Publications Details

Interim storage environment: Considerations for corrosion testing of SNF DRY storage containers

Bryan, Charles R.; Enos, David E.

Once sufficiently cool, spent nuclear fuel is stored in dry storage cask systems, most commonly consisting of welded stainless steel containers enclosed in ventilated concrete or steel overpacks. As the United States does not currently have a viable disposal pathway for SNF, these containers may be required to perform their waste isolation function for many decades beyond their original design criteria. Failure by stress corrosion cracking due to deliquescence of deposited salt aerosols is a major concern. Parameters controlling deliquescence include the temperature and RH at the waste package surface, and the composition of deposited salts. The timing and duration of deliquescence under in situ conditions is poorly defined, because of uncertainties in thermal history, the large variability in temperatures over the storage container surface, and uncertainties in the composition of deposited salts. Storage installations in near-marine environments are of greatest concern because of exposure to significant quantities of chloride-rich sea salt aerosols. Published stainless steel corrosion studies with sea salt and sea salt components suggest that conditions conducive to localized corrosion initiation and propagation may exist on the surface of SNF storage containers in such environments at some point in their extended service life, and furthermore, that stress corrosion cracking may occur over a broad range of potentially relevant conditions. However, the studies were carried out with heavy salt loads and limited gas flow, which may limit the beneficial effects of brine/atmosphere exchange (e.g., acid degassing, CO2 exchange, degassing and thermal decomposition of ammonium phases). Gas exchange with the atmosphere will modify brine pH and chloride content, and will modify the deliquescent salt assemblage through precipitation of Ca and Mg carbonates, potentially limiting brine volumes or resulting in dryout. Nitrate-rich inland salt aerosols are considered less corrosive, but may have higher levels of potentially reactive pollutants. Moreover, the compositions of inland salt deposits on hot storage containers may have greater uncertainty, as ammonium- and nitrate-rich salt assemblages are subject to thermal decomposition and potential reactions with organics. For both inland and near-marine sites, little information is available on the dust/salt deposition rates, or the quantity of salt present on existing storage container surfaces. A sampling program for in situ dust deposits on current storage containers will provide critical compositional data for new stress corrosion cracking studies, and will allow evaluation of the applicability of existing studies of stainless steel stress corrosion cracking under conditions of dust deliquescence.