Temperature dependence of actinide solubilities and speciation

Most of the available thermodynamic data on the nature and solubilities of compounds, as well as solution species, of the actinides U, Np, Pu and Am come from measurements made at or near 250C. Elevated temperatures are, however, expected in a nuclear waste repository. Since oxidation state distributions, solubility product constants and formation constants for solution complexes are in general a function of temperature, one would expect the solubilities, and perhaps even the nature of stable compounds and solution species, of the actinides to change with increasing temperature in a complex way. Unfortunately, not nearly enough thermodynamic data are available at temperatures above 250C to accurately predict solubilities as a function of temperature. However, using available data and estimates of the temperature dependence of reaction constants, one can perform equilibrium calculations that may provide information on the trends in solubilities and speciation and delineate the important parameters. This was done for U, Np, Pu and Am