Separation of isotopes by gaseous diffusion method
Answers
Once the power that was hidden in uranium became evident, the emphasis shifted to methods to separate the much more potent U-235 from its abundant relative, U-238. This question consumed thousands of hours and millions of dollars.
Scientists had concluded that enriched samples of uranium-235 were necessary for further research and that the isotope might serve as an efficient fuel source for an explosive device. "Enrichment" meant increasing the proportion of U-235, relative to U-238, in a uranium sample. This required separating the two isotopes and discarding U-238. Uranium-235 occurred in a ratio of 1:139 in natural uranium ore. Since they were chemically identical, they could not be separated by chemical means. Furthermore, with their masses differing by less than 1 percent, separation by physical means would be extremely difficult and expensive.
Nevertheless, scientists pressed forward on several complicated techniques of physical separation, all based on the small difference in atomic weight between the uranium isotopes. Manhattan Project director General Leslie Groves wanted to investigate as many possibilities as possible, and had the resources to simultaneously pursue multiple speculative projects.
Centrifuge
CentrifugationA centrifuge was the first device to separate chemical isotopes, used by Jesse Beams of the University of Virginia to separate chlorine-35 from chlorine-37 in 1934. In 1940, American physicists thought that the centrifuge was the best possibility for large-scale enrichment, and Beams received government money to attempt uranium enrichment via centrifuge.
Centrifugal force in a cylinder spinning rapidly on its vertical axis would separate a gaseous mixture of two isotopes. This is because the lighter U-235 isotope would be less affected by the action and could be drawn off at the top center of the cylinder. A cascade system composed of thousands of centrifuges could produce a rich mixture. Beams pioneered this method and received much of the early funding.
A high-speed centrifuge initially seemed promising for uranium enrichment, but the Manhattan Project failed to produce a workable model, and research stopped during the war. The centrifuge Beams constructed could separate U-235 from U-238, but required huge amounts of energy and could only sustain a short run before breaking down; in other words, it was not suited for industrial production. Manhattan Project scientists opted to pursue gaseous diffusion over gas centrifuges as the primary method for uranium isotope separation, and in January 1944 Army support for the gas centrifuge method was dropped.
In the early 1950s, however, a German POW in the Soviet Union produced an efficient and durable centrifuge. Because the centrifuge does not require massive capital investment or special equipment, it is the preferred method of separation today, and poses the biggest threat for proliferation.
While centrifuges were thought a lost cause in American nuclear engineering, scientists abroad continued to work on them. Soviet physicists felt, correctly, that gaseous diffusion did not make sense, since it would use more power than it could produce. Fritz Lange, a German émigré working for the Soviet program, set to work on constructing a centrifuge. All his designs, however, used a horizontal centrifuge, which would invariably break due to the pull of gravity on the underside.
Max Steenbeck and Gernot Zippe, prisoners of war from Germany and Austria, were the next to tackle problem. Working in a POW camp, Steenbeck and Zippe made several improvements on Beam's design. The centrifuge rotor was made to spin on the tip of a needle, like a top, thus eliminating friction, while the centrifuge was spun via an electromagnetic field. Though it took more research to adapt this to a scalable design, the basic design problems had been solved.
After release from the prisoner camp, Steeneck returned to East Germany, while Zippe traveled the world, marketing his centrifuge.
In the late 1950s, Zippe worked as a researcher at the University of Virginia, where he built a centrifuge largely from memory. Atomic Energy Commission officials marveled at the simplicity of his design. Subsequent US efforts to build a centrifuge progressed very smoothly. Because a functioning uranium-enrichment centrifuge can be built fairly quickly by a small team without prior experience, there are little technical barriers to proliferation. This design would spread to become the main method for enriching uranium, and remains so today.
