Structure-property relationships in actinide containing molten salts – A review: Understanding and modelling the chemistry of nuclear fuel salts

Transient Covalency in Molten Uranium(III) Chloride

Uranium is arguably the most essential element in the actinide series, serving as a crucial component of nuclear fuels. While U is recognized for engaging the 5f orbitals in chemical bonds under normal conditions, little is known about its coordination chemistry and the nature of bonding interactions at extreme conditions of high temperature. Here we report experimental and computational evidence for the shrinkage of the average U-ligand distance in UCl3 upon the solid-to-molten phase transition, leading to the formation of a significant fraction of short, transient U-Cl bonds with the enhanced involvement of U 5f valence orbitals. These findings reveal that extreme temperatures create an unusual heterogeneous bonding environment around U(III) with distinct inner- and outer-coordination subshells.

HT-NMR Studies of the Be-F Coordination Structure in FNaBe and FLiBe Mixed Salts

Molten NaF-BeF2 salt is widely considered a promising candidate to replace FLiBe in molten salt reactor applications, which is crucial to reducing the operating costs of the molten salt reactor. Studies on beryllium compounds are rarely conducted due to their volatility and high toxicity. Herein, the Be-F coordination structure of NaF/BeF2 mixed salts was investigated in-depth through various HT-NMR and solid-state NMR methods, which are optimized to be appropriate for the detection of beryllium compounds. It was found that Na2BeF4 and NaBeF3 crystals were transformed into amorphous tetrahedral coordinated networks when there was an increase in the BeF2 concentration in the mixed salts. The main coordinate structure comparisons between FNaBe and FLiBe were analyzed, which exhibit high similarity due to the covalent effect of Be-F bonding, demonstrating the theoretical feasibility of applying FNaBe salts as a substitute for FLiBe in MSR systems. In addition, the transition from the crystal phase to the amorphous phase occurred at a lower BeF2 concentration for FNaBe than that for FLiBe. This was further verified by the results of ab initio molecular dynamics (AIMD) simulation that FNaBe melts had more disordered structures, thus causing slight changes in their physical properties.

First-Principles Investigation of the Effects of UF4 and ThF4 Fuels on the Structural, Dynamic, and Thermodynamic Properties of LiF-NaF

An in-depth understanding and characterization of molten salt properties are necessary for the optimized design, efficient operation, and safety assurance of molten salt reactors (MSRs). Investigating molten salt properties in experimental settings can be challenging and time-consuming due to the high temperatures of interest, the salt's corrosiveness, purity and composition control, and health and safety concerns. Therefore, it is beneficial to perform computational screening to assist in the ultimate experimental measurements. Herein, we used first-principles molecular dynamics simulations to calculate several thermophysical, structural, and dynamic properties of eutectic LiF-NaF with fuel additives UF4 and ThF4. We found that with the incorporation of uranium or thorium, a prepeak appears in the structure factor, indicative of a medium-range structural ordering. Furthermore, we explore the mechanism through which these structural changes enhance shear stress correlations, thereby increasing the salt's viscosity. This work highlights the importance of studying the atomic-scale structure of molten salts and how the addition of fuel elements can substantially affect it.

Development of Deep Potentials of Molten MgCl2-NaCl and MgCl2-KCl Salts Driven by Machine Learning

Molten MgCl2-based chlorides have emerged as potential thermal storage and heat transfer materials due to high thermal stabilities and lower costs. In this work, deep potential molecular dynamics (DPMD) simulations by a method combination of the first principle, classical molecular dynamics, and machine learning are performed to systemically study the relationships of structures and thermophysical properties of molten MgCl2-NaCl (MN) and MgCl2-KCl (MK) eutectic salts at the temperature range of 800-1000 K. The densities, radial distribution functions, coordination numbers, potential mean forces, specific heat capacities, viscosities, and thermal conductivities of these two chlorides are successfully reproduced under extended temperatures by DPMD with a larger size (5.2 nm) and longer timescale (5 ns). It is concluded that the higher specific heat capacity of molten MK is originated from the strong potential mean force of Mg-Cl bonds, whereas the molten MN performs better in heat transfer due to the larger thermal conductivity and lower viscosity, attributed to the weak interaction between Mg and Cl ions. Innovatively, the plausibility and reliability of microscopic structures and macroscopic properties for molten MN and MK verify the extensibilities of these two deep potentials in temperatures, and these DPMD results also provide detailed technical parameters to the simulations of other formulated MN and MK salts.