A thermodynamic model of the molten salt system (Na,Cs,Mg,Pu,Nd)(Cl,I) has been developed in this work to assess the effect of CsI on the melting and vaporization behavior of the nuclear fuel in molten salt reactors. Investigation using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) of the binary systems NdCl₃–NdI₃ and MgI₂–NdI₃, as simulant systems for the analogous Pu system, is presented for the first time. Both systems were found to be binary eutectic systems, with solid solutions NdCl_3–3xI_3x (hexagonal in space group P6₃/m) and NdCl_3yI_3–3y (orthorhombic in space group Cmcm) in the NdCl₃–NdI₃ system, and Mg_1–xNd_xI_2+x (hexagonal in space group P 3 m1) in the MgI₂–NdI₃ system. Additionally, the system CsI–MgI₂ was scrutinized using DSC, confirming the available experimental data in the literature. Furthermore, the investigation of the reciprocal diagonals in the systems (Na,Nd)(Cl,I), (Cs,Mg)(Cl,I), (Mg,Nd)(Cl,I), and (Cs,Nd)(Cl,I) is presented, allowing the characterization of the quaternary behavior of these salts. Based on the experimental data obtained in this work, a CALPHAD model is presented using the quasi-chemical formalism in the quadruplet approximation for the liquid solution. With the aim of modeling the complete (Na,Cs,Mg,Pu)(Cl,I) system, the binary systems NaCl–CsCl, CsCl–MgCl₂, CsCl–NdCl₃, NaI–CsI, and CsI–NdI₃ were reassessed based on data from the literature. Furthermore, a CALPHAD model of the PuCl₃ and PuI₃ systems is also presented using Nd as a simulant for Pu in molten halide salts. With the developed thermodynamic models, calculations were finally performed to assess the fission product retention of Cs and I in a molten chloride environment. As opposed to their behavior in molten fluorides, the fission products are well retained in the fuel matrix up to concentrations of at least 5 mol%.

The chemistry following cladding failure in Lead-cooled Fast Reactors involves the interaction between lead (Pb) coolant and the Joint Oxyde Gain (JOG)-phase, mostly composed of dicesium molybdate (Cs₂MoO₄). A thermodynamic analysis of coolant-JOG phase chemical interaction as studied via the scenario of Pb-Cs₂MoO₄ chemical interaction is reported. Measurements of the standard thermodynamic properties of α -Cs₂Pb(MoO₄)₂ are presented. The enthalpy of formation of α -Cs₂Pb(MoO₄)₂ is measured to be -(2570.7 ± 2.3) kJ · mol⁻¹ using solution calorimetry, while the standard entropy is determined to be (399 ± 12) J · K⁻¹·mol⁻¹ using thermal-relaxation calorimetry. Thermodynamic calculations show that Cs₂Pb(MoO₄)₂ is in several cases thermodynamically stable under conditions typical for operation of Lead-cooled Fast Reactors. This means Cs₂Pb(MoO₄)₂ can form in cladding failure scenarios.

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.

A comprehensive thermodynamic assessment of the Cs-Pb system was performed with the CALPHAD method using the experimental thermodynamic and phase diagram data available in literature supplemented by density functional theory (DFT) calculations. The exact nature of the stable compounds in the phase diagrams reported in the literature is uncertain, except for CsPb and Cs₄Pb₉ whose crystalline structures are well known. Therefore, DFT calculations were performed to calculate the energy of formation at 0 K of different possible compounds with various crystalline structures. The enthalpies of formation of the compounds CsPb, Cs₄Pb₉, and CsPb₄, found to be the stable ones by DFT, were then used in the CALPHAD model. The result of this process has enabled the development of a more refined phase diagram comparing to experimental ones, providing more comprehensive insights into the phase equilibria in this system. Moreover, the CALPHAD model succeeded in describing the peculiar behavior of the heat capacity of the liquid phase, by using an ionic two-sublattice model (Cs+1)_P(Pb−1,Va,Pb)_Q which takes into account the short-range ordering taking place at the equimolar composition CsPb, related to the formation of clusters Cs₄Pb₄, modelled as (Cs+1) (Pb−1). The model allows for the prediction of important thermodynamic properties, which are of interest for a range of applications, including lead-cooled fast reactors and perovskite-based photovoltaics.

The structural, thermodynamic, and magnetic properties of Na₂CrCl₄ have been investigated to provide fundamental insights into this ternary halide relevant to chloride-based molten salt reactor systems. Room-temperature powder X-ray and neutron diffraction confirm a monoclinic (P2₁/c) structure and phase purity. Neutron diffraction measurements at 4.6 K reveal additional magnetic reflections indexed with (Formula presented), indicating the onset of long-range antiferromagnetic order. Low-temperature heat capacity measurements in the range 2–300 K show a pronounced λ-type anomaly at T_N = 8.5 ± 0.5 K, with an associated magnetic entropy S_mag = 11.9 ± 0.4 J K^–1 mol ^–1 consistent with antiferromagnetic ordering of high-spin Cr²⁺ (S = 2), a second-order phase transition. The standard molar entropy at 298.15 K, S_m^°(298.15 K) = 256.8 ± 7.7 J K^–1 mol ^–1, is slightly lower than previous CALPHAD assessments of the NaCl-CrCl₂ system. Magnetic susceptibility measurements also confirm antiferromagnetic behavior, with a Curie–Weiss fit giving μ_eff = 5.57 ± 0.05 μ_B and θ_CW = −15.0 ± 1.0 K. Compared to the related ferromagnetic chlorides K₂CrCl₄, Rb₂CrCl₄, and Cs₂CrCl₄, Na₂CrCl₄ exhibits a distinctly lower ordering temperature and antiferromagnetic structure, likely due to variations in lattice geometry and exchange interactions. These results provide the first experimental thermodynamic parameters for Na₂CrCl₄, contributing to refining phase diagrams and corrosion models in chloride salt systems.

In this study, new insights into the solid state chemistry and melting behaviour of the Image 1001 system are presented, building on results in the simulant system Image 1002. Our studies have revealed the solubility of U in the high-temperature β[jls-end-space/]-phase of BaCl₂ (i.e Image 1003 ) and an intermediate compound, Ba₃U₂Cl₁₂, which has led us to revisit or present for the first time the phase diagrams of this system accordingly. Furthermore, we present revised thermodynamic models for the systems Image 1004, Image 1005, Image 1006 and Image 1007 (AE = Sr, Ba) based on existing literature data. With the constructed multi-component database Image 1008, the effect of fission products on the melting behaviour of molten chloride salts containing uranium and thorium is investigated through higher order phase equilibria calculations.

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.

The thermochemistry of the quaternary molten salt system NaCl-NaI-MgCl₂-MgI₂ has been studied using an experimental and thermodynamic modeling approach. The binary subsystems NaCl-NaI and NaCl-MgCl₂ were reassessed based on existing data in the literature. The binary subsystem NaI-MgI₂ was subjected to a renewed experimental investigation, to complement and revisit the data in the literature. The subsystem MgCl₂-MgI₂ was investigated for the first time in this work using Differential Scanning Calorimetry (DSC). Furthermore, the phase equilibria in the pseudobinary phase diagrams of NaCl-MgI₂ and NaI-MgCl₂ in the quaternary system were investigated by DSC, while the condensed phases in the quaternary system were investigated using X-ray diffraction (XRD). A thermodynamic model of the quaternary system was developed using the CALPHAD (CALculation of PHase Diagrams) method with the quadruplet approximation in the modified quasichemical model for the liquid phase, and two-sublattice polynomial models for the solid solution phases. With this model, the liquidus surface of the NaCl-NaI-MgCl₂-MgI₂ quaternary system has been described for the first time.

This article outlines key standardisation needs for specific topics related to molten salt reactors (MSR), namely measurements of thermo-physical properties, safety evaluation, qualification of fuels and fuel cycles, and codes & standards for materials and components. It also explores strategies for bridging gaps in international standardisation, harmonisation and collaboration, and raises the importance of building MSR prototypes and investing in testing facilities. This article is based on a survey and direct inputs from a Putting Science into Standards workshop held on 18 and 19 March 2024 and organised by the European Commissions’ Joint Research Centre (JRC) and European Committee for Standardization (CEN) and European Committee for Electrotechnical Standardization (CENELEC). The workshop gathered 100 experts from research, industry and policy making to discuss the development and standardisation of MSR technologies. By working together at European and international levels, standardisation will be a key instrument to enable the development and commercialisation of mature MSR technologies to support the European Union's goal of achieving fully functioning small modular reactors by 2030.

Thermal properties of actinides in molten salts are linked to the strongly disordered local environment of actinide ions. We illustrate both the limitations of the commonly used fitting method for analysis of extended X-ray absorption fine structure (EXAFS) spectra in molten UF₄ and a possible solution using an “objective neural network-EXAFS” (ONNE) method. ONNE provides both extraction of the pair distribution function, as validated by its application to the EXAFS spectra calculated on molecular dynamics trajectory, and the EXAFS data reconstruction. The ONNE analysis of the molten UF₄ has revealed reduction of the first nearest neighbor U-F coordination number, expansion of the U-F bond length, and smaller contribution to the second shell compared to current molecular dynamics models. This method is therefore an attractive alternative to conventional EXAFS analysis and molecular dynamics simulations for studies of disordered environment of actinides in molten salts.

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.

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.

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.