Ag defects in Σ3 grain boundary of SiC were analyzed to test the hypothesis that Ag release from tri-structural isotropic fuel particles could occur through grain boundary diffusion. Although Σ3 grain boundaries could not provide a connected path through the crystal, they were studied here to provide guidance for overall trends in grain boundary versus bulk Ag transport. Formation energies of Ag defects were found to be 2 to 4eV 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 1600C, which was a temperature relevant for tri-structural isotropic accident conditions, Ag diffusion coefficients were predicted to be 3.7 x 10−18 and 3.9 x 10-29m2/s in the Σ3 grain boundary and bulk, respectively. While at this temperature Σ3 diffusion was still two orders of magnitude slower than diffusion estimated from integral release measurements, the values were close enough to suggest that grain boundary diffusion was a plausible mechanism for release of Ag from intact SiC coatings. The remaining discrepancies in the diffusion coefficients could possibly be bridged by considering high-energy grain boundaries, which were expected to have diffusivity faster than Σ3 and which provided a connected percolating path through polycrystalline SiC.

Diffusion of Ag along Σ3 Grain Boundaries in 3C-SiC. S.Khalil, N.Swaminathan, D.Shrader, A.J.Heim, D.D.Morgan, I.Szlufarska: Physical Review B, 2011, 84[21], 214104