Decomposition and multiphase reactions in the system UO2(NO3)2·6H2O–Ni(NO3)2·6H2O at elevated temperatures

Solid state reactions between uranyl nitrate hexahydrate and nickel nitrate hexahydrate in mixtures of various ratios have been studied at elevated temperatures. The binary system of uranyl nitrate hexahydrate and nickel nitrate hexahydrate was found to form a eutectic of composition 53 mol% uranyl nitrate hexahydrate and 47 mol% nickel nitrate hexahydrate at 40 °C. The overlap of evolution of nitric oxide (NO) and water vapour above 230 °C confirmed the presence of hydroxynitrates of uranium and nickel as intermediate products. These hydroxynitrates began to react above 280 °C to form nickel uranate, NiU3O10, in the case of mixtures containing 75 mol% uranyl nitrate hexahydrate. When the proportion of uranyl nitrate hexahydrate in the mixture was higher than 75 mol%, U3O8 formed along with nickel uranate. For the mixtures containing uranyl nitrate hexahydrate lower than 75 mol%, NiO was observed to form along with NiU3O10.