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Nuclear weapon

A nuclear weapon is a weapon of enormous destructive potential, deriving its energy from nuclear reactions.

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Types of weapons

Fission bombs derive their power from nuclear fission, where heavy nuclei (uranium or plutonium) split into lighter elements when bombarded by neutrons (produce more neutrons which bombard other nuclei, triggering a 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 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.

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 of their primary fuel, or thermonuclear weapons because fusion reactions require extremely high temperatures for a chain reaction to occur.

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. Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the best general term for all types of these explosive devices is "nuclear weapon".

Advanced Thermonuclear Weapons Designs

The largest modern weapons include a fissionable outer shell of uranium. The intense fast neutrons from the fusion stage of the weapon will cause even natural (that is unenriched) uranium to fission, increasing the yield of the weapon many times.


A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bomb which are small thermonuclear weapons 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 the principle destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays are rendered less effective. 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).

For details, refer to "Nuclear weapon design".

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, the other three forms of energy release are immediate.

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 of 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 iteself 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 reradiated 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.

Blast Damage

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 pounds/square inch or 0.35 Atm).

The blast wind may exceed several hundred km/hr. 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 (Kt) yield; 3.2 km for 100 Kt; and 15.0 km for 10 Megatons (Mt).




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.

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.


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

At altitudes above the majority of the air, the x-rays ionize the upper air, moving large numbers of electrons. The moving electric charge causes a single wide-frequency radio pulse. The pulse is powerful enough so that most long metal objects would act as antennas, and generate high voltages when the pulse passes. These voltages and the associated high currents could destroy unshielded electronics and even many wires. There are no known biological effects of EMP except from failure of critical medical and transportation equipment. The ionized air also disrupts radio traffic that would normally bounce from the ionosphere.

One can shield ordinary radios and car ignition parts by wrapping them completely in aluminum foil, or any other form of Faraday cage. Of course radios cannot operate when shielded, because broadcast radio waves can't reach them.


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.


Nuclear fallout



















For more details see: nuclear explosion

Weapons delivery

The term strategic nuclear weapons is often 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 targets such as military, communications, infrastructure.

Basic methods of delivery are:

  • bombers such as the B-52 and V bomber
  • ballistic missiles - a missile using a ballistic trajectory involving a significant ascent and descent including suborbital and partial orbital trajectories. Most commonly ICBM and SLBM. Modern weapons also deliver Multiple Independent Re-entry Vehicles (MIRV) each of which carries a warhead and allows a single launched missile to strike a handful of targets.
  • artillery shells - for tactical use
  • hand held (suitcase)

Related articles


  • Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. (| Available Online (http://nuketesting.enviroweb.org/nukeffct/))
  • NATO Handbook on the Medical Aspects of NBC Defecsive Operations (Part I - Nuclear), Departments of the Army, Navy, and Air Force, Washington, D.C., 1996. (Available Online (http://www.fas.org/nuke/guide/usa/doctrine/dod/fm8-9/1toc.htm))
  • Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945. (Available Online (http://nuketesting.enviroweb.org/hew/Smyth/index))
  • The Effects of Nuclear War, Office of Technology Assessment (May 1979) Available Online (http://www.fas.org/nuke/intro/nuke/7906/index)
  • Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, New York, 1986
  • Carey Sublette's High Energy Weapons Archive is a reliable source of information and has links to other sources: http://nuketesting.enviroweb.org/hew/ or http://gawain.membrane.com/hew/ (Mirror)
  • The Federation of American Scientists (http://fas.org) provide solid information on weapons of mass destruction, including nuclear weapons ( http://fas.org/nuke/ )
  • http://www.oism.org/nwss/ The public-domain text Nuclear War Survival Skills is one of the best sources available on how to survive a nuclear attack.
  • http://www.radius-defense.com/nuclear.php has a table of effects radii, but could probably be replaced with a site which has the info and wasn't selling shelters to make a profit


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