Development of pyrometallurgical partitioning technology of long-lived nuclides. Relationship of distribution behaviour of lanthanides and concentration of lanthanides in molten salt or liquid Cd

High level radioactive waste contains a small amount of long-lived nuclides, mainly of minor actinides, which remain radioactive toxicity for more than one million years. Pyrometallurgical partitioning of the minor actinides is being developed by Central Research Institute of Electric Power Industry. The separation factor between the minor actinides and lanthanides dilutely added in the molten salt/liquid cadmium system was experimentally obtained, which were the most important parameter because the lanthanides, especially light lanthanides, were chemically similar with the minor actinides. According to the thermodynamical estimation, the activity coefficients of these elements are changed along with the increase in the concentration in the molten salt or by the formation of the intermetallic compound in the liquid cadmium. The change of the separation factor along with these change of the activity coefficients were evaluated at 773 K in this study. Consequently, the distribution coefficients of the heavy lanthanides were changed when the concentrations in the molten salt were greater than about 1.5 mol%, and the separation factor between the light lanthanides and the heavy lanthanides were greater by the formation of the intermetallic compounds. Moreover, the saturated solubilities of the lanthanides in the liquid cadmium were significantly smaller under the coexistence of the many lanthanides than the values in the literature. (author)