What Is a Good Simulant for Plutonium(III) in Molten Chloride Salts?

Despite the importance of plutonium compounds in the nuclear energy industry, their properties have rarely been studied directly. This is due to the artificial character as well as the high radioactivity level of this element. Instead, simulants with close physicochemical properties are used, such as cerium(IV) or thorium(IV), for oxide fuel properties. Under the molten chloride salt reactor conditions, Pu, Ce, and Th, respectively, are at the +III, +III, and +IV oxidation states, so that Ce(III) is generally favored. Our work analyzes the relevance of this choice based on quantum chemistry calculations and molecular dynamics (MD) simulations. Using density functional theory, we first demonstrate that among the lanthanide series, the neodymium(III) cation has the most similar interaction with chloride anions when compared to the ones involving Pu(III). Then, we compare the physical and thermodynamic properties as well as the local structure of NaCl-NdCl₃, NaCl-CeCl₃, and NaCl-PuCl₃ mixtures close to the eutectic composition that could be used as nuclear fuel using MD simulations. For all the elements, a polarizable force field obtained using the same procedure is used. The results confirm Nd(III) as the best choice to simulate the properties of Pu(III) with the greatest accuracy, and they allow the estimation of the error made when using Ce(III) instead.