Editing Manhattan Project (section)

==== 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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