Thethree-dimensionalneutronkineticscoupledwiththermal-hydraulicsinRBMKaccidentanalysisOriginalResearchArticle

TheRBMKcoreisacomplexensembleofhigh-pressurehigh-temperaturetubes,graphitebricks,low-pressurelow-temperaturecontrolrodtubes,graphiteinterstitialgaspassages.Anabout7MPaboilinglightwatercrossesthearound19mlongverticaltubes(7mactivelength).Thelatticeconsistingofgraphitecolumnsandhydraulicchannelsisboundedbythereactorcavitywhoseresistantelementsarethemetalcylindricaltankandthickcirculartopandbottomplateswithproperholesforthepassageoftubes. Relatedtoatypicalwatercooledreactor,thepeculiaritiesoftheRBMKcorecanbesummarizedasfollows:(a)largedimensions–theoverallcorevolumeisbyfarthelargestforanuclearpowerplant(NPP)producingelectricity;(b)useofseparatemoderatorandcoolantconstitutedbygraphiteandlightboilingwater,respectively–theboilingwatermostlyabsorbsneutronsinthisenvironmentleadingtothe(small)positivevoidreactivitycoefficient;(c)presenceofwaterchannelsveryclosetoeachothercontainingcoolantatdifferenttemperatures(543–557Kand350Kforfuelchannels(FC)andcontrolandprotectionsystem(CPS)channels,respectively);(d)presenceofcore-wideradial,core-wideaxialandlocaltemperaturegradientsinthegraphitebrickswithtemperaturevaluesintherange330–650Kwiththehigh-temperaturevaluesjustifiedbytheneutronmoderationandgamma-heatingprocesses. Owingtotheabovepeculiarities,thedevelopmentandtheuseofathree-dimensionalneutronkineticscode(3DNK)coupledwithaone-dimensionalthermal-hydraulic(TH)codeisessentialinRBMKsafetyanalyses. Twoapproacheshavebeenusedwithinthepresentcontext,i.e.useofcoupled3DNK-THcodestosupporttheaccidentanalysisintheRBMKasdiscussedinthefirstofthecompanionpapersinthisjournalvolume:applicationofKorsar-BarsmakinguseoftheUnkcodetoderiveλ-matricesneededforBarsandofRelap5/3D-NestlemakinguseoftheHelioscodetoderivethemacroscopiccross-sections. Boundingtransientanalysesofaccidentscenariosincludingcontrolrodwithdrawal,variousLossofCoolantAccident(LOCA)anddischargeofthecontrolrodcircuit,havebeencompleted.Inalloftheanalysedcases,startingfromnominaloperatingconditions,modestfissionpowertimegradientshavebeenfound,i.e.characterizedbytimederivativevaluesforlocalandglobalpowerchangessubstantiallysmallerthancurrentvaluesacceptedinsafetyanalysesoflightwaterreactors.