Extended Storage and Transportation - Evaluation of Drying Adequacy

The U.S. Nuclear Regulatory Commission (NRC) is evaluating the safety and security of spent nuclear fuel (SNF) stored in dry casks for extended time periods before transportation to a location where the SNF is further processed or permanently disposed. Regulations at Title 10 of the Code of Federal Regulations (10 CFR) Part 72, “Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor- Related Greater Than Class C Waste,” require that dry storage casks function to ensure that (1) radioactive releases do not exceed specified limits, (2) there is sufficient shielding to keep direct radiation dose rates below specified limits, (3) subcritical conditions can be maintained under credible scenarios, and (4) SNF assemblies can be retrieved when necessary.
Following accepted procedures, spent fuel is typically loaded into dry storage casks underwater. The cask is then removed from the pool, drained, dried, backfilled with inert gas, and sealed to the atmosphere. These procedures may leave some limited amount of water may remain in the cask, even under normal operations. If residual water remains after drying and sealing, several physicochemical processes can affect the cask and its contents, and potentially compromise the capabilities of the cask to function properly.
Potential effects of residual water include degradation of SNF rod cladding, oxidation of exposed SNF pellets, and corrosion of internal components inside the canister (e.g., SNF basket, neutron absorber plates). Radiolysis of residual water can create reactive oxygen species which can oxidize cladding, other metallic components, and uranium dioxide (UO2) exposed by cladding breaches. This effect would be most pronounced at the higher temperatures present earlier in the storage period. If a significant fraction of UO2 in an SNF rod is oxidized to U3O8, swelling of the SNF pellets can rupture the cladding and release SNF particles into the canister. Contamination of the inside of the canister complicates transport, retrieval, and other handling of SNF assemblies. At later times, when the intensity of the radiation diminishes and the high temperatures generated by radioactive decay heat decrease, potential condensation of liquid water on metallic components could initiate aqueous corrosion. If the canister gas composition includes oxygen generated by radiolysis that was not consumed by chemical reactions and radiolysis-generated hydrogen, the gas mixture in the canister may be flammable if a source of ignition is present.