2. What starts the nuclear chain reaction?

Below

Explanation:

Nuclear chain reactions are series of nuclear fissions (splitting of atomic nuclei), each initiated by a neutron produced in a preceding fission. For example, 21/2 neutrons on the average are released by the fission of each uranium-235 nucleus that absorbs a low-energy neutron. Provided that no more than 11/2 neutrons per fission on the average are lost through leakage and non-fission-producing capture by other nuclei, one neutron per fission on the average remains to sustain the series. Thus the chain reaction is self-sustaining if the ratio of the number of daughter neutrons that cause fission to the number of parent neutrons is 1 (as in nuclear reactors) or greater than 1 (as in nuclear explosions). The Italian-born physicist Enrico Fermi and his coworkers achieved the first self-sustaining fission chain reaction in 1942 at the University of Chicago.