Nuclear fuel assembly

Fuel rods of a nuclear fuel assembly are constituted with long fuel rods and short fuel rods, and an axial region where the amount of nuclear fuels is relatively small and an axial region where the amount is relatively great are formed. The burnable poison-containing fuel rods disposed at the second raw from the outer layer in fuel rod arrangement are positioned such that the axial region with relatively small amount of nuclear fuel materials is apart from four corners of a channel box compared with the axial region with relatively great amount of nuclear fuel materials. With such a constitution, the extent of lowering of reactivity due to burnable poisons is reduced, to provide a flat axial power density distribution. (T.M.)