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At a nuclear security summit last week, word leaked of a perplexing deal: The British will send 1,500 pounds of “nuclear waste” to the US, who in turn will give France and Belgium useful nuclear material to make into medical isotopes for diagnosing cancer. Huh?* Terrible deal!*, as one self-proclaimed dealmaker might say. Losers!

So how to explain this trade? Well first, those initial news articles got it wrong. What the US is getting is not actually nuclear waste, and what Europe is getting is not primarily for making medical isotopes. The deal is not a quid pro quo but a single plank in the US’s longstanding policy of nonproliferation. In other words, this is all about keeping bomb-grade uranium out of the hands of terrorists.

Let’s unpack it.

On the northern coast of Scotland, Britain keeps tons of nuclear material, including highly enriched uranium, the bomb-grade stuff. Highly enriched uranium can be “downblended” into low-enriched uranium, which can still be used for power plants but not for thermonuclear war. The UK doesn’t have the technical ability to downblend this particular batch of highly enriched uranium—which, again, is not “waste”—so the US is doing it for them.

But why did articles citing an unnamed British source call it “waste?” Well, past efforts to ship uranium in Scotland to the US have run afoul of environmental groups concerned about safety in transit, and portraying it as a trade of waste for cancer-fighting material certainly *sounds *better. “It could be someone trying to put an extra good spin on it: ‘We’re saving lives here!,’” Alan Kuperman, a University of Texas professor and coordinator of the Nuclear Proliferation Prevention Project, wrote in an email. (An official press release does call it “nuclear material,” not “waste.”)

And about that whole fighting cancer thing—that’s not the whole story either. Indeed, highly enriched uranium is used to make technetium-99m, a radioactive tracer that aids in the diagnosis of illnesses including cancer and heart disease. But that’s the secondary reason France and Belgium want the bomb-grade material. What this is really about is the US reluctantly meting out supplies of highly enriched uranium—not for weapons programs, of course, but for perfectly peaceful scientific research.

The Mercedes of Nuclear Reactors

Uranium enriched enough to make weapons is also uranium enriched enough to fuel very powerful nuclear reactors—reactors useful for anything from finding trace toxins in the environment to doping silicon in high-performance electronics. In the 1970s, nonproliferation experts looked around and saw hundreds of reactors around the world using highly enriched uranium. Many of these research reactors sat on academic campuses—not exactly the most secure locations.

And so began the US’s push to get those risky reactors to switch to low-enriched uranium. Fewer stockpiles of bomb-grade material meant fewer locations to secure against terrorists or spies. The trick to converting the reactors is reformulating reactor fuel, which is made by embedding uranium in a so-called dispersion matrix made of other material. “You know fruit jello? The fruit is the uranium and the jello is the dispersion matrix,” says Kuperman. The fuel must have the same overall proportion of uranium-235, the naturally occurring but rare isotope that sustains the chain reaction releasing so much energy. Highly enriched uranium is, in fact, enriched with uranium-235, making it the ideal starting material.

Hundreds of reactors have since converted to using low-enriched uranium. Only a handful that use highly enriched uranium remain in the western world: six in the United States and four in the Europe.

The remaining ones are the most stubborn. They required fuel most dense with uranium-235. “The Mercedes of the research reactor world still use highly enriched uranium,” says Miles Pomper, a senior fellow at the James Martin Center for Nonproliferation Studies. Two of them, in Belgium and in France, are getting their highly enriched uranium from the US with this deal. The one in Belgium also uses uranium-235 to make medical isotopes, hence the “fighting cancer” story. But it is possible to make medical isotopes without highly enriched uranium: Australia already has a reactor that only uses low-enriched uranium for medical isotopes, and the Netherlands is converting their's.

A Short Leash

While investigating the Paris attacks, Belgian police made the unsettling discovery that one suspect had security footage of a Belgian nuclear facility employee. And previously, two nuclear plant workers had left Belgium to join ISIS. “It’s probably woken up some people to the threat,” says Kuperman. That’s in part why he has petitioned the US government to send only one year’s supply of highly enriched uranium at a time to Belgium and France. “As other people have said, keep them on a short lease,” he says.

The US has leverage as one of the few suppliers of uranium-235 in the world. The Schumer Amendment in 1992, for example, banned exports of highly enriched uranium to reactors unless a plan to convert to low-enriched uranium was in place. That’s why another reactor in Germany is frozen out of the US’s supply. (The fourth and last such European reactor is going through its stockpile.)

So ultimately, the two parts of the swap of nuclear materials between the US, UK, Belgium, and France are not directly related. But they are both parts of the same nonproliferation policy. Highly enriched uranium has been so persistent because it is so useful for science and medicine. And this year’s deal is a reminder of the uneasy overlap between physics and national security.