Patent Number: 
Section: claims

1. A fuel assembly for use in a core of a nuclear power reactor, the assembly comprising:a frame comprising a lower nozzle that is shaped and configured to mount to the nuclear reactor internal core structure; anda plurality of elongated fuel elements supported by the frame, each of said plurality of fuel elements comprising fissile material, wherein as viewed in a cross section that is perpendicular to an axial direction of the fuel assembly, the plurality of fuel elements are arranged into a mixed grid pattern that comprises a first grid pattern and a second grid pattern, the second grid pattern being different from the first grid pattern, wherein:each of the plurality of fuel elements has a common circumscribed diameter;the first grid pattern comprises a pattern of square rows and columns;the centerline-to-centerline distance between the rows and columns equals the common circumscribed diameter;the second grid pattern comprises a pattern of equilateral triangles; anda length of each side of each triangle equals the common circumscribed diameter. 2. The fuel assembly of claim 1, wherein:the plurality of fuel elements includes non-overlapping first, second, and third subsets, each subset including a plurality of the fuel elements,the plurality of fuel elements of the first subset are disposed within respective grid positions defined by the first grid pattern,the plurality of fuel elements of the second subset are disposed within respective grid positions defined by the second grid pattern,the plurality of fuel elements of the third subset are disposed within respective overlapping grid positions, the overlapping grid positions falling within both the first grid pattern and the second grid pattern. 3. The fuel assembly of claim 1, further comprising additional fuel elements supported by the frame, wherein the additional fuel elements are not disposed within any of the grid positions defined by the first or second grid pattern. 4. The fuel assembly of claim 1, wherein: each of the plurality of fuel elements comprises: a fuel kernel comprising fuel material disposed in a matrix of metal non-fuel material, the fuel material comprising fissile material, and a cladding surrounding the fuel kernel, each of the fuel elements has a multi-lobed profile that forms spiral ribs. 5. A fuel assembly for use in a core of a nuclear power reactor, the assembly comprising:a frame comprising a lower nozzle that is shaped and configured to mount to the nuclear reactor internal core structure; anda plurality of elongated fuel elements supported by the frame, each of said plurality of fuel elements comprising fissile material, wherein as viewed in a cross section that is perpendicular to an axial direction of the fuel assembly, the plurality of fuel elements are arranged into a mixed grid pattern that comprises a first grid pattern and a second grid pattern, the second grid pattern being different from the first grid pattern, each elongated fuel element defining a fuel element cross-section perpendicular to the axial direction of the fuel assembly, wherein each elongated fuel element directly contacts at least one adjacent fuel element, and wherein:the frame comprises a shroud surrounding the plurality of fuel elements such that all of the plurality of fuel elements are disposed inside the shroud;the shroud comprises a first sidewall, a second sidewall, and a third sidewall, wherein the first and second sidewalls meet at a first corner and the second and third sidewalls meet at a second corner; andthe fuel assembly comprises a first corner structure disposed inside the shroud adjacent the first corner and in contact with the first sidewall and the second sidewall and a second corner structure disposed inside the shroud adjacent the second corner and in contact with the second sidewall and the third sidewall, wherein each of the first and second corner structures define a corner structure cross-section perpendicular to the axial direction of the fuel assembly that is different from each fuel element cross-section, wherein the first and second corner structures are separate. 6. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure comprises a burnable poison. 7. The fuel assembly of claim 6, wherein at least one of the first corner structure and the second corner structure abuts at least one of the plurality of elongated fuel elements. 8. The fuel assembly of claim 5, wherein each of the plurality of fuel elements has a common circumscribed diameter. 9. The fuel assembly of claim 5, wherein:the first grid pattern comprises a pattern of square rows and columns,the centerline-to-centerline distance between the rows and columns equals the common circumscribed diameter,the second grid pattern comprises a pattern of equilateral triangles, anda length of each side of each triangle equals the common circumscribed diameter. 10. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure defines an inner contour that is partially-cylindrical and abuts one of the plurality of elongated fuel elements. 11. The fuel assembly of claim 10, wherein the inner contour defines an arc of between about 90 degrees and about 310 degrees. 12. The fuel assembly of claim 10, wherein the inner contour defines an arc of between about 150 degrees and about 310 degrees. 13. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure abuts three adjacent elongated fuel elements of said plurality of elongated fuel elements. 14. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure comprises a tubular structure. 15. The fuel assembly of claim 14, wherein at least one of the first corner structure and the second corner structure abuts the shroud and one of the plurality of elongated fuel elements. 16. The fuel assembly of claim 14, wherein at least one of the first corner structure and the second corner structure further comprises a material helically wrapped around the tubular structure. 17. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure defines three concave, partially-cylindrically shaped surfaces, each of which abuts one of the plurality of elongated fuel elements. 18. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure comprises one or more of: a burnable poison, steel, alloys or ceramics of zirconium, and/or uranium, and/or plutonium, and/or thorium. 19. The fuel assembly of claim 5, wherein at least one of the first corner structure and the second corner structure is attached to the shroud. 20. A fuel assembly for use in a core of a nuclear power reactor, the assembly comprising:a frame comprising a lower nozzle that is shaped and configured to mount to the nuclear reactor internal core structure; anda plurality of elongated fuel elements supported by the frame, each of said plurality of fuel elements comprising fissile material, wherein as viewed in a cross section that is perpendicular to an axial direction of the fuel assembly, the plurality of fuel elements are arranged into a mixed grid pattern that comprises a first grid pattern and a second grid pattern, the second grid pattern being different from the first grid pattern, wherein:the frame comprises a shroud having a first planar sidewall and a second planar sidewall connected by a corner region, wherein all of the plurality of fuel elements are disposed inside the shroud;the fuel assembly comprises a corner structure comprising a burnable poison, wherein the corner structure is located outside of the shroud and extends over the corner region of the shroud from the first planar sidewall to the second planar sidewall. 21. The fuel assembly of claim 20, wherein the corner structure is attached to the shroud. 22. A fuel assembly for use in a core of a nuclear power reactor, the assembly comprising:a frame comprising a lower nozzle that is shaped and configured to mount to the nuclear reactor internal core structure; anda plurality of elongated fuel elements supported by the frame, each of said plurality of fuel elements comprising fissile material, wherein as viewed in a cross section that is perpendicular to an axial direction of the fuel assembly, the plurality of fuel elements are arranged into a mixed grid pattern that comprises a first grid pattern and a second grid pattern, the second grid pattern being different from the first grid pattern, wherein:the frame comprises a shroud such that all of the plurality of fuel elements are disposed inside the shroud, wherein the shroud comprises a first sidewall and a second sidewall separate from one another; andthe fuel assembly comprises a corner structure joining the first sidewall to the second sidewall at a corner of the shroud, wherein the corner structure comprises a burnable poison. 23. The fuel assembly of claim 22, wherein the corner structure is attached to the shroud. 24. A fuel assembly for use in a core of a nuclear power reactor, the assembly comprising:a frame comprising a lower nozzle that is shaped and configured to mount to the nuclear reactor internal core structure; anda plurality of elongated fuel elements supported by the frame, each of said plurality of fuel elements comprising fissile material, wherein as viewed in a cross section that is perpendicular to an axial direction of the fuel assembly, the plurality of fuel elements are arranged into a mixed grid pattern that comprises a first grid pattern and a second grid pattern, the second grid pattern being different from the first grid pattern, wherein:the frame comprises a shroud such that all of the plurality of fuel elements are disposed inside the shroud; andthe fuel assembly comprises four corner structures each comprising a burnable poison, wherein the shroud comprises four separate plates joined by said four corner structures.