Patent Number: 
Section: claims

1. A method of operating a nuclear reactor having a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors are arranged, said reload fuel assemblies including small-number cycle reload fuel assemblies having a larger infinite multiplication factor than a large-number operation cycle reload fuel assemblies having a smaller infinite multiplication factor than the larger infinite multiplication factor of said small-number cycle reload fuel assemblies, the small-number cycle reload fuel assemblies having been operated for a smaller number of operation cycles than the number of cycles of operation of said large-number cycle reload fuel assemblies, said method comprising: inserting control rods alternately in control cells of a first group and of a second group with respect to time, each control cell of the first group and the second group comprising four small-number cycle reload fuel assemblies having the larger infinite multiplication factor for a period longer than half of a period of an operation cycle. 2. A method of operating a nuclear reactor having a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors and a plurality of initial loading fuel assemblies comprising high-enrichment initial loading fuel assemblies and low-enrichment initial loading fuel assemblies with a lower enrichment factor than an enrichment factor of the high-enrichment initial loading fuel assemblies are arranged, said method comprising: inserting control rods alternately in control cells of a first group and of a second group with respect to time, each control cell of the first group and the second group comprising four fuel assemblies including at least one reload fuel assembly and a plurality of high-enrichment initial loading fuel assemblies for a period longer than half of a period of an operation cycle. 3. The method of operating a nuclear reactor according to  claim 1  or  2 , wherein the control cells in which the control rods are inserted include new fuel assemblies loaded into the reactor core for the operation cycle. claim 1 2 4. The method of operating a nuclear reactor according to  claim 1  or  2 , wherein the control rods are inserted in the control cells of each group at time intervals corresponding to a burn-up of several gigawatts day per ton. claim 1 2 5. The method of operating a nuclear reactor according to  claim 4 , wherein the fuel assemblies other than those forming the control cells in which the control rods are inserted are removed from the reactor core after the completion of the operation cycle. claim 4 6. The method of operating a nuclear reactor according to  claim 4 , wherein most of the fuel assemblies arranged in an outer peripheral region of the reactor core are those having small infinite multiplication factors. claim 4 7. The method of operating a nuclear reactor according to  claim 1 , wherein the fuel assemblies other than those forming the control cells in which the control rods are inserted are removed from the reactor core after the completion of the operation cycle. claim 1 8. The method of operating a nuclear rector according to  claim 2 , wherein the fuel assemblies other than those forming the control cells in which the control rods are inserted are removed from the reactor core after the completion of the operation cycle. claim 2 9. The method of operating a nuclear reactor according to  claim 3 , wherein the fuel assemblies other than those forming the control cells in which the control rods are inserted are removed from the reactor core after the completion of the operation cycle. claim 3 10. The method of operating a nuclear reactor according to  claim 1 , wherein most of the fuel assemblies arranged in an outer peripheral region of the reactor core are those having small infinite multiplication factors. claim 1 11. The method of operating a nuclear reactor according to  claim 2 , wherein most of the fuel assemblies arranged in an outer peripheral region of the reactor core are those having small infinite multiplication factors. claim 2 12. The method of operating a nuclear reactor according to  claim 3 , wherein most of the fuel assemblies arranged in an outer peripheral region of the reactor core are those having small infinite multiplication factors. claim 3 13. A method of operating a nuclear reactor having a reactor core in which a plurality of reload fuel assemblies comprising small-number cycle reload fuel assemblies and large-number cycle reload fuel assemblies having been operated for a larger number of operation cycles than a number of operation cycles of the small-number cycle reload fuel assemblies are arranged, said method comprising: inserting control rods alternately in control cells of a first group and of a second group with respect to time, each control cell of the first group and the second group comprising four reload fuel assemblies of the small-number cycle reload fuel assemblies for a period longer than half of a period of an operation cycle. 14. A method of operating a nuclear reactor having a reactor core in which a plurality of reload fuel assemblies and a plurality of initial loading fuel assemblies comprising high-enrichment initial loading fuel assemblies and low-enrichment initial loading fuel assemblies with a lower enrichment factor than an enrichment factor of the high-enrichment initial loading fuel assemblies are arranged, said method comprising: inserting control rods alternately in control cells of a first group and those of a second group with respect to time, each control cell of the first group and the second group comprising four fuel assemblies formed of at least one of the reload fuel assemblies and a plurality of the high-enrichment initial loading fuel assemblies for a period longer than half of a period of an operation cycle. 15. The method of operating a nuclear reactor according to  claim 1  or  13 , wherein the number of operation cycles of the small-number cycle reload fuel assemblies is not greater than three and the number of operation cycles of the large-number cycle reload fuel assemblies is greater than three. claim 1 13 16. The method of operating a nuclear reactor according to  claim 2  or  14 , wherein each control cell of the first group and the second group comprise no more than two of the reload fuel assemblies. claim 2 14