Kinetic study of the UO2/C interaction by high-temperature mass spectrometry

For very high-temperature reactors (V)-HTR, one of Generation IV future systems, the high-level operating temperature of the fuel materials in normal and accidental conditions requires the prediction of the possible chemical interactions between the fuel component (UO2±x) and the structural materials (C, SiC). To predict the thermo-mechanical behaviour of the TRi-ISOtropic (TRISO) particle, it is necessary to better understand the gaseous carbon oxides formation at the fuel–buffer interface that leads to the build up of the internal pressure. High equilibrium CO(g) pressures resulting from the UO2±x/C reaction are predicted using thermodynamic calculations. The kinetic mechanisms involved in this reaction that limit this pressure increase have to be determined by convenient experiments and associated models. Some of the reported data on the kinetics of CO(g) formation due to the UO2±x and carbon interaction have been reviewed. The discrepancies between the reaction mechanisms can be explained (i) by the different geometries and sample types and (ii) by the oxide stoichiometry and the flowing gas used during the experiments. Depending on these characteristics, the phenomena involved in CO(g) formation can be of three different origins: interface, surface or diffusion. Using high-temperature mass spectrometry (HTMS), kinetic measurements of the CO(g) and CO2(g) species evolved during the interaction between UO2±x and carbon were performed. The samples are pressed pellets consisting of a mixture of UO2±x (60% molar) and carbon black (40% molar) powders. CO(g) is the major product above 1200 K. Rates of the CO(g) formation have been established taking into account the oxygen composition of the non-stoichiometric uranium dioxide and temperature. Results underline the upmost importance of kinetic factors for studying the CO(g) pressure variation inside the TRISO particle.