MAAP 4.07 Analysis of Long Term Containment Heat Removal After Reactor Vessel Failure (DEC-B) for Nuclear Power Plant Krško (NEK)

Abstract

The paper presents the MAAP 4.07 analysis of containment heat removal after reactor vessel failure resulting from the initial Station Blackout (SBO) accident. The accident is analysed considering mitigation measures for heat removal from the containment using alternative equipment (Alternative Residual Heat Removal (ARHR) pump and heat exchanger (ARHX) and, also, Alternative Safety Injection (ASI) pump). The mitigation actions are taken according to NEK Severe Accident Mitigation Guidelines (SAMG). There are several possibilities to remove the heat from the containment once the reactor vessel fails and, for all of them, the necessary condition is to have the sufficient source of water (Residual Water Storage Tank (RWST), Alternative Boron Water Tank (ABWT) or other) and the appropriate heat exchanger available. Two options are presented within this paper: 1. Injection to the Reactor Coolant System (RCS) using ASI pump and recirculation (sump to RCS) through ARHR system via ARHX, 2. Spraying the containment through Containment Spray (CI) system using ARHR pump and, then, recirculation (sump to spray) through CI and ARHR systems via ARHX. The results show that the containment heat removal can be done with either of analysed ways if the water is provided for recirculation (assumed containment level 3.9 m ~ 760 m³). However, with the fact that the reactor cavity is not flooded, the cooling using ASI will initially result in considerable containment pressure increase because the water is spilled through the RCS over the hot molten core debris. Therefore, it must be stated that the preferable way of containment pressure reduction, once the vessel has failed, is by using the containment spray. On the other hand, if RWST is not available, then the initial water delivery cannot be made from ABWT via CI system because this option does not exist. It shall also be pointed out that, if the active containment heat removal is started early enough, the Passive Containment Filtered Vent System (PCFVS) opening would be prevented and no fission products shall be released to environment.