atomic bomb


atomic bomb
1. a bomb whose potency is derived from nuclear fission of atoms of fissionable material with the consequent conversion of part of their mass into energy.
2. a bomb whose explosive force comes from a chain reaction based on nuclear fission in U-235 or plutonium.
Also, atom bomb. Also called A-bomb, fission bomb.
[1910-15]

* * *

Weapon whose great explosive power results from the sudden release of energy upon the splitting, or fission, of the nuclei of heavy elements such as plutonium or uranium (see nuclear fission).

With only 11–33 lb (5–15 kg) of highly enriched uranium, a modern atomic bomb could generate a 15-kiloton explosion, creating a huge fireball, a large shock wave, and lethal radioactive fallout. The first atomic bomb, developed by the Manhattan Project during World War II, was set off on July 16, 1945, in the New Mexico desert. The only atomic bombs used in war were dropped by the U.S. on Hiroshima on Aug. 6, 1945, and on Nagasaki three days later. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998). Israel and South Africa were suspected of testing atomic weapons in 1979. See also hydrogen bomb; Nuclear Nonproliferation Treaty; nuclear weapon.

First atomic bomb test, near Alamogordo, New Mexico, July 16, 1945.

Courtesy of Los Alamos National Laboratory, New Mexico

* * *

▪ fission device
also called  atom bomb 
 weapon with great explosive power that results from the sudden release of energy upon the splitting (nuclear fission), or fission (binary fission), of the nuclei of such heavy elements as plutonium or uranium.

      When a neutron strikes the nucleus of an atom of the isotopes uranium 235 or plutonium-239, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In the process of splitting, a great amount of thermal energy, as well as gamma rays (gamma ray) and two or more neutrons, is released. Under certain conditions, the escaping neutrons strike and thus fission more of the surrounding uranium nuclei, which then emit more neutrons that split still more nuclei. This series of rapidly multiplying fissions culminates in a chain reaction in which nearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb.

      Many isotopes of uranium can undergo fission, but uranium-235, which is found naturally at a ratio of about one part per every 139 parts of the isotope uranium-238, undergoes fission more readily and emits more neutrons per fission than other such isotopes. Plutonium-239 has these same qualities. These are the primary fissionable materials used in atomic bombs. A small amount of uranium-235, say 0.45 kg (1 pound), cannot undergo a chain reaction and is thus termed a subcritical mass; this is because, on average, the neutrons released by a fission are likely to leave the assembly without striking another nucleus and causing it to fission. If more uranium-235 is added to the assemblage, the chances that one of the released neutrons will cause another fission are increased, since the escaping neutrons must traverse more uranium nuclei and the chances are greater that one of them will bump into another nucleus and split it. At the point at which one of the neutrons produced by a fission will on average create another fission, critical mass has been achieved, and a chain reaction and thus an atomic explosion will result.

      In practice, an assembly of fissionable material must be brought from a subcritical to a critical state extremely suddenly. One way this can be done is to bring two subcritical masses together, at which point their combined mass becomes a critical one. This can be practically achieved by using high explosives to shoot two subcritical slugs of fissionable material together in a hollow tube. A second method used is that of implosion, in which a core of fissionable material is suddenly compressed into a smaller size and thus a greater density; because it is denser, the nuclei are more tightly packed and the chances of an emitted neutron's striking a nucleus are increased. The core of an implosion-type atomic bomb consists of a sphere or a series of concentric shells of fissionable material surrounded by a jacket of high explosives, which, being simultaneously detonated, implode the fissionable material under enormous pressures into a denser mass that immediately achieves criticality. An important aid in achieving criticality is the use of a tamper; this is a jacket of beryllium oxide or some other substance surrounding the fissionable material and reflecting some of the escaping neutrons back into the fissionable material, where they can thus cause more fissions. In addition, “boosted fission” devices incorporate such fusionable materials as deuterium or tritium into the fission core. The fusionable material boosts the fission explosion by supplying a superabundance of neutrons.

 Fission releases an enormous amount of energy relative to the material involved. When completely fissioned, 1 kg (2.2 pounds) of uranium-235 releases the energy equivalently produced by 17,000 tons, or 17 kilotons, of TNT. The detonation of an atomic bomb releases enormous amounts of thermal energy, or heat, achieving temperatures of several million degrees in the exploding bomb itself. This thermal energy creates a large fireball, the heat of which can ignite ground fires that can incinerate an entire small city. Convection currents created by the explosion suck dust and other ground materials up into the fireball, creating the characteristic mushroom-shaped cloud of an atomic explosion. The detonation also immediately produces a strong shock wave that propagates outward from the blast to distances of several miles, gradually losing its force along the way. Such a blast wave can destroy buildings for several miles from the location of the burst. Large quantities of neutrons and gamma rays are also emitted; this lethal radiation decreases rapidly over 1.5 to 3 km (1 to 2 miles) from the burst. Materials vaporized in the fireball condense to fine particles, and this radioactive debris, referred to as fallout, is carried by the winds in the troposphere or stratosphere. The radioactive contaminants include such long-lived radioisotopes as strontium-90 and plutonium-239; even limited exposure to the fallout in the first few weeks after the explosion may be lethal, and any exposure increases the risk of developing cancer.

   The first atomic bomb was built in Los Alamos, N.M., during World War II under a program called the Manhattan Project. Los Alamos was approved as the site for the main atomic bomb scientific laboratory on Nov. 25, 1942, by Brig. Gen. Leslie R. Groves (Groves, Leslie Richard) and physicist J. Robert Oppenheimer (Oppenheimer, J. Robert) and was given the code name Project Y. One bomb, using plutonium, was successfully tested on July 16, 1945, at a site 193 km (120 miles) south of Albuquerque, N.M. The first atomic bomb to be used in warfare used uranium. It was dropped by the United States on Hiroshima, Japan, on Aug. 6, 1945. (See Sidebar: The decision to use the atomic bomb.) The explosion, which had the force of more than 15,000 tons of TNT, instantly and completely devastated 10 square km (4 square miles) of the heart of this city of 343,000 inhabitants. Of this number, 66,000 were killed immediately and 69,000 were injured; more than 67 percent of the city's structures were destroyed or damaged. The next atomic bomb to be exploded was of the plutonium type; it was dropped on Nagasaki on Aug. 9, 1945, producing a blast equal to 21,000 tons of TNT. The terrain and smaller size of Nagasaki reduced destruction of life and property, but 39,000 persons were killed and 25,000 injured; about 40 percent of the city's structures were destroyed or seriously damaged. The Japanese initiated surrender negotiations the next day.

      After the war, the United States conducted test explosions of atomic bombs in the Pacific at Enewetak (Eniwetok) atoll and in Nevada. In subsequent years, the Soviet Union (1949), Great Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998) tested fission weapons of their own. The great temperatures and pressures created by a fission explosion are also used to initiate fusion and thus detonate a thermonuclear bomb. See also nuclear weapon.

* * *


Universalium. 2010.

Look at other dictionaries:

  • Atomic bomb — Atomic A*tom ic, Atomical A*tom ic*al, a. [Cf. F. atomique.] 1. Of or pertaining to atoms. [1913 Webster] 2. Extremely minute; tiny. [1913 Webster] {Atomic bomb}, see {atom bomb} in the vocabulary. {Atomic philosophy}, or {Doctrine of atoms}, a… …   The Collaborative International Dictionary of English

  • atomic bomb — atomic bombs N COUNT (The form atom bomb is also used, mainly in British English.) An atomic bomb or an atom bomb is a bomb that causes an explosion by a sudden release of energy that results from splitting atoms …   English dictionary

  • atomic bomb — ☆ atomic bomb n. an extremely destructive type of bomb, the power of which results from the immense quantity of energy suddenly released when a very rapid chain reaction of nuclear fission is set off by neutron bombardment in the atoms of a… …   English World dictionary

  • atomic bomb — noun a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239) (Freq. 2) • Syn: ↑atom bomb, ↑A bomb, ↑fission bomb, ↑plutonium bomb • Hypernyms: ↑nuclear… …   Useful english dictionary

  • Atomic bomb —    The atomic bomb was a weapon made using enriched uranium, which on detonation caused a chain nuclear reaction involving the fission of atomic particles. Development of the atomic bomb in the United States began after Albert Einstein wrote to… …   Historical Dictionary of the Roosevelt–Truman Era

  • atomic bomb — atom bomb UK / US or atomic bomb UK / US noun [countable] Word forms atom bomb : singular atom bomb plural atom bombs Word forms atomic bomb : singular atomic bomb plural atomic bombs a bomb that causes a very large nuclear explosion from the… …   English dictionary

  • Atomic bomb — Atom bomb At om bomb , Atomic bomb A*tom ic bomb . a bomb of great power in which the explosive energy is derived from the nuclear fission of a fissionable material, such as plutonium or uranium 235. It is a type of nuclear weapon. The strength… …   The Collaborative International Dictionary of English

  • atomic bomb — Nuclear weapon Nu cle*ar wea pon, n. A weapon of great explosive power, such as an {atomic bomb} or a {hydrogen bomb}, which depends for most of its explosive power on the release of energy from within atomic nuclei by a nuclear reaction. A… …   The Collaborative International Dictionary of English

  • atomic bomb — atominė bomba statusas T sritis apsauga nuo naikinimo priemonių apibrėžtis Aviacinė bomba su atominiu užtaisu, branduolinio šaudmens rūšis. Į taikinį neša naikintuvai, taktiniai naikintuvai, bombonešiai. Pagrindinės atominės bombos dalys:… …   Apsaugos nuo naikinimo priemonių enciklopedinis žodynas

  • atomic bomb — atominė bomba statusas T sritis fizika atitikmenys: angl. atomic bomb vok. Atombombe, f rus. атомная бомба, f pranc. bombe atomique, f …   Fizikos terminų žodynas


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.