Patent Number: 048881520
Section: summary

The invention relates to a fuel assembly grid for retaining in place the fuel rods of light water reactors. In known examples the grids ensuring the lateral and axial retention of the fuel rods are as a rule formed by small plates assembled in a predefined network. The assembly of small plates, welded at their intersections, bounds cells inside which a fuel rod or a guide tube extends. The small plates have springs and bosses ensuring the lateral and axial retention of the fuel rods. European Patent 0033 263 discloses a grid of this kind. The invention discloses cells formed by cylinder portions of polygonal section which have previously been provided with cut-out parts or recessed over a part of their height and which are so arranged that their sections partially overlap one another: the retention of the rods in the cells is therefore ensured by the parts of the adjacent cylinder portions entering the cell in question. Assemblies of high rigidity can be obtained by assembling the different portions, more particularly by welding. Advantageously the parts which enter the cell are projecting and terminate in flat portions against which the rods bear. The general shape of the sections of the cylinder portions, with the exception of the flat portions is square in a square network or triangular in a hexagonal network. The grid according to the invention has many advantages over that disclosed in the European Patent. It is preferably made from materials such as zircaloy, which is advantageous for its low neutron capture properties, even though its mechanical characteristics are mediocre. Its rigidity, more particularly against buckling, is mainly ensured by the small free lengths of the sides of the geometrical figures resulting from the overlapping of the cells; the cells can therefore be made from a thin metal, something which contributes towards reducing the hydraulic resistance of the grid. Another advantage of the invention is to provide an assembly of simple elementary cells not having many variants, unlike the European Patent, in which the small plates have numerous variants. The manufacture of the grid according to the invention is simplified as a result and its cost reduced. A method is also known in which the rods are retained between the outer surface or overlapping circular rings, some of the rods being also enclosed by another ring. This method was provided for a gas-cooled reactor in which the rods are short, their end plugs being borne by the rings. However, the invention has certain other advantages over this Patent. For example, the width of the flat portions can be selected in relation to the spacing pitch of the rods, to prevent the cylinder portions from being too near to the rods, something which would reduce cooling efficiency and at the same time create hot spots, or to prevent the cylinder portions from being too far away from the rods which they enclose, something which would result in poor guiding of the cooling fluid along the rods. The presence of flat portions enables the rods to be conveniently borne since, even if the cylinder portions are eccentric, the rods still bear against a line on the surface of the flat portions and thus remain correctly retained in place. Lastly, the selection of simple polygons enables the number of bearings to be limited, three bearings being provided for each rod in the construction illustrated in FIG. 7. This reduces total hyperstaticity, something which is very advantageous in the case of rods several meters in length.