Patent Number: 052326551
Section: summary

BACKGROUND OF THE INVENTION The invention relates to nuclear fuel assemblies, and more particularly to fuel assemblies comprising a skeleton that constitutes the structure of the assembly and that holds a bundle of fuel elements at nodes in a regular array. Such assemblies are used, in particular, in reactors that are cooled and moderated by light water. The skeleton then comprises two end pieces interconnected by guide tubes which carry grids that are distributed along the guide tubes and that retain the fuel elements at the nodes of the array. The guide tubes are designed to receive elements belonging to absorbent clusters for controlling the reactivity and/or to clusters for varying the energy spectrum of neutrons. To attenuate the shock caused by the clusters dropping in the event of an emergency stop of the reactor, the guide tubes are often designed to act as hydraulic dampers by providing them with bottom plugs that are formed with respective narrow passages. Such a plug may also be used to receive the guide tube to the lower end piece (FR-A-2,469,777). In general, the end pieces are made of a material having high mechanical properties, such as stainless steel or one of the alloys known under the name Inconel, while the guide tubes are made of a zirconium base alloy. Consequently, it is difficult to secure the tubes to the end pieces directly by welding. In addition, it is desirable for the connection of the guide tubes to at least one of the end pieces to be easily disassembled and reassembled so as to allow faulty elements to be replaced. SUMMARY OF THE INVENTION It is an object of the invention to provide an assembly skeleton of the above-defined kind in which connection means can be made in a manner that is relatively simple and of acceptable cost. To this end, the present invention provides, in particular, a skeleton comprising two end pieces interconnected by guide tubes, in which the connection means between each guide tube and the lower end piece comprise a peg formed with a coolant flow hole, having a top portion fixed in the guide tube and a projecting bottom portion, which has a downwardly facing shoulder for bearing on the lower end piece, which passes through a passage formed in the lower end piece, and which is divided into a plurality of resilient fingers each having an upwardly facing shoulder for catching on the bottom end piece. Advantageously, the end portion of the peg beyond the upwardly facing shoulder tapers so that a vertical thrust exerted on the guide tube causes the resilient fingers to move toward one another, thereby enabling the peg to be inserted until its fingers snap into place. The passage through the end piece is constituted by a simple cylindrical machined hole which may open at its top end via a chamferin to facilitate peg insertion. In order to ensure that the fingers are sufficiently resilient and that they retain their resilience under irradiation, the peg is advantageously made of a material having proven mechanical properties, such as Inconel 718. The peg can then be fixed to the guide tube, which is generally made of a zirconium-based alloy, by circularly crimping the guide tube into a groove formed in the peg. To avoid any danger of undesirable peg release, e.g., in the event of a reduction in resilience, means may be provided for installing after snap fastening to prevent the fingers moving towards one another. These means may be constituted, in particular, by a locking ring received in a groove in the peg and having its ends inserted in the slots between the fingers. The connection between the guide tubes and the upper end piece may be implemented by any conventional means, e.g., a connection using threaded sockets of the kind described in French Patent Application No. 7923312 or a connection by crimping the top portion of the guide tube in a recess in the upper end piece, with locking by means of a force fit ring.