The Manhattan Project boiled down to two enormous manufacturing problems: explosive materials and explosive devices. Each of these problems was eventually resolved in its own binary fashion. The explosive material, or, more appropriately, fissile material, came in two flavors: uranium and plutonium. Owing to their different physical properties, it was necessary to create an individual explosive device, or bomb, for each of the two radioactive elements. The gun design, known as Little Boy, was designed for the uranium, and the implosion design, known as Fat Man, was designed for the plutonium weapon.
Although the creation of the bombs, particularly the implosion device, was a fiendishly complex exercise that required some of the greatest physics talent then in existence, the effort to create the processes for the separation of uranium 235 from uranium 238 was every bit the equal intellectual enterprise. As a manufacturing problem, the facilities devoted to the separation of uranium isotopes dwarfed the bomb-making project.
A 1951 AEC (Atomic Energy Commission) report entitled “Liquid Thermal Diffusion” reiterates what we describe:
The primary problem, other than finding circumstances under which a controlled chain reaction could be sustained, which faced scientists engaged in this country’s atomic energy program in early 1940 was the development of methods for separating uranium isotopes on a large scale. Time would not permit a gradual development of individual separation processes, followed by the full exploitation of the best method. Consequently it was necessary to launch a number of separation projects simultaneously.
One of those separation projects was Oak Ridge’s S-50 facility, and it went into full production seventy years ago this month on March 15, 1945. The S-50 uranium production plant used the separation technique known as liquid thermal diffusion.
A good overview of the liquid thermal diffusion technique can be found at the Atomic Archive:
Into the space between two concentric vertical pipes [Philip] Abelson placed pressurized liquid uranium hexafluoride. With the outer wall cooled by a circulating water jacket and the inner heated by high-pressure steam, the lighter isotope tended to concentrate near the hot wall and the heavier near the cold. Convection would in time carry the lighter isotope to the top of the column. Taller columns would produce more separation.
As a graduate student, Philip Abelson worked with Nobel Laureate Ernest Lawrence, himself the developer of another Oak Ridge-based uranium separation technique, electromagnetic separation. Abelson began his pioneering work on using liquid thermal diffusion to enrich uranium in July 1940, and he was one of the editors of the AEC report mentioned above. After several frustrating years of experimental work (part of the frustration resulted from Army vs. Navy squabbles over ownership of the technique), Abelson’s technique was sufficiently refined to build a production plant at Oak Ridge.
Liquid thermal diffusion had already been underway at a pilot plant at the Philadelphia Naval Yard, which was the site of what may have been the largest accidental release of radioactive materials during the Manhattan Project. That accident occurred in September 1944.
The S-50 plant, sited on the Clinch River in Tennessee, was essentially a copy of the Philadelphia Navy Yard pilot plant, which used 102 separation columns (the vertical pipes described in the quote above). S-50 replicated the Philadelphia plant 21 times. S-50 was built so that it could share the steam generated by the power plant that also fed the K-25 gaseous diffusion plant. It was to be completed in ninety days. The primary contractor, H. K. Ferguson Company, missed that deadline, but the plant started operation in October 1944 and, then, went into full production 70 years ago this month.
S-50 was the first step in the uranium enrichment process used during the Manhattan Project, and it took the uranium from 0.72% to 0.85% U-235.
March 1945 was an eventful month beyond the Manhattan Project, of course, with war raging on. Other wartime events that occurred then include a Japanese Fugo balloon that exploded at the Hanford Site for nuclear production, the B-29 firebombing of Japanese cities by the United States, and V-2 rocket attacks against London by the Germans. Though the end of the war was in sight, we shouldn’t forget that much of the world was still enmeshed in battle.