ML17227A424

Special Rept:On 920421,turbine Generator Failed to Trip Following Manual Reactor Trip.Caused by Failures in Redundant & Electrically Independent Turbine Tripping Schemes.I&C Disassembled & Trip Block Inspected

ML17227A424
Person / Time
Site:	Saint Lucie
Issue date:	05/14/1992
From:	SAGER D A FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
L-92-149, NUDOCS 9205180236
Download: ML17227A424 (15)

Text

ACCELERATEDDISTRIBUTIONDEMONSTTIONSYSTEMREGULATORYINFORMATIONDISTRIBUTIONSYSTEM(RIDS)ACCESSIONNBR:9205180236DOC.DATE:92/05/14NOTARIZED:NOFACIL:50-389St.LuciePlant,Unit2,FloridaPower&LightCo.AUTH.NAME.AUTHORAFFILIATIONSAGER,D.A.FloridaPower&LightCo.RECIP.NAMERECIPIENTAFFILIATIONDocumentControlBranch(DocumentControlDesk)

SUBJECT:

Specialrept:on920421,turbinegeneratorfailedtotripfollowingmanualreactortrip.Causedbyfailuresinredundant&electricallyindependentturbinetrippingschemes.I&Cdisassembled&trxpblockinspected.DISTRIBUTIONCODE:IE22DCOPIESRECEIVED:LTRENCLSIZE:TITLE:50.73/50.9LicenseeEventReport(LER),IncidentRpt,etc.NOTESDOCKET05000389RECIPIENTIDCODE/NAMEPD2-2LANORRIS,JINTERNAL:ACNWAEOD/DOAAEOD/ROAB/DSPNRR/DLPQ/LHFB10NRR/DOEA/OEABNRR/DST/SELB8DRR/LB8D1REGFIL02RGN2FILE01EXTERNAL:EG&GBRYCE,J.HNRCPDRNSICPOORE,W.COPIESLTTRENCL1111221-12211111111111133.1111RECIPIENT'IDCODE/NAMEPD2-2PDACRSAEOD/DSP/TPABNRR/DET/EMEB7ENRR/DLPQ/LPEB10NRR/DREP/PRPB11NRR/DST/SICB8H3NRR/DST/SRXB8ERES/DSIR/EIBLSTLOBBYWARDNSICMURPHY,G.ANUDOCSFULLTXTCOPIESLTTRENCL112211111122111111111111NOTETOALL"RIDS"RECIPIENTS:PLEASEHELPUSTOREDUCEV'ASTE!CONTACTTHEDOCUMENTCONTROLDESK,ROOiilPl-37(EXT.20079)TOELlilllNATEYOURNN!EFROivIDISTRIBUTIONLISTSFORDOCUMENTSYOUDONTNEED!TOTALNUMBEROFCOPIESREQUIRED:LTTR32ENCL-32

'P.O.Box128,Ft.Pierce,FL34854-0128FPLMay14,1992L-92-149U.S.NuclearRegulatoryCommissionAttn:DocumentControlDeskWashington,D.C.20555Re:St.LucieUnit2DocketNo.50-389EventDate:April21,1992TurbineTriFailureUdateAttachedpleasefindtheupdatedversionofaplantproblemreportwhichdiscussesaneventwhichoccuredonApril21,1992.Includedisthecurrentstatusofouranalysisoftheevent.Pursuantto10CFR50.73aLicenseeEventReportwillbesubmittedwithin30daysoftheeventdate.Verytrulyyours,D.A.SVicePrSt.LgersidentePlantDAS/JWH/kwAttachmentcc:StewartD.Ebneter,RegionalAdministrator,USNRCRegionIISeniorResidentInspector,USNRC,St.LuciePlantDAS/PSL1697-9292051802369205i4PDRADOCK050003898PDRanFPLGroupcompany ST.LUCIEPLANTINHOUSEEVENTREPORT92-023REV.9TURBINEGENERATORFAILEDTOTRIPFOLLOWINGMANUALREACTORTRIPUNIT'DATE:APRIL21,1992I.INITIALPLANTCONDITIONSOnApril21,1992,St.LucicUnit2wasshuttingdownforascheduledrefuelingoutage.Reactorpowerwasatapproximately12%.II.EVENTDESCRIPTIONAscheduledreactor-turbineshutdownwasinprogressonUnit2.Duringtheshutdown,powermovedtothetopofthereactorcore.Existingproceduralguidanceonaxialshapeindex(ASI)wasbeingemployedtominimizethiseffect.However,whenthreeoutoffourlocalpowerdensityprc-tripsonthereactorprotectionsystemactuated,thcnuclearplantsupervisor(NPS)instructedthereactorcontroloperatorstomanuallytriptheUnit.Amanualreactortripwasinitiated,buttheturbinedidnottrip.Severalunsuccessfulattemptswerethenmadetotriptheturbineusingtheturbinetrippushbutton.Thenuclearwatchengineerwasthendispatchedandtrippedtheturbinelocallyfromtheturbinefrontstandard.III.EVENTSEQUENCESt.LucieUnit2wasperformingascheduledreactorshutdownforrefuelingpurposes.DuringtheplantshutdownASIapproacheditsupper(negative)limit.CoordinationatcontrollingASIwithinlimitsandmaintainingreactorcoolanttemperatureduetoXenonbuildupbecameincreasinglydifficult..Whenreactorpowerwasdecreasedtoabout12%,controlrodinsertionnolongeihadanyeffectonmakingASIlessnegativeandthreeofthefourreactorprotectionsystemlocalpowerdensitypre-tripsalarmed.At0238,theNPSinstructedthereactorcontroloperatorstomanuallytriptheunitandthemanualreactortrippushbuttonsonRTGB(reactorturbinegeneratorboard)204andRTGB.201weredepressed.TwosecondslatertheturbinetrippushbuttononRTGB201wasdepressed;however,thcturbinedidnottrip.StandardposttripactionsfromEmergencyOperatingProcedure(EOP)1wereimplemcnted.Whentheturbinetrippushbuttonwasdepressed,theturbinegovernorvalvesindicatedclosedbutthethrottlevalves,reheatvalves,andinterceptvalvesindicatedopen.Autostopoilandtheemergencytripheaderindicatedpressuresofapproximately100psiand2000psi,respectively.Themegawattrecorderindicatedninetymegawatts.Several"additionalattemptsweremadetotripthcturbineusingthepushbutton,buteachwasunsuccessful.Atthispoint,thereactorcontroloperatoropenedgeneratoroutputbreakers8W52and8W49,securedthevoltageregulator,stoppedthcdigitalelectro-hydraulic(DEH)pumps,closedthemainsteamisolationvalves(MSIV),andbrokecondenservacuum.IHE892-023,5/14/921of5 III.EVENTSEQUENCE(cont'd)TheassistantnuclearplantsupervisorinstructedtheNWEtotriptheturbinefromtheFrontStandard.At0241alltheturbinevalvesclosed.EOP-2,ReactorTripRecovery,wasthencarriedout,twosetsofsatisfactorysafetyfunctionstatuscheckswerccompleted,theplantwasstabilized,andtheTurbineShutdownProcedurewasre-entered.IV.EVENTEVALUATIONA)Withtheexceptionofthefailureoftheturbinetotrip,theplantresponsetothiseventwasasexpected.Uponinitiationofthereactortripsignal,thefollowingshouldhaveoccurred.Thefourcontrolelementdrivemechanism(CEDM)busundervoltagcrelaysshoulddc-energize.Thisshouldenergize(open)the20ETsolenoidvalveanddraintheelectro-hydraulicfluidfromtheemergencytripheader(Seeattachment¹I).The20ASTtripsolenoidcoilshouldsimultaneouslyenergize,pulldownthemechanicaltriplever,anddumptheautostopoilheader.Decreasingautostopoilpressureallowstheinterfacevalvetoopenanddumptheelectro-hydraulicfluidemergencytripheader.Whenautostopoilprcssuredecreasesto45psi,pressureswitches63-1ASTand63-2ASTshouldlockin,actuatingageneratorprimarylockoutwhichtripsthcgeneratoroutputbreakers.Thesameresultshouldbeobtainedwhenattemptingtotriptheturbineviatheturbinetrippushbutton.Theoverspeedprotectionsystem(OPC)didnotandshouldnothaveactuatedbecauseitwasnotchallenged.Themaximumturbinerpmobservedduringthiseventwas1850rpm.ThisisbelowtheOPCsetpointof1854rpm.Amulti-disciplineteamisinvestigatingtodeterminewhytheturbinefailedtotripwhenthetripsignalswereinitiated.RefertoFigures1,2,&3formoredetails.B)FETYIIFfTheplantresponsetothiseventisboundedbysection15.1.5oftheSt.LucieUnit2FinalSafetyAnalysisReport(FSAR),"IncreasedHeatRemovalbytheSecondarySystem"asfurtherdescribedbelow..Analysisofthreescenariossimilartobutwithdifferentparametersthantheactualeventwereconducted.Ineachcasetheeventswerewithintheplant'sdesignbasisandarcsummarizedbelow.1.12%PowerTrip,NoOperatorActionScenario.*AutomaticMSIVclosureat600psig.Cooldownterminated.*CooldownrateboundedbylimitingFSARcooldownevent.IHE¹92-023,5/14/922of5 IV.EVENTEVALUATION(cont'd)B)NIFI2.100%PowerTr'ip,NoOperatorActionScenario.*PossibleSafetyInjectionActuationSignal(SIAS)butnoactualinjection.*AutomaticMSIVclosureat600psig.Cooldownterminated.~CooldownrateisboundedbytheFSARcooldowneventasconfirmedbyin-houseRETRANanalysis.3.100%PowerTripwithFSARChapter15AccidentScenario.*FSARChapter15Analysisconsiderssinglefailureofturbinestopvalvesto~close.Operatorstrainedonthesimulatorfordesignbasisaccidents.MultiplefailuresaresimulatedandprocedurallyaddressedbyEOPs.Theseinclude"ExcessSteamDemand".C)ALYILAAmulti-disciplineinvestigationteamisperformingarootcauseanalysisofthisevent.MembersofthisgroupincludeSystemEngineers,andrepresentativesfromOperations,Maintenance,Engineering,andtheTurbinevendor.Thcredundantdesignofthcturbinetripschemesuggeststhatatleasttwofailureswercnecessarytoexplaintheinabilitytotripobservedduringthe04/21/92event.Accordinglythefailureanalysisplanisevaluatingboththe20ETandthe20ASTturbinetrips.Extensiveturbinevendorinputwasobtainedforthefailureanalysisplan.ParticularattentionwaspaidtothelessonslcarncdintheSalemoverspeedevent.ALetterofInstruction,2-LOI-T-67,"EvaluationofTurbineSolenoidTripFunction,"waswrittentoevaluatetheoperationofalltripfunctions.Attachment1isthefailureanalysis.8QQILJl5-Thefailureoftheturbinetotripwascausedbyunrelatedfailuresintheredundantandelectricallyindependentturbinetrippingschemes.(moredetailslater.....)Turbinetrip20ASTfailedduetoanintermittentlossofcircuitcontinuity.Eitheroftwodeficienciesnotedatrelay62ASTX(item¹6ofattachment1"FailureAnalysisPlan")couldcausesuchafailure.(moredetailslater.....)Resultsofthc20ETsolenoidvalveautopsy.(moredetailslater.....)Becausethesetwounrelatedfailurescouldnotbedetectedwiththcexistingsystemdesignandsurveillanceprogram,atotallossoftripcapabilityresulted.(moredetailslater.....)IHE¹92-023,5/14/923of5 V.CORRECTIVE-ACTIONSA)RAIVITI1.Theoriginal20ET,20OPC-1and20OPC-2Solenoidvalveshavebeenremovedandweresenttoanindependentlaboratoryforanalysis.Resultsareduebackon05/18/922.I&Ciscontinuingtoinvestigatethcrootcauseoftheintermittentelectricalproblemsofthe20ASTtripsolenoidunderPWO7356.Thecircuitsofthe20ETand20OPC1and2solenoidsarealsobeingevaluatedforsimilarproblems3.I&Cwillevaluateprovidingcoilmonitoringlightsfor20ASTand20ETtoindicatecontinuityofthefieldcableandsolenoidcoil.4.OperationsisevaluatingadditionalguidanceforEOP-1,"StandardPostTripActions",onactionstotakeforareactortripwithnosubsequentturbinetrip.5.Engineeringwillevaluateprovidingalatch-incircuitforthe20ETsolenoidvalve.Thelatch-inwillbcprovidedonlyacrossthePB710contacts.6.Engineeringwillevaluateprovidinga30secondtimedelaybetweenaturbinetripandageneratortriptoactivatethereversepowerrelays.7.Engineeringwillevaluateprovidingthecapabilitytoexercisethc20ASTsolenoidvalvewhileonline.8.Operationswillevaluatemodifyingtheirprocedurestoallowindividualtripfeaturetesting.9.PCM032-292M"TurbineLubeOilConditionerConnections"willinstallvalvesandfittingstoallowconnectionofanup-gradedlubeoilconditioner.10.I&CwilldisassembleandinspectthetripblockunderPWO7356.11.Engineeringwillevaluateprovidingimproved20ASTcoilsuppressiontolimitarcingacross62ASTXrelaycontacts.12.NuclearFuelsisevaluatingadditionalguidanceforASIcontrolattheendofcorelife.B)MAIVITI13.EngineeringwillevaluateenhancementstothefiltrationsystemfortheturbineEHfluid.14.Engineeringwillevaluatetheuseofastainlesssteelsolenoidvalvebodytoreplacetheexistingcarbonsteelbody.IHE¹92-023,5/14/924of5 V.CORRECTIVEACTIONS(cont'd)B)AI15.EngineeringwillevaluateuseofanalternativeDEHfluidtoreducematerialincompatibilities.16.Engineeringwillevaluateprovidinganadditional20ASTvalveofdiversemanufacturewhichcanbetestedonlinebytheuseofthemanualtripbuttons.17.Engineeringwillevaluateprovidingameansoftestingthe20ETsolenoidvalvewhileonline.IHE¹92-023,5/14/925of5 Attachment1PFTFAILUREANALYSISPLANFTII1of2Frfr1.20ET(valve)-sticking,binding-removedfromsystemin"asfound"conditiontopreserveanycvidenccoffailuremode;senttoanindependentlab.-resultsduc05/18/922.20ET(circuit)-loosewire-opencoil-impropersupplyvoltage-continuitycheck-resistancecheck~-voltagemeasuredandrecorded-satisfactory/PWO7356-noopen/2-LOI-T-67-acceptable/2-LOI-T-673.20AST(valve)4.20AST(TripBlock)5.20AST(circuit)-sticking,binding'pluggedorifice-loosewire-valveoperatedduringtest-disassembleandinspect-continuitycheck-noindicationofstickingorbinding/2-LOI-T-67-scheduled5/12/92-looseterminalconnectionfoundatposition¹6on63ASTX/PWO7356-opencoil-impropervoltagesupply-resistancecheck-voltagemeasuredandrecorded-noopen/2-LOI-T-67-acceptable/2-LOI-T-676.62ASTX(Relay)-setpointdrift-digitalrecorder-setpointverified/2-LOI-T-67-loosewire-continuitycheck-looseterminalconnectionatposition¹6/2-LOI-T-67*PotentialFailureCause-degradedcontacts-visualexamination-slightarcingonbothsetsofcontactswithburnresidueevident*PotentialFailureCause Attachment1FAILREANALYIPLAN2of27.20-1OPC(valve)-sticking,binding-valveoperatedduringtest-noindicationofsticking,binding/2-LOI-T-67,valvesenttoanindependentlab,resultsduc05/18/928.20-1OPC(circuit)9.20-2OPC(valve)-loosewire-sticking,binding-continuitycheck-valvesenttoanindependentlab-noopens/PWO7356-resultsdue05/18/9210.20-2OPC(circuit)11.FCV-22-148(interface-loosewire-sticking,binding-continuitycheck-valveoperatedduringtest-noopens/PWO7356-noindicationofsticking,binding/2-LOI-T-67valve)12.63-1AST(pressureswitch)13.63-2AST(pressureswitch)14.PB710-contacts-contacts-contacts-contactalignment-digitalrecorderandvisually-acceptable/2-LOI-T-67-digitalrecorderand-visually-acceptable/2-LOI-T-67I'continuitychecks,disassembly,andinspection-topcontactsrequiredmoremovementthanbottomcontacts,switchreplaced/PWO7356-switchoperatedduringtest-acceptable/2-LOI-T-67-NotaCauseoftheFailure15.EFIfluid16.Turbinelubeoil-contamination-contamination-waterandparticulate-waterandparticulate-inspecification/PredictiveMaintenanceperiodictestingprogram-inspecification/PredictiveMaintenanceperiodictestingprogram Attachment2EELLIEKPL1of11.ManualReactortripduetoalossofASIcontrol-IneffectiveASIcontrolmethod-ImprovedASIcontrolprocedure2.Looseleadat62ASTX-Lackofmeanstodetectalossofcontinuity-Designchangestoaddcircuitmonitoringlights-Designchangestoallowperiodicon-linetestingofboth20ASTand20ET3.Burntcontactson62ASTX-Inadequatearcsuppression-Designchangetoupgradearcsuppressionin20ASTcircuit(MORELATER......

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