Molten Salt Reactor Chemistry: Structure and Equilibria

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...RST/Reactor Physics and Nuclear Material

Using the Quasi-chemical formalism beyond the phase Diagram: Density and viscosity models for molten salt fuel systems

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.RST/Reactor Physics and Nuclear Material

Coupled structural-thermodynamic modelling of the molten salt system NaCl-UCl₃

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−, UCl63-/UCl74- at low UCl3 content to a polymeric liquid at high UCl3 content, manifesting itself in the formation of species like U2Cl126-, U3Cl178-, U4Cl2210- 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.RST/Reactor Physics and Nuclear Material

Thermodynamic Description of the ACl-ThCl4 (A = Li, Na, K) Systems

The ACl-ThCl4 (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 orderLi ă Na ă K, in agreement with idealised interactions and structural modelsRST/Reactor Physics and Nuclear Material

A promising fuel for fast neutron spectrum Molten Salt Reactor: NaCl-ThCl₄-PuCl₃

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).RST/Reactor Physics and Nuclear Material

Thermodynamic assessment of the LiF-NiF₂, NaF-NiF₂ and KF-NiF₂ systems

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 (Li1-2xNix)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.Accepted Author ManuscriptApplied SciencesRST/Reactor Physics and Nuclear Material

New insights and coupled modelling of the structural and thermodynamic properties of the LiF-UF4 system

© 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.RST/Reactor Physics and Nuclear MaterialsRST/Technici Poo

Thermodynamic assessment of the KF-ThF₄, LiF-KF-ThF₄ and NaF-KF-ThF₄ systems

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: Cp,mo(K2ThF6,cr,298.15K)=(193.2±3.9) J·K-1·mol-1 and Smo(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.RST/Reactor Physics and Nuclear Material

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 [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 RST/Reactor Physics and Nuclear MaterialsRST/Technici Poo

Experimental and Computational Exploration of the NaF-ThF₄Fuel 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.RST/Reactor Physics and Nuclear Material