An investigation into stress corrosion cracking susceptibility of 304 stainless steel in simulated boiling water reactor start-up environments

Numerous incidents of stress corrosion cracking (SCC) in the pressure boundary components of boiling water reactors (BWRs) have been observed in recent decades. Hydrogen water chemistry (HWC) technology has been widely adopted worldwide for mitigating the SCC problem in the structural components of BWRs. Conventional work temperature of HWC is between approximately 230°C and the normal operating temperature, depending on the HWC system design. The reactor coolant, which typically contains high levels of dissolved oxygen because of the intrusion of atmospheric air during a cold shutdown, can remain relatively oxidizing during start-up operations. Laboratory data have indicated that under oxidizing conditions, the rates of intergranular SCC (IGSCC) at intermediate temperatures during start-up are actually higher than those during normal operations in nuclear power plants. In this research, we evaluated the effect of operating temperatures during plant startup processes on the SCC response of austenitic 304 stainless steel (SS) in a pure-water environment. The SCC behavior was investigated using slow strain rate tensile (SSRT) tests at a strain rate of 3.0 × 10^-7/s in pure water (with a dissolved-oxygen concentration of 300 ppb) at 150, 200, 250, and 288°C. After the SSRT tests, the samples were observed using a detailed characterization of the microstructure and chemical composition of the sensitized 304 SS surface region. The preliminary results revealed that the solution - annealed specimens exhibited a higher SCC resistance than that of the sensitized samples at the same operating temperature. The elongation of the sensitized samples operating at 150, 200, 250, and 288°C were 56.4%, 34.7%, 22.9%, and 31.4%, respectively. The sensitized samples at 250°C yielded significantly larger amounts of IGSCC and total SCC among all of the operating temperatures. Among the operating conditions investigated in this study, an operating temperature would pose a detrimental effect on the samples in the aspects of mechanical property and degree of SCC. (author)