Nuclear
Weapons
I INTRODUCTION
Nuclear Weapons, explosive devices designed to
release nuclear energy on a large scale, used primarily in military
applications. The first atomic bomb (or A-bomb), which was tested on July 16,
1945, at Alamogordo, New Mexico, represented a completely new type of
explosive. All explosives prior to that time derived their power from the rapid
burning or decomposition of some chemical compound. Such chemical processes
release only the energy of the outermost electrons in the atom. See Atom.
Nuclear explosives, on the other hand, involve energy
sources within the core, or nucleus, of the atom. The A-bomb gained its power
from the splitting, or fission, of all the atomic nuclei in several kilograms
of plutonium. A sphere about the size of a baseball produced an explosion equal
to 20,000 tons of TNT.
The A-bomb was developed, constructed, and tested
by the Manhattan Project, a massive United States enterprise that was
established in August 1942, during World War II. Many prominent American
scientists, including the physicists Enrico Fermi and
J. Robert Oppenheimer and the chemist Harold Urey,
were associated with the project, which was headed by a U.S. Army engineer,
then-Brigadier General Leslie R. Groves.
After the war, the U.S. Atomic Energy
Commission became responsible for the oversight of all nuclear matters,
including research on hydrogen bombs. In these bombs the source of energy is
the fusion process, in which nuclei of the isotopes of hydrogen combine to form
a heavier helium nucleus (see Thermonuclear, or Fusion, Weapons below). This
weapons research resulted in the production of bombs that range in power from a
fraction of a kiloton (1,000 tons of TNT equivalent)
to many megatons (1 megaton equals 1 million tons of TNT equivalent).
Furthermore, the physical size of a nuclear bomb was drastically reduced,
permitting the development of nuclear artillery shells and small missiles that
can be fired from portable launchers in the field. Although nuclear bombs were
originally developed as strategic weapons to be carried by large bombers,
nuclear weapons are now available for a variety of both strategic and tactical
applications. Not only can they be delivered by different types of aircraft,
but rockets and guided missiles of many sizes can now carry nuclear warheads
and can be launched from the ground, the air, or underwater. Large rockets can
carry multiple warheads for delivery to separate targets. See also ICBM;
SLBM; MIRV.
II FISSION WEAPONS
In 1905 Albert Einstein published his
special theory of relativity. According to this theory, the relation between
mass and energy is expressed by the equation E = mc2, which
states that a given mass (m) is associated with an amount of energy (E)
equal to this mass multiplied by the square of the speed of light (c). A
very small amount of matter is equivalent to a vast amount of energy. For
example, 1 kg (2.2 lb) of matter converted completely into energy would be
equivalent to the energy released by exploding 22 megatons of TNT.
In 1938 German chemists Otto Hahn and Fritz Strassmann split the uranium atom into two roughly equal
parts by bombardment with neutrons. As a result of these experiments, the
Austrian physicist Lise Meitner,
with her nephew, the British physicist Otto Robert Frisch, went on to explain
the process of nuclear fission in 1939, placing the release of atomic energy
within reach.
III THE CHAIN REACTION
When a uranium or other suitable
nucleus fissions, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast
neutrons, the same type of particle that initiated the fission of the uranium
nucleus. This makes it possible to achieve a self-sustaining series of nuclear
fissions; the neutrons that are emitted in fission produce a chain reaction,
with continuous release of energy.
The light isotope of uranium, uranium-235, is
easily split by the fission neutrons and, upon fission, emits an average of
about 2.5 neutrons. One neutron per generation of nuclear fissions is necessary
to sustain the chain reactions. Others may be lost by escape from the mass of
chain-reacting material, or they may be absorbed in impurities or in the heavy
uranium isotope, uranium-238, if it is present. Any substance capable of
sustaining a fission chain reaction is known as a fissile material.
IV CRITICAL MASS
A small sphere of pure fissile material, such
as uranium-235, about the size of a golf ball, would not sustain a chain
reaction. Too many neutrons escape through the surface area, which is
relatively large compared with its volume, and thus are lost to the chain
reaction. In a mass of uranium-235 about the size of a baseball, however, the
number of neutrons lost through the surface is compensated for by the neutrons
generated in additional fissions taking place within the sphere. The minimum
amount of fissile material (of a given shape) required to maintain the chain
reaction is known as the critical mass. Increasing the size of the sphere
produces a supercritical assembly, in which the successive generations of
fissions increase very rapidly, leading to a possible explosion as a result of
the extremely rapid release of a large amount of energy. In an atomic bomb,
therefore, a mass of fissile material greater than the critical mass must be
assembled instantaneously and held together for about a millionth of a second
to permit the chain reaction to propagate before the bomb explodes. A heavy
material, called a tamper, surrounds the fissile mass and prevents its premature
disruption. The tamper also reduces the number of neutrons that escape.
If every atom in 0.5 kg (1.1 lb) of
uranium were to split, the energy produced would equal the explosive power of
9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process
would be 100 percent. In the first A-bomb tests, this kind of efficiency was
not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical
assembly.
V DETONATION OF ATOMIC BOMBS
Various systems have been devised to detonate the
atomic bomb. The simplest system is the gun-type weapon, in which a projectile
made of fissile material is fired at a target of the same material so that the
two weld together into a supercritical assembly. The atomic bomb exploded by
the United States over Hiroshima, Japan, on August 6, 1945, was a gun-type
weapon. It had the energy of anywhere between 12.5 and 15 kilotons of TNT.
Three days later the United States dropped a second atomic bomb over Nagasaki,
Japan, with the energy equivalent of about 20 kilotons of TNT.
A more complex method, known as implosion, is used
in a spherically shaped weapon. The outer part of the sphere consists of a
layer of closely fitted and specially shaped devices, called lenses, consisting
of high explosive and designed to concentrate the blast toward the center of the bomb. Each segment of the high explosive is
equipped with a detonator, which in turn is wired to all other segments. An
electrical impulse explodes all the chunks of high explosive simultaneously,
resulting in a detonation wave that converges toward the core of the weapon. At
the core is a sphere of fissile material, which is compressed by the powerful,
inwardly directed pressure, or implosion. The density of the metal is
increased, and a supercritical assembly is produced. The Alamogordo test bomb, as well as the one dropped by the United States on Nagasaki,
Japan, on August 9, 1945, were of the implosion type. Each was
equivalent to about 20 kilotons of TNT.
Regardless of the method used to attain a supercritical
assembly, the chain reaction proceeds for about a millionth of
a second, liberating vast amounts of heat energy. The extremely fast
release of a very large amount of energy in a relatively small volume causes
the temperature to rise to tens of millions of degrees. The resulting rapid
expansion and vaporization of the bomb material causes a powerful explosion.
VI PRODUCTION OF FISSILE MATERIAL
Much experimentation was necessary to make the
production of fissile material practical.
A Separation of Uranium Isotopes
The fissile isotope uranium-235 accounts for only
0.7 percent of natural uranium; the remainder is composed of the heavier
uranium-238. No chemical methods suffice to separate uranium-235 from ordinary
uranium, because both uranium isotopes are chemically identical. A number of
techniques were devised to separate the two, all of which depend in principle
on the slight difference in weight between the two types of uranium atoms.
A huge gaseous-diffusion plant was built during World War
II in Oak Ridge, Tennessee. This plant was enlarged after the war, and two
similar plants were built near Paducah, Kentucky, and Portsmouth, Ohio. The
feed material for this type of plant consists of extremely corrosive uranium
hexafluoride gas. The gas is pumped against barriers that have many millions of
tiny holes, through which the lighter molecules, which contain uranium-235
atoms, diffuse at a slightly greater rate than the heavier molecules,
containing uranium-238 (see Diffusion). After the gas has been cycled
through thousands of barriers, known as stages, it is highly enriched in the
lighter isotope of uranium. The final product is weapon-grade uranium,
containing more than 90 percent uranium-235.
B Producing Plutonium
Although the heavy uranium isotope uranium-238 will
not sustain a chain reaction, it can be converted into a fissile material by
bombarding it with neutrons and transforming it into a new species of element.
When the uranium-238 atom captures a neutron in its nucleus, it is transformed
into the heavier isotope uranium-239. This nuclear species quickly
disintegrates to form neptunium-239, an isotope of element 93 (see Neptunium).
Another disintegration transmutes this isotope into an
isotope of element 94, called plutonium-239. Plutonium-239, like uranium-235,
undergoes fission after the absorption of a neutron and can be used as a bomb
material. Producing plutonium-239 in large quantities requires an intense
source of neutrons; the source is provided by the controlled chain reaction in
a nuclear reactor. See Nuclear Chemistry.
During World War II nuclear reactors were
designed to provide neutrons to produce plutonium. Reactors capable of
manufacturing large quantities of plutonium were established in Hanford,
Washington, and near Aiken, South Carolina.
VII THERMONUCLEAR, OR FUSION, WEAPONS
Even before the first atomic bomb was developed,
scientists realized that a type of nuclear reaction different from the fission
process was theoretically possible as a source of nuclear energy. Instead of
using the energy released as a result of a chain reaction in fissile material,
nuclear weapons could use the energy liberated in the fusion of light elements.
This process is the opposite of fission, since it involves the fusing together
of the nuclei of isotopes of light atoms such as hydrogen. It is for this
reason that the weapons based on nuclear-fusion reactions are often called
hydrogen bombs, or H-bombs. Of the three isotopes of hydrogen
the two heaviest species, deuterium and tritium, combine most readily to form
helium. Although the energy release in the fusion process is less per
nuclear reaction than in fission, 0.5 kg (1.1 lb) of the lighter material
contains many more atoms; thus, the energy liberated from 0.5 kg (1.1 lb) of
hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or
almost three times as much as from uranium. This estimate, however, is based on
complete fusion of all hydrogen atoms. Fusion reactions occur only at
temperatures of several millions of degrees, the rate increasing enormously
with increasing temperature; such reactions consequently are known as
thermonuclear (heat-induced) reactions. Strictly speaking, the term thermonuclear
implies that the nuclei have a range (or distribution) of energies
characteristic of the temperature. This plays an important role in making rapid
fusion reactions possible by an increase in temperature.
Development of the hydrogen bomb was impossible
before the perfection of A-bombs, for only the latter could yield that
tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic
scientists regarded the A-bomb as the trigger of the projected thermonuclear
device.
A Thermonuclear Tests
Following developmental tests in the spring of 1951 at the
U.S. Enewetak Proving Grounds in
the Marshall Islands during Operation Greenhouse, a full-scale, successful
experiment was conducted on November 1, 1952, with a fusion-type device. This
test, called Mike, which was part of Operation Ivy, produced an explosion with
power equivalent to several million tons of TNT (that is, several megatons).
The Soviet Union detonated a thermonuclear weapon in the megaton range in
August 1953. On March 1, 1954, the United States exploded a fusion bomb with a
power of 15 megatons. It created a glowing fireball, more than 4.8 km (more
than 3 mi) in diameter, and a huge mushroom cloud, which quickly rose into the
stratosphere.
The March 1954 explosion led to worldwide
recognition of the nature of radioactive fallout. The fallout of radioactive
debris from the huge bomb cloud also revealed much about the nature of the
thermonuclear bomb. Had the bomb been a weapon consisting of an A-bomb trigger
and a core of hydrogen isotopes, the only persistent radioactivity from the
explosion would have been the result of the fission debris from the trigger and
from the radioactivity induced by neutrons in coral and seawater. Some of the
radioactive debris, however, fell on the Lucky Dragon, a Japanese vessel
engaged in tuna fishing about 160 km (about 100 mi) from the test site. This
radioactive dust was later analyzed by Japanese scientists. The results
demonstrated that the bomb that dusted the Lucky Dragon with fallout was
more than just an H-bomb.
VIII FISSION-FUSION-FISSION BOMB
The thermonuclear bomb exploded in 1954 was a
three-stage weapon. The first stage consisted of a big A-bomb, which acted as a
trigger. The second stage was the H-bomb phase resulting from the fusion of
deuterium and tritium within the bomb. In the process helium and high-energy
neutrons were formed. The third stage resulted from the impact of these
high-speed neutrons on the outer jacket of the bomb, which consisted of natural
uranium, or uranium-238. No chain reaction was produced, but the fusion
neutrons had sufficient energy to cause fission of the uranium nuclei and thus
added to the explosive yield and also to the radioactivity of the bomb
residues.
IX EFFECTS OF NUCLEAR WEAPONS
The effects of nuclear weapons were carefully
observed, both after the bombings of Hiroshima and Nagasaki and after many test
explosions in the 1950s and early 1960s.
A Blast Effects
As is the case with explosions caused by
conventional weapons, most of the damage to buildings and other structures from
a nuclear explosion results, directly or indirectly, from the effects of blast.
The very rapid expansion of the bomb materials produces a high-pressure pulse,
or shock wave, that moves rapidly outward from the exploding bomb. In air, this
shock wave is called a blast wave because it is equivalent to and is
accompanied by powerful winds of much greater than hurricane force. Damage is
caused both by the high excess (or overpressure) of air at the front of the
blast wave and by the extremely strong winds that persist after the wave front
has passed. The degree of blast damage suffered on the ground depends on the
TNT equivalent of the explosion; the altitude at which the bomb is exploded,
referred to as the height of burst; and the distance of the structure from
ground zero, that is, the point directly under the bomb. For the 20-kiloton
A-bombs detonated over Japan, the height of burst was about 580 m ( about 1,900 ft), because it was estimated that this height
would produce a maximum area of damage. If the TNT equivalent had been larger,
a greater height of burst would have been chosen.
Assuming a height of burst that will maximize
the damage area, a 10-kiloton bomb will cause severe damage to wood-frame
houses, such as are common in the United States, to a distance of more than 1.6
km (more than 1 mi) from ground zero and moderate damage as far as 2.4 km (1.5
mi). (A severely damaged house probably would be beyond repair.) The damage
radius increases with the power of the bomb, approximately in proportion to its
cube root. If exploded at the optimum height, therefore, a 10-megaton weapon,
which is 1,000 times as powerful as a 10-kiloton weapon, will increase the
distance tenfold, that is, out to 17.7 km (11 mi) for severe damage and 24 km
(15 mi) for moderate damage of a frame house.
B Thermal Effects
The very high temperatures attained in a nuclear
explosion result in the formation of an extremely hot incandescent mass of gas
called a fireball. For a 10-kiloton explosion in the air, the fireball will
attain a maximum diameter of about 300 m (about 1,000 ft); for a 10-megaton
weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (or heat)
radiation is emitted from the fireball and spreads out over a large area, but
with steadily decreasing intensity. The amount of heat energy received a
certain distance from the nuclear explosion depends on the power of the weapon
and the state of the atmosphere. If the visibility is poor or the explosion
takes place above clouds, the effectiveness of the heat flash is decreased. The
thermal radiation falling on exposed skin can cause what are called flash
burns. A 10-kiloton explosion in the air can produce moderate (second-degree)
flash burns, which require some medical attention, as far as 2.4 km (1.5 mi)
from ground zero; for a 10-megaton bomb, the corresponding distance would be
more than 32 km (more than 20 mi). Milder burns of bare skin would be
experienced even farther out. Most ordinary clothing provides protection from
the heat radiation, as does almost any opaque object. Flash burns occur only
when the bare skin is directly exposed, or if the clothing is too thin to
absorb the thermal radiation.
The heat radiation can initiate fires in dry,
flammable materials, for example, paper and some fabrics, and such fires may
spread if conditions are suitable. The evidence from the A-bomb explosions over
Japan indicates that many fires, especially in the area near ground zero,
originated from secondary causes, such as electrical short circuits, broken gas
lines, and upset furnaces and boilers in industrial plants. The blast damage
produced debris that helped to maintain the fires and denied access to
fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary
effect of the blast wave.
Under some conditions, such as existed at Hiroshima but
not at Nagasaki, many individual fires can combine to produce a fire storm
similar to those that accompany some large forest fires. The heat of the fire
causes a strong updraft, which produces strong winds drawn in toward the center of the burning area. These winds fan the flame and
convert the area into a holocaust in which everything flammable is destroyed.
Inasmuch as the flames are drawn inward, however, the area over which such a
fire spreads may be limited.
C Penetrating Radiation
Besides heat and blast, an exploding nuclear bomb
has a unique effect—it releases penetrating nuclear radiation, which is quite
different from thermal (or heat) radiation (see Radioactivity). When
absorbed by the body, nuclear radiation can cause serious injury. For an
explosion high in the air, the injury range for these radiations is less than
for blast and fire damage or flash burns. In Japan, however, many individuals
who were protected from blast and burns succumbed later to radiation injury.
Nuclear radiation from an explosion may be divided into
two categories, namely, prompt radiation and residual radiation. The prompt
radiation consists of an instantaneous burst of neutrons and gamma rays, which
travel over an area of several square miles. Gamma rays are identical in effect
to X rays (see X Ray). Both neutrons and gamma
rays have the ability to penetrate solid matter, so that substantial
thicknesses of shielding materials are required.
The residual nuclear radiation, generally known as
fallout, can be a hazard over very large areas that are completely free from
other effects of a nuclear explosion. In bombs that gain their energy from
fission of uranium-235 or plutonium-239, two radioactive nuclei are produced
for every fissile nucleus split. These fission products account for the
persistent radioactivity in bomb debris, because many of the atoms have
half-lives measured in days, months, or years.
Two distinct categories of fallout, namely, early and
delayed, are known. If a nuclear explosion occurs near the surface, earth or
water is taken up into a mushroom-shaped cloud and becomes contaminated with
the radioactive weapon residues. The contaminated material begins to descend
within a few minutes and may continue to fall for about 24 hours, covering an
area of thousands of square miles downwind from the explosion. This constitutes
the early fallout, which is an immediate hazard to human beings. No early
fallout is associated with high-altitude explosions. If a nuclear bomb is
exploded well above the ground, the radioactive residues rise to a great height
in the mushroom cloud and descend gradually over a large area.
Human experience with radioactive fallout has been
minimal. The principal known case histories have been derived from the
accidental exposure of fishermen and local residents to the fallout from the
15-megaton explosion that occurred on March 1, 1954. The nature of
radioactivity, however, and the immense areas contaminable by a single bomb
undoubtedly make radioactive fallout potentially one of the most lethal effects
of nuclear weapons.
D Climatic Effects
Besides the blast and radiation damage from
individual bombs, a large-scale nuclear exchange between nations could
conceivably have a catastrophic global effect on climate. This possibility, proposed
in a paper published by an international group of scientists in December 1983,
has come to be known as the “nuclear winter” theory. According to these
scientists, the explosion of not even one-half of the combined number of
warheads in the United States and Russia would throw enormous quantities of
dust and smoke into the atmosphere. The amount could be sufficient to block off
sunlight for several months, particularly in the northern hemisphere,
destroying plant life and creating a subfreezing climate until the dust
dispersed. The ozone layer might also be affected, permitting further damage as
a result of the sun’s ultraviolet radiation. Were the results sufficiently
prolonged, they could spell the virtual end of human civilization. The nuclear
winter theory has since become the subject of enormous controversy. It found
support in a study released in December 1984 by the U.S. National Research
Council, and other groups have undertaken similar research. In 1985, however,
the U.S. Department of Defense released a report
acknowledging the validity of the concept but saying that it would not affect defense policies.
E Clean H-Bombs
On the average, about 50 percent of the power of an
H-bomb results from thermonuclear-fusion reactions and the other 50 percent
from fission that occurs in the A-bomb trigger and in the uranium jacket. A
clean H-bomb is defined as one in which a significantly smaller proportion than
50 percent of the energy arises from fission. Because fusion does not produce
any radioactive products directly, the fallout from a clean weapon is less than
that from a normal or average H-bomb of the same total power. If an H-bomb were
made with no uranium jacket but with a fission trigger, it would be relatively
clean. Perhaps as little as 5 percent of the total explosive force might result
from fission; the weapon would thus be 95 percent clean. The enhanced-radiation
fusion bomb, also called the neutron bomb, which has been tested by the United
States and other nuclear powers, does not release long-lasting radioactive
fission products. However, the large number of neutrons released in
thermonuclear reactions is known to induce radioactivity in materials,
especially earth and water, within a relatively small area around the
explosion. Thus the neutron bomb is considered a tactical weapon because it can
do serious damage on the battlefield, penetrating tanks and other armored vehicles and causing death or serious injury to
exposed individuals, without producing the radioactive fallout that endangers
people or structures miles away. See also Arms Control, International;
Guided Missiles; Warfare.
Contributed By: Samuel Glasstone
Microsoft ® Encarta ® Reference Library 2003. ©
1993-2002 Microsoft Corporation. All rights reserved.
nuclear weapon
also called ATOMIC WEAPON, or THERMONUCLEAR
WEAPON, bomb or other warhead that derives its force from either the
fission or the fusion of atomic nuclei and is delivered by an aircraft,
missile, Earth satellite, or other strategic delivery system.
Nuclear
weapons have enormous explosive force. Their significance may best be
appreciated by the coining of the words kiloton (1,000 tons) and megaton (one
million tons) to describe their blast effect in equivalent weights of TNT. For
example, the first nuclear
fission bomb, the one dropped on Hiroshima, Japan, in 1945, released energy
equaling 15,000 tons (15 kilotons) of chemical
explosive from less than 130 pounds (60 kilograms) of uranium. Fusion
bombs, on the other hand, have given yields up to almost 60 megatons.
The first nuclear weapons
were bombs delivered by aircraft; warheads for strategic ballistic missiles, however, have become by far the most important
nuclear weapons. There are also smaller tactical nuclear weapons that include
artillery projectiles, demolition munitions (land mines), antisubmarine depth
bombs, torpedoes, and short-range ballistic and cruise missiles. The U.S.
stockpile of nuclear weapons reached its peak in 1967 with more than 32,000
warheads of 30 different types; the Soviet stockpile reached its peak of about
33,000 warheads in 1988.
Principles of fission weapons
The basic principle of
nuclear fission weapons (also called atomic
bombs) involves the assembly of a sufficient amount of fissile material (e.g.,
the uranium isotope uranium-235 or the plutonium isotope plutonium-239) to
"go supercritical"--that is, for neutrons (which cause fission and
are in turn released during fission) to be produced at a much faster rate than
they can escape from the assembly. There are two ways in which a subcritical assembly of fissionable material can be
rendered supercritical and made to explode. The subcritical
assembly may consist of two parts, each of which is too small to have a
positive multiplication rate; the two parts can be shot together by a gun-type
device. Alternatively, a subcritical assembly
surrounded by a chemical high explosive may be compressed into a supercritical
one by detonating the explosive.
Principles of fusion weapons
The basic principle of the
fusion weapon (also called the thermonuclear or hydrogen
bomb) is to produce ignition conditions in a thermonuclear fuel such as
deuterium, an isotope of hydrogen with double the
weight of normal hydrogen, or lithium deuteride. The Sun
may be considered a thermonuclear device; its main fuel is deuterium, which it
consumes in its core at temperatures of 18,000,000
to
36,000,000 F
(10,000,000 to
20,000,000 C).
To achieve comparable temperatures in a weapon, a fission triggering device is
used.
The development of fission
weapons
Following the discovery of
artificial radioactivity in the 1930s, the Italian physicist Enrico Fermi performed a series of experiments in which
he exposed many elements to low-velocity neutrons. When he exposed thorium and uranium,
chemically different radioactive products resulted, indicating that new
elements had been formed, rather than merely isotopes of the original elements.
Fermi concluded that he had produced elements beyond uranium (element 92), then
the last element in the periodic table; he called them transuranic elements and named two of them ausenium (element 93) and hesperium
(element 94). During the autumn of 1938, however, when Fermi was receiving the
Nobel Prize for his work, Otto
Hahn and Fritz
Strassmann of Germany discovered that one of the
"new" elements was actually barium (element 56).
The Danish scientist Niels Bohr
visited the United
States in January 1939, carrying with him an explanation, devised by the
Austrian refugee scientist Lise Meitner
and her nephew Otto Frisch, of the process behind Hahn's surprising data.
Low-velocity neutrons
caused the uranium nucleus to fission, or break apart, into two smaller pieces;
the combined atomic numbers of the two pieces--for example, barium and
krypton--equalled that of the uranium nucleus. Much energy was released in the
process. This news set off experiments at many laboratories. Bohr worked with
John Wheeler
at Princeton; they postulated that the uranium isotope uranium-235
was the one undergoing fission; the other isotope, uranium-238,
merely absorbed the neutrons. It was discovered that neutrons were produced
during the fission process; on the average, each fissioning
atom produced more than two neutrons. If the proper amount of material were
assembled, these free neutrons might create a chain reaction. Under special
conditions, a very fast chain reaction might produce a very large release of
energy; in short, a weapon of fantastic power might be feasible.
The first atomic bomb
The possibility that such a
weapon might first be developed by Nazi Germany alarmed many scientists and was
drawn to the attention of President Franklin
D. Roosevelt by Albert Einstein, then living in the United States. The
president appointed an Advisory Committee on Uranium; it reported that a chain
reaction in uranium was possible, though unproved. Chain-reaction experiments
with carbon and uranium were started in New York City at Columbia University,
and in March 1940 it was confirmed that the isotope uranium-235 was responsible
for low-velocity neutron fission in uranium. The Advisory Committee on Uranium
increased its support of the Columbia experiments and arranged for a study of
possible methods for separating the uranium-235 isotope from the much more
abundant uranium-238. (Normal uranium contains approximately 0.7 percent
uranium-235, most of the remainder being uranium-238.) The centrifuge
process, in which the heavier isotope is spun to the outside, as in a cream
separator, at first seemed the most useful, but at Columbia a rival process was
proposed. In that process, gaseous uranium hexafluoride is diffused through
barriers, or filters; more molecules containing the lighter isotope,
uranium-235, would pass through the filter than those containing the heavier
isotope, slightly enriching the mixture on the far side. A sequence of several
thousand stages would be needed to enrich the mixture to 90 percent
uranium-235; the total barrier area would be many acres.
During the summer of 1940,
Edwin McMillan
and Philip Abelson of the University of California at Berkeley
discovered element 93, named neptunium;
they inferred that this element would decay into element 94. The Bohr and
Wheeler fission theory suggested that one of the isotopes, mass number 239, of
this new element might also fission under low-velocity neutron bombardment. The
cyclotron at the University of California at Berkeley was put to work to make
enough element 94 for experiments; by mid-1941, element 94 had been firmly
identified and named plutonium,
and its fission characteristics had been established. Low-velocity neutrons did
indeed cause it to undergo fission, and at a rate much higher than that of
uranium-235. The Berkeley group, under Ernest Lawrence, was also considering
producing large quantities of uranium-235 by turning one of their cyclotrons
into a super mass spectrograph. A mass spectrograph employs a magnetic field to
bend a current of uranium ions; the heavier ions (uranium-238) bend at a larger
radius than the lighter ions (uranium-235), allowing the two separated currents
to be collected in separate receivers.
In May 1941 a review
committee reported that a nuclear explosive probably could not be available
before 1945, that a chain reaction in natural uranium was probably 18 months
off, and that it would take at least an additional year to produce enough
plutonium for a bomb and three to five years to separate enough uranium-235.
Further, it was held that all of these estimates were optimistic. In late June
1941 President Roosevelt established the Office
of Scientific Research and Development under the direction of the scientist
Vannevar Bush.
In the fall of 1941 the
Columbia chain-reaction experiment with natural uranium and carbon yielded
negative results. A review committee concluded that boron impurities might be
poisoning it by absorbing neutrons. It was decided to transfer all such work to
the University of Chicago and repeat the experiment there with high-purity
carbon. At Berkeley, the cyclotron, converted into a mass spectrograph (later
called a calutron), was exceeding expectations in separating
uranium-235, and it was enlarged to a 10-calutron system capable of producing
one-tenth of an ounce (about three grams) of uranium-235 per day.
The U.S. entry into World
War II in December 1941 was decisive in providing funds for a massive
research and production effort for obtaining fissionable materials, and in May
1942 the momentous decision was made to proceed simultaneously on all promising
production methods. Bush decided that the army should be brought into the
production plant construction activities. The Corps of Engineers opened an
office in New York City and named it the Manhattan
Engineer District Office. After considerable argument over priorities, a
workable arrangement was achieved with the formation of a three-man policy
board chaired by Bush and the appointment on September 17 of Colonel Leslie
Groves as head of the Manhattan Engineer District. Groves arranged
contracts for a gaseous diffusion separation plant, a plutonium production
facility, and a calutron pilot plant, which might be
expanded later. The day before the success of Fermi's chain-reaction experiment
at the University of Chicago on Dec. 2, 1942, Groves (now a brigadier general)
signed the construction contract for the plutonium production reactors. Many
problems were still unsolved, however. First, the gaseous diffusion barrier had
not yet been demonstrated as practical. Second, Berkeley had been successful
with its empirically designed calutron, but the Oak
Ridge pilot plant contractors were understandably uneasy about the rough
specifications available for the massive separation of uranium-235, which was
designated the Y-12 effort. Third, plutonium chemistry was almost unknown; in
fact, it was not known whether or not plutonium gave off neutrons during
fission, or, if so, how many.
Meantime, as part of the June
1942 reorganization, J. Robert Oppenheimer
became, in October, the director of Project Y, the group that was to design the
actual weapon. This effort was spread over several locations. On November 16
Groves and Oppenheimer visited the former Los
Alamos Ranch School, some 60 miles (100 kilometres) north of Albuquerque,
N.M., and on November 25 Groves approved it as the site for the Los Alamos
Scientific Laboratory. By July two essential and encouraging pieces of
experimental data had been obtained--plutonium did give off neutrons in
fission, more than uranium-235; and the neutrons were emitted in a short time
compared to that needed to bring the weapon materials into a supercritical
assembly. The theorists contributed one discouraging note: their estimate of
the critical mass for uranium-235 had risen over threefold, to something
between 50 and 100 pounds.
The emphasis during the
summer and fall of 1943 was on the gun method of assembly, in which the
projectile, a subcritical piece of uranium-235 (or plutonium-239),
would be placed in a gun barrel and fired into the target, another subcritical piece of uranium-235. After
the mass was joined (and now supercritical), a neutron source would be used to
start the chain reaction. A problem developed with applying the gun
method to plutonium, however. In manufacturing plutonium-239 from uranium-238
in a reactor, some of the plutonium-239 absorbs a neutron and becomes plutonium-240.
This material undergoes spontaneous fission, producing neutrons. Some neutrons
will always be present in a plutonium assembly and cause it to begin
multiplying as soon as it goes critical, before it reaches supercriticality;
it will then explode prematurely and produce comparatively little energy. The
gun designers tried to beat this problem by achieving higher projectile speeds,
but they lost out in the end to a better idea--the implosion method.
In April 1943 a Project Y
physicist, Seth
Neddermeyer, proposed to assemble a supercritical
mass from many directions, instead of just two as in the gun. In particular, a number
of shaped charges placed on the surface of a sphere would fire many subcritical pieces into one common ball at the centre of
the sphere. John
von Neumann, a mathematician who had had experience in shaped-charge, armour-piercing
work, supported the implosion method enthusiastically and pointed out that the
greater speed of assembly might solve the plutonium-240 problem. The physicist Edward
Teller suggested that the converging material might also become compressed, offering the possibility that less material
would be needed. By late 1943 the implosion method was being given an
increasingly higher priority; by July 1944 it had become clear that the
plutonium gun could not be built. The only way to use plutonium in a weapon was
by the implosion method.
By 1944 the Manhattan Project
was spending money at a rate of more than $1 billion per year. The situation
was likened to a nightmarish horse race; no one could say which of the horses
(the calutron plant, the diffusion plant, or the
plutonium reactors) was likely to win or whether any of them would even finish
the race. In July 1944 the first Y-12 calutrons had
been running for three months but were operating at less than 50 percent
efficiency; the main problem was in recovering the large amounts of material
that reached neither the uranium-235 nor uranium-238 boxes and, thus, had to be
rerun through the system. The gaseous diffusion plant was far from completion,
the production of satisfactory barriers remaining the major problem. And the
first plutonium reactor at Hanford, Wash., had been turned on in September, but
it had promptly turned itself off. Solving this problem, which proved to be
caused by absorption of neutrons by one of the fission products, took several
months. These delays meant almost certainly that the war in Europe would be
over before the weapon could be ready. The ultimate target was slowly changing
from Germany to Japan.
Within 24 hours of
Roosevelt's death on April 12, 1945, President Harry
S. Truman was told briefly about the atomic bomb by Secretary of War Henry Stimson. On April 25 Stimson,
with Groves's assistance, gave Truman a more
extensive briefing on the status of the project: the uranium-235 gun design had
been frozen, but sufficient uranium-235 would not be accumulated until around
August 1. Enough plutonium-239 would be available for an implosion assembly to
be tested in early July; a second would be ready in August. Several B-29s had
been modified to carry the weapons, and support construction was under way at Tinian, in the Mariana Islands, 1,500 miles south of Japan.
The test of the plutonium
weapon was named Trinity;
it was fired at 5:29:45 AM (local time) on July 16, 1945, at the Alamogordo
Bombing Range in south central New Mexico. The theorists' predictions of the
energy release ranged from the equivalent of less than 1,000 tons of TNT to
45,000 tons. The test produced an energy, or yield, equivalent to 21,000 tons
of TNT.
A single B-29 bomber, named
the Enola
Gay, flew over Hiroshima,
Japan, on Monday, Aug. 6, 1945, at 8:15 in the morning, local time. The
untested uranium-235 gun-assembly bomb, nicknamed Little Boy, was air-burst
1,900 feet (680 metres) above the city to maximize destruction. Two-thirds of
the city area was destroyed. The population actually present at the time was
estimated at 350,000; of these, 140,000 died by the end of the year. The second
weapon, a duplicate of the plutonium-239 implosion assembly tested in Trinity
and nicknamed Fat Man, was to be dropped on Kokura
on August 11; a third was being prepared in the United States for possible use
in late August or early September. To avoid bad weather, the schedule was moved
up two days to August 9. The B-29, named Bock's Car, spent 10 minutes
over Kokura without sighting its aim point; it then proceeded to the secondary
target of Nagasaki,
where, at 11:02 AM local time, the weapon was air-burst at 1,650 feet with a
force later estimated at 21 kilotons. About half the city was destroyed, and,
of the estimated 270,000 people present at the time, about 70,000 died by the
end of the year.
The spread of atomic weaponsScientists in several countries performed
experiments in connection with nuclear reactors and fission weapons during
World War II, but no country other than the United States carried its projects
as far as separating uranium-235 or manufacturing plutonium-239.The Axis powersBy the time the war began on Sept. 1, 1939, Germany
had a special office for the military application of nuclear fission;
chain-reaction experiments with uranium and carbon were being planned, and ways
of separating the uranium isotopes were under study. Some measurements on
carbon, later shown to be in error, led the physicist Werner Heisenberg to
recommend that heavy water be used, instead, for the moderator. This dependence
on scarce heavy water was a major reason the German experiments never reached a
successful conclusion. The isotope separation studies were oriented toward low
enrichments (about 1 percent uranium-235) for the chain reaction experiments;
they never got past the laboratory apparatus stage, and several times these
prototypes were destroyed in bombing attacks. As for the fission weapon itself,
it was a rather distant goal, and practically nothing
but "back-of-the-envelope" studies were done on it.Like
their counterparts elsewhere, Japanese scientists initiated research on an
atomic bomb. In December 1940, Japan's leading scientist, Nishina
Yoshio, undertook a small-scale research effort supported by the armed forces.
It did not progress beyond the laboratory owing to lack of government support,
resources, and uranium.
Great Britain
The British weapon project
started informally, as in the United States, among university physicists. In
April 1940 a short paper by Otto Frisch
and Rudolf Peierls, expanding on the idea of critical mass, estimated
that a superweapon could be built using several
pounds of pure uranium-235 and that this amount of material might be obtainable
from a chain of diffusion tubes. This three-page memorandum was the first report
to foretell with scientific conviction the practical possibility of making a
bomb and the horrors it would bring. A group of scientists known as the MAUD
committee was set up in the Ministry of Aircraft Production in April 1940 to
decide if a uranium bomb could be made. The committee approved a report on July
15, 1941, concluding that the scheme for a uranium bomb was practicable, that
work should continue on the highest priority, and that collaboration with the
Americans should be continued and expanded. As the war took its toll on the
economy, the British position evolved through 1942 and 1943 to one of full
support for the American project with the realization that Britain's major
effort would come after the war. While the British program was sharply reduced
at home, approximately 50 scientists and engineers went to the United States at
the end of 1943 and during 1944 to work on various aspects of the Manhattan
Project. The valuable knowledge and experience they acquired sped the
development of the British bomb after 1945.
The formal postwar decision to manufacture a British atomic bomb was
made by Prime Minister Clement Attlee's
government during a meeting of the Defence Subcommittee of the Cabinet in early
January 1947. The construction of a first reactor to produce fissile material
and associated facilities had gotten under way the year before. William Penney,
a member of the British team at Los Alamos during the war, was placed in charge
of fabricating and testing the bomb, which was to be of a plutonium type
similar to the one dropped on Nagasaki. That Britain was developing nuclear
weapons was not made public until Prime Minister Winston Churchill
announced on Feb. 17, 1952, plans to test the first British-made atomic bomb at
the Monte Bello Islands off the northwest coast of
Australia. There, on Oct. 3, 1952, the first British atomic weapons test,
called Hurricane, was successfully conducted aboard the frigate HMS Plym. By early 1954 Royal Air Force Canberra bombers
were armed with atomic bombs.
The Soviet
Union
In the decade before the war,
Soviet physicists were actively engaged in nuclear and atomic research. By 1939
they had established that, once uranium has been fissioned,
each nucleus emits neutrons and can therefore, at least in theory, begin a
chain reaction. The following year, physicists concluded that such a chain
reaction could be ignited in either natural uranium or
its isotope, uranium-235, and that this reaction could be sustained and
controlled with a moderator such as heavy water. In June 1940 the Soviet
Academy of Sciences established the Uranium Commission to study the
"uranium problem."
In February 1939, news had
reached Soviet physicists of the discovery of nuclear fission in the West. The
military implications of such a discovery were immediately apparent, but Soviet
research was brought to a halt by the German invasion in June 1941. In early
1942 the physicist Georgy N. Flerov noticed that
articles on nuclear fission were no longer appearing in western journals; this
indicated that research on the subject had become secret. In response, Flerov wrote to, among others, Premier Joseph
Stalin, insisting that "we must build the uranium bomb without
delay." In 1943 Stalin ordered the commencement of a research project
under the supervision of Igor V. Kurchatov, who had been director of the nuclear physics
laboratory at the Physico-Technical Institute in
Leningrad. Kurchatov initiated work on three fronts:
achieving a chain reaction in a uranium pile, designing both uranium-235 and
plutonium bombs, and separating isotopes from these materials.
By the end of 1944, 100
scientists were working under Kurchatov, and by the
time of the Potsdam Conference, which brought the Allied leaders together the
day after the Trinity test, the project on the atomic bomb was seriously under
way. During one session at the conference, Truman remarked to Stalin that the
United States had built a "new weapon of unusual destructive force."
Stalin replied that he would like to see the United States make "good use
of it against the Japanese."
Upon his return from Potsdam,
Stalin ordered that work on the fission bomb proceed at a faster pace. On Aug.
7, 1945, the day after the bombing of Hiroshima, he placed Lavrenty
P. Beria, the chief of secret police, in charge of the
Soviet version of the Manhattan Project. The first Soviet chain reaction took
place in Moscow on Dec. 25, 1946, using an experimental graphite-moderated
natural uranium pile, and the first plutonium production reactor became
operational at Kyshtym, in the Ural Mountains, on
June 19, 1948. The first Soviet weapon test occurred on Aug. 29, 1949, using
plutonium; it had a yield of 10 to 20 kilotons.
France
French scientists, such as
Henri Becquerel, Marie and Pierre Curie, and Frédéric
and Irène Joliot-Curie, made important contributions
to 20th-century atomic physics. During World War II several French scientists
participated in an Anglo-Canadian project in Canada, where eventually a heavy
water reactor was built at Chalk River, Ont., in 1945.
On Oct. 18, 1945, the Atomic
Energy Commission (Commissariat à l'Énergie
Atomique; CEA) was established by General Charles de Gaulle
with the objective of exploiting the scientific, industrial, and military
potential of atomic energy. The military application of atomic energy did not
begin until 1951. In July 1952 the National Assembly adopted a five-year plan,
a primary goal of which was to build plutonium production reactors. Work began
on a reactor at Marcoule in the summer of 1954 and on
a plutonium separating plant the following year.
On Dec. 26, 1954, the issue
of proceeding with a French bomb was raised at Cabinet level. The outcome was
that Prime Minister Pierre
Mendès-France launched a secret program to develop a bomb. On Nov. 30,
1956, a protocol was signed specifying tasks the CEA and the Defense Ministry would perform. These included providing
the plutonium, assembling a device, and preparing a test site. On July 22,
1958, de Gaulle, who had resumed power as prime minister, set the date for the
first atomic explosion to occur within the first three months of 1960. On Feb.
13, 1960, the French detonated their first atomic bomb from a 330-foot tower in
the Sahara in what was then French Algeria.
China
On Jan. 15, 1955, Mao
Zedong (Mao Tse-tung) and the Chinese leadership
decided to obtain their own nuclear arsenal. From 1955 to 1958 the Chinese were
partially dependent upon the Soviet Union for scientific and technological
assistance, but from 1958 until the break in relations in 1960 they became more
and more self-sufficient. Facilities were built to produce and process uranium
and plutonium at the Lan-chou Gaseous Diffusion Plant
and the Chiu-ch'üan Atomic Energy Complex, both in
the northwestern province of Kansu.
A design laboratory (called the Ninth Academy) was established at Hai-yen, east of the Koko Nor in Tsinghai province. A test site at Lop Nor, in far northwestern China, was established in October 1959.
Overall leadership and direction was provided by Nie Rongzhen (Nieh Jung-chen), director of the Defense
Science and Technology Commission.
Unlike the initial U.S. or
Soviet tests, the first Chinese detonation--on Oct. 16, 1964--used uranium-235
in an implosion-type configuration. Plutonium was not used until the eighth
explosion, on Dec. 27, 1968.
Other
countries
On May 18, 1974, India
detonated a nuclear device in the Rajasthan desert near Pokaran
with a reported yield of 15 kilotons. India characterized the test as being for
peaceful purposes and apparently did not stockpile weapons. Pakistan
declared its nuclear program to be solely for peaceful purposes, but it
acquired the necessary infrastructure of facilities to produce weapons and was
generally believed to possess them.
Several other countries were
believed to have built nuclear weapons or to have acquired the capability of assembling
them on short notice. Israel
was believed to have built an arsenal of more than 200 weapons, including
thermonuclear bombs. In August 1988 the South
African foreign minister said that South Africa had "the capability to
[produce a nuclear bomb] should we want to." Argentina,
Brazil,
South
Korea, and Taiwan
also had the scientific and industrial base to develop and produce nuclear
weapons, but they did not seem to have active programs.
The development of fusion
weapons
The United
States
U.S. research on
thermonuclear weapons started from a conversation in September 1941 between Fermi
and Teller.
Fermi wondered if the explosion of a fission weapon could ignite a mass of deuterium
sufficiently to begin thermonuclear fusion. (Deuterium, an isotope of hydrogen
with one proton and one neutron in the nucleus--i.e., twice the normal
weight--makes up 0.015 percent of natural hydrogen and can be separated in
quantity by electrolysis and distillation. It exists in liquid form only below
about -417 F,
or -250 C.)
Teller undertook to analyze the thermonuclear processes in some detail and
presented his findings to a group of theoretical physicists convened by
Oppenheimer in Berkeley in the summer of 1942. One participant, Emil Konopinski, suggested that the use of tritium
be investigated as a thermonuclear fuel, an insight that would later be
important to most designs. (Tritium, an isotope of hydrogen with one proton and
two neutrons in the nucleus--i.e., three times the normal weight--does
not exist in nature except in trace amounts, but it can be made by irradiating
lithium in a nuclear reactor. It is radioactive and has a half-life of 12.5
years.)
As a result of these
discussions the participants concluded that a weapon based on thermonuclear
fusion was possible. When the Los Alamos laboratory was being planned, a small
research program on the Super, as it came to be known, was included. Several
conferences were held at the laboratory in late April 1943 to acquaint the new
staff members with the existing state of knowledge and the direction of the
research program. The consensus was that modest thermonuclear research should
be pursued along theoretical lines. Teller proposed more intensive investigations,
and some work did proceed, but the more urgent task of developing a fission
weapon always took precedence--a necessary prerequisite for a hydrogen bomb in
any event.
In the fall of 1945, after
the success of the atomic bomb and the end of World War II, the future of the
Manhattan Project, including Los Alamos and the other facilities, was unclear.
Government funding was severely reduced, many scientists returned to
universities and to their careers, and contractor companies turned to other pursuits.
The Atomic Energy Act, signed by President Truman
on Aug. 1, 1946, established the Atomic
Energy Commission (AEC), replacing the Manhattan Engineer District, and
gave it civilian authority over all aspects of atomic energy, including
oversight of nuclear warhead research, development, testing, and production.
From April 18 to 20, 1946, a
conference led by Teller at Los Alamos reviewed the status of the Super. At
that time it was believed that a fission weapon could be used to ignite one end
of a cylinder of liquid deuterium and that the resulting thermonuclear reaction
would self-propagate to the other end. This conceptual design was known as the
"classical Super."
One of the two central design
problems was how to ignite the thermonuclear fuel. It was recognized early on
that a mixture of deuterium and tritium theoretically could be ignited at lower
temperatures and would have a faster reaction time than deuterium alone, but
the question of how to achieve ignition remained unresolved. The other problem,
equally difficult, was whether and under what conditions burning might proceed in
thermonuclear fuel once ignition had taken place. An exploding thermonuclear
weapon involves many extremely complicated, interacting physical and nuclear
processes. The speeds of the exploding materials can be up to millions of feet
per second, temperatures and pressures are greater than those at the centre of
the Sun, and time scales are billionths of a second. To resolve whether the
"classical Super" or any other design would work required accurate
numerical models of these processes--a formidable task, since the computers
that would be needed to perform the calculations were still under development.
Also, the requisite fission triggers were not yet ready, and the limited
resources of Los Alamos could not support an extensive program.
On Sept. 23, 1949, Truman
announced that "we have evidence that within recent weeks an atomic
explosion occurred in the U.S.S.R." This first Soviet test stimulated an
intense, four-month, secret debate about whether to proceed with the hydrogen
bomb project. One of the strongest statements of opposition against proceeding
with a hydrogen bomb program came from the General Advisory Committee (GAC) of
the AEC, chaired by Oppenheimer.
In their report of Oct. 30, 1949, the majority recommended "strongly
against" initiating an all-out effort, believing "that extreme
dangers to mankind inherent in the proposal wholly outweigh any military
advantages that could come from this development." "A super
bomb," they went on to say, "might become a
weapon of genocide." They believed that "a super bomb should never be
produced." Nevertheless, the Joint Chiefs of Staff, the State and Defense departments, the Joint Committee on Atomic Energy,
and a special subcommittee of the National Security Council all recommended
proceeding with the hydrogen bomb. Truman announced on Jan. 31, 1950, that he
had directed the AEC to continue its work on all forms of atomic weapons,
including hydrogen bombs. In March, Los Alamos went on a six-day workweek.
In the months that followed
Truman's decision, the prospect of actually being able to build a hydrogen bomb
became less and less likely. The mathematician Stanislaw
M. Ulam, with the assistance of Cornelius J.
Everett, had undertaken calculations of the amount of tritium that would be
needed for ignition of the classical Super. Their results were spectacular and,
to Teller, discouraging: the amount needed was estimated to be enormous. In the
summer of 1950 more detailed and thorough calculations by other members of the
Los Alamos Theoretical Division confirmed Ulam's
estimates. This meant that the cost of the Super program would be prohibitive.
Also in the summer of 1950,
Fermi and Ulam calculated that liquid deuterium
probably would not burn--that is, there would probably be no self-sustaining
and propagating reaction. Barring surprises, therefore, the theoretical work to
1950 indicated that every important assumption regarding the viability of the
classical Super was wrong. If success was to come, it would have to be
accomplished by other means.
The other means became
apparent between February and April 1951, following breakthroughs achieved at
Los Alamos. One breakthrough was the recognition that the burning of
thermonuclear fuel would be more efficient if a high density were achieved
throughout the fuel prior to raising its temperature, rather than the classical
Super approach of just raising the temperature in one area and then relying on
the propagation of thermonuclear reactions to heat the remaining fuel. A second
breakthrough was the recognition that these conditions--high compression and
high temperature throughout the fuel--could be achieved by containing and
converting the radiation from an exploding fission weapon and then using this
energy to compress a separate component containing the thermonuclear fuel.
The major figures in these
breakthroughs were Ulam and Teller. In December 1950 Ulam had proposed a new fission weapon design, using the
mechanical shock of an ordinary fission bomb to compress to a very high density
a second fissile core. (This two-stage fission device was conceived entirely
independently of the thermonuclear program, its aim being to use fissionable
materials more economically.) Early in 1951 Ulam went
to see Teller and proposed that the two-stage approach be used to compress and
ignite a thermonuclear secondary. Teller suggested radiation implosion, rather
than mechanical shock, as the mechanism for compressing the thermonuclear fuel
in the second stage. On March 9, 1951, Teller and Ulam
presented a report containing both alternatives, entitled "On Heterocatalytic Detonations I. Hydrodynamic Lenses and
Radiation Mirrors." A second report, dated April 4, by Teller, included
some extensive calculations by Frederic de Hoffmann and elaborated on how a
thermonuclear bomb could be constructed. The two-stage radiation implosion
design proposed by these reports, which led to the modern concept of
thermonuclear weapons, became known as the Teller-Ulam configuration.
It was immediately clear to
all scientists concerned that these new ideas--achieving a high density in the
thermonuclear fuel by compression using a fission primary--provided for the
first time a firm basis for a fusion weapon. Without hesitation, Los Alamos
adopted the new program. Gordon Dean, chairman of the AEC, convened a meeting
at the Institute for Advanced Study in Princeton, hosted by Oppenheimer, on
June 16-18, 1951, where the new idea was discussed. In attendance were the GAC
members, AEC commissioners, and key scientists and consultants from Los Alamos
and Princeton. The participants were unanimously in favour of active and rapid
pursuit of the Teller-Ulam principle.
Just prior to the conference,
on May 8 at Enewetak atoll in the western Pacific, a
test explosion called George had successfully used a fission bomb to ignite a
small quantity of deuterium and tritium. The original purpose of George had
been to confirm the burning of these thermonuclear fuels (about which there had
never been any doubt), but with the new conceptual understanding contributed by
Teller and Ulam, the test provided the bonus of
successfully demonstrating radiation implosion.
In September 1951, Los Alamos
proposed a test of the Teller-Ulam concept for
November 1952. Engineering of the device, nicknamed Mike, began in October 1951,
but unforeseen difficulties required a major redesign of the experiment in
March 1952. The Mike device weighed 82 tons, owing in part to cryogenic
(low-temperature) refrigeration equipment necessary to keep the deuterium in
liquid form. It was successfully detonated during Operation Ivy, on Nov. 1,
1952 (local time), at Enewetak. The explosion
achieved a yield of 10.4 million tons of TNT, or 500 times larger than the
Nagasaki bomb, and it produced a crater 6,240 feet in diameter and 164 feet
deep.
With the Teller-Ulam configuration proved, deliverable thermonuclear
weapons were designed and initially tested during Operation Castle in 1954. The
first test of the series, conducted on March 1, 1954 (local time), was called
Bravo. It used solid lithium deuteride rather than
liquid deuterium and produced a yield of 15 megatons, 1,000 times as large as
the Hiroshima bomb. Here the principal thermonuclear reaction was the fusion of
deuterium and tritium. The tritium was produced in the weapon itself by neutron
bombardment of the lithium-6 isotope in the course of the fusion reaction.
Using lithium deuteride instead of liquid deuterium
eliminated the need for cumbersome cryogenic equipment.
With completion of Castle,
the feasibility of lightweight, solid-fuel thermonuclear weapons was proved.
Vast quantities of tritium would not be needed after all. New possibilities for
adaptation of thermonuclear weapons to various kinds of missiles began to be
explored.
The Soviet
Union
In 1948 Kurchatov organized a theoretical group, under the
supervision of physicist Igor Y. Tamm, to begin work
on a fusion bomb. (This group included Andrey Sakharov, who, after contributing several important
ideas to the effort, later became known as the "father of the Soviet
H-bomb.") In general, the Soviet program was two to three years behind
that of the United States. The test that took place on Aug. 12, 1953, produced
a fusion reaction in lithium deuteride and had a
yield of 200 to 400 kilotons. This test, however, was not of a high-yield
hydrogen bomb based on the Teller-Ulam configuration
or something like it. The first such Soviet test, with a yield in the megaton
range, took place on Nov. 22, 1955. On Oct. 30, 1961, the Soviet Union tested
the largest known nuclear device, with an explosive force of 58 megatons.
Great Britain
Minister of Defence Harold
Macmillan announced in his Statement of Defence, on Feb. 17, 1955, that the
United Kingdom planned to develop and produce hydrogen bombs. The formal
decision to proceed had been made earlier in secret by a small Defence
subcommittee on June 16, 1954, and put to the Cabinet in July. The decision was
unaccompanied by the official debate that characterized the American experience
five years earlier.
It remained unclear exactly
when the first British thermonuclear test occurred. Three high-yield tests in
May and June 1957 near Malden Island in the Pacific Ocean were probably of
boosted fission designs (see below). The most likely date for the first
two-stage thermonuclear test, using the Teller-Ulam
configuration or a variant, was Nov. 8, 1957. This test and three others that
followed in April and September 1958 contributed novel ideas to modern
thermonuclear designs.
France
Well before their first
atomic test, the French assumed they would eventually have to become a
thermonuclear power as well. The first French thermonuclear test was conducted
on Aug. 24, 1968.
China
Plans to proceed toward a
Chinese hydrogen bomb were begun in 1960, with the formation of a group by the
Institute of Atomic Energy to do research on thermonuclear materials and
reactions. In late 1963, after the design of the fission bomb was complete, the
Theoretical Department of the Ninth Academy, under the direction of Deng Jiaxian (Teng Chia-hsien),
was ordered to shift to thermonuclear work. By the end of 1965 the theoretical
work for a multistage bomb had been accomplished. After testing two boosted
fission devices in 1966, the first Chinese multistage fusion device was
detonated on June 17, 1967.
Refinements in design
From the late 1940s, U.S.
nuclear weapon designers developed and tested warheads to improve their
ballistics, to standardize designs for mass production, to increase yields, to
improve yield-to-weight and yield-to-volume ratios, and to study their effects.
These improvements resulted in the creation of nuclear
warheads for a wide variety of strategic and tactical delivery systems.
Fission
The first advances came
through the test series Operation Sandstone, conducted in the spring of 1948.
These three tests used implosion designs of a second generation, which incorporated
composite and levitated cores. A composite core consisted of concentric shells
of both uranium-235 and plutonium-239, permitting more efficient use of these
fissile materials. Higher compression of the fissile material was achieved by
levitating the core--that is, introducing an air gap into the weapon to obtain
a higher yield for the same amount of fissile material.
Tests during Operation Ranger
in early 1951 included implosion devices with cores containing a fraction of a critical
mass--a concept originated in 1944 during the Manhattan Project. Unlike the
original Fat Man design, these "fractional crit"
weapons relied on compressing the fissile core to a higher density in order to
achieve a supercritical mass. These designs could achieve appreciable yields
with less material.
One technique for enhancing
the yield of a fission explosion was called "boosting." Boosting
referred to a process whereby thermonuclear reactions were used as a source of
neutrons for inducing fissions at a much higher rate than could be achieved
with neutrons from fission chain reactions alone. The concept was invented by
Teller by the middle of 1943. By incorporating deuterium and tritium into the
core of the fissile material, a higher yield could be obtained from a given
quantity of fissile material--or, alternatively, the same yield could be
achieved with a smaller amount. The fourth test of Operation Greenhouse, on May
24, 1951, was the first proof test of a booster design. In subsequent decades
approximately 90 percent of nuclear
weapons in the U.S. stockpile relied on boosting.
Fusion
Refinements of the basic
two-stage Teller-Ulam configuration resulted in
thermonuclear weapons with a wide variety of characteristics and applications.
Some high-yield deliverable weapons incorporated additional thermonuclear fuel
(lithium deuteride) and fissionable material
(uranium-235 and uranium-238) in a third stage. While there was no theoretical
limit to the yield that could be achieved from a thermonuclear bomb (for
example, by adding more stages), there were practical limits on the size and
weight of weapons that could be carried by aircraft or missiles. The largest
U.S. bombs had yields of from 10 to 20 megatons and weighed up to 20 tons.
Beginning in the early 1960s, however, the United States built a variety of
smaller, lighter weapons that exhibited steadily improving yield-to-weight and
yield-to-volume ratios.
A nuclear explosion releases
energy in a variety of forms, including blast, heat, and radiation (X rays,
gamma rays, and neutrons). By varying a weapon's design, these effects could be
tailored for a specific military purpose. In an enhanced-radiation weapon, more
commonly called a neutron
bomb, the objective was to minimize the blast by reducing the fission yield
and to enhance the neutron radiation. Such a weapon would prove lethal to
invading troops without, it was hoped, destroying the
defending country's towns and countryside. It was actually a small (on the
order of one kiloton), two-stage thermonuclear weapon that utilized deuterium
and tritium, rather than lithium deuteride, to
maximize the release of fast neutrons. The first U.S. application of this
principle was an antiballistic missile warhead in the mid-1970s.
Enhanced-radiation warheads were produced for the Lance short-range ballistic
missile and for an eight-inch artillery shell.
Bibliography
The history of nuclear
weapons is the subject of voluminous literature. Richard Rhodes, The Making
of the Atomic Bomb (1986), is an excellent work on the U.S. effort to
develop nuclear weapons. It can be supplemented by the official histories:
Vincent C. Jones, Manhattan, the Army and the Atomic Bomb (1985); David
Hawkins, Edith C. Truslow, and Ralph Carlisle Smith, Manhattan
District History-Project Y, the Los Alamos Project, 2 vol. (1961, reprinted
as Project Y, the Los Alamos Story, in 1 vol. with a new introduction,
1983); and Richard G. Hewlett, Oscar E. Anderson, Jr.,
and Francis Duncan, A History of the United States Atomic Energy Commission,
2 vol. (1962-69); continued by Richard G. Hewlett and Jack M. Hall, Atoms
for Peace and War, 1953-1961 (1989). For the development of thermonuclear
weapons, see Herbert F. York, The Advisors:
Oppenheimer, Teller, and the Superbomb (1976);
and Hans A. Bethe, "Comments on the History of
the H-Bomb," Los Alamos Science, 3(3):43-53 (Fall 1982). Technical
data are compiled in Thomas B. Cochran, William M. Arkin,
and Milton M. Hoenig, U.S. Nuclear Forces and
Capabilities (1984); and Thomas B. Cochran et al., U.S. Nuclear
Warhead Production (1987), and U.S. Nuclear Warhead Facility Profiles
(1987).
The British project is
discussed in the official histories of the U.K. Atomic Energy Authority:
Margaret Gowing, Britain and Atomic Energy,
1939-1945 (1964), and Independence and Deterrence: Britain and Atomic
Energy, 1945-1952, 2 vol. (1974). Little has been published about the
program of the former U.S.S.R., but see David
Holloway, The Soviet Union and the Arms Race, 2nd ed. (1984); and Thomas
B. Cochran, Soviet Nuclear Weapons (1989). No official history is
available for the French project. Bertrand Goldschmidt, Les Rivalités atomiques, 1939-1966
(1967), is a semiofficial account by a participant.
The Chinese project is covered in John Wilson Lewis and Xue
Litai, China Builds the Bomb (1988). David
Irving, The German Atomic Bomb (1968, reprinted 1983), covers the German
program; and Robert K. Wilcox, Japan's Secret War (1985), examines Japanese
work on the atomic bomb. Proliferation developments are followed in Leonard S. Spector, Nuclear Proliferation Today (1984), The
New Nuclear Nations (1985), Going Nuclear (1987), and The
Undeclared Bomb (1988).