'How Safe Is Safe Enough?': A PRA Perspective on GSI-191

摘要

Probabilistic Risk Assessment (PRA) can be used to provide insights to the question of "How Safe is Safe Enough?" The three key traditional keystones of safety are compliance with regulatory requirements; ensuring that defense in depth for accident prevention and mitigation; and maintaining safety margins. The methods used to show compliance with regulatory requirements can significantly impact the design and operation of the plant, especially the conservatisms included in the analysis methods to address uncertainties in knowledge. The PRA can be used to show that, at some point the degree of conservatisms in the analysis methods does not increase safety as measured by the core damage frequency (CDF) and large early release frequency (LERF) risk metrics. A series of PRA analyses have been performed to show the sensitivity of the risk metrics to various key assumptions used to drive the design and operational features of long term core cooling using containment sump recirculation. This directly ties to the NRC acceptance of plant modifications to respond to Generic Issue 191 to ensure long term core cooling via sump recirculation. These sensitivity analyses show that wholesale insulation change-out and further containment sump re-design may not improve safety as measured by risk. Additional focus on other aspects of accident prevention and mitigation such as leak detection and containment water management strategies provide additional defense in depth and decrease overall risk metrics. Thus, the fundamental keystones of safety may not be optimized by only considering conservatisms in methods used for regulatory compliance. This paper describes the analyses and results along with recommendations for improving the probability of successful long term core cooling via sump recirculation and the NRC acceptance of the current plant modifications to address GSI-191.