Thermal and X-ray diffraction studies on the phase equilibria in the system UO2(NO3)2·6H2O–NaNO3

Abstract The interaction between uranyl nitrate hexahydrate and sodium nitrate at high temperatures has been studied using TG–DTA–EGA, infrared spectral and X-ray diffraction (XRD) measurements. Compound formation in the nitrate system was apparent from the differences in dehydration pattern of the nitrate mixtures as the composition was varied and also from the variation in crystallographic nature of the intermediate compounds formed. The results showed that compound formation was favoured in equimolar mixtures and intermediate nitrates of the type Na[UO 2 (NO 3 ) 3 ] · H 2 O and Na 2 [(UO 2 NO 3 ) 2 (O) 2 ] were formed. The latter compound which is essentially a dimerised sodium uranyl oxynitrate has been identified for the first time. XRD, infrared spectral and thermal stability data on this compound have been reported. It decomposes above 435°C to sodium diuranate. When the amount of sodium nitrate in the mixture is less than 50 mol%, the excess uranyl nitrate hexahydrate decomposes to U 3 O 8 through the formation of hydroxynitrates of uranium and amorphous UO 3 rather than forming a mixed nitrate or oxide of lower sodium-to-uranium ratio.