Unavailability and spurious operation probability of k-out-of-n reactor protection systems in consideration of CCF

Many safety–critical applications including nuclear power plants are equipped with k-out-of-n or specific-voting-logic redundant safety signal generation systems for ensuring both safety and economy. In order to determine the configuration of these safety signal generation systems and to analyze the risk from these systems, common cause failures must be considered carefully. The study aims to develop a quantification method of the system unavailability and the spurious operation probability in consideration of the CCF. Analytic equations are developed and three popular configurations for the RPS (2-out-of-3, 2-out-of-4, and selective 2-out-of-4 voting) are investigated as examples. The quantitative results of this study show that the CCF dominates the system unavailability and the SOP for the small channel failure probability.

► We derived equations for the unavailability and the spurious operation probability.
► The combination of independent failures and CCFs is addressed.
► The error from the ‘no CCF multiplication’ approximation is addressed.