Certification of Packages for Transportation of Irradiated Material Transportation and Storage

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Regulatory Challenges of ATF

image of Fuel Cycle with red circles over Spent Fuel

If NRC-licensed power reactors insert full core loads of accident tolerant fuel (ATF), those plants will need licensed storage and transport systems for the irradiated ATF once it is removed from the reactor. Spent fuel storage and transport presents some challenges that are similar to unirradiated material transport. For example, obtaining appropriate material properties for the new cladding material and validating the accuracy of the code(s) used to determine spent fuel criticality safety is still an issue for enrichments greater than 5 weight percent U-235. Additional challenges may exist depending on the licensing or certification strategy, such as performance of the cladding material during vacuum drying1, aging while in dry cask storage, cladding fatigue data for transportation, and benchmarking the relevant analysis codes.

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Regulatory Guidance

The NRC has developed guidance in NUREG-2214, "Managing Aging Processes In Storage (MAPS) Report: Final Report" for reviewing the aging of spent fuel based on uranium dioxide pellets and zirconium-based cladding. The degradation mechanisms described in NUREG-2214 that may be affected by higher burnup and increased enrichment include creep, hydrogen absorption, oxidation, delayed hydride cracking, and irradiation hardening. Other mechanisms may need to be considered as post-irradiation data is generated. In addition, the effects of potential higher end-of-life rod internal pressures and the increased pellet swelling should be evaluated. To resolve the unknowns in the aging effects, the NRC staff will support phenomenon identification and ranking table (PIRT) efforts that focus on the identification and evaluation of material properties and fuel degradation mechanisms used in the review of transportation packages or storage systems for irradiated ATF. These PIRT efforts should help the staff develop additional regulatory guidance for irradiated ATF, if required.

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Confirmatory Analysis

To justify the safety of spent fuel in dry storage, the NRC historically relied on the experimental programs to confirm that the spent fuel in dry storage performs as expected. For low burnup fuel (i.e., average assembly burnups less than 45 gigawatt-days per metric ton of uranium (GWd/MTU)), the results in NUREG/CR-6745, "Dry Cask Storage Characterization Project—Phase 1: CASTOR V/21 Cask Opening and Examination" (Bare and Torgerson, 2001), and NUREG/CR-6831, "Examination of Spent PWR Fuel Rods after 15 Years in Dry Storage" (Einziger et al., 2003) support the determination that low-burnup fuel cladding and assembly hardware should not degrade during the initial storage period and the first renewal term, provided that the cask/canister internal environment is maintained. A similar confirmatory program for high burnup fuel (i.e., average assembly burnups exceeding 45 GWd/MTU) is currently ongoing at the North Anna independent spent fuel storage installation. This program is sponsored by the U.S. Department of Energy in cooperation with the Electric Power Research Institute and fuel and cask vendors. The NRC will review data on irradiated ATF performance and determine whether confirmatory data is needed to support extended dry storage of irradiated ATF.

Irradiated ATF storage systems are expected come into use after 2023.

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In the Nuclear Waste Policy Act of 1982, Congress specified that the permanent disposal of the highly radioactive materials produced by commercial reactors in the United States, comprised of high-level waste and spent nuclear fuel, will be in a deep geological repository. The current approach for ensuring containment within a deep geological repository, both nationally and internationally, is to place the high-level waste and spent nuclear fuel to be contained within long lived waste packages that are emplaced hundreds of meters underground in a suitable geological formation. The NRC is not currently aware of any characteristics of these types of the reactor fuels being developed for future use (e.g., ATF) that would preclude the use of this approach to ensure containment within a deep geological repository.

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1During vacuum drying, the pressure in the container is changed to approximately 3 Torr and held at that pressure for a given length of time. This reduces the boiling point of the residual water to approximately 150°F, which causes the residual water to boil out of the package.