Oak Ridge: Y-12 Pilot Plant (Building 9731) Panoramic Tour

The Y-12 plant in 1944. The man is pointing toward the direction of Building 9731.

US DEPARTMENT OF ENERGY/ED WESTCOTT

Building 9731 is located on the highly-secured grounds of the Y-12 National Security Complex in Oak Ridge, TN. Public visitation is prohibited.

Building 9731, or the Pilot Plant, was the first building constructed at the top-secret Y-12 Electromagnetic Separation Plant in Oak Ridge, Tennessee. Constructed for the Manhattan Project in 1943, Building 9731 houses the prototype equipment for the electromagnetic device known as a calutron, an industrial-sized variety of mass spectrometer invented by University of California scientist Ernest Lawrence. Building 9731 was the pilot building where operations workers and cubicle operators trained to perform uranium separation. Their work yielded enriched uranium-235 to fuel Little Boy, the world’s first uranium gun-type atomic bomb dropped on Hiroshima, Japan on August 6, 1945.

The cubicle operators, known as the Calutron Girls, trained in Building 9731 before operating the arrays at the larger production facilities at Y-12, including the adjacent Beta 3 building. These young women, many of whom were just out of high school, were not aware of the undertaking they were a part of until after the war. 

The calutrons in Building 9731 remained in operation after the Manhattan Project and through a portion of the Cold War, continuing to separate isotopes for scientific research until 1970. Building 9731 houses the only remaining Alpha Calutron magnets from the Manhattan Project.  

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The Alpha Calutrons at Building 9731.

US DEPARTMENT OF ENERGY

Alpha Calutrons

The calutrons operated in unison, using the electromagnetic separation process. This process resulted in the obtainment of highly purified uranium-235. This enriched uranium was ultimately used as fuel in the Little Boy atomic bomb. The total cost of the electromagnetic separation process reached $573 million by the end of the Manhattan Project (over $9 billion today).

The vacuum chamber of an Alpha Calutron being removed in 1944.

US DEPARTMENT OF ENERGY/ED WESTCOTT

The calutrons required a large amount of copper for magnet wiring. Since copper was in high demand for the overall war effort, Manhattan Project administrators borrowed almost 15,000 tons of silver from Treasury Department vaults at West Point, New York. This silver was fabricated and wound onto the coils, acting as an effective alternative to copper.

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Beta Calutrons at Building 9731.

US DEPARTMENT OF ENERGY

Beta Calutrons

The Beta Calutrons were slightly smaller than the Alphas and were rectangular in appearance. The Alpha Calutrons provided partially enriched uranium to the Betas, which would fully enrich the uranium-235.

The Calutron training cubicles in the basement of Building 9731.

US DEPARTMENT OF ENERGY

The Calutron Girls trained to operate the dials in the basement of 9731 before moving to the adjacent Beta 3 building to begin their top-secret work.

A Calutron Girl at Y-12.

US DEPARTMENT OF ENERGY/ED WESTCOTT

The majority of these young women had just graduated high school and were primarily hired from nearby East Tennessee communities, including Knoxville. These women had little to no idea what they were working on at this top-secret facility until after the news broke of the atomic bombings of Hiroshima and Nagasaki, Japan in August, 1945.

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