Microstructural changes induced by post-irradiation annealing of neutron-irradiated austenitic stainless steels

Irradiated EBR-II hexagonal 'hex' duct material fabricated from 304 stainle ss steel(SS) is characterized prior to and following in situ annealing in t he transmission electron microscope (TEM) at temperatures between 400 degre esC and 600 degreesC for various lengths of time. The hex duct samples were irradiated at temperatures between 375 degreesC and 389 degreesC to doses up to 29 dpa over a range of dose rates. The pre-annealed microstructure of the irradiated hex ducts exhibited substantial radiation-induced dislocati on development (networks and loops) and cavity formation (bubbles and voids ). Following annealing at all temperatures, dislocation loop densities decr ease significantly and the dislocation network density increases. Annealing at 500 degreesC and 600 degreesC led to the shrinking or disappearance of many larger faceted voids. In addition to shrinkage of larger voids, small spherical bubbles formed leading to an increase in the overall cavity densi ty and a decrease in the average cavity size. (C) 2000 Elsevier Science B.V . All rights reserved.