Mechanistic calculation of the effective silver diffusion coefficient in polycrystalline silicon carbide: Application to silver release in AGR-1 TRISO particles.

• A mesoscale effective Ag diffusion coefficient in SiC is derived from atomistic calculations of bulk and GB diffusivity. • The effective Ag diffusion coefficient in SiC accounts for temperature and microstructure effects. • Microstructures with grains that are smaller in the direction perpendicular to diffusion promote faster Ag diffusion in SiC. • The multiscale effective Ag diffusivity is within an order of magnitude of the empirical diffusion coefficient currently used in Bison. • Accounting for the microstructure effects on Ag diffusion increases the prediction accuracy of Ag release from AGR-1 TRISO fuel. The silicon carbide (SiC) layer in tristructural isotropic (TRISO) fuel particles serves as a barrier to prevent the escape of fission products produced and not retained in the fuel kernel. The release of silver (Ag) is a concern due to the long half-life of the 110 m Ag isotope. However, accurately determining the fission gas release rate requires knowing the diffusion coefficient through the SiC layer. In this study, we leverage atomistic calculations of Ag diffusivity in SiC bulk and grain boundaries (GBs) to develop a mesoscale effective Ag diffusion coefficient (D e f f) in SiC. Since GBs serve as pathways for Ag diffusion, D e f f is defined as a function of temperature and microstructure variables. In particular, the size of SiC grains in the direction perpendicular to diffusion is shown to significantly affect Ag diffusion. The prediction of the mechanistic, mesoscale approach falls within one order of magnitude of empirical values. The temperature and microstructure-dependent effective Ag diffusivity in SiC is implemented in the fuel performance code Bison with a correction factor to predict Ag release from AGR-1 TRISO fuel particles. We hereby quantify the impact of SiC grain size on Ag release and improve Bison's predictions. [Display omitted] [ABSTRACT FROM AUTHOR]