High Temperature Interaction Between $UO2$ and Carbon: Application to TRISO Particles for Very High Temperature Reactors

For very high temperature reactors, 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 tristructural isotropic particle. The partial pressures of the gaseous carbon oxides formed at the fuel $(UO2)$-buffer (C) interface leading to the build up of the internal pressure in the particle have to be predicted. A good knowledge of the phase diagram and thermodynamic properties of the uranium-carbon-oxygen (UCO) system is also required to optimize the fabrication process of “UCO” kernels made of a mixture of $UO2$ and $UC2$⁠. Thermodynamic calculations using the FUELBASE database dedicated to Generation IV fuels (Guéneau, Chatain, Gossé, Rado, Rapaud, Lechelle, Dumas, and Chatillon, 2005, “A Thermodynamic Approach for Advanced Fuels of Gas Cooled Reactors,” J. Nucl. Mater., 344, pp. 191–197) allow predicting the phase equilibria involving carbide and/or oxycarbide phases at high temperature. Very high levels of $CO(g)$ and $CO2(g)$ equilibrium pressures are obtained above the $UO2±x$ fuel in equilibrium with carbon that could lead to the failure of the particle in case of high oxygen stoichiometry of the uranium dioxide. To determine the deviation from thermodynamic equilibrium, measurements of the partial pressures of $CO(g)$ and $CO2(g)$ resulting from the $UO2/C$ interaction have been performed by high temperature mass spectrometry on two types of samples: (i) pellets made of a mixture of $UO2$ and C powders or (ii) $UO2$ kernels embedded in carbon powder. Kinetics of the $CO(g)$ and $CO2(g)$ as a function of time and temperature was determined. The measured pressures are significantly lower than the equilibrium ones predicted by thermodynamic calculations. The major gaseous product is always $CO(g)$⁠, which starts to be released at 1473 K. From the analysis of the partial pressure profiles as a function of time and temperature, rates of $CO(g)$ formation have been assessed. The influence of the different geometries of the samples is shown. The factors that limit the gas release can be related to interface or diffusion processes as a function of the type of sample. The present results show the utmost importance of kinetic factors that govern the $UO2/C$ interaction.