Patent Number: 046631178
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and IA a nuclear reactor fuel assembly has multiple (typically 271) fuel pins 10 located in a hexagonal tubular wrapper 11 and stabilised by a series of spaced grids 12 (only one of which is indicated) and relative to which the pins 10 are slidable as a result of differential thermal expansion effects. The wrapper has an internal support cage 13 with upturned parts 14 which engage the walls 15 of unit cells 16 at the periphery of grid 12, the whole of the grid also being formed of unit cells, although not essentially. In FIG. 1 one corner of the wrapper 11 is shown. This corner is free of the cage 13 and it is observed (in distinction from previous designs) that a fuel pin lOA is accommodated in the corner position. The fuel pins are located in the unit cells of the grid by dimples 18. The cage 13 is made up from six vertical limbs 20 integral with top and bottom collars. In FIG. 2 one limb 20 of the cage 13 is shown. This also shows the upturned parts 14 and one displaced peripheral unit cell 16 for locating between two upturned parts 14. The cell 16 has tabs 21 which are used for spot welding the unit cell to the upturned parts. In FIG. 3 a limb 20 of the cage is shown but with only one upturned part 14. A single peripheral unit cell 16 with tabs 21 is "exploded" from the upturned part 14. Spot welds 22 for joining tabs 21 and upturned parts 14 are indicated. Access for welding electrodes impose no problems and the welds formed have a good strength to tensile loads. In FIG. 4 the lowermost grid 12 of the fuel assembly is shown spot welded 42 at its unit cells 16 to a bottom hold down grid 40 which is affixed within the wrapper 11, the lower ends of the fuel pins being anchored to the hold down grid 40. The limbs 20 of the cage 13 terminate at line 41 above which they are welded to tabs 21 of the cells of the lowermost grid 12. It will be noted that the only point of anchorage of the cage to the wrapper is at the lower end of the latter, ie. a position where the neutron flux is comparitively low and hence where irradiation embrittlement of the wrapper material is not a significant problem. In FIG. 5 a peripheral unit cell 50 (which may replace cells 16 of FIG. 3) has a cut-away part 51 which effectively produces a pair of joining tabs 52 at which welds 53 can be made to upturned parts 54. A dimple 55 (of which there are three) is shown in the cell wall to locate a fuel pin in the cell. The length dimension of the cell 50 is chosen to be the same as the depth of the grid in which it is located. This cell 50 allows use of existing grid welding equipment to weld the cell 50 to the upstands 54. FIGS. 6-8 are concerned with a modified form of cage structure. In this embodiment, the limbs 20 of the cage are formed with elastically yieldable formations 60 which are arranged to bear against the internal faces of the wrapper and provide sprung lateral support for the cage within the wrapper. Thus, as shown, the formations 60 may comprise pressed indentations extending longitudinally of the side limb 20 adjacent the edges thereof. The limbs 20 are also provided with flanges 62 at their edges for facilitating spot welding at the junctions between each corner cell 64 and the adjacent edge cells 16 each side thereof. As described previously, the edge cells 16 are provided with joining tabs 21 which are spot-welded at 22 to the upturned parts 14. At the corner/edge cell junctions, the edge cell and corner cell joining tabs 21A, 66 are of differing lengths so that one tab projects beyond the other and overlaps with the flange 52. This arrangement allows the spot welds 22A between the joining tabs 21A, 66 and the spot welds 68 between the joining tab 21A and flange 62 to be made through only double rather than triple thicknesses of material. A similar double thickness arrangement may be adopted between adjacent edge cells by producing them with different length joining tabs and by modifying the upturned parts 14 so that these only overlap the longer joining tabs. This is illustrated in FIG. 8A in which, it will be noted, the upturned parts 14 are interrupted so that the shorter sides of the edge cells can fit between the upper and lower parts 14A, 14B whereas the longer sides of the edge cells overlap the parts 14A, 14B. Thus, each longer joining tab 80 is spot welded at 82 to a respective part 14A or 14B whilst the shorter tab 84 is spot welded at 86 to the adjacent longer tab 80. In each of the embodiments described above it will be seen that the parts 14 of each side limb 20 project inwardly of the wrapper and into the cellular structure of the grids without encroaching on the open cross-sections of the cells. Although not shown in the drawings, those zones of the side limbs 20 extending between the sets of upturned parts 14 may be formed with strengthening ribs. FIG. 9 illustrates another modification in which a two-part cage structure is employed: a lower cage including side limbs 20A and an upper cage including side limbs 20B. The lower cage is secured to the wrapper 11 by bottom support hold down grid 40 whereas the upper cage is secured by a support structure 70 located beneath a mixer pin support grid 72. The lower and upper cages are affixed to the wrapper at only their lower and upper ends respectively and are separated by a gap 74 for allowing differential expansion of the cages. In addition the gap provides a convenient location at which the wrapper 11 can be cut during subsequent dismantling of the sub-assembly for the purpose of reprocessing the irradiated fuel.