NRC’s Significance Determination Process: Current Status and Future Trends in Assessing Risks Attributed to External Events

Abstract

Until the late 1990s, inspection and enforcement practices at the U.S. Nuclear Regulatory Commission (NRC) were minimally influenced by probabilistic risk assessment (PRA) results. In the late 1990s, the NRC noted various shortcomings of the licensee assessment process [commonly known as the systematic assessment of licensee performance (SALP)] established in 1980. For example, the SALP process was found to lack focus on the most safety important issues and was subjective. The resulting new reactor oversight process is a risk-informed performance-based framework that uses PRA insights. Consequently, the NRC set up a significance determination process (SDP) that would rely on the risk significance of performance deficiencies of the licensees. The SDP assesses contributions from both internal events and external events using the best available information. Over the last two to three decades, the NRC has completed the development of high-quality PRA models that are capable of assessing risks due to internal events. Several recent regulatory actions, some of which were prompted by the events at Fukushima, have provided the impetus for the NRC and the licensees to enhance the methods and information related to assessing risks associated with external events such as earthquakes and floods. This paper describes the current status on how the NRC staff uses the best available information to assess risk associated with external events and notes a plethora of regulatory actions that may provide inertia for the development of high-quality models for external events. The paper then points to past trends on how the regulatory actions in the fire PRA area contributed to significant advancements in fire PRA technology and points to tangible evidence on how the same trend has begun in the area of seismic- and flood-related risk assessments.