<P> The concept of a nuclear chain reaction was reportedly first hypothesized by Hungarian scientist Leó Szilárd on September 12, 1933 . The neutron had been discovered in 1932, shortly before . Szilárd realized that if a nuclear reaction produced neutrons, which then caused further nuclear reactions, the process might be self - perpetuating . Szilárd, however, did not propose fission as the mechanism for his chain reaction, since the fission reaction was not yet discovered or even suspected . Instead, Szilárd proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts . He filed a patent for his idea of a simple nuclear reactor the following year . </P> <P> In 1936, Szilárd attempted to create a chain reaction using beryllium and indium, but was unsuccessful . Nuclear fission was discovered and proved by Otto Hahn and Fritz Strassmann in December 1938 . A few months later, Frédéric Joliot, H. Von Halban and L. Kowarski in Paris searched for, and discovered, neutron multiplication in uranium, proving that a nuclear chain reaction by this mechanism was indeed possible . </P> <P> On May 4, 1939 Joliot, Halban et Kowarski filed three patents . The first two described power production from a nuclear chain reaction, the last one called "Perfectionnement aux charges explosives" was the first patent for the atomic bomb and is filed as patent n ° 445686 by the Caisse nationale de Recherche Scientifique . </P> <P> In parallel, Szilárd and Enrico Fermi in New York made the same analysis . This discovery prompted the letter from Szilárd and signed by Albert Einstein to President Franklin D. Roosevelt, warning of the possibility that Nazi Germany might be attempting to build an atomic bomb . </P>

Where do the neutrons for nuclear fission come from in a nuclear chain reaction