Today, solvent extraction techniques like PUREX are exclusively employed in the reprocessing processes for spent nuclear fuels. However, we believe that there is still room for innovating the fundamentals of such a chemical process in the nuclear engineering field. In the former time, our colleague found that U(VI) is precipitated from HNO3(aq) by simply adding an N-alkylated γ-butyrolactams like N-cyclohexyl-2-pyrrolidone (NCP). In contrast, tetravalent actinides and other simulated fission products can be retained in the supernatant even after this operation, if an appropriate precipitating agent is employed. After these findings, we proposed a new concept for the simple reprocessing process to selectively recover U(VI) from HNO3(aq) dissolving the spent nuclear fuels. The most important point exerting the selectivity is the characteristic chemical form of U(VI), uranyl, which shows a linear [O=U=O]2+ structure. The equatorial plane of the uranyl ion is fully occupied by two bidentate nitrates and two molecules of the precipitant to form a hexagonal bipyramidal coordination structure around the center U. The former neutralizes the positive charge of uranyl, while the latter gives enough hydrophobicity to precipitate the resulted complex. Our efforts have been dedicated to design and optimize the molecular structures of the precipitants, N-alkylated 2-pyrrolidones (NRPs) and related cyclic ureas (CUs), from multiple viewpoints like hydrophobicity, solubility, coordination chemistry, and crystallography. In this talk, we intend to present a brief introduction about our former works and our strategy for the molecular design of the U(VI)-selective precipitant as well as the latest results of the currently on-going project.

Molecular and crystal structures of uranyl nitrate complexes bearing N-substituted 2-pyrrolidone derivatives: Towards a new aspect of nuclear fuel reprocessing

251st National Meeting (2016)

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