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Question: Answered & Verified by Expert
Assertion : It is desirable to slow down fast moving neutrons to sustain controlled chain reactions.
Reason : Slow moving neutrons efficiently collides with $\mathrm{U}^{235}$.
PhysicsNuclear PhysicsAIIMSAIIMS 2011
Options:
  • A If both assertion and reason are true and reason is the correct explanation of assertion.
  • B If both assertion and reason are true but reason is not the correct explanation of assertion.
  • C If assertion is true but reason is false.
  • D If both assertion and reason are false.
Solution:
2821 Upvotes Verified Answer
The correct answer is: If assertion is true but reason is false.
The average energy of a neutron produced in fission of $\mathrm{U}^{235}$ is $2 \mathrm{MeV}$. These neutrons unless slowed down will escape from the reactor without interacting with uranium nuclei, unless a large amount of fissionable material is used for sustaining the chain reaction. What we needs to do is to slow down the fast neutrons by elastic scattering with light nuclei. Chadwick's experiment showed that in an elastic collision with hydrogen neutron almost come to rest.

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