The U.S. Nuclear Regulatory Commission (NRC) has performed a full-scope, sitewide Level 3 probabilistic risk assessment (PRA) project (L3PRA project) for a two-unit, pressurized-water reactor reference plant. The scope of this project includes all hazards (internal and external) and all radiological sources on the reference site: 1) two, nearly identical reactors; 2) two, hydraulically connected spent fuel pools (SFPs), and 3) a dry cask storage (DCS) facility. The NRC has published a series of reports documenting the results of all of these PRA efforts.

In addition to the single unit PRAs performed for NRC’s L3PRA project, a previously, never-performed task was added: integrated site risk. Integrated site risk is defined as the aggregation of risks from all relevant radiological sources. Of particular interest are those accident scenarios for which both reactors and the spent fuel pools experience simultaneous radiological releases starting from at-power conditions.

To support the ISR task, a sitewide dependency assessment was performed first. The results of this assessment identified that: 1) the two reactors were independent except for unavoidable dependencies (e.g., identical components, sharing of electric power) and 2) there were potential dependencies between the reactors and the SFPs with respect to shared resources (e.g., water supplies, operators). Consequently, the focus of the integrated site risk task was limited to the most challenging accident scenarios that involved the two reactors and the SFPs simultaneously (i.e., a multi-source scenario).
Due to limited time and resources, the ISR task focused on demonstrating a proof-of-concept, rather than full multi-source quantification. Using the results of the sitewide dependency assessment with the multi-unit (MU) results from the Level 1 and Level 2 PRAs, a relatively large seismic event was identified as a good candidate scenario to analyze, since such an event can lead to a severe accident for both the reactors and SFPs. A scenario description was developed, including the timing of reactor and SFP behavior, the timing of plant conditions, and the timing of required operator actions. MU release category definitions were combined with SFP release categories to define release categories for the combination of the reactors and the spent fuel pools. Then, multi-unit consequence results were calculated with the MACCS multi-source capability then combined with release category frequencies to generate multi-unit risk.

Keywords: Integrated site risk (ISR), multi-unit PRA (MUPRA), multi-unit core damage frequency (MUCDF), MU release category frequency (MURCF), multi-source risk, sitewide dependency assessment