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U.S.NUCLEARREGULATORYCOMMISSIONREGULATORYGUIDEREGULATORYGUIDE1.81SHAREDEMERGENCYANDSHUTDOWNELECTRICSYSTEMSFORMULTI-UNITNUCLEARPOWERPLANTSA.INTRODUCTIONGeneralDesignCriterion5,,SharingofStructures,Systems,andComponents,ofAppendixA,GeneralDesignCriteriaforNuclearPowerPlaits,to10CFR.Part50,LicensingofProductionandUtilizationFacilities,rprohibitsstructures,systems,andcomponentsimportanttosafetyfrombeingsharedamongnuclearpowerunitsunlessitcanbeshownthatsuchsharingwillnotsignificantlyimpairtheir'abilitytoperformtheirsafetyfunctions.Thesesafetyfunctionsincludethecapability-toperformanorderlyshutdownandcooldownoftheremainingunitsintheeventofanaccidentinoneunit.ThisregulatoryguidedescribesamethodacceptabletotheNRCstaffforcomplyingwiththeNuclearRegulatoryCommission'srequirementswithrespecttothesharingofonsiteemergencyandshutdownelectricsystemsformulti-unitnuclearpowerplants.Withrespecttothesharingofd.c.systems,thisguideappliestoallmulti-unitplants.Withrespecttothesharingofa.c.systems,thisguideappliestothemulti-unitplantswhosea.c.powerissuppliedbydieselgenerators.TheAdvisoryCommitteeonReactorSafeguardshasbeenconsultedconcerningthisguideandhasconcurredintheregulatoryposition.B.DISCUSSIONThestaffhasdeterminedthat,becauseofthelowprobabilityofamajorreactoraccident,asuitabledesignbasisformulti-unitnuclearpowerplantsistheassumptionthatanaccidentoccursinonlyoneoftheunitsatatime,withallremainingunitsproceedingtoanorderlyshutdownandamaintainedcooldowncondition.UseofthisdesignbasisinconjunctionwiththerequirementsofLinesindicatesubstantivechangesfrompreviousissue.Revision1January1975GeneralDesignCriterion5hasresultedindevelopmentofonsiteemergencyelectricpowersystemsthataresharedamongtheunitsoftheplant.ThedegreeofsharinginthesedesignshasvariedfromsharingofonlyoneSourcetosharingofallsourcesandtheirdistributionsystems.Adeviceisconsideredtobesharedamongunitsifitisdesigniedtoperformthesamefunctioninallunitsasrequired.Sharingofonsitepowersystemsatmulti-unitpowerplantsitesgenerallyresultsinareductioninthenumberandcapacitycftheonsitepowersourcestolevelsbelowthoserequiredforthesamenumberofunitslocatedatseparatesites.Thereducedcapacitycouldcauseunde-.'sirableinteractions.Examplesofsuchinteractionsare(1)theinterconnectionofengineeredsafetyfeature(ESF)controlcircuitsofeachunitsuchthatfailuresandmaintenanceortestingoperationsinoneunitaffectthe'availabilityofESFinotherunits,(2)coordinationrequiredbetweenunitoperatorsinordertocopewithanaccidentinoneunitandsafeshutdownoftheremainingunit(s),and(3)systemoverloadconditionsasaconsequenceofrealaccidentinaunitcoincidentwithafalseorspuriousaccidentsignalinanotherunit.Alloftheapplicantswithsharedonsitepowersystemsatmulti-unitplantsreviewedtodatehavebeenrequiredbythestafftoprovideimprovementsincapacityand/orareductioninsysteminteractionsinordertoresolveconcernswithregardtotheseunde.sirableinteractions.Further,thestaffhas,since1973,advisedseveralapplicantsforconstructionpermitsthatsharingofonsitepowersystemsshouldnotbeincorporatedintotheirdesigns.Inasmuchasmost,ifnotall,oftheapplicationsforconstructionpermitspresentlyunderreviewandthoseintheplanningstagesareformulti-unitplantscomprisingfromtwotosixunits,thestaffhasconcludedthattheonsiteemergencyelectricpowersystemsfortheseplantsshouldbedesignedtominimizeundesirableinteractioneffects.Thisrevisionreflectscommentsreceivedfromthepublicandadditionalstaffreview.USNRCREGULATORYGUIDESCopiesofpublishedguidesmaybeobtainedbyrequestindicatingthedivisionsdesiredtotheUSNuclearRegulatoryCommission.Washington.D.C.M5556.RegulatoryGuidesareissuedtodescribeandmakeavailabletothepublicAttentionDirectorofStandardsDevelopmentCommentsandsuggestionsformethodsacceptabletotheNRCstaffofimplementingspecilicpartsoftheimprovementsintheseguidesareencouragedandshouldbesenttotheCommission'sregulatiot~sstodelineatetechniquesusedbythestaffinSecretaryoftheCommissionUSNuclearRegulatoryCommission.evaluatingspecificproblemsorpostulatedaccidents.ortoprovideguidancetoWashington.DC20566.AttentionDocketingendServiceSection.applicantsRegulatoryGuidesarenotsubstitutesforregulationsandcompliancewiththemisnotrequired.MethodsandsolutionsdifferentfromthosesetoutinTheguidesareissuedinthefollowingtanbroaddivisionstheguideswillbeacceptableiftheyprovideabasisforthefindingsrequisitetotheissuanceorcontinuanceofapermitorlicensebytheCommission1PowerReactors6Products2.ResearchandTestReactors7Transportation3FuelsandMaterialsFacilities8OccupationalHealthPublishedguideswillberevisedperiodically,asappropriate,toaccommodate4.EnvironmentalandSitingIAntitrustReviewcommentsandtoreflectnewinformationorexperienceSMaterialsandPlantProtection10GeneralC.REGULATORYPOSITION1.D.C.systemsinmulti-unitnuclearpowerplantsshouldnotbeshared.2.Multi-unitnuclearpowerplantsnowunderconstructionorforwhichconstructionpermitapplicationwasmadebeforeJune1,1973,willbereviewedonanindividual-casebasis.Fortheseplants,thedesignofsharedonsiteemergencyandshutdowna.c.electricsystemsshouldsatisfythefollowing:a.Thesharinigofonsitea.c.electricsystemsshou