind wave activity, the two-minute average wind speed data were analyzed to find the peak 20-minute average wind speed for each year at the BFN site.

A two-year wind speed is defined as the wind speed that has a 50% chance of being exceeded in any given year. To determine the two-year overland wind speed, the calculated peak 20-minute resultant velocities for each year for each airport reporting site were statistically analyzed to generate a curve of best fit for wind speed versus probability of exceedance using a Pearson Type III transformation. The two-year wind for each airport reporting site was taken from the curve at the point of a 50% probability of exceedance. Using the inverse of the cube of the distance from the BFN site to the airport reporting station, the effective weight the two-year wind speed of each reporting station relative to the BFN site was determined (the near proximity of the Huntsville Airport wind data makes that data predominate for the BFN site). The sum of the weighted wind speeds from each of the five airports reporting station was taken as the two-year overland wind speed for the BFN site. For a bounding BFN site flood elevation

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 34 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 of ((CEII)) ft., the overland two-year wind speed for general structures at the BFN site was up to 25.2 mph. The resultant of the two-year overland wind speed with respect to the topography limited critical fetch at the Reactor, Diesel Generator and Radwaste buildings was 25.2 mph. The Intake Pumping Station, analyzed with respect to the critical fetch from the southeast direction, has a two-year overland wind speed of 24.8 mph.

To account for the increase in wind speed over water, the two-year overland wind speeds were converted to overwater wind speeds with regard to the effective fetch length based on guidance provided in the USACE Engineering Technical Letter (ETL) 1110-2-8. For a bounding BFN site flood elevation of ((CEII)) ft., the resulting over water wind speed at the Reactor, Diesel Generator, and Radwaste Buildings was 31.5 mph, 31.5 mph for the southeast fetch of the Intake Pumping Station, 31.8 mph for the southeast fetch of the Chimney and 32.2 mph and 32.5 mph for the Gate 2 and Gate 3 structures, respectively.

The total wind wave height including run-up and set-up was estimated to be 6.6 ft. at the Reactor, Diesel Generator and Radwaste Buildings, 4.6 ft. at the Intake Pumping Station, and 5.5 ft. and 5.9 ft. at the Gate 2 and Gate 3 structures, respectively.

Justification: These changes were made to update the wind speed reporting information to reflect more current data and to provide more diversity in the wind speed data sources. Guidance from the ASOS Users Guide and USACE ETL 1110-2-8 was utilized.

D. 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 Seismically induced dam failure flooding analyses has been added to the BFN UFSAR to consider the more severe of two combinations: (1) seismic hazard with an annual exceedance probability of 10-4 combined with a 25-year flood, and (2) seismic hazard defined by one-half of the 10-4 annual exceedance probability ground motion combined with a 500-year flood, as given in NRCs current guidance document, JLD-ISG-2013-01.

Seismic hazard attenuation methods and multi-dam failure evaluation techniques were also updated for consistency with JLD-ISG-2013-01.

Existing Design Basis: Seismically induced dam failure flooding evaluations for the BFN site are not included in the BFN UFSAR.

Updated Design Basis: In the updated analysis, the seismic hazard and the basis for analyzing seismic dam failures in combination with floods was revised to be consistent with NRC JLD-ISG-2013-01, Section 1.4.3, which requires consideration of the more severe of the following combinations:

(1) seismic hazard with an annual exceedance probability of 10-4 combined with a 25-year flood.

(2) seismic hazard defined by one-half of the 10-4 annual exceedance probability ground motion combined with a 500-year flood The seismic hazard for key TVA dams whose failure could potentially result in flooding of the BFN site was defined by a probabilistic seismic hazard analysis (PSHA) performed

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 by TV A's Dam Safety organization. A site-specific PSHA and time histories for each dam were developed for 22 major dams upstream of Wilson Dam and the BFN site on the Tennessee River and its tributaries whose failure could potentially impact site flooding at BFN and Wheeler Dam downstream of BFN. The upstream dams were Watts Bar, Fort Loudoun, Chickamauga, Nickajack, Guntersville, and Wheeler dams on the Tennessee River; Watauga, South Holston, Boone, Fort Patrick Henry, Cherokee, and Douglas dams above Fort Loudoun; and Norris, Melton Hill, Fontana, and Tellico Dams between Fort Loudoun and Watts Bar dams; Apalachia, Blue Ridge, Chatuge, Hiwassee, and Nottely Dams with discharge entering the Chickamauga Reservoir through the Hiwassee River tributary, and Tims Ford Dam on the Elk River with discharge entering the Tennessee River upstream of Wheeler Dam.

The TVA Dam Safety PSHA utilized NUREG-2115 (Reference 8) as seismic source characterization (SSC) for the analyzed dams along with EPRls 2004/2006 ground motion prediction models (Reference 9). 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 time histories were 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 were 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 these dams was established, each dam

( except Apalachia, which has uncertain foundation conditions and was not evaluated for stability) was evaluated for stability given the specific seismic hazard at the site.

Multiple dam failures in the Tennessee River and tributary river systems can potentially occur in a single seismic event causing flooding at the BFN site. The guidance in NRC JLD-ISG-2013-01 was applied to determine which combination of dams would potentially fail due to the seismic hazard. Deaggregation of site-specific seismic hazard for the dams was performed for a seismic event with an AFE of 10-4 and a seismic event equal to one-half the ground motion of the 10-4 AFE event. The deaggregation was performed for two ground motion frequencies, 1 O 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% of the seismic hazard. Cutoff distances for an AFE 10-4 event ranged from 50 to 100 km for 10 Hz and 300 to 750 km for 1 Hz. Cutoff distances for one-half the ground motion of a 10-4 AFE ranged from 50 to 125 km for 10 Hz and 300 to 750 km 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 numbers of potential seismic dam failure combinations from the deaggregation analysis were further evaluated to determine the combination of seismic dam failures resulting in the greatest impact to the BFN 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. The potentially controlling events for flooding at the BFN SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 35 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 site consider (1) a seismic hazard ((CEIi))

((CEIi))

The potentially controlling seismically induced flooding events in Table 1 were evaluated using the HEC-RAS unsteady flow stream course model.

Table 1 - Seismically Induced Dam Failure Combinations - Updated Design Basis Simulation One-half of 104 Seismic Ground Motion Failures with 500 Year Flood BFN Flood Elevation ft 1

((CEIi))

2

((CEIi))

((CEIi Simulation o:.i Seismic Grouna rillofion Failure wifn25_Y_e_a_r-+-B-F_N_ F_ loo*tl Ele **v-a-ti_o_n

-i Flood (ft_}

((CE=11

1 3

((CEIi))

Justification: The existing design basis is updated to provide the basis for the seismic dam failure combined with flooding analysis to be consistent with the guidance provided in NRC JLD-ISG-2013-01.

2. Determination of 25-year and 500-year Flood Inflows The methodology applied in the development of 25-year flood and 500-year inflows to be used in combination with the seismic events discussed above is added to the BFN UFSAR. This methodology aligns with the recent work performed for the TV A Clinch River Early Site Permit Application.

Existing Design Basis: Seismically induced dam failure analysis flooding evaluations for the BFN site considering 25-year and 500-year inflows are not included in the BFN UFSAR.

Updated Design Basis: Consistent with NRC JLD-ISG-2013-01, Section 1.4.3, the seismically induced dam failure analysis considers two conditions: (1) seismic hazard with an annual exceedance probability of 104 combined with a 25-year flood and, (2) seismic hazard defined by one-half of the 104 annual exceedance probability ground motion combined with a 500-year flood.

The inflow hydrographs for the 25-year and 500-year floods in the updated design basis were developed by using watershed gaged data to scale prototypical inflow hydrographs to meet estimated 25-year and 500-year volume targets. This methodology uses SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 36 of 49

))

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 historical gaged data across the watershed above the Wheeler Dam aggregated into annual maximum series for one-day to five-day durations to estimate 25-year and 500-year frequency stream flows. In this method, 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 one to five 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

'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 the U.S. Department of Interior's, Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Subcommittee (Reference 4 ), an annual duration series (yearly 'X'-day maximum) was developed for each 'X'-day duration data set. A log-Pearson Type 111 distribution was applied to the resulting annual series following the methodologies described in References 4 and 6. The resulting distributions provide both the 25-year and the 500-year 'X' -day durational average stream flows. The durational volume above the Guntersville project watershed was then selected as the target value for adjustment of the prototype inflow hydrographs for individual Guntersville failure and above the Wheeler project for the multiple dam failure scenario.

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 Wheeler 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 (Reference 5). Rainfall was applied with losses using the National Resource Conservation Service 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-year and 500-year events at each daily duration from one to five days. The final inflow hydrograph ordinates were summed, and volumes calculated to confirm that the target volumes had been met or exceeded. The adjusted surface runoff values were limited to no smaller than the constant baseflow.

The seismic failure of ((CEIi))

was conservatively included in the model seismic flood inflows. A simplified approach was used to account for the ((CEIi))

under these seismic failure conditions. A time to peak of 20 minutes was assumed for the failure hydrographs. A Froehlich approach (Reference

12) was used to postulate the individual failure hydrograph peak flows. The individual hydrographs were then combined into a composite triangular hydrograph based on

((CEIi))

, and the peaks were adjusted to preserve volume ensuring that the ((CEIi))

was included in the seismic failure flows.

Justification: Combining a 25-year and 500-year storm event with the seismic hazard follows the current regulatory guidance in JLD-ISG-2013-01. The 25-year and 500-year inflow hydrograph development uses a methodology consistent with an input format matching the updated HEC-RAS model. Adding inflows to the model from seismic failure of ((CEIi))

increases system volume and downstream impact at the BFN site.

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SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390

3. Dam Stability Design methods and criteria used in the determination of concrete and embankment dam structures, are added to the BFN UFSAR for provide consistency with current regulatory guidance in JLD-ISG-2013-01.

Existing Design Basis: Seismically induced dam failure analysis flooding evaluations and the associated dam stability determinations are not included in the current BFN UFSAR.

Updated Design Basis: The method of analysis of concrete structures was 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 was performed by response spectrum modal analysis or time history analysis. The purpose of the dynamic analysis was 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 was assumed to crack whenever tensile stress normal to the dam/foundation interface was indicated.

After the seismic event damaged state of the concrete dam structure has been determined, the post-earthquake stability of the dam was assessed. Forces applied to the dam include hydrostatic forces due to 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 was conservatively neglected, unless sufficient data were available to justify the use of cohesion. The post-earthquake stability of the concrete structure was confirmed if the sliding factor of safety was 1.3 or greater and the overturning resultant was within the base of the concrete structure. Flood control outlet spillway flow is limited to the original design flow capacity of the spillway; otherwise, failure at the peak headwater elevation was assumed.

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 were 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 was performed using appropriate shear strengths assigned to the potentially liquefiable materials based on standard industry methods. Non-liquefiable materials were evaluated for strain softening and assigned appropriate drained or undrained shear strengths depending on material properties and phreatic surfaces. The post-earthquake analysis was 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 were evaluated. If the search routine develops factor of safety values less than 1.1, then the embankment structure was considered potentially unstable.

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SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 If liquefiable materials were not present in the embankment and/or foundation, then a static equilibrium analysis was 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 were applied from the drained and undrained parameters with the initial effective normal consolidation pressures at normal pool. If the factor of safety was greater than 1.1, the embankment was deemed stable. If the factor of safety was less than 1.1, then a simplified deformation analysis was performed utilizing Newmark Analysis, or other method deemed appropriate. If the deformations by the simplified method were 2 feet or less and less than one-half the thickness of the filter, the dam was considered stable.

Otherwise, additional more sophisticated deformation analysis methods may be considered.

Justification: The stability analysis methods and the factors of safety required for stability of concrete and embankment dam structures have been updated to align with TV A Dam Safety design. For embankments, design criteria, design considerations, material characterization, and stability analysis were consistent with the requirements and guidance contained in the Federal Energy Regulatory Commission (FERC) Draft Chapter 4, Embankment Dams, September 2006. For concrete structures, design criteria were based on Chapter 3 of the FERC Engineering Guidelines as revised in 2002 and additional guidance from the U.S. Army Corps of Engineers (USACE) and U.S.

Bureau of Reclamation (USSR).

E. Warning Time Hydroloqic Basis - Rainfall on Rivers and Streams and Seismically Induced Dam Failure Floods The rivers and streams PMP used in the warning time plan provided in plant procedures was updated to Topical Report TVA-NPG-AWA16-A. An analysis considering multiple PMP nesting sequences is performed to determine the fastest rising PMP which could impact BFN flood protective activity preparation. A target flood elevation less than plant grade is established to ensure plant flood mode activities are completed before these required protective activities are impacted by the flooded Wheeler Reservoir combined with wind wave activity. The updated analysis provides assurance that sufficient time is available to reach Cold Shutdown of all BFN units when considering the fastest rising PMP's. Wheeler Reservoir flooding associated with seismically induced dam failure does not require a warning time evaluation since flood waters for this event do not reach plant grade.

Existing Design Basis: Flood preparation activities for the BFN site are initiated by Plant Operations procedure when the Wheeler Reservoir reaches 558.0 ft. in elevation. When notification is received by Plant Operations that 558.0 ft. elevation has been reached, TVA River Forecast Center is contacted to determine the forecasted flooding elevation at the BFN site. The TVA River Forecast center confirms through modeling (expected to take no more than 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months ) if site flooding at or above plant grade 565.0 ft. is expected. If flooding of plant grade is projected, BFN Plant Operations initiates shutdown of BFN operating units as well as flood protection activities per 0-AOl-100-3, "Flood Above Elevation 558"'. Cold Shutdown of all units and completion of flood protection activities are expected to be complete in 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months following initiation of the procedure or 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months after reaching 558.0 ft.

in Wheeler Reservoir.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 39 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 The warning time available for implementation of protective actions at BFN is defined as the duration of the rise time of the controlling PMF from elevation 558.0 ft. to 565.0 ft. As provided by BFN UFSAR Appendix 2.4A, Figure 16, hydrograph for the BFN site controlling 21,400 sq. mi. downstream-centered storm, the flood rise time is approximately 5.5 days.

Since the flood rise time of 5.5 days is substantially longer than the required time for implementation of the activities in 0-AOl-100-3, the warning plan meets flood protection requirements for the BFN site.

Updated Design Basis: For rivers and streams rainfall flooding, the forecast procedure to assure safe shutdown of BFN for flooding is based upon an analysis of PMP storms from Topical Report TVA-NPG-AWA16-A. 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 above in Section 3.1 A Multiple potentially controlling warning time simulations were reviewed for BFN and formally documented. Each simulation applied a three-day storm having 40 percent of the PMP storm rainfall applied three days before the main storm PMP.

The PMP nesting combinations producing the maximum flood elevation at the BFN site do not produce the quickest warning time PMP nesting combination because the maximum flood elevation PMPs include dam failures upstream of BFN occurring after the flood elevation reached plant grade.

The target elevation used to determine if BFN site flooding conditions exist is 563.5 ft., or 1.5 ft. below plant grade, to include the effects of wind wave activity. Much of the site is protected from wind wave activity by berms or other natural features at elevation 565.0 ft.

or above. The lowest periphery elevation between the Wheeler Reservoir and the safety-related plant structures at plant grade is Gate 3 at a minimum elevation of 562.0 ft.

Wind waves from the Wheeler Reservoir striking Gate 3 will result in a 2.6 ft. wave past Gate 3, heading toward the main plant safety-related reactor, diesel generator, radwaste and standby gas treatment structures at a peak combined elevation of 566.1 ft. Since the flood stillwater remains 1.5 ft. below grade, the waves would break over the wide area at elevation 565.0 between the reservoir and the main plant safety-related structures and drain to the southwest and then to the Wheeler Reservoir without impacting the safety-related structures at grade elevation 565.0 ft. or above.

The HEC-RAS unsteady flow stream course model was used to perform warning time simulations. The analysis of multiple PMF simulations performed shows that the simulations with the highest water surface elevation at BFN result from postulated upstream and cascading dam failures. 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 potentially fastest rising PMP simulations were selected as candidates for detailed analysis.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 40 of 49

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 The most limiting fast-rising storm for BFN is a March PMP on the primary drainage basin above Wheeler Dam to Tims Ford-Guntersville dams combined with secondary PMPs on selected combined drainage areas above Wheeler Dam to Guntersville Dams, above Wheeler Dam to Chickamauga Dam, above Wheeler Dam to Watts Bar-Hiwassee-Blue Ridge dams, above Wheeler Dam to Norris-Cherokee-Douglas-Fontana dams and a secondary PMP on the total drainage areas above Wheeler Dam. Analysis of the PMF hydrograph for this storm demonstrates at least 29 hours3.356481e-4 days 0.00806 hours 4.794974e-5 weeks 1.10345e-5 months is available between the time the fastest-rising storm flood water reaches 558.0 ft. and the time the flood water reaches 563.5 ft. at the BFN site.

As described above in the Design Basis, the required minimum warning time duration includes time for modeling and communication and time to complete activities required to reach Cold Shutdown on the three units and the activities implementing flood protection requirements defined in 0-AOI-100-3.

Since the minimum warning time available, based on an analysis of the fastest-rising storm, is at least 29 hours3.356481e-4 days 0.00806 hours 4.794974e-5 weeks 1.10345e-5 months , exceeding the time necessary to perform Wheeler Reservoir modeling and time to complete the flood mode plant protection activities and to reach Cold Shutdown, the warning plan for BFN is acceptable.

Justification: The key changes in the BFN warning time analysis are (1) updated PMP, (2) replacing SOCH model with HEC-RAS model, and (3) warning time target elevation.

1. PMP Update: TVA-NPG-AWA16-A has been approved by the NRC in a Safety Evaluation Report dated March 18, 2019.

2. Model Software Update: TVA unique modeling software SOCH is replaced with an industry standard modeling software, HEC-RAS.

3. Warning Time Target Elevation Change to 563.5 ft. (1.5 ft. below plant grade): 2.6 ft.

wind generated waves occur between Gate No. 3 and the plant area at a peak elevation of 566.1 ft. or 1.1 ft. above plant grade. These waves break on the wide area at 565.0 ft. and drain to the southwest back to the reservoir without impacting the main plant safety-related structures (Reactor, Diesel Generator, Radwaste, and Standby Gas Treatment buildings) at the plant grade.

Some plant protective activities defined in 0-AOI-100-3 occur at the Intake Pumping Station, requiring access at plant grade elevation 565.0 ft. As discussed above in the Existing Design Basis, activities in 0-AOI-100-3 are complete in 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months following 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months of TVA River Forecasting modeling (or 16 total hours). At 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months following the notification of reaching Wheeler Reservoir elevation 558, flood water has reached 562.5 ft. Combined with a 2.6 ft. wind wave, the maximum flood (562.5 ft.) plus wind wave (2.6 ft.) reaches elevation 565.1 ft. which will not impede the required flood protection activities performed under 0-AOI-100-3.

3.2 UNCERTAINTIES Per American National Standards Institute/American Nuclear Society (ANSI/ANS) 2.8 and Regulatory Guide (RG) 1.59, the techniques applicable to PMF and seismically induced floods for nuclear power plants are estimates. The calculations that support the PMF analysis document assumptions and approaches, which are consistent with ANSI/ANS 2.8 and RG 1.59, as supplemented by guidance in NRC JLD-ISG-2013-01. The PMF analysis is a best estimate SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 41 of 49

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 and is consistent with these standards and guidelines. However, it is realized that various elements of the analysis when modified result in different elevations, some higher and some lower, and those elements discussed in further detail below are consistent with these standards and guidelines demonstrating that the PMF analysis is a reasonable best estimate.

As discussed in NUREG/CR 7046, "Design Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America," the appropriate method to address the uncertainty in the hydrologic analysis is through calibration of the model to historic flood events or sensitivity analyses. TV A calibrated the model to historic flood events using the two highest recent flood events where data exists. The floods used for calibration are the March 1973 and the May 2003 or December 2004 floods with estimated elevations at BFN of approximately 556.4 ft. and 556.6 ft. for those two storms.

Sensitivities of the rainfall-to-runoff transformation (unit hydrographs) were assessed by peaking and lagging the unit hydrographs. When unit hydrographs were peaked by 20 percent and the time to peak was decreased by 1/3, the results show that the maximum flooding elevation at BFN is ((CEIi ft. ((CEIi))

((CEIi)) --

((CEIi))

Therefore, sufficient margin exists in the plant design considerations as described in BFN UFSAR Subsection 2.4.2.2.

The rainfall loss rate is another parameter that has been evaluated through sensitivity analysis.

TVA uses the Antecedent Precipitation Index rain runoff relationship. This is the same loss methodology used by TV A for the daily reservoir operations. This parameter does show sensitivity to the model resulting in several feet added to the PMF elevation when increasing the runoff to 100 percent. However, the rainfall loss rate used in the TV A model is a realistic value for TV A based on regional historic data over more than 60 years, and there is high confidence in this value as the appropriate value for hydrology modeling. In addition, this parameter was tested by comparison to other acceptable methods for determining rain runoff relationships discussed in NUREG/CR 7046. The methodology used by TVA is conservative when compared to the other acceptable methods.

Other parameters in the stream course model such as the Manning's n value or resistance to flow could be increased or decreased during extreme flood events such as the PMF. The values in the model were based on calibration against two of the largest floods of record. If it were postulated that debris in the overbanks would result in an increase in resistance to flow and thus an increase in the Manning's n values, then the elevation at BFN would increase. If it were postulated that Manning's n values would be decreased, then the elevations at BFN would be decreased. Such decreases have been documented for large flood events on the Mississippi River and could have been considered in the updated hydrologic analysis, but Manning's n values were conservatively not decreased. Based on this documented experience, there is conservatism in the applied Manning's n values, but it is difficult to quantify because the flood has never been out of channel to the extreme that it would be in the PMF.

Therefore. TV A uses the best estimate approach with calibration to two of the largest recorded floods with data.

Dam rating curves were developed assuming that the gates would be open. When extreme flood events occur, then the gates will be lifted and left open during the flood. In the controlling PMF simulation for BFN, embankments at the three major dams immediately upstream of BFN fail well before the peak flood elevation is reached at the site. Spillway blockage at these dams SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 42 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 would marginally impact the failure timing but not the failure assumptions for these dams since failure of these dams are conservatively assumed to occur when the reservoir peaks behind the dams. This demonstrates that gate blockage can be accommodated within the plant design considerations described in BFN UFSAR. The model was not tested for loss of gate capacity at Wheeler Dam, downstream of the BFN site, because the controlling simulations result in embankment overtopping regardless of the position of the spillway gates. Wheeler Dam is conservatively considered not to fail in any simulation regardless of overtopping.

The sensitivity of the PMF, seismic-flood and warning-time simulation models to failure of the flood control spillways at flows less than the original design flow capacity of the spillways was assessed by assuming complete failure of the spillways in the controlling simulations when any flow occurs in the spillways. The PMF sensitivity studies demonstrated that PMF flood levels at the BFN site are essentially unchanged and remain less than the BFN OAF elevation. BFN site flooding effects from seismic dam failures are bounded by substantial margins by PMF flooding.

Warning-time simulation model sensitivity studies indicate no impact in the available warning time. Sufficient warning time remains available to complete flood protection activities required by plant procedure before site inundation by flood waters. TV A River Forecasting flood control procedures continue to provide the required warning-time notifications to ensure BFN is properly aligned for flood mode operation prior to site inundation.

3.3 MARGINS In the updated analysis, the calculated PMF still water elevation is ((CEIi ft., a decrease from the existing BFN UFSAR calculated elevation of ((CEIi ft., resulting in a'n increase in the calculated margin of 2.9 ft. For purposes of designing tne flood protection for BFN Units 1, 2, and 3 systems, structures, and components (SSCs), the existing concept of DAF as found in the licensing basis for SQN is proposed to maintain margin. The OAF elevations will be based on the current DBF still water elevation of ((CEIi ft. The existing flooding protection measures are still effective as designed. Thus, design margins at BFN are not adversely impacted by this change.

3.4 CONCLUSION

S The revised margin to the DBF elevations at the BFN site is determined to not impact the safety-related systems, structures, or components required to be available during a plant flood.

Documentation changes have been completed. Also, the warning time for BFN shows that sufficient time remains available in rainfall floods for safe plant shutdown. Seismically induced dam failures do not reach plant grade and do not require a warning program.

The cumulative effects of the proposed changes do not impact the original conclusions of the BFN UFSAR that adequate flooding protection features, and procedures are in place. The hydrologic analysis is considered to be a reasonable best estimate that has accounted for uncertainties based on regulatory guidance using the best data available. The updated hydrologic analysis shows that the design and siting of BFN is adequate to meet the regulatory requirements and criteria specified to be addressed for BFN UFSAR, Section 2.4, Appendix 2.4A, and that BFN is capable of tolerating floods above plant grade in a manner that does not jeopardize public health and safety.

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 43 of 49

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 44 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

4.0 REGULATORY EVALUATION

4.1 APPLICABLE REGULATORY REQUIREMENTS AND CRITERIA Requirements in 10 CFR Part 100 provide for identifying and evaluating hydrologic features of the site.

The criteria set forth in 10 CFR 100.23(d) determines the siting factors for plant design bases with respect to seismically induced floods and water waves at the site.

As discuss in 10 CFR 50, Appendix A, General Design Criteria (GDC) 2, requires consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

In addition to regulatory requirements, acceptable guidance for hydrologic analysis of the site is included in the following:

RG 1.29 identifies seismic design bases for safety-related SSCs.

RG 1.59, as supplemented by best current practices, including regulatory guidance provided in NRC JLD-ISG-2013-01, provides guidance for developing the hydrometeorological design bases.

RG 1.102 describes acceptable flooding protection to prevent the safety-related facilities from being adversely affected.

The BFN Units 1, 2, and 3 hydrologic analysis conforms to the above regulatory requirements and guidance, using the most recent data and updated methodology, which includes use of USACE HEC-HMS and USACE HEC-RAS software.

The BFN Units 1, 2, and 3 hydrologic analyses, as described in this License Amendment Request and as presented in the proposed revision of the BFN Units 1, 2, and 3 UFSAR, contain sufficient substantiated information pertaining to the hydrologic description at the BFN site. The hydrologic analysis meets the requirements of 10 CFR 100 as it relates to:

Identifying and evaluating the hydrology in the vicinity of the site and site regions, including interface of the plant with the hydrosphere, Hydrological causing mechanisms, Spatial and temporal data sets, Alternate conceptual models of site hydrology, Identification and consideration of local intense precipitation and flooding at the site, Identification and consideration of the probable maximum flooding on-streams and rivers at the site and in the surrounding area, Identification and consideration of the effects of hydrological and seismic dam failures at the site and in the surrounding area, The appropriate site phenomena in establishing emergency operations for SSCs important to safety.

CNL-24-006 E5-Page 45 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Further, the hydrologic analysis considers the most severe natural phenomena that have been historically reported for the site and surrounding area, while describing the hydrologic interface of the plant with the site, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

The NRC staff has accepted the methodologies, or similar methodologies, used to determine the severity of the phenomena, the local intense precipitation, flooding causal mechanisms, controlling flooding mechanism reflected in these site characteristics, the probable maximum flooding on streams and rivers, the effects of dam failures, the potential for low water conditions, and consideration of the appropriate site phenomena in establishing emergency operations for SSCs important to safety, as documented in safety evaluation reports for previous licensing actions. Accordingly, the use of these methodologies results in site characteristics and procedures containing sufficient margin for the limited accuracy, quantity, and period of time in which the data have been accumulated. In view of the above, the site characteristics previously identified as described in the proposed changes to the BFN Units 1, 2, and 3 UFSAR are acceptable for use in establishing the design bases for SSCs important to safety and site procedures.

4.2 PRECEDENCE In Reference 11, the NRC approved an amendment revising the Watts Bar Nuclear (WBN)

Plant Unit 1, Updated Final Safety Analysis Report regarding changes to the hydrology analysis. Reference 11 incorporated updates to the hydraulic analysis methodology, including use of the USACE HEC-HMS and HEC-RAS software, and updates for concrete and earthen dams to the TVA Dam Safety dam stability criteria. The HEC-RAS model of the Tennessee River and its tributaries developed in Reference 11, as updated to correct for geometry and overbank storage, is the same model being applied to BFN Units 1, 2, and 3.

4.3 SIGNIFICANT HAZARDS CONSIDERATION The proposed changes modify Browns Ferry Nuclear Plant (BFN), Units 1, 2, and 3 Updated Final Safety Analysis Report (UFSAR) to reflect the updated hydrologic analysis, including changes in the probable maximum precipitation (PMP) used in the local intense precipitation (LIP) and the rivers and streams flooding models, replacement of the Tennessee Valley Authority (TVA) unique hydrological modeling software with an industry-standard HEC-RAS model, update of the wind speed used in the wind wave analysis, addition of a seismically-induced dam failure flooding analysis to current Nuclear Regulatory Commission guidance, and addition of a warning time plan resulting from these changes.

The proposed changes demonstrate adequate margin to the design analysis flood (DAF) elevations considered in the flooding protection of safety-related systems, structures, or components during external flooding events, and verify the adequacy of the warning time for BFN for rainfall floods. The proposed changes do not alter the conclusions presented in the BFN Units 1, 2, and 3 UFSAR that equipment required for operation in the flood is above the Design Basis Flood (DBF), and that there is sufficient time available in rainfall floods for safe plant shutdown. No physical changes to safety-related systems, structures, or components, or any credited flooding protection feature are required to ensure that they remain adequately protected from the effects of external floods.

TVA has concluded that the changes to BFN Units 1, 2, and 3 UFSAR do not involve a significant hazards consideration. TVA's conclusion is based on its evaluation in accordance SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 46 of 49 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 with Title 10 of the Code of Federal Regulations (10 CFR) 50.91(a)(1) of the three standards set forth in 10 CFR 50.92, "Issuance of Amendment," as discussed below:

1.

Does the proposed amendment involve a significant increase in the probability or consequence of an accident previously evaluated?

Response: No The proposed changes reflect the updated hydrologic analysis, including changes in the PMP used in the LIP and the rivers and streams flooding models, replacement of the TVA unique hydrological modeling software with an industry-standard HEC-RAS model, updated wind speed used in the wind wave analysis, addition of dam stability criteria, addition of a seismically induced dam failure flooding analysis to current NRC guidance, and addition of a warning time plan resulting from these changes. The proposed changes demonstrate adequate margin to the DAF relative to the revised DBF elevations and limiting safety-related systems, structures, and components.

Implementation of these changes does not 1) prevent the safety function of any safety-related system, structure, or component during an external flood; 2) alter, degrade, or prevent action described or assumed in any accident described in the BFN Units 1, 2, and 3 UFSAR from being performed, because the safety-related systems, structures, or components remain adequately protected from the effects of external floods; 3) alter any assumptions previously made in evaluating radiological consequences; or 4) affect the integrity of any fission product barrier.

Therefore, this proposed amendment does not involve a significant increase in the probability, or consequences of an accident previously evaluated.

2.

Does the proposed amendment create the possibility of a new or different kind of accident from any accident previously evaluated?

Response: No.

The proposed changes reflect the updated hydrologic analysis, including changes in the PMP used in the LIP and the rivers and streams flooding models, replacement of the TVA unique hydrological modeling software with an industry-standard HEC-RAS model, updated wind speed used in the wind wave analysis, addition of dam stability criteria, addition of a seismically induced dam failure flooding analysis to current NRC guidance, and using the updated PMP, added a warning time plan resulting from these changes.

The proposed changes do not introduce any new accident causal mechanisms, nor do they impact any plant systems that are potential accident initiators.

Therefore, the proposed amendment does not create the possibility of a new or different kind of accident from any accident previously evaluated.

3.

Does the proposed amendment involve a significant reduction in a margin of safety?

Response: No.

The proposed changes reflect the updated hydrologic analysis, including changes in the PMP used in the LIP and the rivers and streams flooding models, replacement of the TVA unique hydrological modeling software with an industry-standard HEC-RAS model, updated wind speed used in the wind wave analysis, addition of dam stability criteria,

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 addition of a seismically induced dam failure flooding analysis to current NRC guidance, and addition of the warning time plan resulting from these changes. The proposed changes do not alter the permanent plant design, including instrument set points, that is the basis of the assumptions contained in the safety analyses. The results of the updated hydrologic analysis show that the proposed changes demonstrate adequate margin to the OAF elevations and limiting safety-related systems, structures, or components during external flooding events. Therefore, the proposed changes do not prevent any safety-related structures, systems, or components from performing their required functions during an external flood. Consistent with existing regulatory guidance, including regulatory recommendations, and discussions regarding calibration of hydrology models using historical flood data and consideration of sensitivity analyses, the hydrologic analysis is considered to be a reasonable best estimate that has accounted for uncertainties using the best data available.

Therefore, the proposed changes do not involve a significant reduction in a margin of safety.

4.4 CONCLUSION

S In conclusion, based on the considerations discussed above, (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public. Based on the above, it is concluded that the proposed changes do not involve a significant hazards consideration under the standards set forth in 10 CFR 50.92(c), and accordingly, a finding of "no significant hazards consideration" is justified.

5.0 ENVIRONMENTAL CONSIDERATION

A review has determined that the proposed amendment would not change a requirement with respect to installation or use of a facility component located within the restricted area, as defined in 10 CFR 20, or would change an inspection or surveillance requirement. Also, the proposed amendment does not involve (i) a significant hazards consideration, (ii) a significant change in the types or significant increase in the amounts of any effluents that may be released offsite, or (iii) a significant increase in individual or cumulative occupational radiation exposure.

Accordingly, the proposed amendment meets the eligibility criterion for categorical exclusion set forth in 10 CFR 51.22(c)(9). Therefore, pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the proposed amendment.

6.0 REFERENCES

1.

TVA letter to NRC, CNL-19-040, "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 )," dated May 21, 2019 (ML19155A043).

2.

NRC Letter to TV A, "Verification Letter of the Approval Version of Tennessee Valley Authority Topical Report 'TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis Calculation CDQ0000002016000041,' Revision 1 (EPID L-2016-TOP-0011 )," dated July 3, 2019 (ML19158A395).

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 47 of 49

SECURITY RELATED INFORMATION-WITHHELD UNDER 10 CFR 2.390 3.

NRC, JLD-ISG-2013-01, Interim Staff Guidance, "Guidance for Assessment of Flooding Hazards Due to Dam Failure," Revision 0, (ML13151A153).

4.

"Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Subcommittee," lnteragency 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.

5.

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.

6.

Hovey, Peter and Thomas DeFiore, "Using Modern Computing Tools to Fit the Pearson Type Ill 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.

7.

J.V. Bonta, "Development and Utility of Huff Curves for Disaggregating Precipitation Amounts," 2004.

8.

NUREG-2115, "Central and Eastern United States Seismic Source Characterization for Nuclear Facilities," January 2012 (ML12048A776).

9.

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.

10.

Whal, Tony, DSO-98-004, "Prediction of Embankment Dam Breach Parameters: A Literature Review and Needs Assessment", Dam Safety Office, Water Resources Research Laboratory, July 1988 (W50240611001) 11.

NRC Letter to TV A, "Watts Bar Nuclear Plant, Unit 1 - Issuance of Amendment to Revise Report Regarding Changes to Hydrology Analysis (TAC NO. ME9130)," dated January 28, 2015 (ML15005A314).

12.

Froehlich, D. C., "Peak Outflow from Breach Embankment Dam", Journal of Water Resources Planning and Management, Volume 121, Issue 1, January 1995 (W50240611001 ).

13.

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 January 14, 2020 (ML20016A396 and ML20016A397).

14.

Garrison, Jack M., Jean Pierre Granju, James T. Price, "Unsteady Flow Simulation in Rivers and Reservoirs," Journal of the Hydraulics Division, ASCE, Volume 95, No. HYS, September 1969.

15.

Tennessee Valley Authority, Calculation CDQ000020080054, "PMF Determination for Tennessee River Watershed," Revision 5, December 18, 2014.

16.

Tennessee Valley Authority, Calculation CDQ0000002017000064, "Guntersville Dam (GuH) Probable Maximum Flood (PMF) Analysis," Revision 1, June 7, 2022.

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 CNL-24-006 ES-Page 48 of 49

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Page 49 of 49 17.

Tennessee Valley Authority, Calculation CDQ0000002017000070, Probable Maximum Flood (PMF) Results Summary, Revision 3, June 11, 2024.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment A SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Att A 1 of 13 Topical Report PMP Evaluation Tool and Application Using GIS Two tools were used to develop the watershed average rainfall over the TVA sub-basins.

The first is the PMP Evaluation Tool provided in the Topical Report. This tool determines point depth-duration values for defined grid points across the Tennessee River watershed.

The second tool provides automation of ArcGIS and Quantum GIS (QGIS) to create a surface from the grid point values and produce weighted average durational PMP depths applied to TVA model sub-basins.

Topical Report PMP Evaluation Tool Setup The processing of gridded PMP depth data produced from the ArcGIS PMP Evaluation Tool provided in the Topical Report provides weighted average PMP depths over the Tennessee Valley Authority project sub-basins. The GIS tool provided in the Topical Report was software dedicated and found to match the test values to less than +/-0.01-inch depth at any grid point.

The tool was used, as provided, as part of the software dedication, with the following minor modifications.

1. As provided, the tool includes provision for the creation of raster files for the results.

Because raster files are not used in the analysis phase, these functions were converted into comments in the Python script so they were non-functional to decrease the computational time and prevent creation of large unused raster files that would require checking or deletion.

2. The second modification was to remove the script used to create TVA storm data.

These events are the maximum probable precipitation (MPP) where the precipitation is used without the maximization step. The script to create these events was separate in the ArcGIS toolbox file and was deleted to prevent use of an undedicated script. This functionality is not anticipated for use under the current quality assurance program.

3. The third modification was to remove the computation to extrapolate values beyond the data set in each SPAS event file. The published PMP Evaluation Tool script allows creation of depth data values at areas beyond the provided depth-area-duration (DAD) data. In discussions with Applied Weather Associates, it was assumed any extrapolated value would be less than the value from another storm with data at that watershed size and should never control (i.e., represent a value that is used for the PMP at a grid point). Due to the large number of SPAS events and an infinite number of possibilities at the overlap points, it was not possible to confirm this condition did not exist. Therefore, the script was modified so that if the input area was larger than the largest DAD data area value, the extrapolation was allowed to run but the value is set to zero at the end of the calculation to prevent extrapolated data from being considered as a controlling event.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment A over each sub-basin. Individual sub-basin uniform cumulative rainfall depths at each selected duration were computed by dividing the volume of precipitation over each sub-basin by the sub-basin area to calculate the PMP cumulative depth-duration data.

The weighted average PMP depths computed within ArcGIS were compared to values calculated in QGIS to ensure that the ArcGIS software platform was operating correctly and with the required degree of precision. Confirmation of the ArcGIS analysis and computations following execution of the PMP Evaluation Tool was provided by direct comparison of computed sub-basin average depths for each storm type and duration for the PMP events with values developed in an alternate methodology that utilized QGIS. Rainfall depth absolute differences between the ArcGIS and QGIS results for individual sub-basins were found to be within one percent.

Local event type storms were computed but not used for evaluation of LIP events. Uniform PMP depth values for the BFN project site LIP specified in the Topical Report were used in calculations.

Storm types included in the PMP data were based on information provided in the Topical Report. General and Tropical event type data were calculated for the events. Review of the Topical Report indicates that the depth-area-duration data developed represent rainfall volume bounding conditions but do not identify all possible areal PMP event combinations that could occur. This means that a smaller (i.e., heavier rainfall) PMP event could occur as part of a larger area event as long as the larger event PMP volume is not exceeded. As a result, it would be possible to have a smaller embedded PMP event that would challenge an upstream project while the more uniform larger watershed PMP event would not impact the smaller project.

Therefore, rainfall data sets developed in the calculations were based on analysis of watersheds above and between modeled TVA projects. Table A-1 shows the 139 candidate events developed for individual and combined project watersheds. Event rainfalls were determined both to provide PMP depths over an entire project watershed and to generate PMP depths concentrated over combined watersheds between projects to allow evaluation of project overtopping potential and flood storage effects.

The Topical Report data shows that for the 20,780-square-mile watershed above Chickamauga, the Topical Report cumulative 72-hour General PMP event depth is 12.06 inches.

PMP Evaluation Tool Dedication The dedication of the PMP Evaluation Tool, which includes the use of associated Python scripts, was performed under the Barge Design Solutions (Barge) Nuclear Quality Assurance (QA) program that complies with 10 CFR 21, 10 CFR 50, Appendix B, ASME NQA 2008/2009 Addenda, and Regulatory Guide 1.28, Revision 4. The Barge QA program has been audited and accepted by TVA and is on the TVA Acceptable Supplier List (ASL). The software dedication report (SDR) for the PMP Evaluation Tool, including the use of associated Python scripts, describes the dedication of the PMP Evaluation Tool.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Att A 3 of 13

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment A SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Att A 4 of 13 While the ArcGIS and QGIS evaluation tools were not dedicated, the calculations performed using these tools were checked by either hand calculations or using alternative software in accordance with Barge procedures for design calculations and computer program applications, under the Barge QA program, which complies with NQA-1 Part II Subpart 2.7 Paragraph 202 and is consistent with similar TVA process control procedures under the TVA QA Program. The ArcGIS software functions that are outside of the functions included in the PMP Evaluation Tool SDR, as noted above, were checked using QGIS as the alternate software in accordance with NQA-1 Part II, Subpart 2.7, Paragraph 202 under the Barge QA Program. Additional information is provided in the Gridded PMP Development calculation and the SDR.

There are portions of the Python scripting that were outside of the PMP Evaluation Tool, but inside a GIS software. While the Microsoft Excel workbook and those portions of the Python scripting that were outside of the PMP Evaluation Tool were not software dedicated, their associated calculations were checked by either hand calculations or using alternative software as discussed above. Additional information is provided in the Gridded PMP Development calculation and the SDR, which are available for NRC review.

Reference A-1.

TVA Calculation, CDQ0000002016000044, Revision 0, Gridded Probable Maximum Precipitation Development, dated September 14, 2018

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment A SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5-Att A 7 of 13 Reference A-1.

TVA Calculation, CDQ0000002016000044, R0, Gridded Probable Maximum Precipitation Development, dated September 14, 2018.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Formulation of Candidate PMP Storms by Applying Nesting Methodology and Primary/Secondary Area of Interest (AOI)

Background

Previous National Weather Service (NWS) documents for estimating probable maximum precipitation (PMP) depths over the Tennessee River watershed have not been updated since publication in 1965 (HMR-41), 1973 (HMR-47), and 1986 (HMR-56). Topical Report TVA-NPG-AWA16-A includes data from recorded significant storm events through 2014 in addition to well documented, up-to-date analysis and application methodologies. Therefore, the gridded PMP data, consisting of the database of storm event depths, including the adjustment factors, the individual storm depth-area-duration (DAD) curves and the automated tool for determination of PMP depths at each of the evenly spaced (2.5km x 2.5km grid) 12,966 grid points above Wheeler Dam were considered the most appropriate rainfall data set for use in the Tennessee River watershed.

As discussed in the Topical Report, the point PMP depths determined using the gridded rainfall methodology represent a worst-case estimated rainfall for the historically largest observed storm events that are transposable to a given grid point for a user defined Area of Interest (AOI). The AOI is critical in defining PMP depths. The DAD relationship curve requires the input of an AOI area to define the PMP rainfall depth for each duration at each of the enclosed grid points. The application of current computer technology allows the combination of a selected AOI with transposed event DAD relationships to produce boundary condition point values representing a variety of PMP events over TVA project sub-watersheds that are independent of isohyetal patterns.

Application of the Topical Report gridded data requires an assessment of concurrent PMP events. Concurrent PMP events are defined as precipitation falling on an adjacent drainage outside that for which the PMP volume has been applied. An example of this would be applying the PMP volume for the 4,514-square-mile Douglas project basin as part of the PMP volume over the 17,310-square-mile BFN project. The 12,769-square-mile watershed between Browns Ferry Nuclear Plant (BFN) and Douglas Dams would be the concurrent PMP area. This type of event was postulated to be possible for PMP events on the furthest upstream tributary projects as well as PMP events localized over a downstream project with less than a PMP depth over projects in upstream concurrent areas. It is postulated that this type of event containing smaller area PMPs could produce controlling water surface elevations (WSE) at a downstream point in a multiple project system by challenging the capacity of an upstream project during that project PMP. Therefore, a nesting methodology was adopted for application of the gridded PMP data such that the gridded data is not utilized independently of the defined watershed DAD curves.

Definitions Nesting is defined as the occurrence of PMP events over smaller areas during a larger area PMP event. Nesting creates scenarios where PMP volumes are concentrated over AOIs to determine if a specific AOI characteristic (e.g., postulated upstream project overtopping failure, concentration of PMP volume nearer to the site, etc.) creates a more severe condition at a given SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 1 of 29

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 3 of 29 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Secondary areas are defined as sub-basins that are between the primary area limits and the next selected downstream or upstream project watershed boundary. An example of a downstream secondary area would be where the Cherokee watershed is selected as the primary (sub-basins 9 through 15), and the Fort Loudoun watershed (sub-basins 1 through 8 and 16 through 18) is selected as the secondary watershed as shown in Figure B-1. An example of an upstream secondary area would be where the Cherokee watershed between Cherokee Dam and Boone Dam (sub-basins 12 through 15) is selected as the primary AOI and sub-basins 9 through 11 would then be considered the secondary area (shown in Figure B-2) for the total watershed above Cherokee Dam.

Figure B Cherokee to Boone PMP as Primary with Upstream Secondary

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SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 5 of 29 project and at successive selected projects moving upstream or downstream as described.

4. Assumption: AOIs may be defined only by selecting the sub-basins between or upstream of TVA projects.

Technical Justification: An example of this is a valid Douglas AOI cannot be subbasins 3 through 6 but must include all sub-basins (1 through 6) above the Douglas project. Based on previous TVA work, maximum unregulated travel times between sub-basins and their respective downstream projects are less than 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months .

Because this travel time is small, and the TVA projects have active flood control operations, these effects will dampen WSE peaks. Because the PMP volume applied to any project watershed must be kept constant to meet the Topical Report criteria, differences between application of a PMP over a portion of the sub-basins immediate to the respective projects compared to application of the same volume distributed differently over the same sub-basins would be negligible.

Event Nesting Directly replacing the PMP values derived for a larger downstream project watershed with PMP values for a smaller upstream project would result in exceedance of the PMP volume for the total project watershed. To match the specified PMP volumes calculated using the Topical Report grid point data, the adjustment of secondary AOI PMP volumes is required to preserve the PMP volume for the primary and secondary nestings selected. This approach has the added advantage of allowing storm event simulation results to be directly compared at projects in the system where the PMP volume is preserved. This required development of a defined nesting sequence to preserve system PMP volumes.

PMP Scenarios The current PMF routing model includes 21 flood control projects operated by TVA upstream of and including Wheeler Dam that have the potential to impact TVA nuclear sites. Figures B-3 and B-4 present the geographic locations of projects in the current Tennessee River model.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Figure B Tennessee River System above Watts Bar Dam and Hiwassee River SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 6 of 29

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 7 of 29 Figure B Tennessee River System between Watts Bar and Wilson Dams

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 8 of 29 TVA Candidate Watersheds Review of the TVAs Tennessee River projects shows they were designed as an integrated flood control system to protect key points (e.g., Chattanooga) through differing project storage volumes, outflow capacities and operating curves. Because these project parameters are non-linear, project interactions under PMF conditions and resulting WSEs at the TVA nuclear sites are not intuitive. TVA has determined that the projects shown in Table B-1 have sufficient storage and are located where they have the potential to impact TVA nuclear sites.

Table B Projects with Potential to Impact TVA Nuclear Sites To identify the most severe, reasonably possible, hypothetical flood event to satisfy the regulatory design basis requirements, the impact of individual Table B-1 project PMPs as well as PMPs applied to combinations of interacting project watersheds must be reviewed for potential to produce maximum WSEs at the TVA nuclear sites.

Based on the definitions and assumptions, the 139 candidate watersheds were defined and are listed in Tables B-2a through B-2e.

T l 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

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 9 of 29 Table B-2a - Candidate Watersheds PMP Event Watershed Sub-basins in PMP Area Watershed 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-Nottely-Blue Ridge 7-8, 16-25, 27, 33-37, 40, 41, 43, 44A, 44B, 45 9,264.3 Chickamauga to Norris-Cherokee-Douglas-Fontana-Chatuge-Nottely-Blue Ridge 7-8, 16-18, 23-25, 27, 33-37, 40, 41, 43, 44A, 44B, 45 7,693.4 Chickamauga to Norris-Cherokee-Douglas-Fontana-Hiwassee-Blue Ridge 7-8, 16-18, 23-25, 27, 33-37, 41, 43, 44A, 44B, 45 7,128.3 Chickamauga to Norris-Cherokee-Douglas-Fontana 7-8, 16-18, 23-25, 27, 33-43, 44A, 44B, 45 8,328.4 Chickamauga to Norris-Cherokee-Douglas-Hiwassee-Blue Ridge 7-8, 16-25, 27, 33-37, 41, 43, 44A, 44B, 45 8,699.2 Chickamauga to Norris-Cherokee-Douglas 7-8, 16-25, 27, 33-43, 44A, 44B, 45 9,899.3 Chickamauga to Norris-Fort Loudoun-Chatuge-Nottely-Blue Ridge 19-25, 27, 33-37, 40, 41, 43, 44A, 44B, 45 7,684.2 Chickamauga to Norris-Fort Loudoun-Hiwassee-Blue Ridge 19-25, 27, 33-37, 41, 43, 44A, 44B, 45 7,119.1 Chickamauga to Norris-Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge 25, 27, 33-37, 40, 41, 43, 44A, 44B, 45 5,058.4 Chickamauga to Norris-Fort Loudoun-Tellico-Hiwassee-Blue Ridge 25, 27, 33-37, 41, 43, 44A, 44B, 45 4,493.3 Chickamauga to Norris-Fort Loudoun-Tellico 25, 27, 33-43, 44A, 44B, 45 5,693.4 Chickamauga to Watts Bar-Chatuge-Nottely-Blue Ridge 40, 41, 43, 44A, 44B, 45 2,852.3 Chickamauga to Watts Bar-Hiwassee-Blue Ridge 41, 43, 44A, 44B, 45 2,287.2

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 10 of 29 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Table B-2b - Candidate Watersheds PMP Event Watershed Basins in PMP Area Watershed Area (sq.mi.)

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

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 11 of 29 Table B-2c - Candidate Watersheds PMP Event Watershed Basins in PMP Area Watershed 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 Boone 1-8, 12-24, 38, 39, 42 10,970.4 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge 1-24, 38, 39, 42 12,809.6 Fort Loudoun-Tellico-Chatuge-Nottely-Blue Ridge to South-Holston-Watauga 1-8, 11-24, 38, 39, 42 11,638.1 Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Boone-Fontana 1-8, 12-18, 23, 24, 38-40, 42 9,964.6 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 Cherokee-Douglas-Fontana 7, 8, 16-18, 23-24, 38-39, 40, 42 3,835.1 Fort Loudoun-Tellico-Hiwassee-Blue Ridge to Cherokee-Douglas 7, 8, 16-24, 38, 39, 40, 42 5,405.9 Above Fort Loudoun-Tellico-Hiwassee-Blue Ridge 1-24, 38-40, 42 13,374.7 Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South Holston-Watauga-Fontana 1-8, 11-18, 23, 24, 38-40, 42 10,632.3 Fort-Loudoun-Tellico-Hiwassee-Blue Ridge to South Holston-Watauga 1-8, 11-24, 38-40, 42 12,203.2

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 12 of 29 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Table B-2d - Candidate Watersheds PMP Event Watershed Basins in PMP Area Watershed 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-Fontana 1-8, 11-18, 23, 24 9,432.2 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 Guntersville to Blue Ridge-Hiwassee-Fontana-Douglas-Cherokee-Norris 7-8, 16-18, 23-25, 27, 33-37, 41, 43, 44A, 44B, 45, 46, 47A, 47B, 48-50 10,799.6 Guntersville to Watts Bar-Hiwassee-Blue Ridge 41, 43, 44A, 44B, 45, 46, 47A, 47B, 48-50 5,958.5 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-Cherokee-Norris 7-8, 16-18, 23-25, 27, 33-37, 41, 43, 44A, 44B, 45, 46, 47A, 47B 8,200.4 Above Blue Ridge-Hiwassee-Fontana-Douglas-Cherokee-Norris 38-40, 42, 19-22, 1-6, 9-15, 26 13,652.5 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

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 13 of 29 Table B-2e - Candidate Watersheds PMP Event Watershed Basins in PMP Area Watershed 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-Douglas-Fontana 7,8,16-18, 23-25, 27, 33-40, 42 6,041.2 Watts Bar-Hiwassee-Blue Ridge to Norris-Cherokee-Douglas 7, 8, 16-25, 27, 33-40, 42 7,612.1 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 Wheeler to Norris-Cherokee-Douglas-Fontana 7-8, 16-18, 23-25, 27, 33-43, 44A, 44B, 45, 46, 47A, 47B, 48-65 17,140.4 Wheeler to Guntersville 51-58, 60-65 4,607.4 Wheeler to Tims Ford-Blue Ridge-Hiwassee-Fontana-Douglas-Cherokee-Norris 7-8, 16-18, 23-25, 27, 33-37, 41, 43, 44A, 44B, 45, 46, 47A, 47B, 48-50, 51-58, 60-65 15,407.0 Wheeler to Watts Bar-Hiwassee-Blue Ridge 41, 43, 44A, 44B, 45, 46, 47A, 47B, 48-65 11,099.2 Above Watauga-Douglas 1-6, 10 5,011.5 Above Watauga 10 468.2

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Event Nesting Sequence Based on the candidate watersheds identified and the assumptions listed, nesting sequences developed must meet the following criteria:

The primary AOI scenario may be a single project set of one or more sub-basins or any combination of adjacent projects set of sub-basins.

Secondary (concurrent) AOI scenarios must include the primary AOI sub-basins.

Once a sub-basin is included in a secondary AOI, it must be included in the subsequent project AOIs selected.

The simulations end with the downstream project representing the model boundary condition at Wheeler Dam.

Event Nesting Calculations The direct application of the smaller watershed PMP depths to larger project watersheds could produce combined total volumes in excess of the project PMP specified by the Topical Report.

An example of this is the selection of the total Cherokee project watershed as the primary AOI.

Above Cherokee (sub-basins 9 though 15), the respective PMP would be applied and the total volume above Cherokee Dam would be correct. However, this PMP volume is larger than the PMP volume specified for the Fort Loudoun-Tellico watershed over sub-basins 9 through 15.

Due to the inverse relationship between rainfall depth and watershed area in the DAD data, this condition would continue as analysis progresses downstream. Therefore, as discussed previously, secondary area volumes must be adjusted.

For a candidate project PMP event, the first step is to select the primary sub-basins of the project watershed. As defined, this primary AOI receives 100 percent of the PMP computed from the Topical Report grid points. If the primary AOI sub-basin(s) are not the entire watershed above the project, then the PMP depth data for each duration from the Topical Report for the entire watershed (i.e., primary and secondary sub-basin areas) above the project is also retrieved. To preserve the 100 percent PMP volume over the selected sub-basins, as well as ensure that the total PMP volume above the project AOI does not exceed the Topical Report PMP values, an adjustment multiplier is applied to the secondary AOI sub-basins for each rainfall duration. The calculation of this adjusted secondary area rainfall is performed using the following equations:

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 14 of 29

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 15 of 29 Vsx Asx Dtx

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

Asx = area of secondary sub-basin x (in sq.mi.)

Dtx = total AOI PMP rainfall depth for sub-basin x (in inches)

Vp x

Apx Dpx

(

)

where: Vp = rainfall volume over primary watershed (in sq.mi.*inches)

Apx = area of primary sub-basin x (in sq.mi.)

Dpx = primary watershed PMP rainfall depth for sub-basin x (in inches)

Vs x

Asx Dtx

(

)

where: Vs = rainfall volume over secondary watershed (in sq.mi.*inches)

Asx = area of secondary sub-basin x (in sq.mi.)

Dtx = total AOI PMP rainfall depth for sub-basin x (in inches)

Vt x

Ax Dtx

(

)

where: Vt = rainfall volume over total watershed (in sq.mi.*inches)

Ax = area of sub-basin x (in sq.mi.)

Dtx = total AOI PMP rainfall depth for sub-basin x (in inches) combining these relationships produces:

Dsx_final Dtx Vt Vp

Vs

where: Dsx_final = rainfall depth for secondary sub-basin x (in inches)

Dtx = total AOI PMP rainfall depth for sub-basin x (in inches)

Vt = rainfall volume over total watershed (in sq.mi.*inches)

Vp = rainfall volume over primary watershed (in sq.mi.*inches)

Vs = rainfall volume over secondary watershed (in sq.mi.*inches)

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B This process is repeated for each rainfall duration and as subsequent project watersheds in the model are added. As each subsequent project watershed is analyzed, the previous total AOI is now considered the primary basin, the additional sub-basins inside the AOI but outside the primary are now the secondary and the calculation process outlined above is repeated.

Adjustment calculations are identical for sub-basins outside the primary area (i.e., projects that are upstream of the next downstream project, but outside the primary area are considered secondary). Adjustments continue through subsequent AOIs until PMP rainfall data for the model sub-basins and durations have been calculated.

Simplified Nesting Example If the entire 1,901.94-square-mile watershed above Fort Patrick Henry (FP) is selected as primary AOI, the tool generates the sub-basin depths on line 1 in Table B-3.

Table B Fort Patrick Henry Example Processing the depths and sub-basin areas generates a 72-hour duration PMP for the 1,901.94-square-mile FP primary AOI of 15.43 inches. If the PMP model is being extended downstream to Cherokee (CR) Dam, secondary area rainfall depths are required. Using the Topical Report PMP Evaluation Tool, we can generate the 72-hour duration PMP depths for sub-basins 9 through 15 in the total 3,425.46-square-mile CR AOI shown on line 2. Processing the depths and sub-basin areas generates an area weighted average for the CR AOI of 14.80 inches, which is the bounding volume allowable for a PMP over the total CR watershed.

If the FP depths for primary sub-basins 9 through 12 are simply combined with the sub-basin 13 through 15 depths with CR as primary, the result is shown on line 3. Note that the resulting area weighted average of 15.03 inches exceeds the allowable PMP depth of 14.80 inches previously identified for the total CR watershed. Therefore, the secondary sub-basin 13 through 15 depths must be adjusted down to achieve the correct PMP volume for the total CR watershed. The adjustment factor of 0.96 on line 4 is calculated by subtracting the area weighted FP PMP volume from the CR PMP area weighted volume and dividing the result by the unadjusted area weighted volume for sub-basins 13 through 15. The adjustment factor calculation is shown in Table B-4.

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 16 of 29 Sub-basin #

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

PMP Depth (inches) 1 FP PMP Depths (inches) 14.30 18.74 14.34 15.07 1,901.94 15.43 2

CR PMP Depths (inches) 14.01 17.92 14.05 15.00 13.63 15.22 3,425.46 14.80 3

Unadjusted FP PMP at CR (inches) 14.30 18.74 14.34 15.07 13.63 15.22 3,425.46 15.03 4

Adjustment Factor = 0.96 5

Adjusted FP PMP at CR (inches) 14.30 18.74 14.34 15.07 13.15 14.68 3,425.46 14.80

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att B 17 of 29 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Table B Fort Patrick Henry Example Adjustment For the adjustment, the sub-basin 13 and 14 & 15 depths are multiplied by the adjustment factor producing 13.15-and 14.68-inch depths, respectively. The adjusted sub-basin PMP rainfalls for both the CR and FP AOIs now meet their respective bounding PMP volumes of 15.43 and 14.80 inches as shown on line 5 of Table B-3.

When the model extends further downstream, each subsequent project watershed is considered a secondary AOI, and sub-basin depths are adjusted using the same methodology to maintain the PMP volume for each successive watershed selected. If the primary AOI is a downstream project watershed, the upstream sub-basins are adjusted.

Note that, in this example, the volumes for the downstream PMPs at FP and CR do not produce a PMP volume above the South Holston or Watauga projects. Simply summing the upstream project PMP depths would exceed the DAD curve relationship at downstream projects. Project PMP volumes for those upstream areas would be reviewed under separate nesting combination scenarios. As a reference, Table B-5 shows the 72-hour PMP depths generated for the watersheds above each respective project (i.e., total project watershed is primary) when the Topical Report PMP Evaluation Tool is used.

Table B Example Total Project Watershed Depths Sub-basin #

9 10 11 12 13 14&15 Sub-basin Area (sq.mi.)

703.25 468.25 667.67 62.77 668.89 854.63 Volume (in sq.mi.-in.)

FP PMP Depths (inches) 14.30 18.74 14.34 15.07 29,350.73 CR PMP Depths (inches) 14.01 17.92 14.05 15.00 13.63 15.22 50,691.04 Unadjusted Secondary Depths (inches) 13.63 15.22 22,127.78 Adjustment Factor (50691.0411 - 29350.7335) / 22127.7772 = 0.96 PMP Event Area Sub-basins Area (sq.mi.)

72hr PMP Depth (inches)

Above South Holston 9

703.25 15.55 Above Watauga 10 468.25 20.25 Above Ft. Patrick Henry 9-12 1,901.94 15.43 Above Cherokee 9-15 3,425.46 14.80

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

Attachment B CNL-24-006 Figure B-6a - Visualization of GUH_01 Nesting 6(&85,7<5(/$7(',1)250$7,21+/-:,7++(/'81'(5&)5

($WWDFKPHQW% 19 RI29

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Attachment B 21 of 29 Conclusions The nesting process described allows the areal distribution of the gridded rainfall data in the Topical Report to challenge smaller project watersheds while maintaining the applied PMP Evaluation Tool volume above larger project watersheds of interest in the Tennessee River system model. The application includes a range of AOIs to be used as a guide in bracketing results of possible scenarios to identify bounding conditions. Review and comparison of resulting water surface elevations from multiple hydraulic routing model simulations confirm the corresponding selected PMP events generate a bounding condition WSE for locations of interest.

Reference B-1.

TVA Letter to NRC, CNL-19-040, 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),

dated May 21, 2019 (ML19155A047).

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment B Table B 72-hour Duration Rainfall for GUH 01 Scenario Note: Yellow highlighted columns represent the sub-basins in the Primary Area of Interest (POI). The other color highlighted groupings represent sub-basins in each Secondary Areas of Interest (SOI) starting below the POI and proceeding down the Tennessee River basin through Wheeler Dam.

CNL-24-006 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 E5 Att B 24 of 29

SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 Attachment B T-= a=b....

le"'""B=--.... 6 __ -.....a.=7 2 -hour Duration Rainfall for GUH 01 Scenario Sub-basin# and-+

60 Area (in sq.mi.)

Sto,m Event Sub-basin Sub-basin Total 293.4 Designation Sub-Basins in Watershed Area Step PMPAOI Watershed Rainfalls Held Rainfalls Adjusted

watershed Data type Weighted Average This Step This Step Arca (sq.mi.)

Rainfa 11 (inches)

I Above Hiwassee & Blue Ridge HIBR 38-40, 42 38-40, 42 1,200.1 Raw 22.48 Final 22.48 2

Above Fontana, Hiwassee, &

FNHIBR 38-40, 42, 19-22 38-40, 42 19-22

2. 770.9 Raw 21.83 Blue Ridge Final 21.83 3

Above Fort Loudoun-Tellico, FTHIBR 1-24, 38-40, 42 19-22, 38-40, 42 1-18, 23-24 13,374.7 Raw 13.93 Hiwassee, &. Blue Ridge Final

.13.93 4

Above \Valls Bar, Hiwassee, &

WBHIBR 1-40, 42 1-24, 38-40, 42 25-37 18,493.6 Raw 12.45 Blue Ridge Final 12.45 5

Above Chickamauga CH 1-45 1-40,42 41, 43-45 20,780.8 Raw 12.06 Final 12.06 6

Above Nickajack NJ 1-478 1-45 46-47B 21,852.9 Raw 11.99 Final 11.99 7

Above GunlersviJle GU 1-50 1-47B 48-50 24,452.1 Raw 11.83 Final 11.83 8

Above Wheeler WE l-65 1-50 5)- 65 29,592.8 Raw 11.47 12.22 Final 11.47 61 62 63 64 65 WE Rese,-voi,*

490.2 488.0 177.0 145.1 1381.1 89.7 12.43 12.50 12.73 12.70 12.96 12.98 Note: "Yellow" highlighted columns represent the sub-basins in the Primary Area of Interest (POI). The other color highlighted groupings represent sub-basins in each Secondary Areas of Interest (SOI) starting below the POI and proceeding down the Tennessee River basin through Wheeler Dam.

CNL-24-006 SECURITY RELATED INFORMATION -WITHHELD UNDER 10 CFR 2.390 ES Att B 29 of 29

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 1 of 7 SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C Applicability of TVA Submittals Responding to NRC Questions Relative to the Sequoyah LAR TS-19-02 (Reference C-1 and C-2)

The TVA submittals listed below provided responses to NRC questions relative to Reference C-1 and Reference C-2. Each of these submittal responses has been reviewed and the listed TVA Responses have been determined to be fully or partially applicable (as noted) to this BFN LAR.

Fully Applicable 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)(EPID L-2020-LLA-004), dated February 18, 2020 (ML20049H184)

TVA Letter to NRC, CNL-20-032, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Change to Hydrologic Analysis

- Response to Request for Additional Information (TS-19-02), dated May 14, 2020 (ML20135H067)

TVA Letter to NRC, CNL-20-057, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Change to Hydrologic Analysis

-Partial Response to Request for Additional Information (TS-19-02), dated August 12, 2020 (ML20225A170)

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 Change to Hydrologic Analysis (TS-19-02), dated June 15, 2021 (ML21203A335)

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 Change to Hydrologic Analysis - Correction of Submitted File Types Contained on Digital Versatile Disc (TS-19-02), dated August 13, 2021 (ML21239A115)

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), dated December 21, 2021 (ML22013A278)

Partially Applicable (As Noted)

TVA Letter to NRC, CNL-20-082, Partial Response to Request for Additional Information Regarding Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Change to Hydrologic Analysis (TS 02), dated November 10, 2020 (ML20328A093) (Note: applicable except for the TVA Responses to RAI 2.4-2, Item 3.b, RAI 2.4-3, RAI 2.4-4, and RAI 2.4-5 which are not applicable to BFN).

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 2 of 7 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 Change to Hydrologic Analysis - Second Partial Response to Additional Request for Additional Request for Additional Information (TS-19-02), dated August 22, 2022 (ML22251A139/ML22251A142) (Note: applicable except for the TVA Responses to RAI 2.4-2, Item 3.b, RAI 2.4-4, Items 1, 2, 3, and 5 and RAI 2.4-5 which are not applicable to BFN.)

For each of the exceptions note above, the following provides the TVA responses that are applicable to BFN:

RAI 2.4-2: PMP Input/Output Files Section 2.4.3, "PMP [Probable Maximum Precipitation] on Streams and Rivers" of the proposed, Updated Final Safety Analysis Report (UFSAR) revision present the revised flood hazard estimations at the plant site due to riverine flooding.

NUREG0800, "Standard Review Plan [SRP} for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR [Light-water Reactor] Edition," Revision 4, Section 2.4.3, "Probable Maximum Flood (PMF) on Streams and Rivers" (Agencywide Documents Access and Management System (ADAMS) Accession No. ML070730405), and Regulatory Guide (RG) 1.59, "Design Basis Floods for Nuclear Power Plants" (ADAMS Accession No. ML003740388), provide guidelines for estimating this flood causing mechanism. After reviewing the license amendment request (LAR) and enclosed documents, the NRC staff has determined that it needs additional information related to estimating this flood causing mechanism to independently verify the licensee's proposed change.

Therefore, the NRC staff requests all PMP input/output files be provided, as summarized in Figure No. 1 of "Response to NRC IQVB Request for Additional Information" (ADAMS Accession No. ML20135H067), including:

3. Non-QA GIS Software Tools - Alternate ArcGIS Calculations

b. ArcGIS sub-basin weighted average PMP depths for controlling PMF scenario TVA Response to RAI 2.4-2, Item 3.b 3.b. PMP Basin Depth following nesting volume adjustments for the BFN Controlling PMF is in calculation CDQ0000002017000064 Revision 001. The precipitation timeseries file for the controlling simulation is located in Appendix_A.zip, Appendix_A-A_PCAT.zip, in folder "GUH_01," in folder "00_Precip," and in the file "GUH_01_HourlyDistributions.csv." The total applied rainfall depth files are in the same "GUH_01" folder in folder "09_OutputGraphics" in the files "Total_Applied_Rainfall_Depths_Antecedent.csv" and "Total_Applied_Rainfall_Depths_Main.csv."

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 3 of 7 RAI 2.4:3: LIP Modeling Input/Output Flies Section 2.4.2, "Floods" of the proposed UFSAR revision presents the revised flood hazard estimations at the plant site due to LIP flooding. SRP Section 2.4.2, "Floods" (ADAMS Accession No. ML070100647), provides guidelines for estimating this flood causing mechanism. After reviewing the submitted LAR and enclosed documents, the NRC staff has determined that it needs additional Information related to estimating this flood causing mechanism to independently verify the licensee's proposed change. Therefore, the NRC staff requests all LIP modeling input/output files be provided, including:

LIP hydrodynamic model input/output files for simulating local intense precipitation surface runoff, routing, and flooding.

TVA Response to RAI 2.4-3 The LIP model input/output files for simulating LIP surface runoff, routing, and flooding are located in calculation CDQ0000002014000249, Revision 9, Appendices C, D, G, I, and K. The LIP rainfall depth input information used in the model is contained in calculation CDQ0000002016000041, Section 5.4.4, Table 13. CDQ0000002016000041 was previously provided to the NRC in CNL-20-082.

RAI 2.4-4: PMF Modeling Input/Output Files Section 2.4.3, "PMF Flood in Rivers and Streams" of the proposed UFSAR revision presents the revised flood hazard estimations at the plant site due to LIP flooding.

SRP Section 2.4.3 provides guidelines for estimating this flood causing mechanism. After reviewing the license amendment request (LAR) and enclosed documents, the NRC staff has determined that it needs additional information related to estimating this flood causing mechanism to independently verify the licensee's proposed change. Therefore, the NRC staff requests all modeling input/output files be provided for controlling PMF (with hydrologic dam failures) scenario, including:

1. Tabular data associated with the controlling primary and secondary area nesting sequence and the controlling PMP rainfall depth spatial distribution over the watershed" (LAR Enclosure, Section 2.4.3.1) as well as executable version of the tools (if any) used to distribute the gridded PMP values into subbasin space and time.

a. SQN LAR Enclosure 3, Figure 2.4.3-1 (BFN LAR Enclosure 3, Figure 14) b.

SQN LAR Enclosure 3, Figure 2.4.3-5a (BFN LAR Enclosure 3, Figure 13)

(as provided in SQN LAR Enclosure 4, Table 2.4.3-1 (BFN LAR Enclosure 3, Table 3))

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 4 of 7

2. Executable version of sub-basin unit hydrograph calculations (as referenced in SQN LAR Enclosure 4, Section 2.4.3.3 (BFN LAR Enclosure 4, Section 2.4, Appendix 2.4A, Subsection 6.3), as well as electronic version of data shown in SQN LAR Enclosure 3, Figure 2.4.3-6 (BFN LAR Enclosure 3, Figure 5).

3. Executable version of calculation files (if applicable) for estimating precipitation excess (as referenced in SQN LAR Enclosure 4, Section 2.4.3.2 (BFN LAR Enclosure 4, Section 2.4, Appendix 2.4A, Subsection 6.2)).

5. Executable version of the runoff and HEC-RAS model files, including HEC-RAS inputs such as calibrated geometry, unsteady flow rules and inflows (as reference in SQN LAR Enclosure 4, Section 2.4.3.3 (BFN LAR Enclosure 4, Section 2.4, Appendix 2.4A, Subsection 6.3)).

TVA Response to RAI 2.4-4, Items 1, 2, 3 and 5 1.

The process used to distribute the gridded PMP values is described in CDQ0000002017000080, Revision 002, Appendix B in Base Calc folder.

1.a. The tabular data associated with LAR Enclosure 3, Figure 14 is in calculation CDQ0000002017000064, Revision 001. The precipitation timeseries file for the controlling simulation is in "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," in folder "00_Precip," and in the file "GUH_01_HourlyDistributions.csv."

The total applied rainfall *depth files are in the same "GUH_01" folder, in folder "09_OutputGraphics," and in the files

Total_Applied_Rainfall_Depths_Antecedent.csv" and "Total_Applied_Rainfall_Depths_Main.csv."

1.b. The tabular data associated with LAR Enclosure 3, Table 3, is in calculation CDQ0000002017000064, Revision, 001. The total applied rainfall depth files for the controlling simulation are in "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," in folder "09_OutputGraphics," and in the files "Total_Applied_Rainfall_Depths_Antecedent.csv" and "Total_Applied_Rainfall_Depths_Main.csv." The runoff determined in the excess computation is in folder "GUH_01," in folder "01_APl_calculation," and in file "GUH_01_API_Outputs.dss." The subbasin areas used in the excess computation are in CDQ0000002016000044, Revision 000 in folder "Appendix B" and in file "Appendix_B_Reservoir_Analysis.xlsx." CDQ0000002016000044, Revision 000 was previously provided to the NRC in CNL-20-082 (ML20328A093).

2.

The sub-basin unit hydrographs were developed in calculations CDQ000020080056 through CDQ0000020080073, and CDQ000020080091 and were also provided for NRC review in the External Hazards Branch (EXHB) Audit for the SQN LAR that began on September 14, 2020. The unit hydrographs calculations for the sub-basins

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 5 of 7 above Chickamauga Dam also supported previous TVA submittals for WBN (Reference C-3) that were reviewed and approved by the NRC in Safety Evaluation issued by letter dated January 28, 2015 (Reference C-4). The unit hydrograph calculations for the sub-basins below Chickamauga Dam were also provided in the EXHB Audit for the SQN LAR because the Hydrologic Engineering Center-River Analysis System (HEC-RAS) model has been extended past Chickamauga Dam to Wheeler Dam below BFN. The unit hydrograph data were reformatted for use in the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) in calculation CDQ0000002016000047 and were also provided for NRC review in the EXHB Audit for the SQN LAR. The sub-basin unit hydrographs before reformatting are provided within Excel spreadsheets in calculation CDQ0000002016000047 in file "Appendix A.zip," "Appendix_A1_NUH_S-Graphs.xlsx." The sub-basin unit hydrographs which were reformatted are provided in Excel spreadsheets within calculation CDQ0000002016000047 in file "Appendix A.zip,"

"Appendix_A2_NUH_Adjusted_S-GraphsR3.xlsm." These Excel files provide electronic versions of the data shown in BFN LAR Enclosure 3, Figure 5. The application of the sub basin unit hydrographs for the controlling event are included in the PMP/PMF Calculation Automation Tool (PCAT) simulation located in calculation CDQ0000002017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," and in folder "02_HEC-HMS_unit_hydrographs."

3.

The process used to compute the precipitation excess is described in calculation CDQ0000002017000080, Revision 002, Appendix C in the Base Calc folder.

Precipitation Excess information for the controlling event is in CDQ0000002017000064, Revision 001 in "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," in folder "01_APl_calculation and in file GUH_01_API_Outputs.dss."

5.

The automation for the hydrologic and hydraulic processes was performed by PCAT and is described in CDQ0000002017000080, Revision 002 as listed below.

The process for rain-on-reservoir calculation is described in CDQ0000002017000080, Revision 002, Appendix E in the Base Calc folder.

Rain-on-reservoir for the controlling event is in CDQ2000000017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," and in folder "03_ROR_calculation."

The process for surface runoff and baseflow calculations is described in CDQ0000002017000080, Revision 002, Appendix F in the Base Calc folder.

Surface runoff and baseflow for the controlling event is in CDQ2000000017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01,"

and in folder "04_Surface_runnoff_scaling_and_baseflow."

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 6 of 7 The process for tributary inflow routing is described in CDQ0000002017000080, Revision 002, Appendix G in the Base Calc folder. Tributary inflow routing for the controlling event is in CDQ2000000017000064, Revision 001, "Appendix_A.zip,"

"Appendix_A-A_PCAT.zip," in folder "GUH_01," and in folder "05_Routing."

The process for inflow hydrograph distribution is described in CDQ0000002017000080, Revision 002, Appendix I in the Base Calc folder. Inflow hydrograph distribution for the controlling event is in CDQ2000000017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_ 01,"

and in folder "07 _lnflow_distribution."

The executable version of the HEC-RAS model for the controlling event is in CDQ2000000017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," and in folder "08_HEC-RAS.

The Controlling PMF Results Data is in CDQ2000000017000064, Revision 001, "Appendix_A.zip," "Appendix_A-A_PCAT.zip," in folder "GUH_01," and in folder "09_OutputGraphics."

RAI 2.4-5: Seismic Dam Failure Modeling Input/Output Files Section 2.4.4, Potential Dam Failures, Seismically Induced of the proposed UFSAR revision in the application presents the reviewed flood hazard estimations at the plant site due to seismic dam failure flooding. SRP Section 2.4.3 provides guidelines for estimating this flood causing mechanism. After reviewing the license amendment requirements (LAR) and enclosed documents, the NRC staff determined that it needs additional information related to estimating this flood causing mechanism to independently verify the licensees proposed change.

Therefore, the NRC staff requests all modeling input/output data files be provided for controlling seismic dam failure scenario, including

1. Hydrologic Engineering Center River Analysis System simulation setup, input and output files for four potentially critical seismic-flood event combinations (BFN LAR has three potentially critical seismic-flood event combinations)(as referenced in SQN LAR Enclosure 4, Section 2.4.4.2 (BFN LAR Enclosure 4, Appendix 2.4A, Section 7.0, Floods Due to Seismically Induced Dam Failures)).

2. Calculations for 25-year and 500-year flood inflows (as referenced in SQN LAR, Section 2.4.4.1 (BFN LAR Enclosure 4, Appendix 2.4A, Section 7.0, Floods Due to Seismically Induced Dam Failures).

SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 Attachment C SECURITY RELATED INFORMATION - WITHHELD UNDER 10 CFR 2.390 CNL-24-006 E5 Att C 7 of 7 TVA Response to RAI 2.4-5

1. The Seismic HEC-RAS files for the input and output files for the three potentially critical seismic-flood event combinations (as referenced in the BFN LAR Enclosure 4, Appendix 2.4A, Section 7.0, Floods Due to Seismically Induced Dam Failures) are located in CDQ0000002014000024, Revision 006, Seismic Dam Failure Simulations Calculation, Appendix_I.zip, Appendix_J.zip, and Appendix_K.zip.

CDQ0000002014000024, R6 was previously provided to the NRC in CNL-22-053 (ML22251A139/ ML22251A142).

2. The 25-year and 500-year flood inflow calculations (as referenced in BFN LAR, Floods Due to Seismically Induced Dam Failures) are contained in calculations, CDQ0000002014000029, Inflow Volume Analysis for Seismic Events Calculation, and CDQ0000002014000030, Inflow Hydrograph Development for Seismic Events. CDQ0000002014000029 and CDQ0000002014000030 were previously provided to the NRC in CNL-20-082 (ML20328A093).

References:

C-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), dated January 14, 2020 (ML20016A396 and ML20016A397).

C-2. TVA Letter to NRC, CNL-23-012, Revision to the Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, dated April 11, 2023 (TS-19-02)

(EPID-2020-LLA-0004) (ML23101A179).

C-3. TVA Letter to NRC, Application to Revise Watts Bar Nuclear Plant, Unit 1 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, TAC No.

ME8300 (BFN-UFSAR-12-01), dated July 19, 2012 (ML122360173).

C-4. Letter from NRC to TVA, Watts Bar Nuclear Plant, Unit 1 - Issuance of Amendment to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrology Analysis (TAC No. ME9130), dated January 28, 2015 (ML15005A314).

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CNL-24-006 E6 - 1 of 2 AFFIDAVIT

1. My name is Kimberly D. Hulvey. I am General Manager, Nuclear Regulatory Affairs &

Emergency Preparedness, for Tennessee Valley Authority (TVA), and as such I am authorized to execute this Affidavit. I am familiar with TVAs Application to Revise Browns Ferry Nuclear Plant Units 1, 2 and 3 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.

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). This submittal 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. 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.

CNL-24-006 E6 - 2 of 2

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

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

8. The information in Enclosures 2, 3, and 4 contain CEII to an extent that a redacted version would be of no value to the public; thus, a redacted version has not been submitted.

9. The foregoing statements are true and correct to the best of my knowledge. I declare under penalty of perjury that the foregoing is true and correct.

Executed on this 12th day of February 2025.

Kimberly D. Hulvey General Manager, Nuclear Regulatory Affairs & Emergency Preparedness Digitally signed by Edmondson, Carla Date: 2025.02.12 10:53:53 -05'00'

v t e Letter Sequence Request
EPID:L-2025-LLA-0034, (Open)
Initiation Request Acceptance Supplement Administration Questions Answers Results Approval Other: ML25345A013
MONTHYEAR2025-03-19 [Table View]

CNL-24-006, Application to Revise Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (BFN-TS-550)

Text

Subject:

Reference:

Subject:

SUMMARY

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

6.0 REFERENCES

SUMMARY

2.1 BACKGROUND

2.3 PROPOSED CHANGE

3.0 TECHNICAL EVALUATION

3.4 CONCLUSION

4.0 REGULATORY EVALUATION

4.4 CONCLUSION

5.0 ENVIRONMENTAL CONSIDERATION

6.0 REFERENCES

Background

References:

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