They came in their thousands: young women from across rural Tennessee, fresh out of high school, drawn by the promise of good pay, cheap rent, and a chance to do their part for the war effort – women like 19-year-old Gladys Owens and Ruth Huddleston. One by one they  stepped off trains and buses and entered the mysterious town of Oak Ridge, a sprawling complex along the Clinch River which had seemingly sprung up overnight and whose purpose was a closely-guarded secret. Indeed, an oppressive atmosphere of secrecy hung over the entire site, from the tall barbed-wire fences to the giant signs at every gate urging residents to keep quiet. The women who came to Oak Ridge had no idea what they had been hired to do; they knew only was that it was vital work that would help secure victory against Nazi Germany and Imperial Japan. Even in training this point was endlessly hammered home, their instructors reminding them:

“What you learn here and what you do here stays here. Don’t tell your family. Don’t tell your friends. All you need to know is that you’re working to help end the war. We can train you how to do what is needed but cannot tell you what you are doing. I can only tell you that if our enemies beat us to it, God have mercy on us!”

The women soon discovered what this war-winning work entailed: sitting for hours in front of massive control panels, carefully monitoring gauges and turning dials:

“If the hand on those meters went too high, we had to get to work and get it back balanced. If it went too low, we had to sit on that stool and watch it. And if it got to the point where we couldn’t control it, we had a person that we could call and they would come help us. And if they couldn’t control it, then they had to shut it down and call for help. I was always afraid to move.”

Though the women were carefully instructed which dials to turn and what settings to maintain, the purpose of the task itself remained a mystery. All they knew was that it somehow involved magnets, for any loose jewelry or bobby pins tended to fly off and stick to the panels. They worked 8-hour days in 7-day rotating shifts, the work carrying on 24 hours a day, 7 days a week. While off-duty, they slept in dormitories like Fostoria Hall which cost only $10/month (about $150 today), ate at the staff cafeteria, and went out to the local cinema or bowling alley. Yet even here the overbearing culture of secrecy made itself known. Residents avoided gathering in groups for fear someone might be listening, and even at the movies security staff patrolled the aisles with flashlights to make sure nobody chatted idly about their work. Those with loose lips were swiftly dealt with, as Gladys Owens recalls:

“They told us that we could immediately be taken out of our position if we were caught talking about it. A time or two, people disappeared, so we didn’t know whether they’d fired them or what had happened. When one young girl didn’t return to her dormitory for her clothes, they told us she died from drinking some poison moonshine.”

And so their mysterious work carried on in silence, week after week, month after month. Then, on the morning of August 6, 1945, someone ran into the dormitory screaming a word no-one had dared utter in over eight months: Uranium. In the control room, a supervisor broke the astonishing news: the United States had just attacked Japan with a powerful new atomic bomb, and the women of Oak Ridge had played a vital role in producing the Uranium used to build that bomb.

When the Manhattan Project, the Allied effort to build an atomic bomb, began in June 1942, there was only one known metal which could sustain a nuclear chain reaction: Uranium-235. This isotope accounts for only 0.72% of natural Uranium, meaning that thousands of tons of Uranium ore would have to be processed to produce the 50 kilograms of enriched U-235 needed to build a single bomb. To carry out this gargantuan task, General Leslie Groves, military director of the Manhattan Project, acquired 59,000 acres of land near the Clinch River in East Tennessee, which soon became known as Oak Ridge. Construction began in February 1943, and within two years the site had grown into a vast complex housing 75,000 people, complete with all the amenities of a large town including dormitories, cafeterias, churches, and cinemas.

Three different Uranium enrichment methods were used at Oak Ridge, all running in parallel in case one method proved unworkable and in order to maximize production. The enrichment facilities were built at breakneck speed and ran 24 hours a day, for the Manhattan Project was in a race against time. So slow was the enrichment process that it was estimated to take three years to produce enough Uranium for a single bomb – long enough, it was feared, for Nazi Germany to complete one of their own.

As U-235 and U-238 are chemically identical, all three enrichment methods worked by exploiting the slight difference in mass between the two isotopes. The first method, known as thermal diffusion, worked on the principle that when a liquid solution is exposed to a thermal gradient, lighter particles tend to collect at the hotter end, and heavier particles at the colder end. In this process, raw Uranium was converted into Uranium Hexafluoride gas and fed into a series of steam-heated steel columns. Gas molecules containing lighter U-235 would rise to the top of the column where they could be tapped off. The thermal diffusion plant, codenamed S50, was built next to the Clinch River to supply water for its steam plant and cooling system.

The second enrichment method used at Oak Ridge was gaseous diffusion. This was carried out in the colossal K-25 complex, which at 152,000 square metres in area was the largest building in the world at the time, surpassing the recently-completed Pentagon. In this process, Uranium Hexafluoride gas was passed through semipermeable nickel-mesh barriers arranged in a series of cascades. As lighter molecules diffuse through membranes faster than heavier ones, as the gas passed through the cascades it became increasingly enriched in U-235. Due to the corrosive nature of Uranium Hexafluoride, gaskets and seals in K-25 had to be made from a newly-discovered material called polytetrafluoroethylene – better known today as Teflon.

The third and final enrichment method at Oak Ridge were the Calutrons, which occupied a complex codenamed Y-12. Named after the University of California Berkeley where they were invented by Nobel laureate Ernest O. Lawrence, Calutrons essentially operated like giant mass spectrometer. Inside a large U-shaped vacuum tank, raw Uranium was vaporized and the resulting hot ions accelerated past a powerful magnetic field. Due to the difference in their mass, U-235 and U-238 atoms would be deflected by different amounts, splitting into two separate beams which were collected at the far end of the tank. To economize on space and energy use, multiple calutron tanks – 1,152 in total – were arranged around a massive ring-shaped electromagnet dubbed a “racetrack.” During construction, technicians at Y-12 ran into a serious problem: due to wartime munitions production, the copper needed to wire the electromagnets was in short supply. Instead, chief engineer Colonel James C. Marshall borrowed 13,3000 tons – or 430 million troy ounces – of silver bullion from the US Treasury, which was extruded into almost a thousand kilometres of wire.

Efficient uranium separation depended on the ion beams remaining tightly focused, a condition that required constant monitoring and adjustments to maintain. At first this task was carried out by technicians from Ernest O. Lawrence’s Berkeley laboratory, but when their skills became urgently needed elsewhere, the job was contracted out to the Tennessee Eastman Company. Due to a shortage of male workers, Eastman was forced to hire thousands of women – mostly high school graduates like Gladys Owens and Ruth Huddleston – reasoning that a job consisting mostly of adjusting dials could be carried out without knowing the exact nature of the work. When Lawrence’s technicians balked, arguing that the delicate work should be carried out by PhDs, a contest was held between the two groups. When the production figures were tallied, the results were indisputable: the “hillbilly girls” – as the technicians disparagingly called them – had beaten the PhDs hands down. According to Ray Smith, historian for the City of Oak Ridge, the reason was simple:

 “[The] young girls were doing what they were told. They were practicing statistical process control, just like they’d been trained. These scientists and engineers, they’d go trying to fix every little thing that was going wrong and the machine never took a set, so it never settled down.”

In the end, the decision to run all three Uranium enrichment processes in parallel proved a wise one, as the S-50 thermal diffusion plant never managed to attain a level of enrichment sufficient to fuel a bomb. But the project could not afford to waste a single gram of Uranium, so the low-enriched product from S-50 was simply used as feedstock for the more efficient K-25 and Y-12 plants. By mid-1944, however, the urgency of the Uranium enrichment program seemed to have faded, as scientists at the Los Alamos laboratory in New Mexico had discovered that another fissile isotope, Plutonium-239, could be bred far more efficiently than Uranium, and the B Reactor at Hanford in Washington state was built to produce it. Nonetheless, Uranium production continued as a backup. This too proved to be a wise decision, as an unfortunate discovery soon brought the  Plutonium program to a screeching halt. The first atomic bomb design, code-named Little Boy, was a gun-type device, wherein a propellant charge launches one subcritical mass of Uranium down a gun barrel into another, creating a supercritical mass that then detonates. When Plutonium was discovered, it was assumed that the same basic design could be used, albeit in a slightly modified form known as Thin Man. But when the first samples of reactor-bred Plutonium from Hanford arrived at Los Alamos, it was discovered that they contained unacceptably high levels of the isotope Plutonium-238. This isotope exhibits a high rate of spontaneous fission, meaning that when the bomb was triggered, the two subcritical masses would start inducing fissions in each other long before they came together, blowing the bomb apart prematurely in what was dubbed a “fizzle.” This discovery sent the whole project into a tailspin, and it seemed once again like the Uranium bomb was the only viable option. Eventually the scientists at Los Alamos would come up with the concept of implosion, where a spherical Plutonium core is compressed to criticality using a system of high-explosive lenses. But this was a radical concept with many unknowns, one which unlike Little Boy – which was almost guaranteed to work – would have to be thoroughly tested. This culminated in the July 16, 1945 Trinity test at Alamagordo, New Mexico – the world’s first nuclear detonation.

By July 1945, the S-50, K-25, and Y-12 enrichment plants had succeeded in producing 64kg of U-235, which was shipped to Los Alamos for integration into the Little Boy weapon. The empty bomb casing was delivered to the Pacific island of Tinian by the USS Indianapolis on July 26, with the Uranium arriving by air four days later. Then at 8:15 AM on August 6, 1945, the B-29 bomber Enola Gay dropped the bomb on Hiroshima, Japan, killing 66,000 people. This was followed three days later by the bombing of Nagasaki, using a plutonium-implosion device code-named Fat Man. Incredibly, due to the inefficient nature of the Little Boy design, of the 64kg of Uranium used only seven tenths of gram – about the mass of a paperclip – was actually converted into energy.

On learning of the role they had played in building the bomb, feelings among the Calutron Girls of Oak Ridge were mixed, as Ruth Huddleston recalled in a 2020 interview:

“They told about all the people that had been killed. It was horrible, but we knew that that would end the war. And my husband-to-be would have been over there soon if the war hadn’t ended. I was happy and then I was sad and then I was happy, and then I got to thinking about it,” she says. “I got to thinking that I had a part in killing all of those people and it really bothered me. It bothered me for a long time after that, but there wasn’t anything I could do about it. There would have been a lot more killed if we hadn’t dropped the bomb. You just have to face reality, so that’s what I did. But I still don’t like to think about that part of it.”

After the war, most of the Calutron Girls were dismissed from Tennessee Eastman to make way for the servicemen returning home. Their patriotic duty done, they scattered back across America and moved on to a variety of careers, Gladys Owens becoming an accountant and Ruth Huddleston a schoolteacher. The strict secrecy of Oak Ridge having been drilled into their heads, neither spoke a word about their wartime experiences – not even to their children or grandchildren – until long after they had retired. In 2004 Gladys Owens visited Oak Ridge for the first time in 59 years, sitting down at the same control panel where she’d worked for 8 months in 1945 and identifying herself in a famous photograph of the Calutron control room taken by Manhattan Project photographer Ed Westcott. On the subject of her wartime service, like Ruth Huddleston Owens’ feelings are conflicted:

“Sometimes I’m proud of what I was involved in, and sometimes I cry about it. We changed the world”.

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