Question

What is the importance of a moderator in a nuclear reactor?
(a). Moderator controls the flow of the water inlet.
(b). Controls the nuclear fission by absorbing the excess neutrons.
(c). Controls the nuclear fission by slowing down the neutrons.
(d). None of these

Answer 1

Hint: For a nuclear reaction to take place we require slow moving neutrons and moderation is a process used in a nuclear reaction in which we slow down the speed of fast moving neutrons for continuation of nuclear reaction.

Complete step by step answer:

In a normal state, neutrons are bound in the nucleus of an atom. Binding energy of a neutron is the energy required to remove a neutron from an atomic nucleus. Its value for neutrons is 7-9 MeV.
In a nuclear reactor, nuclear fission takes place in which a heavy nucleus splits into lighter nuclei. A huge amount of energy is produced by relatively small amounts of reactants and energy required to unbind neutrons is available and free neutrons are produced in these reactions. The neutrons gain energy as they propagate through the reactor.
For the chain reaction to continue we need neutrons but they must be slow for the reaction to occur. So, in order to slow down these fast moving electrons we use a moderator.
Moderation is the process of slowing down fast moving neutrons which is accomplished using a moderator. The neutrons slow down by transferring their energy to the moderator. They reduce the energy of the fast moving neutrons to thermal energy. Thermal neutrons are more efficient in propagating the nuclear chain reaction.
Hence option (c) is correct

Note: For moderation to take place, we need to take into account the cross section of neutrons. The moderator should be chosen in such a way that maximum energy transfer takes place when the moderator collides with neutrons.