Studies of Ni-Cr complexation in FLiBe molten salt using machine learning interatomic potentials.

• Molecular dynamics with a machine learning potential is used to show modest binding interaction of Cr and Ni in FLiBe. • Dissolved Cr and Ni tend to reside in each other's second nearest neighbor. • The existence of Cr in FLiBe increases the solubility of Ni at high salt potentials. In nuclear and/or solar applications that involve molten salts, impurities frequently enter the salt as either fission products or via corrosion. Impurities can interact and make complexes, but the impact of such complexation on the properties of the salts and corrosion rates has not been understood. Common impurities in molten salts, such as FLiBe, include Cr, Ni, and Fe. Here, we investigate the complexation of Cr and Ni in FLiBe using molecular dynamics based on a machine learning interatomic potential (MLIP) fitted using the atomic cluster expansion (ACE) method. The MLIP allows us to overcome the challenges of simultaneously needing accurate energetics and long time scale to study complexation. We demonstrate that impurity behavior is more difficult to capture than that of concentrated elements with MLIPs due to less sampling in training data, but that this can be overcome by using active learning strategies to obtain a robust fit. Our findings suggest that there is a weak but potentially significant binding free energy between CrF 2 and NiF 2 in eutectic FLiBe of - 0.112 e V. Under certain conditions this binding creates a significant short-range order between the species and lowers the redox potential of NiF 2 in the presence of CrF 2 in FLiBe, making Ni dissolution more favorable in the presence of Cr as compared to its dissolution in pure FLiBe. However, we find little impact of this complexation on the diffusivity of Ni and Cr. Overall, the methodology presented here suggests an approach to modeling complexation with MLIPs and suggests that interactions between dissolved cations could be playing a significant role in some salt thermophysical properties. [ABSTRACT FROM AUTHOR]