Electrochemical description of redox equilibria in nuclear glass O. Pinet, S. Mure CEA, DEN, DTCD, SECM, LDMC, F Bagnols sur Cèze Cedex, France Corresponding author: Tel: (+33) , Fax: (+33) 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 [1]. 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, tellurium or 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. Oxide glass melts can be considered as ionized solvents comprising polyoxyanions and polyoxycations. The intrinsic couple in oxide glass melts is O 2 (dissolved gas)/O 2 ‑, at equilibrium with multivalent redox couples Mm+/M(m ‑ n)+ dissolved in the molten glass. The equilibrium reaction between these two redox couples is expressed with allowance for solvation of metal cations by O 2 ‑ oxide ions in the melt: MO x (2x ‑ m) ‑ :oxidized form of the dissolved redox species, MO y (2y ‑ m+n) ‑ :reduced form of the dissolved redox species, n:number of electrons transferred. Considering the Nernst law and the equalization of thermodynamic potentials at equilibrium, the redox ratio follows the relation: E 0M : standard potential of MO x (2x ‑ m) ‑ /MO y (2y ‑ m+n) ‑ redox couple,  : ratio between activity coefficient of MO x (2x ‑ m) ‑ and MO x (2y–m+n)– ions in glass, f O2 :oxygen fugacity in the glass, a(O 2– ):the activity of the O 2– ions in the glass. Schematic representation of test setup for measuring oxygen fugacity in molten oxides Relation between oxygen fugacity at 1200°C and redox ratio measured by chemical analysis for two borosilicate glasses containing iron with different optical basicity values. [1] Ru 0 /Ru(total) redox ratio versus oxygen fugacity [3] (  1000°C  1100°C  1200°C) Modeling the redox equilibrium of the Ce 4+ /Ce 3+ couple in silicate glass by voltammetry. [2] References [1] O. Pinet, R. Guirat, J. Phalippou, T. Advocat - Development of Models to Predict the Redox State of Nuclear Waste Containment Glass- Global - Boise (Idaho) 9 – 13 oct [2] O. Pinet, J. Phalippou, C. Di Nardo, J. of Non-Crystalline Solids, 352, (2006). [3] O. Pinet, S. Mure, J. of Non-Crystalline Solids, 355 (3), (2009).