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 comprisinga 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 a moderator: fuel ratio in a region of the fuel elements is 2.4 or less,wherein the moderator: fuel ratio is an area ratio within a cross-section that is perpendicular to longitudinal axes of the plurality of fuel elements and extends through the plurality of fuel elements, the area ratio is a ratio is of: (1) a total area available for moderator flow for the plurality of fuel elements to (2) a total area of the fuel kernels of the plurality of fuel elements; andwherein a plurality of said plurality of fuel elements each has a spirally twisted, multi-lobed profile that defines a plurality of spiral ribs. 2. The fuel assembly of claim 1, wherein:the plurality of elongated, extruded fuel elements comprise a plurality of elongated, extruded metal fuel elements;the fuel material comprises metal fuel material; andthe fuel kernel comprises a metal fuel alloy kernel comprising an alloy of the metal fuel material and the matrix of metal non-fuel material. 3. The fuel assembly of claim 2, wherein the kernel comprises δ-phase UZr2. 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 plurality of elongated fuel elements provide at least 60% of a total volume of all fuel elements of the fuel assembly. 6. The fuel assembly of claim 1, wherein an average thickness of the cladding is at least 0.6 mm. 7. The fuel assembly of claim 1, wherein the fuel assembly is thermodynamically designed and physically shaped for operation in a land-based nuclear power reactor. 8. A land-based nuclear power reactor comprising the fuel assembly of claim 7. 9. The fuel assembly of claim 7, wherein:the land-based nuclear power reactor comprises a conventional nuclear power plant having a reactor design that was in actual use before 2010; 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. 10. The fuel assembly of claim 1, wherein the respective fuel kernels of the plurality of fuel elements are formed via sintering of the fuel material and metal non-fuel material. 11. The fuel assembly of claim 1, wherein, with respect to a plurality of said plurality of fuel elements:the fuel material of the fuel kernel is enriched to 20% or less by uranium-235 and/or uranium-233 and comprises between a 20% and 30% volume fraction of the fuel kernel; andthe metal non-fuel material of the fuel kernel comprises between a 70% and 80% volume fraction of the fuel kernel. 12. The fuel assembly of claim 11, wherein, with respect to the plurality of said plurality of fuel elements, the fuel material enrichment is between 15% and 20%. 13. The fuel assembly of claim 11, wherein, with respect to the plurality of said plurality of fuel elements, the metal non-fuel material of the fuel kernel comprises zirconium. 14. The fuel assembly of claim 1, wherein, with respect to a plurality of said plurality of fuel elements:the fuel material of the fuel kernel comprises plutonium;the metal non-fuel material of the fuel kernel comprises zirconium; andthe metal non-fuel material of the fuel kernel comprises between a 70% and 97% volume fraction of the fuel kernel. 15. The fuel assembly of claim 1, wherein the fuel material comprises a combination of: uranium and thorium; plutonium and thorium; or uranium, plutonium, and thorium. 16. The fuel assembly of claim 1, wherein the cladding of a plurality of said plurality of fuel elements is metallurgically bonded to the fuel kernel. 17. The fuel assembly of claim 1, wherein the metal non-fuel material of a plurality of said plurality of fuel elements comprises aluminum. 18. The fuel assembly of claim 1, wherein the metal non-fuel-material of a plurality of said plurality of fuel elements comprises a refractory metal. 19. The fuel assembly of claim 1, wherein the cladding of a plurality of said plurality of fuel elements comprises zirconium. 20. The fuel assembly of claim 1, wherein the spiral ribs of adjacent ones of said 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. 21. The fuel assembly of claim 1, wherein the multi-lobed profile comprises concave areas between adjacent lobes. 22. The fuel assembly of claim 1, wherein:the multi-lobed profile comprises lobe tips and intersections between adjacent lobes, and with respect to said plurality of said plurality of fuel elements, the cladding is thicker at the tips than at the intersections. 23. The fuel assembly of claim 1, wherein the fuel assembly comprises a burnable poison. 24. The fuel assembly of claim 23, wherein at least one of the plurality of elongated fuel elements comprises the burnable poison. 25. The fuel assembly of claim 1, further comprising a plurality of UO2 fuel elements supported by the frame, each of said plurality of UO2 fuel elements comprising UO2 fuel. 26. The fuel assembly of claim 25, wherein at least some of the plurality of elongated UO2 fuel elements are positioned laterally outwardly from the plurality of elongated fuel elements. 27. The fuel assembly of claim 26, further comprising a shroud that separates coolant flow past the plurality of elongated UO2 fuel elements from coolant flow past the plurality of elongated fuel elements. 28. The fuel assembly of claim 25, wherein the UO2 fuel has less than 15% U-235 enrichment.