Dependent failure modeling in highly redundant structures—Application to BWR safety valves

Abstract

Abstract Common cause failures and other dependences dominate the failure probability of a group of identical components, particularly when a large number of redundant components are concerned. The currently used models suffer from inconsistencies and practical difficulties at least when structures of more than four redundant components need to be analyzed. This paper will introduce the extension of the common load model (CLM) developed because of the practical need to realistically assess the failure probability of the pressure relief function in the TVO/PRA study. There are twelve safety/relief valves with the success criteria varying from 4/8 to 1–9/12 depending upon the transient case and event scenario. The model is defined through the subgroup failure probabilities, which means that simple but yet exact and consistent expressions for different success criteria are derived. The otherwise-needed truncations and approximations for higher order combinations are avoided. The underlying physical stress-strength model provides understandable interpretations for the model parameters. The paper describes the application on the electromagnetic pilot valves which are critical for the depressurization function, including analysis and utilization of operating experience.