Recently, researchers at the US Idaho National Laboratory (INL) fabricated high-assay low-enriched uranium (HALEU) fuel for next generation nuclear reactors, as a step in the testing and qualification process. They fabricated roughly two dozen pellets of uranium dioxide (UO2) HALEU at the INL Material & Fuels Complex, the Department of Energy (DOE) reported.

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 refuelling. “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. Conventional light water reactor fuels are typically enriched to less than 5% uranium-235, the limit allowed by most existing Nuclear Regulatory Commission operating licences. 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 licences.

In the short term, the HALEU fuel pellets will support a test in INL’s Advanced Test Reactor (ATR) 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. “We’re the best artisanal fuel manufacturer in the world,” he added. “You’ve heard of craft beer. Well, we make craft fuel.”

According to Jennifer Watkins, a nuclear fuels and materials scientist at INL, who led the project, the current effort is another example of INL fulfilling the mission of DOE’s national lab system, namely as a place to develop and test concepts before commercial viability is demonstrated. “INL is the best place in the US 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. DOE says 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.

“Our collaborators were pleased with the outcome,” said Watkins. “The pellets that we produced were extremely high density. Our initial characterisation efforts suggest they will meet all commercial standards for uranium dioxide.” The researchers are now performing tests to further characterise the pellets’ properties and identify impurities. Experts will then fabricate another 100 to 150 pellets for General Electric’s proposed tests in the ATR.

Image: HALEU fuel pellets (courtesy of INL)