Fuel assembly for nuclear reactor

A core of nuclear reactor comprising:

• a first fuel assembly comprising a plurality of fuel rods, each of which is filled with a fuel material, and a large-diameter water rod which occupies a space substantially equal to that occupied by a plural number of fuel rods, wherein said first fuel assembly includes a predetermined amount of a burnable poison;
• a second fuel assembly comprising a plurality of fuel rods, each of which is filled with a fuel material, and a large diameter water rod which occupies a space substantially equal to that occupied by a plural number of fuel rods, wherein said second fuel assembly includes an amount of the burnable poison which is less than that in said first fuel assembly,
• wherein the majority of fuel rods of said second assembly are loaded around the outer periphery of said core and the majority of fuel rods of said first fuel assembly are loaded at the center of said core, and wherein
• said first and second fuel assemblies define a section filled with a fuel material comprising a natural uranium section located at least at the upper or lower end of each of said fuel rods and an enriched uranium section consisting of an upper section, an intermediate section and a lower section, each of which have an average enrichment across a cross section of the fuel assembly, wherein said average enrichment in the cross section of the fuel assembly at the upper and lower sections is less than that at the intermediate section and wherein
• said enriched uranium section includes at least two sub-sections in which the respective amounts of said burnable poison per unit length in the axial direction are different such that the amount of said burnable poison per unit length in the axial direction in said upper section is less than the amount of said burnable poison per unit length in the axial direction in said lower section and,
• wherein said plurality of fuel rods comprise fuel rods without poison having variations in enrichment between natural uranium sections located at the upper and lower ends of these fuel rods, and fuel rods with poison having the same enrichment between natural uranium sections located at the upper and lower ends of these fuel rods.

A fuel assembly for a nuclear reactor comprising:

• a plurality of fuel rods each of which is filled with a fuel material;
• a large-diameter water rod which occupies a space substantially equal to that occupied by a plural number of fuel rods,
• said plurality of fuel rods defining a section filled with a fuel material comprising a natural uranium section located at least at the upper or lower end of each of said fuel rods and an enriched uranium section consisting of an upper section, an intermediate section and a lower section, each of which have an average enrichment across the cross section of the fuel assembly, wherein said average enrichment in the cross section of the fuel assembly at the upper and lower sections is less than that at the intermediate section, wherein said upper and lower sections of said enriched uranium section have a substantially equal average degree of enrichment across a cross section of the fuel assembly, and
• said enriched uranium section having at least two sections which include a burnable poison, wherein the respective amounts of the burnable poison per unit length in the axial direction are different such that the amount of said burnable poison per unit length in the axial direction in said upper section is less than the amount of said burnable poison per unit length in the axial direction in said lower section, and
• wherein said plurality of fuel rods comprise fuel rods without poison having variations in enrichment between natural sections located at the upper and lower ends of these fuel rods, and fuel rods with poison having the same enrichment between natural uranium sections located at the upper and lower ends of these fuel rods.

A fuel assembly for a nuclear reactor comprising:

• a plurality of fuel rods each of which is filled with a fuel material;
• a large-diameter water rod which occupies a space substantially equal to that occupied by a plural number of fuel rods,
• said plurality of fuel rods defining a section filled with a fuel material comprising a natural uranium section located at least at the upper or lower end of each of said fuel rods and an enriched uranium section consisting of an upper section, an intermediate section and a lower section, each of which have an average enrichment across the cross section of the fuel assembly, wherein said average enrichment in the cross section of the fuel assembly at the upper and lower sections is less than that at the intermediate section, and
• said enriched uranium section having at least two sections which include a burnable poison, wherein the respective amounts of the burnable poison per unit length in the axial direction are different such that the amount of said burnable poison per unit length in the axial direction in said upper section is less than the amount of said burnable poison per unit length in the axial direction in said lower section, wherein the amount of burnable poison per unit length in the axial direction in said intermediate section of said enriched uranium section is substantially equal to the amount of said burnable poison per unit length in the axial direction in said lower section of said enriched uranium section, and
• wherein said plurality of fuel rods comprise fuel rods without poison having variations in enrichment between natural uranium sections located at the upper and lower ends of these fuel rods, and fuel rods with poison having the same enrichment between natural uranium sections located at the upper and lower ends of these fuel rods.