Combining Hazards into a Single-Top Fault Tree

Commercial nuclear power plants use probabilistic risk assessment (PRA) models to gain insights into the risks associated with operating the plants. PRA models can be used to assess a variety of hazards such as internal events (transients and loss of coolant accidents), internal flooding, fire, seismic, and other hazards. Each model can provide risk insights, identify vulnerabilities, and identify significant equipment or operator actions. This information can be used to improve plant performance and safety via equipment or operational changes. It is often convenient, and for some risk-informed regulations it may be required, to combine all hazard PRA models into a single calculational model. This “one-top” model provides a single PRA fault tree that can be solved to generate the risk for all hazards. Combining these models can be a challenge, if they were built with different revisions to the internal events model at their core. The one-top model provides a convenient platform for assessing the risk from all of the modeled hazards in a single quantification. Certain software tools, such as the EPRI FRANX software, simplify the process of creating a one-top model. However, quantifying a one-top model has challenges, because each hazard model is built with different assumptions, data, biases, and uncertainty. Furthermore, when one hazard generates a risk value much larger than the risk value from another hazard, combining the results runs the risk of masking risk insights from the hazard with the smaller risk value. In addition to quantifying the one-top model for risk-informed applications that require or benefit from it, hazard models should still be quantified separately to get the risk insights the individual models provide.