Question

In a nuclear reactor, heavy water (D_2O) is used as a moderator. Its main function is to:

• A. absorb neutrons to stop reaction
• B. accelerate neutrons to sustain reaction
• C. slow down fast neutrons to thermal energies
• D. cool the reactor core

Hint:

Moderator ≠ absorber. Control rods absorb neutrons; moderator only slows them.

Answer 1

The Correct Option is C

Step 1: Recall what happens in fission.
When a heavy nucleus like uranium-235 splits, it throws out neutrons that move very fast and carry a lot of energy.

Step 2: Know what a moderator does.
A moderator is a material placed in the reactor to change the speed of these neutrons. Its job is to make the chain reaction keep going steadily.

Step 3: See why fast neutrons are a problem.
Very fast neutrons are not good at causing more fission. Slow, thermal neutrons are much better at being captured by uranium-235 to split it again.

Step 4: How heavy water helps.
Neutrons bounce off the deuterium nuclei in heavy water and lose a little energy with each bounce. After many bounces they slow down to thermal speeds.

Step 5: Why heavy water in particular.
Heavy water slows neutrons well without soaking them up, so plenty of neutrons survive to keep the reaction going.

Step 6: State the answer.
The main function of the moderator is to slow the fast neutrons down to thermal energies. \[ \boxed{\text{slow down fast neutrons to thermal energies}} \]