Patent Number: 046719210
Section: summary

The present invention relates to a process, installation and device for compacting flexible, oblong objects, particularly fuel pencils of a nuclear reactor. In enriched uranium nuclear reactors, the fissile matter is known to be constituted by pellets of uranium oxide stacked in metal cans closed at each end to form what is called "fuel pencils". These are assembled together in large numbers (for example 264), with a certain number of tubes not containing fuel, to form a fuel element. The fuel elements are disposed in the reactor to form the core thereof. Once the required specific burn-up is attained, the irradiated fuel elements are withdrawn from the reactor and immersed for several months in a nearby pond in order to cool and lose the major part of their radioactivity. They are then evacuated to an appropriate site and stored in some manner (pond, storage packaging, etc...) whilst awaiting possible re-processing. Both for transport and storage, in order to save space, it is advantageous to dismantle the fuel elements and to re-assemble the fuel pencils compactly in bundles, i.e. in a more or less compact assembly of oblong, rectilinear objects which are parallel to one another over their whole length. In fact, inside a fuel element, the fuel pencils, which have an outer diameter of the order of 9.5 mm for example, are disposed in a lattice of square meshes whose sides are of the order of 12.6 mm. This results in a section of the fuel element virtually double that corresponding to a compact lattice. It will be readily appreciated that dismantling of the fuel elements, and more particularly the constitution of bundles of pencils and introduction thereof into appropriate containers, presents difficulties since these pencils are very long (about 4 m) with respect to their small diameter (9.5 mm). To overcome this difficulty, a method which may be envisaged at the present time would consist in taking the pencils from inside a fuel element row by row (17 pencils at a time, for example) and in disposing these pencils horizontally on vibrating tables which form a V surface an d which compact the pencils in the point of the V. The bundle of triangular section thus obtained is then pushed into a container, likewise of triangular section. The main disadvantages of such a method, apart from requiring a complex and expensive installation, are that it is slow (only 17 pencils out of the 264 contained in a fuel element are taken at a time) and that one can be sure only after vibration that the pencils will be absolutely parallel to one another. In fact, by reason of their great length, and therefore of their considerable longitudinal flexibility, adjacent pencils may interlace with one another to such an extent that they cannot be returned parallel to one another by vibration. In such a case, the bundle could not penetrate in the triangular container. It is an object of the present invention to overcome all the drawbacks mentioned above. It relates to a process, installation and device for compacting objects of great length and of small transverse dimensions, with a view to disposing them in a container reliably, repetitively and rapidly, with the result that the portion ofthe section of the latter occupied by the transverse sections of said objects is maximum. It is applied more particularly, but not exclusively, to the handling and compacting of fuel pencils with a view to placing them in containers, whatever the purpose of such an operation (final storage, intermediate storage, transport, etc...). To this end, according to the invention, the process for forming a bundle, i.e. an assembly of oblong, approximately rectilinear objects converging at one end, more or less compact, of oblong objects of small transverse dimensions, such as fuel pencils for nuclear reactors, said objects constituting, prior to the change of arrangement, an assembly in which they occupy parallel, transversely spaced apart positions, is noteworthy in that: a plurality of guiding tubes are arranged substantially parallel to the same direction, of which, on one side, the ends are disposed in register with the initial arrangement of said assembly of objects and of which, on the other side, the ends are disposed in register with the desired arrangement, the dimensions of said guiding tubes being such that one of said objects may freely slide, in view of its flexibility, in each of them, each object is introduced by one of its ends in the end disposed on the first side of one of said tubes, and all of said objects are allowed to slide simultaneously towards the ends disposed on the second side of said tubes. Although said tubes may be arranged in any general direction desired, it is advantageous, to utilize gravity, if this direction is substantially vertical. In this case, the upper ends of the guiding tubes are disposed in register with the initial arrangement of said assembly of objects and the lower ends of said tubes are disposed in the desired arrangement; said assembly is brought vertically above the plurality of the guiding tubes and, after introduction of said objects in said tubes, said objects are allowed to slide downwardly under the action of gravity. In this way, at the outlet of the plurality of tubes, i.e. in their lower part, the desired arrangement of the objects is obtained, which may be either more compact or less compact than the initial arrangement of said objects. In the case of compacting, for a minimum diameter of the guiding tubes compatible with a good slide of the oblong objects, the degree of compacting of the objects, obtained at the outlet of the plurality of tubes, depends on the arrangement and the thickness of wall of said tubes in their lower part. If it is desired to obtain an arrangement of the oblong objects in a compact bundle, according to the invention, the plurality of guiding tubes is given the closest possible disposition in their lower part and, downstream,is arranged a plurality of elongated rigid guiding elements of which the ends directed towards the guiding tubes are opposite the interstices between the closely located ends and of which the opposite ends are housed in the interstices left free between said objects, when the latter are in contact with one another. Each of said guiding elements may be constituted by a wire, the assembly of the wires forming a sort of convergent cage. In a variant, said elements may be formed by a thick plate or a multitude of superposed plates in which are formed guiding holes, the matter remaining between said holes constituting said guiding elements. In this way, said guiding elements make it possible transversely to compact the objects to a maximum. When the number of the objects to be compacted is very high, operation can be carried out in several stages. In this case, for the sub-assembly of the objects compacted simultaneously and therefore for the ends for introduction of the guiding tubes, an arrangement is chosen such that, by a few simple relative movements (translation, rotation, etc...) it covers without omission (and possibly without duplication) the arrangement of all the said objects. Such an arrangement may be determined, inter alia, by the conditions of size of the means for gripping the objects. For example, from a fuel element with 264 pencils, four bundles each of 66 pencils may be formed. To carry out the process according to the invention, it is advantageous to provide a compacting device comprising a plurality of guiding tubes of which, on one side, the ends are disposed in register with the initial arrangement of the assembly of objects to be compacted and, on the other side, the ends are disposed in the desired arrangement, the dimensions of said guiding tubes being such that one of said objects. may slide freely in each of them. On both sides, the tubes are preferably rendered fast with one another via transverse flanges pierced with holes disposed respectively in register with the dispositions retained for the ends of the tubes. Downstream, on the narrower side, said compacting device is extended by a plurality of rigid guiding elements fast with the corresponding flange and disposed so as to be housed, on the flange side, in the interstices between the ends of the guiding tubes and, on the side opposite the flange, in the interstices left free between said objects assembled in a bundle. If said elements are wires, at their end opposite the flange, the peripheral wires are rendered fast with a closed bearing surface reproducing the contour of the cross section of the bundle at the most compact point. If, moreover, it is desired to introduce the bundle of objects in a container, this container may be disposed in line with the plurality of the tubes and guiding elements, downstream thereof, then the objects may be slid inside said container. However, this manner of proceeding presents the drawback of necessitating considerable longitudinal dimensions, since, at one stage, the objects are in line with the plurality of guiding tubes and elements which themselves are in line with the container. In certain applications, particularly when fuel pencils are placed in containers vertically in a cooling and deactivation pond, it is impossible to operate in this way, since, in that case, the fuel pencils would at one time be located very close to the surface of the water of the pond. In such a case, according to a variant of the present invention, there is disposed, downstream of the plurality of the guiding elements and in line therewith, a device for provisionally accommodating the oblong objects, this device being extensible, retractable and adapted to maintain said objects in a bundle, the bundle of objects is transferred from the plurality of guiding tubes to the provisional receiving device, by moving said plurality of guiding tubes away and by progressively extending said provisional receiving device towards the guiding tubes, and the bundle of objects is then transferred from the provisional receiving device to the container by pushing the latter coaxially to said bundle, whilst progressively retracting said provisional receiving device. In this way, an advantageous installation for carrying out the process according to the invention comprises a plurality of guiding tubes, disposed in a general vertical direction, of which the upper ends are disposed in register with the initial disposition of said assembly of objects and of which the lower ends are disposed in the desired arrangement, the dimensions of said guidin tubes being such that one of said objects may slide freely in each of them under the action of gravity. If, moreover, the installaticn is provided for taking from a first container an assembly of objects occupying parallel, transversely spaced apart positions, and for placing in a second container the more or less compact bundle obtained from said assembly, it comprises gripping and handling means for hoisting out of the first container, disposed vertically, said assembly of objects, then for introducing the lower ends of said objects in the upper ends of said plurality 0 of guiding tubes, as well as means for transferring said bundle of objects from the plurality of tubes to the second container.