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     The word 'fusion' means 'to join' or 'to combine'. The process in which two nuclei of light elements (like that of hydrogen) combine to form a heavy nucleus (like that of helium), is called nuclear fusion. A tremendous amount of energy is produced during the fusion process. We know that the nuclei of atoms are positively charged. So, when two nuclei are brought together, they repel each other dure to their similar charges. Due to this a lot initial heat energy and high pressure are required to force the lighter nuclei to fuse together to form a bigger nucleus. So, the conditions needed for carrying out nuclear fusion process are 'millions of degrees of temperature' and 'millions of pascals of pressure'. In other words, nuclear fusion is carried out by heating the lighter atoms to extremely high temperatures under extremely high pressure. There is some loss of mass during the fusion process which appears as a tremendous amount of energy. Here is an example of nuclear fusion reaction.

     When deuterium atoms (heavy hydrogen atoms of mass number 2) are heated to an extremely high temperature under extremely high pressure, then two deuterium nuclei combine together to form a heavy nucleus of helium, and a neutron is emitted. A tremendous amount of energy is liberated in this fusion reaction. This fusion reaction can be written as :

 

   

 

     The process of fusion appears to be simple. But it is not easy to fuse the lighter nuclei into a single nucleus. For the fusion two positive nuclei have to be brought close together. This needs extremely high temperature. At high temperatures the lighter nuclei to be fused, possess enough thermal energies to be able to overcome electrostatic force of repulsion between them. It is calculated that fusion is possible at temperatures of the order of 10⁷ to 10⁸ K. Since the reaction can take place only at very high temperature it is called thermonuclear reaction. The energy released in the process is called thermonuclear energy. 

     It is very difficult to obtain temperature of the order of 10⁸ K on earth. Such high temperature can be obtained during the explosion of an atom bomb.