Irradiation creep at 60°C in SUS 316 and its impact on fatigue fracture

Structural materials in fusion reactors will be subjected to irradiation by energetic particles at temperatures widely ranging from liquid He to above 1000°C. Hence, the very large irradiation creep strain at 60°C previously reported in the ORR/ORNL pressurized tube experiment to 8 dpa is important. Computer calculations for the 20% cold-worked SUS 316 demonstrated the transient nature of this radiation-induced creep, caused by the overwhelming flux of excess interstitial atoms lasting nearly one year at 60°C where the diffusivity of vacancies is very low. In order to confirm such a transient nature, continuous creep measurement under irradiation is necessary and was carried out using 17 MeV protons. Development of very significant creep strain, much larger than that at 300°C, and steadily decreasing creep rate were observed at 60°C as the calculation predicted. A significant influence of the dynamic irradiation effect at 60°C on fatigue fracture was also observed.