UPSC Prelims 2001 Q106: Science & Technology – Energy & Environment
Consider the following statements: In a nuclear reactor, self - sustained chain reaction is
possible, because
I. More neutrons are released in each of the fission reactions.
II. The neutrons immediately take part in the fission process.
III. The fast neutrons are slowed down by Graphite.
IV. Every neutron released in the fission reaction initiates further fission.
Which of these statements are correct?
- (a) I, II and III
- (b) I and III
- (c) II and IV
- (d) II, III and IV
Answer: (b) I and III
Explanation
Why a self-sustained chain reaction is possible in a nuclear reactor
| Statement | Verdict |
|---|---|
| I. More neutrons are released in each fission | Correct — about 2-3 neutrons per U-235 fission |
| II. The neutrons immediately take part in fission | Wrong — they are fast and must first be slowed; some are also delayed neutrons |
| III. Fast neutrons are slowed down by graphite | Correct — graphite (or heavy water) acts as moderator |
| IV. Every neutron released causes further fission | Wrong — many are absorbed or escape; control rods absorb some on purpose |
A chain reaction can keep going because each fission releases more than one neutron, and a moderator such as graphite slows the fast neutrons so that U-235 can capture them. Neutrons do not cause fission at once, and not every neutron causes another fission — in a reactor, control rods ensure on average just one does. So I and III are correct. (One third-party key gives (a); statement II is not correct.)
Answer: (b) I and III.
Why each option is right or wrong
- (a) Incorrect. Statement II is wrong; neutrons must be slowed first.
- (b) CORRECT. Extra neutrons per fission and a graphite moderator make the chain reaction possible.
- (c) Incorrect. Statements II and IV are both wrong.
- (d) Incorrect. Statements II and IV are wrong.
Key point
Reactor parts: fuel (U-235), moderator (graphite, heavy water), control rods (boron, cadmium), coolant (water, sodium).
Remember
Chain reaction: 2-3 neutrons per fission + moderator.
Common mistakes
- Thinking every neutron must cause fission; that would be a bomb.
- Thinking fast neutrons fission U-235 easily.
- Mixing up moderator and control rods.
Exam tip
Moderator slows neutrons; control rods absorb them.
Sources
- NCERT Class XII — Physics Part II, chapter 'Nuclei' (nuclear reactor)
- Department of Atomic Energy / NPCIL — How a nuclear reactor works