Uncertainty in External Hazard Probabilistic Risk Assessment (PRA): A Structured Taxonomy

Probabilistic risk assessment (PRA) systematically assesses the risks posed to or by a complex system such as a nuclear power plant. Such systems are often comprised of physical structures, mechanical components, and humans that interact with or control the system or respond to system upsets. This complex interdependence leads to many sources of uncertainty that must be characterized in the PRA. This problem is exacerbated when exploring the impacts of external hazards (e.g., earthquakes, floods, and fires) on complex systems due to the additional need to understand and characterize the hazard and its impacts. This study defines a framework for identifying and categorizing the common sources of uncertainty encountered in performing external hazard PRAs for nuclear power plants. The framework may be more generally applicable to the assessment of a wide range of facilities involving potentially high consequence external hazard events. Commentary on drivers of uncertainty in external hazard PRA (XHPRA) (particularly within the context of external flooding hazards) and current gaps/challenges are also provided.