Patent Number: 046831134
Section: claims

1. A nuclear fuel assembly having a plurality of fuel rods each containing a multiplicity of fuel pellets, comprising: first fuel rods each containing a burnable poison over substantially the entire axial length thereof, said first fuel rod having upper and lower axial regions and a greater mean enrichment in the upper axial region thereof than that in the lower axial region thereof; and second fuel rods containing no burnable poison and having a uniform enrichment distribution over substantially the entire axial length thereof. 2. A nuclear fuel assembly according to claim 1, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 3. A nuclear fuel assembly according to claim 1, wherein said first fuel rods are disposed in the portion of said fuel assembly except the outermost peripheral portion thereof. 4. A nuclear fuel assembly according to claim 3, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 5. A nuclear fuel assembly according to claim 1, wherein the infinite multiplication factor in the upper region of said fuel assembly is greater than that in the lower region thereof. 6. A nuclear fuel assembly according to claim 5, wherein said first fuel rods are disposed in the portion of said fuel assembly except the outermost peripheral portion thereof. 7. A nuclear fuel assembly according to claim 6, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 8. A nuclear fuel assembly having a plurality of fuel rods each containing a multiplicity of fuel pellets, comprising: first fuel rods each containing a burnable poison over substantially the entire axial length thereof, said first fuel rod having upper and lower axial regions and a greater mean enrichment in the upper axial region thereof than that in the lower axial region thereof; and second fuel rods containing no burnable poison and having a uniform enrichment distribution over substantially the entire axial length thereof, wherein the mean enrichment in the outer peripheral portion of a cross-section of said fuel assembly perpendicular to the axis of said fuel assembly is greater than that in the central portion thereof. 9. A nuclear fuel assembly according to claim 8, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 10. A nuclear fuel assembly according to claim 8, wherein said first fuel rods are disposed in the portion of said fuel assembly except the outermost peripheral portion thereof. 11. A nuclear fuel assembly according to claim 10, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 12. A nuclear fuel assembly according to claim 8, wherein the infinite multiplication factor in the upper region of said fuel assembly is greater than that in the lower region thereof. 13. A nuclear fuel assembly according to claim 12, wherein said first fuel rods are disposed in the portion of said fuel assembly except the outermost peripheral portion thereof. 14. A nuclear fuel assembly according to claim 13, wherein the boundary between the said upper axial region and said lower axial region of said first fuel rod is positioned within a range between 1/3 and 7/12 of the overall length of said first fuel rod as measured from the lower end thereof. 15. A nuclear fuel assembly according to claim 2, further comprising third fuel rods, said third fuel rods containing an enriched nuclear fuel material substantially over the entire axial length thereof, said third fuel rods having upper and lower axial regions with a boundary therebetween corresponding to the boundary of said first fuel rods, said third fuel rods containing a burnable poison in said upper axial region and no burnable poison in said lower axial region. 16. A nuclear fuel assembly according to claim 15, wherein each of said first, second and third fuel rods have at least one end region containg natural uranium. 17. A nuclear fuel assembly according to claim 16, wherein each of said second fuel rods have opposite end regions containing natural uranium. 18. A nuclear fuel assembly according to claim 15, further comprising a center portion and an outer peripheral portion surrounding said center portion in a cross-section of said fuel assembly perpendicular to the axis of said fuel assembly, said fuel assembly having a means enrichment which is greater in said outer peripheral portion than in said center portion. 19. A nuclear fuel assembly according to claim 18, wherein said first fuel rods are disposed in the portion of said fuel assembly except the outermost peripheral portion thereof. 20. A nuclear fuel assembly according to claim 19, wherein each of said first, second and third fuel rods have at least one end region containing natural uranium.