Understanding the diffusion and release of fission products in nuclear fuels is of great relevance for ensuring high levels of safety of these materials. The origin of the diffusion of silver in TRISO (Tristructural Isotropic) coated fuel particles has remained an open challenge in the study of this type of fuel despite 40 years of development. In this work, we propose that the existence of grain boundary complexions and complexion transitions might be responsible for the complex diffusion behavior observed with silver. We have identified three types of the Dillon-Harmer complexions in SiC (clean grain boundary, a disordered layer with a similar to 0.6 nm of thickness, and an intergranular amorphous film) through ultra-high resolution transmission electron microscopy.