You have probably read in history books about the atomic
bombs used in World War II. You may also have seen fictional
movies where nuclear weapons were launched or detonated (Fail
Day After, Testament,
Man and Little Boy, The
Peacemaker, just to name a few). In the news, while many
countries have been negotiating to disarm their arsenals of
nuclear weapons, other countries such as India and Pakistan
have been developing nuclear weapons programs.
We have seen that these devices have incredible destructive
power, but how do they work? In this edition of HowStuffWorks,
you will learn about the physics that makes a nuclear bomb so
powerful, how nuclear bombs are designed and what happens
after a nuclear explosion.
Physics of Nuclear Devices Nuclear bombs
involve the forces, strong and weak, that hold the nucleus of
together, especially atoms with unstable nuclei (see How Nuclear
Radiation Works for details). There are two basic ways
that nuclear energy can be released from an atom:
Nuclear fission - You can split the nucleus of an
atom into two smaller fragments with a neutron. This method
usually involves isotopes of uranium (uranium-235,
uranium-233) or plutonium-239.
Nuclear fusion -You can bring two smaller atoms,
usually hydrogen or hydrogen isotopes (deuterium, tritium),
together to form a larger one (helium or helium isotopes);
this is how the sun produces
In either process, fission or fusion, large amounts of heat
energy and radiation
are given off.
Designs of Nuclear Bombs To build an atomic
bomb, you need:
A source of fissionable or fusionable fuel
A triggering device
A way to allow the majority of fuel to fission or fuse
before the explosion occurs (otherwise the bomb will fizzle
The first nuclear bombs were fission devices, and the later
fusion bombs required a fission-bomb trigger. We will discuss
the designs of the following devices:
Fission bombs (in general)
Gun-triggered fission bomb (Little Boy), which
was detonated over Hiroshima, Japan, in 1945
Implosion-triggered fission bomb (Fat Man), which
was detonated over Nagasaki, Japan, in 1945
Fusion bombs (in general)
Teller-Ulam design of a hydrogen fusion bomb,
which was test-detonated on Elugelap Island in 1952
Fission Bombs A fission bomb uses an
element like uranium-235 to create a nuclear explosion. If you
have read How Nuclear
Radiation Works, then you understand the basic process
behind radioactive decay and fission. Uranium-235 has an extra
property that makes it useful for both nuclear-power
production and nuclear-bomb production -- U-235 is one of the
few materials that can undergo induced fission. If a
free neutron runs into a U-235 nucleus, the nucleus will
absorb the neutron without hesitation, become unstable and
This figure shows a uranium-235 nucleus with a neutron
approaching from the top. As soon as the nucleus captures the
neutron, it splits into two lighter atoms and throws off two
or three new neutrons (the number of ejected neutrons depends
on how the U-235 atom happens to split). The two new atoms
then emit gamma radiation as they settle into their new states
Nuclear Radiation Works). There are three things about
this induced fission process that make it interesting:
The probability of a U-235 atom capturing a neutron as
it passes by is fairly high. In a bomb that is working
properly, more than one neutron ejected from each fission
causes another fission to occur. This condition is known as
The process of capturing the neutron and splitting
happens very quickly, on the order of picoseconds (1*10E-12
An incredible amount of energy is released, in the form
of heat and gamma radiation, when an atom splits. The energy
released by a single fission is due to the fact that the
fission products and the neutrons, together, weigh less than
the original U-235 atom.
The difference in weight is converted to energy at a rate
governed by the equation e = m * c^2. A pound of highly
enriched uranium as used in a nuclear bomb is equal to
something on the order of a million gallons of gasoline.
When you consider that a pound of uranium is smaller than a
baseball and a million gallons of gasoline would fill a cube
that is 50 feet per side (50 feet is as tall as a five-story
building), you can get an idea of the amount of energy
available in just a little bit of U-235.
order for these properties of U-235 to work, a sample of
uranium must be enriched . Weapons-grade uranium is
composed of at least 90-percent U-235.
In a fission bomb, the fuel must be kept in separate subcritical
masses, which will not support fission, to prevent premature
mass is the minimum mass of fissionable material
required to sustain a nuclear fission reaction. This
separation brings about several problems in the design of a
fission bomb that must be solved:
The two or more subcritical masses must be brought
together to form a supercritical
mass, which will provide more than enough neutrons to
sustain a fission reaction, at the time of detonation.
Free neutrons must be introduced into the supercritical
mass to start the fission.
As much of the material as possible must be fissioned
before the bomb explodes to prevent fizzle.
the subcritical masses together into a supercritical mass, two
techniques are used:
Neutrons are introduced by
making a neutron generator. This generator is a small
pellet of polonium and beryllium, separated by foil within the
fissionable fuel core. In this generator:
The foil is broken when the subcritical masses come
together and polonium spontaneously emits alpha particles.
These alpha particles then collide with beryllium-9 to
produce beryllium-8 and free neutrons.
The neutrons then initiate fission.
fission reaction is confined within a dense material called a
tamper, which is usually made of uranium-238. The
tamper gets heated and expanded by the fission core. This
expansion of the tamper exerts pressure back on the fission
core and slows the core's expansion. The tamper also reflects
neutrons back into the fission core, increasing the efficiency
of the fission reaction.
Gun-Triggered Fission Bomb The simplest way
to bring the subcritical masses together is to make a gun that
fires one mass into the other. A sphere of U-235 is made
around the neutron generator and a small bullet of
U-235 is removed. The bullet is placed at the one end of a
long tube with explosives behind it, while the sphere is
placed at the other end. A barometric-pressure sensor
determines the appropriate altitude for detonation and
triggers the following sequence of events:
The explosives fire and propel the bullet down the
The bullet strikes the sphere and generator, initiating
the fission reaction.
The fission reaction begins.
The bomb explodes.
Little Boy was this type of bomb and had a
14.5-kiloton yield (equal to 14,500 tons of TNT) with an
efficiency of about 1.5 percent. That is, 1.5 percent of the
material was fissioned before the explosion carried the
Implosion-Triggered Fission Bomb Early in
Project, the secret U.S. program to develop the atomic
bomb, scientists working on the project recognized that
compressing the subcritical masses together into a sphere by
implosion might be a good way to make a supercritical mass.
There were several problems with this idea, particularly how
to control and direct the shock wave uniformly across the
sphere. But the Manhattan Project team solved the problems.
The implosion device consisted of a sphere of uranium-235
(tamper) and a plutonium-239 core surrounded by high
explosives. When the bomb was detonated, this is what
The explosives fired, creating a shock wave.
The shock wave compressed the core.
The fission reaction began.
The bomb exploded.
Fat Man was this type of bomb and had a 23-kiloton
yield with an efficiency of 17 percent. These bombs exploded
in fractions of a second. The fission usually occurred in 560
billionths of a second.
In a later modification of the implosion-triggered design,
here is what happens:
The explosives fire, creating a shock wave.
The shock wave propels the plutonium pieces together
into a sphere.
The plutonium pieces strike a pellet of
beryllium/polonium at the center.
The fission reaction begins.
The bomb explodes.
Fusion Bombs Fission bombs worked, but they
weren't very efficient. Fusion bombs, also called
thermonuclear bombs, have higher kiloton yields and
greater efficiencies than fission bombs. To design a fusion
bomb, some problems have to be solved:
Deuterium and tritium, the fuel for fusion, are both
gases, which are hard to store.
Tritium is in short supply and has a short half-life,
so the fuel in the bomb would have to be continuously
Deuterium or tritium has to be highly compressed at high
temperature to initiate the fusion reaction.
to store deuterium, the gas could be chemically combined with
lithium to make a solid lithium-deuterate compound. To
overcome the tritium problem, the bomb designers recognized
that the neutrons from a fission reaction could produce
tritium from lithium (lithium-6 plus a neutron yields tritium
and helium-4; lithium-7 plus a neutron yields tritium,
helium-4 and a neutron). That meant that tritium would not
have to be stored in the bomb. Finally, Stanislaw Ulam
recognized that the majority of radiation given off in a
fission reaction was X-rays,
and that these X-rays could provide the high temperatures and
pressures necessary to initiate fusion. Therefore, by encasing
a fission bomb within a fusion bomb, several problems could be
Teller-Ulam Design of a Fusion Bomb To
understand this bomb design, imagine that within a bomb casing
you have an implosion fission bomb and a cylinder casing of
uranium-238 (tamper). Within the tamper is the lithium
deuteride (fuel) and a hollow rod of plutonium-239 in the
center of the cylinder. Separating the cylinder from the
implosion bomb is a shield of uranium-238 and plastic foam
that fills the remaining spaces in the bomb casing. Detonation
of the bomb caused the following sequence of events:
The fission bomb imploded, giving off X-rays.
These X-rays heated the interior of the bomb and the
tamper; the shield prevented premature detonation of the
The heat caused the tamper to expand and burn away,
exerting pressure inward against the lithium deuterate.
The lithium deuterate was squeezed by about 30-fold.
The compression shock waves initiated fission in the
The fissioning rod gave off radiation, heat and
The neutrons went into the lithium deuterate, combined
with the lithium and made tritium.
The combination of high temperature and pressure were
sufficient for tritium-deuterium and deuterium-deuterium
fusion reactions to occur, producing more heat, radiation
The neutrons from the fusion reactions induced fission
in the uranium-238 pieces from the tamper and shield.
Fission of the tamper and shield pieces produced even
more radiation and heat.
The bomb exploded.
All of these events happened in about 600 billionths of a
second (550 billionths of a second for the fission bomb
implosion, 50 billionths of a second for the fusion events).
The result was an immense explosion that was more than 700
times greater than the Little Boy explosion: It had a
Consequences of Nuclear Explosions The
detonation of a nuclear bomb over a target such as a populated
city causes immense damage. The degree of damage depends upon
the distance from the center of the bomb blast, which is
called the hypocenter or ground zero. The closer
one is to the hypocenter, the more severe the damage. The
damage is caused by several things:
A wave of intense heat from the explosion
Pressure from the shock wave created by the blast
Radioactive fallout (clouds of fine radioactive
particles of dust and bomb debris that fall back to the
At the hypocenter, everything is immediately
vaporized by the high temperature (up to 500 million
degrees Fahrenheit or 300 million degrees Celsius). Outward
from the hypocenter, most casualties are caused by burns from
the heat, injuries from the flying debris of buildings
collapsed by the shock wave, and acute exposure to the high
radiation. Beyond the immediate blast area, casualties are
caused from the heat, radiation, and fires spawned from the
heat wave. In the long-term, radioactive fallout occurs over a
wider area because of prevailing winds. The radioactive
fallout particles enter the water supply and are inhaled and
ingested by people at a distance from the blast.
Scientists and physicians are still studying the survivors
of the bombs dropped on Japan and expect more results to
appear over time.
In the 1980s, scientists assessed the possible effects of
nuclear warfare (many nuclear bombs exploding in different
parts of the world) and proposed the theory that a nuclear
winter could occur. In the nuclear-winter scenario, the
explosion of many bombs would raise great clouds of dust and
radioactive material that would travel high into Earth's
atmosphere. These clouds would block out sunlight. The
reduced level of sunlight would lower the surface temperature
of the planet and reduce photosynthesis by plants and
bacteria. The reduction in photosynthesis would disrupt the
food chain, causing mass extinction of life (including
humans). This scenario is similar to the asteroid
hypothesis that has been proposed to explain the extinction of
the dinosaurs. Proponents of the nuclear-winter scenario
pointed to the clouds of dust and debris that traveled far
across the planet after the volcanic
eruptions of Mount St. Helens in the United States and
Mount Pinatubo in the Philippines.
Nuclear weapons have incredible, long-term destructive
power that travels far beyond the original target. This is why
the world's governments are trying to control the spread of
nuclear-bomb-making technology and materials and reduce the
arsenal of nuclear weapons deployed during the Cold War.
For more information on nuclear bombs and related topics,
check out the links on the next page.