Patent Number: 063013207
Section: claims

1. A fuel assembly comprising a multiplicity of fuel rods including a first fuel rod type provided with mixed oxide fuel and a second fuel rod type provided with mixed oxide fuel in conjunction with a neutron poison, the mixed oxide and neutron poison being intimately mixed in the second fuel rod type, the fuel rods having peripheral fuel rods wherein between 50 and 100% of the,peripheral fuel rods of the assembly being of the second fuel rod type. 2. A fuel assembly according to claim 1 in which between 0 and 50% of the fuel rods adjacent the peripheral fuel rods are of the second type. 3. A fuel assembly according to claim 1 in which between 0 and 15% of the non peripheral fuel rods are of the second type. 4. A fuel assembly according to claim 1 in which the neutron poison is Gadolinia. 5. A fuel assembly according to claim 1 in which the neutron poison is provided at between 0.5 and 2 weight percent. 6. A fuel assembly according to claim 1 in which only first and second type fuel rods are present. 7. A fuel assembly according to claim 1 in which the first type rod contains mixed oxide fuel with a plutonium content of between 3 and 12% by weight of the total fuel content of the first type rod. 8. A fuel assembly according to claim 7 in which the plutonium content of the second type fuel rod is between 20% and 75% that of the first type. 9. A fuel rod assembly according to claim 1 which is a pressurized water reactor fuel assembly. 10. A system comprising: a pressurized water nuclear reactor; and  the fuel assembly as recited in claim 1 disposed within the pressurized water nuclear reactor. 11. A pressurized water nuclear rector core comprising a fuel assembly having a multiplicity of fuel rods including a first fuel rod type provided with mixed oxide fuel and a second fuel rod type provided with mixed oxide fuel in conjunction with a neutron poison, the mixed oxide and neutron poison being intimately mixed in the second fuel rod type, the fuel rods having peripheral fuel rods wherein between 50 and 100% of the peripheral fuel rods of the assembly being of the second fuel rod type. 12. A pressurized water nuclear reactor core according to claim 11, the reactor core having a peak rod power to an average assembly power ratio between 1:65:1 and 1:4:1.