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In order to fully understand what happens in a nuclear disaster, it is important to have at least a basic understanding of the scientific principles involved. An effort has been made to present the technical material in terms that are easy for the layman to understand. In general an explosion results from the very rapid release of a large amount of energy within a limited space. This is true for a conventional high explosive, such as TNT, as well as for a nuclear explosion, although the energy in each is produced in quite different ways. The sudden liberation of energy causes a considerable increase in temperature and pressure so that all materials present are converted into hot, compressed gases. Since these gases are at very high temperatures and pressures they expand rapidly causing a pressure wave, called a shock wave, in the surrounding medium-air, water, or earth. Characteristic of the ideal shock wave is a sudden increase in pressure at its front with a gradual decrease in pressure behind it. A shock wave in air is generally referred to as a blast wave because it resembles and is accompanied by a very strong wind. In water or in the ground, however, the term shock is used because the effect is like that of a sudden impact. Nuclear weapons are similar to those of more conventional types insofar as their destructive effect is due mainly to blast or shock. There are, however, several basic differences between nuclear and high-explosive weapons. First nuclear explosions can be many thousands or millions of times more powerful than the largest conventional detonations. Second the mass required by a nuclear explosive to produce a given amount of energy is much less than that required by a conventional high explosive. Consequently, in the case of a nuclear explosion, there is a much smaller amount of material available in the weapon itself to be converted into hot, compressed gases. Third the temperatures reached in a nuclear explosion are much higher than in a conventional explosion, and 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. Such radiation is capable of causing skin burns and starting fires at considerable distances-as far away as twelve miles from a one-megaton explosion. Fourth the nuclear explosion is accompanied by highly penetrating and harmful invisible rays of 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 known as residual nuclear radiation or residual radioactivity. It is because of these fundamental differences between a nuclear and a conventional explosion, including the tremendous power of the former, that the effects of nuclear explosions require special consideration. A knowledge and understanding of the mechanical and various radiation phenomena associated with a nuclear explosion are of vital importance.
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