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Nuclear Power. The Moderator and Control Rods.

Fission causes a chain reaction in the core of a nuclear reactor.

The speed of the chain reaction must be controlled
because the ever increasing numbers of splitting nuclei
will very quickly release a large amount of heat energy
and this would cause the nuclear reactor to explode.

The moderator and control rods together
control the rate of reaction in the core of the nuclear reactor.

What is the Moderator in a
Nuclear Reactor?

The moderator is a material which slows down the speed
of the neutrons. The moderator is
usually made from water but some reactors use graphite.

Neutrons released from the chain reaction must be moving
at the right speed for other atoms of uranium to capture them.
If the neutrons are moving too fast then they do not get
captured by the nucleus of another uranium atom and the chain
reaction stops. The moderator slows the neutrons down
so that they can be captured and keep the chain reaction going.

What are the Control Rods in a Nuclear Reactor?

The control rods prevent the chain reaction happening too quickly.
Some control rods are made from an alloy of silver and cadmium.
Some are made from boron mixed with iron or carbon. The control
are very good at absorbing neutrons. They reduce
the number of neutrons available to continue the chain reaction.

The control rods can be moved up and down inside the reactor core.
As they are lifted up and out of the reactor, the less neutrons they absorb,
the faster the chain reaction happens and the reactor gets hotter.
The heat is then used to generate electricity. The further down
the control rods go, the more neutrons they absorb,
the slower the chain reaction happens and the reactor gets colder.
The control rods can used to shut the reactor down completely.

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