A computational model is developed to calculate thermodynamic phase equilibria in aqueous solutions of fluoride, phosphate, and hydroxide up to 100 °C. A variety of data are used, including isopiestic and electromotive force measurements, freezing point data, vapor pressure data at 100 °C, heat capacities, heats of dilution, and solubility measurements. Pitzer's ion-interaction treatment is used to model electrolyte solutions, and many unknown parameters are determined from existing data through nonlinear least-squares fitting. Phase equilibria are determined by minimization of the total Gibbs energy using a modification of the code SOLGASMIX. Results calculated using the model accurately predict phase equilibria from many quantitative experiments. Qualitative experiments are performed to evaluate calculated solubilities in regions of sparse or nonexistent data; the calculated results are reasonable and exhibit a general qualitative agreement with such data. Model predictions are useful in understanding problems that may arise in the treatment of waste streams containing fluoride and phosphate anions in highly caustic solutions.

Despite the obvious importance of understanding the chemistry of oxide glass materials, predictive thermochemical modeis of complex glasses have not yet been developed. Such modeis are important for technologies such as the disposal of high-level nuclear and transuranic waste (HLW), which are currently fore-seen as being incorporated in a host glass for permanen t Sequestration. A large number of glasses have been explored, with a borosilicate glass being the typical base composition. An example of the complexity of such a HLW glass is given in Table I. This article discusses our at-tempts to develop an accurate, easy to understand and use glass Solution model for describing the thermodynamic stability of such HLW glasses. Critical for such a model is the availability of reliable thermodynamic data that can be used in generating accurate values for thermodynamic activities of glass components as a function of temperature and glass composition. Therefore, a major part of this article focuses on developing reliable sets of thermodynamic data for complex HLW glass Systems and Subsystems. With such Information and a model, we can make predictions of the stability of these waste forms, including their volatility, leaching behavior, and corrosion reactions, and understand crystallization behavior during both the initial glass processing and long-term storage.Using an equilibrium thermodynamic model is offen questioned, since HLW is to be stored as part of a glass phase, and glass is a nonequilibrium material. Our model uses a pseudoequilibriu map-proach 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 metas-table 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 critical to understanding the limiting chemi-cal 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 thermodynamic 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 metas-table glass phase.

Additional components such as nitrite and carbonate will be added to the test matrix. The results will be used to develop safe operating windows for safe waste transf=s at Hadord. In addition to the waste transfer, unwanted solids formation can occur during waste processing activities such as the Enhanced Sludge Washing (ESW). Previous work has shown that phosphate gels and particles are formed as the hot leach solutions from the ESW process are permitted to cool. In another study, insoluble aluminosilicates apparently formed during the hot caustic leaching, which significantly reduced the pefiormance of &e ESW process. The aluminosilicates, as well as the phosphate gels from the ESW process, could also forma pipeline plug. This study confirmed the formation of alurninosilicates during the ESWprocess and demonstrated the importance of a thorough water wash prior to the caustic leaches. The water wash improves the performance and the reproducibility of the ESW process. A key difference in the leach tests with washed sludge and unwashed sludge was the higher ionic strength in the unwashed tests due to the presence of water-soluble salts. In response to the alurninosilicate plug at Savannah River, an equilibrium model on the solubilities of silicate vii species at 25 'C was developed. After the solubilities of silicon and aluminum species at higher temperatures are incorporated into the model, safe operating windows for the evaporator system will -be provided to the tank farm operators. Finally, a simulated chemical plug based on the sodium phosphate plug was developed and provided to the Retrieval Program of the Tanks Focus Area to test recovery methods. Additional simulated plugs have been requested.

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