Both Nuclear fission and fusion are used for the production of a tremendous amount of energy but there are some differences between them too. The division of a single nucleus into smaller nuclei is nuclear fission while the combining of two or more small nuclei to form a heavier one is called nuclear fission. A nuclear fission reaction to initiate small energy is required for a nuclear fusion reaction; a large amount of energy is required to combine these small nuclei for the formation of a heavier one. The amount of energy released during a nuclear fusion reaction is very much higher than that of a nuclear fission reaction.