CP-1 Chianti bottle and signatures. Forty-nine people were present in a converted squash-racquet court at the University of Chicago's abandoned Stagg Field on December 2, 1942, when the first nuclear reactor went critical (called CP-1, or Chicago Pile-1, for its pile of graphite.). This is the empty Chianti bottle which participants signed after the celebration. The list contains the signatures of the CP-1 pioneers from a 20th anniversary reunion.