Swelling data from both ion and neutron irradiations of Fe-Cr-Ni alloys are analyzed in terms of the critical radius concept for bubble-to-void conversion. It is shown that while the dependence of critical radius on temperature, displacement rate, and helium can explain many features of the data, one must also consider the dependence of vacancy diffusion on nickel content in order to be able to explain the compositional dependence of void nucleation.