Separating Uranium Isotopes for War and Peace Time Use
In December 1938, just a few months before Germany invaded Poland and triggered World War II, a pair of German chemists made a startling discovery: they split an atom, a feat previously considered impossible. To understand the nature of radioactive elements, Otto Hahn and Fritz Strassmann had been bombarding uranium with neutrons. To their surprise, the resulting sample contained barium, a much lighter element and not an element in the original sample. Unable to explain their findings, Hahn wrote to a former physicist colleague, Lise Meitner.聽
With help from her nephew Otto Frisch, Meitner realized the staggering importance of the discovery. In January 1939, they described this new type of nuclear reaction and named it fission. In nuclear fission, the nucleus of an atom captures neutrons, which destabilizes the atom. The unstable nucleus splits roughly in half and releases more neutrons. If the neutrons emitted from one fission event triggers further fission events, a self-sustaining chain reaction could occur, generating a tremendous amount of energy. This is the process the German chemists witnessed.
Contents
- The competition is on
- New neighbors
- Mastering separation
- Putting the steps together
- Quick and impactful
- Current R&D
The competition is on
Scientists realized the energy from a nuclear fission chain reaction could serve as the basis for an atomic bomb.
Then-U.S. President Roosevelt was warned and urged聽
to stockpile uranium and encourage U.S. researchers to investigate nuclear chain reactions.聽
Meanwhile, back in England, Austrian-born Frisch and German-born Rudolf Peierls, both physicists, were developing a method for a self-sustaining fission chain reaction. In 1940, they authored the Frisch-Peierls memorandum, describing the feasibility of creating a nuclear weapon using a particular uranium isotope: 235U.
Frisch and Peierls theorized that 235U could sustain a chain聽reaction and that the critical mass required was light enough to be carried in an aircraft.聽
At this point in history, researchers around the world raced to create fission reactors and discover how to separate isotopes. To maximize the energy output in the fission process, scientists needed to figure out how to increase the percentage of 235U in a sample.聽
In August 1942, the U.S. Army Corps of Engineers formed the Manhattan Engineer District to manage a large-scale construction project that would eventually make a 235U bomb.聽
The project would succeed in isolating pounds of 235U,聽 a challenge that involved many large manufacturing companies. It was, at the time, the largest enclosed building ever constructed, part of the largest industrial project ever completed. If it still stood today, it would be among the top five largest buildings by area in the U.S., top 50 in the world鈥攖his includes airports, factories, and warehouse facilities.聽
What is an isotope?
Atoms contain electrons, protons, and neutrons. All neutral atoms of a particular element contain the same number of protons and electrons, but the number of neutrons can vary. This results in isotopes, which are atoms of the same element that have different atomic masses. A natural sample of uranium, for example, contains about 99.28% of the isotope uranium-238 (238U) and 0.714% uranium-235 (235U). So, the vast majority of uranium atoms are slightly heavier than others. 235U atoms contain 92 protons and 143 neutrons, while 238U atoms contain 92 protons and 146 neutrons.
New neighbors
On September 19, 1942, General Leslie R. Groves approved a land purchase of nearly 60,000 acres in east Tennessee, amid whispers of a classified engineering project. Some families were discreetly relocated, and a flurry of activity ensued.聽
Approximately 25 miles west of Knoxville, new roads, wires, and pipelines snaked through the once sleepy farmland. In fewer than three years, the area would become an unprecedented complex of industrial facilities and a city with a population of 75,000.聽
This secret complex was dubbed Site X, also known as the Clinton Engineer Works. It was funded in December 1942 with $500 million, the equivalent of $9.7 billion in 2025. Eventually the city was named Oak Ridge, simply because it was located on the south slope of Black Oak Ridge.聽
The covert project may not have seemed like a military operation so much as a public engineering project. In fact, General Groves, a senior engineer in the U.S. Army Corps of Engineers, was assigned on September 17, 1942, to undertake the task of producing an atomic bomb. The project at Site X would play a critical role in this groundbreaking feat of nuclear science and engineering.
One portion of the massive complex, codenamed K-25, was initially designed to produce uranium isotope 235U to fuel nuclear weapons. If the U.S. and its allies could create an atomic bomb before German counterparts, defense officials believed they could force an end to World War II.聽
Isolating 235U remained the most difficult scientific and engineering obstacle.聽
Mastering separation
Isotopes can鈥檛 be separated by chemical means, but scientists realized gaseous diffusion鈥攁kin to a molecular race鈥攚as a viable process to consider. Uranium, a solid at room temperature, readily reacts to form solid uranium hexafluoride (UF6). When heated to 134聽oF (56.5 oC), UF6 becomes a gas.
So, when a sample of uranium, which is composed of a lot of聽238U and a minimal amount of 235U, becomes UF6, most of the gaseous molecules are slightly heavier than others (238UF6 is heavier than 235UF6).听
In gaseous diffusion, two gases with different masses are together in one chamber, which is separated from a second empty chamber by a membrane. Lighter gas molecules move faster and therefore travel through the membrane more readily. Scientists stringed together thousands of membranes to allow for the mixture of gas to聽become more concentrated with the lighter uranium isotope molecule. But the process had several challenges to overcome.聽
The most technically challenging component of the gaseous diffusion process was developing a membrane with small pores. The membranes had billions of holes with a diameter of only 100 nanometers. Pushing a gas through so many small openings required pressure, so the membrane material had to be strong enough to withstand those forces. And the properties of UF6 presented聽challenges: In its gaseous form it is highly corrosive, capable of reacting with engineering components like pipes, valves, and instruments. UF6 also reacts with water, so any leak would create a solid that could clog the pores.
To withstand UF6鈥檚 corrosive effects, the piping, converter shells, and tanks at K-25 were made of nickel-plated steel. Fluorocarbon coolants and lubricants were among the few materials that could handle the harsh chemistry. For gaskets and seals, the team used polymerized tetrafluoroethylene (PTFE), a聽synthetic material developed by DuPont in 1938. After extensive testing, General Groves authorized the government to take over DuPont鈥檚 research facility to produce and ship PTFE components to Oak Ridge. After the war, PTFE became more widely known by its brand name, Teflon.
Advances in membranes and corrosion resistance made the dream of gaseous diffusion at K-25 a reality. Thanks to that preliminary work, researchers could turn their attention to fine-tuning the process. The goal was to isolate a uranium sample that was 90% 235U, which would minimize the weight and maximize energy output, and thus make a bomb possible.
The construction of the gaseous diffusion plant began in June 1943, even though no suitable membrane had been identified. A nickel membrane was the most promising option, and in 1944 construction of working diffusion cascades began.聽
The [gaseous diffusion] method was completely novel. It was based on the theory that if uranium gas was pumped against a porous barrier, the lighter molecules of the gas, containing U-235, would pass through more rapidly than the heavier U-238 molecules. The heart of the process was, therefore, the barrier, a porous, thin metal sheet or membrane with millions of submicroscopic openings per square inch.鈥�
鈥� General Leslie R. Groves, Now It Can Be Told
Putting the steps together
Once the team converted solid uranium to gaseous UF6, the gas passed through multiple聽porous nickel membranes. Each stage of this separation process resulted in higher concentration of lighter UF6 molecules with 235鲍.听
The process of isolating 235U is called enriching. U-shaped, spanning a half-mile in length, each leg 400 feet wide, and rising four stories tall, K-25 was the largest structure under one roof at the time of its completion in 1945, just 16 months after construction began. Thousands and thousands of membrane transits were required to reach the desired enrichment.
Under one 44-acre roof, the building fit hundreds of miles of vacuum-tight nickel or nickel-plated piping, 4 million miles of copper tubing, and 6,000 compressors with huge cooling needs. It would take 60,000 trips by railroad cars to transport materials, including cannisters of UF6.
The massive research and construction job required the industrial might of the U.S., ultimately involving a peak of 25,000 workers. The large, secretive team of Manhattan Project researchers collaborated with firms including Carbide and Carbon Chemicals Company, M.W. Kellogg Company, Kellex Construction Company, J. A. Jones, Chrysler, DuPont, Allis Chalmers, and Houdaille-Hershey Corporation鈥攁ll overseen by the Corps of Engineers.
Quick and impactful
After K-25 went online on January 20, 1945, it didn鈥檛 take long to prove its worth. Initial runs alongside other Manhattan Project enrichment facilities, known as S-50 and Y-12, took place quickly. The S-50 facility enriched natural uranium by separating isotopes based on a temperature gradient, dialing up K-25鈥檚 feedstock concentration from 0.71% to 0.89%. And the Y-12 electromagnetic separation plant added significant enrichment to the K-25 product. The bulk of the separation of the 235U isotope to 80% happened at Y-12.聽
All the plants began ramping up operations by March 1945. Enrichment ticked up from 12% to 23% by summertime, sufficiently above the threshold for a weapon. K-25鈥檚聽gaseous diffusion process made the U.S. the winner in the race to build an atomic bomb.
Germany surrendered to Allied forces in May 1945, but fighting continued with Japan. On August 6, 1945, the U.S. detonated the world鈥檚 first atomic bomb used in warfare above Hiroshima, Japan, using enriched uranium from the Manhattan Project. It is estimated the bomb killed 140,000 people by December 1945. A different type of atomic bomb was detonated above Nagasaki on August 9, killing an estimated 80,000 by the end of the year. Japan surrendered in September. Many credit nuclear warfare for bringing a swift end to the war, possibly preventing the loss of many more lives by making a ground invasion of Japan unnecessary.聽
After the war, the U.S. rapidly expanded its stockpile of uranium and nuclear weapons, spurred by fears of the Soviet Union during the prolonged Cold War era. The capacity of K-25, although unprecedented, was insufficient. Four more process buildings were built in Oak Ridge (K-27, K-29, K-31, and K-33), along with two other plants in Ohio and Kentucky. The new facility at Paducah, Kentucky, began operating in 1952, and in Portsmouth, Ohio, in 1954. However, K-25 was the flagship plant and supplied all the membrane barriers needed in the U.S. And it was part of the production of a significant amount of the weapons-grade 235U for the U.S. until 1964, when the K-25 building was shut down.
In the late 1970s, the U.S. government spent $1.5 billion ($12.3 billion in 2025) on a program to increase the productivity of the government鈥檚 gaseous diffusion plants. The upgrades involved barrier efficiency improvements and improvements to converters, compressors, and coolers. The programs finished in 1980鈥攁head of schedule and under budget鈥攁nd improved the overall efficiency of the gaseous diffusion sites. These programs also reduced the amount of power the processes required.
But the legacy of K-25聽broadened beyond atomic bombs. Reliable access to uranium enriched to more than 90% allowed the U.S. to power the world鈥檚 first nuclear submarine in 1954. Fuel provided by Y-12 continues to power the U.S. Navy鈥檚 nuclear ships to this day.聽
On the other hand, low enrichment鈥攁 purity of 3% to 5%鈥攚as ideal to fuel nuclear power plants. K-25 produced fuel for the first full-scale commercial nuclear power plant for peacetime use, located in Shippingport, Pennsylvania. By the time it closed, K-25 had generated nearly 500 metric tons of low-enriched 235U.
The influx of the material gave rise to a global market for 235U to fuel power plants. This allowed countries to diversify their sources of energy and rely less on fossil fuels. K-25鈥檚 enrichment program produced nuclear fuel for reactors in at least 14 countries other than the U.S. for years, providing energy to millions of homes and businesses.
At the time of its creation, most of the K-25 workers did not even know what they were building. This massive wartime effort was shrouded in complete secrecy. They were part of a team building the first atomic weapon, racing against Nazi Germany to bring an end to World War II. At the K-25 plant, a gaseous diffusion process was used to enrich uranium and ultimately create fuel for one of two atomic bombs. The K-25 facility played a major role and altered the global landscape during World War II and the Cold War. Its contributions to defense, energy, and technology advancements through the 1990s have left a historic impact.鈥�
鈥� K-25 Atomic History Center
Current R&D
K-25 was also the center of research and development for advanced methods to separate uranium, including work on using centrifuges instead of gaseous diffusion. In 1967, Oak Ridge scientists applied the centrifuge technology developed at K-25 to obtain ultra-pure vaccines. Virologist Jonas Salk used the centrifuges to purify early batches of polio vaccine, and pharmaceutical company Eli Lilly & Co. used them to purify large batches of influenza vaccines.
K-25 gaseous diffusion operations ended on August聽27, 1985, as the nuclear arsenal began to shrink, and the construction of nuclear power reactors ceased in the U.S. The facility鈥檚 colossal presence symbolized the monumental effort poured into the Manhattan Project. The five gaseous diffusion buildings at Oak Ridge were ultimately demolished in 2017, and remediation of the site was completed in 2024.聽
The American Museum of Science & Energy and its K-25 Atomic History Center (located at the site) as well as the Oak Ridge History Museum all tell various aspects of the story of a project that changed the world, in war and peace.聽
Landmark dedication and acknowledgments
Landmark dedication
The American Chemical 中国365bet中文官网 (ACS) designated the development of the gaseous diffusion method for uranium isotope separation as a National Historic Chemical Landmark (NHCL) in a ceremony at the K-25 History Center in Oak Ridge, Tennessee, on May 6鈥�7, 2025. The commemorative plaque reads:
The K-25 Plant was built during the World War II Manhattan Project to separate uranium isotopes needed to provide fuel for the first atomic bomb. The process, housed in the largest building in the world at that time, was one of the most complex industrial plants ever built. It required major advances in manufacturing of over 500 miles of ultra clean vacuum piping, installing over 3000 compressors and producing several million square feet of separation membranes that incorporated millions of microscopic pores. After the War, the plant helped give birth to the Nuclear Age by providing fuel for the U.S. Nuclear Navy and tons of uranium to feed nuclear power plants across the world thereby providing electricity to billions of homes.
Acknowledgments
Written by Max Levy.
The author wishes to thank contributors to and reviewers of this booklet, all of whom helped improve its content, especially members of the ACS NHCL Subcommittee.
The nomination for this Landmark designation was prepared by ACS鈥� East Tennessee Local Section.聽
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