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: A nuclear reactor core is provided with a fuel assembly which comprises channel boxes respectively separated with spaces into which control rods are arranged and a plurality of fuel rods arranged in the channel box, at least a part of the fuel rods being charged with uranium-plutonium mixture fuel and an enrichment of U-235 or a concentration of plutonium, or both being distributed in an axial direction of the fuel rods. The concentration of a burnable poison packed in the fuel rod is distributed in the axial direction of the fuel assembly. The ratio of the occupying region of a moderator of the fuel assembly is increased with respect to the fuel occupying region. The number of the fuel rods containing the burnable poison is variously changed with respect to the total number of the fuel rods and the arranging mode thereof is also varied with the concentration thereof.

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.