Thermal behaviour of caesium implanted in UO2: A comparative study with the xenon behaviour

Abstract Xenon and caesium are among the most impacting fission products when studying the nuclear fuel: xenon for its role on the fuel rod thermomechanical behaviour during reactor operation and caesium in the case of atmospheric radioactive release during an accident in a nuclear power plant. This paper focuses on the comparison of caesium and xenon thermal behaviour in polycrystalline uranium dioxide (UO2) pellets. Caesium-133 or xenon-136 stable isotopes were introduced in depleted UO2 samples by ion implantation at a maximum concentration of 0.08 at. % at a depth of around 140 nm below the sample surface. Annealing under reducing atmosphere (Ar/H2 5 %) was performed at 1000°C or 1600°C, which corresponds respectively to a representative temperature during nuclear reactor operation (at the center of the fuel pellets) and during an accident. The caesium migration in UO2 was investigated by Secondary Ion Mass Spectrometry and compared to the thermal behaviour of xenon in UO2 at 1600°C. Transmission Electron Microscopy was performed in order to characterise UO2 microstructure before and after annealing. The results indicate that caesium has a different behaviour than xenon with which it is often compared for its release from the nuclear fuel. In particular, we highlight a difference between the growth kinetics of caesium and of xenon bubbles at 1600°C which can be correlated to the availability of thermal vacancies in UO2 and to the different ability of Xe and Cs atoms for thermal resolution.