Impact of probabilistic risk assessment and severe accident research in reducing reactor risk

The development of probabilistic risk assessment (PRA) as a safety analysis tool and the implementation of lessons learned from risk studies in the design, operation and regulation of nuclear power plants has resulted in a substantial reduction in reactor risk. The lack of a strong technical basis for realistically assessing severe accident behavior, including the release and transport of radionuclides to the environment, resulted in some conservatism in early risk studies that distorted the true nature of severe accident risk. This paper describes the evolution of PRA over the past four decades, the benefits that have been achieved in the reduction of reactor risk, and the changes in the perspective of the nature of severe accident risk associated with the development of a strong technical basis for assessing severe accident consequences. Based on these developments, we conclude that the probability of early containment failure leading to a large, early release of radioactive material to the environment was over stated in these early risk studies. Although it is not possible to preclude the possibility of offsite early fatalities in a severe accident, the probability is extremely small, perhaps below the level at which it should be a key consideration in regulatory oversight. Conversely, as highlighted by the Fukushima accident, the potential for the societal impacts of land contamination represents an important element of reactor accident risk that has received insufficient consideration in the past. These findings have implications regarding preferred strategies for emergency planning and appropriate metrics for risk-informed regulation.