Molecular dynamics simulation study on behavior of metallic ion in molten salt mixture bath

Behavior of very small amount of metallic ion in molten alkali chloride mixture melt is very important information in pyrochemical reprocessing of spent nuclear fuels. In the Synchrotron Radiation Research Center, we have investigated the local structure around metallic ion in molten salt mixtures by using a high energy XAFS (X-ray absorption fine structure) technique. In addition, a molecular dynamics (MD) simulation technique has been used to analyze XAFS output and to simulate mixing behavior from microscopic viewpoint. In the present work, mixing behavior of molten LaCl3 in LiCl-KCl eutectic melt was simulated by MD simulations using polarizable ionic model (PIM). Similarly, simulations of molten YCl3 systems, which have a smaller Y3+, were performed to compare with the LaCl3 simulations. It has been reported that molten pure LaCl3 has an 8-fold (LaCl8)5- structure. The coordination number of the 1st La3+-Cl- correlation in molten LaCl3 decreases by mixing with LiCl-KCl eutectic mixture melt and indicates a value close to 6 less than 31%LaCl3 composition. It suggests that molten LaCl3 has an octahedral coordination structure in the mixture, as shown in molten YCl3 systems. In the MD simulation, the mixing effect was different between LiCl and KCl. By the mixing with KCl, the octahedral coordination (LaCl6)3- structure is formed and stabilized. On the other hand, LiCl does not promote the formation of the octahedral structure. The coordination number of the 1st La3+-Cl- pair is always more than 7 by the mixing with LiCl. It is due to high number density of Cl- ion in molten LiCl comparable to molten LaCl3 and strong Coulomb force introduced by a small Li+ ion. (author)