Patent Number: 049869558
Section: summary

FIELD OF THE INVENTION The invention relates to a device and a method for removing peripheral rods from a fuel assembly of a nuclear reactor and, in particular, fuel rods which have at least one zone of breakage and comprise several pieces along their length. BACKGROUND OF THE INVENTION The fuel assemblies of water-cooled nuclear reactors, such as pressurized-water nuclear reactors, comprise a framework in which fuel rods of great length are disposed in order to form a bundle. The framework comprises spacer grids which are spaced relative to one another along the length of the assembly and connected together by guide tubes. End joining pieces are connected to the end of the guide tubes, which are longer than the fuel rods. Each of the spacer grids comprises an assembly of cells each intended to receive a fuel pencil and disposed in a regular network, generally a squared mesh. At the level of each of the cells intended to receive a fuel rod, the spacer grids comprise means for gripping the rod, while also ensuring both transverse retention and longitudinal retention of such rod. These gripping means generally consist of bosses projecting inwards relative to the walls of the cell of the grid and of springs consisting of resilient elements formed in the metal of certain walls of the cells of the grid or, alternatively, connected to these walls. After a greater or lesser residence time in the tank of a light-water nuclear reactor, the rods of the fuel assemblies may become brittle due to oxidation phenomena or due to transformations suffered by their sheaths under irradiation. The sheaths of certain rods may be perforated or torn by a peripheral guide fin of a spacer grid which has accidentally been folded inwards or by a foreign body conveyed by the fluid for cooling the reactor circulating in contact with the fuel rods. The fins or the foreign bodies may be encrusted in the sheath which is then deformed or perforated in the corresponding zone. It is necessary to perform repairs on fuel assemblies comprising rods whose sheath is perforated or torn, by removing these rods and replacing them with new ones. Removal is performed by pulling the rod via its upper end plug after having removed the upper joining piece of the assembly. Rods may also be removed and replaced by removing the lower joining piece. To this end, fuel assemblies of recent design comprise joining pieces whose elements for fixing on guide tubes may be removed without difficulty. All the operations for removing and replacing rods in the spent assemblies of a nuclear reactor are performed under a certain depth of water which is greater than three meters, inside a storage pool in which the fuel assemblies are placed in a vertical position. The operators carrying out the repair operations from the edge of the pool are thus protected against radiation from the spent assemblies. The fuel rods whose sheath is perforated or torn are liable to break during their removal, the gripping elements of the spacer grids exerting a certain axial retention force on the rods which it is necessary to overcome in order to perform the removal. There then remain pieces of rod inside the fuel assembly which can no longer be removed by the usual means. The peripheral rods of the assembly, i.e., the rods placed in the four outer rows which are adjacent to the frame of the spacer grids, are the most exposed to impacts with foreign bodies and to the effects of penetration by the guide fins of the spacer grids. These rods are thus the most exposed to breakage during the removal operations. To date, there was no known device or method making it possible to remove the peripheral rods of a fuel assembly comprising several successive pieces in the direction of their length, the assembly being placed under water in a storage pool. The spent and damaged assembly must be replaced by a new assembly and dismantled or repaired by using complicated equipment which is available only in nuclear fuel reprocessing plants. This results in increased operating costs for the nuclear reactor. SUMMARY OF THE INVENTION The invention thus aims to propose a device for removing peripheral rods from a fuel assembly of a nuclear reactor comprising a framework consisting of a plurality of spacer grids retaining the fuel rods in a uniform network in the transverse directions and in the axial direction of the rods, by gripping means associated with the cells of the grids in which the rods are inserted as well as of guide tubes replacing certain rods in the network and of end joining pieces fixed on the ends of the guide tubes, removal being performed remotely and under a certain depth of water in a pool for storing fuel assemblies after removal of an end joining piece of the assembly, by the device which comprises a rod of great length on which is mounted a means for the support and displacement of a work tool which is movable in an axial direction of the rod and in two directions perpendicular to this axial direction, this device making it possible to remove rods which have suffered breakage and comprising several pieces along their length. To this end, the work tool comprises: tongs consisting of two arms articulated together about a shaft fixed on the means for support and displacement in a direction parallel to the axis of the rod and comprising end jaws which are capable of gripping a fuel rods; PA1 the means for remotely controlling the tongs comprising an element which is movable in translation and an element for guiding the movable element, each connected to an arm of the tongs in the vicinity of the end of the corresponding arm opposite to the end jaw; and PA1 at least one video camera carried by the device for support and displacement in order to supply an image of the zone in which the rod is gripped, removal being performed by displacing the means for support and displacement in the vertical direction after positioning and gripping of the jaws of the tongs on a part of the pencil.