Conversion of UF₆ to UO₂ in a Flame Reactor

The feasibility of preparing UO₂ powder in a flame reactor was demonstrated by reacting UF₆ with hydrogen and oxygen. In the flame reactor, the reactants combine in a flame at the end of an injector contained within a reaction chamber. The reaction was conducted with stoichiometric quantities of reactants and with mixtures containing excess hydrogen or oxygen. The reaction was found to be self-sustaining after ignition with an HF flame, which was used in initial experiments to supply heat for the reaction. X-ray diffraction of the powder product indicated that UO₂ and U₃O₈ were the principal reaction products, occurring sometimes with other compounds. The crystallite size of as-prepared powder ranged from about 0.012 to 0.038 µm; however, sieve analyses showed that the powder was highly agglomerated. The as-prepared powder contained ∼ 4 to 14 wt% F, but heat treatment at 1000°C in hydrogen reduced the fluorine content to ≤ 30 ppm. The defluorination heat treatment increased the crystallite size to about an order of magnitude. Pellets pressed from defluorinated powder sintered to 95% of theoretical density at 1400°C.