Irradiation hardening and microstructure evolution under neutron irradiation have been investigated for pure Fe, Fe-0.15Cu, Fe-0.28Cu and Fe-0.46Cu alloys. All the alloys were annealed at 780°C for 20 min and quenched in iced water. Neutron irradiations were performed in the Japan Material Test Reactor (JMTR) up to a fluence of about 1 × 10²²n/m² at different fluxes (3.0 × 10¹⁶ and 1.5 × 10¹⁵ n/m²s) at 290°C utilizing a so-called “multi-division temperature control irradiation rig” to investigate flux effects under controlled irradiation temperature and flux. The irradiation hardening increased with increasing copper concentration. Positron annihilation lifetime spectrometry (PAS) revealed that the second lifetime component (τ₂) was observed only in pure iron and Fe-0.15Cu alloy irradiated at the low flux condition. As for the flux effect, the irradiation hardening was larger at the lower flux condition in all the model alloys and A533B steel. Post-irradiation annealing experiments indicated that there were two recovery stages: the first was above 350°C and the second was around 550°C. The amount of hardening recovery in the first stage decreased with increasing copper concentration and depended on the flux; a larger recovery was observed at the lower flux condition. In contrast, the amount of recovery in the second stage increased with increasing copper concentration and was independent of the flux. It is considered that the first recovery is related to the annealing out-of-matrix defects and the second one is due to dissolution of copper precipitates. The τ₂ of PAS disappeared after the annealing at 350°C, indicating that microvoids decomposed during the annealing.