Variability of irradiation creep and swelling of HT9 irradiated to high neutron fluence at 400-600°C

Sets of pressurized tubes constructed from a combination of three lots of HT9 and two heat-treatments were examined after irradiation in FFTF-MOTA at four different temperatures between 400 and 600 degrees C. Diametral measurements indicated that small amounts of swelling occurred at 400 degrees C, with the magnitude of the swelling varying from tube-set to tube-set. At 495, 550, and 600 degrees C, no indications of swelling were observed. Over a limited stress range that decreased with increasing irradiation temperature, the irradiation creep was found to be proportional to the stress level and mirrored the swelling behavior, confirming the B-o + D(S) over dot creep model. The creep compliance, B-o, appears to be about half of that observed in austenitic steels, whereas the creep-swelling coupling coefficient, D, appears to be comparable to that of austenitics. At the highest stress levels examined, the value of the stress exponent became much greater than 1.0. Higher irradiation temperatures resulted in creep behavior with pronounced thermal creep characteristics. A primary creep transient was observed at the three higher irradiation temperatures and increased in magnitude with increasing irradiation temperature, while a stress exponent between 1.0 and 2.0 was observed at 550 degrees C.