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1、InternationalWorkshoponLevel2PSAandSevereAccidentManagement29thtothe31stofMarch2004,Köln,GermanyMCDETandMELCORAnExampleofaStochasticModulecoupledwithanIntegralCodeforPSALevel2M.Sonnenkalb,J.Peschke,M.Kloos,B.Krzycacz-HausmannGesellschaftfürAnlagen-undReaktorsicherheit(mbH),Germany1IntroductionAc
2、cidentsequencesevolveovertimethroughtheinteractionofdynamicsandstochasticsinthesystemof“Man,Machine,ProcessandEnvironment”.InthecustomaryeventtreeanalysisofLevel1PSAandintheaccidentsequenceanalysisofLevel2PSA,theanalystsprescribethestochasticeventstogetherwiththeorderinwhichtheyoccur.Whiletempor
3、alinformationmaybeavailableforfewselectedsequencesinLevel1andonaneventscaleinLevel2,timeisusuallynotconsideredintheconventionaleventtree.Thesecustomarytreeslargelydevelopalongaso-calledeffectlineratherthanatimeline.BranchingpointsofLevel1eventtreesareprescribedbytheorderofsafetysystemdemandsatse
4、tpoints.Usually,thereareonlytwobranchesperpoint,namelyoneeachfor“systemstarts”andfor“failstostart”.Duetolimitationsofthecustomaryeventtreemethodologynoconsiderationcanbegiven,forinstance,totheconsequencesofgradualfailuretorunfortheintendedtimeand/orwiththerequiredcapacity.BranchesinLevel2acciden
5、tsequencetreesarefrequentlyusedtoaccountforclassesofpossibleconsequencemagnitudesfromcomplexphysico-chemicalprocesses.Consequently,thenumberofbranchesoftenexceedstwo.Presently,thespectrumofpossibleaccidentsequencesinLevel2PSAisreducedtothedegreesoffreedomofarathercoarsegridintime(i.e.“early”,“la
6、te”or“before”,“after”),inspace(i.e.“top”,“bottom”)andinmagnitude(i.e.“small”,“medium”,“large”),etc.Thisdoesnotpermittomodeltheinteractionsofphenomena,processes,componentbehaviorandhumanactionsasclosetorealityasisdesirable.Inherenttothecoarsegridisthedangerthat:-importantsequences,resultingfromde
7、tailsintime,space,magnitudeandorderofevents,remainunknown,-unrealisticsequencesaregenerated,basedonanalystspecifiedconditionswhichotherwisewouldresultfromprecedingevents.Themethodsofprobabilisticdynamicsenableustofullyaccoun
