Nuclear Weapons: Nuclear weapons may be of the fission type (atomic weapons) or the fusion type (thermonuclear or hydrogen weapons). Fission weapons get their destructive power from the splittings of atomic nuclei. Only three kinds of atoms are known to be suitable for fissioning in such weapons. These atoms are of the uranium (U) isotopes U-235 and U-238 and of the plutonium (Pu) isotope, Pu-239. An accelerating uncontrolled chain reaction occurs when, for example, two pieces of U-235 come together and exceed the critical mass. Thermonuclear weapons get their power from the fusion atomic nuclei under intense heat. The nuclei fused in thermonuclear weapons are of the hydrogen isotopes, deuterium and tritium. Fusion reactions require temperatures equal to, or greater than, those found in the sun’s core. The only practical way to achieve such temperature si by means of a fission explosion. Thus, thermonuclear explosions are triggered by an implosion-type fission device. (In the implosion method, a subcritical mass is made supercritical by compressing it into a smaller volume.) The first nuclear weapons were two fission bombs used by the United States during World War II (1939-1945). In the war, one was dropped on each of Japanese cities of Hiroshima and Nagasaki. Nuclear explosive devices can have a wide variety of yields. Some older bombs had yields of about 20 megatons, or 1,540 Hiroshima bombs. A megaton is the amount of energy released by 907,000 metric tons of TNT. Today, because of the higher accuracy of missiles, most nuclear devices have yields of less than 1 megaton.