Tc and Re Behavior in Borosilicate Waste Glass Vapor Hydration Tests
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0985-NN06-03
Tc and Re Behavior in Borosilicate Waste Glass Vapor Hydration Tests David A. McKeown1, Andrew C. Buechele1, Wayne W. Lukens2, David K. Shuh2, and Ian L. Pegg1 1 Vitreous State Laboratory, Catholic University of America, 620 Michigan Ave., N.E., Washington, DC, 20064 2 Actinide Chemistry Group, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 ABSTRACT Technetium (Tc), found in some nuclear wastes, is of particular concern with regard to longterm storage, because of its long-lived radioactivity and high mobility in the environment. Tc and rhenium (Re), commonly used as a non-radioactive surrogate for Tc, were studied to assess their behavior in borosilicate glass under hydrothermal conditions in the Vapor Hydration Test (VHT). X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM) measurements were made on the original Tc- and Re-containing glasses and their corresponding VHT samples, and show different behavior for Tc and Re under VHT conditions. XAS indicates that, despite starting with different Tc(IV) and Tc(VII) distributions in each glass, the VHT samples have 100% Tc(IV)O6 environments. SEM shows complete alteration of the original glass, Tc enrichment near the sample surface, and Tc depletion in the center. Perrhenate (Re(VII)O4 ) is dominant in both Re-containing samples before and after the VHT, where Re is depleted near the VHT sample surface and more concentrated toward the center. INTRODUCTION A problem associated with the disposition of nuclear wastes is the long-term immobilization 99 of the radionuclides produced during fission, such as technetium ( Tc). Tc is a β-emitter with a 5
half-life of 2.13x10 years. The stable form of Tc in aerobic environments is highly mobile -
pertechnetate (Tc(VII)O4 )[1,2], which is an environmental risk contributor in the performance assessment of nuclear waste disposal repositories. The preferred method of treatment for highly radioactive nuclear wastes is vitrification to produce a durable glass matrix. Determining the fate of waste glasses in the environment requires studying the processes by which they alter on exposure to environmental conditions, especially involving water. Since waste glasses will have low alteration rates, test methods, such as the Vapor Hydration Test (VHT), employ hydrothermal conditions to accelerate the rate of alteration, where glass is exposed to a watersaturated atmosphere at elevated temperatures. Water diffuses into the glass ahead of the alteration zone and new phases can develop in place of the glass or grow out from the sample surface [3–7]. Tc behavior is unknown under these conditions. In view of the practical importance of Re, the preferred non-radioactive surrogate for Tc, the VHT was also performed on two Re-containing glasses. The speciation and local environments of Tc and Re, in the glasses and the corresponding VHT samples, were determined by X-ray absorption spectroscopy (XAS). Tc K-edge X-ray absorption near edge structure (XANES) data are distinctive for reduced Tc(IV)O6 and o
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