BWR type reactor

In a BWR type reactor, a fuel assembly of A type has a distance (W gap) between the outer surface of a channel box on the side where a control rod is inserted and the boundary surface of unit cell of the fuel assembly being greater than the distance (N gap) between the outer surface of the channel box on the side where a control rod is not inserted and the boundary surface of a unit cell of a fuel assembly. The fuel assembly of A type having the ratio of the N gap width to the W gap width being smaller than 1 is exchanged with the fuel assembly of B type having a greater ratio of the N gap width to the W gap width and having greater number of fuel rods on every fuel exchange batch. Then, the reduction of thermal margin due to change of gap width of the fuel assembly of B type can be compensated by increase of the number of fuel assemblies. In this case, since the W gap and N gap are somewhat averaged, moderators are distributed uniformly in view of fuel design, thereby enabling to increase the average content of fission materials of the fuel assemblies of B type, and improve the fuel economy. (N.H.)