ML12226A561

Application to Revise Sequoyah Nuclear Plant Units I and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (SQN-TS-12-02)

ML12226A561
Person / Time
Site:	Sequoyah
Issue date:	08/10/2012
From:	Shea J W Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
TAC ME8200
Download: ML12226A561 (42)

Text

Attachment 4 to Enclosure I is to be withheld from public disclosure under 10 CFR 2.390.When separated from this attachment, this letter is decontrolled.Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402August 10, 201210 CFR 50.9010 CFR 2.390(d)(1)ATTN: Document Control DeskU.S. Nuclear Regulatory CommissionWashington, D.C. 20555-0001Sequoyah Nuclear Plant, Units 1 and 2NRC Docket Nos. 50-327 and 50-328Facility License Nos. DPR-77 and DPR-79Subject: Application to Revise Sequoyah Nuclear Plant Units I and 2 Updated FinalSafety Analysis Report Regarding Changes to Hydrologic Analysis,(SQN-TS-1 2-02)References: 1. Tennessee Valley Authority (TVA) Submittal to NRC Document ControlDesk, "Application to Revise Watts Bar Nuclear Plant Unit 1 Updated FinalSafety Analysis Report Regarding Changes to Hydrologic Analysis, TACNo. ME8200 (WBN-UFSAR-12-01)," dated July 19, 20122. Tennessee Valley Authority (TVA) Submittal to NRC Document ControlDesk, "Commitments Related to Updated Hydrologic Analysis Results forSequoyah Nuclear Plant, Units 1 and 2, and Watts Bar Nuclear Plant,Unit 1," dated June 13, 2012 (ADAMS Accession No. ML12171A053)3. NRC Meeting Summary, "Summary of March 29, 2012, Pre-ApplicationMeeting with Tennessee Valley Authority on Changing the Licensing Basisfor Hydrologic Engineering (TAC No. ME8200)," dated April 11, 2012(ADAMS Accession No. ML12097A306)In accordance with the provisions of 10 CFR 50.90, "Application for amendment of license,construction permit, or early site permit," the Tennessee Valley Authority (TVA) is submitting arequest for an amendment to Facility Operating License Nos. DPR-77 and DPR-79 forSequoyah Nuclear Plant (SQN), Units 1 and 2.This license amendment request seeks approval to revise the SQN Units 1 and 2 UpdatedFinal Safety Analysis Report (UFSAR) to adopt a revised hydrologic analysis for theSQN Units 1 and 2 site. These changes to the SQN Units 1 and 2 UFSAR are proposed to beconsistent with the latest approved calculations. The proposed technical changes to theSQN Units 1 and 2 UFSAR include changes, in part, currently under review by the NRC in theWatts Bar Nuclear Plant (WBN) Unit 1 UFSAR submitted in the Reference 1 letter. ."Printed on recycled paper U.S. Nuclear Regulatory CommissionPage 2August 10, 2012The proposed changes in the updated hydrologic analysis include updated input information,and updates to methodology which includes use of the U.S. Army Corps of Engineers(USACE) Hydrologic Modeling System (HEC-HMS) and River Analysis System (HEC-RAS)software. As a result of these proposed changes, the design basis flood (DBF) elevations atthe SQN Units 1 and 2 site are revised.These changes are determined to impact existing flooding protection requirements for severalSQN Units 1 and 2 safety-related systems, structures, or components, which include the SpentFuel Pit Cooling Pump Motors and applicable equipment required for flood mode operationlocated in the Diesel Generator Building. To restore margin for the Spent Fuel Pit CoolingPump Motors, the Spent Fuel Pit Cooling Pump Enclosure caps are required to be in place inthe event of a Stage I flood warning as a compensatory measure. For the Diesel GeneratorBuilding, staged sandbags to be constructed into a berm at any time prior to or during theevent of a Stage I flood warning has been established as a compensatory measure.As committed to in the Reference 2 letter, TVA will implement a documentation change torequire the Spent Fuel Pit Cooling Pump Enclosure caps as a permanent plant feature forflooding protection, and will install permanent plant modifications to provide adequate floodingprotection with respect to the DBF level for the Diesel Generator Building, by March 31, 2013.Enclosure 1 provides a description, technical evaluation, regulatory evaluation andenvironmental consideration of the proposed technical changes. TVA is requesting NRCreview and approval of the technical changes to the SQN Units 1 and 2 UFSAR as describedin Enclosure 1 to incorporate the cumulative effects that have occurred in the SQN Units 1and 2 hydrologic analyses since issuance of the Operating License. Specific technicalchanges are proposed in SQN Units 1 and 2 UFSAR Sections 2.4, 2.4.1, 2.4.2, 2.4.3, 2.4.4,2.4.11, 2.4.14, and Appendix 2.4A. Additional editorial changes are shown for SQN Units 1and 2 UFSAR Section 2.4 and do not require NRC review and approval.Attachment 1 to Enclosure 1 provides the existing SQN Units 1 and 2 UFSAR text marked upto show the proposed changes.Attachment 2 to Enclosure 1 provides the proposed replacement SQN Units 1 and 2 UFSARSection 2.4 tables.Attachment 3 to Enclosure 1 provides the proposed replacement SQN Units 1 and 2 UFSARSection 2.4 figures suitable for public disclosure.Attachment 4 to Enclosure 1 provides the proposed replacement SQN Units 1 and 2 UFSARSection 2.4 figures that contain security-related information identified by the designation"Security-Related Information -Withhold Under 10 CFR 2.390." TVA hereby requests thisinformation be withheld from public disclosure in accordance with the provisions of10 CFR 2.390(d)(1).Enclosure 2 provides responses to the issues identified by the NRC Staff in the Reference 3letter regarding the pre-application meeting held between TVA and the NRC Staff on March29, 2012. This meeting involved the License Amendment Request that was under U.S. Nuclear Regulatory CommissionPage 3August 10, 2012development for proposed changes to the WBN Unit 1 UFSAR submitted in the Reference 1letter. The description, technical evaluation, regulatory evaluation and environmentalconsideration of the proposed technical changes provided in Enclosure 1 address these issueswhere applicable to SQN Units 1 and 2.TVA requests that the NRC approve this amendment by August 10, 2013.TVA has determined that there are no significant hazards considerations associated with theproposed changes and that the changes qualify for a categorical exclusion from environmentalreview pursuant to the provisions of 10 CFR 51.22(c)(9).The SQN Plant Operations Review Committee and the SQN Nuclear Safety Review Boardhave reviewed the proposed changes and determined that operation of SQN in accordancewith the proposed changes will not endanger the health and safety of the public.Additionally, in accordance with 10 CFR 50.91 (b)(1), TVA is sending a copy of this letter andthe enclosure to the Tennessee Department of Environment and Conservation.There are no new regulatory commitments in this letter. Please address any questionsregarding this request to Terry Cribbe at 423-751-3850.I declare under penalty of perjury that the foregoing is true and correct. Executed on this10th day of August 2012.Respe ly,J.W StieaVic Ptesident, Nuclear LicensingEn losures:1. Evaluation of Proposed Changes2. Evaluation of Issues from Pre-Application Meetingcc (Enclosures):NRC Regional Administrator -Region IINRC Senior Resident Inspector -Sequoyah Nuclear Plant, Units 1 and 2Director, Division of Radiological Health -Tennessee State Department of Environmentand Conservation ENCLOSURE IEVALUATION OF PROPOSED CHANGESTENNESSEE VALLEY AUTHORITYSEQUOYAH NUCLEAR PLANT, UNITS I AND 2Subject: Application to Revise Sequoyah Nuclear Plant (SQN) Units I and 2 UpdatedFinal Safety Analysis Report Regarding Changes to Hydrologic Analysis(SQN-TS-1 2-02)1.0 SUMMARY DESCRIPTION2.0 DETAILED DESCRIPTION2.1 Proposed Changes2.2 Need for Proposed Changes3.0 TECHNICAL EVALUATION3.1 Evaluation3.2 Uncertainties3.3 Margins3.4 Conclusions4.0 REGULATORY EVALUATION4.1 Applicable Regulatory Requirements and Criteria4.2 Precedent4.3 Significant Hazards Consideration4.4 Conclusions5.0 ENVIRONMENTAL CONSIDERATIONATTACHMENTS1. Proposed SQN Units 1 and 2 UFSAR Text Changes (Markups)2. Proposed SQN Units 1 and 2 UFSAR Tables3. Proposed SQN Units 1 and 2 UFSAR Figures (Public)4. Proposed SQN Units 1 and 2 UFSAR Figures (Non-Public)Page 1 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGES1.0 SUMMARY DESCRIPTIONThe probable maximum flood (PMF) for SQN Units 1 and 2 at the time of Operating Licenseissuance was elevation 722.6 ft, and included assumptions based on the existing understandingof dam structural stability and capability during seismic and extreme flood events in the 1970's.In the 1980's and 1990's, TVA implemented a Dam Safety Program (DSP) that resulted in damsafety modifications that increased dam structural stability and capability Between 1995 and1998, TVA completed a hydrologic reanalysis to credit the results of the dam safetymodifications that had been completed. This reanalysis resulted in lowering the SQN Units 1and 2 calculated probable maximum flood (PMF) to elevation 719.6 ft, although no physicalchanges to SQN Units 1 and 2 site flooding protection features were implemented as a result ofthe decreased design basis flood (DBF) elevations.On October 30, 2007, WVA submitted an application for a combined operating license (COLA)for the proposed Bellefonte Nuclear Plant (BLN) Units 3 and 4, in accordance with 10 CFR 52.During review of the BLN Units 3 and 4 Final Safety Analysis Report (FSAR), the NRCperformed an audit of the hydrologic analysis which resulted in the issuance of three Notice ofViolations (NOVs) on March 19, 2008 (Reference: NRC Letter to TVA, Bellefonte CombinedLicense Application -Nuclear Regulatory Commission Inspection of the Implementation of theQuality Assurance Program Governing the Simulated Open Channel Hydraulics Model -Inspection Report Numbers 05200014/2008--01 and 05200015/2008-001 and Notice ofViolation, Accession No. ML080640487). In response to these NOVs, TVA completed a revisedhydrologic analysis to support the BLN Units 3 and 4 COLA.As a result of the revised BLN Units 3 and 4 hydrologic analysis, the hydrologic analysis forSQN Units 1 and 2 was revised to incorporate updated input information, and updates tomethodology which includes use of the U.S. Army Corps of Engineers (USACE) HydrologicModeling System (HEC-HMS) and River Analysis System (HEC-RAS) software. OnMarch 1, 2010, TVA submitted Licensee Event Report 327 and 328/2009-009, "UnanalyzedCondition Affecting Probable Maximum Flood Level," Revision 0 (ML100610673) followed onApril 14, 2010 with Licensee Event Report 327 and 328/2009-009, "Unanalyzed ConditionAffecting Probable Maximum Flood Level," Revision 1 (ML101090017), providing detailsconcerning an unanalyzed condition affecting the PMF level for the plant. As stated in thesesubmittals, on December 30, 2009, the issuance of an updated calculation titled "PMFDetermination for Tennessee River Watershed" increased the SQN design basis PMF level fromelevation 719.6 ft to elevation 722.0 ft. This increase in calculated PMF elevation resulted fromseveral calculational changes including updated dam rating curves using model data andchanges in reservoir operating policy. A previous change had decreased the SQN PMF levelfrom elevation 722.6 ft to elevation 719.6 ft. However, SQN remains designed for a PMF levelof elevation 722.6 ft with the current exception of the applicable equipment required for floodmode operation located in the Diesel Generator Building and the Spent Fuel Pit Cooling PumpMotors. Because of the unanalyzed condition, the potential existed for SQN to exceed its DBFdesign basis and adversely affect plant safety. The affected calculation and supportingcalculations have since been updated as a result of other necessary changes, and the currentPMF level is elevation 722.0 ft.The update of the hydrologic analysis for SQN Units 1 and 2 includes, but is not limited to,changes to the description of the current hydrosphere, use of more recent flood historyinformation, changes to the inputs used for determining probable maximum precipitation (PMP)and resulting PMF and DBF elevations at the plant site including changes to the runoff andPage 2 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESstream course model, changes to the determination of seismically induced dam failure floodimpacts at the plant site, changes to the analysis for determining that adequate water isavailable for operation of SQN Units 1 and 2, and updates to flooding protection requirements.The update to the runoff and stream course model includes updated discharge rating curves toaddress recently identified rim leaks for Fort Loudoun Reservoir, Watts Bar Reservoir, andNickajack Reservoir which result in bypass flow around the respective dams. As a result of theissues and updates associated with the SQN Units 1 and 2 hydrologic analysis, the PMFelevation at the SQN site is increased from elevation 719.6 ft to 722.0 ft, and the resulting DBFelevations affecting the safety-related SQN Units 1 and 2 systems, structures, and components(SSCs) are increased. Most of the SSCs that are required to be protected from a flood are notimpacted by the increased DBF elevations, because either margin remains between the DBFelevations and the elevation of the SSCs, or the existing flooding protection measures are stilleffective. However, there are exceptions that require compensatory measures to ensureadequate flooding protection in the interim, with documentation changes and permanent plantmodifications planned to restore or gain additional margin between the revised DBF elevationsand limiting safety-related SSCs.TVA is requesting NRC review and approval of the technical changes to the SQN Units 1and 2 UFSAR described in this enclosure to address the cumulative effects that have occurredin the SQN Units 1 and 2 hydrologic analysis since issuance of the Operating License. Specifictechnical changes are proposed in SQN Units 1 and 2 UFSAR Sections 2.4, 2.4.1, 2.4.2, 2.4.3,2.4.4, 2.4.11, 2.4.14, and Appendix 2.4A. Additional editorial changes are shown forSQN Units 1 and 2 UFSAR Section 2.4, and do not require NRC review and approval.Attachment 1 to this enclosure provides the existing SQN Units 1 and 2 UFSAR text pagesmarked up to show the proposed changes. Attachment 2 provides the proposed replacementSQN Units 1 and 2 UFSAR Section 2.4 tables. Attachment 3 provides the proposedreplacement SQN Units 1 and 2 UFSAR Section 2.4 figures (public version). Attachment 4provides the proposed replacement SQN Units 1 and 2 UFSAR Section 2.4 figures (non-publicversion).2.0 DETAILED DESCRIPTION2.1 Proposed ChangesSection 2.4, Hydrological EngineeringSeveral technical changes are proposed to be added for SQN Units 1 and 2 UFSAR Section 2.4to reflect the changes described further in the remaining sections. These include the following:" Adding a discussion of the maximum flood elevation that would result from an occurrence ofthe probable maximum storm, which is revised from elevation 719.6 ft to elevation 722.0 ft.* Adding information regarding coincident wind wave activity that results in wind waves of4.2 ft high (trough to crest)." Adding information regarding run up on the 4:1 and 15:1 slopes approaching the DieselGenerator Building reaching elevation 723.2 ft, and wind wave run up on the walls of theAuxiliary, Control and Shield Buildings reaching elevation 726.2 ft.Page 3 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESAdditional editorial changes are shown in the mark-ups provided in Attachment 1.Section 2.4.1, Hydrological DescriptionThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.1 toupdate the hydrological description to reflect the most current information available regardingoperations of the WVA system of reservoirs and dams that affect the hydrologic analysis forSQN Units 1 and 2. Additional editorial changes are shown in the mark-Ups provided inAttachment 1.HydrosphereAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.1.2, there are 17 major reservoirs inthe TVA system upstream from the plant, 13 of which have substantial reserved flood detentioncapacity during the main flood season. This change proposes to update the descriptions ofthese reservoirs with updated detention areas and capacities. The flood detention capacityreserved in the TVA system varies seasonally, with the greatest amounts during the Januarythrough March flood season. SQN Units 1 and 2 UFSAR Figures 2.4.1-4 (16 Sheets) are theoriginal reservoir seasonal operating guides for the reservoirs above the plant site. This changeprovides updated figures showing the current reservoir seasonal operating guides.Section 2.4.2, FloodsThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.2 toupdate the discussion of floods to reflect the most current information available regardinghistorical floods that affect the hydrologic analysis for SQN Units 1 and 2. Additional editorialchanges are shown in the mark-ups provided in Attachment 1.Flood HistoryAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.2.1, flood records for the period1952 to date can be considered representative of prevailing conditions for the Tennessee RiverValley watershed under existing TVA river operations procedures. This change proposes toupdate the highest flow at Chickamauga Dam tailwater located downstream of the SQN Units 1and 2 site to add additional historical flood events that have occurred since issuance of theoriginal SQN Units 1 and 2 Operating License and corrects historical flood data. This historicalinformation is used to calibrate the hydrologic models used for the hydrologic analysis.Flood Desiqn ConsiderationsAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.2.2, the maximum PMF plant siteflood level was elevation 719.6 ft, with wind waves of 4.2 ft high (trough to crest) predicted. Thischange proposes to increase the PMF level to elevation 722.0 ft with no change to thecoincident wind wave activity that results in wind waves of 4.2 ft high (trough to crest).As described in SQN Units 1 and 2 UFSAR Subsection 2.4.2.2, wind wave run up during thePMF at the Diesel Generator Building was postulated to reach elevation 721.8 ft which is 0.2 ftbelow the operating floor elevation of 722.0 ft. The change proposed increases the DBFincluding wind wave run up during the PMF to elevation 723.2 ft, which is 1.2 ft above the DieselGenerator Building operating floor elevation of 722.0 ft.Page 4 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.2.2, the design basis external floodlevel for the Auxiliary, Control and Shield Buildings was 723.8 ft, and the DBF surge level insidethe buildings was elevation 721.1 ft. The change proposed increases the design basis externalflood level for the Auxiliary, Control and Shield Buildings to elevation 726.2 ft, and the DBFsurge level inside the buildings to elevation 722.5 ft.Section 2.4.3, Probable Maximum Flood (PMF) on Streams and RiversThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.3 toupdate the discussion of PMF on streams and rivers to reflect the most current informationavailable as inputs, and to use updated methodologies such as the USACE HEC-HMS andUSACE HEC-RAS software for elements of the hydrologic analysis for determining the PMF forstreams and rivers for SQN Units 1 and 2. Additional editorial changes are shown in themark-ups provided in Attachment 1.PMF on Streams and RiversAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3, the PMF was determined fromPMP for the watershed above the plant with consideration given to seasonal and arealvariations in rainfall. This change proposes revisions to inputs and use of differentmethodologies for determining PMF as further described below. As a result of these changes,the PMF elevation at the plant is increased from elevation 719.6 ft to 722.0 ft, excluding windwave effects.Probable Maximum Precipitation (PMP)Also as described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.1, the PMF was determinedfrom PMP for the watershed above the plant with consideration given to seasonal and arealvariations. Two basic storm situations were found to have the potential to produce a maximumflood at Sequoyah Nuclear Plant. These are (1) a sequence of March storms producingmaximum rainfall on the 21,400-square-mile watershed above Chattanooga, hereafter called the21,400-square-mile storm, and (2) a sequence of March storms centered and producingmaximum rains in the basin to the west of the Appalachian Divide and above Chattanooga,hereafter called the 7,980-square-mile storm. In the most recent analysis prior to this proposedchange, a PMF of elevation 719.6 ft was produced by the 21,400 square-mile, with the7,980 square-mile storm producing a PMF of elevation 718.9 ft. In the proposed change, a PMFelevation 722.0 ft is produced by the 21,400 square-mile storm, which remains the controllingPMP event. This change is due to the various input changes to the hydrologic analysis, andupdates to methodology.Precipitation LossesAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.2, a multi-variable relationship,used in the day-to-day river operations of the TVA system, has been applied to determineprecipitation excess directly. The relationships were developed from observed data. Theyrelate precipitation excess to the rainfall, week of the year, geographic location, and antecedentprecipitation index (API). For the original study, a median API, as determined from pastrecords, was used at the start of the antecedent storm. This change proposes revising thePage 5 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESinputs for defining API using an 11-year period of historical rainfall records (1997-2007) at thestart of the antecedent storm.Runoff and Stream Course ModelAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.3, the original runoff model used todetermine Tennessee River flood hydrographs at SQN was divided into 45 unit areas andincluded the total watershed above Chickamauga Dam downstream. Unit hydrographs areused to compute flows from the unit areas. The unit area flows are combined with appropriatetime sequencing or channel routing procedures to compute inflows into the most upstreamreservoirs which in turn are routed through the reservoirs using standard techniques. Resultingoutflows are combined with additional local inflows and carried downstream using appropriatetime sequencing or routing procedures including unsteady flow routing. This change proposesrevising the runoff model to use 40 unit areas as a result of removing additional detail inmodeling the subbasins on the Clinch and Holston Rivers that was not needed. The additionalunit areas are not required, and validation of the runoff model is more efficient by eliminatingthese unnecessary modeling details. The results remain essentially the same due to thecalibration of the model to the large flood events.TVA developed the Simulated Open Channel Hydraulics (SOCH) model for flood routingcalculations for the Tennessee River and selected tributaries. The SOCH computer model isthe hydraulic model used to determine flood elevations at each TVA operating nuclear plant site.The SOCH model has been calibrated for main stem reservoirs, and Melton Hill and Tellicotributary reservoirs, to reasonably replicate observed river discharges and elevations for knownhistoric events. Once calibrated, the SOCH model can be used to reliably predict floodelevations and discharges for events of other magnitudes. Additional details including specificchanges to the SOCH model analysis are described below.Tributary reservoir routings: In the original routing model, the Goodrich semigraphical methodand flat pool storage conditions (except Tellico) were used. In the proposed change, the MeltonHill routing is revised to adopt unsteady flow for better refinement for dam seismic failure casesas further described in the next discussion. The Goodrich semigraphical method described inthe SQN Units 1 and 2 UFSAR is the same as standard reservoir routing described in theproposed SQN Units 1 and 2 UFSAR text.'Discharge rating curves: In the original hydrologic analysis, initial dam rating curves weredeveloped based on the existing geographical information available. In the proposed change,temporary flood barriers have been installed on the earthen embankments of four dams(Cherokee, Fort Loudoun, Tellico, and Watts Bar Reservoirs) to increase the height ofembankments and are included in the discharge rating curves for these four dams. Increasingthe height of embankments at these four dams prevents embankment overflow and failure of theembankment. The vendor supplied temporary flood barriers were shown to be stable for themost severe PMF headwater/tailwater conditions using vendor recommended base frictionvalues. These temporary flood barriers are discussed in greater detail in the discussions forproposed changes to SQN Units 1 and 2 UFSAR Subsection 2.4.3.4 below. Also, there areadditional rim leaks for Fort Loudoun Reservoir, Watts Bar Reservoir, and Nickajack Reservoiridentified through use of the latest available geographical information system (GIS) information.These rim leaks result in bypass flow around the respective dams that are addressed in updateddischarge rating curves. A single postulated Fort Loudoun Reservoir rim leak north of theMarina Saddle Dam was added as an additional discharge component for the Fort LoudounPage 6 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESDam which discharges into the Tennessee River at Tennessee River Mile (TRM) 602.3. ForWatts Bar Dam, flow is considered through seven Watts Bar Reservoir rim leaks. Three of therim leak locations discharge to Yellow Creek, entering the Tennessee River three milesdownstream of Watts Bar Dam. The remaining four rim leak locations discharge to WattsCreek, which enters Chickamauga Reservoir just below Watts Bar Dam. A single postulatedNickajack Reservoir rim leak just northeast of the dam is discharged at Nickajack Damtailwaters. The changes are made to update and refine the model.Unsteady flow model: In the original routing model, the main river and Tellico were modeledwith unsteady flow techniques, with calibration of the unsteady flow model performed using thesteady flow profiles from the USACE HEC-2 backwater computer code. In the proposedchange, the main river reservoirs, Tellico, and Melton Hill are modeled with unsteady flowtechniques and calibrated using profiles computed from the more recent USACE HEC-RAScomputer code. The change is made to adopt the most recent computer code.Unsteady flow model -Fort Loudoun Reservoir specific discussion: In the original routing modelfor Fort Loudoun Reservoir, there were 24 cross-sections, verified at three gauged points using1963 and 1973 flood data, and five cross-sections used for the Fort Loudoun and Tellico canalphysically connecting the reservoirs. In the proposed change, there are 33 cross-sections withadditional sections interpolated between each for a total of 59 cross-sections. The FortLoudoun and Tellico canal was modeled using nine cross-sections with an averagecross-section spacing of about 0.18 mile. The Fort Loudoun unsteady flow model was verifiedusing the March 1973 flood data. Tellico Dam was not closed until 1979, and thus was not inplace during the March 1973 flood for verification. The unsteady flow model for the FortLoudoun-Tellico complex, which includes both reservoirs and the Fort Loudoun and Tellicocanal, is verified using the May 2003 flood data. The Tellico reservoir SOCH model is also usedto replicate the Federal Emergency Management Agency (FEMA) published 100-year and500-year flood profiles as an additional calibration step.Unsteady flow model -Cherokee and Douglas Dams specific discussion: In the original routingmodel, the model extended up to Douglas Dam and Cherokee Dam on the French Broad Riverand Holston River, respectively, with the models verified at one gauged point each using the1963 and 1973 flood data. In the proposed change, French Broad River is modeled using 33cross-sections with additional sections interpolated between the original cross-sections for atotal of 49 cross-sections, and the Holston River is modeled using 29 cross-sections withadditional sections interpolated between the original cross-sections for a total of 53cross-sections. The French Broad River and Holston River models are verified at two gaugedpoints each using the March 1973 flood and at one point each using the May 2003 flood. Themodels are also verified by replicating the FEMA published 500-year flood profiles.Unsteady flow model -Little Tennessee River specific discussion: In the original routing model,the Little Tennessee River was modeled from Tellico Dam, mile 0.3, through Tellico Reservoir toChilhowee Dam at mile 33.6 and upstream to Fontana Dam at mile 61.0. The model for TellicoReservoir to Chilhowee Dam was tested for adequacy by comparing its results with steady-stateprofiles at 1,000,000 and 2,000,000 cfs computed by the standard-step method. Minordecreases in conveyance in the unsteady flow model yielded good agreement. 'The averageconveyance correction found necessary in the reach below Chilhowee Dam to make theunsteady flow model agree with the standard-step method was also used in the river reach fromChilhowee to Fontana Dam. In the proposed change, the Little Tennessee River was modeledfrom Tellico Dam, Little Tennessee River mile (LTRM) 0.3 to Chilhowee Dam at LTRM. 33.6.Page 7 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESThe Little Tennessee River from Tellico Dam to Chilhowee Dam at LTRM 33.6 is described by23 cross-sections with additional sections being interpolated between the original sections for atotal of 49 cross-sections in the SOCH model, with a variable cross-section spacing of up toabout 1.8 miles.Unsteady flow model -Watts Bar reservoir specific discussion: In the original routing model,34 reaches were used. In the proposed change, 39 cross-sections with two additionalcross-sections in the upper reach (a total of 41 cross-sections) are used with a variablecross-section spacing of up to about 2.8 miles. The model also includes a junction with theClinch River up to Melton Hill Dam with one additional cross-section being interpolated betweeneach of the original 13 cross-sections.Unsteady flow model -Junction at Tennessee River mile 601.1 to Tellico Dam at LittleTennessee River mile 0.3: This short segment of stream was not considered in the originalanalysis. In the proposed change, five cross-sections with spacing of 0.08 miles are added.The change is made to refine the model.Unsteady flow model -Chickamauga reservoir specific discussion: In the original routing model,28 reaches were used and verified 'at four gauged points using 1973 flood data. In theproposed change, 29 cross-sections with additional cross-sections interpolated between theoriginal cross-sections for a total of 53 are used. The model includes a junction with the DallasBay arm and the Hiwassee River arm. The model is verified using both the March 1973 andMay 2003 flood data. The change is made to refine the model and add unsteady flow modelingfor Dallas Bay and Hiwassee River arms.Model calibration: In the original analysis, the TVA standard-step backwater program orUSACE HEC-2 software for river hydraulics (both solve the same equations, although notspecified in the SQN Units 1 and 2 UFSAR) was calibrated using the March 1963 and March1973 flood data. This model was then used to compute steady state profiles for flows up to1,500,000 cfs. In the proposed change, steady-state profiles are computed using the USACEHEC-RAS software, using March 1973 and May 2003 flood data for verification.Reservoir Operating Guides: In the original routing model, normal operating procedures at thetime were used during the antecedent storm, including turbine and sluice discharge intributaries. Turbine discharges were not used in main river reservoirs after large flood flowsdevelop, because the head differentials were considered to be too small. Normal operatingprocedures were also used during the main storm except for turbine discharge for tributariesand main river dams. Gates were considered operable without failure. In the proposed change,turbine discharges are included in the analysis (main river and tributaries) until head differentialsare too small or the respective powerhouse is flooded. Gates remain operable without failure.Previously, the point where turbine discharge was eliminated was an assumption and was not acalculated value. In the updated analysis, these points are determined using actual elevationdata.Median initial reservoir elevations: In the original routing model, median initial reservoirelevations were used at the start of the storm sequence. While not specifically stated in theoriginal analysis, the initial median reservoir levels for the appropriate season were used. In theproposed change, the updated analysis uses the same method to determine the initial medianreservoir levels. However, these median levels are different as a result of changes to thereservoir operating guidelines.Page 8 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESTemporary flood barriers: In the routing model described in the SQN Units 1 and 2 UFSAR,temporary flood barriers were not used and the earth embankments at the main river dams werenot overtopped as a result of dam safety modifications that have been implemented sinceoriginal licensing of SQN Units 1 and 2. In the proposed change, the height of embankmentsare physically increased using temporary flood barriers to prevent earth embankmentovertopping at Cherokee, Fort Loudoun, Tellico and Watts Bar Reservoirs. While the floodbarriers are "temporary structures," there is a structural analysis for the headwater loadingbehind the temporary flood barriers that verifies that failure of the barriers themselves would notoccur. Experience data on the use of the selected temporary flood barriers during historicfloods and the vendor documentation on barrier testing were evaluated prior to selection anduse. Additionally, although not credited in the seismically induced dam failure analyses, aseismic evaluation completed on the flood barriers (without headwater behind the barriers)verifies that failure of the flood barriers themselves would not occur. A potential exists forrunaway barges to float downstream and impact the temporary flood barriers at two of the fourdams where the barriers are in place. Barges along these reservoirs are typically tied off atbarge terminals or mooring cells during high flow events, such as a PMF event. The mooringfacilities, however, are not designed for PMF elevations and velocities, so barges could breakloose. There is no barge traffic on Cherokee Reservoir, so no potential for impact exists. TheFort Loudoun Reservoir has limited to moderate barge traffic. Using typical barge dimensions, abarge would have to weigh less than 70-80% of full load capacity in order to strike the barriers.However, the earthen embankments of the dam where the temporary flood barriers are placedare located at a distance from the main channel. The stream flow during a high flow event isdirected toward the concrete overflow portion of the dam, and the barges would likely be carriedby the current away from the temporary flood barriers. At the Tellico Reservoir, there is veryinfrequent barge traffic. Conservatively assuming there will be a barge on the reservoir, andusing typical barge dimensions, a barge would have to weigh less than 40-50% of full loadcapacity in order to strike the barriers. However, the earthen embankments of the dam wherethe temporary flood barriers are placed are located at a distance from the main channel. Thestream flow during a high flow event is directed toward the concrete overflow portion of the dam,and the barges would likely be carried by the current away from the temporary flood barriers.There is limited to moderate barge traffic at the Watts Bar Reservoir. An evaluation usingtypical barge dimensions for the Tennessee River, and conservatively assuming barges areempty (less draft allows for the barge to run closer to the top of the dam), demonstrates thatbarges are not likely to impact the temporary flood barriers. A spatial analysis shows that theclosest edge of the temporary flood barrier would have to be at least 9.0 ft away from theupstream edge of the earthen embankment in order to prevent impact. The temporary floodbarriers are located at least this distance from the edge of the earthen embankment, ensuringthat there is no potential for barge impact. In summary, this qualitative evaluation of thepossibility of barge impacts affecting the temporary flood barriers during a PMF event concludesthat due to the physical features of the earthen embankments including width and slope, theexpected direction of flows towards the dam spillway gates during the PMF event, as well as theplacement of the temporary flood barriers at a sufficient distance from the main channel andfrom the shoreline, it is unlikely that a barge would impact any of the barriers. The temporaryflood barriers are not credited in the analysis of seismically induced dam failure combinations,but are credited in the hydrologic analysis for determining the PMF.Page 9 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESProbable Maximum Flood FlowAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.4, the analysis to determine thePMF flow included evaluation of PMP over the total watershed with consideration of criticalseasonal and areal variations. In the most recent analysis prior to this proposed change, thecontrolling PMF discharge was 1,236,000 cfs from the 21,400 square-mile storm in March with adownstream storm pattern. In the proposed change, PMF discharge is 1,331,623 cfs for the21,400 square-mile storm in March with a downstream storm pattern. Discharge differences aredue to input changes. Additional details including specific changes to the PMF flow analysis aredescribed below.Watts Bar and Chickamauga Dams: In the original analysis, the West Saddle Dike at Watts BarDam was considered to be overtopped and breached with the discharge input at Watts BarDam, and the Chickamauga Dam was considered to be overtopped but not postulated to fail. Inthe proposed change, the West Saddle Dike at Watts Bar Dam is overtopped and breached withthe discharge input at the mouth of Yellow Creek, and Chickamauga Dam is overtopped but notpostulated to fail. Therefore, the proposed change also revises the location where thedischarge from Watts Bar West Saddle Dike breach is added back to the river. Also, additionalrim leakage for Watts Bar Reservoir has been addressed in the updated hydrologic analysis. Asa result, rim leakage is input at the mouth of Watts Creek, whose confluence with theTennessee River is at TRM 528.0, or at the mouth of Yellow Creek, whose confluence with theTennessee River is at TRM 526.82. The update is to provide a more realistic modelingconfiguration.Concrete section analysis: In the original analysis, comparisons were included between designheadwater and tailwater levels and those that prevail during the PMF for each dam. Ifoverturning and horizontal forces were not increased by more than 20%, then the structureswere considered safe against failure and were then excluded from further consideration.Because overturning and horizontal forces were increased greater than 20% for Douglas, FortLoudoun and Watts Bar Dams, they were further examined and judged stable. In the proposedchange, factors of safety in sliding are determined by comparison of design headwater andtailwater levels to PMF headwater and tailwater levels for each dam including those previouslyconsidered safe against failure. If the factor of safety is greater than 1.0, then the structures areconsidered safe against failure. Therefore, the proposed change evaluates the possibility offailure for each of the upstream dams instead of just those whose headwater/tailwatercomparison were greater than 20%. This is a more comprehensive evaluation of the dams forthe updated PMF levels.Spillway gates: In the original analysis, there was limited discussion involving the radial spillwaygates at Fort Loudoun and Watts Bar Dams. In the proposed change, the Watts Bar Damspillway gates are described in general, with the yield stress and stress in trunnion pins noted tobe less than allowable design stress. This is a change to add details to the SQN Units 1and 2 UFSAR.Waterborne Objects: In the original analysis, discussion was included for barge (end on)impacts on spillway gates and bents and broadside impacts. This information is revised in theSQN Units 1 and 2 UFSAR to more accurately reflect the engineering judgment used inevaluating potential impacts. A new subsection discussing the potential for barge impacts to thetemporary flood barriers at Cherokee, Fort Loudoun, Tellico, and Watts Bar Reservoirs is alsoadded in UFSAR Section 2.4.3.4 as previously discussed.Page 10 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESLock Gates: In the original analysis, Fort Loudoun, Watts Bar and Chickamauga lock gateswere examined for possible failure. This information is revised in the SQN Units 1and 2 UFSAR to more accurately reflect the evaluation of the lock gate structural elements.Embankment Breaching: In the original analysis, detailed discussion on the methodology usedfor earth embankments breach was included. The first part of the paragraph was since revisedto discuss that the potential for embankment breaching was examined and no breaching wouldoccur except Watts Bar West Saddle Dike which is completely failed. In the proposed change,this discussion is deleted since there is no other embankment breach in the current hydrologicanalysis. This is a change to delete obsolete information.Water Level DeterminationAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.5, elevations from the potentialcontrolling PMF events were evaluated to determine the limiting PMF for the SQN site. In themost recent analysis prior to this proposed change, a PMF of elevation 719.6 ft was producedby the 21,400 square-mile storm, with the 7,980 square-mile storm producing a PMF ofelevation 718.9 ft. In the proposed change, a PMF elevation 722.0 ft is produced by the21,400 square-mile storm, which remains the controlling PMP event. This change is due to thevarious input changes to the hydrologic analysis, and updates to methodology.Coincident Wind Wave ActivityAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.3.6, wind waves are likely when thePMF crests. In the original analysis, wind wave and runup elevations (as well as fetch lengthsto determine those levels) were given for safety-related structures. In the proposed change,updated wind wave and runup elevations are given. Methodology used is the same as theoriginal analysis and inputs are the same except for the updated PMF stillwater elevation.Section 2.4.4, Potential Dam Failures, Seismically InducedThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.4 toupdate the discussion of potential flood levels from seismically induced dam failures to reflectthe most current information available as inputs, and to use updated methodologies such as theUSACE HEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis fordetermining dam failure outflows from tributary dams for SQN Units 1 and 2. Additional editorialchanges are shown in the mark-ups provided in Attachment 1.As described in SQN Units 1 and 2 UFSAR Subsection 2.4.4, procedures described in AppendixA of Regulatory Guide (RG) 1.59 were followed when evaluating potential flood levels fromseismically induced dam failures. Site flooding levels at SQN from potential seismically induceddam failures are determined using the SOCH model, with changes previously discussedregarding the proposed change to the runoff and stream course model.Also as described in SQN Units 1 and 2 UFSAR Subsection 2.4.4, the original discussionincluded general information concerning the TVA inspection and maintenance program. In theproposed change, the TVA DSP, which is consistent with the Federal Guidelines for DamSafety, is described in detail. As part of the TVA DSP, inspection and maintenance activitiesPage 11 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESare carried out on a regular schedule to confirm the dams are maintained in a safe condition.This is a change to add details to the SQN Units 1 and 2 UFSAR.Dam Failure PermutationsAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.4.1, analyses to determine damintegrity during seismic events were performed to determine site flooding levels at SQN frompotential seismically induced dam failures.In the analyses, two basic conditions are used, including:1. Determination of the water level at the plant during one-half the PMF during an operatingbasis earthquake (OBE).2. Determination of the water level at the plant during a 25-year flood during a safe shutdownearthquake (SSE).The OBE and SSE are defined in SQN Units 1 and 2 UFSAR Subsection 2.5.2.4 as havingmaximum horizontal rock acceleration levels of 0.09 g and 0.18 g respectively. The one-halfPMF as used in the analyses is developed by taking half of the rainfall-induced PMF inflowscalculated from the controlling 21,400 square mile March downstream centered design storm,which consists of a three-day antecedent storm, three-day dry period, and a three-day mainstorm. At the start of the antecedent storm, the reservoirs are at the initial median levels usedas inputs to the rainfall-induced PMF analysis. This is consistent with the guidance of RG 1.59,Revision 2.In the most recent analysis prior to this proposed change, only two combinations of potentialseismically induced dam failures were determined to cause a flood elevation above plant gradeelevation. These include the OBE failure of Fontana, Hiwassee, Apalachia, and Blue RidgeDams in the one-half SSE concurrent with a one-half PMF and the SSE failure of Norris,Cherokee and Douglas Dams concurrent with the 25-year flood. In the proposed change, theanalyses are updated to include Tellico Dam failure in the originally controlling combinations asa conservative assumption, and a reduced partial failure of Fontana Dam.In the updated analysis, there are four controlling combinations of potential seismically induceddam failures during one-half the PMF during an OBE, including:* Norris and Tellico Dams;" Fontana and Tellico Dams;* Fontana, Tellico, Hiwassee, Apalachia, and Blue Ridge Dams; and* Cherokee, Douglas, and Tellico Dams.In the updated analysis, there is one controlling combination of potential seismically induceddam failures during a 25-year flood during a SSE, which is failure of the Norris, Cherokee,Douglas, and Tellico Dams.Page 12 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESBased on the updated hydrologic analysis, the peak water surface elevation at SQN is producedby the controlling combination of failure of Cherokee, Douglas, and Tellico Dams during one-halfthe PMF during an OBE at elevation 708.6 ft. The controlling combination for shortest wavetravel time is the Fontana, Tellico, Hiwassee, Apalachia, and Blue Ridge Dams during a one-half PMF during an OBE at 32 hour3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br />sCombinations of seismic induced dam failures have changed due to the input updates and theinclusion of Tellico into the failure scenario. Specific changes include the following:Fontana Dam failure: In the original analysis, partial failure was postulated to occur usingengineering judgment. In the proposed change, partial failure to a higher elevation is postulateddue to modifications of the dam, and additional analysis.Tellico Dam failure: In the original analysis, Tellico Dam was judged to be stable for OBEseismic events. In the proposed change, Tellico Dam is postulated to fail for seismic events dueto lack of supporting analysis. Tellico Dam failure is combined with each seismic failure caseresulting in a bounding case.Seismic Outflow Hydrograph for Norris, Cherokee, Douglas and Fontana Dams: In the originalanalysis, outflow from Norris, Cherokee, Douglas and Fontana Dams was based on a SOCHunsteady flow model developed in sufficient detail to define outflow from postulated dam failure.In the proposed change, outflow from Norris, Cherokee, Douglas, and Fontana Dams is basedon USACE HEC-HMS software model using the same dam failure rating curves for Norris,Cherokee, and Douglas Dams, and a revised dam failure rating curve for Fontana Dam forpartial failure at a higher elevation, with results validated by comparing results with TRBROUTE.Use of USACE HEC-HMS storage routing versus unsteady flow is a. different method, butprovides essentially the same result.Unsteady Flow Analysis of Potential Dam FailuresAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.4.2, unsteady flow routingtechniques were used to evaluate plant site flood levels from postulated seismically induceddam failures wherever their inherent accuracy was needed. In the proposed change, theHEC-HMS storage routing is used to compute the outflow hydrograph from the postulatedfailure of each dam except main river dams. For Tellico Dam, the complete failure is analyzedwith the SOCH model. The failure time and initial reservoir elevations for each dam aredetermined from a pre-failure TRBROUTE analysis. HEC-HMS is used to develop the' postfailure outflow hydrographs based on the previously determined dam failure rating curves. Theoutflow hydrographs are validated by comparing the HEC-HMS results with those generated bysimulations using TRBROUTE. This additional detail for the unsteady flow routing techniques isprovided for completeness.Water Level at Plant SiteAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.4.3, the unsteady flow analyses ofthe postulated combinations of seismic dam failures coincident with floods analyzed yielded amaximum elevation of 707.9 ft excluding wind wave effects. In the proposed change, themaximum elevation excluding wind wave effects is increased to elevation 708.6 ft from the onecontrolling combination of failure of Cherokee, Douglas, and Tellico Dams during one-half thePMF during an OBE. Coincident wind wave activity is required by the guidance in RG 1.59 toPage 13 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESbe addressed in determining whether rainfall induced PMF events or seismically induced damfailure flood events are the bounding event for design of flooding protection features. However,wind wave activity on the order calculated for rainfall induced PMF events if added on top of thelimiting elevation of 708.6 ft for seismically induced dam failure flood events would still result inwater surface elevations several feet below the rainfall induced PMF elevation of 722.0 ftdescribed in UFSAR Section 2.4.3. Therefore, based on this qualitative analysis, the rainfallinduced PMF elevation of 722.0 ft is bounding for SQN Units 1 and 2.Sections 2.4.5 throuqh 2.4.10Editorial changes for SQN Units 1 and 2 UFSAR Sections 2.4.5 through 2.4.10 are shown in themark-ups provided in Attachment 1. In addition, the name of the TVA Water ManagementOrganization is changed to TVA River Operations (RO) to reflect the current organization name.There are no technical changes proposed for these sections.Section 2.4.11, Low Water ConsiderationsThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.11 toupdate the discussion of low water considerations to reflect the most current information for thehydrologic analysis for SQN Units 1 and 2. Additional editorial changes are shown in themark-ups provided in Attachment 1.Low Flow in Rivers and StreamsAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.11.1, analyses are performed todetermine probable minimum water level at SQN Units 1 and 2 and the minimum flowrequirement at the essential raw cooling water (ERCW) intake. In the original analysis, waterlevel at the SQN site upon complete dam failure of the south embankment of ChickamaugaDam resulting in a breach width of five times the dam height from an initial ChickamaugaReservoir pool elevation 682.5 ft (normal summer level) began to drop in one hour and reachedelevation 641.0 ft (minimum river elevation originally assumed for required minimum level in theERCW intake canal) in 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br />. In the proposed change, water level at the SQN site uponcomplete dam failure of the south embankment of Chickamauga Dam resulting in a breachwidth of 400 ft from an initial Chickamauga Reservoir pool elevation of 681.0 ft (slightly belowthe normal summer pool elevation and the normal summer pool elevation in NickajackReservoir) begins to drop in 0.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and reaches elevation 641.0 ft (minimum river elevationfor required minimum level in the ERCW intake canal) in 51 hours5.902778e-4 days <br />0.0142 hours <br />8.43254e-5 weeks <br />1.94055e-5 months <br />. This change includesupdates to the routing model cross-sectional data using new bathymetry and recalibration of themodels. The timing changes are due to these updates to the model. This value is determinedby postulating loss of Chickamauga Dam and no flow from Watts Bar Dam.Historical Low WaterAs described in SQN Units 1 and 2 UFSAR Subsection 2.4.11.3, historical low water records forSQN Units 1 and 2 at the site intakes are provided. In the original analysis, average daily flowsless than 5,000 cfs were determined to occur 0.65% of the time, and average daily flows lessthan 10,000 cfs were determined to occur 5.19% of the time. In the proposed change, averagedaily flows less than 5,000 cfs are determined to occur 0.70% of time, and average daily flowsless than 10,000 cfs are determined to occur 7.30% of the time. This update is based onanalysis of additional years of record.Page 14 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESSections 2.4.12 and 2.4.13Editorial changes for SQN Units 1 and 2 UFSAR Sections 2.4.12 and 2.4.13 are shown in themark-ups provided in Attachment 1. There are no technical changes proposed for thesesections.Section 2.4.14, Floodingq Protection RequirementsThe following technical changes are proposed for SQN Units 1 and 2 UFSAR Section 2.4.14and Appendix 2.4A to update the discussion of flooding protection requirements to reflect themost current information for the hydrologic analysis for SQN Units 1 and 2. The content ofSQN Units 1 and 2 UFSAR Appendix 2.4A as proposed to be revised is relocated toSQN Units 1 and 2 UFSAR Section 2.4.14 to be consistent with NUREG-0800, StandardReview Plan (SRP). Additional editorial changes are shown in the mark-ups provided inAttachment 1.Flooding Protection RequirementsAs described in SQN Units I and 2 UFSAR Appendix 2.4A, Subsection 2.4A. 1, an evaluation isperformed to determine the methods by which the SQN is capable of tolerating floods aboveplant grade without jeopardizing public safety. The DBF is the calculated upper-limit flood thatincludes the PMF plus the wave runup caused by a 45 mph overland wind as discussed inSQN Units 1 and 2 UFSAR Subsection 2.4.3.6. As a result of the changes to inputs andmethodology discussed previously, the DBF elevations at various plant locations that wouldresult for the controlling PMF are increased from those currently provided in the SQN Units 1and 2 UFSAR as follows:Plant Location Current DBF Level (ft.) New DBF Level (ft.)Probable Maximum Flood (still reservoir) 719.6 722.0DBF Runup on Diesel Generator Building 721.8 723.2DBF Runup on vertical external,unprotected walls 723.8 726.2DBF Surge level within flooded structures 720.1 722.5Warninq PlanAs described in SQN Units 1 and 2 UFSAR Appendix 2.4A, Subsection 2.4A.3, plant gradeelevation 705.0 ft can be exceeded by rainfall floods and closely approached by seismicallyinduced dam failure floods. A warning plan is needed to assure plant safety from these floods.In the proposed change, the warning time for SQN has been reevaluated because the initialmedian reservoir levels and flood operational guides have been revised, dam rating curves havechanged at some dams, and the SOCH model of the Tennessee River has been updated tomeet current quality assurance standards. Based on the current hydrologic analysis, thefollowing specific changes are required to the warning plan analysis and results:Rainfall Floods: As described in SQN Units 1 and 2 UFSAR Appendix 2.4A,Subsection 2.4A.3.1, protection of SQN from rainfall floods that might exceed plant gradeutilizes a flood warning issued by TVA's Water Management. TVA's climatic monitoring andflood predicting systems and flood control facilities permit early identification of potentiallyPage 15 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGEScritical flood producing conditions and reliable prediction of floods which may exceed plantgrade well in advance of the event. In the proposed change, the organizational title is changedto TVA River Operations (RO), and the forecasted levels for issuing Stage I and Stage IIwarnings are changed to reflect the updated hydrological basis for the warning plan asdescribed further in SQN Units 1 and 2 UFSAR Subsection 2.4.14.9.4.Seismically-Induced Dam Failure Floods: As described in SQN Units 1 and 2 UFSARAppendix 2.4A, Subsection 2.4A.3.2, only two postulated combinations of seismically induceddam failures and coincident storm conditions were shown to result in a flood which could exceedelevation 705.0 ft at the plant. In the proposed change, four postulated combinations ofseismically induced dam failures are considered to reflect the updated hydrological basis for thewarning plan as described further in SQN Units 1 and 2 UFSAR Subsection 2.4.14.9.4.Basis for Flood Protection Plan in Rainfall FloodsAs described in SQN Units I and 2 UFSAR Appendix 2.4A, Subsection 2.4A.8, largeTennessee River floods can exceed plant grade elevation 705.0 ft at SQN, and plant safety insuch an event requires shutdown procedures which may take 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to implement. TVA floodforecast procedures are used to provide at least 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> of warning before river levels reachelevation 703.0 ft. Use of elevation 703.0 ft, 2.0 ft below plant grade, provides enough marginto prevent wind generated waves from endangering plant safety during the final hours ofshutdown activity. As previously stated, this information as proposed to be revised is relocatedto SQN Units 1 and 2 UFSAR Subsection 2.4.14. Based on the current hydrologic analysis, thefollowing specific changes are required to the warning plan analysis and results for rainfallfloods:Overview: As described in SQN Units 1 and 2 UFSAR Appendix 2.4A, Subsection 2.4A.8, theestimated probability is less than 0.0026 that a Stage I warning could possibly be issued duringthe 40-year life of the plant. In the proposed change, no probability is quoted since there is noregulatory requirement or regulatory guidance to determine the probability.TVA Forecast System: As described in SQN Units 1 and 2 UFSAR Appendix 2.4A,Subsection 2.4A.8, TVA has in constant use an extensive, effective system to forecast flow andelevation as needed in the Tennessee River basin monitored by TVA RO. This permits efficientoperation of the reservoir system and provides warning of when water levels could possiblyexceed critical elevations at selected, sensitive locations. In the original SQN Units 1and 2 UFSAR text, there was an extensive description of process, gage network, and forecastprocedures. In the proposed change, updates are included to reflect current processes, gagenetwork and forecast procedures. These changes reflect refinements which have beenimplemented into the process over time.Basic Analysis: As described in SQN Units 1 and 2 UFSAR Appendix 2.4A, Subsection 2.4A.8,the forecast procedure to assure safe shutdown of SQN for flooding is based upon an analysisof 17 hypothetical PMP storms, including their antecedent storms. In the proposed change, theprocedure is based upon an analysis of nine of the 17 hypothetical storms up to PMP magnitudejudged to be controlling. This change reflects the updated hydrological basis for the warningplan as described further in SQN Units I and 2 UFSAR Subsection 2.4.14.9.4.Hydrologic Basis for Warning System: As described in SQN Units 1 and 2 UFSARAppendix 2.4A, Subsection 2.4A.8, a minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> has been allowed for preparation ofPage 16 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESthe plant for operation in the flood mode. An additional 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> for communication andforecasting computations is provided to translate rain on the ground to river elevations at theplant. Hence, the warning plan provides 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> from arrival of rain on the ground untilelevation 703.0 ft could be reached. The 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> allowed for shutdown at the plant consists ofa minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> of Stage I preparation and an additional 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> for Stage IIpreparation that is not concurrent with the Stage I activity. The threshold river levels at SQN forinitiating Stage I and Stage II preparations differ with the season. In the original analysis, duringthe October 1 -April 15 "winter" season, Stage I threshold elevation was 697.0 ft.Corresponding Stage I threshold river elevation of the April 16 -September 30 "summer"season was elevation 703.0 ft. In the proposed change, during the "winter" season, Stage Ithreshold elevation is 694.5 ft. Corresponding Stage I threshold river elevation for the "summer"season at SQN is elevation 699.0 ft. The specific periods (October 1 -April 15 and April 16 -September 30) were judged to be too rigid and elevations changes are a result of updatedanalysis which change the curve and subsequently change the threshold elevations. Thischange is due to the various input changes to the hydrologic analysis, and as a result of theupdates to methodology.Hydrologic Basis for Threshold Flood Warning Levels: As described in SQN Units 1and 2 UFSAR Appendix 2.4A, Subsection 2.4A.8, predicted threshold flood warning levelswhich assure adequate shutdown times are evaluated. In the original analysis, the procedurefor establishing the threshold levels for SQN shutdown was described with a threshold level inwinter at elevation 697.0 ft (Stage I) and 703.0 ft (Stage II), and summer at elevation 703.0 ft(Stage I) and 703.0 ft (Stage II). In the proposed change, the procedure for establishing thethreshold levels for SQN shutdown is described with a threshold level in winter at elevation694.5 ft (Stage I) and 703.0 ft (Stage II), and in summer at elevation 699.0 ft (Stage I) and703.0 ft (Stage II). This change is due to the various input changes to the hydrologic analysis,and as a result of the updates to methodology.Communications Reliability: As described in SQN Units 1 and 2 UFSAR Appendix 2.4A,Subsection 2.4A.8, communication between projects in the TVA power system is via: (a)TVA-owned microwave network, (b) Fiber-Optics System, and (c) by commercial telephone. Inemergencies, additional communication links are provided by Transmission Power Supply radionetworks. The four networks provide a high level of dependability against emergencies. In theoriginal description of the communications systems, the original technology was described. Inthe proposed change, the description is updated to reflect current technology.Basis for Flood Protection Plan in Seismic-Caused Dam Failures: As described in SQN Units 1and 2 UFSAR Appendix 2.4A, Subsection 2.4A.9, floods resulting from combined seismic andflood events can closely approach plant grade, thus requiring emergency measures. In themost recent analysis previous to this update, only two seismic dam failure combinationscoincident with a flood, i.e., the failure of Fontana, Hiwassee, Apalachia, and Blue Ridge Damsin the one-half SSE (i.e., OBE) concurrent with a one-half PMF and the SSE failure of Norris,Cherokee and Douglas Dams concurrent with the 25-year flood, would result in a floodapproaching plant grade. In the proposed change, plant grade would be exceeded by four offive candidate combinations. This change is due to the various input changes to the hydrologicanalysis, and as a result from the updates to methodology.Page 17 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESSection 2.4 TablesIn support of the technical changes proposed for the SQN Units 1 and 2 UFSAR, and to reflectthe most current information for the hydrologic analysis for SQN Units 1 and 2, the tablesassociated with SQN Units 1 and 2 UFSAR are proposed to be revised as follows:* SQN Units 1 and 2 UFSAR Table 2.4.1-1, Facts About Major TVA Dams and Reservoirs(Historical Information), is revised to incorporate the latest information for the TVA systemdams and reservoirs. The revised table is also renamed to delete the "HistoricalInformation" reference and to delete the word "Major," reformatted using a more recent wordprocessing program, and renumbered as Table 2.4.1-2 to be in the correct sequence for thissection." SQN Units 1 and 2 UFSAR Table 2.4.1-2, Facts About Non-TVA Dams and ReservoirProjects (Historical Information), is renamed to delete the "Historical Information" reference,reformatted using a more recent word processing program, and renumbered asTable 2.4.1-5 to be in the correct sequence for this section. There are no technical changesto the table." SQN Units 1 and 2 UFSAR Table 2.4.1-3, Flood Detention Capacity TVA Projects AboveSequoyah Nuclear Plant Storage Reserved for Flood Control in Acre -Feet, is revised toincorporate the latest information for the TVA system projects above the SQN site. Therevised table is also reformatted using a more recent word processing program, andrenumbered as Table 2.4.1-6 to be in the correct sequence for this section.* A new SQN Units 1 and 2 UFSAR Table 2.4.1-3, TVA Dams -River Mile Distances to SQN,is added." SQN Units 1 and 2 UFSAR Table 2.4.1-4, Public and Industrial Surface Water SuppliesWithdrawn from the 98.6 Mile Reach of the Tennessee River Between Dayton Tennesseeand Meade Corp. Stevenson Ala. (Historical Information), is revised to incorporate the latestinformation for these surface water supplies. The revised table renamed to delete the"Historical Information" reference and is also reformatted using a more recent wordprocessing program, and renumbered as Table 2.4.1-1 to be in the correct sequence for thissection." A new SQN Units 1 and 2 UFSAR Table 2.4.1-4, Facts about TVA Dams AboveChickamauga, is added." SQN Units 1 and 2 UFSAR Table 2.4.1-5, Dam Safety Modification Status (Hydrologic), isdeleted because the information provided is not appropriate information for the SQN Units 1and 2 UFSAR." A new SQN Units 1 and 2 UFSAR Table 2.4.2-1, Peak Streamflow of the Tennessee Riverat Chattanooga, TN (USGS Station 03568000) 1867 -2007, is added., SQN Units 1 and 2 UFSAR Table 2.4.3-1, Probable Maximum Storm Rainfall andPrecipitation Excess, is revised to incorporate the latest information for the determination ofthese inputs to the hydrologic analysis. The revised table is also renamed to replacePage 18 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES"Rainfall" with "Precipitation," reformatted using a more recent word processing program,and renumbered as Table 2.4.3-2 to be in the correct sequence for this section.* A new SQN Units 1 and 2 UFSAR Table 2.4.3-1, Seasonal Variations of Rainfall (PMP), isadded.* SQN Units 1 and 2 UFSAR Table 2.4.3-2, Unit Hydrograph Data, is revised to incorporatethe latest information for the unit hydrograph data used in the hydrologic analysis. Therevised table is also reformatted using a more recent word processing program, andrenumbered as Table 2.4.3-4 to be in the correct sequence for this section." A new SQN Units 1 and 2 UFSAR Table 2.4.3-3, Historical Flood Events, is added.* SQN Units 1 and 2 UFSAR Table 2.4.4-1 Floods from Postulated Seismic Failure ofUpstream Dams (Plant Grade is Elevation 705), is revised to incorporate the latestinformation for the results of the flood analysis. The revised table is also reformatted usinga more recent word processing program.* SQN Units 1 and 2 UFSAR Table 2.4.13-1, Well and Spring Inventory Within 2-mile Radiusof Sequoyah Nuclear Plant Site (Historical Information), is renamed to delete the "HistoricalInformation" reference and replace with "1972 Survey Only," and reformatted using a morerecent word processing program. There are no technical changes to the table.* SQN Units 1 and 2 UFSAR Table 2.4.13-2, Ground Water Supplies Within 20 Mile Radius ofthe Plant Site (Historical Information), is renamed to delete the "Historical Information"reference and replace with "1972 Survey Only," and reformatted using a more recent wordprocessing program. There are no technical changes to the table.* SQN Units 1 and 2 UFSAR Table 2.4A-2, Critical Cases -Seismic Caused Dam FailuresTime Between Seismic Event and Selected Plantsite Flood Elevation, is revised toincorporate the latest information for the results of the flood analysis, is reformatted using amore recent word processing program, and is renumbered as Table 2.4.14-1 to be in thecorrect sequence for this section.Section 2.4 FiguresIn support of the technical changes proposed for the SQN Units 1 and 2 UFSAR, and to reflectthe most current information for the hydrologic analysis for SQN Units 1 and 2, the figuresassociated with SQN Units 1 and 2 UFSAR are proposed to be revised as follows:" SQN Units 1 and 2 UFSAR Figures 2.4.1-1 through 2.4.1-3 are updated to reflect currentinformation, with titles changed and renumbered if necessary to be in proper order. Theinformation provided by the SQN Units 1 and 2 UFSAR figures is not necessary to supportthe updated hydrologic analysis.* New SQN Units 1 and 2 UFSAR Figures 2.4.1-3 and 2.4.1-5 are added to includeinformation of the watershed above SQN including reservoir elevation and storagerelationships used as inputs to the updated hydrologic analysis.Page 19 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES* SQN Units 1 and 2 UFSAR Figure 2.4.2-1 is updated to reflect current information, with titlechanged. The information provided by the figure is not necessary to support the updatedhydrologic analysis.* SQN Units 1 and 2 UFSAR Figures 2.4.3-1 through 2.4.3-3, 2.4.3-5 through 2.4.3-9,2.4.3-11, and 2.4.3-12 are updated to reflect current information, with titles changed andrenumbered if necessary to be in proper order. The information provided by the figuresreflect current TVA RO operating guidelines and revised input assumptions and modelchanges in support of the updated hydrologic analysis." SQN Units 1 and 2 UFSAR Figures 2.4.3-4, 2.4.3-10, and 2.4.3-14 are deleted and notused. The information provided by the figures is not necessary in the updated hydrologicanalysis, and the figures are not referenced in the proposed SQN Units 1 and 2 UFSAR text.* New SQN Units 1 and 2 UFSAR Figures 2.4.3-7 through 2.4.3-11, 2.4.3-13, 2.4.3-14,2.4.3-16, 2.4.3-17, and 2.4.3-20 are added to include information regarding the models usedin the updated hydrologic analysis." SQN Units 1 and 2 UFSAR Figures 2.4.3-13a, 2.4.3-15, 2.4.3-16, and 2.4.3-17 arerenumbered as 2.4.3-22, 2.4.3-24, 2.4.3-25, and 2.4.3-26, respectively, without anytechnical changes.* SQN Units I and 2 UFSAR Figure 2.4.4-1 title is added without any technical changes.* SQN Units 1 and 2 UFSAR Figures 2.4.4-3, 2.4.4-6, 2.4.4-8, 2.4.4-17, 2.4.4-21, 2.4.4-30,and 2.4.4-37 through 2.4.4-39 are deleted and not used. The information provided by thefigures is not necessary in the updated hydrologic analysis, and the figures are notreferenced in the proposed SQN Units 1 and 2 UFSAR text." SQN Units 1 and 2 UFSAR Figures 2.4.4-5, 2.4.4-11, 2.4.4-16, 2.4.4-25, and 2.4.4-27 areupdated to reflect current information, with titles changed and renumbered to be in properorder. The information provided by the figures reflects current analysis of dam andembankment stability in support of the updated hydrologic analysis.* New SQN Units 1 and 2 UFSAR Figures 2.4.4-18 and 2.4.4-20 are added to includeinformation regarding the dam failure permutations and assumptions used in the updatedhydrologic analysis.* SQN Units 1 and 2 UFSAR Figures 2.4.4-29 and 2.4.4-31 are renumbered as 2.4.4-17 and2.4.4-19, respectively, without any technical changes.* SQN Units 1 and 2 UFSAR Figures 2.4.4-2, 2.4.4-7, 2.4.4-9, 2.4.4-10, 2.4.4-12 through2.4.4-15, 2.4.4-18, 2.4.4-24, 2.4.4-26, and 2.4.4-28 are renumbered as 2.4.4-22, 2.4.4-23,2.4.4-4, 2.4.4-5, 2.4.4-7 though 2.4.4-10, 2.4.4-12, 2.4.4-13, 2.4.4-15, and 2.4.4-16,respectively, with the titles changed as necessary.* SQN Units 1 and 2 UFSAR Figure 2.4.8-1 is not revised.* SQN Units 1 and 2 UFSAR Figures 2.4.13-1 and 2.4.13-2 are not revised.Page 20 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES* SQN Units 1 and 2 UFSAR Figures 2.4A-2 and 2.4A-3 are renumbered as 2.4.14-1 and2.4.14-2, respectively, without any technical changes.* SQN Units 1 and 2 UFSAR Figure 2.4A-4 is updated to reflect current information, with titlechanged and renumbered as Figure 2.4.14-3. The information provided by the figurereflects current analysis of flood warning times in support of the updated hydrologic analysis.2.2 Need for Proposed ChangesOn October 30, 2007, TVA submitted an application for a combined operating license (COLA)for the proposed Bellefonte Nuclear Plant (BLN) Units 3 and 4, in accordance with 10 CFR 52.During review of the BLN Units 3 and 4 Final Safety Analysis Report (FSAR), the NRCperformed an audit of the hydrologic analysis which resulted in the issuance of three NOVs onMarch 19, 2008 (Reference: NRC Letter to TVA, Bellefonte Combined License Application -Nuclear Regulatory Commission Inspection of the Implementation of the Quality AssuranceProgram Governing the Simulated Open Channel Hydraulics Model -Inspection ReportNumbers 05200014/2008-001 and 05200015/2008-001 and Notice of Violation, Accession No.ML080640487). In response to these NOVs, TVA completed a revised hydrologic analysis tosupport the BLN Units 3 and 4 COLA. The revised TVA hydrologic analysis includedreconstitution of the inputs and updates to calculations and software, and is documented in WVAquality-related calculations prepared, reviewed, and approved in accordance with an approved10 CFR 50, Appendix B Quality Assurance program.As a result of the revised BLN Units 3 and 4 hydrologic analysis, the hydrologic analysis forSQN Units 1 and 2 was revised to incorporate updated input information, and updates tomethodology which includes use of the USACE HEC-HMS and USACE HEC-RAS software. OnMarch 1, 2010, TVA submitted Licensee Event Report 327 and 328/2009-009, "UnanalyzedCondition Affecting Probable Maximum Flood Level," Revision 0 (ML100610673) followed onApril 14, 2010 with Licensee Event Report 327 and 328/2009-009, "Unanalyzed ConditionAffecting Probable Maximum Flood Level," Revision 1 (ML101090017), providing detailsconcerning an unanalyzed condition affecting the PMF level for the plant. As stated in thesesubmittals, on December 30, 2009, the issuance of an updated calculation titled "PMFDetermination for Tennessee River Watershed" increased the SQN design basis PMF level fromelevation 719.6 ft to elevation 722.0 ft. This increase in calculated PMF elevation resulted fromseveral calculational changes including updated dam rating curves using model data andchanges in reservoir operating policy. A previous change had decreased the SQN PMF levelfrom elevation 722.6 ft to elevation 719.6 ft. However, SQN remains designed for a PMF levelof elevation 722.6 ft with the current exception of the applicable equipment required for floodmode operation located in the Diesel Generator Building and the Spent Fuel Pit Cooling PumpMotors. Because of the unanalyzed condition, the potential existed for SQN to exceed its DBFdesign basis and adversely affect plant safety. The affected calculation and supportingcalculations have since been updated as a result of other necessary changes, and the currentPMF level is elevation 722.0 ft. The proposed changes are also necessary to address thecumulative effects that have occurred in the SQN Units 1 and 2 hydrologic analysis sinceissuance of the Operating License. TVA is requesting NRC review and approval of the technicalchanges to the SQN Units 1 and 2 UFSAR described in this enclosure, specifically the technicalchanges proposed in SQN Units 1 and 2 UFSAR Sections 2.4, 2.4.1, 2.4.2, 2.4.3, 2.4.4, 2.4.11,Page 21 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESand 2.4.14, and Appendix 2.4A, to incorporate the cumulative effects that have occurred in theSQN Units I and 2 hydrologic analysis since issuance of the Operating License.3.0 TECHNICAL EVALUATION3.1 EvaluationSection 2.4, Hydrologic EngineeringThis section provides a summary of information that is more explicitly discussed in theapplicable subsections. The justification for the changes in these sections is provided in theapplicable subsections below.Section 2.4.1, Hydrological DescriptionThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.1 update the hydrologicaldescription of the TVA system upstream of the SQN site, and update figures showing thecurrent TVA reservoir seasonal operating guides. These changes are updates to the inputsused for the hydrologic analysis to reflect the most current information available regardingoperations of the TVA system of reservoirs and dams, and do not represent any changes to themethodologies used in the updated hydrologic analysis. These changes represent the mostcurrent, complete, and substantiated information relative to the hydrologic description in thevicinity of the site and site regions important to the design and siting of SQN Units 1 and 2 asexpected for review by the NRC in NUREG-0800, Standard Review Plan (SRP), Section 2.4.1.Section 2.4.2, FloodsThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.2 update the discussion ofhistorical floods that affect the hydrologic analysis for SQN Units 1 and 2. These changes areupdates to the inputs used for the hydrologic analysis to reflect the most current informationavailable, and do not represent any changes to the methodologies used in the updatedhydrologic analysis. These changes represent the most current, complete, and substantiatedinformation relative to the local intense precipitation, flooding causal mechanisms, and thecontrolling flooding mechanism important to the design and siting of SQN Units 1 and 2 asexpected for review by the NRC in SRP Section 2.4.2.Section 2.4.3, Probable Maximum Flood (PMF) on Streams and RiversThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.3 update the discussion ofPMF on streams and rivers to reflect the most current information available as inputs, and touse updated methodologies such as the USACE HEC-HMS and USACE HEC-RAS software forelements of the hydrologic analysis for determining the PMF for streams and rivers forSQN Units 1 and 2. As a result of these changes, PMF elevation at the plant is increased fromelevation 719.6 ft to 722.0 ft, excluding wind wave effects.The design basis PMF event for SQN is based on storms with the heavy rainfall occurring in themiddle of the three-day main storm (adopted distribution). This time distribution is supported byevaluation of actual storm events which have occurred in the region and is consistent withregulatory guidance and accepted practice.Page 22 of 39 ENCLOSURE IEVALUATION OF PROPOSED CHANGESInputs to the simulations include calibrated SOCH models (geometry files and Manning's nvalues) of each reservoir, operational guides and initial median reservoir levels, initial damrating curves, as well as inflow hydrographs. The model is divided into three segments for easeof computation. Segment 1 is the model comprising Fort Loudoun-Tellico, Melton Hill and WattsBar reservoirs. Segment la is the model comprising the Melton Hill, Watts Bar andChickamauga reservoirs. Segment 2 is the model comprising the Chickamauga, Nickajack, andGuntersville reservoirs. Watts Bar Dam is an appropriate location to divide Segments 1 and 2 ofthe model, because there is not a significant submergence effect by the tailwater on thedischarge from Watts Bar Dam that would require modeling simultaneously with thedownstream dam unless the concrete wall at Watts Bar Dam fails due to overtopping. If theconcrete wall failure occurs, Segment la is required to model the submergence effects at WattsBar Dam.Initial dam rating curves have been developed for six main stem dams and one tributary dam(Melton Hill Dam) to be used as inputs to the SOCH models. The initial dam rating curves weredeveloped using an average tailwater to determine outflow from the dam based on data fromsteady-state profiles at varying flows. In modeling the design storms it is necessary to adjustsome of the initial dam rating curves to more accurately account for the submergence effect ofthe tailwater on the discharge that may occur at the dams during a large flood event.A SOCH model analysis of hypothetical storms on the Tennessee River Watershed above theGuntersville Dam was conducted using the methodology described above. The current lockconfiguration with 18 spillway bays was used for modeling Chickamauga Dam. Additionaldetails including specific changes to the SOCH model analysis are described in Section 2.1 ofthis enclosure.These changes represent the most current, complete, and substantiated information relative tothe probable maximum flooding on streams and rivers important to the design and siting ofSQN Units 1 and 2 as expected for review by the NRC in SRP Section 2.4.3.Section 2.4.4, Potential Dam Failures, Seismically InducedThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.4 update the discussion ofpotential flood levels from seismically induced dam failures to reflect the most currentinformation available as inputs, and to use updated methodologies such as the USACEHEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis fordetermining dam failure outflows from tributary dams for SQN Units 1 and 2.As described in SQN Units 1 and 2 UFSAR Subsection 2.4.4, procedures described in AppendixA of RG 1.59 were followed when evaluating potential flood levels from seismically induced damfailures. Site flooding levels at SQN from potential seismically induced dam failures aredetermined using the SOCH computer hydraulic model, with changes previously discussedregarding the proposed change to the runoff and stream course model in SQN Units 1and 2 UFSAR Subsection 2.4.3.In the updated hydrologic analysis, the following bounding combinations are used for evaluatingpotential flood levels from seismically induced dam failures:A Seismic failures of Tellico Dam and Norris Dam during one-half the PMF during an OBE.Page 23 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESB Seismic failures of Tellico Dam, Norris Dam, Cherokee Dam, and Douglas Dam during a25-year flood during a SSE.E Seismic failure of Tellico Dam and partial seismic failure of Fontana Dam during one-half thePMF during an OBE.F Partial seismic failure of Fontana Dam and seismic failures of Tellico Dam, Hiwassee Dam,Apalachia Dam, and Blue Ridge Dam during one-half the PMF during an OBE.H Seismic failures of Cherokee Dam, Douglas Dam, and Tellico Dam during one-half the PMFduring an OBE.A SOCH model analysis of these bounding dam failure combinations was conducted using themethodology described above for S.QN Units 1 and 2 UFSAR Section 2.4.3. Additional detailsincluding specific changes to the SOCH model analysis are described in Section 2.1 of thisenclosure. Table 1 provides a summary of the peak elevations and discharges at TVA damsand SQN produced by the five potential seismically induced dam failure combinations. Asshown in Table 1, the peak water surface elevation at SQN from seismically induced damfailures is produced by Seismic Dam Failure Combination H, elevation 708.6 ft.Table 1 -Summary of Maximum Elevationsand Discharges in the Tennessee River WatershedSeismic Seismic Seismic Seismic SeismicDam Dam Dam Dam DamFailures Failures Failures Failures FailuresCombination Combination Combination Combination CombinationLocation Parameter A B E F HFort Headwater 817.2 833.3 817.2 817.2 836.2Loudoun DischargeDam Disc 443,594 2,573,677 383,877 383,877 2,101,716Headwater 818.5 813.2 818.4 818.4 818.6Tellico (feet)Dam Discharge 1,829,012 1,855,019 3,549,639 3,549,639 1,731,678(cfs)Headwater 817.4 817.0 795.0 795.0 795.0Melton Hill (feet)Dam Discharge 1,053,348 876,224 88,269 88,269 88,275(cfs)Headwater 763.0 765.6 756.2 756.2 763.1Watts Bar (feet)Dam Discharge 1,074,582 1,195,727 744,786 744,786 1,059,008(cfs)Headwater 706.3 706.0 702.2 706.3 708.6Sequoyah (feet)Nuclear Discharge 912,939 974,937 775,899 918,880 930,585(cfs) IPage 24 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESThese changes represent the most current, complete, and substantiated information relative tothe effects of dam failures important to the design and siting of SQN Units 1 and 2 as expectedfor review by the NRC in SRP Section 2.4.4.Sections 2.4.5 through 2.4.10There are no technical changes proposed for these sections requiring NRC review andapproval.Section 2.4.11, Low Water ConsiderationsThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.11 update the discussion oflow water considerations to reflect the most current information for the hydrologic analysis forSQN Units 1 and 2.As described in SQN Units 1 and 2 UFSAR Subsection 2.4.11.1, analyses are performed todetermine probable minimum water level at SQN Units 1 and 2 and the minimum flowrequirement at the ERCW intake. In the proposed change, water level at the SQN site uponfailure of the south embankment of Chickamauga Dam resulting in a breach width of 400 ft froman initial Chickamauga Reservoir pool elevation 681.0 ft (slightly below the normal summer poolelevation and the normal summer pool elevation in Nickajack Reservoir) begins to drop in0.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and reaches elevation 641.0 ft (minimum river elevation for required minimum level inthe ERCW intake canal) in 51 hours5.902778e-4 days <br />0.0142 hours <br />8.43254e-5 weeks <br />1.94055e-5 months <br />. These changes are the result of updates to the routingmodel cross-sectional data using new bathymetry and recalibration of the models, asdetermined by postulating loss of Chickamauga Dam and no flow from Watts Bar Dam. In theproposed change, flow required to maintain an elevation of 641.0 ft is the same as the originalanalysis, 14,000 cfs within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of Chickamauga Dam failure.These changes represent the most current, complete, and substantiated information relative tothe low water effects important to the design and siting of SQN Units 1 and 2 as expected forreview by the NRC in SRP Section 2.4.11.Sections 2.4.12 and 2.4.13There are no technical changes proposed for these sections requiring NRC review andapproval.Section 2.4.14, Flooding Protection RequirementsThe proposed changes to SQN Units 1 and 2 UFSAR Section 2.4.14 and Appendix 2.4A updatethe discussion of flooding protection requirements to reflect the most current information for thehydrologic analysis for SQN Units 1 and 2. As described in SQN Units 1 and 2 UFSARSubsection 2.4.14 and Appendix 2.4A, an evaluation is performed to determine the methods bywhich the SQN is capable of tolerating floods above plant grade without jeopardizing publicsafety. The DBF is the calculated upper-limit flood that includes the PMF plus the wave runupcaused by a 45 mph overland wind as discussed in SQN Units 1 and 2 UFSAR Subsection2.4.3.6. As a result of the changes to inputs and methodology discussed previously, the DBFelevations at various plant locations that would occur by the limiting large rainfall and seismicallyinduced dam failure floods are increased to the following:Page 25 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESPlant Location DBF Level (ft.)Probable Maximum Flood (still reservoir) 722.0DBF Runup on Diesel Generator Building 723.2DBF Runup on vertical external, unprotected walls 726.2DBF Surge level within flooded structures 722.5SQN is designed in accordance with the Regulatory Position 2 of RG 1.59, Revision 2,August 1977, which specifies that at least those structures, systems, and componentsnecessary for cold shutdown and maintenance thereof are designed with hardened protectivefeatures to remain functional while withstanding the entire range of flood conditions up to andincluding the worst site-related flood probable (e.g., PMF, seismically induced flood, hurricane,surge, seiche, heavy local precipitation) with coincident wind-generated wave action asdiscussed in Regulatory Position 1 of the RG.Although the DBF elevations at various plant locations that would occur by the limiting largerainfall and seismically induced dam failure floods are increased from those currently providedin the SQN Units 1 and 2 UFSAR, there are only two distinct changes to the physical floodingprotection features of SQN Units 1 and 2 required. The other safety-related systems,structures, and components identified in Regulatory Guide 1.29 are designed to withstand theflood conditions associated with the updated DBF elevations, and would remain functionalduring external floods.The UFSAR currently requires the Reactor Building, Diesel Generator Building, and the ERCWIntake Station to remain dry during flood mode. The Reactor Building and ERCW Intake Stationremain protected from the updated DBF levels.The lowest floor of the common SQN Units 1 and 2 Diesel Generator Building is at elevation722.0 ft with its doors on the uphill side facing away from the main body of flood water. Thiselevation is lower than the updated DBF level of elevation 723.2 ft. Therefore, flood levelsexceed the floor level at elevation 722.0 ft. The entrances into safety-related areas andmechanical and electrical penetrations into safety-related areas are sealed to prevent majorleakage into the building for water up to the grade elevation 722.0 ft. Additionally, redundantsump pumps are provided within the building to remove minor leakage. As a result of thisincrease, staged sandbags to be constructed into a berm at the entrances to the DieselGenerator Building at any time prior to or during the event of a Stage I flood warning has beenestablished as a compensatory measure. These sandbags will be constructed into a berm atleast three ft in height (elevation 725.0 ft) to prevent water intrusion inside the building.Permanent plant modifications are planned to provide adequate flooding protection features forthe common SQN Units 1 and 2 Diesel Generator Building.The Service, Turbine, Auxiliary, and Control Buildings are permitted to flood as the waterexceeds the plant level entrances. No permanent barriers to specifically protect flood sensitiveplant equipment exist in any of these structures except, as discussed further in Section 3.3 ofthis enclosure, the SQN Units 1 and 2 Spent Fuel Pit Cooling Pump Enclosure caps in theAuxiliary Building are now required to maintain adequate flooding protection of the Spent FuelPit Cooling Pump Motors during flood mode. The DBF surge level within flooded structures iselevation 722.5 ft. The Spent Fuel Pit Cooling Pump Motors platform is located at elevation721.0 ft, but is located in an enclosure that provides flooding protection up to elevation 724.5 ft.However, the Spent Fuel Pit Cooling Pump Enclosure caps were not originally intended to bepermanently installed. To restore margin for the Spent Fuel Pit Cooling Pump Motors,Page 26 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESinstallation of the caps at any time prior to or during the event of a Stage I flood warning hasbeen established as a compensatory measure. A documentation change is planned to requirethe SQN Units 1 and 2 Spent Fuel Pit Cooling Pump Enclosure caps as a permanent plantfeature for flooding protection.Regulatory Position 2 of RG 1.59, Revision 2, August 1977, also specifies that sufficient warningtime is shown to be available to shut the plant down and implement adequate emergencyprocedures.TVA RO is responsible for operations of the TVA dams and reservoirs for the Tennessee Riverwatershed, including the controls and scheduling of the releases from flood storage dams abovethe nuclear sites as an integrated system. The USACE is responsible for operation of the lockson the Tennessee River. TVA RO operates the TVA dams and reservoirs for purposes thatinclude:" Flood control and storm water management of the Tennessee River and major tributaries," Irrigation of land along the Tennessee River," Generation of hydroelectric power,* Cooling of fossil and nuclear power plants," Navigation of recreational and commercial vessels," Public and industrial water supplies, and" General recreation of the public.As a part of TVA RO's flood control responsibilities, forecast and warning procedures have beenestablished that reflect the updated hydrologic analyses. The safety of the plant in extremeevents requires shutdown procedures which may take 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> to implement. TVA's calculationdemonstrates that the time is available for WVA RO's forecast and warning procedures toprovide at least 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> before river levels reach elevation 703.0 ft. Use of elevation 703.0 ft,2.0 ft below plant grade, provides enough margin to prevent wind generated waves fromendangering plant safety during the final hours of shutdown activity. Flood warning is basedupon rainfall already reported to be on the ground on the watershed above SQN. Although thewarning time for SQN has been reevaluated, the only significant change in the results of theanalysis of warning time is the use of a revised forecasted plant site water levels where Stage Iactions are required to begin. Use of these revised Stage I action levels does not reduce theeffectiveness of the warning plan, as there is still a minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> to prepare foroperation in the flood mode.The warning time for SQN has been reevaluated because the initial median reservoir levels andflood operational guides have been revised, dam rating curves have changed at some dams,and the SOCH model of the Tennessee River has been updated to meet current qualityassurance standards. Details of the warning plan analysis follows:The design basis PMF event for SQN is based on storms with the heavy rainfall occurring in themiddle of the three-day main storm (adopted distribution). This time distribution is supported byPage 27 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGESevaluation of actual storm events which have occurred in the region and is consistent withregulatory guidance and accepted practice. In order to address the warning time issue, differenttime distributions have been evaluated.The original analysis tested the effects of varied time distribution of rainfall by alternativelyplacing the maximum daily rainfall on the first, middle, and the last day of the three-day mainstorm to ensure that the shortest warning times were captured for the hypothetical storms. Thisanalysis showed that the fastest rising floods occur when the heavy rainfall is applied at the endof the storm. The current analysis consists of nine hypothetical storms ranging from slightlymore than five inches (equivalent to the largest flood event since regulation) up to PMP andenveloped potentially critical areal, seasonal variations and time distribution of rainfall. Thewarning time is based on those storm situations which resulted in the shortest time intervalbetween watershed rainfall and elevation 703.0 ft.The warning time is divided into two stages: Stage I, a minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> long and Stage II, aminimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> long so that unnecessary economic consequences can be avoided, whileadequate time is allowed for preparing for operation in the flood mode. Stage I allowspreparation steps causing minimal economic consequences to be sustained but postponesmajor economic damage to the plant until a Stage II warning predicts a likely forthcoming floodabove plant grade. If the flood does not develop beyond a Stage I warning, major economicdamage is avoided.To be certain of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> for pre-flood preparation, flood warnings with the prospect of reachingelevation 703.0 ft must be issued early when lower threshold flood warning levels are forecast.Consequently, some of the Stage I warnings may not progress into a Stage II warning. For thisreason pre-flood preparations are divided into two stages. Stage I steps requiring 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> areeasily revocable and cause minimum economic consequences to the plant.Additional rain and stream-flow information obtained during Stage I activity determines if themore serious steps of Stage II need to be taken with the assurance that at least 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> isavailable before elevation 703.0 ft is reached.Considering results of the original analysis, six storms were identified in the current analysis toenvelop the potentially critical variations of rainfall. Separate controlling winter and summerstorm events were selected for use with the critical time distribution. The controlling winterevent selected was the 21,400 square-mile March downstream centered storm. The controllingsummer event selected was the 7,980 square-mile June Bulls Gap centered event. The sixselected storms represent average basin rainfall from 5.2 inches up to the PMP. The selectedevents are described below.1. 21,400 square-mile March downstream centered PMP storm with heavy rainfall on the lastday (HLD)2. 7,980 square-mile June Bulls Gap centered PMP storm (HLD)3. 21,400 square-mile March downstream centered PMP storm scaled to 10 inches aboveChickamauga Dam, (HLD)4. 7,980 square-mile June Bulls Gap centered PMP storm scaled to 10 inches aboveChickamauga Dam, (HLD)5. 21,400 square-mile March downstream centered PMP storm scaled to 5.2 inches aboveChickamauga Dam, (HLD)Page 28 of 39 ENCLOSURE1EVALUATION OF PROPOSED CHANGES6. 7,980 square-mile June Bulls Gap centered PMP storm scaled to 5.2 inches aboveChickamauga Dam, (HLD)In addition to the six storms above, the three PMF candidate events listed below are analyzed todetermine the design basis flood at SQN.7. 21,400 square-mile, March, downstream centered PMP storm with the adopted distribution8. 7,980 square-mile, March, Bulls Gap centered PMP storm with the adopted distribution9., 7,980 square-mile, June, Bulls Gap centered PMP storm with the adopted distributionFor events 7 to 9 listed above, the heavy rainfall is in the middle of the three-day main stormand is referred to as the adopted distribution.The results of the six simulations performed as part of this analysis, together with the results ofthe three additional simulations, were used to develop the relationship between basin averagerainfall and peak river elevation at SQN.The maximum calculated water surface elevation at SQN for each storm was plotted againstaverage basin rainfall depth. Summer and winter relationships were developed using apolynomial curve. This approach did not result in curves which passed through each of thecalculated points. To ensure that the curve would envelop the calculated points, a systematicadjustment of the polynomial coefficients was applied until the curve passed thru the calculatedpoints or was within 0.1 ft above.The adopted warning time curves envelop the routing simulation results for their respectiveseasons using inflows from the selected worst case storm events. Therefore, these curves area bounding condition for determining the warning times at SQN.The warning time to assure safe shutdown of SQN for flooding resulting from seismic damfailures coincident with flood events is based upon analysis of potentially critical combinations ofdam failures.Flood warnings are issued in real-time by TVA RO. Flood control operations for a major stormthat spans the majority of the Tennessee Valley would necessitate the integrated operation ofthe reservoirs in the system. The flood storage available to TVA for minimizing flood damagesis finite, and does not allow TVA to eliminate flooding at every area along the regulated rivers.Thus, TVA efforts are directed toward using the available flood storage to minimize downstreamflooding, rather than eliminating downstream flooding. During extreme flood events, TVA wouldfocus on minimizing downstream flood damage to the extent possible, operating the projects toensure the safety and integrity of the dams and appurtenant structures, and providing frequentflood warning time and elevation forecasts.The one-half PMF, developed as part of the seismic flood analysis, addresses item 2 ofRegulatory Position 2, Regulatory Guide 1.59, Revision 2. This storm was developed by takingone-half the runoff ordinates of the design basis flood including the antecedent flood(21,400 square-mile PMP storm) plus base flow and routing them through the reservoir system.Based on use of the warning time methodology described above, a minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> hasbeen allowed for preparation of the plant for operation in flood mode. An additional 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> isallowed for communication and forecasting computations by the TVA RO organization toPage 29 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGEStranslate rain on the ground to river elevations at the plant. Hence, the warning time provides aminimum of 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> from arrival of rain on the ground until elevation 703.0 ft could be reached.A minimum of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> is allowed for shutdown at the plant which consists of a minimum of 10hours of Stage I preparation and a minimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> for Stage II preparation that is notconcurrent with the Stage I activity.Although reservoir elevation 703.0 ft, 2.0 ft below plant grade to allow for wind waves, is thecontrolling elevation for determining the need for plant shutdown, lower forecast thresholdwarning flood elevations are used in some situations to assure that the 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> pre-floodtransition interval is always available. The threshold warning flood elevations differ with seasonof the year.Determination of the warning time and flood elevations at SQN requires the following:1. The elevation hydrograph at the plant for each simulated rainfall event;2. A plot of the cumulative storm rainfall for each rainfall event; and3. The relationship between average basin rainfall and plant elevation.The shortest warning time scenario for the winter was determined to be produced by the March21,400 square mile downstream centered PMP with the heavy rainfall on the last day. Theshortest warning time for the summer flood was produced by the 7,980 square mile June BullsGap centered PMP with the heavy rainfall on the last day.Inasmuch as the hydrologic procedures and threshold flood warning levels have beenestablished to provide adequate shutdown time in the flood producing the shortest warning time,longer times are available in other floods. In such cases there is a waiting period after theStage I 10-hour shutdown activity during which activities shall be in abeyance until TVA ROdetermines, based upon weather conditions, that plant operation can be resumed, or if Stage IIshutdown should be implemented.For rainfall induced floods, the available warning times are adopted as results and have beenevaluated for the flood conditions producing the shortest warning time. Therefore, more timewould be available in other flood situations. Table 2 includes the predicted threshold floodwarning levels for the shortest warning time flood conditions which assure adequate warningtime for plant shutdown.Table 2 -Warning Threshold Flood Warning LevelsStage I Shutdown Stage II ShutdownSeason Elevation (ft) Rainfall* (inches) Elevation (ft) Rainfall* (inches)Winter 694.5 7.4 703.0 9.8Summer 699.0 8.2 703.0 9.5* Rainfall in table refers to "inches of rain on the ground above Chickamauga Dam."For seismically induced dam failure floods, Table 3 provides the maximum elevations andwarning times at SQN for the five seismic combinations evaluated. The following four seismicdam failure combinations would result in flood levels above plant grade elevation (705.0 ft).1. SSE failure of Norris, Cherokee, Douglas and Tellico Dams coincident with the 25-yearflood;Page 30 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES2. OBE failure of Norris and Tellico Dams coincident with one-half PMF;3. OBE failure of Tellico, Hiwassee, Apalachia and Blue Ridge Dams and partial failure ofFontana Dam coincident with one-half PMF, and4. OBE failure of Cherokee, Douglas, and Tellico Dams coincident with one-half PMF.As shown in Table 3, the other candidate combination of events would create flood levels belowplant grade elevation (705.0 ft). The times from seismic occurrence to arrival of failure surge atthe SQN are shown in Table 3. The failure of Tellico, Hiwassee, Apalachia and Blue RidgeDams and partial failure of Fontana Dam in an OBE event coincident with one-half PMFproduces the shortest arrival time at 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br /> and is adequate to permit safe plant shutdown inreadiness for flooding.Dam failure during non-flood periods was not reevaluated, but would be bounded by the fourcritical failure combinations described above.The warning time for safe plant shutdown in seismically induced dam failure flood events isbased on the fact that a failure combination of critically centered large earthquake conditionsmust exist before the flood waves from seismically induced dam failures could exceed plantgrade.Table 3 -Floods from Postulated Seismic Failure of Upstream Dams(Plant Grade is Elevation 705.0 ft)SQN Plant Flood Wave TravelElevation (ft) Timec (hr)OBE Failure with One-Half Probable Maximum FloodNorris -Tellico 706.7 34Cherokee -Douglas -Tellico 708.6 46Partial Fontana -Tellico -Hiwassee -Apalachia -Blue 706.3 32RidgeaPartial Fontana -Tellicoa 702.2 NASSE Failure with 25-Year FloodNorris -Cherokee -Douglas -TellicoD 706.0 53a. Includes failure of four ALCOA dams and one Duke Energy dam -Nantahala (Duke Energy, formerlyALCOA), upstream; Santeetlah, on a downstream tributary; and Cheoah, Calderwood, andChilhowee, downstream. Fort Loudoun gates are inoperable in open position.b. Gate opening at Fort Loudoun prevented by bridge failure.c. Time from seismic dam failure to arrival of failure wave at SQN elevation 703.0 ft (2.0 ft below plantgrade).For flood conditions resulting from one-half PMF, RG 1.59 specifies that safety-related facilitiesdesigned in accordance with Regulatory Position 2 must be designed to withstand the floodconditions resulting from a Standard Project event (defined as flow rates generally 40 percent to60 percent of the PMF) with attendant wind-generated wave activity that may be produced bythe worst winds of record and remain functional.The one-half PMF (magnitude in range of Regulatory Guide specifications) would produce amaximum elevation at SQN of 701.6 ft. This is 3.4 ft below plant grade elevation 705.0 ft.RG 1.59 specified attendant wind-generated wave activity that may be produced by the worstwinds of record would not present a problem due to the short wind fetch lengths (0.23 toPage 31 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES1.54 miles) and the elevation margin of 3.4 ft available to plant grade elevation. The modelingconventions used in this simulation produce conservative results at SQN.These changes represent the most current, complete, and substantiated information relative tothe emergency operations important to the design and siting of SQN Units 1 and 2 as expectedfor review by the NRC in SRP Section 2.4.14.As previously stated, the warning time is divided into two stages: Stage I, a minimum of10 hours long; and Stage II, a minimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> long, so that unnecessary economicconsequences can be avoided, while adequate time is allowed for preparing for operation in theflood mode. The plant procedure governing preparations for operation in the flood modeincludes the following initial assumptions:1. SQN Units 1 and 2 is at 100% power.2, Stage I Flood Warning is issued by TVA RO at the threshold flood warning levels during the"winter" season of elevation 694.5 ft, or during the "summer" season of elevation 699.0 ft.Upon issuance of the Stage I Flood Warning, the following actions are taken in accordance withthe appropriate procedures:1. A "Notification of Unusual Event" would be declared per the Emergency Plan, and supportstaffing of the Technical Support Center would be initiated.2. Shutdown to Mode 3 (Hot Standby) would be commenced using AOP-C.03, RapidShutdown.3. Cooldown of the Reactor Coolant System (RCS) to Mode 4 (less than 350°F) by dumpingsteam and then placing Residual Heat Removal (RHR) Shutdown Cooling in service wouldbe performed.4. The RCS would be borated to maintain shutdown margin.5. Preparations would be made to place the Auxiliary Charging/Flood Mode Boration System inservice.6. Various other preparations would be completed such as moving necessary supplies abovethe maximum flood elevation and filling tanks to prevent floating.Upon issuance of the Stage II Flood Warning or if RO confirms that elevation 703.0 ft is likely tobe exceeded, the following actions are taken in accordance with the appropriate procedures:1. An "Alert" would be declared per the Emergency Plan.2. If off-site power is still supplying the shutdown boards, then the shutdown boards would bemanually transferred to the diesel generators.3. Auxiliary Feedwater pumps would be stopped and High Pressure Fire Protection water tothe steam generators would be established, including aligning valves and installation ofspool pieces. This requires steam generator pressure less than 90 psig.Page 32 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES4. When decay heat removal is established by High Pressure Fire Protection water to thesteam generators, RHR Shutdown Cooling would be removed from service.5. Other spool pieces would be installed to establish ERCW cooling to Spent Fuel Pit Coolingheat exchangers and reactor coolant pump thermal barriers.6. Heat sink would be maintained by releasing steam via steam generator atmospheric reliefvalves with secondary makeup water supplied by High Pressure Fire Protection.7. RCS inventory would be maintained using Auxiliary Charging pumps.8. RCS pressure would be maintained less than 500 psig with RCS temperature less than3280F to assure that Fire/Flood Mode pumps can supply High Pressure Fire Protection tosteam generators.Flood mode operation would continue until conditions as described in the appropriate plantprocedures allow either further plant cooldown or restart.3.2 UncertaintiesAs stated in RG 1.59, Revision 2, Probable Maximum Water Level is defined by the Corps ofEngineers as "the maximum still water level (i.e., exclusive of local coincident wave runup)which can be produced by the most severe combination of hydrometeorological and/or seismicparameters reasonably possible for a particular location. Such phenomena are hurricanes,moving squall lines, other cyclonic meteorological events, tsunami, etc., which, when combinedwith the physical response of a body of water and severe ambient hydrological conditions,would produce a still water level that has virtually no risk of being exceeded." The PMF forstreams and rivers for sites like SQN Units 1 and 2 is the hypothetical flood (peak discharge,volume, and hydrograph shape) that is considered to be the most severe, reasonably possible,based on comprehensive hydrometeorological application of PMP and other hydrologic factorsfavorable for maximum flood runoff such as sequential storms and snowmelt. The primarystandards followed for development of the PMF are American National StandardsInstitute/American Nuclear Society (ANSI/ANS) 2.8 and RG 1.59, Revision 2. These guidancedocuments state that the PMF be derived from the combination of circumstances thatcollectively represent a risk probability that is acceptable for nuclear plant accidents. Eachelement in the development of the PMF is based on best available data including PMPestimates from the National Weather Service, rain-runoff relationships developed from historicalstorms, time distribution of PMP consistent with storms in the region, seasonal and arealconsiderations of rainfall, current reservoir operations, and verification of runoff and streamcourse models against large historic floods. Per ANSI/ANS 2.8 and RG 1.59, the techniquesapplicable to PMF and seismically induced floods for nuclear power plants are estimates. Thecalculations which support the PMF analysis document assumptions and approaches which areconsistent with ANSI/ANS 2.8 and RG 1.59. The PMF analysis is a best estimate and isconsistent with these standards and guidelines. However, it is realized that various elements ofthe analysis when modified result in different elevations, some higher and some lower, andthose elements discussed in further detail below are consistent with these standards andguidelines demonstrating that the PMF analysis is a reasonable best estimate.Page 33 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESAs discussed in NUREG/CR-7046, "Design-Basis Flood Estimation for Site Characterization atNuclear Power Plants in the United States of America," the appropriate method to address theuncertainty in the hydrologic analysis is through calibration of the model to historic flood eventsor sensitivity analyses. TVA calibrated the model to historic flood events using the two highestrecent flood events where data exists. The floods used for calibration are March 1973 andMay 2003 with elevations at SQN of approximately 687.0 ft and 687.8 ft for those two storms. Inaddition to the calibration using historic data, sensitivity analyses for the Bellefonte model havebeen completed. While the input assumptions regarding failure of Chickamauga Dam and theChickamauga Dam spillway gate configurations differ between the two models, insights may beapplied to SQN based on these sensitivity analyses. The rainfall-to-runoff transformation (unithydrographs) is completed. The unit hydrographs are peaked by 25% and the results show thatthe model is not sensitive to this parameter.The rainfall loss rate is another parameter that has been evaluated. TVA uses the AntecedentPrecipitation Index rain-runoff relationship. This is the same methodology used by TVA for thedaily reservoir operations. This parameter does show sensitivity to the model resulting inseveral feet added to the PMF elevation when increasing the runoff from 89% to 100%. Muchof this increase was caused by over-topping failure of dams above SQN. However, the rainfallloss rate used in the TVA model is a realistic value for TVA based on regional historic data overmore than 60 years, and there is high confidence in this value as the appropriate value forhydrology modeling. In addition, this parameter was evaluated by comparison to otheracceptable methods for determining rain-runoff relationships discussed in NUREG/CR-7046.The methodology used by TVA is conservative when compared to the other acceptablemethods.Other parameters in the stream course model such as the Manning's n value or resistance toflow could be increased or decreased during extreme flood events such as the PMF. Theadopted values in the model are based on calibration against two of the largest floods of record.If it is postulated that debris in the overbanks would result in an increase in resistance to flowand thus an increase in the Manning's n values, then the elevation at SQN would increase. If itis postulated that Manning's n values would be decreased, then the elevations at SQN would bedecreased. Such decreases have been documented for large flood events on the MississippiRiver and could have been considered in the updated hydrologic analysis, but Manning's nvalues were conservatively not decreased. Based on this documented experience, there isconservatism in the applied Manning's n values but it is difficult to quantify since the flood hasnever been out of channel to the extreme that it would be in the PMF.Therefore, TVA uses the best estimate approach with calibration to the two largest recordedfloods with data.Dam rating curves are developed assuming that gates will be open, and TVA RO has committedto making this occur during the PMF event. For the flood simulations, the spillway gates areoperated when and as needed for flood regulation up to and including the fully open position.The model has not been tested for loss of gate capacity although it may be assumed that loss ofgate capacity could result in an increase in PMF elevation. However, quantification of thechange is not easily determined.While not tested during sensitivity analyses, it is known that a conservative assumption is maderegarding the downstream dam, Chickamauga Dam. This dam is overtopped during the PMFbut is assumed to not fail.Page 34 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESFinally, the TVA hydrology model has been reviewed by an expert panel and the panel agreedthat due to the complexity of the system, it would be very difficult to highlight conservatisms inthe model. The panel concluded after review that the analysis was rigorous and technicallysound. Based on this discussion, the hydrologic analysis is considered to be a reasonable bestestimate that has accounted for uncertainties based on regulatory guidance using the best dataavailable.3.3 MarginsAs previously discussed in the TVA submittal to the NRC Document Control Desk,"Commitments Related to Updated Hydrologic Analysis Results for Sequoyah Nuclear Plant,Units 1 and 2, and Watts Bar Nuclear Plant, Unit 1," dated June 13, 2012 (ADAMS AccessionNo. ML12171A053), the limiting safety-related components required to be available during aplant flood affected by the increase in DBF elevations are the Spent Fuel Pit Cooling PumpMotors and applicable equipment required for flood mode operation located in the DieselGenerator Building. To restore margin for the Spent Fuel Pit Cooling Pump Motors, installationof the Spent Fuel Pit Cooling Pump Enclosure caps at any time prior to or during the event of aStage I flood warning has been established as a compensatory measure. For the DieselGenerator Building, staged sandbags to be constructed into a berm at any time prior to or duringthe event of a Stage I flood warning has been established as a compensatory measure. Ascommitted to in the June 13, 2012 submittal, TVA will implement a documentation change torequire the Spent Fuel Pit Cooling Pump Enclosure caps as a permanent plant feature forflooding protection, and will install permanent plant modifications to provide adequate floodingprotection with respect to the DBF level for the Diesel Generator Building, by March 31, 2013.3.4 ConclusionsThe revised DBF elevations at the SQN Units 1 and 2 site are determined to impact some of thesafety-related systems, structures, or components required to be available during a plant flood.However, temporary compensatory measures are in place to ensure adequate floodingprotection if a PMF event were to occur. Except for the limited cases of the Spent Fuel PitCooling Pump Motors and applicable equipment required for flood mode operation located inthe Diesel Generator Building, no physical change to the systems, structures, or components isnecessary to ensure that they remain adequately protected from the effects of external floods.Documentation changes and permanent plant modifications are planned to restore or gainadditional margin between the revised DBF elevations and limiting safety-related systems,structures, and components. Also, the warning time for SQN shows that there is sufficient timeavailable in both rainfall and seismically induced dam failure floods for safe plant shutdown. Inaddition, the updated low water level analysis demonstrates that there is sufficient flow tosupport operation of SQN Units 1 and 2. Although there are numerous changes to inputs for thehydrological analysis, the cumulative effects of these changes do not impact the originalconclusions of the SQN Units 1 and 2 UFSAR that adequate flooding protection features andprocedures are in place. The hydrologic analysis is considered to be a reasonable bestestimate that has accounted for uncertainties based on regulatory guidance using the best dataavailable. The updated hydrologic analysis shows that the design and siting of SQN Units 1and 2 is adequate to meet the regulatory requirements and criteria specified to be addressed forSQN Units 1 and 2 UFSAR Section 2.4 and that SQN Units 1 and 2 are capable of toleratingfloods above plant grade in a manner that does not jeopardize public health and safety.Page 35 of 39 ENCLOSURE IEVALUATION OF PROPOSED CHANGES4.0 REGULATORY EVALUATION4.1 Applicable Regulatory Requirements and Criteria10 CFR Part 100, requires identifying and evaluating hydrologic features of the site.10 CFR 100.23(d) sets forth the criteria to determine the siting factors for plant design baseswith respect to seismically induced floods and water waves at the site.10 CFR 50, Appendix A, General Design Criteria (GDC) 2, requires consideration of the mostsevere of the natural phenomena that have been historically reported for the site andsurrounding area, with sufficient margin for the limited accuracy, quantity, and period of time inwhich the historical data have been accumulated.10 CFR 50, Appendix A, GDC 44, requires providing an ultimate heat sink for normal operatingand accident conditions.In addition to regulatory requirements, acceptable guidance for hydrologic analysis of the site isincluded in the following:" Regulatory Guide 1.27 describes the applicable ultimate heat sink capabilities.* Regulatory Guide 1.29 identifies seismic design bases for safety-related SSCs.* Regulatory Guide 1.59, as supplemented by best current practices, provides guidance fordeveloping the hydrometeorological design bases.* Regulatory Guide 1.102 describes acceptable flooding protection to prevent thesafety-related facilities from being adversely affected.The SQN Units 1 and 2 hydrologic analysis conforms to the above regulatory requirements andguidance, using the most recent data and updated methodology which includes use of USACEHEC-HMS and USACE HEC-RAS software.The SQN Units 1 and 2 hydrologic analysis as described in this License Amendment Request,and as presented in the proposed revision of the SQN Units 1 and 2 UFSAR, contains sufficientsubstantiated information pertaining to the hydrologic description at the proposed site. Thehydrologic analysis meets the requirements of 10 CFR 100 as it relates to:1. Identifying and evaluating the hydrology in the vicinity of the site and site regions, includinginterface of the plant with the hydrosphere,2. Hydrological causing mechanisms,3. Surface and ground water uses,4. Spatial and temporal data sets,5. Alternate conceptual models of site hydrology,Page 36 of 39 ENCLOSURE IEVALUATION OF PROPOSED CHANGES6. Identification and consideration of local intense precipitation and flooding at the site,7. Identification and consideration of the probable maximum flooding on .streams and rivers atthe site and in the surrounding area,8. Identification and consideration of the effects of dam failures at the site and in thesurrounding area,9. Low water effects important to the design and siting of this plant, and10. The appropriate site phenomena in establishing emergency operations for SSCs importantto safety.Further, the hydrologic analysis considers the most severe natural phenomena that have beenhistorically reported for the site and surrounding area while describing the hydrologic interface ofthe plant with the site, with sufficient margin for the limited accuracy, quantity, and period of timein which the historical data have been accumulated.The NRC staff has generally accepted the methodologies used to determine the severity of thephenomena, the local intense precipitation, flooding causal mechanisms, controlling floodingmechanism reflected in these site characteristics, the probable maximum flooding on streamsand rivers, the effects of dam failures, the potential for low water conditions, and considerationof the appropriate site phenomena in establishing emergency operations for SSCs important tosafety, as documented in safety evaluation reports for previous licensing actions. Accordingly,the use of these methodologies results in site characteristics and procedures containingsufficient margin for the limited accuracy, quantity, and period of time in which the data havebeen accumulated. In view of the above, the site characteristics previously identified asdescribed in the proposed changes to the SQN Units 1 and 2 UFSAR are acceptable for use inestablishing the design bases for SSCs important to safety and site procedures.4.2 PrecedentTVA evaluated license amendment requests and requests for issuance of Combined OperatingLicenses in which the NRC had reviewed and approved changes to or initial hydrologic analysisfor existing and proposed new nuclear power plants. Although there are similar requests forvarious changes to the hydrologic analysis or for a new hydrologic analysis for these othernuclear power plants, no precedent was identified for a rebaselining of an existing hydrologicanalysis similar to this request.4.3 Significant Hazards ConsiderationThe proposed changes modify SQN Units 1 and 2 UFSAR hydrologic analysis and results,including the DBF elevations required to be considered in the flooding protection ofsafety-related systems, structures, or components during external flooding events, and verifythe adequacy of the warning time for SQN for both rainfall and seismically induced dam failurefloods. The proposed changes do not alter the conclusions presented in the SQN Units 1and 2 UFSAR that equipment required for operation in the flood mode is either above the DBFor suitable for submerged operation considering the temporary compensatory measures inplace and upon completion of planned documentation changes and permanent plantPage 37 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGESmodifications, and that there is sufficient time available in both rainfall and seismically induceddam failure floods for safe plant shutdown. No physical changes to safety-related systems,structures, or components, or any credited flooding protection feature, are required to ensurethat they remain adequately protected from the effects of external floods except for the limitedcases of the Spent Fuel Pit Cooling Pump Motors and applicable equipment required for floodmode operation located in the Diesel Generator Building. However, temporary compensatorymeasures are in place to ensure adequate flooding protection if a PMF event were to occur, anddocumentation changes and permanent plant modifications are planned to restore or gainadditional margin between the revised DBF elevations and limiting safety-related SSCs.TVA has concluded that the changes to SQN Units 1 and 2 UFSAR do not involve a significanthazards consideration. TVA's conclusion is based on its evaluation in accordance with10 CFR 50.91(a)(1) of the three standards set forth in 10 CFR 50.92, "Issuance of Amendment,"as discussed below:1. Does the proposed amendment involve a significant increase in the probability orconsequence of an accident previously evaluated?Response: NoAlthough the proposed changes require some documentation and physical changesto plant systems, structures, or components to add flooding protection features torestore or gain additional margin between the revised DBF elevations and limitingsafety-related systems, structures, and components; implementation of thesechanges does not 1) prevent the safety function of any safety-related system,structure, or component during an external flood; 2) alter, degrade, or prevent actiondescribed or assumed in any accident described in the SQN Units 1 and 2 UFSARfrom being performed since the safety-related systems, structures, or componentsremain adequately protected from the effects of external floods; 3) alter anyassumptions previously made in evaluating radiological consequences; or 4) affectthe integrity of any fission product barrier.Therefore, this proposed amendment does not involve a significant increase in theprobability or consequences of an accident previously evaluated.2. Does the proposed amendment create the possibility of a new or different kind ofaccident-from any accident previously evaluated?Response: No.The proposed changes do not introduce any new accident causal mechanisms, nordo they impact any plant systems that are potential accident initiators.Therefore, the proposed amendment does not create the possibility of a new ordifferent kind of accident from any accident previously evaluated.Page 38 of 39 ENCLOSUREIEVALUATION OF PROPOSED CHANGES3. Does the proposed amendment involve a significant reduction in a margin of safety?Response: No.The proposed changes do not alter the permanent plant design, including instrumentset points, that is the basis of the assumptions contained in the safety analyses.However, documentation changes and permanent plant modifications are planned torestore or gain additional margin between the revised DBF elevations and limitingsafety-related systems, structures, and components. Although the results of theupdated hydrologic analysis increase the DBF elevations required to be consideredin the flooding protection of safety-related systems, structures, or components duringexternal flooding events, the proposed changes do not prevent any safety-relatedSSCs from performing their required functions during an external flood consideringthe temporary compensatory measures in place and upon completion of planneddocumentation changes and permanent plant modifications. Consistent with existingregulatory guidance, including regulatory recommendations and discussionsregarding calibration of hydrology models using historical flood data andconsideration of sensitivity analyses, the hydrologic analysis is considered to be areasonable best estimate that has accounted for uncertainties using the best dataavailable.Therefore, the proposed changes do not involve a significant reduction in a margin ofsafety.4.4 ConclusionsIn conclusion, based on the considerations discussed above, (1) there is reasonable assurancethat the health and safety of the public will not be endangered by operation in the proposedmanner, (2) such activities will be conducted in compliance with the Commission's regulations,and (3) the issuance of the amendment will not be inimical to the common defense and securityor to the health and safety of the public.5.0 ENVIRONMENTAL CONSIDERATIONA review has determined that the proposed amendment would not change a requirement withrespect to installation or use of a facility component located within the restricted area, as definedin 10 CFR 20, or would change an inspection or surveillance requirement. Also, the proposedamendment does not involve (i) a significant hazards consideration, (ii) a significant change inthe types or significant increase in the amounts of any effluents that may be released offsite, or(iii) a significant increase in individual or cumulative occupational radiation exposure.Accordingly, the proposed amendment meets the eligibility criterion for categorical exclusion setforth in 10 CFR 51.22(c)(9). Therefore, pursuant to 10 CFR 51.22(b), no environmental impactstatement or environmental assessment need be prepared in connection with the proposedamendment.Page 39 of 39