Patent Number: 
Section: claims

1. A fuel assembly for a nuclear reactor comprising:a parallel, spaced array of a plurality of elongated nuclear fuel rods supported between a lower nozzle and an upper nozzle and having an axial length along the elongated dimension of the nuclear fuel rods;a plurality of spaced support grids arranged in tandem along the axial length of the fuel rods, between the upper nozzle and the lower nozzle, at least partially enclosing an axial portion of the circumference of each fuel rod within a corresponding support cell of the support grids to maintain a lateral spacing between fuel rods, at least one of the support grids comprising;a plurality of elongated, intersecting straps that define the support cells at the intersection of each four adjacent straps that surround the nuclear fuel rods, a length of each strap along its elongated dimension, between the intersections of the four adjacent straps forming a wall of the corresponding support cell, with each wall of the cells that surround fuel rods having a lower leading edge and an upper trailing edge that are substantially in the plane of the corresponding strap with at least one of the leading edge and the trailing edge extending in between the intersections of adjacent straps at an angle substantially deviating from an angle of an axis of the elongated dimension of the straps and having substantially the same elevation at adjacent intersections. 2. The fuel assembly of claim 1 wherein the intersection of the straps are interleaved in an egg-crate arrangement. 3. The fuel assembly of claim 1 wherein the walls of the intersecting straps are at substantially a same height at the intersection. 4. The fuel assembly of claim 1 wherein the angle of deviation comprises a first angle and a second angle and the lower leading edge and the upper trailing edge extend from the intersection between straps respectively at the first and second angles. 5. The fuel assembly of claim 4 wherein the first and second angles are in opposite directions. 6. The fuel assembly of claim 4 wherein the first and second angles are equal. 7. The fuel assembly of claim 1 wherein one or both of the leading edge or the trailing edge of at least one wall of at least some of the cells that surround fuel rods starts at a first elevation within proximity of a first of the intersections between straps and extends along the elongated dimension of the corresponding strap to a second elevation before intersecting an adjacent strap. 8. The fuel assembly of claim 7 wherein the second elevation is lower than the first elevation. 9. The fuel assembly of claim 7 wherein the second elevation is higher than the first elevation. 10. The fuel assembly of claim 7 wherein the one or both of the leading edge or the trailing edge extends from the second elevation to a third elevation before the intersection with an adjacent orthogonal strap wherein the third elevation is different than the second elevation. 11. The fuel assembly of claim 10 wherein the second elevation is higher than the first and third elevations on the leading edge and the second elevation is lower than the first and third elevations on the trailing edge. 12. The fuel assembly of claim 1 wherein at least some of the walls of the support cells include a dimple that has a saw tooth opening cut in the wall below the dimple. 13. The fuel assembly of claim 12 wherein the dimple is substantially rounded at a plurality of corners at which it changes direction into the support cell. 14. The fuel assembly of claim 12 wherein at least some of the walls of the support cells that include a dimple that has a saw tooth opening below the dimple also include a saw tooth ligament as the trailing edge. 15. The fuel assembly of claim 1 wherein either the leading edge, the trailing edge or both the leading edge and the trailing edge are formed as a cutout in the wall of the support cell that has at least a first side that is inclined towards a second side with the first and second sides connected at the bottom with a smooth curved transition. 16. The fuel assembly of claim 15 wherein the cutout extends substantially over a width of the wall. 17. The fuel assembly of claim 15 wherein the cutout that forms the leading edge is above a bottom edge of the strap. 18. The fuel assembly of claim 17 wherein the wall has either a dimple or spring protruding into the support cell and the cutout that forms the leading edge is formed between the bottom edge of the strap and the dimple or spring. 19. The fuel assembly of claim 15 wherein the cutout is in the form of a triangle having a relatively flat horizontal base at the top and a rounded lower tip. 20. The fuel assembly of claim 19 wherein the triangle is substantially an isosceles triangle. 21. The fuel assembly of claim 19 wherein an enclosed angle of the first and second sides is between 20 and 160 degrees. 22. The fuel assembly of claim 21 wherein the enclosed angle of the first and second sides is between 60 and 90 degrees. 23. The fuel assembly of claim 19 wherein the radius at the rounded lower tip is defined by a ratio of the radius to the depth which is the height of the triangle and the ratio is between 0.1 and 0.9. 24. The fuel assembly of claim 23 wherein the ratio is between 0.5 and 0.7. 25. The fuel assembly of claim 19 wherein a ratio of a width of the cutout to a width of the wall is between 0.1 and 0.9. 26. The fuel assembly of claim 25 wherein the ratio is between 0.5 and 0.85. 27. The fuel assembly of claim 1 wherein either the leading edge, the trailing edge or both the leading edge and the trailing edge are formed substantially as a half circle with an open end at the top. 28. The fuel assembly of claim 27 wherein a ratio of a height of the half circle to a width of the half circle at its widest point is approximately 0.5. 29. The fuel assembly of claim 27 wherein a ratio of a width of the half circle at its widest point to a width of the support cell is between 0.2 to 0.9. 30. The fuel assembly of claim 29 wherein the ratio is between 0.4 to 0.6. 31. A support grid for a nuclear fuel assembly comprising:a plurality of elongated, intersecting straps some of which define a support cell at the intersection of four adjacent straps that surround a nuclear fuel rod, a length of each strap along its elongated dimension, between the intersections of the four adjacent straps forming a wall of the corresponding support cell, with each wall of the cells that surround fuel rods having a lower leading edge and an upper trailing edge that are substantially in the plane of the corresponding strap with at least one of the leading edge and the trailing edge extending in between the intersections of adjacent straps at an angle substantially deviating from an angle of an axis of the elongated dimension of the straps: andwherein at least some of the walls of the support cells include a dimple or a spring that has a saw tooth, “V” shaped or Triangle shaped opening cut in the wall below the dimple or spring. 32. A fuel assembly for a nuclear reactor comprising:a parallel, spaced array of a plurality of elongated nuclear fuel rods supported between a lower nozzle and an upper nozzle and having an axial length along the elongated dimension of the nuclear fuel rods;a plurality of spaced support grids arranged in tandem along the axial length of the fuel rods, between the upper nozzle and the lower nozzle, at least partially enclosing an axial portion of the circumference of each fuel rod within a corresponding support cell of the support grids to maintain a lateral spacing between fuel rods, at least one of the support grids comprising;a plurality of elongated, intersecting straps that define the support cells at the intersection of each four adjacent straps that surround the nuclear fuel rods, a length of each strap along its elongated dimension, between the intersections of the four adjacent straps forming a wall of the corresponding support cell, with each wall of the cells that surround fuel rods having a lower leading edge and an upper trailing edge that are substantially in the plane of the corresponding strap with at least one of the leading edge and the trailing edge extending in between the intersections of adjacent straps at an angle substantially deviating from an angle of an axis of the elongated dimension of the straps: andwherein at least some of the walls of the support cells include a dimple or a spring that has a saw tooth opening cut in the wall below the dimple or spring. 33. The fuel assembly of claim 32 wherein the angle of deviation comprises a first angle and a second angle and the lower leading edge and the upper trailing edge extend from the intersection between straps respectively at the first and second angles. 34. The fuel assembly of claim 33 wherein the first and second angles are in a same direction. 35. The fuel assembly of claim 32 wherein one or both of the leading edge or the trailing edge of at least one wall of at least some of the cells that surround fuel rods starts at a first elevation at a first of the intersections between straps and extends along the elongated dimension of the corresponding strap to a second elevation before intersecting an adjacent strap. 36. The fuel assembly of claim 35 wherein the second elevation is at the intersection of the adjacent second strap and the second elevation is one of either greater or lesser than the first elevation. 37. The fuel assembly of claim 36 wherein the one or both of the leading edge or the trailing edge extend along the elongated dimension of the strap from the adjacent strap to a third elevation at the intersection of a third orthogonal strap where the third elevation is the other of either the greater or lesser than the second elevation. 38. The fuel assembly of claim 37 wherein the first and third elevations are substantially the same. 39. The fuel assembly of claim 37 wherein one or both of the leading edge or the trailing edge on adjacent parallel straps have a same undulating pattern, but opposing walls are 180 degrees out of phase.