EPR~(TM) engineered features for core melt mitigation in severe accidents

摘要

For the prevention of accident conditions, the EPR~(TM) relies on the proven 3-level safety concepts inherited from its predecessors, the French "N4" and the German "Konvoi" NPP In addition, a new, fourth "beyond safety" level is implemented for the mitigation of postulated severe accidents (SA) with core melting. It is aimed at preserving the integrity of the containment barrier and at significantly reducing the frequency and magnitude of activity releases into the environment under such extreme conditions. Loss of containment integrity is prevented by dedicated design measures that address short- and long-term challenges, like: the melt-through of the reactor pressure vessel under high internal pressure, energetic hydrogen/steam explosions, containment overpressure failure, and basemat melt-through. The EPR~(TM) SA systems and components that address these issues are: (1) the dedicated SA valves for the depressurization the primary circuit; (2) the provisions for H_(2) recombination, atmospheric mixing, steam dilution; (3) the core melt stabilization system; (4) the dedicated SA containment heat removal system. The core melt stabilization system (CMSS) of the EPR~(TM) is based on a two-stage ex-vessel approach. After its release from the RPV the core debris is first accumulated and conditioned in the (dry) reactor pit by the addition of sacrificial concrete. Then the created molten pool is spread into a lateral core catcher to establish favorable conditions for the later flooding, quenching and cooling with water passively drained from the Internal Refueling Water Storage Tank. Long-term heat removal from the containment is achieved by sprays that are supplied with water by the containment heat removal system. Complementing earlierpublications focused on the principle function, basic design, and validation background of the EPR~(TM) CMSS, this paper describes the state achieved after detailed design, as well as the technical solutions chosen for its main components, including their industrial realization and implementation in the plant layout.