Patent Number: 050930702
Section: claims

1. A boiling water reactor core having a plurality of cells each containing four fuel assemblies adjacent to a control rod, comprising a radially central region of the core and a peripheral region surrounding said central region, said fuel assemblies including first fuel assemblies containing burnable poison which substantially burns out in a first operation cycle of operation of the core and second fuel assemblies which have completed the first operation cycle to effect burn out of the burnable poison therein, a loading fraction of the first fuel assemblies in the central region being larger than a loading fraction thereof in the peripheral region, and a loading fraction of the second fuel assemblies in the peripheral region being larger than a loading fraction thereof in the central region. 2. A boiling water reactor core according to claim 1, wherein said first and second fuel assemblies are disposed in the diagonal direction in each of said cells containing them. 3. A boiling water reactor core according to claim 1, wherein said second fuel assemblies are fuel assemblies each having no burnable poison. 4. A boiling water reactor core according to claim 1, wherein each of said first fuel assemblies is divided into an upper portion and a lower portion in the axial direction thereof, the average enrichment of said upper portion is greater than that of said lower portion, the amount of burnable poison contained in said upper portion is greater than the amount of burnable poison contained in said lower portion. 5. A boiling water reactor core according to claim 1, wherein a plurality of first cells each containing said first fuel assemblies and a plurality of second cells an average infinite multiplication factor of which is smaller than that of said first cells and into each of which a control rod for controlling the power of reactor is inserted, are disposed in said central region. 6. A boiling water reactor core according to claim 5, wherein one of said second cells is disposed at the center of core, and the other second cells are disposed around said second cell disposed at said center of core in a circular form. 7. A boiling water reactor core according to claim 1, wherein a distance between a boundary between the central region and the peripheral region and a center of the core is greater than 2/5 of a radius of the core. 8. A boiling water reactor core according to claim 1, wherein each of the fuel assemblies has a plurality of fuel rods and two water rods arranged between the fuel rods, outer diameters of the water rods being larger than a pitch of the fuel rods, and the two water rods occupying a space capable of receiving seven fuel rods. 9. A boiling water reactor core according to claim 1, wherein said central region receives a plurality of first cells each of which includes two of said first fuel assemblies and two of said second fuel assemblies, a number of operation cycles where one of the two second fuel assembles was operated being different from a number of operation cycles where another of the two second fuel assemblies was operated, said peripheral region receives a plurality of second cells each of which includes four of said a second fuel assemblies, two of said four second fuel assemblies operating in a second operation cycle after the first operation cycle, numbers of operation cycles where the other two of said four second fuel assemblies were operated being different from a number of operation cycles where said two of said four second fuel assemblies were operated. 10. A boiling water reactor core according to claim 9, wherein a plurality of first cells each containing said first fuel assemblies and a plurality of second cells an average infinite multiplication factor of which is smaller than that of said first cells and into each of which a control rod for controlling the power of reactor is inserted, are disposed in said central region. 11. A method of charging a reactor with fuel when fuel assemblies mounted in a core are exchanged after said reactor has been shut down, comprising the steps of, withdrawing burnt-out fuel assemblies having no life, rearranging fuel assemblies remaining in said core to make a loading fraction of second fuel assemblies which have completed a first operation cycles of operation of said core to effect burn out of burnable poison therein for operation in at least a second operation cycle in a radially central region of said core to be smaller than a loading fraction of the second fuel assemblies operating in at least the second operation cycle in a peripheral region of said core surrounding said central region, and loading new first fuel assemblies containing burnable poison in said core to make a loading fraction of said new first fuel assemblies mounted in said central region to be greater than a loading fraction of said new first fuel assemblies loaded in said peripheral region, wherein the burnable poison contained by the new first fuel assemblies substantially burns out in the first operation cycle. 12. A method according to claim 11, wherein the reactor is a boiling water reactor.