Critical review of carbon monoxide pressure measurements in the uranium–carbon–oxygen ternary system

For high temperature reactors (HTR), the high level of fuel operating temperature in normal and accidental conditions requires to predict the possible chemical interactions between the fuel components. Among the concerns of the TRISO fuel particle thermomechanical behavior, it is necessary to better understand the carbon monoxide formation due to chemical interactions at the UO2 kernel and graphite buffer’s interface. In a first step, the thermodynamic properties of the U–C–O system have to be assessed. The experimental data from literature on the equilibrium CO gas pressure measurements in the UO2–UC2–C ternary section of the U–C–O system are critically reviewed. Discrepancies between the different determinations can be explained – (i) by the different gaseous flow regimes in the experiments and – (ii) by the location of the measuring pressure gauge away from the reaction site. Experimental values are corrected – (i) from the gaseous flow type (molecular, transition or viscous) defined by the Knudsen number and – (ii) from the thermomolecular effect due to the temperature gradient inside the experimental vessels. Taking account of the selected and corrected values improves greatly the consistency of the original set of measurements.