Chemical solubility of alkali metal uranate(VI) species in molten carbonates under basic and acidic conditions

A detailed investigation of the dissolution of alkali metal uranates in carbonate melts under the influence of CO2 and O2 partial pressures has been performed between 450 and 800°C. At low temperatures or high CO2 pressure an acidic melt is formed. Acidic dissolution of uranates releases oxide ions and the possible dissolution routes of mono- and diuranates are discussed. At high temperatures carbonates begin to decompose, lose CO2, and become basic melts. Under these conditions, basic dissolution of uranates occurs and again the possible routes and products are discussed. Oxygen partial pressure influences uranate dissolution due to the formation of peroxycarbonate complexes. Under acidic conditions and using pure O2 under pressure a maximum solubility of K2UO4 of ca. 37 mol ppm in (Li–Na–K)2CO3 eutectic was evaluated. This system, when under CO2 and below 600°C, did not produce the expected yellow coloured (quenched) melts from the dissolution of uranates, but gave grey melts that became yellow only on heating above 600°C. The reproducible grey colour is due to a minor contribution from intervalence bands resulting from the formation of very low concentrations of oxygen-deficient polyuranates under these conditions.