Assessment of Uncertainty in Scenario Development for External Hazard Probabilistic Risk Assessment for Nuclear Power Plants

Probabilistic Risk Assessment (PRA) has been widely accepted and implemented by nuclear power plants (NPPs) as an important tool for risk assessment and decision-making. Recent events such as the Fukushima Daiichi accident have increased the focus on external hazards as potentially significant risk contributors to the core damage frequency of NPPs. However, limited models and tools are available to support probabilistic assessment of external hazards at NPP PRAs, particularly for natural hazards (e.g. floods), for which it is important to consider warning time, event progression, and event duration. Since the spatially and temporally dynamic nature of physical event impacts leads to significant sources of uncertainty associated with hazard impacts and event progression, limited access to relevant information poses considerable challenges to modeling these impacts within a PRA structure. In this paper, we develop an integrated assessment strategy to overcome these limitations in external hazard PRAs for NPPs. The plant response model development is the primary focus of this paper. A hybrid mechanistic computational model was developed to estimate the NPP system's event progressions in response to external flooding events. The external flooding PRA response model is currently in development following the proposed flood propagation path to identify the critical plant structures, systems, and components as well as their failure modes that are significant to the risk and would potentially be affected by the external flooding. For demonstration, the loss of offsite power (LOOP) initiated short-term and long-term station blackout (SBO) scenarios under external flooding events for a generic PWR are established and assessed with the developed integrated framework. The results offer valuable insights into the significance of flood assumptions and the consequences of external flooding events on nuclear power plants. The integrated framework will facilitate identifying and characterizing the sources of uncertainties associated with external flooding hazards to ensure the safety and reliability of NPPs.