Patent Number: 051805480
Section: description

DETAILED DESCRIPTION FIG. 1 shows an assembly in which grids of the invention can be used. This assembly 10 comprises a support structure having two end fittings or endpieces 12 and 14 interconnected by guide tubes 16 for receiving the rods of a control cluster (not shown). The assembly also includes fuel rods 18, of which only one is shown. These rods 18 are held at the nodes of a regular array and are supported by grids. At least one of the grids holds the rods 18 longitudinally. This grid may be, in particular, the bottom grid 20 fixed to the guide tubes and provided for this purpose with springs that urge the rods against bosses, which springs may be cut out from the plates constituting the grid 20 or may be fixed thereto. The other grids may include firstly zirconium-based alloy grids 22 including respective belts and providing mechanical strength to the assembly, secondly grids 24 having only a thermohydraulic function of stirring and mixing the coolant, and thirdly a structural grid 20 which holds the fuel rods but which is itself free relative to the guide tubes. The grids 24 are often shorter than the grids 22 and/or not provided with a belt. The stirring grids 24 generally alternate with structural grids 22 in the downstream portion of the assembly 10, i.e., in the top portion thereof. For an assembly in which the fuel rods 18 are distributed at the nodes of a square array, each grid 22 may have the general structure shown in FIG. 2. Such a grid is made up of two crossed sets of plates assembled to half-depth and welded together at their cross-points. The grids 22, or at least those of the grids that are furthest downstream in the flow direction, are provided with fins 26 for generating turbulence or transverse flows. Each of these fins 26 is advantageously made in such a manner as to engage only one cell in the grid, as shown in FIG. 2 or in the above-mentioned French patent. For example, each plate may be provided with fins 26 that all slope to the same side, and that are distributed at intervals along a plate equal to twice the size of a cell, each extending from a cross-point towards the middle of the wall of the associated cell. The belt 28 is generally made up of four external plates that are assembled together and provided with sloping tongues 30 and rounded corners to avoid catching between the grids of two adjacent assemblies when an assembly is being installed. In accordance with the invention, the internal cells for receiving respective rods 18 include inwardly-projecting abutment means on their four faces, the abutment means delimiting a passage which is larger than the dimensions of a rod. On each face separating two internal cells of this type, these abutment means are constituted by two scoop-shaped portions that are cut out and stamped. In the particular case shown in FIGS. 3 and 4, the abutment means formed in a portion of plate constituting the wall between two cells are constituted by two portions 28 and 30 shaped like half-buttons on a base that is approximately semicircular, and they are in alignment in the coolant flow direction, while projecting in opposite directions. The two scoops are of the same size. Their semicircular bases point downstream. The downstream scoop 28 is delimited by a slot 32 cut out in the plate. The upstream scoop 30 terminates in a deformed portion of the upstream edge of the plate. As can be seen in FIG. 4, the downstream scoop 28 projects in the direction opposite to the slope direction of the fin 26 carried on the same wall. The embodiment shown in FIGS. 5 and 6 differs from the preceding embodiment as follows: The downstream scoop 28a opens in the direction opposite to the upstream scoop 30a and extends to the downstream edge of the plate, which plate is consequently not slotted; and the scoops 28a and 30a are offset relative to the center line 33 of the wall so as to prevent the scoop 28a from interfering with the fin 26. As before, the downstream scoop 28a projects in the direction opposite to the fin 26. In the variant embodiment shown in FIG. 5a, the scoops 28a and 30a are identical to those of FIG. 5, but they are aligned in a direction 36 which slopes relative to the general flow direction. In the embodiment shown in FIGS. 7 and 8, the scoops 28b and 30b are disposed head-to-tail in the direction opposite to the scoops shown in FIGS. 5 and 6. Their open ends face each other. They are separated by a slot 32b which is generally wider than the slot shown in FIG. 3. Instead of being orthogonal to the general flow direction, the slot 32b could slope relative thereto in order to put the scoops in a relative disposition comparable to that of FIG. 5a. As mentioned above, the invention is also applicable to grids having a thermohydraulic stirring function only, and not having a belt. Under such circumstances, the outermost cells may be provided with abutment means in the form of fractions of a scoop only. FIGS. 9 and 11 show one possible structure for abutment means suitable for use on a grid 24 that has a thermohydraulic function only and does not have a belt, with its plates projecting beyond the last plate that they cross, and projecting over a length that is less than the normal size 40 for the wall of an internal cell. This plate shortening has the advantage of preventing any catching between the thermohydraulic grids of two adjacent assemblies since the plane "footprint" of the grid is smaller than the envelope of the rods 18. In the embodiment of FIGS. 9 to 11, the terminal portion includes a single partial scoop 28c having the same disposition as the scoop 28b of FIGS. 7 and 8 but offset therefrom and truncated by the shortening of the plate. This scoop points in the same direction as the fin 26 so as to prevent the rod 18 from coming into contact with the fin. It would also be possible to provide a second partial scoop, similar to the scoop 30b of FIGS. 7 and 8 and separated from the scoop 28c by the slot 32c. However, in most cases there is no need for a scoop that points in the direction opposite to the fin. The embodiment shown in FIGS. 12 and 13 differs from the preceding embodiment in that the entire terminal portion of each plate situated beneath the slot 32c is eliminated. In contrast, the plate is a little longer than the plate shown in FIG. 9, so that the scoop 28d constitutes a complete half-button. By way of example, it may be mentioned that the scoops may project by about 1.1 mm to about 1.2 mm having a base diameter of about 7 mm in a grid that provides a thermohydraulic function only, and a height of 10 mm. The scoop may be in the form of a truncated circular cone terminated by a flat having a diameter of 1.5 mm to 2 mm.