Fuel Fragmentation, Relocation, and Dispersal

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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.

These smaller pieces of fuel may axially relocate within the fuel rod if the rod balloons outward, and may be ejected into the reactor coolant flow if the rod bursts.

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.

The industry understands that FFRD will have to be addressed in their applications to the NRC and is currently developing their technical and regulatory approach and any necessary research on the phenomenon.

For additional information, see the Nuclear Energy Agency's report on FFRD