Mitigation of internally initiated severe accidents for a BWR Mark-II power plant

A probabilistic risk assessment and consequence analysis for a BWR with a Mark-II containment design has shown that overpressurization is the dominant containment failure mode for a wide range of potential core meltdown accidents. This failure mode is a major contributor to the predicted off-site health consequences. Mitigation of this failure mode is aimed at maintaining the containment integrity by using a wetwell venting system and a drywell spray system. Detailed analyses with the MARCH 2 code were used to determine the effectiveness of the mitigating features. The results show that the wetwell venting system is capable of preventing overpressurization in the containment and the drywell spray system is needed to reduce the corium/concrete interactions. However, additional mitigating systems are required for long-term cooling of the corium and ultimate removal of the decay heat for the ATWS case.