Time-Dependent Thermal Recovery of Irradiation Hardening in EP-450 Steel from BN-350 Spent Fuel Assembly Wrappers During Isothermal Annealing at 470 °C

Ferritic–martensitic EP-450 steel has been widely used for fast-reactor fuel assembly wrappers because of its high resistance to irradiation-induced swelling; however, its radiation-hardened state formed under low-temperature, high-dose neutron exposure may evolve during subsequent thermal exposure. This study examines the time dependence of post-irradiation thermal softening in EP-450 steel taken from two BN-350 spent fuel assembly wrappers irradiated at approximately 351 °C to 61 and 71 dpa. Samples were annealed in static argon at 470 °C for cumulative durations of 10, 30, and 100 h. Vickers hardness was measured for both wrappers, while miniature tensile tests were performed for the 71 dpa material. The initial hardness values were similar, 356 and 360 HV0.2, but the thermal response differed markedly: after 100 h, hardness decreased by 10.2% and 17.9%, respectively. For the 71 dpa material, the 0.2% proof stress decreased from 815 to 752 MPa between 10 and 100 h, while total and localized strains increased from 15.33% to 17.94% and from 8.84% to 11.45%, respectively. These results demonstrate a gradual, time-dependent reduction of irradiation hardening at 470 °C and reveal that materials with similar initial hardness can exhibit substantially different thermal stability.