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Progress in Plasma Processing of Materials, 1999

ISBN Druckformat: 1-56700-126-2



Properties and composition of uranium oxide powders produced by pulverising of uranyl-nitrate solutions into plasma flow depend to a large degree on the temperature of a reaction mixture. For the preliminary evaluation of experimental conditions needed for the production of oxides with desired properties, mathematical model was created on the basis of equations of condensed phase movement related to reactor, evaporation water from the solution droplets, conservation of energy and the balance of mass. The model allows to appreciate the temperature changes of the gas medium, of the solution droplets and of the oxide particles along the length of the reactor. It was ascertained, for instance, that when using installation with the capacity 10 kW and the high-frequency inductive generation of plasma, the change of the temperature of a reactional mixture on coming out of the reactor from 350 to 850°C can be reached by reduction of the solution feed from 1.3 to 0.5 l/hour. By the lower temperature it is expected the formation of ultra-dispersed oxide powders, by the higher one powders with moderate specific area.
In order to define reducing conditions necessary for obtaining oxides of the type U02+x calculations of the oxygen thermodynamic potential were made keeping in mind the possible interval of temperatures in the reactor and different ratios of amounts of reducing agent (e.g. hydrogen), nitrate ions and water steam. The ratio H2/H20 must be not lower than 1/100 in the finished gas flow in order to receive and keep the ratio O/U in oxides near to 2.00.
A number of experiments was carried out at higher temperatures with intention to prepare oxides suitable for fuel pellets fabrication. Powders with satisfying properties were obtained: ratio of O/U = 2.05−2.18, the structure of fluorite, tapped density 1.8−2.4 g/cm3, specific area 3.8−6.8 m2/g.
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