UPSC Prelims 2001 · General Studies
Question 106: Science and Technology
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?
AnswerThe answer is (b).
Statements I and III are correct. Each fission releases more than one neutron, so a chain reaction can be sustained, and a moderator such as graphite slows the fast neutrons so that they can cause further fission in uranium-235. II and IV are wrong: not every neutron causes fission, since some escape or are absorbed, and fast neutrons do not take part effectively until they are slowed. Remember that a self-sustained chain needs a multiplication factor of at least one.
- I. Each fission releases more than one neutron.
- II. Fast neutrons are not immediately effective until slowed down.
- III. Graphite acts as a moderator and slows fast neutrons.
- IV. Not every neutron causes a further fission, as some escape or are absorbed.
Why each option is right or wrong
- (a) II is wrong, as neutrons must first be slowed.
- (b) Right: I and III.
- (c) Both II and IV are wrong.
- (d) II and IV are wrong, and I is left out.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.