PNNL

Slow-decaying technetium (Tc) in nuclear waste needs to be held tightly in the vitrification process that creates glass logs for long-term storage. Scientists showed that modifying the glass with cobalt (Co) creates solid logs that mark a significant increase in the amount of technetium held.

A long-lived part of nuclear waste, it takes more than 211,000 years for the radioactivity associated with one atom of technetium to decay by half. It can also migrate, either by moving through groundwater or becoming a gas when heated. Heat is an issue because technetium-containing waste and special chemicals are heated—to 1200 degrees Celsius—to prepare glass wasteform for long-term storage.

Through a project sponsored by the Department of Energy, a team from PNNL, Berkeley Lab, and the DOE Office of River Protection developed a way to retain more technetium in the glass. They added cobalt. Mixed with an iron oxide, the cobalt forms “thorns,” or spinels, in the glass. The result? The modified glass marks a 50 to 60 percent increase in the technetium retention rate compared to plain glass.

“It is another unique example of how theory and experiment can make huge leaps of progress when working together,” said PNNL’s Vanda Glezakou, who led the theory effort. “This work highlights the power of modern, state-of-the-art simulations to provide essential insights and generate theory-inspired design criteria of complex materials at elevated temperatures.”

Hitting the Target

Previous approaches could not meet DOE’s targets for technetium retention in glass wasteforms. However, by using complex calculations and simulations as well as experiments to understand what happens inside the glass, this study accelerated the incorporation of spinel materials to help meet the federal targets.

“It is a novel breakthrough to increase technetium retention in stable material at high temperatures,” said PNNL’s Wooyong Um, who leads the experimental efforts.

Um has been leading a world-class experimental effort in technetium remediation using magnetite and spinel. Their idea uses an iron oxide known as magnetite as a way to prevent technetium from volatilizing during nuclear waste vitrification. The magnetite forms spinels that are encased in the glass wasteforms. The problem is that heat used to form the glass wasteforms causes most of the technetium to lose electrons and turn gaseous. The spinels hold about 15 percent of the technetium. The rest still escapes.

The team wanted to see if they could improve the magnetite’s retention ability by adding other metals. The best way to figure out what was happening on the atomic level in these fast-moving reactions was to conduct advanced computer simulations.

“This was an extremely challenging computational feat,” said PNNL’s Mal-Soon Lee. “This work wouldn’t have been possible without advanced computational power.”

The team simulated four variants. They added zinc, nickel, or cobalt to the technetium-magnetite mixture. The results showed that adding cobalt created a material that retains half or more of the technetium. To validate these results, the experimental team prepared magnetite samples loaded with technetium and doped with nickel, zinc, or cobalt. The experiments matched the results from the simulations.

The work underscores the impact of complex calculations and simulation—that include the electronic structure and temperature effects—to reveal crucial variables for material design. The team will continue to apply simulations and the underlying complex equations to a host of problems that face our world today.

PNNL Research Staff: Wooyong Um, Mal-Soon Lee, Guohui Wang, Roger Rousseau, and Vassiliki-Alexandra Glezakou

Reference: Lee MS, W Um, G Wang, AA Kruger, WM Lukens, R Rousseau, and VA Glezakou. “Impeding 99Tc(IV) Mobility in Novel Waste Forms.” Nature Communications 7:12067. DOI: 10.1038/ncomms12067