Patent Number: 
Section: claims

1. A fuel assembly for use in an internal core structure of a nuclear power reactor, the assembly comprising:a frame shaped and configured to fit within the nuclear power reactor internal core structure; anda plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle, each of the fuel elements comprising fissile material;wherein as viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle define a substantially circular perimeter,wherein the plurality of fuel elements are arranged into a mixed grid pattern that includes a first, rectangular grid pattern and a second, triangular grid pattern,wherein each of the plurality of fuel elements comprises a longitudinal centerline, andwherein the longitudinal centerlines of the fuel elements of the second, triangular grid pattern are separated from the longitudinal centerlines of adjacent fuel elements of the second, triangular grid pattern by a centerline-to-centerline distance, and a circumscribed diameter of the fuel elements in the second, triangular grid pattern equals the centerline-to-centerline distance. 2. The fuel assembly of claim 1, wherein the frame is shaped and configured to fit within a pressure tube of a CANDU reactor. 3. The fuel assembly of claim 1, wherein each of the plurality of fuel elements has substantially the same circumscribed diameter. 4. The fuel assembly of claim 1, wherein the plurality of fuel elements are arranged in concentric circles. 5. The fuel assembly of claim 1, wherein the fuel elements comprise extruded fuel elements. 6. The fuel assembly of claim 1, wherein the first, rectangular grid pattern and the second, triangular grid pattern at least partially alternate with one another. 7. The fuel assembly of claim 1, wherein the longitudinal centerlines of the fuel elements of the first, rectangular grid pattern are separated from the longitudinal centerlines of adjacent fuel elements of the first, rectangular grid pattern by a common centerline-to-centerline distance, anda circumscribed diameter of the fuel elements in the first, rectangular grid pattern equals the centerline-to-centerline distance. 8. The fuel assembly of claim 1, wherein each of the fuel elements has a multi-lobed profile that includes ribs. 9. The fuel assembly of claim 8, wherein the ribs comprise spiral ribs. 10. The fuel assembly of claim 9, wherein the ribs of adjacent fuel elements periodically contact one another over the axial length of the fuel elements to at least partially maintain the spacing of the fuel elements relative to each other. 11. The fuel assembly of claim 1, wherein the plurality of fuel elements consists of 61 fuel elements. 12. The fuel assembly of claim 1, wherein the frame comprises a structure circumscribing the fuel rod bundle, such that all of the fuel elements are located inside the structure. 13. The fuel assembly of claim 12, wherein the structure comprises a shroud. 14. The fuel assembly of claim 13, wherein as viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the shroud defines a cross-section substantially defining a circle or dodecagon. 15. The fuel assembly of claim 1, further comprising a tube circumscribing the plurality of helically twisted fuel elements,wherein as viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the plurality of helically twisted fuel elements occupy greater than about 64% of the internal cross-sectional area of the tube. 16. The fuel assembly of claim 15, wherein the plurality of helically twisted fuel elements occupy greater than about 83% of the internal cross-sectional area of the tube. 17. The fuel assembly of claim 15, wherein the plurality of helically twisted fuel elements occupy between about 83% and about 95% of the internal cross-sectional area of the tube. 18. 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 of a nuclear power plant having a reactor design that was in actual use before 2014; andthe frame is shaped and configured to fit into the land-based nuclear power reactor in place of another fuel assembly for said reactor. 19. The fuel assembly of claim 18, wherein the land-based nuclear power reactor is a CANDU reactor. 20. A nuclear reactor, comprising:a core; andone or more fuel assemblies disposed within the core, wherein the one or more fuel assemblies comprises:a frame shaped and configured to fit within the core; anda plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle, each of the fuel elements comprising fissile material;wherein as viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle define a substantially circular perimeter,wherein the plurality of fuel elements are arranged into a mixed grid pattern that includes a first, rectangular grid pattern and a second, triangular grid pattern,wherein each of the plurality of fuel elements comprises a longitudinal centerline, andwherein the longitudinal centerlines of the fuel elements of the second, triangular grid pattern are separated from the longitudinal centerlines of adjacent fuel elements of the second, triangular grid pattern by a centerline-to-centerline distance, and a circumscribed diameter of the fuel elements in the second, triangular grid pattern equals the centerline-to-centerline distance. 21. The nuclear reactor of claim 20, wherein the nuclear reactor is a CANDU reactor comprising pressure tubes, and the frame is shaped and configured to fit within the pressure tubes. 22. The nuclear reactor of claim 20, wherein each of the plurality of fuel elements has substantially the same circumscribed diameter. 23. The nuclear reactor of claim 20, wherein the plurality of fuel elements are arranged in concentric circles. 24. The nuclear reactor of claim 20, wherein the first, rectangular grid pattern and the second, triangular grid pattern at least partially alternate with one another. 25. The nuclear reactor of claim 20, wherein each of the fuel elements has a multi-lobed profile that includes spiral ribs. 26. The nuclear reactor of claim 25, wherein the ribs of adjacent fuel elements periodically contact one another over the axial length of the fuel elements to at least partially maintain the spacing of the fuel elements relative to each other. 27. The nuclear reactor of claim 20, wherein the fuel elements comprise extruded fuel elements. 28. The nuclear reactor of claim 20, wherein the frame comprises a structure circumscribing the fuel rod bundle, such that all of the fuel elements are located inside the structure. 29. The nuclear reactor of claim 28, wherein the structure comprises a shroud. 30. The nuclear reactor of claim 29, wherein as viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the shroud defines a cross-section substantially defining a circle or dodecagon.