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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 fission ("atomic") bomb test released the same amount of energy as approximately 20,000 tons of TNT (see Trinity (nuclear test)). The first thermonuclear ("hydrogen") bomb test released the same amount of energy as approximately 10,000,000 tons of TNT.<ref>See Trinity (nuclear test) and Ivy Mike.</ref>

A thermonuclear weapon weighing little more than {{safesubst:#invoke:convert|convert}} can produce an explosive force comparable to the detonation of more than 1.2 million tons (1.1 million tonnes) of TNT.<ref>Specifically the 1970 to 1980 designed and deployed US B83 nuclear bomb, with a yield of up to 1.2 megatons.</ref> 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 6 August 1945, the U.S. Army Air Forces detonated a uranium gun-type fission bomb over the Japanese city of Hiroshima; three days later, on 9 August, 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 it does not acknowledge having them.<ref name="nuclearweapons1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>See also Mordechai Vanunu</ref> One state, South Africa, fabricated nuclear weapons in the past, but as its apartheid regime was coming to an end, it disassembled its arsenal, acceded to the Nuclear Non-Proliferation Treaty, and accepted full-scope international safeguards.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Federation of American Scientists estimated there were more than 17,000 nuclear warheads worldwide as of 2012, with around 4,300 of them considered "operational" (ready for immediate use).<ref name="nuclearweapons1" />


Nuclear weapon sections
Intro  Types  Weapons delivery  Nuclear strategy  Governance, control, and law  Controversy  Costs and technology spin-offs  Non-weapons uses  See also  Notes and references  Bibliography  External links  

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