Supervised by Compton, Whitaker, and Fermi, the reactor went critical on November 4, 1943, with about of uranium. A week later the load was increased to , raising its power generation to 500 kW, and by the end of the month the first 500 mg of plutonium was created. The reactor normally operated around the clock, with 10-hour weekly shutdowns for refueling. During startup, the safety rods and one shim rod were completely removed. The other shim rod was inserted at a predetermined position. When the desired power level was reached, the reactor was controlled by adjusting the partly inserted shim rod.