China is building two secret nuclear reactors. Scientists are worried.

On the tiny, nondescript island of Changbiao, in the Fujian province, the China National Nuclear Corporation is building two mysterious nuclear reactors that are drawing plenty of international attention—and concern.

The reactors, which are scheduled to power up in 2023 and 2026, are both a type called China Fast Reactor 600 (CFR-600). This kind of reactor is a “breeder,” meaning its nuclear reaction generates more fuel than it uses. And that’s what has scientists from around the world scratching their heads, according to a new report from Al Jazeera.

The goal with most nuclear power plants is to use as much of their fuel up as possible—not make more. That’s especially true when the reactor produces plutonium, which is easy to turn into nuclear weapons…

In a new paper, experts from the Nonproliferation Policy Education Center (NPEC) say with the amount of weapons-grade plutonium the fast reactors will make, China could have 1,270 nuclear warheads by 2030. That’s as many as the U.S. has in its intercontinental ballistic missile arsenal…

Why would China be stockpiling plutonium behind closed doors? “China is presently engaged in a large nuclear weapons build-up that U.S. intelligence officials publicly estimate will result in at least a doubling (or more) of the size of Beijing’s nuclear arsenal,” the experts explain, and accumulating plutonium gives them even more of these critical resources.