Patent Number: 055442110
Section: claims

1. A fresh fuel assembly for loading in a nuclear reactor, charged with uranium and no plutonium and having a plurality of fuel rods and at least one water rod surrounded by said fuel rods, said fuel assembly comprising: a fuel-charged zone including axially upper and lower end regions charged with natural uranium and an enriched uranium region between said upper and lower end regions, said enriched uranium region having three sections of an upper section, a middle section and a lower section of different levels of enrichment;  said middle section having an average enrichment of a level higher than an average enrichment level of said upper and lower sections;  a difference in the average enrichment level between said middle section and said lower section being smaller than a difference in the average enrichment level between said middle section and said upper section;  wherein the fuel assembly comprises said fuel rods arranged in nine columns and nine rows;  wherein a pair of water rods are disposed adjacent to each other in a central portion of the cross-section of said fuel assembly so as to occupy an area substantially equal to an area to be occupied by seven fuel rods;  wherein a part of said plurality of fuel rods comprises burnable-poison-containing fuel rods containing uranium and a burnable poison and the rest of said plurality of fuel rods comprises uranium fuel rods containing uranium and no burnable poison;  wherein a part of said uranium fuel rods have axially upper and lower end regions charged with natural uranium, and an enriched uranium region between said upper and lower end regions, said enriched uranium region having three sections of an upper section, a middle section and a lower section of different levels of enrichment, said middle section having an enrichment of a level higher than an enrichment level of said upper and lower sections, a difference in enrichment level between said middle section and said lower section being smaller than a difference in enrichment level between said middle section and said upper section; and  wherein a burnable poison content per unit axial length in the upper section of each burnable-poison-containing fuel rod is smaller than a burnable poison content per unit axial length in other sections of said enriched uranium region of the burnable-poison-containing fuel rod.  said first fuel rods including a plurality of kinds of uranium fuel rods each having upper and lower end regions charged with natural uranium and an intermediated enriched uranium region of a uniform axial enrichment distribution, and one kind of uranium fuel rods each having upper and lower end regions charged with natural uranium and an intermediate enriched uranium region composed of three sections of different levels of enrichment,  the uranium fuel rods having said three sections of different enrichment levels being disposed in the outermost layer of fuel rods at portions between adjacent corners of the fuel assembly, as viewed in the cross-section of the fuel assembly, and other portions of the outermost layer of fuel rods, as viewed in the fuel assembly cross-section, being constituted by the first fuel rods having the uniform axial enrichment distributions in said enriched uranium regions.  a first fuel assembly having a plurality of fuel rods charged with a nuclear fuel material and arranged in nine columns and nine rows and a pair of water rods which are arranged adjacent to each other at the central portion of the cross-section of said first fuel assembly and occupying an area which can accommodate seven fuel rods, said first fuel assembly containing a burnable poison; and  a second fuel assembly having a plurality of fuel rods charged with a nuclear fuel and arranged in nine columns and nine rows and a pair of water rods which are arranged adjacent to each other at the central portion of the cross-section of said second fuel assembly and occupying an area which can accommodate seven fuel rods, said second fuel assembly containing a burnable poison of an amount greater than an amount of said burnable poison contained in said first fuel assembly;  wherein a part of said plurality of fuel rods of said first and second fuel assemblies comprises burnable-poison-containing fuel rods containing uranium and a burnable poison and the rest of said plurality of fuel rods comprises uranium fuel rods containing uranium and no burnable poison;  wherein each of said first and second fuel assemblies before loading in the nuclear reactor is charged with uranium and no plutonium and comprises a fuel-charged zone including axially upper and lower end regions charged with natural uranium, and an enriched uranium region between said upper and lower end regions, said enriched uranium having an upper section, a middle section and a lower section of different enrichment;  said middle section having average enrichment of a level higher than an average enrichment level of said upper and lower sections;  a difference in the average enrichment level between said middle section and said lower section being smaller than a difference in the average enrichment level between said middle section and said upper section;  wherein a part of said uranium fuel rods has axially upper and lower end regions charged with natural uranium, and an enriched uranium region between said upper and lower end regions, said enriched uranium region having an upper section, a middle section and a lower section of different levels of enrichment, said middle section having an enrichment of a level higher than an enrichment level of said upper and lower sections, a difference in the enrichment level between said middle section and said lower section being smaller than a difference in the enrichment level between said middle section and said lower section; and  wherein a burnable poison content per unit axial length in the upper section of each burnable-poison-containing fuel rod is smaller than a burnable poison content per unit axial length in other sections of said enriched uranium region of the burnable-poison-containing fuel rod. 2. A fresh fuel assembly according to claim 1, wherein said uranium fuel rods include first fuel rods, second fuel rods having an axial length shorter than an axial length said first fuel rods, the uranium fuel rods having said three sections, in said uranium region, of different levels of enrichment are included in said first fuel rods and not in said second fuel rods, the upper end of the nuclear-fuel-charged zone in each said second fuel rod being located within said middle section, the average enrichment, as viewed in the cross-section of the fuel assembly, of the portion of said middle section above said upper end of the fuel-charged zone in each second fuel rod being smaller than the average enrichment of the portion of said middle section below said upper end of the fuel-charged zone of said second fuel rods. 3. A fresh fuel assembly according to claim 2, wherein the difference in the average enrichment, said viewed in the cross-section of the fuel assembly, between the portion of said middle section below the upper end of said fuel-charged zone of said second fuel rod and said lower section thereof is lower than the difference in the average enrichment between the portion of said middle section above said upper end of the fuel-charged zone of said second fuel rod and said upper section thereof. 4. A fresh fuel assembly according to claim 2, wherein the enrichment of said enriched uranium region of said second fuel rod is higher than the average enrichment of the whole fuel assembly. 5. A fresh fuel assembly according to claim 4 wherein the content of the burnable poison per unit axial length is smallest in said upper section, medium in said middle section and greatest in said lower section. 6. A fresh fuel assembly according to claim 1, wherein said uranium fuel rods include first fuel rods, second fuel rods having axial lengths shorter than an axial length of said first fuel rods, 7. A fresh fuel assembly according to claim 1, wherein the axial length of the natural uranium regions ranges from 1/24 to 1/12 of the axial length of the whole fuel-charged zone. 8. A fresh fuel assembly according to claim 1, wherein the axial length of said upper section of said enriched uranium region ranges from 1/12 to 1/8 of the axial length of the whole fuel-charged zone. 9. A fresh fuel assembly according to claim 1, wherein the boundary between said lower section and said middle section is positioned within the range between 1/3 and 7/12 of the axial length of the whole fuel-charged zone as measured from the lower end of said fuel-charged zone. 10. A fresh fuel assembly according to claim 1, wherein fuel rods having higher enrichment than the average enrichment over the whole fuel assembly are disposed to form the outermost layer of fuel rods in the cross-section of the fuel assembly. 11. A fresh fuel assembly according to claim 1, wherein the rest of said uranium fuel rods includes at least one uranium fuel rod having axially upper and lower end regions charged with natural uranium and an enriched uranium region of an enrichment level substantially uniform axially thereof. 12. A nuclear reactor having a reactor core, said reactor core comprising: 13. A nuclear reactor according to claim 12, wherein said uranium fuel rods include first fuel rods and second fuel rods having axial lengths shorter than an axial length of said first fuel rods. 14. A nuclear reactor according to claim 12, wherein the content of the burnable poison per unit axial length is the smallest in said upper section, medium in said middle-section and the greatest in said lower section. 15. A nuclear reactor according to claim 12, wherein the average concentration of said burnable poison in said first fuel assembly is greater than in said second fuel assembly. 16. A nuclear reactor according to claim 12, wherein the first and second fuel assemblies which have stayed in the reactor core and burnt are disposed at four positions surrounding each of control rods which are disposed at the outermost portion and central portion of the cross-section of said reactor core.