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 are rarely studied directly due to plutonium's artificial character and its high radioactivity level. Instead, simulants with close physico-chemical properties are used, such as, cerium(IV) or thorium(IV) for oxide fuels. In the molten chloride salt reactors, Pu, Am and Th respectively are at the +III, +III and +IV oxidation states, so that Ce(III) is generally favored. Our work analyses 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 lanthanides, the neodymium(III) cation has the most similar interactions with chloride anions when compared to the ones involving Pu(III). Then, we compare the physical and thermodynamic properties and the local structure of NaCl-NdCl3, NaCl-CeCl3 and NaCl-PuCl3 mixtures close to the eutectic composition using polarisable MD simulations. The results confirm Nd(III) as the best choice to simulate the properties of Pu(III) with the greatest accuracy, and they allow to estimate the error made when using Ce(III) instead.