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. Image credit: Pacific Northwest National Laboratory Enlarge image.

A long-lived part of nuclear waste, it takes more than 210,000 years for half of any amount of technetium to decay away. It is also able to migrate, either moving through groundwater or becoming a gas when heated. Heat is an issue because technetium-containing waste and special chemicals are heated-to 1150 degrees Celsius-to yield solid logs for long-term storage. Scientists would prefer the technetium stay in the glass logs. A team from Pacific Northwest National Laboratory, Lawrence Berkeley National Lab, and the Department of Energy's Office of River Protection devised a way to retain more technetium. 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 amount of technetium held over baseline glass formulas.

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

Why It Matters: Previous approaches could not meet DOE's targets for technetium retention in the glass logs, known as vitrified waste. This study accelerated a technology, using spinel materials that will help meet the federal targets by using complex calculations and simulations as well as experiments to understand what happens inside the glass.

"It is a novel breakthrough to increase technetium retention in a stable mineral even at the high-temperature vitrification process," said Dr. Wooyong Um, who led the experimental efforts at PNNL.

Play the video (play in Chrome or IE) Researchers integrated molecular simulations and experimental techniques to determine the fate of long-lived technetium and found solutions to mitigate the associated environmental impact.

Methods: Um, who works at PNNL and is a visiting professor at Pohang University of Science and Technology, has been leading a world-class experimental effort in technetium remediation using magnetite and spinel. Their idea uses an iron oxide known as magnetite (formula: Fe3O4) as a way to prevent technetium from volatilizing during nuclear waste vitrification, the process that creates the glass long-term waste form. The magnetite forms spinels that are encased in the glass logs. The problem is that the heat used to form the glass logs 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, to help keep more technetium locked in the glass, 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 simulations that looked at more than the electronic structure.

"This was an extremely challenging computational feat requiring the description of atoms under high temperature," said Dr. Mal-Soon Lee, who worked on this study at PNNL. "This work wouldn't have been possible without advances in computational power."

The simulations consider the heat as well as the electronic structure. The team simulated four variants. They added zinc, nickel, or cobalt to the technetium-incorporated magnetite. While zinc and nickel mostly give two electrons to their surroundings, cobalt may give up one more to the spinel. These electrons keep technetium in its more stable state. Adding cobalt could create a material that retains half or more of the technetium.

To validate the result of the simulations, the experimental team prepared magnetite samples loaded with technetium and doped them with nickel, zinc, or cobalt. They heated the samples for an hour at 700 degrees Celsius, or about seven times that of boiling water, and measured the remaining technetium. The experiments matched the results from the simulations.

As a corollary of this effort, these simulations also provide useful input to catalytic processes. Catalysts are often supported on an iron oxide, like spinels. In addition to providing a physical platform, there is electron traffic between supports and catalysts. Independent studies into the supports complement these ideas and machinery for how to simulate iron oxides. They all tie together as to how to modify supports to channel catalytic activity.

Further, cobalt's behavior could shed new light on supports for catalysts driving reactions that refine crude oil, produce hydrogen for fuel cells, and more. Dr. Roger Rousseau at PNNL plans to utilize these ideas as he advances the catalysis efforts on projects he leads for the DOE Office of Science's Office of Basic Energy Sciences.

What's Next? The work underscores the impact of complex calculations and simulation, which  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.

Acknowledgments

Sponsors: This work was supported by the U.S. Department of Energy, Office of River Protection, Waste Treatment and Immobilization Plant Federal Project and the Office of Science, Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences (RR, VAG).

Research Area: Chemical Sciences

Facilities: Pacific Northwest National Laboratory's Platform for Institutional Computing (PIC), the Environmental Molecular Sciences Laboratory (EMSL), and the National Energy Research Scientific Computing Center (NERSC)

Research Team: Albert A. Kruger, DOE Office of River Protection; Wayne W. Lukens, Lawrence Berkeley National Laboratory; Wooyong Um, Pacific Northwest National Laboratory and Pohang University of Science and Technology; Mal-Soon Lee, Guohui Wang, Roger Rousseau, Vassiliki-Alexandra Glezakou, Pacific Northwest National Laboratory

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