Structural materials during a use in a fission reactor are subjected to irradiation by high energy neutrons, and therefore the properties, functions, and performances of materials become gradually degraded. Since keeping material’s integrity is a key to success towards a stable operation of reactors, materials degradation due to irradiation should be taken into account when reactor design and maintenance are considered. One of the issues to be solved for realization of a long term operation is how the integrity of materials in use is ensured for a long time beyond an actually-experienced operation period. To do this, the future ageing behavior of materials should be understood in advance. Usually, an attempt is made to understand the behavior using the existing irradiation facilities with accelerated irradiation environments. In many cases, however, material’s behavior depends much on acceleration coefficients and its dependency is not so simple. To overcome such a difficulty, a numerical simulation study is employed. In the present study, reaction rate analysis is performed to investigate material microstructure changes under various irradiation conditions. Our results have shown that volume swelling due to irradiation is very much different depending on irradiation conditions provided by such irradiation facilities as HFIR, JOYO, KUR, 14MeV fusion neutrons. Based on the results, irradiation correlation methodology is discussed to predict material’s degradation under arbitrary irradiation conditions using the actual existing data obtained with accelerated irradiation conditions.