Ion Specific Effects on the Structure of Molten AF-ZrF4 Systems (A+ = Li+, Na+, and K+)

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.

Compositional transferability of deep potential in molten LiF-BeF2 and LaF3 mixtures: prediction of density, viscosity, and local structure

The accumulation of lanthanide fission products carries the risk of altering the structure and properties of the nuclear fuel carrier salt LiF-BeF2 (Flibe), thereby downgrading the operating efficiency and safety of the molten salt reactor. However, the condition-limited experimental measurements, spatiotemporal-limited first-principles calculations, and accuracy-limited classical dynamic simulations are unable to capture the precise local structure and reliable thermophysical properties of heterogeneous molten salts. Therefore, the deep potential (DP) of LaF3 and Flibe molten mixtures is developed here, and DP molecular dynamics simulations are performed to systemically study the densities, diffusion coefficients, viscosities, radial distribution functions and coordination numbers of multiple molten Flibe + xLaF3, the quantitative relationships between these properties and LaF3 concentration are investigated, and the potential structure-property relationships are analyzed. Eventually, the transferability of DP on molten Flibe + LaF3 with different formulations as well as the predictability of structures and properties are achieved at the nanometer spatial scale and the nanosecond timescale.

Structure and Dynamics of NaCl/KCl/CaCl2-EuCln (n = 2, 3) Molten Salts

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu²⁺ and Eu³⁺ ions in CaCl₂, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K⁺ to Na⁺ to Ca²⁺, the coordination number (CN) of Cl^- in the first solvation shell increases from 5.6 (Eu²⁺) and 5.9 (Eu³⁺) in KCl to 6.9 (Eu²⁺) and 7.0 (Eu³⁺) in CaCl₂. This coordination change is validated by the EXAFS measurements, in which the CN of Cl^- around Eu increases from 5 in KCl to 7 in CaCl₂. Our simulation shows that the fewer Cl^- ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu²⁺/Eu³⁺ are related to the rigidity of their first coordination shell of Cl^-: the more rigid the first coordination shell is, the slower the solute cations diffuse.

Transferable Deep Learning Potential Reveals Intermediate-Range Ordering Effects in LiF-NaF-ZrF4 Molten Salt

LiF-NaF-ZrF₄ multicomponent molten salts are promising candidate coolants for advanced clean energy systems owing to their desirable thermophysical and transport properties. However, the complex structures enabling these properties, and their dependence on composition, is scarcely quantified due to limitations in simulating and interpreting experimental spectra of highly disordered, intermediate-ranged structures. Specifically, size-limited ab initio simulations and accuracy-limited classical models used in the past are unable to capture a wide range of fluctuating motifs found in the extended heterogeneous structures of liquid salt. This greatly inhibits our ability to design tailored compositions and materials. Here, accurate, efficient, and transferable machine learning potentials are used to predict structures far beyond the first coordination shell in LiF-NaF-ZrF₄. Neural networks trained at only eutectic compositions with 29% and 37% ZrF₄ are shown to accurately simulate a wide range of compositions (11-40% ZrF₄) with dramatically different coordination chemistries, while showing a remarkable agreement with theoretical and experimental Raman spectra. The theoretical Raman calculations further uncovered the previously unseen shift and flattening of bending band at ∼250 cm^-1 which validated the simulated extended-range structures as observed in compositions with higher than 29% ZrF₄ content. In such cases, machine learning-based simulations capable of accessing larger time and length scales (beyond 17 Å) were critical for accurately predicting both structure and ionic diffusivities.

Computational methods to simulate molten salt thermophysical properties

Molten salts are important thermal conductors used in molten salt reactors and solar applications. To use molten salts safely, accurate knowledge of their thermophysical properties is necessary. However, it is experimentally challenging to measure these properties and a comprehensive evaluation of the full chemical space is unfeasible. Computational methods provide an alternative route to access these properties. Here, we summarize the developments in methods over the last 70 years and cluster them into three relevant eras. We review the main advances and limitations of each era and conclude with an optimistic perspective for the next decade, which will likely be dominated by emerging machine learning techniques. This article is aimed to help researchers in peripheral scientific domains understand the current challenges of molten salt simulation and identify opportunities to contribute.

In Situ Determination of Speciation and Local Structure of NaCl-SrCl2 and LiF-ZrF4 Molten Salts

Understanding the local environment of the metal atoms in salt melts is important for modeling the properties of melts and predicting their behavior and thus helping enable the development of technologies such as molten salt reactors and solar-thermal power systems and new approaches to recycling rare-earth metals. Toward that end, we have developed an in situ approach for measuring the coordination of metals in molten salt coupling X-ray absorption spectroscopy (XAS) and Raman spectroscopy. Our approach was demonstrated for two salt mixtures (1.9 and 5 mol % SrCl₂ in NaCl, 0.8 and 5 mol % ZrF₄ in LiF) at up to 1100 °C. Near-edge (X-ray absorption near-edge structure, XANES) and extended X-ray absorption fine structure (EXAFS) spectra were measured. The EXAFS response was modeled using ab initio FEFF calculations. Strontium's first shell is observed to be coordinated with chlorine (Sr²⁺-Cl^-) and zirconium's first shell is coordinated by fluorine (Zr⁴⁺-F^-), both having coordination numbers that decrease with increasing temperature. Multiple zirconium complexes are believed to be present in the melt, which may interfere and distort the EXAFS spectra and result in an anomalously low zirconium first shell coordination number. The use of boron nitride (BN) powder as a salt diluent for XAFS measurements was found to not interfere with measurements and thus can be used for investigations of such systems.

Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures

The structure of a concentrated solution of NaCl in D₂O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl-O and O-O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D-O or O⋯D-O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion-water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl⁰ units, and coexist with solvent-separated Na⁺-Na⁺ and Cl^--Cl^- ion pairs. The exchange of water molecules with Na⁺ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes.

Experimental and Computational Exploration of the NaF-ThF4 Fuel System: Structure and Thermochemistry

The structural, thermochemical, and thermophysical properties of the NaF-ThF4 fuel system were studied with experimental methods and molecular dynamics (MD) simulations. Equilibrium MD (EMD) simulations using the polarizable ion model were performed to calculate the density, molar volume, thermal expansion, mixing enthalpy, heat capacity, and distribution of [ThFn]m- complexes in the (Na,Th)Fx melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF4 and 85-95 mol % ThF4 regions of the NaF-ThF4 phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF4) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na2ThF6 and NaTh2F9 compounds were synthesized and subsequently analyzed with the use of X-ray diffraction. The heat capacities of both compounds were measured in the temperature ranges (2-271 K) and (2-294 K), respectively, by thermal relaxation calorimetry. Finally, a CALPHAD model coupling the structural and thermodynamic data was developed using both EMD and experimental data as input and a quasichemical formalism in the quadruplet approximation. Here, 7- and 8-coordinated Th4+ cations were introduced on the cationic sublattice alongside a 13-coordinated dimeric species to reproduce the chemical speciation, as calculated by EMD simulations and to provide a physical description of the melt.

Examination of the short-range structure of molten salts: ThF4, UF4, and related alkali actinide fluoride systems

The short-range structures of LiF-ThF₄, NaF-AnF₄, KF-AnF₄, and Cs-AnF₄ (An = Th, U), were probed using in situ high temperature Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Signally, the EXAFS spectra of pure molten ThF₄ and UF₄ were measured for the first time. The data were interpreted with the aid of Molecular Dynamics (MD) and standard fitting of the EXAFS equation. As in related studies, a speciation distribution dominated by [AnF_x]^4-x (x = 7, 8, 9) coordination complexes was observed. The average coordination number was found to decrease with the increasing size of the alkali cation, and increase with AnF₄ content. An average coordination number close to 6, which had not been detected before in melts of alkali actinide fluorides, was seen when CsF was used as solvent.

Investigation of the local structure of molten ThF4-LiF and ThF4-LiF-BeF2 mixtures by high-temperature X-ray absorption spectroscopy and molecular-dynamics simulation

The microscopic structures of ThF₄-LiF and ThF₄-LiF-BeF₂ molten salts have been systematically investigated by in situ high-temperature X-ray absorption fine-structure (XAFS) spectroscopy combined with molecular-dynamics (MD) simulations. The results reveal that the local structure of thorium ions was much more disordered in the molten state of the ThF₄-LiF-BeF₂ salt than that in ThF₄-LiF, implying that the Th and F ions were exchanged more frequently in the presence of Be ions. The structures of medium-range-ordered coordination shells (such as Th-F_2nd and Th-Th) have been emphasized by experimental and theoretical XAFS analysis, and they play a significant role in transport properties. Using MD simulations, the bonding properties in the molten ThF₄-LiF and ThF₄-LiF-BeF₂ mixtures were evaluated, confirming the above conclusion. This research is, to the best of our knowledge, the first systematic study on the ThF₄-LiF-BeF₂ molten salt via quantitative in situ XAFS analysis and MD simulations.

In situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th-F and Th-Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF₄) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF₄) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF₄) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations.

Polarizable force field parameterization and theoretical simulations of ThCl4 -LiCl molten salts

Recycle of thorium is an essential process in the thorium-uranium closed fuel cycle of molten salt reactor (MSR). Pyrochemical treatment of spent nuclear fuel using chloride molten salts as medium has been considered as a promising method. In this article, we performed molecular dynamics simulations on the ThCl₄ LiCl molten salts using a polarizable force field parameterized by us from first-principles calculations. The microscopic structures and macroscopic properties at different compositions were investigated using the developed force field to understand the structure/property relationship in the mixture. The differences between ThCl₄ LiCl and ThF₄ LiF MSs are compared to understand the behaviors of Th⁴⁺ in the fluoride-chloride mixed media. In the molten fluorides, the coordination number of Th⁴⁺ is larger, and the resulting more shared anions lead to lower ThF dissociation barrier and shorter lifetime of the Th⁴⁺ first solvation shell. Our results also indicate the Pauling's structural rules for crystals can be used to rationalize the local structures in molten salts. © 2018 Wiley Periodicals, Inc.

Coordination numbers and physical properties in molten salts and their mixtures

Mixtures of trivalent metal halides with alkali halides are involved in many technologies but, from a more fundamental and general perspective, are worthy of study as interesting systems in which to examine the relationship between atomic-scale structure and physical properties. Here we examine the relationship between the viscosity and local and longer range structural measures in such mixtures where the trivalent metal cations span a significant size range and exhibit different behaviours in the dependence of their viscosity on the mixture composition. We characterise the structure and dynamics of the first coordination shell and the relationship between its structural relaxation time and the shear relaxation time of the mixture (the Maxwell relaxation time). We are then led to an examination of the structure of the networks which progressively form between the trivalent metal cations as their concentration increases in the mixtures. Here we find significant differences between small and larger cations, sufficient to explain the different behaviour of their viscosities. We draw attention to the similarities and differences of these networks with those which form in highly viscous, glass-forming materials like BeF₂:LiF.

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF-LaF3-Li2O systems

The local structure of molten LaF3-LiF-Li2O has been investigated by high temperature NMR spectroscopy. The (139)La and (19)F chemical shifts have been measured as a function of temperature and composition. The NMR spectra show that Li2O reacts completely with LaF3 to form a LaOF compound in the solid state below the melting temperature of the sample. LaOF is not completely dissolved in the fluoride melt and solid LaOF is observed in the (19)F spectra for Li2O concentrations above 10 mol%. We discuss the local environment of lanthanum ions in molten LaF3-LiF-Li2O and compare the results to those with the LaF3-LiF-CaO system. The analysis of the temperature and Li2O concentration dependences of the (139)La and (19)F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaOxFy(3-x-y) units are present in the melt.

Chemical interactions between zirconium and free oxide in molten fluorides

The chemical interactions between zirconium and free oxide in FLiNaK melts at different zirconium to oxide (n_Zr/n_O) molar ratios were studied by means of a carbothermal-reduction technique (LECO oxide analyzer) and Raman spectroscopy.