Which of the following is NOT used as a moderator in nuclear reactors?

Correct option is C

​The correct answer is (c) Boron

Explanation:

In a nuclear reactor, a moderator is a substance that slows down the speed of neutrons, allowing them to effectively sustain a chain reaction with the nuclear fuel (such as uranium-235).

• Heavy water (D₂O), Water (H₂O), and Graphite are commonly used as moderators in nuclear reactors because they reduce the energy of fast neutrons without absorbing too many of them.
• Boron, however, is not used as a moderator. Instead, boron is a neutron absorber, meaning it captures neutrons instead of slowing them down. This property makes boron useful in control rods that regulate the fission process by absorbing excess neutrons.
• Boron Carbide (B₄C)is commonly used in control rods due to its high neutron absorption capacity.

Information Booster:

Common Moderators Used in Nuclear Reactors:

1. Heavy Water (D₂O) – Used in CANDU reactors (Canada Deuterium Uranium reactors).
2. Light Water (H₂O) – Used in pressurized water reactors (PWRs) and boiling water reactors (BWRs).
3. Graphite – Used in RBMK (Chernobyl-type) and Magnox reactors.

Why is a Moderator Important?

• A moderator slows down fast-moving neutrons, making them more likely to cause fission in the nuclear fuel (e.g., Uranium-235).
• Without a moderator, the neutrons would move too fast and escape without effectively sustaining the chain reaction.