NUCLEAR REACTOR :-

• A nuclear reactor is a nuclear device in which nuclear fission can be carried out through a sustained and a controlled chain reaction.
• It is also called as atomic pile. 
• It is thus a source of controlled energy which is utilised for many purposes like electric power generation, in production of radioactive isotopes for their use in medical field , agriculture, etc. 

MODERATOR :-

• Moderator is used to slow down the fast moving neutrons. 
• Most commonly used moderators are graphite and heavy water (D2O) 
• Heavy water is considered to be best moderator. 
• Some of the characteristics are :- its atomic weight must be low,  it should not absorb neutrons,  it should undergo elastic collisions with neutrons and reduce their velocity. 

If you still have any queries feel free to ask in the comment section down below. 

hello sai,

graphite and heavy water used as a moderator in the nuclear reactor.

 Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium and beryllium oxide (BeO) has been used occasionally, but they are very costly.

hope this is helpful for you.

thank you.