VOLATILITY OF PERTECHNETATES

German, K.E.; Peretrukhin, V.F.; Obruchnikova, Y.A.; Afanasiev, A.V.; Ryagin, S.N.

Авторы: German K.E., Peretrukhin V.F., Obruchnikova Ya A., Afanasiev A.V., Ryagin S.N.
Сборник: 10th International Symposium on Technetium and Rhenium - Science and Utilization Proceedings and selected lectures of the 10th International Symposium. Moscow
Том: 10
Тезисы
Год издания: 2018
Место издания: Publishing House Granica Moscow
Первая страница: 254
Последняя страница: 256
Аннотация: Technetium volatility have been widely discussed in a number of reviews [1 – 3] the principle attention given to Tc(VII), pertechnetate (TcO4 -), the most dominant species in oxidizing conditions. But the direct experimental data on this item is very scattered. Volatility data on 99Tc(VII) compounds are of high importance for two main reasons. The first is the understanding of Tc behavior in the procedures of radioactive wastes vitrification. The second is the basis for estimating of possible environmental contamination during accidental nuclear reactors. The data on the Tc rejects from Chernobyl accident are sometimes contradictory [1] and indicate the importance of more systematic study of sublimation and volatilization of different 99Tc species under high temperature conditions. Among the Tc compounds, the volatility of oxides is the best studied. At low concentrations of Tc formation of TcO3 was postulated [2], while at relatively high concentrations, Tc forms Tc2O7 with high volatility starting from 100 °C. During vitrification of radioactive wastes the Tc should be considered semi-volatile, a large fraction of this radionuclide evaporates in the melter, which operates at a nominal temperature of 1150 °C [5,8,9]. Small-scale melter tests with simulated Hanford LAW glass feeds have shown that the fraction retained in glass (referred to as “retention”) varied from 18% to 66% for Tc (tested with Tc-99m, a short-lived isotope as a surrogate for Tc-99) and from 1% to 56% for I (tested with nonradioactive I) depending on the feed composition [10–13]. A large fraction of volatilized I is expected to pass through the primary WTP off-gas treatment system (i.e., submerged bed scrubber and wet electrostatic precipitator) [5]. This I fraction will be captured downstream in other off-gas treatment system components (e.g., carbon beds) and sent to the IDF [14]. On the other hand, almost all (> 98%) of the Tc that leaves the melter is expected to be captured by the primary offgas system. Some kinetic data are available on oxidation of Tc metal [3] and sublimation of TcO3 from UO2 ceramics [4]. Volatile Tc oxides and hydroxides are discussed in [5]. The data on sublimation of KTcO4 at 1000°C without decomposition is due to terminology misusing in [6] (in fact they observed evaporation) because the congruent fusion temperature of KTcO4 is 803 K [7]. For NaTcO4 and CsTcO4 fusion points are at 1063 K [8] and 863 K [9] correspondingly. Considerable information is found on volatility of Tc during radioactive wastes vitrification [10, 11, 12]. Data on the volatility of technetium salts are limited by a few works: evaporation rate for NaTcO4 was reported to be 0.37 mg∙cm-2∙min-1 at 700 °C and increased to 3.2 mg∙cm-2∙min-1 at 1050°C [13]. Kuranov with coauthors has determined by effusion mass-spectrometry from Knudsen camera that ΔHevap (KTcO4) = 213 kJ/M and ΔHevap ((KTcO4)2) = 282 kJ/M [14]. Gibson has confirmed that evaporation gives gaseous KTcO4 and (KTcO4)2 in vacuum at 500 - 550 °C [15]. We studied the high-temperature evaporation of MTcO4 (M = K, Cs) from chemically pure compounds by thermogravimetry analyses (TGA) using a Q-1500D derivatograph (system F.Paulik-J.Paulik-L.Erday) in 300 – 1300 K temperature region. Dynamic and quazi-isothermal modes were used. Some experiments were also carried out by effusion mass-spectrometry (EMS). In TGA tests (with lower sensitivity if compared to EMS), no evaporation of solid KTcO4 and CsTcO4 from crucible was noted. The detectable changes in the KTcO4 and CsTcO4 sample masses were noted only after 995 K and 950 K correspondingly (that means after salt fusion) while the Tc2O7 oxide is known to exhibit noticeable volatility even at 475 K.X-ray diffraction test of the rest in the crucible was done after evaporation 20% of the sample. The patterns were identified as identical to initial compounds, i.e. KTcO4 and CsTcO4. The same composition was found by X-ray diffraction after condensation of vapors. The KTcO4 and CsTcO4 evaporation rate increase monotonically with temperature. For comparison similar tests were carried out with CsI sample.
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