Nuclear fusion is a type of nuclear reaction in which two atomic nuclei combine to form a heavier nucleus, releasing energy. For a fusion reaction to take place, the nuclei, which are positively charged, must have enough kinetic energy to overcome their electrostatic force of repulsion. Thermonuclear fusion of deuterium and tritium will produce a helium nucleus and an energetic neutron. This is one basis of the Hydrogen Bomb, which employs a brief, uncontrolled thermonuclear fusion reaction. A great effort is now under way to harness thermonuclear fusion as a source of power.

See Hydrogen Bomb, Thermonuclear.