This work examines the thermochemistry of the chromium difluoride CrF₂ corrosion product in the molten [Formula presented] fuel salt system. Through a combination of experimental investigations and thermodynamic modeling assessment, the study elucidates the thermodynamic properties, phase diagram equilibria, and overall thermodynamic behavior of CrF₂ corrosion product, following dissolution from a structural material to the molten salt fuel environment. In this work, two different synthesis methods were developed for pure CrF₂, further allowing to experimentally measure the phase equilibria in the [Formula presented], [Formula presented], and [Formula presented] systems. Then, thermodynamic models were developed using the CALPHAD method based on the quasichemical model in the quadruplet approximation.

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The vibrational spectrum of the vapour phase above YI3 was measured by infrared spectroscopy. The spectrum revealed four strong bands, three of which could be assigned to the fundamantal infrared active vibrations of the YI3 monomer molecule. The observed frequencies agree very well with quantum-chemical calculations reported in literature. The presence of dimer molecules, the fraction of which is significant in the vapour according to literature, is also discussed in the context of assigning the fourth strong band in the experimental spectrum. Although its position matches well with a strong fundamental of the dimer, its intensity suggest a dimer fraction that is far outside the range for the other rare-earth triiodides.@en

Lithium-based fluoride salts are one of the leading options as fuel matrix in Molten Salt Reactors. The understanding of their thermodynamic behavior, e.g. chemical stability, activity, as well as heat transfer properties, in the reactor’s environment is crucial for the safety assessment. In this work, the chemical equilibria of lithium fluoride with two important fission products dissolved in the salt matrix, namely barium fluoride and zirconium fluoride, are considered. The phase diagrams of the binary systems LiF-BaF2, LiF-ZrF4 and BaF2-ZrF4, as well as the ternary system LiF-BaF2-ZrF4 are assessed by the CALPHAD (CALculation of PHase Diagram) method using the quasichemical model in the quadruplet approximation for the liquid solution. These models are based on literature and new experimental data, and provide a good overview of the stability of intermediate compounds formed in the various systems and of the liquid solution.@en

We measure the thermal conductivity of solid and molten tungsten using steady state temperature differential radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz law to capture the electronic thermal conductivity of metals in their molten phase. We further support this conclusion using ab initio molecular dynamics simulations with a machine-learned potential. Our results show that at these high temperatures, the vibrational contribution to thermal conductivity is negligible compared to the electronic component.

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The thermodynamic and thermo-physical properties of the molten salt system [Formula presented] have been investigated using an experimental and modelling approach. This molten salt system includes a single intermediate compound [Formula presented], whose structure has been investigated using X-ray and neutron diffraction. Furthermore, this system exhibits solubility of [Formula presented] in [Formula presented] at high temperatures up to a concentration of around 25% [Formula presented] at 1060 K. Additionally, our measurements show solubility of [Formula presented] in [Formula presented] up to about 5% [Formula presented] at 973 K. The investigation of these solid solutions has been performed using quenching experiments and subsequent post-characterisation by X-ray diffraction (XRD). Phase diagram equilibria have also been investigated using differential scanning calorimetry (DSC). Using the aforementioned information on phase transitions, intermediate compound formation, and mutual solid solubility, a thermodynamic assessment of the system has been performed using the CALPHAD method. The model for the Gibbs energy of the liquid solution is the quasi-chemical formalism in the quadruplet approximation, while the model for the Gibbs energy of the solid solutions is a two-sublattice polynomial model.

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The thermochemistry of the ternary system CsI-PbI₂-BiI₃, of interest for applications in photovoltaics, memory devices, and nuclear applications, among other things, is investigated in this work. The binary phase diagrams CsI-PbI₂ and CsI-BiI₃ were subjected to renewed experimental investigation, and the compounds CsPbI₃, Cs₄PbI₆, and Cs₃Bi₂I₉ were found to be the only stable phases in the investigated temperature window. The liquidus lines and invariant equilibria were determined. The phase equilibria in the BiI₃-PbI₂ system were measured for the first time by using Differential Scanning Calorimetry (DSC). The end-members form a solid solution over the entire composition range. The pseudobinary section CsPbI₃-Cs₃Bi₂I₉ of the CsI-PbI₂-BiI₃ ternary system was moreover measured by DSC, as well as the ternary eutectic points. A thermodynamic model of the complete CsI-PbI₂-BiI₃ system was developed by using the Compound Energy Formalism (CEF) for the solid phases and the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) for the liquid phase. The binary systems were modeled first, and no ternary interaction parameters were found necessary to reproduce accurately the phase equilibria in the ternary system. With our model, the whole liquidus surface of the field CsI-PbI₂-BiI₃ is described for the first time.

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The heat capacities of CsPbI₃, Cs₄PbI₆, and Cs₃Bi₂I₉ were studied using low-temperature thermal relaxation calorimetry in the temperature range of 1.9-300 K. The three compounds are insulators, with no electronic contribution to the heat capacity. None of them show detectable anomalies in the studied temperature window. Thermodynamic properties at standard conditions are derived. Previously reported results on Cs₃Bi₂I₉ are not fully consistent with the present findings. Moreover, the magnetic susceptibilities of the three title compounds were measured.

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The quaternary compound Cs₂Pb(MoO₄)₂ was synthesized and its structure was characterized using X-ray and neutron diffraction from 298 to 773 K, while thermal expansion was studied from 298 to 723 K. The crystal structure of the high-temperature phase β-Cs₂Pb(MoO₄)₂ was elucidated, and it was found to crystallize in the space group R3̅m (No. 166), i.e., with a palmierite structure. In addition, the oxidation state of Mo in the low-temperature phase α-Cs₂Pb(MoO₄)₂ was studied using X-ray absorption near-edge structure spectroscopy. Phase diagram equilibrium measurements in the Cs₂MoO₄-PbMoO₄ system were performed, revisiting a previously reported phase diagram. The equilibrium phase diagram proposed here includes a different composition of the intermediate compound in this system. The obtained data can serve as relevant information for thermodynamic modeling in view of the safety assessment of next-generation lead-cooled fast reactors.

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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.

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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@en

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).

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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.

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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.

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© 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.@en