A nuclear weapon is a weapon that derives its energy from nuclear reactions and has enormous destructive power—even the smallest nuclear weapons are much more powerful than most conventional explosives, while the largest can destroy entire metropolitan regions. Nuclear weapons have been used twice for war, when the United States dropped two such bombs on the Japanese cities of Hiroshima and Nagasaki during World War II. They have been used many hundreds of times since then, but only for the nuclear testing undertaken by many countries.
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 almost certainly has nuclear weapons, though it refuses to publicly state whether it possesses them or not (see Israel and weapons of mass destruction). North Korea has claimed to American diplomats that it possesses nuclear weapons (though this has not been substantiated); Ukraine may possess a nuclear stockpile due to a clerical error; and Iran is allegedly developing the capacity to produce its own nuclear arsenal. See the list of countries with nuclear weapons for more details.
Non-weaponized nuclear explosives have also been proposed for various civilian uses.
Types of weapons
- For more technical details see: Nuclear weapon design
Fission bombs derive their power from nuclear fission, where heavy nuclei (uranium or plutonium) split into lighter elements when bombarded by neutrons (producing more neutrons which bombard other nuclei, triggering a nuclear chain reaction). These are historically called atom bombs or A-bombs, though this name is not precise due to the fact that chemical reactions release energy from atomic bonds too, and fusion is no less atomic than fission. Despite this possible confusion, the term atom bomb has still been generally accepted to refer specifically to nuclear weapons, and most commonly to pure fission devices.
In general, fission bombs are powered by using chemical explosives to compress a sub-critical amount of either uranium-235 or plutonium into a dense, super-critical mass, which is then subjected to a source of neutrons. This begins an uncontrollable nuclear chain reaction, and produces a very large amount of energy.
One pound of U-235 can release over 37 million million joules of energy. This is 82 terajoules per kilogram (TJ/kg). A typical duration of the chain reaction is 1 μs, so the power is 82 EW/kg (30 μW or 200 MeV/s per atom; related to the duration of one generation of the chain reaction: 3mW/atom, i.e., the power of a chain reaction just at criticality is 3mW in the case of consecutive fissions, one at a time).
Fusion bombs are based on nuclear fusion where light nuclei such as hydrogen and helium combine together into heavier elements and release large amounts of energy. Weapons which have a fusion stage are also referred to as hydrogen bombs or H-bombs because their fusion fuel is often a form of hydrogen, or thermonuclear weapons because fusion reactions require extremely high temperatures for a chain reaction to occur. This latter name can be somewhat confusing, as thermonuclear reactions can take place in nuclear weapons which are not considered "true" fusion bombs (the United States' George test of 1951 was one such device, the Soviet Union's Joe 4 device was another, both of which were fission bombs utilizing some fusion fuel to increase their yield).
Generally speaking, hydrogen bombs work by having a "primary" device (a fission bomb) detonate and begin the fusion reactions in the "secondary" device (fusion fuel). A virtually limitless number of large "secondaries" can be chained together (each fusion reaction beginning the next) in this fashion, creating weapons with far larger yields than could be achieved with simple fission alone.
Dirty bomb is now a term for a radiological weapon, a non-nuclear bomb that disperses radioactive material that was packed in with the bomb. When the bomb explodes, the scattering of this radioactive material causes radioactive contamination, a health hazard similar to that of nuclear fallout. One of the most publicly stated fears of Western governments since the September 11, 2001 attacks has been the terrorist detonation of a dirty bomb in a populated area. Dirty bombs, similar to other enhanced fallout weapons of more technologically sophisticated design, are area denial weapons that can potentially render an area unfit for habitation for years or decades after the detonation. In the estimation of most analysts, though, the effect would be primarily psychological, and potentially economic if a costly clean-up effort was called for.
Nuclear weapons are often described as either fission or fusion devices based on the dominant source of the weapon's energy. The distinction between these two types of weapon is blurred by the fact that they are combined in nearly all complex modern weapons: a smaller fission bomb is first used to reach the necessary conditions of high temperature and pressure to allow fusion to occur. On the other hand, a fission device is more efficient when a fusion core first boosts the weapon's energy. Finally, a fusion weapon may include a fission core (in addition to being externally compressed by fission explosion) in order to achieve more complete fusion (see nuclear weapon design for some description of all these variants). Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the most accurate general term for all types of these explosive devices is "nuclear weapon."
Advanced thermonuclear weapons designs
The most powerful modern weapons include a fissionable outer shell of uranium. The intense fast neutrons from the fusion stage of the weapon will cause natural (that is unenriched) uranium to fission, increasing the yield of the weapon many times.
The cobalt bomb uses cobalt in the shell, and the fusion neutrons convert the cobalt into cobalt-60, a powerful long-term (5 years) emitter of gamma rays. In general this type of weapon is referred to as a salted bomb and variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months), and cobalt for long term contamination (years). The primary purpose of this weapon is to create excess radioactive fallout making a large region uninhabitable. No cobalt or other salted bomb has been built or tested publicly.
A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bomb, which is a small thermonuclear weapon in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. The X-ray mirrors and shell of the weapon are made of chromium or nickel so that the neutrons are permitted to escape. This intense burst of high-energy neutrons is a highly destructive mechanism, although the bomb will still produce damaging thermal and shock effects, only with a lower magnitude than a standard thermonuclear weapon. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays are less effective against neutrons. They are also more biologically harmful than gamma rays, and this knowledge led some to envision a weapon that would do little physical damage while killing all the people in a certain area (a so-called "landlord bomb"). This appears to be somewhat of an exaggeration, as the bomb would still create at least some significant blast and fire damage. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout (as in the salted bombs discussed above).
Though it would not be a nuclear weapon in the traditional sense of using fission or fusion reactions, there has been some speculation as to the use of antimatter as the source for a weapon of some sort. Antimatter reactions give off more energy even than fusion reactions, and would not produce radioactive nuclear fallout. There have been indications that the U.S. Air Force has pursued research in this direction, but as there are as of yet no technologies for production and storage of antimatter in measurable quantities, the whole affair is viewed by many with considerable scepticism. See Antimatter weapons for more information.
Effects of a nuclear explosion
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.
A radioactive fireball tops the smoke column from a nuclear weapon test.
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.
When a nuclear weapon explodes, the bomb's material comes to an equilibrium temperature in about a microsecond. At this time about 75% of the energy is emitted as primary thermal radiation, mostly soft X-rays. Almost all of the rest of the energy is kinetic energy in rapidly-moving weapon debris. The interaction of the x-rays and debris with the surroundings determines how much energy is produced as blast and how much as light. In general, the denser the medium around the bomb, the more it will absorb, and the more powerful the shockwave will be.
When a nuclear detonation occurs in air near sea-level, most of the soft X-rays in the primary thermal radiation are absorbed within a few feet. Some energy is re-radiated in the ultraviolet, visible light and infrared, but most of the energy heats a spherical volume of air. This forms the fireball.
In a burst at high altitudes, where the air density is low, the soft X-rays travel long distances before they are absorbed. The energy is so diluted that the blast wave may be half as strong or less. The rest of the energy is dissipated as a more powerful thermal pulse.
In 1945 there was some initial speculation among the scientists developing the first nuclear weapons that there might be a possibility of igniting the earth's atmosphere with a large enough nuclear explosion. This was, however, quickly shown to be mathematically unlikely enough to be considered impossible, though the notion has persisted as a rumor for many years.
The explosive yield of a nuclear weapon is expressed in the equivalent mass of trinitrotoluene (TNT), either in kilotons (thousands of tons of TNT) or megatons (million of tons of TNT). Examples of nuclear weapon yields:
- Davy Crockett tactical nuclear weapon: variable yield 0.01-1 kt — mass only 23 kg (51 lb), lightest ever deployed by the United States (same warhead as Special Atomic Demolition Munition)
- Hiroshima's "Little Boy": 12-15 kt — gun type uranium-235 fission bomb (the first of the only two nuclear weapons that have ever been used in warfare)
- Nagasaki's "Fat Man": 20-22 kt — implosion type plutonium-239 fission bomb (the second of the two nuclear weapons used in warfare)
- W-76 warhead 100 kt (10 of these may be in a MIRVed Trident II missile)
- B-61 Mod 3 gravity bomb: 4 yield options ("dial-a-yield"): 0.3 kt, 1.5 kt, 60 kt, and 170 kt
- W-87 warhead: 300 kt (10 of these are in a MIRVed LG-118A Peacekeeper)
- W-88 warhead: 475 kt (8 of these may be in a Trident II missile)
- Castle Bravo device: 15 Mt — largest tested by the US
- EC17/Mk-17, the EC24/Mk-24, and the B41 (Mk41) (largest nuclear weapons ever built by the United States): 25 Mt — gravity bombs carried by B-36 bomber (retired by 1957)
- Tsar Bomba device: 50 Mt — USSR, largest yield explosive device ever, mass of 27 short tons (24 metric tons), in its "full" form it would have been 100 Mt
As a comparison, the Oklahoma City bombing, using a truck-based fertilizer bomb, was a mere 0.002 kt.
The "yield per ton", the amount of weapons yield compared to the mass of the weapon, is for current US weapons 600 kt/t (2.5 TJ/kg) to 2.2 Mt/t (9.2 TJ/kg). By comparison, for the Davy Crockett it was 40 kt/t (0.167 TJ/kg) and for the Tsar Bomba it was 2 Mt/t (8 TJ/kg).
Much of the destruction caused by a nuclear explosion is due to blast effects. Most buildings, except reinforced or blast-resistant structures, will suffer moderate to severe damage when subjected to moderate overpressures. The blast wind may exceed several hundred kilometers per hour. The range for blast effects increases with the explosive yield of the weapon.
Two distinct, simultaneous phenomena are associated with the blast wave in air:
- Static overpressure, i.e., the sharp increase in pressure exerted by the shock wave. The overpressure at any given point is directly proportional to the density of the air in the wave.
- Dynamic pressures, i.e., drag exerted by the blast winds required to form the blast wave. These winds push, tumble and tear objects.
Most of the material damage caused by a nuclear air burst is caused by a combination of the high static overpressures and the blast winds. The long compression of the blast wave weakens structures, which are then torn apart by the blast winds. The compression, vacuum and drag phases together may last several seconds or longer, and exert forces many times greater than the strongest hurricane.
Nuclear weapons emit large amounts of electromagnetic radiation as visible, infrared, and ultraviolet light. The chief hazards are burns and eye injuries. On clear days, these injuries can occur well beyond blast ranges. The light is so powerful that it can start fires that spread rapidly in the debris left by a blast. The range of thermal effects increases markedly with weapon yield.
Since thermal radiation travels in straight lines from the fireball (unless scattered) any opaque object will produce a protective shadow. If fog or haze scatters the light, it will heat things from all directions and shielding will be less effective.
When thermal radiation strikes an object, part will be reflected, part transmitted, and the rest absorbed. The fraction that is absorbed depends on the nature and color of the material. A thin material may transmit a lot. A light colored object may reflect much of the incident radiation and thus escape damage. The absorbed thermal radiation raises the temperature of the surface and results in scorching, charring, and burning of wood, paper, fabrics, etc. If the material is a poor thermal conductor, the heat is confined to the surface of the material.
Actual ignition of materials depends on how long the thermal pulse lasts and the thickness and moisture content of the target. Near ground zero where the light is most intense, what can burn, will. Farther away, only the most easily ignited materials will flame. Incendiary effects are compounded by secondary fires started by the blast wave effects such as from upset stoves and furnaces.
In Hiroshima, a tremendous fire storm developed within 20 minutes after detonation. A fire storm has gale force winds blowing in towards the center of the fire from all points of the compass. It is not, however, a phenomenon peculiar to nuclear explosions, having been observed frequently in large forest fires and following incendiary raids during World War II.
Gamma rays from a nuclear explosion produce high energy electrons through Compton scattering. These electrons are captured in the earth's magnetic field, at altitudes between twenty and forty kilometers, where they resonate. The oscillating electric current produces a coherent EMP (electromagnetic pulse) which lasts about 1 millisecond. Secondary effects may last for more than a second.
The pulse is powerful enough so that long metal objects (such as cables) act as antennas and generate high voltages when the pulse passes. These voltages, and the associated high currents, can destroy unshielded electronics and even many wires. There are no known biological effects of EMP. The ionized air also disrupts radio traffic that would normally bounce off the ionosphere.
One can shield electronics by wrapping them completely in conductive mesh, or any other form of Faraday cage. Of course radios cannot operate when shielded, because broadcast radio waves can't reach them.
The largest-yield nuclear devices are designed for this use. An air burst at the right altitude could produce continent-wide effects.
About 5% of the energy released in a nuclear air burst is in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products.
The intensity of initial nuclear radiation decreases rapidly with distance from the point of burst because the radiation spreads over a larger area as it travels away from the explosion. It is also reduced by atmospheric absorption and scattering.
The character of the radiation received at a given location also varies with distance from the explosion. Near the point of the explosion, the neutron intensity is greater than the gamma intensity, but with increasing distance the neutron-gamma ratio decreases. Ultimately, the neutron component of initial radiation becomes negligible in comparison with the gamma component. The range for significant levels of initial radiation does not increase markedly with weapon yield and, as a result, the initial radiation becomes less of a hazard with increasing yield. With larger weapons, above 50 kt (200 TJ), blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.
The residual radioactive contamination hazard from a nuclear explosion is in the form of radioactive fallout and neutron-induced activity. Residual ionizing radiation arises from:
- Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta and gamma radiation. Approximately 60 grams of fission products are formed per kiloton of yield (14 g/TJ). The estimated activity of this quantity of fission products 1 minute after detonation is equal to that of 1.1 × 1021 Bq (30 gigagrams of radium) in equilibrium with its decay products.
- Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays slowly by the emission of alpha particles and is of relatively minor importance.
- Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by various elements, such as sodium, manganese, aluminum and silicon in the soil. This is a negligible hazard because of the limited area involved.
In an explosion near the surface large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses, mixed with fission products and other radiocontaminants that have become neutron-activated. The larger particles will settle back to the earth's surface near ground zero (depending on wind and weather conditions of course) within 24 hours, while fine particles will rise to the stratosphere and be distributed globally over the course of weeks or months.
Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. In detonations near a water surface, the particles tend to be lighter and smaller and produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding effect causing local rainout and areas of high local fallout.
The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and caesium-137, in the body as a result of ingestion of foods incorporating these radioactive materials. Chemically, both isotopes are recognized as similar to calcium and deposited in bone structure throughout the body. These highly-radioactive substances then interfere with white blood cell production, which is a prime effect of radiation sickness. The hazard of worldwide fallout is much less serious than the hazards which are associated with local fallout.
Blast and thermal injuries in many cases will far outnumber radiation injuries. However, radiation effects are considerably more complex and varied than are blast or thermal effects and are subject to considerable misunderstanding. A wide range of biological changes may follow the irradiation of animals, ranging from rapid death following high doses of penetrating whole-body radiation to essentially normal lives for a variable period of time until the development of delayed radiation effects, in a portion of the exposed population, following low dose exposures.
For more technical details see: nuclear explosion
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: 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. Early weapons were so big and heavy that they could only be carried by bombers such as the B-52 and V bombers, but by the mid-1950s smaller weapons had been developed that could be carried and deployed by fighter-bombers.
- ballistic missiles: Missiles using a ballistic trajectory, usually to 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. Since the 1970s modern ballistic weapons often use multiple independent reentry vehicles (MIRVs) with up to a dozen warheads, usually of kiloton-range yield. This allows a single launched missile to strike a handful of targets, or inflict maximum damage on a single target by encircling the target with warheads.
- cruise missiles: 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. Cruise missiles have shorter range and smaller payloads than ballistic missiles. At present most do not have multiple warheads, although conventional cruise missiles sometimes use cluster munition payloads. Cruise missiles may be launched from mobile launchers on the ground, from naval ships, or from aircraft.
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. Most of the air-defense weapons were out of service by the end of the 1960s, and nuclear depth bombs were taken out of service by 1990. 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.
Nuclear weapons in culture
Nuclear weaponry has become a part of pop culture; the decades post-WWII can be termed the atomic age. The stunning power and the astonishing visual effects have been the topic of art including Andy Warhol's silkscreen Atomic Bomb (1965) and James Rosenquist's F-111 (1964-65) to Gregory Green's mockups of atomic devices and the efforts of artist James Acord to use uranium in his sculptures.
Films featuring nuclear war or the threat of it include Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (1964), Fail-Safe (1964), On the Beach (1959), The Day After (1983), The War Game (1966), When the Wind Blows (1982), Testament (1983), The Terminator (1984), Threads (1985), WarGames (1983), Miracle Mile (1988), By Dawn's Early Light (1990), The Sum of All Fears (2002),Taiyo o nusunda otoko / The Man Who Stole the Sun (1979), True Lies (1994), Broken Arrow (1996), The Peacemaker (1997), and the Planet of the Apes and Mad Max movies. Godzilla (1954) is considered by some to be an analogy to the nuclear weapons dropped on Japan, and was the start of a more general genre of movies about creatures mutated or awakened by nuclear testing. The number of movies with references to nuclear weapons is too large to attempt to list.
Nuclear weapons are a staple element in science fiction novels. The chain reaction type nuclear bomb was predicted in a 1944 science fiction story by Cleve Cartmill titled "Deadline" which caused him to be investigated by the FBI, concerned that there had been a breach of security on the Manhattan Project. The phrase "atomic bomb" dates back even further to H. G. Wells's The World Set Free from 1914, when scientists had discovered that radioactive decay implied potentially limitless energy locked inside of atomic particles (Wells's atomic bombs, however, were only as powerful as conventional explosives, but would continue exploding for days on end). Many of the characteristics of nuclear weapons themselves have played on ages-old human themes and tropes (penetrating rays, persistent contamination, virility, and, of course, apocalypse), giving their standing in popular culture and politics a particularly emotional valence (both positive and negative).
Nuclear weapons are also one of the main targets of peace organisations. The CND (Campaign for Nuclear Disarmament) was one of the main organisations campaigning against the 'Bomb'. Its symbol, a combination of the semaphore symbols for "N" (nuclear) and "D" (disarmament), entered modern popular culture as an icon of peace.
- More Technical Details
- Related Technology and Science
- Military strategy
- Proliferation and Politics
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