Insights for Risk Informed Approaches to Sizing Emergency Planning Zones

Emergency preparedness provides the final layer of defence in depth for the protection of public health and safety in the event of a radiological emergency at a nuclear power plant. Emergency plans may include an emergency planning zone (EPZ) when predetermined, prompt protective actions are warranted. The size of an EPZ can be scaled to reflect the risk profile of a particular nuclear facility as determined by a risk informed analysis of the consequences from a spectrum of accidents with considerations given to accident frequency and other factors. The U.S. Nuclear Regulatory Commission and designers of evolutionary and innovative reactors are seeking to use a risk informed approach to determine the appropriate EPZ size for a particular design and site. The paper discusses the merits of a risk informed methodology for determining the EPZ size, an approach that can be supported by contemporary probabilistic risk assessments. The advantages and challenges of using these assessments to support EPZ sizing are also described. The paper also identifies strategies to address prominent challenges, such as the treatment of uncertainty, justifying screening thresholds for accident sequences, consideration of cliff edge effects, and performance monitoring. These strategies are shown to be consistent with the principles of risk informed regulation.