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the process of liberation of a large quantity of energy in a limited volume. As a result of the explosion, the material filling the volume in which the energy liberation takes place is transformed into a very hot gas under high pressure. This gas acts with great force on the surrounding medium, causing its motion. An explosion in a solid medium is accompanied by the destruction and fragmentation of the medium.
The motion generated by the explosion, which is accompanied by sharply increased pressure, density, and temperature of the medium, is called the blast wave. The blast wave front propagates through the medium with a high velocity, which results in rapid expansion of the region enveloped by the explosion. The generation of a blast wave is a characteristic consequence of explosions in various mediums. If a medium is absent—that is, if the explosion takes place in a vacuum—the explosive energy is transformed into the kinetic energy of the products of the explosion, which fly out in all directions at high speed. Through the blast wave (or the products flying apart in a vacuum), the explosion exerts a mechanical action on objects placed at various distances from its center. As the distance from the explosion center increases, the mechanical effect of the blast wave weakens. The distances at which the blast waves from explosions of differing energy generate identical forces increase in proportion to the cube root of the explosion energy, as does the time interval of the action of the blast.
The various types of explosions are distinguished according to the physical nature of the energy source and the mode of energy liberation. The blast of chemical explosives is a typical example of an explosion. Explosives are capable of rapid chemical decomposition, during which the energy of intermolecular bonds is given off in the form of heat. An increase in the rate of chemical decomposition with increasing temperature is characteristic for explosives. At relatively low temperatures, chemical decomposition proceeds very slowly, and the explosive may not undergo any noticeable changes in its state over a long period. In this case thermal equilibrium is established between the explosive and the surrounding medium; under these conditions the continuously liberated small quantities of heat are transmitted out of the material by thermal conductivity. If conditions are created under which the heat given off cannot be transmitted out of the material, the temperature increase leads to a self-accelerating chemical decomposition reaction, which is called thermal explosion. Owing to the fact that the heat is conducted away through the outside surface of the explosive, whereas its liberation takes place throughout the entire volume of the material, the thermal equilibrium may also be upset by increasing the total mass of the explosive. This circumstance is taken into consideration in storing explosives.
Another type of explosive process, in which the chemical transformation propagates through the explosive sequentially from layer to layer in the form of a wave, is also possible. The leading edge of such a wave, which moves at high velocity, is the shock wave—an abrupt (jumplike) transformation of the material from the initial state into a state with very high pres-sure and temperature. The explosive, which is compressed by the shock wave, is in a state in which the chemical de-composition proceeds very rapidly. Consequently, the region in which the energy is liberated is concentrated in a thin layer adjacent to the shock wave’s surface. The liberation of energy leads to the preservation of a constant high pressure in the shock wave. The chemical transformation of the explosive, which is induced by the shock wave and is accompanied by rapid liberation of energy, is called detonation. Detonation waves propagate through the explosive with very high velocities, which always exceed the speed of sound in the initial material. For example, the velocities of the detonation waves in solid explosives are several km/sec. A ton of solid explosive may be transformed in such a way into a dense gas under very high pressure in 10-4 sec. The pressure in gases formed in this way reaches several hundred thousand atmo-spheres. The explosive action of a chemical explosive may be amplified in a predetermined direction by using charges with specific shapes.
Explosions related to the more fundamental transformations of materials include nuclear explosions. A nuclear explosion involves the transformation of atomic nuclei of the initial material into the nuclei of other elements; the trans-formation is accompanied by the liberation of the binding energy of the elementary particles (protons and neutrons) that make up the atomic nucleus. Nuclear explosions are based on the capability of certain isotopes of the heavy elements uranium or plutonium to undergo fission, which involves the disintegration of the nuclei of the initial material and the formation of the nuclei of lighter elements. The fission of all the nuclei contained in 50 g of uranium or plutonium results in the liberation of the same amount of energy as that resulting from the detonation of 1,000 tons of trinitrotoluene (TNT). This comparison shows that nuclear transformations are capable of producing very large explosions.
The fission of uranium or plutonium nuclei may result from the capture of one neutron by the nucleus. The fact that fission is accompanied by the emission of several new neutrons, each of which may initiate fission of other nuclei, is essential. As a consequence, the number of fissions will in-crease rapidly (according to the law of geometric progression). If it is assumed that during each fission event the number of neutrons capable of initiating the fission of other nuclei is doubled, then less than 90 fission events are sufficient for the production of the quantity of neutrons required for the fission of nuclei contained in 100 kg of uranium and plutonium. The time required for the fission of this quantity of matter is ~ 10-6 sec. Such a self-accelerating process is called a chain reaction.
In reality, not all of the neutrons formed during fission initiate the fission of other nuclei. If the total quantity of the fissionable material is small, the majority of neutrons will be ejected from the material without initiating fission. Fissionable material always contains a small number of free neutrons, but the chain reaction develops only when the number of newly formed neutrons exceeds the number of neutrons that do not produce fission. Such conditions are generated when the mass of fissionable material exceeds the so-called critical mass. An explosion takes place following the rapid combination of the separate parts of fissionable material (the mass of each part is less than critical) into a single entity with a total mass exceeding the critical mass, or in the case of rapid compression, which decreases the surface area of the material and at the same time decreases the number of neutrons that are ejected to the outside. These conditions are usually generated by the detonation of chemical explosives.
There is another type of nuclear reaction, the fusion reaction of light nuclei, which is accompanied by the liberation of a large quantity of energy. The forces of repulsion of like electrical charges (all nuclei have positive electrical charges) resist the occurrence of the fusion reaction; therefore, efficient nuclear transformations of this type require that the nuclei have sufficiently high energy. Such conditions may be generated by heating materials to very high temperatures. In connection with this, the fusion process occurring at high temperatures is called a thermonuclear reaction. During the fusion of deuterium nuclei (hydrogen isotope 2H), almost three times as much energy is liberated as during the fission of an equal mass of uranium. The temperature required for fusion is achieved by a nuclear explosion of uranium or plutonium. Thus, if fissionable material and hydrogen isotopes are placed in the same device, a fusion reaction resulting in a very powerful explosion may be produced. In addition to the powerful blast wave, nuclear explosions are accompanied by intense emission of light and penetrating radiation.
In the types of explosions described above, the liberated energy was initially present in the form of the energy of the molecular or nuclear bond in the material. There are types of explosions in which the energy liberated is supplied by an external source. A powerful electrical discharge in any medium serves as an example of such an explosion. The electrical energy in the discharge gap is liberated in the form of heat, which transforms the medium into a high-pressure, high-temperature ionized gas. An analogous phenomenon takes place during the flow of a powerful electrical current in a metal conductor if the current strength is sufficiently high to cause a rapid conversion of the metallic conductor to vapor. Explosions also occur during the action of focused laser radiation on a substance. The process of rapid liberation of energy owing to a sudden destruction of an envelope containing gas under high pressure (for example, the explosion of a compressed-gas cylinder) may be regarded as another form of explosion.
Explosions may result from the collision of solid bodies moving in opposite directions at high speeds. The kinetic energy of the bodies is transformed into heat upon collision as the result of the propagation through the substance of a powerful shock wave that arises at the instant of collision. The velocities of relative approach of solid bodies required for the complete transformation of solid matter into vapor are of the order of dozens of km/sec, and the pressures generated during the collision are of the order of millions of atmo-spheres.
Many natural phenomena are accompanied by explosions. Powerful electrical discharges in the atmosphere during storms (lightning), sudden volcanic eruptions, and the impact of large meteorites on the earth’s surface are examples of various forms of explosions. The impact of the Tungus meteorite (1907) generated an explosion equivalent to the energy liberated by the explosion of ~ 107 tons of TNT. An even larger amount of energy was apparently liberated during the eruption of Krakatoa (1883).
Chromospheric flares on the sun are explosions on a huge scale. The energy liberated during such flares attains a magnitude of ~ 1017 joules (J); by comparison, the explosion of 106 tons of TNT would liberate 4.2 x 1015 J.
The flare-ups of novas are gigantic explosions taking place in space. These flare-ups apparently liberate energy equal to 1038-1039 J over a period of several hours. This energy is liberated by the sun over a period of 10,000 to 100,000 years. Finally, even more gigantic explosions, by far exceeding the limits of human imagination, are the flare-ups of supernovas, with the liberation of energies of ~ 1043 J, and explosions in galactic nuclei, which are estimated at ~ 1050 J.
The explosion of chemical explosives is used as one of the basic means of destruction. Nuclear explosions have a very large destructive capability. The explosion of a single nuclear bomb may be equivalent to the energy of tens of millions of tons of a chemical explosive.
Explosions have found numerous peaceful uses in scientific research and in industry. Explosions made it possible to achieve significant progress in the studies of the properties of gases, liquids, and solids at high pressures and temperatures. Research on explosions is important in the development of the physics of nonequilibrium processes, which is devoted to studies of the mass, momentum, and energy transfer in various mediums, phase transition mechanisms in matter, the kinetics of chemical reactions, and other processes. The action of explosions makes it possible to attain states of matter that are unattainable by other experimental methods. Powerful compression of the channel of an electrical discharge by the detonation of chemical explosives makes it possible to generate magnetic fields of extreme intensities (up to 1.1 gigaamperes per meter, or up to 14 million oersteds) for short time periods. The intense emission of light during the detonation of chemical explosives in gases may be used for the excitation of optical quantum generators (lasers). The action of the high pressure created by the detonation of an explosive is utilized for explosive forming, blast welding, and explosive hardening of metals.
The experimental study of explosions consists of measuring the rate of propagation of blast waves and the rate of mass transfer, as well as the rapidly changing pressure, density distribution, intensity, and spectral composition of the electromagnetic and other types of radiation emitted during explosions. These data make it possible to obtain information concerning the rate of various processes accompanying explosions and to determine the total amount of energy liberated. The pressure and density of material in the shock wave are connected by definite relationships with the rate of propagation of the shock wave and the rate of mass transfer. This fact makes it possible, for example, to calculate the pressures and densities on the basis of the rate measurements in cases where their direct determination is for some reason impossible. The determination of basic parameters that characterize the state and the rate of mass transfer involves various sensors, which transform a particular type of action into an electrical signal, which is in turn recorded by an oscillograph or other recording instrument. Modern electronic instrumentation makes it possible to record phenomena occurring in time intervals of ~ 10-11 sec. Measurements of the intensity and spectral composition of the emitted light by means of special photoelements and spectrographs are a source of information concerning the temperature of the material. High-speed photography, which may be performed at a rate of up to 109 frames per second, is being used widely for recording the phenomena that accompany explosions.
A special piece of apparatus, the shock tube, is used in laboratory studies of shock waves. The shock wave in this tube is generated by the rapid destruction of a membrane separating a gas under high pressure from a gas under low pressure. (This process may be regarded as the simplest type of explosion.) Interferometers and half-shade optical devices, whose operation is based on the changes in the index of refraction of the gas as a consequence of changes in its density, are used effectively in the study of waves in shock tubes.
Blast waves propagating over great distances from their origin serve as sources of information concerning the structure of the atmosphere and the inner layers of the earth. Waves are recorded at long distances from the point of origin using highly sensitive instruments that permit the determination of atmospheric pressure fluctuations of as little as 10-6 atmosphere (0.1 newton per square meter) or ground displacements of ~ 10-9 m.
Explosions are widely used in prospecting for minerals. The seismic waves (elastic waves in the earth’s crust) reflected by various layers are recorded by the seismographs. The analysis of the seismograms makes it possible to draw conclusions concerning the presence of the deposits of oil, natural gas, and other minerals. Explosions are also being used widely in opening up and developing mineral deposits. Most construction projects involving the building of dams, mountain tunnels, and roads would be impossible without blasting operations.
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K. E. GUBKIN
What does it mean when you dream about an explosion?
An explosion may indicate the forceful breakthrough of unconscious feelings into consciousness, particularly repressed rage. More generally, explosions in dreams often reflect an upheaval in one’s life. More positively, explosions may represent the breaking down of barriers.