Kinetics of the carbothermic reduction of a ThO2+UO2+C mixture to prepare (Th, U)C

Carbothermic reduction of mechanically mixed ThO2 + UO2 + C compact to (Th,U)C has been studied in the temperature range between 1743 and 2043 K with emphasis on reaction kinetics. The rate-limiting step of this reaction was attributed to the diffusion of CO gas in the outermost layer of the reaction products. By X-ray diffraction, ThO2 and UO2 were found to react with graphite respectively to produce two nearly separate dicarbide phases, both of which then reacted with residual ThO2 to form a monocarbide phase. An apparent activation energy of about 320 kJ mol-1 was obtained for this carbothermic reduction. Such a high activation energy can be explained by the CO gas diffusion through micropores in that product layer, by taking into account the standard enthalpy changes of the related reactions to produce CO gas. (orig.)