Patent Number: 053176090
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to installing fuel rods in a nuclear fuel assembly skeleton, and it is particularly applicable to fuel assemblies in which the skeleton includes guide tubes interconnecting removable end fittings and carrying grids that delimit cells distributed at the nodes of a regular array and designed to receive the rods. Such assemblies are used in nuclear reactors that are cooled and moderated by water, in particular by pressurized water. An apparatus is already known for installing fuel rods in such a skeleton, of the type which comprises receiving means for receiving a skeleton without end nozzles, interposed between a magazine for storing fuel rods in alignment with locations in the skeleton for receiving the rods, and a displacement mechanism for displacing pull bars longitudinally towards the magazine and away from the magazine, the pull rods being terminated by clamps for grasping the rods to be inserted in the skeleton. This applies in particular when the cells in the grids are distributed in a square array, with the displacement mechanism often being provided with the same number of pull bars as there are cells in a layer and enabling an entire layer of rods to be installed simultaneously in the skeleton. After the rods in one layer have been inserted, the mechanism is displaced transversely relative to the layer through a distance equal to the pitch between layers and is used to load a new layer of rods. The shape of the clamps on the pull bars is such that, while they are being displaced towards the magazine through the grids, they run the risk of damaging the springs and/or the projections provided on the plates that constitute the grids for the purpose of holding the rods in place. To avoid this risk, it is common practice to place caps having an externally tapered shape on the clamps before inserting the pull bars into the skeleton. The caps are removed when the clamps are in an intermediate position between the outlets from the skeleton and the magazine. At present, these operations are performed manually. They are lengthy, particularly the operation of installing the caps. They require an operator to remain in the vicinity of the fuel rods. Unfortunately, greater and greater use is being made of rods that contain reprocessed fuel and/or plutonium, thereby running the risk of irradiating the operators. SUMMARY OF THE INVENTION It is an object of the present invention to provide rod- ling apparatus that satisfies practical requirements better than prior art apparatuses, and in particular that eliminate any need for manual placement of the caps on the clamps. To this end, the invention provides apparatus of the kind defined above, further comprising a cap-placing assembly interposed between the skeleton-receiving means and the displacement mechanism, the assembly having a support fixed to these means and a removable plate that is formed with cap-retaining housings disposed in an array that reproduces the array of rod locations, the shape of the caps being such that the clamps, when closed, engage inside the caps when the pull bars are moved forwardly toward the magazine. This disposition makes it possible to load the caps into the removable plate in a zone that is protected against radiation. The manual operation that remains necessary consists merely in fixing the plate or the receptacle that includes the plate on the fixed support prior to installing the rods of an assembly, and then in removing it once empty in order to replace it with another that is full, after the rods have been installed. In an advantageous embodiment, making it possible to further reduce the exposure time of an operator, the apparatus further includes a cap-removing assembly comprising a frame interposed between the skeleton-receiving means and the magazine, having a frame carrying retaining jaws that are displaceable transversely to the displacement direction of the pull bars between a position in which they hold at least some of the caps and a position in which they release the caps. Such unit makes it possible to retain the terminal caps of a set of pull bars (e.g corresponding to a layer of rods) and to remove the caps, by slightly pulling the pull bars back and then releasing the caps. This can be done merely by displacing the frame transversely to the direction of the pull bars in order to leave room for the pull bars to pass, and to enable the clamps that terminate them to advance and grasp rods placed in the magazine. The invention also provides a method suitable for being implemented with the above-defined apparatus, comprising the steps of placing a skeleton without end nozzles horizontally between a storage magazine that stores rods in alignment with rod-receiving emplacements in the skeleton and a mechanism for longitudinally displacing pull bars terminated by rod-grasping clamps towards the magazine and away from the magazine; passing the pull bars through the skeleton up to the magazine and grasping respective rods; and pulling the rods into the skeleton. The method is characterized in that caps are loaded, at a distance from the magazine, into housings of a receptacle, the housings being distributed in the same array as the rods; the receptacle is interposed between the displacement mechanism and the skeleton; the pull bars are advanced for the clamps to be engaged inside the caps as they pass through the receptacle prior to passing through the skeleton; and on leaving the skeleton the caps are removed without manual intervention prior to bringing the clamps up to the magazine. The invention will be better understood on reading the following description of a particular embodiment given by way of example.