Weapons in which an explosion is caused by nuclear fission, nuclear fusion, or a combination of both. In the fission bomb (atomic bomb or A-bomb) two subcritical masses (see critical mass) of a fissile material (uranium–235 or plutonium–239) are brought together by a chemical explosion to produce one supercritical mass. The resulting nuclear explosion is typically in the kiloton range with temperatures of the order 10⁸ K being reached. The fusion bomb (thermonuclear weapon, hydrogen bomb, or H-bomb) relies on a nuclear-fusion reaction, which becomes self-sustaining at a critical temperature of about 35×10⁶ K. Hydrogen bombs consist of either two-phase fission–fusion devices in which an inner fission bomb is surrounded by a hydrogenous material, such as heavy hydrogen (deuterium) or lithium deuteride, or a three-phase fission–fusion–fission device, which is even more powerful. The megaton explosion produced by such a thermonuclear reaction has not yet been used in war. A special type of fission–fusion bomb is called a neutron bomb, in which most of the energy is released as high-energy neutrons. This neutron radiation destroys people but provides less of the shock waves and blast that destroy buildings.