Fermi's Chicago Pile-1 reactor, constructed under the west viewing stands of the original Stagg Field at the University of Chicago, "went critical" on December 2, 1942. This graphite-moderated reactor only generated up to 200 W, but it demonstrated that "k" was higher than anticipated. This not only removed most of the objections to air-cooled and water-cooled reactor designs, it greatly simplified other aspects of the design. Wigner's team submitted blueprints of a water-cooled reactor to DuPont in January 1943. By this time, the concerns of DuPont's engineers about the corrosiveness of water had been overcome by the mounting difficulties of using helium, and all work on helium was terminated in February. At the same time, air cooling was chosen for the reactor at the pilot plant. Since it would be of a quite different design from the production reactors, the X-10 Graphite Reactor lost its value as a prototype, but its value as a working pilot facility remained, providing plutonium needed for research. It was hoped that problems would be found in time to deal with them in the production plants. The semiworks would also be used for training, and for developing procedures.