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 India—have detonated, or have attempted to detonate, 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; such a device 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 weapons delivery

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.
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 re-entry 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.

Fallout is the residual radiation hazard from a nuclear explosion, so named because it "falls out" of the atmosphere into which it is spread during the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes. This radioactive dust, consisting of hot particles, is a kind of radioactive contamination. It can lead to contamination of the food chain. Fallout can also refer to the dust or debris that results from the nuclear explosion.

Above info taken from: http://en.wikipedia.org/wiki/Atomic_bomb#Weapons_delivery