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 RuO ₂ within the glass melt. During vitrification, RuO ₂ 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 RuO ₂ in the waste glass, was comprehensively investigated. The results of X-ray diffraction and X-ray spectroscopy indicate that lamellar and granular Na ₃RuO ₄ do not form directly through the reaction between NaNO ₃ and RuO ₂ but rather through an intermediate stage from Na ₂RuO ₄. This reaction critically affects the morphology of RuO ₂ within the waste glass. The uncalcined mixture of NaNO ₃ and RuO ₂ was found to interact with the glass melt, forming granular RuO ₂ crystals, whereas the reaction of Na ₃RuO ₄ with the glass melt was found to lead to the formation of acicular RuO ₂ 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 RuO ₂, which was attributed to the presence of trace amounts of Na ₃RuO ₄ in the glass.