Fuel assemblies for use in nuclear reactor

Purpose: To flatten the axial power distribution of highly enriched MOX fuel assemblies. Constitution: In a fuel assembly in which about 1/2 portion or more thereof comprises fissionable plutonium, the number or the diameter of water rods is increased such that the ratio for the number of hydrogen and uranium atoms during power operation is increased about by more than 5 % or 15 % than that of the uranium fuel assembly of the same reactivity life. As a result, hardening of the neutron spectrum can be suppressed and the nuclear property can be approached to that of the uranium fuel assembly. In addition, burnable poisons are disposed to the lower portion in the water rod of the greatest outer diameter to flatten the axial power distribution in the reactor core. That is, the above purpose can be attained only by a simple hard change of disposing gadolinia rods without complicating the enrichment degree or the gadolinia distribution in the MOX fuel rods. Plutonium can be utilized effectively without changing the structures in the reactor core or operation method. (Kamimura, M.)