Chrominum-molybdenum ferritic steels have a better resistance to void swelling than austenitic steels and were considered for use as first wall structural materials in future fusion reactors. One of the major alloying elements, Mo, is deleterious for the reduction of radioactivity; reduced activation steels are under development, where Mo is replaced by W. In this work, four kinds of reduced activation ferritic steels were dual or single ion irradiated to 125 dpa. In the dual ion irradiated steels with a 7–12% Cr content, the dimensional stability of the martensite phase was superior to the other phases. The cavity microstructures of bainitic steels with 2.25% Cr content were affected by the He dpa ratio (and by the He injection rate) even when they had a high damage level (up to 125 dpa). Hitherto, the low activation steels developed by replacing Mo by W suggest excellent stability of microstructures and thus a high resistance to swelling.