When the researchers in the Manhattan Project sought to build an atomic or nuclear bomb from 1942 to 1945, they soon recognized that there were two elements that could represent the fissionable core of the weapon. One fissionable material was plutonium,which could be made in nuclear reactors.The second was an isotope [V] of uranium with an atomic weight of 235. However, uranium 235 or 235U was extremely rare, found with the more common 238U in refined uranium. Since the two isotopes were chemically identical and only different at the atomic level, separating the fissionable 235U from the 238U became a major technical and scientific challenge of the project.
Like many other aspects of the work on the Manhattan Engineer District in the design of the weapons and the manufacture of the materials needed, it is often not realistic to attribute the invention of particular processes and gadgets to specific individuals. Some individuals were given major credit for various aspects of the work, but it is more accurate to regard all of the research as the product of team effort.
Three methods of uranium separation were developed during the war. When the proportion of fissionable 235U was increased above its naturally occurring rate of 0.7 percent, the product was designated as enriched uranium. An enrichment that approached 90 percent 235U was regarded as sufficient for a fissionable core. Thus, even though the methods were thought of as separation, they were actually designed to separate the isotopes only partially, reducing the proportion of 238U and increasing the proportion of the fissionable 235U.
The three methods were electromagnetic separation, gaseous diffusion, and thermal diffusion. In the electromagnetic separation method, large devices called calutronswere built at Oak Ridge, Tennessee, so named because the concept had been developed at the University of California. In a calutron, a sample of uranium with both isotopes was heated and vaporized and then projected in a stream of ionized gas through a strong magnetic field. The heavier 238U isotope stream would hit the wall of the calutron chamber lower than the lighter 235U isotope. On the wall of the chamber, separate slots would collect the two isotopes into two collection containers. Use of the calutron was an extremely expensive and slow process, yielding only small amounts of the fissionable material.
In the second method, gaseous diffusion, uranium was compounded with fluorine into uranium hexafluoride, and as a gas it was passed through a series of pipes, or cascades, each of which had a filter made of a classified material. On each pass through the filter, the amount of 238U would be slightly diminished. As the material circulated through another cascade, the proportion of 235U would increase slightly. The gaseous diffusion plant at Oak Ridge was a vast building, not completed during the war. However, the product of the first cascades, put in operation during the summer of 1945, was somewhat enriched and could be used. It was calculated that if the ideal separating effect between the feed and the output of a single stage were approached, to achieve 99 percent pure 235U hexaflouride, roughly 4,000 stages would be required. Of the gas that passed through the barrier at any stage, only half would pass to the next higher stage, and the other half returned to an earlier stage in the cascade. Most of the material that emerged had been recycled over and over.
The third method, thermal diffusion, was initiated outside the Manhattan Project at the Philadelphia Navy Yard. There researchers participated in a completely independent research project that had as its focus the development of enriched uranium for small reactors that could be used to power ships and submarines. In thermal diffusion, the concept
was very much like that in a fractionating tower used in catalytic cracking of petroleum [V].A column would be filled with gaseous uranium, and the lighter compounds would be removed at the top of the column. Partially enriched uranium produced by this method was used as feedstock for the other two methods.
All three methods were very slow and cumbersome. By July 1945 the project had enough enriched uranium for one weapon and enough plutonium for two. Since the plutonium design was technically more complicated and had a greater chance of failure, one of the two plutonium weapons was tested on July 16, 1945, and the second held for use against Japan. The uranium weapon was dropped on Hiroshima on August 6, and the second plutonium weapon was dropped on Nagasaki on August 8. It was estimated that the United States would be able to produce one new weapon about every five or six weeks through 1946.