Patent Number: 039490262
Section: summary

This invention relates to a method of fabrication of nuclear reactor fuel elements which are primarily intended for use in high-temperature reactors. In general, the fuels currently employed in reactors of this type are made up of spherical particles of uranium oxide or uranium carbide which are bonded together by a mixture of graphite and thermosetting resin. Fuels of this type are placed within a graphite sheath or jacket. The methods of fabrication of these elements are attended by a number of drawbacks, in particular the need to subject the fuel to high pressures in order to permit polymerization of the resin and shaping of the element. Furthermore, a substantial temperature difference is developed between the jacket and the graphite at the time of operation of said elements. Among the different methods of fabrication of fuel elements which have been known up to the present time, mention can be made of the method described in French Pat. No. 1,588,611 filed on Sept. 23, 1968 by Commissariat a l'Energie Atomique, in which a graphite jacket obtained by extrusion is filled with fuel particles coated with a layer of graphite by spraying a mixture containing the graphite and an organic diluent. The practical application of the method according to the present invention is more straightforward. The high degree of porosity of the raw fuel element is such that the gas can readily penetrate throughout the fuel element. Finally, the method permits the fabrication of fuel elements of substantial length (lengths of 50 to 100 cm can readily be achieved). The method according to the invention meets technical requirements more effectively than those referred-to in the foregoing, particularly insofar as fuel elements having good characteristics can accordingly be obtained by virtue of the initial porosity of the semi-finished products. The method under consideration is characterized in that a jacket is fabricated by moulding in the dry state and without application of pressure a mixture of graphite and powdered resin and said jacket is subjected to a coking treatment; said jacket is then filled with a mixture of fissile particles and of graphite; the complete assembly thus obtained is then closed and treated with a gaseous hydrocarbon. According to one advantageous feature of the invention, the treatment which consists in coking the jacket is followed by a treatment of impregnation with a stream of gaseous hydrocarbon prior to filling of said jacket with the fuel particles. This treatment permits better consolidation of the jacket. According to another feature of the invention, the resin used as a constituent of the mixture which is intended to form the jacket is a phenolic resin. The method according to the invention is carried out in the following manner. A mixture of graphite having a suitable particle size and of powdered resin is placed in an aluminium mould. The mould is heated in an oven to 300.degree.C in order to obtain polymerization of the mixture. The jacket which is thus obtained is then heated to 900.degree.C in a nitrogen atmosphere in order to ensure that coking of the jacket is carried out. This coking treatment can be extended by impregnation of the jacket at the same temperature in a stream of gaseous hydrocarbon such as methane, for example. At this stage, the majority of the pores of the jacket have a diameter within the range of 30 to 50 .mu.. Said jacket is then filled with a selected mixture of fuel particles and of graphite; the particle size distribution of said graphite is comprised between 0 and 125 .mu. and can have different values for the same mixture. This mixture is carried out in the dry state or in wet phase, for example in an alcohol. In order to facilitate the obtainment of a fuel mixture having contiguous particles, a solid or hollow core having the same composition as the jacket can be placed at the center of this latter if so required. In the case of a given ratio of fissile material to graphite, the diameter of the core is chosen so as to have particles which are in close contact with each other. After the fuel mixture has been placed within the jacket, said mixture is subjected to light vibrations without application of pressure. The jacket thus obtained is then closed by a plug formed from a paste which exhibits a low degree of shrinkage at the time of baking. This paste consists of graphite and of an aqueous binder. The fuel element thus constituted is then heated to a temperature within the range of 700.degree.C and 1000.degree.C in a stream of gaseous hydrocarbon (such as methane, propane, propene, for example). The method according to the invention makes it possible to obtain fuel elements of homogeneous density within the range of 1.5 to 2 without any marked discontinuity between the jacket and the fuel. Moreover, the raw fuel elements considered as semi-finished products, that is to say prior to densification, which are obtained by means of the method according to the invention have a uniformly distributed porosity. This permits uniform impregnation of the element with the gaseous hydrocarbons without any filling of pores with excessive deposits of pyrolytic carbon. Moreover, the total residual coke yield remains of low value, namely of the order of 3 to 5 %. Since the jacket is obtained by moulding, and depending on the application which is contemplated, it is an easy matter to fabricate fuel elements having centering fins, external cooling fins, internal heat distribution fins, or either a solid or hollow core.