Patent Number: 039403122
Section: description

Uranium carbide with 1.5 atoms % vanadium monocarbide is melted in the flame arc until an homogeneous molten carbide mixture is obtained. The molten mass is then left to solidify, which occurs at a temperature of about 2500.degree.C. The product is then annealed by maintaining the solidified mass at a temperature of about 2000.degree.C during approximately one hour. 1.5 atoms % vanadium carbide have thus been introduced as solution in the uranium carbide. As the vanadium carbide should remain in solution, the annealed mass is cooled very rapidly to room temperature. The cooling is performed for example in a matter of a few minutes. The resulting mass is comminuted in a ball mill until the maximum size of the broken parts is about 400 .mu.. The useful fractions are sieved out from the resulting powder. By useful fractions is meant here particles with a maximum size between 20 and 400 .mu.. Said particles are slightly moistened with liquid paraffin and brought in a mixer together with a very fine powder-like mixture of vanadium monocarbide and vanadium hemicarbide. The mean cross-section of the powdered mixture grains is not critical but lies for example around 0.06 .mu.. The weight ratio between the vanadium monocarbide amount and the vanadium hemicarbide amount is not critical but lies for example about 1:1. The density of the thin layer should be kept low. A density of for example about 65 percent of the theoretical density may be considered as desirable. As the density of VC is 5.65 g/cc and the density of V.sub.2 C is 5.66 g/cc, it is desirable to obtain a density of about 3.67 g/cc for the thin layer. The coated particles are pressed in the shape of a rod or a pellet. The resulting rod or pellet is brought to the sintering temperature. Said sintering temperature does not lie above 1400.degree.C. The fuel has thus taken the shape of monocrystalline particles of uranium carbide in which vanadium monocarbide is present as supersaturated solid solution. The monocrystalline particles 1 are surrounded by a layer 2 which is comprised of a mixture of vanadium hemicarbide and vanadium monocarbide. Said layer is porous and becomes slightly plastic by the high temperatures which are reached in the nuclear reactor. During the irradiation, the vanadium carbide precipitates out of the uranium carbide wherein it is present as supersaturated solid solution. The precipitated vanadium carbide forms a collector for the gaseous fission fragments. Due to the particles of uranium carbide which contain vanadium carbide being monocrystalline, said particles comprise no grain boundaries. This already avoids swelling. When the precipitates from the vanadium carbide in the particles 1 may not completely prevent swelling, said particles will show a slight swelling. The layer 2 comprised of vanadium monocarbide and vanadium hemicarbide will however neutralize said swelling by plastic distortion. The thin layers 2 around the particles form a network of channels along which the formed gaseous fission fragments may escape. The gaseous fission fragments thus leave the fuel. There results from the above that the chemical stabilizing is obtained through the addition of vanadium carbide to the fuel carbide. The stabilizing against swelling results from the fuel being present as monocrystalline particles, from the fuel being prepared as supersaturated solid solution from which precipitates are formed during the irradiation and from the monocrystalline particle being surrounded by a thin porous layer of vanadium carbide which acts as mechanical buffering layer by plastic distortion and as network for releasing the fission gases. Both the dispersoid structure (the vanadium carbide as supersaturated solution in the uranium carbide) and the cell structure (the thin layers of vanadium carbide around the uranium carbide particles) are very important. The invention is in no way limited to the above embodiments and many changes may be brought therein without departing from the scope of the invention as defined by the appended claims. While uranium carbide has been mentioned in the above example, the fuel carbide may also be comprised of uranium-plutonium carbide. The carbide which is introduced as supersaturated solid solution in the fuel carbide particles may be comprised of a mixture of vanadium monocarbide and vanadium hemicarbide. The layer thickness, the layer density and the various dimensions may substantially differ from the above-mentioned values.