Graphite Processing With Carbon Retention in a Waste Form
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565 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
to radioactive graphite waste: C(graphite) +Al + SiO 2 (1), C(graphite) +Al + TiO 2 (2), C(graphite) + Ti + SiO 2 (3). When combustion of the mixture is initiated, metallothermic oxide reduction and interaction of elements reduced by graphite occur. As a result, graphite is chemically bound into stable metal carbides, which form a matrix for radionuclide oxide immobilization. High temperatures are developed from the metallothermic oxide reduction and carbide formation reactions. As a consequence, partial evaporation and dissociation of the reaction products and deviation from the stoichiometric equilibrium inevitably occur. The detailed thermochemical analysis performed in this paper for reactions in mixtures (1) - (3) in item 3 above is aimed at determining the reaction temperature and phase composition of the reaction products. The technological aspects are not discussed. THERMODYNAMIC SIMULATION OF THE THERMOCHEMICAL TREATMENT PARAMETERS Thermodynamic simulation is, in essence, a numerical experiment and, at present, is widely used for prediction and analysis of the characteristics of high-temperature processes and reactions in multicomponent systems, including thermochemical treatment of radioactive waste (see, e.g., [6,71). For thermodynamic simulation, the complex program ASTRA.4 [8] involving a database of thermodynamic characteristics of chemical compounds, comprising extensive domestic and foreign reference data [9,10], was used. In the thermodynamic calculations, the formation of the following compounds due to reactions in mixtures (1)-(3) was considered: in the gas phase: 0, 02, C, C2, C3, CO, C0 2, C20, C2 0 3, Al, A12, AIO, A10 2 , A120, A120 2 , A120 3 , AIC, A1C 2 , A12C2 , Ti, TiO, TiO 2, Si, Si 2, Si3 , SiO, SiO 2, SiC, SiC 2, Si 2C, Si2C2 , and Si3 C; in the condensed phase: C, Al, A120 3, A14 C3 , Si, SiC 2 , SiC, Ti, TiO, TiO 2, Ti2 0 3, Ti3O5 , Ti 4 0 7, TiC, TiSi, TiSi 2, and Ti5 Si 3. The calculation results are shown in Figures 1-3 as ternary diagrams. Equilibrium reaction temperature as a function of mixture composition is plotted in Fig. 1. The highest temperatures of about 2200 K and a wide area of 2000 K were observed in mixture (1) (Fig. la). A maximum temperature of 2300 K was generated in the mixtures (2), and for these mixtures, 2200 K is seen in a very wide region of compositions (Fig. lb,d). The mixtures (3) develop the lowest temperatures of the three mixtures - the maximum temperature is only 1900 K (Fig. Ic). However, the real temperature might be much lower than the calculated equilibrium temperature because of reaction incompleteness. Earlier, it was shown that the mixtures (2) produce the best results [11]. Therefore we shall only further consider further the phase composition of Al-C-TiO 2 mixtures. The equilibrium phase composition of the combustion products of Al-C-TiO 2 mixtures is shown on Fig. 2. As can be seen, the region of mixture formulations that forms stable chemical compounds such as TiC,
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