Patent Number: 048896632
Section: description

EXAMPLE 1 The starting oxide, obtained by a dry process in a furnace of the kind described in U.S. Pat. No. 4,397,824 is a UO.sub.2 powder. The powder obtained is separated into two portions. One of the portions, which represents 19.1% by weight of the total, is oxidized by passing in boats through a furnace at 330.degree. C. through which air flows. The stay time in the furnace is 3 hours. The remaining UO.sub.2 powder and the U.sub.3 O.sub.8 powder thus obtained are calibrated by passing through a rotary sifter, of turbo-calibrator type. The powders obtained are mixed in a 5000 liter "NAUTAMIX" mixer. The mesh size of the sifter may range from 200 to 350 microns. The homogenized mixture is granulated by precompression, then crushed into grains of about 800 microns, using a conventional technique. The granulates are then compressed into pellets of a diameter of 10 mm and a height of 17 mm. A comparison between the green pellets thus obtained, without addition of binder, and pellets manufactured from UO.sub.2 coming directly from the conversion furnace and also sifted, was carried out. Tests have shown that the green pellets manufactured from the intimate UO.sub.2 --U.sub.3 O.sub.8 mixture have a much higher crushing strength. The crushing test consisted in measuring the force required to break a given pellet by compressing it between two parallel surfaces (Brazilian test). For UO.sub.2 powder pellets, about 20 daN was obtained with O/U=2.04. For pellets made from a UO.sub.2 --U.sub.3 O.sub.8 mixture, it required 67 daN for O/U=2.15. EXAMPLE 2 Tests were carried out to determine the influence of the temperature for oxidizing the powder and the unfavorable effect of high temperature oxidation. For that, mixtures were prepared by sifting together the same powders as those described in Example 1, but with U.sub.3 O.sub.8 powders obtained by oxidation at different temperatures. The results obtained are given in the following table 1. They show that the advantages of the method of the invention decrease if the oxidation temperature increases. TABLE I ______________________________________ % powder O/U of strength of oxidized oxidation t.degree. mixture pellets (daN) ______________________________________ 24.6 300.degree. C. 2.22 65 24.6 550.degree. C. 2.22 45 24.6 900.degree. C. 2.22 23 UO.sub.2 powder 2.05 18 ______________________________________ EXAMPLE 3 12,000 kg of UO.sub.2 powder obtained by the "dry" process described in the document FR-A-2 060242 was sifted using a depression sifter having a mesh size of 104 microns. An amount of 25% of the powder to be sifted was oxidized at 350.degree. C. for 4 hours, in air, then mixed with the non-oxidized powder in a plough share mixer having a disagglomeration turbine, for 60 minutes. The UO.sub.2 --U.sub.3 O.sub.8 mixture obtained had an O/U ratio of about 2.22. The powder mixture was granulated by pre-compacting, then compacting using a conventional procedure. The granulates were compressed into pellets 10 mm in diameter and 15.2 mm in height, having a green density of 6.1 g/cm.sup.3, in a rotary press. The pellets were then sintered at 700.degree. C. for 3 hours in a hydrogen atmosphere. The fragility of the green pellets was determined by abrasion of the pellets in a squirrel cage and by measuring the loss in weight after rotating for more than 600 revolutions. This method of measurement characterizes the fragility of the edges, determinant during handling of the green pellets. The fragility test carried out on the green pellets prepared from the UO.sub.2 --U.sub.3 O.sub.8 mixture revealed a loss of material of 6.5%, whereas pellets manufactured from UO.sub.2 suffered a loss of 35%. In addition, the sinterability of the mixture was improved since a density of 97.39% of the theoretical density was obtained instead of 97.19% for the UO.sub.2 powder. EXAMPLE 4 700 kg of powder formed of: 80% by weight of UO.sub.2 powder obtained by a dry process, and PA1 20% of powder obtained by oxidation at 330.degree. C. for 4 hours of UO.sub.2 powder obtained by a dry process, were mixed for 60 minutes in a twin screw orbital mixer having a disagglomeration turbine. These powders had been previously sifted using a depression sifter with a mesh of 150 microns. The mixture was granulated by precompacting and crushing. The granulates were compressed into pellets with different rates of compression, in a die of 9.90 mm in diameter so as to produce green pieces 16 mm in height. The pellets were then sintered in a continuous furnace, being kept at the sintering temperature of 1760.degree. C. for 3.5 hours. The same compacting and sintering tests were carried out with the UO.sub.2 powder. The results of the comparative tests are given in the following table 2. TABLE 2 ______________________________________ With the UO.sub.2 powder Compacting Density Fragility of Density pressure (T) (g/cm3) the edges (%) (% TD) ______________________________________ 2.5 5.77 25.4 97.42 3.5 6.02 19.1 97.63 4.5 6.29 20.0 97.81 5.5 6.44 15.5 97.94 ______________________________________ With the UO.sub.2 --U.sub.3 O.sub.8 powder Compacting Density Fragility of Density pressure (T) (g/cm3) the edges (%) (% TD) ______________________________________ 2.5 5.60 11.05 97.31 3.5 5.90 10.0 97.49 4.5 6.11 8.3 97.75 5.5 6.29 6.1 97.90 ______________________________________ It can be seen that the green pellets made from the UO.sub.2 --U.sub.3 O.sub.8 mixture are appreciably stronger. In addition, the sinterability is slightly improved. Depending on the intrinsic characteristics of the oxides to be formed into pellets and of the pellet forming installation, optimum strength for green pellets can be obtained by adjusting the U.sub.3 O.sub.8 content and the compacting pressure. Similar tests were made on pellets including, in addition to uranium oxide, an absorbant material oxide (gadolinium). These tests showed that the same favorable results are obtained from the dual point of view of reduction of green fragility and the absence of degradation of the sintered product.