The International Thermonuclear Experimental Reactor (ITER)—which aims to prove the commercial viability of fusion power—is slated to be built in France by 2016. Here’s how it will work.

Two hydrogen isotopes—deuterium and tritium—are heated in a doughnutshaped chamber to more than 100 million °C, at which point they form a plasma, or ionized gas. Superconducting coils surrounding the chamber wall create a magnetic field that confines the plasma, forcing the deuterium and tritium nuclei to collide; when they do, they fuse to form helium nuclei, releasing neutrons. The mass of a helium nucleus and a neutron is less than that of a deuterium nucleus and a tritium nucleus; the excess mass is converted into a tremendous amount of energy, which is imparted to the helium nuclei and the neutrons. When the fast-moving neutrons hit the “blanket” that lines the chamber, they generate heat within it, which can be harnessed to produce electricity. Since there’s no plentiful natural source of tritium, ITER will test ways of using some of the neutrons to create tritium from lithiumbearing materials in the blanket.