Manhattan Project Science at Oak Ridge

Sign at Oak Ridge depicting secrecy
Sign at Oak Ridge during the Manhattan Project warning workers and residents to keep the project secret.


The creation of Oak Ridge, Tennessee, called Kingston Demolition Range and Clinton Engineer Works during the Manhattan Project, centered around one main goal; the development of enriched uranium for atomic weapons. The three facilities that achieved this goal, the Y-12 Electromagnetic Isotope Separation Plant, the K-25 Gaseous Diffusion Plant, and the S-50 Liquid Thermal Diffusion Plant, did so in markedly different ways. A fourth facility, the X-10 Graphite Reactor, did not enrich uranium; rather it was the first reactor built after the successful Chicago Pile experiment. Its purpose was to convert enriched uranium into plutonium for experimental purposes, something that Hanford, Washington performed on an industrial scale for weapons making purposes. X-10 plutonium samples shipped to Los Alamos, New Mexico greatly influenced bomb design.
Beta racetrack at Y-12
Beta racetrack at Y-12.


Y-12 Electromagnetic Isotope Separation Plant

The construction of the Y-12 Plant was founded on the need for a facility that could perform electromagnetic isotope separation. Developed by University of California at Berkeley scientist Ernest Lawrence, electromagnetic isotope separation was based on the concept that lighter uranium atoms (U-235) would move more tightly in a circle than heavier uranium atoms (U-238). The creation of a cyclotron, a vacuum tank with a strong magnetic field, could separate the atoms into two paths as they revolved. The lighter U-235 was captured in a separate container from the heavier U-238. Lawrence had created a mass spectrometer cyclotron, or calutron, at his Berkeley lab in late-1941, but Manhattan Project scientists determined that a much larger facility would be required to produce the separated U235 needed to develop an atomic bomb in a timely fashion. This necessitated construction of the much larger Y-12 Plant, which began on February 18, 1943.

At its peak, the Y-12 Plant employed 22,000 people to operate the calutrons, nicknamed “racetracks” for their oval shape. While the facility at Berkeley had only been operated by scientists, a wartime labor shortage meant that young women, many of whom had just graduated high school, were assigned the task of operating the calutrons at Oak Ridge, Tennessee. These women, who earned the nickname “Calutron Girls”, were vital to the development of enriched uranium, a key component of the Little Boy atomic bomb.
Aerial photo of K-25
Aerial photo of K-25.


K-25 Gaseous Diffusion Plant

The K-25 Gaseous Diffusion Plant, the largest building in the world at the time, was founded on the need for a facility that could also separate lighter U-235 atoms from heavier U-238 atoms. However, K-25 achieved this goal through a much different process than Y-12. While Y-12 centered on isotope separation, K-25 relied on over 3,000 gaseous diffusion steps through which uranium hexafluoride gas could pass through. In this process, only the smaller U-235 atoms could pass through the tiny pores in the barrier of the steps, enriching the gas and creating a higher concentration of U-235 as they diffused through each step, much in the same way smaller particles fall through a colander in a kitchen sink.

K-25, which began construction in the fall of 1943 and was in full operation by the spring of 1945, had one major advantage over Y-12. The gaseous diffusion plant could produce more enriched uranium at less cost. This greater efficiency of K-25 over Y-12 led to the shutdown of many of the Y-12 calutrons by the end of 1946, causing the loss of over 20,000 jobs.
Aerial photo of S-50
Aerial photo of S-50.


S-50 Liquid Thermal Diffusion Plant

The S-50 Liquid Thermal Diffusion Plant was designed primarily to be the little sibling of K-25; uranium would be slightly enriched at S-50 before being enriched further at K-25. In order to enrich uranium using the liquid thermal diffusion process, 2,142 rounded columns were installed at S-50, each 48 feet high. These columns had a tiny layer of uranium hexafluoride spacing within to allow for the liquid thermal diffusion process.

Although the S-50 plant was constructed in record time (69 days), the liquid thermal diffusion process proved too costly and consumed too much power per enriched uranium output and was far less effective than K-25. The S-50 Liquid Thermal Diffusion Plant was deactivated in 1946.
Knoxville newspaper with Hiroshima bombing headline
Front page of Knoxville News-Sentinel on August 6, 1945.


Although each of the uranium enrichment facilities at Oak Ridge had their flaws, working together they were vital to the development of U-235 which would ultimately be used in the Little Boy bomb that was dropped on Hiroshima on August 6, 1945. Along with the X-10 Graphite Reactor’s plutonium samples discreetly shipped to Los Alamos for further research and testing, the processes enacted at Y-12, K-25, and S-50 were instrumental in the Manhattan Project being a scientific success. The combination of hundreds of scientists, thousands of workers, and massive facilities led to the production of a top secret weapon the size of a small car that would forever alter the course of history

  • Johnson, Charles W., and Charles O. Jackson. City Behind a Fence: Oak Ridge, Tennessee 1942-1946. Knoxville: The University of Tennessee Press, 1981.
  • Kelly, Cynthia C., ed., The Manhattan Project. New York: Black Dog & Leventhal Publishers, 2007.
  • Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster Paperbacks, 1986.

Manhattan Project National Historical Park

Last updated: April 4, 2023