Silver and cesium diffusion dynamics at the β-SiC Σ5 grain boundary investigated with density functional theory molecular dynamics and metadynamics

The diffusion and release of silver-110m, a strong γ-radiation emitter, through silicon carbide in coated nuclear fuel particles has remained an unsolved topic since it was first observed 40 years ago. The challenge remains to explain why, contrary to other elements, silver is capable of escaping the ceramic diffusion barriers. The current work investigates the underlying differences in the diffusion of silver and cesium along a symmetric tilt Σ5 grain boundary of β-SiC through accelerated density functional theory molecular dynamics simulations. The energy barriers extracted from the simulations give diffusion coefficients that are in reasonable agreement with experiment for silver (2.19 × 10^-19 to 1.05 × 10^-17 m² s^-1), but for cesium the equivalent calculated coefficients for this mechanism are much smaller (3.85 × 10^-23 to 2.15 × 10^-21 m² s ^-1) than those found experimentally. Analysis of the simulated structures and electron densities and comparisons with the calculations of other researchers suggest that diffusion of silver and cesium in β-SiC proceeds via different mechanisms. The mechanisms of cesium diffusion appear to be dominated by its relatively large size and repulsive interactions with the silicon and carbon atoms; β-SiC grain boundaries still offer higher energy barriers to diffusion. Silver, on the other hand, is not only smaller in size but, as we show for the first time, can also participate in weak bonding interactions with the host atoms where favorable geometries allow, thus reducing the energy barrier and enhancing the rate of diffusion.