Editing Manhattan Project (section)

==== 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>