Einstein's relation obtained from the principle of relativity establishes the equivalence of mass and
energy.
Mass of a nucleus is less than the sum of the masses of constituent nuclei, the difference is the mass defect and
the equivalent energy is known as the binding energy of the nucleus.
Nucleus is held together by a force that is strong, short ranged and attractive.
The nuclear binding energy is more for heavier nuclei than for lighter nuclei, as a result of which when a heavier
nucleus splits into smaller fragments (fission), energy gets released.
Fission is effected by a thermal neutron striking a heavy nucleus such as U and disturbing the delicate balance
of forces. Nucleus thereby fissions and in the process liberates more neutrons, which, in turn, can cause more fission. A nuclear reactor is based on the principle of controlling this chain reaction.
When two light nuclei coalesce to form a heavier nucleus, the process is called fusion. Thermonuclear fusion can
also be a source of great energy provided the process can be controlled.
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obtained from the principle of relativity establishes the equivalence of mass and 
U and disturbing the delicate balance 
