Fermi's experiments revealed that slowing down neutrons dramatically increased their effectiveness in initiating nuclear reactions. This was a critical insight for reactor design, allowing for a more controlled and efficient chain reaction.

News of the discovery of nuclear fission by Hahn and Strassmann reached Fermi. He quickly realized the potential for a self-sustaining nuclear chain reaction, where neutrons released from one fission event would trigger further fissions.

Before moving to Chicago, Fermi and his team conducted preliminary experiments at Columbia University to determine the optimal configuration of materials for a nuclear reactor. They explored different neutron moderators, like graphite, to slow down neutrons.

The project was moved to the University of Chicago's Metallurgical Laboratory. The goal was to build the first self-sustaining nuclear reactor and prove the feasibility of generating nuclear power.

At the University of Chicago, under the leadership of Fermi, the Chicago Pile-1 (CP-1) achieved criticality. This was the first demonstration of a controlled and sustained nuclear chain reaction. The reactor used natural uranium fuel and graphite as a moderator.