Development of Thermodynamic Databases for Hyperalkaline, Argillaceous Systems

Arthur, R. C,*; Sasamoto, Hiroshi   ; Oda, Chie  ; Honda, Akira; Shibata, Masahiro ; Yoshida, Yasushi*; Yui, Mikazu

This report describes three thermodynamic databases for minerals, gases, aqueous species and reactions of relevance to bentonite and cement barriers in geologic repositories for transuranic (TRU) and other nuclear wastes. The databases support calculations using the SUPCRT, PHREEQC and GWB geochemical codes, and are referred to as SPRONS.TRU (SUPCRT), JNC-TDB.TRU (PHREEQC) and THERMO_ JNCTRU (GWB). The reliability of these databases should be considered in the context of basic limitations in current conceptual models of clay-bearing, high-pH systems. Minerals that are important in such systems tend to be complex, possibly metastable solid solutions having highly variable compositions that have not been well characterized experimentally or thermodynamically. There is also considerable uncertainty regarding important aqueous species in such systems due to the paucity of relevant experimental data presently available. With these limitations in mind, the databases developed in this study are considered to be reliable insofar as they include data for minerals that are known to be important or representative of hyperalkaline, argillaceous systems, and because the data are in general demonstrably accurate in cases where comparisons can be made between calculated values and their experimental counterparts. The databases may not meet the ideal standard of an internally consistent database because more weight has been given in this study to the accuracy and completeness of the data than to their internal consistency. An approach recommended for future improvements and revisions of the databases described in this report includes the development of a consistent set of empirical methods permitting routine estimation of thermodynamic properties in cases where reliable experimental data are lacking, combined with uncertainty analyses to assess the importance of conceptual and parameter uncertainties on models of the long-term behavior of bentonite and cementitious barriers.