Axial Heterogeneity and Post-Irradiation Thermal Response of EI-847 Austenitic Steel from a BN-350 Fuel Assembly Displacer

Axial variations in irradiation conditions along elongated fuel assembly components can produce locally different irradiation-induced material states and, consequently, different post-irradiation thermal responses. The present study investigates EI-847 austenitic steel from a BN-350 reactor’s spent fuel assembly displacer irradiated to approximately 50 dpa. Material was sampled at axial positions of −175, 0, and +175 mm relative to the reactor core midplane, corresponding to irradiation temperatures of approximately 326, 351, and 377 °C, respectively. The three axial states were compared in the as-irradiated condition and after identical post-irradiation annealing at 470 °C for 100 h. Density, Vickers microhardness, tensile properties, microstructure, and fracture surfaces were examined to assess axial heterogeneity and subsequent thermal response. In the as-irradiated condition, the lowest microhardness and yield strength (YS) occurred at +175 mm, whereas the minimum density was measured at the core midplane. After annealing, microhardness decreased by 13.5%, 15.1%, and 6.3% at −175, 0, and +175 mm, respectively. The YS decreased by 20–28%, while the ultimate tensile strength (UTS) decreased by 10–15%. Total elongation (TE) increased from 10% to 29% at −175 mm, from 16% to 27% at the core midplane, and only from 19% to 21% at +175 mm. Predominantly ductile dimpled fracture was retained in all conditions. These results show that identical post-irradiation annealing produces different degrees of softening and ductility recovery depending on axial position and the irradiation-induced state established during reactor operation.