2011 GS Paper I, all questions
UPSC Prelims 2011 · GS Paper I
Question 32: Science and Technology
The function of heavy water in a nuclear reactor is to
aSlow down the speed of neutrons
bIncrease the speed of neutrons
cCool down the reactor
dStop the nuclear reaction
AnswerThe answer is (a).
Neutrons released in fission are fast, and fast neutrons are poor at causing further fission in uranium-235. A moderator slows them down. Heavy water, which is deuterium oxide, serves as a moderator in Indian pressurised heavy water reactors because deuterium absorbs very few neutrons. Control rods, not heavy water, absorb neutrons to stop or regulate the reaction. Remember: moderator slows neutrons; control rods absorb them.
Why each option is right or wrong
- (a) Right: heavy water slows down fast neutrons, acting as a moderator.
- (b) A moderator slows neutrons; it does not speed them up.
- (c) Heavy water can also carry heat in some designs, but its defining role is moderation.
- (d) Stopping the reaction is the job of control rods and shutdown systems.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.