Exploring the Local Structure of Molten NaF-ZrF4 through In Situ XANES/EXAFS and Molecular Dynamics.

Abstract

Molten salts are critical materials for advanced energy systems, particularly in molten salt reactors (MSRs), due to their exceptional thermophysical and chemical properties. While significant progress has been made in understanding their macroscopic behaviors, detailed knowledge of their atomic structures remains limited, particularly in fluoride-based salts with high zirconium concentrations. This study investigates the atomic structure and thermophysical properties of NaF-ZrF₄ salt mixtures (53-47 and 56-44 mol %) using an integrated experimental and computational approach. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were employed to probe the local environment of Zr atoms across temperatures from 530 to 700 °C, revealing changes in coordination states and bond distances. Complementary ab initio molecular dynamics (AIMD) and neural network-based molecular dynamics (NNMD) simulations were validated against experimental data to elucidate short- and intermediate-range ordering in the melt. The results highlight a temperature-driven transition toward lower Zr coordination numbers and increased structural distortion, providing insights into the fluoroacidity and potential corrosiveness of these salts. This comprehensive understanding of the NaF-ZrF₄ structure supports the development of more reliable models for molten salts, aiding advancements in next-generation nuclear reactors and energy systems.