Fuel assembly

The fuel assembly of the present invention comprises a plurality of long fuel rods and shorter fuel rods having a length shorter than the former, and the effective upper end of the shorter fuel rod situate at or lower than the height of 14/24 of the most longest effective portion of the long fuel rod. With such a constitution, the properties of the critical power of the upper portion of a fuel assembly can be improved without causing boiling transition at a position lower than the upper end portion of the short fuel rod. The peak of neutron fluxes in a reactor shut down state can be minimized, and the reactor shut down margin can be improved remarkably. In addition, since the area of the flow channel of the upper half of the fuel assembly at a region having high quality and a high void coefficient is enlarged to increase the ratio of water to uranium, the pressure loss of fuel assemblies is reduced to increase reactor stability and the reactivity of fuels is enhanced during operation thereby enabling to improve utility efficiency of nuclear materials. (N.H.)