Uranium diboride: Understanding and optimizing arc-melt synthesis

Abstract

Uranium diboride (UB₂) is considered as a nuclear fuel candidate or burnable absorber for advanced reactors. The present study investigates the arc-melt synthesis of UB₂ and aims to improve understanding and reproducibility of the fabrication method. UB₂-U, UB₂, and UB₂-UB₄ 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 UB₂-U, while addition of >∼10 wt% boron excess leads to the formation of UB₂-UB₄. 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 UB₂ and UB₄. To mitigate material loss during the melt and facilitate the fabrication of pure UB₂, 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 UB₂-U composites can be analyzed to determine the amount of additional boron needed to obtain the UB₂ line compound. In short, this paper details the best practices for the reproducible fabrication of pure UB₂ using the arc-melt method. These methods can be extended to other line compounds where volatilization and high melt temperatures complicate synthesis.