Iron Mössbauer redox and relation to technetium retention during vitrification

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Technetium-99 is a long half-life radioactive fission product whose high volatility during waste vitrification (National Research Council, 2001; Migge, Mat Res Symp Proc 127:205–213, 1989) is a factor in the safe disposal of radioactive waste. Oxidation states of +7, +4 mostly, and some traces of metal have been observed in the glass (Migge, Mat Res Symp Proc 176:411–417, 1990; Darab and Smith, Chem Mater 8:1004–1021, 1996) with Tc7+ more volatile than Tc4+, leading to a range of technetium retention in the glass varying from 0.8% to 92%. In the present experiments, a simulated high-sulfate Hanford tank waste was vitrified using a slurry fed joule heated furnace producing about 30 kg of glass per day. Various combinations of sucrose and urea were used as denitration and reducing agents in order to optimize sulfate and technetium retention. Since iron oxide is used as a glass additive, Fe-Mössbauer spectroscopy was a vital tool to monitor the melt redox through its Fe2+/Fetot ratio. A Mössbauer glass redox scale was designed to relate the technetium speciation to its retention in the glass.

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