Ruthenium solubility and its impact on the crystallization behavior and electrical conductivity of MoO3-containing borosilicate-based model high-level nuclear waste glasses

Abstract

The present study focuses on investigating the solubility of RuO2 in a borosilicate-based model high-level waste glass and understanding its impact on the crystallization behavior and electrical conductivity of the resulting vitrified waste forms. The solubility limit of RuO2 in the investigated glass composition has been determined to be 460 ppm by weight. The higher concentration of RuO2 results in sub-micron sized Ru-rich inclusions in the glassy matrix, which eventually agglomerate to form needle-like and polyhedral RuO2 crystals. It is observed that RuO2 selectively promotes the crystallization of the rare-earth apatite phase over the powellite phase. The as-synthesized RuO2-containing glasses exhibit semiconducting behavior with a similar level of electrical conductivity below the glass transition. The percolation of non-uniformly distributed RuO2 inclusions may result in a formation of short-range low-resistive conduction pathways in the host glass matrix leading to an apparent metallic-like behavior of selected thin samples with the highest ruthenium content.