Investigation of reactor graphite processing with the carbon-14 retention.
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II3.13.1
Investigation of reactor graphite processing with the carbon-14 retention. 1
M.I. Ojovan, 2O.K. Karlina, 2V.L. Klimov, 2G.A. Bergman, 2G.Yu. Pavlova, A.Yu. Yurchenko, 2S.A. Dmitriev 1 Department of Engineering Materials, University of Sheffield, UK, E-mail: [email protected] 2 Moscow Scientific & Industrial Association “Radon”, Moscow, Russia, E-mail: [email protected] 2
ABSTRACT The system C – Al – TiO2 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 – TiO2 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 Al4O4C and Al2OC 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 – TiO2 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/(cm2.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. INTRODUCTION There is an increasing interest in developing thermochemical treatment approach suggested in [1] for spills of reactor graphite contaminated with nuclear fuel elements, nuclear fission and corrosion products. Application of self-sustaining reactions in the blend of crushed graphite with powder aluminium and titanium dioxide (rutile) results in formation of a corundum-carbide ceramics efficiently confining practically all carbon in the carbide constituent of ceramics. The corundum-carbide ceramics is a rather durable and thermal stable material, which safely immobilises the biologic hazardous carbon-14. Thermodynamic properties and the phase diagram of C– Al– TiO2 system is a subject of great interest for reactor graphite processing. Oxycarbides of aluminium Al4O4C exp
