Manhattan Project -
Atomic Bombs
U.S.
government research project (1942–45) that produced the first atomic bombs.
In 1939 American
scientists, many of them refugees from Fascist regimes in Europe,
took steps to organize a project to exploit the newly recognized fission
process for military purposes. The first contact with the government was made
by G.B. Pegram of Columbia University, who arranged a
conference between Enrico Fermi and the Navy
Department in March 1939. In the summer of 1939 Albert Einstein was persuaded
by his fellow scientists to use his influence and present the military
potential of an uncontrolled fission chain reaction to President Franklin D.
Roosevelt. In February 1940, $6,000 was made available to start research under
the supervision of a committee headed by L.J. Briggs, director of the National
Bureau of Standards. On Dec. 6, 1941, the project was put under the direction
of the Office of Scientific Research and Development, headed by Vannevar Bush. After the United States' entry into the war,
the War Department was given joint responsibility for the project, since by
mid-1942 it was obvious that a vast array of pilot plants, laboratories, and
manufacturing facilities would have to be constructed by the U.S. Army Corps of
Engineers so that the assembled scientists could carry out their mission. In
June 1942 the Engineers' Manhattan District was initially assigned management
of the construction work (because much of the early research had been performed
at Columbia University, in Manhattan), and in September 1942 Brigadier General
Leslie R. Groves was placed in charge of all Army activities (chiefly
engineering activities) relating to the project. The “Manhattan Project” became
the code name for research work that would extend across the country.
It was known in 1940
that German scientists were working on a similar project and that the British
were also exploring the problem. In the fall of 1941 Harold C. Urey and Pegram visited England
to attempt to set up a cooperative effort, and by 1943 a combined policy
committee with Great Britain and Canada was established. In that year a number
of scientists of those countries moved to the United States to join the project
there.
If the project were to
achieve success quickly, several lines of research and development had to be
carried on simultaneously before it was certain whether any might succeed. The
explosive materials then had to be produced and be made suitable for use in an
actual weapon.
Uranium 235, the
essential fissionable component of the postulated bomb, cannot be separated
from its natural companion, the much more abundant uranium 238, by chemical
means; the atoms of these respective isotopes must rather be separated from
each other by physical means. Several physical methods to do this were
intensively explored, and two were chosen—the electromagnetic process developed
at the University of California at Berkeley under Ernest Orlando
Lawrence and the diffusion process developed under Urey
at Columbia University. Both of these processes, and particularly the diffusion
method, required large, complex facilities and huge amounts of electric power
to produce even small amounts of separated uranium 235. Philip Hauge Abelson developed a third
method called thermal diffusion, which was also used for a time to effect a
preliminary separation. These methods were put into production at a
70-square-mile tract near Knoxville, Tenn.,
originally known as the Clinton Engineer Works, later as Oak Ridge.
Only one method was
available for the production of the fissionable material plutonium 239. It was
developed at the metallurgical laboratory of the University of Chicagounder the direction of Arthur Holly Compton and
involved the transmutation in a reactor pile of uranium 238. In December 1942 Fermi
finally succeeded in producing and controlling a fission chain reaction in this
reactor pile at Chicago.
Quantity production of
plutonium 239 required the construction of a reactor of great size and power
that would release about 25,000 kilowatt-hours of heat for each gram of
plutonium produced. It involved the development of chemical extraction
procedures that would work under conditions never before encountered. An
intermediate step in putting this method into production was taken with the
construction of a medium-size reactor at Oak Ridge. The large-scale production
reactors were built on an isolated 1,000-square-mile tract on the Columbia
River north of Pasco, Wash.—the Hanford Engineer Works.
Before 1943 work on the
design and functioning of the bomb itself was largely theoretical, based on
fundamental experiments carried out at a number of different locations. In that
year a laboratory directed by J. Robert Oppenheimer was created on an isolated
mesa at Los Alamos, N.M., 34 miles (55 km) north of Santa Fe. This laboratory
had to develop methods of reducing the fissionable products of the production
plants to pure metal and fabricating the metal to required shapes. Methods of
rapidly bringing together amounts of fissionable material to achieve a
supercritical mass (and thus a nuclear explosion) had to be devised, along with
the actual construction of a deliverable weapon that would be dropped from a
plane and fused to detonate at the proper moment in the air above the target.
Most of these problems had to be solved before any appreciable amount of
fissionable material could be produced so that the first adequate amounts could
be used at the fighting front with minimum delay.
By the summer of 1945,
amounts of plutonium 239 sufficient to produce a nuclear explosion had become
available from the Hanford Works, and weapon development and design were
sufficiently far advanced so that an actual field test of a nuclear explosive
could be scheduled. Such a test was no simple affair. Elaborate and complexequipment had to be assembled so that a complete
diagnosis of success or failure could be had. By this time the original $6,000
authorized for the Manhattan Project had grown to $2,000,000,000.
The first atomic bomb
was exploded at 5:30 AM on July 16, 1945, at a site on the Alamogordo air base
120 miles (193 km) south of Albuquerque, N.M. It was detonated on top of a
steel tower surrounded by scientific equipment, with remote monitoring taking
place in bunkers occupied by scientists and a few dignitaries 10,000 yards (9
km) away. The explosion came as an intense light flash, a sudden wave of heat,
and later a tremendous roar as the shock wave passed and echoed in the valley.
A ball of fire rose rapidly, followed by a mushroom cloud extending to 40,000
feet (12,200 m). The bomb generated an explosive power equivalent to 15,000 to
20,000 tons of TNT; the tower was completely vaporized and the surrounding
desert surface fused to glass for a radius of 800 yards (730 m). The following
month, two other atomic bombs produced by the project, the first using uranium
235 and the second using plutonium, were dropped on Hiroshima and Nagasaki.
atomic bomb
also called atom bomb
weapon with great explosive power that results from the sudden release of
energy upon the splitting, or 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 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. Since the
radioactive contaminants include such long-lived radioisotopes as strontium-90
and plutonium-239, they can have lethal effects for weeks after the explosion.
The first atomic bombs
were built in the United States during World War II under a program called the
Manhattan Project. One bomb, using plutonium, was successfully tested on July
16, 1945, at a site 193 km (120 miles) south of Albuquerque, New Mexico (see
photograph). The first atomic bomb to be used in warfare used uranium. It was
dropped by the United States on Hiroshima, Japan, on August 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 completelydevastated
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 August 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 dozens of 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.
Atomic Bomb
Atomic Bomb, powerful explosive nuclear weapon fueled
by the splitting, or fission, of the nuclei of specific isotopes of uranium or
plutonium in a chain reaction. The strength of the explosion created by an
atomic bomb is on the order of the strength of the explosion that would be
created by thousands of tons of TNT (see Trinitrotoluene).
An atomic bomb must provide enough mass of plutonium or uranium to
reach critical mass, the mass at which the nuclear reactions going on
inside the material can make up for the neutrons leaving the material through
its outside surface. Usually the plutonium or uranium in a bomb is separated
into parts so that critical mass is not reached until the bomb is set to
explode. At that point, a set of chemical explosives or some other mechanism
drives all the different pieces of uranium or plutonium together to produce a
critical mass. After this occurs, there are enough neutrons bouncing around in
the material to create a chain reaction of fissions. In the fission reactions,
collisions between neutrons and uranium or plutonium atoms cause the atoms to
split into pairs of nuclear fragments, releasing energy and more neutrons. Once
the reactions begin, the neutrons released by each reaction hit other atoms and
create more fission reactions until all the fissile material is exhausted or
scattered.
This process of fission releases enormous energy in the form of
extreme heat and a massive shock wave; this is the intense explosion. In
addition to its nearly unimaginable destructive force, consisting of pressure
waves, flash burns, and high winds, a nuclear explosion also produces deadly
radiation in the form of gamma rays and neutrons. The radiation destroys living
matter and contaminates soil and water.
Atomic bombs were the first nuclear weapons to be
developed, tested, and used. In the late 1930s physicists in Europe and the
United States realized that the fission of uranium could be used to create an
extremely powerful explosive weapon. In August 1939, German American physicist
Albert Einstein sent a letter to U.S. president Franklin D. Roosevelt that
described this discovery and warned of its potential development by other
nations. The U.S. government established the top secret Manhattan Project in
1942 to develop an atomic device. The leader of the Manhattan Project was U.S.
Army Brigadier General Leslie R. Groves. His team, working in several locations
but in large part at Los Alamos, New Mexico, under the direction of American
physicist J. Robert Oppenheimer, designed and built the first atomic bombs.
The first atomic explosion was conducted, as a test, at Alamogordo,
New Mexico, on July 16, 1945. The energy released from this explosion was
equivalent to that released by the detonation of 20,000 tons of TNT. Near the
end of World War II, on August 6, 1945, the United States dropped the first
atomic bomb on the Japanese city of Hiroshima. It followed with a second bomb
against the city of Nagasaki on August 9. According to U.S. estimates, 60,000
to 70,000 people were killed by the Hiroshima bomb, called “Little Boy,” and
about 40,000 by the bomb dropped on Nagasaki, called “Fat Man.” Japan agreed to
Allied terms of surrender on August 14th. These are the only times that a
nuclear weapon has been used in a conflict between nations.
Fusion bombs, also called hydrogen or thermonuclear bombs, were
developed and tested in the early 1950s, but these have never been used in
warfare. A thermonuclear device depends on a fission reaction to produce
extreme heat that causes hydrogen isotopes of deuterium and tritium to come
together, or fuse, but the main energy source for thermonuclear devices comes
from the fusion reaction, not the triggering fission reaction. For more
information on this type of bomb, see Hydrogen Bomb.
Several nations have exploded nuclear devices in tests in the
atmosphere, under the earth, and under the sea. Only the United States, Russia,
the United Kingdom, France, China, India, and Pakistan admit to possessing
nuclear weapons. South Africa admitted to having built and then dismantling a
number of bombs. Other nations, however, including Israel, are thought to have
them as well or to have the capability to assemble them quickly. See also Arms
Control, International.
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