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 an internal core structure of the nuclear power reactor; anda plurality of elongated, extruded fuel elements supported by the frame, each of said plurality of fuel elements comprising:a fuel kernel comprising fuel material disposed in a matrix of metal non-fuel material, the fuel material comprising fissile material, anda cladding surrounding the fuel kernel;wherein each of the fuel elements has a multi-lobed profile that forms spiral ribs,wherein the plurality of fuel elements provide all of the fissile material of the fuel assembly,wherein each of the plurality of fuel elements is disposed in a different grid position of a grid pattern defined by the frame such that a subset of the plurality of fuel elements are disposed along an outer perimeter of the grid pattern,wherein the cladding at a tip of at least one spiral rib of at least one outer side of the cladding on at least some of the fuel elements disposed along an outer perimeter of the grid pattern is thinner than the cladding at other tips of the same fuel element. 2. The fuel assembly of claim 1, wherein:the frame comprises a shroud such that all of the plurality of fuel elements are disposed inside the shroud, andthe laterally shortened outer sides of the cladding contact the shroud. 3. The fuel assembly of claim 1, wherein, in a cross section of the fuel assembly that is perpendicular to an axial direction of the fuel elements, an area of each of the fuel kernels of the at least some of the fuel elements disposed along an outer perimeter of the grid pattern is smaller than an area of at least one of the fuel kernels of in a remainder of the plurality of fuel elements. 4. The fuel assembly of claim 1, wherein the fuel material comprises ceramic fuel material disposed in the matrix of metal non-fuel material. 5. The fuel assembly of claim 1, wherein the cladding is at least 0.4 mm thick throughout each of the plurality of fuel elements. 6. The fuel assembly of claim 1, wherein:the fuel assembly is thermodynamically designed and physically shaped for operation in a conventional land-based nuclear power reactor of a conventional nuclear power plant having a reactor design that was in actual use before 2013; andthe frame is shaped and configured to fit into the land-based nuclear power reactor in place of a conventional uranium oxide fuel assembly for said reactor. 7. The fuel assembly of claim 1, wherein the spiral ribs of adjacent ones of the plurality of fuel elements periodically contact each other over the axial length of the fuel elements, such contact helping to maintain the spacing of the fuel elements relative to each other. 8. The assembly of claim 1, wherein a portion of the fuel assembly that supports the subset of the elongated fuel elements is inseparable from a portion of the fuel assembly that supports the rest of the plurality of fuel elements. 9. The fuel assembly of claim 1, wherein:the grid pattern defines a 17×17 pattern of grid positions; andguide tubes occupy grid positions at row, column positions: 3,6; 3,9; 3,12; 4,4; 4;14; 6,3; 6,15; 9,3; 9,15; 12,3; 12,15; 14,4; 14,14; 15,6; 15,9; and 15,12. 10. 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 an internal core structure of the nuclear power reactor; anda plurality of elongated, extruded fuel elements supported by the frame, each of said plurality of fuel elements comprising:a fuel kernel comprising fuel material disposed in a matrix of metal non-fuel material, the fuel material comprising fissile material, anda cladding surrounding the fuel kernel;wherein each of the fuel elements has a multi-lobed profile that forms spiral ribs,wherein the plurality of fuel elements provide all of the fissile material of the fuel assembly,wherein each of the plurality of fuel elements is disposed in a different grid position of a grid pattern defined by the frame such that a subset of the plurality of fuel elements are disposed along an outer perimeter of the grid pattern,wherein the cladding at a tip of at least one spiral rib of at least one outer side of the cladding on at least some of the fuel elements disposed along an outer perimeter of the grid pattern is thinner than the cladding at other tips of the same fuel element,wherein at least some of the plurality of fuel elements are separated from adjacent fuel elements by a common centerline-to-centerline distance, andwherein a circumscribed diameter of the at least some of the plurality of fuel elements equals the centerline-to-centerline distance.