Theory:

Fission and fusion reaction:

At present, all commercial nuclear reactors are based on nuclear fission. But, there is another possibility of nuclear energy generation by a safer process called nuclear fusion. Fusion means joining lighter nuclei to make a heavier nucleus, most commonly hydrogen or hydrogen isotopes to create helium, such as
 
H2+HHe(+n)32
 
According to the Einstein equation, because the mass of the product is slightly less than the sum of the masses of the original individual nuclei, it releases a tremendous amount of energy. The Sun and other stars get their energy from nuclear fusion reactions. Forcing the nuclei to fuse takes a lot of energy. This process requires extreme conditions, such as millions of degrees of temperature and millions of pascals of pressure. The thermonuclear fusion reaction is the basis of the hydrogen bomb.
 
1200px-Mark_7_nuclear_bomb_at_USAF_Museum.jpg
Nuclear bomb at USAF Museum
 
nuclear-g716b2b434_1920.jpg
An explosion of a nuclear atom bomb
Reference:
https://commons.wikimedia.org/wiki/File:Mark_7_nuclear_bomb_at_USAF_Museum.jpg