K-25 Gaseous Diffusion Plant

Overlook view of large valley with scattered structures
Here once stood the largest building in the world during the Manhattan Project.


Quick Facts
Oak Ridge, TN

Imagine arriving to the first day of a new job in a remote part of East Tennessee in 1944, not knowing what the job even entails. You soon find out your new workplace is a massive, top-secret facility, a giant U-shaped building longer than two Empire State Buildings laid side by side. The massive K-25 building demonstrates the lengths to which the country went to protect itself from foreign adversaries and the determination to be the first country in the world with atomic weaponry. 

Constructed in 1943 by the New York-based Kellex corporation, the K-25 Gaseous Diffusion Plant was the largest building in the world at the time. It covered a 44-acre tract (178,062 sq. meters) over one-half mile long (0.8 km) and 1,000 yards wide (914 meters). This top-secret facility with its distinctive U shape employed over 10,000 workers at it’s the height of its operation in 1945. No facility like K-25 had ever been built. Although smaller laboratory experiments indicated that the gaseous diffusion process would efficiently produce enriched uranium, there was no initial assurance that the process would work on such a large scale.

The K-25 gaseous diffusion plant separated lighter uranium 235 from heavier uranium 238. This uranium separation also occurred at nearby Y-12 and S-50. Unlike the processes used at Y-12 and S-50, K-25 relied on over 3,000 gaseous diffusion steps through which uranium hexafluoride gas passed through. This process allowed for smaller uranium 235 atoms to pass through the tiny pores in the barriers of each step, enriching the gas and creating a higher concentration of uranium 235. This gaseous diffusion method proved cheaper and more efficient than Y-12's electromagnetic separation process and S-50's liquid thermal diffusion process. Uranium 235 is an unstable uranium isotope that will sustain a nuclear chain reaction, making it an ideal fuel for an atomic bomb. The Little Boy atomic bomb used enriched uranium produced at Oak Ridge. The US Army Air Force dropped Little Boy on Hiroshima, Japan on August 6th, 1945, the first time an atomic bomb was used in war. Three days later on August 9th, 1945, the US dropped the plutonium-fueled Fat Man bomb on Nagasaki, Japan, thus far the last atomic weapon used in war. Japan surrendered on August 14, 1945, bringing World War II, the deadliest conflict in human history, to an end. By the end of 1945, over 200,000 people had died as a result of the atomic bombings of Hiroshima and Nagasaki.

K-25 produced enriched uranium for defense and commercial purposes until 1987. Its benchmark gaseous diffusion method was the only uranium-enrichment method used by the United States during the Cold War. In 2013, due to its deteriorated condition, US Department of Energy contractors demolished K-25.

Continue Your Journey

Even though the K-25 Gaseous Diffusion Plant is demolished, you can still see remnants of various outbuildings throughout the complex. Demolition is still occurring at the K-25 complex, so use caution when visiting and heed all warnings and closures. In addition, the newly-opened K-25 History Center rests near the plant’s original footprint. Inside, you can enjoy various exhibits about the history of the K-25 complex and its global impact throughout the Manhattan Project and the Cold War. 

Manhattan Project National Historical Park

Last updated: February 22, 2023