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The effect of chromium, phosphorus,, silicon and sulfur on the stress corrosion cracking of 304L stainless steel in CERT tests in high purity water or argon at 288°C following irradiation with 3.4 MeV protons at gl0*C to 1 dpa, has been investigated using ultrahigh pur:ty alloys (UHP) with controlled impurity additions. Grain boundary segregation of phosphorus or silicon due to proton irradiation was quantified using both Auger electron spectroscopy and scanning transmission electron microscopy, and the alloys with impurity element additions were observed to have greater grain boundary chromium depletion and nickel enrichment than the UHP alloy. The UHP alloy suffered severe cracking in CERT tests in water. Less cracking was found after CERT tests of irradiated UHP+ P or UHP+Si alloys, despite greater chromium depletion. This suggests a mitigating effect of phosphorus and silicon at grain boundaries. No cracking was fou-.~ in argon tests, eliminating a purely mechanical embrittlement mechanism, but not eliminating a contribution from radiation hardening. Implanted hydrogen was not a factor in the intergranular cracking found.

A research program has been undertaken to determine the origins of irradiation-assisted stress corrosion cracking (IASCC) in austenitic alloys in light water reactors, and the effect of impurities on IASCC susceptibility. Controlled purity alloys of 304L stainless steel were irradiated with protons at 400°C to a dose of 1 dpa and analyzed via Auger electron spectroscopy @ES) and scanning transmission electron microscopy (STEM). The alloys investigated were an ultra-high purity (UHPl alloy and UHP alloys containing phosphorus (UI-IP + P), sulfur (UHP + S), or silicon (UHP + Si). Microstructural and microchemical changes were quantified and compared with literature results for other irradiating species. Following irradiation, the alloys showed dislocation loop formation and growth, "black dot" loops, and a change in the nature of the dislocation network. AES and STEM microchemical analysis of the alloys revealed Cr depletion of up to 6 at% and Ni enrichment of up to 6.6 at% at the grain boundaries of the alloys, with more segregation observed in the alloys containing impurities than in the UHP alloy. Significant gram boundary enrichment of P and Si in the UHP +P and UHP + Si alloys, respectively, was also observed. The results of the analyses of proton-irradiated samples are shown to compare favorably with previous studies on samples irradiated with neutrons at or near LWR conditions.

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