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 car bonate melts under the influence of CO2 and O-2 partial pressures has been performed between 450 and 800 degrees C. At low temperatures or high CO2 pr essure an acidic melt is formed. Acidic dissolution of uranates releases ox ide ions and the possible dissolution routes of mono- and diuranates are di scussed. At high temperatures carbonates begin to decompose, lose CO2, and become basic melts. Under these conditions, basic dissolution of uranates o ccurs and again the possible routes and products are discussed. Oxygen part ial pressure influences uranate dissolution due to the formation of peroxyc arbonate complexes. Under acidic conditions and using pure O-2 under pressu re a maximum solubility of K2UO4 of ca. 37 mol ppm in (Li-Na-K)(2)CO3 eutec tic was evaluated. This system, when under CO2 and below 600 degrees C, did not produce the expected yellow coloured (quenched) melts from the dissolu tion of uranates, but gave grey melts that became yellow only on heating ab ove 600 degrees C. The reproducible grey colour is due to a minor contribut ion from intervalence bands resulting from the formation of very low concen trations of oxygen-deficient polyuranates under these conditions.