Nuclear weapon

The  of the  , rose some 18 km (11 mi) above the .
The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the epicenter.

A nuclear weapon is a weapon that derives its energy from the nuclear reactions of fission and/or fusion. Even the smallest nuclear weapons are more powerful than all but the largest of conventional explosives. A ten-megaton weapon can destroy an entire city. A hundred-megaton weapon (although judged impractical) would set wooden houses and forests afire in a circle 60-100 miles (100-160 km) in diameter1. Nuclear weapons have been delivered only twice in the history of warfare – both in the ending days of World War II ; the first such bombing was 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, and the last nuclear bombing occurred three days later; this second bomb was a plutonium implosion-type device code-named "Fat Man", dropped on the city of Nagasaki.

Testing accounts for the rest of more than two thousand nuclear detonations, chiefly by the following seven nations: the U.S., Soviet Union, France, United Kingdom, China, India and Pakistan.

The declared nuclear powers are the United States, Russia, the United Kingdom, France, the People's Republic of China, India, and Pakistan. In addition, Israel has both modern aerial delivery systems and there is evidence of an extensive nuclear program, though such has never been publically admitted (see: Israel and weapons of mass destruction). North Korea has stated recently that it has nuclear capabilities; Ukraine may possess an obsolete Soviet nuclear stockpile due to a post-Cold War clerical error. Iran and others may be attempting to develop indigenous nuclear capabilities. See the list of countries with nuclear weapons for more details.

Non-weaponized nuclear explosives have been proposed for various non-military uses.


Types of nuclear weapons

Missing image
The two basic fission weapon designs.

Main article: Nuclear weapon design

The simplest nuclear weapons derive their energy from nuclear fission. A mass of fissile material is rapidly assembled into a critical mass, in which a chain reaction begins and grows exponentially, releasing tremendous amounts of energy. This is accomplished either by shooting one piece of subcritical material into another, or compressing a subcritical mass into a state of supercriticality. A major challenge in all nuclear weapon designs is ensuring that a significant fraction of the fuel is consumed before the weapon destroys itself. These are colloquially known as atomic bombs.

More advanced nuclear weapons take advantage of nuclear fusion to derive more energy. In such a weapon, the X-ray thermal radiation from a nuclear fission explosion is used to heat and compress a capsule of tritium, deuterium, or lithium, in which fusion occurs, releasing even more energy. These weapons, colloquially known as hydrogen bombs, can be many hundreds of times more powerful than fission weapons.

More exotic nuclear weapons also exist, designed for special purposes. 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) can result in the production of exceptionally large quantities of radioactive contamination. A nuclear weapon may also be designed to permit as many neutrons as possible to escape; such a weapon is called a neutron bomb. Hypothetical antimatter weapons, which would use matter-antimatter reactions, would not technically be nuclear weapons (as they would not be using energy derived from either nuclear fission or fusion), but bear noting due to a potentially higher potential energy by weight than conventional or nuclear explosives.

Effects of a nuclear explosion

Main article: Nuclear explosion

Missing image
A radioactive fireball tops the smoke column from a nuclear weapon test.

The energy released from a nuclear weapon comes in four primary categories:

  • Blast—40-60% of total energy
  • Thermal radiation—30-50% of total energy
  • Ionizing radiation—5% of total energy
  • Residual radiation (fallout)—5-10% of total energy

The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy, while the other three forms of energy release are immediate.

The energy released by nuclear weapons is generally measured in its equivalence to kilotons and megatons—thousands and millions of tons, respectively—of TNT. The first fission weapons had yields measureable in the tens of kilotons, while the largest practical hydrogen bombs had yields around 10 megatons. In practice, nuclear weapon yields can be highly variable, from the sub-kiloton power of the man-portable Davy Crockett warheads developed by the United States, to the impractical 54 megaton Tsar Bomba created by the Soviet Union as a display of political power.

The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives. The primary difference is that nuclear weapons are capable of releasing much larger amounts of energy at once. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, but would be present for any explosion of the same magnitude.

The damage done by each of the three initial forms of energy release differs with the size of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more important this effect becomes. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation.

Weapons delivery

The term strategic nuclear weapons is generally used to denote large weapons which would be used to destroy large targets, such as cities. Tactical nuclear weapons are smaller weapons used to destroy specific military, communications, or infrastructure targets. By modern standards, the bombs that destroyed Hiroshima and Nagasaki in 1945 may perhaps be considered tactical weapons (with yields between 13 and 22 kilotons (54 to 92 TJ)), although modern tactical weapons are considerably lighter and more compact.

Basic methods of delivery for nuclear weapons are:

Gravity bombs

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The first nuclear weapons, such as the "Fat Man" device, were large and cumbersome gravity bombs.

No nuclear weapon qualifies as a "wooden bomb" — US military slang for a bomb that is trouble-free, maintenance-free, and danger-free under all conditions. Gravity bombs are designed to be dropped from planes, which requires that the weapon can withstand vibrations and changes in air temperature and pressure during the course of a flight. Early weapons often had a removable core for safety, installed by the air crew during flight. They had to meet safety conditions, to prevent accidental detonation or dropping. A variety of types also had to have a fuse to initiate detonation. US nuclear weapons that met these criteria are designated by the letter "B" followed, without a hyphen, by the sequential number of the "physics package" it contains. The "B61", for example, was the primary bomb in the US arsenal for decades.

Various air-dropping techniques exist, including toss bombing, parachute-retarded delivery, and laydown modes, intended to give the dropping aircraft time to escape the ensuing blast.

The first gravity nuclear bombs could only be carried by the B-29 Superfortress. The next generation of weapons were still so big and heavy that they could only be carried by bombers such as the B-52 Stratofortress and V bombers, but by the mid-1950s smaller weapons had been developed that could be carried and deployed by simple fighter-bombers.

Ballistic missile warheads

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A MIRVed missile (such as the LG-118A Peacekeeper) can hold multiple nuclear warheads on one missile bus.

Missiles using a ballistic trajectory usually deliver a warhead over the horizon. Mobile ballistic missiles may have a range of tens to hundreds of kilometers, while larger ICBMs or SLBMs may use suborbital or partial orbital trajectories for intercontinental range. Early ballistic missiles carried a single warhead, often of megaton-range yield. Due to accuracy considerations, this kind of high yield was considered necessary in order to ensure a particular target's destruction.

Since the 1970s modern ballistic weapons have seen the development of far more accurate targeting techologies. This set the stage for the use of multiple independent re-entry vehicles (MIRVs) with up to a dozen independently targetable warheads, usually in the hundreds-of-kilotons-range yield, on one ballistic platform. This allows for a number of advantages over a missle with a single warhead. It allows a single missile to strike a variety of apparently unrelated targets, or it can inflict maximum damage on a single target by encircling the target with warheads, as well as providing such an onslaught of warheads in conjunction with other tactical weapons that any form of defensive technology would be rendered useless. Soviet plans in the '70s were said to entail dropping one MIRV based warhead every ninety seconds to three minutes on major US targets for up to an hour.

Missile warheads in the American arsenal are indicated by the letter "W"; for example, the W61 missile warhead would have the same physics package as the B61 gravity bomb described above, but it would have different environmental requirements, and different safety requirements since it would not be crew-tended after launch and remain atop a missile for a great length of time.

Cruise missile warheads

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Cruise missiles have a shorter range than ICBMs, but would be harder for an enemy to detect or intercept.

A jet engine or rocket-propelled missile that flies at low altitude using an automated guidance system (usually inertial navigation, sometimes supplemented by either GPS or mid-course updates from friendly forces) to make them harder to detect or intercept could carry a nuclear warhead. Cruise missiles have shorter range and smaller payloads than ballistic missiles, so their warheads are smaller and less powerful. Rather than multiple warheads, which would have to be dropped separately as though the cruise missile were itself a bomber, each cruise missile carries its own warhead, although the B-1 Lancer bomber was designed to carry in its bomb-bay a rotating fixture for cruise missiles which resembles a set of MIRV warheads. Conventional cruise missiles sometimes use cluster munition payloads, though. Cruise missiles may be launched from mobile launchers on the ground, from naval ships, or from aircraft.

There is no letter change in the US arsenal to distinguish the warheads of cruise missiles from those for ballistic missiles.

Other delivery systems

The  artillery shell was the smallest nuclear weapon developed by the USA.
The Davy Crockett artillery shell was the smallest nuclear weapon developed by the USA.

Other potential delivery methods include artillery shells, mines such as Blue Peacock, and nuclear depth charges and torpedoes for anti-submarine warfare. An atomic mortar was also tested. In the 1950s the U.S. developed small nuclear warheads for air defense use, such as the Nike Hercules. Further developments of this concept, some with much larger warheads, showed promise as anti-ballistic missiles. Most of the United States' nuclear air-defense weapons were out of service by the end of the 1960s, and nuclear depth bombs were taken out of service by 1990. However, the USSR (and later Russia) continues to maintain anti-ballistic missiles with nuclear warheads. Small, two-man portable tactical weapons (erroneously referred to as suitcase bombs), such as the Special Atomic Demolition Munition, have been developed, although the difficulty of balancing yield and portability limits their military utility.

See list of nuclear weapons for a list of the designs of nuclear weapons fielded by the various nuclear powers.


Main article: History of nuclear weapons

The aftermath of the .
The aftermath of the atomic bombing of Hiroshima.

The first nuclear weapons were created by the United States, with assistance from the United Kingdom, 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 first develop them, 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 espionage obtained from spies in the USA, and both the USA and USSR developed fusion weapons 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 state of Russia inherited the weapons of the former USSR, and along with the USA pledged to reduce their stockpile for increased international safety. Nuclear proliferation has continued, though, with Pakistan testing their first weapons in 1998, and the state of North Korea claiming to have developed nuclear weapons in 2004. 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.


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