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.