Reliability of Multiple Redundant Systems in Consideration of Common Cause Failure

A reactor protection system (RPS) in a nuclear power plant includes multiple processing channels for ensuring both safety and economy. Lu and Lewis suggested a method of unavailability and spurious operation probability (SOP) estimation for this kind of multiple redundant safety system. They treat independent failures as the main focus of study since they consider that sufficient diversities including physical and technical separation among channels are effective in circumventing common cause failures (CCF) in a CANDU-type nuclear plant. However, most of pressurized water reactors (PWR), including OPR1000 plants in Korea, have single shutdown mechanism for an emergency, excluding some parameters covered by diverse protection system. Authors had developed some formulas which considered the CCFs as additional reason of system failures for simple a k-out-of-n system. This study aims to enlarge the scope of previous study to specific voting logics (including selective 2-out-of-4) and to develop the equations to accommodate the possible combinations of independent events and CCF events