 Nuclear
weapons have kept the world in an uneasy balance of peace and
terror since the only two ever used forced Japan to surrender
on September 2, 1945, ending the Second World War. The awesome
power of these weapons comes from the release of huge amounts
of energy from the hearts (nuclei) of atoms by the reactions
of fission and fusion.
Fission is the process by which an atom divides; it is the basis
of the atomic bomb. Fusion is the opposite, the combination
of atoms to form a larger one. It release even larger amounts
of energy than fission, and provides hydrogen bombs with their
power. It is also believed to be the source of the Sun’s
energy. Most modern nuclear weapons make use of both processes.
Fission bombs must have one of two ingredients, uranium or plutonium.
The bomb dropped on Hiroshima on August 6, 1945, used uranium
and plutonium are fissile materials, with nuclei that can be
split by subatomic particles called neutrons.
Every time a nucleus is split it produces at least two new neutrons.
With a small lump of fissile material these neutrons with fly
off into the air harmlessly. But if the lump of plutonium or
uranium is large enough (about the size of a grapefruit, and
known as the critical mass), the neutrons collide with other
nuclei before they can escape into the air. This results in
two new fissions, which in turn produce eight neutrons and so
on.
Each stage happens in about one hundred millionth of a second.
In well under a millionth of a second the chain reaction has
multiplied so rapidly that there is an explosive release of
energy.
The source of the energy comes from the fact that the light
atoms produced weigh less than the heavy atom that is split.
So matter appears to be destroyed. Instead, the matter is converted
to energy when the mass combines with light going at its maximum
speed. This sets off a chain reaction, which stops when the
original matter is used up or when it blows apart, so the neutrons
can no longer cause fission.
About half the energy is taken up by a blast, a bomb equal to
20,000 tons of TNT can destroy buildings up to half a mile (80m)
away. Just over a third of the energy is in the form of heat,
so intense that it sets light to anything combustible within
a range of 4 miles (6.4km). the rest is released as radiation,
gamma rays and X-rays. After a nuclear explosion, millions of
small radioactive particles, known as fallout, float to the
ground.
 Although
the chain reaction only need a split second to take place, the
bomb designer still has to make sure that a critical mass of
uranium of plutonium stays together long enough before it blows
itself apart. The Hiroshima bomb used a conventional explosive
charge to drive one piece of uranium down a tube into the other
piece. On its own, neither piece was large enough to explode:
but thrown together they exceeded the critical mass and exploded
with a force of 12-13,000 tons of TNT. The Nagasaki bomb took
advantage of the fact that the critical mass is reduced if the
fissile material (plutonium in this case) is compressed to increase
its density. A subcritical mass of plutonium was surrounded
by conventional explosive charges. When they were detonated
they compressed the plutonium so that it became supercritical
and produced a blast equal to 22,000 tons of TNT. |
