Nuclear energy is released through fission, fusion, and radioactivity. There are two types: nuclear fission and nuclear fusion. Nuclear fission occurs when a nucleus splits and releases energy, such as when a Uranium-235 nucleus is bombarded with a neutron, resulting in a chain reaction. This was used in the atomic bombs dropped on Hiroshima and Nagasaki in 1945. Nuclear reactors produce energy through controlled fission reactions using uranium fuel. Nuclear fusion is the merging of light nuclei to form heavier nuclei and was an energy source for the sun.