Moderator nuclear
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.
There are several material swhich 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.
Table Moderator nuclear
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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