Nuclear fission is a kind of nuclear reaction. It is when an atom splits apart into smaller atoms. Some fission reactions give off a lot of energy, and are used in nuclear weapons and nuclear reactors. Nuclear fission was discovered in December 1938 by physicists Lise Meitner and Otto Frisch, who observed a uranium nucleus splitting in two. Although they originally thought the two tiny resulting nuclei were barium isotopes, what Meitner and Frisch truly saw was a process that would revolutionize nuclear chemistry: nuclear fission