Common mode failure analysis

Standard Reliability analysis techniques have demonstrated the capability to evaluate system vulnerability to random component failures. Recently, in the nuclear power industry, interest has been developing in methods to consider systematic, nonrandom, so-called "Common-mode" failures which are not tractable in quantitative terms using the usual fault tree or reliability block diagram approaches. This paper summarizes the state of the art as reliability engineers seek to extend the techniques to include multiple failures from a common cause (or in a common mode). Qualitative extensions of both the fault tree and the reliability block diagrams have been used to evaluate the adequacy of reactor protection circuitry for commercial power reactors.