Patent Number: 
Section: claims

1. A fuel assembly for use in an internal core structure of a nuclear power reactor, the fuel assembly comprising:a plurality of fuel elements, each fuel element of the plurality of fuel elements having a longitudinal centerline,wherein the plurality of fuel elements are arranged into a predetermined mixed grid pattern that includes a first grid pattern and a second grid pattern different from the first grid pattern, the first grid pattern and the second pattern each being defined by the longitudinal centerlines of the plurality of fuel elements, wherein the first grid pattern does not encompass the pattern of the second grid pattern and the second grid pattern does not encompass the pattern of the first grid pattern,wherein the longitudinal centerlines of a subset of the plurality of fuel elements are separated from the longitudinal centerlines of adjacent fuel elements by a common centerline-to-centerline distance, and a circumscribed diameter of each of the fuel elements of the subset is equal to the centerline-to-centerline distance, the circumscribed diameter being equal to the largest cross-sectional dimension of the fuel element taken along a length of the entire respective fuel element, the length of the fuel element extending parallel to the longitudinal centerline. 2. The fuel assembly of claim 1, wherein the longitudinal centerline of each of the plurality of fuel elements of the first grid pattern is separated from the longitudinal centerline of adjacent fuel elements by the common centerline-to-centerline distance, and a circumscribed diameter of each of the fuel elements of the first grid pattern is equal to the centerline-to-centerline distance. 3. The fuel assembly of claim 2, wherein the longitudinal centerline of each of the plurality of fuel elements of the second grid pattern is separated from the longitudinal centerline of adjacent fuel elements by the common centerline-to-centerline distance, and a circumscribed diameter of each of the fuel elements of the second grid pattern is equal to the centerline-to-centerline distance. 4. The fuel assembly of claim 1, wherein a fuel element located in the first grid pattern with a first set of fuel elements is simultaneously located in the second grid pattern with a second set of fuel elements. 5. The fuel assembly of claim 1, wherein the first grid pattern and the second grid pattern alternate with one another. 6. The fuel assembly of claim 1, wherein the plurality of fuel elements are arranged in concentric circles and wherein the first grid pattern and the second grid pattern alternate with another along one or more of the concentric circles. 7. The fuel assembly of claim 6, wherein the first grid pattern and the second grid pattern alternate along one or more of the concentric circles in a three-to-one basis. 8. The fuel assembly of claim 1, wherein the first grid pattern is one of a rectangular grid pattern, an isometric grid pattern, a parallelogram grid pattern, a triangular grid pattern, and an equilateral triangular grid pattern. 9. The fuel assembly of claim 1, wherein the first grid pattern is a rectangular grid pattern and the second grid pattern is a triangular grid pattern. 10. The fuel assembly of claim 1, wherein the plurality of fuel elements are four-lobe fuel rods, pelleted cylindrical fuel rods, or burnable poison bearing fuel rods. 11. The fuel assembly of claim 1, wherein the plurality of fuel elements are helically twisted fuel elements having a multi-lobed profile that includes spiral ribs. 12. The fuel assembly of claim 1, wherein the plurality of fuel elements further comprise a perimeter circumscribing the plurality of fuel elements. 13. The fuel assembly of claim 12, wherein the perimeter is a shroud. 14. The fuel assembly of claim 12, wherein the plurality of fuel elements consists of 61 fuel elements and wherein the perimeter is a shroud circumscribing the 61 fuel elements, the shroud having a cross-sectional shape of a circle or dodecagon. 15. The fuel assembly of claim 12, wherein the plurality of fuel elements consists of 19 fuel elements and wherein the perimeter has a shape of a circle circumscribing the 19 fuel elements. 16. The fuel assembly of claim 1, wherein the plurality of fuel elements occupy at least 83% of a cross-sectional area of a circle that circumscribes the fuel assembly.