Nuclear Fission is a process where the nucleus of an atom is split into two or more other nuclei, and sometimes other products come out as well. The fuel is a radioactive element, for example Uranium235, although polonium can be used as well as other elements.
One method is where a neutron is fired into uranium 235, to create a chain reaction and to create new elements, which have roughly 100 neutrons each. These elements, as shown are Krypton and Barium. The by-products are three neutrons.
Nuclear fission is usually used in chain reactions – this means that the three neutrons that are given out in the reaction are fired into three more U235 nuclei. This means that the energy given in can be relatively small, as the fission provides the energy to fire the neutrons after the first one.
Nuclear reactions can either give out energy, or take in energy. Iron and nickel have the most stable nuclei, as their binding energy per neutron is the highest out of all the elements. Because of this, for nuclear fission, the elements above these two will release energy, and those below will take in energy. For nuclear fusion this is vice versa. This is why the elements used in nuclear fission are ones with over 100 neutrons, whereas in nuclear fusion, the elements have very few neutrons in comparison.
Nuclear fission power stations are not great big atom bombs, ready to go off, as many people now believe. This is because uranium has different isotopes, and the one needed for reacting, U235, is only 0.7% of all the uranium on Earth; the rest is U238. The atom bombs that were dropped on Hiroshima and Nagasaki were both fission bombs. They used polonium-239 and uranium-235 in the same way that uranium is used to create energy.