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cylinder, contains a pile of pure graphite, the material from which pencil leads are
made. It has many hundreds of vertical or vertical and horizontal channels; in some of
them the fuel – rods of uranium metal in magnesium alloy sheaths – are stacked, in
the rest there are the control rods made of boron or cadmium; these can be pushed in
and pulled out. A very thick concrete or steel wall around the reactor vessel–-the
'biological shield' – prevents the escape of radio-activity.
As soon as the control rods are pulled out the chain reaction begins; uranium
nuclei split up under neutron bombardment, and release more neutrons. These
neutrons bounce off the graphite atoms so that they shoot to and fro through the
reactor until they hit and split more uranium nuclei: the graphite acts as the moderator
in this process, helping to keep the chain reaction going and preventing the 'capture'
of fast neutrons by the nuclei through slowing them down. The uranium rods get hot
(up to 400° C, in Calder Halland Chapelcross), and this heat is removed by the
'coolant', carbon dioxide gas under pressure. It circulates through the reactor vessel in
tubes entering at the bottom at 140° С and leaving it at the top at about 340° С The
coolant gas, after leaving the 'core' of the reactor, is conducted to the heat exchangers.
They are basically ordinary boilers in which water is turned into steam. The water is
contained in steel pipes around which the hot coolant gas is blown. The resulting
steam is directed into the turbines which rotate the electric generators. Calder Hall
and Chapelcross have eight of them each, generating 180,000 and 140,000 kW
respectively of electricity, which is fed into the national grid.
If the chain reaction gets too fast and the reactor becomes too hot, the control
rods are lowered into the core automatically, thus slowing down the process; if
pushed in completely they will stop it altogether.
Uranium as the fuel, graphite as the moderator, and carbon dioxide gas as the
coolant are only one possible combination.
Some nuclear engineers believe that organic substances can be used as
moderators and coolant fluids, others that the fuel should be given the form of a
ceramic. A good deal of research work is done with various types of homogeneous
reactors, in which fuel, moderator, and coolant circulate as a single, fluid mixture.
Nuclear power is still in roughly the same early phase as steam power was at
the beginning of the nineteenth century, and it may not reach maturity until the end of
our own century. By that time, however, we shall not only have fission but also
fusion as a basic energy-producing nuclear process.
The theory of nuclear fusion was discovered in the early 1930's – years before
that of fission – by John Cockeroft at the Cavendish Laboratory, Cambridge, where
he worked under Lord Rutherford. Here they built a simple machine, which looked
more like a couple of stove-pipes than an atom-smashing tool, for shooting
electrically speed-up protons at the nuclei of light elements, such as lithium. The
result was that the lithium nuclei turned into nuclei of helium. This was strange; for
helium is heavier than lithium. Somehow the helium atoms must have been formed
not only by splitting but by subsequent accumulation of protons and neutrons. It was
only later that it dawned on the physicists that some such process is responsible for
the way in which the stars, including our own sun, produce their tremendous energy.
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