Oak Ridge: The Secret City, the Calutron Girls and the Race for Uranium-235

In 1944 a young woman starting a shift at the Y-12 plant in Oak Ridge, Tennessee, would show her badge at the gate, change into work clothes, walk down a long, brightly lit corridor of steel cabinets and climb onto a wooden stool in front of a panel of meters and knobs. For the next eight hours her job was to keep a needle where her supervisor had told her it should be. She did not know what the machine behind the panel did, and she had been told not to ask. Outside, in a valley that two years earlier had held farms, churches and a few small villages, some 75,000 people now lived behind a fence, in a city that did not appear on maps. What the machines were doing was separating uranium-235, a few atoms at a time, for the bomb that would be dropped on Hiroshima.

Why it mattered

Of the two kinds of atomic bomb built in 1945, the uranium bomb depended entirely on Oak Ridge. Its fissile material, uranium-235, had to be separated from the far more common uranium-238, and in 1942 nobody knew how to do that on an industrial scale. The Manhattan Project’s answer was to try every promising method at once, and Oak Ridge was where it did so: a giant electromagnetic plant (Y-12), the largest industrial building of its day (K-25), a thermal diffusion plant thrown up in a summer (S-50), and the world’s first reactor designed to run continuously (X-10), all within a few miles of each other in an East Tennessee valley. None of them alone produced enough material in time. Together, working as a chain, they did.

Oak Ridge also shows the human scale of the project. At its peak, in May 1945, the Clinton Engineer Works employed about 82,000 people and the town had about 75,000 residents, making it the fifth-largest city in Tennessee, though it was not on any map. Most of the workers were local: farm families, high-school graduates, construction men from across the South. Most did not know what they were making until 6 August 1945. The city they lived in was also a segregated Southern town of 1940s America, and its Black workers lived in conditions that the official histories mention only briefly.

Site X: a valley in East Tennessee

The uranium plants needed a great deal of land, electric power, water and labor, and they needed to be inland, away from the coasts, and far enough from towns to be safe and secret. An OSRD team looked at the Knoxville area in April 1942, and on 1 July Col. James Marshall and Lt. Col. Kenneth Nichols surveyed sites there with the Tennessee Valley Authority and Stone & Webster. They favored a stretch of ridges and valleys along the Clinch River, in Roane and Anderson counties, about 18 miles west of Knoxville and 8 miles south of the town of Clinton. The TVA’s dams, especially Norris Dam upstream on the Clinch, could supply the power; the river supplied water; the parallel ridges would separate the plants from one another. Marshall hesitated over the purchase. Groves, on taking command in September 1942, ordered it at once, and on 29 September Under Secretary of War Robert Patterson authorized the acquisition of about 56,000 acres (Jones).

More than a thousand families lived on the land, in the communities of Elza, Robertsville, Scarboro and Wheat and on scattered farms; some had already been moved once, for the Great Smoky Mountains National Park in the 1920s or for Norris Dam in the 1930s. The government obtained a court order of possession on 6 October 1942 and began taking tracts in November. Many families had only weeks to leave. They were paid on appraisals that averaged about $47 an acre, and many considered them unfair; landowners petitioned, hired lawyers and won a congressional subcommittee hearing in 1943, but the subcommittee’s report that December produced no extra compensation (Jones; Johnson and Jackson, City Behind a Fence, 1981). In the end the reservation covered about 58,900 acres, bought for about $2.6 million. The site, first called the Kingston Demolition Range as a cover, became the Clinton Engineer Works in January 1943. Its town took the name Oak Ridge in 1943, chosen from names suggested by employees.

A city built in months

Stone & Webster’s first plan for the town, in October 1942, was for 13,000 people. Every forecast after that was too low. The Chicago architects Skidmore, Owings & Merrill were brought in to design a town of prefabricated family houses, built of cemesto, a panel of cement and fiberboard, and set along the ridges in neighborhoods with schools and shopping centers; about 3,000 houses of the first phase were finished in early 1944. The demand outran them. By the end of the war the town held houses, apartments, dormitories, trailers, barracks, plywood “flattops” and the crude one-room plywood huts called hutments, and the construction camps alone, like Happy Valley beside K-25 with about 15,000 people, were towns in their own right. The population reached about 75,000 in May 1945, when the Clinton Engineer Works employed about 82,000 people (Jones; Manhattan District History).

The town had no elected government; the Army ran it through a contractor, the Roane-Anderson Company, which operated the buses, the utilities, the housing and the cafeterias. Mail was censored, workers signed security declarations, and badges were color-coded to show where each holder could go. Ed Westcott, the Army’s photographer and one of the few people allowed to carry a camera, photographed the billboards that reminded everyone, at every turn, not to talk.

Y-12: Lawrence’s calutrons

The electromagnetic method came from Ernest O. Lawrence’s Radiation Laboratory in Berkeley, where in 1941–42 his staff converted the giant magnet of the unfinished 184-inch cyclotron into a mass spectrograph able to separate measurable amounts of uranium-235. They called the device a calutron, for “California University cyclotron.” Its great advantages were that it worked, and that its product could be highly enriched in two stages; its great disadvantage was that each unit handled only tiny amounts of material, so that the plant would need well over a thousand of them. Stone & Webster was assigned to build it, and the Tennessee Eastman Corporation, a subsidiary of Eastman Kodak with a chemical works at Kingsport, to run it. Construction began in Bear Creek Valley in February 1943.

The electromagnets needed enormous amounts of conducting metal for their windings, and copper was one of the scarcest materials of the war. Nichols went to the Treasury, which held the nation’s silver reserves. When he asked the Under Secretary, Daniel W. Bell, for thousands of tons, Bell, by Nichols’s account, answered that in the Treasury one spoke not of tons of silver but of troy ounces (Nichols, The Road to Trinity, 1987). The project borrowed about 395 million troy ounces, roughly 13,500 short tons, from the West Point Bullion Depository. It was cast into bars, rolled into strips at Bayway, New Jersey, and wound into the magnet coils under special guards and accounting, and after the war almost all of it was returned.

The first Alpha racetrack was started in November 1943 and failed almost at once. Its huge magnets shorted out, and when they were opened in December, the coils were found to be wound too tightly and fouled by rust and dirt in the cooling oil. Groves ordered the magnets shipped back to Allis-Chalmers in Milwaukee to be rebuilt, a delay of a month and more that he had no way to make up. The racetracks came into service one by one through 1944; the fourth Alpha I track was running in April, and the four Alpha II tracks were finished between July and October. Y-12’s first enriched uranium, of 13 to 15 percent uranium-235, went to Los Alamos in March 1944 (Jones; Hewlett and Anderson).

The Calutron Girls

Lawrence’s Berkeley staff ran the first calutrons themselves, and the plan had been to staff the plant with scientists and engineers. There were not nearly enough of them, and Tennessee Eastman recruited instead among young local women, most of them just out of high school, who could be trained in a short course to operate the control cubicles. By most estimates about 10,000 women worked as cubicle operators between 1943 and 1945. Each sat on a stool before a panel and watched meters showing the state of the ion beam and the vacuum, turning knobs to keep the readings where they belonged and calling a supervisor when something went wrong. They worked shifts around the clock. They were told what to do, not why.

Nichols later told the story of a contest. Lawrence’s physicists complained that the young operators were not qualified, and Nichols, who had noticed that the women’s production figures were higher, proposed a week-long production race between the cubicles run by scientists and those run by the Tennessee operators. The operators won. Nichols’s explanation was that they had been trained like soldiers to follow instructions, while the scientists could not resist investigating every small fluctuation (Nichols, 1987). The story rests on Nichols’s memoir, written more than forty years later, but it agrees with the recollection of the Y-12 staff interviewed by the plant’s historian, who attributed the women’s success to disciplined adherence to procedure. By August 1945 Y-12 had 22,482 employees.

K-25: the half-mile building

Gaseous diffusion was the method that would dominate uranium enrichment for forty years after the war, but in 1943 it was the riskiest of all. Uranium hexafluoride, a corrosive gas, had to be pumped through thousands of porous barriers in series, each stage enriching it very slightly. The M. W. Kellogg Company set up a subsidiary, the Kellex Corporation, to design the plant, and by the end it had more than 3,700 employees. The prime construction contractor was the J. A. Jones Construction Company of Charlotte, North Carolina, which also built the plant’s coal-fired power station. The plant was to be operated by the Carbide and Carbon Chemicals Corporation, a unit of Union Carbide. Construction of the main building began in the autumn of 1943 on a site near the Clinch River, west of the town.

The building was a four-story U about half a mile long, the largest industrial building in the world at the time, according to the Department of Energy’s K-25 history. Its construction used about 200,000 cubic yards of concrete and 100 miles of process piping, and at the peak of construction, in May 1945, 25,266 people worked on the site. The hardest problem was the barrier itself, which had to have billions of microscopic holes, resist corrosion by the gas and be made by the acre. The Norris–Adler barrier developed at Columbia proved too brittle; in January 1944 Groves chose a design by Clarence A. Johnson of Kellex, and by April the pilot plants were producing acceptable barriers. Chrysler, under K. T. Keller, built and nickel-plated more than 3,500 diffusers in a converted Detroit factory, and Groves obtained 80 tons of nickel from the International Nickel Company (Hewlett and Anderson; Jones).

The first section of K-25 was tested in April 1944, but the plant did not begin production until February 1945, and its first product went to the calutrons in March. By June it was enriching to about 7 percent; the last of its 2,892 stages came on line in August, and by September it was producing material of about 23 percent. In the summer of 1945 K-25 was not yet a complete enrichment plant; its role was to feed the Y-12 calutrons with partly enriched uranium, which multiplied their output. With the K-27 extension built after the war, the complex cost about $480 million (Jones; Manhattan District History).

S-50: the plant built in a summer

The fourth uranium process came from outside the Army’s project. Philip Abelson, a Berkeley-trained physicist working for the Naval Research Laboratory, had developed liquid thermal diffusion: liquid uranium hexafluoride was held between two concentric pipes, a hot one inside and a cold one outside, and the lighter isotope tended to drift toward the hot wall and rise. The Navy built a pilot plant of 102 columns, each 48 feet tall, at the Philadelphia Navy Yard. In the spring of 1944 Oppenheimer, worried about the slow pace of uranium enrichment, pointed out to Groves that even a slight enrichment of the calutrons’ feed would raise their output. Groves approved a full-scale plant on 24 June 1944.

The H. K. Ferguson Company of Cleveland broke ground beside the K-25 power plant, whose steam it would use, on 9 July 1944. Groves gave it four months; the first rack of columns was operating by 16 September, 69 days later, with about a third of the plant complete. When finished in early 1945 the S-50 plant had 21 racks with 2,142 columns, each 48 feet tall, run by Ferguson’s subsidiary Fercleve. It raised natural uranium from 0.71 percent uranium-235 to about 0.85–0.89 percent, a small step, but one that fed both K-25 and Y-12 and was estimated to have advanced the completion of Little Boy’s uranium by about a week (Hewlett and Anderson; Reed, 2011). The process carried its own hazards: on 2 September 1944 a cylinder of uranium hexafluoride exploded at the Philadelphia pilot plant, killing two men, Peter N. Bragg Jr. and Douglas P. Meigs, and injuring others with hydrofluoric acid.

X-10: the Graphite Reactor

Oak Ridge also held the first step on the plutonium road after Chicago Pile-1 (Chapter 2). In Bethel Valley, about 10 miles southwest of the town, DuPont built a pilot plutonium plant designed by the Metallurgical Laboratory: an air-cooled reactor of graphite with a concrete face, into which aluminum-clad slugs of uranium were pushed through hundreds of channels, and a chemical separation plant beside it. Construction began on 2 February 1943, and the reactor went critical early on 4 November 1943. It was run by the University of Chicago as the Clinton Laboratories. Its power rose to 500 kilowatts within a week and to 4,000 kilowatts by July 1944, and it produced the first gram quantities of reactor plutonium.

X-10’s plutonium went to Los Alamos in 1944, where it had an unexpected consequence. Measurements by Emilio Segrè’s group showed that reactor-made plutonium contained enough of the isotope plutonium-240 to make the gun-type design planned for it unworkable, which forced the laboratory to turn to the far more difficult implosion design, a story told in Chapter 5. X-10 also trained the operators for the Hanford reactors (Chapter 4) and, after the war, produced the radioisotopes for medicine and research that became Oak Ridge National Laboratory’s peacetime business.

The chain of enrichment

By the spring of 1945 the Oak Ridge plants were working as a chain. Natural uranium went first to S-50, which raised it slightly; the product went to K-25, which raised it further; K-25’s product went to the Alpha calutrons, or directly to the Betas, which brought it up to weapons grade. The finished material was shipped from Oak Ridge to Los Alamos by couriers, in the custody of Army officers travelling by train and car. All of the uranium-235 in Little Boy passed through Y-12’s Beta calutrons; the last of it was finished in July 1945 and, after fabrication at Los Alamos, crossed the Pacific aboard USS Indianapolis and by air to Tinian (Chapter 7).

By the numbers

Where Oak Ridge’s electricity went, 1945

Electric power allocated to each part of the Clinton Engineer Works in 1945, in kilowatts. The total supply, from TVA hydroelectric dams and the K-25 steam plant, reached about 310,000 kW; the peak load was 298,800 kW on 1 September 1945.

Y-12 electromagnetic plant200,000

K-25 gaseous diffusion plant80,000

Oak Ridge township23,000

S-50 thermal diffusion plant6,000

X-10 Graphite Reactor site1,000

Separate and unequal: life behind the fence

Oak Ridge was built by the federal government in Tennessee in 1943 and followed the segregation of its time and place. Black workers, recruited mainly for construction and for the hardest and lowest-paid jobs in the plants, were not given family housing in the town’s neighborhoods. A district planned for Black families, the East Village, was assigned to white residents. Most Black workers lived in hutments, one-room plywood shelters of about 16 feet square with a coal stove and no plumbing, grouped in segregated compounds, where married couples were often housed separately, men in one area and women in another, and families with children were generally not accommodated at all (Johnson and Jackson, 1981; Kiernan, 2013). The Gamble Valley trailer camp later became the main Black neighborhood, and the community of Scarboro, built on its site from 1948, housed Oak Ridge’s Black residents until the 1960s.

One episode has come to stand for the project’s indifference to the people at the bottom of it. In April 1945 Ebb Cade, a 53-year-old Black construction worker injured in a car accident, was taken to the Oak Ridge army hospital, where he was injected with plutonium, without his knowledge or consent, in a study of how the body retained it; bone and tooth samples were taken from him. He was the first of eighteen people injected with plutonium in such studies, which were not made public until 1993–94 (Welsome, The Plutonium Files, 1999; U.S. Advisory Committee on Human Radiation Experiments, 1995). Oak Ridge’s white workers, too, lived under rules that would be unthinkable in an ordinary American town: no elected government, censored mail, searches, dismissal for careless talk. Yet many of them, and many Black residents, later remembered the wartime city with pride and affection, and the archive of oral histories collected since the 1990s holds both memories side by side.

6 August 1945 in Oak Ridge

On 6 August 1945 President Truman’s statement announcing the bombing of Hiroshima, and the War Department releases issued with it, identified the Clinton Engineer Works and Hanford as the places where the material had been made, and the people of Oak Ridge learned what their city was for. The news was greeted with celebration, newspaper extras and church services, and with shock; many workers had assumed they were making something for a weapon, few had imagined that. Three days later the second bomb, made of Hanford plutonium, destroyed much of Nagasaki (Chapter 9). On 29 August Groves came to Oak Ridge to thank the men and women of the project.

The wartime plants did not survive the peace in their original form. The Alpha calutrons were shut down on 22 September 1945 and S-50 the same month; by the end of 1946 all the racetracks had stopped except a few Beta units, kept for producing isotopes. K-25 and the diffusion plants that grew around it enriched uranium for reactors and weapons until 1985. Oak Ridge’s gates were opened on 19 March 1949, and the town became a self-governing city in 1959. Y-12 remains a nuclear weapons production plant of the National Nuclear Security Administration, and X-10 grew into Oak Ridge National Laboratory.

Timeline

Oak Ridge, 1942–1949

1. Apr 1942An OSRD team looks at the Knoxville area for the uranium plants.
2. 1 Jul 1942Marshall and Nichols survey sites with the TVA and Stone & Webster.
3. 19 Sep 1942Groves orders the Tennessee site acquired.
4. 29 Sep 1942Patterson authorizes the purchase of about 56,000 acres.
5. 6 Oct 1942Court order of possession; families begin receiving notices to leave.
6. Jan 1943The reservation is named the Clinton Engineer Works.
7. 2 Feb 1943DuPont begins construction of X-10 in Bethel Valley.
8. 18 Feb 1943Construction of Y-12 begins.
9. 1 Apr 1943Access control begins: fences, gates, badges.
10. Mid-1943The town takes the name Oak Ridge.
11. Aug 1943The Manhattan District headquarters moves to Oak Ridge under Col. Kenneth Nichols.
12. Autumn 1943Construction of the K-25 main building begins.
13. 4 Nov 1943The X-10 Graphite Reactor goes critical.
14. Nov–Dec 1943The first Alpha racetrack starts and fails; its magnets go back to Allis-Chalmers.
15. Mar 1944Y-12’s first enriched uranium is shipped to Los Alamos.
16. 24 Jun 1944Groves approves the S-50 thermal diffusion plant.
17. 9 Jul 1944Ground is broken for S-50.
18. 2 Sep 1944Explosion at the Navy’s Philadelphia thermal diffusion pilot plant kills two men.
19. 16 Sep 1944The first S-50 rack begins operation.
20. Feb–Mar 1945K-25 begins production and sends its first product to Y-12.
21. Apr 1945Ebb Cade is injected with plutonium at the Oak Ridge army hospital.
22. May 1945Peak: about 82,000 employed at the CEW; town population about 75,000.
23. Jul 1945The last of the uranium for Little Boy is completed at Y-12.
24. 6 Aug 1945Hiroshima; the announcement identifies the Clinton Engineer Works.
25. 29 Aug 1945Groves addresses service personnel at Oak Ridge.
26. 22 Sep 1945The Y-12 Alpha calutrons are shut down.
27. 1 Jan 1947The Atomic Energy Commission takes over the plants.
28. 19 Mar 1949The gates of Oak Ridge are opened to the public.

Questions readers ask

Did the Calutron Girls know they were building a bomb?

Almost none of them did. They were told only how to read and adjust their instruments, and were warned not to ask questions or discuss their work. Most learned the purpose of the plant from President Truman’s announcement on 6 August 1945.

Why was Oak Ridge chosen?

For its combination of cheap electricity from the Tennessee Valley Authority, abundant water from the Clinch River, a rail line, an available labor force, a location far inland, and a terrain of parallel ridges and valleys that separated the plants from each other and from the town in case of accident. The land was also sparsely populated and could be bought quickly, though at a heavy cost to the families who lived there.

Did Oak Ridge make the uranium for both atomic bombs?

No. Oak Ridge made the enriched uranium for Little Boy, the bomb used at Hiroshima. The Nagasaki bomb, Fat Man, used plutonium made in the Hanford reactors in Washington State, although Oak Ridge’s X-10 reactor made the first plutonium used in research at Los Alamos.

Was Oak Ridge really kept off the map?

Yes, in the sense that the town did not appear on published maps during the war and was not openly identified, although tens of thousands of people knew it existed and Tennessee politicians complained about it. Its existence and purpose were revealed in August 1945, and the public could enter only from March 1949.

Is there a North Carolina connection?

Yes. The J. A. Jones Construction Company of Charlotte was the prime construction contractor for the K-25 gaseous diffusion plant and built its power station, among the largest construction jobs of the war. Many workers at Oak Ridge also came from the mountain counties of western North Carolina.

How it connects

The decisions that put the uranium plants in Tennessee are traced in Chapter 1 and Chapter 2. Chapter 4 follows the plutonium road to Hanford, where reactors descended from X-10 made the material for Trinity and Nagasaki; Chapter 5 describes what Los Alamos did with Oak Ridge’s uranium; and Chapter 7 and Chapter 8 follow it across the Pacific, aboard USS Indianapolis and the B-29 Enola Gay, to Hiroshima. The sea route of the uranium’s last journey also figures in the Submarine War collection, since Indianapolis was sunk by the Japanese submarine I-58 days after delivering her cargo, and the B-29s that carried the bombs are part of the story in Army Air Forces. The Special Units hub covers other wartime formations like the Special Engineer Detachment.

Seeing it today

Oak Ridge is one of the three units of the Manhattan Project National Historical Park, established in 2015. The American Museum of Science and Energy in Oak Ridge is the starting point for the park’s bus tours, which, when running, take visitors to the X-10 Graphite Reactor, a National Historic Landmark at Oak Ridge National Laboratory, and past the Y-12 and K-25 sites. Y-12’s Beta 3 building, which still holds original calutron racetracks, is inside a secure weapons plant and closed to the public, but the National Park Service describes it, and the New Hope Center visitor exhibit covers Y-12’s history. At the former K-25 site, the K-25 History Center, opened in 2020, tells the story of the gaseous diffusion plant. In town, the Alexander Inn, the wartime guest house for official visitors, has been restored, and some of the original cemesto houses still stand on the ridges. The Atomic Heritage Foundation’s Voices of the Manhattan Project includes many Oak Ridge oral histories, and Ed Westcott’s photographs can be seen in the DOE Oak Ridge collection and at the National Archives.

Sources

• Jones, Vincent C. Manhattan: The Army and the Atomic Bomb. Washington: U.S. Army Center of Military History, 1985. CMH Pub 11-10 (PDF).
• Hewlett, Richard G., and Oscar E. Anderson Jr. The New World, 1939–1946. University Park: Pennsylvania State University Press, 1962.
• Johnson, Charles W., and Charles O. Jackson. City Behind a Fence: Oak Ridge, Tennessee, 1942–1946. Knoxville: University of Tennessee Press, 1981.
• Kiernan, Denise. The Girls of Atomic City: The Untold Story of the Women Who Helped Win World War II. New York: Touchstone / Simon & Schuster, 2013.
• Nichols, Kenneth D. The Road to Trinity. New York: William Morrow, 1987.
• Groves, Leslie R. Now It Can Be Told. New York: Harper & Row, 1962.
• Smyth, Henry DeWolf. Atomic Energy for Military Purposes. Princeton: Princeton University Press, 1945.
• Reed, B. Cameron. “Liquid Thermal Diffusion during the Manhattan Project.” Physics in Perspective 13 (2011): 161–188.
• Welsome, Eileen. The Plutonium Files: America’s Secret Medical Experiments in the Cold War. New York: Dial Press, 1999. Advisory Committee on Human Radiation Experiments, Final Report, Washington, 1995.
• Westcott, Ed. Oak Ridge. Images of America. Charleston: Arcadia Publishing, 2005; and Through the Lens of Ed Westcott: A Photographic History of World War II’s Secret City, University of Tennessee, 2005.
• National Park Service, Manhattan Project National Historical Park. “Beta 3 at Y-12.”
• Atomic Heritage Foundation / National Museum of Nuclear Science & History. “Y-12 Plant,” Voices of the Manhattan Project.
• Blue Ridge Country. “Girl Power, Circa 1940: Building the Bomb (and Not Knowing It) in East Tennessee,” with interviews by Y-12 historian Ray Smith.
• Wikipedia articles “Clinton Engineer Works,” “Y-12 National Security Complex,” “K-25,” “S-50 (Manhattan Project),” “Calutron Girls” and “Ed Westcott,” used as pointers to the sources above.

Photographs: almost all by James Edward “Ed” Westcott, official photographer of the Clinton Engineer Works, taken for the U.S. Army and the Department of Energy and in the public domain, via the DOE Oak Ridge collection and Wikimedia Commons; one photograph of calutron operators from the Atomic Energy Commission records at the National Archives, public domain; and a DOE portrait of Westcott, public domain. Dates are those recorded by DOE; where the record gives only an approximate year, the caption says so. The Calutron Girls insignia drawn for this collection is an illustration, not an official emblem.