Question

Heavy water is used to moderate in nuclear reactor because

• A. It controls the energy released in the reactor
• B. It absorbs neutrons and stops chain reaction
• C. it cools the reactor faster
• D. It slows down the fast-moving neutrons

Answer 1

The Correct Option is D

To solve the problem, we need to understand the role of heavy water in a nuclear reactor.

1. Function of a Moderator in a Nuclear Reactor:
In a nuclear reactor, a moderator is a substance used to slow down the fast-moving neutrons produced during fission. Slower (thermal) neutrons are more likely to cause further fission reactions with fissile material like Uranium-235, thus maintaining a sustained and controlled chain reaction.

2. Why Heavy Water?
Heavy water ($\text{D}_2\text{O}$) is an excellent moderator because: - It slows down neutrons without capturing them (unlike ordinary water which absorbs more neutrons). - It helps sustain the chain reaction even with natural (unenriched) uranium due to its low neutron absorption cross-section.

3. Eliminating Incorrect Options:
- "It controls the energy released in the reactor" → Incorrect: control rods do this, not the moderator.
- "It absorbs neutrons and stops chain reaction" → Incorrect: moderators must not absorb neutrons.
- "It cools the reactor faster" → Incorrect: that’s the function of a coolant, not a moderator.
- "It slows down the fast moving neutrons" → Correct.

Final Answer:
Heavy water is used as a moderator because it slows down the fast moving neutrons.