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 ~0.6 nm of thickness, and an intergranular amorphous film) through ultra‐high resolution transmission electron microscopy.

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One of the main challenges in the study of TRISO (Tristructural Isotropic) coated fuel particles is the understanding of the diffusion of fission products through SiC. Among the elements produced inside the uranium kernel, it has been suggested that Pd might enhance the diffusion of other fission products. In this work, we have studied the interaction between Pd and SiC. We have observed that as Pd diffuses it can change the chemical composition and microstructure of SiC. Electron Backscattered Diffraction (EBSD) analysis showed that Pd increased the amount of high angle grain boundaries from 47% to 59%. Furthermore, we have observed that as Pd diffused, it changed the composition of SiC by leaving a trail of excess carbon at the grain boundary. This change in localized chemical composition and microstructure suggests a grain boundary complexion transition induced by Pd and a new way in which Pd can lead to faster diffusion routes for other fission products.