Rhenium Volatilization From Molten Glass: Relationship Between Volatilization and Melt Viscosity

ABSTRACT Rhenium (Re) is a nonradioactive surrogate for technetium‐99 (Tc‐99), which is a highly volatile nuclide at elevated temperatures and poses critical challenges for nuclear waste vitrification. Consequently, understanding Re volatilization behavior is crucial for improving vitrification efficiency. This study investigated Re loss in borosilicate glasses as a function of melt viscosity, which revealed that a reduction in viscosity from ∼15.9 to ∼5.2 Pa·s increased Re volatilization by ∼40% over 8 h at 1200°C. A nonoxidizing volatilization model (NOX model), which integrated Fick's law, reaction kinetics, and partition coefficients, characterized the relationship between reaction and diffusion and accounts for unique localized states of Re in the glass melt to describe this behavior. The NOX model effectively described Re volatilization from molten glass across a wide range of melt viscosity. The coefficient analysis showed that Re volatilization was governed by its diffusion coefficient, which increased from ∼1.5 × 10 −8 to ∼1.5 × 10 −7 cm 2 /s as melt viscosity decreased. Moreover, a modified Stokes–Einstein equation employing the effective radius of Re showed that the theoretical radius of Re (∼5.6 × 10 −11 m) exceeded its effective radius (∼2.1 × 10 −11 m). A ∼60% reduction in the effective radius of Re with lower viscosity confirmed that glass network depolymerization enhanced Re mobility. Concurrently, increased partition coefficients under reduced viscosity further validated the role of shorter diffusion pathways in promoting Re volatilization.