ML17059B488

Non-proprietary Rev 0 to Gene B13-01739-40, Shroud Repair Anomalies NMP1,RF014.

ML17059B488
Person / Time
Site:	Nine Mile Point
Issue date:	04/30/1997
From:	DEAVER G A, RODABAUGH J F GENERAL ELECTRIC CO.
To:
Shared Package
ML17059B487	List: ML17059B486 ML17059B488 ML17059B489 ML17059B493 ML18040A254
References
GENE-B13-01739, GENE-B13-01739-40, GENE-B13-1739, GENE-B13-1739-40, NUDOCS 9704100247
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'GENuclearEnergyGENEB13-01739<0Revision0ClassIIIApril1997ShroudRepairAnomaliesNineMilePointUnit1,RFO14Preparedfor:NiagaraMohawkPowerCorporationPreparedby:GENuclearEnergyReactorModificationServices175CurtnerAvenue,MIC571SanJose,California95125g0500022047970408PDRADQCP

GE26u&m'Eruqp&oPrictory~inruxHoeGHVEB1341739-40Revision0April199ShroudRepairAnomaliesNineNilePointUnit1,RF014J.6Cfiarnley,PnacipalEngineerA.Nafiadevan,PrincipalEngineerT.EGleason,PrincipafEngineerReactorModIflcatfonServicesReviewedbyGA.Deaver,EngineeringLeader~ReactorNodificatInnServicesApprovedby:dabaugh,MinionManagerInVesselRepairsReactorModificationServices

GENEB13-01739QORevision0April1997REVISIONSTATUSSHEETRevision0ApprovalG.A.Deaver'IDate4/5/97InitialIssueDescription11

GENEB13-01739-40Revision0April1997IMPORTANTNOTICEREGARDINGCONTENTSOFTHISREPORTPleasereadcarefullyTheonlyundertakingsoftheGeneralElectricCompany(GE)respectinginformationinthisdocumentarecontainedinthecontractbetweenNiagaraMohawkPowerCorporationandGE,asidentifiedinPO15247,andnothingcontainedinthisdocumentshallbeconstruedaschangingthecontract.TheuseofthisinformationbyanyoneotherthanNiagaraMohawkPowerCorporation,orforanypurposeotherthanthatforwhichitisintendedisnotauthorized;andwithrespecttoanyunauthorizeduse,GEmakesnorepresentationorwarranty,expressorimplied,andassumesnoliabilityastothecompleteness,accuracy,orusefulnessoftheinformationcontainedinthisdocument,orthatitsusemaynotin&ingeuponprivatelyownedrights.

GENEB13-01739<0Revision0April1997EXECUTIVESUMML4RYDuringtheSpring1997refuelingoutageofNineMilePointUnit1,thenuclearcoreshroudrepairassemblieswerefoundtobedegraded.Theshroudrepairassemblieshadbeeninstalledduringthe1995outage.Thisreportdescribestheasfoundcondition,theconsequencesofthedegradedconditiononpreviousplantoperation,therootcauseofthedegradedcondition,andtherepairsimplementedtoassurecontinued,safeandreliablefutureplantoperation.Thedegradationconsistedofloosetierodsandfailedlowerspringcontactwedgelatches.Therootcauseofthedegradedshroudrepairconditionwasunacceptablemovementoftheshroudrepairassembliesduringplantoperationcausedbyfailuretorecognizetheimpactofclearancesbetweentoggleboltsandtheholes,andanincorrectdesignassumptionregardingslidingatthevesseltowedgeinterface.Therepairstobeimplementedthisoutageassurethatunacceptableconsequencesofmovementoftherepairassemblieswillnotoccurinthefuture.

GENEB13-01739<0Revision0April1997LISTOFFIGURESFigureNo.ShroudRepairAssembliesSubjectPage262PhotographofFailureSurface27ToggleBoltMovementinShroudSupportCone28 0

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71.0INTRODUCTION

DuringtheSpring1997refuelingoutageatNineMilePointUnit1(NMPl),anomalieswerefoundwiththeshroudrepairhardware.Thisreportdescribesthoseanomaliesanddiscussestherootcauseandcorrectiveaction.Theshroudrepairhardwarewasinserviceforapproximatelytwoyears.Theanomaliesconsistedofloosetierodsandfailedlowerspringwedgelatches.Theanomalieswerefoundduringplannedvisualinspectionsoftheshroudrepairhardwareandduringtheplannedreplacementofashroudrepairassemblyat270degrees.

GENEB13-01739<0Revision0April19972.0SVMMARYAllfourshroudrepairassemblieswerefoundtohavelostverticalpreloadandthreeofthelatchesthatpreventrelativemotionbetweenthelowerspringandthewedgeweredamaged.Onelatchhadfailedinservice,anotherfailedduringtheremovalprocess,andathirdhasvisualevidenceofdamage.Similarlatchesonthemid-supportsandontheupperspringswerefoundtobenormal.Theselatchesaresimilarinphysicalfeaturesbuthavedifferentappliedloadings.Theevaluationoftheas-foundconditionshowsthatboththelatchfailureandtierodloosenesswererelated.ThedesignofthelowerspringcontactimplicitlyassumedthatthelowerspringcontactwouldslidealongtheReactorPressureVessel(RPV)wall.Ifslidingalwaysoccurredatthisinterface,thennoadditionalstresswouldbeinducedinthelatches.Ontheotherhand,ifthe&ictiononthelowerspringcontactareapreventssliding,thiscouldcausehighstressesandyieldinginthelatch.ThisinturncouldcauseSCCofthematerial.Giventhatthereisnoslidingonthelowerspringcontactareaonthevessel,stressesinthelatchcouldbedevelopedasaresultoftwoconditions:~Ifthelowersupport/toggleboltassemblieswereinstalledsuchthattheseassembliesmoveduptheshroudsupportcone,towardtheshroud,whentheplantreachednormaloperatingconditions,theresultingverticaldisplacementcouldcausehighstressesinthelatch.~DifferentialmotioncouldalsobecausedbythedeflectionoftheC-springundertierodloadforheatup.Thiscouldalsocausestressesinthelatch,althoughsomewhatlessthaninthepreviouscase.Theevaluationshowedthatifslidingdoesnotoccur,thestresses&omthemovementassociatedwithinstallationandsubsequentthermaldisplacement,couldcausesufficientlyhigherstresses.ThisexplainstheobserveddeformationandthesubsequentSCCfailure.Motionofthetierodassemblyrelativetothevesselsurfacecanalsocausethethermalpreloadtobeovercomeandcausetierodlooseness.

0 GENEB13-0l739<ORevision0Aprilf997Therootcauseofthelatchfailureandthetierodloosenessisrelatedtothedesignassumptionofslidingonthevesselsurface.Whilethisappearedreasonableinitially,theobserveddeformationonthelatchcon6rmsthatslidingdidnotoccur,andthattheoriginalassumptionofslidingwasincorrect.Anevaluationwasperformedtoshowthatoperationduringthepreviousfuelcycle,withthedegradedshroudrepairassemblies,didnotresultinoperationoftheplantinanunsafemanner.Theshroudrepairassemblieshavebeenrepairedbyremovingtheloosenessbypushingthelowersupporttoggleboltassembliestotheshroudsideoftheholesintheshroudsupportcone.Thelatcheshavebeenreplacedwithanewdesignwhichismoretolerantofdifferentialmotion.Thesetwochangesassurethattheshroudrepairassemblieswillfunctionasoriginallyintendedduringallmodesofplantoperation.

GENEB13-01739<0Revision0Ap17I19973.0EVALUATION'heasfoundcondition,designdescription,additionalinspections,andtheloadsappliedtotheshroudrepairassembliesarediscussedinthissection.3.1AsFoundConditionFigure1showsanelevationviewofonesetofshroudrepairhardware.Therearefoursuchsetsofhardwareatazimuths90,166,270,and350degreesaroundthecoreshroud.Briefly,thetierodisthemaincomponentforreactingaxialloads.Thelowerspringisthelinearspringforsupportingtheshroudatthecoreplateelevation.ThelowerwedgeisacomponentthatwasmachinedbasedonactualsitemeasurementtofitbetweentheRPVandthelowerspringwithasmall(0.010inch)compressionofthelowerspringatroomtemperature.Thelatchisawishboneshapedpiece,thatisintendedtopreventrelativemotionbetweenthelowerwedgeandthelowerspring.Similarlatchesarealsousedtopreventrelativemotionatthemid-supportandattheupperspring.Thelowersupportisanassemblythatconnectstheshroudrepairhardwaretothe'shroudsupportcone.Thetierodnutisatthetopofthetierodandisusedtotightentheassembly.Duringinstallation,thetierodnutwastorquedclockwisetopreloadtheassembliestoassureminimaltightnessofcomponents.Themid-supportisusedtolimitrelativemotionbetweenthemiddleoftheshroudandtheRPV.Theupperspringisahnearspringforsupportingtheshroudatthetopguideelevation.3.1.1TieRodNutsDuringtheplannedreplacementoftheshroudrepairassemblyat270degrees,thetierodnutwasfoundtobeloose.Thenutlockingdevicewasnormalandthenutwasnotabletobemovedwithoutremovalofthelockingfeature.However,therewasnopreloadbetweenthenutandthetierod.Afterremovalofthelockingfeature,thenutwasturnedwithlessthan25footpounds.Therotationofthenutpriortotighteningat25footpoundswasequivalenttoanaxialclearance(i.e.verticalmovement)of0.08inch.

GENEB13-01739<0Revision0April19973,1.2LatchesThelowersupportwedgelatchat90degreeswasfoundduringthevisualinspectiontobebroken.Apieceofthelatchwasmissingandlaterfoundonthelowersupportconeatapproximatelyazimuth330degrees.Figure2isaphotographofthefailuresurface.Basedonanexaminationofthisphotographandothervideotapes,thefailureisnotconsistentwithafatiguemechanism.Thereisnovisibleevidenceofplasticdeformation,whichwouldbenecessaryforasingleoverloadtypeoffailure.Thefailuresurfaceappearsconsistentwithastresscorrosionfailureunderhighstress.Basedonthevisualinformation,highstresscausingstresscorrosionismorelikelythananoverload,butuntilresultsofametallurgicalevaluationareavailable,overloadcannotbeeliminated.Videotapeinspectionoftheotherthreelowerwedgelatchesshowedthemalltobeinonepiece,butthe350degreelatchappearedtobebent.Inaddition,thelowerspringwedgeshaveevidenceoflocalhardcontact,duetoverticalloads,withthelatches.SincethelatchesarealloyX-750andthelowerspringwedgesareType316lowcarbonstainlesssteel,thelowerspringwedgeswillshowsurfacewearbeforethelatches.Onesimilarlatchisusedineachmid-supportassemblyandtwosimilarlatchesareusedineachuppersupportassembly.Thelatchesonthemid-supportandontheuppersupporthavebeenvisuallyexaminedandalltwelvearenormal.Becauseofdesigndifferences,theselatchescannotbeloadedasseverelyasthelowerwedgelatches.ThecontactforcebetweentheRPVandtheshroudrepairismuchsmallerattheselocationsascomparedtothecontactforceatthelowerwedge.Inaddition,theselatchesarenotloadedduringplantheat-up.3.1.3LowerSpringWedgesThelowerwedgeat90degreeshaddroppedtothebottomofthepostonthelowerspring.Thelowerwedgeat350degreesappearedtobeapproximately1/8inchbelowitsnormalposition.Theothertwowedgeswereintheirnormalposition.

e GENEB13-01739<0Revision0April1997~~3.1.4MarksonRPVWallContactslidingmarkshavebeenobservedontheRPVwallat166degreesabovetheloweranduppercontactpointsoftheupperspringassembly.Slidingmarksarenotevidentatallothercontactpointsfortheupperspringslocatedatthe90,270,and350degreelocations.3.1.5ShroudSupportConeVisualinspectionoftheshroudsupportconedidnotrevealanydifferences&omtheoriginalinstallation.3.1,6Mid-SupportsVisualinspectionofallmid-supportcontactsconfirmedthattherewascontactwiththeRPVsurfaceinthecoldcondition.Duringnormaloperation,themid-supportcompressionontheRPVincreasesduetothermalconditions.Thusthefunctionofprovidingaloadpath&omthetierodtotheRPV(alsointendedtoincreasethenatural&equencyofthetierodassemblies)wasmaintained.Followingthisinspection,in-vesselworkrelatedtoverificationoftherootcauseofthelatchfailure,involvedapplicationofforcesthatledtodisplacementoftwomid-supports.Thisleftaconditionwithagapatazimuths90and166degreesrelativetothevesselwall.Newmid-supportsarebeingfabricatedandwillbeinstalledwiththeoriginaldesignpreload.Themid-supportat350degreeswillbeverifiedforproperpreload.The270degreemid-supportwillbeverifiedforproperpreloadduringinstallationofthenewtierod.Therefore,therequiredsupportconfigurationwillstillbemaintainedforfutureoperation.3.2DesignDescriptionTheshroudrepairwasdesignedtostructurallyreplacethecircumferentialweldsinthecoreshroud.Fourassembliesareplacedapproximatelyuniformlyaroundtheshroud(azimuths90,166,270,and350).Eachassemblyfunctionstoverticallyholdtheshroudtotheshroudsupportconeandtohorizontallysupporttheshroudatthetopguideandcoreplateelevations.Inaddition,thereareotherhorizontalsupportsthatwouldpreventunacceptablehorizontalmovementofanyshroudcylindricalsegmentthatcouldbeproducedbyfailureofthehorizontalshroudwelds.

GENEB13-01739%0Revision0April19973.2.1VerticalRestraint,Verticalrestraintisprovidedbyanalternateloadpathbetweenthetopoftheshroudandtheshroudsupportcone.Thisloadpathconsistsoftheuppersupport,tierod,Cspring,lowersupport,andtogglebolt.Differentialthermalexpansionduetothedifferentmaterialsusedforcomponentsofthisloadpath,provideathermalpreloadatplantoperatingconditions.Thethermalpreloadissufficienttoholdtheshroudinplaceforallnormaloperatingconditions,suchastheverticalupwardforceappliedtotheshroudbythecoolantfiowandpressure.Theverticalloadpathisalsodesignedtohaveaverticalspringratethatbothpreventsunacceptableverticalloadduringplantupsetthermalconditionsandprovidesacceptabledynamicresponseduringaplantseismicevent.>>3,2.2HorizontalRestraintTheshroudissupportedbylinearspringsatthetopguideandthecoreplateelevations.Atthetopguideelevationthelinearspringconsistsoftheupperspring,upperwedge,uppercontact,andtheuppersupport.Atthecoreplateelevationthespringconsistofthelowerwedge,lowercontact,andlowerspring.Thehorizontalspringrateofthesespringsweredesignedtoproduceacceptablehorizontaldynamicresponseduringaplantseismicevent.Thehorizontaldisplacementoftheshroudduringalleventsmustbelimitedbythesespringstoassurecontrolrodinsertionandpreventionofunacceptableleakage.Inaddition,unacceptabledisplacementofothercylindricalsectionsoftheshroudbetweenpostulatedcracksinhorizontalshroudweldsarepreventedbydisplacementlimiters,suchasthemidsupportandtopsupport.

I GENEB13-01739<0Revision0April19973.3AdditionalInspectionsBasedontheasfoundcondition,additionalinspectionsweredeterminedtoberequired.Adetailedprocedurewasdevelopedforperformingtheinspectionsaswellastoreplacethelatchesonthelowerwedges.ThatprocedureisgiveninReference3.1.Thepurposesoftheprocedureareto:removethelatchesfromtheotherlowerwedges,determineiftheothertierodnutsarealsolooseasthe270degreeonewas,identifythesourceoftheloosenessofthetierodnuts,andinstallnewlowerwedgelatchesandretorquethetierodnutstotherequiredvalue.ThefollowinginformationwasobtainedbyfollowingtheReference3.1procedure.1.Threelowerwedgelatchesweresuccessfullyremoved.The350degreelatchbrokeduringtheremoval.Thefourthlatchat270degreeisstillinstalledintherepairassembly.Thisrepairassemblywascompletelyremovedforreplacement.2.Thetierodnutat90degreescouldnotbeturnedwith25footpoundsuntilthelowersupportwaspusheduptheshroudsupportcone.Afterajackwasinstalledtopushthelowersupportuptheshroudsupportconetowardtheshroud,thetierodnutcouldthenbeturned.Thetotalnut 4

GENEB13-01739<0Revision0April1997rrotation,atthefinalinstallationtorqueof190footpounds,was272degrees.Thisisequivalenttoanaxialclearance(i.e,verticalmovement)of0.151inch.3.Priortojackingthelowersupport/toggleboltassemblies,thetierodnutat350degreesturned38degreeswith25footpoundsapplied.Afterajackwasinstalledtopushthelowersupportuptheshroudsupportcone,towardtheshroud,thetierodnutcouldbeturnedevenfurther.Thetotalnutrotationatthefinalinstallationtorqueof190footpoundswas98degrees.Thisisequivalenttoanaxialclearanceof(i.e.verticalmovement)0.054inch.4.Priortojackingthelowersupport/toggleboltassemblies,thetierodnutat166degreescouldbeturned67degreeswith25footpoundsapplied.Afterajackwasinstalledtopushthelowersupportuptheshroudsupporttowardtheshroudthetierodnutcouldbeturnedevenfurther.Thetotalnutrotationatthefinalinstallationtorqueof190footpoundswas168degrees.Thisisequivalenttoanaxialclearance(i.e.verticalmovement)of0.093inch.Inconclusion,allfourtierodnutswerefoundtobeloose.Theamountofaxialclearance,foundRomthenutrotation,variedbetween0.054and0.151inches.Theclearancebetweenthetoggleboltsandtheupperportionoftheholeintheshroudsupportconewaseliminated.Allfourtierodsweretorquedtotheoriginalrequiredinstallationvalue.AlloftheverificationactivitieswereperformedinaccordancewiththeprocedureinReference3.1.

GENEB13-01739<0Revision0Ap17I1997 40 GENEB13-01739<0Revision0April1997

GENEB13-01739<0Revision0April19974.0CONSEQUENCESTOPREVIOUSPLANTOPERATIONTheNMP1planthadbeenoperatingforonecyclesincetheinstallationoftheshroudrepairhardware.Uponinspection,asidentifiedearlier,thefollowingwasdetermined:Thetierodswereloose,theloosenessranging&om0.054to0.151inchesaxialinthefourtierods.Thedifferentialthermalexpansionbetweentheshroudandthetierodassemblyduringnormaloperationis0.155inches.Thus,thethermalpreloadislostbytheamountequivalenttotheloosenessofthetierodassembly.Thecumulativeloosenesswas(.054+.080+.093+.151)=0.378inches.Theremainingthermalpreloadis(0.155x4-0.378)x514=124.4kips.2.Theretainerlatchinoneofthelowerstabilizerspringhadbroken,resultinginnocontactbetweenthelowerspringandtheRPV.Therefore,itwasrenderedineffective.Thisassessmentwasperformedtodetermineifthefailureswouldhaveimpairedthesafeoperationoftheplantduringthepastoperatingcycle.Forpurposeofthisassessment,itisconservativelyassumedthat:1.Alllowerspringorstabilizersareineffectiveinprovidinglateralrestraint2.Allhorizontalweldshavethrough-wallcracks3.Allhorizontalweldcracksare360',withnoligamentremaining.Thecriticalhardwareloadswererecomputedandcomparedtothehardwaredesignloads,(References3.2,3.3).Ifthenewloadissmallerthanthedesignload,thehardwareremainsqualified.Ifthenewloadislargerthanthedesignload,thenewstresseswillbecomparedtotheavailablestressmargins/codeallowables.Inaddition,impactonleakageisaddressed.12

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GENEBf3-01739<0Revision0Ap/7/19974.5FlowInducedVibration(FIV)Natural&equencyevaluationwithnolowerspringcontactandnopreloadandnopreloadoftierodassembliesshowedthe&equencywaslower(15Hz)comparedtotheoriginaldesignvalue(28Hz).Thisisstillwellinexcessofthevortexshedding&equency(7Hz)andwillnotcausevibrationorhighcyclefatigue.Thiswassupportedbystressanalysiswhichshowsthatthestressamplitudewasbelowtheendurancelimit.IfFIVhadoccumedtherewouldhavebeendamageontheRPVcladdingatthemid-supportlocation.Inspectionofthoseareasshowednoevidenceofdamage.ThisconfirmsthatFIVwasnotanissueinthepreviousplantcycleofoperation.4.6ConservatismTheaboveloadcalculationsarebasedontheconservativeassumptionslistedinSection4.0.Itisknownbasedonactualinspectionoftheshroudthat,intheregionswhereinspectionhasbeenperformed,nothroughwallcrackhadbeenfoundinanyofthehorizontalwelds.Thus,considerableconservatismisbuiltintotheabovecalculations/model.Also,basedonthescreeningcriteria,therequiredligamentsizesaremuchsmallerthanwhatareactuallyavailable,whichmeansthatbysatisfyingtheminimumrequiredligamentrequirement,theshroudisstillsafelyoperableevenwithouttherepairinplace.18

GENEB13-01739%0Revision0April19974.7ConclusionInconclusion,evenwiththedegradedrepairhardwareandexistingshroudweldcracking,therewasnosafetyconcernintermsofsafeshutdowncapabilityorcorecoolingfunctionability,duringthepastoperationalcycleoftheplant.19

GENEB13-01739<0Revision0April19975.0ROOTCAUSEOFFAILUREBasedontheinitialobservationsoftheloosetierodat270degreesandthefailureofthelatchat90degrees,differentpotentialcauseswerepostulated.Thesecauseswerepossiblevibrationleadingtofatigueofthelatch,oryieldingofthetierods,orotherunexpecteddisplacements.IAreviewofthestressanalysesshowedthatthetierodscouldnothavebeenoverloadedtoyield.EvaluationssolelybyIVVItechniquesandphotomacrographsofthe&acturefaceofthelatch,areofcourseinsufficientevidencetoestablishtheactualcauseoflatch&acture.However,theevidenceobtainedbymacroscopicobservationstronglysuggeststhatthelatch&acturewasduetoastresscorrosionmechanismratherthanafatigueormechanicaloverloadfailure.Thelogged,irregularfailuresurfaceofthebroken90degreelatchagaintendstoruleoutfatigueasapossiblefailuremechanism.Thefailuresurfacedoesnotshowevidencetypicalofasingleoverloadingasthereisnovisibleplasticdeformation.Thesurfacedoeshowever,havecharacteristicssuggestiveofstresscorrosionunderhighstress.TheonlyknownsourceofhighstressisduetorestraintofdifferentialverticalmotionbetweentheRPVandthelowerspringwedge.IfthelowerspringwedgedidnotslideverticallyalongtheRPV,thenthedifferentialdisplacementmustoccurbetweenthelowerspringandthelowerwedge.Suchmovementwillcausehighstressinthelatch.SourcesofsuchdifferentialdisplacementaretheverticalloosenessofthetierodsandthedifferentialdisplacementstabulatedinSection3.4.Atplantoperatingconditions,theentirevalueoftierodloosenesswouldaddtothedifferentialdisplacementatthelowerspringwedgeresultinginatotaldifferentialmovementofatleast0.121inch(.054inch+.067inch).Suchadisplacementwouldresultinstressesinthelatchofwelloveryield.CrackgrowthratesinalloyX-750,withappliedstressesoveryieldstress,canbequitehigh.Valueswellinexcessof0.2inchperyear(thicknessoflatch)havebeenreported.Therefore,therootcauseofthelatch'ailureandthetierodloosenessisrelatedtothedesignassumptionofslidingonvesselsurface.Whilethisappearedreasonableinitially,theobserveddeformationonsomeofthelatchesconfirmsthatslidingdidnotoccurandthattheoriginalassumptionofslidingwasincorrect.Giventhatfrictiononthelowerspringcontactareacanpreventsliding,theshroudsupport/toggleboltassembliesshouldhavebeeninstalledasclosetotheshroudasItpossible,andasallowedbytheholesintheshroudsupportcone.Withtheincorrectassumption,theimportanceoftheclearancebetweenthetoggleboltsandtheholewasnotrecognizedandnot20

GENEB13<<01739-40Revision0Ap17'I1997incorporatedintotheinstallationengineeringdocumentation.Therefore,allfourshroudrepairassemblieshadaninstalledloosenessof0.054to0.151inch.Thisloosenesswasremovedbythe79,600poundforceappliedbydifferentialthermalexpansionof0.155inchatplantoperatingconditions,This0.054to0.151displacementcombinedwiththeunexpectednoslippageofcontactIdisplacement,over-stressedthelatch(assumingnoslippagebetweentheRPVandthewedge).Thehighstressislikelytohaveresultedinstresscorrosionandlatchseparationintwoyears.21

GENEB13-01739<0Revision0April19976.0CORRECTIVEACTIONSTherearetwocorrectiveactions.Thefirstistoremovetheloosenessbetweenthetoggleboltsandtheshroudsupportcone.Thishasbeen,orwillbe,accomplishedwiththeReference3.1procedure.Thesecondistoinstallnewlatcheswhicharemoretolerantofdifferentialverticaldisplacement.Thedesignofthenewlatchesmaintainstheoriginaldesignfunction,whichistolockthewedgetothelowerspringwheneveritisnotsupported,butmodiGesthelatchmechanismtoincorporateanotherspringmechanismwhichcantolerateverticaldisplacements.Therefore,theoriginal.functionalrequirementisaccomplishedwhileaddingmoreQexibilityintheverticaldirectiontoaccommodatethenowrecognizedverticaldisplacements.Thenewlatchisagainmade&omX-750materialbecauseofitshighstrengthcapabilities.TestingofX-750hasshownthatitisresistanttostresscorrosioncrackingforstressesupto75ksi.Incomparison,theonlyotherhighstrengthmaterialwithexcellentcorrosionresistancepropertiesandwithin-vesselexperienceisXM-19.However,forthismaterialtheyieldstrengthisonly38ksi.Likewise,theultimatestrengthofX-750issignificantlyhigherthanXM-19(142ksivs.88ksi).Therefore,X-750continuestobethebestchoice.ThedesignofthelatchwillaccommodateallpotentialverticaldisplacementswithoutexceedingtheASMEcodelimits.Underthemostprobableoperatingandslidingconditionsthenewlatchdesignisexpectedtoperformsatisfactorilyfortheremaininglifeoftheplant.Evenforworstcasepostulatedconditions,thelatchiscapableofoperatingwithoutfailure.22

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78.0REFERENCES

3.1NMP-SHD-003Revision1"LowerWedge.LatchReplacementandTieRodTorqueChecks"includingSpecialProcessControlSheetSPCS¹01Revision1.3.2GENE-B13-01739-04,"NineMilePoint1-ShroudRepairHardwareStressAnalysis",includingSupplement2.3.3GENE-B13-01739-25,April,1996,"CoreShroudRepair-270'ieRodAssemblyAssessment"24

GENEBf3-Of739<0Revision0April1997Figure1ShroudRepairAssemblies25

QEgEB13-01739-4PRevision0Aprilf587~,I'/4%VPQ~~>>'lA~C~r,I'II~~'aql~lLg~'I~~l~y~PLhh\Figure2PhotographofFailureSurface26

GENE813-Orm-eRevision0April1997VERTIl'.ALWOVEHeNTI,LOWERSVHt7RTI)IAT8orloeoiIHOLEtHtQNG,IIIIIIIILD+EgSUPPogrArTopOiHOLEINCONEFigure3ToggleBoltMovementlnShroudSupportCone27