EFFECTS OF LARGE FAST-NEUTRON FLUENCES ON STRUCTURE OF STAINLESS STEEL

The changes in microstructure which occur when type 304L stainless steel is irradiated at temperatures between 370 and 472 °C and fast-neutron fluences between 0.8 and1.4 × 1022 n/cm2 have been investigated by electron microscopy. The fuel cladding from an Experimental Breeder Reactor-II driver fuel element served as the experimental material. Voids and dislocation loops formed by the precipitation of irradiation-produced vacancies and interstitials were observed in each specimen. As the irradiation temperature was increased the density of each type of defect decreased and the average size increased. Upon postirradiation annealing at 600 °C, the dislocation loops converted to a dislocation network and the smallest voids (diameters < 150 A ̊) began to disappear. As the annealing temperature was increased the dislocation and void densities both decreased. After annealing 1 h at 900 °C, the voids were completely removed and the dislocation density was reduced to that of an unirradiated annealed specimen. © 1969.