Abstract: A fuel assembly comprises a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as a burnable poison. At least one of the fuel rod having gadolinium contains gadolinium enriched in at least one kind of isotope of odd mass number more than an isotopic abundance of natural gadolinium. In the enriched gadolinium, a ratio of a content of Gd-155 to that of Gd-157 is 0.1 or less. An average concentration (wt %) G0 of enriched gadolinia is, with M denoting the number of month under rated power operation per one cycle of an equilibrium core, P power density of a nuclear reactor (kw/l unit) and W a sum of isotopic composition, is set in the range shown by the following expression. G0<0.25·P·M/W. Thereby, in the fuel assembly, residual reactivity of a burnable poison at a cycle end can be decreased and thermal performance can be improved.

Abstract: Gadolinium is provided which is adapted for nuclear fuel as a burnable poison, having a plurality of isotopes in an isotopic composition such that the content of at least one even mass numbered isotope is smaller than the content of the same isotope in natural gadolinium. A fuel assembly is also provided having a plurality of nuclear fuel rods arrayed as a lattice in which at least one of the fuel rods contains the gadolinium burnable poison of the present invention. Also, a fuel assembly is described which has a plurality of nuclear fuel rods arrayed as a lattice which includes at least a first group and a second group of nuclear fuel rods containing gadolinium. The content of Gd-157 in the gadolinium is larger than that found in natural gadolinium. Further, the gadolinium concentrations in the first and second groups are different from each other.

Abstract: A moderator rod provided with a burnable poison and disposed in a fuel assembly of a reactor core comprises an outer tube, an inner tube disposed in the outer tube, the outer and inner tubes constituting a double tube structure between which an annular section is formed, and a burnable poison charged in said annular section. A containing amount of the burnable poison per unit area of the annular section is distributed at least one of peripheral direction and axial direction of the double tube structure of the inner and outer tubes. A fuel assembly of a nuclear reactor comprises an upper tie plate, a lower tie plate, a number of fuel rods supported at their upper ends by the upper tie plate and at their lower ends by the lower tie plate and filled up in their inner spaces with a plurality of fuel pellets, at least one of moderator rods containing burnable poison in various states and a channel box. The moderator has an outer tube and an inner tube disposed coaxially in the outer tube.

Abstract: A fuel assembly for a boiling water reactor comprising an upper and lower tie plates, a plurality of fuel rods filled therein with a plurality of fuel pellets, a fuel bundle comprising the fuel rods retained with a plurality of spacers in spaced relation and disposed between the upper and lower tie plates, and a polygonal tube-like channel box which encloses the fuel bundle and constitutes flow paths for coolant. The fuel rods comprise long fuel rods and short fuel rods which are each shorter in the effective portion than each of the long fuel rods. The concentration of fissile material in each of the short fuel rods is lower than the mean concentration of fissile material in the cross section of the fuel bundle.