A vanadium-iron binary alloy has shown exceptionally large swelling after irradiation in the Fast Flux Test Facility/Materials Open Test Assembly (FFTF/MOTA)⁴ to about 30 dpa. The volumetric swelling for irradiation temperatures of 520 and 600°C is more than 80%. The effect of preimplanted helium was to enhance the swelling in all conditions. The mechanism of the large swelling has been discussed in terms of the cavity and dislocation bias factors. It has been suggested that the bias factors modified by undersized iron solute in more than one way promote the large swelling.