The K-25 plant, located on the southwestern end of the Oak Ridge reservation, used the gaseous diffusion method to separate uranium-235 from uranium-238. Based on the well-known principle that molecules of a lighter isotope would pass through a porous barrier more readily than molecules of a heavier one, gaseous diffusion produced through myriads of repetitions a gas increasingly rich in uranium-235 as the heavier uranium-238 was separated out in a system of cascades. Although producing minute amounts of final product measured in grams, gaseous diffusion required a massive facility to house the hundreds of cascades and consumed enormous amounts of electric power.
Begun in June 1943 and completed in early 1945 at a cost of $500 million, the K-25 plant employed 12,000 workers. The U-shaped K-25 building measured half a mile by 1,000 feet and is larger than the Pentagon. Construction began before completion of the design for the process. Due to construction needs at K-25 and elsewhere on the reservation, the town of Oak Ridge, originally designed for 13,000 people, grew to 50,000 by summer 1944.
Gaseous diffusion was one of three isotope separation processes that provided uranium-235 for the Hiroshima weapon (Little Boy)-the other two being electromagnetic separation and liquid thermal diffusion. The S-50 liquid thermal diffusion plant, using convection to separate the isotopes in thousands of tall columns, was built next to the K-25 power plant, which provided the necessary steam. Much less efficient than K-25, the S-50 plant was torn down after the war. Gaseous diffusion was the only uranium enrichment process used during the Cold War. K-25 was the prototype for later Oak Ridge plants and those at Paducah and Portsmouth. The K-25 plant ceased production in 1987. Due to environmental hazards and decreasing structural integrity, the K-25 plant is being demolished.