Patent Number: 
Section: claims

1. A fuel assembly comprising: at least, a first fuel rod containing plutonium and not containing burnable poison; a plurality of second fuel rods containing uranium and burnable poison and not containing plutonium; a water rod; and a channel box having a rectangular shape in horizontal section and accommodating the first fuel rod, the plurality of second fuel rods, and the water rod in a bundle, whereinthe plurality of second fuel rods is disposed on an outermost periphery and/or adjacent to the water rod, of a fuel rod array in the horizontal section,N2<N1 (N2 is a positive integer or zero) is satisfied where a number of the second fuel rods arranged on the outermost periphery is N1 and a number of the second fuel rods arranged adjacent to the water rod is N2, andof the second fuel rods arranged on the outermost periphery, W2<N2+WO<W1 (W2 is a positive integer) is satisfied where a number of the second fuel rods arranged without being vertically and/or horizontally adjacent to any of other of second fuel rods in the horizontal section is WO, a number of the second fuel rods arranged vertically and/or horizontally adjacent to only one second fuel rod in the horizontal section is W1, and a number of the second fuel rods arranged vertically and/or horizontally adjacent to two second fuel rods in the horizontal section is W2, andthe second fuel rods arranged vertically and/or horizontally adjacent to the two second fuel rods in the horizontal section are arranged at a corner portion of the channel box. 2. The fuel assembly according to claim 1, whereinaverage fissile plutonium enrichment in the horizontal section is 4.0 wt % to 7.8 wt %. 3. The fuel assembly according to claim 2, whereinthe burnable poison contained in the second fuel rod is gadolinium. 4. The fuel assembly according to claim 1, whereinthe burnable poison contained in the second fuel rod is gadolinium. 5. The fuel assembly according to claim 3, whereinthe fuel assembly includes a nine-row by nine-column fuel grid array, andthe first fuel rod includes thirty four fuel rods with fissile plutonium enrichment of 6.1 wt %, twenty fuel rods with fissile plutonium enrichment of 4.2 wt %, five fuel rods with fissile plutonium enrichment of 2.5 wt %, and eight partial length fuel rods with fissile plutonium enrichment of 6.1 wt %. 6. The fuel assembly according to claim 3, whereinthe fuel assembly includes a nine-row by nine-column fuel grid array, andthe first fuel rod includes thirty two fuel rods with fissile plutonium enrichment of 9.3 wt %, twenty one fuel rods with fissile plutonium enrichment of 6.5 wt %, one fuel rod with fissile plutonium enrichment of 3.0 wt %, two fuel rods with fissile plutonium enrichment of 5.5 wt %, four partial length rods with fissile plutonium enrichment of 8.0 wt %, and four partial length fuel rods with fissile plutonium enrichment of 9.3 wt %. 7. The fuel assembly according to claim 3, whereinthe fuel assembly includes a nine-row by nine-column fuel grid array, andthe first fuel rod includes thirty two fuel rods with fissile plutonium enrichment of 10.9 wt %, twenty fuel rods with fissile plutonium enrichment of 7.5 wt %, one fuel rod with fissile plutonium enrichment of 2.5 wt %, three partial length fuel rods with fissile plutonium enrichment of 10.0 wt %, and four partial length fuel rods with fissile plutonium enrichment of 10.9 wt %. 8. The fuel assembly according to claim 3, whereinthe fuel assembly includes a ten-row by ten-column fuel grid array, andthe first fuel rod includes forty fuel rods with fissile plutonium enrichment of 9.3 wt %, twenty four fuel rods with fissile plutonium enrichment of 6.5 wt %, one fuel rod with fissile plutonium enrichment of 3.0 wt %, two fuel rods with fissile plutonium enrichment of 5.5 wt %, eight partial length rods with fissile plutonium enrichment of 8.0 wt %, and six partial length fuel rods with fissile plutonium enrichment of 9.3 wt %. 9. A reactor of a nuclear reactor loaded with a plurality of fuel assemblies, each of the fuel assemblies comprising:at least, a first fuel rod containing plutonium and not containing burnable poison;a plurality of second fuel rods containing uranium and burnable poison and not containing plutonium;a water rod; anda channel box having a rectangular shape in horizontal section and accommodating the first fuel rod, the plurality of second fuel rods, and the water rod in a bundle, whereinthe plurality of second fuel rods is disposed on an outermost periphery and/or adjacent to the water rod, of a fuel rod array in the horizontal section,N2<N1 (N2 is a positive integer or zero) is satisfied wherea number of the second fuel rods arranged on the outermost periphery is N1 anda number of the second fuel rods arranged adjacent to the water rod is N2, andof the second fuel rods arranged on the outermost periphery,W2<N2+WO<W1 (W2 is a positive integer) is satisfied wherea number of the second fuel rods arranged without being vertically and/or horizontally adjacent to any of other second fuel rods in the horizontal section is WO,a number of the second fuel rods arranged vertically and/or horizontally adjacent to only one second fuel rod in the horizontal section is Wi, anda number of the second fuel rods arranged vertically and/or horizontally adjacent to two second fuel rods in the horizontal section is W2, and the second fuel rods arranged vertically and/or horizontally adjacent to the two second fuel rods in the horizontal section are arranged at a corner portion of the channel box. 10. The reactor according to claim 9, whereinaverage fissile plutonium enrichment in the horizontal section is 4.0 wt % to 7.8 wt %. 11. The reactor according to claim 10, whereinthe burnable poison contained in the second fuel rod is gadolinium. 12. The reactor according to claim 9, whereinthe burnable poison contained in the second fuel rod is gadolinium.