Atomic-Scale Dynamics of Molten Salts

Abstract

With growing concerns over climate change and increasing reliance on electronic technologies, demand for clean, sustainable energy sources is surging. Nuclear power offers a low-carbon, highly energy-dense solution but faces challenges such as finite fuel resources, economic viability, safety, radioactive waste, and nuclear proliferation. Molten salt reactors (MSRs), a type of generation IV nuclear reactor, are being developed to address these concerns. MSRs use molten salts as coolants, but these salts are highly corrosive, leading to limited studies on their thermo-fluid-dynamic properties. Determining the self-diffusion coefficients of the ions in molten salts is crucial for understanding and predicting their behaviour in a nuclear reactor environment. This study aimed to develop a nuclear magnetic resonance (NMR) technique using an integrated furnace probe and the superconducting magnets’ stray field to measure isotope-specific self-diffusion of molten salts at high temperatures, with improved temperature control and reduced convection current influence compared to laser-heated systems. To better understand the chemical shift information imparted by NMR, a computational model using molecular dynamics and DFT-based code was developed to investigate its temperature-dependent mechanisms and effects. The computational model isolated and confirmed the shielding effect of lattice expansion from the deshielding effect of lattice vibration, and the determination of the dominating effect in each ion with increasing temperature. The model corroborated an empirically-based literature theory describing the temperature-dependent chemical shifts in the LiCl and KCl solids, as well as provide a basis of understanding for their liquid phases alongside the LiCl-KCl eutectic mixture (LKE). Temperature-dependent trends in 35Cl chemical shift were corroborated by practical NMR using the integrated furnace probe up to at least 700 °C across all samples. Higher temperature experiments were limited by material choices as the resonator and sample containers suffered structural failures. Diffusion measurements were unsuccessful owing to the limited diffusion at the magnetic field gradients achievable in the stray field.