Failure Frequency Estimates for Boiling Water Reactor Pressure Vessels

Abstract

This paper reviews estimates of rupture frequencies for reactor pressure vessels (RPV) at boiling water reactor (BWR) nuclear power plants as reported in the literature. Results permit improved probabilistic risk assessments (PRA) for severe accidents that could cause core damage and/or challenge the capabilities of BWR reactor containment systems. Current and historical estimates of failure frequencies are considered for light water reactors in general and more specifically for BWR plants. The focus is on large ruptures that could give flow rates exceeding the rates associated with double ended breaks of large diameter recirculation piping. Rupture frequencies for BWR vessels as used for PRA evaluations have historically been assigned low values (e.g. 10−7 to 10−6 per vessel per year). The objective of the present work was to establish possible technical bases for more realistic values of rupture frequency (i.e. 10−8). Historical estimates from the early WASH-1400 reactor safety study were first reviewed and used as a point-of-reference. More recent estimates came from various sources such as a U.S. Nuclear Regulatory Commission expert elicitation process that estimated Loss-of-Coolant Accident (LOCA) frequencies. Other studies both by industry and by the USNRC have addressed rupture frequencies for BWR vessels subject to low-temperature-over-pressure (LTOP) events. On the other hand, recent comprehensive evaluations have focused mostly on RPV failure frequencies for pressurized water reactors (PWRs) caused by pressurized thermal shock events. An important consideration was that rupture frequencies for BWR vessels are believed to be lower than those for PWR vessels, because BWR vessels are less embrittled than PWR vessels and are subject to less severe thermal transients. The review concludes that prior studies support an estimate of 10−8 or less for BWR vessel rupture frequencies. Probabilistic fracture mechanics calculations for individual vessels accounting for plant specific conditions are recommended to support even lower estimated frequencies. Use of more realistic vessel rupture frequencies in a plant’s PRA provides an improvement in not only the perceived plants risk of core damage, but also provides better decision making for plant operation and maintenance activities in that a conservative initiating event treatment within a PRA can mask other initiating events of higher importance.