A Reflection on Time and Parameters in Common Mode/Cause Failures

Common cause, or common mode, failures (CCF) are often major contributors in large scale risk analyses of complex systems, such as nuclear probabilistic safety assessment (PSA). In this paper we discuss two challenges and limitations of the most widely used approaches regarding CCF quantification based on parametric CCF models. The first one deals with the time dependent behavior of a system under a staggered testing scheme. We illuminate differences in modeling and quantification which result from the application of different methods for staggered testing of components that share common cause failures. We aim at a simple and flexible way of specifying staggered tests that provides a possibility of a realistic quantification for real-life PSA models. Secondly, we illustrate conditions for using parametric CCF models and their limitations. From this perspective, we investigate alternative approaches for characterizing common cause failures. The goal is to increase flexibility for situations where classical parametric models are unsuitable.

Keywords: Common cause failures, Time-dependent analysis, Staggered testing, Generalized CCF probabilities.

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