( i) Fuel Fissionable materials like 92 U 235, 92 U 238, 94 U 239 are used as fuel.
( ii) Moderator Heavy water, graphite and beryllium oxide are used to slower down fast moving neutrons.
( iii) Coolant The cold water, liquid oxygen, etc. are used to remove heat generated in the fission process.
( iv) Control rods Cadmium or boron rods are good absorber of neutrons and therefore used to control the fission reaction.
Atom bomb working is based on uncontrolled chain reaction.
Nuclear Fusion
The process of combining of two lighter nuclei to form one heavy nucleus, is called nuclear fusion.