Structure and Dynamics of NaCl/KCl/CaCl₂-EuCl _n (n=2, 3) Molten Salts

Modern molten salt reactor design and the techniquesof electrorefiningspent nuclear fuels require a better understanding of the chemicaland physical behavior of lanthanide/actinide ions with different oxidationstates dissolved in various solvent salts. The molecular structuresand dynamics that are driven by the short-range interactions betweensolute cations and anions and long-range solute and solvent cationsare still unclear. In order to study the structural change of solutecations caused by different solvent salts, we performed first-principlesmolecular dynamics simulations in molten salts and extended X-rayabsorption fine structure (EXAFS) measurements for the cooled moltensalt samples to identify the local coordination environment of Eu2+ and Eu3+ ions in CaCl2, NaCl, andKCl. The simulations reveal that with the increasing polarizing theouter sphere cations from K+ to Na+ to Ca2+, the coordination number (CN) of Cl- inthe first solvation shell increases from 5.6 (Eu2+) and5.9 (Eu3+) in KCl to 6.9 (Eu2+) and 7.0 (Eu3+) in CaCl2. This coordination change is validatedby the EXAFS measurements, in which the CN of Cl- around Eu increases from 5 in KCl to 7 in CaCl2. Oursimulation shows that the fewer Cl- ions coordinatedto Eu leads to a more rigid first coordination shell with longer lifetime.Furthermore, the diffusivities of Eu2+/Eu3+ arerelated to the rigidity of their first coordination shell of Cl-: the more rigid the first coordination shell is, theslower the solute cations diffuse. First-principlemolecular dynamics simulations for moltensalts and EXAFS measurements for the cooled molten salt samples toidentify the local coordination environment of Eu2+ andEu(3+) ions in molten salts of CaCl2, NaCl, andKCl, which help us to understand the changes of coordination environmentof Eu, the rigidity of the first coordination shell, and their impacton the diffusivity in the melts.