Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steels Used in Dry Cask Storage Systems in A Simulated Marine Environment

摘要

Austenitic stainless steel is the regular material used in the canister of dry storage casks, including Type 304, 304L, 316L stainless steel. When the spent fuel storage installations located at the coastal sites, these types of austenitic stainless steel are prone to chloride induced stress corrosion cracking (CISCC) in aggressive environment. CISCC has complex interactions associated with environment, stress and material properties. Therefore, the objective of this work is to evaluate the susceptibility to chloride induced stress corrosion cracking of candidate canister materials by using U-bend tests in a simulated marine atmospheric environment containing sodium chloride (NaCl) or magnesium chloride (MgCl_2) and keeping constant relative humidity of 40 percent. Prior to the U-bend tests, samples were prepared and underwent various pretreatments, including solution annealing and thermal sensitization. After the tests, the samples were examined with the scanning electron microscope (SEM) in order to measure and quantify the cracks. According to the results, there are more cracks observed on sensitized samples at higher temperature in the presence of sodium chloride, and the length of cracks measured in all circumstances are in the range of 0-20 um. The results of quantitative analysis indicated that the number of cracks increases with temperature at constant humidity, and there is no significant influence on the test duration. Subsequently, magnesium chloride was also used for investigating the influence of different salt on the tendency of chloride induced stress corrosion cracking. More cracks were induced and observed on the surface due to MgCl_2 deposition. The cross section will be observed with optical microscope to measure the depth of cracks in the future.