Evidence for Neptunium Incorporation into Uranium (VI) Phases

This paper examines the ability of Electron Energy-Loss Spectroscopy (EELS) on the Transmission Electron Microscope (TEM) to both detect low levels of neptunium (Np) in a uranium matrix and to provide evidence for incorporation of Np in U6+ phases. The case for using U6+ secondary minerals as solubility-controlling phases for Np in repository performance assessment models has not been established. Direct evidence for incorporation rather than sorption has been difficult to obtain. The detection of Np in a matrix of uranium (U) can be impeded by the occurrence of a plural scattering event from U (U-M5 + U-O4,5) that results in severe overlap on the Np-M5 edge at 3665 eV. By examining the energy gap between the Np-M5 and Np-M4 edges (184 eV), a method for observing Np independently of the plural scattering event has been established. Clear evidence of Np incorporation into studtite, [(UO2)(O2)(H2O)2](H2O)2 and uranophane, Ca(UO2)2(SiO3OH)2(H2O)5 has been found in laboratory tests using TEM-EELS. This data suggests that U6+ phases, if present, may control Np solubility in a repository environment