Simulant chemistry for uranium and plutonium molten fuel salts: crystallographic investigation and thermodynamic modelling assessment of the NaCl–RECl3 and NaCl-MgCl2-RECl3 (RE = Ce, Nd) systems

Abstract

In this study, new insights into the solid state chemistry of the systems NaCl–RECl₃ (RE = Ce, Nd) are presented, in which the intermediate compound suggested in the literature, i.e. Na₃RE₅Cl₁₈, is investigated more closely. Our studies have revealed a solubility range around the intermediate composition in the form of the stoichiometry, and have allowed us to revisit the phase diagrams of the NaCl–RECl₃ (RE = Ce, Nd) systems accordingly. Furthermore, we demonstrate that among the lanthanide chlorides, NdCl₃ is the prime simulant candidate for the melting behaviour of PuCl₃-based systems, while CeCl₃ is most suited to simulate UCl₃-based systems. This is corroborated in this work by comparing the melting profiles of the NaCl-MCl₃, MgCl₂-MCl₃, and NaCl-MgCl₂-MCl₃ (M = Ce, Nd, U, Pu) systems. In doing so, the binary systems MgCl₂-MCl₃ (M = Ce, Nd) have been re-visited based on existing data in the literature and estimated mixing enthalpies. Extrapolations to the ternary systems NaCl-MgCl₂-RECl₃ (RE = Ce, Nd) have been made and compared to the available data in the literature, showing good agreement.