Patent Number: 043022944
Section: description

The fuel assembly shown in FIG. 1 comprises a set of parallel fuel rods 1 held in a rigid bundle by means of transverse cross-pieces 2, which are arranged at approximately regular intervals over the length of the rods, and end plates 3. As shown in FIG. 2 each cross-piece 2 defines cells 4 through which the fuel rods extend. Some of these cells 4 are occupied by support tubes 5 which are substituted for a certain number of fuel rods. The length of the support tubes 5 is slightly greater than the length of the fuel rods 1, and the tubes 5 are joined at their ends to grids 6. As can be seen in FIG. 3, each grid 6 provides a network of square cells 10. When the grid 6 is in position in the assembly, as shown in FIG. 1, the support tubes 5, which are permanently fixed to the grids 6 either by crimping or by crimping and welding, are positioned in some of the cells 10. Each of the grids 6 comprises an assembly of metal strips or small metal plates 11, of a certain height, which intersect and are fixed to one another. Some of the cells of each grid 6, which cells 10 are indicated by crosses 12 in FIG. 3, are intended to receive sockets for joining the grid to the respective end plate 3. The network consisting of the cells 10 of each grid 6 is arranged, relative to the fuel elements of the assembly, in such a way that each of the cells 12 inside which support tubes are not arranged is in line with a fuel element 1. The network of the grid 6 is thus arranged in substantially the same manner as the cross-piece shown in FIG. 2. Furthermore, the size of the cells 10 of each of the grids 6 is such that the fuel elements 1 can be extracted by displacing them longitudinally and passing them through the cells of the grid. When one of the end plates 3 is dismantled, it is therefore possible to extract the fuel elements from the assembly, even though the grids 6 remain permanently fixed to the support tubes 5. Reference will now be made to FIGS. 4, 5, 6 and 7 in order to describe the method of fixing the grids 6 to the end plates 3. The method is described in respect of the upper grid and end plate. The fixing of the lower grid to the lower end plate is effected in a corresponding way. This fixing is achieved by means of sockets 7, of cylindrical shape, each of which possesses, at its upper end, an enlarged part 14 of which the lower face constitutes a shoulder 15 which can come into contact with the bottom of a cavity 16 of circular section which is provided in the outer upper face of the end plate 3. When the upper end plate 3 is positioned on the assembly, its lower face comes into contact with the upper face of the grid 6, and passages 17 of square section, which are provided in the end plate 3 at the centers of the recesses 16 and pass right through this plate 3, coincide with the square cells 10 of the grid. The dimensions of the transverse sections of the recesses 17 are approximately the same as the dimensions of the cells 10. When the socket 7 is in position it extends through a passage 17 and a cell 10 of the grid, as shown in FIG. 5 and its shoulder 15 comes to rest on the bottom of the recess 16 provided on the upper face of the end plate. The diameter of the cylindrical central part of circular section of the socket 7 is slightly smaller than the side of the cell of the grid. The socket 7 is also provided, at its lower part, with an enlarged part 18 of square transverse section, the dimensions of which are slightly smaller than the dimensions of a cell 10 and of a passage 17, so that the part 18 can, when appropriately orientated, pass through a passage 17 and a cell 10. The enlarged upper part 14 of the socket 7 is provided with a slot 20 for engagement by the blade of a screwdriver in order to orientate the socket by rotating the screwdriver. It is thus possible to arrange the socket 7 in such a way that the enlarged lower part 18 is in the position shown in FIG. 7, in which the corners of the part 18 come to bear on the lower surface of the grid. Once the socket 7 has been introduced into the end plate 3 and the grid 6, as shown in FIGS. 5 and 7, the socket is fixed, with pressure, against the end plate and the inner surface of the grid by expanding the cylindrical part of the socket 7, at the level of the square section passage 17 in the end plate, into the corners of the passage 17, as shown by 19 in FIG. 6. The deformation 19 can be obtained inside the socket 7 by expansion, that is to say by means of an elongate tool comprising, at its end, rollers which deform the metal of the socket. During this deformation, the metal which is pushed out penetrates into the four corners of the square passage 17 and thus prevents the rotation of the socket relative to the end plate and to the grid. The deformation also causes a slight shortening of the socket, and this causes the end plate 3 to be tightened against the grid 6 via the parts 15 and 18 of the socket, which are in contact with the upper part of the end plate and the lower part of the grid respectively. It will be understood that the unit consisting of the support tubes 5, the cross-pieces 2 and the grids 6 which are fixed to the ends of the support tubes retains a certain rigidity when one or both of the two end plates 3 are removed, and that it is then possible to extract fuel elements from this structure, or introduce them into this structure, in order to replace the elements or to remove them for the purpose of checking or testing operations. When it is desired to fit the assembly together again, the fixing of the or each end plate to the corresponding grid is extremely easy because it suffices to place the end plate on the grid so that the passages 17 are in alignment with the cells of the grid, and then to introduce the fixing sockets 7 into some of the passages and cells and to orientate the sockets by means of a screwdriver engaging the slots 20, and finally to produce the deformations 19 inside the sockets 7. Conversely, when it is desired to dismantle the assembly in order to remove a defective element or an element which is intended for checking or testing operations, the assembly then being located in the swimming pool of the reactor, the operations are even simpler and can easily be carried out by remote control with the assembly immersed. In order to dismantle the assembly, it suffices, in fact, to introduce the blade of a screwdriver successively into the slots 20 of the various sockets 7 arranged in the end plate which it is desired to dismantle, and to rotate each socket using the screwdriver. The protuberances 19, made in the socket when it was mounted, are then deformed by contact with the inner surface of the passage 17, and this then permits the rotation of the socket and the release of the tightening force between the end plate and the grid. The sockets can then easily be extracted from the cavities, and the end plate can be removed. The fuel elements are then accessible again. It is seen that the main advantage of the above described assembly is that it has a connection between the end plates and the rest of the assembly, the making and breaking of which is extremely simple, and, in particular, which avoids machining operations, such as grinding or cutting, when the assembly is dismantled, which operations are likely to contaminate parts of the fuel assembly, likewise, any welding operation which presents the same contamination risks. Furthermore, even when the end plates have been dismantled, the assembly retains a certain rigidity and a certain stability. The invention is not intended to be restricted to the embodiment which has been described but, on the contrary, includes all variants thereof, and points of detail can be modified without thereby going outside the scope of the invention. For example, the upper part of the socket 7 may be provided with a hexagonal or square recess into which it is possible to insert a key, of corresponding shape, in place of a slot for a screwdriver blade. The socket may be tubular, as shown in FIGS. 5 and 6, or it may have solid parts obstructing the flow through the end plate. The deformable part of the socket may be located at any point on the socket or it may consist of a part added to the socket. The lower part of the socket may have one of a number of very diverse shapes, provided that the lower part can penetrate through the passage in the end plate and the cell of the grid, and can then, after rotation, present bearing surfaces to the inner surface of the grid. It is also possible to join one or both of the grids to the upper cross-piece or to the lower cross-piece respectively, that is to say to the cross-piece located closest to the upper end or to the lower end of the assembly, depending on whether the upper grid or the lower grid is concerned. The grid can even form part of the upper cross-piece or the lower cross-piece if the height of the cross-piece is increased and if openings are provided therein for the passage of the locking ends of the sockets. The grid may be joined to the cross-piece by a number of strips and small plates of which it is made, and particularly by the small plates arranged at the exterior of the grid. The sockets 7 and the grids 6 are generally made of stainless steel of the same type as the other parts of the assembly, but the use of other materials, chosen, for example, for their useful mechanical properties, is not excluded. However, the grids are preferably made of a material which is identical to the material constituting the end plates, so as to avoid problems due to the thermal expansion of these pieces. Finally, the connection, by means of a grid and sockets, between the assembly and the end plates can be used for the upper end plate or for the lower end plate of the assembly, or for both these end plates.