Patent Number: 050911201
Section: description

EXAMPLE 1 This example illustrates the two activation variants during the process according to the invention. A diameter 15 mm, length 10 cm uranium bar was oxidized in the presence of air at 600.degree. C. for 8 h. This gave a coarse U.sub.3 O.sub.8 powder crushed until a powder with an average grain size of approximately 27 .mu.m was obtained. This powder was subdivided into two parts. The first part (test 1) was activated by passing into a fluidized bed gas jet mill to give a powder with an average grain size of approximately 2 .mu.m. It was then reduced to UO.sub.2 by a mixture of H.sub.2 (20 l/min) and N.sub.2 (8 l/min) at 600.degree. C. for 5 h. The UO.sub.2 powder obtained was passivated at 50.degree. C. for 31/2 hours by an air-nitrogen mixture progressively enriched with air to 37% by volume. Its specific BET surface was then 1.74 m.sup.2 /g, its average grain size 2.1 .mu.m (laser grain size distribution carried out with the aid of the CILAS apparatus) and its apparent specific mass was 1.48 g/cm.sup.3. The uranium oxide powder obtained with the characteristics described hereinbefore was shaped by compacting and then fritted at 1740.degree. C. for 3 h under hydrogen. The density results of the fritted pellets are between 96.62 and 97.11% of the theoretical density of the uranium oxide and appear in table 1 (test 1). The second part (test 2) of the crushed U.sub.3 O.sub.8 powder was firstly reduced to the UO.sub.2 state and passivated under the same conditions as hereinbefore, followed by activation with the gas jet mill under the following conditions: Milling pressure, 6 bars relative at the end of the test, turbine rotation speed: 15,000 r.p.m. The uranium oxide powder obtained had a specific surface of 2.04 m.sup.2 /g, an average grain size of 0.8 .mu.m (CILAS laser granulometer) and an apparent specific mass of 2.03 g/cm.sup.3. The powder was shaped and fritted under the same conditions as in test 1. The density results of the fritted pellets are between 96.85 and 97.03% of the theoretical density and appear in table 1 (test 2). TABLE 1 ______________________________________ UO.sub.2 before Crude Fritted pellet compacting: Compacting pellet density Test specific surface pressure density % of theoretical No. m.sup.2 /g MPa t/cm.sup.2 g/cm.sup.3 density ______________________________________ 1 1.74 290 2.96 6.45 96.62 368 3.75 6.65 96.95 425 4.34 6.81 97.11 2 2.04 290 2.96 6.49 96.85 368 3.75 6.65 96.94 425 4.34 6.78 97.03 ______________________________________ It should be noted that in this example the compacting pressures are particularly low, but still correspond to crude and fritted densities well above the normal. EXAMPLE 2 This example compares pellets obtained without an activation treatment and those obtained with an activation treatment, the results being given in table 2. Firstly a metallic uranium bar was oxidized with air at a temperature between 450.degree. and 550.degree. C. for 8 h. This powder was crushed to 10 .mu.m in the manner described in example 1 and then divided into two parts (batches 1 and 2). The first part (batch 1) was reduced under 50% hydrogen: 50% nitrogen at 600.degree. C. for 41/2 hours and then screened to 250 .mu.m. Its specific surface was 1.05 m.sup.2 /g. Its average grain size was 7 .mu.m (Sedigraph sedimentation granulometer) and its apparent specific mass was 2.06 g/cm.sup.3. The thus obtained uranium oxide powder was subdivided into two further batches: Batch 3 was directly shaped without granulation and then fritted under hydrogen at 1700.degree. C. for 4 h. The densities of the fritted pellets were between 87.35 and 89.26% of theoretical density, despite the good crude densities (table 2, test 3). Batch 4 was granulated and then shaped and fritted under the same conditions as batch 3. Thus, optimum conditions existed for obtaining good densities of the fritted pellets. The fritted densities were between 89.26 and 89.58% of the theoretical density (table 2, test 4). They are better than the previous values, but are still inadequate. The second part (batch 2) of the coarsely milled U.sub.3 O.sub.8 powder was firstly reduced to the state UO.sub.2 by treatment under hydrogen identical to that carried out for batches 3 and 4. It was then activated by an oxidation-reduction treatment under the following conditions: ______________________________________ Oxidation: temperature 400.degree. C. atmosphere air (10 m.sup.3 /h) residence time 6 h Reduction: temperature 600.degree. C. atmosphere H.sub.2 (3 m.sup.3 /h) + N.sub.2 (3 m.sup.3 /h) residence time 6 h. ______________________________________ The powder obtained had an average grain size of 4.8 um (Sedigraph granulometer) a BET specific surface of 3.49 m.sup.2 /g and an apparent specific mass of 1.38 g/cm.sup.3. It was then directly shaped without prior granulation and then fritted in the manner described hereinbefore (H.sub.2, 1700.degree. C., 4 h). The fritted densities obtained were between 95.83 and 96.73% of the theoretical density (table 2, test 5). Table 2 also shows the strength of the crude pellets, which are generally shaped like cylinders of limited height. The test consists of inserting the pellet between two parallel planar jaws bearing on two diametrically opposite generatrixes of the pellet and measuring the force necessary for breaking, the value being given in table 2. TABLE 2 ______________________________________ Fritted UO.sub.2 be- pellet fore com- density, pacting: Crude % of Strength of specific Compacting pellet theoret- crude Test surface pressure density ical pellets No. m.sup.2 /g MPa (t/cm.sup.2) g/cm.sup.3 density daN ______________________________________ 3-1 1.05 200 (2.04) 5.60 87.35 3.4 3-2 1.05 242 (2.47) 5.80 88.17 4.3 3-3 1.05 303 (3.09) 5.80 88.92 7.0 3-4 1.05 345 (3.52) 6.10 89.26 10.0 4-1 1.05 187 (1.91) 5.60 89.26 3.4 4-2 1.05 229 (2.34) 5.80 89.47 4.6 4-3 1.05 290 (2.96) 6.00 89.34 6.5 4-4 1.05 332 (3.39) 6.20 89.58 7.5 5-1 3.49 479 (4.89) 5.50 95.87 58.3 5-2 3.49 526 (5.37) 5.80 96.30 92.6 5-3 3.49 588 (6.00) 6.00 96.49 115.8 5-4 3.49 684 (6.98) 6.20 96.73 124.2 ______________________________________ The powder of example 2, test 5, according to the invention, led to pellets with remarkable characteristics. The crude pellets are very strong, which is an advantage from the handling standpoint. Most of the fritted densities are above 96% and in all cases exceed the minimum required, whereas, without activation, it was not possible to obtain 90% of the theoretical density, even with a granulation operation.