Study on mixing behavior of rare earth chloride in molten alkali chloride by molecular dynamics (MD) calculation

From the viewpoint of the effective utilization of nuclear energy and reduction of radioactive waste, recovery of uranium (U), plutonium (Pu), and minor actinide (MA) and their recycling as fuel of fast breeder reactors (FBR) are very important. And pyrochemical reprocessing with the use of molten salt is regarded as a strong candidate for such recovery technology. It is predicted that metallic ion in molten salt influences the electrolysis process directly. Therefore, clarifying the local structure of metallic ion in molten salt and its physico-chemical properties is very important information in pyrochemical reprocessing of spent nuclear fuels. We have investigated the local structure around metallic ion in molten salt by XAFS (X-ray absorption fine structure) and MD (molecular dynamics) simulation techniques. In this study, we investigated the mixing behavior of molten TbCl3 in LiCl-KCl eutectuic by MD simulation. At the same time, simulation of molten MCl3 (M=Y, La) systems were performed to elucidate the difference of the structural change by the difference in cation size (Y3+<Tb3+<La3+). In addition, to elucidate the mixing effect with LiCl-KCl in detail, we also examined MCl3-LiCl and -KCl systems (M=Y, Tb, La). The molten pure TbCl3 has mainly 7-fold structure. By mixing with LiCl-KCl, the network structure by Cl- is broken and the stable 6-fold structure is formed. Especially, the drastic change of the coordination number is observed between 40mol% and 15mol% TbCl3. Comparison of MCl3 systems shows that the change of coordination number and the Tb3+-Cl- distance by the mixing with LiCl-KCl are between those of YCl3 and LaCl3 systems. Independent of cation size, the molten MCl3 tend to be formed the stable 6-fold structure by the mixing with LiCl-KCl. MD simulations on MCl3-LiCl and -KCl systems revealed that the mixing effect was different between LiCl and KCl. By the mixing with LiCl, the formation of the stable 6-fold structure is suppressed. On the other hand, by the mixing with KCl, the formation of the stable 6-fold structure is promoted. We consider that the number density of Cl- and the distance of M3+-Li+,-K+ are related to the difference of the formation of the stable 6-fold structure. (author)