In metallic alloys of fission or fusion reactors, microstructural evolution results from a dynamic equilibrium between thermodynamic forces and the production of defects by irradiation. The migration of defects can lead to the formation of clusters of defects (nano-cavities, dislocation loops, nano-precipitates) or variations in chemical composition close to the defect sinks, a phenomenon known as Radiation Induced Segregation (RIS). To predict the effect of irradiation conditions (type of irradiation particle, dose rate, temperature), phenomenological diffusion models exist that have to be assembled to give an overall description. Our objective is to describe these models and to propose numerical implementations to solve them starting at the atomic scale (DFT energy calculations, Kinetic Monte Carlo, Self-Consistent Mean Field approaches) to reach the mesoscopic one using the phase field modeling. This multi-scale approach is illustrated by a short review of recent studies focusing on dilute and concentrated iron based alloys.