After the Japanese attacked Pearl Harbor in December of 1942, the Manhattan project ramped up with millions of dollar and thousands of staff. Four major deposits of uranium ore had already been identified and efforts were being made to obtain ore from the three that were in Allied hands. In November of 1942 it had been determined that there should be sufficient ore available to produce an atomic weapon. Ore from the Belgian Congo, Ontario, Canada and a mine in Colorado was being collected during 1942.
The ore was dissolved in nitric acid to produce uranyl nitrate. Ether was added to the solution to remove impurities. The solution was then heated to produce uranium trioxide which was ultimate reduced to pure uranium dioxide. The Ames process was developed to produce pure uranium metal after other methods failed. Uranium dioxide was converted to uranium tetraflouride which was mixed with powdered magnesium and heated in a sealed metal tube to produce pure uranium metal.
The big problem was separating the U-235 isotope from U-238. Only .7 % of uranium ore is U-235. It was estimated that the percentage of U-235 needed to create a bomb was around 90%. Research and development of separation methods proceeded during the early 1940s.
Converting uranium to uranium hexafluoride gas was a necessary first step in isotope separation. In a centrifuge, gas with the lighter isotope would move further than the heavier isotope. Feeding the output of one centrifuge into the next would theoretically allow the needed enrichment. Unfortunately, attempts to use centrifuges for separation were unsuccessful due to technical problems with running the big centrifuges at very high speeds for extended periods of time.
Electromagnetic separation was a known technology which used magnetic fields to deflect charged particles based on mass. Copper was in short supply so tons of silver were used instead to build production systems. Despite mechanical problems and efficiency, this process was used to enrich uranium to 15% U-235.
The third process was based on the idea that gases of different molecular weight will pass through membranes at different rates. With one chamber feeding the next, a cascade of these cells could enrich uranium hexafluoride gas up to 7% U-235. This resultant product could be used to feed other processes such as the electromagnetic separation system.
The final process developed was referred to as thermal diffusion. When there is a vertical thermal gradient in a chamber full of a mixture of gases, the heavier gas will concentrate in the lower cooler part of the chamber and the lighter gas will collect in the warmer upper part of the chamber. This was a new idea and was not part of the original attempts to separate isotopes of uranium. Fifty foot columns with three tubes were constructed. Steam and water created the thermal gradient. This process was able to enrich uranium from .7% U-235 to .9% U-235.
In 1945, all three of these processes were used in series to enrich uranium. The thermal diffusion plant enriched uranium from .7% U-235 to .9% U-235. This was fed to the gaseous diffusion plant where the enrichment reached 23% U-235. The gaseous diffusion plant fed the electromagnetic separator which enriched to 89% U-235 which was sufficient for weapons production.
Electromagnetic isotope separation “racetrack”:
