ML20366A023
| ML20366A023 | |
| Person / Time | |
|---|---|
| Issue date: | 12/29/2020 |
| From: | NRC/OCIO |
| To: | |
| Shared Package | |
| ML20366A007 | List: |
| References | |
| FOIA, FOIA/PA-2017-0, NRC-2017-000688 | |
| Download: ML20366A023 (71) | |
Text
ENCLOSURE 1 NEAR-TERM TASK FORCE (NTTF)-RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT FOR WATTS BAR NUCLEAR PLANT
NEAR-TERM TASK FORCE (NTTF) - RECOMMENDATION 2.1 MITIGATING STRATEGIES FLOOD HAZARD EVALUATION REPORT Response t o United St at es Nuclear Regulat ory Commission (USNRC) -
Code of Federal Regulat ions 10 CFR Part 50, Section 50.54 (f)
W atts Bar Nuclear Plant Tennessee Valley Authority M arch 12, 2015 Revision 0
TABLE OF CONTENTS 1 PURPOSE ...................................................................................................................................... 5 1.1 Requested Actions ................................................................................................................ S 1.2 Requested Information ........................................................................................................ 5 2 BACKGROUND ............... .................................................. ............................................................. 6 3 PLANT SITE DESCRIPTION ............................................................................................................. 8 3.1 Cu rrent Site Layout ............................................................................................................... 8 3.2 Site Topography .................................................................................................................... 8 3.3 Bathymetry in Vicinity of Plant - Tennessee River ............................................................... 8 3.4 Current Design Basis Flood Elevations ............................................................................... 16 3.4.1 Local Intense Precipitation .................................................................................................. 16 3.4.2 Flooding from Rivers and Streams ...................................................................................... 16 3.4.3 Flooding from Dam Breaches or Failures ............................................................................ 16 3.4.4 Flooding from Storm Surge and Seiche ............................................................................... 16 3.4.5 Flooding from Tsunami ....................... ................................................................................ 16 3.4.6 Flooding from Ice-Induced Events ...................................................................................... 17 3.4.7 Channel Migration or Diversion .......................................................................................... 17 3.4.8 Flooding from Combined Effects ........................................................................................ 17 3.5 Current Flood Protection and Mitigation Features ............................................................ 17 3.5.1 Dam and Reservoir System ................................................................................................. 18 3.5.2 Watts Bar Nuclear Site Protective Structures ..................................................................... 18 3.5.3 TVA River Operations Forecasting and Warning ................................................................. 18 3.5.4 WBN Flood Response Procedures ....................................................................................... 19 4 CHRONOLOGY OF FLOOID RELATED CHANGES SINCE LICENSING .................................................. 20 4.1 Watershed Changes since Licensing ................................................................................... 20 4.2 Summary of Clnanges to Design Basis Flood Elevations ..................................................... 23 4.2.1 Local Intense Precipitation .................................................................................................. 23 4.2.2 Flooding in Rivers and Streams ........................................................................................... 23 5
SUMMARY
OF PLANT WALKDOWN RESULTS AND MODIFICATIONS ............................................ 33 6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS ................................................ 35 6.1 Local Intense Precipitation ................................................................................................. 35 6.2 Flooding from Rivers and Streams ..................................................................................... 35 6.3 Flooding from Dam Breaches or Failures ........................................................................... 35 6.3.1 Project Specific PMF ............................................................................................................ 35 6.3.2 Sunny Day Failure of Upstream Dams ..................................................................... ........... 36 6.3.3 Seismic Failure of Upstream Dams ..................................................................................... 36 6.3.4 Sediment Transport ............................................................................................................ 36 6.4 Flooding from Storm Surge and Seiche .............................................................................. 36 6.5 Flooding from Tsunami. ...................................................................................................... 38 6.6 Flooding from Ice-Induced Events ...................................................................................... 38 6.7 Channel Migration and Diversion ....................................................................................... 38 6.8 Flooding from Combined Effects ........................................................................................ 39 6.8.1 Floods Caused by Precipitation Events ............................................................................... 39 6.8.2 Floods Caused by Seismic Dam Failures .............................................................................. 39 7 DESCRIPTION OF MODELS USED FOR REEVALUATION ...... ........................................................... 40 Page 2 of 70
7.1 HEC-RAS .............................................................................. ................................................ 40 7.1.1 Description of HEC-RAS Model Verification ........................................................................ 40 7.1.2 Description of HEC-RAS Model Ext ents............................................................................... 40 7.2 HEC-HMS ........................................................................ .................................................... 42 7.2.1 Description of HEC-RAS M odel Verification ........................................................................ 43 7.2.2 Description of HEC-HMS Model Extents .. ........................................................................... 43 8 JUSTIFICATION OF INPUTS .......................................................................................................... 44 8.1 HEC-RAS Model Geometry Development and Calibration ....................................... .. ........ 44 8.2 Dam Rating Curves ................................................................. ............................................ 44 8.3 Unsteady Flow Rules ............................ .............................................................. .... ............ 45 8.4 Probable M aximum Flood lnflows............................................................................... ....... 45 8.4.1 Hydrometeorologica l Report ..................................... ......................................................... 46 8.4.2 Critical Storm Selection .............. .............................. ........................................................... 46 8.4.3 National Inventory of Dams (NI D) Inflows .......................................................................... 47 8.5 Seismic Inflows ................................................................................................................... 47 8.5.1 National Invento ry of Dams (NID) Seismic Inflows ............................................................. 47 8.6 Sunny Day and Watauga Project Specific PMF Inflows ...................................................... 47 9 APPLICABLE FLOOD CAUSING MECHANISMS .............................................................................. 49 9.1 Local Intense Precipitation .. ..... .. ........................................................................................ 49 9.1.1 Previous Analysis .... ................................................................................................ ............. 49 9.1.2 Technical Approach ................................................................................... .......................... 49 9.2 Flood ing from R1ivers and Streams ............................ .. ........... ............................................. 50 9.2.1 Previous Analysis ................................................................................................................. 50 9.2.2 Technical Approach ........................ ..................... ... ............................................................. 50 9.3 Flooding from Dam Breach or Failures .................. ...... .. ..................................................... 52 9.3.1 Project Specific PMF .... ..................................... ....................................... ............................ 52 9.3.2 Sunny Day Dam Failure ...... ................................................................................................. 53 9.3.3 Single Seismic Dam Failure ....................................... ........................................................... 54 9.4 Flooding from Combined Effects ....................................................................................... . 54 9.4.1 Floods Caused by Precipitation Events ......................................................................... ...... 54 9.4.2 Multiple Seismic Dam Failures with Combined Flood Event .............................................. 56 10 EVALUATION OF UNCERTAINTIES ........................................................................................ ....... 58 10.1 100% Runoff .................. ..................................................................................................... 58 10.2 Peaked and Lagged Unit Hydrographs ............................................................................... 58 10.3 Gate Operability/Blockage ................................................................................................. 59 10.4 Breach Size .... .................................. .................................................................................... 60 10.5 Initial Reservoir Conditions .............. .............................. ............ ........................................ 60 11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS .................. 61 12 IDENTIFICATION AND EVALUATION OF ANY INTERIM ACTIONS TAKEN TO MITIGATE HIGHER FLOOD HAZARD RELATIVE TO DESIGN BASIS ............................*.................................................. ....... 62 12.1 Local Intense Precipitation ................................................................................................. 62 12.2 Flooding in Rivers and Streams .......................................................................................... 64 12.3 Combined Effect s Floods Caused by Precipitation Events ................................................. 65 13 References .............................................................................................................. .................. . 67 Page 3 of 70
FIGURES Figure 2-1 River System Schematic .. ................................................................................................ ... .......... 7 Figure 3-1 Watts Bar Nuclear Plant .. ............................................................................................................. 9 Figure 3-2 Watts Bar Nuclear Plant Site ...................................................................................................... 10 Figure 3-3 Watts Bar Nuclear Plant Site Layout .... ... ................................................................................... 11 Figure 3-4 Watts Bar Nuclear Plant Topogra phy ............................................................................. ... ........ 12 Figure 3-5 Location of Tennessee River Cross-Sections .............................................................................. 13 Figure 3-6 Bathymetry - Tennessee River in the Vicinity of the Watts Bar Nuclear Plant ......................... 14 Figure 4-1 Land Cover for the Tennessee River Watershed above Guntersville Dam ............... ................. 21 Figure 4-2 Timeline of Flood !Related Changes Since Licensing ................................................................. 23 Figure 6-1. Landslide Incidence M ap of United States ............................................................................... 37 Figure 7-1 Upper HEC-RAS Model Extents .................................................................................................. 41 Figure 7-2 Lower HEC-RAS Model Extents ................................. ............................... .................................. 41 Figure 7-3 Local Intense Precipitation HEC-RAS Model Extents ................................................................. 42 Figure 7-4 Local Intense Precipitation HEC-HMS Model Extents ................................................................ 43 Figure 12-1 WBN Access Doors ................................................... ................................................................ 63 TABLES Table 3.4-1 Combined Effects of Flood and Wind ...................................................................... ..... ........... 17 Table 4.1-1 Land Use above Guntersville {2001- 2011) ............................................................................ 21 Table 4.1-2 Impervious Area above Guntersville {2001 - 2011) .... ..................................................... ....... 22 Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11) .......................................................... 27 Table 6-4.2-1 Potential Flood Causing M echanisms or Causal Phenomena .............................................. 35 Table 9.1-1 Results of WBN LIP Analysis ..................................................................................................... SO Table 9.2-1 PMF Elevations and Discharges at WBN (TRM 528) Resulting from Reevaluation .................. 51 (b)(3) 16 USC
§ B24o-1(d} (o~
. . . .T~?.1~ ~:?=?~f'.!1F ~IE!y~tig~-~r:i.ctQi?Charges atWBN l t Resulting from 1 ~~~(7~)w)~Ff C § 8240"1(d), (b)
Dam .............................. ............................................................................................... ................................. 51 I
(4) (b)(7)(F)
Table 9.3-1 Elevation and Discharge at WBN Resulting from Project Specific Dam Failures ..................... 52 Table 9.3-2 Elevations and Discharges at WBN Resulting from Sunny Day Dam Failures .......................... 53 Table 9.3-3 Elevations and Discharges at WBN Resulting from Single Seismic Failure of Upstream Dams54 Table 9.4-1 Wind Wave Elevation Results at Dams ................................ .................................................... 55 Table 9.4-2 Wind Wave Elevation Results at Critical Structures ................................................................ 55 Table 9.4-3 Elevations and Dishcharges at WBN Resulting from Seismic Failure of Upstream Dams ........ 57 Table 11-1 Comparison of Current Design Basis Elevations and Reevaluation Results .............................. 61 Page 4 of 70
1 PURPOSE In response to the accident at the Fukushima Dai-ichi nuclear power plant resulting from the March 11, 2011, Great Tohoku Earthquake and subsequent tsunami, the United States Nuclear Regulatory Commission (NRC) established the Near Term Task Force (NTTF) to conduct a systematic and methodical review of NRC processes and regulations, and to make recommendations to the Commission for its policy direction. The NTTF reported a set of recommendations that were intended to clarify and strengthen the regulatory framework for protection against natural phenomena.
On March 12, 2012, the NRC issued an information request pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) herein after referred to as the 50.54(f) letter which included six enclosures.
(Reference 1)
In Enclosure 2 of the 50.54(f) letter, the NRC requests that licensees reevaluate the flooding hazards at their sites. (Reference 1)
This report provides the information for Watts Bar Nuclear Plant (WBN) requested in the 50.54(f} letter; specifically, the information listed under the Requested Information section of Enclosure 2, paragraph 1 (a through e). The Requested Information section of Enclosure 2, paragraph 2 (a through d), Integrated Assessment Report, will be addressed separately.
1.1 Requested Actions Per Enclosure 2 of the 50.54(f) letter, addressees are requested to perform a reevaluation of all appropriate external flooding sources. The reevaluation applies present-day methodologies and regulatory guidance, supplemented with interim staff guidance developed for review of the reevaluations. This includes current techniques, software, and methods used in present-day standard engineering practice to develop t he 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 to submit an interim action plan that documents actions planned or taken to address the reevaluated hazard. Subsequently, addressees should perform an integrated assessment of the plant to identify vulnerabilities and actions to address them.
1.2 Requested Information Per Enclosure 2 of the 50.54(f) letter, the final Hazard Reevaluation Report should document results, as well as pertinent site information and detailed analysis, including the following:
- a. Site information related to the flood hazard, including relevant structures, systems, and components (SSCs) important to safety and the ultimate heat sink (UHS). Pertinent data concerning SSCs and the UHS should be included. Other relevant site data includes the following:
- 1. Detailed site information (both designed and as -built), including present-day site layout, elevation of pertinent SSCs important to safety, site topography, and pertinent spatial and temporal data sets;
- 2. Current design basis flood elevations for all flood causing mechanisms;
- 3. Flood-related changes to the licensing basis and any flood protection changes (including mitigation) since license issuance; Page 5 of 70
- 4. Changes to t he 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 trans.port, 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 causi ng 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 S0.54(f) letter (i.e.,
Recommendation 2.3 flooding walkdowns) support this determination. If the current design basis flood bounds t he 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 l.d taken or planned to address flooding hazards, if any. (Reference 1)
Z BACKGROUND The Watts Bar Nuclear plant is located on the west bank of Chickamauga Lake at Tennessee River Mile (TRM) 528 with plant grade at elevation 728.0 ft. as shown in Figure 2-1. The Tennessee River above WBN site drains 17,319 square-miles. Watts Bar Dam, 1.9 miles upstream, has a drainage area of 17,310 square-miles. Chickamauga Dam, the next dam downstream, has a drainage area of 20,790 square-miles. The watershed is about 70 percent forested with much of the mountainous area being 100 percent forested. (Reference 2)
There are 12 major TVA dams (South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, and Watts Bar) in the TVA system upstream from WBN. Figure 2-1 presents a simplified flow diagram for the Tennessee River system. In addition, there are six major dams not owned by TVA {Mission (not shown), Calderwood, Chilhowee, Santeetlah, Cheoah, and Nantahala Dams). These reservoirs often contribute to flood reduction, but they do not have dependable reserved flood detention capacity. (Reference 2)
Page 6 of 70
Bnst~ Pro1*cts I}) NohchUC"'Y Holston Rrv&r honer, 810()(/
Clmcn RNer rt Paine, Hoory Boone
- Soulh Holston
- wetauga Wilbur RNer
.. lhOrpe (N)
~ Oougles N811lahola IN)
~ ~ Fonl:nna Oio,eoai.
catoo,wooo iri- - Ser1ee11a11 (T)
Norr** ~ n~
Melton H4I - :;':,fc~lhowee {TI - <'hatlJ!I" HNV9SS8e RNer Watts Bu Plont - - - - waas Bar "'="" - Nottely
~
.., H1wsssee Aealscr,,a CfkRIWJf O,tci<emauga N1ckJ1~
- Oco** I, 1, 3 Raccoon Mountain
'Cl"r,ms Fonl
- (,untersvile Duel. Rrver Cumoerl6nd River ._ Normanoy - Wheeler
- Bear Croes Pl'Ofects (4)
TenneS$ae-Tomblgbe* Weterv,ay Pld<WJCI<
Barldey(CI Mote 011'0RIV&f IC) U S Army Corps of Engineers Dams (NI Nentehala Power & L191'1l Company (subs1diery of Duke Energy)
!Tl 81ook11e10 Smo1<oy Moun1oin Hyorc, Power (Formertylapooo)
M,ssiss,pp1 RNe1 Figure 2-1 River System Schematic Page 7 of 70
3 PLANT SITE DESCRIPTION 3.1 Current Site Layout WBN is located in Rhea County, Tennessee on the west bank of Chickamauga Reservoir at TRM 528 with plant grade elevation at 728.0 ft. As shown in Figure 3-1, WBN is 44 miles from Chattanooga and 54 miles from Knoxville. The site is approximately 1.9 river miles sout h of Watts Bar Dam (TRM 529.9), as shown in Figure 3-2, and approximately 31 miles north-northeast of Sequoyah Nuclear Plant (SQN). Details of the current site layout and plant structures are shown in Figure 3-3. (Reference 2) 3.2 Site Topography WBN plant site and the Watts Bar Dam Reservation comprise approximately 1,770 acres on the west bank of Chickamauga Reservoir. As shown in Figure 3-4, the site is on high ground with the Tennessee River being the major potential source of flooding. (Reference 2) 3.3 Bathymetry in Vicinity of Plant - Tennessee River Between April 2007 and June 2008, t he U.S. Army Corps of Engineers (USACE) conducted a hydrographic survey ,of Chickamauga Reservoir. This survey includes only t he part of the reservoir that was below the water-surface at the time of the survey and was focused primarily on t he navigation channel. A Triangular Irregular Network (TIN) was constructed from the USACE Hydrographic Survey data. Using this TIN, cross-sections were cut at various locations on the Tennessee River in Chickamauga Reservoir in the vicinity of WBN, as shown in Figure 3-5. The actual cross-sections taken at these locations are shown in Figure 3-6. As shown, t he depth of the river in the vicinity of WBN ranges between 18 and 26 feet at normal summer pool elevation, 682.5 ft. (Reference 3)
Page 8 of 70
Tennessee North Carolina Georgia 10 N A
Mile$
Tennessee Counties Figure 3-1 Watts Bar Nuclear Plant c::J states Date l 1 Joi --.* 1 3 Page 9 of 70
B,ng Maps Aena1
,.:m l- llt~;'=~'itJ;:l~r°'
N A Figure 3-2 Watts Bar Nuclear Plant Site + River Mile Marker Bw:scr:::............
- .: *; RhH Cout11Y, fennenee
) ',
Page 10 of 70
ESRI WC~d Imagery Figure 3-3 Watts Bar Nuclear Plant Site Layout RIie-a Count:y, T,n,.,.n**
Page 11 of 70
Figure 3-4 Watts Bar Nuclear Plant Topography ""'-- Contour Page 12 of 70
Rhea Countv, fennessee Page 13 of 70
U(
... ~ 60) eoo l :>00 IEOO a<<
'"'(ft) 5tdcan SrM.ionlftj Watts Bar Nuclear Plant TRM S28
-l
.___r-J J Bathymetry
/
(i(C River *n 1 the Vicinity of Tennessee N clear Plant Watt s Bar u (Sheet 1 of 2) 1>> -00 800 l(XJ'.) uoo ltw sut10n Ut)
Figure 3 - 6 Bathymetry -Tennessee Rive
- rm . of the Watts Bar Nuclear Plant
. the Vicinity Page 14 of 70
TRM 527.75
... TRM 527.S Nr.o-r>.11~,,r>.., P -
on 3
l' f,ti"I
""-' 06'
- " M" 0 ,00
$t.atio1t(ft 1000 ! XlO ,,.,, ..,.
)0) ,,, 11)0 lt,O Bat hymetry Tennessee River in the Vicinity of Watts Bar Nuclear Plant (Sheet 2 of 2)
Figure 3-6 Bathymetry -Tennessee River in the Vicinity of the Watts Bar Nuclear Plant Page 15 of 70
3.4 Current Design Basis Flood Elevations On July 19, 2012, TVA submitted a revised hydrological licensing basis for WBN (Reference 4) for flooding on the Tennessee River and its tributaries. On September 30, 2014, TVA submitted a revision to the 2012 submittal {Reference S). 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 t he Probable Maximum Precipitation {PMP), Hydrometeorological Report (HMR) 56 is used to define the storm event {Reference 7). The 1-hour maximum precipitation in t he CLB is 16.8 inches. In the analysis of the PMP, the underground drains are assumed clogged and runoff is assumed to equal rainfall. The computed maximum surface elevation is 728.9 ft. which is below the critical floor elevation 729.0 ft. {Reference 2) 3.4.2 Flooding from Rivers and Streams The CLB analysis utilizes the U.S. Army Corps of Engineers (USACE) Hydrology Engineering Center River Analysis System (HEC-RAS). The stillwater probable maximum flood {PMF) elevation is (b)(3) 16 USC
§ 824o-1(dr(bj' LJ* (Reference 6).
3.4.3 Flooding from Dam Breaches or Failures Postulated failure of the {b)(3) 16 USC § 824o-1(d). (b)(4). (b)(?)(F)
Dams is evaluated in the CLB analysis for the PMF. The CLB flood elevation associated with this event is provided in the eva luation of the flooding from rivers and streams {Section 3.4.2).
Postulated failure of multiple dams upstream from WBN is evaluated in the CLB analysis for the combined effects of seismic and flooding. The CLB flood elevation is reported in the combined effects flooding for seismic plus flooding conditions {Section 3.4.8).
~b§(il~~~)~f~) .---
(4) (b)(7)(F)
Postulatedfailureofl
- Idam is considered in the CLB analysis for loss of ultimate heat sink during non-flood conditions. This loss of downstream dam failure does not present a flooding hazard at WBN and is not considered in the CLB.
3.4.4 Flooding from Storm Surge and Seiche Surges and seiches are not considered applicable in the WBN CLB because of the size and configuration of the lake and because of the elevation difference between the normal lake level and the plant grade is approximately 45 feet.
3.4.S Flooding from Tsunami Tsunami is not considered applicable in the WBN CLB because of the inland location of the plant.
Page 16 of 70
3.4.6 Flooding from Ice-Induced Events Ice-induced flooding is not considered applicable in the WBN CLB because of the temperate zone location of the pla nt.
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 piroduce 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.)
(O)(JJ16US--C § Probable Maximum Flood (still reservoir) 8240-1 (d) (b)(4)
(b)(7)(F)
Run-up on 4:1 sloped surfaces on the Diesel Generator Bui lding Run-up on critical vertical wall of the Intake Pumping St ation 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 u 5 c § 8240-1 (ct). (b)(4) (b)(7) F) The CLB maximum flood elevation for floods caused by seismic dam failures is . Refer~11c:~ ~L (b)(3) 16 U SC
§824<f1(d) (b) 3.5 Current Flood Protection and Mitigation Features Flood protection and mitigation for the WBN site are provided by 4 key elements: dams upstream of WBN, structures and structural features at the WBN site protecting equipment required for flood mode operation, 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.
Page 17 of 70
3.5.1 Dam and Reservoir System Flood control above the plant is provided largely by eight tributary reservoirs. Near the end of the flood season, these provide a minimum of 3,937,400 acre-feet of detention capacit y. This is 89% of the total available above the plant. Additionally, two main river reservoirs, Fort Loudoun and Watts Bar, provide 490,000 acre-feet of storage above Watts Bar Dam. (Reference 6)
Chickamauga Dam, the headwater elevation of which affects flood elevations at the plant, has a drainage area of 20,790 square miles, 3,480 square miles more than Watts Bar Dam. There are seven major tributary dams (Chatuge, Nottely, Hiwassee, Apalachia, Blue Ridge, Ocoee No. 1 and Ocoee No. 3) in the 3,480 square-mile-intervening watershed, of which four have substantial reserved capacity. Near the end of the flood season, these provide a minimum of 379,300 acre-feet of storage on the 1,200 square mile controlled area. Chickamauga Dam contains 345,300 acre-feet of detention capacity, on the remaining 2,280 square miles.
(Reference 6) 3.5.2 Watts Bar Nuclear Site Protective Structures At the WBN site, equipment required during flood mode operation, discussed in Section 3.5.4, is either located above the DBF, is within a non-flooded structure, is designed for submerged operation, or is otherwise protected. The Reactor Building will be maintained dry during flood mode. Walls and penetrations are designed to withstand static and dynamic forces imposed by the DBF. (Reference S)
The Diesel Generator Building also will remain dry during the flood mode since its lowest floor is at elevation 742.0 ft. With the PMF elevation ofr-:::=lft;,windwave....mn:.UP atJheDie~ (b)(3) 16 USC (b)(3) 16 USC _____G = =en~_e=r=aftt=
_o~(rR. . eBfyeJlrdeJnncgeJs
. ..e).Jevatior:C7ft. Therefore, ~ vels do not exceed floor elevation of §8240:J(d), (b)
§ 824o-1(cl):(b) l,H 1L,t..,.\I,...\ 74 2 0 5 l=--.J "' ,w 7
Hr, 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 ~ creens, and support equipment. During a DBF event, the internal flood
~b~~i~~~J!(~.----- . elevati'oni5L:...J ft. While this does not wet any flood-sensitive equipment on elevation 741.0 ft., the ERCW strainers and support equipment are located on elevation 722.0 ft. of the IPS, connected to elevation 741.0 ft. via stairwells. Passive flood barriers installed in the stairwell protect the equipment on elevation 722.0 ft. in the IPS. (Reference S)
Other structures, including the service, turbine, au xiliary, and control buildings, would be allowed to flood as the water exceeds their grade level entrances. Equipment that is located in these structures and required for flood mode operation is either above the DBF, is suitable for submerged operation, or is otherwise protected. (Reference 5) 3.5.3 TVA River Operations Forecasting and Warning Protection of WBN from rainfall floods that might exceed plant grade utilizes a flood warning issued by TVA's River Operations (RO). TVA's climatic monitoring and flood forecasting systems and flood control facilities permit early identification of potentially critical flood producing conditions and reliable prediction of floods which may exceed plant grade well in advance of the event. (Reference 5)
Because the plant grade elevation at WBN can be exceeded by large rainfall (PMP) and seismically-induced dam failure floods, plant flood preparations are required to cope with the Page 18 of 70
"fastest rising" calculated flood, including seismically induced dam failure floods that can exceed plant grade with the shortest warning time. (Reference 5)
Flood preparations at WBN are divided into two stages~ f reparation. By dividing the pre-flood preparation steps into two stages, a minimum of a
- hou, pre-flood .transition.....interva.L.is (b)(3) 16 USC
. §'824o:1(d), (b) available between the time a flood warning is receive and the time the flood waters ,exceed plant grade. I (b)(3) 16 U.S C § 824o.1(d) (b)(4) (b)(7)(F)
..._ ________________ __,\(Reference S)
The plant preparation s.tatus is held at Stage I until either Stage II begins or TVA's RO determines (b)(3) 16 USC JhatJloodwaterswilLnot.exceed .elevattonE:]ft. at the plant. Stage I shutdown is initiated
§ 824o-1(d); (br .. upon notification that a critical dam failure combination has occurred or loss of communication prevents determining a critical case has not occurred. Stage I shutdown continues until it has been determined positively that critical combinations do not exist. If communications do not document this certainty, shutdown procedures continue into Stage II activity. Stage II shutdown continues to completion or until lack of critical combinations is verified. The Sta e II warning is issued only when enough additional rain has fallen to forecast that elevatio ..... ft,. . w.inte.r.o.r .................~(~):(~.1~~)Ss(~
summer) is likely to be reached. Use of elevation ~ ft;, (b)(3)"16 U SC § 824o-1(d}, provides ~A.,'o\'1 v adequate margin to prevent wind-generated wa~ om en angering pant sa ety uring the ,:,2,~~~\~: (b) final hours of plant shutdown activity. Forecast will be based upon rainfall already reported to be on the ground. (Reference 5)
During t he winter season, Stage I shutdown procedures will be started as soon as target river elevation 715.S ft. has been forecast. Stage II shutdown will be initiated and carried to completion if and when target river elevation C::]tt-atWBN.hasbeen..fo.r.e.c.ast. C9Ir!:!~PC>ricJirig (b)(3) 16 U SC target river elevations for the summer season at WBN are elevation 720.6 ft. and elevation §824o:1(d), (b)
'" 11 \n\,r\
(b)(3) 16 USC E J ft. (Reference S)
§ 824o*1(d) (b) .
(4) (b)(?)(F) 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 damag*e 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 critical dam failure condition has occurred which could cause a flood wave to approach elevationr = l t;***at.the.. plantsiteor..it.cannot.be.... (b)(3) 16 USC determined t hat it has not occurred, Stage I shutdown ~ initiated. Once initiated, the flood l~~i?;;,~~: (b) preparation procedures will be carried to completion unless it is determined that the critical case has not occurred . The time from seismic failure to t he time elevation [3tt,. is.. i:eached..at. (b)(3) 16 USC (b)(3) 16 USC WBN jnthemost.critical eventisaboutO hours. This t ime is adequate to permit safe plant .§824o:1(d}, (b)
§ 8240- f(df (b). . shutdown in readiness for flooding. (Reference S) 3.5.4 WBN Flood Response Procedures Plant operation during t he flood response period is governed by Abnormal Operating Instructions (AOI). Maintenance and operations activities are directed by the AOls to align systems and compop.e.a,ts for flood mode operation. These activities are required by the AOI to
.. beco, ple~ dwithi'L.Jhours of notification of a Stage II flood alert. Coupled with the mi..n. i.m.um timeo **
- ours for Stage I, preparations result in a required total preparation time ofo ~?-~r-~
for the AOI actions.
t~11~~~):(~
,., ,.,m,.-,
Page 19 of 70
If the reactor is operating at power at the time the flood warning is received, Stage I and then, if necessary, Stage II procedures are initiated. St age I procedures consist of a controlled reactor shutdown and other easily revocable steps, such as moving flood mode supplies above the PMF elevation and making load adjustments to the onsite power supply. After the controlled reactor shutdown, the reactor coolant system (RCS) is cooled by the auxiliary feedwater system and the RCS pressure is reduced to less than 350 psig.
Stage II procedures a re the less easily revocable steps necessary to have the plant in the flood mode when the flood exceeds plant grade. HPFP system water (raw) will replace auxiliary feedwater for steam generator makeup water. RCS makeup is accomplished using the auxiliary charging system. Other essential plant cooling loads are transferred from the Component Cooling Water Systenn to the ERCW System and the ERCW replaces raw cooling water to the ice condensers. The radioactive waste system will be secured by filling tanks below DBF level with enough water to prevent flotation. One exception is the waste gas decay tanks, which are sealed and anchored against flotation. Power and communication cables below the DBF level that are not required for submerged operation are disconnected, and batteries beneath the DBF level are disconnected.
For a reactor in refueling status, if time permits, fuel is removed from the unit undergoing refueling and placed in the spent fuel pool; otherwise fuel cooling is accomplished using the following strategy. If the refueling canal is not already flooded, the mode of cooling requires that the canal be flooded with borated water from th,e refueling water storage tank. The Spent Fuel Pool Cooling System (SFPCS) Pumps will take suction from the spent fuel pool by their normal intakes and will discharge to the spent fuel pool heat exchangers. The outflow of the spent fuel pool heat exchangers is aligned, via spool piece installation during Stage II preps, to the Residual Heat Removal (RHR) system upstream of the RHR heat exchangers. Once the cooling water passes through the RHR heat exchangers it will enter the RCS via the RHR cold leg injection paths. The water flows through and exits the top of the vessel into the refueling cavity.
The head generated will force the water through the fuel transfer canal back to the spent fuel pool. ERCW provides the secondary side cooling for the spent fuel pool and RHR heat exchangers. (Reference S) 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 cha nge 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)
Page 20 of 70
-~
l!mr9~~
WalerM'elland/Barren (2.8%)
c:J Urban (10.0%)
CJ ForoSled (83.Gl&)
0 Gmsland/Shrub (5.6%)
0 Agriculuro (18.0%)
eima
@:d'1)
N
({
A 100 Hi n 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 Proj ect . The rast er product from this project is converted t o polygon vector data t hat cou ld be displayed, manipulated, and analyzed in a geographic information system (GIS) to determine change in impervious area (land use) between 2001 and 2011 as follows:
Table 4.1-1 Land Use above Guntersville {2001 - 2011)
Land Use Classificatio n 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/Wet land/Barren 2.82% 2.85% 0.03%
Page 21 of 70
Table 4.1-2 Impervious Area above Guntersville (2001 - 2011)
I 2001 2011 Change I Impervious Area 1.74% 1.96% 0.22%
Based on this assessment the Tennessee Valley watershed is experiencing a change in impervious area at a rate of about 0.02% per year in a 10 year period. Thus it is judged at this rate that potential impacts of watershed changes due to urbanization would have minimal impact on runoff from t he 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 Protect ed Areas Database, comprise one third of the entire watershed above Gunt ersville Dam.
These lands are set aside for public use and include prohibitive development rest rictions. Further, a computat ion 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 t here is good agreement with coefficients used in the flood hazard reevaluation. In general the computed runoff coefficients are lower than those used in this analysis and result in less runoff.
Page 22 of 70
4.2 Summary of Changes to Design Basis Flood Elevations The original licensing basis is documented in t he 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 NRCQA Original Safety 1999 Review of Licensing Modifications UFSAR Hydrology 2012 and 2014 Basis complete Change Documentation WBN Ul LAR
- 1982 Dam 1997*98 Flood 2004 River 2008-2012 January 28, Safety Reassessment Operations Hydrology 2015 SER Review Study (ROS) Reanalysis began Implemented Figure 4-2 Timeline of Flood Related Changes Since Licensing 4.2.1 Local Intense Precipitation The original licensing basis for the WBN LIP, as established in the 1982 FSAR, concluded that the local PM F would not reach elevation 729.0 ft. in the channels and pools surrounding the tu rbine, reactor, service, auxiliary, and diesel generator buildings.
The LIP has been reeva luated throughout the life of the plant for site configuration changes, as necessary. For any access exposed t o 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 complet ed in the 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 cent ered at Bulls Gap, Tennessee (about 50 miles northeast of Knoxville) below the major t ributary dams (e.g. Cherokee, Douglas, etc.) and one produced by a sequence of M arch storms producing maximum rainfall on the 21,400-square-mile watershed above Chatt anooga. The controlling event at WBN was determined to be the March 7,980-square-mile event as shown below:
St orm Event Maximum Dis.charge Maximum Elevation (b)(3) 16 U SC March 7,980-square-mile Bulls Gap 1,478,000 cfs §824<f1(d), (b) 4.2.2.1.1.1 Key Elements and Assumptions of the Original PMF Analysis
- a. Comput er codes used in the analysis were developed by TVA (UNITGRAPH, THIESSEN, FLDHYDRO, TRBROUTE, CHANROUT, DBREACH, CONVEY, WWIDTH, and SOCH)
Page 23 of 70
- b. Sub-basin uni1t 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
- e. Simulations were start ed 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)(3) 16 U SC § 8240-1
- g. Embankment failures from overtopp,ng were postulated at (d). (b)(4). (b)(?)(F) and 0
- because dams were not designed for the PMF 1(d) (b)(4) (b){7)(F)
- h. Plant grade elevation 728 ft. exceeded by design flood event - Regulatory Position 2 -
flood warning plan in place for safe plant shutdown - minimum 09 ours11v1;1Jl1;1\)lg ~b~ii~~~):(~)
- i. Embankments overtopped as a result of flood wave from upstream dam failure were postulated to fail instantaneously and completely with time of failure based on DBREACH results. (bX3l 16 us c § (b)(3) 16 U SC 824 4
§ 8240--l(d), (b} ***********------****r.--~
- -- Dam and (b)(7)(F) 0-l(d) (b)( l Dam postulated failure outflows combined and routed ownstream
- k. No correction for tailwater submergence was applied (b)(3) 16 U S.C § I. 824o-1(d) (b)(4l (b) Dam, downstream of WBN, was overtopped during PMF event simulation u pos u a eel not to fail 4.2.2.1.2 Seismically Induced Failure of Upstream Dam s The maximum pl ant site elevations at WBN for the different postulated combinations of seismic dam failures coincident with floods were as follows:
Operating Basis Earthquake (OBE) Failure with 1/2 PMF Maximum Elevation at WBN (ft.)
{b)(3) 16 USC § B24o 1{d) (b)(4} (b)(7)(F)
SSE Failures with 25-year Flood rb){3)16 u SC § 82401{d) {b)(4) {bJl/l(FJ Page 24 of 70
r)(3) 16 u S C § 82 <o- Hdl lbI<,I <bx71<fI 4.2.2.1.2.1 Key Elements and Assumptions of the Original PMF Analysis
- a. Seismic stability determined by engineering analysis and judgment
- b. Outflows at failed dams were generated by approximate models of the reservoirs using the SOCH model c.
u Warning t ime available greater tha rJhoursforallfaillfret6mbihat=,o =n~s~ --- - -~ (b~~1~:6i~)s(~
- d. Any embankments overtopped as a result of a flood wave from an upstream dam IA\ IL\r7\/rl 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 t he Federal Guidelines for Dam Safety and similar efforts of other Federal agencies. Technical studies, engineering analyses, and modifications were performed to ensure hydro logic and seismic integrity of TVA dams.
Table 4.2-1 provides the modification status {hydrologic) of the dam safety effort as of 1998.
The reassessment addressed the effects of these dam safety modifications on maximum flood levels at WBN and on warning time available for safe plant shutdown.
The reassessment of t he 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 t he original analysis. The March 12,030 square-mile event became a candidate on the lower main river after!(bl(3) 16 Us C § 8 240-1 (d) (b)(4 l (b)(7}(F) !dams were modified to preve*nt 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 t he March 7,980-square-mile event centered at Bulls Gap as shown below:
Page 25 of 70
St orm Event Maximum Discharge Maximum Elevation (b)(3) 16 WBN (March 7,980-square-mile) 1,288,000 cfs USC § eet 8240-1 (d}, (b)
(4), (b)
(7)(F)
Page 26 of 70
Table 4.2-1 Dam Modifications Completed by 1998 (Reference 11)
Dam Dam Modification Year Modifications Comoleted (b)(3) 16 USC§ 824o-1(d) (b)(4}, (b}(7}(F}
Page 27 of 70
4.2.2.2.1.1 Summary of Differences Between 1970's Licensing Basis and 1997 - 1998 Reassessment for PMF
- a. 4o-1 (d) (b)(4 , (b )( )
b.
c.
- d. An unst eady flow model was added for t he Fort Loudoun - Telli
- e. Controlling PMF elevation lowered by* * *
- eet(from ............
4.2.2.2.2 Seismically Induced Failure o pstream Dams Reference During t he 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:
QBE Failure with1/2 PMF Maximum Elevation at WBN (ft.)
r,1,,,,, 0 s C § .,4o.11,1 (bl(.),(bXI)( F)
SSE Failures with 25-year Flood
[O)(J) 16 U SC § tlZ40- 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.
Page 28 of 70
f (5x3) 16 0 SC § ,,.~ 1(d)(b)(4I, (b)(7)("
L...,_* * - - - - - - - ' / (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 operat ing license (COLA) for the proposed Bellefonte Nuclear Plant (BLN) Units 3 and 4, in accordance with 10 CFR 52.
During review of the BLN Units 3 and 4 FSAR, the NRC performed an audit of the hydrologic analysis which resulted in the issuance of three Notice of Violations (NOVs) on March 19, 2008 (Reference 12). In response to these NOVs, TVA completed a revised hydrologic analysis to support the BLN Units 3 and 4 COLA.
While the February 2008 QA inspection was for the BLN licensing request, it directly impacted WBN because the analysis is similar for TVA nuclear plants located along the Tennessee River. As a result of the NOV, TVA initiated a confirmator anal sis of the PMF
,,,.,.,,,.,..,.,,.,.,.,,~ ,l,HI;~~
- ns and installed temporary flood barriers at (b)(3) 16 u.s c § 824o-1(d), lb)(4), (bl(7J(Fl and b F Dams to increase the height of the emban~men s. ncreasmg e e1g o the em an ments at these dams prevents embankment overflow and failure during the PMF.
(Reference 4) 4.2.2.3.1 PMF The hydrologic analysis revision converts the model to the U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center River Analysis System (HEC-RAS) as well as updates to input information to address current reservoir operating guides, revised flow coefficients and dam modifications.
The revised licensing basis also updates t he evaluation criteria for dam stability to current industry standards. In this evaluation, dams which did not comply with the revised criteria are either modified or assumed to fail.
As a result of the reassessment the controlling event at WBN is the same as in the original analysis and results from the March 7,980-square-mile event centered at Bulls Gap as shown below:
Storm Event Maximum Discharge Maximum Elevation (b)(3) 16 WBN (March 7,980-square-mile) 1,160,000 cfs The calculated PMF elevation is combined with 0.3 feet. of additional mar (b)(3} 16 U SC .......... P~~ignbasis PMF.elevationot E Jft.
§ 824o-1(dJ; (br IA\ l&-.\n\Jr\
4.2.2.3.1.1 Summary of Differences between 1997 -1998 and 2008- 2014 Analyses for PMF
- a. Cross section bathymetry was updated based on recent USACE Doppler profiler navigation surveys Page 29 of 70
- b. Dam operating guides were updated in the hydrology model to reflect current reservoir operating policy
- c. The stream course model used was changed from the Simulated Open Channel Hydraulics (SOCH) suite of software, including TRBROUTE, CONVEY, WWIDTH, and SOCH, to the USACE HEC-RAS model.
- d. Dam rating curves updated using model test data
- e. Turbine discharges were used at all river reservoirs up to the point where the head differentials were too small and/or the powerhouse would fl ood
- f. Model refinements have been made at the Fort Loudoun-Tellico canal and Dallas Bay rim leak.
- g. The model was extended on t he Elk River to Tims Ford Dam, along t he Hiwassee River to Chatuge Dam, along the Nottely River to Nottely Dam, along the Ocoee River to Blue Ridge Dam, along the Clinch River to a gage at CRM 65.4, along the Little Tennessee River to LTRM 92.9, along the Tuckasegee River to RM 12.6, along t he 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 tailwat er submergence applied at dams as appropriate
" rn osc § .,,,_, ,a1.,sx,1.,6k1xF1
- j. .
I (b)(3) 16 USC
§ 8240 Hd}*(e} *
(4) (b)(7)(F)
(b)(3) 16 USC EJ
§ B24o-1(d}/o} - - - - -i-: .... ********** dams in the tributary system, that were not evaluated and are postulated to fail, (4) (b)(7)(F) were included:
.,,r..,.)(""3)...,1.,.
6.,., § .,..,
U"""S'"'C.-..,.. 82""'4-o-"1'"('"'
d,...
) .,,..
(b..,.
)(4,.,.)""'(b..,.)(7
"')"(F
"'""')- - - - - - - - - - - ,
Page 30 of 70
(b)(3) 16 USC § 8240 1(d) (b)(4) (b)(7)(F)
(b)(3) 16 USC E]
§ 824o-1{d}{tl**f -- - - - - m*:* ********* dams that were evaluated for stability, but were not credited due to low marnin :
(4) (b)(7)(F) (b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F)
- n. A license condition for Watts Bar is included i11 the NRC SER Reference 6 that states modifications to dams will be completed. These modifications are as follows:
- 1. (b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F) 2.
3.
4.
s.
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 U SC § 8240-1(d) (b)(4) (b)(7)(F)
Page 31 of 70
(b)(3) 16 U.S.C § 824o-1(d), (b)(4). (b)(7)(F)
SSE Failures with 25-year Flood (b)(3).16 U S C § 8240-1 (d), (b)(4), (b)(7)(F) 4.2.2.3.2.1 Summary of Differences between 1997 - 1998 and 2008 - 2014 Analyses for Seismically Induced Failures
- a. Failure outflow from tributary dams (b)(3 ) 16 us c § 8240- 1 (d) (b)( 4 ) (b)(J)(F) developed using HEC-HMS model (b)(3) 16 USC
§ 8240-1{-d} (b) ************ b; ***I_
...* _____.ladded because seismic stability analyses were not conclusive (4) (b)(?)(F)
- c. Cross section bathymetry was updated based on recent USACE Doppler profiler navigation surveys
- d. Dam operating guides were updated in the hydrology model to reflect current reservoir operating policy
- e. Dam rating curves updated using model test data
- f. Turbine discharges were used at all river reservoirs up to t he point where the head differentials were too small and/or the powerhouse would flood
- g. Model refinements made at the Fort Loudoun-Tellico canal, Dallas Bay rim lea'k, and model extended on the Hiwassee River arm of Chickamauga reservoir
- h. Operational Allowance approach developed t o allow flood simulat ions to more nearly mimic the integrated operation of the reservoir system
- i. Correction for tailwater submergence applied at dams as appropriate
. (b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F)
J.
- k. Temporary barriers and permanent dam modifications were not credited in the seismic analysis Page 32 of 70
5
SUMMARY
OF PLANT WALK.DOWN 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 t he guidance document and TVA procedure CTP-FWD-100.
The walkdown team was made up of four engineers consist ing of one nuclear engineer, one structural engineer, one mechanical engineer, and one civil engineer with extensive experience that met the qualifications provided in the guidance document. The walkdown team was supported by a retired TVA SRO, active maintenance personnel and active assistant unit operators in planning and performance of t he 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 wat er 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 floodin g, TVA implemented the following additional flood protection improvements:
- a. Thermal Barrier Booster Pumps (TBBP) Flood Protection Barrier Permanent flood protection barriers surrounding the WBN Unit 1 and Unit 2 TBBPs have been installed. This barrier is designed to protect the TBBPs and their respective motors in the event of a PMF event.
- b. Spent Fuel Pit (SFP) Pumps and Skimmer Pump Motors Barrier A permanent plant modification to provide flood protection for the common Spent Fuel Pit Cooling pumps and motors in the event of a PMF event has been installed.
- c. Intake Pumping Station (IPS) Personnel Access Door Temporary Barriers Temporary flood barriers are employed at the IPS to prevent floodwater intrusion through the elevation 741 ft. stairwells into t he elevation 722 ft. mechanical equipment rooms.
Syst em Chilled Water Circulat ing 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 Build ing from enter ing the non-flooded Shield Building. This deficiency was entered into the corrective action program and a Page 33 of 70
design change notice (DCN) is planned to replace the existing expansion joints with new qualified expansion joints to perform the flood barrier function.
Page 34 of 70
6 IDENTIFICATION OF POTENTIAL FLOOD CAUSING MECHANISMS The sections that follow discuss previous and proposed analyses or provide justification for exclusion for each of the flood -causing mechanisms. The hierarchical hazard assessment approach recommended in NUREG/CR-7046 is employed in these analyses. This approach allows a stepwise, progressively refined series of analyses that demonstrates that SSCs important to safety are protected from the a,dverse effects of severe flooding at the site.
Guidance, in addition to NUREG/CR-7046 (Reference 14), for potential flood-causing mechanisms, or causal phenomena, is provided in Table 6-4.2-1:
Table 6-4.2-1 Potential Flood Causing Mechanisms or Causal Phenomena Flood Causing Mechanism Guidance Reference Local intense precipit ation HMR 52 and HMR 56 1
15 & 7 Flooding from rivers and streams ANSI/ANS-2.8-1992 16 Flooding from upstream dam breaches or failures Dam Failure ISG 17 Flooding from storm surges or seiches Not Applicable NA Flooding from tsunamis Not Applicable NA Flooding from ice-induced events Not Applicable NA Flooding from channel diversion or migration toward the site Dam Failure ISG 17 ANSI/ANS-2.8-1992 and Flooding from combined effects Dam Failure ISG 16 and 17 6.1 Local Intense Precipitation The LIP was previously evaluated for WBN and is included in the reevaluation for WBN. The analysis assumes fully functional site grading and partially blocked drainage channels.
6.2 Flooding from Rivers and Streams The PMF was previously evaluated for WBN and is included in the reevaluation for WBN. The PMF in rivers and streams adjoining the site is determined by applying the PMP to the drainage basin of t hese rivers and streams. The model inputs and assumptions, as well as the previous analysis, technical approach, and results are described in subsequent sections.
6.3 Flooding from Dam Brea1ches or Failures 6.3.1 Project Specific PMF The project specific PMF was previously evaluated and determined to be non-governing. The proj ect 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 predict ed to occur at a site as a result of severe hydrometeorological conditions. The model inputs, assumptions, technical approach and results of this analysis are presented in subsequent sections.
Page 35 of 70
6.3.2 Sunny Day Failure of Upstream Dams Sunny day failure of upstream dams has not previously been analyzed for WBN. Inputs, assumptions, t echn ical 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 det ermined 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 hypothetica (b)(3) 16 USC.§ 824o-1 Reference 18). The following was evaluated:
d b4 b7F The incipient motion results for various flows between 26,000 and 210,000 cfs were assessed.
The latter being t he peak flow for the Sunny Day dam breach. Relatively large size particles (d >
3mm) will be able to move throughout the reach between Watts Bar and Chickamauga Dams.
During the peak flow, particles between 20 and 25mm (coarse gravel) will be able to miove in the upper 10 miles of the reach. As the dam breach hydrograph attenuates with time, the average size particle lowers t o approximately 5mm.
These silts and clay particles will be moving through the system in suspension during t he 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 t han 1 inch. Sedimentation would not resu lt in a loss of storage in the reservoir, which would lead to flooding, because of the small amount of settlement of suspended sediment (less than 1 inch) 6.4 Flooding from Storm Surge and Seiche Flooding from storm surge and seiche has not previously been evaluated for WBN and is not considered a credible flood -causing mechanism at this site. The WBN site is located on the west bank of the TRM 528 approximately 1,530 River miles inland (Tennessee River Miles 528, Ohio River Miles 48, and Mississippi River Miles 953) from the Gulf of Mexico at grade elevation 728 ft.
The Chickamauga Reservoir level during non-flood conditions would not exceed approximate elevation 685.0 ft. {Chickamauga headwater 682.5 ft. plus 2.4 feet for full turbine flow from Watts Bar Dam located immediately above the plant) at the plant, for any significant period of time. The plant grade at elevation 728.0 ft. is approximately 45 feet above the normal maximum pool levels.
Page 36 of 70
While seismic seiche has been recorded in the Tennessee Valley area it has been of very small magnitude. For example the March 1964 Alaska Earthquake which was a 9.2 magnitude event resulted in seiche being observed on about 25% of the 130 gages available in Tennessee at the time. The largest amplitude of seiche recorded on lakes, reservoirs, and/or ponds in Tennessee was 0.14 feet and in Kentucky 0.57 feet (Reference 19).. Reference 20 indicates that WBN is within the Eastern Tennessee Seismic Zone. However, there has been no recorded seiche of any significant magnitude reported as a result of earthquake events in the Tennessee Valley area.
Further examination of landslide activity as taken from the United States Geologic Survey (USGS) in the vicinity of the plant indicate t hat landslide susceptibility is considered to be moderate to high with low incidence as shown in Figure 6-1. (Reference 21)
USGS map of re Jllve Jndsboe ,nc,oence anct susceptibl 1y Ji:toss the D Low(lossth*n 1.5 °' of aru Involved) con1emw,ous United D Moderate (1 .5'4-15% of area involved)
Stales ~ell and ;,ink areas ha\/e lhe h,gt>est
- High (grutor lh*n 115 °' of aru lnvolvtd) lllC*dence an<l Landslide Suscepllbililyl lneidenoe suscepllbl ,1 Sl.-
- ,*ap Enlarge lmaqe D Moderate susceptibility/low Incidence D High susctptlbillly/low lnclde nco D High susceptibility/moderate incidence Figure 6-1. Landslide Incidence Map of United States Examination of the sloipes 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 t he 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 condit ions and plant grade.
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6.5 Flooding from Tsunami Flooding from tsunami was not previously evaluated for WBN and is not a credible flood causing mechanism at this site. WBN is located about 7.5 miles southeast of the town of Spring City, in Rhea County, approximately 44 miles northeast of Chattanooga, TN, and approximately 54 miles southwest of Knoxville, TN. At this location WBN is approximately 1,530 river miles inland (Tennessee River Miles 528, Ohio River Miles 48, and Mississippi River Miles 953) from the Gulf of Mexico. The Gulf of Mexico is the nearest body of open water directly downstream from Chickamauga Lake that is subject to seismically generated tsunamis. Further, the site is more than 320 miles inland from the Atlantic coast and more than 430 miles inland from the Great Lakes. The WBN site wit h plant grade at an elevation of 728 ft. will not be subject t o t he 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 ma ny 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 t he 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 D.am 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 t o initiate a jam and an ice dam would need to reach more than 50 feet above normal winter levels to reach plant grade elevation 728.0 ft. Thus, it is judged that an ice jam sufficient to cause plant flooding is not credible.
6.7 Channel Migration and Diversion Channel migration anid diversion was not previously evaluated for WBN and is not a credible flood causing mechanism at this site. The reservoir in t he 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 t he vicinity of the plant ranges from a low of around 2,000 feet to over 3,000 feet with stable slopes. It is judged that the potential for a channel diversion is not credible.
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6.8 Flooding from Combined Effects The following combinations will be considered in the reevaluation for WBN:
6.8.1 Floods Caused by Precipitation Events Floods caused by precipitation events were previously evaluated and will be included in the reevaluation for WBN. This scenario evaluates the effects of wind-wave activity during fl oods 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, PM'P 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 grou nd motion coincident with a 25-year flood.
- b. 1/2 of the 10,000-year ground motion combined with the lesser of 1/2 PMF or 500-year flood.
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7 DESCRIPTION OF MODELS USED FOR REEVALUATION 7.1 HEC-RAS HEC-RAS is an integrated system of hydraulic analysis programs, designed for interactive use in a multi-t asking environment. The HEC-RAS modeling system was developed by the Hydrologic Engineering Center (HEC) which is a division of the Institute for Water Resources, U.S. Army Corps of Engineers (Reference 23). It was designed to simulate one-dimensional steady and unsteady flow in subcritical, supercritical or mixed flow regime in open channels.
TVA is using the HEC-RAS model for flood analyses performed in response to the post -Fukushima request for information letters under 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 simu lation. Additionally, use of internal and lat eral st ructure 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. St eady-flow backwater profiles
- b. Unsteady flow for subcritlcal flow regime
- c. Unsteady flow for mixed flow
- d. Dam breach modeling
- e. Hydraulic design computations
- f. Sediment transport computations
- g. Water quality ana lysis
- h. RAS mapper graphical inundation mapping In steady flow mode, HEC-RAS explicitly solves the energy eq uation 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 t he 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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~
,,.,,...111,t ID N
A--=;;::-=
Figure 7-1 Upper HEC-RAS Model Extents
- 0.-
Nu<-... lo\-,1,A oftC.U\.f1Nlil\Jffllol N
A --=-=
Figure 7-2 Lower HEC-RAS Model Extents Page 41 of 70
HEC-RAS models calculat e water surface elevations for defined channels. The contributing subareas' discharges that were computed in HEC-HMS models are input s 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 t o 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 mod eling tool developed by the US Army Corps of Engineers (USACE), to model the hydrologic cycle and understand the behaviors and implications of wat ershed, channel, and water-control structure by simulating watershed precipitation and evaporat ion, runoff volume, direct run-off (overland and interflow), base flow and channel flow. The results of this software are used as an aid in decision making for: (a) planning and designing new flood-damage reduction facilities, (b) operating and/or evaluating existing hydraulic conveyance and water-control facilities, (c) preparing for and responding t o flood s, (d) regulating floodplain activities and (e) restoring or enhancing the environment.
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7.2.1 Description of HEC-RAS Model Verification The software is dedicated in accordance with QA procedures. Dedication is based on completion of the procedures in the HEC-HMS Validation Guide and parallel t esting through alternative software. This Validation Gu ide is provided by the US Army Corps of Engineers- Hyd rologic Engineering Center and out lines 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 compa red t o 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. l he 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 t he WBN East , WBN West, and WBN Southwest drainage areas. The extents of the HEC-HMS model used in the LIP ana lysis are presented in Figure 7-4.
Figure 7-4 Local lntens*e Precipitation HEC-HMS M odel Extents Page 43 of 70
8 JUSTIFICATION OF INPUTS The verified HEC-RAS unst eady flow model of the Tennessee River System developed for the September 30, 2014 WBN Unit 1 LAR (Reference 5) is used in the hazard reeva luation. It is utilized to predict flood elevations and discharges for flood s of varying magnitudes including the PMF and flooding from dam failures. Input s t o t he 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 t he 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 t he HEC-RAS geometry was developed, the HEC-RAS model for each reach was calibrated to historic events. The main stem is calibrated to the March 1973 and May 2003 events and the t ributaries are calibrated to two large flood events of record for t he 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 evenits used in ca libration is from TVA, National Weather Service, and United Stat es Geological Survey data as well as FEMA flood profiles. The calibrated reaches were linked together to form a continuous model for flood simulation. The HEC-RAS model calibration produced final geometry files to be used in simulations of flood s at TVA nuclear plant sites. The calibration process is described in References 26 and 27.
8.2 Dam Rating Curves Initial dam rating (headwater rating) curves are required as input s 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 f unction of headwater elevation. Final dam rating curves are simulation specific and determined in the HEC-RAS model w hich incorporates tailwater effects.
The Dam Safety group of TVA's RO division evaluated the stability of TVA's dams under various load condit ions in accordance with TVA Dam Safet y accept ance criteria; they also considered load conditions that comply with nuclear guidelines. The load conditions studied included PMF level headwat er elevations with varying tailwater elevations and multiple seismic load conditions.
Each stability analysis report was examined and the results provided the basis for t he dam failure cases presented in the dam rating curves. Earthen embankment breaches were determined from empirically based methods as recommended in Reference 17. The dam rating curves are documented in References 28 and 29.
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8.3 Unsteady Flow Rules HEC-RAS uses unsteady flow rules to control complex releases from hydraulic structures. In the reevaluation, unsteady flow rules were developed in HEC- RAS to represent the operations of the reservoirs in the Tennessee River system upstream of Wilson Dam. The rules blend the flood operational guides (Refe rence 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 t he TVA system. The rules reflect the flexibility provided in the guides to respond to unusual or extreme circumstances through t he use of elbow recovery curves; seasonal variability in the operational guides is also included in the unsteady flow rules.
Elbow recovery curves are used during the antecedent storm to expedite the recovery of the reservoir to a more normal state. The use of elbow curves is explained in detail in the Flood Operational Guides (Reference 30). The antecedent storm, in which t he elbow recovery curves are implemented, is a three-day storm occurring prior to a three-day dry period and the three-day main storm. Once the antecedent storm is complete and the surcharge elevation is exceeded the discharges will be calculated using the dam rating curves for the applicable cases.
The surcharge elevation is the elevation at which gates are fully open and discharge through the dam is computed by t he dam rating curve.
The dam rat ing curves are used in concert with the flood operational guides in the unsteady flow rules to define total dam discharge as a function of headwater elevation, tailwater elevation, and outlet configuration. If, as during a probable maximum flood (PMF) event, headwater exceeds the normal operating range, the dam rating curves determine flow over ot her components such as non-overflow sections, navigation locks, the tops of open spillway gates, tops of spillway piers, saddle dams, and rim leaks. If the operating deck is not exceeded, operations return to the Flood Operation Guides (Reference 30); if the operating deck is exceeded then gates remain in the open position. The unsteady flow rules are documented in References 31 and 32.
8.4 Probable Maximum Flood Inflows To determine t he inflows for the PMF event, rainfall depths for the 21,400 square-mile downstream centered March and the 7,980 squa re-mi le 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 ident ified and included.
The application of t he following approach was adopted for inflow development (Reference 34) in sub-basins above Wheeler Dam for use in the subsequent Tennessee River routing model:
- 1. transform rainfall to runoff using available UHs and using UHs adjust ed for non-linear basin response;
- 2. develop sub-basin surface runoff hydrographs both with no losses and using applicable loss rates;
- 3. Include sub-basin monthly average constant baseflow (Reference 35);
- 4. develop total event inflow hydrographs for all sub-basins in the Tennessee Valley watershed; and
- 5. as necessary, translate developed tributary sub-basin surface runoff hydrographs to model input points.
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8.4.1 Hydrometeorological Report The applicability of the National Weather Service (NWS) HMR to the development of the PMF at TVA nuclear projects was reviewed and documented in Reference 36. This was done in accorda nce 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 HMRs. The process involves an expert panel review of the product in its entirety, with specific attention to storm selection and storm transposition. The replacement for 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 50.54(f), and are appropriate for use in the re-analysis.
8.4.2 Critical Storm Selection The critical storm selection for the PMF event on the Tennessee River for WBN is reviewed and documented in Reference 37. As defined in Section 1.1 of the NUREG/CR-7046, the probable maximum event is " ...the event that is considered to be the most severe reasonably possible at the location of interest and is thought to exceed the severity of all historically observed events.
For example, a PMF is the hypothetical flood generated in the drainage area by a PMP event."
The PMP for the Tennessee Valley at the aforementioned nuclear projects is currently defined by the NWS HMR 41 (Reference 10).
The HMR 41 guidance defines two general PMP configurations. The first configuration is a 21,400-square-mile PMP event with either an upstream or a downstream centering, which has a fixed location over tlhe Tennessee Valley watershed. The second pattern type is a moveable 7,980-square-mile PMP event that may be slid roughly from southwest to northeast along the long axis of the published pattern.
As stated in Reference 38, currently analyzed storms are the 21,400-square-mile downstream centered event, the 7,980-square-mile Bulls Gap centered event and the 7,980-square-mile Sweetwater centered event. Selection of these events was based on the original TVA plant licensing analysis. The original analysis was based on the ANSI N170-1976/ANS-2.8, Section 5.2.6 guidance recommending downstream placement and use of NWS proposed methods. As expected, the previous and current TVA modeling efforts show that the critical PMP storm event producing the PMF when routed maximizes the rainfall volume over the total watershed. A re-analysis was performed using GIS software to allow direct comparison of the reviewed events.
The GIS analysis of the 21,400 square-mile PMP event showed that the downstream centered event produced higher rainfall depths at all locations reviewed. An analysis of the 7,980-square-mile PMP event was also performed. It was determined that PMP depth is maximized at WBN by the 7,980-square-mile, Station 5 (Bulls Gap) centered event.
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An independent analysis performed by Pacific Northwest National Laboratory confirmed that the crit ical storm centerings are identified for use in computing PMP rainfall (Reference 39).
8.4.3 National Inventory of Dams {NID) Inflows The USACE maintains the National Inventory of Dams (NID), which provides characteristics for each dam (location, height, and volume). The guidance for assessment of flooding hazards due to dam failure {Reference 17) requires a screening process to identif y 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 t he peak main storm precipitation.
8.5 Seismic Inflows Staff positions listed in Section 5.6 of Reference 17 specify that the coincident inflow from the 25-4 year flood be applied during the 10* exceedance probability seismic hazard and either the 500-year flood or the half PMF be applied as the coincident inflow during half the 10"' exce,edance 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 st arts with the selection of streamflow event durations sufficient to allow maximization of the headwater elevation at the hypothetical failure location. These durations are then used in probabilistic analyses to develop the required return period volumes from historical streamflow data . A candidate historical or synthetic rainfall event sufficiently large to reflect the watershed translation of rainfall to runoff is then selected. This rainfall is distributed across the watershed, losses are applied and the surface runoff is generated based on available unit hydrograph data. The candidate surface runoff ordinates are then scaled to produce the calculated probabilistic volumes at the selected durations.
8.5.1 National Inventory of Dams {NID) Seismic Inflows During postulated single and multiple project failure events, the concurrent failure of NID identified projects outside the model is considered possible. The NID volumes are located across the sub-basins with conveyances having differing sinuosity, length, slope, cross sectional and roughness characteristics. As a result, t he postulated failure waves are expected to pass through a variety of supercritical, critical and subcritical flow regimes as they traverse the respective reaches starting at the failure location andl ending at the respective model input points. The resulting translation reduces the peak flows and spreads the time base of the volume input. A simplified calculation approach, as described in Reference 41, is used to account for the NID volumes under these failure conditions.
8.6 Sunny Day and Watauga Project Specific PMF Inflows Sunny day project failures are postulated to occur due to non-hydrologic and non-seismic causes as required by Reference 17. Simplified volume analyses are used and TVA projects having the potential to cause flooding at the plant sites were identified Reference 42. The Watauga Project Specific PMF analysis is included in the sunny day failure analysis to provide a bounding scenario for the sunny day failures on the Holston tributary. Inflows for both the Sunny day failures and the Watauga Project Specific PMF are documented in Reference 41.
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The development of two inflow scenarios for use in these model calculations is necessary.
Inflows for use concurrent with sunny day failures identified in Reference 42 are included in Reference 41. Constant June baseflows from Reference 35 are applied for both. Watauga project PMP rainfall was taken from Reference 41 as recommended by Reference 17 and is convoluted in a spreadsheet using SCS methodology. June curve numbers were ta ken from Reference 43 and uni1t hydrograph data were taken from Reference 34. NID inflows are not included in sunny day simulations.
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9 APPLICABLE FLOOD CAUSING MECHANISMS 9.1 Local Intense Precipitation 9.1.1 Previous Analysis Previous evaluation of the effect of LIP on water surface elevations in the WBN plant sit e 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 Referen,ce 44. The LIP analysis is a measure of the extreme precipitation (high intensity and short duration) at a specific location, in this case WBN. According to NU REG/CR-7046 (Reference 14) LIP should be equivalent to the 1-hr, 1-mi2, PMP at the location of the site. The analysis assumes fully functional site grading and partially 1
blocked drainage channels.
2 The rainfa ll hyetograph of the 1-hr, 1-mi PMP for WBN is determined according to HM Rs 52 and 56 (References 14 and 7, respectively). The drainag,e 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 to a peak runoff. Additionally, the HEC-RAS model is used to produce t he water surface elevation for defined channels utilizing the peak discharges computed in the HEC-HMS models.
This analysis conservatively assumes that drainage features carrying offsite drainage toward WBN are fully f unctional 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 SO percent of their original length thus increasing flows to the steady-state model.
Additionally, this analysis assumes partially, but significantly, blocked drainage channels. Each cross-section in the HEC-RAS model is assumed to be blocked by a 40-foot long, 10-foot high obstruction in the bottom of a channel (which would be similar to a trailer building).
Obstruction of drainage channels by multiple large t railers is considered a conservatism.
(Reference 44)
The results of t his analysis are presented below in Table 9.1-1:
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Table 9.1-1 Results olf WBN LIP Analysis Critical Plant W ater Surface Location Elevation Elevation (ft.)
(ft.)
WBN East HEC-HIMS 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 t he results of dam sta bility analyses and migration of the analysis from the SOCH suite of programs to the HEC-RAS unsteady flow model to determine the PMF elevation. The inflows for this analysis are determined using the FLDHYDRO and the Antecedent Precipitation Index (API) runoff methodology.
9.2.2 Technical Approach The reevaluation of the PMF is in accordance with the guidance in Reference 14. The PMP is applied to the drainage basin of t he rivers and streams adjoining WBN. Inflows to the model were generated using the SCS runoff methodology. The SCS runoff parameters are calibrated to the inflow model presented in the September 30, 2014 LAR (Reference 5). Curve numbers (CNs) used were validated against the TVA API met hod results for the 21,400-sq.-mi. March event.
When compared to National Resources Conservation Service (NRCS) soils and USGS Multi-Resolution Land Charact eristics (MRLC) data for sub-basins above the Wheeler project, the area weighted, validated CNs for the antecedent event are approximately 15. 7% higher and for the main storm event are 4.3% higher. Inflow development, storm selection, as well as additional inputs and assumptions are described in Sect ion 8. Three HEC-RAS simulations were performed:
- a. 21,400 square-mile downstream-centered March storm
- b. 7,980 square-mile Bulls Gap-centered March st orm
. . (b)(3)16USC §824o-1(d) (b)(4)
- c. 7,980 square-mile Bulls Gap-cent ered March storm with (b)(7)(F) '
Dam failures Result s 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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Table 9.2-1 PMF Elevations and Discharges at WBN (TR:M 528) Resulting from Reevaluation PMF Event Elevation (ft) Discharge (cfs) 21,400 Sq.-Mi. Event 739.2 1,125,000 7,980 Sq.-Mi. Bulls Gap Event 744.9 1,454,000 The 7,980 square-mile Bulls Gap centered storm rn:1"7'l'Ml'!!"T'r"'l"""'l'l"'"l"'!!""l"lr::"'Tr.!l"'"?rn':i,----,
overtopping and subsequent failure of s a result an additional PMF analysis assuming rn:1"7'l'MM!'T1r"'l"""'l'l"'"l"~~ r,1t-=l'rll~T-::-:::::--::::-r.-::--::r-.t-: o prevent overtopping is presented in Table 9.2-2.
Table 9.2- 2 PMF Elevations and Discharges at WBN ~ l Resultingfrom 24o-1(d) (b)(4)
(b)(3) 16 USC
- §8240°1(d) (b)
(b (3 16 USC § - -- (b)(7)(F) (4), (b)(7)(F) 824o-1(d) (b)(4) Dam PMF Event Elevation (ft.) Discharge (cfs) 7,980 Sq.-Mi. Bulls Gap E (b)(3) 16 USC ................ EmergencyAction*Ptan-a *
- am
§ 824o-1(d}(b)
(b)(3) 16 USC§ 824o-1(d) (b)(4), (b)(7)(F)
I As a result t IP 7 QR() qua re-mile Bulls Gap cent ered storm becomes the controlling event with the EAP at (b)(3) 16 Dam and a 1ft . USC § maximum e Ieva t',on o,, [lb))
3 16 8240-1 USC (d), (b)
§ 824o-1(d) (b)
(4), (b)(?)
(4). (b) (F)
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9.3 Flooding from Dam Breach or Failures Upstream dam failures by seismic events, structural defects, as well as other failure modes have the potential to impact WBN.
A simplified volume analysis identifies individual dams that have the potential to cause flooding at WBN if failed during a sunny day (Reference 42). The analysis conservatively assumes that
!(b)(3) 16 us c § 824o-1(<J). (6)(4) (6)(7)(F) Iproviding a bounding scenario for cascading dam failures as a result of a sunny day dam failure. The results of the analysis identified
~b§(il~~t~);i~) . .. .. . r=-ifams whose failures could lead to flooding at the plant. These dams are as follows:
(4). (b)(7)(F) (b)(3).16US.C §824o-1(d)(b)(4) (b)(7)(F)
~b§~l~-~{~}~(;) *TheO dams identified were evaluated in one of three ways: under a project specific PMF (4) (b)(7)(F) gle seismic dam failure combined with a flood event, or failure during a su nny day. A (b)(3): 16 U.S C .... ... . *** * ** * * * *
- PMF simulation bounds a sunny day failure of the I ........... ldam, (b)(3) 16 U.S C.
~.~2 ,~?:~,(~: (b) (~)(~l 1~ sFc § 7
8240
- 1(d) dams have low margin for seismic stability and are analyzec:11 as single 1~ 2~~:~,(~; (b) seismic dam failure simulations paired with a flooding event. These seismic simulations bound 1 4
~l,!,IJ,l.~~~1,!.1,-~ ai,lure simulations for these dams. The remaining dams, (b (4 ) (b ?) Fi8 o-l
~bl( 3 1 us c 8240 are analyzed
- in sunny day simulations. Th )(3) 16 U SC
- 1
- I
- b d h (bl(3l 15 . . f 'I . I . b .__......,.,---1 §8246~1 (d), (b) f a, ure s1mu at,on oun s t e us c § e1sm1c a, ure s,mu at,on ecause results in only a minor loss of th 24o-1(d) reservoir volume.
(b)(4) (b) 9.3.1 Project Specific PMF (?)(F)
(b)(3).16 U SC
§ 824o-1(d); (b)
IA\ 11..\/?\lr\
Table 9.3-1 Elevation and Discharge at WBN Resulting from Project Specific Dam Failures Dam Failure I Elevation (ft.) I Discharge (cfs) r b)(3) 16 USC § 8240-1 (d). (b)(4) (b)(I)(1-)
Page S2 of 70
9.3.2 Sunny Day Dam Failure 9.3.2.1 Previous Analysis No previous analyses were completed on the failure of a dam upstream of WBN on a sunny day.
9.3.2.2 Technical Approach Potential failure modes were evaluated for each dam to determine the sunny day simulations breach parameters.
~b~;~~~,~;!1~ . . . . . . .. AsunnydayfaHureof
- Dam was not included since the resulting elevation would tQKklM ~ 'C ~~J>ounded by.th.e.. ........ .. . single seismic failure during a 500-year event. The results of
~Qfll9ls{t:!J
!J+i:i,~~Y*(b)*"*"*
t he I ***
I single seismic failure 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)
(0)(3)16 Us t; § tlL4o-1(d) (b)(4) (b)(/)(f-)
Page 53 of 70
9.3.3 Single Seismi-c Dam Failure The results of t he 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 o f Upstream Dams Elevation Discharge Seismic Dam Failure Combination (ft.) (cfs)
(b)(:J) 16 USC S 8240-1 (d) (b)(4), (0)(7)(~)
9.4 Flooding from Combined Effects 9.4.1 Floods Caused by Precipitation Events 9.4.1.1 Previous Analysis In the previous flood evaluation, described in Section 3.4.8.1, coincident wind wave run-up was* comput ed 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). l he 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 scena rio evaluates the effects of wind-wave activity du ring 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 § 8240-1 (d). (b)(4) (b)(7)(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 50.
Page 54 of 70
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)(3) 16 USC § 824o-1(d), (b)(4), (b)(7)(F)
Table 9.4-2 Wind Wave Elevation Results at Critical Structures Total Wind Wave Height added (Wave ru11up + wind Stillwater PMF setup) Final PMF Elevation Location Elevation (ft.) (ft.) (ft.)
(b)(3) 16 USC § 824o-1(d), (b)(4) (b)(?)(F)
Page 55 of 70
9.4.2 Multiple Seismic Dam Failures with Combined Flood Event 9.4.2.1 Previous Analysis The results of the previous seismic analysis are presented in Section 3.4.8.2. This analysis uses the SOCH suite of codes to determine the critical combinations for flood caused by OBE coincident with a 1/2 PMF event and SSE coincident with a 25-year flood. Wind wave run-up was not applied to seismic flood events because this combination did not produce the controlling flood elevation.
9.4.2.2 Technical Approach The seismic dam failure reevaluation is in accordance with the guidance in References 17 and 14. Staff positions listed in Section 5.6 of Reference 17 specify that the coincident inflow 4
from t he 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 scr,eening 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 eart hen 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
~~~l~~(~)~(~t ... .. .. .. .. indicate-thata! !centered seismic event results in I !dam failure.swithJhe (b)(3) 16 USC (4) (b)(7)(F)
(b)(3 ) 16 U s.c largest volume of water that could be released for a 10,000 year ground motion. Ar-:::7 centeredseismiceventresults+nf* ** ldam failures with the largest volume of w ~
- ,t~§~lw{i~
(4), (b)n)(F)
§ 824o-1(dh (b}* -------- * * ,,
(4) (b)fl)(F) could be released for a1/2 10,000 year ground motion.
The reevaluation considered the following scenarios:
(b)(3) 16 USC § 824o-1{d) (b)(4) (b)(7)(F) a.
b.
c.
d.
e.
Page 56 of 70
f.
Results of the seismic failure of upstream dam simulations are presented in Table 9.4-3 (Reference 48).
Table 9.4-3 Elevations and Dishcharges at WBN Resulting from Seismic Failure of Upstream Dams Elevation Discharge Seismic Dam Failure Combination (ft.) (cfs)
(b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F)
Page 57 of 70
10 EVALUATION OF UNCERTAINTIES Inherent uncertainties exist in the analysis of the PMF in rivers and streams. Flooding simulations require many assumptions while determining input parameters for the analysis as well as during simulation. These assumptions are based on available data and industry accepted practice. NRC Interim Staff Guidance recommends several sensitivity analyses be performed t o 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, in itial 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.
Ta t s of the 100% runoff sensitivity simulations produced an increase in elevation at WBN (b)(3) 16 USC of feet for the 21,400 square mile storm event, and an increase ofn eetfoi: :thel,980 (b)(3) 16 USC.
§ 8240 1(d)". (b)
IA\ IL.\rt\Jr\ sq LJ' ile Bulls Gap Storm event because of additional cascading dam failures. (Reference 52)
§8"24o:1(d), (b)
'" ,um,.-,
These increases are not realistic and need not be* considered in the re-evaluation. This conclusion is based on:
- Section 4.1 indicates the impervious area of the watershed above Guntersville Dam is extremely small (1.96%) and it is unrealistic to assume 100% runoff for the entire watershed during a nine day event based on land use.
- Rainfall to runoff transformation rates calculated are approximately 88%.
- Unit hydrographs used in determining inflow to streams have been calibrated to actual storm run-off levels for larger historical events.
- Reservoir flow models are biased to predict flood levels at or above historical storm levels.
10.2 Peaked and Lagged Unit Hydrographs Peaking and lagging of the unit hydrographs is performed for the 21,400 sq.-mi. downstream centered March event and the 7,980 sq .-mi. Bulls Gap Centered March event. These simulations utilize unit hydrographs that were adjusted t o reflect the non-linearity of the runoff generation process under field condit ions. 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 sensit ivity simulations produce an increase in elevation at WBN of 1.5 feet. for the 21,400 square mile storm event, and ain increase of 0.6 feet for the 7,980 square mile Bulls Gap Storm event. The results of the sensitivity analysis Page 58 of 70
shift the controlling storm to the 21,400 storm rather than the 7,980 storm. Even with this increase in stillwater flood elevation, t he floor elevation of the Diesel Generator Building (742.0 ft.) and t he 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 ma rgin 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 M ain Control Room and Shutdown Board Room chilled water pump motors are addressed in Section 12 of t his report.
Adjusting the unit hydrog raphs 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) N RC performed a confirmatory check of TVA's unit hydrogra phs utilizing Snyder's unit hydrograph method. The NRC not ed that the hydrographs computed by TVA have shorter peak time and higher peak flows w hen 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 analysis assumed (b)(3 1 u (b)(3) 16 USC ........j.................. ~o eva lua.,. t e__,., t ....e- se-n-s""
1t..
1v""'
1t_y_o..,...,.
t ,..,,....
s-a-ss_u_m_p""'t'""10-n-, -s-en- s-1t"1"-v,t.,..y- t=o_g
- _a_,.t_
e _o_p_e-ra.....,1""
1t_y _o_,
r
§ 824o-1(d)M ** *
(4) (b)(7)(F) gate blockage due to debris accumulation at dams credited for stability was performed for both the 21,400 sq.-mi. downstream centered March event and the 7,980 sq.-mi. Bulls Gap Centered M arch event. Multiple simulations were performed using the HEC-RAS unsteady flow m odel to compute peak water surface elevations at various dams whose outlet capacities were ostulat ed to be reduced during the two PMF storm events. For each dam analyzed, th (b)(3) 16 USC§ 82401(d) (b)(4) (b)(7)(F) e s1mu at1ons or eac ana ze . e dams considered ab I in 3 16 USC 52) in the 7,980 square mile Bulls Gap centered event. These two
...d,-a_m_s---:- h-av_e_ a_
n _o_u-:-t~e-:-
t -u....,n...,.
a-va.,...,.,.,a" -ility margin less than 5% as measured by reaching a potential embankment overtopping elevation.
Spillway gate blockage due to maintenance issues or debris has been evaluated and determined to not represent a significant hazard for WBN . The conclusion is based on:
- There is no hist ory of debris blocking spillways at these dams in historically large flooding events Page 59 of 70
- There is no barge traffic above t hese two dams
- River Operations (RO) monitors gates daily for operation and the maintenance program for gates assures high reliability
- RO has the means and resources to resolve gate issues if needed to respond to flood events.
- The embankments at the dams are not threat ened 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 select ed, Von Thune and Gillette is the best suited for TVA based on the size of the dams in the system and the volume-driven nat ure of the Tennessee River System. Total failures of concrete structures are conservatively assumed for dam failures above Chickamauga Dam when the dam stability evaluation demonstrates low margin (Reference 45).
10.5 Initial Reservoir Conditions TVA controls and/or schedules the releases from all TVA dams and some non-TVA dams.
Providing the scheduled releases from these dams enables TVA t he opportunity to control the river syst em as an integrated syst em. TVA monitors t he dams 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> a day, 7 days a week. The releases are based on the needs of the entire Tennessee River system while maintaining reservoir levels within the operating guide. The reservoir operating guidelines are implemented as prescribed operating ranges of reservoir levels throughout the year. Operating within the prescribed operating ranges provides consistency in the normal reservoir levels, therefore also providing consistency in the headwater and tailwater elevations at the dams. Reference 17 presents recommendations to eva luate 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 t he reevaluation. Tailwater variability is limited by the reservoir level relationsh ips and is simulated within the HEC-RAS model. Thus no additional sensitivities for initial reservoir conditions were evaluated Page 60 of 70
11 COMPARISON - CURRENT DESIGN BASIS ELEVATIONS VS. REEVALUATION RESULTS Table 11-1 presents a comparison of the design basis flood elevations and the reevaluated results.
Table 11-1 Comparison of Current Design Basis Elevations and Reevaluation Results Bounded Flood Causing Mechanism Design Basis Reevaluation Comments (Yes/No)
Critical floor elevation is 729 Local Intense ft. Case 2 exceeds 729 ft. in
<729 ft. 729.2 ft. No Precipitation some portions of the protected area.
(b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(?)(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 this site.
Not a credible flood-causing Flooding from Tsunamis N/A N/A N/A mechanism at this site Flooding from Ice-Induced Not a credible flood-causing N/A N/A N/A Events mechanism at this site Flooding From Channel Not a credible flood-causing Diversion or Migration N/A N/A N/A mechanism at this site Toward t he Site (b)(3) 16 us c § t:24o-1(d), (b)(4) (bJ1 )(F)
Flooding from Combined Effects N/A - Not applicable a Dam modifications in place as described in Reference 5 (b)(3) 16 U SC b Dam modifications in place as described in Reference 5 and an § 824o-1(d), (b) Dam (4), (b)(7)(F)
Page 61 of 70
12 IDENTIFICATION AND EVALUATION OF ANY INTERIM ACTIONS TAKEN TO MITIGATE HIGHER FLOOD HAZARD RELATIVE TO DESIGN BASIS As identified in Table 11-1, reevaluation results for three flood causing mechanisms, local intense precipitation, flooding from rivers and streams and flooding from combined effects of PMF and wind are not bounded by the current design basis for WBN. Each of these mechanisms is evaluated below and interim actions defined if needed.
12.1 Local Intense Precipitation The WBN critical elevation, as related to the LIP flooding at the site, is the floor elevation of the exterior doors leading to the Auxiliary Building and Control Building, which is equal to 729.0 ft.
The calculation prepal"ed for the flooding at the WBN site due to the LIP determined that the flood water will exceed the plant critical elevation by as much as 0.2 feet (2.4 inches).
Per the CLB, the LIP flood at the site will not exceed the 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 the Integrated Assessment, in order to mitigate potential impacts of the LIP flood hazard, interim actions are evaluated as discussed in the following sections.
The results of the reevaluated LIP flood show a small increase in flood levels above the crit ical 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 expe,cted to significantly affect the groundwater levels around the structures because of its short duration and due to impermeable land cover at the majority of the site area.
The results of the HEC-RAS model in t he reevaluated LIP calculation indicate that flood water will be above the critical elevation only on the plant east side (Subareas K2 and K4) from channel cross sections 21 to 17 and drops below the critical elevation somewhere prior to cross-section
- 16. Reference LIP Figure Page 62 of 70
(b)(3) 16 U SC § 824o-1(d), (b)(4), (b)(7)(F)
Page 63 of 70
(b)(3) 16 U.S.C § 824o-1(d), (b)(4), (b)(?)(F)
No i_n terim actions are required for WBN. Based on the performed eva luations t here is no impact on equipment relied upon for safe plant operation.
12.2 Flooding in Rivers and Streams [ ] ~ - ~ ~ _(b)(3) 16 u s c (b)(3) 16 USC
§ 824o-1(d}:(bj The re-evaluation of the PMF indicates a maximum st illwater elevation of a
- verflow fai lure of!(b)(3)*16 USC § 824o-1(d), (b)(4),
Dams is not prevented, the maximum stillwater elevation on results exceed the WBN design basis flood elevation of o
!Dam are prevente~
ft. at WBN §8246~1(d), (b) erflow of "' ,wwr,
ft,aspresentedin Reference 5. For Flooding in Rivers and Streams an Integrat ed Assessment w, be performed.
~~~~l, t
?
isL_:jft; -:rhese .. (?)(3_) __)_ U)S C s(~_
,L?,..,_{.J: ( )
(b)(3) 16 USC § 824o-1(d) (b)(4)
The overtopping of (b)(?)(F) is being prevented by the (b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(?)(F)
In the Integrated Assessment, *
- ions for addressin the in elevation of Page 64 of 70
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 (~ illed water motors are the most critical. The protected elevation for this equipment isL = jft**Since -a ...........~?~~1;~(~)~(~
(b)(3) 16 USC - - ~flood..w.ater..surge insidethestructUfe *f*****d *** eet, t~ r - valuation flood level at this equipment
§ 8240-f(d);(b)
-,,r, -
is expected to be 740.3 ft. which excee s the ********** ft,protected elevation. The other (b)(3) 16 USC.
,.' 'L' . . §B240:1(d), (b) equipment required for plant operation during flood mode operation has critical protection elevations greater than 740.3 ft.
Since the protective barriers for the MCR and SDBR chilled water motors is exceeded in the re-evaluated PMF, the cooling capability of the MCR and SOBR air handling units is assumed to be lost. If the MCR air handling units are lost during the re-evaluation PMF event, the MCR can be abandoned and operation of the plant for the duration of the event would be from the Auxiliary Control Room (ACR). Heat load calculations show that the ACR temperatures do not exceed 114.3 degrees during the event. Heat load calculation also show that the SDBR temperatures remain below the required 104 degree temperature limit.
Although the temperatures exceed the required environmental qualification requirements, WBN calculations have determined that electrical equipment in mild areas can withstand either an increasing ambient temperature to 140°F for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> followed by a period of 99 days at less than or equal to 120°F or a slow ramp to 135°F followed by a temperature of 135°F for 100 days. The result of this calculation bounds the temperatures determined by the model run for the areas served by the SDBR air conditioning system. Therefore, the electrical equipment in the area required for the mitigation ofthe PMF event remains operable.
12.3 Combined Effects Floods Caused by Precipitation Events he : Aasis minimum PMF plus wind effects elevation as (b)(3) 16 U.S C su mittedto theNRCin References is ************ ft. The critical elevation at the Diesel Generator
§ 824o-1(d}:(b) ..
Ill IL\1"7\lr"'\ Building is the floor elevation at 742.0 . Since the re-evaluation results exceed the critical (b)(3) 16 U S C elevation .byl="lfeet, interim actions are required to prevent water ingress into the Diesel
§ 8240 1(df (5) .. Generator BUng. As an interim action, sandbags stacked a minimum of 12 inches high will be utilized to protect doors and the exterior fuel oil fill ports. Sufficient warning time exist to install the sandbag protective barriers prior to the beginning of the main storm of the PMP. Procedures will be revised as needed to implement this interim action by May 31, 2015.
During the Integrated Assessment, options will be evaluated for permanent barriers to protect the Diesel Generator Building for flood plus wind wave run-up elevations 0[ 3ft., . .. "****--- .... ~?*~F~~)S(~
2!~~*~*
5 The re-evaluation PMF combined with wind effects results in a maximum elevation of " 7ft, at .. -~~ (bi' the exterior wall of the Intake Pumping Station (IPS). As discussed in Section 3.5.2~ RCW '" ,u,..,,,r; pumps, fire protection pumps, and screen wash pumps are located on the upper deck at elevation 741.0 ft. The 741.0 ft. elevation is enclosed by a 13 feet. high concrete wall. The critical component required at the 741.0 ft. elevation, inside the concrete wall, is a junction box for the High Pressure Fire Protection (HPFP) pump. The critical elevation of the junction box is 742.4 ft.
There is a small (2.2 ft2 ) opening on the exterior wall of the IPS for the trash sluice which extends below the 741.0 ft. elevation. A sluice backsplash concrete wall with a top elevation of 741.0 ft.
Page 65 of 70
direct entry into building. The re-evaluation PMF combined with wind effects elevation of (b)(3) 16 USC ** ft. would overtop the sluice backsplash wall and could ent er elevation 741.0 ft. Flood
§ 824o-1(d): (br .
a e s that enter the structure through this small opening would gravity drain out of the structure as the wave action on the structure recedes. This circuitous entry into the structure due to the external wave action would dampen the waves such that the junction box for the HPFP pump is not impacted. No interim actions are required.
Page 66 of 70
13 REFERENCES
- 1. U.S. Nuclear Regulatory Commission. Letter to Licensees. Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3 of the Near Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. March 12, 2012. ,
- 2. Tennessee Valley Authority, Watts Bar Nuclear Plant Updated Final Safety Analysis Report (Amendment 11).
- 3. Tennessee Valley Authority, SOCH Geometry Verification Chickamauga Reservoir, CDQ000020080030, Revision 2, EDMS 11841 110720 005.
- 4. Tennessee Valley Authority, Watts Bar Nuclear Plant, License Amendment Request, Final Safety Analysis Report, July 19, 2012, ML12236A167.
- 5. Tennessee Valley Authority, Watts Bar Nuclear Plant, License Amendment Request, September 30, 2014, ML14289A106.
- 6. U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation Letter to the Tennessee Valley Authority, Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Unit 2 (NUREG-0847, Supplement 27), Docket Number 50-391, Tennessee Valley Authority, January 28, 2015, ML15005A314.
- 7. National Oceanic and Atmospheric Administration and Tennessee Valley Authority, "Hydrometeorological Report No. 56", October 1986.
- 8. USA. United States Geological Survey. United States Department of the Interior. Multi-Resolution Land Characteristics Consortium (MRLC). n.d. Web. Feb. - Mar. 2013.
http://www.mrlc.gov/ind ex.php .
- 9. Tennessee Valley Authority Calculation CDQ0000002014000014, Revision 000, BWSC TVAGENQ13003, Revision 0, Curve Number Determination.
- 10. U.S. Department of Commerce, Weather Bureau, Probable Maximum and TVA Precipitation over the Tennessee River Basin above Chattanooga, Hydrometeorological Report No. 41. June 1965.
- 11. Harrington, Bruce C. and Ramon G. Lee, Flood Reassessment for the Effects of Dam Safety Modifications, CDQ0999-98001, Revision 0, March 1998, 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, Marrch 19, 2008, ML080640487.
- 13. Tennessee Valley Authority, Letter to U.S. Nuclear Regulatory Commission, Fleet Response t o NRC Request for Information Pursuant to Title 10 of Code of Federal Regulations 50.54(f) Regarding the Flooding Walkdown Resuilts of Recommendation 2.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. Enclosure 4, November 27, 2012, ML12335A340.
- 14. U.S. Nuclear Regulatory Commission, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, NUREG/CR-7046, November 2011.
- 15. National Oceanic and Atmospheric Administration and U.S. Department of the Army Corps of Engineers, "Hydrometeorological Report No. 52", October 1982.
Page 67 of 70
- 16. American National Standard, American National Standard for Determining Design Basis Flooding at Power React or Sit es, July 28, 1992, ANSI/ANS 2.8-1992.
- 17. U.S. Nuclear Regulatory Commission. Japan Lessons-Lea rned Project Directorate. Guidance for Assessment of Flooding Hazards Due to Dam Failu re, Revision 0, JLD-ISG-2013-01. July 29, 2013.
- 18. Tennessee Valley Authority Technical Memo, Watts Bar Dam Breach Sedimentation Analysis -
Phase 1 Baseline Analysis, November 18, 2014.
- 19. McGarr, Arthur and Robert C. Vorlis, The Alaska Earthqua ke, March 27, 1964: Effects on the Hydrologic Regimen, Seismic Seiches From the March 19'64 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 m ap).
- 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 Laborat ory , 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 Calcu lation 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 #841140919 001.
- 26. Tennessee Valley Authority Calculation CDQ0000002012000006, Revision 001, BWSC Calculation TVAGENQ114007, Revision 0, HEC-RAS M odel Calibration and M odel Set -up - M ain Stem , EDMS
- 841140919 002.
- 27. Tennessee Valley Authority Calcu lation CDQ0000002014000018, Revision 000, BWSC Calculation TVAGENQ13007, Revision 0, HEC-RAS Tributary Model Ca libration, EDMS #841140919 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, EDM S #841110718 003.
- 31. Tennessee Valley Authority Calculation CDQ0000002012000005, Revision 002, BWSC Calculation TVAGENQ14012, Revision 1, HEC-RAS Unsteady Flow Ru les - M ain 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 COQ0000002014000012, Revision 000, BWSC Calculation TVAGENQ13005, Revision 0, Unit Hydrograph Adjustment.
- 34. Tennessee Valley Authority Calculation COQ0000002014 000015, Revision 000, BWSC Calculation TVAGENQ13008, Revision 0, Inflows.
Page 68 of 70
- 35. Tennessee Valley Authority Calculation CDQ0000002014000013, Revision 000, BWSC Calculation TVAGENQ13004, Revision 0, Baseflow.
- 36. Tennessee Valley Authority Technical M emorandum, " HMR Applicability." December 19, 2012.
- 37. Tennessee Valley Authority Technical Memorandum, "Critical Storm Selection." December 21, 2012.
- 38. Tennessee Valley Authorit y Calcu lation CDQ000020080053, Revision 1, PMF Inflows, EDMS #841 120628 004.
- 39. U.S. Department of Energy, Det ermining the Best Estimate of Maximum Wat er 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 Memorandum, " National Inventory of Dams Inflows."
December 19, 2014, EDMS #WSO 150107 001.
- 41. Tennessee Valley Authority Calculation CDQ0000002014000030, Revision 000, BWSC Calculation TVAGENQ14015, Revision 0, Inflow Hydrograph Development for Seismic Events.
- 42. Tennessee Va lley Authority, Simplified Volume Analysis for Multiple Dam Failures, November 24, 2014, EDMS # WSO 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 Va lley Authority Calculation CDQ0000002014000023, Revision 000, BWSC Calculation TVAGENQ14009, Revisiori 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 Recom mendation 2. 1: Sunny Day Dam Failure Simulations
- 47. Tennessee Valley Authority Calculation CDQ0000002014000028, Revision 000, BWSC Ca lculat ion TVAGENQ14016, Revision 0, Fukushima NTTF Recommendation 2.lG ---------- DamSunnv ___Day FaiJ1.1re (b)(3) 16 USC Simulation ' 1~2,~?~,(~, (b)
- 48. Tennessee Valley Authority Calculation CDQ0000002014000024, Revision 000, BWSC Calcul ation 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.
SO. Ten nessee Valley Authority Calculation CDQ0000002014000033, Revision 000, BWSC Calculation TVAGENQ14020, Revision 0, Fukushima NTTF Recommendation 2.1: Wind Waves for Combined-Effect Floods
- 51. Tennessee Valley Authority, Multiple Dam Failure Screening Analysis for the Tennessee Valley Authority's Dams, M arch 31, 2014, EDMS #WSO 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-l, 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.
- 66. DCA-51280-A.
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