Patent Number: 059995858
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows the oxygen potential in kJ/mole calculated on the basis of the Lindemer and Besmann formula for UO.sub.2, as well as for superstoichiometric oxides UO.sub.2+x and substoichiometric oxides UO.sub.2-x, as a function of the temperature in .degree.C. FIG. 2 shows the evolution of the oxygen potential (in kJ/mole) for the Cr/Cr.sub.2 O.sub.3 pair as a function of the temperature (in .degree.C.), and it can be seen that, throughout the temperature range in question, the oxygen potential of the oxide is below that of the superstoichiometric oxides UO.sub.2+x of FIG. 1. FIG. 3 shows the evolution of the oxygen potential (in kJ/mole) for MoO.sub.2 as a function of the temperature, and it can be seen that it is still below that of the superstoichiometric oxides UO.sub.2+x at the same temperatures. Consequently these two elements are suitable as a metal able to trap oxygen for fuel materials based on UO.sub.2 and the following examples illustrate the use of the two elements with UO.sub.2. In all the examples, use is made of a UO.sub.2 powder with an average grain size of 0.5 to 100 .mu.m. EXAMPLE 1 In this example preparation takes place of UO.sub.2 pellets incorporating micrometric, metallic precipitates of Cr. 100 g of UO.sub.2 powder are mixed together with 0.1 g of metallic Cr powder having an average grain size below 2 .mu.m and then the mixture is brought into pellet form by uniaxial compression at 350 MPa, the matrix being lubricated in a hydraulic press. The pellets are then placed in a molybdenum boat and sintered at 1700.degree. C. for 4 h under dry hydrogen. This gives a small UO.sub.2 grain microstructure with micrometric, metallic precipitates of Cr. FIG. 4 is a micrograph illustrating this structure with a 600X magnification. It is clearly possible to see the intergranular or intragranular metallic precipitates (white particles), and the electron diffraction pattern confirms the metallic character of these inclusions. In order to verify the behavior of said fuel for trapping oxygen, a managed oxidation takes place of the pellets by heat treatment at 700.degree. C. in a helium atmosphere having 0.01 vol % oxygen, under conditions making it possible to achieve in the case of pure oxide an average O/U ratio of 2.024. FIG. 5 is a micrograph with a 400.times. magnification illustrating the structure of the fuel material having undergone the oxidation. It can be seen that the fuel material has trapped the oxygen and has no phases other than the previously obtained UO.sub.2 matrix. For comparison purposes, FIG. 6 shows the micrograph of a uranium dioxide pellet obtained under the same conditions as in example 1, but without any chromium addition and when it has undergone the same managed oxidation for obtaining the average O/U ratio of 2.024. FIG. 6 shows that there are U.sub.4 O.sub.9 needles in the UO.sub.2 matrix. Thus, by comparing FIGS. 5 and 6, it is possible to see the effectiveness of the metallic chromium inclusions, which have prevented the transformation of UO.sub.2 into U.sub.4 O.sub.9. EXAMPLE 2 In this example preparation takes place of uranium dioxide nuclear fuel pellets having a small UO.sub.2 grain microstructure with micrometric, metallic Cr precipitates. In this case, 100 g of UO.sub.2 powder are mixed with 0.15 g of Cr.sub.2 O.sub.3 powder (with a grain size below 2 .mu.m), followed by the formation of pellets from the mixture and they are sintered as in Example 1, under a dry hydrogen atmosphere. In this case, the added chromium oxide is reduced to metallic chromium during the sintering under dry hydrogen and has not activated the crystal growth of UO.sub.2 in order to form a large grain microstructure. Thus, a small grain microstructure is obtained with metallic Cr precipitates. FIG. 7 shows this structure. EXAMPLE 3 In this example, preparation takes place of a nuclear fuel having a UO.sub.2 small grain microstructure with metallic Cr precipitates. Preparation takes place of a powder by the atomization-drying of a slip containing 150 g of UO.sub.2, 0.6 g of a soluble chromium salt: (NH.sub.4).sub.2 CrO.sub.4 and 250 g of distilled water. The powder obtained is then calcined for 2 h in an alumina boat at 400.degree. C. in an alumina laboratory tubular furnace under an argon flow (300 ml/min) in order to transform the chromium salt into Cr.sub.2 O.sub.3. This is followed by the shaping of the powder and sintering, as in Example 1, under a dry hydrogen atmosphere. In this case, the oxygenated compound of the chromium is reduced during sintering into metallic chromium, so that it cannot serve as an activator for UO.sub.2 crystal growth. Thus, a UO.sub.2 small grain microstructure is obtained with metallic chromium precipitates. EXAMPLE 4 In this example, preparation takes place of a nuclear fuel having a UO.sub.2 large grain microstructure with nanometric, micrometric, metallic precipitates of Cr. A powder is prepared by atomization-drying, as in Example 3, using 1.5 g of (NH.sub.4).sub.2 CrO.sub.4, i.e., a Cr.sub.2 O.sub.3 content above the Cr.sub.2 O.sub.3 solubility limit in UO.sub.2 at 1700.degree. C. The powder obtained is treated in accordance with Example 3, being calcined for 2 h in an alumina boat at 400.degree. C. in an alumina laboratory tube furnace under an argon flow (300 ml/min). It is then brought into the form of pellets by uniaxial compression at 350 MPa, as in Example 1. Sintering then takes place under a hydrogen atmosphere humidified with 1.7 vol. % water, at 1700.degree. C. and for 4 h in order to keep the chromium in oxide form and assist the increase in the UO.sub.2 grain size. After sintering, an annealing treatment takes place at 1300.degree. C. for 5 h and under dry hydrogen having a water content below 0.05 vol. % in order to reduce the Cr.sub.2 O.sub.3 oxide to metallic chromium. Maintaining the Cr.sub.2 O.sub.3 in oxide form during sintering has made it possible to use it as an activator for crystal growth and in this way to obtain a large grain microstructure and the annealing treatment under dry hydrogen has then made it possible to reduce Cr.sub.2 O.sub.3 to metallic chromium and consequently obtain nanometric, micrometric, metallic precipitates. The microstructure of the material obtained under these conditions is illustrated in FIG. 8, where it is possible to see the large grains 1 of UO.sub.2 and the micrometric chromium inclusions 5. The nanometric chromium inclusions are revealed by electron diffraction. EXAMPLE 5 A powder is prepared as in Example 3 by atomization-drying, but using 0.2 g of (NH.sub.4).sub.2 CrO.sub.4, i.e. a Cr.sub.2 O.sub.3 equivalent content below the solubility limit of Cr.sub.2 O.sub.3 in UO.sub.2 at 1700.degree. C. This is followed by the compression of the powder in the form of pellets and sintering as in Example 4 to obtain a large grain microstructure due to the maintaining of the chromium in oxide form. This is followed by an annealing treatment as in Example 4 for reducing Cr.sub.2 O.sub.3 into metallic chromium. In this case, a large grain UO.sub.2 microstructure is obtained with nanometric metallic precipitates of Cr, because there was no Cr.sub.2 O.sub.3 excess for forming metallic, micrometric precipitates during the reduction. EXAMPLE 6 This example adopts the same operating procedure as in Example 4, but use is made of 1.5 g of (NH.sub.4).sub.2 CrO.sub.4 and 0.04 g of ultrafine SiO.sub.2 in slip containing 150 g of UO.sub.2 and 250 g of distilled water. The powder obtained by atomization-drying is compressed in pellet form and then sintered in a humidified hydrogen atmosphere and subjected to an annealing treatment under dry hydrogen, under the same conditions as in Example 4. This gives a large grain UO.sub.2 microstructure with metallic chromium precipitates and a silica phase at the grain boundaries. EXAMPLE 7 In this example a mixture of 100 g of UO.sub.2 and 0.6 g of MoO.sub.3 is prepared by cogrinding in a metallic uranium ball jar, followed by the compression of the powder mixture to pellet form and sintering under the same conditions as in Example 1. In this case, the molybdenum oxide is reduced to molybdenum during sintering and it is not possible to active the crystal growth of the UO.sub.2 grains. Thus, a small grain UO.sub.2 microstructure is obtained with micrometric, metallic precipitates of Mo. EXAMPLE 8 A powder is obtained by atomization-drying of an aqueous suspension constituted by 150 g of UO.sub.2 and 7.7 g of ammonium heptamolybdate (NH.sub.4).sub.6 Mo.sub.7 O.sub.24, 4H.sub.2 O and 250 g of distilled water. The powder is then treated as in Example 1. This gives a small grain UO.sub.2 microstructure with micrometric, metallic Mo precipitates.