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 nuclear reactor comprising a number of fuel rods arranged in a regular fashion in a channel box which is surrounded by water gap in a reactor core. Some of fuel rods disposed at portions facing the water gap through which no control blade is inserted or drawn out, have atomic number densities of fissionable material contained in the fuel rods. The atomic number density of each of these fuel rods is made smaller than that of the fuel rod disposed in the fuel assembly at portions other than the portions referred to above.

Abstract: Disclosed is a fuel assembly comprises a plurality of cylindrical fuel rods with a plurality of fuel pellets sealed therein, a plurality of small units each having a predetermined number of the fuel rods bundled spaced with a fixed intercentral distance and being arranged in such a manner that the intercentral distance between the component fuel rods falling on the mutually juxtaposed sides of the adjacent small units is larger than the intercentral distance between the adjacent fuel rods in the small units, and at least one water rod near the center of a cluster of the plurality of the small units.