Nuclear weapon

Nuclear weapon, powerful explosive weapon whose power derives from nuclear energy. There are 2 main types of such weapons: fission bombs and fusion, or thermonuclear, bombs. The bombs dropped by the United States on the Japanese cities Hiroshima and Nagasaki in 1945, during World War II, were fission bombs. Fusion bombs (also called hydrogen bombs, or H-bombs) were developed in the early 1950s and have never been used in warfare. Various countries today have fission bombs, but only 5 countries—the United States, the former Soviet Union, Britain, France, and China—have produced and tested fusion bombs.

See also: Nuclear energy.