Spectroscopic characterization of neptunium(vi), plutonium(vi), americium(vi) and neptunium(v) encapsulated in uranyl nitrate hexahydrate

Abstract

The coordination of crystalline products resulting from the co-crystallization of Np(VI), Pu(VI), Am(VI), and Np(V) with uranyl nitrate hexahydrate (UNH) has been revealed through solid-state spectroscopic characterization via diffuse reflectance UV-Vis-NIR, SEM-EDS, and extended X-ray absorption fine structure (EXAFS). Density functional and multireference wavefunction calculations were performed to analyze the An(VI/V)O2(NO3)2·2H2O electronic structures and to help assign the observed transitions in the absorption spectra. EXAFS show similar coordination between the U(VI) in UO2(NO3)2·6H2O and Np(VI) and Pu(VI); while Am resulted in a similar coordination to Am(III), as reduction occurred prior to EXAFS data was obtained. The co-crystallization of the oxidized transuranic species—penta- and hexavalent—with UNH, represents a significant advance from not only a practical standpoint in providing an elegant solution for used nuclear fuel (UNF) recycle, but also as an avenue to expand the fundamental understanding of the 5f electronic behavior in the solid-state.