The moderator material is generally graphite or water. These materials do not absorb or react with the neutrons, they simply act to slow down fast moving neutrons. If neutrons are moving too quickly they are not captured by Uranium nuclei and the chain reaction stops. Once they are moving at the correct speed they will be captured by Uranium nuclei and the chain reaction can continue.It is important to note that the moderator in a nuclear fission reactor is not the same as the control rods. Both the moderator and the control rods can be used to change and maintain the rate of the chain reaction within the reactor. But the control rods are used to absorb neutrons, reducing the number of neutrons available for further chain reactions. Whilst the moderator is installed to slow down neutrons, making them more likely to be captured by Uranian nuclei, increasing the rate of chain reactions.
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Radioactivity
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
rods 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 fasterthe 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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