Thermodynamic Assessment of the NaCl-CrCl₂, NaCl-CrCl₃, and FeCl₂-CrCl₂ Pseudo-Binary Systems for Describing the Corrosion Chemistry Between Molten Salt Fuel and Steel

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.