Patent Number: 
Section: claims

1. A spacer grid for a nuclear fuel assembly including a plurality of elongated fuel rods and a coolant flowing over the fuel rods, the spacer grid comprising:a plurality of interlocking grid straps forming a plurality of four-walled cells structured to support the fuel rods therein, the four-walled cells each including four cell walls having a longitudinal axis; anda plurality of mixing vanes extending from each of the grid straps, each of the mixing vanes including a middle ligament directly extending from a top portion of a corresponding one of the grid straps, an upper piece extending from an upper portion of the middle ligament, and a lower piece extending from a lower portion of the middle ligament,wherein each of the upper piece and the lower piece has at least one of a predetermined bend angle and a predetermined degree of twist, and the middle ligament has at least one of a predetermined rotation angle and a predetermined degree of twist, in order to provide a predetermined affect on the coolant as it flows over the fuel rods and through the fuel assembly. 2. The spacer grid of claim 1, wherein the middle ligament extends in a substantially orthogonal direction with respect to the longitudinal axis of the cell wall. 3. The spacer grid of claim 1, wherein at least one of the upper piece and the lower piece of at least one of the mixing vanes is bent leftwardly or rightwardly with respect to the longitudinal axis of the cell wall, wherein the longitudinal axis is parallel to a vertical coolant flow stream through the fuel assembly. 4. The spacer grid of claim 1, wherein each of the grid straps has a corresponding plane; and wherein the middle ligament of at least one of the mixing vanes is rotated with respect to the plane. 5. The spacer grid of claim 1, wherein the middle ligament includes at least one pivot point; and wherein the middle ligament is twisted with respect to at least one of the at least one pivot point. 6. The spacer grid of claim 1, wherein at least one of the upper piece and the lower piece of at least one mixing vane is twisted. 7. The spacer grid of claim 1, wherein the upper piece is bent towards the four-walled cell structured to support a fuel rod; and wherein the lower piece is bent away from the four-walled cell structured to support a fuel rod, wherein both the upper piece and lower piece are bent with respect to the same longitudinal axis in order to provide the mixing vane with a generally S-shaped configuration. 8. The spacer grid of claim 1, wherein the upper piece is bent towards an adjacent one of the fuel rods; and wherein the lower piece is also bent toward the adjacent one of the fuel rods, in order to provide the mixing vane with a generally parabolic shape. 9. The spacer grid of claim 1, wherein one said mixing vane is extending from each cell wall of the spacer grid. 10. The spacer grid of claim 1, wherein the grid straps interlock in a generally perpendicular configuration in order to form the four-walled cells; and wherein at least one of the four-walled cells of the spacer grid includes four mixing vanes. 11. A nuclear fuel assembly comprising:a plurality of elongated fuel rods;anda plurality of spacer grids for securing the elongated fuel rods in an organized array, at least one of the plurality of spacer grids comprising:a plurality of interlocking grid straps forming a plurality of four-walled cells for supporting the fuel rods therein, each of the four-walled cells including four cell walls having a longitudinal axis, anda plurality of mixing vanes extending from each of the grid straps, each of the mixing vanes including a middle ligament directly extending from a top portion of a corresponding one of the grid straps, an upper piece extending from an upper portion of the middle ligament, and a lower piece extending from a lower portion of the middle ligament,wherein each of the upper piece and the lower piece has at least one of a predetermined bend angle and a predetermined degree of twist, and the middle ligament has at least one of a predetermined rotation angle and a predetermined degree of twist, in order to provide a predetermined affect on a coolant as it flows over the fuel rods and through the fuel assembly. 12. The nuclear fuel assembly of claim 11, wherein the middle ligament extends in a substantially orthogonal direction with respect to the longitudinal axis of the cell wall. 13. The nuclear fuel assembly of claim 11, wherein at least one of the upper piece and the lower piece of at least one of the mixing vanes is bent leftwardly or rightwardly with respect to the longitudinal axis of the cell wall, wherein the longitudinal axis is parallel to a vertical coolant flow stream through the fuel assembly. 14. The nuclear fuel assembly of claim 11, wherein each of the grid straps has a corresponding plane; and wherein the middle ligament of at least one of the mixing vanes is rotated with respect to the plane. 15. The nuclear fuel assembly of claim 11, wherein the middle ligament includes at least one pivot point; and wherein the middle ligament is twisted with respect to at least one of the at least one pivot point. 16. The nuclear fuel assembly of claim 11, wherein at least one of the upper piece and the lower piece of at least one mixing vane is twisted. 17. The nuclear fuel assembly of claim 11, wherein the upper piece is bent towards the four-walled cell structured to support a fuel rod; and wherein the lower piece is bent away from the four-walled cell structured to support a fuel rod, wherein both the upper piece and lower piece are bent with respect to the same longitudinal axis in order to provide the mixing vane with a generally S-shaped configuration. 18. The nuclear fuel assembly of claim 11, wherein the upper piece is bent towards an adjacent one of the fuel rods; and wherein the lower piece is also bent toward the adjacent one of the fuel rods, in order to provide the mixing vane with a generally parabolic shape. 19. The nuclear fuel assembly of claim 11, wherein one said mixing vane is extending from each cell wall of the spacer grid. 20. The nuclear fuel assembly of claim 11, wherein the grid straps interlock in a generally perpendicular configuration in order to form the four-walled cells; and wherein at least one of the four-walled cells of the spacer grid includes four mixing vanes. 21. The spacer grid of claim 1, wherein the bend angle and/or the degree of twist of the upper piece being independent from the bend angle and/or the degree of twist of the lower piece. 22. The nuclear fuel assembly of claim 11, wherein the bend angle and/or the degree of twist of the upper piece being independent from the bend angle and/or the degree of twist of the lower piece. 23. A spacer grid for a nuclear fuel assembly including a plurality of elongated fuel rods and a coolant flowing over the fuel rods, the spacer grid comprising:a plurality of interlocking grid straps forming a plurality of four-walled cells structured to support the fuel rods therein, the four-walled cells each including four cell walls having a longitudinal axis; anda plurality of mixing vanes extending from each of the grid straps, each of the mixing vanes including a middle ligament extending from a top portion of a corresponding one of the grid straps, an upper piece extending from an upper portion of the middle ligament, and a lower piece extending from a lower portion of the middle ligament,wherein each of the upper piece and the lower piece has at least one of a predetermined bend angle and a predetermined degree of twist, the bend angle and/or the degree of twist of the upper piece being independent from the bend angle and/or the degree of twist of the lower piece, and the middle ligament has at least one of a predetermined rotation angle and a predetermined degree of twist, in order to provide a predetermined affect on the coolant as it flows over the fuel rods and through the fuel assembly. 24. The spacer grid of claim 23 wherein the middle ligament is directly extending from a top portion of a corresponding one of the grid straps. 25. A nuclear fuel assembly comprising:a plurality of elongated fuel rods; anda plurality of spacer grids for securing the elongated fuel rods in an organized array, at least one of the plurality of spacer grids comprising:a plurality of interlocking grid straps forming a plurality of four-walled cells for supporting the fuel rods therein, each of the four-walled cells including four cell walls having a longitudinal axis, anda plurality of mixing vanes extending from each of the grid straps, each of the mixing vanes including a middle ligament extending from a top portion of a corresponding one of the grid straps, an upper piece extending from an upper portion of the middle ligament, and a lower piece extending from a lower portion of the middle ligament,wherein each of the upper piece and the lower piece has at least one of a predetermined bend angle and a predetermined degree of twist, the bend angle and/or the degree of twist of the upper piece being independent from the bend angle and/or the degree of twist of the lower piece, and the middle ligament has at least one of a predetermined rotation angle and a predetermined degree of twist, in order to provide a predetermined affect on a coolant as it flows over the fuel rods and through the fuel assembly. 26. The nuclear fuel assembly of claim 25, wherein the middle ligament is directly extending from a top portion of a corresponding one of the grid straps. 27. A spacer grid for a nuclear fuel assembly including a plurality of elongated fuel rods and a coolant flowing over the fuel rods, the spacer grid comprising:a plurality of interlocking grid straps forming a plurality of four-walled cells structured to support the fuel rods therein, the four-walled cells each including four cell walls having a longitudinal axis; anda plurality of mixing vanes extending from each of the grid straps, each of the mixing vanes including a middle ligament extending from a top portion of a corresponding one of the grid straps, an upper piece extending from an upper portion of the middle ligament, and a lower piece extending from a lower portion of the middle ligament,wherein each of the upper piece and the lower piece has at least one of a predetermined bend angle and a predetermined degree of twist, and the middle ligament has at least one of a predetermined rotation angle and a predetermined degree of twist, in order to provide a predetermined affect on the coolant as it flows over the fuel rods and through the fuel assembly, and wherein the upper piece and lower piece are structured such that the upper and lower pieces are not directly extending from a top portion of a corresponding one of the grid straps. 28. The spacer grid of claim 27 wherein the middle ligament is directly extending from a top portion of a corresponding one of the grid straps. 29. The spacer grid of claim 27 wherein the bend angle and/or the degree of twist of the upper piece being independent from the bend angle and/or the degree of twist of the lower piece.