Various scenarios to getting to net zero carbon emissions from power generation by 2050 hinge on the success of some hugely ambitious initiatives in renewable energy, grid enhancements, and other areas. Perhaps none of these are more audacious than an envisioned renaissance of nuclear power, driven by advanced-technology reactors that are smaller than traditional nuclear power reactors.

What many of these reactors have in common is that they would use a kind of fuel called high-assay low-enriched uranium (HALEU). Its composition varies, but for power generation, a typical mix contains slightly less than 20 percent by mass of the highly fissionable isotope uranium-235 (U-235). That’s in contrast to traditional reactor fuels, which range from 3 percent to 5 percent U-235 by mass, and natural uranium, which is just 0.7 percent U-235.

Now, though, a paper in Science magazine has identified a significant wrinkle in this nuclear option: HALEU fuel can theoretically be used to make a fission bomb—a fact that the paper’s authors use to argue for the tightening of regulations governing access to, and transportation of, the material. Among the five authors of the paper, which is titled “The Weapons Potential of High-Assay Low-Enriched Uranium,” is IEEE Life Fellow Richard L. Garwin. Garwin was the key figure behind the design of the thermonuclear bomb, which was tested in 1952.

The Science paper is not the first to argue for a reevaluation of the nuclear proliferation risks of HALEU fuel. A report published last year by the National Academies, “Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors,” devoted most of a chapter to the risks of HALEU fuel. It reached similar technical conclusions to those of the Science article, but did not go as far in its recommendations regarding the need to tighten regulations.

Why is HALEU fuel concerning?

Conventional wisdom had it that U-235 concentrations below 20 percent were not usable for a bomb. But “we found this testimony in 1984 from the chief of the theoretical division of Los Alamos, who basically confirmed that, yes, indeed, it is usable down to 10 percent,” says R. Scott Kemp of MIT, another of the paper’s authors. “So you don’t even need centrifuges, and that’s what really is important here.”

Centrifuges arranged very painstakingly into cascades are the standard means of enriching uranium to bomb-grade material, and they require scarce and costly resources, expertise, and materials to operate. In fact, the difficulty of building and operating such cascades on an industrial scale has for decades served as an effective barrier to would-be builders of nuclear weapons. So any route to a nuclear weapon that bypassed enrichment would offer an undoubtedly easier alternative. The question now is, how much easier?

“It’s not a very good bomb, but it could explode and wreak all kinds of havoc.”

Adding urgency to that question is an anticipated gold rush in HALEU, after years of quiet U.S. government support. The U.S. Department of Energy is spending billions to expand production of the fuel, including US $150 million awarded in 2022 to a subsidiary of Centrus Energy Corp., the only private company in the United States enriching uranium to HALEU concentrations. (Outside of the United States, only Russia and China are producing HALEU in substantial quantities.) Government support also extends to the companies building the reactors that will use HALEU. Two of the largest reactor startups, Terrapower (backed in part by Bill Gates) and X-Energy, have designed reactors that run on forms of HALEU fuel, and have received billions in funding under a DOE program called Advanced Reactor Demonstration Projects.

The difficulty of building a bomb based on HALEU is a murky subject, because many of the specific techniques and practices of nuclear weapons design are classified. But basic information about the standard type of fission weapon, known as an implosion device, has long been known publicly. (The first two implosion devices were detonated in 1945, one in the Trinity test and the other over Nagasaki, Japan.) An implosion device is based on a hollow sphere of nuclear material. In a modern weapon this material is typically plutonium-239, but it can also be a mixture of uranium isotopes that includes a percentage of U-235 ranging from 100 percent all the way down to, apparently, around 10 percent. The sphere is surrounded by shaped chemical explosives that are exploded simultaneously, creating a shockwave that physically compresses the sphere, reducing the distance between its atoms and increasing the likelihood that neutrons emitted from their nuclei will encounter other nuclei and split them, releasing more neutrons. As the sphere shrinks it goes from a subcritical state, in which that chain reaction of neutrons splitting nuclei and creating other neutrons cannot sustain itself, to a critical state, in which it can. As the sphere continues to compress it achieves supercriticality, after which an injected flood of neutrons triggers the superfast, runaway chain reaction that is a fission explosion. All this happens in less than a millisecond.

The authors of the Science paper had to walk a fine line between not revealing too many details about weapons design while still clearly indicating the scope of the challenge of building a bomb based on HALEU. They acknowledge that the amount of HALEU material needed for a 15-kiloton bomb—roughly as powerful as the one that destroyed Hiroshima during the second World War—would be relatively large: in the hundreds of kilograms, but not more than 1,000 kg. For comparison, about 8 kg of Pu-239 is sufficient to build a fission bomb of modest sophistication. Any HALEU bomb would be commensurately larger, but still small enough to be deliverable “using an airplane, a delivery van, or a boat sailed into a city harbor,” the authors wrote.

They also acknowledged a key technical challenge for any would-be weapons makers seeking to use HALEU to make a bomb: preinitiation. The large amount of U-238 in the material would produce many neutrons, which would likely result in a nuclear chain reaction occurring too soon. That would sap energy from the subsequent triggered runaway chain reaction, limiting the explosive yield and producing what’s known in the nuclear bomb business as a “fizzle.“ However, “although preinitiation may have a bigger impact on some designs than others, even those that are sensitive to it could still produce devastating explosive power,” the authors conclude.

In other words, “it’s not a very good bomb, but it could explode and wreak all kinds of havoc,” says John Lee, professor emeritus of nuclear engineering at the University of Michigan. Lee was a contributor to the 2023 National Academies report that also considered risks of HALEU fuel and made policy recommendations similar to those of the Science paper.

Critics of that paper argue that the challenges of building a HALEU bomb, while not insurmountable, would stymie a nonstate group. And a national weapons program, which would likely have the resources to surmount them, would not be interested in such a bomb, because of its limitations and relative unreliability.

“That’s why the IAEA [International Atomic Energy Agency], in their wisdom, said, ‘This is not a direct-use material,’” says Steven Nesbit, a nuclear-engineering consultant and past president of the American Nuclear Society, a professional organization. “It’s just not a realistic pathway to a nuclear weapon.”

The Science authors conclude their paper by recommending that the U.S. Congress direct the DOE’s National Nuclear Security Administration (NNSA) to conduct a “fresh review” of the risks posed by HALEU fuel. In response to an email inquiry from IEEE Spectrum, an NNSA spokesman, Craig Branson, replied: “To meet net-zero emissions goals, the United States has prioritized the design, development, and deployment of advanced nuclear technologies, including advanced and small modular reactors. Many will rely on HALEU to achieve smaller designs, longer operating cycles, and increased efficiencies over current technologies. They will be essential to our efforts to decarbonize while meeting growing energy demand. As these technologies move forward, the Department of Energy and NNSA have programs to work with willing industrial partners to assess the risk and enhance the safety, security, and safeguards of their designs.”

The Science authors also called on the U.S. Nuclear Regulatory Commission (NRC) and the IAEA to change the way they categorize HALEU fuel. Under the NRC’s current categorization, even large quantities of HALEU are now considered category II, which means that security measures focus on the early detection of theft. The authors want weapons-relevant quantities of HALEU reclassified as category I, the same as for quantities of weapons-grade plutonium or highly enriched uranium sufficient to make a bomb. Category I would require much tighter security, focusing on the prevention of theft.

Nesbit scoffs at the proposal, citing the difficulties of heisting perhaps a metric tonne of nuclear material. “Blindly applying all of the baggage that goes with protecting nuclear weapons to something like this is just way overboard,” he says.

But Lee, who performed experiments with HALEU fuel in the 1980s, agrees with his colleagues. “Dick Garwin and Frank von Hipple [and the other authors of the Science paper] have raised some proper questions,” he declares. “They’re saying the NRC should take more precautions. I’m all for that.”