Patent Number: 043022944
Section: summary

The invention relates to improvements in nuclear reactor fuel assemblies. Fuel assemblies, in particular those which are used in water-cooled reactors, generally consist of a bundle of parallel fuel rods held apart by means of cross-pieces which extend transversely relative to the rods. Tubes for supporting the assembly are substituted for some of the fuel rods, and these support tubes, which are longer than the rods, are themselves joined to end plates which make it possible to achieve a good mechanical rigidity of the assembly and to hold the fuel rods in the longitudinal direction. When the nuclear reactor core is recharged, it is extremely useful to be able to remove defective fuel rods and to replace them, so as to be able to continue using the whole assembly, with the removal, from the reactor circuit, of the contamination caused by fission products originating from the defective fuel rods. This ability to remove rods provides the advantage that it economizes in terms of fuel since, once the assembly has been reconstituted, it can be recharged and re-used, after the defective rods have been removed and replaced. Furthermore, replacement of defective rods enables the reactors to be run with greater safety for the installations and the personnel by reducing the contamination risks. However, changing defective rods is an operation which requires considerable precautions; the irradiated element can ony be repaired under radiological protection. In general, this operation is carried out under a certain depth of water in a swimming pool adjacent the reactor. Furthermore, for certain more specific reasons, for example in order to carry out examinations of some of the rods or to perform mechanical or physical tests on these rods, it can be of value for the user to be able to dismantle a fuel assembly in order to recover those bars on which the tests or examinations are to be carried out. In order to have access to these fuel rods and to be able to separate them from the remainder of the assembly, it is necessary to dismantle the end plates; this dismantling operation in the swimming pool presents certain difficulties because the assembly is only accessible by remote control and because the dismantling operation is performed on an immersed assembly. For the purpose of facilitating the operation for dismantling and reassembling the fuel rods in the assembly inside the swimming pool, it has been proposed to detachably fix the end plates to the support tubes so that the end plates can be easily dismantled by remote control when the assembly is inside the swimming pool. To enable this detachable fixing of the end plates to the support tubes, screw-threaded sockets, for example, have been proposed which sockets are screwed inside the support tubes. Each socket has a shaped enlarged portion which fits in a housing, of corresponding shape, provided on the inner face of the end plate. Rotation of the sockets is prevented by expanding part of the sockets inside the housings provided in the end plate. This proposal makes it possible to dismantle the end plate from the support tubes rapidly and easily; however, when the end plates have been dismantled, the ends of the support tubes are no longer held in the transverse direction relative to one another, and, when reassembling, it is necessary to realign the end plate with the set of support tubes which must be inserted in the housings provided in the end plate. According to the invention there is provided a nuclear reactor fuel assembly comprising a bundle of parallel fuel rods held apart by cross-pieces which extend transversely relative to said rods, and tubes, for supporting the assembly, which are substituted for some of said fuel rods, said support tubes being longer than said rods and ensuring, in cooperation with transverse end plates, the rigid assembling of the whole, said plates also ensuring the longitudinal support of said rods, wherein, at one or both of their end, said support tubes are fixed to a grid extending transversely relative to said support tubes, said grid forming a network of cells which approximately correspond, in size and position, to said fuel rods, so that a said rod can pass by longitudinal displacement through a said cell of said grid, said grid being detachably fixed to the corresponding said end plate which is positioned outwardly of said grid, by cylindrical sockets which extend through passages provided in said end plate and through some of said cells of said grid, each said socket comprising: bearing surfaces at the level of the outer face of said end plate and the inner face of said grid, means at the outer end of said socket for cooperation with a tool for rotating said socket between a first position in which said socket can be freely inserted into said cell of said grid, and a second position in which said bearing surface at the level of said inner face of said grid comes into contact with said inner face of said grid to lock said grid to said end plate, and a deformable part which is expandable into one or more housings provided at the level of said passage in said end plate or of said cell of said grid to prevent rotation of said socket relative to said grid.