To suppress local output lowering at the axial position of a pellet- pellet boundary, and reduce the total thermal output loss by making a fuel rod different at the axial position of the pellet-pellet boundary part in the fuel rod.
A fuel assembly comprises fuel rods in which a plurality of pellets of sintered uranium oxide or plutonium oxide or mixed oxide thereof are charged, upper and lower time plates, fuel spacers and the like. In this constitution, the height of the pellet is made in two kinds of pellets 1a, 1b, and respective kinds of the pellets are charged in the separate fuel rods 2a, 2b. Thereby shifting of the axial position in a pellet-pellet boundary is reduced, and a degree of local lowering of the axial output distribution of the fuel assembly can be made small.
NAKAJIMA JUNJIRO
KIDA MITSUKO
