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June 2010

Breaking Good

Study examines durability of glass with ties to nuclear waste storage

In a first-of-its-kind study, scientists at Pacific Northwest National Laboratory determined how durable four-component glass is when aluminum atoms are replaced by boron atoms and vice versa. Enlarge Image

Results: In a first-of-its-kind study, scientists at Pacific Northwest National Laboratory determined how durable four-component glass is when aluminum atoms are replaced by boron atoms and vice versa. By determining the effect of these elements, scientists can better predict how water interacts with minerals and glass. Water-glass interactions matter to people working to glassify liquid and sludge-like waste from nuclear weapons into a safer, solid form. Water-mineral interactions are also of interest to geochemist and agricultural scientists who want to know how minerals weather.

Why it matters: As with any power source, the safety of workers, the environment, and nearby populations must be considered. In nuclear power, this means trapping or immobilizing certain radioactive elements in the waste. In addition, nuclear waste from decades-old weapons sites must be permanently and safely stored. This research supports continuing efforts to manufacture a vitrified form that safely encapsulates the waste.

Methods: Using several spectrometers including EMSL's 900-MHz nuclear magnetic resonance spectrometer, five glass samples with different ratios of boron and aluminum were studied. These samples were four-component glass. These glasses are so named because it contains four major nonradioactive parts of vitrified atomic waste: alumina, boron oxide, sodium oxide, and silica. These four components play a significant role in the determining the chemical durability of glass.

The spectrometry results were combined with flow-through dissolution experiments. The flow-through experiments allowed the scientists to evaluate the relationship between the atomic structure and the rate of glass dissolution.

The scientists found that water determines how fast the glass dissolves. Water breaks apart bonds between aluminum and oxygen as well as silicon and oxygen under certain conditions (noted in previous studies). Determining how substituting other elements weakens or strengthens water's desire to break apart the bonds is critical in understanding the processes that control water's interactions with minerals and glass.

What's next? This research is part of ongoing work at Pacific Northwest National Laboratory and elsewhere to answer the questions about chemically durable waste forms.

Acknowledgments: This work was funded by the Department of Energy's Office of Science Environmental Management Science Program. Additional student funding was provided by the Department of Energy's Community College Initiative and Summer Research Apprenticeship programs.

Resources in the Department of Energy's EMSL, a national scientific user facility, were critical to this research.

This work was done by Eric Pierce, Wendy Shaw, Pete McGrail, Jonathan Icenhower, Charles Windisch, and Elsa Cordova from Pacific Northwest National Laboratory; Lunde Reed from Washington State University; and Johnathan Broady from the Office of Fellowship Programs.

Reference: Pierce EM, LR Reed, WJ Shaw, BP McGrail, JP Icenhower, CF Windisch Jr, EA Cordova, and J Broady. 2010. "Experimental Determination of the Effect of the Ratio of B/Al on Glass Dissolution along the Nepheline (NaAlSiO4)—Malinkoite (NaBSiO4) Join." Geochimica et Cosmochimica Acta 74(9):2634-2654. DOI: 10.1016/j.gca.2009.09.006.

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Turning Waste to Glass

To vitrify waste, add glass-forming materials and apply heat. Vitrifying nuclear waste—from operating reactors or decades-old weapons production—solidifies the waste and reduces the risks of toxic materials entering the environment. However, the vitrified waste also contains a host of nonradioactive elements, such as aluminum, that can impact the durability of the final product. How aluminum, boron, and other elements behave is of keen interest to researchers, regulators, and the public.