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A nuclear weapon is a weapon which derives its destructive force from nuclear reactions of fission or fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives, and a single weapon is capable of destroying an entire city.
In the history of warfare, nuclear weapons have been used only twice, both during the closing days of World War II. The first event occurred on the morning of August 6, 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when the United States dropped a plutonium implosion-type device code-named "Fat Man" on the city of Nagasaki. The use of these weapons, which resulted in the immediate deaths of around 100,000 to 200,000 people and even more over time, was and remains controversial — critics around the world charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war (see Atomic bombings of Hiroshima and Nagasaki for a full discussion).
Since the Hiroshima and Nagasaki bombings, nuclear weapons have been detonated on over two thousand occasions for testing and demonstration purposes. The only countries known to have detonated such weapons are (chronologically) the United States, Soviet Union, United Kingdom, France, People's Republic of China, India, Pakistan, and North Korea.
Various other countries may hold nuclear weapons but have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction), though it officially maintains a policy of "ambiguity" with respect to its actual possession of nuclear weapons. Iran currently stands accused by a number of governments of attempting to develop nuclear capabilities, though its government claims that its acknowledged nuclear activities, such as uranium enrichment, are for peaceful purposes. South Africa also secretly developed a small nuclear arsenal, but disassembled it in the early 1990s. (For more information see List of countries with nuclear weapons.)
The first nuclear weapons were created in the United States by an international team including many displaced scientists from central Europe with assistance from the United Kingdom and Canada, during World War II as part of the top-secret Manhattan Project. While the first weapons were developed primarily out of fear that Nazi Germany would develop them first, they were eventually used against the Japanese cities of Hiroshima and Nagasaki in August 1945. The Soviet Union developed and tested their first nuclear weapon in 1949, based partially on information obtained from Soviet espionage in the United States. Both the U.S. and USSR would go on to develop weapons powered by nuclear fusion (hydrogen bombs) by the mid-1950s. With the invention of reliable rocketry during the 1960s, it became possible for nuclear weapons to be delivered anywhere in the world on a very short notice, and the two Cold War superpowers adopted a strategy of deterrence to maintain a shaky peace.
Nuclear weapons were symbols of military and national power, and nuclear testing was often used both to test new designs as well as to send political messages. Other nations also developed nuclear weapons during this time, including the United Kingdom, France, and China. These five members of the "nuclear club" agreed to attempt to limit the spread of nuclear proliferation to other nations, though at least three other countries (India, South Africa, Pakistan, and most likely Israel) developed nuclear arms during this time. At the end of the Cold War in the early 1990s, the Russian Federation inherited the weapons of the former USSR, and along with the U.S., pledged to reduce their stockpile for increased international safety. Nuclear proliferation has continued, though, with Pakistan testing their first weapons in 1998, and North Korea performing a test in 2006. In January 2005, Pakistani metallurgist Abdul Qadeer Khan confessed to selling nuclear technology and information of nuclear weapons to Iran, Libya, and North Korea in a massive, international proliferation ring. On October 9, 2006, North Korea claimed it had conducted an underground nuclear test, though the very small apparent yield of the blast has led many to conclude that it was not fully successful (see 2006 North Korean nuclear test).
Nuclear weapons have been at the heart of many national and international political disputes and have played a major part in popular culture since their dramatic public debut in the 1940s and have usually symbolized the ultimate ability of mankind to utilize the strength of nature for destruction.
There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the U.S. or USSR/Russia launching its weapons in retaliation for a supposed attack.<ref>Template:Cite web</ref> Additionally, during the Cold War the U.S. and USSR came close to nuclear warfare several times, most notably during the Cuban Missile Crisis. As of 2005, there are estimated to be at least 29,000 nuclear weapons held by at least eight countries, 96 percent of them in the possession of the United States and Russia.
 Types of nuclear weapons
There are two basic types of nuclear weapons. The first are weapons which produce their explosive energy through nuclear fission reactions alone. These are known colloquially as atomic bombs, A-bombs, or fission bombs. In fission weapons, a mass of fissile material (enriched uranium or plutonium) is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction—either by shooting one piece of subcritical material into another, or by compressing a subcritical mass with chemical explosives, at which point neutrons are injected and the reaction begins. A major challenge in all nuclear weapon designs is ensuring that a significant fraction of the fuel is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range between the equivalent of less than a ton of TNT upwards to around 500,000 tons (500 kilotons) of TNT.
The second basic type of nuclear weapon produces a large amount of its energy through nuclear fusion reactions, and can be over a thousand times more powerful than fission bombs. These are known as hydrogen bombs, H-bombs, thermonuclear bombs, or fusion bombs. Only six countries— United States, Russia, United Kingdom, People's Republic of China, France, and possibly India—are known to possess hydrogen bombs. Hydrogen bombs work by utilizing the Teller-Ulam design, in which a fission bomb is detonated in a specially manufactured compartment adjacent to a fusion fuel. The gamma and X-rays of the fission explosion compress and heat a capsule of tritium, deuterium, or lithium deuteride starting a fusion reaction. Neutrons emitted by this fusion reaction can induce a final fission stage in a depleted uranium tamper surrounding the fusion fuel, increasing the yield considerably as well as the amount of nuclear fallout. Each of these components is known as a "stage", with the fission bomb as the "primary" and the fusion capsule as the "secondary". By chaining together numerous stages with increasing amounts of fusion fuel, thermonuclear weapons can be made to an almost arbitrary yield; the largest ever detonated (the Tsar Bomba of the USSR) released an energy equivalent to over 50 million tons (megatons) of TNT, though most modern weapons are nowhere near that large.
There are other types of nuclear weapons as well. For example, a boosted fission weapon is a fission bomb which increases its explosive yield through a small amount of fusion reactions, but it is not a hydrogen bomb. Some weapons are designed for special purposes; a neutron bomb is a nuclear weapon that yields a relatively small explosion but a relatively large amount of prompt radiation; these could theoretically be used to cause massive casualties while leaving infrastructure mostly intact. The detonation of a nuclear weapon is accompanied by a blast of neutron radiation. Surrounding a nuclear weapon with suitable materials (such as cobalt or gold) creates a weapon known as a salted bomb. This device can produce exceptionally large quantities of radioactive contamination. Most variety in nuclear weapon design is in different yields of nuclear weapons for different types of purposes, and in manipulating design elements to attempt to make weapons extremely small.
 Nuclear strategy
Nuclear warfare strategy is a way for either fighting or avoiding a nuclear war. The policy of trying to ward off a potential attack by a nuclear weapon from another country by threatening nuclear retaliation is known as the strategy of nuclear deterrence. The goal in deterrence is to always maintain a second strike status (the ability to respond to a nuclear attack against your country with a nuclear attack of your own) and potentially to strive for first strike status (the ability to completely destroy an enemy's nuclear forces before they could retaliate). During the Cold War, policy and military theorists in nuclear-enabled countries worked out models of what sorts of policies could prevent one from ever being attacked by a nuclear weapon.
Different forms of nuclear weapons delivery (see below) allow for different types of nuclear strategy, primarily by making it difficult to defend against them and difficult to launch a pre-emptive strike against them. Sometimes this has meant keeping the weapon locations hidden, such as putting them on submarines or train cars whose locations are very hard for an enemy to track, and other times this means burying them in hardened bunkers. Other responses have included attempts to make it seem likely that the country could survive a nuclear attack, by using missile defense (to destroy the missiles before they land) or by means of civil defense (using early warning systems to evacuate citizens to a safe area before an attack). Note that weapons which are designed to threaten large populations or to generally deter attacks are known as "strategic" weapons. Weapons which are designed to actually be used on a battlefield in military situations are known as "tactical" weapons.
There are critics of the very idea of "nuclear strategy" for waging nuclear war who have suggested that a nuclear war between two nuclear powers would result in mutual annihilation. From this point of view, the significance of nuclear weapons is purely to deter war because any nuclear war would immediately escalate out of mutual distrust and fear, resulting in mutual assured destruction. This threat of national, if not global, destruction has been a strong motivation for anti-nuclear weapons activism.
Critics from the peace movement and within the military establishment have questioned the usefulness of such weapons in the current military climate. The use of (or threat of use of) such weapons would generally be contrary to the rules of international law applicable in armed conflict, according to an Advisory opinion issued by the International Court of Justice in 1996.
Perhaps the most controversial idea in nuclear strategy is that nuclear proliferation would be desirable. This view argues that unlike conventional weapons nuclear weapons successfully deter all-out war between states, as they did during the Cold War between the U.S. and the Soviet Union. Political scientist Kenneth Waltz is the most prominent advocate of this argument
 Weapons delivery
Historically the first method of delivery, and the method used in the two nuclear weapons actually used in warfare, is as a gravity bomb, dropped from bomber aircraft. This method is usually the first developed by countries as it does not place many restrictions on the size of the weapon, and weapon miniaturization is something which requires considerable weapons design knowledge. It does, however, limit the range of attack, the response time to an impending attack, and the number of weapons which can be fielded at any given time. Additionally, specialized delivery systems are usually not necessary; especially with the advent of miniaturization, nuclear bombs can be delivered by both strategic bombers and tactical fighter-bombers, allowing an air force to use its current fleet with little or no modification. This method may still be considered the primary means of nuclear weapons delivery; the majority of U.S. nuclear warheads, for example, are represented in free-fall gravity bombs, namely the B61.
More preferable from a strategic point of view are nuclear weapons mounted onto a missile, which can use a ballistic trajectory to deliver a warhead over the horizon. While even short range missiles allow for a faster and less vulnerable attack, the development of intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs) has allowed some nations to plausibly deliver missiles anywhere on the globe with a high likelihood of success. More advanced systems, such as multiple independently targetable reentry vehicles (MIRVs) allow multiple warheads to be launched at several targets from any one missile, reducing the chance of any successful missile defense. Today, missiles are most common among systems designed for delivery of nuclear weapons. Making a warhead small enough to fit onto a missile, though, can be a difficult task.
Tactical weapons (see above) have involved the most variety of delivery types, including not only gravity bombs and missiles but also artillery shells, land mines, and nuclear depth charges and torpedoes for anti-submarine warfare. An atomic mortar was also tested at one time by the United States. Small, two-man portable tactical weapons (somewhat misleadingly referred to as suitcase bombs), such as the Special Atomic Demolition Munition, have been developed, although the difficulty to combine sufficient yield with portability limits their military utility.
- Video clips of three test nuclear explosions in Enewetak, Marshall Islands.
 See also
- More technical details
- History of nuclear weapons
- Manhattan Project
- Los Alamos National Laboratory
- Lawrence Livermore National Laboratory
- Nuclear testing
- Soviet atomic bomb project
- Stanislav Petrov
- German nuclear energy project
- Japanese atomic program
- Nuclear and radiation accidents (including nuclear weapons accidents)
- Related technology and science
- Military strategy
- Proliferation and politics
- Nuclear proliferation
- Nuclear Non-Proliferation Treaty
- Comprehensive Test Ban Treaty
- Nuclear disarmament
- International Court of Justice advisory opinion on legality of nuclear weapons
- List of countries with nuclear weapons
- Nuclear weapons and the United States
- Nuclear weapons and the United Kingdom
- List of nuclear weapons
- Popular culture
- Bethe, Hans Albrecht. The Road from Los Alamos. New York: Simon and Schuster, 1991. ISBN 0-671-74012-1
- DeVolpi, Alexander, Minkov, Vladimir E., Simonenko, Vadim A., and Stanford, George S. Nuclear Shadowboxing: Contemporary Threats from Cold War Weaponry. Fidlar Doubleday, 2004.
- Glasstone, Samuel and Dolan, Philip J. The Effects of Nuclear Weapons (third edition). Washington, D.C.: U.S. Government Printing Office, 1977. Available online (PDF).
- NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear). Departments of the Army, Navy, and Air Force: Washington, D.C., 1996.
- Hansen, Chuck. U.S. Nuclear Weapons: The Secret History. Arlington, TX: Aerofax, 1988.
- Hansen, Chuck. The Swords of Armageddon: U.S. nuclear weapons development since 1945. Sunnyvale, CA: Chukelea Publications, 1995. 
- Holloway, David. "Stalin and the Bomb". New Haven: Yale University Press, 1994. ISBN 0-300-06056-4
- The Manhattan Engineer District, "The Atomic Bombings of Hiroshima and Nagasaki" (1946)
- Smyth, Henry DeWolf. Atomic Energy for Military Purposes. Princeton, NJ: Princeton University Press, 1945. (Smyth Report – the first declassified report by the US government on nuclear weapons)
- The Effects of Nuclear War. Office of Technology Assessment, May 1979.
- Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. New York: Simon and Schuster, 1995. ISBN 0-684-82414-0
- Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1986 ISBN 0-684-81378-5
- Weart, Spencer R. Nuclear Fear: A History of Images. Cambridge, MA: Harvard University Press, 1988.
 External links
- Nuclear Weapon Archive from Carey Sublette is a reliable source of information and has links to other sources and an informative FAQ.
- The Federation of American Scientists provide solid information on weapons of mass destruction, including nuclear weapons and their effects
- Alsos Digital Library for Nuclear Issues – contains many resources related to nuclear weapons, including a historical and technical overview and searchable bibliography of web and print resources.
- Everything you wanted to know about nuclear technology — Provided by New Scientist.
- Congressional Research Service (CRS) Reports regarding Nuclear weapons
- The Manhattan Project: Making the Atomic Bomb at AtomicArchive.com
- Los Alamos National Laboratory — History (U.S. nuclear history)
- Race for the Superbomb, PBS website on the history of the H-bomb
- Hans Bethe talking about his shock of seeing the after effects of Hiroshima on Peoples Archive.
- Nuclear weapon simulator for several major US cities
- Fallout Calculator for various regions
- Example Scenarios - Two scenarios of a nuclear explosion on two United States Cities
- Effects of Nuclear weaponsThese tables describe the effects of various nuclear blast sizes. All figures are for 15-mph winds. Thermal burns represent injuries to an unprotected person. The legend describes the data.
- The Effects of Nuclear Weapons
- "The Nuclear Weapons Debate" - OneWorld.net's Perspectives Magazine, May 2005
- "Nuclear Power and Nuclear Weapons: Making the Connections" – an article about the connections between nuclear power and nuclear weapons development by an anti-nuclear group.
- Nuclear War Survival Skills is a public domain text about civil defense.
- IPPNW: International Physicians for the Prevention of Nuclear War – Nobel Peace Prize-winning organization with information about the medical consequences of nuclear weapons, war and militarization.
- Bulletin of the Atomic Scientists – Magazine founded in 1945 by Manhattan Project scientists. Covers nuclear weapons proliferation and many other global security issues. See this page for comprehensive data on nuclear weapons worldwide.
- 50 Facts About U.S. Nuclear Weapons – Largest, smallest, number, cost, etc.
- Nuclear Files.org covers the history of nuclear weapons and explores the political, legal and ethical challenges of the Nuclear Age.
- Union of Concerned Scientists – Nuclear Policy, weapons, testing, technical issues, and arms control.
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