Patent Number: 050080709
Section: claims

1. A fuel assembly comprising a plurality of first fuel rods each of which contains nuclear fuel material but does not contain burnable poison and a plurality of second fuel rods each of which contains nuclear fuel material and burnable poison, characterized in that an amount of the burnable poison in a lower region of said fuel assembly is smaller than that in an upper region of said fuel assembly, and that when each of said second fuel rods is divided into an upper region and a lower region, a region of said divided regions in said second fuel rods containing a maximum burnable poison concentration Gmax and a region of said divided regions in said second fuel rods containing a minimum burnable poison concentration Gmin are located in the lower region of said fuel assembly, the maximum burnable poison concentration Gmax being located only in the lower region of said fuel assembly. 2. The fuel assembly according to claim 1, wherein said minimum burnable poison concentration Gmin is zero. 3. The fuel assembly according to claim 1, wherein said minimum burnable poison concentration Gmin is greater than zero. 4. The fuel assembly according to claim 1, wherein an average enrichment in the lower region of said fuel assembly is smaller than that of the upper region thereof. 5. The fuel assembly according to claim 1, wherein the burnable poison concentration G of the upper region of each of said second fuel rods meet a relationship, Gmin&lt;G&lt;Gmax. 6. The fuel assembly according to claim 1, wherein said fuel assembly further comprising a third fuel rod having an axial length of fuel effective length portion smaller than that of said first fuel rod and smaller than that of said second fuel rod. 7. The fuel assembly according to claim 6, wherein said third fuel rod is a part of said first fuel rod. 8. A fuel assembly comprising a plurality of fuel rods each of which contains nuclear fuel material but does not contain burnable poison, a plurality of fuel rods each of which contains the nuclear fuel material and the burnable poison, and at least one water rod disposed between said fuel rods, characterized in that an amount of said burnable poison in a lower region of said fuel assembly is smaller than that in an upper region thereof, and said plurality of fuel rods containing the nuclear fuel material and the burnable poison include first fuel rods and second fuel rods whose average burnable poison concentration is lower than that of said first fuel rods, and said second fuel rods are arranged close to said water rod, and than when each of said first and said second fuel rods is divided into upper and lower regions, a region of said divided regions containing a maximum burnable poison concentration Gmax and a region of said divided regions containing a minimum burnable poison concentration Gmin are located in the lower region of said fuel assembly, the maximum burnable poison concentration Gmax being located only in the lower region of said fuel assembly. 9. The fuel assembly according to claim 8, wherein the burnable poison concentration of the lower region of said first fuel rods is said maximum burnable poison. concentration Gmax, and the burnable poison concentration of the lower region of said second fuel rods is said minimum poison concentration Gmin. 10. The fuel assembly according to claim 8, wherein said first fuel rods are disposed in a fuel rod arrangement region except for an outermost peripheral fuel rod arrangement region of the fuel assembly and for a fuel rod arrangement region adjacent to said water rod. 11. The fuel assembly according to claim 8, wherein the burnable poison concentration G in the upper regions of said first and said second fuel rods meets a relationship, Gmin&lt;G&lt;Gmax. 12. The fuel assembly according to claim 8, wherein the burnable poison concentration in the lower region of the first fuel rods is higher than that in the upper region thereof, and the burnable poison concentration in the lower region of the second fuel rods is lower than that in the upper region thereof. 13. The fuel assembly according to claim 8, wherein said fuel assembly further comprising a third fuel rod having an axial length of fuel effective length portion smaller than that of said first fuel rod and smaller than that of said second fuel rod. 14. The fuel assembly according to claim 1, wherein both the maximum burnable poison concentration Gmax and the minimum burnable poison concentration Gmin are located only in the lower region of said fuel assembly. 15. The fuel assembly according to claim 1, wherein the upper region and the lower region of said divided regions of said second fuel rods contain enriched uranium, and the regions containing at least one of the maximum burnable poison concentration Gmax and the minimum burnable poison concentration Gmin are located in the lower region which is filled with the enriched uranium. 16. The fuel assembly according to claim 8, wherein both the maximum burnable poison concentration Gmax and the minimum burnable poison concentration Gmin are located only in the lower region of said fuel assembly. 17. The fuel assembly according to claim 8, wherein the upper region and the lower region of said divided regions of said second fuel rods contain enriched uranium, and the regions containing at least one of the maximum burnable poison concentration Gmax and the minimum burnable poison concentration Gmin are located in the lower region which is filled with the enriched uranium.