Phenomena Identification and Ranking Table Process

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The Phenomena Identification and Ranking Table (PIRT) process is a systematic way of gathering information from experts on a specific concept, and ranking the importance of the information, in order to meet some decision-making objective. It has been applied to many nuclear technology issues, including nuclear analysis, to help guide research or develop regulatory requirements. The execution of the PIRT process on a specific concept is called an exercise and these exercises will vary greatly in scope and depth based on the accident tolerant fuel (ATF) concept's departure from the current state of practice and its maturity.

NRC-sponsored PIRT exercises start with a clearly defined technical issue and identified objective(s). Next, a literature review is performed (often by a contractor like the U.S. Department of Energy National Labs) to identify the current knowledge base on the issue. Once the current knowledge base is established, the NRC convenes a panel of experts from around the world (either physically or virtually) to brainstorm possible challenges (phenomena) that might arise from the concept. Then, the expert panel ranks those phenomena according to how much is known about the phenomena and their safety significance. For ATF, the panel subsequently completes a report to help the NRC develop or refine its regulations and guidance. Information gleaned from the PIRT exercises will also inform the development of confirmatory analysis tools used as part of the review of ATF-related licensing actions.

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Completed ATF PIRT Exercises

To date, the NRC has sponsored two PIRT exercises. One PIRT exercise was chromium-coated cladding performance during normal reactor operations, anticipated operational occurrences, and design basis accidents. The PIRT exercise took place in April 2019 and was led by NRC contractors at Pacific Northwest National Laboratory. The final report was completed in June 2019. The PIRT exercise was instrumental in development of NRC interim staff guidance for chromium-coated cladding.

The second NRC-sponsored PIRT exercise covered the performance in severe reactor accidents of the current ATF concepts, higher burnup fuel, and fuel with enrichment above 5%. This PIRT exercise also assessed the impact of ATF, higher burnup, and increased enrichment on accident source terms. The PIRT exercise took place from September 2020 through April 2021 and was led by NRC contractors at Energy Research Inc.  The final report was completed in April 2021.  

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Current ATF PIRT Exercises

Currently, the NRC is sponsoring a PIRT exercise regarding Fuel Fragmentation Relocation and Dispersal (FFRD). The final report is expected in Spring 2024.

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Future ATF PIRT Exercises

Additional PIRT exercises are being considered to cover spent fuel storage and transport of fuel with iron-chromium aluminum (FeCrAl) cladding and with chromium-coated cladding. The NRC will develop timelines, as needed, for subsequent ATF PIRT exercises and additional implementation details through coordination with its external stakeholders.

Completed literature reviews and PIRT exercise reports can be found on the ATF-related Documents page.

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