AnalysisofnaturalcirculationphenomenainVVER-1000OriginalResearchArticle

Inalllightwaterreactors(LWR),naturalcirculationisanimportantpassiveheatremovalmechanism.Inthepresentpaper,thenaturalcirculationphenomenaarestudiedwithreferencetostep-wisecoolantinventoryreductionandasmallbreakloss-of-coolant-accident(SBLOCA)inthecoldlegofVVER-1000.Thenaturalcirculationflowmap(NCFM)approachisconsideredtoevaluatethenaturalcirculationperformanceoftheVVER-1000NPPalsocomparingVVER-1000andPWRsystems.Threedifferentelevationsbetweenheatsource(core)andheatsink(steamgenerators)zoneshavebeenconsideredinordertocharacterizethebuoyancyforceinaVVER-1000.Theinfluenceofpowerandthecoldlegsloopsealuponthenaturalcirculationperformanceisalsoevaluated.Inthesecondpart,aseriesofSBLOCAsimulationswithbreakarearangingfrom0.5to11.7%ofthecoldlegcrosssectionalareaareperformedstartingwiththeVVER-1000systeminnominalconditions.TheeffectofEmergencyCoreCoolingSystem(ECCS)includingpassiveandactivepartsofECCSareevaluated.ThesimulationswereperformedbythehelpofthesystemcodeRELAP5.Withintheframeworkofthequalificationoftheadoptedcomputationaltools,theresultsarecomparedwithexperimentaldatafromKozloduyNPPunit6testandPSB-VVERintegraltestfacilityavailablefromtheliterature.Namely,thequalificationoftheadoptednodalisationinsteadystateconditionsisachievedbyusingexperimentaldata.TheaccuracyofselectedresultshavebeenestimatedinquantitativetermsbyapplyingthefastFouriertransformbasedmethod(FFTBM).Finally,therelevanceandthepotentialfortheoccurrenceoftherefluxcondensationmode,i.e.,oneoftheNaturalCirculationregimes,forcoolingofreactorcoreinVVER-1000arediscussed.