Response to Annealing and Reirradiation of AISI 304L Stainless Steel Following Initial High-Dose Neutron Irradiation in EBR-II

The object of this study was to measure the stability of irradiation-induced microstructure upon annealing and, by selectively annealing out some of these features and reirradiattng the material, it was expected that information could be gained concerning the role of microstructural changes in the void swelling process. Transmission electron microscopic examinations of isochronally annealed (200 to 1050°C) AISI 304L stainless steel, which had been irradiated at approximately 415°C to a fast (E > 0.1 MeV) neutron fluence of approximately 5.1 × 10²⁶neutrons/m², verified that the two-stage hardness recovery with temperature was related to a low-temperature annealing of dislocation structures and a higher-temperature annealing of voids and solute redistribution. Voids which remained after annealing at 1054°C reflected void coalescence and were quite stable due to the development of long diffusion distances between vacancy sinks. Reirradiation of material which had been annealed for I h at 700°C, resulting in dislocation anneal and solute redistribution, while undisturbing the void population, produced the surprising result that the material swelled at rates equal to those of a material which had never been previously irradiated. These results confirmed that void nucleation alone does not determine swelling rates in 304L stainless steel. Moreover, contrasting quantitative microstructural information observed in 304L stainless steel with that previously observed in AISI 316 stainless steel helped to demonstrate that the observed differences in swelling behavior are likely to be related to differences in concentrations of solute elements.