The severe accident control strategy of the KERENA TM BWR

AREVA NP currently finalizes the Basic Design of its KERENA TM advanced BWR. The plants safety concept is founded on highly reliable redundant active and passive systems for water supply and heat removal from the core. As a consequence, the probability to enter a severe accident (SA) with core damage is extremely low, in the range of 10-8 per reactor year. The probability to end up in a non-coolable core configuration is by a factor of ten lower. To cover also the remaining cases, the KERENA TM BWR involves SA design provision that allow the flooding of the reactor cavity. The aim is to prevent RPV melt-through, hence preserve the containment integrity, and thus further reduce the frequency of large activity releases to the environment. The deterministic demonstration of the efficiency of in-vessel melt retention (IVR) considers the state of the art and includes phenomena like: - the focusing effect of the top metal layer and its transient evolution, - the potential formation of a dense metallic phase inside the oxidic pool, - an oxidic crust at the surface of the metal layer. As already a thin oxidic crust can thermally insulate the top surface of the metal layer and aggravate the focusing effect, confirmatory experiments were performed on this issue. Deterministic integral assessments for the KERENA TM plant indicate that the large coolant reservoirs inside the containment are capable of delaying RPV dry-out and melt relocation into the lower head over long periods of time, even in case of a break in the turbine building. Such long delays effectively reduce the decay power in the molten pool at the time of first vessel contact or even prevent core melt within the grace periods. In combination with the large diameter of the KERENA TM vessel and the high mass of internal steel structures the low decay power ensures that heat fluxes to the water on the outside of the RPV remain below critical values. (authors)