Evaluation of activities of carbons in chemical equilibrium with uranium carbonitride

A mixture of uranium sesquinitride and carbon was prepared by the reaction of UC of UC2 with N2 in the temperature range from 700 to 1400degC. When the mixture of uranium sesquinitride and carbon is kept at temperatures above 1200degC in the atmosphere of N2 at low pressure, the state where uranium carbonitride (UC1-xNx) and carbon are present together in chemical equilibrium will be established. A thermodynamic analysis suggests that, in the equilibrium state, the composition of UC1-xNx is determined by the chemical activity of carbon, ac, which is related to the chemical potential of the carbon, μc, by the equation, μc = μcdeg + RT 1n ac. Here μcdeg refers to graphite, which is usually taken as the standard state of carbon (ac = 1). Mixtures of U2N3 and carbon with several degrees of graphitization were heat-treated at 1400degC, and the composition of UC1-xNx in the reaction product was determined. From these experimental results and the thermodynamic analysis, values of the activity of the carbon coexisting with UC1-xNx were estimated. (author)