Chain
Reaction
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A single
neutron collides with a Uranium nucleus |
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The nucleus splits into two major fragments with the release of further neutrons which cause further fission. |
If on average
one of the several neutrons released in a fission
reaction, induce further fission, the reaction is self-sustaining.
If more than one of the fission neutrons induce further fission (on
average) then the reaction is escalating (perhaps dangerously). If
less than one of the fission neutrons induce further fission (on
average) then the chain reaction is fading out. The first
"successful" chain reaction was the atomic bomb. Attempts to get a
controlled chain reaction involved overcoming a number of
difficulties.
Difficulties in obtaining a controlled chain reaction
Natural uranium contains approximately 99% U-238 and 1% U-235. Only
fast neutrons (KE> 1 MeV) are capable of causing fission in U-238
and in general only slow neutrons cause fission in U-235.
A chain reaction is not possible in natural uranium in spite of the
fact that an average of 2.5 neutrons are released each time. The
reason for this is that most of the neutrons released in fission
reactions are not fast enough to cause fission in U -238 and are too
fast to cause fission in U-235. Because of their neutral charge they
cannot be accelerated, so a reduction in speed due to collision is
the only viable option.
The scarcity of neutrons with the right speed combined with the
scarcity of U-235 atoms with which to collide, result in only about
5% of neutrons released through fission, causing further fission.
There are two ways to improve the chances of having a chain
reaction.
1. By using enriched uranium, in which the percentage of U-235 is
greater than normal.
2. By using moderators, which slow down the neutrons to energies of
less than 5eV.
To obtain
enriched uranium, the two isotopes U-235 and U-238 would have to be
separated. However this is a lengthy and costly process. The second
option is much more practical and simply requires that slender rods
of natural uranium be separated by moderators rather than left as a
solid block of uranium.
How does
an atomic bomb work?
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Atomic Bombs dropped on Japan in 1945, during World War
II
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Hiroshima
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Nagasaki
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This is an example of an uncontrolled chain reaction, which is triggered off in either pure 235 U or 239 Pu. If the amount of material present is too small, many of the fission neutrons will escape and so the reaction dies out. There is a critical size below which the reaction won’t be self-sustaining. When the bomb is to be exploded the two sub critical masses are brought together to form a super critical mass within which a single fission will set off a uncontrollable chain reaction.
The chain
reaction begins when a single stray neutron causes a single 235 U
nucleus to fission. Once begun, the chain reaction builds up to
explosive force in a matter of microseconds. The first atomic bomb
was exploded in the New Mexico desert in 1945.
A Natural Fission Reactor
For thirty years it was assumed that the first nuclear chain
reaction to occur on Earth was that set up by Fermi in Chicago in
1942. However, it has now been established that a natural reactor
operated in a natural uranium deposit in west Africa 1.8 billion
years ago. Evidence for this came in an interesting way. Natural
uranium from Gabon was exported to France; an examination of the
isotopic content showed that the proportion of uranium-235 was
slightly lower than normally found This small difference was
investigated and traces of the fission products of uranium were
found in higher proportions than in normal uranium ore. This
suggested that at some time in the geological history of the
uranium, some of it had undergone a fission reaction. But how could
a chain reaction have been established in natural uranium? The seam
of ore, which was being extracted, was unusually rich in uranium–235
(up to 10 per cent). Geological conditions were responsible for
accumulating large quantities in a small area. The water of
crystallisation of the minerals in the ore might have acted as a
moderator. It is now believed that a natural fission chain reaction
must have taken place in the ore approximately 1800 million years
ago. It may have run for just over 100 years, emitting a thermal
power of tens of kilowatts (any greater power would have led to the
evaporation of the water required as a moderator). In the course of
its lifetime, it would have consumed a similar amount of uranium as
a present-day power reactor consumes in a year.