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A nuclear weapon is an explosive device that derives its destructive force from the nuclear reaction of fission or from a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter; a modern thermonuclear weapon weighing little more than a ton can produce an explosion comparable to the detonation of more than a billion kilograms of conventional high explosive. The largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of approximately 50 megatons of TNT and even small nuclear devices with yields equivalent to only a few thousand tons of TNT can devastate a city. Because of their extreme destructive power, nuclear devices are weapons of mass destruction.
In the history of warfare, only two nuclear weapons have been detonated offensively, both by the United States of America during the closing days of World War II. The first was detonated on the morning of 6 August 1945, when the United States dropped a (uranium) gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second was detonated three days later when the United States dropped a plutonium implosion-type device code-named "Fat Man" on the city of Nagasaki. These bombings resulted in the immediate deaths of around 120,000 people from injuries sustained from the explosion and acute radiation sickness, and even more deaths over time from long-term effects of radiation. The use of these weapons was and remains controversial. (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 purposes and demonstration purposes. The only countries known to have detonated such weapons are (chronologically) the United States, the Soviet Union, the United Kingdom, France, the People's Republic of China, India, Pakistan, and North Korea. (For more information see List of states with nuclear weapons.)
Israel also most likely possesses nuclear weapons. Israel, who has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction), officially maintains a policy of "ambiguity" with respect to its actual possession of nuclear weapons. According to some estimates, it possesses as many as 200 nuclear warheads.
Just one state, South Africa used to have nuclear weapons but disarmed completely. South Africa secretly developed a small nuclear arsenal, but disassembled it within 1991 and joined the NPT.
Iran currently stands accused by the United States of intending to develop nuclear weapons capabilities, though its government states that its acknowledged nuclear activities, such as uranium enrichment, are for non-weapons purposes.
Apart from their use as weapons, nuclear explosives have been tested and used for various non-military uses. Synthetic elements, such as einsteinium and fermium, created by neutron bombardment of uranium and plutonium during thermonuclear explosions, were discovered in the aftermath of the first thermonuclear bomb test.
The first nuclear weapons were created in the United States by an international team, including many displaced scientists from central Europe, which included Germany, 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. The first test was conducted on July 16, 1945 at a site near Alamogordo, New Mexico. 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 (thermonuclear 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 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 four other countries (India, South Africa, Pakistan, and Israel) developed or acquired 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 2004, Pakistani nuclear national hero Abdul Qadeer Khan confessed to selling enrichment technology to Iran, Libya, and North Korea, as well as a nuclear weapons design to Libya, in a massive, international proliferation ring, that had been operating for about 20 years. This was the largest proliferation scandale for decades. 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). Additionally, since 9/11 increased attention has been given to the threat of nuclear terrorism, whereby non-state actors manage to develop, purchase, or steal nuclear arms and detonate them against civilians. Post-Cold War discussions of nuclear weapons have focused on the fact that the "rationality" of nuclear deterrence, credited with the lack of use of nuclear weapons during the Cold War, may not apply in a world with only one superpower, or a world where the nuclear actors are stateless.
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. 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 2006, there are estimated to be at least 27,000 nuclear weapons held by at least eight countries, 96 percent of them in the possession of the United States and Russia.
Nuclear weapons have been at the heart of many national and international political disputes, 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. Dozens of movies, books, television shows, plays, and other cultural productions have been made with nuclear weapons as either the explicit subject or an implied leitmotiv.
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 called fission bombs. In early news accounts, pure fission weapons were called atomic bombs or A-bombs, a misnomer since the energy comes only from the nucleus of the atom.
In fission weapons, a mass of fissile material (enriched uranium or plutonium) is assembled into a supercritical massâ€”the amount of material needed to start an exponentially growing nuclear chain reactionâ€”either by shooting one piece of sub-critical material into another (the "gun" method), or by compressing a sub-critical sphere of material using chemical explosives to many times its original density (the "implosion" method). The latter approach is considered more sophisticated than the former, and only the latter approach can be used if plutonium is the fissile material.
A major challenge in all nuclear weapon designs is to ensure 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. These weapons are called thermonuclear bombs. In early accounts, weapons involving fusion were called hydrogen bombs or H-bombs, a misnomer since their destructive energy comes mostly from fission. Because fusion material cannot go overcritical no matter the amount used, and because fusion weapons can be staged, these kind of weapons may be made significantly more powerful than fission bombs. Only six countriesâ€”United States, Russia, United Kingdom, People's Republic of China, France and Indiaâ€”have detonated hydrogen bombs. (This Indian claim remains controversial.)
Thermonuclear bombs work by using the energy of a fission bomb in order to compress and heat fusion fuel. In the Teller-Ulam design, which accounts for all multi-megaton yield hydrogen bombs, this is accomplished by placing a fission bomb and fusion fuel (tritium, deuterium, or lithium deuteride) in proximity within a special, radiation-reflecting container. When the fission bomb is detonated, gamma and X-rays emitted at the speed of light first compress the fusion fuel, then heat it to thermonuclear temperatures. The ensuing fusion reaction creates enormous numbers of high-speed neutrons, which then can induce fission in materials which normally are not prone to it, such as depleted uranium. Each of these components is known as a "stage," with the fission bomb as the "primary" and the fusion capsule as the "secondary." In large hydrogen bombs, about half of the yield, and much of the resulting nuclear fallout, comes from the final fissioning of depleted uranium.  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 (50 megatons) of TNT. Most thermonuclear weapons are considerably smaller than this, due for instance to practical constraints in fitting them into the space and weight requirements of missile warheads.
There are many 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 fusion bomb. In the boosted bomb, the neutrons produced by the fusion reactions serve primarily to increase the efficiency of the fission bomb. Some weapons are designed for special purposes; a neutron bomb is a thermonuclear weapon that yields a relatively small explosion but a relatively large amount of neutron radiation; such a device could theoretically be used to cause massive casualties while leaving infrastructure mostly intact and creating a minimal amount of fallout. 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 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 of a country to respond to a nuclear attack with one of its 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 it 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 mutually 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.
It has been claimed that the threat of potentially suicidal terrorists possessing nuclear weapons (a form of nuclear terrorism) complicates the decision process. Mutually assured destruction may not be effective against an enemy who expects to die in a confrontation, as they may feel they will be rewarded in a religious afterlife as martyrs and would not therefore be deterred by a sense of self-preservation. Further, if the initial act is from rogue groups of individuals instead of a nation, there is no fixed nation or fixed military targets to retaliate against. It has been argued, especially after the September 11, 2001 attacks, that this complication is the sign of the next age of nuclear strategy, distinct from the relative stability of the Cold War.
Nuclear weapons deliveryâ€”the technology and systems used to bring a nuclear weapon to its target—is an important aspect of nuclear weapons relating both to nuclear weapon design and nuclear strategy. Additionally, developing and maintaining delivery options is among the most resource-intensive aspects of nuclear weapons: according to one estimate, deployment of nuclear weapons accounted for 57% of the total financial resources spent by the United States in relation to nuclear weapons since 1940.
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.
Governance, control, and law
Because of the immense military power they can confer, the political control of nuclear weapons has been a key issue for as long as they have existed; in most countries the use of nuclear force can only be authorized by the head of government. In the United States, the President and the Secretary of Defense, acting as the National Command Authority, must jointly authorize the use of nuclear weapons.
In the late 1940s, lack of mutual trust was preventing the United States and the Soviet Union from making ground towards international arms control agreements, but by the 1960s steps were being taken to limit both the proliferation of nuclear weapons to other countries and the environmental effects of nuclear testing. The Partial Test Ban Treaty (1963) restricted all nuclear testing to underground nuclear testing, to prevent contamination from nuclear fallout, while the Nuclear Non-Proliferation Treaty (1968) attempted to place restrictions on the types of activities which signatories could participate in, with the goal of allowing the transference of non-military nuclear technology to member countries without fear of proliferation. In 1957, the International Atomic Energy Agency (IAEA) was established under the mandate of the United Nations in order to encourage the development of the peaceful applications of nuclear technology, provide international safeguards against its misuse, and facilitate the application of safety measures in its use. In 1996, many nations signed and ratified the Comprehensive Test Ban Treaty which prohibits all testing of nuclear weapons, which would impose a significant hindrance to their development by any complying country.
Additional treaties have governed nuclear weapons stockpiles between individual countries, such as the SALT I and START I treaties, which limited the numbers and types of nuclear weapons between the United States and the Soviet Union.
Nuclear weapons have also been opposed by agreements between countries. Many nations have been declared Nuclear-Weapon-Free Zones, areas where nuclear weapons production and deployment are prohibited, through the use of treaties. The Treaty of Tlatelolco (1967) prohibited any production or deployment of nuclear weapons in Latin America and the Caribbean, and the Treaty of Pelindaba (1964) prohibits nuclear weapons in many African countries. As recently as 2006 a Central Asian Nuclear Weapon Free Zone was established amongst the former Soviet republics of Central Asia prohibiting nuclear weapons.
In the middle of 1996, the International Court of Justice, the highest court of the United Nations, issued an Advisory Opinion concerned with the "Legality of the Threat or Use of Nuclear Weapons". The court ruled that the use or threat of use of nuclear weapons would violate various articles of international law, including the Geneva Conventions, the Hague Conventions, the UN Charter, and the Universal Declaration of Human Rights.
- Specifically the US B83 nuclear bomb.
- Trinity Site Pamphlet. White Sands Missile Range. URL accessed on 2007-08-15.
- Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986.
- Richelson, Jeffrey. Spying on the bomb: American nuclear intelligence from Nazi Germany to Iran and North Korea. New York: Norton, 2006.
- See, for example: Feldman, Noah. "Islam, Terror and the Second Nuclear Age," New York Times Magazine (29 October 2006).
- Forden, Geoffrey False Alarms on the Nuclear Front. Nova Online. URL accessed on 2006-03-05.
- Norris, Robert S., and Hans M. Kristensen. "Global nuclear stockpiles, 1945-2006", Bulletin of the Atomic Scientists 62, no. 4 (July/August 2006), 64-66.
- Weart, Spencer R. Nuclear Fear: A History of Images. Cambridge, Harvard University Press, 1988; Boyer, Paul S. By the bombâ€™s early light: American thought and culture at the dawn of the atomic age. New York: Pantheon, 1985.
- The best overall printed sources on nuclear weapons design are: Hansen, Chuck. U.S. Nuclear Weapons: The Secret History. San Antonio, TX: Aerofax, 1988; and the more-updated Hansen, Chuck. Swords of Armageddon: U.S. Nuclear Weapons Development since 1945. Sunnyvale, CA: Chukelea Publications, 1995.
- Sublette, Carey The Nuclear Weapon Archive. URL accessed on 2007-03-07.
- Stephen I. Schwartz, ed., Atomic Audit: The Costs and Consequences of U.S. Nuclear Weapons Since 1940. Washington, D.C.: Brookings Institution Press, 1998. See also Estimated Minimum Incurred Costs of U.S. Nuclear Weapons Programs, 1940-1996, an excerpt from the book.
- 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.
- Current World Nuclear Arsenals
- Current World Nuclear Arsenals has estimates of nuclear arsenals in the respective countries.
- 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 â€“ 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 â€” Provided by New Scientist.
- Congressional Research Service (CRS) Reports regarding Nuclear weapons
- Video archive of US, Soviet, UK, Chinese and French Nuclear Weapon Testing at sonicbomb.com
- The Manhattan Project: Making the Atomic Bomb at AtomicArchive.com
- Los Alamos National Laboratory â€” History (U.S. nuclear history)
- Race for the Superbomb, PBS website on the history of the H-bomb
- U.S. nuclear test photographs from the DOE Nevada Site Office
- U.S. nuclear test film clips from the DOE Nevada Site Office
- Hans Bethe talking about his shock of seeing the after effects of Hiroshima on Peoples Archive.
- Nuclear weapon simulator for several major US cities, from Federation of American Scientists
- HYDESim: High-Yield Detonatonation Effects Simulator Another Nuclear weapon simulator with a few more features based on the "The Effects of Nuclear Weapons", 3rd Edition, by Samuel Glasstone and Philip J. Dolan.
- Fallout Calculator for various regions, from Federation of American Scientists
- Example scenarios â€“ Two scenarios of a nuclear explosion on two United States cities, from AtomicArchive.com
- Effects of Nuclear weapons These tables describe the effects of various nuclear blast sizes. All figures are for Template:convert/mi/hTemplate:convert/test/Aon winds. Thermal burns represent injuries to an unprotected person. The legend describes the data.
- Effects of nuclear weapons from AtomicArchive.com
- The Effects of Nuclear Weapons by Samuel Glasstone and Philip J. Dolan (1977 edn.) â€” an official text of the US government on weapons effects which is generally considered definitive
- "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.
- Nuclear Ambitions - The World's Deadly Arsenal - Independent news on issues relating to nuclear weapons and disarmament by the news agency Inter Press Service