In nuclear fission reaction a heavy nuclei breaks apart into two lighter nuclei and in fusion reaction two lighter nuclei fuse to form a heavy nuclei. Nuclear fusion produces tremendous amount of energy which is several times the amount produced by any nuclear fission reaction. It is the nuclear fusion reaction that powers the sun. Nuclear fission produces radioactive, toxic waste which nuclear fusion does not. The resources of nuclear fusion are abundantly available on Earth, such as lithium and hydrogen isotopes.