Effect of Spinel Crystals on the Electrical Conductivity of RuO ₂ ‐Bearing Nuclear Waste Glass Melts

ABSTRACT Spinel and platinum‐group‐element (PGE) crystals are commonly present in nuclear waste glass melts at high temperatures and their accumulation at the bottom of the melts can affect high‐temperature processing. However, the high‐temperature electrical behavior of spinel‐bearing nuclear waste melts, and particularly systems containing both spinel and PGE crystals, remains insufficiently understood. In this study, we investigate the electrical conductivity of molten nuclear waste glasses containing RuO 2 crystals, NiFe 2 O 4 spinel crystals, and mixtures of both phases. The results show that RuO 2 crystals significantly increase melt conductivity and can trigger a transition from ionic to electronic conduction, whereas spinel crystals alone produce only a modest increase in conductivity and maintain predominantly ionic conduction. When both crystalline phases coexist, spinel crystals appear to interrupt the continuous conductive pathways formed by RuO 2 crystals and reduce the conductivity relative to RuO 2 ‐only systems. The extent of this effect depends on both the RuO 2 and spinel contents. These findings indicate that spinel crystals can influence the electrical response of RuO 2 ‐bearing nuclear waste glass melts by modifying the connectivity of RuO 2 ‐based conductive networks. This behavior has implications for understanding conductivity changes and potential short‐circuiting risks during vitrification processes.