Thermochemical Comparison of the Systems Re-O and Tc-O

Abstract

Technetium is a hazardous fission product with a long half-life. In vitrification of nuclear waste, technetium tends to be lost substantially by evaporation [1], and the formation of gaseous Tc oxides is assumed to be the reason. Reliable thermochemical treatment of the problem is difficult, since data on the Tc-O system are surprisingly scarce [2]. Therefore, the system Re-O is treated for comparison. Key thermodynamic data for the condensed rhenium oxides exist [3,4,5] as well as measurements on the sublimation and the evaporation of the oxides [6–11]. Consistency of the different data is investigated by assessing the sublimation data of solid Re2O7, using them to calculate other sublimation equilibria and to compare the results with published measurements. Then a predominance area diagram is constructed and discussed with respect to the pressures of the gaseous oxides and their dependence on the temperature, oxygen partial pressure, and condensed oxide phase present. Predominance area diagrams of the Tc-O system are calculated and critically discussed. Owing to the small amout of available data, the possible existence of solid TcO3 is discussed. Comparison with the system Re-O is used to clarify, where further investigations need to be done.

Spent fuel dissolution studies FY 1991 to 1994

“…After the first few days, the 99Tc/U dissolution ratio was only 0.1 to 0.2. It was first thought that the higher temperatures needed to generate U308 during the short-time laboratory exposure to air may have vaporized a large fraction of the 99Tc, perhaps as the volatile compound T9O7 (Migge 1989 . To evaluate that possibility, a portion of the U308 was completely dissolved and analyzed for U, ' 7Cs, 99Tc, and 90Sr.…”

Dissimilar Behavior of Technetium and Rhenium in Borosilicate Waste Glass as Determined by X-ray Absorption Spectroscopy

“…[6][7][8][9][10] During vitrification, the volatile technetium compounds that are lost from the glass melt must be captured in the off-gas treatment equipment. Secondary wastes from the off-gas treatment systems must then be either recycled or stabilized in alternative waste forms, which increases the amount of waste created.…”

“…A number of Tc(VII) compounds have significant volatility at the temperatures (typically 1100-1200 °C) at which waste is vitrified. 6,9,10 On the other hand, lower valent Tc compounds are less volatile under these conditions since reducing conditions reduce the amount of Tc lost during vitrification. 6,11,12 Consequently, one of the major factors in controlling Tc volatility during vitrification is the redox state of the melt.…”

“…For this reason, Re is often used as a non-radioactive surrogate for Tc in such large-scale experiments. 6,9 In analogy to Tc chemistry, Re(VII) and Re(IV) are assumed to be stable in the glass melt. Although ReO 3 is known, it is unstable towards disproportionation above 300 °C.…”

Thermodynamic modeling of volatile hazardous metal behavior in the Vortec Vitrification System

“…Technetium is a hazardous fission product with a half-life of 200,000 years. In the vitrification of nuclear waste, technetium oxides are known to be lost substantially by vaporization, especially under normal oxidizing conditions (13). The highest oxidation state of Tc is 7, such as in Tc 2 O 7 .…”