Patent Number: 
Section: claims

1. A nuclear fuel assembly spacer grid having an axial dimension along a vertical axis of an elongated nuclear fuel assembly in which it is to be deployed, the nuclear fuel assembly spacer grid comprising:a plurality of fuel rod support cells having four walls, generally square in cross section, having a length along the axial dimension that is longer than a width of the walls and corners that connect with adjacent fuel rod support cells or a control rod support cell, with an interior of each of the walls of the fuel rod support cells supporting a vertical spring; anda mixing vane connected to an exterior of at least one of the fuel rod support cells, wherein each of the fuel rod support cell walls are bowed inwardly, along an entire length thereof, into an interior of the fuel rod support cell. 2. The nuclear fuel assembly spacer grid of claim 1 wherein the fuel rod support cell walls are bowed symmetrically around an axis parallel to the vertical axis. 3. The nuclear fuel assembly spacer grid of claim 2 wherein at least one of the vertical springs is formed from two vertical slits in a center portion of at least one of the walls of at least one of the fuel rod support cells. 4. The nuclear fuel assembly spacer grid of claim 1 wherein each vertical spring is formed from two vertical slits in a center portion of each of the walls of the fuel rod support cells. 5. The nuclear fuel assembly spacer grid of claim 1 wherein at least one of the vertical springs is coated with a material. 6. The nuclear fuel assembly spacer grid of claim 5 wherein the material is a carbon coating. 7. An elongated nuclear fuel assembly, comprising:a spacer grid having an axial dimension along a vertical axis of the elongated nuclear fuel assembly, the spacer grid comprising:a plurality of fuel rod support cells having four walls, generally square in cross section, having a length along the axial dimension that is longer than a width of the walls and corners that connect with adjacent fuel rod support cells or a control rod support cell, with an interior of each of the walls of the fuel rod support cells supporting a vertical spring; anda mixing vane connected to an exterior of at least one of the fuel rod support cells, wherein each of the fuel rod support cell walls are bowed inwardly, along an entire length thereof, into an interior of the fuel rod support cell; anda fuel rod positioned in contact with at least one of the vertical springs. 8. The elongated nuclear fuel assembly of claim 7 wherein the fuel rod support cell walls are bowed symmetrically around an axis parallel to the vertical axis. 9. The elongated nuclear fuel assembly of claim 7 wherein each vertical spring is formed from two vertical slits in a center portion of each of the walls of the fuel rod support cells. 10. The elongated nuclear fuel assembly of claim 7 wherein the at least one of the vertical springs is coated with a material having a hardness that is equal to or greater than the hardness of a SiC cladding on the fuel rod. 11. The nuclear fuel assembly of claim 10 wherein the material is a carbon coating. 12. The nuclear fuel assembly of claim 8 wherein the at least one of the vertical springs is formed from two vertical slits in a center portion of at least one of the walls of at least one of the fuel rod support cells.