CNL-15-043, Flood Hazard Reevaluation Report for Watts Bar, Response to NRC Request for Information Per Title 10 of CFR 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 Acc

Flood Hazard Reevaluation Report for Watts Bar, Response to NRC Request for Information Per Title 10 of CFR 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 Accid

ML15084A324
Person / Time
Site:	Watts Bar
Issue date:	03/25/2015
From:	James Shea Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
Shared Package
ML15084A323	List: CNL-15-043, Flood Hazard Reevaluation Report for Watts Bar, Response to NRC Request for Information Per Title 10 of CFR 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 Acc
References
CNL-15-043A
Download: ML15084A324 (76)

Text

Security Related Information -Withhold Under 10 CFR 2.390 This letter is decontrolled when separated from Enclosure 1 Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-15-043A March 25, 2015 10 CFR 2.390 10 CFR 50.54(f)

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington , D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No. 50-390 Watts Bar Nuclear Plant, Unit 2 Construction Permit No. CPPR-92 NRC Docket No. 50-391

Subject:

Flood Hazard Reevaluation Report for Watts Bar 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) Recommendation 2.1, Flooding , of the Security Related Information

Security Related Information -Withhold Under 10 CFR 2.390 Th is letter is decontrolled when separated from Enclosure 1 U.S. Nuclear Regulatory Commission CNL-15-043A Page 2 March 25, 2015 Near Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated March 12, 2013 (ML13080A073)

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 Watts Bar Nuclear Plant (WBN) , 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 WBN site, WBN 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 WBN , Units 1 and 2. Specifically, Enclosure 1 of this letter provides the WBN Flooding HRR. The enclosed flooding HRR describes the approach , methods and results from the reevaluation of flood hazards at WBN, 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 WBN, Units 1 and 2.

Security Related Information

Security Related Information - Withhold Under 10 CFR 2.390 This letter is decontrolled when separated from Enclosure 1 U.S. Nuclear Regulatory Commission CNL-15-043A Page 3 March 25, 2015 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. It is noted that WBN , Unit 2 is under construction. The CLB flood levels presented in the Enclosure for WBN, Unit 1 will be the same CLB flood levels for WBN, Unit 2 when the Unit 2 licensing application is approved.

In .accordance with Reference 6, an Integrated Assessment (IA) 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 IA be completed and a report submitted within two years of submitting the Flooding HRR An IA 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 WBN, Unit 1 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 WBN, Unit 1 are available, functional and implementable and any degraded or nonconforming flood protection features were entered in TVA's Corrective Action Program.

The Reference 1 walkdowns were not required for WBN, Unit 2. Flood protection requirements are being addressed as an integral part of the WBN, Unit 2 completion .

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

Specifically, TVA continues to develop the Flood Mitigation System for WBN as described in TVA letters to the NRC dated April 16, 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 3 of this letter provides a list of new regulatory commitments.

Security Related Information

Security Related Information - Withhold Under 10 CFR 2.390 This letter is decontrolled when separated from Enclosure 1 U.S. Nuclear Regulatory Commission CNL-15-043A Page 4 March 25, 2015 The WBN HRR was originally submitted on March 12, 2015. Enclosure 1 of this letter, the WBN HRR, is being resubmitted as containing "Security-Related Information - Withhold Under Title 10 CFR 2.390." The TVA hereby requests Enclosure 1 be withheld from public disclosure in accordance with the provisions of 10 CFR 2.390. A redacted version is being provided in Enclosure 2. No other changes were made from the March 12, 2015, WBN HRR submittal. This submittal replaces the March 12, 2015, WBN HRR submittal in its entirety.

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 25th day of March 2015.

Respectfully,

/~/-7(;i Jwyleq J. W . Shea Vice President, Nuclear Licensing

Enclosures:

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

2. Near-Term Task Force (NTTF) - Recommendation 2.1 Mitigating Strategies Flood Hazard Evaluation Report for Watts Bar Nuclear Plant (Redacted Version)

3. List of Commitments cc (Enclosures):

NRR Director - NRC Headquarters NRO Director - NRC Headquarters NRR JLD Director - NRC Headquarters NRC Reg ional Administrator - Region II NRC Project Manager - Watts Bar Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant, Unit 1 NRC Senior Resident Inspector - Watts Bar Nuclear Plant, Unit 2 Security Related Information

ENCLOSURE 2 NEAR-TERM TASK FORCE (NTTF) - RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT FOR WATTS BAR NUCLEAR PLANT (REDACTED VERSION)

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)

Watts Bar Nuclear Plant Tennessee Valley Authority March 12, 2015 Revision 0

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 ............................................................................... 16 3.4.1 Local Intense Precipitation .................................................................................................. 16 3.4.2 Flooding from Rivers and Streams ...................................................................................... 16 3.4.3 Flooding from Dam Breaches or Failures ............................................................................ 16 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 ...................................................................................... 17 3.4.7 Channel Migration or Diversion .......................................................................................... 17 3.4.8 Flooding from Combined Effects ........................................................................................ 17 3.5 Current Flood Protection and Mitigation Features ............................................................ 17 3.5.1 Dam and Reservoir System ................................................................................................. 18 3.5.2 Watts Bar Nuclear Site Protective Structures ..................................................................... 18 3.5.3 TVA River Operations Forecasting and Warning ................................................................. 18 3.5.4 WBN 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 ..................................................... 23 4.2.1 Local Intense Precipitation .................................................................................................. 23 4.2.2 Flooding in Rivers and Streams ........................................................................................... 23 5

SUMMARY

OF PLANT WALKDOWN RESULTS AND MODIFICATIONS ............................................ 33 6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS ................................................ 35 6.1 Local Intense Precipitation ................................................................................................. 35 6.2 Flooding from Rivers and Streams ..................................................................................... 35 6.3 Flooding from Dam Breaches or Failures ........................................................................... 35 6.3.1 Project Specific PMF............................................................................................................ 35 6.3.2 Sunny Day Failure of Upstream Dams ................................................................................ 36 6.3.3 Seismic Failure of Upstream Dams ..................................................................................... 36 6.3.4 Sediment Transport ............................................................................................................ 36 6.4 Flooding from Storm Surge and Seiche .............................................................................. 36 6.5 Flooding from Tsunami ....................................................................................................... 38 6.6 Flooding from Ice-Induced Events ...................................................................................... 38 6.7 Channel Migration and Diversion ....................................................................................... 38 6.8 Flooding from Combined Effects ........................................................................................ 39 6.8.1 Floods Caused by Precipitation Events ............................................................................... 39 6.8.2 Floods Caused by Seismic Dam Failures.............................................................................. 39 7 DESCRIPTION OF MODELS USED FOR REEVALUATION ................................................................. 40 Page 2 of 70

7.1 HEC-RAS .............................................................................................................................. 40 7.1.1 Description of HEC-RAS Model Verification ........................................................................ 40 7.1.2 Description of HEC-RAS Model Extents............................................................................... 40 7.2 HEC-HMS ............................................................................................................................ 42 7.2.1 Description of HEC-RAS Model Verification ........................................................................ 43 7.2.2 Description of HEC-HMS Model Extents ............................................................................. 43 8 JUSTIFICATION OF INPUTS .......................................................................................................... 44 8.1 HEC-RAS Model Geometry Development and Calibration ................................................. 44 8.2 Dam Rating Curves ............................................................................................................. 44 8.3 Unsteady Flow Rules .......................................................................................................... 45 8.4 Probable Maximum Flood Inflows...................................................................................... 45 8.4.1 Hydrometeorological Report .............................................................................................. 46 8.4.2 Critical Storm Selection ....................................................................................................... 46 8.4.3 National Inventory of Dams (NID) Inflows .......................................................................... 47 8.5 Seismic Inflows ................................................................................................................... 47 8.5.1 National Inventory of Dams (NID) Seismic Inflows ............................................................. 47 8.6 Sunny Day and Watauga Project Specific PMF Inflows ...................................................... 47 9 APPLICABLE FLOOD CAUSING MECHANISMS .............................................................................. 49 9.1 Local Intense Precipitation ................................................................................................. 49 9.1.1 Previous Analysis ................................................................................................................. 49 9.1.2 Technical Approach ............................................................................................................. 49 9.2 Flooding from Rivers and Streams ..................................................................................... 50 9.2.1 Previous Analysis ................................................................................................................. 50 9.2.2 Technical Approach ............................................................................................................. 50 9.3 Flooding from Dam Breach or Failures ............................................................................... 52 9.3.1 Project Specific PMF............................................................................................................ 52 9.3.2 Sunny Day Dam Failure ....................................................................................................... 53 9.3.3 Single Seismic Dam Failure.................................................................................................. 54 9.4 Flooding from Combined Effects ........................................................................................ 54 9.4.1 Floods Caused by Precipitation Events ............................................................................... 54 9.4.2 Multiple Seismic Dam Failures with Combined Flood Event .............................................. 56 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 Conditions ................................................................................................ 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 .......................................................................................... 64 12.3 Combined Effects Floods Caused by Precipitation Events ................................................. 65 13 References ................................................................................................................................. 67 Page 3 of 70

FIGURES Figure 2-1 River System Schematic ............................................................................................................... 7 Figure 3-1 Watts Bar Nuclear Plant............................................................................................................... 9 Figure 3-2 Watts Bar Nuclear Plant Site ...................................................................................................... 10 Figure 3-3 Watts Bar Nuclear Plant Site Layout .......................................................................................... 11 Figure 3-4 Watts Bar Nuclear Plant Topography ........................................................................................ 12 Figure 3-5 Location of Tennessee River Cross-Sections .............................................................................. 13 Figure 3-6 Bathymetry - Tennessee River in the Vicinity of the Watts Bar Nuclear Plant ......................... 14 Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam ................................ 21 Figure 4-2 Timeline of Flood Related Changes Since Licensing ................................................................. 23 Figure 6-1. Landslide Incidence Map of United States ............................................................................... 37 Figure 7-1 Upper HEC-RAS Model Extents .................................................................................................. 41 Figure 7-2 Lower HEC-RAS Model Extents .................................................................................................. 41 Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents ................................................................. 42 Figure 7-4 Local Intense Precipitation HEC-HMS Model Extents ................................................................ 43 Figure 12-1 WBN Access Doors ................................................................................................................... 63 TABLES Table 3.4-1 Combined Effects of Flood and Wind ...................................................................................... 17 Table 4.1-1 Land Use above Guntersville (2001 - 2011) ............................................................................ 21 Table 4.1-2 Impervious Area above Guntersville (2001 - 2011) ................................................................ 22 Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11) .......................................................... 27 Table 6-4.2-1 Potential Flood Causing Mechanisms or Causal Phenomena .............................................. 35 Table 9.1-1 Results of WBN LIP Analysis ..................................................................................................... 50 Table 9.2-1 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Reevaluation .................. 51 Table 9.2-2 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Alternative Douglas Saddle Dam ............................................................................................................................................................. 51 Table 9.3-1 Elevation and Discharge at WBN Resulting from Project Specific Dam Failures ..................... 52 Table 9.3-2 Elevations and Discharges at WBN Resulting from Sunny Day Dam Failures .......................... 53 Table 9.3-3 Elevations and Discharges at WBN Resulting from Single Seismic Failure of Upstream Dams 54 Table 9.4-1 Wind Wave Elevation Results at Dams .................................................................................... 55 Table 9.4-2 Wind Wave Elevation Results at Critical Structures................................................................ 55 Table 9.4-3 Elevations and Dishcharges at WBN 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 Watts Bar Nuclear Plant (WBN) 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 Watts Bar Nuclear plant is located on the west bank of Chickamauga Lake at Tennessee River Mile (TRM) 528 with plant grade at elevation 728.0 ft. as shown in Figure 2-1. The Tennessee River above WBN site drains 17,319 square-miles. Watts Bar Dam, 1.9 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 12 major TVA dams (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 WBN. 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)

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Figure 2-1 River System Schematic Page 7 of 70

3 PLANT SITE DESCRIPTION 3.1 Current Site Layout WBN is located in Rhea County, Tennessee on the west bank of Chickamauga Reservoir at TRM 528 with plant grade elevation at 728.0 ft. As shown in Figure 3-1, WBN is 44 miles from Chattanooga and 54 miles from Knoxville. The site is approximately 1.9 river miles south of Watts Bar Dam (TRM 529.9), as shown in Figure 3-2, and approximately 31 miles north-northeast of Sequoyah Nuclear Plant (SQN). Details of the current site layout and plant structures are shown in Figure 3-3. (Reference 2) 3.2 Site Topography WBN plant site and the Watts Bar Dam Reservation comprise approximately 1,770 acres on the west bank of Chickamauga Reservoir. As shown in Figure 3-4, 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 WBN, as shown in Figure 3-5. The actual cross-sections taken at these locations are shown in Figure 3-6. As shown, the depth of the river in the vicinity of WBN ranges between 18 and 26 feet at normal summer pool elevation, 682.5 ft. (Reference 3)

Page 8 of 70

Figure 3-1 Watts Bar Nuclear Plant Page 9 of 70

Figure 3-2 Watts Bar Nuclear Plant Site Page 10 of 70

Figure 3-3 Watts Bar Nuclear Plant Site Layout Page 11 of 70

Figure 3-4 Watts Bar Nuclear Plant Topography Page 12 of 70

Figure 3-5 Location of Tennessee River Cross-Sections Page 13 of 70

Figure 3-6 Bathymetry - Tennessee River in the Vicinity of the Watts Bar Nuclear Plant Page 14 of 70

Figure 3-6 Bathymetry - Tennessee River in the Vicinity of the Watts Bar Nuclear Plant Page 15 of 70

3.4 Current Design Basis Flood Elevations On July 19, 2012, TVA submitted a revised hydrological licensing basis for WBN (Reference 4) for flooding on the Tennessee River and its tributaries. On September 30, 2014, TVA submitted a revision to the 2012 submittal (Reference 5). On January 28, 2015 the License Amendment Request (LAR) was approved by the NRC (Reference 6).

In the balance of this report, the WBN current licensing basis (CLB) flood elevation values are those presented in the January 28, 2015 Safety Evaluation Report (SER) (Reference 6). The LIP CLB was not changed in the 2012 and 2014 submittal and is defined in the WBN UFSAR.

3.4.1 Local Intense Precipitation The effects of LIP are documented in the Final Safety Analysis Report (FSAR) (Reference 2). For the Probable Maximum Precipitation (PMP), Hydrometeorological Report (HMR) 56 is used to define the storm event (Reference 7). The 1-hour maximum precipitation in the CLB is 16.8 inches. In the analysis of the PMP, the underground drains are assumed clogged and runoff is assumed to equal rainfall. The computed maximum surface elevation is 728.9 ft. which is below the critical floor elevation 729.0 ft. (Reference 2) 3.4.2 Flooding from Rivers and Streams The CLB analysis utilizes the U.S. Army Corps of Engineers (USACE) Hydrology Engineering Center River Analysis System (HEC-RAS). The stillwater probable maximum flood (PMF) elevation is 739.2 ft. (Reference 6).

3.4.3 Flooding from Dam Breaches or Failures Postulated failure of the Boone, Fort Patrick Henry, Melton Hill, Watts Bar West Saddle, Calderwood, Cheoah, Chilhowee, Ocoee 1, Ocoee 2, Ocoee 3, John Sevier, Mission, and Wilbur Dams is evaluated in the CLB analysis for the PMF. The CLB flood elevation associated with this event is provided in the evaluation of the flooding from rivers and streams (Section 3.4.2).

Postulated failure of multiple dams upstream from WBN is evaluated in the CLB analysis for the combined effects of seismic and flooding. The CLB flood elevation is reported in the combined effects flooding for seismic plus flooding conditions (Section 3.4.8).

Postulated failure of Chickamauga dam is considered in the CLB analysis for loss of ultimate heat sink during non-flood conditions. This loss of downstream dam failure does not present a flooding hazard at WBN and is not considered in the CLB.

3.4.4 Flooding from Storm Surge and Seiche Surges and seiches are not considered applicable in the WBN CLB because of the size and configuration of the lake and because of the elevation difference between the normal lake level and the plant grade is approximately 45 feet.

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

Page 16 of 70

3.4.6 Flooding from Ice-Induced Events Ice-induced flooding is not considered applicable in the WBN 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 WBN CLB because the configuration of the flood plain would not produce major channel meanders or cutoffs.

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 21 miles-per-hour overland wind (Reference 4). The combined effects of the flood plus wind are provided in Table 3.4-1 for WBN.

Table 3.4-1 Combined Effects of Flood and Wind Design Basis Flood (DBF)

Plant Location Elevation (ft.)

Probable Maximum Flood (still reservoir) 739.2 Run-up on 4:1 sloped surfaces on the Diesel 741.6 Generator Building Run-up on critical vertical wall of the Intake Pumping 741.7 Station Surge level within flooded structures 739.7 3.4.8.2 Floods Caused by Seismic Dam Failure Events The maximum seismic-induced flood elevation at WBN is due to a Safe Shutdown Earthquake (SSE) combined with a 25-year flood. Dams upstream of WBN which are postulated to fail in this combined event are Norris, Douglas, Cherokee, and Tellico. The CLB maximum flood elevation for floods caused by seismic dam failures is 731.2 ft. (Reference 6) 3.5 Current Flood Protection and Mitigation Features Flood protection and mitigation for the WBN site are provided by 4 key elements: dams upstream of WBN, structures and structural features at the WBN site protecting equipment required for flood mode operation, TVAs River Operations forecasting and flood warning capability and WBN flood response procedures. Together, these elements ensure safe operation of WBN for flooding above plant grade. Each of these elements is described below.

Page 17 of 70

3.5.1 Dam and Reservoir System Flood control above the plant is provided largely by eight tributary reservoirs. Near the end of the flood season, these provide a minimum of 3,937,400 acre-feet of detention capacity. This is 89% of the total available above the plant. Additionally, two main river reservoirs, Fort Loudoun and Watts Bar, provide 490,000 acre-feet of storage above Watts Bar Dam. (Reference 6)

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 6) 3.5.2 Watts Bar Nuclear Site Protective Structures At the WBN site, equipment required during flood mode operation, discussed in Section 3.5.4, is either located above the DBF, is within a non-flooded structure, is designed for submerged operation, or is otherwise protected. The Reactor Building will be maintained dry during flood mode. Walls and penetrations are designed to withstand static and dynamic forces imposed by the DBF. (Reference 5)

The Diesel Generator Building also will remain dry during the flood mode since its lowest floor is at elevation 742.0 ft. With the PMF elevation of 739.2 ft., wind wave run-up at the Diesel Generator Building is elevation 741.6 ft. Therefore, flood levels do not exceed floor elevation of 742.0 ft. (Reference 5)

The IPS structure contains various equipment required to support flood mode. The IPS contains the essential raw cooling water (ERCW) and high pressure fire protection (HPFP) pumps, travelling water screens, and support equipment. During a DBF event, the internal flood elevation is 741.7 ft. While this does not wet any flood-sensitive equipment on elevation 741.0 ft., the ERCW strainers and support equipment are located on elevation 722.0 ft. of the IPS, connected to elevation 741.0 ft. via stairwells. Passive flood barriers installed in the stairwell protect the equipment on elevation 722.0 ft. in the IPS. (Reference 5)

Other structures, including the service, turbine, auxiliary, and control buildings, would be allowed to flood as the water exceeds their grade level entrances. Equipment that is located in these structures and required for flood mode operation is either above the DBF, is suitable for submerged operation, or is otherwise protected. (Reference 5) 3.5.3 TVA River Operations Forecasting and Warning Protection of WBN from rainfall 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 5)

Because the plant grade elevation at WBN can be exceeded by large rainfall (PMP) and seismically-induced dam failure floods, plant flood preparations are required to cope with the Page 18 of 70

"fastest rising" calculated flood, including seismically induced dam failure floods that can exceed plant grade with the shortest warning time. (Reference 5)

Flood preparations at WBN are divided into two stages of preparation. By dividing the pre-flood preparation steps into two stages, a minimum of a 27 hour3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> pre-flood transition interval is available between the time a flood warning is received and the time the flood waters exceed plant grade. The first stage, a minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> long, commences upon receipt of a flood warning. The second stage, a minimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> long, is based on a confirmed estimate that conditions will produce a flood above plant grade. An additional 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> for communication and forecasting computations is provided to allow TVAs RO to translate rain on the ground to river elevations at the plant. Hence, the warning plan provides 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> from arrival of rain on the ground until elevation 727.0 ft. could be reached. (Reference 5)

The plant preparation status is held at Stage I until either Stage II begins or TVA's RO determines that floodwaters will not exceed elevation 727.0 ft. at the plant. Stage I shutdown is initiated upon notification that a critical dam failure combination has occurred or loss of communication prevents determining a critical case has not occurred. Stage I shutdown 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. The Stage II warning is issued only when enough additional rain has fallen to forecast that elevation 727.0 ft. (winter or summer) is likely to be reached. Use of elevation 727.0 ft., one foot below plant grade, provides adequate margin to prevent wind-generated waves from endangering plant safety during the final hours of plant shutdown activity. Forecast will be based upon rainfall already reported to be on the ground. (Reference 5)

During the winter season, Stage I shutdown procedures will be started as soon as target river elevation 715.5 ft. has been forecast. Stage II shutdown will be initiated and carried to completion if and when target river elevation 727.0 ft. at WBN has been forecast. Corresponding target river elevations for the summer season at WBN are elevation 720.6 ft. and elevation 727.0 ft. (Reference 5)

Protection of WBN from seismically induced dam failure floods that might exceed plant grade also utilizes a flood warning issued by TVA's River Operations (RO). If loss of or damage to an upstream dam is suspected based on monitoring by TVAs RO, efforts will be made by TVA to determine whether dam failure has occurred. If a critical dam failure condition has occurred which could cause a flood wave to approach elevation 728.0 ft. at the plant site or it cannot be determined that it has not occurred, Stage I shutdown will be initiated. Once initiated, the flood preparation procedures will be carried to completion unless it is determined that the critical case has not occurred. The time from seismic failure to the time elevation 727.0 ft. is reached at WBN in the most critical event is about 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />. This time is adequate to permit safe plant shutdown in readiness for flooding. (Reference 5) 3.5.4 WBN Flood Response Procedures Plant operation during the flood response period is governed by Abnormal Operating Instructions (AOI). Maintenance and operations activities are directed by the AOIs to align systems and components for flood mode operation. These activities are required by the AOI to be completed within 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> of notification of a Stage II flood alert. Coupled with the minimum time of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> for Stage I, preparations result in a required total preparation time of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> for the AOI actions.

Page 19 of 70

If the reactor is operating at power at the time the flood warning is received, Stage I and then, if necessary, Stage II procedures are initiated. Stage I procedures consist of a controlled reactor shutdown and other easily revocable steps, such as moving flood mode supplies above the PMF elevation and making load adjustments to the onsite power supply. After the controlled reactor shutdown, the reactor coolant system (RCS) is cooled by the auxiliary feedwater system and the RCS pressure is reduced to less than 350 psig.

Stage II procedures are the less easily revocable steps necessary to have the plant in the flood mode when the flood exceeds plant grade. HPFP system water (raw) will replace auxiliary feedwater for steam generator makeup water. RCS makeup is accomplished using the auxiliary charging system. Other essential plant cooling loads are transferred from the Component Cooling Water System to the ERCW System and the ERCW replaces raw cooling water to the ice condensers. The radioactive waste system will be secured by filling tanks below DBF level with enough water to prevent flotation. One exception is the waste gas decay tanks, which are sealed and anchored against flotation. Power and communication cables below the DBF level that are not required for submerged operation are disconnected, and batteries beneath the DBF level are disconnected.

For a reactor in refueling status, if time permits, fuel is removed from the unit undergoing refueling and placed in the spent fuel pool; otherwise fuel cooling is accomplished using the following strategy. If the refueling canal is not already flooded, the mode of cooling requires that the canal be flooded with borated water from the refueling water storage tank. The Spent Fuel Pool Cooling System (SFPCS) Pumps will take suction from the spent fuel pool by their normal intakes and will discharge to the spent fuel pool heat exchangers. The outflow of the spent fuel pool heat exchangers is aligned, via spool piece installation during Stage II preps, to the Residual Heat Removal (RHR) system upstream of the RHR heat exchangers. Once the cooling water passes through the RHR heat exchangers it will enter the RCS via the RHR cold leg injection paths. The water flows through and exits the top of the vessel into the refueling cavity.

The head generated will force the water through the fuel transfer canal back to the spent fuel pool. ERCW provides the secondary side cooling for the spent fuel pool and RHR heat exchangers. (Reference 5) 4 CHRONOLOGY OF FLOOD RELATED CHANGES SINCE LICENSING 4.1 Watershed Changes since Licensing The potential impacts of land use change in the Tennessee River basin are 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)

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Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam The data for this urban land use change assessment are derived from the NLCD 2001 and 2011 Retrofit Change Project. 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%

Page 21 of 70

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 about 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.

Page 22 of 70

4.2 Summary of Changes to Design Basis Flood Elevations The original licensing basis is documented in the 1982 Watts Bar Nuclear Plant FSAR. Figure 4-2 provides a timeline for the changes that have occurred between the 1982 FSAR and the January 28, 2015 SER (Reference 6).

1982 1997 Dam 2008 NRC QA Original Safety 1999 Review of Licensing Modifications UFSAR Hydrology 2012 and 2014 Basis complete Change Documentation WBN U1 LAR 1982 Dam 1997-98 Flood 2004 River 2008-2012 January 28, Safety Reassessment Operations Hydrology 2015 SER Review 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 WBN LIP, as established in the 1982 FSAR, concluded that the local PMF would not reach elevation 729.0 ft. in the channels and pools surrounding the turbine, reactor, service, auxiliary, and diesel generator buildings.

The LIP has been reevaluated throughout the life of the plant for site configuration changes, as necessary. For any access exposed to the environment and located at grade elevation, sufficient drainage is provided to prevent water from entering the opening. The WBN CLB LIP has been maintained below critical floor elevation 729.0 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 WBN 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 (e.g. 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 WBN was determined to be the March 7,980-square-mile event as shown below:

Storm Event Maximum Discharge Maximum Elevation March 7,980-square-mile Bulls Gap 1,478,000 cfs 738.1 ft.

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)

Page 23 of 70

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

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

d. Probable Maximum Precipitation (PMP) defined for TVA by HMR 41 (Reference 10)

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 728 ft. exceeded by design flood event - 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 WBN, was overtopped during PMF event simulation but postulated not to fail 4.2.2.1.2 Seismically Induced Failure of Upstream Dams The maximum plant site elevations at WBN for the different postulated combinations of seismic dam failures coincident with floods were as follows:

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

1. Fontanaa 732.1

2. Norris 731.1

3. Cherokee, Douglas 732.0 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasb 732.8

2. Norris, Douglas, Fort Loudoun, Tellico 731.1 Page 24 of 70

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 PMF 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 all 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 11)

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 WBN and on warning time available for safe plant shutdown.

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 WBN would be the same as in the original analysis and result from the March 7,980-square-mile event centered at Bulls Gap as shown below:

Page 25 of 70

Storm Event Maximum Discharge Maximum Elevation WBN (March 7,980-square-mile) 1,288,000 cfs 734.9 feet Page 26 of 70

Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11)

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 27 of 70

4.2.2.2.1.1 Summary of Differences Between 1970s Licensing Basis and 1997 - 1998 Reassessment for PMF

a. Dam safety modifications at tributary dams, Douglas and Watauga, eliminated overtopping and breach

b. Dam safety modifications at main river dams, Fort Loudoun - Tellico and Watts Bar as shown in Table 4.2-1, eliminated failure from overtopping

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

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

e. Controlling PMF elevation lowered by 3.2 feet (from 738.1 ft. to 734.9 ft.)

4.2.2.2.2 Seismically Induced Failure of Upstream Dams (Reference 11)

During the 1997-1998 reassessment, the maximum plant site elevations at WBN 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 WBN (ft.)

1. Fontanaa 725.2

2. Norris 721.5

3. Cherokee, Douglas 723.1 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasb 727.5

2. Norris, Douglas, Fort Loudoun, Tellico 722.8 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.

4.2.2.2.2.1 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.

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 Page 28 of 70

would not fail. The Watts Bar West Saddle Dam would fail completely but Watts Bar Dam would not fail. (Reference 11) 4.2.2.3 2008 - 2014 (CLB) Updated Analysis of PMF and Seismically Induced Floods 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 12). In response to these NOVs, TVA completed a revised hydrologic analysis to support the BLN Units 3 and 4 COLA.

While the February 2008 QA inspection was for the BLN licensing request, it directly impacted WBN 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 and installed temporary flood barriers at Cherokee, Fort Loudoun, Tellico, and Watts Bar Dams to increase the height of the embankments. Increasing the height of the embankments at these dams prevents embankment overflow and failure during the PMF.

(Reference 4) 4.2.2.3.1 PMF The hydrologic analysis revision converts the model to the U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center River Analysis System (HEC-RAS) as well as updates to input information to address current reservoir operating guides, revised flow coefficients and dam modifications.

The revised licensing basis also updates the evaluation criteria for dam stability to current industry standards. In this evaluation, dams which did not comply with the revised criteria are either modified or assumed to fail.

As a result of the reassessment the controlling event at WBN is the same as in the original analysis and results from the March 7,980-square-mile event centered at Bulls Gap as shown below:

Storm Event Maximum Discharge Maximum Elevation WBN (March 7,980-square-mile) 1,160,000 cfs 738.9 ft.

The calculated PMF elevation is combined with 0.3 feet. of additional margin, providing the design basis PMF elevation of 739.2 ft.

4.2.2.3.1.1 Summary of Differences between 1997 - 1998 and 2008 - 2014 Analyses for PMF

a. Cross section bathymetry was updated based on recent USACE Doppler profiler navigation surveys Page 29 of 70

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

c. 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 HEC-RAS model.

d. Dam rating curves updated using model test data

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

f. Model refinements have been made at the Fort Loudoun-Tellico canal and Dallas Bay rim leak.

g. The model was extended on the Elk River to Tims Ford Dam, along the Hiwassee River to Chatuge Dam, along the Nottely River to Nottely Dam, along the Ocoee River to Blue Ridge Dam, along the Clinch River to a gage at CRM 65.4, along the Little Tennessee River to LTRM 92.9, along the Tuckasegee River to RM 12.6, along the Holston River to the confluence of the South Fork Holston River and the North Fork Holston River, along the South Fork Holston River to South Holston Dam, along the Watauga River to Watauga Dam, along the French Broad River to RM 77.5, along the Nolichucky River to RM 10.3, along Cove Creek to RM 12.2, along Big Creek to CRM 11.8, and along North Chickamauga Creek to RM 12.82.

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

k. Eighteen 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 15. South Holston

7. Douglas 16. Tellico

8. Fontana 17. Watauga

9. Fort Loudoun 18. Watts Bar

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

1. Ocoee 1 Page 30 of 70

2. Ocoee 2

3. Ocoee 3

4. Chilhowee

5. Calderwood

6. Cheoah

7. Mission

8. John Sevier

9. Wilbur

m. 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)

n. A license condition for Watts Bar is included in the NRC SER Reference 6 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 and raising embankment saddle dam overtopping elevation; adding saddle dam toe berms

3. Fort Loudoun - Post-tensioning non-overflow dam (remaining HESCO barriers will be removed 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.

4.2.2.3.2 Seismically Induced Failure of Upstream Dams (Reference 4)

The maximum plant site elevations at WBN 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 WBN (ft.)

a b

1. Fontana , Tellico 720.7

2. Fontana, Tellico, Hiwassee, Apalachia, Blue Ridge 722.0

3. Norris, Tellico 728.7 Page 31 of 70

4. Cherokee, Douglasc, Tellicob 729.1 SSE Failures with 25-year Flood

1. Norris, Cherokee, Douglasc, Tellicob 731.2 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.

4.2.2.3.2.1 Summary of Differences between 1997 - 1998 and 2008 - 2014 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 all 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. West saddle dam failure discharge input at the mouth of Yellow Creek downstream of WBN where it would enter Chickamauga Reservoir instead of combining with Watts Bar Dam discharge and routing downstream

k. Temporary barriers and permanent dam modifications were not credited in the seismic analysis Page 32 of 70

5

SUMMARY

OF PLANT WALKDOWN RESULTS AND MODIFICATIONS TVA completed flooding walkdowns in accordance with the NEI 12-07 walkdown guidelines. In Reference 13, TVA provided the results of the flooding walkdowns in response to Recommendation 2.3, item 2 in Enclosure 4 of Reference 1.

The WBN 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 one nuclear engineer, one structural engineer, one mechanical engineer, and one civil engineer with extensive experience that met the qualifications provided in the guidance document. The walkdown team was supported by a retired TVA SRO, active maintenance personnel and active assistant unit operators in planning and performance of the walkdowns.

Walkdowns were performed in the safety related buildings and structures at WBN, as well as in the turbine building. TVA evaluated the findings from the walkdowns for both deficiencies and observations.

One deficiency was identified at WBN during the walkdowns. Two main control room chilled water circulating pumps and two shutdown board room chilled water circulating pumps were determined to be partially submerged during a PMF. This deficiency was entered into the corrective action program and resolved as described below.

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

a. Thermal Barrier Booster Pumps (TBBP) Flood Protection Barrier Permanent flood protection barriers surrounding the WBN Unit 1 and Unit 2 TBBPs have been installed. This barrier is designed to protect the TBBPs and their respective motors in the event of a PMF event.

b. Spent Fuel Pit (SFP) Pumps and Skimmer Pump Motors Barrier A permanent plant modification to provide flood protection for the common Spent Fuel Pit Cooling pumps and motors in the event of a PMF event has been installed.

c. Intake Pumping Station (IPS) Personnel Access Door Temporary Barriers Temporary flood barriers are employed at the IPS to prevent floodwater intrusion through the elevation 741 ft. stairwells into the elevation 722 ft. mechanical equipment rooms.

d. Main Control Room (MCR) and Shutdown Board Room (SDBR) Air Conditioning (AC)

System Chilled Water Circulating Pumps and Ancillary Equipment Protection Flood protection barriers and sealing of other ancillary equipment has been implemented to protect the MCR and SDBR AC system.

Subsequent to the reported walkdown, an additional deficiency was identified in the lack of qualification documentation for expansion joints in the Containment Purge Exhaust System. The expansion joints perform a flood barrier function to prevent flood waters from the Auxiliary Building from entering the non-flooded Shield Building. This deficiency was entered into the corrective action program and a Page 33 of 70

design change notice (DCN) is planned to replace the existing expansion joints with new qualified expansion joints to perform the flood barrier function.

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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 14), for potential flood-causing mechanisms, or causal phenomena, is provided in Table 6-4.2-1:

Table 6-4.2-1 Potential Flood Causing Mechanisms or Causal Phenomena Flood Causing Mechanism Guidance Reference Local intense precipitation HMR 52 and HMR 56 15 & 7 Flooding from rivers and streams ANSI/ANS-2.8-1992 16 Flooding from upstream dam breaches or failures Dam Failure ISG 17 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 17 ANSI/ANS-2.8-1992 and Flooding from combined effects Dam Failure ISG 16 and 17 6.1 Local Intense Precipitation The LIP was previously evaluated for WBN and is included in the reevaluation for WBN. 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 WBN and is included in the reevaluation for WBN. 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 WBN. 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 WBN. Inputs, assumptions, technical approach and results of this analysis are presented in subsequent sections.

6.3.3 Seismic Failure of Upstream Dams The seismic failure of single dams combined with flood events was previously evaluated and determined to be non-governing. 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 are 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 18). 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 WBN and is not considered a credible flood-causing mechanism at this site. The WBN site is located on the west bank of the TRM 528 approximately 1,530 River miles inland (Tennessee River Miles 528, Ohio River Miles 48, and Mississippi River Miles 953) from the Gulf of Mexico at grade elevation 728 ft.

The Chickamauga Reservoir level during non-flood conditions would not exceed approximate elevation 685.0 ft. (Chickamauga headwater 682.5 ft. plus 2.4 feet for full turbine flow from Watts Bar Dam located immediately above the plant) at the plant, for any significant period of time. The plant grade at elevation 728.0 ft. is approximately 45 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 19). Reference 20 indicates that WBN 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 21)

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 43 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 WBN and is not a credible flood causing mechanism at this site. WBN is located about 7.5 miles southeast of the town of Spring City, in Rhea County, approximately 44 miles northeast of Chattanooga, TN, and approximately 54 miles southwest of Knoxville, TN. At this location WBN is approximately 1,530 river miles inland (Tennessee River Miles 528, 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 430 miles inland from the Great Lakes. The WBN site with plant grade at an elevation of 728 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 WBN site have been stable for many years and no landslides into the reservoir have been documented for Rhea County.

(Reference 21) 6.6 Flooding from Ice-Induced Events Flooding from ice-induced events was not previously evaluated for WBN and is not a credible flood causing mechanism at this site. The WBN 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 22)

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 1.9 miles above the plant.

There are no safety-related facilities at the Watts Bar 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 1.9 mile reach below Watts Bar Dam to initiate a jam and an ice dam would need to reach more than 50 feet above normal winter levels to reach plant grade elevation 728.0 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 WBN and is not a credible flood causing mechanism at this site. The reservoir in the vicinity of WBN 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.

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

6.8.1 Floods Caused by Precipitation Events Floods caused by precipitation events were previously evaluated and will be included in the reevaluation for WBN. 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 (Reference 14) 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 23). 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 September 30, 2014 WBN Unit 1 LAR (Reference 5) and is used in this hazard reevaluation. Figure 7-1 and Figure 7-2 show the extent of the model, as well as the location of dams.

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Figure 7-1 Upper HEC-RAS Model Extents Figure 7-2 Lower HEC-RAS Model Extents Page 41 of 70

HEC-RAS models calculate water surface elevations for defined channels. The contributing subareas discharges that were computed in HEC-HMS models are inputs for HEC-RAS models.

Only the highest peak HEC-HMS discharges, were applied as inflows to the HEC-RAS models. HEC-RAS channel modeling was employed to model the WBN East and WBN West areas. Summaries of these models follow. A schematic of the HEC-RAS model can be seen in Figure 7-3. In the red lines represent cross-section locations.

Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents 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.

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7.2.1 Description of HEC-RAS 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 WBN LIP HEC-HMS model comprises of twelve sub-basins totaling 328 acres. Reservoir modeling is used where no clear drainage channel exists and as an alternate method to determine water surface elevations. HEC-HMS reservoir modeling is employed to model the WBN East, WBN West, and WBN Southwest drainage areas. The extents of the HEC-HMS model used in the LIP analysis are presented in Figure 7-4.

Figure 7-4 Local Intense Precipitation HEC-HMS Model Extents Page 43 of 70

8 JUSTIFICATION OF INPUTS The verified HEC-RAS unsteady flow model of the Tennessee River System developed for the September 30, 2014 WBN Unit 1 LAR (Reference 5) is used in the 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 24 and 25.

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 described in References 26 and 27.

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 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 17. The dam rating curves are documented in References 28 and 29.

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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 30) and dam rating curves (References 28 and 29). 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.

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 30). 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 probable maximum flood (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 30); if the operating deck is exceeded then gates remain in the open position. The unsteady flow rules are documented in References 31 and 32.

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 Probable Maximum Precipitation (PMP) events, as described in HMR 41 (Reference 10), were determined; hydrographs were developed using validated unit hydrographs (UHs) as well as UHs that were adjusted for non-linear basin response (Reference 33); 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 34) 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 35);

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

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

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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 36. 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.

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 10). 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 WBN is reviewed and documented in Reference 37. 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 10).

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 38, 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 all locations reviewed. An analysis of the 7,980-square-mile PMP event was also performed. It was determined that PMP depth is maximized at WBN by the 7,980-square-mile, Station 5 (Bulls Gap) centered event.

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An independent analysis performed by Pacific Northwest National Laboratory confirmed that the critical storm centerings are identified for use in computing PMP rainfall (Reference 39).

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 17) requires a screening process to identify all dams that are inconsequential. In order to identify the number of structures upstream of WBN the NID was queried for the Tennessee Valley watershed and approximately 700 dams were included in the analysis. As documented in Reference 40, 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 17 specify that the coincident inflow from the 25-year flood be applied during the 10-4 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 exceedance probability seismic hazard ground motion. To develop these inflows, a methodology for production of scaled hydrographs was developed in Reference 41. 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 41, 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 17. Simplified volume analyses are used and TVA projects having the potential to cause flooding at the plant sites were identified Reference 42. 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 41.

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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 42 are included in Reference 41. Constant June baseflows from Reference 35 are applied for both. Watauga project PMP rainfall was taken from Reference 41 as recommended by Reference 17 and is convoluted in a spreadsheet using SCS methodology. June curve numbers were taken from Reference 43 and unit hydrograph data were taken from Reference 34. NID inflows are not included in sunny day simulations.

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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 WBN plant site area is documented in Section 3.4.1.

PMP for the plant drainage systems is defined by HMR 56 (Reference 7). All 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 at the WBN plant site utilized HEC-HMS and HEC-RAS simulations and is documented in Reference 44. The LIP analysis is a measure of the extreme precipitation (high intensity and short duration) at a specific location, in this case WBN. According to NUREG/CR-7046 (Reference 14) 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 WBN is determined according to HMRs 52 and 56 (References 14 and 7, 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 peak flow rates from each drainage basin to a peak runoff. Additionally, the HEC-RAS model is used to produce the water surface elevation for defined channels utilizing the peak discharges computed in the HEC-HMS models.

This analysis conservatively assumes that drainage features carrying offsite drainage toward WBN are fully functional and drainage features carrying on-site drainage away from WBN 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 in the HEC-RAS model 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 a conservatism.

(Reference 44)

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

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Table 9.1-1 Results of WBN LIP Analysis Critical Plant Water Surface Location Elevation Elevation (ft.)

(ft.)

WBN East HEC-HMS 728.5 WBN East HEC-RAS 729.2 WBN West HEC-HMS 729.0 728.6 WBN West HEC-RAS 728.7 WBN Southwest HEC-HMS 728.4 9.2 Flooding from Rivers and Streams 9.2.1 Previous Analysis The results of the previous PMF analysis are presented in Section 4.2.2.3.1. This analysis incorporates the results of dam stability analyses and migration of the analysis from the SOCH suite of programs to the HEC-RAS unsteady flow model 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 14. The PMP is applied to the drainage basin of the rivers and streams adjoining WBN. Inflows to the model were generated using the SCS runoff methodology. The SCS runoff parameters are calibrated to the inflow model presented in the September 30, 2014 LAR (Reference 5). 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 (NRCS) soils and USGS Multi-Resolution Land Characteristics (MRLC) 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. Inflow development, storm selection, as well as additional inputs and assumptions are described in Section 8. 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 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 (Reference 45). This analysis assumes modifications to dams as presented in Section 4.5.1.1.m.

Page 50 of 70

Table 9.2-1 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Reevaluation PMF Event Elevation (ft) Discharge (cfs) 21,400 Sq.-Mi. Event 739.2 1,125,000 7,980 Sq.-Mi. Bulls Gap Event 744.9 1,454,000 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 Elevations and Discharges at WBN (TRM 528) Resulting from Alternative Douglas Saddle Dam PMF Event Elevation (ft.) Discharge (cfs) 7,980 Sq.-Mi. Bulls Gap Event with 739.8 1,189,000 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 7,980 square-mile Bulls Gap centered storm becomes the controlling event with the EAP at Douglas Dam and a maximum elevation of 739.8 ft.

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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 WBN.

A simplified volume analysis identifies individual dams that have the potential to cause flooding at WBN if failed during a sunny day (Reference 42). 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

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 reservoir volume.

9.3.1 Project Specific PMF A project specific PMF at Watauga Dam serves as the upper bound of any sunny day failure at South Holston or Watauga dams. The postulated failure of Watauga Dam results in the cascading failure of Wilbur, Boone, Fort Patrick Henry, and John Sevier Dams. The results are presented in Table 9.3-1 (Reference 46).

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

Watauga Dam Project Specific PMF 704.7 356,000 Page 52 of 70

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 WBN 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.

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 WBN Resulting from Sunny Day Dam Failures Dam Failure Elevation (ft.) Discharge (cfs)

Cherokee Dam Sunny Day Failure 712.2 562,000 Fontana Dam Sunny Day Failure 717.7 703,000 Norris Dam Sunny Day Failure 711.1 528,000 Nottely Dam Sunny Day Failure 684.1 45,000 Page 53 of 70

9.3.3 Single Seismic Dam Failure The results of the single seismic failures combined with a 500-year flood event are presented in Table 9.3-3 (Reference 48).

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

Chatuge Dam Single Seismic Failure During a 500-711.4 497,060 Year June Flood Event Watts Bar Dam Single Seismic Failure During a 500-722.8 1,346,000 Year June Flood Event 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 Technical Letter (ETL) 1110-2-8 (Reference 49). 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 14) 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 17) 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, Tims Ford, 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 PMF elevation is evaluated. The wind wave heights to be added to the PMF elevation at dams are displayed in 50.

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

Cherokeea 1095.2 1.8 1095.8 Douglas Saddle Dam 1b 1023.4 1.5 1024.9 Fort Loudoun 834.6 1.1 837.0 Hiwassee 1536.7 0.8 1537.5 Melton Hill 812.2 0.6 805.0 Norris (Main) 1056.4 0.8 1061.0 Nottely 1782.9 1.3 1807.5 South Holston 1756.3 1.1 1765.0 Tellico 831.7 2.2 834.9 Watauga 1990.9 0.7 2012.0 Watts Bar 768.8 1.3 772.0 a

The volume of water overtopping is evaluated in Reference 50 and does not result in damage to Cherokee embankments.

b This crest elevation includes temporary engineered barriers assumed in the EAP c

EAP elevation at Douglas Dam.

The combined effects flood consisting of the effects of wind wave, from a 26 mile per hour wind speed, on PMF elevation is evaluated. The wind wave heights to be added to the PMF elevation at critical structures are displayed in Table 9.4-2 (Reference 45).

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

Diesel Generator 739.8 2.9 742.7 Building Intake Pumping 739.8 2.7 742.5 Station Unit 1 Shield 739.8 2.9 742.7 Building Unit 2 Shield 739.8 2.9 742.7 Building Page 55 of 70

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 caused by 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 17 and 14. Staff positions listed in Section 5.6 of Reference 17 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 51. 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 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 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

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 Page 56 of 70

Bar Dams and the Watts Bar West Saddle Dam coincident with a 500-year June flood event - Watts Bar Dam 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 Results of the seismic failure of upstream dam simulations are presented in Table 9.4-3 (Reference 48).

Table 9.4-3 Elevations and Dishcharges at WBN Resulting from Seismic Failure of Upstream Dams Elevation Discharge Seismic Dam Failure Combination (ft.) (cfs) 10,000-Year Fort Loudoun Centered Seismic 720.6 1,292,000 Failure During a 25-Year June Flood Event 10,000-Year Fort Loudoun Centered Seismic Failure During a 25-Year June Flood Event - 727.7 1,493,000 Watts Bar fails at peak headwater 10,000-Year Fort Loudoun Centered Seismic Failure During a 25-Year June Flood Event - 720.2 1,292,000 Chickamauga Dam fails seismically Half-10,000 Year Douglas Centered Seismic 723.3 1,346,000 Event During a 500-Year June Flood Event Half-10,000 Year Douglas Centered Seismic Event During a 500-Year June Flood Event - 729.9 1,609,000 Watts Bar fails at peak headwater Half-10,000 Year Douglas Centered Seismic Event During a 500-Year June Flood Event - 723.1 1,346,000 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 17).

Recommended sensitivities included evaluation of dam breach configuration, debris accumulation, gate failures, initial reservoir levels, reservoir inflow, and tailwater conditions. Sections 10.1 - 10.5 discuss the sensitivity analyses performed to support the flood hazard reevaluation for WBN.

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 WBN 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 WBN of 11.9 feet for the 21,400 square mile storm event, and an increase of 15 feet for the 7,980 square mile Bulls Gap Storm event because of additional cascading dam failures. (Reference 52)

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 17.

The results of the Peaked and Lagged Unit Hydrographs sensitivity simulations produce an increase in elevation at WBN of 1.5 feet. for the 21,400 square mile storm event, and an increase of 0.6 feet for the 7,980 square mile Bulls Gap Storm event. The results of the sensitivity analysis Page 58 of 70

shift the controlling storm to the 21,400 storm rather than the 7,980 storm. Even with this increase in stillwater flood elevation, the floor elevation of the Diesel Generator Building (742.0 ft.) and the elevation at the ERCW Intake Pumping Stations (741.0 ft.) are not exceeded. In addition, this increase does not adversely impact flood mode equipment (specifically those with lower margin including Thermal Barrier Booster Pump motors, Spent Fuel Pit Cooling pump motors and ERCW motor operated valves) in the Auxiliary/Control Building. Impacts associated with the Main Control Room and Shutdown Board Room chilled water pump motors are addressed in Section 12 of this report.

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

a. 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.

b. In the 2014 evaluation of the WBN LAR (Reference 5) and approved in the 2015 SER (Reference 6) 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 all 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 WBN, only at the dam being analyzed. The dams considered above the WBN site include Cherokee, Douglas, Fontana, Fort Loudoun, Norris, South Holston, Tellico, Watauga, and Watts Bar Dams. (Reference 52)

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

except Cherokee and Douglas in the 7,980 square mile Bulls Gap centered event. These 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 WBN. 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 Page 59 of 70

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 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 45).

10.5 Initial Reservoir Conditions TVA controls and/or schedules the releases from all 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 17 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 Page 60 of 70

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 Bounded Flood Causing Mechanism Design Basis Reevaluation Comments (Yes/No)

Critical floor elevation is 729 Local Intense ft. Case 2 exceeds 729 ft. in

<729 ft. 729.2 ft. No Precipitation some portions of the protected area.

Controlling event was 7,980 739.2 ft. 744.9a ft. No Flooding from Rivers and Sq.-Mi. Bulls Gap Event Streams Controlling event was 7,980 739.2 ft. 739.8b ft. No Sq.-Mi. Bulls Gap Event Watts Bar Dam Single Flooding from Dam

<728 ft. 722.8 ft. Yes Seismic Failure During a 500-Breaches or Failures Year June 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 741.7 ft. 747.8a ft. No Design Basis was PMF plus wind waves Reevaluation was PMF plus 741.7 ft. 742.7b ft. No 3.1 feet of wind wave Controlling combination was Flooding from Combined the Half-10,000 Year Douglas Effects Centered Seismic Event During a 500-Year June Flood 731.2 ft. 729.9a ft. Yes Event when Watts Bar Dam fails at peak headwater. Not a direct comparison with design basis.

N/A - Not applicable a

Dam modifications in place as described in Reference 5 b

Dam modifications in place as described in Reference 5 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, local intense precipitation, flooding from rivers and streams and flooding from combined effects of PMF and wind are not bounded by the current design basis for WBN. Each of these mechanisms is evaluated below and interim actions defined if needed.

12.1 Local Intense Precipitation The WBN critical elevation, as related to the LIP flooding at the site, is the floor elevation of the exterior doors leading to the Auxiliary Building and Control Building, which is equal to 729.0 ft.

The calculation prepared for the flooding at the WBN site due to the LIP determined that the flood water will exceed the plant critical elevation by as much as 0.2 feet (2.4 inches).

Per the CLB, the LIP flood at the site will not exceed the plants critical elevation. 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, interim actions are 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. 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 site area.

The results of the HEC-RAS model in the reevaluated LIP calculation indicate that flood water will be above the critical elevation only on the plant east side (Subareas K2 and K4) from channel cross sections 21 to 17 and drops below the critical elevation somewhere prior to cross-section

16. Reference LIP Figure Page 62 of 70

Security Related Information Removed Page 63 of 70

Security Related Information Removed No interim actions are required for WBN. Based on the performed evaluations there is no impact on equipment relied upon for safe plant operation.

12.2 Flooding in Rivers and Streams The re-evaluation of the PMF indicates a maximum stillwater elevation of 739.8 ft. at WBN assuming overflow failure of Saddle Dams 1 and 3 at Douglas Dam are prevented. If overflow of the Sadd le Dams is not prevented, the maximum stillwater elevation at WBN is 744.9 ft. These re-evaluation results exceed the WBN design basis flood elevation of 739.2 ft. as presented in Reference 5. 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. The maximum stillwater exceeds the design basis elevation of 739.2 ft.

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

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A review of the most limiting elevations of plant equipment required for flood mode operation indicates that the Main Control Room (MCR) and Shutdown Board Room (SDBR) chilled water motors are the most critical. The protected elevation for this equipment is 739.75 ft. Since a flood water surge inside the structure is 0.5 feet, the re-evaluation flood level at this equipment is expected to be 740.3 ft. which exceeds the 739.75 ft. protected elevation. The other equipment required for plant operation during flood mode operation has critical protection elevations greater than 740.3 ft.

Since the protective barriers for the MCR and SDBR chilled water motors is exceeded in the re-evaluated PMF, the cooling capability of the MCR and SDBR air handling units is assumed to be lost. If the MCR air handling units are lost during the re-evaluation PMF event, the MCR can be abandoned and operation of the plant for the duration of the event would be from the Auxiliary Control Room (ACR). Heat load calculations show that the ACR temperatures do not exceed 114.3 degrees during the event. Heat load calculation also show that the SDBR temperatures remain below the required 104 degree temperature limit.

Although the temperatures exceed the required environmental qualification requirements, WBN calculations have determined that electrical equipment in mild areas can withstand either an increasing ambient temperature to 140°F for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> followed by a period of 99 days at less than or equal to 120°F or a slow ramp to 135°F followed by a temperature of 135°F for 100 days. The result of this calculation bounds the temperatures determined by the model run for the areas served by the SDBR air conditioning system. Therefore, the electrical equipment in the area required for the mitigation of the PMF event remains operable.

12.3 Combined Effects Floods Caused by Precipitation Events The re-evaluation PMF combined with maximum wind effects results in worst case elevation of 742.7 ft. if failure of the Douglas Saddle Dams 1 and 3 is prevented and a worst case elevation of 747.8 ft. if failure is not prevented. The design basis minimum PMF plus wind effects elevation as submitted to the NRC in Reference 5 is 741.7 ft. The critical elevation at the Diesel Generator Building is the floor elevation at 742.0 ft. Since the re-evaluation results exceed the critical elevation by 0.7 feet, interim actions are required to prevent water ingress into the Diesel Generator Building. As an interim action, sandbags stacked a minimum of 12 inches high will be utilized to protect doors and the exterior fuel oil fill ports. Sufficient warning time exist to install the sandbag protective barriers prior to the beginning of the main storm of the PMP. Procedures will be revised as needed to implement this interim action by May 31, 2015.

During the Integrated Assessment, options will be evaluated for permanent barriers to protect the Diesel Generator Building for flood plus wind wave run-up elevations of 742.7 ft.

The re-evaluation PMF combined with wind effects results in a maximum elevation of 742.5 ft. at the exterior wall of the Intake Pumping Station (IPS). As discussed in Section 3.5.2, the ERCW pumps, fire protection pumps, and screen wash pumps are located on the upper deck at elevation 741.0 ft. The 741.0 ft. elevation is enclosed by a 13 feet. high concrete wall. The critical component required at the 741.0 ft. elevation, inside the concrete wall, is a junction box for the High Pressure Fire Protection (HPFP) pump. The critical elevation of the junction box is 742.4 ft.

There is a small (2.2 ft2) opening on the exterior wall of the IPS for the trash sluice which extends below the 741.0 ft. elevation. A sluice backsplash concrete wall with a top elevation of 741.0 ft.

Page 65 of 70

blocks direct entry into building. The re-evaluation PMF combined with wind effects elevation of 742.5 ft. would overtop the sluice backsplash wall and could enter elevation 741.0 ft. Flood waters that enter the structure through this small opening would gravity drain out of the structure as the wave action on the structure recedes. This circuitous entry into the structure due to the external wave action would dampen the waves such that the junction box for the HPFP pump is not impacted. No interim actions are required.

Page 66 of 70

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, Watts Bar Nuclear Plant Updated Final Safety Analysis Report (Amendment 11).

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

4. Tennessee Valley Authority, Watts Bar Nuclear Plant, License Amendment Request, Final Safety Analysis Report, July 19, 2012, ML12236A167.

5. Tennessee Valley Authority, Watts Bar Nuclear Plant, License Amendment Request, September 30, 2014, ML14289A106.

6. 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, ML15005A314.

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

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. 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.

11. Harrington, Bruce C. and Ramon G. Lee, Flood Reassessment for the Effects of Dam Safety Modifications, CDQ0999-98001, Revision 0, March 1998, B45980326001.

12. 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.

13. 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 4, November 27, 2012, ML12335A340.

14. 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.

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

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

17. 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.

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

Phase 1 Baseline Analysis, November 18, 2014.

19. 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.

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

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

22. 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:

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

24. Tennessee Valley Authority Calculation CDQ0000002012000004, Revision 001, BWSC Calculation TVAGEN14006, Revision 0, HEC-RAS Geometry Development-Main Stem, EDMS #B41 140918 001.

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

26. 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.

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

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

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

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

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

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

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

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

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

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

37. Tennessee Valley Authority Technical Memorandum, Critical Storm Selection. December 21, 2012.

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

39. 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.

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

December 19, 2014, EDMS #W50 150107 001.

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

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

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

44. Tennessee Valley Authority Calculation CDQ0000002013000163, Fukushima NTTF Recommendation 2.1: Watts Bar Local Intense Precipitation Analysis, Revision 1, EDMS #T71 140606 801.

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

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, Fukushima NTTF Recommendation 2.1: Norris Dam Sunny Day Failure Simulation

48. Tennessee Valley Authority Calculation CDQ0000002014000024, Revision 000, BWSC Calculation TVAGENQ14010, Revision 0, Fukushima NTTF Recommendation 2.1: Seismic Dam Failure Simulations

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

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

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

53. Tennessee Valley Authority WBN Drawing 46W501-3, Architectural Plan El. 729.0 & 737.0 ABSCE Boundary, Revision K.

54. Tennessee Valley Authority WBN Drawing 46W501-1, Architectural Plan El. 676.0 & 692.0 ABSCE Boundary, Rev. J, Note 8.

55. Tennessee Valley Authority WBN Drawing 44N360, Pressure Confining Personnel Doors Arrangements & Details Sheet 1, Revision H.

56. Tennessee Valley Authority WBN Drawing 44N361, Pressure Confining Personnel Doors Arrangements & Details Sheet 2, Revision L.

57. TVA WBN Drawing 0-47E235-39, Environmental Data Environment - Harsh El. 729.0, Rev. 0.

58. TVA WBN Drawing 0-47E235-40, Environmental Data Environment - Harsh 692.0, Rev. 0.

59. TVA WBN Drawing 0-47E235-10, Environmental Data Environment - Mild El. 729.0, Rev. 0.

60. TVA WBN Drawing 46W455-22, Architectural Frames and Details, Rev. 2

61. TVA WBN Drawing 46W462-2, Architectural Louvers, Rev. E.

62. TVA WBN Drawing 48W1259-1, Miscellaneous Steel Heating and Ventilation Grilles, Rev. G.

63. TVA WBN Drawing 41N333-2, Concrete U-Line Wall Outline, Rev. 17.

64. TVA WBN Drawing 46W404-8, Architectural Wall Sections, Rev. 7.

65. TVA WBN Drawing 46W401-2, Architectural Plan - El. 729.0, Rev. D.

66. DCA-51280-A.

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ENCLOSURE 3 LIST OF COMMITMENTS

1. TVA will complete the March 12, 2012, 50.54(f) Request for Information required Integrated Assessment for Watts Bar 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.

3. As an interim action until the Watts Bar Nuclear Plant Integrated Assessment is complete, TVA will implement procedures to install sandbags to protect doors and equipment access pathways into the Diesel Generator Building. The procedures to install sandbags to protect doors and equipment pathways into the Diesel Generator Building will be revised by May 31 , 2015.