Low‐Temperature Sintering of Uranium Dioxide

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Abstract
Studies of the fabrication of uranium oxide fuel pellets by the low‐temperature sintering of nonstoichiometric oxide are described. Completer reversion to stoichiometric UO2 in the sintered pellets was attained by two methods: (1) Sintering in a nitrogen atmosphere which contained a small amount of hydrogen and (2) sintering in pure nitrogen and then exposing the pellets to hydrogen at the sintering temperature. Large variations in sinterability were found in commercially procured ceramic‐grade UO2 powder. In studying these variations, it was discovered that fluorine was a powerful inhibitor of low‐temperature sintering. This impurity could be readily removed by oxidation in air at 500°C. The data strongly indicated that the primary mechanism responsible for the removel of residual fluorine was pyrohydrolysis. It was found that a preliminary oxidation‐reduction cycle activated the less‐sinterable oxides so that in every case densities of at least 95% of theoretical were obtained by sintering at 1200°C.

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