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Which substance is used as moderator in Nuclear Reactor?
1. H2O
2. D2O
3. Graphite
4. Both D2O Graphite

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Correct Answer - Option 4 : Both D2O Graphite


Nuclear Reactor 

A nuclear reactor is a device in which nuclear reactions are initiated, controlled, and sustained at a steady rate. 

Nuclear reactor basic principles 

  1. Neutron-induced fission releases energy with extra fast neutrons.
  2. These fast neutrons are then slowed down by a moderator, allowing a chain reaction to take place.
  3. The Chain reaction is controlled by controlling the condition of the moderator, or by use of the neutron absorbing materials such as cadmium control rods. 
  4. Heat is removed by some form of heat exchanger where it is used to run a heat engine.



  • Moderator is required in a Nuclear reactor to slow down the neutrons produced during the fission reaction.
  • Heavy Water is an excellent moderator due to its high moderating ratio and low absorption cross-section for neutrons.
  • Also, it does not capture any neutrons.
  • Graphite and heavy water (D2O) can also be used as a moderator in a nuclear reactor. 

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