Fission
(A-Bomb) & Fusion (H-Bomb)
There are two types of atomic explosions that can be facilitated
by U-235: fission and fusion. Fission, simply put, is a nuclear
reaction in which an atomic nucleus splits into fragments, usually
two fragments of comparable mass, emitting 100 million to several
hundred million volts of energy. This energy is expelled explosively
and violently in the atomic bomb. A fusion reaction is usually
started with a fission reaction, but unlike the fission (atomic)
bomb, the fusion (hydrogen) bomb derives its power from the
fusing of nuclei of various hydrogen isotopes into helium nuclei.
This article discusses the A-bomb or atomic bomb.
The massive power behind the reaction in an
atomic bomb arises from the forces that hold the atom together.
These forces are akin to, but not quite the same as, magnetism.
Atoms are comprised of various numbers and
combinations of the three sub-atomic particles: protons, neutrons
and electrons. Protons and neutrons cluster together to form
the nucleus (central mass) of the atom while the electrons orbit
the nucleus much like planets around a sun. It is the balance
and arrangement of these particles that determine the stability
of the atom.
Most elements have very stable atoms which
are impossible to split except by bombardment in particle accelerators.
For all practical purposes, the only natural element whose atoms
can be split easily is uranium, a heavy metal with the largest
atom of all natural elements and an unusually high neutron-to-proton
ratio. This higher ratio does not enhance its "splitability,"
but it does have an important bearing on its ability to facilitate
an explosion, making uranium-235 an exceptional candidate for
nuclear fission.
There are two naturally-occurring isotopes
of uranium. Natural uranium consists mostly of isotope U-238,
with 92 protons and 146 neutrons (92+146=238) per atom. Mixed
with this is a 0.6% accumulation of U-235, with only 143 neutrons
per atom. The atoms of this lighter isotope can be split, thus
it is "fissionable" and useful in making atomic bombs.
Neutron-heavy U-238 has a role to play in the atomic bomb as
well since its neutron-heavy atoms can deflect stray neutrons,
preventing an accidental chain reaction in a uranium bomb and
keeping neutrons contained in a plutonium bomb. [U-238 can also
be "saturated" to produce plutonium (Pu-239), a man-made,
radioactive element also used in atomic bombs.]
Both isotopes of uranium are naturally radioactive;
their bulky atoms disintegrating over time. Given enough time
(hundreds of thousands of years) uranium will eventually lose
so many particles that it will turn into lead. This process
of decay can be greatly accelerated in what is known as a chain
reaction. Instead of disintegrating naturally and slowly, the
atoms are forcibly split by bombardment with neutrons.
A blow from a single neutron is enough to
split the less-stable U-235 atom, creating atoms of smaller
elements (often barium and krypton) and releasing heat and gamma
radiation (the most powerful and lethal form of radioactivity).
The chain reaction occurs when "spare" neutrons from
this atom fly out with sufficient force to split other U-235
atoms they come in contact with. In theory, it is necessary
to split only one U-235 atom, which will release neutrons which
will split other atoms, which will release neutrons ... and
so on. This progression is not arithmetic; it is geometric and
takes place within a millionth of a second.
The minimum amount to start a chain reaction
as described above is known as super critical mass. For pure
U-235, it is 110 pounds (50 kilograms). No uranium is ever quite
pure, however, so in reality more will be needed. U-235, U-238
and Plutonium
Uranium is not the only material used for
making atomic bombs. Another material is the Pu-239 isotope
of the man-made element plutonium. Plutonium is only found naturally
in minute traces, so useable amounts must be produced from uranium.
In a nuclear reactor, uranium's heavier U-238 isotope can be
forced to acquire extra particles, eventually becoming the plutonium.
Plutonium will not start a fast chain
reaction by itself, but this difficulty is overcome by having
a neutron source, a highly radioactive material that gives off
neutrons faster than the Plutonium itself. In certain types
of bombs, a mixture of the elements Beryllium and Polonium is
used to bring about this reaction. Only a small piece is needed
(super critical mass is about 32 pounds, though as little as
22 can be used). The material is not fissionable in and of itself,
but merely acts as a catalyst to the greater reaction.
Top
Principles
of atomic bombs
To explode, the bomb must first be imploded:
compress a subcritical spherical fissionable mass (a ball of
normal density uranium and other metals) with specially designed
explosives. Implosion is the detonation of explosives on the
outer surface, instead of the inner surface, which causes the
detonation/shock wave to move inward. The engineers working
on the bomb had to carefully design a smooth, symmetrical implosion
setup so that the shock waves would reach each part of the core
at the same time, and that was a very difficult task. Once the
shock wave is transmitted to the fissionable core it compresses
the core and raises the density to the point of superciticality.
Which then leads to a great explosion, which in the case of
"Fat Man" is equivalent to 10,000 tons of TNT. Essentially
what is happening here is that the fissionable mass is crushed
to a great density, and once the mass has reached that supercritical
density it goes boom!
There are four main problems that must be taken care of for
an atomic bomb to explode. They are all related with creating
a fission chain reaction:
The fissionable material must be kept in a subcritical state
before detonation.
The fissionable material must be brought into a supercritical
state while keeping it free of neutrons. Otherwise most of the
fissionable mass would be used up and it would not generate
a large explosion (if any). The neutrons must be added to the
critical mass when it is at maximum supercriticality, meaning
at the most "explosive" point. This can be compared
to releasing a rubber-band when it is fully stretched so it
will travel with the most speed.
The fissionable mass must be kept together until a large amount
of it has gone through fission, making it efficient. If the
fissionable mass does not stay together, the fission reaction
would immediately be stopped. When the atomic nuclei in the
center of an atomic bomb, which is composed of fissile materials,
are split, an enormous amount of energy is released as dangerously
high levels of heat and radiation. Atomic bombs use this energy
as a weapon for killing.
Other explanation: When a single neutron strikes the nucleus
of a fissile material such as uranium 235 (or plutonium 239),
two or three more neutrons are released. When those neutrons
are ejected, enormous energy is released. The flying neutrons
then hit other nuclei of the uranium and cause them to split
in a similar manner, releasing more energy and neutrons. When
this fission spreads, a huge amount of energy is generated instantaneously.
Estimated damages of atomic bombs
Atomic: (-800 m): Deadly
(-1000 m): Deadly for 50% of all persons
(-2000 m): Shock- and Heatwawe, Neutrons- and Gammaemission
(-2200 m): Destruction of buildings
(-3000 m): Heavy fires
Neutronic: (-200 m): Destruction of buildings
(-800 m): Deadly within 1-2 days
(-1000 m): Deadly within 4-6 days
(-1200 m): Mostly deadly within a few weeks
(-1400 m): Deadly for 50% of all persons
(-3000 m): Neutronic emission
Top
This is a model of a basic Teller-Ulam atomic
bomb.
Top
Nuclear bomb factory in Negev Desert near Dimona
described as "chocolate" or "textiles" factory.
The domed structure glimmering on the sun to the right is the
reactor built in 1958 by France, or Machon (block) No. 1. Cooling
towers are clearly visible to the left. Palms and gardens are
planted to obscure the facility from the road. The picture was
taken from Rd. 25 at around 7:00 a.m. The Middle East, April
26, 2000
Top
|
