The displacements per atom (DPA) concept was adopted in Revision 2 to Regulatory Guide 1.99 to account for the through-wall attenuation of radiation damage in the reactor vessel beltline. Models have been developed that are capable of predicting atom displacements, but the overall effect of these models to predict the mechanical properties of the material is not exact. A review of strengthening mechanisms in reactor vessel steels demonstrates that the creation and motion of dislocation is the basis of all mechanisms. Regulatory Guide 1.99, Revision 2, in estimating radiation damage attenuation, does not consider the inherent metallurgical strengthening mechanisms in modelling reactor vessel wall attenuation. Data in the literature shows that experimental, surveillance capsule, and actual pressure vessel data are not modeled well by the use of DPA. At best, DPA is able to model certain stainless steels while not being able to model the more complex low-alloy reactor vessel steels.