Thermal and X-ray diffraction studies on the phase equilibria in the system UO2(NO3)(2)center dot 6H(2)O-NaNO3

The interaction between uranyl nitrate hexahydrate and sodium nitrate at hi gh temperatures has been studied using TG-DTA-EGA, infrared spectral and X- ray diffraction (XRD) measurements. Compound formation in the nitrate syste m was apparent from the differences in dehydration pattern of the nitrate m ixtures as the composition was varied and also from the variation in crysta llographic nature of the intermediate compounds formed. The results showed that compound formation was favoured in equimolar mixtures and intermediate nitrates of the type Na[UO2(NO3)(3)]. H2O and Na-2[(UO2NO3)(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 decompos es above 435 degrees C to sodium diuranate. When the amount of sodium nitra te in the mixture is less than 50 mol%, the excess uranyl nitrate hexahydra te decomposes to U3O8 through the formation of hydroxynitrates of uranium a nd amorphous UO3 rather than forming a mixed nitrate or oxide of lower sodi um-to-uranium ratio. (C) 2000 Elsevier Science B.V. All rights reserved.