Pure and Applied Chemistry

Abstract: If a true thermodynamic equilibrium with a well-known solid is expected to establish, chemical equilibrium thermodynamics allows estimation of the maximum concentration of a given radionuclide in a specified pore fluid of an underground repository. However, in the course of the review process for the Nagra/PSI Chemical Thermodynamic Data Base 01/01, important cases of insufficient chemical knowledge were identified, leading to gaps in the database. First, experimental data for the ThO₂–H₂O and UO₂–H₂O systems cannot be interpreted by a unique set of thermodynamic constants. There, a pragmatic approach was chosen by including parameters in the database that are not thermodynamic constants in a strict sense, but that reproduced relevant experimental observations. Second, potentially important thermodynamic constants are missing because of insufficient experimental data. Estimations of these missing constants led to problem-specific database extensions. Especially constants for ternary mixed carbonato-hydroxo complexes of tetravalent actinides have been estimated by the “backdoor approach”, i.e., by adjusting thermodynamic constants to maximum feasible values that are still consistent with all available experimental solubility data.