Assessment of rhenium as a surrogate for technetium in Hanford low activity waste borosilicate glasses: Speciation, solubility, and redox effects

Abstract

AbstractRe is often used as a nonradioactive surrogate for Tc in borosilicate melts because both elements have similar valences, ionic radii, and coordination environments. Re solubility in high sodium borosilicate waste glasses is sensitive to melter redox conditions; at 1150°C the solubility is ∼4130 ppm under oxidizing atmospheres. Raman, X‐ray absorption spectroscopy, and X‐ray diffraction show that Re dissolved in waste glasses under oxidizing conditions is in perrhenate tetrahedra; under more reducing conditions, Re0 separates from the melt. From air to ∼ 10−4 atm, both Re7+ and Tc7+ are dominant species in borosilicate melts. However, Tc and Re behaviors deviate between < 10−4 atm and > 10−8 atm, where Tc4+ becomes dominant, whereas Re7+ remains stable. Unlike Tc4+, Re4+ is not observed in borosilicate melts at 1150°C and associated glasses. Near = 10−8 atm, Tc4+ reduces to Tc0, whereas below = 10−8 atm, Re7+ reduces to Re0. Re is found to be a good surrogate for Tc under oxidizing conditions (> 10−4 atm) and under highly reducing conditions (< 10−9 atm). Redox and solubility behaviors of Tc and Re are also sensitive to glass chemistry, such that the fair assessment of Re modeling Tc behavior should be glass specific.