As the world clamors for carbon-free power, U.S. nuclear reactor developers have responded with a multitude of advanced reactor designs.
These reactors would produce efficient, flexible heat and electricity. Nuclear energy from light water reactors already ranks among the safest forms of energy production, and most advanced reactors will use safety systems that rely on the laws of physics to virtually eliminate the possibility of a serious accident.
To move these designs from the drawing board to commercial deployments around the world, researchers and industry must subject them to rigorous testing that proves their performance and safety. This is especially true of the new, high-performance nuclear fuels that will power these reactors.
Recently, U.S. researchers have fabricated HALEU fuel for next generation nuclear reactors, an important step in the testing and qualification process.
In early 2023, researchers fabricated roughly two dozen pellets of uranium dioxide (UO2) HALEU — which stands for high-assay low-enriched uranium — at Idaho National Laboratory’s Experimental Fuels Facility at the Material and Fuels Complex.
HALEU fuel has some big advantages over conventional light water reactor fuel including longer cycle times in reactor, less waste production and less downtime for refueling. “With HALEU, advanced reactors can get increased fuel in-core lifetimes because you have higher enrichment,” said Adrian Wagner, a metallurgical engineer and INL’s Advanced Manufacturing group lead. “In simple terms, higher enrichment means more uranium-235 atoms in each pellet.”
The HALEU fuel pellets made at INL contain fuel enriched to 15% uranium-235, the form of uranium that undergoes fission during a nuclear reaction. Conventional light water reactor fuels are typically enriched to less than 5% uranium-235, the limit allowed by most existing Nuclear Regulatory Commission operating licenses.
Demonstrating the capability to fabricate a commercial quality of HALEU UO2 provides options for industry and other government agencies to make fuel samples with a wider range of enrichment without impacting existing operating licenses.
A COLLABORATION WITH GENERAL ELECTRIC
In the short term, the HALEU fuel pellets will support a test in INL’s Advanced Test Reactor in collaboration with General Electric. The work will test the endurance of a prototype of cladding material that could improve the performance of today’s existing light water reactors and tomorrow’s advanced reactors.
“The bigger picture is that it will help the licensing of advanced reactor designs, almost all of which will use some form of HALEU fuel,” Wagner said.
NASA is also looking at HALEU fuels to power spacecraft via thermal-nuclear propulsion, and for fission-based surface power for the moon and Mars.
NUCLEAR FUEL FOR THE NEXT GENERATION
The project’s success establishes INL as the go-to custom HALEU fuel fabricator for reactor companies.
“We’re the best artisanal fuel manufacturer in the world,” Wagner said. “You’ve heard of craft beer. Well, we make craft fuel.”
Jennifer Watkins, a nuclear fuels and materials scientist at INL, led the project and sees this effort as another example of INL fulfilling the mission of DOE’s national lab system, i.e., a place to develop and test concepts before commercial viability is demonstrated.
“INL is the best place in the U.S. to develop fabrication processes for unique and novel fuel concepts,” said Watkins. “INL has one of the widest ranges of feedstock options at a variety of enrichment levels and an extremely flexible DOE-based enrichment license allowing for adaptability to user needs.”
INL experts made the pellets at lab scale using a traditional powder metallurgy process — a pressure-less sintering technique similar to that used by industry to make light water reactor fuel.
Demonstrating UO2 HALEU fabrication opens the door for other types of HALEU, both metallic and ceramic, and highlights INL’s ability to tailor enrichments to customer and experiment requirements.
Nitride, boride, carbide and silicide fuels have higher uranium densities that could provide even higher levels of performance for advanced reactors. But first, researchers and industry must test the fuel and obtain its approval from regulators.
“Currently, regulations restrict commercial fuel to less than 5% enrichment. This work can assist in advancing approval for higher enriched materials for nuclear power plants,” Watkins said.
For now, the researchers are performing tests to further characterize the pellets’ properties and identify impurities. Experts will then fabricate another 100 to 150 pellets for General Electric’s proposed tests in the Advanced Test Reactor.
“Our collaborators were pleased with the outcome,” said Watkins. “The pellets that we produced were extremely high density. Our initial characterization efforts suggest they will meet all commercial standards for uranium dioxide.”
