Next-generation nuclear reactors are heating up a debate over whether their fuel could be used to make bombs, jeopardizing efforts to prevent the proliferation of nuclear weapons. 

Uranium in the fuel could theoretically be used to develop a nuclear weapon. Older reactors use such low concentrations that they don’t really pose a weapons proliferation threat. But advanced reactors would use higher concentrations, making them a potential target of terrorist groups or other countries wanting to take the fuel to develop their own nuclear weapons, some experts warn.

They argue that the US hasn’t prepared enough to hedge against that worst-case scenario and are calling on Congress and the Department of Energy to assess potential security risks with advanced reactor fuel.

Other experts and industry groups still think it’s unfeasible for such a worst-case scenario to materialize. But the issue is starting to come to a head as nuclear reactors become a more attractive energy source, garnering a rare show of bipartisan support in Congress.

Nuclear reactors generate electricity without producing the greenhouse gas emissions causing climate change. And unlike solar and wind energy, which fluctuate with the weather and time of day, nuclear reactors provide a steady source of electricity similar to gas and coal power plants. Earlier this month, President Joe Biden signed bipartisan legislation into law meant to speed the development of next-generation nuclear reactors in the US by streamlining approval processes.

Next-generation reactors are smaller and modular, meant to make them cheaper and easier to build than old-school nuclear power plants. Aside from generating electricity, small reactor designs could also be used to produce high-temperature heat for industrial facilities.

The US Nuclear Regulatory Commission (NRC) certified an advanced small modular reactor design for the first time last year. And we’re likely still years away from seeing commercial plants in action. But if the US ever wants to get there, it’ll also have to build up a supply chain for the fuel those advanced reactors would consume. The Inflation Reduction Act includes $700 million to develop that domestic fuel supply.

Today’s reactors generally run on fuel made with a uranium isotope called U-235. Naturally occurring uranium has quite low concentrations of U-235; it has to be “enriched” — usually up to a 5 percent concentration of U-235 for a traditional reactor. Smaller advanced reactors would run on more energy-dense fuel that’s enriched with between 5 to 20 percent U-235, called HALEU (short for high-assay low-enriched uranium).

That higher concentration is what has some experts worried. “If the weapons usability of HALEU is borne out, then even a single reactor would pose serious security concerns,” says a policy analysis penned by a group of nuclear proliferation experts and engineers published in the journal Science last month (including an author credited with being one of the architects of the first hydrogen bomb).

Fuel with a concentration of at least 20 percent is considered highly enriched uranium, which could potentially be used to develop nuclear weapons. With HALEU designs reaching 19.75 percent U-235, the authors argue, it’s time for the US to think hard about how safe the next generation of nuclear reactors would be from malicious intent.

“We need to make sure that we don’t get in front of ourselves here and make sure that all the security and safety provisions are in place first before we go off and start sending [HALEU] all around the country,” says R. Scott Kemp, associate professor of nuclear science and engineering and director of the MIT Laboratory for Nuclear Security and Policy.

That 20 percent threshold goes back to the 1970s, and bad actors ostensibly have more information and computational tools at their disposal to develop weapons, Kemp and his coauthors write in the paper. It might even be possible to craft a bomb with HALEU well under the 20 percent threshold, the paper contends.

Fortunately, that would still be incredibly difficult to do. “This is not minor theft,” says Charles Forsberg, a principal research scientist at MIT and previously a corporate fellow at Oak Ridge National Laboratory. A group might have to steal a couple years’ worth of fuel from a small advanced reactor to make the kind of bomb described in the paper, he says.

Even with a working weapons design, he says it would take a sophisticated team of at least several hundred people to go through all of the steps to turn that fuel into uranium metal for a viable weapon. “Unless they’re a whole lot better than I am, and the colleagues I work with, a subnational group [like a terrorist group] doesn’t have a chance,” he tells The Verge.

An adversarial nation would have more capacity than a small group. But he still doesn’t think it would be worth it for them. With their resources, they could go ahead and build a plant to produce weapons-grade uranium, typically enriched above 90 percent U-235. 

A more credible risk, he says, would be if another country starts to produce and stockpile HALEU for future reactors — but actually has more nefarious intentions in mind. Once they’re enriching uranium for HALEU, they’ve already started to build up their capacity to reach weapon-grade uranium. “That’s the concern we have with any nation-state that decides to produce HALEU,” Forsberg says. “They’ve taken some of the steps … they’re edging right up to the race line.”

Aside from asking Congress for an updated security assessment of HALEU, the paper suggests setting a lower enrichment limit for uranium based on new research or ramping up security measures for HALEU to more closely match those for weapons-usable fuels. 

Unlike the authors of the Science paper, Forsberg thinks the appropriate precautions are already in place to keep next-generation nuclear reactors and HALEU secure in the US. The security risks have been well understood and discussed for decades, he says, although much of that is classified information. That’s part of what makes it difficult to assuage fears. 

“The views from the authors of this study do not present any new information that should discourage the development and deployment of HALEU in accordance with already strict requirements set by U.S. and international regulatory bodies,” Jennifer Uhle, vice president of technical and regulatory services at the Nuclear Energy Institute, said in an emailed statement to The Verge.

Some of the fears surrounding nuclear energy in the wake of disasters in Chernobyl in 1986 and Fukushima in 2011 have faded with the need to find energy sources that don’t contribute to climate change and with promises of more advanced technologies. But not everyone’s convinced, and the security concerns cropping up with HALEU dovetail with other issues critics take with nuclear energy.

“Unless there’s a really good reason to switch to fuels that pose greater risks of nuclear proliferation, then it’s irresponsible to pursue those,” says Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists and another author of the paper. Lyman has also raised concerns about the radioactive waste from nuclear reactors over the years. “There is no good reason.”