Patent Number: 052079791
Section: claims

1. A nuclear fuel assembly having upper and lower regions and an array of fuel rods extending through both said regions, said fuel rods comprising a plurality of first fuel rods each containing nuclear fuel material but not containing burnable poison and a plurality of further fuel rods each containing both nuclear fuel material and burnable poison, said further fuel rods comprising at least one second fuel rod and at least one third fuel rods, wherein said at least one second fuel rod has at a lower region of the fuel assembly a burnable poison concentration which is a lowest burnable poison concentration other than zero of said burnable poison concentration contained in said further fuel rods at said lower region of the fuel assembly, and said at least one third fuel rod has at said lower region of the fuel assembly a burnable poison concentration higher than said lowest burnable poison concentration of said at least one second fuel rod, and wherein as seen in plan view said first fuel rods are disposed nearer to said second and third fuel rods than said second and third fuel rods are disposed with respect to one another in said array, and the burnable poison concentration of said second and third fuel rods of said further fuel rods at the lower region of the fuel assembly is determined at the same horizontal section extending through said further fuel rods in said lower region of the fuel assembly average burnable poison concentration of said further fuel rods is equal or higher in said lower region than in said upper region. 2. A nuclear fuel assembly according to claim 1 having a plurality of said second fuel rods. 3. A nuclear fuel assembly according to claim 1 wherein said lowest burnable poison concentration is less than half of an average burnable poison concentration of said further fuel rods in said lower region. 4. A nuclear fuel assembly according to claim 1 wherein having a plurality of said third fuel rods and at least one of said third fuel rods has at said lower region of the fuel assembly a burnable poison concentration which is a highest burnable poison concentration in said further fuel rods. 5. A nuclear fuel assembly according to claim 1 further having water rods and wherein said at least one second fuel rod is adjacent said water rods. 6. A nuclear fuel assembly according to claim 1 wherein said array of fuel rods as seen in plan view is a rectangular array having rows and columns at right angles, and each said further fuel rod has said first fuel rods as adjacent fuel rods in the row direction and the column direction. 7. A nuclear fuel assembly according to claim 6 wherein a lower tie plate supports lower ends of said fuel rods, said rows and columns of said array being parallel to respective side edges of said lower tie plate. 8. A nuclear fuel assembly according to claim 1 wherein said lowest burnable poison concentration in said lower region of the assembly is lower than the burnable poison concentration in any of said further fuel rods at said upper region of the assembly. 9. A nuclear fuel assembly according to claim 1 wherein said burnable poison is Gd and said minimum burnable poison concentration is in the range 1 to 2% by weight (calculated as Gd.sub.2 O.sub.3) of the fuel content of the rod at the lower region. 10. A reactor core having at least one fuel assembly according to claim 1. 11. A nuclear fuel assembly having a plurality of first fuel rods each containing nuclear fuel material but not containing burnable poison and a plurality of further fuel rods each containing both nuclear fuel material and burnable poison, said further fuel rods comprising (a) at least one second fuel rod having at a lower region of the fuel assembly a burnable poison concentration which is a lower burnable poison concentration other than zero in said further fuel rods, (b) a plurality of third fuel rods having at said lower region of the fuel assembly a burnable poison concentration higher than said lowest burnable poison concentration, the fuel rods being arranged so that said first fuel rods are adjacent fuel rods of each of said further fuel rods, and wherein the burnable poison concentration of said second and third fuel rods of said further fuel rods at said lower region of said fuel assembly is determined at the same horizontal section extending through said further fuel rods in said lower region of the fuel assembly the average burnable poison concentration of said further fuel rods is equal or higher in said lower region than in said upper region.