Patent Number: 053596340
Section: claims

1. A reactor core for a boiling water nuclear reactor with a plurality of vertical fuel assemblies, each fuel assembly containing a plurality of fuel rods with enriched nuclear fuel material which are arranged between a bottom tie plate and a top tie plate in a surrounding vertical fuel channel, wherein each fuel assembly is formed with an inlet for water for conducting water in through the bottom tie plate, through the space between the fuel rods in the vertical fuel channel, and out through the top tie plate, and each fuel assembly is arranged with intermediate gaps with respect to adjacent fuel assemblies for conducting water through the core, wherein at least some fuel assemblies are arranged with fuel rods with different enrichment at different levels, wherein each one of at least a main part of those fuel rods, central rods, which in a fuel assembly are surrounded by fuel rods, edge rods, which are located adjacent to a water gap, are arranged with a ratio of the enrichment in the central rod in question to the medium enrichment for the central rods and the edge rods, in a horizontal section, which is lower for an upper part than for a lower part of the central rod, and each one of at least a main part of the edge rods is arranged with a ratio of the enrichment in the edge rod in question to the medium enrichment for the central rods and the edge rods, in a horizontal section, which is higher for the upper part than for the lower part of the edge rod. 2. A reactor core according to claim 1, wherein the length of the upper part constitutes one-fourth to two-thirds of the active length of the fuel rod. 3. A reactor core according to claim 2, wherein the medium enrichment of the central rods and the edge rods in a horizontal section is 5-15% lower in the area of the upper part of the fuel rods than in the area of the lower part of the fuel rods. 4. A reactor core according to claim 1, wherein the medium enrichment of the central rods and the edge rods in a horizontal section is 5-15% lower in the area of the upper part of the fuel rods than in the area of the lower part of the fuel rods. 5. A reactor core for a boiling water nuclear reactor with a plurality of vertical fuel assemblies, each fuel assembly containing a plurality of fuel rods with enriched nuclear fuel material which are arranged between a bottom tie plate and a top tie plate in a surrounding vertical fuel channel, wherein each fuel assembly is formed with an inlet for water for conducting water in through the bottom tie plate, through the space between the fuel rods in the vertical fuel channel, and out through the top tie plate, and each fuel assembly is arranged with intermediate gaps with respect to adjacent fuel assemblies with an internally arranged channel for conducting water through the gaps and through the internal channel in a vertical direction from below and upwards through the core, wherein at least some fuel assemblies are arranged with fuel rods with different enrichment at different levels, wherein each one of at least a main part of those fuel rods, central rods (10a), which in a fuel assembly are surrounded by fuel rods, edge rods (10b), which are located adjacent to a water gap or adjacent to the internal water channel, are arranged with a ratio of the enrichment in the central rod in question to the medium enrichment for the central rods and the edge rods, in a horizontal section, which is lower for an upper part than for a lower part of the central rod, and each one of at least a main part of the edge rods (10b) is arranged with a ratio of the enrichment in the edge rod in question to the medium enrichment for the central rods and the edge rods, in a horizontal section, which is higher for the upper part than for the lower part of the edge rod. 6. A reactor core according to claim 5, wherein the length of the upper part constitutes one-fourth to two-thirds of the active length of the fuel rod. 7. A reactor core according to claim 6, wherein the medium enrichment of the central rods and the edge rods in a horizontal section is 5-15% lower in the area of the upper part of the fuel rods than in the area of the lower part of the fuel rods. 8. A reactor core according to claim 5, wherein the medium enrichment of the central rods and the edge rods in a horizontal section is 5-15% lower in the area of the upper part of the fuel rods than in the area of the lower part of the fuel rods.