Coolant flow mixer grid for a nuclear reactor fuel assembly

In a nuclear reactor fuel assembly including a number of longitudinally extending control rod guide thimbles, and transverse grids axially spaced along the thimbles for supporting an array of upstanding fuel rods, a flow mixing nonsupport grid structure is described which is mounted on the thimbles and disposed between a pair of adjacent fuel rod support grids for promoting the mixing of a coolant as it flows longitudinally upwardly along and between the fuel rods. The grid structure comprising: (a) inner interleaved straps arranged in an egg-crate configuration in defining inner cell openings for receiving respective ones of the fuel rods; (b) at least one mixing vane mounted on one of the inner straps associated with each inner cell opening having a fuel rod extending therethrough. The mixing vane projects upwardly and inwardly toward the longitudinal axis of the cell opening for deflecting the upwardly flowing coolant laterally from one fuel rod to an adjacent fuel rod; and (c) a protrusion associated with each mixing vane and mounted on the inner strap and extending into the cell opening toward the longitudinal axis of the cell to a greater extent than the mixing vane so as to prevent damaging impact of the fuel rod with the vane, but not so far as to normally contact the fuel rod. The distance across each inner cell opening between the interleaved straps with the protrusion mounted thereon is greater than the diameter of the one fuel rod received through the inner cell opening such that the fuel rod is unsupported within the cell opening