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# Phenomena Identification and Ranking Technique (PIRT) Exercise for Ranking Low-Power Shutdown Plant Operating States and Outage Types. Appendices A – K (NUREG/CR-7265, Volume 2)

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## Publication Information

**Manuscript Completed:** December 2018**Date Published:** October 2020

**Prepared by:**GA Coles*, SM Short*, AM White*, and MY Toyooka*

Pacific Northwest National Laboratory* P.O. Box 999, Richland, Washington 99352

Jeff Wood, NRC Project Manager

Office of Nuclear Regulatory ResearchU.S. Nuclear Regulatory CommissionWashington, D.C. 20555-0001

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## Abstract

The U.S. Nuclear Regulatory Commission (NRC) is performing a full-scope, site Level 3 probabilistic risk assessment (PRA), using a four-loop pressurized water reactor (PWR), as the reference plant. During the development of the Level 3 PRA, specifically the low-power shutdown (LPSD) analysis, the need to prioritize the plant operating states, hazards, and outage types to include in the full-scope site Level 3 PRA was identified by the Level 3 PRA project team. This need was further magnified by the fact that realistic LPSD modeling of plant outages involves consideration of the range of types of outages, from planned refueling and maintenance outages to unscheduled maintenance outages, and the significant variation in the types of activities that are performed during these outages. In short, the scope to develop a PRA for each of these LPSD plant configurations is a resource-intensive undertaking.

Because of the significant resources required to develop a full-scope LPSD PRA model, the Level 3 PRA project team decided to use the Phenomena Identification and Ranking Technique (PIRT) process to identify and prioritize the plant operating states, hazards, outage types, and other influences to include in the full-scope site Level 3 PRA. Pacific Northwest National Laboratory (PNNL) was contracted by NRC to coordinate and facilitate this PIRT process.

This report describes the PIRT process developed and used to meet the project objectives and associated results of implementing the process. The objective was to identify the plant operating states (POSs) and plant outage types (POTs), rank them according to their importance to LPSD risk in the context of different hazards, and consider important influences/phenomena (e.g., systems/components out of service, fission product inventory, thermal-hydraulics, status of reactor coolant system pressure and containment boundaries, operator/maintenance activities) associated with an LPSD model in the ranking process. The PIRT process focused on activities that potentially result in damage to fuel during LPSD operations and while the fuel is in the reactor pressure vessel. POSs and POTs specific to LPSD operation at the reference plant were evaluated by the PIRT panel, whose purpose was to identify and rank plant operating states, hazards, and outage types according to their importance to LPSD risk, and to consider important influences/phenomena associated with LPSD in the ranking process. Plant-specific hazards evaluated by the PIRT panel were internal event hazards, internal flooding hazards, internal fire hazards, and seismic hazards. Many of the plant-specific information sources are based on revisions from 2012 and earlier. The information does not necessarily represent the reference plant as currently operated today. However, the information and insights gained are deemed to be generally applicable to the low power and shutdown operation of a four-loop PWR.

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