Thermal–hydraulicperformanceofconfinementsystemofRBMKincaseofaccidentsOriginalResearchArticle

WithintheframeworkofaEuropeanCommissionsponsoredactivity,anoverallassessmentofthedeterministicsafetytechnologyofthe‘post-Chernobylmodernized’ReactorBolshoyMoshchnostyKipyashiy(RBMK)hasbeencompleted.Theaccidentanalysis,limitedtotheareaofdesignbasisaccident,constitutedthekeysubjectforthestudy;therefore,thenotoriousChernobylUnit4eventwasoutsidethescopeoftheinvestigation. ThepresentpaperdealswiththeevaluationoftheaccidentperformanceoftheconfinementoftheRBMK.Ignalina-2andSmolensk-3NuclearPowerPlants(NPP)areconsidered.Thedocumentedactivityincludesfourmainparts:(a)descriptionofkeyfeaturesoftheconfinementforthetwoNPP;(b)identificationandcharacterizationofrelevantaccidentscenariospartofthe(DBA)area;(c)keyfeaturesandqualificationlevelofadoptedcomputationtoolsprimarilyincludingcodesandinputdecks;(d)resultsfromtheanalysisofselectedaccidentscenarios. Inthefirstpartofthepaper,thelayoutcomplexityoftheRBMKconfinementsystems,includingthebuildingonthetopofthereactorcavityispointedouttogetherwithmaindifferencesbetweenSmolensk-3andIgnalina-2units. Accidentscenariossuitablefortheanalysis,secondpartofthepaper,arederivedfromrecentlyissuedInternationalAtomicEnergyAgency(IAEA)reports,fromtheavailableSafetyAnalysisReports(SAR)ofexistingRBMKNPPandfromthelistofboundingaccidentsderivedfromthealreadymentionedECProject. ThefeaturesofCocosysandRelap5thermal–hydraulicsystemcodesdevelopedinGermanyandUS,respectively,areoutlinedinthethirdpartofthepaper,givingemphasistothequalificationlevelintherangeofparametersofinteresttoRBMKconfinementsystems.TheadoptednodalisationsresemblingIgnalina2andSmolensk3NPParealsodescribedincludingthedemonstrationofthequalificationlevel. Theresults,fourthpartofthepaper,basicallyshowtheoverallRBMKconfinementsystemrobustnesswithintheDBAarea.Theimportantroleofthereactorcavityisconfirmedandoftheventing.Thenumberofbrokenchannelscausingthedisplacementoftheupperreactorcavityplateiscalculatedintherange20–25.