Nuclear Weapons

A nuclear weapon derives its destructive force from nuclear reactions of fusion or fission. 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, two nuclear weapons have been detonated — both by the United States, during the closing days of World War II. The first event occurred 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 event occurred three days later when, again, 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 and even more over time because of injuries sustained and long-term 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 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.
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. According to some estimates, it possesses as many as 200 nuclear warheads. Iran currently stands accused by the United Nations 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 states with nuclear weapons).
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 nuclear fission, were discovered in the aftermath of the first hydrogen bomb test.



Types of nuclear weapons

The two basic fission weapon designs:

1.Gun-Type
2.Implosion Assembly Method


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 sub-critical material into another (the "gun" method), or by compressing a sub-critical sphere of material 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 used.
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, and can be over a thousand times more powerful than fission bombs as fusion reactions release much more energy per unit of mass than fission reactions. 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 India—have detonated hydrogen bombs.
Hydrogen 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, and 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 component 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 (megatons) of TNT. Most hydrogen bombs are considerably smaller than this, though, due to constraints in fitting them into the space and weight requirements of missile warheads.
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. 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 nuclear weapon that yields a relatively small explosion but a relatively large amount of prompt 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.