The Savannah River Plant was built from 1950 to 1956 by Du Pont for the Atomic Energy Commission. As a nuclear production plant, it produced tritium and plutonium for national defense during the Cold War. Creating a 310-square-mile site in three counties required moving all residents from their homes in Ellenton, Dunbarton, Meyers Mill, Leigh, and other nearby communities. The first reactor went online in 1953, and the free neutrino, a subatomic particle, was first detected at P reactor in 1956. The plant also produced medical and research isotopes and energy sources for NASA. In 1972 it became the first National Environmental Research Park. Renamed Savannah River Site in 1989, it is owned by the U.S. Department of Energy.