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Nuclei


Effects of Nuclear Explosions

Nuclear Explosion Nuclear weapons are similar to those of more conventional types in so far as their destructive action is due mainly to blast or shock. On the other hand, there are several basic differences between nuclear and high-explosive weapons. In the first place, nuclear explosions can be many thousands (or millions) of times more powerful than the largest conventional detonations. Second, a fairly large proportion of the energy in a nuclear explosion is emitted in the form of light and heat, generally referred to as "thermal radiation". It is capable of causing skin burns and of starting fires at considerable distances. Third, the nuclear explosion is accompanied by highly penetrating and harmful invisible rays, called the "initial nuclear radiation". Finally, the substances remaining after a nuclear explosion are radioactive, emitting similar radiations over an extended period of time. This is know as the "residual nuclear radiation" or "residual radioactivity". Figure 14-15a shows the distribution of energy in a typical nuclear explosion. The detonation of nuclear weapon leads to the liberation

Figure 14-15a Distribution of Energy [view large image]


of a large amount of energy in a very small period of time within the casing. Tre-mendous pressures (over million times the ambient pressure) is produced in the form of shock wave. Damage is done at the shock front by the huge difference in air pressure as well as by the drag force (strong winds) trailing behind. The radiation energy are absorbed within a few feet
in the surrounding to form a hot and highly luminous, spherical mass called fireball. The growth of the fireball becomes the mushroom cloud as shown in Figure 14-15b. It rises to 7200 feet in 10 sec, and eventually attains a height of about
Mushroom Cloud Shock Wave 4.5 miles. As the shock wave travels in the air away from its source, the overpressure at the front steadily decreases, and the pressure behind the front falls off until it develops a "negative pressure", in which a partial vacuum is produced and the air is sucked in reversing the wind direction. Figure 14-15c illustrates the variation of overpressure with distance at successive times. Its effects on a light structure, a tree, and a small animal are indicated with a series of pictures corresponding to the various

Figure 14-15b Mushroom Cloud [view large image]

Figure 14-15c Shock Wave [view large image]

times. Speed of the shock front varies from about 1600 ft/sec initially to 1150 ft/sce (slightly faster than the sound speed of 1115 ft/sec) at later time.
On the microscopic level, the action of radiation is to generate unstable molecular species, or excited molecules by interaction with water or other substances. They are very reactive chemically and soon undergo a number of secondary processes with various molecules present in the living cell. As a result, essential enzyme reactions may be inhibited and
Biological Effects the behavior of DNA and RNA is modified. There are consequently changes in the cells which may have significant detectable effects on the body as a whole. All radiations apparently induce the same general biological consequences, but neutrons are unusual in the respect that they can convert a N-14 atom in an amino acid into one of C-14. Such a change might inactivate an enzyme or affect a nucleic acid. Certain macroscopic phenomena are soon apparent in the living cell. Among these are breaking of the chromosomes. Figure 14-15d shows the normal plant cell,

Figure 14-15d Biological Effects [view large image]

with two groups of chromosomes (left), and changes (right) produced by X-rays. Frequently, the cells are unable to undergo mitosis, so that normal replacement occurring in the living organism is inhibited.

The detonation of a nuclear bomb over a city can cause immense damage. The degree of damage depends upon the distance from the center of the bomb blast, its altitude, and the explosive energy (see Figure 14-15efor the effects on Hiroshima). At the hypocenter (ground zero), everything is immediately vaporized by the high temperature (up to 300 million oC). Outward from the hypocenter, most casualties are caused by burns from the heat, injuries from the flying
Effects Hiroshima debris of buildings collapsed by the shock wave, and acute exposure to the high radiation. Beyond the immediate blast area, casualties are caused from the heat, radiation, and fires spawned from the heat wave. Figure 14-15f presents two views of Hiroshima before and after an atomic-bomb attack. It occurred in the morning (8:16 a.m.) of August 6, 1945. The bomb detonated at an altitude of 580 meters killing or wounding about half of its 350,000 inhabitants with long-term effects on

Figure 14-15e Effects of A-Bomb
[view large image]

Figure 14-15f Hiroshima

incalculable numbers among the survivors.


In the long-term, radioactive fallout occurs over a wider area because of prevailing winds. The radioactive fallout particles enter the water supply and are inhaled and ingested by people at a distance from the blast. Radiation and radioactive fallout affect those cells in the body that actively divide (hair, intestine, bone marrow, reproductive organs). Radiation induced DNA damage would increase the risk of leukemia and cancer.

A global nuclear warfare (many nuclear bombs exploding in different parts of the world) could produce a nuclear winter. In such scenario, the explosion of many bombs would raise great clouds of dust and radioactive material that would travel high into Earth's atmosphere. These clouds would block out sunlight. The reduced level of sunlight would lower the surface temperature of the planet and reduce photosynthesis by plants and bacteria. The reduction in photosynthesis would disrupt the food chain, causing mass extinction of life (including humans). This scenario is similar to the asteroid hypothesis that has been proposed to explain the extinction of the dinosaurs.

  • Aftermath - After the immediate effects of the nuclear explosion are over, certain acts are required to minimize the hazards of the early fallout and from the fires which may result from thermal radiation and secondary blast effects. First, if small fires can be quickly extinguished, extensive conflagrations may be prevented. This must be accomplished before the arrival of the fallout or in areas of low radioactivity levels. Some protection from the fallout may be secured in the basements of buildings or in a quickly constructed shelter, such as placing a sturdy table in a corner adjacent to an unexposed outer wall and covered with 10 to 12 inches of soil, sandbags, solid concrete block, etc., according to what is available. It is important to keep from coming into physical contact with the fallout particles, and to prevent contamination of food and water sources. Monitoring equipment should be used to determine areas which have safe radiation levels and decontamination efforts can proceed to recover necessary equipment, buildings, and areas. Figure 14-16d shows a reinforced precast concrete house before and after a nuclear explosion (under overpressure of 5 psi or wind speed of about 160 mi/hr) at the Nevada Test Site. Note the gas tank, sheltered by the house, is essentially undamaged.
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