ML14253A303

9/10/2014, Meeting Slide for Watts Bar Nuclear Plant, Units 1 and 2 Regarding Hydrology

ML14253A303
Person / Time
Site:	Watts Bar
Issue date:	09/10/2014
From:	Tennessee Valley Authority
To:	Hon A L Watts Bar Special Projects Branch
Hon A
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Download: ML14253A303 (16)

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HdlHydrologySeptember 10, 2014 Agendag*Introduction and PurposeJoe Shea*BackgroundJoe Sheag*TVAs ApproachGary Mauldin*ScheduleGary Mauldin*ConclusionJoe Shea

• QuestionsGary Mauldin/Joe Shea2Shea Hydrology Background*Describe TVA's Approach to resolving NRC questions regarding dam stability analyses as part of the Watts Bar Nuclear Plant (WBN) hydrology evaluation*DibTVA'ltbitdttthdiWBNUit1Li*Describe TVA's plans to submit an update to the pending WBN Unit 1 License Amendment Request and WBN Unit 2 Final Safety Analysis Report that:-ReflectrevisedapproachtoevaluatingdamstabilityaspartofTVA'snuclearplantReflect revised approach to evaluating dam stability as part of TVAs nuclear plant hydrology analyses-Revise the hydrology modeling tools used by TVA to evaluate probable maximum flood scenarios*Discuss schedule for submitting updated information3Shea Hydrology Background*TVA submitted a request to amend the WBN Unit 1 license on July 19, 2012-TVA proposed to revise the hydrologic analyses for Watts Bar siteTVAdNRCJl222014diifhbilTh*TVA and NRC met on July 22, 2014 to discuss portions of the submittal. The discussion surrounded TVA's dam stability analyses.In2012TVAcompletedre-evaluationofthedesignbasisProbableMaximumIn 2012, TVA completed re-evaluation of the design basis Probable MaximumFlood (PMF) affecting the TVA Nuclear SitesAs a result of the re-evaluation, PMF levels increased from 734.9' to 739.2' at Watts BarThe increase in PMF levels resulted in a corresponding increase in the headwater and tailwater levels seen at the dams of the WBN and Sequoyah (SQN) plants.The increase in headwater and tailwater required the structural stability of the dams 4to be validatedShea Hydrology Background*July 22, 2014 meeting with NRC (continued): DuringanauditofTVA'sUpdatedFinalSafetyAnalysisReport(UFSAR)submittalinDuring an audit of TVAs Updated Final Safety Analysis Report (UFSAR) submittal in April of 2014, the NRC staff noted that the "no dam failure" assumption in the PMF analysis for a licensing basis change was not supported.BasedonTVA'sunderstandingoftheissuethecriteriausedtoevaluatetheBased on TVAs understanding of the issue, the criteria used to evaluate the acceptability of dams impacting WBN and SQN for PMF will be clearly defined and should be consistent with current industry techniques*TVAhasperformedanalysisofapplicabledamsusingcurrentindustrytechniques*TVA has performed analysis of applicable dams using current industry techniquesDetails will be discussed in subsequent slidesTVA is designing and implementing improvements to certain dams consistent with current industry techniques5Shea Timeline of Design Basis Hydrology Events6Shea Tennessee River System Overview *The existing WBN and SQN licensing basis regarding dams and hydrology are based on dam safety criteria in place at the time. Those techniques, while meeting the requirements of the licensing basis, do not meet the documentation expectations of today*TVAhasdeterminedthatthemostrobustwayto*TVA has determined that the most robust way to demonstrate the stability of its dams and associated hydraulic analysis is to convert to modern industry methodologyTVAhdlliPMPiTVA can then document all its PMP assumptions, methods, and conclusions in a way that will best facilitate the NRC's review to support the licensing timeline for Watts Bar Unit 2This methodology will also more clearly demonstrate the margins in PMF for WBN and SQN.*Simulated Open Channel Hydraulics (SOCH) is not an industryrecognizedstandardtool.7industry recognized standard tool.Mauldin TVA's Approach*Two major elements of converting TVA to modern industry techniques:1)TVA has developed, in its RO Dam Safety program, a procedure for evaluating dams that )pypgpgis consistent with other federal dam safety agencies2)TVA will convert to the United States Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) tool for performing hydraulic analysis.*The new TVA analytical approach is to perform hydraulic analysis using HEC-RAS. UsingthedevelopednuclearPMFelevationsTVAwillevaluatestabilityofupstreamdamsUsing the developed nuclear PMF elevations, TVA will evaluate stability of upstream dams credited in the current licensing basis using TVA RO procedures. Any dam not meeting the requirements of TVA's current procedures based on nuclear PMF inputswillnotbecreditedinitshydraulicPMFanalysesinputs will not be credited in its hydraulic PMF analyses HEC-RAS will be re-run crediting only those dams meeting current TVA RO dam stability requirements 8Mauldin TVA's Approach*The basis for TVA to demonstrate dam safety under PMF conditions will be the combination of upgraded dam stability criteria and HEC-RAS crediting only fully compliant dams:dams:Current approach will require completing in-process modifications and additional modifications. Design and implementation of the new modifications is underwayEvaluate warning times to ensure the updated PMF analysis continues to meet design basisgpygEvaluate the seismic events associated with flooding to ensure updated PMF analysis continuestomeetdesignbasisCurrentseismicdesignbasisisnotrevisedaspartofthiscontinues to meet design basis. Current seismic design basis is not revised as part of this updated submittal9Mauldin TVA's Approach*Submittal (Approach and Actions):TVA proposes a submittal that moves to a full HEC-RAS model, and requests NRC staff's review of that model. Stability assumptions of model:oStability for PMF to modern industry standards analysis complete for initial flood numbers (will require validation after HEC-RAS model runs are completed)o11 dams are credited in the evaluation -Cherokee, Douglas (main/concrete dam), Fontana, Hiwassee, Norris, South Holston, Watauga, Nottely, Chatuge, Blue Ridge, Chickamauga10Chickamauga Mauldin TVA's Approach*Submittal (Approach and Actions) continued:o4 dams are not credited in the analysis -Boone, Fort Patrick Henry, Apalachia, Melton HilloTVA will upgrade 4 reduced margin dams to dam stability standards*Additional dam modifications will be performed -Watts Bar and Tellico pnon-overflow necks, Fort Loudoun non-overflow, Watts Bar west saddle dam, Watts Bar east flood wall, Douglas saddle damDam stability calculations will be available for audit consistent with TVA's other upgraded calculations (Cherokee and Douglas).11Mauldin TVA's Approach*Submittal (Approach and Actions) -continued:Assumptions of HEC-RAS modelV&V model -main stream and tributariesInput assumptions for main river consistent with SOCH modelTributary reservoir routingAssumptions consistent with July 2012 submittalMain Stem GeometryStorm DataRunoffdata12Runoff dataMethodology for discharge ratingMauldin TVA's Approach*Basis (Why Approach Works):Removes any question about the stability of dams credited in the PMF analysisMove to a full HEC-RAS model provides more consistent and reliable results, which will not resultinmodificationstotheplantresult in modifications to the plant.HEC-RAS tool provides a robust analysis using current industry acknowledged methodologiesFacilitatestimelyNRCreviewusinganalysismethodologyfamiliartothestaffFacilitates timely NRC review using analysis methodology familiar to the staffTool is consistent with TVA analysis for its Fukushima submittals*Result:PMF level is bounded by July 2012 License Amendment Request submittal level13Mauldin Schedule*Complete initial evaluation of affected dams -Completepp*Commence Independent Evaluation of reduced margin dams -Completepgp*Complete development and implementation of modifications to reduced margin dams -Prior to Fuel Load*CompletedevelopmentofWBNUFSARAmendmentSeptember302014*Complete development of WBN UFSAR Amendment -September 30, 2014 14Mauldin Conclusion*NRC approval of TVA's proposal to transition to a HEC-RAS based model for evaluating dam stability will maintain the 2012 proposed design basis flood levels and facilitatethelicensingofWattsBarUnit2facilitate the licensing of Watts Bar Unit 2.15Shea Questions