Method of changing fuel in nuclear reactor

Purpose: To obtain a large cycle degree of burn-up and to increase the thermal margin. Method: Fuel assemblies are switched with new fuel assemblies in such a manner that the number of years in the pile of four fuel assemblies at the center of a reactor core and the fuel assemblies except the fuel assemblies at the periphery of the reactor are 3 years, any four adjacent fuel assemblies in more than 60% of the area of the reactor core of the fuel assemblies having 3 years of the in-core ages include 3 fuel assemblies having different in-core age in a balanced reactor core and yet the fuel assemblies of the same ages are always disposed along diagonal lines. (Yoshihara, H.)