Otto Hahn and Lise Meitner , with their colleague Fritz Strassman , bombarded uranium nuclei with slow speed neutrons.  Meitner, after fleeing the Nazis and working with Otto Frisch , interpreted the Hahn-Strassman results to be 'nuclear fission,' the term fission being borrowed from biology.  They explained what happened in the nucleus by reference to the liquid drop model: "As the nucleus gets bigger, with more and more protons, the protons are farther apart, and the repulsive strength grows in comparison with the strong nuclear force, [and eventually] just enough to tip the balance in favor of the repulsive forces and split the nucleus" also calculated that vast amounts of energy would be released by a sustained chain reaction.