Energy is released when two atomic nuclei combine to form a larger atomic nuclei. There's a threshold of energy required to attain this fusion reaction (otherw

Energy is released when two atomic nuclei combine to form a larger atomic nuclei. There's a threshold of energy required to attain this fusion reaction (otherwise there would be no light nuclei in the universe), and once the nuclei combine there's energy that is released, similar to how some chemical reactions can be exothermic in nature.