Probing higher valences of uranium in nuclear materials using diffuse reflectance spectroscopy

Understanding uranium valences in nuclear materials is critical as it is known to influence both their physical and chemical properties. As such, the synthesis of uranium-containing materials with varied uranium oxidation states was achieved and their uranium valences, as determined by diffuse reflectance spectroscopy (DRS), herein reported. It was revealed that DRS was well suited to differentiating between U(IV), U(V) and U(VI) in these materials. The U(IV), visible as broad vibronic absorptions in the UV-vis region, U(V), present as sharp peaks arising from f-f transitions and thus restricted to the near-infrared region and U(VI), featured by broad charge transfer bands in the ultra-violet range, were successfully identified in mono-valent and mixed-valence materials. A particular sensitivity to U(V) was revealed, even when present as a minor species. Of particular note was the capability for DRS to determine the coordination environment about the U(V) centres, with a shift of the diagnostic peak to higher wavelengths when the uranium coordination number was changed from 6-fold to 7- and 8-fold. Highlighted in this work is the potential for DRS to be applied to the analysis of nuclear materials across the entirety of the nuclear fuel cycle, towards better understanding and improving their applications.