Fuel assembly

The diameter of a water rod is decreased at a position where a spacer is disposed than that at a position where a spacer is not disposed, in at least one position. With such a constitution, since the region where the diameter of the water rod is decreased forms a bypass region for reactor water, the pressure loss in the spacer portion is decreased without lowering the power. A plurality of T-shaped protrusions having a length equal with or slightly shorter than the diameter for the large diameter portion are disposed to the water rod at the portion having the decreased diameter. Alternately, recesses are disposed to the water rod, and the top portion thereof has a cross sectional shape of a triangle or a rectangle having the diameter equal with or slightly smaller than that of the diameter for the large diameter portion. Then, these protrusions or the top portions of the cross section are brought into contact with springs and the cell stops of the spacers. With such a constitution, the water rods can be engaged and fixed to the spacers by using an existent spacer structure. (I.N.)