Abstract: A fuel assembly includes a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns and at least one water rod. In this fuel assembly, the fuel rod pitch of the plurality of fuel rods is in a range of 14.15 mm to 14.65 mm, and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section of the fuel bundle is offset from the center in a cross section of the lower tie plate toward the channel fastener side. With this configuration, it is possible to provide a fuel assembly for a D-lattice core, which is capable of achieving the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and of using the existing fuel spacers.

Abstract: A boiling water type nuclear reactor core, in which a plurality of fuel assemblies, each enclosed in a channel box, are loaded and a plurality of control rods, each having control blades, are arranged between the channel boxes. Long blade control rods, each having control rod blades which extend in four directions, are arranged between channel boxes on diagonals of square bundle regions each formed by a plurality of fuel assemblies, and short blade control rods, each having a control rod blade length of about half of the width of one of the square bundle regions, are arranged between the channel boxes in the center of each of the square bundle regions.

Abstract: A method of operating a nuclear reactor reduces the number of reload fuels to be loaded into the nuclear reactor in the second and the following operation cycles. The nuclear reactor has a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors are arranged. The method operates the nuclear reactor with control rods inserted in control cells each comprising four reload fuel assemblies having relatively large infinite multiplication factors among the plurality of reload fuel assemblies for a period longer than half of a period of an operation cycle.

Abstract: A boiling water type nuclear reactor core, in which a plurality of fuel assemblies, each enclosed in a channel box, are loaded and a plurality of control rods, each having control blades, are arranged between the channel boxes. Latitudinal long blade control rods, each having control rod blades which extend latitudinally in four directions, are arranged between channel boxes on diagonals of square bundle regions each formed by a plurality of fuel assemblies, and latitudinal short blade control rods, each having control rod blades which extend latitudinally in four directions with each control rod blade having a latitudinal length of about half of the width of one of the square bundle regions, are arranged between the channel boxes in the center of each of the square bundle regions. The long blade control rods have a latitudinal blade length which is about twice as long as the latitudinal blade length of the short blade control rods.

Abstract: Several unit loading patterns are arranged in the central area of an initial core to which the present invention is applied. The unit loading pattern is composed of one square-shaped unit cell and four cross-shaped control rods 3 which surround the unit cell. The unit cell is composed of one low enrichment fuel assembly 7, two high enrichment fuel assemblies 8 and one high enrichment fuel assembly 9. The low enrichment fuel assemblies 7 of each unit loading pattern adjoin each other and are arranged to constitute the first control cell 2a being square-shaped. The high enrichment fuel assemblies 9, obliquely adjoining the low enrichment fuel assembly 7 in each unit loading pattern, adjoin each other and are arranged to constitute the second control cell 2b being square-shaped.

Abstract: In an arrangement of an initial core, the core is loaded with 572 high enrichment fuel assemblies H with 4.2 wt % average enrichment and 300 low enrichment fuel assemblies L with 1.5 wt % average enrichment. The average enrichment of the core is about 3.3 wt %. In this core, only the low enrichment fuel assemblies are loaded into the most outer position of the core and the high enrichment fuel assemblies are loaded into an area other than the most outer position. In this arrangement, the average enrichment of reload fuel assemblies is 3.7 wt %.

Abstract: In a fuel loading method of the present invention, a nuclear reactor is operated up to the second cycle without exchanging the fuel. Low-enrichment fuel is discharged from a core at the end of the second cycle and at the end of the third cycle. Moreover, high-enrichment fuel with an average enrichment higher than that of replacement fuel is discharged from the core at the end of the third cycle and reloaded into the core at the end of the fifth cycle.According to the present invention, because the high-enrichment fuel discharged from the core at the end of the third cycle normally has a high enrichment, a short combustion period, and a low burnup compared to the replacement fuel burned for 4 to 5 cycles, a lot of fissionable material are left in the high-enrichment fuel. By reloading the high-enrichment fuel to the core after one cycle or more passes, it is possible to greatly increase the discharge exposure of initially loaded fuel.

Abstract: In an initial core of the present invention, a low enrichment fuel assembly having the lowest average enrichment factor and three fuel assemblies having a higher average enrichment factor than that of the low enrichment fuel assembly are arranged in a square shape, control rods of a cross shape are arranged at each of four corners of the square shape to constitute a unit loading pattern, and a plurality of the unit loading patterns are provided in the central region of the core, the fuel assemblies having the higher average enrichment factor are divided by a diagonal line into a first region of the side of the control rods and a second region of the side opposite to the control rods, and the number of the gadolinia-containing fuel rods is greater in the second region by at least two than in the first region.

Abstract: In an initial core constituted by various kinds of fuel assemblies having a different average enrichment of uranium 235 four fuel assemblies having the lowest average enrichment constitute a square shaped cell. Three fuel assemblies having the highest average enrichment constitute an L-shaped cell, and the L-shaped cell is arranged at corners of the square shaped cells, whereby each assembly of the L-shaped cell adjoins a square shaped cell. The ratio of the number of assemblies of the L-shaped cell to the total number of fuel assemblies in the core is 10% or more.

Abstract: A fuel assembly has part length and full length fuel rods, and a pair of large-diameter water rods which occupy an area which can accommodate 7 fuel rods. Natural uranium regions are provided in the upper and lower end portions of the effective fuel zone of the fuel assembly. An intermediate region between these upper and lower natural uranium regions provides an enriched uranium region which has three axial sections: an upper section, a middle section and a lower section. The middle section has the highest average enrichment, the lower section has the medium average enrichment and the upper section has the smallest average enrichment. The difference in the average enrichment between the middle section and the lower section is smaller than that between the middle section and the upper section. The upper section has a lower concentration of burnable poison than other sections of the enriched uranium region.

Abstract: A mean uranium enrichment of fuel rods is set to not less than 4 wt% and preferably 4.25%, a percentage in number of Gd rods to all the fuel rods is set in the range of 20 to 30% and preferably 23%, and an enrichment 4.45 wt% of the Gd rods is between a pellet maximum uranium enrichment and a pellet minimum uranium enrichment. A percentage in number of those fuel rods having a maximum uranium enrichment of 5.0% to all the fuel rods except the Gd rods is set to not less than 75% and preferably 82%. A mean uranium enrichment in the enriched fuel section except blanket regions at upper and lower end portions is 4.75 wt% and a ratio e.sub.max /e.sub.mean of the pellet maximum uranium enrichment to that mean uranium enrichment is not larger than 1.16 and preferably 1.105. Accordingly, when the maximum uranium enrichment is limited to 5.0 wt%, the mean uranium enrichment can be raised to attain mean discharged exposure not less than 45 GWd/t without causing any problems in gadolinia containing fuel rods.

Abstract: A fuel assembly of the type having a plurality of a fuel material characterized in that a water rod disposed in a channel has a large diameter so that it has a space into which a plurality of fuel rods could be loaded and liquid water boil flows through the water rod and which has a section filled with a fuel material comprising a natural uranium section defined at least one of the upper and lower ends of each fuel rod and an enriched uranium section comprising the uppermost, intermediate and lowermost sections each having a different degree of average enrichment across a cross section of the fuel assembly and the upper and lowermost sections have the degree of enrichment across a section of the fuel assembly is lower than that of the intermediate section, and the enriched uranium section has two sections each of which has a different content of a burnable poison per unit length in the axial direction and the content of the burnable poison per unit of length in the axial direction in the uppermost section is

Abstract: A fuel assembly for use in a boiling water reactor has a multiplicity of fuel rods. In the fuel assembly, among the fuel rods located in the periphery (for example, the outermost periphery) of the cross section of the fuel assembly, the proportion of fuel rods whose enrichment in their respective lower regions are greater than the average enrichment in the lower region of the fuel assembly is less than the proportion of fuel rods whose enrichment in their respective upper regions are greater than the average enrichment in the upper region of the fuel assembly.