Extension of the Modified Associate Species Thermochemical Model for High-Level Nuclear Waste: Inclusion of Chromia
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Extension of the Modified Associate Species Thermochemical Model for High-Level Nuclear Waste: Inclusion of Chromia Theodore M. Besmann1, Karl E. Spear2, and John D. Vienna3 1 Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6063, USA 2 Materials Science and Engineering Department, Pennsylvania State University University Park, PA 16802 3 Pacific Northwest National Laboratory Richland, WA 99352 ABSTRACT The successful thermochemical model based on the modified associate species approach for the Na2O-Al2O3-B2O3-SiO2 base glass system has been extended to include a critical constituent, Cr2O3. This includes the Cr2O3- Al2O3 solid solution. For the liquid, and potentially glass phase when undercooling is allowed to occur, the model uses the relative simple, modified associate species method to allow accurate determination of phase relations, including liquidus surfaces. It also allows prediction of chemical activities and vapor pressures, which can be important in both processing and in modeling long-term waste form stability. INTRODUCTION High-level nuclear and transuranic wastes are currently foreseen as being incorporated in a host glass for permanent disposal. A large number of glasses have been explored, with borosilicate glass as the typical base composition. Glass compositions are under development at Pacific Northwest National Laboratory, Savannah River Laboratory and other sites that will allow dissolution of the waste species in a glass matrix. Issues of glass stability are important in that the glass must remain mechanically intact and retain a low leach rate on exposure to moisture. A somewhat opposing goal is to maximize waste loading of the glass, with a significant economic gain associated with incremental increases in waste content. At Hanford, for example, the loading of high-level waste (HLW) in glasses will be limited primarily by the liquidus temperature of the glass in the primary phase fields of eskolaite (Cr2O3), spinel ([Mn,Ni,Fe][Cr,Fe]2O4), and zirconia-containing phases such as baddelyite, zircon (ZrSiO4), and parakeldyshite (Na2ZrSi2O7). A recent evaluation of waste loading constraints for Hanford HLW found that glass volume would nearly double if the solubility of Cr2O3 is half of that currently expected [1]. In order to provide a sufficient thermochemical understanding of the liquid and glass system used for sequestering HLW, an approach using the associate species technique was chosen. It is attractive because it (a) accurately represents the thermodynamic behavior of very complex chemical systems over wide temperature and composition ranges, (b) accurately predicts the activities of components in metastable equilibrium glass phases, (c) allows logical estimation of unknown thermodynamic values with an accuracy much greater than that required for predicting useful engineering limits on thermodynamic activities in solutions, and (d) is relatively easy for non-specialists in thermochemistry to understand and use.
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Ideal mixing of associate
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