Recent advances in analysis of PWR containment bypass accidents

The Reactor Safety Study identified and quantified the contribution to off-site radiological risks of accident sequences at pressurized water reactors (PWRs) in which the release of fission products may be released by bypassing the containment building. These so-called bypass accidents were also referred to as interfacing systems loss-of-coolant accidents (LOCAs) or Event 5 sequences due to the postulated failure of valves separating the high-pressure reactor coolant system (RCS) from low-pressure piping located outside containment. Containment bypass sequence risks constitute a large fraction of the total pressurized water reactor (PWR) in NUREG-1150 in large part because estimates of competing risks from early containment failures have been greatly reduced since WASH-1400. Rigorous analyses of both SGTR and V sequence bypass sequences result in reductions in fission product release to such an extent that in-containment sequences are expected to dominate PWR risks at levels substantially lower than reported in NUREG-1150. It is important that these findings be confirmed by other investigators, particularly in light of the NRC's ongoing study of the frequency of occurrence of interfacing systems. LOCAs based on extensive investigations at operating plants. Progress in this latter effort should be matched by progress in the knowledge and understanding of the progressionmore » of bypass sequences, once initiated.« less