In a world that is increasingly paying attention to the need to reduce, reuse, and recycle, the nuclear power industry may be in for a boost.
Researchers from PNNL and Argonne National Laboratory have developed and tested a new chemical process that successfully captures certain radioactive byproducts from used nuclear fuel in one step instead of two. These hazardous byproducts—actinides such as americium, curium, and neptunium—could be sent to advanced reactors to be destroyed while also being used to produce more electrical power.
In laboratory tests with simulated dissolved nuclear fuel, the researchers’ actinide-lanthanide separation, or ALSEP, process separated nearly 100 percent of the target byproducts from the waste stream. The results were validated in small batch tests with genuine used nuclear fuel.
“The successful tests give us confidence that the process can be implemented in an industrial setting and that there’s nothing funky in the fuel that would mess with the chemistry,” said Gregg Lumetta, a PNNL lab fellow and principal investigator for the team’s ALSEP research.
The team’s single-step approach holds promise for simplifying plant designs and for smaller, less expensive plant footprints that involve less secondary waste generation.
The team’s research, “Closing the Nuclear Fuel Cycle with a Simplified Minor Actinide Lanthanide Separation Process (ALSEP) and Additive Manufacturing” appeared in Scientific Reports in September. The Department of Energy’s Office of Nuclear Energy funded the work.
A Complex Minority
Lanthanides and actinides reside in the stew of nuclear fuel byproducts. In nature, the lanthanide elements are nonradioactive, but during the nuclear fission process, they form radioactive isotopes. Most of these isotopes have relatively short half-lives and decay into nonradioactive lanthanide isotopes.
Actinides, on the other hand, are inherently radioactive and most of them are manmade. Because of their long half-lives and the heat generated during their radioactive decay, these byproducts pose most of the long-term risks associated with disposal of used nuclear fuel.
The so-called major actinides—uranium and plutonium—hold about 95 percent of the power-producing potential in the used nuclear fuel byproducts. Technology for extracting and recycling these two major elements has been around since the 1950s.
To recycle the remaining actinides into the nuclear fuel cycle, they must be separated from the lanthanides, which interfere with the nuclear fission process. The research team focused on a way to extract the minor actinides left in the process liquids, or raffinate, after removing the uranium and plutonium.
Oil and Water
The key to the ALSEP process is the right chemical combination of two extractants—one neutral and one acidic.
The neutral extractant pulls the actinides from the raffinate, a highly acidic water-based solution, into an oil-like phase. Nearly all the undesired components remain in the water layer, except for some lanthanide elements. The oil-like phase then mixes with a different, weakly acidic water-based solution. This solution contains a reagent that selectively binds the actinide ions and transfers them to the water-based solution. The acidic extractant keeps the lanthanide elements in the oil-like phase, separate from the actinides.
It took the researchers about four years and numerous iterations with six different combinations of neutral and acidic extractants to find the right recipe. With decontamination factors over 1000 and more than 99.9 percent recovery of the minor actinides, Lumetta said it was clear when they found the right combination.
“It doesn’t take long to see the potential; the math tells you everything you need to know,” Lumetta said.
Shaken and Stirred
While the engineering scale of solvent extraction is very well understood, a new chemical process can’t be just dropped in. Not only must the chemistry work in the laboratory, it must work under industrial conditions.
At Argonne, researchers performed tests with a simulant, spiked with key radionuclides, in a prototype industrial setting. The researchers used devices known as centrifugal contactors to mix and separate the water-based and oil-based solutions.
Art Gelis, now at the University of Nevada Las Vegas as an associate professor and director of the radiochemistry program, led the testing at Argonne. He said that although the test equipment was much smaller than what industry would use, results indicated great promise for scaleup given the maturity of solvent extraction engineering and operations.
Meanwhile, Lumetta and colleagues Emily Campbell, Gabriel Hall, Tatiana Levitskaia, and Vanessa Holfeltz designed and completed small batch experiments using actual dissolved fuel in hot cells at PNNL’s Radiochemical Processing Laboratory. Tests in the harsh radioactive environment showed that no component in the fuel matrix interfered with the separation chemistry.
In addition to simplifying the approach to fuel management, the team’s extraction concept could lead to a simplified means of reusing the byproducts from the nuclear fuel cycle. Closing the loop on this cycle could expand the use of low-carbon nuclear power by improving the use of uranium resources. It could also help optimize disposal options for the low-value byproducts from nuclear fission.
Published: December 4, 2019