The three forms of nuclear reactions known to mankind were nuclear fission, nuclear fusion, and nuclear decay.
Among these, nuclear fission and nuclear fusion provided energy for humans. According to Einstein's mass-energy equation, E=mc^2, as long as there was mass attenuation after the nuclear reaction, it meant that a certain amount of energy had been released.
Nuclear fission was the production of energy through mass loss caused by the fission of heavy atoms.
On the contrary, with nuclear fusion, the aggregation of light atomic nuclei produced heavier atomic nuclei and mass attenuation would occur, thereby generating energy.