Patent Number: 046577335
Section: summary

FIELD OF THE INVENTION The invention relates to a fuel assembly for a nuclear reactor, in which the fuel rods are fixed on the lower end plate of the assembly. BACKGROUND OF THE INVENTION Fuel assemblies for nuclear reactors and in particular the fuel assemblies for water nuclear reactors consist of a bundle of parallel fuel rods held by a structure consisting of spacer grids, two end plates, and guide tubes which connect the spacer grids and the end plates. The fuel rods, which are cylindrical and of great length, are engaged in the spacer grids which hold them in position in the bundle. The spacer grids generally provide both the transverse positioning and the axial positioning of the rods which are shorter than the guide tubes which are substituted for certain rods of the assembly. The ends of the rods are therefore free and are located at a certain distance from the end plates. In order to provide both the axial positioning and the transversal positioning of the rods by virtue of the spacer grids, it is necessary to use springs which exert a high transversal force on the rods. PRIOR ART To improve the neutron yield of the reactor core, use is generally made of a zirconium alloy with a low neutron absorption for forming the grids of the assembly. However, the springs must be made of a nickel alloy to make it possible to obtain adequate elastic and mechanical properties of the spring under irradiation. Thought has therefore been given to separating the functions of transverse positioning and of axial positioning of the rods by fixing the latter at one of their ends on the lower end plate of the assembly. This, however, has the disadvantage of complicating the structure of the lower end plate, of making its disassembly more awkward, and of reducing the cross-section of flow of the primary fluid in the assembly. SUMMARY OF THE INVENTION The aim of the invention is therefore to propose a fuel assembly for a nuclear reactor, which consists of a bundle of fuel rods which are cylindrical over at least a part of their length, are parallel and are held by a structure which consists of spacer grids which are transverse relative to the rods, by two transverse end plates and by guide tubes which are connected to the spacer grids and to the end plates and which are substituted for certain rods of the bundle, the rods being fixed at one of their ends on one of the end plates arranged in the lower part of the assembly, when this assembly is in a vertical working position in the reactor core, this fuel assembly needing to have a lower end plate of a simple structure which permits easy dismantling and an adequate flow cross-section of primary fluid, while permitting efficient axial and transverse positioning of the rods. To this end, the assembly comprises, in addition an attachment plate which is superimposed on the lower end plate and attached to the latter against its upper face, comprising openings throughout its thickness each of which corresponds, in diameter and position, to a fuel rod, and opening into parallel grooves, each corresponding to a row of rods, which are machined on the lower surface of the plate in contact with the end plate which is itself pierced with holes opposite each of the rods, and each rod comprises a lower part the transverse section of which has three branches at 120.degree. angles to each other, each of the corresponding radial expansions terminating at the bottom in a shoulder projecting in the radial direction relative to the outline of the cylindrical part of the rod, the rods passing through the attachment plate in such a way that their lower part projecting by a height which is substantially equal to the depth of the grooves is arranged in the corresponding groove for their locking in rotation and their axial fixing between the end plate and the attachment plate.