ML20366A022

NRC-2017-000688 (Formerly FOIA/PA-2017-0690) - Resp 5 - Final, Agency Records Subject to the Request Are Enclosed, Part 14 of 15

ML20366A022
Person / Time
Issue date:	12/29/2020
From:	NRC/OCIO
To:
Shared Package
ML20366A007	List: ML20366A008 ML20366A009 ML20366A010 ML20366A011 ML20366A012 ML20366A013 ML20366A014 ML20366A015 ML20366A016 ML20366A017 ML20366A018 ML20366A019 ML20366A020 ML20366A021 ML20366A022 ML20366A023
References
FOIA, FOIA/PA-2017-0690, NRC-2017-000688
Download: ML20366A022 (71)

Text

Deem ity Relateel l11fon ii0li0II lVitl ,I ,old U, ,de, IO err: Z.3~0 ENCLOSURE 1 NEAR-TERM TASK FORCE (NTTF)- RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT FOR WATTS BAR NUCLEAR PLANT REVISION 1 Geeurit) Re letee lr,ferr, ,etior,

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 NEAR-TERM TASK FORCE (NTTF) - RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT Response to United States Nuclear Regulatory Commission (USNRC) -

Code of Federa l Regulations 10 CFR Part SO, Section 50.54 (f)

Watts Bar Nuclear Plant Tennessee Valley Authority May 29, 2015 Revision 1 Sccm ity Related l11fu1111a tio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 RECORD OF REVISION Revision DESCRIPTION OF REVISION No.

0 Total Number of Pages: 71 Total Number of Pages: 71 Description of Revision:

The WBN Flood Hazard Reevaluation was revised to incorporated revisions made to Reference 50, "Fukushima NTTF Recommendation 2.1: Wind Waves for Combined-Effect Floods." The revised calculation incorporated on-site topography in the evaluation of fetch lengths and wave runup that were not previously considered. Additionally, refinements wer,e made to the resultant wind calculation. The changes resulted in lower wind wave heights at critical structures which do not exceed the critical elevation at the Diesel Generator Building as was the case in Revision 0. As a result, the interim actions to mitigate higher flood elevations from combined effects flooding at the Diesel Generator Building no longer apply. Section 12.3 is revised to remove the interim actions as a result of coincident wind wave activity.

All major text changes are denoted by a right- hand revision bar.

Pages Revised: 57, 63, 67, 69, and 70 Pages Added: Page 2 (Record of Revision)

Pages Removed: NA Sccm ity Related l11fu1111a tio11

SccmitJ fi.tclatcd t;:fo1111atio11 001u,1101d tJ11de1 1e errt z.see TABLE OF CONTENTS 1 PURPOSE ....................................................................................................................................7 1.1 Requested Actions ................................................................................................................ 7 1.2 Requested Information ........................................................................................................ 7 2 BACKGROUND ............................................................................................................................8 3 PLANT SITE DESCRIPTION .......................................................................................................... 10 3.1 Current Site Layout ............................................................................................................. 10 3.2 Site Topography .................................................................................................................. 10 3.3 Bathymetry in Vicinity of Plant - Tennessee River ............................................................. 10 3.4 Current Design Basis Flood Elevations ............................................................................... 18 3.4.1 Local Intense Precipitation .................................................................................................. 18 3.4.2 Flooding from Rivers and Streams ...................................................................................... 18 3.4.3 Flooding from Dam Breaches or Failures............................................................................ 18 3.4.4 Flooding from Storm Surge and Seiche ............................................................................... 18 3.4.5 Flooding from Tsunami ....................................................................................................... 18 3.4.6 Flooding from Ice-Induced Events ...................................................................................... 19 3.4.7 Channel Migration or Diversion .......................................................................................... 19 3.4.8 Flooding from Combined Effects ........................................................................................ 19 3.5 Current Flood Protection and Mitigation Features ............................................................ 19 3.5.1 Dam and Reservoir System ................................................................................................. 20 3.5.2 Watts Bar Nuclear Site Protective Structures ..................................................................... 20 3.5.3 TVA River Operations Forecasting and Warning ................................................................. 20 3.5.4 WBN Flood Response Procedures ....................................................................................... 21 4 CHRONOLOGY OF FLOOD RELATED CHANGES SINCE LICENSING ................................................. 22 4.1 Watershed Changes since Licensing ................................................................................... 22 4.2 Summary of Changes to Design Basis Flood Elevations ..................................................... 25 4.2.1 Local Intense Precipitation .................................................................................................. 25 4.2.2 Flooding in Rivers and Streams ........................................................................................... 25 5

SUMMARY

OF PLANT WALKDOWN RESULTS AND MODIFICATIONS............................................ 35 6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS ................................................ 37 6.1 Local Intense Precipitation ................................................................................................. 37 6.2 Flooding from Rivers and Streams ..................................................................................... 37 6.3 Flooding from Dam Breaches or Failures ........................................................................... 37 6.3.1 Project Specific PMF............................................................................................................ 37 6.3.2 Sunny Day Failur e of Upstream Dams ................................................................................ 38 6.3.3 Seismic Failure of Upstream Dams ..................................................................................... 38 6.3.4 Sediment Transport ............................................................................................................ 38 6.4 Flooding from Storm Surge and Seiche .............................................................................. 38 6.5 Flooding from Tsunami ....................................................................................................... 40 6.6 Flooding from Ice-Induced Events ...................................................................................... 40 6.7 Channel Migration and Diversion ....................................................................................... 40 6.8 Flooding from Combined Effects ........................................................................................ 41 6.8.1 Floods Caused by Precipitation Events ............................................................................... 41 6.8.2 Floods Caused by Seismic Dam Failures.............................................................................. 41 7 DESCRIPTION OF MODELS USED FOR REEVALUATION ................................................................42 Page 4 of 71 SccmitJ Related l11fu1111atio1 1

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 7.1 HEC-RAS ..............................................................................................................................42 7.1.1 Description of HEC-RAS Model Verification ........................................................................ 42 7.1.2 Description of HEC-RAS Model Extents ............................................................................... 42 7.2 HEC-HMS ............................................................................................................................44 7.2.1 Description of HEC-RAS Model Verification ........................................................................ 45 7.2.2 Description of HEC-HMS Model Extents ............................................................................. 45 8 JUSTIFICATION OF INPUTS ........................................................................................................46 8.1 HEC-RAS Model Geometry Development and Calibration ................................................. 46 8.2 Dam Rating Curves .............................................................................................................46 8.3 Unsteady Flow Rules .......................................................................................................... 47 8.4 Probable Maximum Flood Inflows ..................................................................................... 47 8.4.1 Hydrometeorological Report ..............................................................................................48 8.4.2 Critical Storm Selection .......................................................................................................48 8.4.3 National Inventory of Dams (NID) Inflows .......................................................................... 49 8.5 Seismic Inflows ................................................................................................................... 49 8.5.1 National Inventory of Dams (NID) Seismic Inflows ............................................................. 49 8.6 Sunny Day and Watauga Project Specific PMF lnflows ...................................................... 49 9 APPLICABLE FLOOD CAUSING MECHANISMS .............................................................................51 9.1 Local Intense Precipitation ................................................................................................. 51 9.1.1 Previous Analysis ................................................................................................................. 51 9.1.2 Technical Approach ............................................................................................................. 51 9.2 Flooding from Rivers and Streams ..................................................................................... 52 9.2.1 Previous Analysis ................................................................................................................. 52 9.2.2 Technical Approach ............................................................................................................. 52 9.3 Flooding from Dam Breach or Fa ilures ............................................................................... 54 9.3.1 Project Specific PMF ............................................................................................................ 54 9.3.2 Sunny Day Dam Failure ....................................................................................................... 55 9.3.3 Single Seismic Dam Failure.................................................................................................. 56 9.4 Flooding from Combined Effects ........................................................................................ 56 9.4.1 Floods Caused by Precipitation Events ............................................................................... 56 9.4.2 Multiple Seismic Dam Failures with Combined Flood Event .............................................. 58 10 EVALUATION OF UNCERTAINTIES .............................................................................................. 60 10.1 100% Runoff ....................................................................................................................... 60 10.2 Peaked and Lagged Unit Hydrographs ............................................................................... 60 10.3 Gate Operability/Blockage ................................................................................................. 61 10.4 Breach Size .......................................................................................................................... 62 10.5 Initial Reservoir Conditions ................................................................................................ 62 11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS ................... 63 12 IDENTIFICATION AND EVALUATION OF ANY INTERIM ACTIONS TAKEN TO MITIGATE HIGHER FLOOD HAZARD RELATIVE TO DESIGN BASIS ..................................................................................... 64 12.1 Local Intense Precipitation ................................................................................................. 64 12.2 Flooding in Rivers and Streams .......................................................................................... 66 12.3 Combined Effects Floods Caused by Precipitation Events ................................................. 67 13 References ..................... .......................................................................................................... 68 Page 5 of71 Sccm ity Related l11fu1111a tio11

Sccmily !i(clotcd t.1fc111:atio:1 uoltltliold IS11de1 le eP"R ! .39e FIGURES Figure 2-1 River System Schematic ............................................................................................................... 9 Figure 3-1 Watts Bar Nuclear Plant............................................................................................................. 11 Figure 3-2 Watts Bar Nuclear Plant Site ...................................................................................................... 12 Figure 3-3 Watts Bar Nuclear Plant Site Layout .......................................................................................... 13 Figure 3-4 Watts Bar Nuclear Plant Topography ........................................................................................ 14 Figure 3-5 Location of Tennessee River Cross-Sections .............................................................................. 15 Figure 3*6 Bathymetry- Tennessee River in the Vicinity of the Watts Bar Nuclear Plant ......................... 16 Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam ................................ 23 Figure 4-2 Timeline of Flood Related Changes Since Licensing ................................................................. 25 Figure 6-1. Landslide Incidence Map of United States ............................................................................... 39 Figure 7-1 Upper HEC-RAS Model Extents .................................................................................................. 43 Figure 7-2 Lower HEC-RAS Model Extents ..................................................................................................43 Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents ................................................................. 44 Figure 7-4 Local Intense Precipitation HEC-HMS Model Extents ................................................................ 45 Figure 12-1 WBN Access Doors ................................................................................................................... 65 TABLES Table 3.4-1 Combined Effects of Flood and Wind ...................................................................................... 19 Table 4.1-1 Land Use above Guntersville (2001- 2011) ............................................................................ 23 Table 4.1-2 Impervious Area above Guntersville (2001- 2011) ................................................................ 24 Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11) .......................................................... 29 Table 6-4.2-1 Potential Flood Causing Mechanisms or Causal Phenomena .............................................. 37 Table 9.1-1 Results of WBN LIP Analysis ..................................................................................................... 52 Table 9.2-1 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Reevaluation .................. 53 (b)(3) 16 USC J:c11:ll~Q,?:?PMfJtevationsandDischargesatWBN(J IJ Resulting from i~~iF\ 1 § 40 1 4

• (d) (b)( ), (b

§ 8240-l(d}. (b)

(4) (b)(7)(F) Dam ............................................................................................................................................................ .

Table 9.3-1 Elevation and Discharge at WBN Resulting from Project Specific Dam Failures ..................... 54 Table 9.3-2 Elevations and Discharges at WBN Resulting from Sunny Day Dam Failures .......................... 55 Table 9.3-3 Elevations and Discharges at WBN Resulting from Single Seismic Failure of Upstream Dams56 Table 9.4-1 Wind Wave Elevation Results at Dams .................................................................................... 57 Table 9.4-2 Wind Wave Elevation Results at Critical Structures ................................................................ 57 Table 9.4-3 Elevations and Dishcharges at WBN Resulting from Seismic Failure of Upstream Dams........ 59 Table 11-1 Comparison of Current Design Basis Elevations and Reevaluation Results .............................. 63 Page 6 of 71 Sccmity Related l11fc111:atio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 recommenda1tions 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 S0.54(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 staff' s 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 t o submit an interim action plan that documents actions planned or taken to address t he reevaluated hazard. Subsequently, addressees should perform an integrated assessment of the plant to identify vulnerabilities and actions to address t hem.

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 7 of 71 Sccm ity Related l11fu1111a tio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390

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 i n 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-mi les. The watershed is about 70 percent forested with much of t he 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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OUltllil0IU tJII0GI 10 er ft ! .390 Bristol Projects (2) Noltchucky Holston Rwer F19ncn Bro9d Clinch River Ft. Pa Irick Henry Boone er

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Doakes C/89k t:t- J:,hn Sevier Wilbur

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Cherokee ~ ~ Doug!~ Fontana (T)Cheoali,,

Norris Calderwood (Tl - - senteellah (Tl Ft ~

Melt on Hill - - Chilhowee IT) .r HNVassoo RNer Tellico Chetuge WattsBarPlant ~

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Raccoon Mountain Apalachie Ocoee 1, 2, 3 Blue Ridge

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Tennesse&-Tomb1gbee Waleiwey Nole Oh,o River (C) U S Army Corps of Engineers Dams (N) Nantahala Power & Light Company (subsidiary or Duke Energy)

(T) Brookfield Smokey Mountain Hydro Power (Forme~y T apoco)

Miss1ssipp1 River Figure 2-1 River System Schematic Page 9 of 71 Sccmity Related l11fo111:atio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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) conduct ed a hydrographic survey of Chickamauga Reservoir. Th is 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 Figur,e 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)

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OUltllil0IU tJII0GI 10 er ft ! .390 ESRI World Imagery

, 10 N Figure 3-3 Watts Bar Nuclear Plant Site Layout Rhea county, Ttnnus u Page 13 of 71 Sccmity Related l11fo111:atio11

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OUltllilOIU tJIIOGI 10 er ft ! .390 USGS Topo Figure 3-5 Location of Tennessee River Cross-Sections C.ross-Section Rh ea County, Tennessee Oa~ 16 Janua,y 2012 Page 15 of 71 Sccmity Related l11fo111:atio11

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Figure 3-6 Bathymetry-Tennessee River in the Vicinity of the Watts Bar Nuclear Plant Page 17 of 71 Security fitclatcd l 11Fu111:atiu11

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OUltllil0IU tJII0GI 10 er ft ! .390 3.4 Current Design Basis Flood Elevat ions On July 19, 2012, TVA submitted a revised hydrologicall 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 utilize*s the U.S. Army Corps of Engineers (USACE) Hydrology Engineering Center River Analysis System (HEC-RAS). The stillwater probable maximum flood (PMF) elevation is (b)(3) 16 USC C::--,ft. (Reference 6).

§ 824o-1(d): (bf .. c__J 3.4.3 Flooding from Dam Breaches or Failures Postulated failure of the (b)(3) 16 USC § 824o-1(d), (b)(4), (b)(7)(F)

Dams 1s 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 s *

• oding conditions (Section 3.4.8).

(b)(3) 16 USC Postulated failure of § 8240*1(d) (b) am is considered in the CLB analysis for loss of ultimate heat

. . 4 b 7 F . .

sink during non-flo 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 lalke 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.

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OUltllil0IU tJII0GI 10 er ft ! .390 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.)

(OJ(;iJ 16 use§ Probable Maximum Flood (still reservoir) a24o-1(d) (b)(4)

(b)(7)(F)

Run-up on 4:1 sloped surfaces on the Diesel Generator Building Run-up on critical vertical wall of the Intake Pumping Station Surge level within flooded structures 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 u stream of WBN which are postulated to fail in this combined event are (b)(3) 16 us c § 8240- 1(d) (b)(4 ) (b)(?)(F) The CLB maximum flood elevation for floods caused by seismic dam failures is .......... ft Reference 6) (b)(3) 16 USC

.. ... . §82401(d) (b) 3.5 Current Flood Protect ion 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, TVA's 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.

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OUltllil0IU tJII0GI 10 er ft ! .390 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 t he 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 t he 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 d~ e flood mode since its lowest floor is at elevation 742.0 ft. With the PMF elevation of * ****** * * : wind wave run-up at the Diesel (b)(3) 16 USC

' . §824 :1(d) ()

(b)(3) 16 U SC Generc1tc.>rl:}yjlqiog.Js.eJevation[==itt. Therefore, flood levels do not exceed floor elevation of ,., ,L?,.,.,,-; b

§ 824o-1(d):(b) .. 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 islE]tt. 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 mon itoring 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 20 of 71 Sccmity Related l11fo111:atio11

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OUltllil0IU tJII0GI 10 er ft ! .390 "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 stage~ reparation. By dividing the pre-flood preparation steps into two stages, a minimum of aLJhour *pre-flood- -transition.intervaLi s ~~~~p~ ~ S ~

available between the time a flood warning is received and the time the flood waters exceed '" ,c?,..,,',J; ( )

plant grade.I (b)(3) 16 USC§ B24o-1(d), (b)(4). (b)(7)(F)

.___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.....J f (Reference 5)

The plant preparation status is held at Stag either Stage II begins or TVA's RO determines

~b~ll~~f(~)~(~ - --hatfloodwaters.willnotexceedelevation *** * *** ** ft. at the plant. Stage I shutdown is initiated upon notification that a critical dam failure com mation 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 elevationC ]ft.{winteLQJ(b)(~) )? USC 0

1

~;q::~~i~~k~~t~ob:r~:aecn~e!in~~:e :e~~~:~ti: t es***t ;! ~ *=~~~~~s~~i~!f~ i~~t(b~~*f ~~y -*J ~;r~ii~~!***~ ~~~,~~i~-

(,I.\ IL\1-,\1r1 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[ : Jtt. atWBNhasbeenforecast.,....~.Q.r_resQ_£ndi~g (~)(3) 1? U SC target river elevations for the summer season at WBN are elevation 720.6 ft. and elevation ~.~2~?~,'~; (b)

(b)(3) 16 U SC . .. .J. .

• Jt. (Reference 5)

§ 824o-1(d), (bj .

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 TVA's RO, efforts will be made by TVA to determine whether dam failure has occurred. If a cri~ m failure condition has occurred which could cause a flood wave to approach elevation (b) ft. at the plant site or it cannot be determined that it has not occurred, Stage I shutdown wi e 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 elevationL it. is reached at (b)(3) 16U s c___________ WBNinJhe mostcriti.cal event is aboutEJhours. This time is adequate to permit safe plant

~}2,~?~.{~, (b) 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 A0ls to align systems and components for flood mode operation. These activities are required by the AOI to becom~ edwithinO hours of notification of a Stage II flood alert. Coupled with then

• imum timeofl:_Jhours for Stage I, preparations result in a required total preparation time o ... . hours t~;l~~(~)\~

for the AOI actions. "' ,._,,.wr:

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 ot her 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 st eps necessary to have the plant in the flood mode when the flood exceeds plant grade. HPFP system water (raw) will replace auxilia ry 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 f lotation. 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 t ake suction from the spent fuel pool by their normal intakes and will discharge to the spent fuel pool heat exchangers. The outflow of t he 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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OUltllil0IU tJII0GI 10 er ft ! .390

-~

l!ml:9~~'mnii&im

• water/Welland/Barren (2.8%)

Urban (10.0%)

0 Forested (63.6%)

Grassland/Shrub (5.6%)

~

0 Agriculure (18.0%)

~

@m1lbm

~

Clmlllm)

~

N Aso 100 Min 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) t o 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%

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 t his 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 aind result in less runoff.

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 NR<:OA Original Safety 1999 Review of Licensing M odificatio ns UFSAR Hydrology 2012 and 2014 Basis com plete Change Documentation W BN Ul LAR

• 1982 Dam Safety 1997-98 Flood Reassessment 2004 Riv er Op erations 2008-2012 Hy drology January 28, 2015 SER Review Study (ROS) Reanalysis began Implemented Figure 4-2 Timeline of Flood Relat ed 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 t he 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 t he late 1970's 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 G 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)

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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

"'(b"""J("JJ

",..,1..,,.

6""" u""'s'""'c,.....,,..

§ ..,

8 2,..,4-o--, ....

g. Embankment failures from overtopping were postulated at (d) (b)(4) (b)(7)(F) and (b)(3) 16 us c § 8240- lbecause dams were not designed for the PMF l Wl lb}(4} lb}l7}/f}

.l .

h. Plant grade elevation 728 ft. exceeded by design flood event - Rf-8,latory Position 2 -

flood warning plan in place for safe plant shutdown - minimum ofLJhoursavaUic!l:1J1;L ~~~;{~~(~)s(~

i. Embankments overtopped as a result of flood wave from upstream dam failure were '" 'L,rrnr, postulated to fail instantaneously and completely with time of failure based on DBREACH results. bX3).16 USC § (b)(3t16 USC 4

§ 824o-1fd}:' (b) *l l ~~=~strJ ~am an ~~~(~d). (bl( l, am postulated failure outflows combined and routed

k. ~~li!-\ii~~for tailwater submergence was applied I. ....,......,........_ _ am, downstream of WBN, was overtopped during PMF event simulation not to fail 4.2.2.1.2 Seismically Induced Failure of Upstream Dams The maximum plaint site elevations at WBN for the different postulated combinations of seismic dam failures coincident with floods were a,s follows:

Operating Basis Earthquake (OBE) Failure with1/2 PMF Maximum Elevation at WBN (ft.)

(b)(3) 16 USC § 8240-1 (d) (b)(4) (b)(7)(F)

SSE Failures with 25-year Flood (b)(3)16 USC § 8l4o- 1(d) (b)(4) (b)(7)(F)

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 (b)(3) 16 USC § 8240-1 (dJ (b)(4) (b)(7)(F) 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 rJ (b)(3) 16 USC

c. Warning time available greater thau ho(H'if fot allfailure combinaticins _ _ _ _ 824o:1(d), (b)

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 oni 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!(b)(3l 16 us c § B240-1 (dl (b)(4), (b)(l)(F) !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:

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Bccw ity Related Ii ,fo,, 11atio1, OOilihiUIU biidbi 10 6 1 it 2.396 Storm Event Maximum Discharge Maximum Elevation (b)(3):16 WBN (March 7,980-square-mi le) 1,288,000 cfs U.SC § eet 824o-1(d),

(b)(4), (b)

(7)(F)

Page 28 of 71 Sttdl a, ~elated Ii rfm I I iUliDI I

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11)

Dam Dam Modification Year Modifications Completed (b)(3) 16 USC. § 824o-1(d) (b)(4), (b)(7)(F)

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OUltllil0IU tJII0GI 10 er ft ! .390 4.2.2.2.1.1 Summary of Differences Between 1970's Licensing Basis and 1997 - 1998 Reassessment for PMF

a. (b)(3) 16 USC§ 824o-1(d) (b)(4) (b)(7)(F) b.

c.

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.)

SSE Failures with 25-year Flood (b)(3) 16 USC § 824o-1(d), (b)(4) (b)(7)(F) 4.2.2.2.2.1 Summary of Differences Between 1970's Licensing Basis and 1997 - 1998 Reassessment for Seismically Induced Failures

a. (b)(3) 16 USC § 824o-1(d), (b)(4), (b)(7)(F) b.

c.

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OUltllilOIU tJIIOGI 10 er ft ! .390 l lb)OI I" I SG § iZ!Ol *1 lb#) lbl/71/FI

~, - - - - - - - ' (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 t hree 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 m tafons and installed temporary flood barrie*rs at~g~3l 16 0 C § B2 4o.i(~ (b: ) (~ (7) p nd s

(b)(3) 16 USC

§ 8240-1*(0}:'"(bf ............................ . ......... Dams to increase the height of the emban ments. increasing t e e1g t of the ents at these dams prevents embankment overflow and failure during the PMF.

(Reference 4) 4.2.2.3.1 PMF The hydrologic analysis revision convert s 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 t he 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 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 31 of 71 Sccmity Related l11fo111:atio11

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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 t est data

e. Turbine discharges were used at all river reservoirs up to t he 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 Noittely 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. llbll3)16 U S C § 8240-1(d)(b)l4) [b)I/)(F; (b)(3) 16 USC

§ 824o-1{d}{b}* k. j ITVA dams were evaluated for stability, including:

(4) (b)(7)(F) .___ (b)(3) 16 U SC § 8240-1 (d), (b)(4) (b)(7)(F)

(b)(3) 16 USC

§ 824o-1(d); (b) I: E Jdams in the tributary system, t hat were not evaluated and are postulated to fail, were included:,.,,..,..,.,,,.....,...,....,..,...,,....,,...,,--....,

(b)(3) 16 USC § 824o-1(d), (b)(4), (b)

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OUltllil0IU tJII0GI 10 er ft ! .390 (b)(3) 16 USC§ 824o-1(d), (b)(4), (b)(7)(F)

(b)(3) 16 USC []

§ 8240-l(dJ(o} -----<m ; * * * * * *

• dams that were evaluated for stability, but were not credited due to low margin:

4

( ) (b}(?}(F) (b)(3) 16 U SC § !l24o-1 (d} (b)(4) (b)(7)(F)

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. (b}( 1 2.

3.

4.

5.

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 with1/2 PMF Maximum Elevation at WBN (ft.)

(b}(3) 16 USC § 8240-1 (d) (b}(4}, (b)(?)(F)

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OUltllil0IU tJII0GI 10 er ft ! .390 r b)(JJ 16 0 S C § 8240-1(dJ(b)(4) (b){l){F)

SSE Failures with 25-year Flood (b)(J) 16 u s c § tiL4o-1(a), (o)(4) (o)(f)(r )

4.2.2.3.2.1 Summary of Differences between 1997 - 1998 and 2008 - 2014 Analyses for Seismically Induced Failures

a. Failu re outflow from tributary dams l(b)( 3) 16 DSC § 824o-1(d) (6)(4) (b)(?)(F) developed using HEC-HMS model .

b. § 8240_1(d) (b) added because seismic stability analyses were not conclusive

c. ross section bathymetry was updated based on recent USACE Doppler profi ler 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 t he 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. l(b)(JJ 16 U S C § 824o 1(d)(b)(4)(b)(7)(8

k. Temporary barriers and permanent dam modifications were not credited in the seismic analysis Page 34 of 71 Sccmity Related l11fo111:atio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 r etired 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 insta lled. This barrier is designed to protect the TBBPs and their respect ive 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 35 of 71 Sccm ity Related l11fu1111a tio11

Bccw ity Related Ii ,fo,, 11atio1, OOilihiUIU biidbi 10 6 1 it 2.396 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 demonst rates 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 M echanisms 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 adj oining 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 i n 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 present ed in subsequent sections.

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OUltllil0IU tJII0GI 10 er ft ! .390 6.3.2 Sunny Day Failure of Upstream Dams Sunny day failure of upstream dams has not previously been analyzed for WBN. Inputs, assumptions, technicail 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 anal sis was erformed to determine the impact of sediment released from a hypothetical (b)(3) 16 USC § 8240-1 (Reference 18). The following was evaluated:

Sediment core sam les of the embankment were examined.

(b)(3) 16 USC§ 824o-1(d . (F)

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 bet ween 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 at tenuates 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 ait elevation 728.0 ft. is approximately 45 feet above the norma l maximum pool levels.

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Sccmily !i(clotcd t.1fc111:atio:1 uoltltliold IS11de1 le eP"R ! .39e While seismic seiche has been recorded in the Tennessee Valley area it has been of very small magnitude. For example the March 1964 Alaska Earthq uake 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)

USGS map or relative Landslide Inci dence landslide madeoce and susrep1 blity across the D Low(less than 1.5 % of area involv*d) contenmnous Umted D Mod orate (1.5%,15% of area Involve d)

Slates Red and pink

- High (greater than 15 % of aru i nvolved) areas have the highest incidence and Landslide Suscepti bility/ Incidence suscepl b 11, USGS D Moderate suscepti bi lity/low incidence Map Enlarge Image D High susceptibility/ low incidenoe D High suscept ibility/ moderate incidence 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 S28, 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 downstr eam 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 credilble.

6.7 Channel M igration 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 incl uded 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 (Re.ference 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 10CFRS0.54(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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Sccmily !i(clotcd t.1fc111:atio:1 uoltltliold IS11de1 le eP"R ! .39e Figure 7-1 Upper HEC-RAS Model Extents N

HfC.Wf"kMMtU!t!M, A , .:.

• Figure 7-2 lower HEC-RAS Model Extents Page 43 of 71 Sccmity Related l11fc111:atio11

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OUltllil0IU tJII0GI 10 er ft ! .390 HEC-RAS models calculate water surface elevations for defined channels. The contribut ing 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 a1nd 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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OUltllil0IU tJII0GI 10 er ft ! .390 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 45 of 71 Sccmity Related l11fo111:atio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 t o 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 hist oric events. The main stem is calibrated to the M arch 1973 and May 2003 events and the tribut aries 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 contin uous model for flood simulation. The HEC-RAS model calibration produced final geometry files to be used in simulations of floods at TVA nuclear plant sit es. 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 TVA's 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 t ailwater effects.

The Dam Safety group of TVA's RO division evaluated the stability of TVA's dams under various load conditions in accor dance with TVA Dam Safety acceptance criteria; t hey also considered load conditions that comply with nuclear guidelines. The load conditions studied included PMF level headwater elevations with varying tailwat er 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 H EC-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 cu rves; seasonal variabi lity 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 diam 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 det ermine flow over other components such as non-overflow sections, navigation locks, t he 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 t he 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 lJHs 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 t o model input points.

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 HM Rs had sound methodology and data basis at the time of the analysis.

TVA is currently reevaluating the PMP and developing a replacement for the HM Rs. 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 HM Rs 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 S0.54(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 t ype is a moveable 7,980-square-mile PMIP event that may be slid rough ly 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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) Inflow s 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 Inflow s Staff positions listed in Section 5.6 of Reference 17 specify that the coincident inflow from the 25-yea r flood be applied during the 10*4 exceedance probability seismic hazard and either the 500-4 year flood or the half PMF be applied as the coincident inflow during half the 10* 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 o rdinates are then scaled to produce the calculated probabilistic volumes at the selected durations.

8.5.1 National Inventory of Dams (NID) Seismic lnfllow s 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 expect ed 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 failur e 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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Bccw ity Related Ii ,fo,, 11atio1, OOilihiUIU biidbi 10 6 1 it 2.396 The development of two inflow scenarios for use in these model calculations is necessary.

Inflows for use concurrent with sunny day failu res 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.

Page 50 of 71 Sttdl a, ~elated Ii rfm I I iUliDI I

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 2

WBN . According to NUREG/CR-7046 (Reference 14) LIP should be equivalent to the 1-hr, 1-mi ,

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-mi 2 PMP for WBN is determined according to HM Rs 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, Manning's 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 t o 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 f eatures 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-st ate model.

Additionally, this analysis assumes partially, but significantly, blocked drainage channels. Each cross-section in t he 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 t railer 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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SccmitJ fi.tclatcd t;:fo1111atio11 001u,1101d tJ11de1 1e errt z.see Table 9.1-1 Results of WBN LIP Analysis Critical Plant W ater 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 St reams 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 t he 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 aigainst 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, st orm 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 M arch storm

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

. . 1(6)(3) i6 0 s c § 8240-1 (dj (6)(4)

c. *7,980 square-mile Bulls Gap-centered M arch storm w1th (b)(7)(F)

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.

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OUltllilOIU tJIIOGI 10 er ft ! .390 Table 9.2-1 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Reevaluation PMF Event Elevation (ft) Discharge (cfs) 21,400 Sq.-M i. 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 cen~~~WJ~~~..QJ;~~~in Table 9.2-1 result in 3 1 overtopping and subsequent failure of (b){ ) As a result an additional PMF analysis assumin,g ( ( 1 4o-1{d,{ )4 (b are raised to prevent overtopping is presented in Table 9.2-2,_(7_l(_F)_ _ _ _ _ _ _ _ ___,

~~~'!'--:!",+/-,J-IW, Elevations and Discharges at WBN ~...__ ........_......._,l,Resulting-from '""~bdi

"""=~_..,.

f,,..

ct).~.,.,...

~).,,.

~"""

l§---. ;~~~l;!.~)s(~)

l (b)(4) (b)(7)\F) am (b)(7)(F) (4), (b)(7)(F)

PMF Event Elevation (ft.) Discharge (cfs) 7,980 Sq.-Mi. Bulls Gap Event with (b)(3) 16 USC ___ Emergency.Ac.tio.n. .P.lan. at . . Dam

§ 824o-1(dJ;(b)

(b)(3) 16 USC § 824o-1(d), {b)(4) (b)(7)(F)

J"":, a result me 7,980 square-mile 111 B

!!!"u-:l:--

ls""'.G

-a-p_c_e_n"'."te_r_e-:

d-s"'."

to_r_m--:-

b-ec_o_m_e_s_t'."7h_e_c_o_n"'tr

.'"-07."

ll:--

in_g_e_v_e-nt~ wit* h t he EAP at ~lf616§ Dam and a (b)(3) 16 U.SC

§ 824o-1(d}, (b) . ,n_axin,u,n *~*~~!ionofE l i:;'~/,

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OUltllil0IU tJII0GI 10 er ft ! .390 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 (b)(3)*16 us c § 824o-1(d} (b}(4) (b)(7}(F} providing a bounding scenario for casca mg am a1 ures as a resu o a sunny ay am a1 ure. The results of the analysis identified (b)(3) 16 USC ams whose failures could lead to flooding at the plant. These dams are as follows:

§ 8240-l{d)*(b-) *

(4) (b)(?)(F) (b)(3} 16 USC § 8240-1 (d), (b)(4}, (b}(?)(F)

(b)(3) 16 USC

§ 8240-1(-d) (b) * ** * . TheE Jdams identified were evaluated in one of three ways: under a project specific PMF (4) (b)(7)(F) event sin~le seismic dam failure combined with a flood event, or failure d~rjng a sunny qay. A (b)(3) 16 U SC

§ 8240- Hd),-(b)

• +

• p roject specific PMF simulation bounds a sunny day failure of thel  ?

j dam,J~)(~1 1 USC (0j(3~4MJ6 C l !dams have low margin for seismic stability and are analyzed as single ~.~,.?:~,(~: (b)

§ 824o-1(d), (b) seismic dam failure simulations paired with a flooding event. These seismic simulations bound 0

sunny day dam failure simulations for these dams. The remaining dams, *

(b}(3) 16 us c 8240 1 J . . . ... .... ... . lare analyzed for failure in sunny day simulations. The *-****-** Dam.SY.O.DYc:l~y(~)(~) 1? USC

§ * {-dJ.(o) fa ilure simu lation bounds the (b)(3} 16 eismic failure simulation because the seismic failure § 824o-1(d), (b)

(4) (b)(?)(F) I . I . I f h u SC § "' ,.,,.,.,,r, resu ts in on ya minor ass o t 8240 _1(d) reservoir volume.

(b)(4}, (b) 9.3.1 Project Specific PMF (?)(Fl Table 9.3-1 Elevation and Discharge at WBN Result ing from Proj ect Specific Dam Failures Dam Failure I Elevation (ft.) I Discharge (cfs) r b)(3} 16 USC § 824o-1(d) (b}(4}, (b)(?)(F)

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OUltllilOIU tJIIOGI 10 er ft ! .390 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. (b)(3)16 us C § 824o-1(d) (b)(4) (b)(7)(F) b.

C.

d.

(~bl)(~j).;~~!:@.** **** Asunnyday-failur-e-ofj -- loam was not included since the resulting elevation would

'~WJ' - - - - - - 'b"""'ec...-l:l.9JJJ1dedbythef ~mgle seismic failure during a 500-year event. The results of

~Ji)(&~~~ . ... the+ .* ~ingle se1sm1c railure simulation are in Table 9.3-3. Results of the sunny day (4) (b)(7)(F) 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 I Elevation (ft.) I Discharge (cfs)

(b)(3) 16 USC § 824o-1(d) (b)(4), (b)(l)(F)

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OUltllil0IU tJII0GI 10 er ft ! .390 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 Upst ream Dams Elevation Discharge Seismic Dam Failure Combination (ft.) (cfs)

(b)(3)16 USC § 8i4o-1(d) (b)(4), (b)(7)(F) 9.4 Flooding from Combined Effects 9.4.1 Floods Caused by Precipitat ion 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 Commission's 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(

(b)(3) 16 USC§ 824o-1(d) (b)(4), (b)(?)(F)

I 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 Reference 50.

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Sccmily !i(clotcd t.1fc111:atio:1 uoltltliold IS11de1 le eP"R ! .39e Table 9.4-1 Wind Wave Elevation Results at Dams Maximum Maximum Wave Stillwater Height, Hmax Dam Crest Elevation Elevation Dam (ft.) (ft.) (ft.)

b (J) 1 ti USC !i tll40-l (d) {b)(4) (I )(1 /\r)

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

(b)(3)16USC §824o-1(d), (b)(4) (b)(7)(F)

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OUltllil0IU tJII0GI 10 er ft ! .390 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 4

from the 25-year flood be applied during the 10* 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 (b)(3) 16 us c indicatethatal . .... !centered seismic event results in! * ~ amfailureswiththe (bl(3 l,1_6 u s(c)

[4~~~f~)lrt 0

) largest volume of water that could be released for a 10,000 year ground motion. AD M~lit)J}(~)

(b)(3 ) 16 u 5 C C:E:?J1teredseismiceventresults.in l*** * * ... ldam faillures with the largest volume of water that (4), (b)(7)(F)

§ 824o-1(d}(b) * ********~--- . .

(4) (b)(7)(F) could be released for a1/2 10,000 year ground motion.

The reevaluation considered the following scenarios:

a. (b)(3) 16 USC § 8240 1(d) (0)(4), (b)(/)(F) b.

c.

d.

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OUltllil0IU tJII0GI 10 er ft ! .390 (b)(3) 16 USC§ 824o-1(d), (b)(4) (b)(f)(F) e.

f.

Results of the seismic failure of upst ream dam simulations are presented in Table 9.4-3 (Ref erence 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)

(b)(3) 16 U SC § 8240 1(d) (b)(4) (b)(?J(r)

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OUltllil0IU tJII0GI 10 er ft ! .390 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.

T~ lts of the 100% runoff sensitivity simulations produced an increase in elevation at WBN (b)(3) 16 USC . o *********** feet for the 21,400 square mile storm event, and an increase ofr7feetforthe7,980 (~)(3) 16 USC

§ 824o-1(d):(bj ..

square mile Bulls Gap Storm event because of additional cascading dam failb-rnt. (Reference 52) ~}

2

~?:~,(~: (b)

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 inf low 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 60 of 71 Sccmity Related l11fo111:atio11

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OUltllil0IU tJII0GI 10 er ft ! .390 shift the cont rolling 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 TVA's unit hydrographs util izing Snyder's 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 TVA's 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 a al sis assumed !(b)(3) 16 USC § 824o-1(d}, (b)(4}, (b)(?)(F)

~b~~I~~-~)!(~* .. .. .__..,....,..,,_,..--,,_, To evaluate the sensitivity of this assumption, sensitivity to gate operability or gate oc age ue 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 capaciti s were ostulated to be reduced during the two PMF storm events. For each dam analyzed, the (b)(3)*16 USC § 824o-1(d) (b)(4) (b)(?

The simulations for each dam did not provide results at WBN, only at am considered above the WBN site include 52)

(b)(3) 16 USC § 824o-1(d) (b)(4) ,;; (b,a:

l(;.:,

7),.;. ) ---=-::-;:=----...,,..--:::--:-:----:::---'

(F,:...

m t e 7,980 square mi e Bulls Gap ce- n-:t-e-re-d~e-v_e_n-:-

t _- - - - - - - - - - - - - - - - '

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 61 of 71 Sccmity Related l11fo111:atio11

Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390

• There is no barge traffic above these two dams

• River Operations (RO) monit ors 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 syst em and t he volume-driven nature of the Tennessee River System. Total failures of concrete structures are assumed for dam failures above Chickamauga Dam. 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 wit hin 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 62 of 71 Sccm ity Related l11fu1111a tio11

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OUltllil0IU tJII0GI 10 er ft ! .390 11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS Table 11-1 presents a comparison of t he design basis flood elevat ions and the reevaluated results.

Table 11-1 Comparison of Current Design Basis Elevat ions 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.

(DJ(J) 16 U SC § ts '.40- 1(d) (b)(4 ). (b)(1)(F)

Flooding from Rivers and Streams Flooding from Dam Breaches or Failures Flooding from Storm Not a credible flood-causing N/A N/A N/A Surges or Seiches mechanism at t his 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 M igration N/A N/A N/A mechanism at this site Towa rd t he Site (b)(3) 1b USC§ tl240-1(d), (0)(4) (b)(()(F)

Flooding from Combined Effect s N/ A- Not applicable

• Dam modifications in p lace as described in Reference 5 (b)(3) 16 U.S C § b Dam modifications in place as described in Reference 5 and an 824o-1(d), (b)(4), Dam (b)(7)(F)

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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390 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 t he 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 plant's 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 plant's safety-related SSCs will be examined in detail. Prior to the completion of t he 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 majorit y 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 64 of 71 Sccm ity Related l11fu1111a tio11

SccmitJ fi.tclatcd t;:fo1111atio11 001u,1101d tJ11de1 1e errt z.see (b)(3) 16 U SC § 8240-1 (d) {b)(4) (b)(?)(F)

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OUltllil0IU tJII0GI 10 er ft ! .390 (bJlJJ 16 U SC § 8240-1 (d), (bJ(4), (b)(7)(F)

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

12.2 Flooding in Rivers and Streams 3 16 U.S C.

The re-eva Iuat .ton of th e PMF .in d.1cat es a maximum . st 'II t wa t er eIevat*ton of ~ *-**** ft*, at --WBN (b)(

• *** * *§ a24) :l(d) (b) assuming overflow failure of (b)(3) 16 USC § 824o-1(d), (b)(4), (b) Dam are prevente . If overflow of ,., ,L~n"r;

~b~~~~~~):(~ . .,. I* .. !Dams is not prevente 't e maximum sti water e evation ~ is[ 3ft. Ih~~~ ~igue us C

,., '~"..,

11

.-, re-evaluation results exceed the WBN design basis flood elevation of * * * * * *

• ftaspresentedin ~

824

~~(f~

Reference 5. For Flooding in Rivers and Streams an Integrated Assessment wi be performed. '" 'L~..,_,r:

The overto (b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(?)(F)

In the Integrated Assessment, TVA will review the long term options for addressing the I overtopping elevation of! (b)(3) 1,..,,1,J""g...,c...,.

s .. s'.l~4~o.i.1(11,od~)(i,i;ib,14j){ai1,1,).,JJlhw_)(..c.Z.iu)IF:..1,}_ _ _ _ _ _ _ _ _ _ __J Page 66 of 71 Sccmity Related l11fo111:atio11

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OUltllil0IU tJII0GI 10 er ft ! .390 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 (l°BR) cr lled water motors are the most critical. The prot elevation for this equipment is * ****** *ft,*******.Since a (~)(3.)16 U SC (b)(3):16 USC 2

§ 824o-1fd}:(b) floodwater.surge inside the structure i * * * *

• eet, the re-evaluation flo*o* d.. le.ve. I. at t..hi.s eq.u..i.p..m.*e***n****t****t~. ,~.?.;-,

..\(~; (b) is expected to be 740.3 ft. which excee s thei--=:l ft. protected elevation. .The othE!r (~)(3) 16 USC equipment required for plant operation during ~ ode operation has critical protection ~}2 ,i?;;,'~; (b) elevations greater than 740.3 ft.

Since the protective barriers for the MCR and SDBR chil led 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 M CR 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-evaluated PMF combined with wind effects results in[ 3tt: atthe Diesel-Generator ~~ ;i~~(~)S(~

Building. The d~ sis maximum PMF plus wind effects elevation as submitted to the NRC in },f ,u,-,,,r;

~b~~~~~(~)~(~ Reference 5 ist=_j ft. The critical elevation at the Diesel Generator Building is the floor elevation at 742.0 ft. Since the re-evaluation results do not exceed the critical elevation, interim actions are not required to prevent water ingress into the Diesel Generator Building.

1 The re-evaluated PMF combined with wind effects results in a maximum elevation ofr 7ft, at1J~I~~(~)S(~

the exterior wall of the Intake Pumping Station (IPS). As discussed in Section 3.5.2~ RCW "' ,..?,.,"r; pumps, fire protection pumps, and screen wash pumps are located on the upp,er deck at elevation 741.0 ft. The 741.0 ft. elevation is enclosed by a 13 foot 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.

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OUltllil0IU tJII0GI 10 er ft ! .390 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, Revi sion 2, EDMS #B41110720 005.

4. Tennessee Valley Authority, Watt s 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 M aximum 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, 845980326001.

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 Fukushi ma 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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Scca:ilJ Ji(clatcd l11fo1111atio11 UOltiliiOlcl tJIIGGI 10 er f\ ! .390

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, Chickamauga Reservoir Volume Storage Capacity, March 6, 2015, EDMS# W50 150309 002.

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 User's 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 #B41140918 001.

25. Tennessee Valley Authority Calculation CDQ0000002014000017, Revision 000, BWSC Calculation TVAGENQ12002, Revision 0, HEC-RAS Tributary Geometry Development, EDMS #B41140919 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. B41140919 002.

27. Tennessee Valley Authority Calculation CDQ0000002014000018, Revision 000, BWSC Calculation TVAGENQ13007, Revision 0, HEC-RAS Tributary Model Calibration, EDMS #B41140919 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 #B41110718 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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OUltllil0IU tJII0GI 10 er ft ! .390

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 Watershed's Configuration Modifications and Impact on Operability, PNNL-20256. March 2011.

40. Tennessee Valley Authority Technical M emorandum, " 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 2.1f ---=:Jpam Sunny Day Failure (b)(3)16 U.S C.

Simulation I____] ********** t~ i~:J,(~;

2 (b)

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 001, BWSC Calculation TVAGENQ14020, Revision 1, Fukushima NTTF Recommendation 2.1: Wind Waves for Combined-Effect Floods

51. Tennessee Valley Authority, Multiple Dam Failure Screening Analysis for the Tennessee Valley Authority's 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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