An effort has been made to investigate defect accumulation processes in the reactor structure steel during irradiation. Simultaneous reaction rate equations were constructed on a basis of the reaction rate theory, and solved numerically on a computer to understand such microstructural changes as the formation of voids and dislocation loops in the steel. The effect of cascade cluster formation on defect accumulation processes is especially focused. When the production of cascade clusters is considered, voids and dislocation loops created during irradiation become small in size and their number densities become large, which results in an increase in total volume swelling. The dependence of defect accumulation processes during irradiation on incident neutron energy is discussed, which may lead to an establishment of the irradiation correlation rule.