ML17325A125

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Steam Generator Tube Integrity - Apr 1987:Assessment of Next Operating Interval Length. Related Info Encl,Including Description of Steam Generator Manway Cover Closure Repairs for Mar-Apr 1987 & Westinghouse Nuclear Safety Check List
ML17325A125
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INDIANA6MICHIGANELECTRICCOMPANYDONALDC.COOKNUCLEARPLANTUNIT2STEAMGENERATOR TUBEINTEGRITY

-APRIL1987AnAssessment oftheNextOperating IntervalLengthAttachment 1toAEP:NRC:0936J Preparedby:AmericanElectricPowerServiceCorporation OneRiverside PlazaColumbus, Ohio43216May18,1987870bOCR050003ib0201b9870522pDRADPDR8i TABLEOFCONTENTSLISTOFABBREVIATIONS LISTOFTABLESANDFIGURESREFERENCE SUBMITTALS

1.0INTRODUCTION

1.1ReportObjectives 1.2Operating Experience Overview1.2.11~2.2Background MostRecentOperating Period2.0CONDITXON OFTUBEBUNDLES2.1SteamGenerator Inspection andTubePlugging-March19872.1.12.1.2EddyCurrentAnalysisCriteria/Tube PluggingCriteria6EddyCurrentInspection Results-------72.2TubeDegradation GrowthRateEvaluation 2.2.12.2.22'.3TubePluggingComparison 9GrowthRateDetermination 10Probabilistic ModelVerification

ll3.0EVALUATION OFOPERATION THROUGHTHEENDOFFUELCYCLE6123.1USNRCRegulatory Guide1.121Basis123.1.13.1.2F1.3MinimumAllowable WallDetermination

---12LeakBeforeBreakVerification

14EddyCurrentTestingUncertainty

153.2Operating IntervalJustification

-SafetyAssessment 154.0OPERATING INTERVALDETERMINATION 4.1Operational Considerations 4.2TubeBundleCondition Projection

5.0CONCLUSION

S 16161718 LISTOFABBREVIATIONS ASMEDIEC,ECTEFPDEFPMgpdgpmIGMECoIGA/SCCMWt-hrNDDR.G.RxxCxxSGxxSLB/FLBAmericanSocietyofMechanical Engineers Distorted indication Eddycurrent,eddycurrenttestingEffective fullpowerdayEffective fullpowermonthGallonsperdayGallonsperminuteIndiana&MichiganElectricCompanyIntergranular attack/stress corrosion crackingKilohertz

Megawatt, thermalMegawatthours,thermalNodetectable degradation USNRCRegulatory GuideRowandcolumndesignation ofsteamgenerator tubeSteamGenerator No.21,22,23,or24SteamlinebreakorfeedlinebreakaccidentscenarioSQRSuSquirrelUltimatetensilestressTWUDSThrough-wall penetration Undefined signal LISTOFTABLESANDFIGURESTable1Table2ListofTablesIndications ofHotLegSecondary SideCorrosion-March1987A.Including onlythemostsignificant indication pertube.B.Including multipleindications pertube.TubesPluggedDuetoIGA/SCC-GeneralComparison Table3Table4Table5Table6TubesPluggedDuetoIGA/SCC,Compensated forChangesinAnalysisandPluggingCriteria-Comparison byLocationCook2SteamGenerator Tubing-MinimumAcceptable WallRequirements A.Tubesheet creviceandtubesheet surfaceregions.B.Tubesupportplateintersections.

Cook2SteamGenerator Tubing-Allowable WallLossDetermination Operating IntervalJustification

-Remainder ofFuelCycle6-R.G.1.121BasisListofFiuresFigure1Figure2Figure3SG21Tubesheet Map:March1987SG22Tubesheet Map:March1987SG23Tubesheet Map:March1987ECInspection Results-ECInspection Results-ECInspection Results-Figure4SG24Tubesheet Map:ECInspection Results-March1987 ListofFiuresCont'dFigure5SGs21,22,23,and24:ECInspection Results-March1987;Indications ofIGA/SCCvs.Generator Elevations Figure6SGComposite:

"Start-of-Interval TubeBundleCondition

-July1986"comparedto"End-of-Interval TubeBundleCondition

-March1987Figure7Figure8Figure9Comparison ofModelPrediction toActualECInspection Results,Tubesheet CreviceRegionComparison ofModelPrediction toActualECInspection Results,Tubesheet SurfaceRegionComparison ofModelPrediction toActualECInspection Results,TubeSupportPlateIntersections Figure10FigurellFigure12A.Fullrange,0to1004TW.B.Blowupof40to100%TWrange.Projected End-of-Interval Condition, Tubesheet CreviceRegionProjected End-of-Interval Condition, Tubesheet SurfaceRegionProjected End-of-Interval Condition, TubeSupportPlateIntersections REFERENCE SUBMITTALS AEP:NRC:0936G, lettertoMr.HaroldR.Denton,NRR-USNRC, "SteamGenerator TubeLeakandReturntoPowerOperation,"

datedApril9,1987.AEP:NRC:0936E, lettertoMr.HaroldR.Denton,NRR-USNRC, "SteamGenerator TubeIntegrity,"

datedNovember24'986;transmittal ofWCAP-11329 (proprietary version)andWCAP-11330 (non-proprietary version).

AEP:NRC:0936C, lettertoMr.HaroldR.Denton,NRR-USNRC, "SteamGenerator TubeIntegrity

-InterimStatusReport,"datedFebruary7,1986;transmittal ofWCAP-11055 (Proprietary version)andWCAP-11056 (non-proprietary version).

AEP:NRC:0936A, lettertoMr.HaroldR.Denton,NRR-USNRC, "SteamGenerator TubePlugging-InterimStatusReport,"datedOctober10,1985.

STEAMGENERATOR TUBEINTEGRITY

-APRIL1987AnAssessment oftheNextOperating IntervalLength

1.0INTRODUCTION

1.1ReortObectivesReference Submittal 1addressed thecourseofactiontakenasaresultoftheDonaldC.CookNuclearPlantUnit2(Cook2)steamgenerator tubeleakwhichoccurredonMarch3,1987.Theletterdocumented thepreliminary tubeinspection results,addressed restoration oftubebundleintegrity tothesamelevelasatthebeginning ofthepreviousoperating period,andpresented qualitative justification forreturntopowerandoperation forastaff-recommended initialperiodofthreemonths.Thepurposeofthisfollow-up reportistoprovideamorecompleteevaluation ofrecentevents,andtopresentquantitative justification foranoperating intervalinexcessoftheinitialthreemonths.Historically, assessment ofanoperating intervalbetweensteamgenerator tubeinspections hasconsidered onlythesafetyissuesofUSNRCRegulatory Guide1.121.Suchanassessment waspresented forCook2tojustifyoperation throughtheentirecurrentfuelcycle(Reference Submittal 2).However,evenminorsteamgenerator tubeleakage,althoughnotasafetyissueandinfactallowable uptothelimitsetbytheplantTechnical Specifications, isundesirable frombothregulatory andoperating perspectives.

Inrecognition ofthisfact,determination ofthenextoperating intervalwillfocusonminimizing thepotential foraforcedoutageduetoexcessive steamgenerator leakage.ThesafetyissuesofR.G.1.121willofcoursebeagainmetbythismoreconservative approach.

1.20eratinExerienceOverviewl.2.1Background Cook2incorporates anuclearsteamsupplysystemmanufac-turedbyWestinghouse, andislicensedfor3411MWt.Initialcriticality occurredonMarch10,1978.Theunitis D.C.CookUnit2AEP:NRC:0936J Attachment 1currently operating initssixthfuelcycle;at,theendofMarch1987,about5.7effective fullpoweryearsofoperation havebeenaccrued.Cook2hasfourWestinghouse Series51steamgenerators.

Adescription ofsignificant featuresandareviewofthetypesoftubedegradation experienced priortoNovember1983arecontained inReference Submittal 3.Alloftheearlytubedegradation wasunrelated tosecondary sidecorrosion.

Thefirstsignificant indication ofsecondary sidetubecorrosion intheCook2steamgenerators occurredinNovember1983,whentheunitwasremovedfromserviceduetosteamgenerator tubeleakage.Detailsofthatandsubsequent eventshavebeendiscussed intwomeetingswiththestaff(December 4,1985andSeptember 16,1986)andaredocumented inReference Submittals 2,3,and4.Forconvenience, however,following isachronology ofsignifi-cantsteamgenerator eventsuptoMarch1987.oNovember7,1983ForcedOutae-firststeamgenerator tubeleakduetosecondary sidecorrosion.

Leakrateof0.29gpmLeakidentified inSG21,TubeR16C40ECTof1225tubesintwosteamgenerators Pluggedthreetubes,allduetoindications ofsecondary sidecorrosion Restarted onNovember22,1983oMarch10,1984RefuelinOutae100percentECTineachsteamgenerator seventubesamplesremovedforanalysis; confirmed intergranular corrosion intubesheet regionPlugged402tubes,68ofwhichwereduetoindications ofsecondary sidecorrosion Restarted onJuly7,1984 D.C.CookUnit2AEP'NRC0936JAttachment 1oJuly15,1985ForcedOutae-steamgenerator tubeleakLeakrateof0.22gpmLeakidentified inSG23,TubeR16C56ECTof25tubesinSG23Pluggedtwotubes,bothduetoindications ofsecondary sidecorrosion Attempted restartonAugust2,1985oAugust2,1985ForcedOutae-steamgenerator tubeleakduringstart-upLeakratemeasurements notpossibleLeaksidentified inSG23,TubesR7C28andR14C70ECTof1500tubesinSG23Plugged35tubes,allduetosecondary sidecorrosion Initiated boricacidtreatment program(30percentpowersoakandon-lineaddition)

Restarted onAugust22,1985oAugust23,1985ForcedOutae-steamgenerator tubeleakduring30percentpowersoakLeakrateof0.2gpmLeaksidentified inSG22,TubeR14C41andSG24,TubeR19C52100percentECTinSGs21,22,and24;ECTofalltubesinSG23nottestedduringAugust2,outageFirstECindications notedathotlegtubesupportplateintersections Fivetubesamplesremovedforanalysis; confirmed intergran-ularcorrosion attubesupportplateintersections D.C.CookUnit2AEP:NRC:0936J Attachment 1Plugged110tubes,104ofwhichwereduetosecondary sidecorrosion Decidedtoadministratively limitunitpowertoabout,80percentRestarted onOctober23,1985oDecember4,1985Presentation toNRCStaffJustified continued operation untilscheduled refueling outage,approximately 90effective fullpowerdaysfromtheOctober23restartoFebruary28,1986RefuelinOutaeMinorsteamgenerator leakage,about0.04gpm,attimeofshutdownLeakidentified inSG22,R16C45ECTinaccordance withTechnical Specification surveillance requirements oninitialsampleof550tubes;classification ofSGs22and24asC-3requiredexpansion ofprogramtoalltubesineachsteamgenerator Plugged151tubes,149ofwhichwereduetosecondary sidecorrosion Boricacidtreatment programcontinued (creviceflushing, 30percentpowersoak,andon-lineaddition)

RestartonJuly7,1986Unitpoweragainadministratively limitedtoabout80percentoSeptember 16,1986Presentation toNRCStaffJustified continued operation throughentirefuelcyclewithoutshutdownforsteamgenerator surveillance-D.C.CookUnit2AEPNRC0936JAttachment 1InformedstaffofintenttoreplaceCook2steamgenerators 1.2.2MostRecentOperating PeriodCook2beganoperation inFuelCycle6onJuly7,1986.Thermalpoweroutput,throughout thecyclehasagainbeenadministratively limited-typically to80percent,althoughtherehavebeenbriefperiodsofoperation at90percentinordertoperformcertaintestsandtomeethighsystem'loaddemandduringthesummerpeakperiod.Thermalgeneration

.throughtheendofMarch1987hasbeen14,990,974 MWt-hrs,orabout183EFPDs.Onebriefoutageunrelated tosteamgenerator tubedegrada-tionoccurredearlyinCycle6.Following that,Cook2rancontinuously foraperiodof226calendardaysuntilbeingremovedfromserviceonMarch3,1987duetoanindicated primary-to-secondary leakinSG22.Themeasuredleakratewas0.247gpm,wellbelowtheTechnical Specification leakratelimitof500gpd(0.347gpm).TheleakingtubeinSG22wasidentified byhydrostatic testingasTubeR28C53,andwassubsequently confirmed byeddycurrenttestingtohaveathrough-wall defectinthehotlegtubesheet creviceabout3.7inchesbelowthetubesheet surface.Thisdefectistypicalofthesecondary sideIGA/SCCpreviously identified intheCook2steamgenerators.

Toverifytubeintegrity priortoreturning toservice,aneddycurrentinspection programconsistent withtherequirements ofTechnical Specification 3/4.4.5wasperformed.

Testingresultsarepresented anddiscussed inSection2.0ofthisreport.Afterrestoring tubebundleintegrity bypluggingdefective tubes,theunit,returnedtoserviceonApril21,1987.BeforechangingtoMode3,creviceflushingwithboricacid(1000-2000 ppmboron)wasperformed; a32-hoursoakatabout30percentpowerwithboricacid(50ppmboron)wasconducted; on-lineadditionofboricacid(5-10ppmboron)willcontinueduringpoweroperation.

Unitthermalpowerwillagainbeadministratively limitedtoabout80percent,althoughbriefperiodsofhigherpoweroperation maybenecessary fortestingortomeetsystemloaddemand.

D.C.CookUnit2AEP:NRC:0936Z Attachment 12.0CONDITION OFTUBEBUNDLES2.1SteamGenerator InsectionandTubePluin-March1987Althoughnotmandatory sincetheTechnical Specification leakratelimitwasnotexceededpriortoshutdown, IGMECoelectedtoverifythecondition oftheCook2steamgenerator tubebundlesbyperforming aneddycurrentinspection consistent withtherequirements ofTechnical Specification 3/4.4.5.TestingresultsofaninitialsampleofaboutsixpercentofthetubesineachofSGs22and24necessitated expanding theinspection topotentially affectedareasofalltubesineachsteamgenerator.

2.1.1EddyCurrentAnalysisCriteria/Tube PluggingCriteriaThecriteriausedtoanalyzeeddycurrentdataduringtheMarch1987inspection werethesameasthoseusedduringtheMay1986inspection.

Thesecriteriaweredeveloped fromacorrelation offieldbobbincoileddycurrentdatawithmetallography resultsoftubesamplesremovedin1984and1985,andarediscussed indetailinReference Submittals 2and3.Forconvenience, following isabriefsummaryofpertinent eddycurrentsignalclassifications:

oClearIndication reortedinercentthrouh-wallenetration or4TW-Asignalwithanunequivocal phaseanglemeasurable at400kHz,confirmed at100kHz;industrypracticeistouseathreshold voltage,usuallyabout1volt,todiscriminate betweenreportable andnon-reportable clearin'dications; asaconservatism, however,allclearindications, regard-lessofvoltage,werereportedfordisposition duringtheMay1986andMarch1987inspections.

oDistorted Indication DI-Asignalvisibleat400kHzbelievedbytheinterpreter torepresent tubedegrada-tion,butwithanunquantifiable phaseangle;expectedcorrelation inmixedfrequencies orothersinglefre-quenciesisnotnecessarily present.Qindication inthetubesheet creviceregionwhosesignaltraceat400kHziscomplexwithanunquantifiable phaseangle;theseindications havehistorically beenshowntocompromise tubewallintegrity.

D.C.CookUnit2AEPNRC09367Attachment 1oUndefined SinalUDS-Ananomalous signal,notnecessarily indicative oftubedegradation, but.whichtheinterpreter believesshouldbenotedforconsidera-tionanddisposition.

oNoDetectable DeradationNDD-Asignalwithnoevidenceoftubewalldegradation; eitherthereisnodegradation oritisbelowthedetection threshold.

TubepluggingcriteriausedduringtheMarch1987inspection werebasically thesameasthoseusedduringtheMay1986inspection, althoughanadditional conservatism wasincorporated forindications attubesupportplateintersections, asnotedbelow.Development andrationale forthesecriteriaarecontained inReference Submittal 2.Forconvenience, following isabriefsummaryofthepluggingcriteriaimplemented forsecondary sidecorrosion ineachofthethreeareasofconcern:oTubesheet crevicereionhotle-Allclearindications, DIs,SQRs,andUDSsinthetubesheet creviceregion(fromthetubesheet rolltransition tothesecondary faceofthetubesheet) wereconsidered pluggable, regardless ofvoltageorphaseangle.oTubesheet surfacereionhotle-Allclearindications, DIs,andUDSsinthetubesheet surfaceregion(fromthesecondary faceofthetubesheet uptoabout6inchesintothefreespanoftubing)wereconsidered pluggable, regardless ofvoltageorphaseangle.oTubesuortlateintersection hotle-Clearindications meetingathreshold voltageof1.75voltsandhavinganindicated through-wall penetration of>40percentwereconsidered pluggable.

Inaddition, someindications notmeetingthevoltagethreshold werepluggedonphaseanglealonebased'nrecommendations ofthedatainterpreter.

Thisrepresents anaddedconservatism overthecriteriausedinMay1986.ITheTechnical Specification pluggingcriteriaof>40percentthrough-wall penetration wasappliedtoallotherareasofthesteamgenerator tubing.2.1.2EddyCurrentInspection ResultsSummaries ofpertinent hotlegeddycurrentindications, bytypeandlocation, aregiveninTables1-Aand1-B.

D.C.CookUnit2AEP:NRC:09368 Attachment 1Quantities inTable1-Arepresent individual tubes;fortubeswithmultipleindications, onlytheindication deemedmostsevereislisted.Pluggingcriteriaareillustrated bytheboundarylinedrawninthetable.The107tubesinsidetheboundarywereremovedfromservicebyplugging.

Inaddition, threetubeswerepluggedduetoreasonsunrelated tosecondary sidecorrosion (twobecauseeddycurrenttestingcouldnotbeperformed andoneasaprecautionary measureduetoaDIatthetubesheet rolltransition).

InTable1-B,allindications havebeentabulated.

ThelargertotalcomparedtoTable1-Areflectsthefactthatsometubeshavemultipleindications, particularly at.tubesupportplateintersections.

Thistotalpopulation ofindications isusedinlaterdegradation growthrateevaluations.

Figures1,2,3,and4aretubesheet mapsforeachCook2steamgenerator showingthelocationandextentofwalldegradation inthesteamgenerator tubing.Indications plottedarethosecontained inTablel-A.Figure5graphically depictsthedataofTable1-Bforeachsteamgenerator.

Figure6isacomposite forallfoursteamgenerators, andgivesagraphical comparison oftotalindications reportedduringtheMarch1987inspection tothetotalindications leftinservicefollowing the1986inspection.

Thisprovidesanoverviewoftubedegradation progression duringthepastoperating period.2.2TubeDeradationGrowthRateEvaluation Theobjectives ofthissectionaretodetermine ifthetubedegradation observedduringthemostrecentoperating intervalisconsistent withaveragegrowthratespreviously developed, andtoattempttoidentifycharacteristics ofthestatistical distribution ofpreviousgrowthratedatawhichcouldbeusedintheevaluation offutureoperating intervals.

Threepastoperating intervals areofinterestinthissection,andforconvenience arereferredtoas84-85,85-86,and86-87.Pertinent, factorsineachintervalareasfollows:

D.C.CookUnit2AEP:NRC:0936Z Attachment 1IntervalDurationEFPDsBoricNominalAcid?Power84-857/07/84to7/15/85291.2No100485-8610/23/85to2/28/8685.0Yes86-877/07/86to3/03/87183.1Yes804804Thegeneral,averagegrowthratesincurrentuseweredeveloped afterthe85-86interval, andarebasedonapplyingidentical analysiscriteriatothe1985and1986inspection data;specifics ofthismethodology arediscussed inSection2.2.2.The85-86intervalprovidedauniqueopportunity todevelopagrowthratemethodology, becausemanytubesleftinserviceafterthe1985inspection wouldhavebeenpluggedhadthelateranalysisandpluggingcriteriabeeninuseatthetime.Thevalidityoftheexistinggeneralgrowthratemethodology willbeexaminedin,twoways.First,tubeplugginghistorywillbereviewedtoseeiftherecentpluggingisconsistent withpluggingexperience duringtheearlierinterval.

Thiscomparison cannotdemonstrate thatthemethodology isvalid,butcanbeusedtoshowthatthemethodology isnotnecessarily invalid.Second,thepopulation ofindications fromtherecentinspection willbeevaluated toseeifitstatistically fitsthedistribution ofthe85-86intervalgrowthratedata.2.2.1TubePluggingComparison Abroadindicator oftubedegradation growthrateisthetubepluggingrequiredattheendofeachoperating inter-val,asshowninItem1ofTable2.'Anobviousfallacywiththisgrosscomparison isthatitdoesnotaccountfordif-ferentoperating intervallengthsorforchangesindataanalysisandpluggingcriteria.

Accounting foroperating intervals of9.7,2.8,and6.1EFPMsforthe84-85,85-86,and86-87intervals, respectively, yieldsthecomparison showninItem2ofTable2.Furthercompensation forchangesinanalysisandpluggingcriteriaresultsinthemoremeaningful comparison giveninItem3.Thislastcomparison reflectsthe107tubespluggedattheendofthe85-86intervalwhichwouldhavebeenpluggedattheendofthe84-85intervalifthelatercriteriahadbeenapplied,andthe10tubespluggedattheendofthe86-87interval'hichneednothavebeenplugged.(Theselatter10tubes D.C.CookUnit2AEP:NRC:0936ZAttachment 110hadsupport,plateindications belowthe1.75voltthreshold forplugging, butwerepluggedasanaddedconservatism basedonrecommendations ofthedatainterpreter).

ReviewofTable2showsthatthecompensated tubepluggingrateduringthe86-87intervalcloselymatchesthat.ofthe85-86interval(15.9vs.14.8tubes/EFPM),

whichindicates thatthegeneraltubedegradation growthrateobservedinthe86-87intervalisconsistent withthat.observedinthe85-86interval.

Thisisanexpectedresultsincepowerlevelandchemistry parameters wereconsistent duringeachinterval.

Sincethegrowthratemethodology incorporates different generalgrowthratesforthetubesheet creviceregion,tubesheet surfaceregion,andtubesupportplateinter-sections, aslightlymorerefinedtestistoevaluatetubepluggingratesateachofthesethreeareas.Table3providesacomparison ofthecompensated tubepluggingrateforeachareaduringthe85-86and86-87intervals.

Reviewofthetableshowsthatthetubepluggingrateineachareaisfairlyconsistent forthetwointervals, andfurthersuggeststhatthegrowthratemetholodogy isvalid.Fromareviewoftubeplugginghistory,itcanbeconcluded thatthepluggingrequiredinMarch1987isconsistent withthepreviousoperating period.Therefore, thenumerical degradation growthratedatadeveloped duringthatpriorperiodmaybevalidforassessing thenextoperating interval.

2.2.2GrowthRateDetermination Quantitative generalgrowthrateshavebeenevaluated afterpastoperating intervals forthethreeareasofinterest.

Thedetermination oftheaveragegrowthrateforeachareahasbeenmadebycomparing eddycurrentinspection resultsbeforeandafteranoperating interval.

Tubeswithoutevidenceofdegradation orwithverylow,non-quantifiable degradation havebeenexcludedfromthecalculations.

Thus,thegrowthratesdetermined reflectthegeneral,averagedegradation growthrateoftubesundergoing observable degradation

-nottheentiretubebundle.Severalmethodsfordetermining numerical growthrateshavebeenused.Themostobjective andreliableisadirectcomparison ofclearindications fromoneintervaltothenext("4TW-4TW"Method).Theothermethodsareregardedaslessdependable sincetheyutilizeassumptions oninitialconditions.

Theyareuseful,however,becausetheyallowacomparison tothe"~TW-4TW"Methodresultsandbecausetheyprovidealargersamplesize.Asreportedinprior' D.C.CookUnit:2AEP:NRC:0936J Attachment 1submittals, thealternate methodsandthe"4TW-4TW"Methodyieldconsistent results.Asnotedearlier,the85-86intervalprovidedauniqueopportunity toassessgrowthratesusingthe"4TW-4TW"Method.Becauseofnewanalysisandpluggingcriteriathatevolvedafterthe1985outage(fromtubesamplesremovedduringthe1985outage),anumberofnow-pluggable indica-tionswereleftinserviceandgivenanopportunity togrowduringthe85-86operating interval.

Comparison ofthereevaluated 1985datawiththe1986dataresultedindevelopment ofthegeneralgrowthratemethodology described inReference Submittal 2,andsummarized belowforconvenience:

85-86IntervalIGASCCGrowthRatesLocationMeanGrowthRateSampleSizeTubesheet CreviceRegionTubesheet SurfaceRegion1.600.821918TubeSupportPlateInter-sections0.6638Theabilitytodetermine newgrowthratesduringthe86-87intervalforthetubesheet creviceandtubesheet surfaceregionsusingthe"<TW-~TW"Methodhasbeeneffectively eliminated becauseofpluggingcriteriawhichremovedallpreviousindications fromservice.Thusthepopulation'f 4TWsfromthisoperating intervalrepresents theextremeingrowthratepossibilities, i.e.tubesclassified previously asNDDwhichgrewtohigh4TWs.Inessence,allthatcanbeobservedisthetailofthestatistical distribution ofgrowthrates.Iftheextremesofthepopulation canbeshowntofitthedistribution ofthepreviousgrowthratedata,theassumption canthenbemadethatthedistribution asawholehasnotchangedandaprobabilistic growthratemodeldeveloped fromthe85-86intervaldatawillbevalid.2.2.3Probabilistic ModelVerification Toevaluatetheextremesintubeconditions observedinthemost.recentoperating interval1)astart-of-interval tubecondition probability distribution wasdetermined, 2)thegrowthrateprobability distribution fromthe85-86interval D.C.CookUnit2AEP'NRC0936ZAttachment 112wasassumed,3)thetwodistributions werecombinedtodefineanhypothetical end-of-interval tubebundlecondition, and4)thehypothetical condition wasthencomparedtotheMarch1987inspection resultstoconfirmthemodel.Thestart-of-interval tubecondition probability distribu-tionwasestablished fromthe1986inspection resultsandtheprobability ofdetection/non-detection forvariousindication sizes.Non-quantifiable indications (DIs,SQRs,andUDSs)wereincludedinthepopulation inanappropriate 4TWsizerangebasedondetection threshold andothereddycurrentinformation independent of"sizing"parameters whichweredeveloped fromcorrelation ofprevioustubesampleanalysisandeddycurrentdata(seeReference Submittal 3,WCAP-11055, Figure4.1andReference Submittal 2,WCAP-11329, Figure2.2.4).Theendresultsoftheabove-described comparison areshowninFigure7forthetubesheet creviceregion,Figure8forthetubesheet surfaceregion,andFigures9-Aand9-Bfortubesupportplateintersections.

Inthetubesheet surfaceregionandattubesupportplateintersections, themodeldatawasfitwitha"bestestimate" curve.Inthetubesheet creviceregion,themodeldatawasfitwithamoreconserva-tive"over-prediction" curveinrecognition ofthefactthatcrevicecorrosion hasbeenthelimitingfactorforcontinued operation.

Reviewofthesefiguresshowsverygoodagreement betweenthemodel'sprediction andtheactualinspection results.Fromthisitisconcluded thatgrowthratedatafromthe85-86intervalisvalidforassessing thelengthofthenextoperating interval.

3.0 EVALUATION

OFOPERATION THROUGHTHEENDOFFUELCYCLE63~1USNRCReulatorGuide1.121Basis3.l.1MinimumAllowable WallDetermination Minimumwallrequirements fortheCook2steamgenerator tubingwerecalculated inaccordance withthecriteriaofR.G.1.121,entitled"BasesforPluggingDegradedPWRSteamGenerator Tubes".Confirmation thattherecommendations oftheguidearemetintheCook2steamgenerators wasdemonstrated inReference Submittals 2and3,andisrestatedhereforconvenience.

D.C.CookUnit2AEPNRC0936ZAttachment 113Thebasicrecommendations ofR.G.1.121areoutlinedbelow.I.Allowable minimumwalldetermination perthefollowing:

Fornormalplantoperation, primarytubestressesarelimitedsuchthatamarginofsafetyof3isprovidedagainstexceeding theultimatetensilestressofthetubematerial, andtheyieldstrengthofthematerialisnotexceeded, considering normalandupsetcondition loadings.

2.Foraccidentcondition

loadings, therequirements ofparagraph NB-3225ofSectionIIIoftheASMECodearetobemet.Inaddition, itmustbedemonstrated thattheappliedloadsarelessthantheburststrengthofthetubesatoperating temperature asdetermined bytesting.3.Foralldesigntransients, thecumulative fatigueusagefactormustbelessthanunity.II.Leak-Before-Break Verification, i.e.,thatasinglethrough-wall crackwithaspecified leakagelimit(Technical Specification leakratelimit)duringnormaloperation wouldnot;propagate andresultintuberuptureduringpostulated accidentcondition loadings.

Inestabishingthesafelimitingcondition ofoperation ofatubeintermsofitsremaining wallthickness, theeffectsofloadingsduringbothnormaloperation andpostulated accidentconditions mustbeevaluated.

ItemI.3isaddressed indetailinbothReference Submittals 2and3.Briefly,fromtheviewpoint offatigueandrelatedimplications ofcracking, thecausesofcrackingareaccounted forintheverification ofleak-before-break.

Inthecalculation oftubeminimumwall,threedistinctareasoftubedegradation withintheCook2steamgenerators wereaddressed:

thetubesheet creviceregion,thetubesheet.

surfaceregion,andthetubesupportplateintersections.

Basedonpreviousmetallography, tubeminimumwalldetermination forlocalized tubedegradation occurring inthetubesheet creviceoratthetopofthetubesheet assumed:

D.C.CookUnit2AEP:NRC0936JAttachment 1142~Tubedegradation tobecharacterized aseithermultipleSCCorIGA/SCC(intergranular SCCcombinedwithshallower, morewidelyspreadIGA).Tubewalldegradation canbeevaluated asequivalent thinning(asaresultofIGA)withasuperimposed crack.3.Theaxialextentoftheequivalent thinnedlengthoftubedegradation is1.5inches.Also,theIGA(equivalent thinning) wasuniformaroundthetubecircumference.

Likewise, thetubeminimumwalldetermination forthe'localized tubedegradation occurring atthetubesupportplateelevations assumed:1.Tubedegradation tobemultipleSCC,withindividual cracks0.1to0.2inchinaxialextent.2.Partialthrough-wall crackingcanbeevaluated assingleandmultiplecracks.3~Astubesupportplatedegradation wasconfinedtothethickness ofthetubesupportplate,themaximummacrocrack lengthisequaltothesupportplatethickness, or0.75inch.4.Link-upofmultipleSCCisimprobable atpostulated accidentcondition pressuredifferential asreflected inthetubespecimenbursttests.Resultsofthesecalculations areprovidedinTable4foreachoftheaboveareasoftubedegradation.
Moreover, Table5providesasummaryofminimumwalldetermination forthethreeregionsoflocalized tubedegradation occurring intheD.C.CookUnit2steamgenerators.

Ineachcase,thelimitingcriterion fordetermining theallowable wallreduction istheR.G.1.121criterion fornormaloperation thatrequiresamarginofsafetyof3againstexceeding theultimatetensilestressofthematerial.

3.1.2Leak-Before-Break Verification Theleak-before-break rationale istolimittheallowable primary-to-secondary leakrateduringoperation suchthattheassociated cracklengthwhichTechnical Specification leakageoccursisthecriticalcracklengthcorresponding totubemaximumnormalthroughlessthanburst.atthe D.C.CookUnit2AEPNRC0936JAttachment 115maximumpostulated pressurecondition loading(SLB/FLB)

.Again,Reference Submittals 2and3showonthebasisofnormaloperation thatunstablecrackgrowthinatubeisnotexpectedtooccurinthetubesheet crevice,topofthetubesheet, ortubesupportplateintersections oftheCook2steamgenerators intheunlikelyeventofalimitingaccident.

Itisdemonstrated thatgrowthofpartialthrough-wall cracksexhibitalimitedaspectratio.Thischaracteristic resultsincrackextension through-wall priortoreachingtheSLB/FLBcriticalcracklength.3.2IGMECo'sutilization ofaprimary-to-secondary leakmonitoring, policywhichemphasizes bothabsoluteleakratemeasurement andrateofchange,andwhichincludestheinitiation ofactionpriortoreachingtheTechnical Specification limit,yieldsadditional safetymargin.3.1.3EddyCurrentTestingUncertainty Comparison ofinsitueddycurrentinspection resultswithlaboratory destructive analysisoftubesamplesremovedfromtheCook2steamgenerators hasprovidedagoodbasisfordetermining theeddycurrent.testinguncertainty associated withtheparticular tubedegradation experienced.

onCook2.Fortubesamplesinwhichmetallography revealedtubewallpenetration tobeatleast40percentthrough-wall, theinsitueddycurrenttestsyieldamaximumunder-prediction of16percent.Aswallpenetration getsdeeper,theeddycurrenttestsmorecloselypredicttheactualdepthofpenetration (seeReference Submittal 3,Figure4-3).Tobeconservative, a16percenteddycurrenttestinguncertainty isusedtoevaluateoperating intervallength.0eratinIntervalJustification

-SafetAssessment Theinfluence oftheoperating environment mayaffectsomeofthetubesinthesteamgenerator andresultinlocalized walldegradation.

Aspartofapreventive programtodetecttubedegradation, in-service inspection usingeddycurrenttechniques wasperformed.

Affectedtubeswitharemaining wallthickness greaterthantheminimumrequiredwallthickness areacceptable forcontinued service,providededdycurrentmeasurement uncertainty isaccounted forandanoperational allowance forcontinued degradation untilthenextscheduled inspection isconsidered.

Table6summarizes theprojected safetymarginsforlocallydegradedsteamgenerator tubing,bytubeelevation, uponcompletion ofCycle6operation ofCook2(about240EFPDsor8.0EFPMsfromstart-uponApril21,1987).Itisdemonstrated fromasafetyperspective that,operating intervalmarginexistsat D.C.CookUnit2AEP:NRC:0936J Attachment 116allthreetubeareasinquestionwithrespecttotubeminimumallowable wall.Thesemarginsarebasedonthemaximumpermissible walllosscalculated inaccordance withR.G.1.121criteria, aneddycurrenttestinguncertainty of16percent,andthegeneraldegradation growthratesdescribed inSection2.2.2.Whiletheaboveevaluation demonstrates thattherecommenda-tionsofR.G.1.121aremetforanoperating intervalof8.0EFPMs,theincidence ofprimary-to-secondary leakageduringthatintervalisnotprecluded.

I&MECohasconservatively chosentoestablish anoperating intervalwhichminimizes thepotential forforcedoutagesduetosteamgenerator tubeleaks.4.0OPERATING INTERVALDETERMINATION 4.10erational Considerations Asnotedearlier,anoperating intervalbetweensteamgenerator inspections willbeselectedsuchthatthepotential foraforcedoutageduetosteamgenerator leakageisminimized.

However,becauseofthehighcostandhighoccupational radiation exposureassociated withsteamgenerator inspections, theoperating intervalshouldbeaslongaspossibletominimizethenumberofintermediate inspections requiredpriortoreplacement oftheCook2steamgenerators.

The.selectedintervalshouldalsobeconsistent withfuelcycleconsiderations, andshouldofferIGMECosomeflexibility forscheduling basedonsystemloadrequirements.

Atstart-uponApril21,1987,Cook2hadabout240EFPDsoffuelremaining inCycle6.Sincetherecenttubeleakoccurredafteronly183EFPDsofoperation, theneedforanintermediate inspection isapparent.

Anobviousintervaltolookatwouldbethemid-point oftheremaining fuel,orabout120EFPDs.At80percentpower,theearliestthiscouldoccurismid-September 1987,whichwouldnotconflictwiththescheduled Cook1refueling

,andshouldbeafterthesummerpeakloadperiod.However,choosingtheexactmid-point oftheremaining fuelaffordsIGMEConoflexibility astowhentoremovetheunitfromservice;alateshutdownwouldviolatethejustified intervalandanearlyshutdownwouldmakethesecondintervallongerthanjustified.

Anallowance ofaboutthreeweeksshouldbeaddedtoprovidethisneededscheduling flexibility.

Therefore, anoperating intervalof140EFPDs,or4.7-EFPMs, D.C.CookUnit2AEP'NRC'0936Z Attachment 117isacceptable fromanoperational perspective.

Thepotential forsteamgenerator leakageduringthisintervalisassessedinSection4.2.4.2TubeBundleCondition Pro'ection Duringthemostrecentoperating

interval, asteamgenerator tubeleakofsufficient magnitude toinitiateunitshutdownoccurredsoonerthanexpectedbasedonthepriorsafetyanalysiswhichjustified operation throughCycle6.AlthoughtheleakwasbelowTechnical Specification limitsandwaswellwithinoperatorcontrolcapabilities topreventanoff-siteradiation release,theelementofsignificant currentinterestiswhytheleakoccurredinsuchashorttimeframe.Xnanefforttoaddressthisconcern,severalpossibilities wereidentified.

Eachpossibility, alongwithitsassociated responserelativetoselecting thenextoperating intervalandanevaluation ofitslikelihood ofbeingtrue,isoutlinedbelow:higherthanduringpreviousperiods.~Resonse-Usethehighermeangrowthratestoadjusttheoperating intervaltocomplywithsafetyanalysisconsiderations.

Evaluation

-Littleornoevidencecouldbefoundtosupportthispossibility; asdescribed inSection2.2,growthratesareconsistent withthe85-86interval.

generaldz.stributions ofgrowthratesandinitialconditions, andistherefore arandomevent.~Resonse-Maintaintheprioroperating intervaljusti-fication, andaccommodate leakagefromanyadditional "outliers" throughleakratemonitoring andmaintenance shutdowns asrequired.

Evaluation

-Someevidencesupporting thispossibility isfoundinthefactthatthereisalownumberofveryhighlevelindications separated fromthemaindistri-butionofindications.

combining theextremesofthegeneraldistributions ofgrowthratesandinitialconditions.

D.C.CookUnit2AEPNRC0936JAttachment 118~Resonse-Adjusttheoperating intervaltoreducethepotential forleakagebyconsidering thestatistical distribution ofthegrowthratedata.Evaluation

-Evidenceinsupportofthispossi-bilitywasdeveloped throughaprobabilistic modelcombining start-of-interval tubeconditions andgrowthrates,asdescribed inSection2.2.3.Theresultsoftheevaluation havelargelyeliminated thefirstpossibility.

Whilethesecondandthirdcasesarestillpossible, thepresentinformation favorsthethird.Therefore, undertheassumption thatextremedegradation conditions areafunctionofoperating intervalandnotarandomoccurrence, itseemsprudenttoadjusttheoperating intervaltominimizethepotential forleakage.Consistent withtheSection4.1discussion ofreasonable operating intervallengths,anoperating intervalof4.7EFPMswasconsidered.

Toassessthereduction inpotential forleakage,theprobabilistic modeldescribed inSection2.2.3wasappliedinthesamemannerasusedtoassessgrowthrate.Theanalysisincludednewstart-of-interval conditions resulting fromtheMarch1987inspection andplugging, andusedthegrowthratedistribution derivedfromthe85-86interval.

Theprojected end-of-interval conditions forthetubesheet creviceregion,tubesheet surfaceregion,andtubesupportplateintersections areshowninFigures10,ll,and12.Sincetheend-of-interval projections shownoappreciable numberoftubesatextremewallpenetrations, suchasmightresultinleakage,the4.7EFPMintervalisconsidered appropriate.

5.0CONCLUSION

S Thefollowing conclusions havebeendrawnfromreviewandevaluation oftheMarch1987Cook2steamgenerator tubeleakeventandsubsequent eddycurrentinspection results:oTheleakwastypicalofpreviousIGA/SCCdegradation experienced intheCook2steamgenerators.

Anadequateunderstanding ofthisdegradation mechanism hasbeenacquiredthroughpreviousmetallographic examination andbursttestingoftubesamples,sonofurtherdestructive testingisnecessary.

oTherecentoveralleddycurrentinspection resultsand D.C.CookUnit2AEP:NRC:0936J Attachment 119thenumberofpluggable indications areconsistent withexperience intheprioroperating

interval, andcanbeusedtoshowthat.thegeneral,averagedegradation growthratemethodology developed fromthe85-86operating intervalisstillvalid.oAR.G.1.121safetyevaluation basedontubestructural limitsfortheCook2steamgenerator tubing,generaltubedegradation growthrates,andaconservative eddycurrentuncertainty margincouldbeusedtojustifyoperation throughtheremaining 8.0EFPMsofCycle6.However,thereisadistinctprobability ofatubeleakoccurring duringthatinterval.

oAprobabilistic gr'owthratemodeldeveloped fromthegeneralgrowthratedatabasecanbeusedtopredictextremeconditions ofthetubebundlesfollowing aspecified operating interval.

Determination ofanoperating intervalbasedonextremeratherthangeneraltubeconditions shouldgreatlyreducetheprobability ofaprimary-to-secondary steamgenerator tubeleakduringthatinterval, althoughthepossibility ofarandom(outlier) eventcannotbeprecluded.

Selection ofaconservative operating intervalbasedonextremetubeconditions shouldalsoincludeoperational considerations toreasonably limittheeconomicpenal-tiesandincreased personnel radiation exposureassociated withmorefrequentsteamgenerator inspec-tions.Anoperating intervalofabout4.7EFPMsmeasuredfromthereturn-to-power inApril1987appearsmostappropriate whenconsidering bothextremetubeconditions andremaining fuelinthecurrentfuelcycle.IGMECowillremoveCook2fromservicewithinthatintervaltoverifyandrestoreasnecessary theintegrity ofthesteamgenerator tubebundles.Thesubsequent operating intervalwouldendattheCycle7refueling outage.oSelection ofoperating intervals beyondCycle6shouldconsideroperating experience duringthenexttwointervals, theresultsofthenexttwotubeinspection

programs, lengthofthenextfuelcycle,andscheduling ofthesteamgenerator replacement outage.I&MECorecognizes thatexcessive steamgenerator tubeleakageresulting inunscheduled shutdowns isnotacceptable onacontinuing basis,andhasadoptedaconservative.

D.C.CookUnit.2AEP:NRC:0936J Attachment 120approachtoselecting thenextoperating intervalwhichshouldgreatlyreducetheprobability ofaforcedshutdownduetoleakage.Previously instituted remedialmeasures(e.g.-bettersecondary waterchemistry, boricacidtreatment, andadministrative powerreduction) willbecontinued during.theinterval.

Intheunlikelyeventthattheincidence ofextremewallpenetration isarandomeventandisnotpredicted bytheforegoing probabilistic

analysis, thenIGMECo'sleakratemonitoring programandtheTechnical Specification leakratelimitwillensureleak-before-break conditions andthatanorderlyshutdowncanbeaffected.

IGMECo'sadministrative policyofshuttingdownbeforereachingtheactualleakratelimitaddsadditional margintoleak-before-break considerations.

Table1Indications ofHotLeSecondarSideCorrosion

-March1987A.Including onlythemostsignificant indication pertube,totalforall4SGs.Location<404>404DIUDSSQRTotalTubesheet Crevice64255Tubesheet Surface1931TubeSupportPlatesTotal15182159433615042630716B.Including multipleindications pertube,totalforall4SGs.Location<404>40<DIUDSSQRTotalTubesheet CreviceTubesheet Surface196425531TubeSupportPlatesTotal16192383035851042869955 Table2TubesPluedDuetoIGASCC-GeneralComarison0eratinInterval84-8585-8686-872~3~Tubespluggedduetosecondary sideIGA/SCC(totaltubes)Tubespluggedduetosecondary sideIGA/SCC(tubes/EFPM)

Tubespluggedduetosecondary sideIGA/SCC,compensated forchangesinanalysisandpluggingcriteria(tubes/EFPM) 14114.525.514952.714.810717.515.9Table3TubesPluedDuetoIGASCCComensatedforChanesinAnalsisandPluinCriteria-ComarisonbLocationLocation0eratinInterval85-8686-87Tubesheet Crevice(tubes/EFPM)

Tubesheet Surface(tubes/EFPM)

TubeSupportPlateIntersections (tubes/EFPM) 10.92.81~114.89.05.11.815.9 Table4Cook2SteamGenerator TubinMinimumAccetableWallReirementsA.Tubesheet creviceandtubesheet surfaceregions.CriteriaCondition MinimumWallinchesyieldASMECodeSu/3normalfaultednormal0.0150.0170.019B.Tubesupportplateintersections.

CriteriaCondition MinimumWallinchesyieldASMECodeSu/3normalfaultednormal0.0120.0130.015Table5Cook2SteamGenerator TubinAllowable WallLossDetermination LocationGeometric Condition BasisAllowable Wall~LossTubesheet CreviceRegionAxialextent>1.5inchesSu/362Tubesheet SurfaceRegionAxialextent>1.5inchesSu/362TubeSupportPlateIntersections Axialextent<0.75inchesSu/370 Table60eratinIntervalJustification Remainder ofFuelCcle6-R.G.1.121BasisItemAllowable tubewallloss(>)ECTuncertainty (4).Growth(4/EFPM)Projected growth(+o/8'EFPM)Plugginglevelrequired(4)Tubesheet Crevice62*161.612.833.2Tubesheet Surface62160.826.639.4TubeSupportPlates70*160.665.348.7Plugginglevelimplemented

(%)AllAll40.0*Tubeburstwithinthetubesheet creviceregionorattubesupportplateintersections isconsidered tobeincredible.

ECINSPECTION RESuLTS-MARCHf987SECONDARY SIDECORROSION, HOTLEGPLANT:DCCOOKUNIT20~PLUGGASLE INOICATIONS.

TSCREVICE(2)w~teN-PLUGGASLE INOICATIOHS.

TSPS092)GENERATOR:

2i0~PLUGGABLE IHOICATIOHS.

TSSURFACE(T)TOTALTUBES:3388OUTOFSERVICE(N):

f42v~PLUGGASLE IHOICATIOHS.

TSPo(5)TOTALTUSESASSIGNER209~~aoOJJ~~JJJ25C/lD~~~++~20IIVAd'DCll~J4~JJJJ~JJJ~JJ~JTJJ~joO~4~~~~~~~~J~~~~~~~~~~~~~~~~~~~~~J~~~~~~~~~~~~~~~~~~~o5IIrgNAHNAYIIIP.INLET(NotLog)IIIIICOLUHHSIIIIIIR88oFigure1I ECINSPECTION RESULTS-MARCHl987SECONDARY SIDECORROSION, HOTLEGPLANT:DCCOOKUNIT2GENERATOR:

22TOTALTUBES:3388OUTOFSERVICE(B):2lOo-pLUBBABLE ZNorcAnoNB.

TscREvrcEtao)ksNOIR.USSABLE ZNOZCATZONS.

TSPs(2fB)0~PLUSSABLE ZNOZCATZONS.

TSSURFACEBO)T>PLUSSABLE ZNOZCATZONS.

TSPs0)70TALTUBEsAssrsNEo:

247k00~~JkJkkkkkJkkkkJ~JkJJ~kkJkJkkJkSkJJJ~~k~~~~~~s~~~~4~~~k~~~~0~JksJJoJSJk20V-XOJkkkkkJ0okkk~J~JkSk~ks4kos~kkk0kkkJ0~J0J0J000~J~~~~~~s~~~~~~~~~~s~~~~s~k~~~sJJk~~~~kkJ~~~~~~~~~~~~J~~~~~~~~~~~~~s~~s~~~~~~~~~~~~s~~IIgIII8RZtLETtNotLay)IIIII=IIIIIIIIIt88800RR-880COLUNNSNOZZLEFigure2 ECINSPECTION RESULTS-MARCH1987SECONDARY SIDECORROSION, HOTLEGPLANT:DCCOOKUNIT2GENERATOR 23TOTALTUBES'388 OUTOFSERVICE(0):2100~PUNSABLEINOICATIOHS.

TSCREVICE(27)NNRVISSABLE IINICATIONS, TSPOt64)0~PLUSBA))LE IHOICATIONS, TSENFACE(9)v~PLU66ABLE INOICATIONS, TSPs(1)TOTALTLNESASSISNEL121J~~J~JJ~~ssr~~00~0r0~J~J0~~J0~ss~rrr~os~~~0~~~~r44or00~~~~0~JIQR20V10r~~r~~~~00~~~r~~rr~~~~r~~~r~~~r~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~r~~~srr~r~~~~~JJ~re~IIgNAWAYIIIm8RI%ETOhtLsg)IIIIIIIIIIIIII)'38m)800R888880c-COLUMNSHOZZLEFigure3 ECINSPECTION RESULTS-MARCH1987SECONDARY SIDECORROSION, HOTLEGPLANT:OCCOOKUNIT2CI~PLUSSABLE Z)QICATZOMS, TSCREVICE(8)A~~U68ABLEINOZCATZONS, TSPr(Zfd)GENERATOR 24TOTALTUBES:3388OUTOFSERVICE(s)'010~PLUBSASLE I)NICATIONS, TSSNFACE(8)T~PLUSSABLE IHOZCATZNBL TSPs(1i)TOTALT(NESASSZSNE(k f39AA~AAA~~~A~AAAVAAAAAAATA~AAAAAAAA~~~~~~~~~~s~~~~~s~r~~~~s~~~~~20V~~s~ArAss~4A~~4s~~ArA4~sLO8s~ssburrsburrs~s~rsr'~~~r~~~~~rer~~~~~~~~~~~~~~~~~~s~~~~rs~~srsss~r~~~~~rsvpIIIIIIIIIIIIIIII888P888$8))88888)IANNAYI)LET(HotLoO)COL(NNSFigure4 OCCOOKUHIT2SteamGenerator I2fECINSPECTION RESULTS-MARCHi987cvCaOCCOOKUIIIT2SteamGenetator:

22ECINSPECTION RESULTS-MARCHi98700to8H820HI"0Ha2HTM4TSiH~aHsHSH~~~sHTHTRHTSHtHSHSH~OHeHVHGENERATOR ELKVATZONS 4R0088HIt.0OI20HI-0HD2HTlOI'tmfHCHtkHsHSH~~~sHTHTRHTSH4HaHSM~sH4MVHGENERATOR ELEVATZONS

>40%TW~DZ<<40KTW~UDSSQRQ>40%TW+DZQ<<40KTWQUDSQSQROCCOOKUIIIT2SteamGenetstoaI23ECINSPECTION RESULTS-MARCHi987OCCOOKUNIT2SteamGenetstot':24ECINSPECTION RESULTS-MARCHi98700to8H10000oI8HCOO820HI-0HD2HVlOITSIH4HeHaHatt~~~OH7HTEHT4HCHRHSH~sHOHvHGENERATOR ELEVATZONS 20HI-0HD2HTIHTSfHfHsttSH5H~~~eH7HTCHTSHkHsttaHAHsHOHVHGENERATOR ELKVATZONS

%40XTW~DZ<<40KTW~UDSggSQR>40%TW~DZ<<40%TW~UDSSQRFigure DCCOOKUNIT2SteamGenerator:

00START-OF-INTERVAL TUBEBUNDLECONDITION

-JUL198640000N8HQ.0NZ0HI"00ZH350300250200150100ao0TMHTS-1H1H-2H2H3HSH4H4H-5HSH-SH6H7HTEHTSH1H2HSH4HSHSH7HGENERATOR ELEVATIONS

>40KTW+DI<40KTW+UDSDCCOOKUNIT2SteamGenerator

00END-OF-INTERVAL TUBEBUNDLECONDITION

-MARCH198740000N8HU0NZ0Hl-0H0ZHSSO900250200150100500THHTs-1H1H-2H2H-SH9H-4H4H-aHSH-SHSH-7HTSHTSH1H2HSH4HSHSH7HGENERATOR ELEVATIONS>40XTW+DI<40KTW+UDSFigure6 40C0~rfg)30O~IH200Inspection Model1005060708090100TubeWallPenetration,10KIntervalsFigure7Comparison ofModelPrediction toActualECInspection Results,Tubesheet CreviceRegion i6Co14U12HiO8O6z2'~Y'Inspection Model405060708090iooTubeWallPenetration,iOXIntervalsFigure8Comparison ofModelPrediction toActualECInspection Results,Tubesheet SurfaceRegion (o500C04003000200z~Inspection Model0iO2030405060708090100TubeWallPenetration, KOXIntervalsFigure9-AComparison ofModelPrediction toActualECInspection Results,TubeSupportPlateIntersections FullRange,0toKOOKTW 25C0~He20U~HC150c1054'Inspection Model405060708090100TubeWallPenetration, 10KIntervals Figure9-BComparison ofModelPredictiontoActualECInspection Results,TubeSupportPlateIntersectionsBlowupof40to100KTWRange

10010EDz.01TubeWallPenetration,5%IntervalsFigure10PredictedEnd-of-Interva1Condition, Tubesheet CreviceRegion 10010*Sr.14TubeWallPenetration, 5XIntervals Figure11Predicted End-of-Interval Condition, Tubesheet SurfaceRegion 100TubeWallPenetration,5XIntervals Figure12Predicted End-of-Interva1Condition, TubeSupportPlateIntersections 03 Attachment 2toAEP:NRC:0936J SteamGenerator ManwayCoverClosureRepairsMarch-April 1987 h

D.C.CookUnit2AEP'NRC:0936j'ttachment 21ESTEAMGENERATOR MANWAYCOVERCLOSUREREPAIRSMarch-April 1987Eachsteamgenerator channelheadhalf(hotlegandcoldleg)hasa16-inchmanway;designoftheboltedclosureisshowninFigure1.Whenopeningthemanwaystoperformtubeinspections following theMarch1987steamgenerator tubeleak,difficulty inremovingtheboltsonbothlegsofSGs22and23wasencountered.

Therewasevidenceofgallingundertheboltheadatsomelocations, andanobservation wasmadethatinsufficient threadlubricant mayhavebeenusedduringthepreviousinstallation.

FiveboltsonSG23couldnotberemovedbyde-torquing andweredrilledout.TheboltsonSGs21and24wereremovedwithoutdifficulty.

ActionstakenbyI&MECoasaresultoftheboltremovalproblemincluded:

oAdesignchange(RFC)toallowuseofhardenedsteelwashersundertheboltheadswasapproved.

Thischangeisintendedtoprovideamoreuniformfrictionfactorunderthehead,andtherefore introduce moreuniformbolttension.oThenewly-approved washersandnewmanwaycoverboltswereprocuredforuseinre-installing themanwaycovers.oWestinghouse washiredtoinspectandgaugetheboltholes.A"go/notgo"gaugewasusedtodetermine theacceptability oftheholepitchdiameter.

Thegaugetolerances werethoseofanewholeandweretherefore veryconservative.

ResultsoftheboltholegaugingprogramonSGs22and23wereasfollows:oSG22-Fiveholesonthehotlegandfiveholesonthecoldleghadoversizepitchdiameters andrequiredrepair.oSSZZ-Thirteenholesonthehotlegandeightholesonthecoldleghadoversizepitchdiameters andrequiredrepair

D.C.CookUnit2AEP:NRC:0936J'ttachment 2Eventhough:nodifficulty wasexperienced onSGs21and24,themanway.coverboltholesonthosetwosteamgenerators weregaugedasanaddedprecaution.

Resultsofthatinspection areasfollows:oSG21-Allsixteenholesonbothhotandcoldlegswereslightlyoversizeandcouldnotbedispositioned byWestinghouse.

Znalllikelihood, theholeswereacceptable andacompleteanalysiswouldhavealloweddisposition oftheminthe"as-found" condition.

However,duetotheinherentdifficulty inmeasuring insitufemalethreadparameters (e.g.-threadform,threadangle,andactualpitchdiameter),

sufficient datatodoacompleteanalysiscouldnotbereadilyacquired, soitwasdecidedtorepairthesealso.oSG24-Allholeswereacceptable.

Twomethodsoffemalethreadrepairareincommonuse:replacement oftheexistingthreadswithaHeli-coil andinstallation ofathreadedinsert.TheHeli-coil methodwasselectedfortheUnit2repairs,withthethreadedinsertmethodheldasaback-upintheeventtheHeli-coil technique wasunsuccessful onaparticular hole.Westinghouse providedasafetyevaluation andinstallation procedure foreachmethod;anRFCtoallowtheuseofeitherwasapproved.

However,useofthreadedinsertswasnotnecessary.

TheHeli-coil repairtechnique consistsofdrillingtheexistingboltholeabout1/8inchoverthenominalsizetoremovetheoldthread,threading theresultant holewithanappropriate sizedthreadtap,andthenscrewinginastainless steelHeli-coil (tradenameforahelicalthreadwhoseoutersurfacemateswiththenewly-tapped holethreadsandwhoseinnersurfaceformsfemalethreadsforthebolthole).Thenewholeacceptsthesamesizedboltasbefore,andactuallyhas"better"threads(closertolerances, moreexactthreadform,and-inthiscase-bettermaterial).

Heli-coils areconsidered apermanent repair.TheHeli-coil repairsweremadetoallaffectedboltholesasnotedabove,andthemanwaycoverswereputinplace-usingwashersandnewbolts-withoutfurtherincident.

Weareevaluating thecauseofthisproblemandwewillinformtheNRCoftheresultsofthisevaluation whenitiscompleted Dia.ofCover26.75D~23bcFigure1PrimaryManveyArrangement GasketDia.16.~~Dia.18.1350.25--"16Bolts1-7/8in,Gasket:I.D.16.0625.O.D.=18.0625 Attachment 3toAEP:NRC:0936J Westinghouse NuclearSafetyEvaluation ofLooseMechanical PluginSteamGenerator 22 SECLr87-229CustomerReference No(s).Westinghouse Reference No(s).~NS-RCS~L-87-450WJ%TEAHOUSE NUCLEARSAFETYEVAIIJATION CKXZIZST1)NUCLZARPRATE(S)D.C.COOKUNIT22)CKKKLESTAPPIZCABIZ TO:IlXSEMESCALPIDGPH'MIGENERATOR 22(SubjectofChange)3)Ihesafetyevaluation oftherevisedprocedure, designchangeormodification requiredby10CFR50.59 hasbeenpreparedtotheextentrequiredandisattached.

Ifasafetyevaluation isnotre@~orisimxmplete foranyreason,explainonPage2.PartsAandBofthisSafetyEvaluation CheckListaretobecompleted onlyonthebasisofthesafetyevaluation performed.

CHECKIZST-PARDA(3.1)YesNoXAchangetotheplantasdescribed intheFSAR?(3.2)'esNoXAc1mngetoprocedures asdescribed intheFSAR?(3.3)YesNoXAtestorexperiment notdescribed intheFSAR?(3.4)YesNoXAchangetotheplanttechnical specifications (Appendix AtotheOperating License)?

4)CHECKIZST-PARPB(Justification forPartBanswersmustbe'included onpage2.)(4.1)YesNoX(4.2)YesNoX(4.3)YesNoX(4.4)YesNoX(4.5)YesNoX(4.6)YesNoX(4.7)YesNoXWilltheprobability ofanaccidentpreviously evaluated intheFSARbeincreased?

Willtheconsequences ofanaccidentpreviously evaluated intheFSARbeincreased?

Maythepossibility ofanaccidentwhichisdifferent thananyalreadyevaluated intheFSARbecreated?Willtheprobability ofamalfunction ofequipment important tosafetypreviously evaluated intheFSARbeincreased?

Willtheconsequences ofamalfunction ofequipment important tosafetypreviously evaluated intheFSARbeinn~sed?Maythepossibility ofamalfunction ofequipment important tosafetydifferent than,anyalreadyevaluated intheFSARbecreated?Willthemarginofsafetyasdefinedinthebasestoanytechnical specification bereduced?PAGE1OF7

'h SECL-87-229 Iftheanswerstoanyoftheabovequestions areun)mown,indicateunder5)R12%RESandexplainbelow.Iftheanswertoanyoftheabovequestions in4)cannotbeansweredinthenegative, basedonwrittensafetyevaluation, thechangecannotbeapprovedwithoutanapplication forlicenseamen(:hnent submitted totheNRCpursuantto10CFR50.59.

5)REMARKS:None'Ihefollowing mamarizes thejustification uponthewrittensafetyevaluation,

(*)foranswersgiveninPartBoftheSafetyEvaluation CheckList:SeeattachedSafetyEvaluation.

(*)Reference todocument(s) containing writtensafetyevaluation:

Section:PagesTables:Figures:Reasonfor/Description ofChange:NonePreparedby(NuclearSafety):MATIHENS Ccordinated withEngineer(e):NIESCNR.+Ccord(hated GroupManager(e):KEATING kPNuclearSafetycroupNanager:NNIsrC.Date.S~)~~~~Date:~~~87te:PAGE2OF7 SECL-87-229 NS-RCS~/L-87-450 PAGE3OF7D.C.COOKUIGT2ZDOSEMECHANICAL PIIJGSTEAMGENER%)Rf22SAHH'YEVAIIJATION

'Ihisevaluation isprovidedtoact]ressthesafetyimpactofanobjectfoundlodgedinatubeonthehotlegsideofsteamgenerator 422ofD.C.CookUnit2.'Iheitemhasbeenidentified asamechanical plugoriginally installed inthehotlegtubeendofanothertubeinthesamesteamgenerator.

1hisevaluation considers theeffectofdisengagement oftheplugfrcanthetubeinwhichitwasoriginally installed, theeffectoftheplugonthetubeinwhichitbecamelodgedandtheimpactoftheplugonthehotlegchannelheadccarponents whiletheplugwasmobileandnotlodgedinanytube.Duringtherec~t100%eddycurrentprogramatD.C.CookUnitg2,aforeignobjectwasreportedtobelodgedinthehotlegofsteamgenerator 522.'leobjectwaslocatedapproximately 0.75inchesabovethetubeendofRear3Column5.'Xheforeignobjectwasreportedtoberoundanditappearedtocloselyfillthetubeinnerdiameter(ID).Afterprelimir~

attemptsweremadetodislodgeandrezmvetheforeignobject,anattemptwasmadebysitepersonnel todrivetheobjectfurtherintothetube.i%iswasintendedtoallerenougha~toinstallamech-mical plugbehindit.Finallytheforeignobjectwassuccessfully rawvedbyinitially drillingapilothole,followedbydrillinga3/8incha~holethroughthematerial, inserting aslidehairandthenpullingitfreefromthetubeID.Onceremved,theforeignobjectwasidentified asaWestinghouse mechanical plugthathadlodgedinthetubeendinaninvert~position.

Athoroughreviewofvideotapesofthetubesheet inthehotlegofsteamgenerator 422showedthatthetubeendatRow40Column39wasmissingamechanical plug.'ibistubeendwasdocun~ted ashavingbeenpluggedintheApril,1986outage,wasdetexnuned tobeopenandwastheapparentsourceofthemechanical plugfoundinR3-C5.Toinvestigate thepossiblecauseoftheplugmovingfromthetubeendintowhichithadbeeninstalled, theremovedmechanical plugandthetubeendatR40-C39werevisuallyandmechanically inspected including theexpandeddiameters andthem~ndertranslation.

Visualexamination oftheplugbyexperienced meclmnical pluggingandqualityassurance personnel revealedthattheplugexhibited scratches onthesurfaceaswellasthepluglandshadbeenrounded SECXr87-229'S-RCS~/L-87-450 PAGE4OF7off.,'IhetubeID(R40~9)inthee1evation rangewheretheplugisdesignedtoseal,wasmeasureat0.5inchintervals attwoazimtuths.

Therecordeddiameters areconsistent withthencaninalroll.exparded diameters forsteamgenerators with7/8inchdiametertubing.%hetubeendwasvisuallyexaminedtocheckforanycircumferential indentations thatareoccasionally leftinthetubeIDafterasuccessful installation andsubsecpent

~ncnralandnonewereevident.%hetubeerdinwhichtheplugbecamelodged(R3-C5)wasinspected inaccord-mce withtheproperacceptance criteriaasspecified intheprocedure formechanical pluggingofsteamgenerator tubes.Itwasevaluated asacoeptable formeclmnical plugging.

WehotlegtubeendatR40C39wasa1soevaluated asacceptable forinstallation ofanewplug.Botherdsofthetubeinwhichtheplughadbeccalodged(R3-C5)weremechanically pluggedandthetubevznovedfromserviceasaprecautionary measure.%hehotlegtubeendofR40-C39,thatwasmissingthemechmicalplugwasalsomchtanically plugged.theprocessparameters forthesepluggingoperations werewitnessed, verified'arxlrecorded.

%hecondition forwhichtheR40-C39tubehadbeenpluggedintheApril1986outagewasaneddycurrent.irdication termedasqau~l.Suchanirdication isasignalinthetubesheet regionwhosetraceat400KHziscomplexandphaseangleunclear,butwhosepresencerepresents change.Kheseirdications havebeenhistorically pmventocompromise tubewallintegrity ifthetuberemainsinserviceandthushavebeenclassified astubedegradation.

IntheD.C.CookUnit2steamgenerators theseirdications areassociated withdegradation ontheoutsidesurfaceofthetubeinthetubetotubesheet crevice.'Ihecorrosion resistance ofasteamgenerator tubepluggedonthecoldlegonlywasevaluated.

Generalformsofcorrosion aretypically enviroranentally ard/ormaterially controlled.

Mostsecond-uy sideinitiated tubingcorrosion foundinrecirculating steamgenerators hasoccurredinlocalized regions(mostcananonly crevices) ofasteamgenerator tubeinwhichdissolved chemicalspeciescanbeconcentrated tolevelsfargreaterthanthoseinthebulkprimaryorsecondary fluid.Heattr<msferisnecessary suchthattheavailable superheat (localwalltemperature minusfluidsaturation temperature) isincreased comparedtovaluesassociated withconventional nucleateboilirgprocesses astheyexistonthetubesurface.'Iheelevatedtemperatures providethedrivingforceforpromoting chemicalconcentration i.e.,thepotential fortheformation ofalocallyconcentrated soluti.on canbecorrelated withtheexpectedavailable superheat withintheregion.Astheprimlyfluidwithinatubepluggedonthecoldlegonlywouldbeat SECXr87-229NS-RCS~/L-87-450 PAGE5OF7approximately secondary sidebulkfluidsaturation temperature andinasubcooled state,noheattransferwouldbeexpectedacrossthetubesurfaceandanylocalized tubedegradation including continuing degradation atthesiteofthepreviously locatededdycurrentsignalwouldbeexpecttobeminhnal.'Ihesafetyimpactofoperation oftubeR40-C39withwhatisnormallyapluggable indication ismitigated bythegeometryoftheregion.%hetubetotubesheet creviceisthespacebetweenthetubesheet andtheunexparxled tubesandisontheorderofafewmills.Tubeplugginglimitsare,established inpartbasedonpredicted performance ofadegradedtubeunderpostulated faultedconditions, specifically steamlinebreakconditions.

Forindications inthetubesheet creviceregion,tuberuptureisnotpossibleduetothepresenceofthetubesheet aroundthetubewhichwouldcontainthemnrementofthetubewallrequiredtoeffectabursttubecondition.

'lherefore, intheeventofapostulated steamlinebreakwiththemechanical plug,missingfmmoneendoftheR40-C39tubeandthepreviously observededdycurrentindication wouldnotbeexpectedtoresultinprimarytosecorxtey leakageinexcessofthatusedforaccidentanalyses.

'Iheeffectofplantoperation onplugintegrity foruptooneyearwiththesteamgenerator tubepluggedononlythecoldlegsidehasbeenevaluated.

'Ihemechanical plugwasdesignedtoacxxzmnodate thedesignconditions specified forthesteamgenerator.

'lhedesignconditions enveloptheapproximate 10psipressuredifferential whichoccursacrossthechannelheadinatubewhichhasbeenpluggedonthecoldlegonlybutnotonthehotleg.'Ihedesignverification p~msimulated thesteamgenerator serviceconditions ofter~rature andpressureaswellasthermalcyclingassociated withthevariousplantconditions.

'Ihedesignverification progranfortheexpandedmeat~calplugdemonstrated pressurebourxho~integrity undersimulated faultedcondition loadingsinadditiontootherplantops~tingconditions.

%hedesignoftheSeries51steamgenerators atD.C.CookUnit2includesasmallextension ofthetubeendpastthebottomofthetubesheet surface.Aforeignobjectramvedframthechannelheadduringapreviousoutagehadresultedinsomedeformation ofthetubeends.Noneofthetubeendsoftheothertubeshadarestriction thatwouldpreventinsertion ofaneddycurrentprobearxlthetubeendshadnoapparentadditional damageduetothelooseplug.%hetubetotubesheet weldsarepartially shieldedfromimpactofanobjectofthesizeofamechanical plugandtheweldshadnoapgm~tdamage.thecladdingofthechannelheadandthetubesheet alsoshowednoapp-~tdamage.'Lhetubes,channelheadandtubesheet

cladding, weldmetalandthemechanical plugareallcarposedofveryductilematerial.

Reputedimpactoftheplugonthecladding, tubeends,andtubetotubesheet weldwouldnotbeexpecttocause SECL-87-229 NS-RCS~/L-87-450 PAGE6OF7crackirgorsmallpiecestobreaklooseframthesurfacesimpactedbythelooseplug.Evaluations ofmeresignificant deformation oftubeendsinathersteamgenexators ofsimilardesignhaveshownthatdeformation ofthetubeerdwillnotsignificantly degradethestructural integrity ofthetubeorthetubetottd~sheet weldorcauseasignificant increaseintherestriction toflawthroughthesteamgenerator

'lhemechanical pluglandouterdiameters approximate thetubeinnerdiameters intheseatingareaofR40~9.Inorderforthemechanical plugtohavep~~sealirg,theplugshouldhavebeenlargerthanthetubeIDtoallawforaninterference fit.Dherewasnovisuallydicmernible evidenceontheIDofthetubeatR40-C39thattheplughadapositiveinterference fit,withthetube,althoughitisnotmardatory tohavethisforapraperlyinstalled plug.Insametubeerdsthatareapproximately ashardasthepluglands,however,therearenointerference marksandplugsaresuccessfully installed.

'lheestimateoftheactualtranslation oftheexparderintherepavedmechanical plugwouldindicatethatinsufficient

~~nsionhadoccurred.

'Iheestimated expardertranslation distancedidnotmeetthepzocech.xe installation minirttum reguixement.

%hepossibile ananelies inthetube-to-tubesheet jointcontributing tothedisengagement ofthemechanical plugwerereviewed.

%heavality,notapernoranyotherproblem(suchasalackofrollexpansion intheplugsealingarea),whichwouldirdicatethattheconfiguration ofthetubejointcontributed totheasinstalled condition ofthemechanical plug.Basedonthefirdirgsoftheinvestigation outlinedaboveithasbeenconcluded thatsuccessful installation pamneters formectmu.cal plugwerenotachievedanditwaseventually displaced fmmthetubeerdduringtheoperating periodpreyingthediscovery ofthemisplaced plug.RelevantWestinghouse logbooks,datasheets,notesardprol.xxiures werereviewedindetailframtheApril,1986outageinanattend%toidentifyapotential areatoaccauntfortheasinstalled cordition oftheplug.%hejobsitecoordinator, shiftsupervisors andotherkeypeL~nnelwerequeriedtoattempttoidentifyacausative factor'.Inallcasestherewasnothingidentified.

BasedonpriorWestinghouse mcpmience ofvirtually 100%su~fulinstallations overaneightyearperiodofover25,000previousaedmnical pluginstallations, coupledwithotherinstallation datacollected onsurveillance reportsfromalargepercentage ofmechanical plug

~87-229NS-EKS~/L-87-450 PAGE7OF7installations duringtheApril,1986progmn,thejudcpnent hasbeenmadethattheprobability that.theothermechanical plugsinstalled atD.C.CookVnit42duringtheApril,1986outagewereinstalled correctly approximates 1004.CONCWSIONS Onthebasisoftheinvestigation andevaluation asoutlinedabove,ithasbeenconcluded thatthemechanical pluglodgedinthehotlegofsteamgenerator 422inR3-C5isthesamemechanical plugthatwasoriginally installed intheR40~9inthesamelegofthesamesteamgenerator duringthe4/86outage.Duetotheconditions ofthefluidinthepartially pluggedtube,significant.

additional orcontinuing corrosion wouldnotbeexpectedtooccur.Operation ofthesteamgenerator withoneplugintheR40~9tubeisnotexp~tohaveresultedinacondition whichwouldhavecausedprimarytosecordary leakageintheeventofapostulated steamlinebreakinexcessofthatassumedforaccidentanalyses.

Theintegrity oftheplugonthecoldlegofthetubeframwhichthehotlegsideplugwasdisplaced, wasmaintained undernormaloperating andpostulated accidentcondition loadings.

Theimpactofthelooseplugpriortobeel.'ming lodgedintubeR3-C5causednoapparentdamagetothetubeendsorothersurfacesinthechannelhead.Theapparentcauseoftheasinstalled condition ofthesubjectmechanical plugistheteritunation oftheinstallation processpriortoreachingsuccessful installation parameters.

Therefore thedisplacement ofamechanical plugframthehotlegendoftubeR40-C39,theimpactofthelooseplugonthechannelheadsurfaces, andthesubsequent lodgingoftheplugintubeR3~didnotresultinthepossibility ofapreviously unanalyzed accidentorincreasetheabilityofapreviously analyzedaccident.

Themarginofsafetywasnotreduced.Basedontheinformation outlinedabove,theloosepluginthehotlegofD.C.Cooksteamgenerator f22didnotresultinanunrevised safetyegestionasdefinedinthecriteriaof10CFR50.59.

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