Assessment of candidate materials for fusion power plants provides one of the major structural materials challenges of the next decades. Both plasma and the release of high-energy neutrons during the reaction pose a challenge for the development of materials constituting the reactors. Nanoscale defect clusters such as voids, solute-atom precipitates, and dislocation loops can form in metals irradiated by high-energy atomic particles. As they are obstacles to dislocation glide, they can affect plasticity, substantially changing the yield and flow stresses, and ductility. Computer simulation provides a useful alternative to experiments on real-life irradiated materials, by simulating the microscopic processes responsible for radiation hardening. Within the framework of a multiscale modeling approach, atomic-scale studies by molecular dynamics and statics are of importance, as they enable understanding of atomic interaction mechanisms occurring at the nanoscale while being invisible at coarser scales. Research presented in this chapter consists of atomic-scale computer modeling of interactions between edge dislocations and nanoscale radiation-induced defects in bcc α-Fe, a simplified model system for reactor first wall materials. The goals are (1) to explain irradiation hardening qualitatively by describing the mechanisms that control them and (2) to emphasize the effect nanoscale entities can have on bulk material properties.