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.

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.

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.

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.

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.

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).

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.

Molten salts have recently received increased attention worldwide as key materials for sustainable and low-carbon energy supply technologies (e.g. thermal energy storage, concentrated solar power technologies, fission and fusion nuclear reactors) thanks to their appealing thermo-physical properties. In particular, they are used as fuel and coolant in the Molten Salt Reactor (MSR), considered a breakthrough technology for the next generation of nuclear fission reactors. This review focuses on the thermochemical and thermo-physical properties of actinide bearing fluoride and chloride salts, used as nuclear fuel in MSRs, and more specifically on the structure–property relationships. In the last 15 years, the knowledge on the structural properties of actinide containing salts has grown quite significantly thanks to the development of dedicated high temperature in situ X-ray Absorption Spectroscopy measurements, and to the advancement of atomistic simulations. These have evidenced the formation of short-range order in the liquid, which contributes to storing energy in the salt and directly influences transport and thermodynamic excess properties. This interrelationship is illustrated in the present article, which covers both experimental and computational information, as well as most recent developments in the modelling methods at the mesoscale of the structural, thermodynamic, density and viscosity properties using the CALPHAD methodology.