Solubility-driven process could simplify used fuel reprocessing

Researchers at Oregon State University have developed an aqueous reprocessing technique that could provide a new way to recycle used nuclear fuel or simplify existing reprocessing methods.

As published in Communications Chemistry, the technique uses differences in solubility between actinide fluoride clusters and used nuclear fuel contaminants in a concentrated ammonium fluoride solution to remove a majority of fission products from samples simulating used nuclear fuel.

The case for recycling: When nuclear fuel is removed from a fission reactor, only about 5 percent-10 percent of the fuel's fissile material has been consumed. If reprocessed, the remaining uranium is still useful as fuel.

"The catch is that you can't simply take fuel and put it into a nuclear reactor and run it 20 times as long," said Alexander Chemey, assistant professor of nuclear science and engineering at Oregon State and one of the paper's coauthors. There are fission products. These are little pieces of the nucleus that split off from the much bigger nucleus of uranium or plutonium inside of a nuclear reactor, and some of these induce unfavorable nuclear properties," he told Nuclear News.

So, processing used fuel requires chemically separating these fission products, some of which are themselves useful for medical and technology applications.

Searching for radiation resistance: Established recycling techniques such as the Plutonium Uranium Redox Extraction (PUREX) process are highly effective, but they have drawbacks.

"They all have one thing in common: they are very susceptible to radiation damage," Chemey said.

Radiation can break apart molecules in the processing solutions, creating highly reactive species that can damage containers and degrade other chemicals. As a result, those processes often require excess volumes of solution, increasing costs.

Chemey said a starting point for this study was considering what chemicals don't take much radiation damage, which is how they ended up exploring salt.

The research team settled on a highly concentrated ammonium fluoride solution, which features a unique solubility profile: very little is soluble in the solution to any appreciable degree, said Chemey, with the exception of a class of chemicals that includes uranium, neptunium, and plutonium. While still not very soluble, they are dramatically more soluble than most used nuclear fuel contaminants.

"So now you have a radiation-resistant solution that you can dissolve your desirable materials up in, and most other things shouldn't dissolve up. That could be the basis for really good separation," Chemey said.

The experiment: For the study, Chemey and his team created simulated used nuclear fuel containing materials in proportions similar to those found in real used nuclear fuel. They reacted the samples at low temperature with ammonium bifluoride, which makes a product that is soluble in the highly concentrated ammonium fluoride solution.

"Anything that is soluble tends to stay soluble, even when adding an antisolvent, except for the actinides. So, we get a two-step separation that takes weakly soluble actinide materials and recovers them separate from most other elements," he said.

The recovered actinide products can then be heat treated to produce either actinide fluorides, which are important for molten salt reactors, or oxides, which are the basis for most nuclear fuels.

"This is very much a proof-of-concept stage, but we removed about 90-something percent of the total radioactivity from our solution, if we assume that everything that is chemically similar enough behaves like the model species we use, and that's actually quite a lot," said Chemey.

He said that even if the approach never becomes a standalone recycling method, it could potentially be used to simplify existing reprocessing techniques such as PUREX.

Future work: As the work is in its early stages, there are several potential next steps. The team would need to determine how the process scales up to using larger amounts of material, and tests would need to be done using actual used nuclear fuel.

Chemey said that the technique also potentially is advantageous from a nonproliferation standpoint. While PUREX must separate out plutonium, he believes this demonstration could form the basis for a reprocessing method where the actinides are kept together.

He noted work by Jonathan Burns at the University of Alabama-Birmingham, who has taken a solubility-based approach in the opposite direction by using uranyl nitrates, which are very soluble, and evaporating down the solution such that most other things precipitate out first, leaving behind an enriched solution.

"There are a lot of challenges this would have to overcome before it could be a practical solution, but I think it still is worthwhile, because there's a strength when you have a consensus to zig when the group is zagging. It gives you an alternative perspective," he said. "There is great value in optimizing existing methods, but there's also really great value in finding very different alternatives that can strengthen the whole."