An assessment of the core damage frequency of a PWR at shutdown

Abstract

Abstract The core damage frequency of a Pressurized Water Reactor (PWR) at shutdown was assessed for three types of initiating events: loss of residual heat removal, loss of offsite power, and loss of coolant inventory. The assessment takes into consideration different plant conditions in different types of outages, and uses failure data collected specifically for shutdown conditions. In particular, events occuring during mid-loop operations in which the reactor vessel level is lowered to the hotleg mid-plane were analyzed and found to be the dominant contributor to core damage. A total core damage frequency of approximately 5·22 × 10 −5 per year was obtained. The dominant cause of core damage was found to be human errors during mid-loop operations. The reduction in core damage frequency due to three plant improvements was also estimated. The improvements are (1) upgraded instrumentation with emergency procedures for shutdown conditions, (2) upgraded vessel level indication, and (3) removal of auto-closure interlock on the RHR suction valves.