Nuclear weapon

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

A nuclear weapon is a weapon which derives its destructive force from the nuclear reactions of nuclear fission and/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 on two occasions, both 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 a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of the weapons, which resulted in the immediate deaths of at least 120,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics 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 that time, nuclear weapons have been detonated on over two thousand occasions, mostly for testing purposes, chiefly by the following seven countries: the United States, Soviet Union, France, United Kingdom, People's Republic of China, India and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse).

Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airbourne delivery systems and appears to have an extensive nuclear program (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has now stated that it will abandon all of its nuclear weapons programs); Ukraine may possess an obsolete Soviet-era nuclear stockpile due to a post-Soviet administrative error; and Iran is believed to be attempting to develop nuclear capabilities (for more information see List of countries with nuclear weapons).

Nuclear weapons in modern times have been used primarily as a method of creating a strategic threat. For example, the worry that North Korea will use nuclear weapons has dominated the relations between the United States and North Korea.

Apart from their use as weapons, nuclear explosives have been proposed for various non-military uses.


Types of nuclear weapons

The two basic fission weapon designs.
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. The so-called "Teller-Ulam design" is thought to be responsible for megaton range thermonuclear 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.

Effects of a nuclear explosion

Main article: Nuclear explosion
A radioactive fireball tops the smoke column from a nuclear weapon test.
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 occur immediately.

The damage from each of the three initial forms of energy release differs with the size (or "yield", see below) of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more significant the impact of this effect. 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.

The energy released by a nuclear weapon is generally measured by the explosive power of an equivalent amount of trinitrotoluene, known as the weapon's yield. The yield of nuclear weapons may be rated as equivalent to several kilotons or megatons of TNT. The first fission weapons had yields measurable in the tens of kilotons, while the largest practical hydrogen bombs had yields around 10 megatons. In practice, nuclear weapon yields will vary significantly, from fractional kiloton weapons designed for tactical use on the battlefield (eg. the man-portable Davy Crockett warheads developed by the United States), to the record Tsar Bomba created by the Soviet Union which had a theoretical maximum design yield of around a hundred megatons.

Although a nuclear weapon is capable of causing the same destruction as conventional explosives through the effects of blast and thermal radiation, it does so by releasing much larger amounts of energy in a much shorter period of time. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, and would be present for any explosion of the same magnitude.

In human terms, nuclear weapons are enormously destructive. A weapon with a ten-megaton yield can destroy most of the buildings of a modern city, while a weapon with a hundred-megaton yield (although the deployment of such a weapon would be considered impractical) would set wooden structures and forests alight up to 60-100 miles (100-160 km) from ground zero1. A nuclear weapon detonated in the upper atmosphere will also generate an electromagnetic pulse which can disrupt or disable electronic communications and instruments over a wide area, causing more difficulties for those who survive the effects of a detonation.

Since most of the effects of nuclear weapons are blast, thermal, or fallout, well-known civil defense efforts could greatly reduce the total loss of life in a nuclear war.

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)), though they were not used in a tactical manner.

Basic methods of delivery for nuclear weapons are:

Gravity bombs

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

A MIRVed missile (such as the LG-118A Peacekeeper) can hold multiple nuclear warheads on one missile bus.
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. Some ballistic missiles may have a range of tens to hundreds of kilometers, while larger intercontinental ballistic missiles (ICBMs) or submarine-launched ballistic missiles (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 technologies. This set the stage for the use of Multiple independently targetable reentry 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 missile 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

Cruise missiles have a shorter range than ICBMs, but would be harder for an enemy to detect or intercept.
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 Davy Crockett 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 atomic bombing of Hiroshima.
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 Russian Federation 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.

There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the US or Russia launching its weapons in retaliation for a supposed attack.[1] As of 2005, there are estimated to be at least 29,000 nuclear weapons held by at least seven countries, though 96% of these are in the possession of just two (the United States and the Russian Federation).


Eniwetok nuclear detonation tests (info)
Video clips of three test nuclear explosions in Eniwetok, Marshall Islands.
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Related topics

Weapons of
mass destruction
By Type
Biological weapons
Chemical weapons
Nuclear weapons
Radiological weapons
By Country
China (PRC)
North Korea
South Africa
Taiwan (ROC)
United Kingdom
United States
Nuclear weaponry
Nuclear countries
Nuclear proliferation
Nuclear strategy
Nuclear terrorism
Nuclear warfare
Nuclear weapon history
Nuclear weapon design
Nuclear explosion
Nuclear testing
See also
Dirty bomb
Radiological warfare


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