Patent Number: 044514271
Section: claims

1. In a pressurized water nuclear power reactor having a multiplicity of elongated, square fuel assemblies supported side-by-side to form a generally cylindrical, stationary core which must be periodically refueled between burnup cycles by removing a fraction of the burned assemblies, rearranging the remaining burned assemblies, and inserting fresh assemblies, each of said cycles having a core average burnup of at least about 10,000 Mw-days per tonne, (MWD/T), the arrangement of assemblies to begin a new burnup cycle after refueling, comprising: (a) a generally cylindrical inner core region consisting of approximately two-thirds the total assemblies in the core and forming a figurative checkerboard array having  (b) a generally annular outer region consisting of the remaining assemblies and including at least some but less than two-thirds of the fresh assemblies.  (1) a third component consisting of fresh assemblies and assemblies having burned through only one previous burnup cycle,  (2) a fourth component consisting of assemblies having burned through at least one previous burnup cycle.  (a) permanently removing from the core assemblies having experienced at least three burnup cycles;  (b) figuratively dividing the core into a generally cylindrical inner region consisting of about 145 assembly locations and a generally annular outer region consisting of the remaining assembly locations;  (c) inserting fresh assemblies having permanent burnable poison lattice shims into the inner region and relocating burned assemblies to form a checkerboarded inner region having  (d) inserting fresh assemblies into the outer region and relocating burned assemblies to form a checkerboarded outer region having  (a) figuratively dividing the core into a generally cylindrical inner region consisting of approximately two-thirds the total assembly location in the core, and a generally annular outer region consisting of the remaining assembly locations in the core;  (b) forming a checkerboard array of assemblies in the inner region having  (c) placing the remainder of the assemblies including some but less than two-thirds of the fresh assemblies in the outer region. 2. The assembly arrangement of claim 1 wherein the first component of the inner region checkerboard consists of fresh assemblies and assemblies having burned through only one previous burnup cycle. 3. The assembly arrangement of claim 1 or 2 wherein at least one but less than one-half of the fresh assemblies are on the core periphery. 4. The assembly arrangement of claim 3 wherein at least some of the fresh assemblies in the outer region are unshimmed. 5. The assembly arrangement of claim 3 wherein the shims in the fresh assemblies are permanent lattice shims. 6. The assembly arrangement of claim 3 wherein the burnable poison distribution and reactivity worth in the fresh assemblies are sufficient to deplete at least one reactivity percent during said new burnup cycle and contribute a residual reactivity worth of less than about 0.5 percent at the end of said new cycle. 7. The assembly arrangement of claim 6 wherein at least two-thirds of the fresh assemblies in the inner region contain permanent lattice burnable poison shims. 8. The assembly arrangement of claim 7 wherein the contribution to end of cycle residual reactivity worth is substantially zero. 9. The assembly arrangement of claim 7 wherein the core average burnups of the preceding and said new burnup cycles is between about 10,000 and 14,000 MWD/T. 10. The assembly arrangement of claim 7 wherein the core contains 241 assemblies and the inner region contains 145 assemblies. 11. The assembly arrangement of claim 1 wherein the core average burnup of the preceeding and said new burnup cycles is no greater than about 14,000 MWD/T. 12. The assembly arrangement of claim 2 wherein no more than half the core peripheral locations contain fresh assemblies. 13. The arrangement of claim 12 wherein the core periphery consists only of fresh assemblies and assemblies having burned through only one previous burnup cycle. 14. The assembly arrangement of claim 2 wherein the second checkerboard component in the inner region consists only of assemblies having burned through at least two previous burnup cycles. 15. The assembly arrangement of claim 14 wherein the assemblies in the outer region form a checkerboard having 16. The assembly arrangement of claim 15 wherein the outer region checkerboard third component consist only of fresh assemblies. 17. The assembly arrangement of claim 16 wherein the core has about 241 assembly locations and the inner region consists of about 145 assemblies. 18. In a pressurized water power reactor having about 241 elongated, square fuel assemblies supported side-by-side to form a generally cylindrical stationary core, the core having produced power for at least three previous burnup cycles of at least 10,000 MWD/T each, a method for refueling the core comprising the steps of: 19. In a pressurized water nuclear power reactor having a multiplicity of elongated, square fuel assemblies supported side-by-side to form a generally cylindrical, stationary core which must be periodically refueled between burnup cycles by removing a fraction of the burned assemblies, rearranging the remaining burned assemblies, and inserting fresh assemblies, each of said cycles having a core average burnup of at least about 10,000 MWD/T, the method of arranging the assemblies during refueling, comprising the steps of: