A new method for determining oxygen solubility in molten carbonates and carbonate–chloride mixtures using the oxidation of UO2 to uranate reaction

One of the possible pyrochemical reprocessing procedures for spent ceramic nuclear fuels may involve the oxidation of UO2 in alkali metal carbonate and carbonate-based melts, and this is controlled by the level of dissolved oxygen in the melt. A quantitative relationship has been derived between the extent of UO2 oxidation and the concentration of oxygen (peroxide/superoxide) species formed upon oxygen dissolution in carbonates. A novel sensitive method for determining oxygen solubility in molten carbonates and carbonate-based melts has thus been developed. The concentrations of the alkali metal uranates(VI) formed can then be accurately determined without interference from unreacted UO2. Oxygen solubilities at temperatures from 450°C to 800°C have been determined. The solubility of oxygen in a range of carbonate–chloride melts was also determined and found to increase with decreasing radius of the cation of the alkali metal chloride added. Measurements at various partial pressures of oxygen allowed the determination of the predominant oxygen species formed in the melt, and preliminary experiments showed that in the ternary carbonate melt, at 450°C, oxygen dissolves forming mainly superoxide ions. The applicability of Henryʹs Law in this situation is examined.