Nuclear weapon

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Nuclear weapons
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A nuclear weapon^[a] is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon^[b]), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.

Nuclear bombs have had yields between 10 tons (the W54) and 50 megatons for the Tsar Bomba (see TNT equivalent). Yields in the low kilotons can devastate cities. A thermonuclear weapon weighing as little as 600 pounds (270 kg) can release energy equal to more than 1.2 megatons of TNT (5.0 PJ).^[1] Apart from the blast, effects of nuclear weapons include firestorms, extreme heat and ionizing radiation, radioactive nuclear fallout, an electromagnetic pulse, and a radar blackout.

The first nuclear weapons were developed by the Allied Manhattan Project during World War II. Their production continues to require a large scientific and industrial complex, primarily for the production of fissile material, either from nuclear reactors and nuclear reprocessing plants or from enrichment facilities. Nuclear weapons have been used twice in war, both times by the United States against the Japanese cities of Hiroshima and Nagasaki in 1945. Nuclear deterrence, sometimes based on mutually assured destruction, aims to prevent nuclear warfare through the potential to inflict unacceptable damage, including the danger of uncontrolled escalation to nuclear holocaust. A nuclear arms race for weapons and their delivery systems was a defining component of the Cold War.

Strategic nuclear weapons are targeted against civilian, industrial, and military infrastructure, while tactical nuclear weapons are intended for battlefield use. Strategic weapons led to the development of dedicated intercontinental ballistic missiles, submarine-launched ballistic missile, and nuclear strategic bombers, collectively known as the nuclear triad. Tactical weapons options have included shorter-range ground-, air-, and sea-launched missiles, nuclear artillery, nuclear land mines, nuclear torpedoes, and nuclear depth charges, but they have become less salient since the end of the Cold War.

As of 2025^[update], nine countries possess nuclear weapons and delivery systems, and six more agree to nuclear sharing. Nuclear weapons are weapons of mass destruction, and their control is a focus of international security through measures to prevent their proliferation, limit their numbers, or eliminate them altogether. Key international agreements and organizations include the Non-Proliferation Treaty, the Comprehensive Test Ban Treaty and Organization, the International Atomic Energy Agency, the Treaty on the Prohibition of Nuclear Weapons, and Nuclear-weapon-free zones.

Nuclear weapons testing is used to validate and improve designs and to understand the effects of nuclear explosions. Between 1945 and 2017, over 2,000 tests were conducted, ranging from fully weaponized to experimental designs. A science-based stockpile stewardship is usually adopted by countries that have ceased testing.

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