Hardening of low copper steels irradiated at 288°C increased with increasing flux in the range from about 0.6 to 50 × 1016 n/m2-s. In contrast, the hardening of high copper steels was generally less effected and, in some cases, decreased with increasing flux. Thus there are at least two competing effects of flux. The behavior of low copper alloys is attributed to a population of thermally unstable matrix defects that anneal during irradiation, hence, produce hardening that increases with flux. However, the unstable matrix defects also act as point defect sinks reducing radiation enhanced solute diffusion rates. In high copper steels, lower diffusion rates delay hardening due to precipitation. These conclusions were confirmed by low-temperature post-irradiation annealing treatments which produced rapid recovery of the unstable matrix defects. The observed data trends are consistent with a model that treats the unstable matrix defects as small cascade vacancy clusters.