Advanced nuclear reactors need a different type of uranium. Here’s 4 things to know 

Questions about the fuel, called high-assay low-enriched uranium, remain

Laboratory equipment used to produce high-assay low-enriched uranium.

Nuclear engineers are making fuel for advanced nuclear reactors known as HALEU, using equipment at Idaho National Laboratory (shown).

Idaho National Laboratory

Nuclear power of the future is going to need fuel. That has governments, energy companies and nuclear engineers clamoring to get their hands on HALEU: high-assay low-enriched uranium.

HALEU (pronounced like “Hey, Lou”) was previously a niche material, used mainly in nuclear reactors conducting scientific research. But now, multiple companies in the United States have proposed newfangled types of nuclear reactors that they claim will generate electric power more efficiently and safely. Such reactors, many of which will run on HALEU, are a key part of the United States government’s plan to meet future demands for clean energy (SN: 12/14/22).

On June 10, TerraPower, a company founded by Bill Gates, broke ground on what is to be one of the first of this new generation of HALEU-fueled reactors. But currently, the United States doesn’t make that fuel in the amounts that will be needed by that cohort. So while the U.S. Department of Energy is funding the development of such advanced reactors, it is also working to secure an ample supply of HALEU fuel.

But some scientists are raising concerns about the rise of HALEU. According to a commentary in the June 7 Science, HALEU could be used to make a nuclear weapon, something not possible with current reactor-grade fuel.

HALEU’s potential for providing power, and the weapons worries that may come along with it, raise pressing questions. Here are four things to know about HALEU.

What is HALEU?

Compared with standard reactor fuel, HALEU contains a larger proportion of a key variety of uranium, the isotope uranium-235. U-235 is fissile: Its nucleus splits into two upon absorbing a low-energy neutron, releasing energy in the process. 

Naturally occurring uranium contains only about 0.7 percent U-235. Most of the remainder is the isotope U-238. To be used in a nuclear power plant, uranium must be enriched to contain more U-235. Standard reactor-grade uranium contains about 3 to 5 percent U-235. Uranium enriched to 20 percent or above is known as highly enriched uranium, which, unlike reactor-grade uranium, can be used to make nuclear weapons.

HALEU falls between those two extremes, with around 5 to 20 percent U-235. That means it can be used in ways that reactor-grade uranium can’t, but the United States and other countries don’t restrict its use as tightly as highly enriched uranium.

Three cylindrical pellets of HALEU fuel stand in front of laboratory equipment
High-assay low-enriched uranium fuel (shown) is being made at Idaho National Laboratory using spent fuel from a retired nuclear reactor.Idaho National Laboratory

Why are people so interested in it?

The HALEU hoopla has been fueled by the interest in advanced nuclear reactors. That term lumps together a wide variety of reactor designs that don’t fit the standard mold for reactors in the United States. Advanced reactors are often smaller than typical reactors and may use a substance other than normal water for cooling, such as liquid sodium. And advanced reactors commonly require HALEU, typically enriched to just under 20 percent.

With HALEU, “you’re able to make the core smaller and more energy-efficient in the space that you have, thus reducing construction costs,” says nuclear engineer Josh Jarrell of Idaho National Laboratory in Idaho Falls. And HALEU fuel can be used in forms that differ from the uranium dioxide fuel used in current reactors (SN: 11/20/14). Some reactor designs use a metallic fuel, or poppy seed–sized coated pellets of uranium called TRISO. The different fuel options and different reactor designs can be a plus for safety, Jarrell says. “Depending on the design, they don’t actually require human involvement to shut down safely.”

At the moment, most advanced reactors in the United States exist only on paper. But DOE is funding two advanced reactor demonstration projects: TerraPower’s Natrium Reactor in Kemmerer, Wyo., and X-energy’s Xe-100 Reactor in Seadrift, Texas. Both require HALEU. 

Where does HALEU come from?

There’s no established, large-scale commercial supplier of HALEU in the United States. And no matter how advanced a reactor is, it’s useless without fuel. Russia produces HALEU, but a U.S. law passed in May will prohibit most importation of uranium from Russia.

To ensure that advanced reactor projects have fuel, the U.S. government has been supporting efforts to produce the material. A Maryland-based company, Centrus Energy Corp., has begun producing some HALEU as part of a demonstration project in collaboration with DOE at an enrichment facility in Piketon, Ohio. 

A uranium processing facility with tall tubes stretching up to the ceiling
Centrifuges are used to create high-assay low-enriched uranium at a facility in Piketon, Ohio.Centrus Energy Corporation

As commercial enrichment operations get up to speed, a stopgap technique takes preexisting highly enriched uranium and blends it with other uranium to lower its enrichment. Idaho National Laboratory is currently performing this process using spent fuel from a retired nuclear reactor, with the target of producing 10 metric tons of HALEU. “The goal is to make sure we have a reasonable HALEU supply to allow some of these advanced reactor companies to demonstrate those first reactors,” Jarrell says. DOE has projected that more than 40 metric tons of HALEU will be needed by 2030, and additional HALEU will be required each year thereafter.

Other countries such as the United Kingdom are likewise making plans to produce HALEU.

What are the concerns?

Historically, HALEU has not been considered useful for weapons. But now that HALEU appears poised for widespread use, scientists are looking closer. A bomb made of HALEU with 19.75 percent enrichment could match the yield of the one that the United States dropped on Hiroshima in 1945, physicist Edwin Lyman and colleagues report in the Science commentary (SN: 8/6/20).

HALEU is not as easy to work with as highly enriched uranium — significantly larger quantities of the material would be needed to make a weapon. But the amount contained in a single reactor could be enough, says Lyman, of the Union of Concerned Scientists. “If you have a reactor that requires 300 or 400 kilograms of HALEU, that would be sufficient probably to make a crude nuclear weapon with a significant yield.” 

If HALEU use becomes more widespread, Lyman and colleagues worry, countries that currently don’t have nuclear weapons could squirrel that HALEU away to make them, or terrorist organizations could steal HALEU and put it to nefarious use. Security standards for HALEU should be beefed up to consider this risk, they say.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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