Composition and Evolution of Sea-Salt Deliquescent Brines on SNF Storage Canister Surfaces

For long-term storage, spent nuclear fuel (SNF) is placed in dry storage systems, commonly consisting of welded stainless steel containers enclosed in ventilated cement or steel overpacks. At near-marine sites, a significant fraction of aerosols deposited on the canisters may be sea-salts, which as the canister cools, will deliquesce to form chloride rich brines, potentially leading to chloride-induced stress corrosion cracking (SCC). Here, we have used thermodynamic modeling to predict the evolving composition of sea-salt deliquescent brines as the canisters cool and the surface relative humidity increases. Representative brine compositions have been mixed, and we have characterized their physical and electrochemical properties, including density, viscosity, conductivity, and oxygen concentrations. These parameters are necessary for predicting the corrosion behavior of the storage canisters over time.When predicting corrosion behavior, it is common to assume that sea-salt brines form and persist unchanged over time. However, here we show that the magnesium chloride rich-brines that form when sea-salt aerosols initially deliquesce are not stable at elevated temperatures. Elevated temperatures increase brine HCl(g) partial pressures, driving chloride loss through degassing of HCl. Concomitantly, pH rises until precipitation of less-deliquescent phases such as magnesium carbonate and magnesium hydroxychloride. These reactions buffer the pH, allowing continued degassing of chloride; ultimately, complete brine dry-out may occur. In the absence of magnesium chloride, the remaining sea-salts will not deliquesce until temperatures cool further, allowing higher RH values on the canister surface. These data suggest that initiation of corrosion may be delayed for longer than previously thought.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2019-0018 A.