disposed slot 48 these mixing vanes are disposed so as to provide the desired directional flow of the fluid coolant as explained heretofore each vane shields a small opening or window 35 which according to the preferred embodiment is formed under the bottom end in each vane and directly above and adjacent to the junction of the intersecting strips while the windows 35 are shown as oval other shapes and configurations such as rectangular semi circular square etc can be employed it is also possible to locate the window 35 and weld nugget 26 at different elevations within the grid 14 the function of the window 35 in each case is to create material and provide clearance for welding the strips 46 6 39 together the placement of the weld 26 shielded by the vane 28 substantially eliminates flow separation on the downstream side of the mixing vane this results in an improvement of the vane 39 s fluid mixing capability between subchannels and rod heat transfer ability downstream of the vanes the intersecting joints are formed in the usual manner by providing strips 46 46 39 with complementary slots 48 which are oriented as shown in fig7 11 for engagement slots 48 may be tapered at their ends if intermediate welds are required for additional grid strength the vane strips 46 46 39 are provided with an.