Nuclear Fission The word ‘nuclear’ refers to the center or nucleus of an atom. Nuclear fission is the chief method of producing nuclear energy. It involves using a neutron to split the nucleus of a heavy element, such as uranium, into fission fragments. Besides heat energy, fission releases neutrons and such radiation as gamma rays. Nuclear fusion is another reaction that produces nuclear energy. Fusion occurs when atoms become fused together through heat. Both fission and fusion create nuclear energy. However, with the technology we have today, the fission reaction is much easier to produce and we can control the amount of energy that is produced. The nuclear fission reaction occurs when a n