Controlled growth of ruthenium dioxide nanostructures in borosilicate glass melts

Ruthenium, a fission product generated during the fission of uranium oxide fuel in a reactor, interacts with alkali metals such as sodium in the upper layer of the cold cap, forming a sodium ruthenate intermediate (Na_xRu_yO_z), which promotes the crystallization of acicular RuO₂ within the glass melt. During vitrification, RuO₂ predominantly settles at the bottom of the melt owing to its high density and low solubility, significantly increasing both the conductivity and viscosity of the glass melt. Herein, the formation of Na_xRu_yO_z, along with the chemical reactions promoting the crystallization of RuO₂ in the waste glass, was comprehensively investigated. The results of X-ray diffraction and X-ray spectroscopy indicate that lamellar and granular Na₃RuO₄ do not form directly through the reaction between NaNO₃ and RuO₂ but rather through an intermediate stage from Na₂RuO₄. This reaction critically affects the morphology of RuO₂ within the waste glass. The uncalcined mixture of NaNO₃ and RuO₂ was found to interact with the glass melt, forming granular RuO₂ crystals, whereas the reaction of Na₃RuO₄ with the glass melt was found to lead to the formation of acicular RuO₂ crystals. The results of X-ray absorption fine structure analysis indicate that the valence state of Ru in NaRu-BSG is slightly higher than that in reference RuO₂, which was attributed to the presence of trace amounts of Na₃RuO₄ in the glass.