Uranium diboride: Understanding and optimizing arc-melt synthesis

Uranium diboride (UB 2 ) is considered as a nuclear fuel candidate or burnable absorber for advanced reactors. The present study investigates the arc-melt synthesis of UB 2 and aims to improve understanding and reproducibility of the fabrication method. UB 2 -U, UB 2 , and UB 2 -UB 4 compounds are synthesized and characterized by X-ray diffraction and scanning electron microscopy. Direct addition of < ∼7 wt% boron excess, in a single step, results in formation of UB 2 -U, while addition of >∼10 wt% boron excess leads to the formation of UB 2 -UB 4 . Diffraction data shows that material loss during the melt, previously assumed to be volatilization of boron, comprises both uranium and boron in the forms of UB 2 and UB 4 . To mitigate material loss during the melt and facilitate the fabrication of pure UB 2 , a stoichiometric amount of boron is incrementally dissolved into uranium metal using relatively low current (<100 A for 2 g of uranium); after all the boron is incorporated, the ingot is fully melted. Alternatively, microscopy images of UB 2 -U composites can be analyzed to determine the amount of additional boron needed to obtain the UB 2 line compound. In short, this paper details the best practices for the reproducible fabrication of pure UB 2 using the arc-melt method. These methods can be extended to other line compounds where volatilization and high melt temperatures complicate synthesis.