Patent Number: 040381358
Section: description

The fuel plate which is illustrated in FIG. 1 comprises two external cladding plates 1 and a central layer 2 formed by a plurality of fuel wafers 3 arranged in several rows so as to cover the surface of the fuel plate while nevertheless leaving lateral clearance spaces. Said lateral spaces are taken up by a metallic frame 4 which surrounds the assembly of fuel wafers 3. The fuel wafers 3 are separated from each other by sheets 5 of metallic foil which serve to wrap each fuel wafer 3 and form the partitions which isolate the fuel compartments. Referring now to FIG. 2 which illustrates an alternative form of construction of a fuel plate in accordance with the invention, there are again shown the external cladding plates 1 and the central layer 2 formed by the fuel wafers 3 which are arranged in a number of rows. In this example, metallic partition strips 6 are interposed between the adjacent fuel wafers and serve to reinforce the partitions of the fuel compartments containing the wafers 3. In the case of FIG. 2, said metallic partition-strips 6 are disposed on a lattice having two perpendicular directions whereas on the contrary, in the mode of execution shown in FIG. 4, all the metallic partition-strips 6 are parallel and extend along the entire length of the fuel plate. The method of fabrication of a fuel plate as illustrated in FIG. 1 will now be described by taking as a non-limitative example the case of a zircaloy plate containing a fuel having a base of enriched uranium oxide. The method first consists in providing wafers of square cross-section and having a thickness of 9 to 4 mm, said wafers being formed of sintered uranium oxide having a density of approximately 10.3. After passing out of the sintering furnace, said wafers are first covered with a layer of graphite which constitutes an anti-diffusion barrier in order to prevent the zircaloy-UO.sub.2 reaction, deposition of said graphite layer being carried out by any known method such as deposition of pyrolytic graphite from a gaseous phase, the Aquadag processes and the like. Said fuel wafers are then wrapped in a sheet of ductile zirconium foil having a thickness of approximately 0.05 to 0.1 mm in a manner which can be likened to a toffee. FIG. 3 shows the fuel wafer 3 which is placed on the sheet 5 of zirconium foil. By folding back the corners of said sheet 5, the wafer is thus wrapped in much the same fashion as a toffee. The next operation consists in assembling these wrapped wafers on a support plate of zircaloy having a thickness of 0.40 mm and constituting one of the fuel element cladding plates 1. The wafers 3 are laid side by side, thus forming rows so as to cover the surface of the cladding plate except for the edges of the plate 1 which are left free and along which are placed the zirconium members 4 which have a substantial length and a thickness of 4 mm, said members being intended to form a frame which surrounds the fuel wafers. The assembly thus obtained is covered by a zircaloy plate 0.40 mm in thickness which constitutes the other cladding plate of the fuel element. The cladding plates and the frame which surround the fuel core are then welded in vacuo so as to close the fuel element and ensure that the fuel wafers are maintained in position within the cladding. The final step of the method concerns the rigid interassembly of the complete structure so as to form the metallurgical bonds between the fuel element cladding plates 1, the fuel 3 contained in each compartment and the sheets 5 of metallic foil which delimit these compartments. The above-mentioned operation is performed by diffusion bonding, the entire assembly being maintained for a period of 4 hours at a temperature of 830.degree. C. in a gaseous atmosphere such as halium under a pressure of 1000 bars. The method of fabrication of the fuel plate illustrated in FIG. 2 is wholly similar. The starting material consists of fuel wafers which are covered and then wrapped like toffees in ductile zirconium foil. These wrapped wafers are then assembled on one of the zircaloy support plates 1 which constitute the cladding plates of the fuel element by interposing between the fuel wafers metallic partition-strips 6 having the same thickness as the wafers so as to reinforce the partitions of the fuel compartments. The metallic partition-strips are of two types. Both types have the same width and the same thickness but differ in length. The first type has a length equal to one side of a fuel wafer in the case of wafers of square cross-section. The second type has a length equal to the total length of the fuel plate. The first row of fuel wafers is formed on the zircaloy support plate by placing thereon in alternate sequence a fuel wafer 3 and then a zircaloy partition-strip 6 of the first type. When this row is completed, a zircaloy partition-strip 6 of the second type is placed along said row and the second row is then formed in the same manner as the first, the operations being continued so as to cover the surface of the cladding plate. The entire assembly is covered by the second cladding plate, whereupon closure of the fuel plate unit and bonding of the complete structure are performed in the same manner as before. It is wholly apparent that this example of assembly of fuel wafers and metallic partition-strips is not given in any sense by way of limitation and that other types of assemblies can be employed while remaining within the scope of the present invention. In particular, the fuel wafers can have any geometrical shape provided that their juxtaposed assembly serves to constitute a fuel surface which does not have any vacant spaces. There is shown in FIG. 5 a wafer 3 of ceramic fuel material fitted in accordance with the second mode of execution of the invention with a protective metallic strip 7 of small thickness which is welded at a certain number of points such as the point 8. In the case of FIG. 5, the thin metallic strip is of zircaloy and has been formed by means of the strip components 9 and 10 having a thickness of 0.30 mm and bent in the shape of a U, said components being represented diagrammatically in the figure in dashed outline prior to interengagement around the wafer 3; FIG. 5 also shows in a highly diagrammatic form the electric welding circuits 11 and 12 and the electric generator 15 which are employed for the purpose of forming the spot welds 8 of the metallic strip 7 of the fuel wafer 3. In accordance with the invention, two different modes of execution of the interengagement of the U-shaped strip components can be adopted and are shown respectively in FIGS. 5a and 5b which do not show the wafers for the sake of enhanced simplicity of the drawing. In the first form of construction shown in FIG. 5a, one of the two strip components, namely the inner component 9, is entirely covered by the other strip component, namely the component 10. In the second form of construction shown in FIG. 5b, the strip component 9 and the strip component 10 each have an outer arm 9a and 10a and an inner arm 9b and 10b. In the first mode of execution of the method, the electrodes 13 and 14 are aligned along the arrows F1 and F2 so as to apply a compressive force on the lateral walls 7 of the wafer 3. In the example herein described, said force of application which is necessary in order to reduce the contact resistance between the two strip components 9 and 10 and the effects of Joule heating in the outer strip component 10 (FIG. 5a) which short-circuits the two electrodes 13 and 14 is of the order of 14 kg. This mode of procedure has given the best results but a fairly high energy is required in order to take account of the fact that a substantial proportion of this energy is inevitably expended in short-circuit within one of the U-shaped strip components. On the other hand, it is possible by means of this procedure to make two simultaneous welds on the opposite sides of the two arms of each U and this represents an advantage. In the second mode of execution of the method described with reference to FIG. 5, the electrical circuit is shown in dashed lines and the generator is shown in 15a. The electrodes 13 and 14 are again applied against the lateral walls of the fuel wafer 3 in oppositely-facing relation to each other so as to exert requisite compressive force; however, the two electrodes 13 and 14 are brought to the same potential and the supply of current is effected through a circuit 12 which is connected electrically to the lateral arms of the inner U-shaped strip component. The electrical connection may be established by means of a copper strip, for example. The advantage of this alternative form lies in the fact that a result which is similar to the preceding is obtained with lower electric power and a lower compressive force since the short-circuit problems no longer arise by virtue of the arrangement of the strip components as shown in FIG. 5b ; on the other hand, the industrial application of this embodiment on a large scale is clearly a matter of greater difficulty. In the case of FIG. 6, the wafer of ceramic fuel material has been omitted for the sake of enhanced visibility of the drawing in which there is simply shown the zircaloy strip 16 which is folded and wound around itself. Once it has been wound so as to surround the wafer of fuel material, said strip is in turn welded at a certain number of points such as the point 8 as in the previous example. In the example of FIG. 6, the thin metallic strip which constitutes the lateral cladding of the wafer has a thickness of the order of 0.2 mm. In FIG. 7, there is shown in cross-section a fragment of a fuel element plate unit 17 formed by assembling together a single row of ceramic fuel wafers such as the wafer 3 which are fitted with their independent covering-strip components 9 and 10 and which are intended to be clad by two zircalogy plates, namely a top plate 18 and a bottom plate 19. To this end, the plates 18 and 19 have curved extremities so as to enclose two zircaloy wires 20 and 21 which extend along the entire length of the plate unit. The assembly being thus completed, the cladding plates 18 and 19 are welded along their entire junction lines, namely along the two lines 22 and 23, for example by means of the electronic beam welding process. Finally, cladding by diffusion bonding at high temperature and under pressure effects the leak-tight assembly of the different elements. The final product obtained as shown in FIG. 8 is a fuel element plate unit 17 entirely clad with zircaloy in which each wafer of ceramic fuel material is clad within a unitary compartment. Fuel element plate units of this type can very advantageously be employed in fuel assemblies each consisting of a large number of these narrow plate units which are maintained by the spacer grids of a fuel element casing; these fuel assemblies are employed in particular as fuel elements in nuclear reactors of the water-cooled type.