A comparison of Pitzer databases for nuclear waste disposal modelling

For the modelling of different chemical aspects of a nuclear waste repository in salt rock, the Pitzer formalism is necessary. Therefore, a comprehensive database with the relevant species reaction constants and associated ion-ion interaction parameters including temperature-dependencies is required. A number of different tailored Pitzer databases are available. To judge their capabilities and limitations we performed comparative calculations for well-defined chemical systems (binary or ternary solubility diagrams). To avoid possible deviations due to different speciation codes all databases where transformed into the format specific for Geochemist’s Workbench. Additionally, model results are compared to experimental values from the literature.
Most results for the Oceanic Salt Systems (Na+, K+, Ca2+, Mg2+ / Cl−, SO42− – H2O) at 25 °C show a good agreement between experiment and model. At higher temperatures, sparse temperature-dependent data causes strong differences in the results.
For the radionuclides (e.g. Nd, Np), the solubility of their amorphous hydroxides in high salinar solutions was calculated as a function of pH. Missing anionic hydroxo-complex species or less reliable data produce inadequate predictions of the increasing solubility of mineral phases (e.g. fresh amorphous Nd(OH)3 or NpO2OH) in the strongly alkaline medium. This demonstrates the importance of complete chemical speciation data.
Caesium was chosen as example for a fission product. Only minor differences can be found in the calculated solubility diagrams and the occasional absence of solubility data for Cs phases is not significant for real-world scenarios due to the high solubility of these phases in comparison to other salts.
The revealed discrepancies illustrate the need for further database work. Joint benchmark activities could help to identify missing or weak data, enhance the quality of all databases and consequently increase the confidence in modelling results.