United States Nuclear Regulatory Commission - Protecting People and the Environment

Reviewing Accident Tolerant Fuel: In-Reactor Performance | Fuel Cycle, Transportation, and Storage | Probabilistic Risk Assessment | Independent Confirmatory Calculation

How the NRC Reviews Probabilistic Risk Assessment Changes for ATF

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The NRC uses probabilistic risk assessments (PRAs) to estimate risk to investigate what can go wrong, how likely it is, and what the consequences could be. The results of PRAs provide the NRC with insights into the strengths and weaknesses of the design and operation of a nuclear power plant. PRAs cover a wide range of NRC regulatory activities, including many risk informed licensing and oversight activities (e.g., risk-informed technical specification initiatives, the significance determination process portion of the Reactor Oversight Process). These activities make use of both plant-specific licensee PRA models and plant-specific NRC PRA models. The NRC uses the former models predominantly for licensing and operational activities and the latter models predominantly for oversight activities. A key tenet of risk informed decision-making is that these models reflect the as-designed, as-operated plant. For this reason, these models should be updated to reflect significant plant modifications. The introduction of significantly different fuel into the reactor core has the potential to affect these models, particularly once the reactor core composition significantly influences the plant's response to a postulated accident (e.g., time to fuel heat up and degradation, amount of total hydrogen generation).

For ATF, the NRC may need to perform two important activities:

  1. Prepare for, and review, PRA related-information submitted as part of the licensing process for both batch loading of ATF and incorporation of any significant plant modifications for ATF.

  2. Update PRA-related capabilities to effectively:

    • Review risk-informed licensing applications and ensure that applicants are using acceptable PRA models once ATF is implemented.
    • Perform risk-informed oversight evaluations (e.g., significance determination process) once ATF is implemented.

The first activity is highly dependent on the approach taken by each vendor or licensee, or both, in its licensing application. The insights gained from these reviews will assist the NRC in determining necessary updates to the Standardized Plant Analysis Risk (SPAR) models, so that they represent the as-built, as-operated plant. In general, the PRA modeling that may be impacted include the following:

  • selection of core damage surrogates used in defining PRA end states (e.g., peak nodal clad temperature changes, water level at two-thirds active fuel height)

  • accident sequence modeling assumptions used to create event tree models that define the high-level successes and failures that can prevent core damage (e.g., late containment venting is required for avoiding core damage)

  • system success criteria used in fault trees for defining the minimum hardware needed to fulfill specific mitigation functions (e.g., two relief valves are needed to prevent injection pump deadhead when feed and bleed cooling is used for a transient with no feedwater)

  • sequence timing assumptions used in accident sequence modeling, success criteria determinations, and human reliability analysis to establish relevant time windows (e.g., feed and bleed cooling initiated within 20 minutes of –low steam generator water level).

For the second activity, licensees' PRAs should continue to use acceptable models and assumptions as part of the implementation of ATF and update the NRC's models (as necessary) to reflect the ATF plant modifications. Licensees should incorporate modifications affecting a plant's risk profile (e.g., ATF or other plant modifications) into their plant's PRA models under their existing PRA maintenance programs.

PRA-related impacts may be assessed using the general knowledge being developed in ATF areas in conjunction with one or more pilot efforts using the existing SPAR models. Such pilots may help gain risk insights, assess the potential changes in core damage frequency (CDF) and large early release frequency (LERF), and highlight areas where existing guidance or methods may require refinement to address the implementation of ATF.

Risk Insights gained from these activities may be applied by the NRC to perform risk-informed oversight evaluations (e.g., significance determination process) once ATF is implemented.

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Page Last Reviewed/Updated Thursday, July 23, 2020