Courtesy of the Department of Energy

January 1945
The first stage of the K-25 gaseous diffusion plant at Oak Ridge comes on line.

The K-25 building ended up being 4 stories high and almost a half mile long. The building enclosed some 2 million square feet of space, making it the largest building in the world at the time. The final stages of the plant did not come on line until mid-August 1945. Maximum enrichment during the war was about 20%. The eventual cost of the K-25 complex would be over $500 million. Because the uranium hexafluoride gas which was pumped through the building attacked grease, new pump seals had to be developed that were both gas tight and greaseless. The new seal material developed came to be known after the war as Teflon. Even though the plant was fully automated, 9,000 employees working in three shifts were needed to operate the plant. In April 1945, a second gaseous diffusion plant, K-27, was started. It would come on line in Jan. 1946 and make all of the other uranium separation processes obsolete. After the war additional gaseous diffusion buildings were built to increase production of enriched uranium. The original K-25/K-27 facility was shut down in 1964 after operating without error for 20 years. The rest of the gaseous diffusion plant was shut down in 1987.

K-25 control room - Courtesy of the National Archives

This is a schematic flow diagram for isotope separation by gaseous diffusion showing three stages. The gas passing each barrier will be enriched in U-235 and the recycled gas enriched in U-238. The K-25 plant had 3122 stages. The barrier was the porous material through which the uranium hexafluoride molecules needed to pass to achieve a separation. Finding an acceptable barrier proved to be very difficult. The pores of the barrier had to be less than one millionth of an inch, uniform in size, could not become plugged up, and had to be rugged enough to withstand high pressure uranium hexafluoride gas, which is extremely corrosive to almost all materials. A barrier material was finally chosen in January 1944, well after construction of K-25 had begun. It is believed that the barrier was composed of sintered nickel powder, although the actual design is still classified. The barrier was used in the form of tubes about one inch in diameter. In all 5,174,000 individual barrier tubes having a total length of 6659 miles were used. The tubes were placed within 3122 barrel shaped cylinders called diffusers. Associated with each diffuser were two powerful blowers, one to carry the enriched portion of the gas forward to the next stage, the other to take the impoverished portion back down the line to begin a new series of reprocessings. Each diffuser and its blowers was known as a stage. The stages came in four different sizes ranging in dimensions from 11 feet 3 inches by 6 feet 8 1/2 inches for the largest (bottom stages), to 5 feet 6 inches by 3 feet 8 inches for the smallest (top stages). The length of the barrier tubes within the cylinders ranged from 89 3/8 inches to 38 3/8 inches.

From Lapp/Andrews, NUCLEAR RADIATION PHYSICS, copyright 1954.
Electronically reproduced by permission of Prentice Hall, Upper Saddle River, New Jersey.

K-25 diffusers - Courtesy of the Department of Energy

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