Nuclear explosion
nuclear test.]]
A
nuclear explosion ('''nuclear detonation''') has occurred:
Potential other applications (not yet applied, or the idea was considered but abandoned):
Nuclear explosions are associated with "
mushroom clouds" although mushroom clouds can occur with ground based chemical explosions and it is possible to have an air burst nuclear explosion without these clouds. Nuclear explosions produce large amounts of
radiation and can also produce large amounts of radioactive materials.
In the case of a
fizzle the effects of the first part of the
nuclear chain reaction prevent it to be continued, by blowing the material apart too soon.
Effects of a nuclear explosion
The energy released from a nuclear weapon comes in four primary categories:
- Blast
- Thermal radiation
- Ionizing radiation
- Residual radiation (fallout)
The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated.
The energy produced by a nuclear explosive is millions of times more per gram than that produced by a chemical (conventional) explosive and the temperatures reached are in the tens of millions of degrees.
The energy of a nuclear explosive is initially released in the form of gamma rays and
neutrons. When there is a surrounding material such as air, rock, or water, this radiation interacts with the material, rapidly heating it to an equilibrium temperature in about a
microsecond. The hot material emits thermal radiation, mostly soft
X-rays, which accounts for 75% of the energy of the explosion. In addition, the heating and vaporization of the surrounding material causes it to rapidly expand and the
kinetic energy of this expansion accounts for almost all of the remaining energy.
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.
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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 a
fireball and its associated effects.
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.
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 pounds/square inch or 0.35 Atm).
The blast wind may exceed several hundred km/h. 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).
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/cm
2, 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.
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.
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, blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.
Surviving A Nuclear Blast, A Historical Perspective
Nuclear blast range vary. In the 1960's and 1970's, survival techniques taught in schools to children speculate that its blast range could be as wide as 40 to 50 square miles. In the event of such a blast, survivors were instructed to be prepared by stocking canned food goods and plastic trash bags in bomb or fallout shelters. The concept of bottled water was not available at the time, but one can assume that bottled water would have been added to the list of important items. If the blast were within visual range, eyes were to be covered quickly; shelter and medical assistance sought immediately (if available).
See also
References
- Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. PDF Version
- NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear), Departments of the Army, Navy, and Air Force, Washington, D.C., 1996.
- Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945. (Smyth Report)
- The Effects of Nuclear War, Office of Technology Assessment (May 1979).
External links
Category:Nuclear weapons
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de:Atombombenexplosion