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 Tc2O7above 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 CsTcO4was assumed [3,4]. Gaseous CsTcO4or 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 CsTcO4and other solid alkalipertechnetates are well known [7]. By using the thermochemical data of the alkaliperrhenates, free energies of condensed as well as of gaseous CsTcO4are 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 CsTcO4above 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.