Nuclear fission or fusion fractal illustration. Conceptual fractal illustration of a heavy element, such as uranium, undergoing fission. In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts. The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a very large amount of energy even by the energetic standards of radioactive decay. Nuclear fusion is the joining (fusing) of light elements to form heavier elements, which releases large amounts of energy. It is hoped that fusion will be a clean, renewable energy source for the future