Helium bubble formation behavior in ODS ferritic steels with and without simultaneous addition of Al and Zr

•He effects were studied for two ODS ferritic steels with/without simultaneous addition of Al and Zr.•Trapping effects of oxide particles, grain boundaries, dislocations on He atoms were discussed.•Oxide particles with a high sink strength contribute to enhanced swelling resistance. Two 14.5wt.% Cr-ODS ferritic steels with and without simultaneous addition of 3.5wt.% Al and 0.27wt.% Zr were irradiated with He+ up to 0.2dpa/3500appm at 300°C, 550°C, 700°C in order to investigate the effects of Al and Zr addition on He bubble formation behavior in 14.5Cr-ODS ferritic steel. The mean grain size of each ODS steel was about 370nm. Al addition resulted in the formation of larger oxide particles with a lower number density in 14.5Cr-ODS ferritic steel. After irradiation to 0.2dpa/3500appm He, dislocation densities in both ODS ferritic steels showed no change. However, the size and number density of He bubbles were increased and decreased, respectively, by the simultaneous addition of Al and Zr. Although both ODS steels showed good resistance to He-bubble induced swelling, the 14.5Cr-ODS steel possessed a better swelling resistance than 14.5Cr-3.5Al (Zr)-ODS steel. It was considered that the oxide particles were the main contributor for the swelling resistance of ODS steels by providing a number of defect sinks with a high sink strength. The swelling increased with increasing irradiation temperature in both ODS steels, indicating that the de-trapping He enhanced He-bubble swelling.