Formation of Technetium Salts in Hanford Low‐Activity Waste Glass

Abstract

The distribution and physical form of technetium in a Hanford low‐activity waste (LAW) glass was examined with scanning electron microscopy (SEM) and X‐ray diffraction (XRD). A simulated Hanford LAW glass was spiked with varying amounts of potassium pertechnetate and melted at 1000°C. The glass was melted in a sealed quartz ampoule with the air pumped out, so that volatile material could leave the glass but would not be lost from the system. Previous studies have shown that technetium remains in the glass up to about 2000 ppm, but rises to the top of the melt as a separate salt phase above this concentration. Examination by SEM shows that crystals of technetium compounds appear to grow out of the hot glass, which implies that the hot glass was supersaturated in technetium salts. Some of the technetium compound crystals had apparently melted, but other crystals had obviously not melted and must have formed after the glass had partially cooled. The technetium compounds in the salt layer are KTcO4 and NaTcO4, according to SEM and XRD. No TcO2 was found in the salt phase, even though Tc(IV) has been previously reported in the glass.

Chemical Trends in Solid Alkali Pertechnetates

“…According to current baseline plans for the disposal of legacy defense waste from Cold War plutonium production in the United States, a significant portion of the ∼1200 kg of 99 Tc inventory at the Hanford site in Washington state will be immobilized in high sodium oxide, aluminum borosilicate glasses. 75 Additionally, a recent paper published by Soderquist and co-workers 76 describes the crystallization of Na and K pertechnetate out of a nuclear waste glass surface. Their results suggest that there is a high concentration of dissolved pertechnetate at the glass surface when it is quenched.…”

Vitrification of wastes: from unwanted to controlled crystallization, a review

“…Work is ongoing to understand the glass compositional effects of sulfate salt formation [Bingham et al, 2017, Kruger et al, 2010, Skidmore et al, 2019, its relation to solubility [Vienna et al, 2004[Vienna et al, , 2014, and its structural role in the glass [McKeown et al, 2001, 2004. There are many similarities between the tendency for salt phase formation with molybdate, sulfate, and other oxyanions like those of Re and Tc [Riley et al, 2013, Soderquist et al, 2016.…”

Use of WetSEM® capsules for convenient multimodal scanning electron microscopy, energy dispersive X-ray analysis, and micro Raman spectroscopy characterisation of technetium oxides

“…Consequently, we have considered how analysis of such specimens could be achieved with multi-user instruments, outside of a controlled area 1 whilst adequately managing the radiological risk [11]. Previous studies, such as Soderquist et al have prepared Tc samples for SEM-EDX by depositing powdered material on a sticky carbon tab [12], although generally effective for powdered material, the possibility of Tc-bearing material becoming dislodged during measurement renders this method unsuitable for use outside of controlled areas or dedicated equipment for radioactive specimens. McKeown et al used silica vials as a method of containment for Raman analysis of Tc-bearing glasses [13], however, this method of containment is unsuitable for SEM study and requires the preparation of an additional sample for both characterisation methods to be used.…”