K-25

thumb|upright=1.3|The K-25 building of the Oak Ridge Gaseous Diffusion Plant aerial view, looking southeast. The mile-long building, in the shape of a "U", was completely demolished in 2013.

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 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 caused by the uranium-235 isotope, while at high energies it was mainly a reaction with 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 had Alfred O. C. Nier from the University of Minnesota 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 they 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

thumb|Gaseous diffusion uses semi-permeable membranes to separate enriched uranium.

thumb|right|Stages are connected together to form a cascade. A, B and C are pumps.

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. Fluorine consists of only a single natural isotope , so the 1percent difference in molecular weights between and is solely 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:

:<math>{\mbox{Rate}_1 \over \mbox{Rate}_2}=\sqrt{M_2 \over M_1}=\sqrt{352 \over 349} \approx 1.0043</math>

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. Kellex operated as a self-contained and autonomous entity. Percival C. Keith, Kellogg's vice president of engineering,

Codename

The codename "K-25" was a combination of the "K" from Kellex, and "25", a World War II-era code designation for uranium-235 (an isotope of element 92, mass number 235). The term was first used in Kellex internal reports for the end product, enriched uranium, in March 1943. By April 1943, the term "K-25 plant" was being used for the plant that created it. That month, the term "K-25 Project" was applied to the entire project to develop uranium enrichment using the gaseous diffusion process. When other "K-" buildings were added after the war, "K-25" became the name of the original, larger complex.

Research and development

Diffusers

thumb|right|A gaseous diffusion cell, showing the diffuser

The highly corrosive nature of uranium hexafluoride presented several technological challenges. Pipes and fittings that it came into contact with had to be made of or clad with nickel. This was feasible for small objects but impractical for the large diffusers, the tank-like containers that had to hold the gas under pressure. Nickel was a vital war material, and although the Manhattan Project could use its overriding priority to acquire it, making the diffusers out of solid nickel would deplete the national supply. The director of the Manhattan Project, Brigadier General Leslie R. Groves Jr., gave the contract to build the diffusers to Chrysler. In turn Chrysler president K. T. Keller assigned Carl Heussner, an expert in electroplating, the task of developing a process for electroplating such a large object. Senior Chrysler executives called this "Project X-100".

Electroplating used one-thousandth of the amount of nickel needed for a solid nickel diffuser. The SAM Laboratories had already attempted this and failed. Heussner experimented with a prototype in a building built within a building, and found that it could be done, so long as the series of pickling and scaling steps required were done without anything coming in contact with oxygen. Chrysler's entire factory at Lynch Road in Detroit was turned over to the manufacture of diffusers. The electroplating process required over of floor space, several thousand workers and a complicated air filtration system to ensure the nickel was not contaminated. By the war's end, Chrysler had built and shipped more than 3,500 diffusers. So Keith and Groves met with executives at Allis-Chalmers, who agreed to build a new factory to produce the pumps, even though the pump design was still uncertain. The SAM Laboratories came up with a design, and Westinghouse built some prototypes that were successfully tested. Then Judson Swearingen at the Elliott Company came up with a revolutionary and promising design that was mechanically stable with seals that would contain the gas. This design was manufactured by Allis-Chalmers.

Barriers

Difficulties with the diffusers and pumps paled in significance beside those with the porous barrier. To work, the gaseous diffusion process required a barrier with microscopic holes, but not subject to plugging. It had to be porous but strong enough to handle the high pressures. And, like everything else, it had to resist corrosion from uranium hexafluoride. The latter criterion suggested a nickel barrier. Foster C. Nix at the Bell Telephone Laboratories experimented with nickel powder, while Edward O. Norris at the C. O. Jelliff Manufacturing Corporation and Edward Adler at the City College of New York worked on a design with electroplated nickel. Norris was an English interior decorator who had developed a very fine metal mesh for use with a spray gun. The design appeared too brittle and fragile for the proposed use, particularly on the higher stages of enrichment, but there was hope that this could be overcome.

thumb|left|Setting up a process pump

In 1943, Urey brought in Hugh S. Taylor from Princeton University to look at the problem of a usable barrier. Libby made progress on understanding the chemistry of uranium hexafluoride, leading to ideas on how to prevent corrosion and plugging. Chemical researchers at the SAM Laboratories studied fluorocarbons, which resisted corrosion and could be used as lubricants and coolants in the gaseous diffusion plant. Despite this progress, the K-25 Project was in serious trouble without a suitable barrier, and by August 1943 it was facing cancellation. On 13 August Groves informed the Military Policy Committee (the senior committee that steered the Manhattan Project) that gaseous diffusion enrichment in excess of fifty percent was probably infeasible, and the gaseous diffusion plant would be limited to producing product with a lower enrichment which could be fed into the calutrons of the Y-12 electromagnetic plant. Urey therefore began preparations to mass-produce the Norris-Adler barrier, despite its problems.

Meanwhile, Union Carbide and Kellex had made researchers at the Bakelite Corporation, a subsidiary of Union Carbide, aware of Nix's unsuccessful efforts with powdered nickel barriers. To Frazier Groff and other researchers at Bakelite's laboratories in Bound Brook, New Jersey, it seemed that Nix was not taking advantage of the latest techniques, and they began their own development efforts. Both Bell and Bound Brook sent samples of their powdered nickel barriers to Taylor for evaluation, but he was unimpressed; neither had come up with a practical barrier. At Kellogg's laboratory in Jersey City, New Jersey, Clarence A. Johnson, who was aware of the steps taken by the SAM Laboratories to improve the Norris-Adler barrier, realized that they could also be taken with the Bakelite barrier. The result was a barrier better than either, although still short of what was required. At a meeting at Columbia with the Army in attendance on 20 October 1943, Keith proposed switching the development effort to the Johnson barrier. Urey balked at this, fearing this would destroy morale at the SAM Laboratories. The issue was put to Groves at a meeting on 3November 1943, and he decided to pursue development of both the Johnson and the Norris-Adler barriers.

Groves summoned British help, in the form of Wallace Akers and fifteen members of the British gaseous diffusion project, who reviewed the progress made thus far. Their verdict was that while the new barrier was potentially superior, Keith's undertaking to build a new facility to produce the new barrier in just four months, produce all the barriers required in another four and have the production facility up and running in just twelve "would be something of a miraculous achievement". On 16 January 1944, Groves ruled in favor of the Johnson barrier. Johnson built a pilot plant for the new process at the Nash Building. Taylor analyzed the sample barriers produced and pronounced only 5percent of them to be of acceptable quality. Edward Mack Jr. created his own pilot plant at Schermerhorn Hall at Columbia, and Groves obtained of nickel from the International Nickel Company. With plenty of nickel to work with, by April 1944 both pilot plants were producing barriers of acceptable quality at a 45 percent rate.

Construction

The project site chosen was at the Clinton Engineer Works in Tennessee. The area was inspected by representatives of the Manhattan District, Kellex and Union Carbide on 18 January 1943. Consideration was also given to sites near the Shasta Dam in California and the Big Bend of the Columbia River in Washington state. The lower humidity of these areas made them more suitable for a gaseous diffusion plant, but the Clinton Engineer Works site was immediately available and otherwise suitable. Groves decided on the site in April 1943.

Under the contract, Kellex had responsibility not just for the design and engineering of the K-25 plant, but for its construction as well. The prime construction contractor was J. A. Jones Construction from Charlotte, North Carolina. It had impressed Groves with its work on several major Army construction projects, such as Camp Shelby, Mississippi. There were more than sixty subcontractors. Kellex engaged another construction company, Ford, Bacon & Davis, to build the fluorine and nitrogen facilities, and the conditioning plant. Construction work was initially the responsibility of Lieutenant Colonel Warren George, the chief of the construction division of the Clinton Engineer Works. Major W. P. Cornelius became the construction officer responsible for K-25 works on 31 July 1943. He was answerable to Stowers back in Manhattan. He became chief of the construction division on 1March 1946. J. J. Allison was the resident engineer from Kellex, and Edwin L. Jones, the General Manager of J. A. Jones.

Power plant

thumb|K-25 power plant (the building with three smoke stacks) in 1945. The dark building behind it is the [[S-50 (Manhattan Project)|S-50 thermal diffusion plant.]]

Construction began before completion of the design for the gaseous diffusion process. Because of the large amount of electric power the K-25 plant was expected to consume, it was decided to provide it with its own electric power plant. While the Tennessee Valley Authority (TVA) believed it could supply the Clinton Engineer Works' needs, there was unease about relying on a single supplier when a power failure could cost the gaseous diffusion plant weeks of work, and the lines to TVA could be sabotaged. A local plant was more secure. The Kellex engineers were also attracted to the idea of being able to generate the variable frequency current required by the gaseous diffusion process without complicated transformers.

A site was chosen for this on the western edge of the Clinton Engineer Works site where it could draw cold water from the Clinch River and discharge warm water into Poplar Creek without affecting the inflow. Groves approved this location on 3May 1943. Surveying began on the power plant site on 31 May, and J. A. Jones started construction work the following day. Because the bedrock was below the surface, the power plant was supported on 40 concrete-filled caissons. Installation of the first boiler commenced in October 1943. Construction work was complete by late September. To prevent sabotage, the power plant was connected to the gaseous diffusion plant by an underground conduit. Despite this, there was one act of sabotage, in which a nail was driven through the electric cable. The culprit was never found but was considered more likely to be a disgruntled employee than an Axis spy.

Electric power in the United States was generated at 60 hertz; the power house was able to generate variable frequencies between 45 and 60 hertz, and constant frequencies of 60 and 120 hertz. This capability was not ultimately required, and all but one of the K-25 systems ran on a constant 60 hertz, the exception using a constant 120 hertz. The first section was ready for test runs on 17 April 1944, although the barriers were not yet ready to be installed.

The main process building surpassed The Pentagon as the largest building in the world,

Operations

thumb|right|The K-25 control room

The preliminary specification for the K-25 plant in March 1943 called for it to produce per day of product that was 90 percent uranium-235. As the practical difficulties were realized, this target was reduced to 36 percent. On the other hand, the cascade design meant construction did not need to be complete before the plant started operating. In August 1943, Kellex submitted a schedule that called for a capability to produce material enriched to 5percent uranium-235 by 1June 1945; 15 percent by 1July; and 36 percent by 23 August. This schedule was revised in August 1944 to 0.9 percent by 1January 1945; 5percent by 10 June; 15 percent by 1August; 23 percent by 13 September; and 36 percent as soon as possible after that.

A meeting between the Manhattan District and Kellogg on 12 December 1942 recommended the K-25 plant be operated by Union Carbide. This would be through a wholly owned subsidiary, Carbon and Carbide Chemicals. A cost-plus-fixed-fee contract was signed on 18 January 1943, setting the fee at $75,000 per month. This was later increased to $96,000 per month to operate both K-25 and K-27. Union Carbide did not wish to be the sole operator of the facility; Union Carbide suggested the conditioning plant be built and operated by Ford, Bacon & Davis. The Manhattan District found this acceptable, and a cost-plus-fixed-fee contract was negotiated with a fee of $216,000 for services up to the end of June 1945. The contract was terminated early on 1May 1945, when Union Carbide took over the plant. Ford, Bacon & Davis was therefore paid $202,000. The other exception was the fluorine plant. Hooker Chemical was asked to supervise its construction of the fluorine plant and initially to operate it for a fixed fee of $24,500. The plant was turned over to Union Carbide on 1February 1945.

thumb|left|A worker on a bicycle in the K-25 operating level

Part of the K-300 complex was taken over by Union Carbide in August 1944 and was run as a pilot plant, training operators and developing procedures, using nitrogen instead of uranium hexafluoride until October 1944, and then perfluoroheptane until April 1945. The design of the gaseous diffusion plant allowed for it to be completed in sections and for the sections to be put into operation while work continued on the others. J. A. Jones completed the first 60 stages by the end of 1944. Before each stage was accepted, it underwent tests by J. A. Jones, Carbide and Carbon, and SAM Laboratories technicians to verify that the equipment was working and there were no leaks. Between four and six hundred people devoted eight months to this testing. Perfluoroheptane was used as a test fluid until February 1945, when it was decided to use uranium hexafluoride despite its corrosive nature.

Manhattan District engineer Colonel Kenneth Nichols placed Major John J. Moran in charge of production at K-25. Production commenced in February 1945, and the first product was shipped to the calutrons in March. By April, the gaseous diffusion plant was producing 1.1 percent product. It was then decided that instead of processing uranium hexafluoride feed from the Harshaw Chemical Company, the gaseous diffusion plant would take the product of the S-50 thermal diffusion plant, with an average enrichment of about 0.85 percent. Product enrichment continued to improve as more stages came online and performed better than anticipated. By June product was being enriched to 7percent; by September it was 23 percent. The S-50 plant ceased operation on 9September, and Kellex transferred the last unit to Union Carbide on 11 September. Highly enriched uranium was used in the Little Boy atomic bomb used in the bombing of Hiroshima on 6August.

thumb|right|Air compressors and water pumps in the K-1101 air conditioning building

With the end of the war in August 1945, the Manhattan Project's priority shifted from speed to economy and efficiency. The cascades were configurable, so they could produce a large amount of slightly enriched product by running them in parallel, or a small amount of highly enriched product through running them in series. By early 1946, with K-27 in operation, the facility was producing per day, enriched to 30 percent. The next step was to increase the enrichment further to 60 percent. This was achieved on 20 July 1946. This presented a problem, because Y-12 was not equipped to handle feed that was so highly enriched, but the Los Alamos Laboratory required 95 percent. For a time, product was mixed with feed to reduce the enrichment to 30 percent. Taking the concentration up to 95 percent raised safety concerns, as there was the risk of a criticality accident.

After some deliberation, with opinions sought and obtained from Percival Keith, Norris Bradbury, Darol Froman, Elmer E. Kirkpatrick, Kenneth Nichols and Edward Teller, it was decided that this could be done safely if appropriate precautions were taken. On 28 November 1946, the K-25 plant began producing 94 percent product. At this point, they ran into a serious flaw in the gaseous diffusion concept: enrichment in uranium-235 also enriched the product in the unwanted and fairly useless uranium-234, making it difficult to raise the enrichment to 95 percent. On 6December 1946, production was dropped back to a steady per day enriched to 93.7 percent uranium-235, along with 1.9 percent uranium-234. This was regarded as a satisfactory product by the Los Alamos Laboratory, so on 26 December 1946 enrichment activity at Y-12 was curtailed. The Manhattan Project ended a few days later. Responsibility for the K-25 facility then passed to the newly established Atomic Energy Commission on 1January 1947. Workers at the plant were represented by the Oil, Chemical and Atomic Workers International Union.

Closure and demolition

thumb|left|The K-25 complex in 2006

K-25 became a prototype for other gaseous diffusion facilities established in the early post-war years. The first of these was the K-27 completed in September 1945. It was followed by the K-29 in 1951, the K-31 in 1951 and the K-33 in 1954. and Portsmouth, Ohio, in 1954. The K-25 plant was renamed the Oak Ridge Gaseous Diffusion Plant in 1955.

Today, uranium isotope separation is usually done by the more energy-efficient ultra centrifuge process, developed in the Soviet Union after World War II by Soviet and captured German engineers working in detention. The centrifuge process was the first isotope separation method considered for the Manhattan Project but was abandoned due to technical challenges early in the project. When German scientists and engineers were released from Soviet captivity in the mid-1950s, the West became aware of the ultra centrifuge design and began shifting uranium enrichment to this much more efficient process. As centrifuge technology advanced, it became possible to carry out uranium enrichment on a smaller scale without the vast resources that were necessary to build and operate 1940s and 1950s "K" and "Y" style separation plants, a development which had the effect of increasing nuclear proliferation concerns.

thumb|right|Demolition of K-25 in progress in April 2012

Centrifuge cascades began operating at Oak Ridge in 1961. A gas centrifuge test facility (K-1210) opened in 1975, followed by a larger centrifuge plant demonstration facility (K-1220) in 1982. In response to an order from President Lyndon B. Johnson to cut production of enriched uranium by 25 percent, K-25 and K-27 ceased production in 1964, but in 1969 K-25 began producing uranium enriched to 3to 5percent for use in nuclear reactors. Martin Marietta Energy replaced Union Carbide as the operator in 1984. Gaseous diffusion ceased 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. Presently all commercial uranium enrichment in the United States is carried out using gas centrifuge technology.

The United States Department of Energy contracted with British Nuclear Fuels Ltd in 1997 to decontaminate and decommission the facilities. Its subsidiary Reactor Sites Management Company Limited was acquired by EnergySolutions in June 2007. Initially K-29, K-31 and K-33 were to be retained for other uses, but it was subsequently decided to demolish them. Bechtel Jacobs, the environmental management contractor, assumed responsibility for the facility in July 2005. Demolition of K-29 began in January 2006 and was completed in August. Demolition of K-33 began in January 2011 and was completed ahead of schedule in September. It was followed by the demolition of K-31, which began in October 2014 and was completed in June 2015.

Bechtel Jacobs was contracted to dismantle and demolish the K-25 facility in September 2008. The contract, valued at $1.48&nbsp;billion, was made retrospective to October 2007 and ended in August 2011. Demolition work was then carried out by URS | CH2M Hill Oak Ridge. Demolition was completed in March 2014 Demolition of K-27, the last of the five gaseous diffusion facilities at Oak Ridge, began in February 2016. US Senator Lamar Alexander and US Congressman Chuck Fleischmann joined 1,500 workers to watch the final wall come down on 30 August 2016. Its demolition was completed in February 2017. Since 2020, the K-25 site is being redeveloped in part into a general aviation airport to service the city of Oak Ridge. Several small private nuclear facilities are also planned on the site.

Commemoration

On 27 February 2020, the K-25 History Center, a 7,500-square foot museum opened at the site. The museum is a branch of the American Museum of Science and Energy and features hundreds of original artifacts and interactive exhibits related to the K-25 site.

Notes

References

External links

• K-25 History Center on-site museum
• K-25 Virtual Museum
• Historic photos of K25 by Ed Westcott
• Demolition of the north end of the K-25 building (Video)
• Historic American Engineering Record (HAER) documentation, filed under State Highway 58, Oak Ridge, Anderson County, TN: