Forecasting of changes in the physical properties of steels under irradiation is usually achieved through a detailed knowledge of the source in terms of atomic displacements. This source is extended to the secondary and tertiary atoms of the cascade, with Lindhard’s inelastic collision description. On this basis, it is possible to establish a model for the creation of “displacement zones” by evaluating their effectiveness through a nonoverlapping criterion between zones. The A 533 B steel yield strength increases after irradiation at 100°C (373 K) in two different spectra (Saclay—H2O at Osiris, D2O at EL.3) made it possible to compare the efficiency of this model with existing ones. Furthermore, preparation of the model leads to a normalized damage function closely approaching the one hitherto recommended by Euratom.