UPSC Prelims 2011 Q32: Science & Technology – Energy & Environment
The function of heavy water in a nuclear reactor is to
- (a) Slow down the speed of neutrons
- (b) Increase the speed of neutrons
- (c) Cool down the reactor
- (d) Stop the nuclear reaction
Answer: (a) Slow down the speed of neutrons
Explanation
The role of heavy water in a nuclear reactor
Heavy water (D2O) is widely used as a moderator in nuclear reactors, especially in India's PHWR (Pressurised Heavy Water Reactor) design.
| Option | Verdict | Reason |
|---|---|---|
| (a) Slow down the speed of neutrons | Correct | A moderator's job is to slow fast neutrons to thermal speeds so they can sustain the fission chain reaction efficiently |
| (b) Increase the speed of neutrons | Incorrect | This is the opposite of a moderator's function |
| (c) Cool down the reactor | Incorrect | Coolant, not moderator, is the term for the heat-removal function (though heavy water can double as coolant in some designs, its defining function here is moderation) |
| (d) Stop the nuclear reaction | Incorrect | That is the job of control rods (e.g., boron, cadmium), not the moderator |
Moderator vs coolant vs control rod:
| Component | Function |
|---|---|
| Moderator (heavy water) | Slows down fast neutrons to sustain fission |
| Coolant | Removes heat from the reactor core |
| Control rods | Absorb neutrons to control/stop the reaction |
Answer: (a) Slow down the speed of neutrons.
Why each option is right or wrong
- (a) CORRECT. Heavy water acts as a moderator, slowing fast neutrons to sustain the chain reaction.
- (b) Incorrect. This is the reverse of moderation; a moderator slows neutrons down, not up.
- (c) Incorrect. Cooling is a separate reactor function, not the defining role being asked about here.
- (d) Incorrect. Stopping the reaction is the job of control rods, not the moderator.
Key point
Heavy water in a nuclear reactor: acts as a MODERATOR, slowing down fast neutrons to sustain the fission chain reaction (India's PHWR design).
Remember
Heavy water = moderator, slows fast neutrons to sustain fission (used in India's PHWRs).
Common mistakes
- Confusing the moderator's role with the coolant's role.
- Thinking heavy water stops the reaction (that's control rods).
- Reversing the direction — moderators slow down, not speed up, neutrons.
Exam tip
Moderator = slows neutrons. Control rod = stops/controls reaction. Coolant = removes heat. Keep these three separate.
Sources
- Department of Atomic Energy, Government of India — PHWR technology note
- Nuclear Power Corporation of India Limited — Technology: Pressurised Heavy Water Reactor (PHWR)