Editing Manhattan Project (section)

=== Isotope separation ===
Natural uranium consists of 99.3% uranium-238 and 0.7% uranium-235, but as only the latter is [[fissile]] it must be physically separated from the more plentiful isotope. Various methods were considered for [[uranium enrichment]], most of which was carried out at Oak Ridge.<ref>{{harvnb|Smyth|1945|pp=154–156}}.</ref> The most obvious technology, the centrifuge, failed, but electromagnetic separation, gaseous diffusion, and thermal diffusion technologies were all successful and contributed to the project. In February 1943, Groves came up with the idea of using the output of some plants as the input for others.<ref>{{harvnb|Jones|1985|p=157}}.</ref>

[[File:Clinton Engineer Works.png|thumb|center|upright=3.2|Oak Ridge hosted several uranium separation technologies. The Y-12 electromagnetic separation plant is in the upper right. The K-25 and K-27 gaseous diffusion plants are in the lower left, near the S-50 thermal diffusion plant. The X-10 was for plutonium production.|alt=Contour map of the Oak Ridge area. There is a river to the south, while the township is in the north.]]

==== Centrifuges ====
The centrifuge process was regarded as the only promising separation method in April 1942.<ref>{{harvnb|Hewlett|Anderson|1962|pp=22–23}}.</ref> [[Jesse Beams]] had developed such a process in the 1930s, but had encountered technical difficulties. In 1941 he began working with [[uranium hexafluoride]], the only known gaseous compound of uranium, and was able to separate uranium-235. At Columbia, [[Karl P. Cohen]] produced a body of mathematical theory making it possible to design a centrifugal separation unit, which Westinghouse undertook to construct.<ref>{{harvnb|Hewlett|Anderson|1962|p=30}}.</ref>

Scaling this up to a production plant presented a formidable technical challenge. Urey and Cohen estimated that producing a kilogram (2.2&nbsp;lb) of uranium-235 per day would require up to 50,000 centrifuges with {{convert|1|m|adj=on|sp=us}} rotors, or 10,000 centrifuges with {{convert|4|m|adj=on|sp=us}} rotors, assuming that 4-meter rotors could be built. The prospect of keeping so many rotors operating continuously at high speed appeared daunting,<ref>{{harvnb|Hewlett|Anderson|1962|p=64}}.</ref> and when Beams ran his experimental apparatus, he obtained only 60% of the predicted yield, indicating that more centrifuges were required. Beams, Urey and Cohen then began work on a series of improvements which promised to increase efficiency. However, frequent failures of motors, shafts and bearings at high speeds delayed work on the pilot plant.<ref>{{harvnb|Hewlett|Anderson|1962|pp=96–97}}.</ref>

In November 1942 the centrifuge process was abandoned by the Military Policy Committee.<ref>{{harvnb|Nichols|1987|p=64}}.</ref> Successful gas centrifuges of the [[Zippe-type centrifuge|Zippe-type]] design were instead developed in the Soviet Union after the war. It eventually became the preferred method of uranium isotope separation, being far more economical.<ref>{{harvnb|Kemp|2012|pp=281–287, 291–297}}.</ref>

==== Electromagnetic separation ====
{{Main|Y-12 Project}}
Electromagnetic isotope separation was developed at the University of California Radiation Laboratory. This method employed devices known as [[calutron]]s. The name was derived from the words ''California'', ''university'' and ''cyclotron''.<ref name="Jones, pp. 117-119">{{harvnb|Jones|1985|pp=117–119}}.</ref> In the electromagnetic process, a magnetic field deflected charged particles according to mass.<ref>{{harvnb|Smyth|1945|pp=164–165}}.</ref> The process was neither scientifically elegant nor industrially efficient.<ref name="Fine & Remington, p. 684">{{harvnb|Fine|Remington|1972|p=684}}.</ref> Compared with a gaseous diffusion plant or a nuclear reactor, an electromagnetic separation plant would consume more scarce materials, require more manpower to operate, and cost more to build. Nonetheless, the process was approved because it was based on proven technology and therefore represented less risk. Moreover, it could be built in stages, and rapidly reach industrial capacity.<ref name="Jones, pp. 117-119" />

[[File:Alpha 1 racetrack, Uranium 235 electromagnetic separation plant, Manhattan Project, Y-12 Oak Ridge.jpg|thumb|left|Alpha I racetrack at Y-12|alt=A large oval-shaped structure]]
Marshall and Nichols discovered that the electromagnetic isotope separation process would require {{convert|5000|ST|abbr=off}} of copper, which was in desperately short supply. However, silver could be substituted, in an 11:10 copper to silver ratio. On 3 August 1942, Nichols met with [[United States Deputy Secretary of the Treasury|Under Secretary of the Treasury]] [[Daniel W. Bell]] and asked for the transfer of 6,000 tons of silver bullion from the [[West Point Bullion Depository]].<ref>{{harvnb|Nichols|1987|p=42}}.</ref> Ultimately {{convert|14700|ST|t ozt|abbr=off}} were used.<ref name="Jones, p. 133" /> The {{convert|1000|ozt|kg|adj=on}} silver bars were cast into cylindrical billets, extruded into strips, and wound onto magnetic coils.<ref name="Jones, p. 133">{{harvnb|Jones|1985|p=133}}.</ref><ref>{{harvnb|Hewlett|Anderson|1962|p=153}}.</ref>

[[File:Y12 Calutron Operators.jpg|thumb|The [[Calutron Girls]] were young women who monitored calutron control panels at Y-12. Gladys Owens, seated in the foreground, was unaware of what she had been involved in.<ref>{{cite web |url=http://smithdray1.net/angeltowns/or/go.htm |publisher=SmithDRay |title=The Calutron Girls |access-date=22 June 2011}}</ref>|alt=A long corridor with many consoles with dials and switches, attended by women seated on high stools]]
Responsibility for the design and construction of the electromagnetic separation plant, which came to be called [[Y-12 National Security Complex|Y-12]], was assigned to Stone & Webster in June 1942. The design called for five first-stage processing units, known as Alpha racetracks, and two units for final processing, known as Beta racetracks. In September 1943 Groves authorized construction of four more racetracks, known as Alpha II. Construction began in February 1943.<ref>{{harvnb|Jones|1985|pp=126–132}}.</ref> The second Alpha I was operational at the end of January 1944, the first Beta and first and third Alpha I's came online in March, and the fourth Alpha I was operational in April. The four Alpha II racetracks were completed between July and October 1944.<ref>{{harvnb|Jones|1985|pp=138–139}}.</ref> [[Tennessee Eastman]] was contracted to manage Y-12.<ref>{{harvnb|Jones|1985|p=140}}.</ref> The calutrons were turned over to trained Tennessee Eastman operators known as the [[Calutron Girls]].<ref>{{harvnb|Nichols|1987|p=131}}.</ref>

The calutrons initially enriched the uranium-235 content to between 13% and 15%, and shipped the first few hundred grams of this to Los Alamos in March 1944. Only 1 part in 5,825 of the uranium feed emerged as product. Much of the rest was splattered over equipment in the process. Strenuous recovery efforts helped raise production to 10% of the uranium-235 feed by January 1945. In February the Alpha racetracks began receiving slightly enriched (1.4%) feed from the new S-50 thermal diffusion plant, and the next month they received enhanced (5%) feed from the K-25 gaseous diffusion plant. By August, K-25 was producing uranium sufficiently enriched to feed directly into the Beta tracks.<ref>{{harvnb|Jones|1985|pp=143–148}}.</ref>

==== Gaseous diffusion ====
{{Main|K-25}}
The most promising but also the most challenging method of isotope separation was gaseous diffusion. [[Graham's law]] states that the rate of [[effusion]] of a gas is inversely proportional to the square root of its [[molecular mass]], so in a box containing a semi-permeable membrane and a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules. The idea was that such boxes could be formed into a cascade of pumps and membranes, with each successive stage containing a slightly more enriched mixture. Research into the process was carried out at Columbia University by a group that included Harold Urey, [[Karl P. Cohen]], and [[John R. Dunning]].<ref>{{harvnb|Hewlett|Anderson|1962|pp=30–32, 96–98}}</ref>

[[File:K-25 aerial view.jpg|thumb|left|Oak Ridge K-25 plant|alt=Oblique aerial view of an enormous U-shaped building]]
In November 1942 the Military Policy Committee approved the construction of a 600-stage gaseous diffusion plant.<ref>{{harvnb|Hewlett|Anderson|1962|p=108}}.</ref> On 14 December, [[M. W. Kellogg]] accepted an offer to construct the plant, which was codenamed K-25. A separate corporate entity called Kellex was created for the project.<ref>{{harvnb|Jones|1985|pp=150–151}}.</ref> The process faced formidable technical difficulties. The highly corrosive gas uranium hexafluoride had to be used as no substitute could be found, and the motors and pumps had to be vacuum tight and enclosed in inert gas. The biggest problem was the design of the barrier, which had to be strong, porous and resistant to corrosion. Edward Adler and Edward Norris created a mesh barrier from electroplated nickel. A six-stage pilot plant was built at Columbia to test the process, but the prototype proved to be too brittle. A rival barrier was developed from powdered nickel by Kellex, the [[Bell Telephone Laboratories]] and the [[Bakelite]] Corporation. In January 1944, Groves ordered the Kellex barrier into production.<ref>{{harvnb|Jones|1985|pp=154–157}}.</ref><ref>{{harvnb|Hewlett|Anderson|1962|pp=126–127}}.</ref>

Kellex's design for K-25 called for a four-story {{convert|0.5|mi|km|adj=on}} long U-shaped structure containing 54 contiguous buildings. These were divided into nine sections containing cells of six stages. A survey party began construction by marking out the {{convert|500|acre|km2|adj=on}} site in May 1943. Work on the main building began in October 1943, and the six-stage pilot plant was ready for operation on 17 April 1944. In 1945 Groves canceled the upper stages, directing Kellex to instead design and build a 540-stage side feed unit, which became known as K-27. Kellex transferred the last unit to the operating contractor, [[Union Carbide]] and Carbon, on 11 September 1945. The total cost, including the K-27 plant completed after the war, came to $480&nbsp;million.<ref>{{harvnb|Jones|1985|pp=158–165}}.</ref>

The production plant commenced operation in February 1945, and as cascade after cascade came online, the quality of the product increased. By April 1945, K-25 had attained a 1.1% enrichment, and the output of the S-50 thermal diffusion plant began being used as feed. Some product produced the next month reached nearly 7% enrichment. In August, the last of the 2,892 stages commenced operation. K-25 and K-27 achieved their full potential in the early postwar period, when they eclipsed the other production plants and became the prototypes for a new generation of plants.<ref>{{harvnb|Jones|1985|pp=167–171}}.</ref>

==== Thermal diffusion ====
{{Main|S-50 Project}}
The thermal diffusion process was based on [[Sydney Chapman (mathematician)|Sydney Chapman]] and [[David Enskog]]'s [[Chapman–Enskog theory|theory]], which explained that when a mixed gas passes through a temperature gradient, the heavier one tends to concentrate at the cold end and the lighter one at the warm end.<ref>{{harvnb|Smyth|1945|pp=161–162}}.</ref> It was developed by US Navy scientists, but was not one of the enrichment technologies initially selected for use in the Manhattan Project. This was primarily due to doubts about its technical feasibility, but the inter-service rivalry between the Army and Navy also played a part.<ref>{{harvnb|Jones|1985|p=172}}.</ref> The Naval Research Laboratory continued the research under Philip Abelson's direction, but there was little contact with the Manhattan Project until April 1944, when [[Captain (United States O-6)|Captain]] [[William S. Parsons]], the naval officer in charge of ordnance development at Los Alamos, brought Oppenheimer news of encouraging progress on thermal diffusion. Oppenheimer informed Groves, who approved construction of a thermal plant on 24 June 1944.<ref>{{harvnb|Jones|1985|pp=175–177}}.</ref>

[[File:S50plant.jpg|thumb|The S-50 plant is the dark building to the upper left behind the Oak Ridge powerhouse (with smokestacks).|alt=A factory with three smoking chimneys on a river bend, viewed from above]]
Groves contracted with the H. K. Ferguson Company of [[Cleveland]], Ohio, to build the thermal diffusion plant, which was designated S-50.<ref>{{harvnb|Hewlett|Anderson|1962|pp=170–172}}.</ref> Plans called for the installation of 2,142 {{convert|48|ft|m|adj=mid|-tall}} diffusion columns arranged in 21 racks. Inside each column were three concentric tubes. Steam, obtained from the nearby K-25 powerhouse{{efn|It is necessary to distinguish between the K-25 gaseous diffusion plant and the K-25 power plant. The latter provided energy to both the K-25 gaseous diffusion plant and the S-50 thermal diffusion plant.}} at a pressure of {{convert|100|psi}} and temperature of {{convert|545|F|C}}, flowed downward through the innermost {{convert|1.25|in|adj=on}} nickel pipe, while water at {{convert|155|F|C}} flowed upward through the outermost iron pipe. The uranium hexafluoride flowed in the middle copper pipe, and isotope separation of the uranium occurred between the nickel and copper pipes.<ref>{{harvnb|Jones|1985|pp=178–179}}.</ref> Work commenced on 9 July 1944, and S-50 began partial operation in September. Leaks limited production and forced shutdowns over the next few months, but in June 1945 the S-50 plant produced {{convert|12,730|lb}} of slightly enriched product.<ref>{{harvnb|Jones|1985|pp=180–183}}.</ref>

By March 1945, all 21 production racks were operating. Initially the output of S-50 was fed into Y-12, but starting in March 1945 all three enrichment processes were run in series. S-50 became the first stage, enriching the uranium from 0.71% to 0.89% uranium-235. This was then fed into the gaseous diffusion process in the K-25 plant, which produced a product enriched to about 23%. In turn, this was fed into Y-12,<ref>{{harvnb|Hewlett|Anderson|1962|pp=300–302}}.</ref> which boosted it to about 89%, sufficient for use in nuclear weapons. About {{convert|50|kg}} of uranium enriched to 89% was delivered to Los Alamos by July 1945. The entire 50&nbsp;kg, along with some 50%-enriched, averaging out to about 85% enriched, were used in the first [[Little Boy]] bomb.<ref name="Hansen, p. 112">{{harvnb|Hansen|1995b|p=V-112}}.</ref>

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