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 centre 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.

氙和铯是研究核燃料时影响最大的裂变产物：氙在反应堆运行期间对燃料棒热机械行为的作用，以及在核电站事故期间在大气中释放铯的铯。本文着重比较多晶二氧化铀（UO ₂）颗粒中铯和氙的热行为。通过离子注入，在样品表面以下约140 nm处，以最大浓度0.08 at％的离子注入将铯133或氙136稳定同位素引入到贫化的UO ₂样品中。在还原气氛下退火（Ar / H ₂5％）是在1000°C或1600°C下执行的，这分别对应于核反应堆运行期间（在燃料芯块的中心）和事故期间的代表温度。通过二次离子质谱研究了铯在UO ₂中的迁移，并与氙在1600°C下在UO ₂中的热行为进行了比较。进行透射电子显微镜以表征UO ₂退火前后的显微组织。结果表明铯与氙的行为不同，经常将其与从核燃料中释放的氙进行比较。特别是，我们着重指出了铯和氙气气泡在1600°C的生长动力学之间的差异，这可能与UO ₂中热空位的可用性以及Xe和Cs原子的热分解能力不同有关。