Was Fermi's demonstration of a controlled chain reaction in 1942, the mouse trap demonstration or the actual nuclear reactor which he set up. Need Help. Help will be greatly rewarded.
Fermi wasn't the first to achieve fission, but he was the first to make a working controlled fission reaction in his "atomic pile" which used uranium oxide and uranium as a fuel, with graphite as a moderator. (he made it in a squash court)