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Q106 (IAS/2001) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Atomic and nuclear physics Answer Verified

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 realeased in the fission reaction initiates further fission. Which of these statements are correct ?

Result
Your answer: —  Â·  Correct: B
Explanation

Statement I is correct because each fission yields on average more than one neutron (typically two or three), providing the source neutrons required for chain multiplication. Statement III is correct since reactors use moderators to slow fast neutrons to thermal energies where fission cross-sections are higher; materials such as graphite are commonly used to moderate or scatter neutrons and shift the neutron spectrum to the thermal region [1]. Statement II is not generally correct for thermal reactors: fast neutrons produced are not immediately effective for sustained fission until moderated. Statement IV is false because not every neutron from fission causes another fission; neutrons are lost by absorption in nonfissile materials and by leakage, determining keff and whether a chain is sustained [2].

Sources

  1. [1] https://www.aerb.gov.in/storage/uploads/documents/regdocF9RS8.pdf
  2. [2] https://www.nrc.gov/docs/ml1125/ml11258a296.pdf
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