Thermonuclear fusion is a phenomenon that occurs when light atomic nuclei, such as those of hydrogen, collide at extremely high speeds. The collision transforms the nuclei into heavier nuclei, such as those of helium, releasing massive heat energy through the transformation process. But such reactions can't occur unless the atomic nuclei are accelerated to at least 1,000 kilometers per second at a temperature of 100 million degrees or more. The International Thermonuclear Experimental Reactor (ITER) project plans to use heavy hydrogen and tritium as fuel in its experimental nuclear fusion reactor. Theoretically, thermonuclear fusion using 1 gram of this fuel generates energy equivalent to that produced by burning 8 tons of oil. In addition to requiring far less fuel to produce more power than conventional nuclear power generation, which uses the reverse process of splitting heavier nuclei into lighter ones, thermonuclear fusion has the advantage of using fuel that is abundantly available.

nuclear reaction in which light atoms are coalesced into heavier atoms with the release of energy.

occurs in a gas of sufficient temperature that its atoms in collision will fuse in significant numbers (see nuclear fusion). A thermonuclear process is purported to provide the power radiated by the stars.