SAM Laboratories

K-25 was the codename given by the Manhattan Project to the program to produce enriched uranium for atomic bombs using the gaseous diffusion method. Originally the codename for the product, over time it came to refer to the project, the production facility located at the Clinton Engineer Works in Oak Ridge, Tennessee, the main gaseous diffusion building, and ultimately the site. When it was built in 1944, the four-story K-25 gaseous diffusion plant was the world's largest building, comprising over of floor space and a volume of .

Construction of the K-25 facility was undertaken by J. A. Jones Construction. At the height of construction, over 25,000 workers were employed on the site. Gaseous diffusion was but one of three enrichment technologies used by the Manhattan Project. Slightly enriched product from the S-50 thermal diffusion plant was fed into the K-25 gaseous diffusion plant. Its product in turn was fed into the Y-12 electromagnetic plant. The enriched uranium was used in the Little Boy atomic bomb used in the atomic bombing of Hiroshima. In 1946, the K-25 gaseous diffusion plant became capable of producing highly enriched product.

After the war, four more gaseous diffusion plants named K-27, K-29, K-31 and K-33 were added to the site. The K-25 site was renamed the Oak Ridge Gaseous Diffusion Plant in 1955. Production of enriched uranium ended in 1964, and gaseous diffusion finally ceased on the site on 27 August 1985. The Oak Ridge Gaseous Diffusion Plant was renamed the Oak Ridge K-25 Site in 1989, and the East Tennessee Technology Park in 1996. Demolition of all five gaseous diffusion plants was completed in February 2017.

Background

The discovery of the neutron by James Chadwick in 1932, followed by that of nuclear fission in uranium by the German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation (and naming) by Lise Meitner and Otto Frisch soon after, opened up the possibility of a controlled nuclear chain reaction with uranium. At the Pupin Laboratories at Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be achieved. Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein-Szilard letter to the President of the United States, Franklin D. Roosevelt. This prompted Roosevelt to initiate preliminary research in late 1939.

Niels Bohr and John Archibald Wheeler applied the liquid drop model of the atomic nucleus to explain the mechanism of nuclear fission. As the experimental physicists studied fission, they uncovered puzzling results. George Placzek asked Bohr why uranium seemed to fission with both fast and slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope. The former makes up just 0.714 percent of the uranium atoms in natural uranium, about one in every 140; natural uranium is 99.28 percent uranium-238. There is also a tiny amount of uranium-234, which accounts for just 0.006 percent.

At Columbia, John R. Dunning believed this was the case, but Fermi was not so sure. The only way to settle this was to obtain a sample of uranium-235 and test it. He got Alfred O. C. Nier from the University of Minnesota to prepare samples of uranium enriched in uranium-234, 235 and 238 using a mass spectrometer. These were ready in February 1940, and Dunning, Eugene T. Booth and Aristid von Grosse then carried out a series of experiments. They demonstrated that uranium-235 was indeed primarily responsible for fission with slow neutrons, but were unable to determine precise neutron capture cross sections because their samples were not sufficiently enriched.

At the University of Birmingham in Britain, the Australian physicist Mark Oliphant assigned two refugee physicists—Otto Frisch and Rudolf Peierls—the task of investigating the feasibility of an atomic bomb, ironically because their status as enemy aliens precluded their working on secret projects like radar. Their March 1940 Frisch–Peierls memorandum indicated that the critical mass of uranium-235 was within an order of magnitude of , which was small enough to be carried by a bomber aircraft of the day.

Gaseous diffusion

In April 1940, Jesse Beams, Ross Gunn, Fermi, Nier, Merle Tuve and Harold Urey had a meeting at the American Physical Society in Washington, D.C. At the time, the prospect of building an atomic bomb seemed dim, and even creating a chain reaction would likely require enriched uranium. They therefore recommended that research be conducted with the aim of developing the means to separate kilogram amounts of uranium-235. At a lunch on 21 May 1940, George B. Kistiakowsky suggested the possibility of using gaseous diffusion.

Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas through a porous barrier is inversely proportional to the square root of the gas's molecular mass. In a container with a porous barrier containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules. The gas leaving the container is slightly enriched in the lighter molecules, while the residual gas is slightly depleted. A container wherein the enrichment process takes place through gaseous diffusion is called a diffuser.

Gaseous diffusion had been used to separate isotopes before. Francis William Aston had used it to partially separate isotopes of neon in 1931, and Gustav Ludwig Hertz had improved on the method to almost completely separate neon by running it through a series of stages. In the United States, William D. Harkins had used it to separate chlorine. Kistiakowsky was familiar with the work of Charles G. Maier at the Bureau of Mines, who had also used the process to separate gases.

Uranium hexafluoride () was the only known compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Before this could be done, the ''Special Alloyed Materials (SAM) Laboratories'' at Columbia University and the Kellex Corporation had to overcome formidable difficulties to develop a suitable barrier. Fortunately, fluorine consists of only a single isotope , so that the 1percent difference in molecular weights between and is due solely to the difference in weights of the uranium isotopes. For these reasons, was the only choice as a feedstock for the gaseous diffusion process. Uranium hexafluoride, a solid at room temperature, sublimes at at . Applying Graham's law to uranium hexafluoride:
:$=\sqrt=\sqrt \approx 1.0043$
where:
:''Rate₁'' is the rate of effusion of ²³⁵UF₆.
:''Rate₂'' is the rate of effusion of ²³⁸UF₆.
:''M₁'' is the molar mass of ²³⁵UF₆ ≈ 235 + 6 × 19 = 349g·mol⁻¹
:''M₂'' is the molar mass of ²³⁸UF₆ ≈ 238 + 6 × 19 = 352g·mol⁻¹

Uranium hexafluoride is a highly corrosive substance. It is an oxidant and a Lewis acid which is able to bind to fluoride. It reacts with water to form a solid compound, and is very difficult to handle on an industrial scale.

Organization

Booth, Dunning and von Grosse investigated the gaseous diffusion process. In 1941, they were joined by Francis G. Slack from Vanderbilt University and Willard F. Libby from the University of California. In July 1941, an Office of Scientific Research and Development (OSRD) contract was awarded to Columbia University to study gaseous diffusion. With the help of the mathematician Karl P. Cohen, they built a twelve-stage pilot gaseous diffusion plant at the Pupin Laboratories. Initial tests showed that the stages were not as efficient as the theory would suggest; they would need about 4,600 stages to enrich to 90 percent uranium-235.

A secret contract was awarded to M. W. Kellogg for engineering studies in July 1941. This included the design and construction of a ten-stage pilot gaseous diffusion plant. On 14 December 1942, the Manhattan District, the US Army component of the Manhattan Project, as the effort to develop an atomic bomb became known, contracted Kellogg to design, build and operate a full-scale production plant. Unusually, the contract did not require any guarantees from Kellogg that it could actually accomplish this task. Because the scope of the project was not well defined, Kellogg and the Manhattan District agreed to defer any financial details to a later, cost-plus contract