Patent Number: 
Section: claims

1. A method comprising:propagating a nuclear fission traveling wave burnfront along first and second dimensions within a plurality of nuclear fission fuel subassemblies in a reactor core of a nuclear fission traveling wave reactor;determining an existing shape of the nuclear fission traveling wave burnfront;determining a desired shape of the nuclear fission traveling wave burnfront, andin response to the determined desired shape, controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations in the reactor core to respective second locations in the reactor core to alter the existing shape to resemble the desired shape, wherein the respective first locations and the respective second locations are on opposite sides of the nuclear fission traveling wave burnfront. 2. The method of claim 1, wherein the first dimension is substantially orthogonal to an elongated axis of the plurality of nuclear fission subassemblies. 3. The method of claim 1, wherein:the first dimension includes a radial dimension; andthe desired shape defines the nuclear fission traveling wave burnfront in an axial dimension. 4. The method of claim 1, wherein:the first dimension includes an axial dimension; andthe desired shape defines the nuclear fission traveling wave burnfront in a radial dimension. 5. The method of claim 1, wherein:the first dimension includes a lateral dimension; andthe desired shape defines the nuclear fission traveling wave burnfront in an axial dimension. 6. The method of claim 1, wherein:the first locations include outward locations; andthe second locations include inward locations. 7. The method of claim 1, wherein:the first locations include inward locations; andthe second locations include outward locations. 8. The method of claim 1, wherein controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations includes rotating at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 9. The method of claim 1, wherein controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations includes inverting at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 10. The method of claim 1, wherein the desired shape is based on a selected set of dimensional constraints including a predetermined maximum distance along the second dimension. 11. The method of claim 1, wherein the desired shape is based on a selected set of dimensional constraints and the selected set of dimensional constraints is a function of at least one burnfront criteria. 12. The method of claim 11, wherein the burnfront criteria includes neutron flux. 13. The method of claim 12, wherein the neutron flux is associated with at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 14. The method of claim 11, wherein the burnfront criteria includes neutron fluence. 15. The method of claim 14, wherein the neutron fluence is associated with at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 16. The method of claim 11, wherein the burnfront criteria includes burnup. 17. The method of claim 16, wherein the burnup is associated with at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 18. The method of claim 11, wherein the burnfront criteria includes burnfront location within at least one of the selected ones of the plurality of nuclear fission fuel subassemblies. 19. The method of claim 1, wherein controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations includes controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies spirally along the first dimension from respective first locations to respective second locations. 20. The method of claim 1, wherein the desired shape of the nuclear fission traveling wave burnfront includes a shape chosen from a substantially spherical shape, a shape conforming to a selected continuously curved surface, a shape that is substantially rotationally symmetrical around the second dimension, and a shape having substantial n-fold rotational symmetry around the second dimension. 21. The method of claim 1, wherein the shape of the nuclear fission traveling wave burnfront along the second dimension is asymmetrical. 22. The method of claim 21, wherein the shape of the nuclear fission traveling wave burnfront is rotationally asymmetrical around the second dimension. 23. The method of claim 1, further comprising initiating a nuclear fission traveling wave burnfront with a plurality of nuclear fission traveling wave igniter assemblies. 24. The method of claim 23, further comprising removing at least one of the plurality of nuclear fission traveling wave igniter assemblies prior to controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations. 25. The method of claim 24, wherein removing at least one of the plurality of nuclear fission traveling wave igniter assemblies prior to controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations includes removing at least one of the plurality of nuclear fission traveling wave igniter assemblies from the second locations prior to controllably migrating selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations to respective second locations. 26. The method of claim 1, wherein the desired shape of the nuclear fission traveling wave burnfront includes approximating a Bessel function. 27. The method of claim 1, wherein controllably migrating further includes:controllably migrating a first nuclear fission fuel subassembly of the plurality of nuclear fission fuel subassemblies along the first dimension from a first location of the respective first locations in the reactor core to a second location of the respective second locations in the reactor core, wherein the first nuclear fission fuel subassembly includes fertile fuel to be bred up at the second location and wherein the first location has a lower burn-up rate than the second location. 28. The method of claim 1, wherein controllably migrating further includes:controllably migrating a first nuclear fission fuel subassembly of the plurality of nuclear fission fuel subassemblies along the first dimension from a first location of the respective first locations in the reactor core to a second location of the respective second locations in the reactor core, wherein the first nuclear fission fuel includes spent fuel at the second location and wherein the first location has a higher burn-up rate than the second location. 29. The method of claim 1, wherein controllably migrating further includes migrating each individual one of the plurality of nuclear fission fuel subassemblies based on relative burn-up rates at a first location of the respective first locations on a first side of the nuclear fission traveling wave burnfront and a second location of the respective second locations on opposite sides of the nuclear fission traveling wave burnfront.