Shutdown Probabilistic Safety Assessment of Boiling Water Reactor

The aim of the safety of the nuclear power plant is to assure that risk from nuclear power plant to public and plant personnel is insignificant during all plant operating states. Shutdown Probabilistic Safety Assessment (SPSA) is a systematic methodology for assessment of the safety of nuclear power plant at shutdown mode of operation. A comprehensive Shutdown PSA has been conducted for a typical boiling water reactor (BWR) to demonstrate the safety of the reactor during the shutdown mode of operation. The study is carried out in accordance with procedures outlined in various IAEA documents, NUREGs and AERB documents, etc. Refueling and associated maintenance is a dynamic process. It can have various configurations of the plant during the entire span of the refueling shutdown. In this study, a typical refueling shutdown (RSD) plan of BWR Plant at Tarapur Site has been used to derive the plant operating states. To reduce the effects of dynamic nature, refueling shutdown (RSD) duration is divided into 7 plant operating states (POS) and plant configuration is assumed constant during each POS. Six transients are considered as initiating events (IEs) in this study. For each IE, a separate event tree with various possible accident sequences has been developed considering the availability of systems/components in the POS considered. Dynamic human actions modeled in different event trees are quantified using Accident Sequence Evaluation Program (ASEP) model based on NUREG/CR-4772. The Level-1 Shutdown PSA developed for TAPS-1&2, represents a complete model of the safety of the plant enclosing design, maintenance, and safety practices during refueling shutdown, component reliabilities, common cause failures (CCFs), and operator errors. This model finds out the main contributors to most likely severe accidents which can occur during refueling shutdown as a combination of postulated initiating events, component malfunction, and operator errors. The operation and maintenance procedure at TAPS-1&2 can be developed based on the outcome of the present study. With this integrated model available, the effect of proposed changes in maintenance practices can be examined by studying their impact on changes in accident frequencies. The results of the Shutdown PSA indicate that a fairly high level of defense-in-depth exists in TAPS-1&2 design during shutdown mode of operation. This is evident from the final CDF value, as well as from the predominantly higher order minimal cut-sets observed in the core damage sequences. The various tasks of SPSA are discussed in this paper.