Neutron irradiation often introduces severe changes in the mechanical properties of austenitic and martensitic steels (e.g., reduction of elongation below 400°C). This affects the mechanical responses of the reactor components. A large amount of hot cell work, however, is required in order to examine the mechanical response of intensely irradiated components experimentally, as well as to obtain materials irradiation data for the estimation of the component behavior. The development of a methodology with which to estimate the mechanical response of such components based on knowledge of the irradiation-induced microstructural changes and models of the post-irradiation mechanical properties is therefore an effective way to evaluate the structural integrity of an intensely irradiated structural component with minimal effort. A methodology for simulating the microstructural changes of face-centered cubic metals during irradiation using molecular dynamics and rate equation (RE) calculations has been developed. For RE calculation, the capture radius of the point defect clusters has been obtained through in situ ion irradiation experiments. The flow stress level is estimated from the dispersed barrier hardening equation with calculated microstructural data. By means of correlating the limited data from irradiation experiments with the calculated results (relation between the flow stress level and the microstructure of a heat), the flow stress levels of the heat can be estimated accurately as functions of the damage level and temperature. A Swift type constitutive equation with the concept of an equivalent plastic strain of irradiation hardening is proposed from the work hardening behavior of irradiated steels. By using this equation, it is possible to estimate the deformation and ductile fracture conditions of intensely irradiated components. This is a preliminary multi-scale method for estimating the mechanical response of irradiated components.