Recent trends in risk-informed safety margin characterization

The design and maintenance of adequate safety margins has served as a foundational principle for the safe operation of commercial nuclear power plants since the inception of the commercial nuclear power industry. During the original licensing of the current fleet of plants, adequate safety margins were established by performing conservative analyses and using conservative engineering judgment to specify appropriate safety limits for critical plant parameters. However, over time, plant operation and ageing of plant structures systems and components (SSCs) has the potential to impact these original design margins. Due to the recent emphasis on extended plant operation, it will become imperative that effective methods be developed to manage age-related degradation of plant SSCs, prevent the occurrence of safety-significant operational events, and demonstrate maintenance of acceptable (and even improved) nuclear safety risk. In this paper, we summarize the current state of research to develop a risk-informed approach to characterize and manage nuclear plant safety margins. We describe the basic safety margin concept and summarize research performed under the Nuclear Energy Agency Committee on the Safety of Nuclear Installations Safety Margins Working Group to investigate such an approach for use by regulatory authorities. We also describe collaborative safety margin research sponsored by the Electric Power Research Institute Long Term Operation initiative and the United States Department of Energy's Light Water Reactor Sustainability program being conducted to support decision making by plant owner/operators. Finally, we provide some preliminary conclusions and suggestions for further investigation.