Patent Number: 052215156
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a method for manufacturing grids to be incorporated in nuclear reactor fuel assemblies. It is particularly suitable for the manufacture of grids for assemblies whose framework comprises end pieces connected by guide tubes imprisoned in some of the cells of grids spaced apart evenly along the guide tubes, the other cells of the grids supporting the fuel rods of the assembly. Among such grids, reference will particularly be made to those whose major faces have a square shape and which comprise two sets of inner plates disposed in two orthogonal directions and interlocked together so as to define the elementary cells through which the fuel rods and the guide tubes pass. The inner plates are connected at their peripheries, to plates forming a belt. These component parts are secured together at their intersections to provide cohesion of the grids. Grids are also known, particularly for undermoderated reactor assemblies, which have a hexagonal cross-section and which comprise three sets of intersecting plates. Although particularly advantageous in the case of grids whose major faces have a square shape and whose cells are distributed above the nodes of the square lattice, the invention also applies to grids whose major faces have a hexagonal shape. The component parts of the grids have generally been secured together by manual brazing, which is a long and tedious operation and which, in addition, does not guarantee fully satisfactory reproducibility. Different methods and devices have however been proposed for welding the component parts together in a more or less automatic way. The invention relates to a method of the kind described in French Patent No. 2,522,560, in which the grid whose component parts are to be secured together is placed in a mechanical shaping and holding frame having passages for access to the points to be welded on the two major faces and on the sides of the grid; the frame containing the grid is gripped by a device, placed in a gas-tight chamber containing an inert gas atmosphere; weldings is carried out on one face at a time, using a laser beam orthogonal to the axis of rotation of the device, delivered by a laser generator placed outside the chamber and penetrating thereinto through a transparent window. The operation is then repeated on the other faces after manually repositioning the grid. By welding in an inert gas protective atmosphere, oxidation of the welds is avoided, which makes it possible, in particular, to apply the method not only to grids made of a high strength alloy, such as INCONEL, but also to grids whose components are made of a zirconium-based alloy. From a reading of French Patent No. 2,522,560, it seems that the protective atmosphere is obtained by scavenging, which involves considerable inert gas losses. The laser generator is held fixed during welding of a face, except for the welds carried out on the belt, for which the beam is deflected by means of a set of mirrors. The variety of accurate movements of the grids inside the chamber does not guarantee sufficient accuracy to be compatible with the very small size of the focus spot of the laser beam. In addition, the chamber must have a very large volume. FR-A-2532216 and EP-A-0102252 also describe a method for welding using a fixed laser generator. The laser beam is directed and focussed in an open chamber, subjected to argon scavenging, in which a grid is driven with orthogonal displacement movements. This method requires very considerable gas flows, collection of the gases polluted by the laser shots is difficult and controlling leaks in the open chamber requires very reduced clearances, which are difficult to obtain. U.S. Pat. No. 4,710,606 describes a welding robot in which a laser beam must be moved along five degrees of freedom by mirrors, which implies a great complexity of the mechanism and accumulation of tolerances. SUMMARY OF THE INVENTION An object of the invention is to provide a method for welding grids by laser beam of the type defined above, for sequentially and automatically making the required welds under conditions ensuring mechanical holding of the parts and absence of deformations and oxidation, while avoiding complex movements of a kind likely to adversely affect the welding accuracy due to cumulative lost motions. To this end, there is provided a method of the above-defined type, wherein, for making the welds on one face of the grid, the laser beam is moved in two directions orthogonal to each other and to the direction of the beam, while the frame is held stationary in the inert gas atmosphere chamber. It is consequently possible to distribute the movements between the laser beam, the required focussing optical system and the frame. Although this method is of particular advantage in the case of grids whose plates and belts are made of zirconium-based alloy, it can also be used in the case of grids whose component parts are made of a nickel-chrome base alloy or of steel. In a typical embodiment of the invention, each grid is placed first of all in a first chamber having a device for orientation of the grid about a first axis, and all welds are effected on the faces parallel to this axis; then the grid still contained in its frame, is placed in another chamber having a device for orientation about an axis which is parallel to the first one, but on which the grid is placed in an angular position perpendicular to that which it had in the first device, and welding is carried out on the major faces. This arrangement considerably simplifies the orientation devices, and thus provides greater positioning accuracy. On each face, welding is carried out in the same plane, i.e. without modification of the distance to the laser generator, except for particular welds, such as the corner welds of the grid. Since the grid remains in its frame from the beginning to the end of the welding operations, transfer from one chamber to the other does not disturb the relative position of the component parts of the grid. Unloading of one grid, loading of another grid to be welded and provision of inert atmosphere in a chamber may take place during welding of the grid occupying the other chamber, i.e. in overlapping time. In practice, the inert gas atmosphere may be created by pumping out the chamber until a primary vacuum is obtained, then filling with inert gas so that the atmosphere in which welding takes place contains less than 50 vpm of oxygen. To protect the windows from the alternating pressure stresses, they may be protected by lids during evacuation. It is often of advantage to form some at least of the welds by shots focussed in two points located on two parallel lines at a short distance from each other. To that end, the optical path of the laser beam may be defined by optical means, such as mirrors or lenses, which are vibrated. Then the vibrations of the focussing lens are synchronized with the laser pulses, so that each shot takes place when the optical path is oriented towards the point which is to receive the energy. The invention also provides an installation for laser welding the different component parts of the grid, comprising a controlled atmosphere enclosure having a device for angular adjustment about an axis, receiving a frame containing a grid to be welded and having a transparent window and comprising a welding laser source carried by a table with crossed movements in two orthogonal directions one of which is parallel to the axis of angular adjustment. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description of a particular embodiment, given by way of example.