Microstructure evolution in proton -irradiated austenitic Fe-Cr-Ni alloys under LWR core conditions.

Abstract

Irradiation-induced microstructure of austenitic stainless steel was investigated using proton irradiation. High-purity alloys of Fe-20Cr-9Ni (UHP 304 SS), Fe-20Cr-24Ni and Ni-18Cr-9Fe were irradiated using 3.2 MeV protons at a dose rate of 7 x 10<super>-6</super> dpa/s between 300&deg;C and 600&deg;C. The irradiation produced a microstructure consisting of dislocation loops and voids. The dose and temperature dependence of the number density and size of dislocation loops and voids were investigated. The changes in yield strength due to irradiation were estimated from Vickers hardness measurements and compared to calculations using a dispersed-barrier hardening model. The dose and temperature dependence of microstructure and hardness change for proton irradiation follows the same trend as that for neutron irradiation at comparable irradiation conditions. Commercial purity alloys of CP 304 SS and CP 316 SS were irradiated at 360&deg;C to doses between 0.3 and 3.0 dpa. The irradiated microstructure consists of dislocation loops. No voids were detected at doses up to 3.0 dpa. Loop size distributions are in close agreement with that in the same alloys neutron-irradiated in a LWR core. The loop density also agrees with neutron irradiation data. The yield strength as a function of dose in proton irradiated commercial purity alloys is consistent with the neutron-data trend. A fast-reactor microstructure model was adapted for light water reactor (LWR) irradiation conditions (275&deg;C, 7 x 10<super> -8</super> dpa/s) and then applied to proton irradiation under conditions (360&deg;C, 7 x 10<super>-6</super> dpa/s) relevant to LWRs. The original model was modified by including in-cascade interstitial clustering and the loss of interstitial clusters to sinks by cluster diffusion. It was demonstrated that loop nucleation for both LWR irradiation condition and proton irradiation are driven by in-cascade interstitial clustering. One important result from this modeling work is that the difference in displacement cascade between neutron irradiation (275&deg;C, 7 x 10<super>-8</super> dpa/s) and proton irradiation (360&deg;C, 7 x 10<super>-6</super> dpa/s) has little effect on the final irradiated microstructure. This is because the reduced level of in-cascade interstitial clustering in proton irradiation can be balanced by the higher cascade efficiency and dose-rate and the lower sink strength due to higher temperature.