Constructing CP-1

In November 1942, in the squash court beneath the West Stand of Stagg Field, construction began on a nuclear reactor that was designated Chicago Pile-1 (CP-1). Layers of graphite blocks containing slugs of uranium metal and uranium oxide alternated with layers of solid graphite blocks, gradually forming a roughly spherical shape supported by a wooden framework. A specially ordered cubical balloon-cloth bag encased the reactor to reduce absorption of neutrons by nitrogen in the air. Higher grades of uranium became available as work progressed, allowing the pile to be redesigned in a somewhat reduced size with a flattened top.