Question

Moderator is used in nuclear reactor for:
A. Slowing neutrons
B. Accelerating neutrons
C. Stopping neutrons
D. Heating the neutrons

Answer 1

Recall the fission reaction happens inside the nuclear reactor. The neutrons accelerated in the nuclear reactor sometimes have high kinetic energy. If they are not scattered by the light nuclei, they will escape the nuclear reactor.

Complete answer:
We know that nuclear reactors work on fission reactions that are continuously happening inside the reactor. We have the fission reaction,
${}_{92}^{235}{\text{U}} + {}_0^1{\text{n}} \to {}_{92}^{236}{\text{U}} \to {}_{56}^{144}{\text{Ba}} + {}_{36}^{89}{\text{Kr}} + 3{}_0^1{\text{n}}$

When Uranium isotope ${}_{92}^{235}{\text{U}}$ is bombarded by a neutron, it breaks it into two intermediate mass fragments ${}_{56}^{144}{\text{Ba}}$ and ${}_{36}^{89}{\text{Kr}}$. In this reaction, three neutrons are released. Thus, we can see that, from a single neutron we get three neutrons. This reaction continuously happens inside the nuclear reactor. Since we get more neutrons as time passes, if the reactions are uncontrollable, it will lead to explosive damage. Some of the neutrons generated in the chain reactions have high energy, and therefore, will escape the reactor if not slowed down. We can slow down the fast moving neutrons by interacting them with light nuclei known as moderators.

Therefore, in nuclear reactors, light nuclei such as water, heavy water and graphite are provided along with the fissionable element. Therefore, the moderators are used in the nuclear reactors to slow down the fast moving neutrons,We are all familiar with the sad incident that happened in Ukraine in 1986 of the Chernobyl reactor.

So, the correct answer is option (A).

If the neutrons are stopped completely, there will not be any nuclear fission in the nuclear reactor. Therefore, the option (C) is incorrect. Also, the neutrons should not be accelerated otherwise they will escape the nuclear reactor. Therefore, the option (B) is incorrect.

Note: In nuclear reactors, the factor K determines the growth rate of neutrons in the reactor. It has a constant value of 1 and if in case it becomes greater than 1, the reactions in the reactor will be uncontrollable and it will lead to catastrophe. If the value of K becomes less than 1, the nuclear fission reaction will eventually come to cease.