Cs diffusion in SiC high-energy grain boundaries

TL;DR

This study models cesium diffusion in high-energy grain boundaries of cubic-SiC using ab-initio and kinetic Monte Carlo methods, revealing dominant grain boundary diffusion pathways relevant for nuclear fuel safety.

Contribution

It introduces a novel ab-initio based kinetic Monte Carlo model for Cs diffusion in SiC grain boundaries, highlighting the role of amorphous SiC environments.

Findings

Cs diffusion in HEGBs follows Arrhenius behavior between 1200-1600°C.

HEGB diffusion is faster than bulk diffusion but slower than in- and out-of-pile measurements.

Other factors like radiation may enhance Cs diffusion beyond the modeled mechanisms.

Abstract

Cesium (Cs) is a radioactive fission product whose release is of concern for Tristructural-Isotropic (TRISO) fuel particles. In this work, Cs diffusion through high energy grain boundaries (HEGBs) of cubic-SiC is studied using an ab-initio based kinetic Monte Carlo (kMC) model. The HEGB environment was modeled as an amorphous SiC (a-SiC), and Cs defect energies were calculated using density functional theory (DFT). From defect energies, it was suggested that the fastest diffusion mechanism as Cs interstitial in an amorphous SiC. The diffusion of Cs interstitial was simulated using a kMC, based on the site and transition state energies sampled from the DFT. The Cs HEGB diffusion exhibited an Arrhenius type diffusion in the range of 1200-1600{\deg}C. The comparison between HEGB results and the other studies suggests not only that the GB diffusion dominates the bulk diffusion, but also that the HEGB is one of the fastest grain boundary paths for the Cs diffusion. The diffusion coefficients in HEGB are clearly a few orders of magnitude lower than the reported diffusion coefficients from in- and out-of- pile samples, suggesting that other contributions are responsible, such as a radiation enhanced diffusion.