Enriching Uranium

A building with long rectangular wings.
An aerial view of K-25, one of three uranium-enrichment facilities in Oak Ridge, 1945.



The three massive Manhattan Project facilities at Oak Ridge— the Y-12 Electromagnetic Isotope Separation Plant, the K-25 Gaseous Diffusion Plant, and the S-50 Liquid Thermal Diffusion Plant—operated for one purpose: to enrich uranium for use in an atomic bomb. These three facilities separated the rare uranium-235 isotope from the more common uranium-238 isotope, each using a different mass separation method.  

Though the three facilities began construction and went into operation at different points during World War II, they were designed to operate simultaneously, with uranium hexaflouride and uranium tetrachloride passing from S-50 to K-25 and finally Y-12, each stage raising the concentration of U-235. Y-12 began operating in the fall of 1943, using calutrons to separate the uranium isotopes. S-50 went online in late fall of 1944.  

By the spring of 1945, sections of the K-25 plant began using the more efficient gaseous diffusion method. When all plants were in operation, the uranium would be slightly enriched at S-50 (up to .9% U-235), enriched more at K-25 (1.1%, 7%, or 23% as more of the facility came online), and final enrichment at Y-12 (up to 90% U-235). The enriched uranium was transported to Los Alamos and used as fuel for Little Boy, the atomic bomb dropped on Hiroshima, Japan, August 6, 1945. Learn more about the uranium enrichment process at Oak Ridge below.

Loading results...

    Last updated: March 26, 2023

    Park footer

    Contact Info

    Mailing Address:

    Manhattan Project National Historical Park
    c/o NPS Intermountain Regional Office
    P.O. Box 25287

    Denver, CO 80225-0287


    Hanford: 509.376.1647
    Los Alamos: 505.661.6277
    Oak Ridge: 865.482.1942

    Contact Us