Evolution of the radiation-induced defect structure in 316 type stainless steel after post-irradiation annealing

•TEM study of irradiated CW316 steel after post-irradiation annealing.•Frank loops were removed after annealing at 550°C, by unfaulting and growing.•The cavity density decreases after annealing at 550°C, but not completely removed.•Frank loop and cavity removal is controlled by the annealing temperature.•The dissolution of γ' precipitates is controlled by the iron diffusion length. The thermal stability of Frank loops, black dots, cavities and γ′ precipitates in an irradiated 316 stainless steel was studied by transmission electron microscopy. The samples were retrieved from a thimble tube irradiated at around 320°C up to 80dpa in a commercial nuclear power reactor, and thermally annealed, varying both annealing temperature and time. With increasing annealing temperature the density of all defects gradually decreased, resulting in the complete removal of Frank loops at 550°C. In contrast to other defects, the density of the γ′ precipitates sharply decreased with increasing annealing time, which indicates that the dissolution of the γ′ precipitates is governed by the iron diffusion length.