On the product phases and the reaction kinetics of carbothermic reduction of UO2+C at relatively low temperatures

Abstract

The synthesis of UC using carbothermic reduction of UO₂ and C mixtures has been well studied at high temperatures. However, the product phase behavior of carbothermic reduction at low temperatures (≤1773 K) is not well studied. Such a study is important as low temperatures permit single phase UC synthesis without forming secondary higher carbides, and it further supports the knowledge base of the process that needs to be used for transuranic elements such as plutonium that have high vapor pressures at elevated temperatures. Therefore, a low temperature carbothermic reduction of two different C/UO₂ molar ratios under inert and reducing environments have been studied here. Two different sample holding crucibles, alumina (Al₂O₃) and graphite, were also used here to differentiate the hypostoichiometric (UC_1-a) and oxygen dissolved (UC_1-xO_x) uranium monocarbide phases adding more details on the two systems. Also, the reaction kinetics involved in the formation of UC via the carbothermic reduction of UO₂+C using product phases instead of evolved gases such as carbon monoxide is reported here. Under inert atmospheres but with significant oxygen partial pressures, the low temperature carbothermic reduction of UO₂+C produced up to 90 wt.% UC_1-xO_x type oxycarbides as was confirmed by Xray powder diffraction. Reducing Ar-4%H₂ environments at these temperatures were not successful in synthesizing UC as it reduces the amount of C required for the carbothermic reduction, leaving UC phase at a non-equilibrium state. Inert atmospheres with low or negligible oxygen partial pressures on the other hand produced near stoichiometric UC at high phase purity, especially at 1673 – 1773 K temperature range. An activation energy of 377±75 kJmol^-1 was also calculated using product phase concentrations of the carbothermic reduction of UO₂+C under these inert Ar_(g) atmospheres.