Question

Which of the following controls the chain reaction in a nuclear reactor?
A) Molecules of high energy
B) Ions
C) Active Hydrogen
D) Moderator

Answer 1

We need to understand how the chain reaction works, what is the mechanism of nuclear fission reactions. And how does a nuclear reactor operate and control the chain reaction.

Complete step by step answer:
First let’s have a look at the nuclear fission.
Nuclear fission is a process in which a heavy nucleus splits into two or more lighter nuclei of almost equal mass. It can be initiated when the heavy nucleus captures a slow neutron, and it generates high amounts of energy. The neutron is often bombarded to the nucleus of the heavy nucleus.
But the combined mass of the daughter nuclei is less than the mass of the parent nucleus.
Why does this happen?
The reason behind this is that the reaction also produces some neutrons along with daughter nuclei.
A typical example for nuclear fission is of $U_{92}^{235}$

$n_0^1 + U_{92}^{235} \to Ba_{56}^{141} + Kr_{36}^{92} + 3(n_0^1)$

These neutrons can be absorbed by the remaining nuclei of the heavy element, resulting in exponential increase in reaction rate and production of a high amount of energy. This phenomenon is termed as uncontrollable chain reactions.

The reason behind chain reaction is that the neutrons released from a fission reaction can cause further fission of the heavy nucleus.

In the uncontrolled fission of ${U^{235}}$ three neutrons are produced which are again absorbed by three other ${U^{235}}$ nuclei in the vicinity which releases nine more neutrons resulting in exponential increase in the rate of fission reaction.
This fission reaction lasts for a very short amount of time $({10^{ - 12}}s)$, so it is crucial to control these neutrons for our benefits.

In a nuclear reactor the chain reaction is steady and controlled so that on average only one neutron from each fission produces another fission.
The reaction rate is adjusted by inserting neutron absorbing rods of boron steel into the Uranium 235 and to slow the neutrons down, moderators are used.

Note: In nuclear reactors, for manifesting energy from fission, we have to stop the spontaneous chain reaction and control it using moderators. Sometimes questions can be asked about the materials used in the moderator. Mainly graphite core is used as a moderator to slow down neutrons in a reactor.