It is known that the degradation of mechanical properties of reactor pressure vessel steels caused by neutron irradiation is partly due to the formation of nanometer-size solute and point-defect (PD) clusters. Depending on temperature, PDs produced by collision cascades under neutron irradiation can migrate and either recombine or agglomerate to form larger defect clusters, greatly affecting the microstructure evolution and thus the mechanical properties of the material. Therefore, studying the rationalization of radiation-induced effects on the microstructure and their influence on the material properties through the development of predictive models is of great importance. A cluster dynamics (CD) simulation based on rate equations has been used to estimate the long-term evolution of point-defect clusters, i.e., clusters of self-interstitial atoms (SIAs) and those of vacancies and precipitations containing solute atoms. We have extended a CD simulation code to account for the possibility of all SIA clusters migrating three dimensionally, to reproduce the agglomeration of point-defects to form clusters during irradiation with collision cascades in austenitic stainless steel. We have also performed a parametric study of a production bias model, which can take into account the effects of fast one-dimensional motion of SIA loops, of defect accumulation processes in neutron-irradiated alpha-iron. It is found that formulations that take into account proper reaction kinetics for different materials can successfully reproduce the microstructure evolution under neutron irradiation.