Thermodynamics of Actinide-Doped Glass Leachates.

Current plans for permanent disposal of highly radioactive wastes involve incorporating these wastes in borosilicate glass matrices and disposal in deep underground repositories.  Several actinides and fission products in the wastes have extremely long half lives.  No real-time experimental studies can be conducted to assess the safety of the repositories.  Lacking such information, thermodynamic data are being developed to use in setting upper limits of radionuclide concentrations that can potentially leach out of the repositories.  The advantage of the thermodynamic approach is that it can help predict scientifically defensible aqueous concentrations of different elements, especially those forming sparingly soluble solids, for any given groundwater composition, independent of glass dissolution kinetics and independent of time.  A criticism of this approach has been that the glasses are chemically very complex and the solubility-controlling solids are likely to be more complex as well.  Many studies have been conducted to determine leach rates, but they are empirical and highly dependant on many variables that have not been systematically evaluated.  An alternative to leach rate studies is to experimentally evaluate the solubilities of pulverized doped glasses in a wide range of well-controlled important variables such as pH and pe, and compare them to predicted solubilities of known solid phases from the thermodynamic data in order to reach reliable predictions of leachate concentrations.  This presentation will discuss the data based on those few glass/water solubility studies and reinterpretations of these data to ascertain the solubility-controlling solids.  Particularly, the presentation will address thermodynamic interpretations of the doped-glass/water solubility data and show that thermodynamic data can be used to reliably predict/set upper-limit concentrations of the thorium, uranium, neptunium, plutonium, americium, and rare earth elements present in leachates.  This discussion will show one variation of the use of thermodynamics to understand complex systems as taught by Dr. Lindsay.