An in-core constant load test was carried out to examine the effect of pre-irradiation on the intergranular stress corrosion cracking (IGSCC) behavior of thermally sensitized Type 304 stainless steel ([SS] UNS S30400) in a simulated boiling water reactor (BWR) water environment. It was shown that, in constant load tests, preirradiated, thermally sensitized Type 304 SS had a lower IGSCC susceptibility, a larger time to failure, than unirradiated, thermally sensitized SS for a given stress level. An out-of-core constant load test and interrupt slow strain rate tensile (SSRT) tests also were carried out to discuss the in-core test result on unirradiated, thermally sensitized Type 304 SS, simulating radiation hardening by work hardening. The out-of-core tests, with surface observations, suggested that an increase in proof strength by neutron irradiation would be the main cause of the lower IGSCC susceptibility of the preirradiated Type 304 SS, since the increase in proof strength would prevent a formation of coarse slip lines that destroy the protective films on the SS surface during the initial loading in the corrosive environment before the constant load condition is achieved.

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