MCQ Detailed View

In a nuclear reactor, controlled nuclear fission takes place to release energy. Fission occurs when a heavy nucleus, such as Uranium-235 or Plutonium-239, absorbs a neutron and splits into smaller nuclei, releasing heat and more neutrons. These newly released neutrons travel at very high speeds. If they remain too fast, they are less likely to be captured by other fuel nuclei, reducing the efficiency of the chain reaction.

A moderator is a material placed inside the reactor core that is used to slow down the speed of neutrons without absorbing them. By reducing their kinetic energy, the neutrons become thermal neutrons, which are much more effective in causing further fission reactions in nuclear fuel. This process ensures a sustained and controlled chain reaction, preventing the reactor from shutting down or becoming unstable.

Common moderator materials include light water (H₂O), heavy water (D₂O), and graphite. These substances have light nuclei that effectively collide with fast neutrons, slowing them down through elastic scattering while keeping their number intact.

Now, evaluating the options:

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  Accelerate the neutrons: This would make fission less efficient, not more.

  
  


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  Slow down the speed of the neutrons: ✅ This is the correct and essential function of a moderator.

  
  


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  Increase or decrease the number of electrons: Electrons are not directly involved in nuclear fission, so these options are incorrect.

  
  


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  None of these: Incorrect, since one option is valid.

  
  

In conclusion, the moderator in a nuclear reactor slows down the speed of neutrons, making fission more efficient and keeping the chain reaction under safe, controlled conditions.