Patent Number: 047939621
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a process for placing a bundle of rods arranged in the form of a square pitch spacing lattice in a nuclear fuel assembly in a case. The invention also also relates to an installation for performing this process. In nuclear reactors, particularly of the pressurized water type, the fissile material is constituted by fritted uranium dioxide (U0.sub.2) pellets, which are stacked in metal cans for forming fuel rods. The fuel rods are themselves arranged in bundles in accordance with a square spacing lattice in nuclear fuel assemblies, which are then juxtaposed for forming the reactor core. FIG. 1 is a perspective view diagrammatically showing a nuclear fuel assembly. Apart from the bundle of rods a, it comprises a certain number of guide tubes b occupying the space of certain rods a in the square pitch or spacing lattice. In order to clarify the drawing, only some rods and some guide tubes are shown. Most of the guide tubes are used for slidingly guiding the absorbing rods of the not shown control groups. The relative positioning between the rods and the tubes is ensured by spacing grids 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 pieces constituted by an upper fitting d and a lower fitting e, said fittings being respectively used for the handling of the assembly and the fixing thereof in the reactor core. They are fixed to the ends of the guide tubes b. When such an assembly has been irradiated in the nuclear reactor core, it has to be dismantled to enable the fuel to be removed. A first stage in the dismantling of assemblies consists of cutting up the guide tube b at a level below that of the upper end of the rods a, so as to give access to said rod end, following the removal of the upper fitting d from the upper grid and the parts of the guide tubes attached thereto. During a second stage, rods a are grasped by their ends, so as to be extracted from the remainder of the framework of the assembly. When the rods have been extracted, the actual dismantling is completed and the rods are generally stored in a pool to enable their activity to decay. For this purpose they are placed in cases. The invention specifically relates to a process and installation making it possible to place the bundle of rods in a case following the extraction thereof. When the decay in the activity of the nuclear fuel contained in the rods is considered to be adequate, the latter are cut and the fuel contained therein is dissolved prior to removal. The function of placing the rods in the case prior to the storage thereof serves to reduce to the maximum the overall dimensions of the rods stored, whilst making it easily possible to manage said rods. To this end, all information concerning the life of the rods (irradiation time in the reactor, date of placing in the case, etc) generally appear on the case. The operation of placing the rods in the case must consequently make it possible to occupy in an optimum manner the internal volume of the cases. In the present state of the art, said operation is performed by introducing each of rods vertically into an individual guide, whose diameter decreases towards the bottom, the lower ends of the guides also approaching one another towards to the bottom, above the vertically positioned case to be filled. However, installations of this type suffer from the disadvantage of making it necessary to individually push on each of these rods, which is very difficult to carry out, in view of the deformations of some of these after irradiation. Moreover, the passage of each of the rods into an individual guide having at its lower end a diameter which is scarcely larger than the nominal diameter of the rods leads to a risk of the latter jamming, due to their swelling under irradiation. In EP-A-0 06695, the rods are moved together by means of a complex device, within which the rods are vertically pushed. This device is in three parts successively ensuring the shape change of the bundle, the moving together of the rods contained in each row and then the moving together of the rows of rods. However, the first part of this device has individual guide tubes into which the rods pass, so that the aforementioned disadvantages still exist. The present invention relates to a process and installation for placing in a case a bundle of rods, whilst eliminating any risk of these rods jamming and whilst still ensuring a completely effective filling of the cases. SUMMARY OF THE INVENTION The present invention more specifically relates to a process for placing in a case a bundle of rods arranged in n parallel rows of at least r rods, in accordance with a square pitch lattice in a nuclear fuel assembly, wherein it comprises placing the bundle of rods horizontally in accordance with said lattice in a waiting position above a cassette having p recesses separated by partitions, p being an integer at least equal to n, so that each row of rods is located above a recess in the cassette, introducing the rods into the cassette recesses, placing the cassette in the horizontal extension of a case, by interposing between cassette and case a transformation member, whose width decreases progressively from cassette to case by a value equal to the cumulative thickness of the cassette partitions and transferring the rods from the cassette into the case through the transformation member by simultaneously pushing all the rods. The essential originality of this process is based on the use of an intermediate cassette. By introducing the rods of the bundle into said cassette, the rods are moved together in a first direction of the square pitch lattice formed by the bundle. The transfer of the rods from the intermediate cassette into the case ensures the moving together of the rods in a second direction of said lattice, without it being necessary to use at any time an individual guide for each of the rods. During this transfer, the rods are constantly in contact with one another, so that to the thrust exerted on the rods is added a displacement by friction between adjacent rods. To take account of the fact that in practice the number of rods in certain rows exceeds r, because the locations previously occupied by the guide tubes remain empty, use is made of a cassette, whereof each recess can contain at the most s rods, s being an integer at the most equal to r and having a number p of the recesses exceeding n, so that the product p.multidot.s is at least equal to the number of rods of the bundle and in that in order to introduce the rods into the recesses, after lowering s rods of each row into each of the n recesses positioned below the n rows of rods, a relative horizontal displacement takes place perpendicular to the rods between the cassette and the rods of the bundle not introduced into the aforementioned n recesses, called the remaining rods, in order to bring the (p-n) remaining recesses successively below each row of rods and to lower said remaining rods into said (p-n) recesses. According to a preferred embodiment of the invention, the bundle of rods is positioned horizontally in the waiting position by maintaining the rods in accordance with the square pitch lattice by at least one set of horizontal combs or rakes and one set of vertical combs or rakes and the set of horizontal combs is retracted before introducing the rods into the cassette recesses. Preferably, before placing the bundle of rods horizontally above the cassette, part of the assembly is eliminated to free one of the ends of the rods of the bundle, the assembly being placed horizontally in the extension of said waiting position and the bundle of rods of the assembly is extracted in order to bring it directly into said waiting position, the horizontal and vertical combs being put into place corresponding to the displacements of the bundle towards said position. According to another feature of the invention, in order to facilitate the introduction of the rods into the case, at the entrance to the latter is previously positioned a base in which are located the ends of the rods turned towards the case, said base being able to slide in the case with the rods during the introduction of the latter. The process according to the invention also makes it possible to transfer the rods into cases which can have different sections, e.g. triangular, hexagonal or trapezoidal. For this purpose, before introducing the rods into the cassette recesses, the face of at least certain of the recesses is moved upwards, so as to reduce the number of rods which can be received in each recess, whose base is displaced. Moreover, if required by the shape of the case, after introducing the rods into the cassette recesses, there is a downwards displacement of the base of at least certain of the recesses, whose base has been previously upwards, so that the rods contained in these recesses are at the same level as that part of the case into which they have to be introduced. The invention also relates to an installation for placing in a case a bundle of rods arranged in n parallel rows of at least r rods, according to a square pitch lattice in a nuclear fuel assembly, wherein it comprises means for maintaining the bundle of rods horizontally in accordance with said lattice in a waiting position above a cassette having p recesses separated by partitions, p being an integer at least equal to h, in such a way that each row of rods is positioned above a cassette recess, means for introducing the rods into the cassette recesses and means for simultaneously pushing all the rods contained in the cassette so as to introduce them into a case positioned horizontally in the extension of the cassette, via a transformation member interposed between cassette and case and whose width decreases progressively from the cassette towards the case by a value equal to the cumulative thickness of the cassette partitions. According to a preferred embodiment of the invention, the means for introducing the rods into the cassette recesses comprise at least two horizontal support members extending perpendicularly with respect to the rods and on which the latter can rest by gravity and means for displacing said member vertically between a top position in which said members are located above the cassette and a bottom position in which said members are located below the base of the recesses in vertical notches formed in the cassette to the right of the support members.