Diffusion behavior of cesium in graphite matrix for a High-temperature Reactor Pebble-bed Module: a first-principles study

Graphite is one of the main materials of the fuel element in High Temperature Reactor Pebble-bed Module (HTR-PM). Knowledge of the behavior of Cesium (Cs) in graphite is necessary for reliable source term analysis under normal and accidental conditions for HTR-PMs. In this study, the diffusion behavior of Cs atoms in graphite was calculated to estimate the diffusivity of Cs using the first-principles density functional theory. The results show that the energy barriers for vacancy and interstitial diffusion are significantly different (2.257 eV and 0.051 eV, respectively), suggesting that interstitial diffusion is more likely to occur in high-temperature graphite. However, experimental data in literatures indicate that the activation energy for Cs diffusion in graphite falls between 1.5 to 3.3 eV, aligning with the migration energy for vacancy diffusion identified in this study. It is hypothesized that experimental observations of diffusion may be primarily influenced by vacancies. The results may help designers to select appropriate parameters or make physical assumptions and to consider the radiation risks caused by the residual fission product (Cs) in graphite.