As stated in the External Events HRA EPRI guidance [Ref. 1], there are inherent differences from internal events PRA that impact the ability of operators to perform actions during an external event.
“Although the process steps and concepts are the same for an external event PRA, a fire PRA and an internal (non-spatial) PRA, there are several key differences needing to be addressed in the development of an external event HRA. Differences include the impact of the initiating event on the site structures, systems, and components (SSCs). Other differences, such as procedures and the impact on timeline development, should also be considered.”
A damage context “hybrid approach” has been developed and utilized for Seismic Probabilistic Risk Assessment (SPRA) that takes advantage of the binning system developed in the EPRI guidance [Ref. 1], while also applying site and context-specific parameters. In addition, the hybrid approach does not provide overly conservative results, like some scaling and factor based methods. This paper describes the methodology of the Seismic and External Hazards (EH) HRA hybrid approach and provides an example of how it has been applied in a SPRA. This approach meets the Capability Category II quality requirements in Part 5 of the ASME/ANS PRA Standard RA-Sa-2009 [Ref. 2].