Toward a More Risk-Informed and Performance-Based Framework for the Regulation of the Seismic Safety of Nuclear Power Plants   [open pdf - 2MB]

"Dozens of seismic probabilistic risk assessments (SPRAs) exist that have analyzed large nuclear power plants (NPPs). A major insight from these SPRAs is that, although generally the plants are adequately safe against earthquake threats, the way the industry and the United States Nuclear Regulatory Commission (NRC) currently design, build, operate, analyze, and regulate seismic NPP safety may not be optimal. This suboptimal situation means that for both operating NPPs and new NPP designs not yet built, they may fail to take advantage of possible additional safety insights and improvements, may be more difficult to analyze and to regulate than they need to be, and may cost more than they otherwise would. There is room for improvement in several areas. The major topics covered herein are the variations in residual seismic risk from plant to plant; the unbalanced risk profiles and incomplete defense in depth achieved at many plants; the impact of structures and components being designed individually for seismic performance rather than taking a systems approach; the variations in margin that exist among the industry consensus codes and standards for seismic design and analysis; and the observation that the design codes and the NRC regulations do not work together well. Each of these topics is analyzed in detail, and case-study examples based on the seismic risk profiles of two operating plants are used to illustrate the issues. Suggestions are presented that would improve the seismic framework in each of these areas."

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