Density functional theory metadynamics of silver, caesium and palladium diffusion at β-SiC grain boundaries

The use of silicon carbide in coated nuclear fuel particles relies on this materials impermeability towards fission products under normal operating conditions. Determining the underlying factors that control the rate at which radionuclides such as Silver-110m and Caesium-137 can cross the silicon carbide barrier layers, and at which fission products such as palladium could compromise or otherwise alter the nature of this layer, are of paramount importance for the safety of this fuel. To this end, DFT-based metadynamics simulations are applied to the atomic diffusion of silver, caesium and palladium along a Σ5 grain boundary and to palladium along a carbon-rich Σ3 grain boundary in cubic silicon carbide at 1500 K. For silver, the calculated diffusion coefficients lie in a similar range (7.04 × 10−19-3.69 × 10−17 m2 s−1) as determined experimentally. For caesium, the calculated diffusion rates are very much slower (3.91 × 10−23-2.15 × 10−21 m2 s−1) than found experimentally, suggesting a different mechanism to the simulation. Conversely, the calculated atomic diffusion of palladium is very much faster (7.96 × 10−11-7.26 × 10−9 m2 s−1) than the observed penetration rate of palladium nodules. This points to the slow dissolution and rapid regrowth of palladium nodules as a possible ingress mechanism in addition to the previously suggested migration of entire nodules along grain boundaries. The diffusion rate of palladium along the Σ3 grain boundary was calculated to be slightly slower (2.38 × 10−11-8.24 × 10−10 m2 s−1) than along the Σ5 grain boundary. Rather than diffusing along the precise plane of the boundary, the palladium atom moves through the bulk layer immediately adjacent to the boundary as there is greater freedom to move.