Simultaneous Evaporation of CS and TC During Vitrification-A Thermochemical Approach

Abstract

In vitrification of nuclear waste, technetium tends to be lost substantially by evaporation [1,2,3,4,5]. If Cs is not present, 63% of the Tc volatilized from the molten borolilicate glass at 1420K under oxidizing conditions and about 40% under reducing conditions achieved by adding 0.5 wt% graphite to the dried frit mixture[5]. This behaviour could be correlated to thermochemistry in the simple Tc-O system considering the predominance areas of the different oxides and the partial pressures of Tc2O7 above them[6]. If Cs is present(0.544 g Cs/l together with 1.61 g Na/l in the synthetic HAW solution), Tc was found to start a genuine volatilization at about 800K which proceeded rapidly in the range 875 to 950K[2]. At 1300K between 60 and 80% of the Tc and 10% of the Cs had volatilized. Because of the ratio Tc:Cs≉7:l volatility by formation of gaseous CsTcO4 was excluded[2]. On the other hand, for the same glass and HAW solution as reported in Ref.[2], Tc was found to produce a synergistic effect on the volatility of Cs[3,4]: between 2 and 5% Cs volatilized without Tc but about 22% in the presence of Tc and temperatures up to 1400K. The amount of volatilized Tc was about 50%. Furthermore[3,4], Tc was identified together with Cs in crystals deposited in the aerosolfilters after continuous heating to about 1170K. Since the aerosol crystals showed a constant ratio of Tc:Cs =1:1 and since the valency of Tc in the crystals was found to be seven the formation and the condensation of gaseous CsTcO4 was assumed [3,4]. Gaseous CsTcO4 or other gaseous pertechnetates of the alkaline metals have not yet been identified experimentally. Its existence may be concluded, however, from the existence of the gaseous perrhenates which form with every alkaline metal. Solid CsTcO4 and other solid alkalipertechnetates are well known [7]. By using the thermochemical data of the alkaliperrhenates, free energies of condensed as well as of gaseous CsTcO4 are estimated. Together with the results of the thermochemical treatment of the Tc-O system[6] and experimental data of the Cs-O system[8] they are used to calculate tentative isothermal predominance area diagrams of the ternary Cs-Tc-O system. Isobars of the gases Cs2O,CsO,Tc2O7, and CsTcO4 above the different condensed phases are also cal culated and used to understand the experimental results[2,3,4] on the simultaneous evaporation of Cs and Tc during vitrification.

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

“…The first is disproportionation of M(IV) to M(0) and M(VII), which has already been analyzed in detail by Migge. 9,10 Since the glass melt is sodium-rich, the second reaction may be a better approximation of the behavior of Tc and Re in the melt.…”

“…[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.…”

Final Report, Research Program to Investigate the Fundamental Chemistry of Technetium

“…During waste vitrification, the stability of heptavalent technetium species leads to the volatilization of technetium (Darab 1996). Technetium forms volatile oxides, Tc 2 O 7 and CsTcO 4 , when heated under oxidizing conditions (Darab 1996, Migge 1990. Such conditions are present during the normal vitrification of both low-level and high-level waste and low level waste glasses.…”

Research Program to Investigate the Fundamental Chemistry of Technetium

“…Technetium forms volatile oxides, Tc 2 O 7 and CsTcO 4 , when heated under oxidizing conditions. (Migge 1990;Darab and Smith 1996) Such conditions are present during the normal vitrification of both low-level and high-level waste and low level waste glasses. Volatilization of the technetium proses two challenges.…”