Thermochemical Modeling of Glass: Application to High-Level Nuclear Waste Glass
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MRS BULLETIN/APRIL 1999
to be stored as part of a glass phase, and glass is a nonequilibrium material. Our model uses a p s e u d o e q u i l i b r i u m approach in which we thermochemically treat the glass as a supercooled liquid. This is a more accurate approach than assuming a global System equilibrium, as it describes the behavior of the metastable glass phase using thermodynamic
data for the liquid phase and excludes the formation of crystalline species. As a result, developing an accurate model and data for representing the thermodynamic properties of oxide liquid phases is criti cal to understanding the limiting chemical behavior of the nuclear waste glass. The methodology requires that a critically assessed thermodynamic database be created for binary and ternary combinations of the major constituents in a typical waste glass. These data can then be combined to represent the thermody namic behavior of the more complex multicomponent HLW glass Systems. If a crystalline phase is experimentally observed to precipitate from the glass under certain conditions, a thermodynamic description can be used to calculate the composition-temperature conditions under which this specific crystalline phase can exist in equilibrium with the metastable glass phase. The change in glass phase composition immediately adjacent to the precipitated crystals can also be calculated and the properties of this new composition contrasted with that of the bulk glass. A major concern with radioactivewaste storage is the possibility of active components leaching from the waste should it come into contact with groundwater. Accurate predictions of leaching from a chemically complex nuclear waste glass require accurate thermodynamic activities of its component oxides. However, these have not been generally
Table I: An Example of Components in High-Level Nuclear Waste Glass.1 Major
mol%
Trace
mol%
Si0 2 Li 2 0 Na 2 0 Fe 2 0 3 FeO K20 Al 2 0 3 MgO MnO CaO NiO Ti0 2
54.82 9.66 9.24 7.55 2.86 2.84 2.69 2.56 2.20 1.88 1.13 0.78 0.74
U308 Th0 2 Cr 2 0 3 CuO ' CaS0 4 BaS0 4 NaCI Na 2 S0 4
0.167 0.047 0.052 0.363 0.037 0.116 0.213 0.055
Subtotal trace
1.050
Subtotal major
98.95
Group B Ag, Cd, Cr, Pd, V, La, Ce, Pr, Pm, Nd , Srr), Sn , Sb, Co, Zr, Nb, Eu, Np, Am, Cm
B2O3
Others Group A Tc, Se, Te, Rb, Mo
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Thermochemical Modeling of Glass: Application to High-Level Nuclear Waste Glass
available to date, and over-simplistic assumptions for the activities of glass components or correlations of the free energy of hydration with the bonding of the glass constituents are currently being incorporated in most leaching predictions. 2 Figure 1 schematically illustrates the critical dependence of leachate concentrations on the assumptions about the thermodynamic activities of metal oxide constituents in the glass.
Developing Reliable Thermodynamic Data Sets A major focus of our efforts to thermochemically describe glass—and HLW glass Systems in particular—has been to develop internally consistent sets of ther mochemical information for
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