Thermodynamic properties of chemical species of waste radionuclides

The object of the experimental program at Lawrence Berkeley Laboratory is to identify gaps or conflicts in thermodynamic data on the solubilities of compounds and on the formation of solution complexes of waste radionuclides needed for the reliable prediction of solution concentrations. It involves laboratory measurements necessary to (1) generate specific new data, where none exists, in order to demonstrate the importance of a particular solution species, compound or solution parameter (e.g., temperature, Eh) and to (2) resolve conflicts in existing thermodynamic data on important species or compounds. The measurement of the solubility of AmOHCO3 in 0.1 M NaClO4 at 250C and 1 atmosphere pressure has been completed. From the experimental data, an average solubility product quotient, Qsp, was evaluated for the reaction, AmOHCO3 (S) + 2H+ = Am3+ + HCO3- + H2O. The logarithm of Qsp was calculated to be 2.74 +/- .17. Speciation calculations, using this new data plus reported data on the solubility of Am(OH)3 and the hydrolysis and carbonate complexation of Am3+, indicate that the presence of carbonate can have a substantial effect on the nature of compounds and solution species formed by americium in ground waters. Since actinides in a given oxidation state tend to exhibit similar chemical properties, this result should apply to other actinides in the trivalent state. Thus, the effect of carbonate on the solubilities and complexation of trivalent actinides should be included in any predictive modelling studies required for licensing. 27 references, 4 figures, 5 tables