Image Number 11 for United States Patent #5416813.
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. An area of a metal, ceramics or thermet containing the burnable poison is formed at a portion except for portions having axial length corresponding to 1/24 to 1/12 length of the fuel effective length from the upper and lower ends thereof.