Redox Systematics in Model Glass Compositions from West Valley
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REDOX SYSTEMATICS IN MODEL GLASS COMPOSITIONS FROM WEST VALLEY HENRY D. SCHREIBER, CHARLOTTE W. SCHREIBER, AND CARY C. WARD Department of Chemistry, Virginia Military Institute, Lexington, VA 24450 ABSTRACT At a processing temperature of 1150'C for model West Valley glass compositions, the prescribed range of oxygen fugacities needed to achieve an [Fe 2+]/[Fe3 +] of 0.1 to 0.5 is 10' to 10-' atm. Establishment of the Fe2+-Fe° equilibrium, resulting in metal precipitation from the melt, occurs at oxygen fugacities lower than 1011 atm at this temperature. The target processing range as defined by the iron redox ratio is equally valid at both lower and higher temperatures (± 100°C). Elevations of the concentrations of redox-active components to I wt% Cr 20 3, 1 wt% NiO, 1 wt% CuO, 1 wt% CeO 2, and 4 wt% Mn 20 3 in the waste glass will not affect the redox limits as established by the iron redox ratio of 0.1 to 0.5; these limits provide sufficiently large margins of safety to assure no stabilization of reduced or oxidized forms of these elements. INTRODUCTION The processing and immobilization of high-level nuclear waste into a glass waste form in the United States will be accomplished at several sites, one of which is the West Valley Demonstration Project. Effective redox control has to be maintained in this immobilization strategy in order to dissolve the waste into the glass melt, to prevent melt foaming and metal precipitation, and to produce a durable product [1]. Previous studies have defined the redox control in terms of a target iron redox state, [Fe 2 +]/[Fe3 +] = 0.1 to 0.5, of the resulting glass [2,3]. If the ratio is too low, problems such as melt foaming may be encountered in an overlyoxidized environment; if the ratio becomes too high, representative of reducing conditions,
metals and metal sulfides may precipitate from the glass melt [1]. An additional concern is the possible introduction of substantial amounts of redox-active components such as chromium or manganese in the waste stream, and thus in the glass melt. Subsequent interactions with the iron redox states may upset the iron redox ratio and render ineffective the direct monitoring of the melt by this ratio. OBJECTIVES The objective of this paper is to define the redox constraints on the processing of highlevel nuclear waste in model West Valley glass compositions. The redox state of the model glasses as represented by the [Fe2 +]/[Fe3 +] ratio will be calibrated to the oxygen fugacity imposed on the glass melt at three temperatures (1050'C, 1150"C, and 1250 0C). Two methods for the determination of the [F&+] content of the glass, a near-infrared absorption and the total reducing power, will be tested for their suitability as redox state indicators. The effects of other redox-active components such as nickel, cerium, copper, manganese, titanium, and chromium on the processing limitations will be established, since the potential exists for such additives to "destroy" the redox buffering capacity of the melt as defined by the Fe?+/Fe3 + equilibriu
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