}
2
NUCLEAR REACTORS
In an oil or coal fired power station the fuel is burnt to produce heat. What is actually happening is that the fuel is combining chemically with oxygen in the air and it is this chemical reaction that gives out heat energy. This heat energy is used to convert water to steam. The steam is then used to drive a turbine which is connected to a generator that produces electricity.
In a nuclear reactor large atoms of uranium or plutonium are split or 'fissioned' into smaller atoms and it is this fission reaction which results in a great heat output. As with a fossil fuel power station the heat is used to convert water to steam. The steam drives the turbine and electricity is generated.
It is essentially only the different heat source that distinguishes a nuclear power station from a coal or oil fired
station.
Figure 2.1 illustrates the essential features of a pressurised water reactor (PWR) similar to the reactors being constructed at Daya Bay.
FIG. 2.1
PRESSURISED
WATER REACTOR
Pressuriser
Reactor
pressure
vessel
Primary coolant
Steam generator
Feed
water
pump
Pressurised water circuit
Reactor building
Steam
Condenser
Turbine
Generator
Sea
Cooling
water
2.1 The Fission Process
A small number of the larger types of atoms are capable of being split into two or occasionally three smaller atoms. One particular type of uranium atom, known as uranium-235, is capable of undergoing this fission process and it is the most commonly used fuel in nuclear power stations. The centre, or nucleus, of a uranium-235 atom is made up of 92 particles known as protons and a further 143 particles known as neutrons, a total of 235 particles (hence the name—uranium-235). Neutrons, being electrically neutral, can move freely around in the spaces between atoms. If such a free neutron, travelling relatively slowly, hits the uranium-235 nucleus then that nucleus can split, usually
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