Patent Number: 042971705
Section: summary

The present invention relates to a device for transversely holding the fuel rods of a nuclear reactor assembly. In a nuclear reactor assembly, the fuel bars, or rods, are arranged parallel to one another in the form of a bundle, within which they are held in a uniform arrangement in order to permit the passage of cooling fluid. In operation, the fuel rods are subjected to vibrations due to the flow of the coolant, to thermal expansion and to swelling due to irradiation. The fuel rods are held in uniform bundles by means of a number of devices providing transverse support, which devices consist of cross-pieces and are produced in the form of grids spaced more or less uniformly along the bundle. The main functions which a grid of this kind must fulfill are: to hold the fuel rods in a precise and uniform transverse network, even under the action of transverse forces, to permit the axial lengthening of the rods, and to hold the rods with a sufficient transverse force to restrict the vibrations of the rods under the effect of the coolant which flows around the rods. Additionally, the grids must oppose flow of the coolant with minimum resistance and, in order to do this, they must present a minimum cross-section in the direction of flow of the coolant. Furthermore, in order to disturb the neutron flux as little as possible, it is necessary to avoid the use of materials with high neutron absorption in the construction of the grids. For this purpose, grids have been envisaged, for holding the fuel rods in nuclear reactor assemblies, comprising pieces for holding or steadying the fuel bars, these pieces being added inside the cells of the grid or cut out of the material constituting the walls of the cells of the grid. The use of different metals for forming the frame of the grid and elements for elastically holding the fuel rods has also been envisaged. However, these devices are of complicated construction and do not make it possible to exert transverse forces, of exactly controlled magnitude, on the rods in order to restrict the vibrations of the rods. According to the invention there is provided a device for transversely holding the fuel rods of a bundle of fuel rods for a nuclear reactor assembly, which rods are arranged parallel to one another, in a uniform arrangement which comprises support means for exerting forces on the rods in transverse directions to restrict vibrations thereof, while permitting axial lengthening of the rods, said support means comprising three subunits comprising grids arranged adjacent one another in the longitudinal direction of the assembly, said subunits comprising: two end grids each formed by small metal plates which are assembled in a uniform network delimiting cells inside which the rods pass, and whose side walls, parallel to the longitudinal axis of the assembly, are formed by said small metal plates arranged transversely, relative to the assembly, so that their width constitutes the thickness of said grid in the longitudinal direction, said end grids being identical and made integral with one another with a longitudinal spacing along the fuel rods, and an intermediate grid comprising small, elastic metal plates, the width of which is less than the width of said plates of said end grids, and which are assembled in a uniform network delimiting cells inside which the rods pass and arranged between said end grids in said longitudinal space provided therebetween, in a staggered manner in a transverse direction relative to said end grids, so that said elastic walls of the cells of said intermediate grid exert on the fuel rods transverse forces which press the rods against some of said walls of said cells of said end grids, said intermediate grid resting on at least one of said end grids. In order to provide a clear understanding of the invention, an embodiment of a device according to the invention will now be described, by way of example only, with reference to the accompanying drawings.