SANS study of hierarchical structure assembled by coordination species in biphasic solvent extraction

Motokawa, Ryuhei   

We present a hierarchical aggregate model of the organic phase containing the coordination species as a fundamental building unit for essential comprehension of the microscopic structure and phase separation in ionic separation and recovery systems during solvent extraction. Tri-n-butyl phosphate (TBP) is an important extractant for separating hexavalent uranium and tetravalent plutonium from used nuclear fuel by solvent extraction. In such solvent extractions, the organic phase occasionally separates into two organic phases, namely, light and heavy organic phases. The heavy organic phase is called the third phase, which contains high concentrations of uranium and plutonium, and TBP. In essence, the third phase is induced by a growth of the aggregation due to a lot of extracted coordination species, where the third phase formation in the extraction process could lead to a criticality accident and hinder the safe and stable operation of used fuel reprocessing facilities. Accordingly, understanding the mechanism of third phase formation, in conjunction with the microscopic structure of aggregation, is important. Through small-angle neutron scattering observations of the organic phase before and after extraction of the zirconium ions, this work contributes to provide a contemporary knowledge of third phase formation mechanisms as well as the microscopic structure of the hierarchical aggregates in the organic phases.