  The diffusion of silver (Ag) impurities in high energy grain boundaries
(HEGBs) of cubic (3C) silicon carbide (SiC) is studied using an ab initio based
kinetic Monte Carlo (kMC) model. This study assesses the hypothesis that the
HEGB diffusion is responsible for Ag release in Tristructural-Isotropic fuel
particles, and provides a specific example to increase understanding of
impurity diffusion in highly disordered grain boundaries. The HEGB environment
was modeled by an amorphous SiC. The structure and stability of Ag defects were
calculated using density functional theory code. The defect energetics
suggested that the fastest diffusion takes place via an interstitial mechanism
in a-SiC. The formation energy of Ag interstitials and the kinetic resolved
activation energies between them were well approximated with Gaussian
distributions that were then sampled in the kMC. The diffusion of Ag was
simulated with the effective medium model using kMC. At 1200-1600C, Ag in a
HEGB is predicted to exhibit an Arrhenius type diffusion and with a diffusion
prefactor and effective activation energy of (2.73+-1.09)*10-10 m2s-1 and
2.79+-0.18 eV, respectively. The comparison between HEGB results to other
theoretical studies suggested not only that GB diffusion is predominant over
bulk diffusion, but also that the HEGB is one of fastest grain boundary paths
for Ag diffusion in SiC. The Ag diffusion coefficient in the HEGB shows a good
agreement with ion-implantation measurements, but is 2-3 orders of magnitude
lower than the diffusion coefficients extracted from integral release
measurements. The discrepancy between GB diffusion and integral release
measurements suggests that other contributions are responsible for the fast
release of Ag in some experiments and we propose that these contributions may
arise from radiation enhanced diffusion.
