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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, while the largest can destroy an entire city. Nuclear weapons have been employed only twice in warfare- first on the morning of August 6, 1945, when the United States dropped a uranium gun-type device entitled Little Boy on the Japanese city of Hiroshima, and three days later a plutonium implosion-type device named Fat Man on the city of Nagasaki during World War II. 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 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

Common types

Main article: Nuclear weapon design

Fission bombs

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). With each of those splits, an amount of energy thousands of times greater than that available from a chemical reaction is released. 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 (implode) 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. A more crude design for such a weapon is to have two sub-critical amounts of uranium-235 simply shot into each other inside a gun barrel. This approach, used in the weapon dropped on Hiroshima during World War II, is conceptually easier but inefficient and inherently more dangerous to maintain than an implosion weapon.

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

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.

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.

Thermonuclear devices can be phenomenally energetic; easily capable of releasing a thousand times the energy of a fission bomb (megaton range). Consequently, the power of a fusion bomb can achieve staggering levels, representing the highest power levels achievable by humans. For instance, the Tsar bomba released 50 megatons of energy, almost all produced by its final fusion stage. Since 50 Mt is 2.1x1017 J the power produced during the burn is around 5.3x1024 watts (5.3 yottawatts). This represents a power just greater than one percent of the entire power output of the Sun (3.86x10^26 watts)!

Dirty bombs

Main article: Dirty bomb

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.

Cobalt bombs

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, which produces major radioactive contamination. In general this type of weapon is 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). To be useful for salting, the parent isotopes must be abundant in the natural element, and the neutron-bred radioactive product must be a strong emitter of penetrating gamma rays.

The primary purpose of this weapon is to create extremely radioactive fallout to deny a region to an advancing army, a sort of wind-deployed mine-field. No cobalt or other salted bomb has ever been atmospherically tested, and as far as is publicly known none has ever been built. In light of the ready availability of fission-fusion-fission bombs, it is unlikely any special-purpose fallout contamination weapon will ever be developed. The British did test a bomb that incorporated cobalt as an experimental radiochemical tracer (Antler/Round 1, 14 September 1957). This 1 kt device was exploded at the Tadje site, Maralinga range, Australia. The experiment was regarded as a failure and not repeated.

The thought of using cobalt, which has the longest half-life of the feasible salting materials, caused Leó Szilárd to refer to the weapon as a potential doomsday device. With a 5yr half-life people would have to remain shielded underground for many years, effectively wiping out humanity. However this would require a massive (unrealistic) amount of such bombs, yet the public heard of it and there were numerous stories involving a single bomb wiping out the planet.

Neutron bombs

Main article: Neutron bomb

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).

Antimatter bombs

Main article: Antimatter weapon

Though weapons using matter-antimatter reactions would not technically be nuclear weapons (as they would not be using energy derived from either nuclear fission or fusion), they bear noting due to a potential higher potential energy by weight than conventional or nuclear explosives. As an antimatter annihilation event would produce similar radiological effects as a nuclear weapon, they are often classified together. The annihilation of a single gram of antimatter would release 90TJ (terajoules ) of energy, more than a 20KT nuclear device. Fortunately, the extreme difficulty in creating, capturing, and holding antimatter particles dictate that such a quantity would take at least tens of millions of years to capture, even in the unlikely event a containment mechanism was developed. Barring a revolution in physics, this relegates antimatter weapons to the annals of science-fiction.

Effects of a nuclear explosion

Main article: 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.
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 delivered an energy of 12,000 tons of , leveling buildings and killing over 100,000.
The bombing of Hiroshima delivered an energy of 12,000 tons of TNT, leveling buildings and killing over 100,000.

Blast Damage

The high temperatures and pressures cause gas to move outward radially in a thin, dense shell called "the hydrodynamic front." The front acts like a piston that pushes against and compresses the surrounding medium to make a spherically expanding shock wave. At first, this shock wave is inside the surface of the developing fireball, which is created in a volume of air by the X-rays. However, within a fraction of a second the dense shock front obscures the fireball, making the characteristic double pulse of light seen from a nuclear detonation.

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 overpressures of only 35.5 kilopascals (kPa) (5 lbf/in² or 0.35 atm).

The blast wind may exceed several hundred kilometers per hour. The range for blast effects increases with the explosive yield of the weapon. In a typical air burst, these values of overpressure and wind velocity noted above will prevail at a range of 0.7 km for 1 kiloton of TNT (kt) yield; 3.2 km for 100 kt; and 15.0 km for 10 megatons (Mt).

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.
Positive and negative blast pressures.

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.

Acting on the human body, the shock waves cause pressure waves through the tissues. These waves mostly damage junctions between tissues of different densities (bone and muscle) or the interface between tissue and air. Lungs and the gut, which contain air, are particularly injured. The damage causes severe hemorrhaging or air embolisms, either of which can be rapidly fatal. The overpressure estimated to damage lungs is about 68.9 kPa. Some eardrums would probably rupture around 22 kPa (0.2 atm) and half would rupture between 90 and 130 kPa (0.9 to 1.2 atm).

Blast Winds: The drag energies of the blast winds are proportional to the cubes of their velocities multiplied by the durations. These winds may reach several hundred kilometers per hour.

Thermal radiation

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.

There are two types of eye injuries from the thermal radiation of a weapon:

Flash blindness is caused by the initial brilliant flash of light produced by the nuclear detonation. More light energy is received on the retina than can be tolerated, but less than is required for irreversible injury. The retina is particularity susceptible to visible and short wavelength infrared light, since this part of the electromagnetic spectrum is focused by the lens on the retina. The result is bleaching of the visual pigments and temporary blindness for up to 40 minutes.

A retinal burn resulting in permanent damage from scarring is also caused by the concentration of direct thermal energy on the retina by the lens. It will occur only when the fireball is actually in the individual's field of vision and would be a relatively uncommon injury. Retinal burns, however, may be sustained at considerable distances from the explosion. The apparent size of the fireball, a function of yield and range will determine the degree and extent of retinal scarring. A scar in the central visual field would be more debilitating. Generally, a limited visual field defect, which will be barely noticeable, is all that is likely to occur.

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 the how long the thermal pulse lasts and the thickness and moisture content of the target. Near ground zero where the light exceeds 125 joules/cm2, 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.

Electromagnetic pulse (EMP)

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.

Ionizing radiation

About 5% of the energy released in a nuclear air burst is in the form of neutrons, gamma rays, alpha particles, and electrons moving at incredible speeds. 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 neutron radiation serves to transmute the surrounding matter, often rendering it radioactive. When added to the dust of radioactive material released by the bomb itself, a large amount of radioactive material is released into the environment. This form of radioactive contamination is known as nuclear fallout and poses the primary risk of exposure to ionizing radiation for a large nuclear weapon.

Nuclear fallout

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 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 and GAR-11 Nuclear Falcon missile)
  • 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).

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

No nuclear weapon qualifies as a "wooden bomb" - US slang for one trouble-free, maintenance-free, and danger-free under all conditions. This method of delivery requires that the weapon be capable of withstanding 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. Also, they had to meet safety conditions were they dropped accidentally. They also had to have a fuze for a variety of types for 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 main such 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 weapons could only be carried by the B-29. 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 missile warheads

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. 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.

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

Cruise missile warheads

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

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.

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