Case Study Analyses of Flaw Growth and Tolerance of Stainless Steel Canisters in Marine Environment

Abstract

The majority of dry cask storage systems used in the world confine nuclear fuel within austenitic stainless steel canisters. Past experience indicates that stainless steel may become susceptible to stress corrosion cracking (SCC) in the presence of stress and chloride salts. Recently, a cracking growth rate (CGR) model was developed and applied to evaluate the flaw depth of stainless steel canisters over the timeframe of the storage at independent spent fuel storage installations. This paper presents the results of case study analyses on stainless steel canisters of dry storage systems in the 1st and 2nd nuclear power plants in Taiwan. In detail, the flaw depth was first evaluated using the CGR model, site climate data, and surface temperature at the location of interest on the canister. The critical flaw sizes and depth were then determined from the structural tolerance assessment of canister shells to flaws of varying sizes. It was found that the difference in thermal-hydraulic behaviors of drystorage canisters can cause a great variation in the SCC initiation time. But that may not cause a significant difference in the surface temperature at initial salt deliquesce (ranging from 55 to 60 °C). The CGR at the SCC initiation is greater, and the flaw growth takes a great part. The surface temperature and activation energy get low as the time increases. The total flaw depth therefore may not vary greatly with the SCC initiation time. Overall, the drystorage canisters have shown relatively high structural tolerance to flaw sizes and depths.