Nuclear Power: On PRA and Protective System Maintenance

The processes used in managing protective system equipment failures in the commercial nuclear power setting are reviewed. We assert that efficacy of protection is governed by maintenance policy that includes system modification, maintenance inter-arrivals as a function of time, and upset inter-arrivals as a function of time; we further assert that such a maintenance policy is the one used in nuclear power plant protective systems. Observations described in this article bear on the impact of time-dependent activities, associated with maintenance policy, as they relate to endogenous and exogenous upset inter-arrival times. We describe why methods evaluating maintenance policy reliant on combinatorial logic, such as Probabilistic Risk Assessment (PRA), fault trees, or event trees, may lead to ineffective maintenance policy decision-making for protective system efficacy, and we show why this is true. Recommendations for maintaining effective protections, and connections to engineering maintenance practice and regulations are made based on the implications that come from our observations. The importance of the issues described is that relationship of design, maintenance, and repair policies must be properly understood and taken into account by process owners, operators, and investors as well as regulators, who manage protections in hazardous processes.