Molecular dynamics study of fission product behaviour in molten [formula omitted]-[formula omitted] salt

Abstract

The solvation of fission products in fuel salts and the effects due to their accumulation are important factors to understand in molten salt reactor design. In this contribution, we use ab initio Molecular Dynamics (AIMD) and polarizable ion model (PIM) based MD to study the structural and dynamic characteristics of the fission products (Cs, I and Xe) in the model fuel salt ThF4-LiF with different compositions at the infinite dilution limit. We first develop or refine existing PIM potentials to better reproduce the results of AIMD simulations. The AIMD and PIM-MD results are then compared in terms of the structures present in the salt, and the dynamics of their coordination. The results of this study indicate that, at 1200 K and 1400 K the fission products studied here have weak interactions with all composition ranges of fuel salt. This is evident, as first coordination shell associations being generally short-lived, as determined by cage correlation times. The diffusion coefficients determined through PIM-MD match the AIMD results reasonably well. The three fission products studied here are diffuse at similar rates; however, the Cs+ diffusion is slightly lower likely due to its more tightly bound associated F−. With all solutes, the diffusion coefficients decrease with a greater ThF4 fraction. Additionally, the behaviour of these solutes was studied at finite concentration ranges, up to 4 atom %. Here, it was found that the presence of Cs+ has little effect on the density, heat capacity and viscosity. However, these properties were affected by higher I+ and Xe concentrations.