Patent Number: 053373375
Section: claims

1. A fuel assembly comprising a plurality of first fuel rods containing a nuclear fuel material free from a burnable poison and a plurality of second fuel rods containing the nuclear fuel material and the burnable poison, the plurality of the second fuel rods including (A) a plurality of second fuel rods containing a first burnable poison, located at positions in an outermost row in a cross-section of the fuel assembly, and (B) a plurality of second fuel rods containing a second burnable poison having a larger thermal neutron absorption cross-section than a thermal neutron absorption cross-section of the first burnable poison, located at positions in an inner region of the cross-section of the fuel assembly other than the outermost row. 2. A fuel assembly according to claim 1, wherein (A) the second fuel rods containing the first burnable poison are located at corners in the outermost row. 3. A fuel assembly according to claim 15, wherein the first burnable poison is boron and the second burnable poison is gadolinium. 4. A nuclear reactor core, comprising a plurality of fuel assemblies, at least one of the fuel assemblies including a plurality of first fuel rods containing a nuclear fuel material free from a burnable poison and a plurality of second fuel rods containing the nuclear fuel material and the burnable poison, the plurality of the second fuel rods comprising (A) a plurality of second fuel rods containing a first burnable poison, located at positions in an outermost row in a cross-section of the fuel assembly, and (B) a plurality of second fuel rods containing a second burnable poison having a larger thermal neutron absorption cross-section than a thermal neutron absorption cross-section of the first burnable poison, located at positions in an inner region of the cross-section of the fuel assembly other than the outermost row. 5. A nuclear reactor core according to claim 4, wherein (A) the second fuel rods containing the first burnable poison are located at corners in the outermost row. 6. A nuclear reactor core according to claim 4, wherein the first burnable poison is boron and the second burnable poison is gadolinium.