Manufacture of Uranium Dioxide Powder

The present invention concerns the manufacture of uranium dioxide powder by the ammonium uranyl carbonate method, also called the AUC method, in which UF.sub.6 is first transformed into the gaseous phase and the latter is hydrolyzed in a water receiver with the simultaneous addition of NH.sub.3 and CO.sub.2 ; the AUC precipitated thereby is separated from the mother liquor by filtering and washing, and the UO.sub.2 is formed by decomposition and reduction of the AUC at 500.degree.C in a water vapor/hydrogen atmosphere. The uranium dioxide powder prepared in this manner is put into pellet form, for instance, by pressing and sintering and is enclosed as nuclear fuel in the fuel cladding tubes of nuclear reactor fuel elements. This manufacture of uranium dioxide is described in detail in German Pat. Nos. 1,126,363; 1,592,477 and 1,924,594.

The thermal decomposition of the AUC and the reduction of the hexavalent oxide into UO.sub.2 in hydrogen gas is normally carried out in a furnace or in two separate furnace units, such as so-called vortex-bed furnaces.

The thermal decomposition of the AUC is an endothermic process step which, together with the adhering moisture, limits the capacity of any furnace, as a substantial part of the heat that is to be supplied by the furnace to the powder is first required to evaporate the residual water. In order to increase the throughput of these vortex-bed furnaces, it would, therefore, be desirable to reduce the moisture content of the AUC as much as possible.

According to the invention, the above objective is reached by washing the AUC, after separation from the mother liquor, with an ammonium hydrogen carbonate or an NH.sub.4 OH solution and then post-treating it with a liquid which reduces the surface tension of the residual water in the AUC.

With the former process technique, the AUC filter cake had a residual water content of about 7 to 10 percent after washing and a subsequent 1 hour drying period, using the suction-drying principle. A further extension of the drying period resulted in no appreciable reduction of the water content. The method according to the present invention now makes a reduction of the water content to 0.5 to 0.8 percent possible. For this purpose, the filter cake is washed several times with a liquid, which is water-soluble and reduces the surface tension. A suitable liquid is alcohol, and preferably methyl alcohol. It is easy to understand that drying according to this treatment is relatively simple and that the evaporation of the residual alcohol in the vortex-bed furnace requires substantially less thermal energy than the water-moist AUC used heretofore. Surprisingly, it has been found that washing with alcohol leads to a further lowering of the fluorine content by displacement of the mother liquor already in the AUC, so that the pyrohydrolysis time required for the reduction of the fluorine content in carrying out the AUC process can be reduced, or that, with the pyrohydrolysis time being kept the same, the fluorine content in the UO.sub.2 powder becomes smaller. Such a reduction of the fluorine content, however, is particularly important, as the useful life of the fuel rods in a nuclear reactor depends to a very large extent on the fluorine content of the nuclear fuel manufactured from this powder.

To explain the present invention further, the manufacturing process of uranium oxide powder according to this invention, is disclosed in more detail as follows:

The suspension containing the AUC precipitate is placed on a rotary vacuum filter. The motor liquor, approximately 1 m.sup.3, is suctioned off for about one hour, whereupon the filter cake of approximately 500 kg weight, with about 0.3 to 0.4% by weight of fluorine, is washed three times with 50 liters each of a 1-molar ammonium hydrogen carbonate solution or an NH.sub.4 OH solution. Then drying suction is again applied for one hour. After this, the filter cake contains still about 7 to 10% of water, and its fluorine content, referred to the uranium, is approximately 300 ppm. Now the cake is washed twice with 35 liters each of methyl alcohol (methanol), where a reaction time of about 20 minutes is interposed before suctioning off, so that the ammonium hydrogen carbonate solution or the NH.sub.4 OH solution can be taken up by the methyl alcohol. Drying-suction is then applied again, for which about one-half hour is sufficient. The residual content of water, methanol and fluorine have thereby been reduced very substantially, as may be seen from the following Table, in which the depletion of the water and fluorine content in the AUC filter cake is shown in ten tests before and after washing with methanol.

From this will be seen that the throughput in the vortex-bed furnaces, in which the UO.sub.2 is formed, is substantially greater with the pretreatment of this invention. If the production rate of UO.sub.2 without washing the AUC with methanol is about 40 kg per hour, it can be increased for the same furnace to 60 to 80 kg per hour when this invention is applied.

In conclusion, it should be pointed out once more that in addition to this quite substantial increase in production capacity, an exceedingly advantageous reduction of the residual fluorine content is achieved.