Diffusion of Ag along Σ3 grain boundaries in 3C-SiC

Ag defects in Sigma 3 grain boundary of SiC were analyzed to test the hypothesis that Ag release from tristructural isotropic (TRISO) fuel particles can occur through grain boundary diffusion. Although Sigma 3 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 Sigma 3 was found to be dramatically faster than through the bulk. At 1600 degrees C, which is a temperature relevant for TRISO accident conditions, Ag diffusion coefficients are predicted to be 3.7 x 10(-18) m(2)/s and 3.9 x 10(-29) m(2)/s in the Sigma 3 grain boundary and bulk, respectively. While at this temperature Sigma 3 diffusion is still two orders of magnitude slower than diffusion estimated from integral release measurements, the 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 Sigma 3 and which provide a connected percolating path through polycrystalline SiC.