COOLABILITY OF SEVERELY DEGRADED CANDU CORES

Analytical and experimental studies have shown that the separately cooled moderator in a CANDU reactor provides an effective heat sink in the event of a loss-of-coolant accident (LOCA) accompanied by total failure of the emergency core cooling system (ECCS). Should the moderator heat sink could fail in such an accident, early studies indicated that the resulting core debris would be retained in the calandria vessel, cooled by the independently cooled shield-tank water. Thus, the calandria vessel would act inherently as a "core-catcher" as long as shield tank water cooling was maintained. The present paper describes an empirically based mechanistic model of core disassembly in a severe accident sequence in a CANDU reactor; the end state consists of a porous bed of dry solid, coarse debris, irrespective of the initiating event and the core disassembly process. The paper also describes an improved model for the subsequent heat-up of the debris bed and a detailed model for the thermal behaviour of the molten pool that might form by eventual melting of the solid debris bed. The results of this work confirm and provide more confidence in the conclusions of the early studies that the calandria vessel will retain its integrity in such severe accidents in a CANDU reactor and will contain a disassembled or partially molten core for a long period without operator intervention, thus acting as an effective core-catcher.