For Very High Temperature Reactors (V-HTR), the study of the Uranium-Carbon-Oxygen system is of major importance to predict the high temperature behaviour of the TRISO fuel particle. Firstly, the high level operating temperature of the fuel materials in normal and accidental conditions requires studying the possible chemical interaction between the UO2 fuel kernel and the surrounding structural materials (C, SiC) that could damage the particle. The formation of the gaseous carbon oxides at the fuel (UO2)-buffer (C) interface that leads to the build up of the internal pressure in the particle has to be predicted. Secondly, the U-C-O ternary system is also involved in the fabrication process of “UCO” kernels made of a mixture of UO2 and UC2. For the fabrication of such mixture of uranium oxide and carbide, the phase diagram and thermodynamic properties of the U-C-O system are necessary to investigate in order to perform adequate heat treatments. For both reasons, a new study of the U-C-O ternary system has been undertaken. Firstly, some thermodynamic calculations (equilibrium CO(g) and CO2(g) pressures, phase diagrams) were performed using the thermodynamic FUELBASE database dedicated to generation IV fuels [1]. The results allow representing the different phase equilibria involving carbide and oxicarbide phases at high temperature. They also show the high level of CO(g) and CO2(g) equilibrium pressures above the UO2±x fuel in equilibrium with carbon which could lead to the failure of the particle for high oxygen stoichiometry of the uranium dioxide. In a second step, the partial pressures of CO(g) and CO2(g) resulting from the UO2/C interaction have been measured by high temperature mass spectrometry. Two types of samples were used (i) pellets made of a mixture of UO2 and C powders or (ii) UO2 kernels disseminated in a carbon bed. The kinetic measurements of the release of CO(g) and CO2(g) lead to measured pressures that are lower than the equilibrium pressures predicted from thermodynamic calculations. This discrepancy can be explained by limitations due to distinct kinetic mechanisms. Rates of CO(g) formation have been established taking into account the oxygen stoichiometry of uranium oxide and temperature. The major gaseous product is always CO(g) which release significantly starts at 1473 K. The influence of the different geometries is shown. The limitative kinetic step can be an interface or a diffusion process as a function of the type of sample. These results underline the up most importance of kinetic factors for studying the UO2 / C interaction to determine realistic CO(g) pressure levels inside a TRISO particle or to improve the fabrication process of the “UCO” kernels.

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