Patent Number: 055240330
Section: claims

1. Gadolinium for use as a burnable poison for nuclear fuel, comprising: a plurality of isotopes of gadolinium,  wherein a content of at least one even mass numbered isotope of said plurality of isotopes is smaller than a content of said at least one even mass numbered isotope in natural gadolinium, wherein said at least one even mass numbered isotope is both Gd-156 and Gd-158.  a plurality of nuclear fuel rods arrayed in a lattice; wherein at least one of the fuel rods comprises gadolinium having a plurality of isotopes,  wherein a content of at least one even mass numbered isotope of said plurality of isotopes is smaller than a content of said at least one even mass numbered isotope in natural gadolinium, wherein said at least one even mass numbered isotope is both Gd-156 and Gd-158.  a plurality of nuclear fuel rods arrayed in a lattice,  said plurality of fuel rods including at least first and second groups of fuel rods each comprising gadolinium isotopes wherein a content of Gd-157 is larger than a content of Gd-157 in natural gadolinium;  wherein said first group has a gadolinium concentration different from a gadolinium concentration of said second group. 2. A fuel assembly, comprising: 3. The fuel assembly according to claim 2, wherein the content of gadolinium is higher in a lower portion of the fuel assembly than in an upper portion of the fuel assembly. 4. The fuel assembly according to claim 2, wherein the content of gadolinium is larger in an upper portion of the fuel assembly than in a lower portion of the fuel assembly. 5. The fuel assembly according to claim 2, wherein said at least one of the fuel rods comprising gadolinium is positioned at an outer peripheral portion of the fuel assembly. 6. A fuel assembly, comprising: 7. The fuel assembly according to claim 6, wherein a ratio of said gadolinium concentration of the first group to said gadolinium concentration of the second group is from 1.2 to 1.3.