Study on SCC growth behavior of BWR core shroud

Recent studies suggest that material hardening should cause enhancement in Stress Corrosion Cracking (SCC) growth rate of Stainless Steels (SS). While the hardening of BWR core shroud of high fluence is governed mainly by neutron irradiation, the hardening of HAZ of BWR core shroud of low fluence is dominated by welding condition. Therefore, in this study, SCC growth rates of SS irradiated up to the middle of 10²⁴ n/m² and un-irradiated SS were measured in order to obtain reasonable estimation for SCC growth behavior in the core shrouds. The SCC growth rates of actual BWR core shrouds made of 304 SS were measured using 1T-CT specimens in BWR simulated environment. The cumulative neutron fluences of the specimens were 1.5-5.1×10 ²⁴ n/m². The stress intensity factor, K-value for the SCC growth tests were from 10.5 to 24.2MPa m^0.5. The test results showed that the SCC growth rates of the specimen from actual core shrouds were enveloped by the current upper limit, 9.2×10^-10 m/s of the K-da/dt disposition curve of JSME NA1-2002 standard for Sensitized 304 SS in NWC Reactor Water, suggesting that the limit is reasonably applicable even for SS irradiated up to those fluences. The SCC growth rates of the BWR shroud mock-ups made of 316L SS, taking the welding procedures simulating actual components, were also measured using 1/2T-CT specimens from HAZ. In spite of substantial hardness of HAZ, 200 HV, all SCC growth rates from the specimens were below the K-da/dt disposition curve of the JSME NA1-2002 standard for Low Carbon Austenitic SS in NWC Reactor Water. This fact suggests that the degree of hardening assumed in actual shrouds' HAZ should bring little enhancement effect in SCC growth rates.