Safety Assessment for Ensuring Core Catcher Performance During Severe Accident Scenarios for VVER-1000 Reactors

VVER is a Pressurized Water Reactors (PWRs) which uses light water as coolant and moderator and uses enriched Uranium as fuel which is housed inside Reactor Pressure Vessel (RPV). Modern NPP designs require a device for core melt localization within the containment as a key element of severe accident management during a core melt accident. In VVER-1000 reactors a crucible type core catcher has been installed for retention of molten corium inside the reactor containment. A crucible type catcher is basically a steel vessel placed below RPV to accommodate molten corium coming to catcher vessel after RPV failure. Heat from molten corium is initially removed from walls of core catcher to the cooling water in the reactor sump. For safe operation of core catcher, sufficient water inventory should be ensured in the reactor cavity. For most of core melt scenarios sufficient inventory in reactor cavity is accumulated passively from primary leak and hydro accumulators. But there are some core melt scenarios for which extra water from fuel pool is to be diverted by operator action into the reactor cavity for ensuring adequate water level. Also in the later phase, water is supplied directly on corium melt surface for melt quenching. Water should be supplied on the top of corium surface only after inversion of layers has taken place inside core catcher. Water is supplied on the top of melt surface as an operator action by the operator. In the present study, above mentioned phenomenon and safety assessment for core catcher have been carried out using in-house developed computer code ‘KKCAT’. The various phenomenon related to core catcher has been modelled using ‘KKCAT’ and assessment for critical safety parameters related to heat transfer have been carried out in the present analysis.