Nuclear Explosion


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nuclear explosion

[′nü·klē·ər ik′splō·zhən]
(nucleonics)
An explosion for which the energy is produced by a nuclear transformation, either fission or fusion.

Nuclear Explosion

 

an explosion of enormous scale and destructive force caused by the release of nuclear energy.

By the beginning of World War II (1939–45), physicists had already come close to the possibility of harnessing nuclear energy. The first atomic bomb was built in the United States as a result of the joint efforts of a large group of eminent scientists, many of whom had emigrated from Europe to escape the Hitlerite regime. The first nuclear explosion was set off on July 16, 1945, near Alamogordo, N.M.; on Aug. 6 and 9, 1945, two US atomic bombs were dropped on the Japanese cities of Hiroshima and Nagasaki. The energy of the first nuclear explosions was estimated at approximately 1021 ergs (1014 joules), which is equivalent to the energy released upon the explosion of about 20,000 tons (20 kilotons) of TNT (the energy of a nuclear explosion is usually characterized by its TNT equivalent). In the USSR, the first atomic blast was detonated in August 1949, and the considerably more powerful hydrogen bomb was first tested on Aug. 12, 1953. The nuclear powers later detonated nuclear explosions with energies reaching tens of millions of tons (megatons) of TNT equivalent.

A nuclear explosion may be caused either by the nuclear chain reaction resulting from the fissioning of heavy nuclei, such as 235U and 239Pu, or by the thermonuclear reaction of the fusion of helium nuclei from lighter nuclei. The nuclei of 235U and 239Pu split up upon the capture of a neutron into two fragment nuclei of intermediate mass; when this happens, several neutrons, usually two or three, are also produced. The sum of the masses of all the daughter particles is less than the mass of the initial nucleus by an amount Δm, called the mass defect (seeMASS DEFECT). According to A. Einstein’s relation, the mass defect has a corresponding energy ΔE = Δm × c2 (c is the speed of light), which is the binding energy of the fission products in the parent nucleus. The release of this energy in a rapidly developing nuclear fission chain reaction is what leads to the explosion. The energy ΔE comes to about 200 megaelectron volts (MeV) for one fissioning nucleus. One kg of M5U or 239Pu contains 2.5 × 1024 nuclei. An enormous amount of energy, approximately equal to 1021 ergs, is released upon the fission of all these nuclei.

The possibility of a fission chain reaction arises because more than one neutron is produced in a single fission event. Each of these neutrons may also split up the nuclei; the next generation of neutrons splits more nuclei, and so on. For example, if two neutrons of each generation produce fission, a reaction that begins with a single neutron will lead to the decay of all nuclei of 1 kg of fissionable material within 80 generations. Usually, not all the neutrons cause the splitting of nuclei; some are lost. If the losses are too great, a chain reaction cannot develop. The probability of the loss of an individual neutron increases with a reduction in the linear dimensions and mass of the fissionable material. The limiting conditions under which a chain reaction may develop in the material are called critical conditions. They are characterized by the density, geometry, and mass of the material; for example, there exists a critical mass (seeCRITICAL MASS). The fissionable material in a nuclear warhead is arranged such that it is in subcritical conditions (for example, so that the mass is dispersed). At the necessary moment, the supercritical conditions are brought about (the entire mass is brought together) and the chain reaction initiated. It is necessary to bring the entire mass together very rapidly so that the reaction will proceed at the highest possible degree of supercriticality and there will be time for as much as possible of the heated material to react before dispersal. From a practical standpoint, the possibilities for intensifying the power of a nuclear explosion based on a nuclear fission chain reaction are limited, since it is very difficult to convert a large mass of initially subcritical fissionable material sufficiently rapidly to the supercritical state.

High-yield nuclear explosions, with a TNT equivalent in the millions and tens of millions of tons, are based on the thermonuclear fusion reaction. The basic reaction here is the conversion of two nuclei of heavy hydrogen isotopes (deuterium 2H and tritium 3H) to a nucleus of 4He and a neutron. An energy of 17.6 MeV is released in a single event. Upon the complete transformation of 1 kg of heavy hydrogen, the energy released exceeds by a factor of approximately 4 the amount of energy resulting from the fission of 1 kg of 235U or 239Pu. In order that the positively charged 2H and 3H may collide and undergo conversion, they must overcome the electrical forces of repulsion between them; that is, they must have considerable velocity (kinetic energy). Therefore, the thermonuclear reaction used in the hydrogen bomb proceeds at very high temperatures, of the order of tens of millions of degrees, which is achieved upon the explosion of an atomic bomb, used as “primer” in a hydrogen bomb. Since hydrogen in the ordinary state is a gas, the solid hydrogen-containing substances 6Li2H and 6Li3H are used to initiate a thermonuclear explosion. The lithium nuclei themselves also participate in the thermonuclear reaction, raising the energy yield of the thermonuclear explosion.

Immediately upon completion of the nuclear reaction, which occurs within 10–7 sec, the energy released is concentrated in a very limited mass and volume (of the order of 1 ton and 1 m3). The temperature and pressure in this case reach enormous values, of the order of 10 million degrees and 1 billion atmospheres. A considerable portion of the energy is emitted by the heated substance as soft X-radiation; however, this radiation can propagate to a considerable distance only upon a nuclear burst in an extremely rarefied atmosphere, at altitudes of the order of 100 km or more. In all other cases, upon air bursts, that is, explosions at fairly low altitudes, and upon underground and underwater bursts, nearly all the energy of the explosion is transferred to the medium directly surrounding the substance of the nuclear explosive charge: air, earth, or water. A strong shock wave arises in the surrounding medium under the action of the high pressure. A nuclear explosion also produces penetrating radiation—fluxes of gamma quanta and neutrons that carry away several percent of all the energy of the explosion and that travel through the air at atmospheric pressure to a distance of many hundreds of meters.

The air in the shock wave generated by a nuclear explosion is heated to hundreds of thousands of degrees; it begins to glow brightly, giving rise to what is referred to as the fireball. At first, the surface of the fireball coincides with the shock-wave front, and they expand together rapidly. For example, in a 20-kiloton nuclear explosion in air at atmospheric pressure, the radius of the fireball within 10−4 sec after the explosion is approximately 14 m, and after 0.01 sec, 100 m. At this stage, the shock wave separates from the boundary of the fireball. No longer causing luminescence, it proceeds far ahead, while the expansion of the fireball slows down and then stops altogether. Within 0.1 sec after the explosion, the radius of the fireball attains its maximum extent of approximately 150 m; the luminescence temperature at this stage is about 8000°K. Within 1 sec after the explosion, the luminescence begins to fade, and within 2–3 sec it has virtually stopped. Luminous radiation accounts for approximately one-third of all the energy of the explosion. Brighter than the sun, it is highly destructive, causing fires even at a distance of 2 km, charring objects, and burning people and animals. Within 10 sec after the explosion, the shock wave reaches a distance of 3.7 km from the center of the explosion. The shock wave from a 20-kiloton nuclear burst has a strong destructive effect on buildings, industrial facilities, and military installations at a distance of up to 1 km.

After luminescence has ceased, the heated air of the fireball, being less dense than the surrounding air, is lifted by buoyancy force. As it rises, the heated air expands and cools, and water vapor condenses within it. Thus, the characteristic mushroom cloud of the nuclear explosion is formed, with a diameter of hundreds of meters. Within 1 min, it reaches an altitude of 4 km, and within 10 minutes, an altitude of 10 km. Subsequently, the cloud, containing products of nuclear reactions, is dispersed by the wind and air currents to distances of tens or hundreds of km.

Nuclear fission products are radioactive, emitting gamma quanta and electrons. Because of the radioactivity and the radioactive fallout, the terrain in the vicinity of the trail of the cloud becomes contaminated; this is one of the most dangerous consequences of a nuclear explosion, causing radiation sickness in people and animals. A nuclear explosion at low altitudes is the most dangerous with respect to radioactivity, since the fireball, upon its expansion, touches the surface of the earth, an enormous column of dust and dirt is lifted skyward, and radioactive products subsequently fall out together with the dust. The radius of action of the shock wave is approximately proportional to the cube root of the energy released in the explosion. For example, the radius of very strong destructive action of a 20-megaton nuclear explosion is ten times as great as for a 20-kiloton blast, that is, of the order of 10 km. Such an explosion can annihilate a large city.

The shock wave and fireball also arise in high-altitude nuclear explosions, that is, explosions above 100–200 km, but a considerably smaller fraction of the energy of the explosion is converted to luminous radiation, since the air radiates light much more weakly because of the strong rarefaction. Among the most important consequences of high-altitude nuclear bursts are the formation of very large regions of increased ionization, with a radius of tens and even hundreds of km, and the perturbation of the atmosphere. The ionization is caused by the action of X rays and gamma radiation, as well as neutrons, and leads to serious disruptions in the operation of radar and radio-communication facilities. High-altitude nuclear explosions detonated in the United States in the period 1958–62 revealed that stable radio communications can be disrupted for as long as tens of minutes.

In an underwater burst, approximately one-half of all the energy is contained in the primary shock wave, which also causes the most destruction. Typical of the underwater explosion is the formation of a large bubble around the center of the burst, which undergoes pulsating motions that subside with time. The secondary waves emitted as a result of the pulsations have much less effect than the primary shock waves. For a 20-kiloton nuclear explosion at shallow depth, the radius of strong destructive action, causing ships to sink, is ~0.5 km. In an underwater nuclear explosion, an enormous column of water vapor and spray rises into the air above the water. Intense surface waves also arise, which spread for many km: the height of the wave crest reaches 3 m at a distance of 3 km from the epicenter of a 20-kiloton underwater burst.

The shock wave also causes destruction in the case of an underground nuclear burst. A high-pressure gas bubble arises at the center, as in the underwater burst. In the case of a burst at small depths, an enormous funnel is formed, and a column of dust and dirt is carried into the air. An underground burst causes a surge similar to an earthquake in its effect. With respect to energy, a 20-kiloton nuclear explosion can be compared to an earthquake measuring five points on the Richter scale. A nuclear explosion of a 20-megaton hydrogen bomb corresponds to an earthquake measuring seven points on the Richter scale. Seismic waves generated by an underground burst have been recorded at distances of thousands of km from the blast itself.

IU. P. RAIZER

Underground nuclear explosions have been used for peaceful purposes, for example, in large-scale mining operations and the mining of minerals. A distinction is made between a buried nuclear explosion of external action and a subsurface (contained) blast, in which the radius of destructive action does not reach the surface of the earth. Nuclear explosions of external action, which can be used to move enormous masses of rock in opening up new mineral deposits and building canals, earthen dams, reservoirs, artificial harbors, and the like, require nuclear devices and methods of detonation that guarantee no radioactive contamination of the atmosphere and the total safety of the biosphere. Contained nuclear explosions are detonated by charges buried several km deep. These charges make it possible to further exploit depleted petroleum and gas deposits; to construct storage reservoirs in plastic rocks for natural gas, petroleum products, and the burial of wastes; to break up stubborn ore bodies for removal; and to stop accidental oil and gas flows.

REFERENCES

Deistvie iadernogo oruzhiia. Moscow, 1960. (Translated from English.)
Zel’dovich, Ia. B., and Iu. P. Raizer. Fizika udarnykh voln i vysoko-temperaturnykh gidrodinamicheskikh iavlenii, 2nd ed. Moscow, 1966.
Cole, R. Podvodnye vzryvy. Moscow, 1950. (Translated from English.)
Podzemnye iadernye vzryvy. Moscow, 1962. (Translated from English.)
Iadernyi vzryv v kosmose, na zemle i pod zemlei. Moscow, 1974. (Translated from English.)
Atomnye vzryvy v mirnykh tseliakh. Moscow, 1970.
Izrael’, Iu. A. Mirnye iadernye vzryvy i okruzhaiushchaia sreda. Leningrad, 1974.
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