Patent Number: 040381358
Section: summary

This invention relates to a ceramic nuclear fuel element and to a method of fabrication of said element. It has already been proposed in the nuclear technique, especially for reactors of the light-water system, to design nuclear fuel in the form of a clad plate unit in which the ceramic fuel material is subdivided and placed within compartments which are isolated from each other by partitions made of metallic material, the intended function of said partitions being to ensure that the fuel compartments afford a certain degree of leak-tightness with respect to each other. The use of a fuel structure of this type in nuclear power plants and especially light-water plants considerably improves the resistance of the fuel to power cycling, the behavior of the fuel in the event of stoppage of the cooling system, the reliability of the cladding and the safety features of the nuclear power plant. The above-mentioned structure of the compartment type in fact improves heat-transfer processes between the fuel core and the cladding by virtue of the partitions which isolate the compartments since said partitions constitute paths for the flow of heat from each fuel compartment towards the fuel element cladding. As a consequence, in the case of an identical coolant outlet temperature, the temperature at the center of the fuel element is maintained at a lower value than that of fuels which are not divided into compartments, this being achieved both by virtue of the small thickness of the fuel wafers and by virtue of the division into compartments. In the case of zirconium-clad fuel having a uranium oxide base, the maintenance of a lower temperature improves the behavior of the fuel under irradiation. Since differential expansions between the fuel and the cladding are of lower magnitude, there is a much slower development of potential fault conditions. Similarly, in the event of accidental stoppage of the cooling system, re-wetting by means of injected emergency water is facilitated since the equilibrium temperature of oxide and cladding is considerably lower and there is less danger of any zirconium-water reaction. Furthermore, at the time of failure of a fuel element cladding, the quantity of fission products released into the coolant is much smaller since it is limited to the contents of a single compartment of the fuel element. Unfortunately, it has not proved possible up to the present time to make full use of the advantages which have been described in the foregoing and which are normally expected in this technique. The reasons for this lie primarily in the imperfections of the technology which has been employed hitherto. In the first place, the plate-type fuel elements fabricated up to the present time have not permitted wholly leak-tight partitioning during operation in the case of the different fuel compartments, with the result that the advantages of this structure are considerably reduced in the event of a failure of the cladding. In the second place, the method of fabrication of plate-type elements is relatively difficult in practice. One known technique for constructing clad fuel plate units divided into compartments in fact consists in inserting between two metallic cladding plates a perforated metallic plate in which the perforations have been filled with powdered nuclear fuel. The fabrication of the fuel core proper therefore calls for machining of the metallic plate in order to obtain the perforations prior to the filling of these latter with nuclear fuel. The assembly of the fuel core and the cladding plates is then performed by welding, rolling or hot compression. The precise aim of this invention is to provide a novel plate-type ceramic fuel element which offers much greater safety than the structures of the prior art from the viewpoint of in-pile behavior and which entails a much simpler method of manufacture since it does not require costly intermediate machining operations. By means of a single heat treatment under pressure, it is in fact possible by means of this method and starting from a fuel core formed by previously fabricated and juxtaposed elements to form a cladding on the fuel element and at the same time to form a perfect metallurgical bond between the partitions of each fuel compartment. To this end, the plate-type nuclear fuel element in accordance with the invention comprises a core having a base of ceramic fuel material enclosed between two metallic cladding plates and is distinguished by the fact that the fuel core is constituted by the juxtaposed array of a plurality of wafers of ceramic fuel material, at least a number of said wafers being provided with an individual metallic protection which has a contributory function in the cladding of the wafers and in the division of the fuel element into compartments. It is therefore apparent that in accordance with this essential feature of the invention, the division of the fuel into compartments as mentioned earlier is achieved by means of the individual and independent metallic protection or cladding components of each fuel wafer and that these latter are conducive to the advantageous performances of this type of fuel element. Moreover, not all the fuel wafers need necessarily be provided with an individual protection in order to obtain this result. Thus it is only necessary to ensure that one unprotected wafer is surrounded within the plate element by four wafers which are provided with this type of protection. In accordance with the invention, the individual metallic protection of the fuel wafers can be carried out in a number of different ways. In a first mode of execution of the invention, each fuel wafer is wholly covered with a sheet of thin metallic foil which serves as a wrapper for said wafer and is formed especially of zirconium or a zirconium alloy. In some cases, the ceramic nuclear fuel wafers are advantageously covered with a layer of graphite before being wrapped in the thin sheet of metallic foil. In a variant of the mode of execution aforesaid, metallic partition-strips are interposed between the covered fuel wafers in such a manner as to ensure that two adjacent wafers are separated by the whole or a part of a metallic partition-strip. In a second mode of execution, the individual metallic protection of each wafer of ceramic fuel material is provided laterally by means of a thin metallic strip which is placed as a hoop around said wafer. One of the advantages offered by this second mode of execution lies in the fact that it dispenses with the need to provide a system of guide strips for setting the fuel wafers in rows between the two cladding plates prior to diffusion bonding since the lateral metallic protection of said wafers endows these latter with a sufficient degree of strength and independence to make it possible simply to align them side by side. In this mode of execution, a preferred embodiment of the invention consists in forming the fuel plates by means of a single row of unitary wafers. This accordingly makes it possible to form narrow fuel-plate units of substantial length which are maintained in position by a system of grids placed within the fuel element casing in such a manner as to determine the respective distances between fuel-plate units. A fuel element as thus constituted is particularly advantageous in reactors of the light-water type. The above-mentioned fuel element which is fabricated from unitary fuel wafers hooped individually by means of a thin metallic strip can be constructed in accordance with the invention in two different ways. In some cases, the thin metallic strip is constituted by two strip components which are bent in the shape of a U, said components being fitted one inside the other along the sides of the wafer and secured by welding at separate points, for example by the known spot-welding technique. In other cases, the thin metallic strip is formed by winding at least one turn of metallic ribbon around the lateral perimeter of the wafer, said metallic ribbon being of small thickness within the range of 0.05 to 0.2 mm, for example. As in the first mode of execution, the metallic protection of each wafer of fuel material is preferably provided by one of the metals usually selected for cladding the fuel elements of water reactors, that is to say zirconium and its alloys, for example. The metallic strip thus employed for the purpose of hooping the wafers of ceramic fuel material has a variable but relatively small thickness within the range of 0.1 to 0.5 mm, for example. When the thin metallic strip is formed by means of U-shaped strip components, said components can be fitted one inside the other in two different ways. Thus, either the arms of one strip component or outer component completely cover the arms of the other strip component or inner component or, on the contrary, each strip component has an inner arm and an outer arm. In accordance with one characteristic feature of the invention, the ceramic fuel material is uranium dioxide and the cladding material is selected from the group comprising zirconium and the zirconium alloys; the metallic partition-strips are also formed of identical material. In accordance with another characteristic feature of the invention, the ceramic fuel material is loaded with a burnable neutron poison in order to compensate for the drop in reactivity during the lifetime of the reactor. The method of fabrication of a nuclear fuel plate in accordance with the invention essentially comprises the construction of ceramic nuclear fuel wafers protected individually by independent cladding, the positioning between two metallic cladding plates of a layer of said fuel wafers placed within a metallic frame, then the diffusion bonding of the complete assembly thus obtained under pressure and at high temperature. In an alternative embodiment of the invention, thin metallic partition-strips each forming a separation between two adjacent fuel wafers are placed between said wafers at the time of assembly. In accordance with a characteristic feature of the method which forms the subject of the invention, the diffusion bonding operation is performed while maintaining the assembly thus obtained at a temperature in the vicinity of 830.degree. C. under a pressure of the order of 1000 bars for a period of approximately 4 hours. During the operation just mentioned, the metallic frame, the individual and independent protection means and the two top and bottom metallic cladding plates fuse together to form a single piece, thus encapsulating the unitary fuel wafers within separate and independent compartments which are perfectly isolated from each other. The metallic cladding plates can be either flat or curved along their lateral edges and are always welded so as to enclose the assembly of plates in leak-tight manner prior to the aforesaid diffusion bonding operation.