Evidence for Neptunium Incorporation into Uranium (VI) Phases
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Evidence for Neptunium Incorporation into Uranium (VI) Phases Edgar C. Buck, Brady D. Hanson, Judah I. Friese, Matt Douglas and Bruce K. McNamara Environmental Technology Division, Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, [email protected]
ABSTRACT This paper examines the ability of electron energy-loss spectroscopy (EELS) combined with transmission electron microscopy (TEM) to detect both low concentrations of Np in a U matrix and to provide evidence for incorporation of Np in U(VI) phases. The case for U(VI) secondary minerals acting as solubility-controlling phases for Np in repository performance assessment models has not been fully established. Direct evidence for incorporation, rather than sorption, continues to be difficult to obtain. Detection of Np with TEM-EELS is hampered by the occurrence of a plural (multiple) scattering event from the more abundant U atoms (U-M5 + UO4,5), 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 synthetic studtite {(UO 2)(O2)(H2O)2](H2O)2} and uranophane {Ca(UO2)2(SiO3OH)2(H2O)5} has been found with TEM-EELS. The EELS technique continues to remain an important tool for examining the potential for transuranic behavior in nuclear waste materials.
INTRODUCTION Several research groups are examining the potential for U(VI) phases for incorporating radionuclides that would lead to improvements in performance assessment models [1-7]. A fundamental problem addressing the issue of Np behavior in weathered waste forms is the low concentration of this actinide in the starting materials. Typical spent nuclear fuel (SNF) contains only about 400-600 ppm 237Np, although this amount will increase with the decay of 241Am that is also present in SNF. Preliminary data from Buck et al. [1] suggested that Np is sequestered in a uranyl oxy-hydroxide phase during vapor-phase corrosion tests on SNF, confirming the theoretical prediction by Burns et al. [8]. Significant experimental problems with the quantification of Np in a U matrix challenges the veracity of previous electron energy-loss spectroscopy (EELS) results and additional validation will be necessary before models can consider the possibility of Np incorporation and/or up-take into U(VI) phases. Detection of Np in a matrix of U is hindered by plural scattering from uranium, involving the combination of two scattering events from the U-O4,5 edge and the U-M edges. This results in a small, broad, peak occurring nearly coincident with the Np-M5 edge at 3665 eV. The degree of plural scattering will depend on the thickness of the sample [9]. This effect tends to reduce the sensitivity of EELS in this energy region for detecting Np in a U matrix. Recent work by Burns et al. [6], has suggested that ppm levels of Np can be incorporated into uranophane {Ca(UO2)2(SiO3OH)2(H2O)5} and com
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