Patent Number: 
Section: claims

1. A fuel assembly for a nuclear reactor comprising:a parallel 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 support cell of the support grids to maintain a lateral spacing between fuel rods, at least one of the support grids comprising;an egg-crate base grid having a plurality of orthogonal 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 between the intersections of the four adjacent straps forming a wall of the corresponding support cell, anda lock-support sleeve configured to fit within at least one of the support cells and in a first orientation loosely receive the nuclear fuel rod therethrough and in a second orientation place a transverse pressure on the fuel rod to restrain the fuel rod axially, the lock-support sleeve being rotatable within the at least one of the support cells between the first orientation and the second orientation. 2. The nuclear fuel assembly of claim 1 including means for restraining rotation of the lock-support sleeve when the lock-support sleeve is rotated to the second orientation. 3. The nuclear fuel assembly of claim 2 wherein the means for restraining rotation of the lock-support sleeve comprises one of a male or female lock member on at least one wall of the support cell and another of the male or female lock member on at least one wall of the lock-support sleeve. 4. The nuclear fuel assembly of claim 3 wherein the male and female lock member are respectively a protrusion and a hole wherein the protrusion is sized to fit within the hole when aligned. 5. The nuclear fuel assembly of claim 2 wherein the means for restraining rotation of the lock-support sleeve when the lock-support sleeve is rotated to the second orientation also restrains the axial movement of the lock-support sleeve relative to the support cell when the lock-support sleeve is in the second orientation. 6. The nuclear fuel assembly of claim 1 wherein the lock-support sleeve is a quasi four sided sleeve having generally rounded corners that are bulged out radially with walls of the lock-support sleeve extending between the bulged out corners and configured so that when the lock-support sleeve is rotated from the first orientation where the corners are substantially aligned with the intersection between straps to the second orientation wherein the corners are substantially aligned with a midsection in the walls of the support cells between the intersection between the adjacent straps, at least two walls of the lock-support sleeve bend radially inward to place a lateral force on the corresponding fuel rod and restrain the fuel rod axially. 7. The nuclear fuel assembly of claim 6 wherein when the lock-support sleeve is rotated to the second orientation all four walls of the lock-support sleeve bend radially inward to place a lateral force on the corresponding fuel rod and restrain the fuel rod axially. 8. The nuclear fuel assembly of claim 6 wherein at least one wall of the lock-support sleeve that bends radially inward makes contact with the fuel rod over the entire height of the lock-support sleeve wall. 9. The nuclear fuel assembly of claim 1 wherein a height of a wall of the support cells in the axial direction is longer than the corresponding height of a wall of the lock-support sleeve. 10. The nuclear fuel assembly of claim 1 wherein the support cells having a lock-support sleeve fit therein have a removable stop appended to a lower portion of at least one wall of the support cells outside of a path of the fuel rod and in an axial path of the lock-support sleeve, the removable stop supporting the lock-support sleeve in the axial direction when the lock-support sleeve is in the first orientation. 11. The nuclear fuel assembly of claim 10 wherein the removable stop is a positioning bar that passes through and is supported by openings in the lower portion of two walls of the support cell. 12. The nuclear fuel assembly of claim 11 wherein the positioning bar passes through and is supported by openings in the lower portion of two opposing walls of the support cell. 13. The nuclear fuel assembly of claim 10 wherein the removable stop comprises at least two positioning bars, one on either side of the fuel rod path.