Otto Hahn discovered nuclear fission, the method used in nuclear reactors to extract energy from nuclear materials. Enrico Fermi is credited as the first person to successfully create a nuclear chain reaction, which would be used in reactors in the future.
While Otto Hahn won a Nobel Prize for his discovery, he also worked closely with Fritz Strassmann and Lise Meitner. They found that by shooting neutrons at a uranium atom, the nucleus could be split and energy would be released as heat. The fission process also released more neutrons, which Fermi discovered could initiate fission in other nearby uranium atoms.