Patent Number: 048896632
Section: summary

BACKGROUND OF THE INVENTION 1. Technical Field The invention relates to methods of manufacturing uranium oxide base pellets for use as nuclear fuel and more particularly to manufacture of "green" pellets by cold compression of a powder whose major component is uranium dioxide. The invention makes use of uranium dioxide powder obtained by a dry process (i.e., by direct reduction of UF.sub.6 into dioxide). The powders obtained by the dry conversion process, described in French No. 2,060,242 and U.S. Pat. No. 4,397,824, including water vapor hydrolysis followed by pyrohydrolysis of the uranyl fluoride UO.sub.2 F.sub.2 obtained, have the advantage of being readily sinterable. On the other hand, the green pellets obtained by compression are relatively fragile. Handling thereof is delicate; the rejects are numerous if special care is not exercised. 2. Prior Art Different methods have been proposed for reducing the fragility of green pellets. Attempts have been made to increase the density in green condition by increasing the compacting pressure, which has the drawback of causing premature wear of the compression dies. It has been proposed to add a binder and to granulate the powder and/or to subject the powder to different treatments (French 2,561,026). All of these solutions have drawbacks. Those which use water or a binder raise criticality problems. The methods are generally applicable only to small batches, practically not exceeding 50 kg. The fragility of the green pellets obtained by compression of UO.sub.2 powder obtained by dry conversion may probably be attributed to the fact that the powder is formed of fine crystallites more or less rounded, with a very low oxygen over-stoechiometry (ratio O/U of from 2.02 to 2.06). Reoxidation of the powder improves the strength of the pellets. Surface oxidization of the uranium oxide UO.sub.2 redivides the grains into jagged aggregates with intertwining ramifications. The powder then has properties comparable to those of a powder obtained by a wet process: during compression of the pellets into shape, the grains engage each other and provide coherence. But limited surface oxidization to UO.sub.2 powder is difficult to achieve industrially. Oxidation tends to bolt until the whole of the oxide has oxidized to U.sub.3 O.sub.8 which is difficult to sinter into pellets free of cracks, unless a slow controlled reduction step is added to the manufacture during presintering. Moreover, oxidation generates islets of U.sub.3 O.sub.8 which, during sintering, shrink more than the UO.sub.2 grains, whence a heterogeneous texture. It is moreover practically impossible to carry out oxidation in a reactor for direct conversion of UF.sub.6, such as described in U.S. Pat. No. 4,397,824. It is, moreover, current practice to recycle waste from manufacture of the pellets, such as the oxidized grinding muds, the rejected pellets, the splinters and residues of powders in the form of U.sub.3 O.sub.8 (French 2,001,113). These oxidized powders, while they slightly improve the strength of the green pellets if mixed with UO.sub.2 before compacting and sintering, have a very unfavorable effect on the density and texture of the sintered pellets, for the U.sub.3 O.sub.8 is in the form of dense and coarse grains. The percentage of recycled U.sub.3 O.sub.8 in the green pellets is generally limited to 12% wt. at most. It has further been suggested to mix UO.sub.2 obtained by a wet process with an amount of U.sub.3 O.sub.8 prepared for that purpose and whose function is to generate pores which collect fission gases in the sintered pellets (French No. 1,412,878). SUMMARY OF THE INVENTION It is an object of the invention to provide a manufacturing method which appreciably increases the strength of the green pellets without having an unfavorable effect on the properties of the sintered pellets. With that purpose in mind, there is provided a method in which fine powder of uranium oxide UO.sub.2 obtained by dry conversion is mixed with a proportion less than 40% by weight of uranium oxide powder substantially in the state of reactive U.sub.3 O.sub.8 and having a grain size less than 350 microns. The total content of U.sub.3 O.sub.8 in the mixture is preferably higher than 5%, typically from 15 to 25% wt. The fine powder typically consists of elementary particles whose size is in the micron and sub-micron range, agglomerated into grains which pass across a 350 .mu.m sieve. The fine powder representing 60% wt. at least of the mixture may include up to 10% wt. of an oxide of at least another element, such as Pu, Th and Gd. The U.sub.3 O.sub.8 powder is advantageously obtained by oxidation in air of UO.sub.2 at a temperature lesser than 800.degree. C. It must be continued until the O/U ratio corresponds substantially to the stoechiometry of the U.sub.3 O.sub.8 oxide. To avoid sintering of the U.sub.3 O.sub.8 powder and obtaining a good desaggregation of this U.sub.3 O.sub.8 during subsequent mixing with the UO.sub.2 powder, the oxidation is preferably carried out at a temperature between 250.degree. C. and 350.degree. C. When so prepared, U.sub.3 O.sub.8 has no pore forming action comparable to that of the U.sub.3 O.sub.8 used in the process according to French No. 1,412,878. The improvement in the strength of the green pellets increases with the content of oxidized powder. In practice, it is desirable to use a U.sub.3 O.sub.8 content of at least 5% by weight. Thus the rejects and the faults caused by handling are reduced and the manufacturing yield is increased. The method of the invention is applicable not only to the manufacture of uranium oxide pellets but also to that of combined oxide UO.sub.2 base pellets, the best known of which are of UO.sub.2 --PuO.sub.2, UO.sub.2 --ThO.sub.2, UO.sub.2 --Gd.sub.2 O.sub.3 types. Recycled UO.sub.3 may also be incorporated in an amount not exceeding 10% wt. In all cases, it is necessary to achieve intimate mixing of the powders before compacting to obtain pellets. This mixing may particularly be achieved: from UO.sub.2 and U.sub.3 O.sub.8 powders crushed together in a hammer crusher, PA1 from UO.sub.2 and U.sub.3 O.sub.8 powders mixed in an arm mixer and equipped with a disaggglomeration turbine. Before mixing, the powders may be disagglomerated by crushing and/or sifting, the desired result being the absence of U.sub.3 O.sub.8 agglomerates. In all cases, the mixing may be carried out in industrial equipment so as to obtain large homogeneous batches, which may exceed two tons of powder when the enrichment is sufficiently low for there to be no problem of criticality. The invention also provides a complete method for manufacturing uranium dioxide based pellets in which green pellets are prepared by the above described steps, so as to obtain pellets formed by an intimate mixture of UO.sub.2 and U.sub.3 O.sub.8 without isolated U.sub.3 O.sub.8 agglomerates. Then the green pellets are sintered at a temperature allowing consolidation to be obtained, usually from 1500.degree. to 1800.degree. C., when the sintering takes place in a reducing atmosphere, from 1200.degree. to 1350.degree. C. when the sintering takes place in a slightly oxidizing atmosphere. Different embodiments of the invention will now be given, it being understood that they are in no wise limitative.