Thermal analysis studies of the reactions occurring during the decomposition of ammonium uranyl carbonate in different atmospheres

Abstract

The thermal decomposition of ammonium uranyl carbonate (AUC) has been studied in H 2, H 2 + 10 volume% CO 2, N 2, Ar, CO 2 and O 2 atmospheres. Auc decomposes at around 190°C, giving off CO 2, NH 3 and H 2O with formation of an amorphous phase. This phase absorbs water, both from the decomposition of AUC and from the ambient atmosphere. The amorphous phase contains UO 3, H 2O and some carbon and nitrogen. These elements are driven off upon further heating (above 300°C), together with some of the water. In N 2, Ar and CO 2 atmosphere the amorphous phase crystallizes to α-UO 3 before decomposing to U 3O 8. The temperature for this decomposition varies with the atmosphere used. In an O 2 atmosphere the amorphous phase is retained to higher temperatures (580°C), where it slowly decomposes to U 3O 8 via an intermediate phase, UO 2.9. In H 2 and H 2 + 10 volume% CO 2, U 3O 8 is formed from the amorphous phase in the temperature range 400–450°C, and UO 2 between 450–500°C. Experiments in which AUC was heated to 300°C in one atmosphere, e.g. H 2, and subsequently up to 800°C in another one, e.g. O 2, are also reported.