Method can detect harmful salts forming in nuclear waste melters

(Press-News.org) PULLMAN, Wash. –  A new way to identify salts in nuclear waste melters could help improve clean-up technology, including at the Hanford Site, one of the largest, most complex nuclear waste clean-up sites in the world.

Reporting in the journal Measurement, Washington State University researchers used two detectors to find thin layers of sulfate, chloride and fluoride salts during vitrification, a nuclear waste storage process that involves converting the waste into glass. The formation of salts can be problematic for waste processing and storage.

“We were able to demonstrate a technique to see when the salts are forming,” said John Bussey, a WSU undergraduate who is one of the paper’s lead authors. “By doing that, the melters could be monitored to know if we should change what is being put in the melt.”

Vitrification entails putting the nuclear waste into large melters that are then heated to high temperatures. The resulting glass is then poured into cylinders and solidified for long-term safe storage.

The U.S. Department of Energy is building a vitrification plant at the Hanford Site. Because Hanford was used to make plutonium for the very first nuclear bomb, the waste there is particularly complex, containing nearly all of the elements of the periodic table, said Bussey. A total of 55 million gallons of chemical and nuclear waste are stored in 177 tanks at the site.

In the processing of the nuclear waste, salts can form. They can be corrosive and ruin very expensive vitrification equipment. Furthermore, since they dissolve in water, salts in the final glass waste form could lead to leaks and contamination if the waste form becomes exposed to water during storage. The wide variety of waste components at Hanford makes salt formation more likely.

“Salt formation is very undesirable during the vitrification process,” said Bussey.

With a system that was developed at Pacific Northwest National Laboratory and the Massachusetts Institute of Technology, the researchers used optical and electrical components to look at light between infrared and microwave wavelengths that are naturally emitted during the melting process. They looked at samples of glass melts that are similar to those found at the Hanford site. Using two types of detectors, they were able to study the thermal emissions of the samples as well as the change over time.

“The brightness is really interesting for identifying all of the melting, solidification and salt formation,” said Ian Wells, co-lead author and a graduate student in the WSU School of Mechanical and Materials Engineering. “What is really unique about this is you don’t have to add any additional lighting or additional systems -- Purely based on the heat that is coming off the melt, you are able to look at the brightness of one-pixel images, and you can tell what’s happening.”

The researchers were able to see when there’s a large change in the melt. Whether because a salt is forming or if there’s melting or solidification, there is also a sharp change in the intensity. The researchers compared different melts and were able to identify behavior indicative of salts.

“We can clearly identify what is happening based on that behavior,” said Wells. “We were surprised by how sensitive a probe it was even with very small amounts of salt.”

The system can discriminate between salt types. The sensors can also sense the salts remotely, without having to be dipped in the radioactive molten glass, thus avoiding additional challenges.

“This work takes this monitoring technology a good step of the way closer to being able to be used inside the vitrification plant,” said Bussey. “This piece of equipment without too much modification could be put straight into the vitrification plant.”

The researchers think the work has other potential applications in molten salt nuclear reactors or in different types of manufacturing processes, such as glass, epoxies or carbon fiber processing, in which manufacturers want to better understand phase changes and the formation of different compounds during those phases. They hope to next move from lab-scale testing to larger scale melt tests. This research was funded through the United States Department of Energy Office of Environmental Management.

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