The system C -Al -TiO 2 has been demonstrated to be a strong candidate for the processing of irradiated reactor graphite waste with the retention of biologic hazardous carbon-14 in chemically and thermal stable corundum-carbide ceramics. The corundum-carbide ceramics is obtained from the powdered precursors blend through self-sustaining thermochemical reactions. Investigations of the system C -Al -TiO 2 were carried out both theoretically and experimentally. The refining thermodynamic calculations of the phase composition of resulting end product were performed for a wide variety of components content in the system being investigated. Aluminium oxycarbides production was taken into account in the calculations. Thermodynamic functions of aluminium oxycarbides Al 4 O 4 C and Al 2 OC have been calculated for this purpose using currently available literature evidences and own assessments of missing data. On the basis of thermodynamic simulation the proportions of the source substances were determined, which result in the aluminium oxycarbides production. These simulation results have been supported by XRD-analysis of produced specimens. The experimental processing of reactor graphite was conducted by the use of self-sustaining reactions in C -Al -TiO 2 powder blends. Test specimens were produced by mass ranging from 0.1 to 3 kg in the argon atmosphere. Various techniques were used to characterize the produced specimens. The compressive strength of specimens of corundum-carbide matrices produced ranges from 7 to 13 MPa. The leaching rates of Cs-137 and Sr-90 from specimens ranged between 10 -4 and 10 -5 g/(cm 2 .day) respectively. The carry-over of the carbon combined in carbon monoxide from the reacting mixtures during exothermic process may run up to 1% wt. that appropriates roughly to less than 0.01% wt. of the carbon-14 in the irradiated reactor graphite.

A thermodynamic analysis and laboratory investigations are performed of the thermochemical processes occurring in the system C-Al-TiO 2 during the reprocessing of reactor graphite. The conditions under which hydrolytically unstable aluminum oxycarbides are formed in the final product are determined. The influence of air and moisture in the initial charge and the composition of the surrounding medium on the characteristics of the process leading to the synthesis of the final product is investigated.It has been shown that the self-maintaining high-temperature synthesis technology based on the reaction 3C(graphite) + 4Al + TiO 2 = 3TiC + 2Al 2 O 3(1)is promising for reprocessing irradiated reactor graphite [1-3]. Preliminary laboratory investigations and a thermodynamic analysis of the reactions in the system C-Al-TiO 2 have been completed [3]. Thermodynamic calculations of the composition of the reaction products in the system indicated have established that carbon monoxide forms in the gas phase. Analysis of samples of the gas phase also showed other constituents besides oxide: H 2 and CH 4 . x-Ray diffraction analysis of the condensed reaction products in the system C-Al-TiO 2 showed that hydrolytically unstable aluminum oxycarbide is present for certain ratios of the initial components [3]; this oxycarbide was neglected in the thermodynamic analysis because the required thermodynamic functions are not avaialble in the reference literature. In this connection, additional thermodynamic and experimental investigations of the thermodynamic processes occurring in the system C-Al-TiO 2 are required. Thermodyamic Analysis. It is known that the aluminum oxycarbides Al 4 O 4 C and Al 2 OC are formed when aluminum oxide interacts with carbon [4]. They also form when the oxide interacts with aluminum carbide [5]. Thus, aluminum oxycarbide forms in the system C-Al-TiO 2 under appropriate conditions, as has been observed experimentally [3]. In order to take account of the possibility of the formation of aluminum oxycarbides when performing calculations, it is necessary to know their thermodynamic properties. At the present time, there are no experimental data on the thermodynamic properties of aluminum oxycarbides. There are only estimates of the formation enthalpy, heat capacity, and entropy for tetraaluminum tetraoxycarbide Al 4 O 4 C and dialuminum oxycarbide Al 2 OC [6,7]. However, the thermodynamic functions calculated for aluminum oxycarbides on the basis of these data [8] do not permit reproducing the phase diagram of the system Al 2 O 3 -Al 4 C 3 [9].The thermodynamic functions presented in Table 1 have been recommended for peforming calculations [10]. These functions were determined from a re-examination of the estimates of the thermodynamic properties of aluminum oxycarbides using the phase diagram of the system Al 2 O 3 -Al 4 C 3 .

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