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For implementation of the near-term ATF concepts (chromium-coated cladding, doped pellets, FeCrAl cladding), fuel vendors and power reactor licensees are exploring the possibility of an increase in the burnup limit to 75 or 80 gigawatt-days per metric ton of uranium (GWd/MTU). The current burnup upper limit for a nuclear fuel rod varies slightly among the three fuel vendors, but it corresponds to roughly 62 GWd/MTU when averaged over the rod. This limit is contained within the NRC-approved fuel analysis methods of each individual NRC-issued license. To increase this burnup limit, licensees would need a license amendment and provide a technical justification that the fuel remains safe during operation and after removal from the reactor.
Higher Burnup Advantages
Increasing the burnup limit may provide several potential advantages for licensees, including:
- Longer reactor cycles – reactors can operate longer between refueling outages
- Need to purchase fewer fuel assemblies from fuel vendors
Higher Burnup Implementation
Higher burnup may have significant effects on power operation and the back-end of the nuclear fuel cycle. In addition to license amendments, several of the other specific tasks necessary for higher burnup to be licensed may include:
- Update of fuel analytical models which predict fuel behavior and performance to capture higher burnup effects
- Update of design bases and safety analyses at higher burnup levels
- Changes to source term calculations
- Development of transportation and dry storage casks for higher burnup
- Criticality safety analysis of spent fuel pools and modifications for higher heat loads
- Address technical challenges (e.g., Fuel Fragmentation Relocation and Dispersal)
The NRC staff is currently preparing to review topical reports and license amendments regarding higher burnup. Higher burnup-related licensing actions received by the NRC can be found on ATF-related Licensing Actions.
Fuel Fragmentation, Relocation, and Dispersal (FFRD)
During power reactor operation, fuel pellets undergo microscopic and macroscopic changes. Under accident conditions, fuel pellets may fracture due to expanding fission gas bubbles, thermal stress, loss of mechanical constraint, and/or mechanical loading. Experiments performed by United States and international laboratories demonstrated that the higher the burnup of the fuel during accidents, the smaller each fuel fragment becomes.
NRC FFRD Research Efforts
This phenomenon in fuel was first recognized as a potential technical challenge in the mid-2000s and the NRC staff took action. In May 2008, the NRC Office of Nuclear Regulatory Research (RES) issued Research Information Letter (RIL)-0801, "Technical Basis for Revision of Embrittlement Criteria in 10 CFR 50.46", which provided a technical basis for revising the loss-of-coolant accident (LOCA) cladding embrittlement criteria found in Title 10 of the Code of Federal Regulations (10 CFR) 50.46, "Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors". RIL-0801 discussed axial fuel relocation and the loss of fuel particles through a rupture opening and recommended further research in these areas. As a follow up in 2012, the NRC staff issued NUREG-2121, "Fuel Fragmentation, Relocation, and Dispersal During the Loss-of-Coolant Accident," to further explore this phenomenon.
The NRC staff provided information regarding fuel fragmentation, relocation, and dispersal (FFRD) to the Commission in November 2015 through SECY-15-0148, "Evaluation of Fuel Fragmentation, Relocation and Dispersal Under Loss-Of-Coolant Accident (LOCA) Conditions Relative to the Draft Final Rule on Emergency Core Cooling System Performance During a LOCA (50.46c),". At that time, the NRC staff concluded that regulatory action was not needed to address FFRD phenomena. This conclusion was closely linked with fuel design limits in place at that time, especially considering the allowable fuel rod burnup of 62 GWd/MTU.
Industry is currently looking to receive approval to increase fuel burnup limits from 62 GWd/MTU to 75 or 80 GWd/MTU. In order to support the review of fuel designs, methodologies related to fuel burnup extension and associated licensing reviews, RES issued RIL 2021-13, “Interpretation of Research on Fuel Fragmentation, Relocation, and Dispersal (FFRD) at High Burnup.” The RIL provides NRC technical reviewers with timely interpretations of over 10 years of research on a complex technical issue important to high burnup fuel safety in a way that is easy to use. The RIL does not counter prior work discussed in SECY-15-0148 that concluded that existing burnup limits for fuel in operating reactors are acceptable.
Higher Burnup Licensing Actions
The NRC staff is currently preparing to review topical reports and license amendments regarding higher burnup. Higher burnup-related licensing actions received by the NRC can be found on ATF-related Licensing Actions.
Page Last Reviewed/Updated Tuesday, July 11, 2023