Ab-Initio Simulation of Fission Product Diffusion in Graphite*

Graphite has been used for neutron moderation from the beginning of the nuclear reactor era. While research activities have abated over the years, there is renewed interest in graphite motivated by its use in Very High Temperature Reactors (VHTRs) and Molten Salt Reactors (MSRs). Retention of the activated fission products is paramount during normal operating and accident conditions, and a mechanistic understanding of the bonding and diffusion properties of fission products is imperative for predicting the release rates and designing appropriate barriers.

While the simulation, via Density Functional Theory, of the diffusion and adsorption of a number of elements on graphene has received some attention, to the best of our knowledge little work has been conducted on the bonding and diffusion properties of many elements in bulk graphite. We have performed high-accuracy DFT simulations of nuclear fission products on graphene and in bulk graphite using LDA, GGA and van der Waals exchange-correlation functionals. The bonding, structural and diffusiive properties have been extracted. These simulations form a basis for the understanding of the diffusion and retention of such products in nuclear graphite.