This year a multinational team is scheduled to begin constructing ITER, the International Thermonuclear Experimental Reactor, a project designed to demonstrate that fusion can generate almost limitless amounts of electricity without the risks and long-lived radioactive waste linked with nuclear fission reactors.

Fusion reactions power the sun and stars. ITER aims to re-create that energy here on Earth by heating hydrogen gas to 100 million˚C. In this inferno, hydrogen ions smash together, fuse into a larger ion—helium—and release energy. Physicists have already built reactors that can achieve fusion, but none has yet produced more energy than it consumes.

If all goes as planned, ITER will change that. By building a bigger, more powerful reactor, scientists hope to produce 500 megawatts of power from just 50 megawatts input. Fuel in the form of the hydrogen isotope deuterium is extracted from water, and the small amount of radioactive waste it yields decays to a safe level in decades. In contrast, today’s nuclear fission reactors generate waste that can stay hot for thousands of years.

The project will take 10 years and cost $6 billion to complete, with the goal of producing fusion electricity by the middle of the century. There’s just one holdup. As we went to press, the international collaboration backing ITER—China, the European Union, Japan, South Korea, Russia and the U.S.—was still arguing over where to build it. For the past year it has been split between sites in France and Japan (China and Russia favor France; the U.S. and South Korea back Japan), and no amount of negotiation seems able to break the impasse. It’s an inauspicious start to a collaborative endeavor second in scope only to the International Space Station.

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