Irradiation creep in type 316 stainless steel and U

Irradiation creep was investigated in type 316 stainless steel (316 SS) and US Fusion Program PCA using a tailored spectrum of the Oak Ridge Research Reactor (ORR), in order to achieve a He:dpa value characteristic of a fusion reactor first wall. Pressurized tubes with stresses of 20 to 470 MPa were irradiated at temperatures of 330, 400, 500 and 6000C. It was found that irradiation creep was independent of temperature in this range and varied linearly with stress at low stresses, but the stress exponent increased to 1.3 and 1.8 for 316 SS and PCA, respectively, at higher stresses. Specimens of PCA irradiated in the ORR and having helium levels up to 200 appm experienced a 3 to 10 times higher creep rate than similar specimens irradiated in the Fast Flux Test Facility (FFTF) and having helium levels below 20 appm. The higher creep rates are attributed to either a lower flux of the presence of helium. A mechanism involving interstitial helium-enhanced climb is proposed. The enhanced creep rates observed in the presence of helium might serve to relieve swelling stresses in a fusion reactor first wall, and hence, must be considered in reactor design. 17 references, 5 figures, 1 table