ML14056A195

South Texas Project, Units 1 and 2, Second Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation

ML14056A195
Person / Time
Site:	South Texas
Issue date:	02/13/2014
From:	Powell G T South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-14003085, TAC MF1110, TAC MF1111
Download: ML14056A195 (11)

Text

Nuclear Operating CompanySouth Teas eProe Elecftic Generating Station PO. Box 289 Wadstvorth, Texas 77483-February 13, 2014NOC-AE-14003085 10 CFR 50.54(f)U.S. Nuclear Regulatory Commission Attention:

Document Control DeskWashington, DC 20555-0001 South Texas ProjectUnits 1 & 2Docket Nos. STN 50-498, STN 50-499STPNOC Second Partial Response to Request for Additional Information Associated With Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation (TAC Nos. MF1110 and MF1111)

References:

1. Letter from NRC to All Power Reactor 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 ofthe Insights from the Fukushima Dai-ichi Accident" March 12, 2012(ML12056A046).

2. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk,"Response to NRC Request for Information Pursuant to 10 CFR 50.54(f)Regarding Recommendation 2.1 Flooding of the Near-Term Task ForceReview of Insights from the Fukushima Dai-ichi

Accident, Enclosure 2,Required Response 2, Flood Hazard Reevaluation Report",

March 11, 2013(ML13079A806)

3. Letter from B. K. Singal, NRC, to D.L. Koehl, STPNOC, "South Texas Project,Units 1 and 2 -Request for Additional Information Regarding Fukushima Lessons Learned -Flooding Hazard Reanalysis Report ", January 14, 2014(ML13358A065)

4. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk,"STPNOC Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard -Reevaluation",

January 22, 2014 (NOC-AE-1 4003074)STI: 33824629 NOC-AE-14003085 Page 2 of 3On March 12, 2012 (Reference 1), the U.S. Nuclear Regulatory Commission (NRC) staff issueda letter requesting additional information per Title 10 of the Code of Federal Regulations, Section 50.54(f)

(hereafter called the 50.54(f) letter).

The 50.54(f) letter requested that licensees reevaluate the flooding hazards at their sites against present-day regulatory guidance andmethodologies.

STPNOC submitted the requested Flood Hazard Reevaluation Report to theNRC on March 11, 2013 (Reference 2).By letter dated January 14, 2014 (Reference 3), the NRC requested additional information (RAI)related to the flooding reevaluations.

Per an agreement with the NRC, STPNOC has responded to the RAls in two parts. The first part of the STPNOC response addressed RAls 1, 2, 7, 8 and 9(Reference 4). The second part of the requested information addressing RAIs 3, 4, 5 and 6 isincluded in the attachment and enclosures submitted with this letter.There are no commitments in this letter.If there are any questions regarding this letter, please contact Wendy Brost at (361) 972-8516 orme at (361) 972-7566.

I declare under penalty of perjury that the foregoing is true and correct.Executed on ,. .Zo,G.T. PowellSite Vice President web

Attachment:

STPNOC Second Partial Response to Request for Additional Information Related to Fukushima Lessons Learned Flooding Hazard Reevaluation Report

Enclosure:

Supplemental Information for RAI 5 (CD labeled NOC-AE-14003085 Disc 1)

NOC-AE-14003085 Page 3 of 3cc:(paper copy)(electronic copy)Regional Administrator, Region IVU. S. Nuclear Regulatory Commission 1600 East Lamar Boulevard Arlington, TX 76011-4511 Balwant K. Singal *Senior Project ManagerU.S. Nuclear Regulatory Commission One White Flint North (MS 8 B1)11555 Rockville PikeRockville, MD 20852NRC Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16Wadsworth, TX 77483Jim CollinsCity of AustinElectric Utility Department 721 Barton Springs RoadAustin, TX 78704A. H. Gutterman, EsquireMorgan, Lewis & Bockius LLPBalwant K. SingalU. S. Nuclear Regulatory Commission John RaganChris O'HaraJim von SuskilNRG South Texas LPKevin PolioRichard PefiaL.D. BlaylockCity Public ServicePeter NemethCrain Caton & James, P.C.C. MeleCity of AustinRichard A. RatliffRobert FreeTexas Department of State Health Services* Digital media enclosure is only being transmitted to the NRC Document Control Desk and toSTPNOC's NRC Project Manager, Balwant K. Singal, for distribution to the NRC staff reviewers Attachment 1NOC-AE-14003085 Page 1 of 8STPNOC Second Partial Response to Request for Additional Information Related to Fukushima Lessons Learned Flooding Hazard Reevaluation ReportNote: References described in this Attachment are found at the end of the Attachment.

RAI 1: Local Intense Precipitation and Associated Site DrainagePlease provide electronic versions of the input files used for HEC-HMS analysis in the flood hazardreevaluation report (FHRR) related to the local intense precipitation analyses.

STPNOC Response:

The requested information was provided in the first partial RAI response (Reference 1).RAI 2: Local Intense Precipitation and Associated Site DrainagePlease provide electronic versions of the input files used for HEC-RAS analysis in the FHRR relatedto local intense precipitation analyses.

STPNOC Response:

The requested information was provided in the first partial RAI response (Reference 1).RAI 3: Local Intense Precipitation and Associated Site DrainagePlease provide descriptions of the sources of elevation data, the methods used to incorporate elevation measurements into local intense precipitation flood analysis, and the likely magnitude ofthe errors associated with these elevations.

STPNOC Response:

Ground topography within the South Texas Project (STP) property boundary was obtained from anaerial photogrammetric survey. At least 90% of all elevation data obtained from elevation contourswas required to have an accuracy of +/-0. 5ft according to the survey specification and the remaining 10% of the data was required to have an accuracy of +/- 1. Oft. Additionally, a ground survey wasperformed to compare field measurements with aerial survey elevations at selected locations.

Theroot-mean-square error for the difference in elevation between aerial survey and ground survey datawas found to be approximately

0. 196ft, demonstrating that the elevations used in the local intenseprecipitation evaluation are of reasonable accuracy.

Of the 311 comparison points, there are onlytwo locations where the difference was above 0.5ft (one at 0.538ft and one at 1.043ft).

For boththese instances, elevations from the aerial survey were higher than the elevations from fieldmeasurements.

Outside of the STP property boundary ground elevation data were obtained from theUnited States Geological Survey (USGS) topographic map for the Blessings SE Quadrangle.

Theaerial survey and USGS topographic data were used to develop the HEC-RAS cross sections asshown on Figure 2.1-7 of the Flooding Hazard Reevaluation Report (FHRR).

Attachment 1NOC-AE-1 4003085Page 2 of 8In addition to using the accurate ground elevation data to develop the local intense precipitation flood model, the HEC-HMS and HEC-RAS models incorporate highly conservative assumptions.

These include conservative Manning's roughness coefficients, the entire site considered impervious and modeled with a curve number of 98, reduced time of concentration values, and all storm drainsconsidered to be non-operational.

The combined effect of these conservative assumptions morethan adequately compensates for the small uncertainty in the ground elevations such that estimated flood levels can be considered as the maximum possible flood levels due to the local intenseprecipitation over the site. Moreover, as indicated in the FHRR, the maximum local intenseprecipitation flood elevation (33ft MSL) would not adversely impact the safety functions of the plantas this elevation is much lower than the design basis flood elevations associated with the MainCooling Reservoir embankment breach, which range between 44.5ft and 50. 8ft MSL at the powerblock structures and 40. 8ft MSL at the emergency cooling water intake structure.

This conclusion remains accurate even with the consideration of the small uncertainty in the elevation measurements.

No FHRR revision is required as a result of this RAI response.

RAI 4: Local Intense Precipitation and Associated Site DrainagePlease provide a description of the basis used to classify Probable Maximum Flood (PMF) flow asshallow concentrated flow used in the Natural Resources Conservation Service (NRCS) TR-55methodology.

STPNOC Response:

The NRCS TR-55 methodology (Reference

2) was used to estimate the time of concentration foreach sub-basin.

Time of concentration, which is defined as, "the time for runoff to travel from thehydraulically most distant point of the watershed to a point of interest within the watershed" (Reference 2), is the sum of flow travel times from consecutive flow segments of the drainageconveyance system. According to Reference 2, the time of concentration flow paths are divided intothree segments:

sheet flow, shallow concentrated flow, and ditch flow. The equations used todetermine the travel time for each segment are described in Reference 2.The three flow segments delineated for the flow path in each sub-basin are summarized below. Thesub-basin areas and corresponding flow paths are shown on Figure 2.1-3 of the FHRR.Sheet Flow:The sheet flow occurs over plane surfaces at the headwater of streams (Reference

2) andrepresents the upstream-most segment of the flow path. The travel time in this segment isaffected by the flow path length (ft), overland flow roughness coefficient, surface slope (ft/ft) and2-year 24-hour rainfall intensity (inch). The sheet flow segments for sub-basins STPI, STP3b,STP3c, STP4a, and STP5a are over short grass-covered areas; therefore, a roughness coefficient of 0. 15 representing this condition was selected per Reference 2 (also see FHRRFigure 2.1-3 for sub-basin locations).

A roughness coefficient of 0. 011 (Reference

2) wasselected for the sheet flow segments for sub-basins STP2b, STP4b, STP5b, and SW torepresent asphalt and gravel covered area conditions.

A roughness coefficient of 0. 06(Reference

2) was selected for the sheet flow segments for sub-basins STP2a, STP3a, North B,North I and North 2 to represent cultivated soil covered area conditions.

Slopes along the sheetflow paths were determined from the topographic data. McCuen and Spiess (Reference

3)

Attachment 1NOC-AE-1 4003085Page 3 of 8argued that to use a specific fixed length (e.g. 300ft) for all sub-basins could lead toinaccuracies, and proposed the following relationship based on empirical results:

L= (I 0N/S)/n,where L is the flow path length (ft), S is the flow path slope (ft/ft),

and n is the overland flowroughness coefficient as defined in Reference

2. McCuen and Spiess (Reference

3) relationship was used to verify sheet flow lengths in the FHRR. In general, the maximum sheet flow lengthvalue used was 5Oft, with the exception of sub-basins North 1 and North 2, where a sheet flowlength of lOOft was used.Shallow Concentrated Flow:Beyond the sheet flow length, flows convert to shallow concentrated flow where the travel timesare defined based on empirical velocity (ft's) relationships for unpaved and paved surfaces andflow path lengths (ft) (Reference 2). In the FHRR, the velocity equation for unpaved surfaces wasused for all sub-basins, except for sub-basins STP3c, STP4b and STP5b. The paved surfaceequation was used for the latter sub-basins because the shallow concentrated flow segments atthose locations are characterized by paved and gravelly surfaces due to the development at theS TP I & 2 power block and surroundings.

Ditch Flow:Ditch or open channel flow begins where channels are visible on aerial photographs, wheresurveyed cross section information is available, or where stream information appears on USGSquadrangle maps (Reference 2). The ditch flow travel time is estimated based on the flow pathlength (ft) to the end of the sub-basin or a point of interest and average ditch flow velocity.

Theflow velocity (ft/s) was initially estimated based on a ditch-full condition, which was verified bycomparing with the corresponding average channel velocity obtained from the HEC-RAS model.For the FHRR, a constant velocity of 3 fps was designated for the ditch flow segment, with theexception of sub-basins North 1 and North 2 where a velocity of 6 fps was conservatively used.The ditch flow velocities were found to be higher than those computed in the HEC-RAS model,and, therefore, resulted in a shorter ditch flow travel time and conservatively higher runoffdischarge.

In accordance with the US Army Corps of Engineers (Reference 4), for extreme storm runoffanalysis the computed time of concentration values should be reduced to account for the non-linear response for extreme rainfall events. Accordingly, the time of concentration values were decreased by 25% for the local intense precipitation event. The numerical rainfall runoff model HEC-HMSrequires the 'Lag Time' value instead of the time of concentration value as an input parameter.

LagTime values are estimated as 0.6 times the time of concentration values (Reference 5). Theestimated length for each flow path segment, time of concentration and lag time for all sub-basins are shown in Table 1.No FHRR revision is required as a result of this RAI response.

Attachment 1NOC-AE-1 4003085Page 4 of 8Table 1 -Time of Concentration Estimates Sheet Flow Shallow Concentrated Flow Ditch Flow Ca/c. 25%Sub- (TcI1) (Tc2) (Tc3) Calc. Time ofRed LagBasinTime of CadcBasin Length Manning's-Slope Length Slope2 T2 LengthTL Vel 3 CTinL (ft) n1 S (ft/ft) L ( V (fps) (hr) (ft) V (fps) (hr) Conc. (hr) (mi)STPI 50 0.15 0.18 0.03 380 0.08 4.6 0.02 7955 3 0.74 0.79 0.59 21STP2a 46 0.06 0.04 0.03 1947 0.0007 0.4 1.27 5870 3 0.54 1.84 1.38 50STP2b 47 0.011 0.12 0.004 91 0.024 2.5 0.01 2026 3 0.19 0.20 0.15 5STP3a 49 0.06 0.0007 0.14 2059 0.003 0.9 0.65 2036 3 0.19 0.97 0.73 26STP3b 24 0.15 0.0015 0.12 192 0.0015 0.6 0.09 810 3 0.08 0.28 0.21 8STP3c4 48 0.15 0.2 0.03 2898 0.004 1.3 0.63 619 3 0.06 0.71 0.53 19STP4a 27 0.15 0.002 0.12 1937 0.002 0.7 0.75 840 3 0.08 0.94 0.71 25STP4b4 49 0.011 0.001 0.03 597 0.00006 0.2 1.05 1471 3 0.14 1.22 0.91 33STP5a 47 0.15 0.04 0.06 1199 0.0006 0.4 0.84 4566 3 0.42 1.32 0.99 36STP5b4 49 0.011 0.0007 0.04 3147 0.0013 0.7 1.19 783 3 0.07 1.30 0.98 35SW 51 0.011 0.0008 0.04 1339 0.0008 0.5 0.82 4626 3 0.43 1.28 0.96 35North B 51 0.06 0.001 0.12 1274 0.0006 0.4 0.90 3001 3 0.28 1.30 0.97 35North 1 100 0.06 0.001 0.21 4760 0.000315 0.3 4.62 10200 6 0.47 5.30 3.98 143North 2 100 0.06 0.001 0.21 5000 0.0004 0.3 4.30 4060 6 0.19 4.70 3.53 1271 Reference 2.2 Slopes for areas North 1, North 2 and North B are estimated from Ift interval contours developed from the USGS 7.5 minute series quadmap (Blessing SE Quad); for the rest of the sub-basin areas Ift interval contour from aerial survey data are used.Assumed and compared to HEC-RAS hydraulic model's average velocities.

The assumed 3 fps velocity is conservative, as it is greater thanthe average velocity from the HEC-RAS model.Paved surfaces velocity equation (Reference

2) was used to calculate shallow concentrated flow on sub-basins STP3c, STP4b and STP5b.

Attachment 1NOC-AE-1 4003085Page 5 of 8RAI 5: Failure of Dams and Onsite Water Control/Storacie Structures Please provide details of the ineffective flow areas and levees that were removed from the HalffAssociates, Inc. HEC-RAS model while developing the HEC-RAS model used to reevaluate theflood hazard from upstream dam failures at the South Texas Project (STP), Units 1 and 2 site. Also,please provide a justification for removal of these features.

STPNOC Response:

Enclosed on a CD with this response are: (1) PDF files of the channel cross-sections from Halff'sHEC-RAS models (Reference 6), which depict the locations of the ineffective flow areas and leveesthat were specified for different reaches of Colorado River; and (2) an electronic spreadsheet filethat provides details on the Left and Right Stations and Elevations of the ineffective flow areas andlevees that were specified in Ha/ff's HEC-RAS models. The PDF files are 13 in total and Table Iprovides the reaches of Colorado River represented by the PDF files. The tabs in the electronic spreadsheet file, Ineffective-areas-and-Levees.xls, correspond to the reaches represented in thePDF files.Table 2 -Colorado River reaches represented by the PDF filesNo PDF File ReachName From ToI Inks.pdf Buchanan Dam Inks Dam2 LBJ.pdf Inks Dam Wirtz Dam3 Marble.pdf Wirtz Dam Starcke Dam4 Travis.pdf Starcke Dam Mansfield Dam5 LakeAustin.pdf Mansfield Dam Tom Miller Dam6 TownLake.pdf Tom Miller Dam Longhorn Dam7 Bastrop.pdf Longhorn Dam Bastrop Gauge8 LaGrange.pdf Bastrop Gauge La Grange Gauge9 Columbus.pdf La Grange Gauge Columbus Gauge10 Garwood.pdf Columbus Gauge Garwood Gauge11 Wharton.pdf Garwood Gauge Wharton Gauge12 BayCity.pdf Wharton Gauge Bay CityDownstream 13 Matagorda.pdf Bay City BoundaryThe setup of the Halff HEC-RAS model, including the designation of ineffective areas in the channel,was developed to evaluate flow conditions up to the Standard Project Flood (SPF). For thepostulated dam break scenario analyzed in the Flooding Hazard Reevaluation Report (FHRR), thepeak flood discharges and flood levels are higher than those associated with the SPF condition, inundating floodplains and areas that were previously assigned as ineffective including the areasbehind the levees in the Halff model.

Attachment 1NOC-AE-1 4003085Page 6 of 8As a result, these ineffective areas were removed in the HEC-RAS model to more accurately reflectthe actual effective flow areas that will be available for the propagation of dam break floodflow. Further, in the FHRR dam break analysis, all bridges in the path of the flood wave wereassumed to be washed out to provide more conservative peak flows. As such, the ineffective areasassociated with these bridge sections in the Halff model were removed as well to properly represent the washout of the bridges.No FHRR revision is required as a result of this RAI response.

RAI 6: Failure of Dams and Onsite Water Control/Storage Structures Please provide details of the intra-basin flows that were allowed to occur in the HEC-RAS model.Also, because allowing intra-basin flows would reduce the discharge at STP, Units 1 and 2 site, thelicensee is requested to provide a justification how the flood hazard from upstream dam failureswould still be conservative.

STPNOC Response:

As stated in Subsection 2.3.1.2.1.3 (Channel Geometry) of the Flooding Hazard Reevaluation Report (FHRR), the initial dam break model runs showed that the simulated water level elevations along the drainage divide were higher than the local ground elevations at the divide, indicating thepotential for overflow to the adjacent basin. Therefore for the locations where inter-basin spillagewas expected to occur, the HEC-RAS cross-sections were extended to better simulate the effect ofthe inter-basin flow. Without the extension of the cross-sections HEC-RAS would assume by defaulta vertical wall at both ends of the cross-sections.

This approach would be highly conservative butnot realistic, in that the actual flow condition would not be simulated.

Therefore, this approach wasnot adopted.Based on the USGS topographic data, it was found that inter-basin spillage would occur nearGarwood, Texas. The cross-sections, from the Ha/ff HEC-RAS models (Reference

6) downstream ofthis location were extended on both sides to cover the areas where inter-basin spillage is expectedto occur. Extending the HEC-RAS cross-sections, and hence considering inter-basin

spillage, provided a more accurate model of the physical hydraulic condition and therefore produced a morerepresentative dam break peak discharge at STP 1 & 2 site.Overall, the modeling results are still conservative in that many conservative assumptions were usedin setting up the HEC-RAS model. Some of the conservative assumptions include relatively highManning's n values for the river channel and flood plains; simultaneous arrival of all upstream damstorage volumes at Lake Buchanan before its failure; initial water level at Buchanan Dam higherthan the crest of dam; and removal of all dams and bridges between Buchanan and Mansfield Damsand farther to downstream boundary of the HEC-RAS model.No FHRR revision is required as a result of this RAI response.

Attachment 1NOC-AE-1 4003085Page 7 of 8RAI 7: Failure of Dams and Onsite Water Control/Storage Structures Please provide electronic versions of the input files used for HEC-RAS analysis in the FHRR relatedto upstream dam failures.

STPNOC Response:

The requested information was provided in the first partial RAI response (Reference 1).RAI 8: Failure of Dams and Onsite Water Control/Storage Structures Please provide the electronic version of National Weather Service (NWS) BREACH model input filesused in the recent Main Cooling Reservoir (MCR) breach analyses of the three postulated breachlocations described in FHRR Section 2.3.STPNOC Response:

The requested information was provided in the first partial RAI response (Reference 1).RAI 9: Failure of Dams and Onsite Water Control/Storage Structures Please provide a description of model configuration, boundary conditions, and model parameters forthe three RMA2 simulations.

Also, Please provide the RMA2 input files, including the computational grids, used for the three simulations.

STPNOC Response:

The requested information was provided in the first partial RAI response (Reference 1).

Attachment 1NOC-AE-1 4003085Page 8 of 8References

1. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk, "STPNOC PartialResponse to Request for Additional Information Associated with Near-Term Task ForceRecommendation 2.1, Flood Hazard Reevaluation" January 22, 2014 (NOC-AE-14003074)

2. U.S. Department of Agriculture, Soil Conservation

Services, "Urban Hydrology for SmallWatersheds",

June 1986 (Technical Release 55)3. R.H. McCuen and J.M. Spiess, American Society of Civil Engineers, "Assessment ofKinematic Wave Time of Concentration",

Journal of Hydraulic Engineering, March 1, 19954. U.S. Army Corps of Engineers, "Flood-Runoff Analysis',

August 31, 1994 (Engineer Manual1110-2-1417)

5. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-HMS, Hydrologic Modeling System, Technical Reference Manual, August 2010.6. Halff Associates, Inc., "Flood Damage Evaluation Project",

Chapter 1-6, Volume I/-C, VolumelI-B, July 2002