( 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 .