CNL-15-042, Flood Hazard Reevaluation Report for Response to NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3 and 9.3 of the Near-Term Task Force Review.

Flood Hazard Reevaluation Report for Response to NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3 and 9.3 of the Near-Term Task Force Review.

ML15071A462
Person / Time
Site:	Sequoyah
Issue date:	03/12/2015
From:	James Shea Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CNL-15-042, L44 150312 002
Download: ML15071A462 (76)

Text

L44 150312 002 Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-15-042 March 12, 2015 10 CFR 50.54(f)

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Unit 1 and Unit 2 Facility Operating License No. DPR-77 and DPR-79 NRC Docket No. 50-327 and 50-328

Subject:

Flood Hazard Reevaluation Report for Sequoyah Nuclear Plant, Response to NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3 and 9.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident

References:

1. NRC Letter, "Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated March 12, 2012 (ML12053A340)

2. NRC Letter, "Prioritization of Response Due Dates for Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Flooding Hazard Reevaluations for Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated May 11 , 2012 (ML12097A509)

3. Letter from TVA to NRC, "Tennessee Valley Authority (TVA)- Extension Request Regarding the Flooding Hazard Reevaluation Report Required by NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendation 2.1, Flooding ,

of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated March 12, 2013 (ML13080A073)

U.S. Nuclear Regulatory Commission CNL-15-042 Page 2 March 12, 2015

4. Letter from TVA to NRC, "Revised Commitments for the Extension Request Regarding the Flooding Hazard Reevaluation Report Required by NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendation 2.1, Flooding ,

of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated May 9, 2013 (ML13133A004)

5. Letter from NRC to TVA, " Sequoyah Nuclear Plant, Units 1 and 2, and Watts Bar Nuclear Plant, Units 1 and 2 - Relaxation of Response Due Dates Regarding Flooding Hazard Reevaluations for Recommendation 2.1 of the Near-Term Task Force Review of the Insights from the Fukushima Dai-ichi Accident ," dated July 1, 2013 (ML13163A296)

6. Letter from NRC to NEI , "Trigger Conditions for Performing an Integrated Assessment and Due Date for Response ," dated December 3, 2012 (ML12326A912)

7. Letter from TVA to NRC, "Tennessee Valley Authority (TVA)- Fleet Response to NRC Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding the Flooding Walkdown Results of Recommendation 2.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated November 27, 2012 (ML12335A340)

On March 12, 2012, the NRC issued Reference 1 to all power reactor licensees and holders of construction permits in active or deferred status. Enclosure 2 of Reference 1 requested that each licensee perform a reevaluation of external flooding sources and report the results in accordance with the NRC's prioritization plan (Reference 2).

The report due date established for Sequoyah Nuclear Plant (SON) , Units 1 and 2, was March 12, 2013. Due to the amount of time required to complete evaluation of upstream and downstream dams to fully assess the flood hazard for the SON site, SON submitted References 3 and 4 requesting an extension of the due date from March 12, 2013 to March 12, 2015. The NRC authorized the due date extension in Reference 5.

The purpose of this letter is to provide the Flooding Hazard Reevaluation Report (HRR) for SON , Units 1 and 2. Specifically, Enclosure 1 of this letter provides the SON Flooding HRR.

The enclosed flooding HRR describes the approach, methods and results from the reevaluation of flood hazards at SON, Units 1 and 2. The eight flood-causing mechanisms, and a combined effect flood identified in Attachment 1 to Enclosure 2 of Reference 1, are described in the report along with the potential effects on SON , Units 1 and 2.

The Flooding HRR shows that some flood levels exceed the Current Licensing Basis (CLB) levels. The increased levels are the results of newer methodologies and guidance which typically exceed the methodologies and guidance which were used to establish the CLB for existing plants.

U.S. Nuclear Regulatory Commission CNL-15-042 Page 3 March 12, 2015 In accordance with Reference 6, an Integrated Assessment (lA) is required if flood levels determined during the flood hazard reevaluation are not bounded by the CLB flood levels. of Reference 1 specifies that the lA be completed and a report submitted within two years of submitting the Flooding HRR. An lA will be completed and a report submitted no later than March 12, 2017 .

As discussed in Reference 1, the NRC stated that the current regulatory approach , and the resultant plant capabilities, gave the NRC the confidence to conclude that an accident with consequences similar to the Fukushima accident is unlikely to occur in the United States.

The NRC concluded that continued plant operations and the continuation of licensing activities did not pose an imminent risk to public health and safety. The flooding walkdowns for SON CLB flood protection features have been performed and the results were provided in Reference 7. The flood walkdowns verified that the flood protection systems for SON are available, functional and implementable and any degraded or nonconforming flood protection features were entered in TVA's Corrective Action Program .

Section 12 of the Flooding HRR provides a discussion of the interim actions taken or planned to address the higher flooding levels relative to the CLB flood levels. These actions will enhance the current capability to maintain the plant in a safe condition during the beyond-design-basis external flooding events that exceed the CLB flood levels and as a result, continued plant operation does not impose an imminent risk to the public health and safety while completing the lA.

In parallel with development of the required lA for SON , TVA is continuing with several additional actions associated with understanding and mitigating potential flood hazards.

Specifically, TVA continues to develop the Flood Mode Mitigation System for SON as described in TVA letters to the NRC dated April16 , 2013 (ML13108A107) and July 1, 2013 (ML13189A135) and as described in periodic updates to the NRC. In addition, TVA is developing updates to site specific precipitation data using advanced techniques and may engage in further dialogue with the NRC in the future .

Enclosure 2 of this letter provides a list of new regulatory commitments.

If you have any questions regarding this submittal, please contact Beth Wetzel at (423) 751-2403.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 12th day of March 2015.

Respectfully,

et::~~ 9W~~

Vice President, Nuclear Licensing

Enclosures:

cc: See Page 3

U.S. Nuclear Regulatory Commission CNL-15-042 Page 4 March 12, 2015 Enclosures

1. Near-Term Task Force (NTTF)- Recommendation 2.1 Mitigating Strategies Flood Hazard Evaluation Report for Sequoyah Nuclear Plant

2. List of Commitments cc (Enclosures):

NRR Director- NRC Headquarters NRO Director- NRC Headquarters NRR JLD Director- NRC Headquarters NRC Regional Administrator- Region II NRC Project Manager- Sequoyah Nuclear Plant NRC Senior Resident Inspector - Sequoyah Nuclear Plant

ENCLOSURE 1 NEAR-TERM TASK FORCE (NTTF)- RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT FOR SEQUOYAH NUCLEAR PLANT

NEAR-TERM TASK FORCE (NTTF) - RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT Response to United States Nuclear Regulatory Commission (USNRC) -

Code of Federal Regulations 10 CFR Part 50, Section 50.54 (f)

Sequoyah Nuclear Plant Tennessee Valley Authority March 12, 2015 Revision 2

TABLE OF CONTENTS 1 PURPOSE ....................................................................................................................................5 1.1 Requested Actions ........................................................................................................................ 5 1.2 Requested Information ................................................................................................................. 5 2 BACKGROUND ............................................................................................................................6 3 PLANT SITE DESCRIPTION ............................................................................................................8 3.1 Current Site Layout ....................................................................................................................... 8 3.2 Site Topography ............................................................................................................................ 8 3.3 Bathymetry in Vicinity of Plant - Tennessee River ....................................................................... 8 3.4 Current Design Basis Flood Elevations ........................................................................................ 15 3.4.1 Local Intense Precipitation.................................................................................................. 15 3.4.2 Flooding from Rivers and Streams ...................................................................................... 15 3.4.3 Flooding from Dam Breaches or Failures ............................................................................ 15 3.4.4 Flooding from Storm Surge and Seiche............................................................................... 16 3.4.5 Flooding from Tsunami ....................................................................................................... 16 3.4.6 Flooding from Ice-Induced Events ...................................................................................... 16 3.4.7 Channel Migration or Diversion .......................................................................................... 16 3.4.8 Flooding from Combined Effects ........................................................................................ 16 3.5 Current Flood Protection and Mitigation Features..................................................................... 17 3.5.1 Dam and Reservoir System ................................................................................................. 17 3.5.2 Sequoyah Nuclear Site Protective Structures ..................................................................... 17 3.5.3 TVA River Operations Forecasting and Warning................................................................. 18 3.5.4 SQN Flood Response Procedures ........................................................................................ 19 4 CHRONOLOGY OF FLOOD RELATED CHANGES SINCE LICENSING ................................................. 20 4.1 Watershed Changes since Licensing ........................................................................................... 20 4.2 Summary of Changes to Design Basis Flood Elevations .............................................................. 22 4.2.1 Local Intense Precipitation.................................................................................................. 22 4.2.2 Flooding in Rivers and Streams ........................................................................................... 22 5

SUMMARY

OF PLANT WALKDOWN RESULTS AND MODIFICATIONS............................................ 31 5.1 Diesel Generator Building Flood Protection Barriers.................................................................. 31 5.2 Spent Fuel Pit Cooling Pump Enclosure ...................................................................................... 31 5.3 ERCW Intake Pumping Station Flood Protection Barriers........................................................... 31 5.4 Conduit Penetration Seals........................................................................................................... 32 5.4.1 ERCW Intake Pumping Station Conduit Penetration Seals ................................................. 32 5.4.2 Shield Building Conduit Penetration Seals .......................................................................... 32 5.5 Diesel Generator Drain Plugs ...................................................................................................... 32 5.6 Flood Mode Preparation Improvements .................................................................................... 32 6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS ................................................ 33 6.1 Local Intense Precipitation.......................................................................................................... 33 6.2 Flooding from Rivers and Streams .............................................................................................. 33 6.3 Flooding from Dam Breaches or Failures .................................................................................... 33 6.3.1 Project Specific PMF............................................................................................................ 33 6.3.2 Sunny Day Failure of Upstream Dams ................................................................................ 34 6.3.3 Seismic Failure of Upstream Dams ..................................................................................... 34 6.3.4 Sediment Transport ............................................................................................................ 34 6.4 Flooding from Storm Surge and Seiche....................................................................................... 34 6.5 Flooding from Tsunami ............................................................................................................... 36 6.6 Flooding from Ice-Induced Events .............................................................................................. 36 6.7 Channel Migration and Diversion ............................................................................................... 36 Page 2 of 70

6.8 Flooding from Combined Effects ................................................................................................ 36 6.8.1 Floods Caused by Precipitation Events ............................................................................... 37 6.8.2 Floods Caused by Seismic Dam Failures.............................................................................. 37 7 DESCRIPTION OF MODELS USED FOR REEVALUATION ................................................................ 38 7.1 HEC-RAS ...................................................................................................................................... 38 7.1.1 Description of HEC-RAS Model Verification ........................................................................ 38 7.1.2 Description of HEC-RAS Model Extents............................................................................... 38 7.2 HEC-HMS ..................................................................................................................................... 41 7.2.1 Description of HEC-HMS Model Verification ...................................................................... 41 7.2.2 Description of HEC-HMS Model Extents ............................................................................. 41 8 JUSTIFICATION OF INPUTS ........................................................................................................ 42 8.1 HEC-RAS Model Geometry Development and Calibration ......................................................... 42 8.2 Dam Rating Curves ...................................................................................................................... 42 8.3 Unsteady Flow Rules ................................................................................................................... 42 8.4 Probable Maximum Flood Inflows .............................................................................................. 43 8.4.1 Hydrometeorological Report .............................................................................................. 43 8.4.2 Critical Storm Selection ....................................................................................................... 44 8.4.3 National Inventory of Dams (NID) Inflows .......................................................................... 45 8.5 Seismic Inflows ............................................................................................................................ 45 8.5.1 National Inventory of Dams (NID) Seismic Inflows ............................................................. 45 8.6 Sunny Day and Watauga Project Specific PMF Inflows............................................................... 45 9 APPLICABLE FLOOD CAUSING MECHANISMS ............................................................................. 46 9.1 Local Intense Precipitation.......................................................................................................... 46 9.1.1 Previous Analysis................................................................................................................. 46 9.1.2 Technical Approach ............................................................................................................. 46 9.2 Flooding from Rivers and Streams .............................................................................................. 49 9.2.1 Previous Analysis................................................................................................................. 49 9.2.2 Technical Approach ............................................................................................................. 49 9.3 Flooding from Dam Breach or Failures ....................................................................................... 51 9.3.1 Project Specific PMF............................................................................................................ 52 9.3.2 Sunny Day Dam Failure ....................................................................................................... 52 9.3.3 Single Seismic Dam Failure.................................................................................................. 53 9.4 Flooding from Combined Effects ................................................................................................ 53 9.4.1 Floods Caused by Precipitation Events ............................................................................... 53 9.4.2 Multiple Seismic Dam Failures with Combined Flood Event .............................................. 55 10 EVALUATION OF UNCERTAINTIES .............................................................................................. 58 10.1 100% Runoff ................................................................................................................................ 58 10.2 Peaked and Lagged Unit Hydrographs ........................................................................................ 58 10.3 Gate Operability/Blockage .......................................................................................................... 59 10.4 Breach Size .................................................................................................................................. 60 10.5 Initial Reservoir Level .................................................................................................................. 60 11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS ................... 61 12 IDENTIFICATION AND EVALUATION OF ANY INTERIM ACTIONS TAKEN TO MITIGATE HIGHER FLOOD HAZARD RELATIVE TO DESIGN BASIS ..................................................................................... 62 12.1 Local Intense Precipitation.......................................................................................................... 62 12.2 Flooding in Rivers and Streams ................................................................................................... 66 12.3 Combined Effects Flood Caused by Precipitation Events ........................................................... 66 13 References ............................................................................................................................... 67 Page 3 of 70

FIGURES Figure 2-1 River System Schematic .............................................................................................................. 7 Figure 3-1 Sequoyah Nuclear Plant ............................................................................................................... 9 Figure 3-2 Sequoyah Nuclear Plant Site Layout .......................................................................................... 10 Figure 3-3 Sequoyah Nuclear Plant Topography ........................................................................................ 11 Figure 3-4 Location of Tennessee River Cross-Sections.............................................................................. 12 Figure 3-5 Bathymetry - Tennessee River in the Vicinity of the Sequoyah Nuclear Plant ......................... 13 Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam ................................ 20 Figure 4-2 Timeline of Flood Related Changes Since Licensing .................................................................. 22 Figure 6-1 Landslide Incidence Map of United States ................................................................................ 35 Figure 7-1 Upper HEC-RAS Model Extents .................................................................................................. 39 Figure 7-2 Lower HEC-RAS Model Extents .................................................................................................. 39 Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents ................................................................. 40 Figure 9-1 Local Intense Precipitation Storage Areas ................................................................................. 48 Figure 12-1 Access Doors to Auxiliary and Control Buildings from 706 Elevation ..................................... 63 TABLES Table 3.4-1 Combined Effects of Flood and Wind ...................................................................................... 16 Table 4.1-1 Land Use above Guntersville (2001 - 2011) ............................................................................ 21 Table 4.1-2 Impervious Area above Guntersville (2001 - 2011) ................................................................ 21 Table 4.2-1 Dam Modifications Completed by 1998 (Reference 13).......................................................... 26 Table 6-5.6-1 Potential Flood Causing Mechanisms or Causal Phenomena ............................................... 33 Table 9.1-1 Results of SQN LIP Analysis ...................................................................................................... 47 Table 9.2-1 PMF Elevations at SQN (TRM 484.5) Resulting from Reevaluation ......................................... 51 Table 9.2-2 PMF Elevations at SQN (TRM 484.5) Including Emergency Action Plan at Douglas Dam ........ 51 Table 9.3-1 Elevation and Discharge at SQN Resulting from Project Specific Dam Failures ...................... 52 Table 9.3-2 Elevations and Discharges at SQN Resulting from Sunny Day Dam Failures ........................... 53 Table 9.3-3 Elevations and Discharges at SQN Resulting from Single Seismic Failure of Upstream Dams 53 Table 9.4-1 Wind Wave Elevation Results at Dams .................................................................................... 54 Table 9.4-2 Wind Wave Elevation Results at Critical Structures ................................................................ 55 Table 9.4-3 Elevations and Discharges at SQN Resulting from Seismic Failure of Upstream Dams ........... 57 Table 11-1 Comparison of Current Design Basis Elevations and Reevaluation Results .............................. 61 Page 4 of 70

1 PURPOSE In response to the accident at the Fukushima Dai-ichi nuclear power plant resulting from the March 11, 2011, Great Tohoku Earthquake and subsequent tsunami, the United States Nuclear Regulatory Commission (NRC) established the Near Term Task Force (NTTF) to conduct a systematic and methodical review of NRC processes and regulations, and to make recommendations to the Commission for its policy direction. The NTTF reported a set of recommendations that were intended to clarify and strengthen the regulatory framework for protection against natural phenomena.

On March 12, 2012, the NRC issued an information request pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) herein after referred to as the 50.54(f) letter which included six enclosures.

(Reference 1)

In Enclosure 2 of the 50.54(f) letter, the NRC requests that licensees reevaluate the flooding hazards at their sites. (Reference 1)

This report provides the information for Sequoyah Nuclear Plant (SQN) requested in the 50.54(f) letter; specifically, the information listed under the Requested Information section of Enclosure 2, paragraph 1 (a through e). The Requested Information section of Enclosure 2, paragraph 2 (a through d), Integrated Assessment Report, will be addressed separately.

1.1 Requested Actions Per Enclosure 2 of the 50.54(f) letter, addressees are requested to perform a reevaluation of all appropriate external flooding sources. The reevaluation applies present-day methodologies and regulatory guidance, supplemented with interim staff guidance developed for review of the reevaluations. This includes current techniques, software, and methods used in present-day standard engineering practice to develop the flood hazard. The requested information is gathered in Phase 1 of the NRC staffs two phase process to implement Recommendation 2.1, and is used to identify potential vulnerabilities.

For the sites where the reevaluated flood exceeds the design basis, addressees are requested to submit an interim action plan that documents actions planned or taken to address the reevaluated hazard. Subsequently, addressees should perform an integrated assessment of the plant to identify vulnerabilities and actions to address them.

1.2 Requested Information Per Enclosure 2 of the 50.54(f) letter, the final Hazard Reevaluation Report should document results, as well as pertinent site information and detailed analysis, including the following:

a. Site information related to the flood hazard, including relevant structures, systems, and components (SSCs) important to safety and the ultimate heat sink (UHS). Pertinent data concerning SSCs and the UHS should be included. Other relevant site data includes the following:

1. Detailed site information (both designed and as-built), including present-day site layout, elevation of pertinent SSCs important to safety, site topography, and pertinent spatial and temporal data sets;

2. Current design basis flood elevations for all flood causing mechanisms;

3. Flood-related changes to the licensing basis and any flood protection changes (including mitigation) since license issuance; Page 5 of 70

4. Changes to the watershed and local area since license issuance;

5. Current licensing basis flood protection and pertinent flood mitigation features at the site;

6. Additional site details, as necessary, to assess the flood hazard (i.e., bathymetry, walkdown results, etc.).

b. Evaluation of the flood hazard for each flood causing mechanism, based on present-day methodologies and regulatory guidance including an analysis of local intense precipitation (LIP) and site drainage, flooding in streams and rivers, dam breaches and failures, storm surge and seiche, tsunami, ice-induced flooding, channel migration or diversion, sediment transport, and combined effects. Mechanisms that are not applicable at the site may be excluded with appropriate justification. Evaluation will provide a basis for inputs and assumptions; methodologies; and models used.

c. Comparison of current and reevaluated flood causing mechanisms at the site. Provide an assessment of the current design basis flood elevation to the reevaluated flood elevation for each flood causing mechanism. Include how the findings from Enclosure 4 of the 50.54(f) letter (i.e., Recommendation 2.3 flooding walkdowns) support this determination. If the current design basis flood bounds the reevaluated hazard for all flood causing mechanisms, include how this finding was determined.

d. Interim evaluation and actions taken or planned to address any higher flooding hazards relative to the design basis, prior to completion of the integrated assessment described below, if necessary.

e. Additional actions beyond Requested Information item 1.d taken or planned to address flooding hazards, if any. (Reference 1) 2 BACKGROUND The SQN site is located on a peninsula on the west bank of Chickamauga Reservoir at Tennessee River Mile (TRM) 484.5 with plant grade at elevation 705 ft. as shown in Figure 2-1. The Tennessee River above SQN site drains 20,650 square-miles. Watts Bar Dam, 45.4 river miles upstream, has a drainage area of 17,310 square-miles. Chickamauga Dam, the next dam downstream, has a drainage area of 20,790 square-miles. The watershed is about 70 percent forested with much of the mountainous area being 100 percent forested. (Reference 2)

There are 17 major TVA dams (Apalachia, Blue Ridge, Hiwassee, Chatuge, Nottely, South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, and Watts Bar) in the TVA system upstream from SQN. Figure 2-1 presents a simplified flow diagram for the Tennessee River system. In addition, there are six major dams not owned by TVA (Mission (not shown),

Calderwood, Chilhowee, Santeetlah, Cheoah, and Nantahala Dams). These reservoirs often contribute to flood reduction, but they do not have dependable reserved flood detention capacity. (Reference 2)

Page 6 of 70

Figure 2-1 River System Schematic Page 7 of 70

3 PLANT SITE DESCRIPTION 3.1 Current Site Layout The SQN is located in Hamilton County, Tennessee on a peninsula on the western shore of Chickamauga Reservoir at Tennessee River Mile (TRM) 484.5 with plant grade elevation at 705 ft.

The site is approximately 45.4 river miles south of Watts Bar Dam (TRM 529.9) and approximately 7.5 miles northeast of the nearest city limit of Chattanooga, Tennessee. The location of SQN is shown in Figure 3-1 and Figure 3-2. Details of the current site layout and plant structures are shown in Figure 3-2. (Reference 2) 3.2 Site Topography The Sequoyah Nuclear plant site comprises approximately 525 acres on a peninsula on the west bank of Chickamauga Reservoir. As shown in Figure 3-3, the site is on high ground with the Tennessee River being the major potential source of flooding. (Reference 2) 3.3 Bathymetry in Vicinity of Plant - Tennessee River Between April 2007 and June 2008, the U.S. Army Corps of Engineers (USACE) conducted a hydrographic survey of Chickamauga Reservoir. This survey includes only the part of the reservoir that was below the water-surface at the time of the survey and was focused primarily on the navigation channel. A Triangular Irregular Network (TIN) was constructed from the USACE Hydrographic Survey data. Using this TIN, cross-sections were cut at various locations on the Tennessee River in Chickamauga Reservoir in the vicinity of SQN, as shown in Figure 3-4. The actual cross-sections taken at these locations are shown in Figure 3-5. As shown, the depth of the river in the vicinity of SQN ranges between 50 and 57 feet at normal summer pool elevation, 682.5 ft. (Reference 3)

Page 8 of 70

Figure 3-1 Sequoyah Nuclear Plant Page 9 of 70

Figure 3-2 Sequoyah Nuclear Plant Site Layout BWSCI1:::~...

Date 24 January 2012 Page 10 of 70

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Figure 3-3 Sequoyah Nuclear Plant Topography "'-" Contour Nonh .trnericaoDatum 1983 Date: 11 Jan uary 2012 Page 11 of 70

Figure 3-4 Location of Tennessee River Cross-Sections Page 12 of 70

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Sequoyah Nuclear Plant TRM 484.5 Bathymetry Tennessee River in the Vicinity of Sequoyah Nuclear Plant (Sheet 1 of 2) 500 1000 1500 2000 2500 30CO 3500 LOOO 4500 5000 St11tlon{ftj Figure 3-5 Bathymetry - Tennessee River in the Vicinity of the Sequoyah Nuclear Plant Page 13 of 70

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Bathymetry Tennessee River in the Vicinity of Sequoyah Nuclear Plant (Sheet 2 of 2)

Figure 3-5 Bathymetry - Tennessee River in the Vicinity of the Sequoyah Nuclear Plant (2 of 2)

Page 14 of 70

3.4 Current Design Basis Flood Elevations On August 10, 2012, TVA submitted a revised hydrological licensing basis for the SQN (Reference

4) for flooding on the Tennessee River and its tributaries. In the balance of this report, this revised hydrological licensing basis, which has not been approved by the NRC, will be referred to as the SQN 2012 LAR and, for the purposes of this report, will be considered as the SQN hydrological Current Licensing Basis (CLB). The exception is the evaluation of Local Intense Precipitation (LIP). The LIP analysis was revised in December 2014 (SAR Change Package No. 26-06, S10150106802) under the 10CFR50.59 process and is included in the SQN Living Final Safety Analysis Report (FSAR).

3.4.1 Local Intense Precipitation The effects of LIP are evaluated in Section 2.4.3.1 and 2.4.3.5 of the SQN Living FSAR. For the probable maximum precipitation (PMP), Hydrometeorological Report 56 (HMR 56, Reference 5) is used to define the storm event. The probable maximum storm used to test the adequacy of the local drainage system produces a maximum one-hour depth of rainfall is 16.21 inches. Three different temporal distributions were applied to the model with peak intensity shifted between early, middle and late occurrence. Structures housing safety-related facilities, systems and equipment are protected from flooding during the LIP by the slope of the plant yard. The yard is graded such that the surface runoff will be carried to the Chickamauga Reservoir. In the analysis of the LIP, underground drains were assumed clogged and runoff was assumed to equal rainfall.

The computed maximum surface elevation of 705.7 ft. is at or below the critical floor elevation of 706 ft. (Reference 6) 3.4.2 Flooding from Rivers and Streams In the SQN 2012 LAR, TVA submitted a revised hydrological licensing basis for SQN (Reference 4) for flooding on the Tennessee River and its tributaries. The changes to the hydrologic analysis included adaptation of more recent flood history information, changes to reservoir operating guides, revised flow coefficients and dam safety modifications, changes to the runoff and stream course model, updated flood protection requirements, and model changes to address identified rim leaks.

As a result of these changes, the revised still water Probable Maximum Flood (PMF) elevation is 722.0 ft. (Reference 4). SQN evaluated the effects of the revised flooding elevation and adequate flood protection is currently provided.

3.4.3 Flooding from Dam Breaches or Failures Breaching failure of the Watts Bar west saddle dam was evaluated in the CLB analysis for the PMF. The CLB flood elevation associated with this event is provided in the evaluation of the stream and river flooding hazard in Section 3.4.2.

Breaching and seismic-induced failures of multiple dams upstream of SQN were evaluated in the CLB analysis for the combined effects of seismic-induced dam failures and flooding and is reported in Section 3.4.8.2.

Breaching failure of Chickamauga dam was considered in the CLB analysis for loss of ultimate heat sink during non-flood conditions. This loss of downstream dam does not present a SQN flooding elevation hazard and is not considered applicable in SQN CLB.

Page 15 of 70

3.4.4 Flooding from Storm Surge and Seiche Surges and seiches are not considered applicable in the SQN CLB because of the size and configuration of the lake and the elevation difference between the normal lake level and the plant grade is approximately 22 feet.

3.4.5 Flooding from Tsunami Tsunami is not considered applicable in the SQN CLB because of the inland location of the plant.

3.4.6 Flooding from Ice-Induced Events Ice-induced flooding is not considered applicable in the SQN CLB because of the temperate zone location of the plant.

3.4.7 Channel Migration or Diversion Channel diversion is not considered applicable in the SQN CLB because the configuration of the flood plain would not produce major channel meanders or cutoffs. Carbon 14 dating of the material at the high terrace levels has shown that the Tennessee River has essentially maintained its present alignment for over 35,000 years (Reference 2).

3.4.8 Flooding from Combined Effects 3.4.8.1 Floods Caused by Precipitation Events The CLB PMF elevation discussed in Section 3.4.2 is combined with a 45 mile-per-hour overland wind (Reference 4). The combined effects of the flood plus wind are provided in Table 3.4-1 for SQN.

Table 3.4-1 Combined Effects of Flood and Wind Plant Location Flood Level (feet)

Probable Maximum Flood (still reservoir) 722.0 Run-up on 4:1 sloped surfaces on the Diesel Generator Building 723.2 Run-up on critical vertical wall of the Essential Raw Cooling Water 726.2 (ERCW) Pumping Station Surge level within flooded structures 722.5 3.4.8.2 Floods Caused by Seismic Dam Failure Events The maximum flood elevations for combined seismic dam failure and flood events are provided in Reference 4. The maximum seismic-induced flood elevation at SQN is due to an Operating Basis Earthquake (OBE) combined with a 1/2 PMF storm. Dams upstream of SQN which are postulated to fail in this combined event are Cherokee, Douglas, and Tellico. The maximum flood elevation caused by seismic dam failures at SQN is 708.6 ft. (Reference 4)

Page 16 of 70

3.5 Current Flood Protection and Mitigation Features Flood protection and mitigation for the SQN site are provided by 4 key elements: dams upstream and downstream of SQN, structures and structural features at the SQN site protecting equipment required for flood mode operation, TVAs River Operations forecasting and flood warning capability and SQN flood response procedures. Together, these elements ensure safe operation of SQN for flooding above plant grade. Each of these elements is described below.

3.5.1 Dam and Reservoir System Flood control above the plant is provided largely by twelve tributary reservoirs. Near the end of the flood season, these provide a minimum of 4,818,500 acre-feet of detention capacity. This is 90% of the total available above Chickamauga Reservoir. Additionally, the two main river reservoirs, Fort Loudoun and Watts Bar, provide 490,000 acre-feet of storage on the remaining area above Chickamauga Dam (Reference 4).

Chickamauga Dam, the headwater elevation of which affects flood elevations at the plant, has a drainage area of 20,790 square miles, 3,480 square miles more than Watts Bar Dam. There are seven major tributary dams (Chatuge, Nottely, Hiwassee, Apalachia, Blue Ridge, Ocoee No. 1 and Ocoee No. 3) in the 3,480 square-mile-intervening watershed, of which four have substantial reserved capacity. Near the end of the flood season, these provide a minimum of 379,300 acre feet of storage on the 1,200 square mile controlled area. Chickamauga Dam contains 345,300 acre-feet of detention capacity on the remaining 2,280 square miles (Reference 4).

3.5.2 Sequoyah Nuclear Site Protective Structures At the SQN site, equipment required during flood mode operation is either located above the design basis flood (DBF), is within a non-flooded structure, is designed for submerged operation, or is otherwise protected. The Reactor Building, as referred to as Shield Building in this report, will be maintained dry during the flood mode. Walls and penetrations are designed to withstand static and dynamic forces imposed by the DBF. (Reference 7)

The lowest floor of the Diesel Generator Building is at elevation 722.0 ft. with its doors on the uphill side facing away from the main body of flood water. With the PMF elevation of 722.0 ft.,

wind wave run-up at the Diesel Generator Building is elevation 723.2 ft. Therefore, flood levels exceed floor elevation of 722.0 ft. The entrances and mechanical and electrical penetrations are sealed prior to flood mode to prevent major leakage into the building for water up to the PMF, including wave run-up. Redundant sump pumps are provided within the building to remove minor leakage. (Reference 7)

The ERCW Pumping Station is designed to remain fully functional for floods up to the PMF, including wind-wave run-up. The upper deck elevation (elevation 720.0 ft.) is below the PMF plus wind wave run-up, but it is protected from flooding by the outside walls. The traveling screen wells extend above the deck elevation up to the design basis surge level. The wall penetration for water drainage from the deck in non-flood conditions is below the DBF elevation, but it is designed for sealing in event of a flood. Other exterior penetrations of the station below the PMF are permanently sealed. Redundant sump pumps are provided on the deck and in the interior rooms to remove rainfall on the deck and water seepage. (Reference 7)

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Other structures, including the service, turbine, auxiliary, and control buildings, will be allowed to flood as the water exceeds their grade level entrances. Equipment that is located in these structures and required for operation in the flood mode is either above the DBF, is designed for submerged operation, or is otherwise protected. (Reference 7) 3.5.3 TVA River Operations Forecasting and Warning Protection of SQN from floods that might exceed plant grade utilizes a flood warning issued by TVA's River Operations (RO). 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.

(Reference 7)

Because the plant grade elevation at SQN can be exceeded by flooding in rivers and streams and seismically-induced dam failure floods, plant flood preparations cope with the fastest rising calculated flood. (Reference 7)

Reservoir levels for large rainfall floods in the Tennessee Valley can be predicted well in advance. A minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />, divided into two stages, is provided for safe plant shutdown by use of this prediction capability. Stage I, a minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> long, will commence upon a prediction that flood-producing conditions might develop. Stage II, a minimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> long, will commence on a confirmed estimate that conditions will provide a flood above plant grade.

This two-stage scheme is designed to prevent excessive economic loss in case a potential flood does not fully develop. (Reference 7)

Seismically induced dam failures and coincident storm conditions were shown to result in floods which could exceed elevation 703 ft. at the plant. SQNs notification of these floods utilizes TVAs RO forecast system to identify when a critical combination exists. Stage I is initiated upon notification that a critical dam failure combination has occurred. Stage I continues until it has been determined positively that critical combinations do not exist. If communications do not document this certainty, shutdown procedures continue into Stage II activity. Stage II shutdown continues to completion or until lack of critical combinations is verified.

Although the river elevation of 703 ft., two feet below plant grade to allow margin for wind wave consideration, is critical during final stages of plant shutdown for flooding, lower forecast target levels are used in most situations to assure that the 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> pre-flood transition interval will always be available. (Reference 7)

The time to elevation 703 ft. is 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br /> as a result of the PMF. The event producing the next shortest time interval to elevation 703 ft. involves the OBE failure of Tellico and Norris during the one-half PMF resulting in a time interval of 34 hours3.935185e-4 days <br />0.00944 hours <br />5.621693e-5 weeks <br />1.2937e-5 months <br />. These times are adequate to permit safe plant shutdown in readiness for flooding. (Reference 7)

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The warning plan for safe plant shutdown is based on the fact that a combination of critically centered large earthquake conditions must coincide before the flood wave from seismically caused dam failures will approach plant grade. In flood situations, an extreme earthquake must be precisely located to fail two or more major dams before a flood threat to the site would exist.

The warning system utilizes TVA's flood forecast system to identify when flood conditions will be such that seismic failure of critical dams could cause a flood wave to exceed elevation 703.0 ft.

at the plant site. In addition to the critical combinations, failure of a single major upstream dam will lead to an early warning. A Stage I warning is declared once failure of (1) Norris, Cherokee, Douglas, and Tellico Dams or (2) Norris and Tellico Dams, or (3) Fontana, Tellico, Hiwassee, and Blue Ridge Dams, or (4) Cherokee, Douglas and Tellico Dams has been confirmed. (Reference 7) 3.5.4 SQN Flood Response Procedures Flood mode operation is defined as the set of conditions described below by means of which the plant will be safely maintained during the time when flood waters exceed plant grade (elevation 705 ft.) and during the subsequent period until recovery is accomplished.

Plant operation during the flood response is governed by an abnormal operating procedure (AOP). Maintenance and operations activities are directed from this AOP to align systems and components to a safe configuration for flood mode. Site grading and building design prevent any flooding before the end of the 27 hour3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> pre-flood period. (Reference 7)

For a reactor at power at the time the flood warning is received, Stage I and then, if necessary, Stage II procedures will be initiated. Stage I procedures will consist of a controlled reactor shutdown and other easily revocable steps such as moving supplies necessary to the flood protection plan above the DBF level and making temporary connections and load adjustments on the onsite power supply. Stage II procedures will be the less easily revocable and more damaging steps necessary to have the plant in the flood mode when the flood exceeds plant grade. The fire/flood mode pumps may supply feedwater to the steam generators for reactor decay heat removal. Other essential plant cooling loads will be transferred from the component cooling water to the ERCW System. (Reference 7)

For a reactor in refueling status, if time permits, fuel is removed from the unit(s) undergoing refueling and placed in the spent fuel pit; otherwise fuel cooling will be accomplished by natural circulation in the Reactor Coolant System (RCS). Heat removal from the steam generators will be accomplished by adding river water from the Fire Protection (FP) system and relieving steam to the atmosphere through the power relief valves. If the refueling canal is not already flooded, the mode of cooling described in SQN Units 1 and 2 LAR Subsection 2.4.14.2.2 (Reference 4) requires that the canal be flooded with borated water from the refueling water storage tank. If the flood warning occurs after the reactor vessel head has been removed or at a time when it could be removed, before the flood exceeds plant grade, the flood mode reactor cooling water will flow directly from the vessel into the refueling cavity. If the warning time available does not permit this, then the upper head injection piping will be disconnected above the vessel head to allow the discharge of water through the four upper head injection standpipes. Additionally, it is required that the prefabricated piping be installed to connect the Residual Heat Removal (RHR) and Spend Fuel Pit Cooling (SFPC) Systems, and that Essential Raw Cooling Water (ERCW) be directed to the RHR and SFPC System heat exchangers (Reference 7).

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If the flood warning occurs after the reactor vessel head has been removed or at a time when it could be removed before the flood exceeds plant grade, the RWST makeup water will flow directly from the vessel into the refueling cavity. If the warning time available does not permit this, then the upper head injection piping will be disconnected above the vessel head to allow the discharge of water through the four upper head injection standpipes. Additionally, it is required that the prefabricated piping be installed to connect the residual heat removal (RHR) and spent fuel pool cooling (SFPC) Systems, and that ERCW be directed to the secondary side of the RHR System and SFPC System heat exchangers. (Reference 7) 4 CHRONOLOGY OF FLOOD RELATED CHANGES SINCE LICENSING 4.1 Watershed Changes since Licensing The potential impact of land use change in the Tennessee River basin is evaluated using the National Land Cover Data (NLCD) to determine the change in impervious area over the watershed. The Landsat data are acquired by satellite sensor at 30 meter resolution. The NLCD data have been used for many applications including national environmental reporting, climate change, Clean Water Act studies and conservation assessments. Using this product the land cover for the watershed was defined as shown in Figure 4-1. (Reference 8)

Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam Page 20 of 70

The data for this urban land use change assessment are derived from the NLCD 2001 and 2011 Retrofit Change Project (Reference 8). The raster product from this project is converted to polygon vector data that could be displayed, manipulated, and analyzed in a geographic information system (GIS) to determine change in impervious area (land use) between 2001 and 2011 as follows:

Table 4.1-1 Land Use above Guntersville (2001 - 2011)

Land Use Classification 2001 2011  % Change Agricultural 18.35% 18.02% -0.33%

Forest 64.47% 63.56% -0.91%

Urban 9.56% 9.99% 0.43%

Grassland/Shrub 4.80% 5.58% 0.78%

Water/Wetland/Barren 2.82% 2.85% 0.03%

Table 4.1-2 Impervious Area above Guntersville (2001 - 2011) 2001 2011 Change Impervious Area 1.74% 1.96% 0.22%

Based on this assessment the Tennessee Valley watershed is experiencing a change in impervious area at a rate of approximately 0.02% per year in a 10 year period. Thus it is judged at this rate that potential impacts of watershed changes due to urbanization would have minimal impact on runoff from the basin over the life of the project. Additional lands owned by federal agencies, including US Fish and Wildlife Service, US Forestry Service, National Park Service, and the Protected Areas Database, comprise one third of the entire watershed above Guntersville Dam.

These lands are set aside for public use and include prohibitive development restrictions. Further, a computation of runoff coefficients using the land use data and soils data is presented in Reference 9. The results of the computed runoff coefficients show that there is good agreement with coefficients used in the flood hazard reevaluation. In general the computed runoff coefficients are lower than those used in this analysis and result in less runoff.

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4.2 Summary of Changes to Design Basis Flood Elevations The original licensing basis for the SQN Design Basis Flood elevation is documented in the 1979 NRC Safety Evaluation Report (NUREG-0011, March 1979). Figure 4-2 provides a timeline for the changes that have occurred between the 1979 SER and this flooding reevaluation.

Late 1970s 1997 Dam 2008 NRC QA Original Safety 2001 Review of Licensing Modifications UFSAR Hydrology 2012 LAR Basis complete Change Documentation Submittal 1982 Dam 1997-98 2004 River 2008-2012 Safety Flood Operations Hydrology Review Reassessment Study (ROS) Reanalysis began Implemented Figure 4-2 Timeline of Flood Related Changes Since Licensing 4.2.1 Local Intense Precipitation The original licensing basis for the SQN LIP, as established in NUREG-0011, March 1979, concluded that the local PMF would not reach or exceed the critical floor elevation, 706 ft. In 1993, the LIP was re-evaluated for the installation of security fences at the site (Reference 10).

The re-evaluation confirmed that the eastern flow paths at the site could successfully pass a 14 inch in one-hour rainfall as defined in HMR 45 (Reference 11) and would not exceed the critical floor elevation, 706 ft. In 2005, the LIP was re-evaluated for the addition of the Multipurpose Building and the Fire Operations Center structures on the site. The 2005 re-evaluation confirmed that the critical floor elevation, 706 ft., would not be exceeded (Reference 10). In 2014, the LIP was re-evaluated to address the addition of a concrete pad and associated trash compactors installed in the yard area of the SQN site (Reference 6). In this re-evaluation the Design Basis PMP was changed to 16.21 inches per hour rainfall following the guidelines set forth in HMR 56 (Reference 5). The 2014 re-evaluation, which is the current licensing basis, confirmed that flood levels would not exceed the critical floor elevation of 706 ft.

4.2.2 Flooding in Rivers and Streams 4.2.2.1 1970s - Licensing Basis 4.2.2.1.1 PMF The original flood analysis for SQN was completed in the late 1970s as documented in the FSAR. Two candidate flood events were evaluated for the original PMF - one produced by a sequence of March storms producing maximum rainfall on a 7,980-square-mile watershed centered at Bulls Gap, Tennessee (about 50 miles northeast of Knoxville) below the major tributary dams (i.e., Cherokee, Douglas, etc.) and one produced by a sequence of March storms producing maximum rainfall on the 21,400-square-mile watershed above Chattanooga. The controlling event at SQN was determined to be the March 21,400-square-mile event as shown below:

Storm Event Maximum Discharge Maximum Elevation March 21,400-square-mile Bulls Gap 1,370,000 cfs 722.6 ft.

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4.2.2.1.1.1 Key Elements and Assumptions of the Original PMF Analysis

a. Computer codes used in the analysis were developed by TVA (UNITGRAPH, THIESSEN, FLDHYDRO, TRBROUTE, CHANROUT, DBREACH, CONVEY, WWIDTH, and SOCH)

b. Sub-basin unit hydrographs were developed using the largest floods of record

c. Probable Maximum Precipitation (PMP) defined for TVA by HMR 41 (Reference 12)

d. Nine day events - three day antecedent storm, three day dry period and three day main storm

e. Simulations were started from median reservoir levels at the time and median moisture conditions were postulated

f. Simulated Open Channel Hydraulic (SOCH) unsteady flow model used for flood simulations on main river and selected tributaries - model calibration based on replication of two largest floods of record in each reservoir, as available

g. Embankment failures from overtopping were postulated at Fort Loudoun-Tellico and Watts Bar because dams were not designed for the PMF

h. Plant grade elevation 705 ft. exceeded by design flood event - Reg. Guide 1.59, Regulatory Position 2 - flood warning plan in place for safe plant shutdown - minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> available

i. Embankments overtopped as a result of flood wave from upstream dam failure were postulated to fail instantaneously and completely with time of failure based on DBREACH results.

j. Watts Bar Dam and West Saddle Dam postulated failure outflows combined and routed downstream

k. No correction for tailwater submergence was applied

l. Chickamauga Dam, downstream of SQN, was overtopped during PMF event simulation but postulated not to fail Page 23 of 70

4.2.2.1.2 Seismically Induced Failure of Upstream Dams The maximum plant site elevations at SQN for the different postulated combinations of seismic dam failures coincident with floods were as follows:

OBE (Operating Basis Earthquake) Failure with 1/2 PMF Maximum Elevation at SQN (ft.)

a

1. Fontana 706.9

2. Norris 704.5

3. Cherokee, Douglas 710.4 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasb 710.3

2. Norris, Douglas, Fort Loudoun, Tellico 704.8 a.

Includes failure of five non-TVA dams - Nantahala, Santeetlah, Cheoah, Calderwood, and Chilhowee. Fort Loudoun spillway gates are inoperable in the open position.

b.

Gate opening at Fort Loudoun prevented by bridge failure.

4.2.2.1.2.1 Key Elements and Assumptions of the Original Seismically Induced Failure of Upstream Dams Analysis

a. Seismic stability determined by engineering analysis and judgment

b. Outflows at failed dams were generated by approximate models of the reservoirs using the SOCH model

c. Warning time available greater than 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> for the failure combinations

d. Any embankments overtopped as a result of a flood wave from an upstream dam failure were postulated to fail instantaneously and completely with time of failure based on DBREACH results.

4.2.2.2 1997 - 1998 Reassessment 4.2.2.2.1 PMF (Reference 13)

A reassessment of maximum flood levels was performed between 1997 and 1998. The reassessment of maximum flood levels was made to address dam safety modifications that had been made subsequent to the flood level determinations in the original licensing basis.

Other inputs to the reanalysis were not changed.

In 1982 TVA established the Dam Safety Program and began a safety review of TVA dams.

This dam safety effort was designed to be consistent with the Federal Guidelines for Dam Safety and similar efforts of other Federal agencies. Technical studies, engineering analyses, and modifications were performed to ensure hydrologic and seismic integrity of TVA dams.

Table 4.2-1 provides the modification status (hydrologic) of the dam safety effort as of 1998.

The reassessment addressed the effects of these dam safety modifications on maximum flood levels at SQN and on warning time available for safe plant shutdown.

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The reassessment of the PMF involved evaluation of three candidate flood events:

1. March 7,980-square-mile

2. March 21,400-square-mile

3. March 12,030-square-mile The March 7,980 square-mile and March 21,400 square-mile events were described in the original analysis. The March 12,030 square-mile event became a candidate on the lower main river after Fort Loudoun, Tellico, Watts Bar, Nickajack, and Guntersville dams were modified to prevent their failure in an extreme flood event. This new storm produces maximum rainfall on the 12,030 square-mile watershed above Pickwick Dam and below Chickamauga Dam.

As a result of the reassessment the controlling event at SQN would result from the March 21,400-square-mile event as shown below:

Storm Event Maximum Discharge Maximum Elevation SQN (March 21,400-square-mile) 1,236,000 cfs 719.6 feet Page 25 of 70

Table 4.2-1 Dam Modifications Completed by 1998 (Reference 13)

Dam Dam Modification Year Modifications Completed Main River Dams Fort Loudoun Dam was raised 3.25 feet with a 1989 Fort Loudoun- concrete wall to elevation 833.25 feet. A 2000-foot Tellico uncontrolled spillway with crest at elevation 817 feet was added at Tellico Dam.

Watts Bar Embankment was raised 10 feet with earth- 1997 fill/concrete wall to elevation 767 feet.

Nickajack South embankment was raised 5 feet with earth- 1992 fill/concrete wall to elevation 657 feet. A 1900-foot roller-compacted concrete overflow dam with top at elevation 634 feet was added below the north embankment.

Guntersville Embankment was raised 7.5 feet with earth-fill to 1996 elevation 617.5 feet.

Tributary Dams Embankment was raised 4.5 feet with earth-fill to 1992 Beech elevation 475.5 feet.

Blue Ridge Three (3) additional spillway bays were added in 1995 1982. Embankment was raised 7 feet with earth-fill/concrete wall to elevation 1713 feet, and a 395-foot uncontrolled spillway with crest at elevation 1691 feet was added in 1995.

Boone Embankment was raised 8.5 feet with earth-fill to 1984 elevation 1408.5 feet.

Cedar Creek Embankment was raised 5.5 feet with concrete wall 1997 to elevation 605 feet.

Chatuge Embankment was raised 6.5 feet with earth-fill to 1986 elevation 1946.5 feet.

Cherokee A portion (600 feet) of the non-overflow dam was 1982 raised 7.75 feet to elevation 1089.75 feet.

Douglas A portion of the non-overflow dam was raised 13.5 1988 feet to elevation 1022.5 feet and eight saddle dams were raised 6.5 feet with earth-fill to elevation 1023.5 feet.

Nottely Embankment was raised 13.5 feet with rock-fill to 1988 elevation 1807.5 feet.

Upper Bear Creek Embankment was raised 4 feet with concrete wall to 1997 elevation 817 feet.

Watauga Embankment was raised 10 feet with rock-fill to 1983 elevation 2012 feet.

Fontana Dam post-tensioned 1988 Melton Hill Dam post-tensioned 1988 Page 26 of 70

4.2.2.2.1.1 Summary of Differences Between 1970s Licensing Basis and 1997 - 1998 Reassessment for PMF Dam safety modifications at main river dams, Fort Loudoun - Tellico and Watts Bar as shown in Table 4.2-1, eliminated failure from overtopping

a. The only postulated embankment failure from rainfall floods that influenced plant site elevations were those at the Watts Bar West Saddle Dam

b. The Dallas Bay rim leak on Chickamauga Reservoir was modeled as a reach with a junction with the Tennessee River and a downstream boundary at the overflow consisting of a rating curve

c. An unsteady flow model was added for the Fort Loudoun - Tellico complex

d. Controlling PMF elevation lowered by 3.0 feet (from 722.6 ft. to 719.6 ft.)

4.2.2.2.2 Seismically Induced Failure of Upstream Dams (Reference 13)

During the 1997-1998 reassessment, the maximum plant site elevations at SQN with dam safety modifications for the various postulated combinations of seismic dam failures coincident with floods were as follows:

OBE Failure with 1/2 PMF Maximum Elevation at SQN (ft.)

a

1. Fontana 702.8

2. Norris 698.1

3. Cherokee, Douglas 701.1

4. Fontana, Hiwassee, Apalachia, Blue Ridge 707.9 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasb 706.0

2. Norris, Douglas, Fort Loudoun, Tellico 699.3 a.

Includes failure of five non-TVA dams - Nantahala, Santeetlah, Cheoah, Calderwood, and Chilhowee. Fort Loudoun spillway gates are inoperable in the open position.

b.

Gate opening at Fort Loudoun prevented by bridge failure.

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4.2.2.2.2.1 Summary of Differences Between 1970s Licensing Basis and 1997 - 1998 Reassessment for Seismically Induced Failures

a. During Fontana and Norris failure analyses Watts Bar West Saddle Dam would fail completely but Watts Bar Dam would not fail due to the modifications made at Watts Bar Dam.

b. An unsteady flow model, developed during the dam safety studies, was used as an adjunct to route the Hiwassee, Apalachia, and Blue Ridge Dam failures.

c. The Cherokee and Douglas failure would result in a very small overtopping of the Fort Loudoun Dam, 0.55 feet, for about 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Breach analysis indicated that the dam would not fail. The Watts Bar West Saddle Dam would fail completely but Watts Bar Dam would not fail due to the modifications made. (Reference 2)

d. At SQN two combination events would exceed plant grade. In both cases, Fontana, Hiwassee, Apalachia and Blue Ridge; and Norris, Cherokee, and Douglas, the time from seismic event to arrival of failure surge at the plant is adequate to permit safe plant shutdown.

4.2.2.3 2012 LAR On October 30, 2007, TVA submitted an application for a combined operating license (COLA) for the proposed Bellefonte Nuclear Plant (BLN) Units 3 and 4, in accordance with 10 CFR 52.

During review of the BLN Units 3 and 4 FSAR, the NRC performed an audit of the hydrologic analysis which resulted in the issuance of three Notice of Violations (NOVs) on March 19, 2008 (Reference 14). In response to these NOVs, TVA completed a revised hydrologic analysis.

While the February 2008 Quality Assurance (QA) inspection was for the BLN licensing request, it directly impacted SQN because the analysis is similar for TVA nuclear plants located along the Tennessee River. As a result of the NOV, TVA initiated a confirmatory analysis of the PMF computations (Reference 4). Above SQN, temporary flood barriers have been installed at Cherokee, Fort Loudoun, Tellico, and Watts Bar Dams to increase the height of embankments and are included in the discharge rating curves for these dams.

Increasing the height of embankments at these dams prevents embankment overflow and failure of the embankment (Reference 4).

It should be noted that TVA is preparing a supplement to the 2012 LAR which is expected to be submitted by the end of April 2015. This supplement will incorporate changes to make the SQN licensing basis consistent with the licensing basis approved for Watts Bar Nuclear Plant (WBN) (Reference 15).

4.2.2.3.1 PMF As a result of the reassessment the controlling event at SQN changed from the original analysis and results from the March 21,400-square-mile downstream-centered event as shown below:

Storm Event Maximum Discharge Maximum Elevation March 21,400-square-mile 1,331,623 cfs 722.0 ft.

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4.2.2.3.1.1 Summary of Differences between 1997 - 1998 and 2012 LAR Analyses for PMF

a. Cross section bathymetry was updated based on recent USACE Doppler profiler navigation surveys

b. Dam operating guides were updated in the hydrology model to reflect current reservoir operating policy

c. Dam rating curves updated to include discharge coefficients derived using model test data

d. Turbine discharges were used at river reservoirs up to the point where the head differentials were too small and/or the powerhouse would flood

e. Model refinements made at the Fort Loudoun-Tellico canal, Dallas Bay rim leak, and model extended on the Hiwassee River arm of Chickamauga reservoir

f. Operational Allowance approach developed to allow flood simulations to more nearly mimic the integrated operation of the reservoir system

g. Correction for tailwater submergence applied at dams as appropriate

h. Watts Bar West Saddle Dam failure discharge input at the mouth of Yellow Creek where it would enter Chickamauga Reservoir instead of combining with Watts Bar Dam discharge and routing downstream

i. Height of embankments modified by use of temporary HESCO barriers at Cherokee, Fort Loudoun, Tellico, and Watts Bar dams to prevent overtopping.

4.2.2.3.2 Seismically Induced Failure of Upstream Dams (Reference 4)

The maximum plant site elevations at SQN with dam safety modifications for the different postulated combinations of seismic dam failures coincident with floods were as follows:

OBE Failure with 1/2 PMF Maximum Elevation at SQN (ft.)

a b

1. Fontana , Tellico 702.2 b

2. Fontana, Tellico , Hiwassee, Apalachia, Blue Ridge 706.3

3. Norris, Tellicob 706.3

4. Cherokee, Douglasc, Tellicob 708.6 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasc, Tellicob 706.0 a.

Includes failure of five non-TVA dams - Nantahala, Santeetlah, Cheoah, Calderwood, and Chilhowee. Fort Loudoun spillway gates are inoperable in the open position.

b.

Tellico failure added because seismic stability analyses were not conclusive c.

Gate opening at Fort Loudoun prevented by bridge failure.

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4.2.2.3.2.1 Summary of Differences between 1997 - 1998 and 2012 LAR Analyses for Seismically Induced Failures

a. Failure outflow from tributary dams (Norris, Cherokee, Douglas, Fontana, etc.)

developed using HEC-HMS model

b. Tellico failure added because seismic stability analyses were not conclusive

c. Cross section bathymetry was updated based on recent USACE Doppler profiler navigation surveys

d. Dam operating guides were updated in the hydrology model to reflect current reservoir operating policy

e. Dam rating curves updated using model test data

f. Turbine discharges were used at river reservoirs up to the point where the head differentials were too small and/or the powerhouse would flood

g. Model refinements made at the Fort Loudoun-Tellico canal, Dallas Bay rim leak, and model extended on the Hiwassee River arm of Chickamauga reservoir

h. Operational Allowance approach developed to allow flood simulations to more nearly mimic the integrated operation of the reservoir system

i. Correction for tailwater submergence applied at dams as appropriate

j. Watts Bar West Saddle Dam failure discharge input at the mouth of Yellow Creek where it would enter Chickamauga Reservoir instead of combining with Watts Bar Dam discharge and routing downstream

k. Temporary barriers were not credited in the seismic analysis

l. At SQN four events as shown above would exceed plant grade elevation 705 ft. In these events the warning time would be greater than the 27 hour3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> time required for safe plant shutdown.

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5

SUMMARY

OF PLANT WALKDOWN RESULTS AND MODIFICATIONS TVA completed flooding walkdowns in accordance with NEI 12-07 walkdown guidelines. In Reference 7, TVA provided the results of the flooding walkdowns in response to Recommendation 2.3 item 2 in of Reference 1.

The SQN external flood protection features were visually inspected. The NEI walkdown record forms included in Appendix B of the guidance document were used as a template for the inspections. Training was provided, as recommended in the guidance document and TVA procedure CTP-FWD-100.

The walkdown team was made up of four engineers consisting of three civil engineers and one mechanical engineer. The walkdown team was supported by a retired TVA Senior Reactor Operator and a retired TVA Shift Manager in planning and performance of the walkdowns.

Walkdowns were performed in the safety related buildings and structures at SQN as well as in the Turbine Building. TVA evaluated the discoveries from the walkdowns for both deficiencies and observations. The deficiencies and observations were entered into the corrective action program.

As a result of plant walkdowns and disposition of other corrective actions related to flooding, TVA has implemented the following flood protection modifications and improvements:

5.1 Diesel Generator Building Flood Protection Barriers Flood protection barriers have been installed in the Diesel Generator Building at elevation 722 for equipment doors D15 - D18, personnel access door D5, and emergency doors D1 - D4 (Reference 7). These barriers will prevent water from entering the building protecting the safety related Diesel Generators. Barriers are permanently installed and/or stored in a storage rack in close proximity to the doors which they protect with procedural actions to install the barriers upon receipt of Stage I flood warning. Procedure actions were added to prevent water intrusion into the building from the sewer drain. The four external fuel fill ports to the seven day oil storage tanks were raised and encased in concrete to prevent intrusion of water and allow filling of tanks during a flood event. These modifications were completed to eliminate compensatory measures.

5.2 Spent Fuel Pit Cooling Pump Enclosure Engineering document design change was implemented to ensure enclosure caps as permanent plant feature with procedural actions to install the caps upon receipt of a Stage I flood warning.

This design change reinstated procedural requirements to eliminate compensatory measures.

5.3 ERCW Intake Pumping Station Flood Protection Barriers The Traveling Water Screen vent lines associated with the instrument wells at the ERCW Intake Pumping Station were extended to prevent intrusion of water to the pump deck during a design basis flood event. A flood protection barrier for personnel access door PS5 on elevation 725 ft.

was installed and stored in storage rack in close proximity to the door it is protecting. Procedural actions are in place to install the barrier upon receipt of a Stage I warning. The pump deck (elevation 720 ft.) is below the DBF, but it is protected from flooding by the outside walls.

Redundant deck drainage sump pumps are designed to remove rainfall and water seepage.

These modifications eliminate potential in leakage paths to minimize water seepage.

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5.4 Conduit Penetration Seals 5.4.1 ERCW Intake Pumping Station Conduit Penetration Seals Previously installed moisture barriers on the conduits in manhole 33 were replaced by mechanical seals. This design has been tested to withstand pressures adequate to protect water ingress for flooding up to the design basis flood elevation. This modification resolved a degraded/nonconforming condition and associated compensatory measures.

5.4.2 Shield Building Conduit Penetration Seals As a result of 5.4.1, an extent of condition analysis confirmed potential for water transport through conduits below the flood elevation in the Shield Building penetrations. This item is being addressed through the corrective action program.

5.5 Diesel Generator Drain Plugs As a result of flood mode barrier inspection, two Diesel Generator Building drain manual isolation valves were found to be inoperable. Drain plugs have been purchased and staged with procedural actions in place to install upon receipt of Stage I flood warning. This resolves the nonconforming condition.

5.6 Flood Mode Preparation Improvements Various corrective actions and improvements were implemented to address Flood Mode Preparation timeline challenges. The External Flooding Procedures were revised to address numerous improvements identified as part of reasonable simulation performed under Recommendation 2.3. Modifications are planned for a new permanent platform, improved bolting material, and permanent rigging attachment points to facilitate ease of installation of 16 ERCW to CCS Flood Mode spool pieces. Procedures require Emergency Preparedness Drills be performed on External Flooding on a four year frequency.

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6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS The sections that follow discuss previous and proposed analyses or provide justification for exclusion for each of the flood-causing mechanisms. The hierarchical hazard assessment approach recommended in NUREG/CR-7046 is employed in these analyses. This approach allows a stepwise, progressively refined series of analyses that demonstrates that SSCs important to safety are protected from the adverse effects of severe flooding at the site.

Guidance, in addition to NUREG/CR-7046 (Reference 16), for potential flood-causing mechanisms, or causal phenomena, is provided in Table 6-5.6-1:

Table 6-5.6-1 Potential Flood Causing Mechanisms or Causal Phenomena Flood Causing Mechanism Guidance Reference Local intense precipitation HMR 52 and HMR 56 17 and 5 Flooding from rivers and streams ANSI/ANS-2.8-1992 18 Flooding from upstream dam breaches or failures Dam Failure ISG 19 Flooding from storm surges or seiches Not Applicable NA Flooding from tsunamis Not Applicable NA Flooding from ice-induced events Not Applicable NA Flooding from channel diversion or migration toward the site Dam Failure ISG 19 ANSI/ANS-2.8-1992 and Flooding from combined effects Dam Failure ISG 18 and 19 6.1 Local Intense Precipitation The LIP was previously evaluated for SQN and is included in the reevaluation for SQN. The analysis assumes fully functional site grading and partially blocked drainage channels.

6.2 Flooding from Rivers and Streams The PMF was previously evaluated for SQN and is included in the reevaluation for SQN. The PMF in rivers and streams adjoining the site is determined by applying the PMP to the drainage basin of these rivers and streams. The model inputs and assumptions, as well as the previous analysis, technical approach, and results are described in subsequent sections.

6.3 Flooding from Dam Breaches or Failures 6.3.1 Project Specific PMF The project specific PMF was previously evaluated and determined to be non-governing. The project specific PMF is included in the reevaluation for SQN. The project specific PMF is the design basis flood level for a dam, and is defined as the most severe flood that may be reasonably predicted to occur at a site as a result of severe hydrometeorological conditions. The model inputs, assumptions, technical approach and results of this analysis are presented in subsequent sections.

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6.3.2 Sunny Day Failure of Upstream Dams Sunny day failure of upstream dams has not previously been analyzed for SQN. Inputs, assumptions, technical approach and results of this analysis are presented in subsequent sections.

6.3.3 Seismic Failure of Upstream Dams Seismic failure of upstream dams was previously evaluated for SQN and is included in the reevaluation for SQN. Seismic failure of single dams combined with flood events is evaluated as part of this analysis. The inputs, assumptions, technical approach, and results are presented in subsequent sections. Seismic failure of upstream multiple dam combinations with coincident flood events as described in Section 6.8.2.

6.3.4 Sediment Transport A sediment transport analysis was performed to determine the impact of sediment released from a hypothetical Watts Bar Dam Breach (Reference 20). The following was evaluated:

Sediment core samples of the embankment were examined. The embankment is composed of an impervious clay core, silty-sand mix for the pervious outer zone of the embankment, and rip-rap protective shell.

The incipient motion results for various flows between 26,000 and 210,000 cfs were assessed.

The latter being the peak flow for the Sunny Day dam breach. Relatively large size particles (d >

3mm) will be able to move throughout the reach between Watts Bar and Chickamauga Dams.

During the peak flow, particles between 20 and 25mm (coarse gravel) will be able to move in the upper 10 miles of the reach. As the dam breach hydrograph attenuates with time, the average size particle lowers to approximately 5mm.

These silts and clay particles will be moving through the system in suspension during the whole simulation, and sand size particles will move in suspension and settle-out at various locations during the simulation.

It was estimated that the amount of suspended sediment that would settle in the dredged area in front of the ERCW intake is minimal, at less than 1 inch. Sedimentation would not result in a loss of storage in the reservoir, which would lead to flooding, because of the small amount of settlement of suspended sediment (less than 1 inch).

6.4 Flooding from Storm Surge and Seiche Flooding from storm surge and seiche has not previously been evaluated for SQN and is not considered a credible flood-causing mechanism at this site. The SQN site is located on a peninsula on the west bank of the Chickamauga Reservoir at TRM 484.5 approximately 1,485 River miles inland (Tennessee River Miles 484.5, Ohio River Miles 48, and Mississippi River Miles 953) from the Gulf of Mexico at grade elevation 705 ft. The Chickamauga Reservoir level during non-flood conditions would not exceed approximate elevation 682.5 ft. at the plant, for any significant period of time. The plant grade at elevation 705 ft. is over 22 feet above the normal maximum pool levels.

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While seismic seiche has been recorded in the Tennessee Valley area it has been of very small magnitude. For example the March 1964 Alaska Earthquake which was a 9.2 magnitude event resulted in seiche being observed on about 25% of the 130 gages available in Tennessee at the time. The largest amplitude of seiche recorded on lakes, reservoirs, and/or ponds in Tennessee was 0.14 feet and in Kentucky 0.57 feet (Reference 21). Reference 22 indicates that SQN is within the Eastern Tennessee Seismic Zone. However, there has been no recorded seiche of any significant magnitude reported as a result of earthquake events in the Tennessee Valley area.

Further examination of landslide activity as taken from the United States Geologic Survey (USGS) in the vicinity of the plant indicate that landslide susceptibility is considered to be moderate to high with low incidence as shown in Figure 6-1. (Reference 23)

Figure 6-1 Landslide Incidence Map of United States Examination of the slopes in the vicinity of the plant does not indicate instabilities or the potential for a landslide. There also have been no recorded incidences where landslides have generated a seiche in the TVA reservoir system.

Because the site is not located on an open or large body of water, surge or seiche flooding will not produce the maximum water levels at the site with over 22 feet of margin between normal non-flood conditions and plant grade.

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6.5 Flooding from Tsunami Flooding from tsunami was not previously evaluated for SQN and is not a credible flood causing mechanism at this site. The SQN site is located about 7.5 miles northeast of the nearest city limits of Chattanooga in Hamilton County, Tennessee, approximately 14 miles west-northwest of Cleveland, Tennessee, and approximately 31 miles south-southwest of TVAs WBN. At this location SQN is approximately 1,485 river miles inland (Tennessee River Miles 484.5, Ohio River Miles 48, and Mississippi River Miles 953) from the Gulf of Mexico. The Gulf of Mexico is the nearest body of open water directly downstream from Chickamauga Lake that is subject to seismically generated tsunamis. Further, the site is more than 320 miles inland from the Atlantic coast and more than 460 miles inland from the Great Lakes. The SQN site with plant grade at an elevation of 705 ft. will not be subject to the effects of tsunami flooding because the site is not adjacent to a coastal area.

The potential for a seismically induced hill-slope failure which could produce a tsunami-like wave in the vicinity of the plant were also examined. The slopes near the SQN site have been stable for many years and no landslides into the reservoir have been documented for Hamilton County.

(Reference 23) 6.6 Flooding from Ice-Induced Events Flooding from ice-induced events was not previously evaluated for SQN and is not a credible flood causing mechanism at this site. The SQN plant is located in a temperate climate where significant amounts of ice do not form on lakes and rivers in the vicinity of the plant and ice jams are not a source of major flooding. On several occasions, ice has formed near the shore and across protected inlets but has not constituted a problem on the main river reservoirs. There has been no recorded incidence of ice near the plant site or of ice-induced flooding. (Reference 24)

The potential for significant surface ice formation is further reduced by the daily water level fluctuation resulting from power operations at Chickamauga Dam located downstream of the plant and Watts Bar Dam located 45.4 river miles above the plant.

There are no safety-related facilities at the SQN site which could be affected by an ice jam flood, wind driven ice ridges, or ice-produced forces. There are no valley restrictions in the 45.4 mile reach below Watts Bar Dam to initiate a jam and an ice dam would need to reach more than 29 feet above normal winter levels, 676 ft., to reach plant the grade elevation of 705 ft. Thus, it is judged that an ice jam sufficient to cause plant flooding is not credible.

6.7 Channel Migration and Diversion Channel migration and diversion was not previously evaluated for SQN and is not a credible flood causing mechanism at this site. The reservoir in the vicinity of SQN and above has been stable for many years with no indication of the potential for migration or diversion. Historic floods have not produced any major changes in the reservoir configuration. The reservoir width in the vicinity of the plant ranges from a low of around 2,000 feet to over 3,000 feet with stable slopes.

It is judged that the potential for a channel diversion is not credible.

6.8 Flooding from Combined Effects The following combinations will be considered in the reevaluation for SQN:

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6.8.1 Floods Caused by Precipitation Events Floods caused by precipitation events were previously evaluated and will be included in the reevaluation for SQN. This scenario evaluates the effects of wind-wave activity during floods at a site along the shore of an enclosed body of water. This combination is described in NUREG/CR-7046 and includes the antecedent, PMP event, and waves induced by 2-year wind speed applied along the critical direction.

6.8.2 Floods Caused by Seismic Dam Failures The load combinations identified for the reevaluation includes present day methodology for probabilistic seismic hazard analysis are as follows:

a. 10,000-year ground motion coincident with a 25-year flood.

b. 1/2 of the 10,000-year ground motion combined with the lesser of 1/2 PMF or 500-year flood.

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7 DESCRIPTION OF MODELS USED FOR REEVALUATION 7.1 HEC-RAS HEC-RAS is an integrated system of hydraulic analysis programs, designed for interactive use in a multi-tasking environment. The HEC-RAS modeling system was developed by the Hydrologic Engineering Center (HEC) which is a division of the Institute for Water Resources, U.S. Army Corps of Engineers (Reference 25). It was designed to simulate one-dimensional steady and unsteady flow in subcritical, supercritical or mixed flow regime in open channels.

TVA is using the HEC-RAS model for flood analyses performed in response to the post-Fukushima request for information letters under 10CFR50.54(f) because the HEC-RAS model is a well-documented and supported industry standard program; it allows the entire watershed to be modeled in a single continuous simulation. Additionally, use of internal and lateral structure rules allows the computation of any correction for submergence that may influence dam discharge directly at each time step during model runs.

HEC-RAS can be used to perform the following functions:

a. Steady-flow backwater profiles

b. Unsteady flow for subcritical flow regime

c. Unsteady flow for mixed flow

d. Dam breach modeling

e. Hydraulic design computations

f. Sediment transport computations

g. Water quality analysis

h. RAS mapper graphical inundation mapping In steady flow mode, HEC-RAS explicitly solves the energy equation with an iterative procedure called the standard step method. In unsteady flow, HEC-RAS implicitly solves for flow and stage at every cross section using a finite difference approximation of the Saint Venant equations called the box scheme. Outputs of the HEC-RAS model include flood stages and flows.

7.1.1 Description of HEC-RAS Model Verification The HEC-RAS model is dedicated in accordance with QA procedures. A Software Dedication Report was prepared that presented a test plan to identify the critical characteristics and limitations of the software. As a result 22 test problems were developed and verified using either hand calculations or other means.

7.1.2 Description of HEC-RAS Model Extents The TVA total watershed HEC-RAS model is documented in the January 28, 2015 WBN SER (Reference 15) and is used in this hazard reevaluation because SQN is located on the same reservoir as WBN. Figure 7-1 and Figure 7-2 show the extent of the model, as well as the location of dams. The model is described in more detail in Section 8.

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Figure 7-1 Upper HEC-RAS Model Extents Figure 7-2 Lower HEC-RAS Model Extents Page 39 of 70

The LIP analysis utilized an unsteady-state HEC-RAS to determine water surface. Storage areas in the HEC-RAS model were interconnected via weirs of variable profile. Some storage areas were connected directly to the channel simulations as upstream stage boundaries, and others were connected to the channel simulation at specific cross-sections via lateral structures also modeled as weirs. A schematic of the HEC-RAS model can be seen in Figure 7-3. In the figure blue outlines represent storage areas, red lines represent storage area weir connections, and green lines represent cross-section locations.

Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents Page 40 of 70

7.2 HEC-HMS The software Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) is a public source numerical modeling tool developed by the US Army Corps of Engineers (USACE) to model the hydrologic cycle and understand the behaviors and implications of watershed, channel, and water-control structure by simulating watershed precipitation and evaporation, runoff volume, direct run-off (overland and interflow), base flow and channel flow. The results of this software are used as an aid in decision making for: (a) planning and designing new flood-damage reduction facilities, (b) operating and/or evaluating existing hydraulic conveyance and water-control facilities, (c) preparing for and responding to floods, (d) regulating floodplain activities and (e) restoring or enhancing the environment.

7.2.1 Description of HEC-HMS Model Verification The software is dedicated in accordance with QA procedures. Dedication is based on completion of the procedures in the HEC-HMS Validation Guide and parallel testing through alternative software. This Validation Guide is provided by the US Army Corps of Engineers- Hydrologic Engineering Center and outlines a protocol for validating the HEC-HMS software on designated hardware. The protocol includes a Test Suite of thirty-four projects to test the ability to properly perform the HEC-HMS simulation commands. In addition, a sample of the projects included in the Validation Guide was completed using alternative software and compared to the HEC-HMS results.

7.2.2 Description of HEC-HMS Model Extents HEC-HMS models are used to calculate runoff hydrographs from each subarea and maximum water surface levels of storage areas in the LIP analysis. The SQN LIP HEC-HMS model comprises twelve sub-basins totaling 171 acres. Reservoir modeling is used where no clear drainage channel exists and as an alternate method to determine water surface elevations.

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8 JUSTIFICATION OF INPUTS The verified HEC-RAS unsteady flow model of the Tennessee River System documented in the January 28, 2015 WBN SER (Reference 15) is used in the SQN hazard reevaluation. It is utilized to predict flood elevations and discharges for floods of varying magnitudes including the PMF and flooding from dam failures. Inputs to the HEC-RAS model are described in the following subsections.

8.1 HEC-RAS Model Geometry Development and Calibration Tennessee River geometry information was developed for the HEC-RAS model with the primary objectives of generating and/or verifying cross-sectional data and augmenting cross-sections to account for reach storage. USACE bathymetric surveys, DTM/DEM data from state and local databases, and USGS topographical maps were used in developing and verifying cross-sections.

The development of the HEC-RAS geometry is detailed in References 26 and 27.

Once the HEC-RAS geometry was developed, the HEC-RAS model for each reach was calibrated to historic events. The main stem is calibrated to the March 1973 and May 2003 events and the tributaries are calibrated to two large flood events of record for the respective reaches. This enabled reliable prediction of flood elevations and discharges at downstream locations in the Tennessee River system. The flow, elevation, and date information for the historic events used in calibration is from TVA, National Weather Service, and United States Geological Survey data as well as FEMA flood profiles. The calibrated reaches were linked together to form a continuous model for flood simulation. The HEC-RAS model calibration produced final geometry files to be used in simulations of floods at TVA nuclear plant sites. The calibration process is further described in References 28 and 29.

8.2 Dam Rating Curves Initial dam rating (headwater rating) curves are required as inputs to TVAs HEC-RAS model used in performing flood-routing calculations for the Tennessee River System. The initial dam rating curves provide total dam discharge as a function of headwater elevation. Final dam rating curves are simulation specific and determined in the HEC-RAS model which incorporates tailwater effects.

The Dam Safety group of TVAs River Operations (RO) division evaluated the stability of TVAs dams under various load conditions in accordance with TVA Dam Safety acceptance criteria; they also considered load conditions that comply with nuclear guidelines. The load conditions studied included PMF level headwater elevations with varying tailwater elevations and multiple seismic load conditions. Each stability analysis report was examined and the results provided the basis for the dam failure cases presented in the dam rating curves. Earthen embankment breaches were determined from empirically based methods as recommended in Reference 19. The dam rating curves are documented in References 30 and 31.

8.3 Unsteady Flow Rules HEC-RAS uses unsteady flow rules to control complex releases from hydraulic structures. In the reevaluation, unsteady flow rules were developed in HEC-RAS to represent the operations of the reservoirs in the Tennessee River system upstream of Wilson Dam. The rules blend the flood operational guides (Reference 32) and dam rating curves (References 30 and 31). The unsteady flow rules incorporate the flood operational guides, as they provide prescribed operating ranges of reservoir levels for the reservoirs in the TVA system. The rules reflect the flexibility provided in the guides to respond to unusual or extreme circumstances through the use of elbow recovery curves; seasonal variability in the operational guides is also included in the unsteady flow rules.

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Elbow recovery curves are used during the antecedent storm to expedite the recovery of the reservoir to a more normal state. The use of elbow curves is explained in detail in the Flood Operational Guides (Reference 32). The antecedent storm, in which the elbow recovery curves are implemented, is a three-day storm occurring prior to a three-day dry period and the three-day main storm. Once the antecedent storm is complete and the surcharge elevation is exceeded the discharges will be calculated using the dam rating curves for the applicable cases.

The surcharge elevation is the elevation at which gates are fully open and discharge through the dam is computed by the dam rating curve.

The dam rating curves are used in concert with the flood operational guides in the unsteady flow rules to define total dam discharge as a function of headwater elevation, tailwater elevation, and outlet configuration. If, as during a PMF event, headwater exceeds the normal operating range, the dam rating curves determine flow over other components such as non-overflow sections, navigation locks, the tops of open spillway gates, tops of spillway piers, saddle dams, and rim leaks. If the operating deck is not exceeded, operations return to the Flood Operation Guides (Reference 32); if the operating deck is exceeded gates remain in the open position. The unsteady flow rules are documented in References 33 and 34.

8.4 Probable Maximum Flood Inflows To determine the inflows for the PMF event, rainfall depths for the 21,400 square-mile downstream centered March and the 7,980 square-mile Bulls Gap centered March PMP events, as described in HMR 41 (Reference 12), were determined; hydrographs were developed using validated unit hydrographs (UHs) as well as UHs that were adjusted for non-linear basin response (Reference 35); and storage from potentially critical projects outside the model limits are identified and included.

The application of the following approach was adopted for inflow development (Reference 36) in sub-basins above Wheeler Dam for use in the subsequent Tennessee River routing model:

1. transform rainfall to runoff using available UHs and using UHs adjusted for non-linear basin response;

2. develop sub-basin surface runoff hydrographs both with no losses and using applicable loss rates;

3. include sub-basin monthly average constant baseflow (Reference 37);

4. develop total event inflow hydrographs for the sub-basins in the Tennessee Valley watershed above Wheeler Dam; and

5. as necessary, translate developed tributary sub-basin surface runoff hydrographs to model input points.

8.4.1 Hydrometeorological Report The applicability of the National Weather Service (NWS) HMR to the development of the PMF at TVA nuclear projects was reviewed and documented in Reference 38. This was done in accordance with the requested actions in the 50.54(f) letter requiring a reevaluation of flood causing mechanisms using present-day regulatory guidance and methodologies.

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The NWS is no longer funded for PMP research and has not updated the HMRs since their publication. While the Bureau of Reclamation references indicate that updated PMP estimates are needed, no evidence was found of any published revision in the PMP estimates applicable to the TVA projects. In review, only one errata was noted in HMR 41 Table 7-2 where two values were assumed swapped and were noted in the calculation where used (Reference 12). The HMRs had sound methodology and data basis at the time of the analysis.

TVA is currently reevaluating the PMP and developing a replacement for the HMRs. The process involves an expert panel review of the product in its entirety, with specific attention to storm selection and storm transposition. The replacement for HMRs was not completed at the time of this reevaluation but preliminary data supports the conservatism of the current HMRs.

Therefore, the current HMRs meet the requirements of the 10 CFR Section 50.54(f), and are appropriate for use in the re-analysis.

8.4.2 Critical Storm Selection The critical storm selection for the PMF event on the Tennessee River for SQN is reviewed and documented in Reference 39. As defined in Section 1.1 of the NUREG/CR-7046, the probable maximum event is the event that is considered to be the most severe reasonably possible at the location of interest and is thought to exceed the severity of all historically observed events.

For example, a PMF is the hypothetical flood generated in the drainage area by a PMP event.

The PMP for the Tennessee Valley at the aforementioned nuclear projects is currently defined by the NWS HMR 41 (Reference 12).

The HMR-41 guidance defines two general PMP configurations. The first configuration is a 21,400-square-mile PMP event with either an upstream or a downstream centering, which has a fixed location over the Tennessee Valley watershed. The second pattern type is a moveable 7,980-square-mile PMP event that may be slid roughly from southwest to northeast along the long axis of the published pattern.

As stated in Reference 40, currently analyzed storms are the 21,400-square-mile downstream centered event, the 7,980-square-mile Bulls Gap centered event and the 7,980-square-mile Sweetwater centered event. Selection of these events was based on the original TVA plant licensing analysis. The original analysis was based on the ANSI N170-1976/ANS-2.8, Section 5.2.6 guidance recommending downstream placement and use of NWS proposed methods. As expected, the previous and current TVA modeling efforts show that the critical PMP storm event producing the PMF when routed maximizes the rainfall volume over the total watershed. A re-analysis was performed using GIS software to allow direct comparison of the reviewed events.

The GIS analysis of the 21,400 square-mile PMP event showed that the downstream centered event produced higher rainfall depths at the locations reviewed. An analysis of the 7,980-square-mile PMP event was also performed. It was determined that PMP depth is maximized at SQN by the 7,980-square-mile, Station 5 (Bulls Gap) centered event.

An independent analysis performed by Pacific Northwest National Laboratory confirmed that the critical storm centerings are identified for use in computing PMP rainfall (Reference 41).

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8.4.3 National Inventory of Dams (NID) Inflows The USACE maintains the National Inventory of Dams (NID), which provides characteristics for each dam (location, height, and volume). The guidance for assessment of flooding hazards due to dam failure (Reference 19) requires a screening process to identify all dams that are inconsequential. In order to identify the number of structures upstream of SQN the NID was queried for the Tennessee Valley watershed and approximately 700 were included in the SQN analysis. As documented in Reference 42, rectangular-shaped hydrographs are used at existing inflow locations to account for the volume of these dams. These hydrographs are distributed across 6 days, from one day after the peak antecedent precipitation to one day after the peak main storm precipitation.

8.5 Seismic Inflows Staff positions listed in Section 5.6 of Reference 19 specify that the coincident inflow from the 25-year flood be applied during the 10-4 annual exceedance probability seismic hazard and either the 500-year flood or the half PMF be applied as the coincident inflow during half the 10-4 excedence probability seismic hazard ground motion. To develop these inflows, a methodology for production of scaled hydrographs was developed in Reference 43. The scaled hydrograph methodology used starts with the selection of streamflow event durations sufficient to allow maximization of the headwater elevation at the hypothetical failure location. These durations are then used in probabilistic analyses to develop the required return period volumes from historical streamflow data. A candidate historical or synthetic rainfall event sufficiently large to reflect the watershed translation of rainfall to runoff is then selected. This rainfall is distributed across the watershed, losses are applied and the surface runoff is generated based on available unit hydrograph data. The candidate surface runoff ordinates are then scaled to produce the calculated probabilistic volumes at the selected durations.

8.5.1 National Inventory of Dams (NID) Seismic Inflows During postulated single and multiple project failure events, the concurrent failure of NID identified projects outside the model is considered possible. The NID volumes are located across the sub-basins with conveyances having differing sinuosity, length, slope, cross sectional and roughness characteristics. As a result, the postulated failure waves are expected to pass through a variety of supercritical, critical and subcritical flow regimes as they traverse the respective reaches starting at the failure location and ending at the respective model input points. The resulting translation reduces the peak flows and spreads the time base of the volume input. A simplified calculation approach, as described in Reference 43, is used to account for the NID volumes under these failure conditions.

8.6 Sunny Day and Watauga Project Specific PMF Inflows Sunny day project failures are postulated to occur due to non-hydrologic and non-seismic causes as required by Reference 19. Simplified volume analyses are used and TVA projects having the potential to cause flooding at the plant sites were identified Reference 44. The Watauga Project Specific PMF analysis is included in the sunny day failure analysis to provide a bounding scenario for the sunny day failures on the Holston tributary. Inflows for both the Sunny day failures and the Watauga Project Specific PMF are documented in Reference 43.

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The development of two inflow scenarios for use in these model calculations is necessary.

Inflows for use concurrent with sunny day failures identified in Reference 44 are included in Reference 43. Constant June baseflows from Reference 37 are applied for both. Watauga project PMP rainfall was taken from Reference 43 as recommended by Reference 19 and is convoluted in a spreadsheet using Soils Conservation Service (SCS) methodology. June curve numbers were taken from Reference 45 and unit hydrograph data were taken from Reference 36.

NID inflows are not included in sunny day simulations.

9 APPLICABLE FLOOD CAUSING MECHANISMS 9.1 Local Intense Precipitation 9.1.1 Previous Analysis Previous evaluation of the effect of LIP on water surface elevations in the SQN plant site area is described in Section 3.4.1.

PMP for the plant drainage systems is defined by HMR 52 (Reference 17) and HMR 56 (Reference 5). Underground drains are assumed clogged, and runoff is assumed to be equal to rainfall. One of two methods of analysis is used on each drainage area according to whether (1) flow conditions controlled or (2) ponding or reservoir-type conditions controlled.

9.1.2 Technical Approach The reevaluation of the LIP on water surface elevations in the SQN plant site utilized HEC-HMS and HEC-RAS simulations and is documented in Reference 6. The LIP analysis is a measure of the extreme precipitation (high intensity and short duration) at a specific location, in this case SQN.

According to NUREG/CR-7046 (Reference 16) LIP should be equivalent to the 1-hr, 1-mi2, PMP at the location of the site. The analysis assumes fully functional site grading and partially blocked drainage channels.

The rainfall hyetograph of the 1-hr, 1-mi2, PMP for SQN is determined according to HMRs 52 and 56 (References 17 and 5, respectively). The drainage basins and storage volume in each drainage basin were computed using topographic contour data. After determination of hydrologic inputs (hydraulic length, Mannings n, time of concentration, channel flow travel time, etc.) the HEC-HMS model is used to produce the discharge hydrographs for each drainage basin utilizing the SCS unit hydrograph method to transform the LIP rainfall from each drainage basin to runoff hydrographs. Additionally, the HEC-RAS model is used to produce the water surface elevation for defined channels and level pool storage areas utilizing the discharge hydrographs computed in the HEC-HMS models.

This analysis conservatively assumes that drainage features carrying offsite drainage toward SQN are fully functional and drainage features carrying on-site drainage away from SQN are not fully functional. Weir flow locations carrying discharge away from the plant were reduced to 50 percent of their original length thus increasing flows to the steady-state model. Additionally, this analysis assumes partially, but significantly, blocked drainage channels. Each cross-section controlling outflow Northeast of the plant is assumed to be blocked by a 40-foot long, 10-foot high obstruction in the bottom of a channel (which would be similar to a trailer building).

Obstruction of drainage channels by multiple large trailers is considered conservative.

(Reference 6)

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The results of this analysis are presented below in Table 9.1-1:

Table 9.1-1 Results of SQN LIP Analysis Max Storage WSE* Description Area (feet)

AREA 4 706.3 Between Shield Buildings and Diesel Generators, spent fuel storage AREA 5 706.3 Multipurpose Building area, North of Unit 1 AREA 6A 706.0 500 kV Switchyard AREA 6BN 706.1 Parking immediately Northwest of main portal AREA 6BS 706.0 Main portal, plant office, TEACP AREA 6C 704.4 161 kV Switchyard AREA 6D 705.7 South of Turbine Building and Unit 2 AREA 6E 702.0 West of 161 kV Switchyard AREA 7 704.5 South of main plant entrance drive

• Water Surface Elevation Page 47 of 70

_,,_,,_,,_,, DRAINAGE / ROUTED STORAGE AREA DIVIlE

~ SEQUOYAH NUCLEAR PLANT

-~ STORAG,AREAS Figure 9-1 Local Intense Precipitation Storage Areas Page 48 of 70

9.2 Flooding from Rivers and Streams 9.2.1 Previous Analysis The results of the previous PMF analysis are described in Section 4.2.2.3.1. This analysis incorporates the dam safety modifications utilizing the SOCH suite of programs to determine the PMF elevation. The inflows for this analysis are determined using the FLDHYDRO and the Antecedent Precipitation Index (API) runoff methodology.

9.2.2 Technical Approach The reevaluation of the PMF is in accordance with the guidance in Reference 16. The PMP is applied to the drainage basin of the rivers and streams adjoining SQN. Inflows to the model were generated using the SCS runoff methodology. The SCS runoff parameters are calibrated to the inflow model presented in the LAR (Reference 4). Curve numbers (CNs) used were validated against the TVA API method results for the 21,400-sq.-mi. March event. When compared to National Resources Conservation Service soils and USGS Multi-Resolution Land Characteristics data for sub-basins above the Wheeler project, the area weighted, validated CNs for the antecedent event are approximately 15.7% higher and for the main storm event are 4.3%

higher. Therefore these CNs are considered conservative. Inflow development, storm selection, as well as additional inputs and assumptions are described in Section 8.

The verified HEC-RAS unsteady flow model of the Tennessee River System developed for the LAR and approved in the NRC January 28, 2015 WBN SER (Reference 15) is used in the hazard reevaluation. The summary of differences between the SQN LAR PMF analysis and the hazard reevaluation include:

a. The stream course model used was changed from the Simulated Open Channel Hydraulics (SOCH) suite of software, including TRBROUTE, CONVEY, WWIDTH, and SOCH, to the USACE Hydrologic Engineering Center River Analysis System (HEC-RAS) model.

b. Eighteen critical TVA dams were evaluated for stability, including:

1. Apalachia 10. Fort Patrick Henry

2. Blue Ridge 11. Hiwassee

3. Boone 12. Melton Hill

4. Chatuge 13. Norris

5. Cherokee 14. Nottely

6. Chickamauga (downstream) 15. South Holston

7. Douglas 16. Tellico

8. Fontana 17. Watauga

9. Fort Loudoun 18. Watts Bar

c. Nine dams in the tributary system, that were not evaluated and are postulated to fail, were included:

1. Ocoee 1

2. Ocoee 2 Page 49 of 70

3. Ocoee 3

4. Chilhowee

5. Calderwood

6. Cheoah

7. Mission

8. John Sevier

9. Wilbur

d. Four dams that were evaluated for stability, but were not credited due to low margin:

1. Apalachia (during the 21,400 sq. mi. storm)

2. Boone

3. Fort Patrick Henry

4. Melton Hill (During the 7,980 sq. mi. storm)

e. A license condition for Watts Bar is included in the NRC SER Reference 15 that states modifications to dams will be completed. These modifications are as follows:

1. Cherokee - Post-tensioning non-overflow dam and raising embankment overtopping elevation (removing HESCO barriers)

2. Douglas - Post-tensioning non-overflow dam, raising embankment saddle dam crests to a minimum of 1022.5 ft., and adding saddle dam toe berms

3. Fort Loudoun - Post-tensioning non-overflow dam and replacing HESCO barriers with permanent modifications. (approximately 1900 ft. of the HESCO barriers will be replaced by the end of 2017 following installation of new bridge)

4. Tellico - Reinforcing the non-overflow dam neck and raising the embankments overtopping elevation (removing HESCO barriers)

5. Watts Bar - Reinforcing the portions of the non-overflow and lock necks; raising the overtopping elevation of embankments and flood walls (removing HESCO barriers);

lowering the west saddle dam elevation to 752.0 ft.

Three HEC-RAS simulations were performed:

a. 21,400 square-mile downstream-centered March storm

b. 7,980 square-mile Bulls Gap-centered March storm

c. 7,980 square-mile Bulls Gap-centered March storm with alternative Douglas Saddle Dam failures The PMF in rivers and streams adjoining SQN is determined by applying the PMP to the drainage basin of these rivers and streams adjoining SQN. Inflows to the model are generated using industry standard codes and the SCS runoff methodology.

Results of the PMF analysis for the two candidate storms, the 21,400 sq.-mi. storm and the 7,980 sq.-mi. Bulls Gap centered storm, are presented in Table 9.2-1. This analysis assumes modifications to dams as presented in Section 9.2.2.e.

Page 50 of 70

Table 9.2-1 PMF Elevation and Discharge at SQN (TRM 484.5) Resulting from Reevaluation PMF Event Elevation (ft.) Discharge (cfs) 21,400 Sq.-Mi. Event 721.2 1,404,000 7,980 Sq.-Mi. Bulls Gap Event 723.7 1,532,000 The 7,980 square-mile Bulls Gap centered storm results presented in Table 9.2-1 result in overtopping and subsequent failure of Douglas Saddle Dams 1 and 3. As a result an additional PMF analysis assuming Douglas Saddle Dams 1 and 3 are raised to prevent overtopping is presented in Table 9.2-2.

Table 9.2-2 PMF Elevation and Discharge at SQN (TRM 484.5) Including Emergency Action Plan at Douglas Dam PMF Event Elevation (ft.) Discharge (cfs) 7,980 Sq.-Mi. Bulls Gap Event with 719.6 1,340,000 Emergency Action Plan at Douglas Dam To prevent overflow of the Douglas Saddle Dams 1 and 3, closure plans have been developed as an interim action under the guidance provided in an existing supplement to the TVA Emergency Action Plan (EAP). The closure plans will install temporary engineered flood barriers across the full length of the crests of both Saddle Dam 1 and 3 when the trigger elevations are reached in the Douglas Dam reservoir. The flood elevation which triggers the closure plans provides sufficient notification time for TVA to install the temporary engineered flood barriers in advance of re-evaluation flood levels having the potential to exceed the current crest elevations at Saddle Dam 1 and 3. The installation time was established based on actual TVA experience with similar installations at other TVA dams. The time between notification and completion of installation for the barriers at Saddle Dam 1 and 3 is seven days. In support of the closure plans, material has been staged off-site with the source suppliers. As a result the 21,400 sq.-mi. storm becomes the controlling event with the EAP at Douglas Dam and a maximum elevation of 721.2 ft.

9.3 Flooding from Dam Breach or Failures Upstream dam failures by seismic events, structural defects, as well as other failure modes have the potential to impact SQN.

A simplified volume analysis identifies individual dams that have the potential to cause flooding at SQN if failed during a sunny day (Reference 44). The analysis conservatively assumes that gates at downstream dams are closed and inoperable providing a bounding scenario for cascading dam failures as a result of a sunny day dam failure. The results of the analysis identified eight dams whose failures could lead to flooding at the plant. These dams are as follows:

1. Chatuge

2. Cherokee

3. Fontana Page 51 of 70

4. Norris

5. Nottely

6. South Holston

7. Watauga

8. Watts Bar The eight dams identified were evaluated in one of three ways: under a project specific PMF event, single seismic dam failure combined with a flood event, or failure during a sunny day. A Watauga project specific PMF simulation bounds a sunny day failure of the South Holston dam.

Chatuge and Watts Bar dams have low margin for seismic stability and are analyzed as single seismic dam failure simulations paired with a flooding event. These seismic simulations bound sunny day dam failure simulations for these dams. The remaining dams, Cherokee, Fontana, Norris, and Nottely are analyzed for failure in sunny day simulations. The Fontana Dam sunny day failure simulation bounds the Fontana seismic failure simulation because the seismic failure results in only a minor loss of the Fontana dam.

9.3.1 Project Specific PMF A project specific PMF above Watauga Dam (Reference 46) could result in overtopping a subsequent failure of main embankments. The postulated failure of Watauga Dam results in the cascading failure of Wilbur, Boone, Fort Patrick Henry, and John Sevier Dams. This event serves as the upper bound of any sunny day failure at South Holston or Watauga dams which are tributaries of the Holston River. The results are presented in Table 9.3-1.

Table 9.3-1 Elevation and Discharge at SQN Resulting from Project Specific Dam Failures Dam Failure Elevation (ft.) Discharge (cfs)

Watauga Dam Project Specific 689.2 325,000 PMF 9.3.2 Sunny Day Dam Failure 9.3.2.1 Previous Analysis No previous analyses were completed on the failure of a dam upstream of SQN on a sunny day.

9.3.2.2 Technical Approach Potential failure modes were evaluated for each dam to determine the sunny day simulations breach parameters.

a. Cherokee -The South Embankment is the longest and tallest embankment at Cherokee and is assumed to fail for the sunny day simulation.

b. Fontana-A total failure of Fontana Dam is assumed in the sunny day simulation. This failure will present the greatest potential for flooding downstream.

c. Norris-The Right Abutment is assumed to collapse and fail in the sunny day simulation.

Page 52 of 70

d. Nottely- The sunny day simulation assumes piping through the right embankment (main embankment) that could lead to sloughing, sliding and a breach of the embankment.

A sunny day failure of Watts Bar Dam was not included since the resulting elevation would be bounded by the Watts Bar single seismic failure during a 500-year event. The results of the Watts Bar single seismic failure simulation are in Table 9.3-3. Results of the sunny day dam failure simulations are presented in Table 9.3-2 (References 46 and 47).

Table 9.3-2 Elevations and Discharges at SQN Resulting from Sunny Day Dam Failures Dam Failure Elevation (ft.) Discharge (cfs)

Cherokee Dam Sunny Day Failure 691.2 471,000 Fontana Dam Sunny Day Failure 694.4 648,000 Norris Dam Sunny Day Failure 691.0 453,000 Nottely Dam Sunny Day Failure 683.6 88,000 9.3.3 Single Seismic Dam Failure Three dams, Chatuge, Fontana, and Watts Bar, have low margin for seismic stability. The Fontana seismic stability analysis results in only a minor failure which does not result in the loss of the entire volume of the reservoir; it is bounded by the Fontana Sunny Day analysis. The results of the single seismic failures combined with a 500-year flood event are presented in Table 9.3-3.

Table 9.3-3 Elevations and Discharges at SQN Resulting from Single Seismic Failure of Upstream Dams Seismic Dam Failure Combination Elevation (ft.) Discharge (cfs)

Chatuge Dam Single Seismic Failure During a 500-Year 693.9 645,000 June Flood Event*

Watts Bar Dam Single Seismic Failure During a 500-696.8 724,000 Year June Flood Event*

• June flood events are chosen because they have higher median reservoir levels 9.4 Flooding from Combined Effects 9.4.1 Floods Caused by Precipitation Events 9.4.1.1 Previous Analysis In the previous flood evaluation, described in Section 3.4.8.1, coincident wind wave run-up was computed for and applied to the controlling PMF event. Coincident wind wave activity was determined based on the guidance provided in the Department of Army, Engineering Page 53 of 70

Technical Letter (ETL) 1110-2-8 (Reference 48). The overland wind speed was determined from ASOS Surface 1-minute data from the National Climatic Data Center for four critical structures.

9.4.1.2 Technical Approach In this reevaluation, this wind wave combination scenario evaluates the effects of wind-wave activity during floods at a site along the shore of an enclosed body of water. This combination is described in NUREG/CR-7046 (Reference 16) and includes the antecedent, PMP event, and waves induced by 2-year wind speed applied along the critical direction.

The results of the analysis are the wind wave heights at the four critical structures to be added to the PMF elevation.

The U.S. Nuclear Regulatory Commissions Guidance for Assessment of Flooding Hazards Due to Dam Failure (Reference 19) requires that wind wave activity be accounted for at all dams. Wind wave activity was calculated at Cherokee, Douglas, Fort Loudoun, Hiwassee, Melton Hill, Norris, Nottely, South Holston, Tellico, Watauga, and Watts Bar Dams. Wind wave activity was calculated at the previously mentioned dams because the top of dam elevations were such that they would not be overtopped by the stillwater flooding elevation, but could conceivably still be susceptible to wind wave damage.

The combined effects flood consisting of the effects of wind wave on elevations generated from the controlling PMF event with the EAP at Douglas in place is evaluated. The wind wave heights (Reference 49) to be added to the elevation at dams are displayed in Table 9.4-1.

Table 9.4-1 Wind Wave Elevation Results at Dams Maximum Maximum Wave Calculated Stillwater Height, Hmax Dam Crest Elevation Flooding Elevation Dam (ft.) (ft.) (ft.)

Cherokeea 1084.4 1.8 1095.8 Douglasb 1022.4 1.5 1024.9c Fort Loudoun 833.9 1.1 837.0 Hiwassee 1540.9 0.8 1537.5 Melton Hill 805.0 0.6 805.0 Norris (Main) 1055.2 0.8 1061.0 Nottely 1789.2 1.3 1807.5 South Holston 1752.7 1.1 1765.0 Tellico 832.6 2.2 834.9 Tims Ford 895.1 1.1 910.0 Watauga 1987.2 0.7 2012.0 Watts Bar Main 767.7 1.3 772.0 a

The volume of water overtopping is evaluated in Reference 49 and does not result in damages to Cherokee embankments.

b Douglas Saddle Dams 2 through 10 are sheltered from the wind.

c Douglas Dam Crest Elevation with EAP.

Page 54 of 70

The wind wave heights to be added to the PMF elevation at critical structures are displayed in Table 9.4-2.

Table 9.4-2 Wind Wave Elevation Results at Critical Structures Total Wind Wave Stillwater Height added (Wave PMF runup + wind setup)

Location Elevation (ft.) (feet) Final PMF Elevation (ft.)

Diesel Generator 721.2 0.8 722.0 Building ERCW Pumping 721.2 2.6 723.8 Station Unit 2 Reactor 721.2 2.4 723.6 Building Unit 1 Reactor 721.2 3.1 724.3 Building 9.4.2 Multiple Seismic Dam Failures with Combined Flood Event 9.4.2.1 Previous Analysis The results of the previous seismic analysis are presented in Section 3.4.8.2. This analysis uses the SOCH suite of codes to determine the critical combinations for flood cause by an OBE coincident with a 1/2 PMF event and SSE coincident with a 25-year flood. Wind wave run-up was not applied to seismic flood events because this combination did not produce the controlling flood elevation.

9.4.2.2 Technical Approach The seismic dam failure reevaluation is in accordance with the guidance in References 19 and 16. Staff positions listed in Section 5.6 of Reference 19 specify that the coincident inflow from the 25-year flood be applied during the 10-4 exceedance probability seismic hazard event and either the 500-year flood or the half PMF be applied as the coincident inflow during half the 10-4 exceedance probability seismic hazard ground motion. This applies to both single and multiple seismic dam failures. An analysis to support the screening of multiple dam failures due to a single seismic event is detailed in Reference 50. The analysis presents deaggregation results for the 10,000 year and 1/2 10,000 year ground motion for both concrete and earthen embankments.

Using seismic stability results, a volume analysis was performed to determine which seismic centering would result in the largest volume of water that could be released. The results indicate that a Fort Loudoun centered seismic event results in multiple dam failures with the Page 55 of 70

largest volume of water that could be released for a 10,000 year ground motion. A Douglas centered seismic event results in multiple dam failures with the largest volume of water that could be released for a 1/2 10,000 year ground motion.

The reevaluation considered the following scenarios:

a. 10-4 Fort Loudoun centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fontana, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 25-year June flood event

b. 10-4 Fort Loudoun centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fontana, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 25-year June flood event

- Watts Bar Dam conservatively fails at peak headwater

c. 10-4 Fort Loudoun centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fontana, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 25-year June flood event

- Chickamauga Dam fails seismically per the stability analysis

d. 1/2 10-4 Douglas centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fort Patrick Henry, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 500-year June flood event

e. 1/2 10-4 Douglas centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fort Patrick Henry, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 500-year June flood event - Watts Bar Dam conservatively fails at peak headwater

f. 1/2 10-4 Douglas centered seismic event resulting in failures of Apalachia, Blue Ridge, Chatuge, Fort Loudoun, Fort Patrick Henry, Melton Hill, Tellico and Watts Bar Dams and the Watts Bar West Saddle Dam coincident with a 500-year June flood event - Chickamauga Dam fails seismically per the stability analysis Page 56 of 70

Results of the seismic failure (Reference 51) of upstream dam simulations are presented in Table 9.4-3.

Table 9.4-3 Elevations and Discharges at SQN Resulting from Seismic Failure of Upstream Dams Seismic Dam Failure Combination Elevation (ft.) Discharge (cfs) 10,000-Year Fort Loudoun Centered Seismic 701.5 868,000 Failure During a 25-Year June Flood Event 10,000-Year Fort Loudoun Centered Seismic Failure During a 25-Year June Flood Event -Watts 703.0 965,000 Bar fails at peak headwater 10,000-Year Fort Loudoun Centered Seismic Failure During a 25-Year June Flood Event - 697.7 883,000 Chickamauga Dam fails seismically Half-10,000 Year Douglas Centered Seismic Event 704.2 943,000 During a 500-Year June Flood Event Half-10,000 Year Douglas Centered Seismic Event During a 500-Year June Flood Event -Watts Bar 705.3 1,013,000 fails at peak headwater Half-10,000 Year Douglas Centered Seismic Event During a 500-Year June Flood Event - 700.8 978,000 Chickamauga Dam fails seismically Page 57 of 70

10 EVALUATION OF UNCERTAINTIES Inherent uncertainties exist in the analysis of the PMF in rivers and streams. Flooding simulations require many assumptions while determining input parameters for the analysis as well as during simulation. These assumptions are based on available data and industry accepted practice. NRC Interim Staff Guidance recommends several sensitivity analyses be performed to understand and account for this inherent uncertainty in key parameters of the flood hazard reevaluation (Reference 19).

Recommended sensitivities included evaluation of dam breach configuration, debris accumulation, gate failures, initial reservoir levels, reservoir inflow, and tailwater conditions. Section 10 discusses the sensitivity analyses performed to support the flood hazard reevaluation for SQN.

10.1 100% Runoff Simulations assuming no losses (100% runoff) were performed for the two PMF storm events, the 21,400 sq.-mi. downstream centered March event and the 7,980 Bulls Gap centered March event. The 100% runoff simulations assumed no precipitation losses and quantified the impacts on flood elevation and discharge at SQN due to the increased runoff volume. The purpose of the 100% runoff simulations was to present the upper bound for runoff volume for the two PMF storm events.

The results of the 100% runoff sensitivity simulations produced an increase in elevation at SQN of 11.4 feet for the 21,400 square mile storm event, and an increase of 16.1 feet for the 7,980 square mile Bulls Gap Storm event because of additional cascading dam failures.

These increases are not realistic and need not be considered in the re-evaluation. This conclusion is based on:

• Section 4.1 indicates the impervious area of the watershed above Guntersville Dam is extremely small (1.96%) and it is unrealistic to assume 100% runoff for the entire watershed during a nine day event based on land use.

Rainfall to runoff transformation rates calculated are approximately 88%.

• Unit hydrographs used in determining inflow to streams have been calibrated to actual storm run-off levels for larger historical events.

• Reservoir flow models are biased to predict flood levels at or above historical storm levels.

10.2 Peaked and Lagged Unit Hydrographs Peaking and lagging of the unit hydrographs is performed for the 21,400 sq.-mi. downstream centered March event and the 7,980 sq.-mi. Bulls Gap Centered March event. These simulations utilize unit hydrographs that were adjusted to reflect the non-linearity of the runoff generation process under field conditions. Adjustments to the unit hydrographs include increasing the peak discharge by 20% and decreasing the time-to peak by 1/3 as recommended in Appendix I of Reference 16.

Page 58 of 70

The results of the peaked and lagged unit hydrographs sensitivity simulations produce an increase in elevation at SQN of 0.9 feet for the 21,400 square-mile storm event, and an increase of 0.5 feet for the 7,980 square-mile Bulls Gap Storm event. The 7,980 square-mile event included protection of the Douglas Dam saddle dams as discussed in Section 9.2.2. SQN equipment required for flood mode operation remains protected for this small elevation increase above the CLB Stillwater flood elevation.

Adjusting the unit hydrographs for SQN is not required based on:

• The unit hydrographs developed for the licensing basis PMF and used in the flood hazard reevaluation are developed from gage data and calibrated to historical events.

• In the 2014 evaluation of the WBN LAR and approved in the 2015 WBN SER (Reference

15) NRC performed a confirmatory check of TVAs unit hydrographs utilizing Snyders unit hydrograph method. The NRC noted that the hydrographs computed by TVA have shorter peak time and higher peak flows when compared to the synthetic Snyder unit hydrographs. Based on reviewing the methodology and procedure used by TVA to develop the unit hydrographs the NRC staff concluded that TVAs unit hydrographs were conservative and acceptable. Therefore adjusting the peak and time to peak of the unit hydrographs is not appropriate.

10.3 Gate Operability/Blockage The PMF analysis assumed each dam is able to fully open gates and no debris blockage of the spillways occurs. To evaluate the sensitivity of this assumption, sensitivity to gate operability or gate blockage due to debris accumulation at dams credited for stability was performed for both the 21,400 sq.-mi. downstream centered March event and the 7,980 sq.-mi. Bulls Gap Centered March event. Multiple simulations were performed using the HEC-RAS unsteady flow model to compute peak water surface elevations at various dams whose outlet capacities were postulated to be reduced during the two PMF storm events. For each dam analyzed, the percentage of gate openings that may be blocked by debris and still pass the PMP without causing other failures at the dam was determined. The simulations for each dam did not provide results at SQN, only at the dam being analyzed. The dams considered include Blue Ridge, Chatuge, Cherokee, Douglas, Fontana, Fort Loudoun, Hiwassee, Norris, Nottely, South Holston, Tellico, Watauga, and Watts Bar Dams. (Reference 52)

The dams analyzed above the SQN site have an outlet unavailability margin greater than 10%

except Cherokee and Douglas in the 7,980 square-mile Bulls Gap centered event. Those two dams have an outlet unavailability margin less than 5% as measured by reaching a potential embankment overtopping elevation.

Spillway gate blockage due to maintenance issues or debris has been evaluated and determined to not represent a significant hazard for SQN. The conclusion is based on:

TVA RO has a debris management program

• There is no history of debris blocking spillways at these dams in historically large flooding events

• There is no barge traffic above these two dams

• River Operations (RO) monitors gates daily for operation and the maintenance program for gates assures high reliability Page 59 of 70

• RO has the means and resources to resolve gate issues if needed to respond to flood events

• The embankments at the dams are not threatened until floods reach PMF levels. At these levels, the flood waters at the larger reservoir dams significantly overtop the spillways gates by several feet. Floating debris would easily pass over the gates and continue downstream without blocking flow paths 10.4 Breach Size The embankment breach size and breach method sensitivity was evaluated for each dam with earthen embankments. The breach method selected, Von Thune and Gillette is the best suited for TVA based on the size of the dams in the system and the volume-driven nature of the Tennessee River System. Total failures of concrete structures are conservatively assumed for dam failures above Chickamauga Dam when the dam stability evaluation demonstrates low margin (Reference 53).

10.5 Initial Reservoir Level TVA controls and/or schedules the releases from TVA dams and some non-TVA dams. Providing the scheduled releases from these dams enables TVA the opportunity to control the river system as an integrated system. TVA monitors the dams 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> a day, 7 days a week. The releases are based on the needs of the entire Tennessee River system while maintaining reservoir levels within the operating guide. The reservoir operating guidelines are implemented as prescribed operating ranges of reservoir levels throughout the year. Operating within the prescribed operating ranges provides consistency in the normal reservoir levels, therefore also providing consistency in the headwater and tailwater elevations at the dams. Reference 19 presents recommendations to evaluate the sensitivity of initial reservoir level, inflow, and tailwater conditions. In the case of the Tennessee River System, the reservoir level is strictly controlled and a conservative starting point based on known reservoir operating levels was used in the reevaluation. Tailwater variability is limited by the reservoir level relationships and is simulated within the HEC-RAS model. Thus no additional sensitivities for initial reservoir conditions were evaluated.

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11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS Table 11-1 presents a comparison of the design basis flood elevations and the reevaluated results.

Table 11-1 Comparison of Current Design Basis Elevations and Reevaluation Results 2012 Bounded Flood Causing Mechanism Reevaluation Comments LAR (Yes/No)

Critical elevation is 706 ft.

Local Intense

<706 ft. 706.3 ft. No Analysis exceeds 706 ft. in some Precipitation portions of the protected area.

Controlling event was 7,980 Sq.-

722.0 ft. 723.7a ft. No Flooding from Rivers and Mi. Bulls Gap Centered Event Streams Controlling event was 21,400 Sq.-

722.0 ft. 721.2b ft. Yes Mi. Downstream Centered Event Watts Bar Dam Single Seismic Flooding from Dam

<705 696.8 ft. Yes Failure During a 500-Year June Breaches or Failures Flood Event Flooding from Storm Not a credible flood-causing N/A N/A N/A Surges or Seiches mechanism at this site.

Not a credible flood-causing Flooding from Tsunamis N/A N/A N/A mechanism at this site Flooding from Ice-Induced Not a credible flood-causing N/A N/A N/A Events mechanism at this site Flooding From Channel Not a credible flood-causing Diversion or Migration N/A N/A N/A mechanism at this site Toward the Site 726.2 ft. 726.8a ft. No Design Basis was PMF plus wind waves Reevaluation was PMF plus 3.1 726.2 ft. 724.3b ft. Yes feet of wind wave Flooding from Combined Effects Controlling combination was the Half-10,000 Year Douglas Centered 708.6 ft. 705.3a ft. Yes Seismic Event During a 500-Year June Flood Event when Watts Bar Dam fails at peak headwater.

N/A - Not applicable a

Dam modifications in place as described in Reference 15 b

Dam modifications in place as described in Reference 15 and an EAP at Douglas Dam Page 61 of 70

12 IDENTIFICATION AND EVALUATION OF ANY INTERIM ACTIONS TAKEN TO MITIGATE HIGHER FLOOD HAZARD RELATIVE TO DESIGN BASIS As identified in Table 11-1, reevaluation results for three flood causing mechanisms, LIP, flooding from rivers and streams and flooding from combined effects of PMF and wind are not bounded by the current design basis for SQN. Each of these mechanisms is evaluated below and interim actions defined if needed.

12.1 Local Intense Precipitation The SQN critical elevation, as related to the LIP flooding at the site, is the elevation of the doors leading to the Auxiliary and Control building which is equal to 706.0 ft. The calculation prepared for the flooding at the SQN site due to the LIP determined that the flood water will exceed the plant critical elevation by as much as 0.33 ft. (4 inches) for a maximum duration of 44 minutes.

Per the CLB, the flood at the site will not exceed plants critical elevations. Because the reevaluated LIP flood hazard is not bounded by the CLB, an Integrated Assessment will be performed where the effect of the exceeded flood hazard on the plants safety-related SSCs will be examined in detail. Prior to the completion of the Integrated Assessment, in order to mitigate potential impacts of the LIP flood hazard, the need for interim actions is evaluated as discussed in the following sections.

The results of the reevaluated LIP flood show a small increase in flood levels above the critical plant elevation for a short duration. The LIP associated effects, such as debris loads, hydrodynamic and hydrostatic loads, are expected to be negligible due to the low flow velocities and shallow water depths. The LIP event is not expected to significantly affect the groundwater levels around the structures because of its short duration and due to impermeable land cover at the majority of the plant area.

The access doors, as well as any other openings at or below the LIP flood height, that would allow water into the safety-related buildings were reviewed using site drawings and were observed during a site walkdown performed in February 2015.

The only safety-related buildings that are potentially subject to the LIP flooding are the Condenser Circulating Water Intake Pumping Structure, Auxiliary Building and Control Building.

The plant structures which have safety-related equipment and systems, as well as the elevations of the major exterior accesses, are specified in the SQN UFSAR (Reference 2). The buildings that could be affected by the LIP flood are the ones that have access at elevation below (or close to) the maximum water level from the LIP flood. The access doors to the Auxiliary and Turbine Buildings and Main Steam Valve Vaults from the ground elevation (706.0 ft.) are highlighted in Figure 12-1 (Reference 54).

Page 62 of 70

Figure 12-1 Access Doors to Auxiliary and Control Buildings from 706 Elevation Additionally to the doors shown in Figure 12-1, access to the Auxiliary Building is available from the Service Building at elevation 690 ft. and to the Control Building from the Turbine Building at elevation 685 ft. Each location where water ingress into the safety-related buildings is possible due to the LIP flooding is analyzed as described in this section.

The main steam valve rooms subject to external flooding are 706.0-A1 (U1), 706.0-A2 (U1) and 706.0-A10 (U2). The doors potentially providing water access into these rooms are A103, A107, A233, A234 and A235 (Reference 54). Doors A103 and A107 are identical and have a 6-inch concrete curb with top elevation of 706.5 ft. (Reference 55). Door A233 also has a 6-inch curb with top elevation of 706.5 ft. (Reference 56). Doors A234 and A235 are similar to door A233 (i.e. have ~ 6-inch curb), as verified via walkdown (Reference 7). The louvers in the main steam valve rooms have a bottom elevation of 706.5 ft. (Reference 57). The LIP maximum flood level is at elevation 706.3 ft. (Reference 6). Therefore, the above referenced openings are located above the maximum LIP water level. Additionally, internal flood elevation for the steam valve vault rooms is 4 inches or more (Reference 58). Therefore, even if any water splashed over, equipment in the rooms is required to be protected up to this elevation, i.e. 706-4.

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The Additional Equipment Building (also called the Upper Head Injection Room) includes rooms 706.0-A14 (U1) and 706-A15 (U2). Access to these rooms is through doors A117 and A118, respectively (Reference 54). These doors are located at elevation 706.0 ft. and have no threshold or curb. The Moderate Energy Line Break (MELB) flood height for these rooms is 66 and 41 inches, respectively (Reference 58). Therefore, the impact of a LIP flood is bounded by the MELB flood height. These rooms do not provide a water pathway to other areas at elevation 706 ft. or levels below. (Reference 59)

External doors exist at rooms 706.38-A1 (U1) and 706.08-A2 (U2) (Particular, Iodine & Noble Gas Monitoring Station rooms). Rooms 706.38-A1 and 706.08-A2 provide access to steam valve instrument rooms 706.0-A12 (U1) and 706.0-A13 (U2), respectively. The floor elevation of room 706.38-A1 is 706.4 ft. (Reference 54 and 60) which is above the LIP maximum flood elevation of 706.3 ft. Therefore, room 706.0-A1 and 706.0-A12 are protected from the LIP flooding. Room 706.08-A2 has an entrance elevation of 706.08 (Reference 61). It does not contain any equipment required for safe shutdown capabilities of the plant (Reference 62). Room 706.0-A13 does not include any equipment located below the LIP flood level of 706.3 ft. Additionally, these two rooms do not provide access to any other areas. Therefore, water ingress due to the LIP flooding into rooms A706.08-A2 and 706.0-A13 will not affect the plants capability to perform a safe shutdown.

The railroad bay (door A112) provides access to the waste package area rooms 706.0-A3 and 706.0-A4 through doors A110 and A111. The railroad bay also provides access to the fuel handling area (room 706.0-A6) through doors A113 and A114 from where an unrestricted pathway exists to the adjacent areas, including the cask decontamination room 705.0-A7 through door A115 and areas below 706.0 ft. via stairwell No. 4. The six railroad access bay covers and their embedded frames and the railroad access door A112 and its embedded frame provide a semi-airtight closure. These covers and either of the two doors leading to the fuel handling area (A113 and A114) or either of the two doors leading to the waste packaging area (A110 and A111) operate in conjunction with the railroad access door to provide an airlock (Reference 54, Note 2). Given the airlock operation, water volume potentially entering the fuel handling area and the waste packaging area will be minimal. The MELB internal flooding height is 2 inches or more in the fuel handling area and the areas below on elevation 669.0 ft.

(Reference 63). Additionally, the Fire Hazard Analysis (Reference 62) indicates there is no equipment required for safe shutdown in rooms 706.0-A6, 705.0-A7, 706.0- A8, 706.0-A9 and 669.0-A14, 669.0-A15 on the floor below. There is no safety-related equipment housed in the railroad bay or in the waste package area. Therefore, water ingress through the railroad bay door A112 will not affect the plants capability to perform a safe shutdown.

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One more access to the Auxiliary Building is available from the Service Building at elevation 690 ft. The LIP flood waters can enter the Service Building through multiple doors and openings at elevation 706 ft. (Reference 64) and then propagate downstairs to elevation 690 ft. where a pathway exist from the Service Building corridor to the Auxiliary Building personnel entrance door A56 providing access to the doors A57 leading into Auxiliary Building and door A58 leading into chemistry laboratory area (Reference 65). Water entering the Service Building will spread through 706 ft. elevation first and then propagate to 690 ft. elevation through staircases and other openings. Similarly, water will spread through 690 ft. elevation of the Service Building and also enter 690 ft. elevation of the Turbine Building with large open areas, thus minimizing the water depths in the 690 ft. elevation corridor leading to the Auxiliary Building access door A56.

The Turbine Building is not a safety-related structure and it does not house any safety-related equipment (Reference 58, Table 4 Footnote 2). Also, the Turbine Building does not provide exterior door access to the safety-related Control Buildings other than through door T52 (discussed further in this report). Due to the torturous pathway and large open areas available to accumulate water in the Service and Turbine Buildings, the amount of water that can potentially reach doors A57 and A58 is very limited. In addition, door A57 is watertight (Reference 65) which will preclude any water propagation into Auxiliary Building. There is no equipment required for plant safe shutdown located in the chemical laboratory area (rooms 690.0-A3, 690.0-A4, and 690.0-A5). An access to the Auxiliary Building from the chemistry laboratory area exists via an airlock (room 690.0-A30) through doors A55 and A60. Door A55 is watertight (Reference 66) which will preclude any water propagation into Auxiliary Building.

Another access available from the Turbine Building into the safety-related areas is through the external door T52 at 706 ft. elevation. The T52 door opens directly to a staircase leading to elevation 685 of the Turbine Building where access to the Control Building is available through doors C14/C15 and through an equipment hatch (Reference 67). Door C14 is a watertight door (Reference 68) and door C15 is a pressure personnel access door (Reference 69). The equipment hatch is provided with a gasketed seal (Reference 70) which will preclude water ingress. The internal flooding calculation (Reference 63) also determined that there is no water ingress through doors C14/C15 and through the equipment hatch (Reference 63, pages C44 and C52).

Water entered through the T52 door will spread across Turbine Building elevation 685 ft. and propagate to the Turbine Building elevations below. However, the Turbine Building does not provide other access points below flood elevation into the Auxiliary or Control Buildings except door C27 as discussed below. Therefore, water entering the Turbine Building through door T52 will not enter to the Auxiliary or Control Buildings.

The only other access available from the Turbine Building into the safety-related areas is through door C27 into the Control Building at elevation 685.0 ft. Door C27 is watertight (Reference 66) which will preclude water propagation into Control Building.

The CCW Intake Pumping Structure is also a safety-related building. However, the pumps housed in the building are designed to operate submerged. Also, the cable tunnel is designed for a submerged operating condition. Therefore, the LIP flooding will not adversely affect safety-related equipment in the CCW Intake Pumping Structure.

No interim actions are required for SQN. Based on the performed evaluations there is no impact on equipment relied upon for safe plant operation.

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12.2 Flooding in Rivers and Streams The re-evaluation of the PMF indicates a maximum stillwater elevation of 721.2 ft. at the SQN site assuming overflow failure of Saddle Dams 1 and 3 at Douglas Dam are prevented. If overflow of these saddle dams is not prevented, the maximum stillwater elevation at the SQN site is 23.7 ft. which exceeds the revised design basis flood elevation of 722.0 ft. submitted by TVA to the NRC in Reference 4. For Flooding in Rivers and Streams an Integrated Assessment will be performed.

The overtopping of Douglas Saddle Dams 1 and 3 is being prevented by the EAP installation of temporary engineered barriers as an interim action, as described in Section 9.2.2. Since overtopping has been prevented at Douglas Saddle Dams 1 and 3, the maximum stillwater does not exceed the design basis elevation of 722 ft.

In the Integrated Assessment, TVA will review the long term options for addressing the overtopping elevation of Douglas Saddle Dams 1 and 3 to replace the temporary engineered barrier employed in the EAP described above.

12.3 Combined Effects Flood Caused by Precipitation Events The re-evaluation PMF combined with maximum wind effects results in worst case elevation of 722.6 ft. if failure of the Douglas Saddle Dams 1 and 3 is prevented and a worst case elevation of 726.7 if failure is not prevented. The design basis minimum PMF plus wind effects elevation as submitted to the NRC in Reference 4 is 723.2 ft. Since overflow failure of Douglas Saddle Dams 1 and 3 is prevented by the interim actions established above for Flooding in Rivers and Streams, the re-evaluation PMF combined with maximum wind effects are bounded. No additional interim actions are required other than already established in the evaluation of the impacts to the SQN from Flooding in Rivers and Streams. An Integrated Assessment of the Combined Effects Flood Caused by Precipitation Events will be performed.

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13 REFERENCES

1. U.S. Nuclear Regulatory Commission. Letter to Licensees. Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3 of the Near Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. March 12, 2012.

2. Tennessee Valley Authority, Sequoyah Nuclear Plant Updated Final Safety Analysis Report ,

(Amendment 25).

3. Tennessee Valley Authority, SOCH Geometry Verification Chickamauga Reservoir, CDQ000020080030, Revision 2, EDMS #B41 110720 005.

4. Tennessee Valley Authority, Sequoyah Nuclear Plant, License Amendment Request, Final Safety Analysis Report, August 10, 2012. ML12226A561

5. National Oceanic and Atmospheric Administration and Tennessee Valley Authority, Hydrometeorological Report No. 56, October 1986.

6. Tennessee Valley Authority Calculation CDQ0000002013000057, Fukushima NTTF Recommendation 2.1: Sequoyah Local Intense Precipitation Analysis, Revision 1.

7. Tennessee Valley Authority, Letter to U.S. Nuclear Regulatory Commission, Fleet Response to NRC Request for Information Pursuant to Title 10 of Code of Federal Regulations 50.54(f) Regarding the Flooding Walkdown Results of Recommendation 2.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. Enclosure 3, November 27, 2012.

8. USA. United States Geological Survey. United States Department of the Interior. Multi-Resolution Land Characteristics Consortium (MRLC). n.d. Web. Feb. - Mar. 2013.

http://www.mrlc.gov/index.php .

9. Tennessee Valley Authority Calculation CDQ0000002014000014, Revision 000, BWSC TVAGENQ13003, Revision 0, Curve Number Determination.

10. Tennessee Valley Authority Calculation SCG-1S-503, R0, PMP Site Drainage Analysis Easter Site Drainage, B87931102003

11. National Oceanic and Atmospheric Administration and Tennessee Valley Authority, Hydrometeorological Report No. 45, Probable Maximum and TVA Precipitation for Tennessee River Basin up to 3,000 Square Miles in Area and Duration 72 Hours, May 1969.

12. U.S. Department of Commerce, Weather Bureau, Probable Maximum and TVA Precipitation over the Tennessee River Basin above Chattanooga, Hydrometeorological Report No. 41. June 1965.

13. Harrington, Bruce C. and Ramon G. Lee, Flood Reassessment for the Effects of Dam Safety Modifications, March 1998, B45980326001

14. U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Bellefonte Combined License Application - Nuclear Regulatory Commission Inspection of the Implementation of the Quality Assurance Program Governing the Simulated Open Channel Hydraulics Model - Inspection Report Numbers 05200014/2008-001 and 05200015/2008-001 and Notice of Violation, March 19, 2008, ML080640487.

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15. U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation Letter to the Tennessee Valley Authority, Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Unit 2 (NUREG-0847, Supplement 27), Docket Number 50-391, Tennessee Valley Authority, January 28, 2015.

16. U.S. Nuclear Regulatory Commission, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, NUREG/CR-7046, November 2011.

17. National Oceanic and Atmospheric Administration and U.S. Department of the Army Corps of Engineers, Hydrometeorological Report No. 52, October 1982.

18. American National Standard, American National Standard for Determining Design Basis Flooding at Power Reactor Sites, July 28, 1992, ANSI/ANS 2.8-1992.

19. U.S. Nuclear Regulatory Commission. Japan Lessons-Learned Project Directorate. Guidance for Assessment of Flooding Hazards Due to Dam Failure, Revision 0, JLD-ISG-2013-01. July 29, 2013.

20. Tennessee Valley Authority Technical Memo, Watts Bar Dam Breach Sedimentation Analysis -

Phase 1 Baseline Analysis, March 6, 2015. EDMS #W50150309002

21. McGarr, Arthur and Robert C. Vorlis, The Alaska Earthquake, March 27, 1964: Effects on the Hydrologic Regimen, Seismic Seiches From the March 1964 Alaska Earthquake, Geological Survey Professional Paper 544-E, 1968.

22. Earthquake Survey for the Eastern Tennessee Seismic Zone http://geology.com/press-release/eastern-tennessee-seismic-zone/ (seismic zone map)

23. Landslide Hazard Information http://geology.com/usgs/landslides/ (Landslide incidence map)

24. United States Army Corps of Engineers, Ice Engineering Research Group Cold Regions Research and Engineering Laboratory , Ice Jam Clearinghouse https://rsgisias.crrel.usace.army.mil/apex/f?p=273:1:

25. US Army Corps of Engineers Hydrologic Engineering Center, HEC-RAS River Analysis System Users Manual, Version 4.1, January 2010.

26. Tennessee Valley Authority Calculation CDQ0000002014000017, Revision 000, BWSC Calculation TVAGENQ12002, Revision 0, HEC-RAS Tributary Geometry Development, EDMS #B41 140919 001.

27. Tennessee Valley Authority Calculation CDQ0000002012000004, Revision 001, BWSC Calculation TVAGEN14006, Revision 0, HEC-RAS Geometry Development - Main Stem.

28. Tennessee Valley Authority Calculation CDQ0000002012000006, Revision 001, BWSC Calculation TVAGENQ114007, Revision 0, HEC-RAS Model Calibration and Model Set-up - Main Stem, EDMS

1. B41 140919 002.

29. Tennessee Valley Authority Calculation CDQ0000002014000018, Revision 000, BWSC Calculation TVAGENQ13007, Revision 0, HEC-RAS Tributary Model Calibration, EDMS #B41 140919 003.

30. Tennessee Valley Authority Calculation CDQ0000002013000007, Revision 001, BWSC Calculation TVAGENQ14001, Revision 1, Main Stem Initial Dam Rating Curves.

31. Tennessee Valley Authority Calculation CDQ0000002014000016, Revision 001, BWSC Calculation TVAGENQ14001, Revision 1, Tributary Initial Dam Rating Curves.

32. Tennessee Valley Authority Calculation CDQ000020080050, Revision 3, Flood Operational Guides, EDMS #B41 110718 003.

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33. Tennessee Valley Authority Calculation CDQ0000002012000005, Revision 002, BWSC Calculation TVAGENQ14012, Revision 1, HEC-RAS Unsteady Flow Rules - Main Stem.

34. Tennessee Valley Authority Calculation CDQ0000002014000019, Revision 001, BWSC Calculation TVAGENQ14003, Revision 1, HEC-RAS Tributary Unsteady Flow Rules.

35. Tennessee Valley Authority Calculation CDQ0000002014000012, Revision 000, BWSC Calculation TVAGENQ13005, Revision 0, Unit Hydrograph Adjustment.

36. Tennessee Valley Authority Calculation CDQ0000002014000015, Revision 000, BWSC Calculation TVAGENQ13008, Revision 0, Inflows.

37. Tennessee Valley Authority Calculation CDQ0000002014000013, Revision 000, BWSC Calculation TVAGENQ13004, Revision 0, Baseflow.

38. Tennessee Valley Authority Technical Memorandum, HMR Applicability. December 19, 2012.

EDMS #W50 150310 001

39. Tennessee Valley Authority Technical Memorandum, Critical Storm Selection. December 21, 2012. EDMS #W50 150310 002

40. Tennessee Valley Authority Calculation CDQ000020080053, Revision 1, PMF Inflows, EDMS #B41 120628 004.

41. U.S. Department of Energy, Determining the Best Estimate of Maximum Water Surface Elevation at the Sequoyah Nuclear Plant to Determine the Significance of Tennessee River Watersheds Configuration Modifications and Impact on Operability, PNNL-20256. March 2011.

42. Tennessee Valley Authority Technical Memorandum, National Inventory of Dams Inflows.

December 19, 2014, EDMS #W50 150107 001.

43. Tennessee Valley Authority Calculation CDQ0000002014000030, Revision 000, BWSC Calculation TVAGENQ14015, Revision 0, Inflow Hydrograph Development for Seismic Events.

44. Tennessee Valley Authority, Simplified Volume Analysis for Multiple Dam Failures, November 24, 2014. EDMS #W50 141126 001

45. Tennessee Valley Authority Calculation CDQ0000002014000014, Revision 000, BWSC Calculation TVAGENQ13003, Revision 0, Curve Number Determination.

46. Tennessee Valley Authority Calculation CDQ0000002014000025, Revision 000, BWSC Calculation TVAGENQ14011, Revision 0, Fukushima NTTF Recommendation 2.1: Sunny Day Dam Failure Simulations.

47. Tennessee Valley Authority Calculation CDQ0000002014000028, Revision 000, BWSC Calculation TVAGENQ14016, Revision 0, Norris Dam Sunny Day Failure Simulation.

48. Department of the Army, Office of the Chief of Engineers, Engineering Technical Letter No 1110-2-8, August 1, 1966.

49. Tennessee Valley Authority Calculation CDQ0000002014000033, Revision 000, BWSC Calculation TVAGENQ14020, Revision 0, Fukushima NTTF Recommendation 2.1: Wind Waves for Combined-Effect Floods.

50. Tennessee Valley Authority, Multiple Dam Failure Screening Analysis for the Tennessee Valley Authoritys Dams. March 31, 2014, EDMS #W50 150108 001.

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51. Tennessee Valley Authority Calculation CDQ0000002014000024, Revision 000, BWSC Calculation TVAGENQ14010, Revision 1, Fukushima NTTF Recommendation 2.1: Seismic Dam Failure Simulations.

52. Tennessee Valley Authority Calculation CDQ0000002014000032, Revision 000, BWSC Calculation TVAGENQ14018, Revision 0, Fukushima NTTF Recommendation 2.1: Uncertainty Simulations.

53. Tennessee Valley Authority Calculation CDQ0000002014000023, Revision 000, BWSC Calculation TVAGENQ14009, Revision 0, Fukushima NTTF Recommendation 2.1: HEC-RAS Probable Maximum Flood Simulations.

54. Tennessee Valley Authority SQN Drawing 46W501-3, Architectural Plan El. 706.0 & 714.0 ABSCE Boundary, Rev. 4.

55. Tennessee Valley Authority SQN Drawing 46W455-30, Architectural Door Frames and Details, Rev.

B

56. Tennessee Valley Authority SQN Drawing 46W455-29, Architectural Door Frames and Details, Rev.

A

57. Tennessee Valley Authority SQN Drawing 46W462-4, Architectural Louvers, Rev. 6

58. Tennessee Valley Authority SQN Calculation SQN-DC-V-21.0, Environmental Design, Rev. 23

59. Tennessee Valley Authority SQN Drawing 41N381-1, Concrete Additional Equipment Building Outline, Rev. 2

60. Tennessee Valley Authority SQN Drawing 41N397-3, Unit 2 Concrete East Steam Valve Room Outline, Revision 5.

61. Tennessee Valley Authority SQN Drawing 41N397-3, Unit 1 Concrete East Steam Valve Room Outline, Revision 1.

62. Tennessee Valley Authority SQN Calculation 26D54EPMABBIMPFHA, Fire Hazard Analysis, Rev. 103

63. Tennessee Valley Authority SQN Calculation 3C37-0686-001, MELB Flood Level, Rev. 2.

64. Tennessee Valley Authority SQN Drawing 46W421-4, Architectural Plan El. 706.0, Powerhouse Service Building, Rev. 19

65. Tennessee Valley Authority SQN Drawing 44W364-2, Watertight Personnel Doors Arrangement &

Details, Sheet 2, Rev. 4

66. Tennessee Valley Authority SQN Drawing 44W364-1, Watertight Personnel Doors and Arrangement and Details, Rev. 1.

67. Tennessee Valley Authority SQN Drawing 46W401-5, Architectural Plan El. 685.0 & 690.0 Powerhouse 1 & 2, Rev. 2.

68. Tennessee Valley Authority SQN Drawing 44W369-1, Watertight Personnel Access Door C14 Arrangement Details, Rev. 0.

69. Tennessee Valley Authority SQN Drawing 44W369-3, Pressure Personnel Door C15 Arrangement and Details, Rev. 0.

70. Tennessee Valley Authority SQN Drawing 48N1284, Miscellaneous Steel Hatch Frames & Covers, Rev. 7.

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ENCLOSURE 2 LIST OF COMMITMENTS

1. TVA will complete the March 12, 2012, 50.54(f) Request for Information required Integrated Assessment for Sequoyah Nuclear Plant, Units 1 and 2 and submit a report no later than March 12, 2017.

2. As an interim action until the permanent solution to prevent overflow of Douglas Saddle Dams 1 and 3 is implemented, TVA will implement an Emergency Action Plan to install HESCO barriers across the crests of both Douglas Saddle Dams 1 and 3 to prevent overtopping of these saddle dams.