Moderation is necessary in all aspects of life if one has to achieve success. Usually extreme of anything is bad, no matter whether it is good or bad. It is no wonder the same principle applies to nuclear reactions as well. Just learn few basic concepts about moderation and moderators in context of nuclear power plants
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The nuclear fission reaction consists of bombarding fuels such as Uranium with energetic neutrons. This makes the target unstable and makes it split into two parts accompanied with the release of energy which is utilized to generate electricity. There is a certain threshold below which the neutron will not be absorbed by the target nucleus, but that does not mean that above that threshold any neutron can cause fission. Infact there is a range of energy within which they can cause fission. Neutrons which fall above that range are known as fast neutrons and they are not readily absorbed by the target nucleus and hence not useful in sustaining a chain reaction. A moderator is one of the important components of nuclear power plant helping to maintain neutron population in the thermal energy range.
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The problem lies in the fact that whenever a thermal neutron causes fission it also leads to the release of fast neutrons. Now these fast neutrons have to be slowed down and brought to lower energy levels if they have to cause successful fission in turn. It is here that the concept of a moderator comes in the picture.
As you must have understood above, a moderator is a medium which is used to absorb a portion of the kinetic energy of fast neutrons so that they come in the category of thermal neutrons which help to sustain a controlled chain reaction. The mechanism of speed control works in such a way that fast moving neutrons strike the nuclei of moderator material which is not efficient at absorbing them but simply slows them down with repeated collisions thus bringing them into the thermal zone.
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Materials for Moderator
There are several materials which are used for the purpose including the following
Normal or Light Water is used in majority of the reactors simply because of its cheap and abundant availability. The only flipside of using light-water is that the fuel has to be enriched to use with water
Deuterium - also known as heavy water in common terminology, Deuterium is costly to manufacture as compared to light water but gives the option of using un-enriched fuel in the reactor which is a big advantage
Miscellaneous - Several materials such as Graphite, Beryllium, Lithium are used in different types of reactors as moderators
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Is it always necessary?
Although moderators are necessary in most nuclear reactors this does not mean to say that all reactors require moderators. There is a special class of reactors known as fast reactors which do not use moderators but depend on the use of fast moving neutrons for causing fission. Even otherwise it must be remembered that fast moving neutrons have lesser probability of getting absorbed and causing fission but it does not mean that they are incapable of causing the fission reaction. Just to give you a relative idea a fast moving neutron travels with a speed which is nearly in the region of 10% of the speed of light, while a thermal neutron travels with a speed which is typically of the order of a few kilometers per second.
There are also other categories of neutrons based on their energy levels such as slow neutrons, cold neutrons, ultra cold neutrons and so forth.