Question

In a nuclear reactor, what is the function of a moderator?

Answer 1

Hint: In a nuclear reactor, nuclear fission takes place, and neutrons are released at high speeds. Some device/structure is needed to control their high seeds into slow speeds to continue the nuclear fission chain reaction.

Complete Step by Step Solution:
A nuclear reactor is used to produce electrical energy by utilising controlled and sustained nuclear fission chain reaction.

A slow moving neutron $(n)$ is made to collide with a Uranium-235 atom, this breaks the heavy $^{235}U$ nucleus breaks into two smaller nuclei and releases three neutrons along with very high energy. These three nuclei are further utilised to collide with three $^{235}U$ atoms and so on.

$n + {}^{235}U \to {}^{92}Kr + {}^{141}Ba + 3n + energy$

The energy released in this chain reaction is used to convert water into steam and use it to run turbines (rotate them) which produces electricity.

Now, the main point to ponder is that the 3 neutrons that are released at very high speeds need to be moderated into slow speeds in order to carry out the chain reaction.
The device/part used to perform this task is called a Moderator.

A moderator allows only slow speed neutrons to continue (sustain) the chain reaction at a constant rate by changing their initial high speeds by slowing them down.
Usually, Graphite moderators are used in nuclear reactors.

Note: The students should know about the nuclear fission and fusion reactions. Nuclear fusion reactions are uncontrollable, so they are not utilised by mankind to produce electricity. Nuclear fission reactions are used to produce electricity by controlling the conditions of the reaction.
If the high speed neutrons are made to collide with the Uranium atom, then the reaction will become uncontrollable. Therefore, their speeds need to be slowed down using the moderator before they collide with the parent atom.