Patent Number: 
Section: claims

1. A method of operating a nuclear core, the method comprising:loading an outer peripheral region extending from an edge of the core toward a center of the core with burnt fuel bundles;loading fuel bundles in a first type of control cell in an inner peripheral region extending from the outer peripheral region toward the center; andloading fuel bundles in a second type of control cell in an inner core region extending from the inner peripheral region toward the center, wherein,the first type and the second type of control cells include only fuel bundles positioned directly adjacent to moveable reactivity control elements, andthe first type of control cell includes,a first fuel bundle having a reactivity that is substantially higher than a reactivity of each fuel bundle of the second type of control cell, anda second fuel bundle having a reactivity that is substantially equal to a fuel bundle of the second type of control cell. 2. The method of claim 1, further comprising:moving control elements in only the second type of control cell to control reactivity in the core for a plurality of operating days while control elements in the first type of control cell remain inserted in the core and stationary. 3. The method of claim 2, further comprising:withdrawing control elements in the first type of control cell only after approximately 3 GWd/ST. 4. The method of claim 1, further comprising:completely withdrawing control elements in only the first type of control cell during the final quarter of the operating cycle. 5. The method of claim 1, wherein the loading the inner periphery creates the first type of control cell including at least two fresh fuel bundles, and wherein the loading the inner core creates the second type of control cell including no fresh fuel bundles. 6. The method of claim 1, wherein the first type of control cell includes a plurality of the first fuel bundles, wherein the first fuel bundles are fresh fuel bundles having different fuel enrichments from each other. 7. The method of claim 1, further comprising:removing approximately half of all fuel bundles in the core before the loadings. 8. The method of claim 1, further comprising:loading the inner core region completely with fuel bundles, wherein adjacent fuel bundles not in the second type of control cell in the inner core region have substantially differing reactivities from each other. 9. The method of claim 8, further comprising:repeating the loading the outer peripheral region and the inner peripheral region until the core is full and so that the core includes more of the second type of control cells than the first type of control cells, wherein the first fuel bundle is a fresh fuel bundle, and wherein the second type of control cells includes no fresh fuel;fully withdrawing all control elements except those in the first and the second type of control cells from the core; andmaintaining all control elements in the core in the first type of control cells while controlling reactivity in the core with only the control elements in the second type of control cells for a plurality of days. 10. The method of claim 8, wherein the first type of control cell is loaded with fuel bundles having a highest reactivity among all fuel bundles in the core. 11. The method of claim 1, wherein the first fuel bundle has a reactivity that is higher by equivalent of approximately 15 to approximately 23 GWd/ST exposure from the second bundle. 12. A method of operating a nuclear core having at least two different types of control cells, with a first type of control cell having a combined reactivity higher than a combined reactivity of the second type of control cell, and with more second type of control cells than the first type of control cells, wherein the first type of control cell includes a fuel bundle having a reactivity that is substantially equal to a fuel bundle of the second type of control cell, the method comprising:moving control elements in only the second type of control cell to control reactivity in the core while control elements in the first type of control cell remain inserted in the core and stationary for a plurality of operating days. 13. The method of claim 12, further comprising:withdrawing control elements in the first type of control cell only after approximately 3 GWd/ST. 14. The method of claim 12, further comprising:completely withdrawing control elements in only the first type of control cell during the final quarter of the operating cycle. 15. The method of claim 12, wherein there are a plurality of the first type of control cells, and wherein all of the first type of control cells are radially outside of the second type of control cells in the core. 16. The method of claim 15, wherein the core is filled with fuel assemblies surrounding the first and the second type of control cells, and wherein adjacent fuel bundles not in the second type of control cell and radially within the first type of control cells in the core have substantially differing reactivities from each other. 17. The method of claim 16, wherein the first type of control cells all include fresh fuel, and wherein the second type of control cells all include only burnt fuel. 18. The method of claim 12, further comprising:fully withdrawing all control elements from the core except for those in the first and the second type of control cells prior to the moving control elements in only the second type of control cell. 19. The method of claim 18, further comprising:partially withdrawing control elements in the first type of control cell only after approximately 3 GWd/ST; andfully withdrawing control elements in the first type of control cell from the core only during the final quarter of the operating cycle. 20. The method of claim 12, wherein the first type of control cells all include fresh fuel, and wherein the second type of control cells all include only burnt fuel.