Question

Which of the following statement regarding $D_{2}O$ is correct?

• A. It is an isotope of hydrogen
• B. It is used as a bleaching agent
• C. It is used as a moderator in atomic reactors
• D. It is used as a coolant in atomic reactors

Answer 1

Answer: (C), (D)

The correct options are
C It is used as a moderator in atomic reactors
D It is used as a coolant in atomic reactors

$D_{2}O$ is used as a moderator in atomic reactors and as coolant in atomic reactors.

The chain reaction in a reactor is sustained by the free neutrons coming from one fission being absorbed by a Uranium nucleus, which then fissions itself. The probability of this absorbtion is increased if the neutrons are moving slowly. The purpose of the moderator is to slow the neutrons down.
Lighter nuclei make better moderators than heavy nuclei. If a neutron were to hit an infinitely heavy nucleus, it would bounce off in a different direction with the same velocity. When a neutron hits a nucleus of atomic mass one head on, it comes to a complete stop.
So, we want to use light nucleus. The reason water $\left(H_{2}O\right)$ is unsuitable is that it is likely to absorb the neutrons instead of slowing them down. The probability of water absorbing a neutron is more than $200$ times higher than the probability of deuterium (which already has a neutron) doing so.

Hence, options C and D are correct.