Guidelines for reduced seismic loads to assess temporary conditions in nuclear power plants. Final report

Utilities do not have uniform guidelines and criteria to treat plant temporary conditions (TCs) or planned changes to safety-related systems. Regulatory review and acceptance criteria and guidelines tend to be overly conservative, leading to costly measures. This report proposes a risk-informed procedure for evaluating TCs using reduced seismic loads and current licensing basis allowables that reduces plant operation and maintenance (O and M) costs, shortens plant outages, and reduces personnel radiation exposure. The project team applied principles in the Nuclear Regulatory Commission (NRC) risk-informed draft regulatory documents to develop reduced seismic loads based on an allowed increase in time-averaged core damage frequency in a refueling cycle. They determined reduction factors as a function of total usage time of the reduction factors within the duration of one refueling cycle. The methodology is capable of application to plants with and without a seismic probabilistic risk assessment (SPRA) model. Models in the guidelines use information available from the seismic IPEEE (Individual Plant Examination for External Events) program for safe shutdown paths. An Expert Task Force provided input to the development of the methodology and reviewed the final product.