Fuel assembly

Fuel rods are arranged by required number in a square lattice together with water rods in the central portion, upper and lower ends thereof are mounted to upper and lower tie plates, middle portions are supported by a plurality stage of spacers and a channel box is put over the outer circumferential surface thereof. All of the fuel rods are constituted with natural uranium or uranium at low concentration similar thereto, and burnable poisons such as gadolinia (Gd2O3) and boron carbide (B4C) are incorporated to a portion of fuel rods entirely or partially in the axial direction. Since this can substitute a portion of excess reactivity with the fuel assembly, it is possible to reduce the amount of the control rods used and the radiation amount to the control rods. Further, since the amount, the kind and the positions to be placed of poisons can properly be selected, flexibility can be provided to the reactor core design. (T.M.)