Experimental study and modeling of the solid-liquid equilibrium of the binary system of H2O-UO2(NO3)(2) and uranyl nitrate

The binary system H2O-UO2(NO3)(2) was studied by solubility measurements an d constant heat flow thermal analysis. Temperature and composition of the e utectic transformation between ice and uranyl nitrate hexahydrate were accu rately defined. A new hydrate with 24 molecules of water decomposes at -21 degrees C according to the peritectoid reaction < UO2(NO3)(2). 24H(2)O> reversible arrow <UO2(NO3)2 . 6H(2)O> + 18 < H2O> The quasi-ideal model was applied to the solid-liquid equilibria, using the following reaction hypothesis: ((UO22+))+2((NO3-))+h((H2O)) reversible arrow ((UO(2)OH(+)aq))+((H(3)O(+)aq +2((NO(3)(-)aq)) A complete calculation of the binary system was carried out with a global i onic hydration number h equal to 9 in the aqueous solutions. It allowed to the melting enthalpies of uranyl nitrate hydrates.