Patent Number: 
Section: claims

1. A fuel assembly, comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as a burnable poison, wherein at least one of the fuel rods that contain gadolinium comprises gadolinium-155 and gadolinium-157,  wherein in the fuel rods that contain gadolinium-155 and gadolinium-157, gadolinium-157 is enriched and is present in an amount more than its isotopic abundance in natural gadolinium, and a ratio of said gadolinium-155 to said gadolinium-157 is 0.1 or less. 2. A fuel assembly, comprising a plurality of fuel rods bundled in grid pattern and containing enriched uranium as a nuclear fuel material, a part of the fuel rods containing gadolinium as a burnable poison, G 0 less than 0.25 xc2x7Pxc2x7M/W  wherein at least one of the fuel rods that contain gadolinium comprises gadolinium-155 and gadolinium-157,  wherein in the fuel rods that contain gadolinium-155 and gadolinium-157, gadolinium-157 is enriched and is present in an amount more than its isotopic abundance in natural gadolinium, and a ratio of said gadolinium-155 to said gadolinium-157 is 0.1 or less,  and wherein an average concentration (wt %) G 0  of an oxide of the gadolinium-155 and an oxide of gadolinium-157 averaged over an entire fuel rod having the gadolinium is set in the range shown by the following expression:  wherein in the expression, M denotes a number of month under rated power operation per one cycle of an equilibrium reactor core, P power density of a nuclear reactor (kw/l unit), W a sum of isotopic composition (% unit) of gadolinium-155 and gadolinium-157. 3. A nuclear reactor loaded with the fuel assembly set forth in  claim 1  and a fuel assembly having natural gadolinium, respectively, wherein a gadolinia concentration averaged over the fuel assembly having natural gadolinium is larger than that of the fuel assembly set forth in  claim 1 . claim 1 claim 1 4. A nuclear reactor loaded with the fuel assembly set forth in  claim 2  and a fuel assembly having natural gadolinium, respectively, wherein a gadolinia concentration averaged over the fuel assembly having natural gadolinium is larger than that of the fuel assembly set forth in  claim 2 . claim 2 claim 2