The Effect of Temperature on the Redox Constraints for the Processing of High-Level Nuclear Waste into a Glass Waste For
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THE EFFECT OF TEMPERATURE ON THE REDOX CONSTRAINTS FOR THE PROCESSING OF HIGH-LEVEL NUCLEAR WASTE INTO A GLASS WASTE FORM
HENRY D. SCHREIBER, CHARLOTTE W. SCHREIBER, MARGARET W. RIETHMILLER, AND J. SLOAN DOWNEY Department of Chemistry, Virginia Military Institute, Lexington, VA 24450 ABSTRACT The oxidation-reduction equilibria of selected multivalent elements in an alkali borosilicate glass melt (Savannah River Laboratory frit #131) were measured as a function0 of the imposed oxygen fugacity over the Redox constraints on the temperature range from 950 C to 1350 0 C. processing of high-level nuclear waste tnto the _ ass melt reguire that thq prevail'ng oxygen fugacity be about 1O-:to 10' Zatm at 950 0 C, 0about 10-4 0 to 10- atm at 1150 C, and about 100 to 1O- atm at 1350 C. Such conditions circumvent foaming under oxidizing situations and metal/sulfide precipitation if the system becomes too reducing. The defined oxygen fugacity ranges correspond to the previously prescribed range of 0.1 to 0.5 3 for the [Fe2+]/[Fe +] ratio in the resulting glass, independent of the processing temperature from 950 0 C to 13500 C. INTRODUCTION The efficient processing of high-level nuclear waste into a borosilicate glass matrix requires consideration of the oxidation-reduction {redox) properties of the system [1-3]. If the conditions that accompany the reaction of the waste with the glass melt are too oxidizing, the melt may foam due to the stabilization of oxidized redox states of chromium, cerium, and/or manganese [4]. Conversely, if the conditions during processing are too reducing, sulfide and metal phases may precipitate from the melt and result in short circuiting between melter electrodes [5,6]. In order to avoid either of these two redox extremes during the reaction of the waste with the melt, an acceptable range for the prevailing oxygen fugagity during the processing of the mixture has been defined as 10- to 1-1 atm3 at 11500 C [7]. This range of oxygen fugacities translates to a [Fe +]/[Fe +] ratio of 0.04 to 1.5 in the glass product [7]. The ferrous/ferric ratio is employed as a convenient internal monitor of the redox conditions during glass processing. In order to allow a safety margin at both extremes, the prescribed iron redox ratio in the glass waste form has been established as 0.1 to 0.5 [8]. The redox constraints, as specified by the oxygen fugacity or the iron redox ratio, have been shown to be applicable to a range of borosilicate compositions under consideration for nuclear waste immobilization at 1150 0C, the probable temperature of melter operation [7]. However, temperature gradients or excursions of the working temperature may be intentionally or unintentionally encountered in the melter, so that an understanding of the thermal dependence of these redox constraints becomes essential. OBJECTIVES The objective of this study is to determine the effect of temperature on the redox equilibria of selected multivalent elements in a reference borosilicate melt. Consequently, the temperature dependence of the redox
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