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.
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.
A thermochemical analysis is performed in the system Cu-In-S at 723 K. Free energies of In6S7, In417S583, "In2S3", Cugsjln^, and CuIn5S8 have been estimated, the numerical values (kJ/mol) of which are -1285, -97780, -481.1, -31680, and -1444. The free energy (kJ/mol) of CuInS2 is calculated from the relation G° = (-306.1 ± 54.4) + 0.50927 -1.397 10~5 T2 -0.09468T In T + 268.27-1, which is obtained from published assessed standard formation enthalpy and specific heat and entropy data. The free energy of the Cu-In melt is taken from very new literature. A consistent set of data is used for the calculation of a tentative Gibbs triangle as well as of the corresponding predominance area diagram. The Gibbs triangle is calculated with the program THERMO, the algorithm of which is given. The results are in agreement with the results of published measurements, also for the equilibria which involve the melt. The compound CuInS2, one of the possible base materials for thin film solar cells, is shown to equilibrate with most of the compounds of the system. Predictions are made how to prepare CuInS2 from Cu-In alloys and H2S/H2 gas mixtures. However, more experiments are necessary to establish the data, the experiments, and/or the results of the calculations.
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