A new thermodynamic approach for phase stability of a nanoscale material under irradiation taking into account Gibbs free energy of phase formation and nucleation is proposed. The influence of powder dispersion and vacancy saturation on the radiation stability and phase changes of spherical Ferrum nanoparticles during irradiation is determined. The paper shows the possibility of a radiation-induced α-Fe → γ-Fe polymorphic transition, and also defines the radiation stability zones of Ferrum nanoparticles. It is shown that the competition between the energy of the accumulated vacancies in the particle, the bulk energy of the phase transformation, and the surface energy of the particle is responsible for the specific behavior of irradiated nanocrystalline Ferrum.