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Ag defects in Σ3 grain boundary of SiC were analyzed to test the hypothesis that Ag release from TRISO fuel particles can occur through grain boundary diffusion. Although Σ3 grain boundaries cannot provide a connected path through the crystal, they are studied here to provide guidance for overall trends in grain boundary vs. bulk Ag transport. Formation energies of Ag defects are found to be 2 − 4 eV lower in the grain boundaries than in the bulk, indicating a strong tendency for Ag to segregate to the grain boundaries. Diffusion of Ag along Σ3 was found to be dramatically faster than through the bulk. At 1600 • C, which is a temperature relevant for TRISO accident conditions, Ag diffusion coefficients are predicted to be 3.7 × 10 −18 m 2 /s and 3.9 × 10 −29 m 2 /s in the Σ3 grain boundary and bulk, respectively. While at this temperature Σ3 diffusion is still two orders of magnitude slower than diffusion estimated from integral release measurements, they values are close enough to suggest that grain boundary diffusion is a plausible mechanism for release of Ag from intact SiC coatings. The remaining discrepancies in the diffusion coefficients could be possibly bridged by considering high-energy grain boundaries, which are expected to have diffusivity faster than Σ3 and which provide a connected percolating path through polycrystalline SiC.

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Understanding of the fission product transport in TRISO fuel particles is fundamental to improving the safety and performance of high temperature gas cooled reactors. Previous experiments showing silver release from TRISO fuel have focused on release measurements and not direct observation of the fission product transport. The possible diffusion of Ag via a grain boundary diffusion mechanism is being examined. By characterizing the SiC grain boundary structure according the coincidence site lattice scheme and detecting diffusion along specific grain boundaries, insight into the relationship between SiC microstructure and Ag release may be obtained. In addition computer modeling is being used to investigate the diffusion of silver through SiC. We employ a multi-scale approach based on ab initio techniques, molecular dynamics, and continuum rate equations in order to establish relationships between complex microstructures and diffusion rates. Initial work has begun on transport through bulk SiC and on building realistic models of grain boundaries in SiC.

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