The site was chosen for the X-10 Graphite Reactor, used to produce plutonium from natural uranium for the Manhattan project. Enrico Fermi and his colleagues developed the world's second self-sustaining nuclear reactor after Fermi's previous experiment, the Chicago Pile-1. The X-10 was the first reactor designed for continuous operation. After the end of World War II the demand for weapons-grade plutonium fell and the reactor and the laboratory's 1,000 employees were no longer involved in nuclear weapons. Instead, it was used for scientific research. In 1946 the first medical isotopes were produced in the X-10 reactor, and by 1950 almost 20,000 samples had been shipped to various hospitals. As the demand for military science had fallen dramatically, the future of the lab was uncertain. Management of the lab was contracted by the US government to Monsanto; however, they withdrew in 1947. The University of Chicago re-assumed responsibility, until in December 1947, when Union Carbide and Carbon Co., which already operated two other facilities at Oak Ridge, took control of the laboratory. Alvin Weinberg was named Director of Research, ORNL, and in 1955 Director of the Laboratory. In 1950 the Oak Ridge School of Reactor Technology was established with two courses in reactor operation and safety; almost 1,000 students graduated. Much of the research performed at ORNL in the 1950s was related to nuclear reactors as a form of energy production, both for propulsion and electricity. More reactors were built in the 1950s than in the rest of the ORNL's history combined. Another project was the world's first light water reactor. With its principles of neutron moderation and fuel cooling by ordinary water, it is the direct ancestor of most modern nuclear power stations. The US Military funded much of its development, for nuclear-powered submarines and ships of the US Navy. The US Army contracted portable nuclear reactors in 1953 for heat and electricity generation in remote military bases. The reactors were designed at ORNL, produced by American Locomotive Company and used in Greenland, the Panama Canal Zone and Antarctica. The United States Air Force (USAF) also contributed funding to three reactors, the lab's first computers, and its first particle accelerators. ORNL designed and tested a nuclear-powered aircraft in 1954 as a proof-of-concept for a proposed USAF fleet of long-range bombers, although it never flew. The provision of radionuclides by X-10 for medicine grew steadily in the 1950s with more isotopes