Nuclear energy is produced by changes in the nucleus of the atom of a radioactive element such as uranium or plutonium. This process is called nuclear fission. The nucleus is split by bombarding the atom with a neutron particle. Each time the uranium atom is split in this way, it releases energy. It also produces three more neutrons, which then go on to split other uranium atoms. This is called a chain reaction because, once started, it will continue the process of nuclear fission while releasing very large amounts of energy.
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