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K-25 was the Manhattan Project codename for the program that produced enriched uranium for atomic bombs using the gaseous diffusion method at the Clinton Engineer Works in Oak Ridge, Tennessee, in the United States. When the production facility was built in 1944, the four-story gaseous diffusion plant (pictured) was the world's largest building, with over 152,000 square metres (1,640,000 sq ft) of floor space. At the height of construction, over 25,000 workers were employed on the site. Slightly enriched uranium from the S-50 thermal diffusion plant in the form of the highly corrosive uranium hexafluoride was fed into the K-25 gaseous diffusion plant; its product in turn was fed into the Y-12 electromagnetic plant. The enriched uranium was used in the Little Boy atomic bomb used in the atomic bombing of Hiroshima. Production of enriched uranium ended in 1964, gaseous diffusion ceased in 1985, and demolition of the facility was completed in 2017. (This article is part of a featured topic: History of the Manhattan Project.)