Splitting the Atom You know that atoms are made up of a nucleus and electrons moving around the nucleus, and the nucleus is made up of protons and neutrons. The principal actor in the fission drama is the uranium atom. The nucleus of the Uranium 238 atom has 92 protons and 146 neutrons. About one atom in 140 atoms of uranium has 143 neutrons in its nucleus. lt is called Uranium 235. It is the Uranium 235 that is mostly used to provide fuel for nuclear furneces. Before the Second World War, two German scientists discovered that if neutrons were shot at the nuclei of Uranium 235, the nuclei split into two and produced other neutrons to repeat the process. This is called nuclear fission.