Third phase formation induced by a growth of hierarchical aggregation structure in biphasic solvent extraction systems

溶媒抽出系において階層的凝集構造の成長が誘起する第三相の生成

元川 竜平   

Motokawa, Ryuhei

Tri-n-butyl phosphate (TBP) is an important extractant for separating hexavalent uranium, U(VI), and tetravalent plutonium, Pu(IV), 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 in particular is called the third phase, which contains high concentrations of U(VI), Pu(IV), 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 X-ray and neutron scattering observations of the organic phase before and after extraction of the zirconium ions (Zr), this work contributes to contemporary knowledge of third phase formation mechanisms as well as to clarifying the microscopic structure of the aggregation in organic phases.