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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.

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Stainless steel (SS) samples were irradiated with protons at 400°C and strained in 288°C water to examine the role of oxide particles in the irradiation-assisted stress corrosion cracking (IASCC) process. Oxides in the matrix acted as the predominant crack initiation sites, and the amount of cracking scaled with oxide density. Intergranular cracking occurred by mechanical failure of oxide particles that created electrochemical crevices and stress concentrators from which intergranular cracks could propagate. Relatively few of the cracked oxide particles actually led to intergranular cracking in the matrix, which was consistent with the requirement that the crack tip solution had to be deaerated for an aggressive crevice chemistry to form and that the cracks in the oxides had to be well aligned with susceptible grain boundaries. Intergranular cracking occurred only when both the SS was irradiated and when straining was conducted in hightemperature water. This observation supported an IASCC mechanism that required an aggressive environment and an irradiated microstructure.