Patent Number: 
Section: claims

1. In a continuous fueling nuclear fission reactor, the improvement comprising:determining an asymptotic core for the nuclear fission reactor; andproviding the nuclear fission reactor with a moderator-to-fuel ratio that is optimally moderated for the asymptotic core of the nuclear fission reactor, the moderator-to-fuel ratio allowing said nuclear fission reactor to be substantially continuously operated in an optimally moderated state. 2. The nuclear fission reactor of claim 1, wherein providing the nuclear fission reactor with a moderator-to-fuel ratio that is optimally moderated for the asymptotic core of the nuclear fission reactor comprises providing a fuel source that is optimally moderated for the asymptotic core of the nuclear fission reactor. 3. The nuclear fission reactor of claim 2, wherein providing a fuel source comprises providing a fuel source with a graphite moderator, said graphite moderator being present in an amount sufficient to allow said nuclear fission reactor to be substantially continuously operated in an optimally moderated state. 4. The nuclear fission reactor of claim 2, wherein providing a fuel source comprises providing a fuel source with a moderator selected from the group consisting of graphite, hydrogen, deuterium, and beryllium, said moderator being present in an amount sufficient to allow said nuclear fission reactor to be substantially continuously operated in an optimally moderated state. 5. A method for designing a continuous fueling nuclear fission reactor, comprising:a) specifying a continuous fueling reactor design, the reactor design including reactor parameters;b) specifying a fuel source design for a fuel source comprising a fuel pebble having a fueled zone and an unfueled zone, wherein specifying a fuel source design comprises determining a radius of an interface between the fueled zone and the unfueled zone of the fuel pebble comprising the fuel source;c) using the specified fuel source design and specified reactor design to determine an effective multiplication factor (keff) for an asymptotic core; andd) repeating (b) and (c) until an optimized fuel source design is developed that yields an asymptotic core with the highest value of the effective multiplication factor (keff); ande) selecting the optimized fuel source design as a fuel source to be used in the continuous fueling nuclear fission reactor so that the continuous fueling nuclear fission reactor can be substantially continuously operated in an optimally moderated state. 6. The method of claim 5, further comprising: verifying that the highest value of the effective multiplication factor (keff) is equal to 1.0 within a specified tolerance; and if the highest value of the effective multiplication factor (keff) is not about equal to one within said specified tolerance, repeating (a), (b), and (c) until the maximum value of the effective multiplication factor (keff) equal to 1.0 within said specified tolerance. 7. The method of claim 5, wherein determining the radius of the interface between the fueled zone and the unfueled zone establishes a number of fuel kernels to be provided in the fueled zone of the fuel pebble. 8. The method of claim 7, wherein the number of fuel kernels in the fueled zone is in a range of about 9,800 to about 10,000 fuel kernels. 9. The method of claim 5, wherein the radius of the interface between the fueled zone and the unfueled zone is in the range of about 2.3 to about 2.4 cm. 10. The method of claim 9, wherein the radius of the interface between the fueled zone and the unfueled zone is about 2.33 cm. 11. The method of claim 9, wherein the radius of the interface between the fueled zone and the unfueled zone is about 2.4 cm. 12. The method of claim 5, wherein said fuel source comprises a fuel pebble having a plurality of fuel kernels provided therein, and wherein determining a fuel source design comprises determining a number of said plurality of fuel kernels to be provided to said fuel pebble. 13. The method of claim 5, wherein said fuel source comprises a moderator, and wherein determining a fuel source design comprises determining an amount of moderator to be provided to said fuel source. 14. The method of claim 5, wherein said fuel source comprises a moderator, and wherein determining a fuel source design comprises determining a composition of moderator to be provided to said fuel source. 15. The method of claim 5, wherein the reactor parameters comprise one or more reactor parameters selected from the group consisting of reactor shape, reactor size, presence of reflectors, location of reflectors, composition of reflectors, size of reflectors, operating power level, refueling rate, fuel source re-circulation pattern, and burn-up cutoff for spent fuel. 16. The method of claim 5, wherein (c) comprises using PEBBED computer code or another code embodying the methods of the PEBBED code. 17. The method of claim 16, further comprising using COMBINE computer code in conjunction with the PEBBED code in an iterative process to determine the effective multiplication factor (keff) for the asymptotic core. 18. The method of claim 5, wherein (c) comprises:i) assigning an initial average burn-up level for the fuel source design;ii) calculating microscopic nuclear data corresponding to the initial average burn-up level;iii) using the microscopic nuclear data to determine corresponding nuclide data;iv) using the corresponding nuclide data to calculate revised microscopic nuclear data; andv) repeating (iii) and (iv) until the nuclide data converge. 19. The method of claim 18, further comprising repeating (iii) and (iv) until the microscopic nuclear data converge. 20. The method of claim 18, wherein said nuclide data comprise a nuclide number density and a nuclide distribution. 21. The method of claim 18, wherein (iii) further comprises using the microscopic nuclear data to determine a corresponding asymptotic loading and burn-up pattern. 22. A method for designing a continuous fueling nuclear fission reactor, comprising:a) specifying a continuous fueling reactor design;b) specifying a fuel source design for a fuel source comprising a fuel pebble having a fueled zone and an unfueled zone, wherein specifying a fuel source design comprises determining a radius of an interface between the fueled zone and the unfueled zone of the fuel pebble comprising the fuel source;c) using the specified reactor design and the specified fuel source design to determine a keff for the fuel source design;d) repeating (b) and (c) until an optimized fuel source design is developed having a keff that is about a maximum for the specified reactor design;e) verifying that the maximum value of keff is equal to one within a specified tolerance and if not equal to one within said tolerance, modifying the design specified in (a) and repeating (b), (c) and (d) until the maximum value of keff is one within the specified tolerance; andf) selecting as an optimal fuel source design the fuel source design having the keff that is about a maximum, the optimal fuel source design allowing said continuous fueling nuclear fission reactor to be substantially continuously operated in an optimally moderated state. 23. The method of claim 22, wherein determining the radius of the interface between the fueled zone and the unfueled zone establishes a number of fuel kernels to be provided in the fueled zone of the fuel pebble. 24. The method of claim 23, wherein the number of fuel kernels in the fueled zone is in a range of about 9,800 to about 10,000 fuel kernels. 25. The method of claim 22, wherein the radius of the interface between the fueled zone and the unfueled zone is in the range of about 2.3 to about 2.4 cm. 26. The method of claim 22 further comprising using PEBBED computer code in determining the keff for the fuel source design. 27. The method of claim 26, further comprising using COMBINE computer code in conjunction with the PEBBED code in an iterative process to determine the keff for the fuel source design. 28. The method of claim 22, wherein specifying an initial reactor design comprises specifying parameters selected from the group consisting of reactor shape, reactor size, presence of reflectors, location of reflectors, operating power level, refueling rate, fuel source re-circulation pattern, and burn-up cutoff for spent fuel. 29. A method for designing a continuous fueling nuclear fission reactor, comprising:a) specifying a continuous fueling reactor design, the reactor design including reactor parameters;b) specifying a fuel source design;c) using the specified fuel source design and specified reactor design to determine an effective multiplication factor (keff) for an asymptotic core; andd) repeating (b) and (c) until an optimized fuel source design is developed that yields an asymptotic core with the highest value of the effective multiplication factor (keff); ande) selecting the optimized fuel source design as a fuel source to be used in the continuous fueling nuclear fission reactor so that the continuous fueling nuclear fission reactor can be substantially continuously operated in an optimally moderated state. 30. A method for designing a continuous fueling nuclear fission reactor, comprising:a) specifying a continuous fueling reactor design;b) specifying a fuel source design;c) using the specified reactor design and the specified fuel source design to determine a keff for the fuel source design;d) repeating (b) and (c) until an optimized fuel source design is developed having a keff that is about a maximum for the specified reactor design;e) verifying that the maximum value of keff is equal to one within a specified tolerance and if not equal to one within said tolerance, modifying the design specified in (a) and repeating (b), (c) and (d) until the maximum value of keff is one within the specified tolerance; andf) selecting as an optimal fuel source design the fuel source design having the keff that is about a maximum, the optimal fuel source design allowing said continuous fueling nuclear fission reactor to be substantially continuously operated in an optimally moderated state.