Oak Ridge's Y-12 Pilot Plant, Building 9731: Where the Calutrons Began

Building 9731, the Pilot Plant, was the first building put up at the top-secret Y-12 Electromagnetic Separation Plant in Oak Ridge, Tennessee. Built for the Manhattan Project in 1943, it housed the prototype of the calutron, an industrial-scale mass spectrometer invented by University of California scientist Ernest Lawrence. Workers trained here to separate uranium, and their work produced the enriched uranium-235 that fueled Little Boy, the world's first uranium gun-type atomic bomb, dropped on Hiroshima, Japan, on August 6, 1945.

The building stands on the high-security grounds of what is now the Y-12 National Security Complex and is closed to the public; the National Park Service's Manhattan Project National Historical Park offers a virtual panoramic tour instead.

The Y-12 plant in 1944; the man is pointing toward Building 9731. U.S. Department of Energy / Ed Westcott

The entryway to Building 9731 at the Y-12 Plant, Oak Ridge: a wraparound panorama shown flattened. NPS.

Alpha calutrons

The calutrons worked together using electromagnetic separation to produce highly purified uranium-235, which ultimately fueled Little Boy. By the end of the Manhattan Project, the electromagnetic separation process had cost $573 million, which the park puts at more than $9 billion today.

Alpha calutrons in Building 9731. U.S. Department of Energy

The magnets needed huge amounts of copper wiring, but copper was in heavy demand for the war. So the project's administrators borrowed almost 15,000 tons of silver from Treasury Department vaults at West Point, New York, and wound it onto the coils instead.

The vacuum chamber of an Alpha calutron being removed in 1944. U.S. Department of Energy / Ed Westcott

The Alpha calutrons in Building 9731: a wraparound panorama shown flattened. NPS.

Beta calutrons

The Beta calutrons were somewhat smaller and rectangular. The Alphas produced partly enriched uranium, which the Betas then fully enriched.

Beta calutrons in Building 9731. U.S. Department of Energy

The Beta calutrons in Building 9731: a wraparound panorama shown flattened. NPS.

The Calutron Girls

The operators of the calutron control panels, known as the Calutron Girls, trained on the dials in the basement of Building 9731 before moving to the production buildings, including the neighboring Beta 3 building. Most were young women just out of high school, hired mainly from nearby East Tennessee communities such as Knoxville. They had little or no idea what they were working on until news of the atomic bombings of Hiroshima and Nagasaki broke in August 1945.

The calutron training cubicles in the basement of Building 9731. U.S. Department of Energy

A Calutron Girl at Y-12. U.S. Department of Energy / Ed Westcott

After the war

The calutrons in Building 9731 kept running after the Manhattan Project and into the Cold War, separating isotopes for scientific research until 1970. The building holds the only surviving Alpha calutron magnets from the Manhattan Project.

Sources

• National Park Service, "Oak Ridge: Y-12 Pilot Plant (Building 9731) Panoramic Tour" (Manhattan Project National Historical Park): https://www.nps.gov/articles/000/oak-ridge-y-12-pilot-plant-building-9731-panoramic-tour.htm The Park Service's panoramas, lost when the page was imported, were recovered from NPGallery, where they are marked public domain (2026-09-26).