Electrochemical Description of Redox Equilibria in Nuclear Glass
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Electrochemical Description of Redox Equilibria in Nuclear Glass O. Pinet, S. Mure CEA, DEN, DTCD, SECM, LDMC, 30207 Bagnols sur Ceze Cedex, France ABSTRACT Glass matrices were selected to immobilize fission products because glass is capable of chemically incorporating a wide spectrum of elements within a single matrix. Some of these elements can be found at different oxidation states. The redox equilibrium constants of multivalent species can be used to develop thermodynamic models for a better description of nuclear glasses. Some of the multivalent species loaded in nuclear glass, such as iron and sulfur, have already been a subject of investigation by conventional glassmakers or geochemists in the earth sciences. Other redox species more specifically related to nuclear glass, including cerium and ruthenium, have also been investigated. These studies have demonstrated the advantages of using electrochemical techniques, voltammetry and potentiometry, to determine the equilibrium constants. Oxygen potential measurements are also particularly suitable for characterizing the redox state of the multivalent dissolved species in molten glass. INTRODUCTION One of the major concerns in the glassmaking industry is the control of oxidation-reduction phenomena, which determine the glass properties—and notably its refinement and color. The development of vitrification processes and vitreous materials for containment of nuclear or nonnuclear wastes also highlighted the advantages of optimizing the glass oxidation state [1]. The redox state of the waste elements, for example, can be related to phase separation, crystallization or volatilization phenomena during the fabrication process; the redox state of the glass therefore requires special attention for containment of the numerous polyvalent elements produced by nuclear fuel reprocessing, as optimization of this parameter can significantly improve the final glass properties. Several techniques are routinely used to characterize the glass redox state. UV, Mossbauer and XANES spectrometry are used to characterize the redox state of some multivalent species. More recently other spectroscopic techniques have proved effective, such as Raman spectroscopy [2]. Techniques based on chemical analysis of the glass and multivalent cation assay are also widely used by glassmakers to determine the redox ratios of multivalent species in the glass, especially iron. However these techniques are not suitable for characterizing complex glasses. They are limited to one or two redox couples in the glass, and concern only a limited number of elements. These limitations are particularly restrictive for the examination of complex formulations such as nuclear glasses. Electrochemical techniques can make up for some of these limits. Electrochemical measurements of the molten glass can not only determine the oxygen fugacity in the glass but also establish general relations between the oxygen fugacity and redox ratio.
THEORY Oxide glass melts can be considered as ionized solvents comprising polyoxyanion
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