Patent Number: 047972483
Section: summary

The invention relates to a gripping apparatus of a group of rods arranged in square grid form in a nuclear fuel assembly. In nuclear reactors, particularly of the pressurized water type, the fissile material is constituted by sintered uranium dioxide (U0.sub.2) pellets. These pellets are placed in the form of piles in metal cans in order to form the fuel rods. The rods are arranged in bundle or group form in accordance with a square grid in nuclear fuel assemblies and are then juxtaposed to form the reactor core. FIG. 1 is a perspective view diagrammatically showing a nuclear fuel assembly. The latter comprises, besides the bundle of rods (a), a certain number of guide tubes (b) occupying the place of certain rods (a) in the square grid. In order to clarify the drawing, only one rod and one guide tube are shown. Most of the guide tubes (b) are used for the sliding guidance of the not shown control member absorbing rods. The relative position in between the rods and tubes is ensured by spacing rods (c), of which there are generally eight and whereof only three are shown in FIG. 1. These grids constitute the framework of the assembly. The assembly also comprises two end parts constituted by an upper end fitting (d) and a lower end fitting (e). These end fittings are respectively used for handling the assembly and fixing the latter in the reactor core, being fixed to the ends of the guide tubes (b). When such an assembly has been irradiated in a nuclear reactor core, it must be dismantled so that the fuel can be reprocessed. A first stage of the dismantling of assemblies consists of cutting the guide tubes (b) at a level below that of the upper end of rods (a), in order to free the access to said end of the rods, following the removal of the upper end fitting (d), the upper grid and portions of the guide tubes attached thereto. During a second stage, the rods (a) are grasped by their ends, so that they can be extracted from the remaining part of the assembly framework. The invention specifically relates to a gripping apparatus for performing this operation. When the rods have been extracted, the actual dismantling is at an end and the rods are generally stored in a pool in order to enable the activity thereof to decay. When this decay is considered to be adequate, the rods are cut and the fuel contained therein is dissolved before being reprocessed. In the present state of the art, there is no gripping apparatus making it possible in single operation to extract the rods from the remainder of the assembly framework during the second dismantling stage. Moreover, when they ensure the simultaneous gripping of several rods, the existing gripping apparatuses are formed by the juxtaposing of several independent gripping members, e.g. of the self-locking ball type, each ensuring the gripping of a single rod. These existing gripping apparatuses consequently lead to a relatively long handling time resulting from the large number of operations necessary for the extraction of all the rods of the same assembly. Moreover, there is a risk of the rods breaking because they may have been locked in the assembly framework as a result of deformation due to irradiation thereof. Thus, the locking or clamping force exerted by the balls of each independent gripping member on the rod which it grasps is not limited. The object of the present invention is a gripping apparatus designed so that it can simultaneously grip all the rods of the bundle or group contained in a nuclear fuel assembly and as shown in FIG. 1, preferably by applying a limited force or stress to each of the rods. It takes into account the differential elongation of the rods of the same assembly following irradiation. Thus, the invention proposes an apparatus for gripping a group of rods arranged in parallel rows of at the most (n) rods in accordance with a square grid in a nuclear fuel assembly, said apparatus being characterized in that it comprises: a body having a central portion recessed according to a rectangle, whereof the dimensions are at least equal to the external dimensions which the group has in section, parallel partitions dividing said central portion into (p) passageways, each of which is able to receive the ends of the rods of at the most two rows of adjacent rods; (p) rows of (n+1) mobile members, which are located in each of the (p) passageways in such a way that the end of each rod can be placed between two adjacent mobile members of the same passageway; locking or clamping means located in the body on at least one side of said central portion, so as to be able to apply to the ends of the rods and the mobile members contained in each of the passageways a force tending to lock each rod between two adjacent mobile members of the same passageway. The essential originality of this appratus is based on the gripping mode, each row of rods being subjected to a gripping or clamping force applied to the ends of said rod and transferred from one rod to the next by mobile members interposed between each of the rods. In this way, it is possible to simultaneously grip all the rods of the assembly. To take account of the fact that in practice the group of rods generally comprises less than (n) rods in certain of the rows, because the locations previously occupied by the guide tubes remain empty, the apparatus also comprises in a preferred manner, rollers having an external diameter essentially equal to that of the rods, said rollers being placed between certain of the mobile members so that they can fill the gaps of the grid. According to a preferred embodiment of the invention in which each of the passageways can receive the ends of rods of two rows of adjacent rods, the mobile members are shaped like crosses and have one branch placed between the two rows of adjacent rods of the same passageway and whereof one end can bear on an end of said branch of the adjacent mobile member, in order to limit the force applied to the rods when the locking means are put into operation. In order to facilitate the automation of the realization of the gripping apparatus, the latter can also comprise an ejector having a plate mobile with respect to the body in a direction orthogonal to the rectangle formed by the recessed central portion, said plate carrying a group of bars arranged in accordance with a grid identical of the rods and able to take the place of the latter, in order to eject them from the gripping apparatus following the loosening of the locking means. Elastic means can then urge the ejector towards a position in which the bars are placed between the mobile members instead of the rods. According to a preferred embodiment of the invention, the locking means comprise two rows of (p) plungers located in the body on two opposite sides of the central portion, one plunger of each row entering each of the passageways at the opposite ends thereof and actuating means able to displace the plungers towards said central portion to exert said force on the ends of the rods and the mobile members. Each plunger can be constructed in two parts and comprise an elastic device by which the force exerted by the actuating means is transferred between these two parts. In the preferred embodiment of the invention, the actuating means comprise two wedge devices acting on each row of plungers, said wedge devices being able occupy a stable opening position and a stable closing position. As a result of this feature, it is possible to control the opening and closing of the gripping device with the aid of two sets of fixed station jacks located respectively level with a gripping station and a release station of the rods. The gripping apparatus by means of which the group of rods is transported from one station to the other is consequently simplified. In practice, each wedge device preferably comprises a first part mobile in the body in a sliding direction whilst moving towards and away from the central portion, said first part bearing on one of the rows of plungers and a second part mobile in the body in a direction orthogonal to the sliding direction of the first part, said two parts cooperating with one another by a system of ramps terminated by flats defining said stable opening and closing positions.