Database / Thermodynamic equilibrium constants / RadionuclidesSummary. The Nagra/PSI Chemical Thermodynamic Data Base has been updated to support performance assessments of the planned Swiss repositories for radioactive waste. The update from version 05/92 to 01/01 involved major revisions for most of the actinides and fission products. Altogether, more than 70% of the database contents have been revised. Data for U, Np, Pu, Am and Tc recommended by the NEA TDB project were considered in the update. Thermodynamic data for Th, Sn, Eu, Pd, Al, and solubility and metal complexation of sulphides and silicates were extensively reviewed. Data for Zr, Ni and Se were examined less rigorously as these elements are currently being reviewed in phase II of the NEA TDB project. Our experiences from this two year team effort can be summarised as follows. Detailed in-house reviews and critical appraisal of NEA recommendations greatly improved the chemical consistency and quality of the selected data. On the other hand, we could discern major gaps in the data, especially missing carbonate complexes. In some systems, e.g. Th(IV)-H 2 O and U(IV)-H 2 O, experimental data could not be described by a unique set of thermodynamic constants. There, a pragmatic approach based on solubility data was chosen for application to performance assessment. The electronic version of our database and information concerning its full documentation is available on our PSI web site (http://www.psi.ch/les).

Reactive mass transport (RMT) simulation is a powerful numerical tool to advance our understanding of complex geochemical processes and their feedbacks in relevant subsurface systems. Thermodynamic equilibrium defines the baseline for solubility, chemical kinetics, and RMT in general. Efficient RMT simulations can be based on the operator-splitting approach, where the solver of chemical equilibria is called by the mass transport part for each control volume whose composition, temperature, or pressure has changed. Modeling of complex natural systems requires consideration of multiphase-multicomponent geochemical models that include nonideal solutions (aqueous electrolytes, fluids, gases, solid solutions, and melts). Direct Gibbs energy minimization (GEM) methods have numerous advantages for the realistic geochemical modeling of such fluid-rock systems. Substantial improvements and extensions to the revised GEM interior point method algorithm based on Karpov's convex programming approach are described, as implemented in the GEMS3K C/C++ code, which is also the numerical kernel of GEM-Selektor v.3 package (http://gems.web.psi.ch). GEMS3K is presented in the context of the essential criteria of chemical plausibility, robustness of results, mass balance accuracy, numerical stability, speed, and portability to high-performance computing systems. The stand-alone GEMS3K code can treat very complex chemical systems with many nonideal solution phases accurately. It is fast, delivering chemically plausible and accurate results with the same or better mass balance precision as that of conventional speciation codes. GEMS3K is already used in several coupled RMT codes (e.g., OpenGeoSys-GEMS) capable of high-performance computing.

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Hydrated calciumsilicates are the main constituents of hydrated Portland cements. Their chemistry will strongly influence the longterm behaviour of a concrete system envisioned in use in radioactive waste repositories.Experimental data show that hydrated calciumsilicates dissolve incongruently, depending on the calcium/silicon ratio of the solid. A model that simulates the incongruent dissolution behaviour of these hydrated calciumsilicates is presented. In the model the hydrated calciumsilicates are represented as a mixture of two congruently soluble components. The dissolution of the particular components is described using the concept of variable activities in the solid state. Each component's activity in the solid state is obtained from a large body of solubility data by applying the Gibbs-Duhem equation for nonideal mixtures. Using this approach a simplified set of equations, which describe the solubility of the components as a function of the calcium / silicon ratio of the solid, is derived.As an application, the degradation of a standard portland cement in pure water and in a carbonate-rich groundwater is modelled.

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