Patent Number: 039403122
Section: summary

The invention relates to nuclear fuel comprised of particles of fuel carbide in which vanadium carbide is dissolved. By fuel carbide is meant within the scope of the invention, any carbide which may be used as fuel in a nuclear reactor. Said fuel carbide will thus mostly be uranium carbide or uranium-plutonium carbide. The use of carbide as fuel for nuclear reactors comes up against two main difficulties. There should first be taken into account the problem of joinability of the fuel carbide and of the stainless steel used as case material. When carbon migrates from the carbide to the case material, the mechanical properties of the steel deteriorate, which may cause a breaking of the case. The fuel carbide has moreover the property to swell strongly during the irradiating. Such swelling is caused by gaseous fission products which are insoluble in the carbide and which collect mostly on the boundaries of the grain-like particles, in the shape of gas bubbles. Due to the growing of such gas bubbles there occurs an undesirable volume increase. It is already known to obviate the chemical incompatibility between the fuel carbide and the case material, to add small amounts of metals or metal carbides, for example vanadium carbide. Due to such an addition, the chemical carbon potential is stabilized to a level where carbon migration to the case becomes thermodynamically impossible. To the contrary the problem of swelling of the fuel due to the formation of gaseous fission products during irradiating has not yet been solved. To oppose the fuel swelling, empty space has already been provided both between the case and the carbide and inside the carbide itself, as a porosity. This arrangement is but partly successful and causes further disadvantages. It has also already been proposed to precipitate finely-divided tungsten during the irradiating, because the elementary gas bubbles would be retained on the precipitate, in such a way that they will not coalesce and thus swelling of the aggregate will not take place. This measure also is not completely successful. The invention now provides a successful and economical solution, both to the fuel carbide swelling and to the incompatibility between said fuel and the stainless steel of the case material, without bringing substantial new disadvantages. For this purpose vanadium carbide is present in the particules as supersaturated solid solution and the particles are coated with a thin porous layer of vanadium carbide. It is also very important that the particles be monocrystalline. In an advantageous embodiment, the thin layer of vanadium carbide is comprised of vanadium monocarbide and vanadium hemicarbide. Preferably the density of the thin layer is low. In a particular embodiment of the invention, the thin layer has a density of about 65 percent of the theoretical density. The invention does not only relate to the above-described fuel, but also to a method for preparing a nuclear fuel. According to the invention, the method comprises preparing a supersaturated solid solution of vanadium carbide in fuel carbide, comminuting the material thus obtained down to a maximum size of the resulting grains of about 400 .mu., sieving out the particles with a maximum size lying between 20 and 400 .mu., coating the resulting particles with a thin layer of vanadium carbide having a maximum thickness of 10 .mu., and pressing together the coated particles. The particles pressed in a mold are preferably sintered. In an advantageous embodiment, the particles are coated with a thin layer of vanadium monocarbide and vanadium hemicarbide. In an useful embodiment, the supersaturated solid solution is prepared with approximately 1.5 atoms % vanadium carbide. In a preferred embodiment, to prepare the supersaturated solid solution, the fuel carbide is molten with the vanadium carbide, the resulting product is left to solidify, the product is annealed and it is very rapidly cooled. In a very particular embodiment, the particles are coated with a thin layer of vanadium monocarbide-vanadium hemicarbide with a maximum thickness of 10 .mu. by slightly moistening the particles and then bringing same in a mixer together with powder-like vanadium monocarbide-vanadium hemicarbide.