What is "nuclear weapon"

Nuclear weapon \Nu"cle*ar wea"pon\, n. A weapon of great explosive power, such as an atomic bomb or a hydrogen bomb, which depends for most of its explosive power on the release of energy from within atomic nuclei by a nuclear reaction. A fission weapon or a fusion weapon. The term includes atomic shells for cannon.

n. A weapon that derives its energy from the nuclear reactions of either fission or fusion.

n. a weapon of mass destruction whose explosive power derives from a nuclear reaction

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission ( fission bomb) or a combination of fission and fusion ( thermonuclear weapon). Both reactions release vast quantities of energy from relatively small amounts of matter. The first test of a fission ("atomic") bomb released the same amount of energy as approximately . The first thermonuclear ("hydrogen") bomb test released the same amount of energy as approximately .

A thermonuclear weapon weighing little more than can produce an explosive force comparable to the detonation of more than . A nuclear device no larger than traditional bombs can devastate an entire city by blast, fire, and radiation. Nuclear weapons are considered weapons of mass destruction, and their use and control have been a major focus of international relations policy since their debut.

Nuclear weapons have been used twice in nuclear warfare, both times by the United States against Japan near the end of World War II. On August 6, 1945, the U.S. Army Air Forces detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the U.S. Army Air Forces detonated a plutonium implosion-type fission bomb codenamed " Fat Man" over the Japanese city of Nagasaki. The bombings resulted in the deaths of approximately 200,000 civilians and military personnel from acute injuries sustained from the explosions. The ethics of the bombings and their role in Japan's surrender remain the subject of scholarly and popular debate.

Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated on over two thousand occasions for the purposes of testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are (chronologically by date of first test) the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, the People's Republic of China, India, Pakistan, and North Korea. Israel is also believed to possess nuclear weapons, though in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium and the Netherlands are nuclear weapons sharing states.

The nuclear non-proliferation treaty aimed to reduce the spread of nuclear weapons, but its effectiveness has been questioned, and political tensions remained high in the 1970s and 1980s. As of 2016, 16,000 nuclear weapons are stored at sites in 14 countries and many are ready for immediate use. Modernisation of weapons continues to occur.