The
fission pile was created by
Enrico Fermi, and it wasn't exactly 'in a football stadium'; it was a squash court, that may have been located by the stadium.
A primitive affaire, this first nuclear reactor, built of interlocking blocks of graphite as a moderator, and uranium, with cadmium rods to control the reaction.
In the event of an uncontrolled chain reaction, there was one person whose job was to use an axe to cut the ropes holding the cadmium rods out of the pile. Then everybody had to scram!