Structure and Dynamics of NaCl/KCl/CaCl-EuCl ( = 2, 3) Molten Salts.

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu and Eu ions in CaCl, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K to Na to Ca, the coordination number (CN) of Cl in the first solvation shell increases from 5.6 (Eu) and 5.9 (Eu) in KCl to 6.9 (Eu) and 7.0 (Eu) in CaCl. This coordination change is validated by the EXAFS measurements, in which the CN of Cl around Eu increases from 5 in KCl to 7 in CaCl. Our simulation shows that the fewer Cl ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu/Eu are related to the rigidity of their first coordination shell of Cl: the more rigid the first coordination shell is, the slower the solute cations diffuse.