Abstract
Some U.S. nuclear power plants are storing spent nuclear fuel in dry cask storage systems (DCSS). In many cases, the DCSS consists of a metal storage canister inside a concrete vault or overpack for radiation shielding. Most canisters are made of austenitic stainless steel, including UNS S30400 (304 SS). The concrete vault or overpack is vented to the atmosphere for passive cooling, allowing interaction between the canister and the ambient environment. In coastal environments, airborne salts could deposit and accumulate on the surface of the canisters over time. Deliquescence of these salts in a humid environment could create a chloride-rich brine on the canister surface. This, in addition to the presence of residual tensile stresses, could make the canister susceptible to chloride-induced stress corrosion cracking.
The objective of this work is to evaluate how salt concentration, temperature, and relative humidity affect stress corrosion cracking initiation for austenitic SS used for dry storage canisters. To that end, simulated sea salt in quantities between 0.1 and 10 g/m2 was deposited on the surface of type 304 SS U-bend specimens. Three material conditions were used in this evaluation: as-received, sensitized, and welded with type 308 SS filler metal. Once the salt was deposited, the specimens were exposed to a range of temperatures and both cyclic and static humidity conditions to examine crack initiation. The results indicate that both as-received and sensitized 304SS are susceptible to crack initiation at all salt concentrations from 0.1 to 10 g/m2, and that cracks initiate at temperatures between 35 and 80 °C.
