New study models diffusion of hazardous fission products through protective barriers

The next generation of nuclear power technology possess multiple barriers to prevent the release of radioactive material. However, some hazardous fission products have been known to leak out under accident conditions. In particular, cesium isotopes remain of high concern because of the large amount produced during operation, as well as a long radioactive half-life. Even though the uranium fuel is encased in silicon carbide (SiC) for protection, cesium can escape into the environment. In Journal of Applied Physics, researchers from the University of Wisconsin-Madison investigate how cesium diffuses through the high energy grain boundaries (HEGBs) of SiC.

The authors first modeled the HEGB structure as amorphous SiC and performed ab initio density functional theory calculations to identify interstitial sites and transition states. Then, a kinetic Monte Carlo simulation was used to statistically sample atomic hopping and evaluate cesium diffusion coefficients on the network of interstitial sites. The kinetic Monte Carlo method was employed with the Bortz-Kalos-Liebowitz algorithm and included the site and transition state energies sampled from density functional theory.

The results, in combination with other studies, point to grain boundary diffusion as the dominant type of transport for cesium. Specifically, HEGBs appear to be some of the fastest grain boundary paths for cesium diffusion. Coauthor Dane Morgan suggests that based on ion implantation studies with irradiation, radiation may even enhance diffusion rates for cesium in SiC.

The work contributes additional understanding to the mechanism of cesium motion, as a first step to reduce fission product leakage. According to Morgan, their findings have implications for creating barrier materials with fewer HEGBs to make cesium escape less likely.

Source: “Cs diffusion in SiC high-energy grain boundaries,” by Hyunseok Ko, Izabela Szlufarska, and Dane Morgan, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4989389 .