The development, operation, and regulation of nuclear reactors that utilize molten salts as fuel or as heat transfer media require knowledge of the thermal properties of the salt systems and quantification of the corresponding uncertainties. Knowledge of molten salt properties is also necessary for applications in material synthesis, processing, separations, solar thermal power generation, and energy storage. A round robin was conducted with national laboratory and university participants from twenty-one laboratories in five countries to compare property measurements, to better understand uncertainties, and to identify possible best practices. Two salt mixtures, each from a common batch, were distributed to participants for evaluation: equimolar NaCl-KCl and 45.0LiF-13.7NaF-41.3KF mol % (FLiNaK). Measurements were performed to determine the major constituent composition, oxygen content, density, thermal expansivity, melting point, and thermal conductivity. Error analysis was performed on each measurement for uncertainty quantification for each type of property that was explored. The resulting discussion of the methodologies used in this work is meant to lay the groundwork for the development of standard methods and reference materials for future high-temperature property measurements on halide melts.

The structural, thermochemical, and thermophysical properties of the AnCl₄, and NaCl-AnCl₄ (An = Th, U) melts were investigated using molecular dynamics (MD) simulations based on the Polarisable Ion Model (PIM). New force fields were proposed and used to compute key properties including density, thermal expansion, enthalpy of mixing, heat capacity, as well as the local structure and chemical speciation in the molten (Na, An)Cl_x (An = Th, U) salts. Thermodynamic models were then developed based on the CALPHAD method, using both PIM-MD and experimental data as input. Employing the modified quasichemical formalism in the quadruplet approximation for the liquid solution, the models account for the chemical speciation in the melt as calculated by MD simulations, and reproduce well phase equilibria in those systems. In particular, the models included monomeric and dimeric species to represent the physical nature of the ionic melt, which shows progressive oligomerisation with increasing AnCl₄ fraction. Our studies confirm that the melt becomes highly volatile at high AnCl₄ fractions, which is discussed in light of the results obtained herein.

A thorough understanding of the corrosion chemistry between molten salt fuel and structural materials (e.g., steel) is key for the advancement of Molten Salt Reactor technology. In this work, we consider more specifically the case of a chloride fuel salt mixture and the thermochemistry of a salt mixture such as (NaCl-MgCl₂-PuCl₃) in interaction with (Fe, Cr, Ni). The present work aims at the development of a thermodynamic model of the key subsystems NaCl-CrCl₂, NaCl-CrCl₃, and FeCl₂-CrCl₂ to predict corrosion products that may form between molten salt and structural materials. The Modified Quasichemical Model in the quadruplet approximation is used to describe the Gibbs energy of the liquid phase. A critical review of the existing phase diagram and thermodynamic data on theses systems is first presented. To alleviate the lack of data, ab initio calculations coupled with a quasi-harmonic approach are performed to estimate the thermodynamic properties for the intermediate solid compounds Na₂CrCl₄ and Na₃CrCl₆, which exist in the NaCl-CrCl₂ and NaCl-CrCl₃ systems, respectively. These atomistic simulation data together with selected experimental data are then used as input for the thermodynamic assessment of the three subsystems.

CALPHAD models to compute the density and viscosity of four keystone systems related to Molten Salt Reactor (MSR) technology have been optimized: NaCl-UCl₃, LiF-ThF₄, LiF-UF₄, and LiF-ThF₄-UF₄. Revised thermodynamic assessments of all four systems, using the modified quasichemical formalism in the quadruplet approximation for the description of the liquid solutions, are reported. In the case of NaCl-UCl₃, phase diagram and mixing enthalpy data available in the literature are taken into account. For the fluoride systems, recently published data on some solid phases are taken into account, while retaining the most recently published descriptions of the liquid solutions. The densities of the liquid solutions are modelled using pressure-dependent terms of the excess Gibbs energy, while the viscosities are then modelled using an Eyring equation. Both state functions are related to the thermodynamic assessments through the quadruplet distributions.

Chloride salts are considered a good alternative to fluoride salts as fuel carrier in the Molten Salt Fast Reactor concepts. The NaCl–ThCl4–PuCl3 fuel salt solution seems very promising, with low melting temperature eutectic compositions, and the potential to be used in a breeder and burner type of reactor design. This work focuses on the first thermodynamic modeling assessment of the ThCl4–PuCl3 binary system and the NaCl–ThCl4–PuCl3 ternary system, using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method and the quasichemical formalism in the quadruplet approximation. The investigated system shows potential for a high flexibility with respect to composition at operating temperatures, which can be beneficial to accommodate the requirements on other essential fuel properties (e.g. neutronic and thermo-hydraulic).

Corrected excess Gibbs energies of the liquid solutions in the ACl-ThCl4 (A = Li, Na, K), as well as revised standard enthalpies of formation and standard entropies of the intermediate phases occurring in the binary systems, are presented. The phase diagrams are reproduced to a similar level of accuracy as in the original publication, and the trends in thermodynamic stability of the liquid solutions are maintained. That is, the main conclusions of the paper are not affected. The original publication has also been updated

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.

Molten salts are a class of ionic liquids which have in recent years been the focus of extensive fundamental research given that they are a versatile class of reaction media with a variety of appealing thermophysical and thermochemical properties (e.g. melting points, heat capacities, vapor pressures, densities, thermal conductivities, etc..) suited for a variety of industrial applications, in particular at high temperature. Themost wellknown is perhaps the production of materials as important as aluminum and sulfuric acid, yet thermal energy storage is also a notable application. One of the most noteworthy application of molten salts, is as fuel and coolant for a type of nuclear fission reactor known as theMolten Salt Reactor (MSR). In its most general sense, aMSR is a class of nuclear reactor in which fissile (235U, 233U, 239Pu) and/or fertile isotopes (e.g. 232Th, 238U) are dissolved in a carrier salt. The resulting mixture acts both as fuel and coolant. The two prototypes which have been built in the past used a fluoride fuel, so historicallymost work has concentrated on fluoride salt mixtures. However,modern day reactor developers are also interested in chloride fuels, so both molten salt fuel families are relevant at present...

The ACl-ThCl (Formula presented.) (A = Li, Na, K) systems could be of relevance to the nuclear industry in the near future. A thermodynamic investigation of the three binary systems is presented herein. The excess Gibbs energy of the liquid solutions is described using the quasi-chemical formalism in the quadruplet approximation. The phase diagram optimisations are based on the experimental data available in the literature. The thermodynamic stability of the liquid solutions increases in the order Li < Na < K, in agreement with idealised interactions and structural models.

Molten chloride salts are ionic liquids in which the anions and cations exhibit network formation. An attractive salt system for use in molten salt reactors is NaCl-UCl₃, an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. The relationship between local structure and thermodynamic properties is investigated in this work, in which molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling by means of the CALPHAD method are combined. The system is simulated for a wide range of temperatures and compositions and various properties are derived derived from molecular dynamics data: density/molar volume, thermal expansion, heat capacity and excess properties including excess molar volume, mixing enthalpy and excess heat capacity. Generally, there is good agreement with previously published experimental data. An in depth analysis of the local structure of the liquid is performed for multiple temperatures. This analysis demonstrates the transition from a molecular liquid consisting of primarily Na⁺, Cl⁻, UCl₆^3-/UCl₇^4- at low UCl₃ content to a polymeric liquid at high UCl₃ content, manifesting itself in the formation of species like U₂Cl₁₂^6-, U₃Cl₁₇^8-, U₄Cl₂₂^10- etc. Exceeding 40% UCl₃, the liquid consists of a three-dimensional network of corner or edge-sharing uranium polyhedra. The output of the MD simulations and experimental data are incorporated into a coupled structural-thermodynamic model for the NaCl-UCl₃ system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of uranium polymeric species predicted from the simulations.

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 [AnFx]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. This journal is

© 2021 The Authors LiF-UF4 is a key binary system for molten fluoride reactor technology, which has not been scrutinized as thoroughly as the closely related LiF-ThF4 system. The phase diagram equilibria in the system LiF-UF4 are explored in this work with X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The short-range ordering in the molten salt solution is moreover surveyed with Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) and interpreted using a combination of standard fitting of the EXAFS data and Molecular Dynamics (MD) simulations with a Polarizable Ion Model (PIM) potential. The density, excess molar volume, thermal expansion, heat capacity, and enthalpy of mixing are extracted from the MD simulations across a range of temperatures and compositions; the behavior is non-ideal, with reasonably good agreement with the experimental data. Also calculated is the distribution of heteropolyanions in the liquid solution, and modelled using the quasi-chemical formalism in the quadruplet approximation taking into account the existence of the single-shell complexes [UF7]3−, [UF8]4−, and the dimeric species [U2F14]6−. Subjecting the optimization of the excess Gibbs energy parameters of the liquid solution to the constraints of the phase diagram data and local structure of the melt as derived from the EXAFS and coupled MD simulations, a CALPHAD-type assessment is proposed, linking structural and thermodynamic properties, with a rigorous physical description of the melt.

A thermodynamic assessment of the KF-ThF₄ binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K₂ThF₆ has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: C_p,m^o(K₂ThF₆,cr,298.15K)=(193.2±3.9) J·K^-1·mol^-1 and S_m^o(K₂ThF₆,cr,298.15K)=(256.9±4.8) J·K^-1·mol^-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF₄ and NaF-KF-ThF₄ using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF₆ are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF₄ system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF₅ and NaF-KThF₅ pseudobinaries, with good agreement.

Using the modified quasi-chemical model in the quadruplet approximation, three new thermodynamic assessments of binary systems useful for the detailed operational design of the Molten Salt Reactor are presented: AF-NiF₂ (A = Li, Na, K). These systems are particularly relevant for the study of the molten salt-structural materials interaction, as the salt containment is made of a Ni-based alloy. Using powder X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC), new experimental data were gathered for two of these systems, LiF-NiF₂ and KF-NiF₂, and compared to previous experimental assessments. Our data have confirmed the formation of a (Li_1-2xNi_x)F solid solution. The three thermodynamic models show a very good agreement with the experimental data. The melting point of NiF₂ was measured for the first time to be T = (1629 ± 5) K, and the thermal expansion coefficient for Li₂NiF₄ was found to be α=27.6·10^-6K^-1 in the temperature range T = (298–773) K.

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.