In Situ X-ray Diffraction Studies on the Reduction of U₃O₈ by Various Reducing Agents

In situ X-ray diffraction studies were conducted to elucidate the reduction pathways of U₃O₈ using various reducing agents, including hydrogen, the fluorinating agent polyvinylidene fluoride (PVDF), and calcium hydride (CaH₂). The reduction processes were characterized by a transition from the orthorhombic, pseudo-hexagonal phase α-U₃O₈ to the hexagonal polymorph, followed by the formation of fluorite-type UO_2+x phases. The reduction temperature and intermediate phases vary with the type of reducing agent, though they all follow a similar sequence of phases. In the presence of pure hydrogen and upon reaction with PVDF and CaH₂, U₃O₈ transformed entirely into fluorite-type phases UO_2+x. By decreasing the chemical potential of hydrogen via its partial pressure, the behavior of U₃O₈ preceding the reduction to UO_2+x can be switched from an orthorhombic-to-hexagonal phase transition followed by disproportionation to two different U₃O₈ phases, presumably with different oxygen content, to a continuous oxygen loss within one U₃O₈ phase without prior phase transition. The detailed analysis of lattice parameters and phase transformations in the course of the investigated reactions offers insights into the reduction pathways of uranium oxides, highlighting the impact of different reducing environments on the reaction pathways and final products.