Thermodynamic Date for Predicting Concentrations of Th(IV), U(IV), Np(IV), and Pu(IV) in Geologic Environments

Rai, D.*; Rao, L.*; Weger, H. T.*; Felmy, A. R.*; Choppin, G. R.*; Yui, Mikazu

This report provides thermodynamic data for predicting concentrations of Th(IV), U(IV), Np(IV), and Pu(IV) in geologic environments, and contributes to an integration of the JNC chemical thermodynamic database, JNC-TDB (previously PNC-TDB), for the performance analysis of geological isolation system for high-level radioactive wastes. Thermodynamic data for the formation of complexes or compounds with hydroxide, chloride, fluoride, carbonate, nitrate, sulfate and phosphate are discussed in this report. Where data for specific actinide(IV) species was lacking, the data were selected based on chemical analogy to other tetravalent actinides. ln this study, the Pitzer ion-interaction model is used to extrapolate thermodynamic constants to zero ionic strength at 25C.