1. Fusion
The source of nearly all the energy we have on earth is the fusion
of hydrogen into helium which takes place inthe sun. Self sustaining
fusion reactions can occur only under conditions of extreme
temperature and pressure. Stellar interiors meet these
specifications. The energy liberated in the fusion of light nucei
into heavier ones is often called thermonuclear energy.
2. Fission
A nuclear reactor is a device in which a controlled chain reaction
involving fission, releases energy. The energy liberated in a
nuclear reactor is manifested as heat. It can be extracted by
circulating a suitable coolant (liquid or gas) around the core of
the reactor.
3.
Radioactivity
The energy liberated during radioactive decay comes from within
individual nuclei without external excitation.