Theft, diversion, and trafficking of nuclear materials is an ongoing problem that poses significant threats to U.S. and international security. Uranium oxides are used as fuel material in reactors and are the most common illicitly trafficked form of uranium. Currently, there are various ways to fabricate UO2 fuel. Many of the multiple synthesis steps involved can be sources of diverted illicit material, yet each provides chemical signatures that could be used to deduce processing and alteration history. However, while baseline efforts have focused on bulk phase unit cell identification, impurity quantification, and isotope systematics to constrain point of origin, molecular signatures that can provide nuanced and highly valuable information on detailed chemical processing remain underdeveloped. There is an ongoing and urgent need for nuanced signatures and new methods that can further constrain forensic problems. The molecular structures of impurities, present in source material or introduced at successive fuel fabrication steps provide potentially valuable signatures of synthesis methods. Here, we demonstrate the applicability of X-Ray Absorption Spectroscopy (XAS) to decode molecular structures of impurity and host elements in U materials. We will focus on the molecular signatures of both U and impurities that can be extracted with XAS. We will show the potential of using a combination of XRF microprobe imaging with XAS, as applied to samples from the 5th Collaborative Materials Exercise (CMX) of the ITWG, to determine differences in synthesis processing history and conditions. These results show the value of XAS-determined molecular signatures to infer the process history and will further advance our ability to understand processes by which interdicted materials were fabricated and the facilities of origin.

X-ray molecular signatures of UO2 chemical processing for nuclear forensic science

ACS Fall 2021

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