The S-50 Project was the Manhattan Project's effort to produce enriched uranium by liquid thermal diffusion during World War II. It was one of three technologies for uranium enrichment pursued by the Manhattan Project.
The liquid thermal diffusion process was not one of the enrichment technologies initially selected for use in the Manhattan Project, and was developed independently by Philip H. Abelson and other scientists at the United States Naval Research Laboratory. This was primarily due to doubts about the process's technical feasibility, but inter-service rivalry between the United States Army and United States Navy also played a part.
Pilot plants were built at the Anacostia Naval Air Station and the Philadelphia Navy Yard, and a production facility at the Clinton Engineer Works in Oak Ridge, Tennessee. This was the only production-scale liquid thermal diffusion plant ever built. It could not enrich uranium sufficiently for use in an atomic bomb, but it could provide slightly enriched feed for the Y-12 calutrons and the K-25 gaseous diffusion plants. It was estimated that the S-50 plant had sped up production of enriched uranium used in the Little Boy bomb employed in the atomic bombing of Hiroshima by a week.
The S-50 plant ceased production in September 1945, but it was reopened in May 1946, and used by the United States Army Air Forces Nuclear Energy for the Propulsion of Aircraft (NEPA) project. The plant was demolished in the late 1940s.
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 Robert Frisch soon after, opened up the possibility of a nuclear chain reaction with uranium. Fears that a German atomic bomb project would develop nuclear weapons, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein–Szilard letter. This prompted preliminary research in the United States 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 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, 0.006 percent.
At the University of Birmingham in Britain, the Australian physicist Mark Oliphant assigned two refugee physicists—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 10 kg, which was small enough to be carried by a bomber of the day. Research into how uranium isotope separation (uranium enrichment) could be achieved assumed enormous importance. Frisch's first thought about how this could be achieved was with liquid thermal diffusion.
Liquid thermal diffusion
The liquid thermal diffusion process was based on the discovery by Carl Ludwig in 1856 and later Charles Soret in 1879, that when a temperature gradient is maintained in an originally homogeneous salt solution, after a time, a concentration gradient will also exist in the solution. This is known as the Soret effect. David Enskog in 1911 and Sydney Chapman in 1916 independently developed the Chapman–Enskog theory, which explained that when a mixture of two gases passes through a temperature gradient, the heavier gas tends to concentrate at the cold end and the lighter gas at the warm end. This was experimentally confirmed by Chapman and F. W. Dootson in 1916.
Since hot gases tend to rise and cool ones tend to fall, this can be used as a means of isotope separation. This process was first demonstrated by Klaus Clusius and Gerhard Dickel in Germany in 1938, who used it to separate isotopes of neon. They used an apparatus called a "column", consisting of a vertical tube with a hot wire down the center. In the United States, Arthur Bramley at the United States Department of Agriculture improved on this design by using concentric tubes with different temperatures.
Research and development Philip H. Abelson was a young physicist who had been awarded his PhD from the University of California on 8 May 1939. He was among the first American scientists to verify nuclear fission, reporting his results in an article submitted to the Physical Review in February 1939, and collaborated with Edwin McMillan on the discovery of neptunium. Returning to the Carnegie Institution in Washington, D.C., where he had a position, he became interested in isotope separation. In July 1940, Ross Gunn from the United States Naval Research Laboratory (NRL) showed him a 1939 paper on the subject by Harold Urey, and Abelson became intrigued by the possibility of using the liquid thermal diffusion process. He began experiments with the process at the Department of Terrestrial Magnetism at the Carnegie Institution. Using potassium chloride (KCl), potassium bromide (KBr), potassium sulfate () and potassium dichromate (), he was able to achieve a separation factor of 1.2 (20 percent) of the potassium-39 and potassium-41 isotopes.
The next step was to repeat the experiments with uranium. He studied the process with aqueous solutions of uranium salts, but found that they tended to be hydrolyzed in the column. Only uranium hexafluoride () seemed suitable. In September 1940, Abelson approached Ross Gunn and Lyman J. Briggs, the director of the National Bureau of Standards, who were both members of the National Defense Research Committee (NDRC) Uranium Committee. The NRL agreed to make $2,500 available to the Carnegie Institution to allow Abelson to continue his work, and in October 1940, Briggs arranged for it to be moved to the Bureau of Standards, where there were better facilities.
Uranium hexafluoride was not readily available, so Abelson devised his own method of producing it in quantity at the NRL, through fluorination of the more easily produced uranium tetrafluoride at . Initially, this small plant supplied uranium hexafluoride for the research at Columbia University, the University of Virginia and the NRL. In 1941, Gunn and Abelson placed an order for uranium hexafluoride with the Harshaw Chemical Company in Cleveland, Ohio, using Abelson's process. In early 1942, the NDRC awarded Harshaw a contract to build a pilot plant to produce of uranium hexafluoride per day. By the spring of 1942, Harshaw's pilot uranium hexafluoride plant was operational, and DuPont also began experiments with using the process. Demand for uranium hexafluoride soon rose sharply, and Harshaw and DuPont increased production to meet it.
Abelson erected eleven columns at the Bureau of Standards, all approximately in diameter, but ranging from high. Test runs were carried out with potassium salts, and then, in April 1941, with uranium hexafluoride. On 1 June 1941, Abelson became an employee of the NRL, and he moved to the Anacostia Naval Air Station. In September 1941, he was joined by John I. Hoover, who became his deputy. They constructed an experimental plant there with columns. Steam was provided by a gas-fired boiler. They were able to separate isotopes of chlorine, but the apparatus was ruined in November by the decomposition products of the carbon tetrachloride. The next run indicated 2.5% separation, and it was found that the optimal spacing of the columns was between . Abelson regarded a run on 22 June with a 9.6% result as the first successful test of liquid thermal diffusion with uranium hexafluoride. In July, he was able to achieve 21%.
Relations with the Manhattan Project The NRL authorized a pilot plant in July 1942, which commenced operation on 15 November. This time they used fourteen columns, with a separation of between them. The pilot plant ran without interruption from 3 to 17 December 1942. Colonel Leslie R. Groves, Jr., who had been designated to take charge of what would become known as the Manhattan Project (but would not do so for another two days), visited the pilot plant with the Deputy District Engineer of the Manhattan District, Lieutenant Colonel Kenneth D. Nichols on 21 September, and spoke with Gunn and Rear Admiral Harold G. Bowen, Sr., the director the NRL. Groves left with the impression that the project was not being pursued with sufficient urgency. The project was expanded, and Nathan Rosen joined the project as a theoretical physicist. Groves visited the pilot plant again on 10 December 1942, this time with Warren K. Lewis, a professor of chemical engineering from MIT, and three DuPont employees. In his report Lewis recommended that the work be continued.
The S-1 Executive Committee superseded the Uranium Committee on 19 June 1942, dropping Gunn from its membership in the process. It considered Lewis' report, and passed on its recommendation to Vannevar Bush, the director of the Office of Scientific Research and Development (OSRD), of which the S-1 Executive Committee was a part. The relationship between the OSRD and the NRL was not good; Bowen criticised it for diverting funds from the NRL. Bush was mindful of a 17 March 1942 directive from the president, Franklin D. Roosevelt, albeit on his advice, that the navy was to be excluded from the Manhattan Project. He preferred to work with the more congenial Secretary of War, Henry Stimson, over whom he had more influence.
James B. Conant, the chairman of the NDRC and the S-1 Executive Committee, was concerned that the navy was running its own nuclear project, but Bush felt that it did no harm. He met with Gunn at Anacostia on 14 January 1943, and explained the situation to him. Gunn responded that the navy was interested in nuclear marine propulsion for nuclear submarines. Liquid thermal diffusion was a viable means of producing enriched uranium, and all he needed was details about nuclear reactor design, which he knew was being pursued by the Metallurgical Laboratory in Chicago. He was unaware that it had already built Chicago Pile-1, a working nuclear reactor. Bush was unwilling to provide the requested data, but arranged with Rear Admiral William R. Purnell, a fellow member of the Military Policy Committee that ran the Manhattan Project, for Abelson's efforts to receive additional support.
The following week, Briggs, Urey, and Eger V. Murphree from the S-1 Executive Committee, along with Karl Cohen and W. I. Thompson from Standard Oil, visited the pilot plant at Anacostia. They were impressed with the simplicity of the process, but disappointed that no enriched uranium product had been withdrawn from the plant; production had been calculated by measuring the difference in concentration. They calculated that a liquid thermal diffusion plant capable of producing 1 kg per day of uranium enriched to 90% uranium-235 would require 21,800 columns, each with a separation factor of 30.7%. It would take 18 months to build, assuming the use of the Manhattan Project's overriding priority for materials. This included of scarce copper for the outer tubes and nickel for the inner, which would be required to resist corrosion by the steam and uranium hexafluoride respectively.