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How
Uranium is Turned into Electricity
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A small handful of uranium provides as much electrical energy
as 70 tons of coal or 390 barrels of oil. A power station
big enough to supply a city of a million people consumes just
6.6lb (3kg) of uranium a day, so it is by far the most concentrated
source of energy used by man.
Uranium is one of the densest naturally occurring elements
and each of its atoms teeters on the edge of instability.
The heart of the atom, called the nucleus, needs only a tiny
‘push’ to cause it to divide. And when a nucleus
splits it releases huge amounts of energy, in a process called
nuclear fission.
The ‘push’ can be provided by neutrons, tiny particles
much smaller then atoms, which strike the nucleus and cause
it to split. In the process of splitting, at least two extra
neutrons are produced, which fly off and cause further fissions,
so that once the process has started it can continue almost
indefinitely.
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Experimental
Reactor |
No ordinary paper could meet these
exacting demands. Teabag paper is made from two strong fibres,nianila
henip, a long natural fibre (used to make rope) for strength,
and thermoplastic fibres, to sual the bags.The two fibres are
not woven together, they are laid down as a watery mixture in
two separate layern. They formpaper when the water drains away
and the damp web remaining is squeezed dry through rollers.
This gives thepaper an irregular. web-like structure will pores
varying in size.
The paper goes through the tea-packing machine in a sandwich
of two strips and the machine measure out the amount of tea
on the lower strip. The thermoplastic: fibres melt to form the
bond, which stays strong whenit solidifies again on cooling.
Its melting point is higher than 212 F (100 C), so the bag will
not come appearwhen boiling water is poured over it.
The energy of fission can be released slowly, bit by bit, and
used to hear water. The steam from the water is then used to
drive a generator, which produces electricity. This is the principle
of the nuclear reactor.
Fuel assemblies
Inside most reactors, the fuel assemblies are made from small
pellets of uranium dioxide, loaded into thin tubes. The tubes
are usually put into vertical bundles with ‘spacers’
to separate them.
Once inside, a fuel assembly may stay there for as long as three
years, but even after that length of time, all the uranium has
not been consumed. But by-products begin to accumulate; some
are gasses like krypton, others are solids like caesium, strontium
and plutonium. Before these by-products have built up too much,
and water corrodes the fuel tubes, the assemblies are removed.
To recover the unburned uranium, the spent fuel may be taken
to a special out uranium, plutonium waste products.
The plutonium is a useful by-products of the nuclear power industry.
It can be used as a fuel in power stations, because plutonium,
like uranium, has nuclei that can split and release energy.
Uranium occurs in several different forms identical chemically
but with different-sized nuclei in their atoms. Of these different
forms, called isotopes, one is uranium-235 particles making
up its nucleus. Only seven atoms out of every 1000 in naturally
occurring uranium are U-235. The rest consist almost entirely
of uranium-238.
When U-238 is struck by neutrons it does not split as readily
as U-235. It may be converted into a completely new element,
plutonium-239. So if a reactor is made using natural uranium
as fuel, the danger is that too many neutrons will be absorbed
by U-238 before they can hit U-235 atoms and cause further fissions.
If this happens the reactor will never get going.
There are two ways around this problem. One is to increase the
amount of U-235 in the reactor fuel, by a process called enrichment,
from seven atoms to between 30 and 40 in every thousand. This
is done before the fuel is manufactured, usually in a centrifuge,
a machine that whirls round, separating U-235 from U-238 by
the outward pushing forces of high-speed rotation. The second
way is to make the very best use of the available neutrons inside
the reactor by slowing them down, which increases their chances
of causing further fissions.
The way to slow down is to make them ricochet to and fro off
light atoms of an element such as hydrogen or carbon, like balls
in a pinball machine. The light elements act as a ‘moderator’,
because their job is to moderate the speed of the neutrons.
Most modern reactors use both enriched fuel and moderators.
Some are moderated by water (which, of course, contains hydrogen),
while others are moderated by carbon in the form of graphite,
which is the hard dark material known as the ‘lead’
in an ordinary pencil.
Nuclear
Power Station |
Obviously, a
nuclear rector produces a great amount of heat, and to
stop the reactors from overheating,coolants have to be
used. Pressurized water reactors use water as a coolant,
so these plants need to be built near river or oceans.
Advanced gas-cooled reactors, first built in Great Britain,
are cooled by carbon-dioxide gas. In Canada, heavy water,
in which hydrogen atoms are replaced with an isotope of
hydrogen called deuterium, cools fast breeder reactors.
France has pioneered the use of liquid sodium as a coolant
for their fast breeders. |
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