Technical Report NTB 02-12

If a true thermodynamic equilibrium with a well-known solid has established, chemical equilibrium thermodynamics allows estimation of the maximum concentration of a given radionuclide in a specified pore fluid of an underground repository. In the course of the review process for the Nagra/PSI Chemical Thermodynamic Data Base 01/01 we identified important cases of insufficient chemical knowledge leading to gaps in the database. First, experimental data for the systems ThO₂ – H₂O and UO₂ – H₂O cannot be interpreted by a unique set of thermodynamic constants. There we chose a pragmatic approach 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 hydroxide-carbonate 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. We conclude that gaps in the Nagra/PSI TDB 01/01 concerning Th, U, Np and Pu data do not lead to grossly wrong estimates of their respective solubility limits in the case of a bentonite pore water defined for the planned Swiss high-level waste repository. Attempts to improve our chemical knowledge should concentrate on the key parameters identified in this study.