A Risk-Based Approach to Assessing CANDU Containment Reliability Requirements

Abstract

The CANDU multi-unit negative-pressure containment system has a number of active and passive features which can play a role in accident consequence mitigation. The AECB has a licensing requirement that containment meet an unavailability target of 10^-3 yr/yr. Compliance is demonstrated by calculating the unavailability of each subsystem and summing each contribution to obtain an overall unavailability. This process has the inherent assumption that each subsystem is of equal importance to the containment function. The paper uses results from a probabilistic risk assessment to show that this assumption is not optimal and proposes an alternative approach to setting unavailability targets based on equalizing the risk posed by failure of each subsystem.