PRA and Protective System Maintenance

Abstract

The processes used to manage protective system equipment failures as they relate to Probabilistic Risk Assessment (PRA) in the commercial nuclear power setting are reviewed. 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. Such a maintenance policy is the one used in nuclear power plant protective systems. Observations described in this article include the impact of time-dependent activities associated with maintenance policy as they relate to endogenous and exogenous upset inter-arrival times. Methods evaluating maintenance policy reliant on combinatorial logic, such as PRA, fault trees, or event trees, may lead to ineffective maintenance policy decision-making for protective system efficacy. 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 herein is that the 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 specify and enforce protections in hazardous processes.