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2.2 Gas Centrifuge
Gas centrifuges are the most commonly used technology today for enriching uranium. The technology was considered in the U.S. during the Manhattan Project, but gaseous diffusion and electromagnetic separation were pursued instead for full scale production. The centrifuge was later developed in Russia by a team lead by Austrian and German scientists captured during the Second World War. The head of the experimentation group in Russia was eventually released and took the centrifuge technology first to the United States and then to Europe where he sought to develop its use in enriching commercial nuclear fuel.
The centrifuge is a common technology used routinely in a variety of applications such as separating blood plasma from the heavier red blood cells. In the enrichment process, uranium hexafluoride gas is fed into rapidly spinning cylinders. In order to achieve as much enrichment in each stage as possible, modern centrifuges can rotate at speeds approaching the speed of sound. It is this feature that makes the centrifuge process difficult to master, since the high rate of revolution requires that the centrifuge be sturdy, nearly perfectly balanced, and capable of operating in such a state for many years without maintenance. Inside the rotating centrifuge, the heavier molecules containing U-238 atoms move preferentially towards the outside of the cylinder, while the lighter molecules containing U-235 remain closer to the central axis. The gas in this cylinder is then made to circulate bottom to top driving the depleted uranium near the outer wall towards the top while the gas enriched in U-235 near the center is driven towards the bottom. These two streams (one enriched and one depleted) can then be extracted from the centrifuge and fed to adjoining stages to form a cascade just as was done with the diffusers in the gas diffusion plants. A schematic diagram of such a centrifuge is shown in Figure 4 below.
Figure 4: A schematic diagram of the cross section of a single gas centrifuge.
The rotating cylinder forces the heavier U-238 atoms towards the outside of the centrifuge while leaving the lighter U-235 more towards the middle. A bottom to top current allows the enriched and depleted streams to be separated and sent via pipes to subsequent stages. Like the gas diffusion process, it requires thousands to tens of thousands of centrifuge stages to enrich commercially or militarily significant quantities of uranium. In addition, like the gas diffusion plants, centrifuge plants require the use of special materials to prevent corrosion by the uranium hexafluoride, which can react with moisture to form a gas of highly corrosive hydrofluoric acid. One of the most important advantages to the gas centrifuge over the gas diffusion process, however, is that it requires 40 to 50 times less energy to achieve the same level of enrichment. The use of centrifuges also reduces the amount of waste heat generated in compressing the gaseous UF6, and thus reduces the amount of coolants, such as Freon, that would be required. A bank of centrifuges from an enrichment plant in use in Europe is shown in Figure 5.
Figure 5: A section of a typical cascade of centrifuge stages in a European uranium enrichment plant. The operative power of each centrifuge increases with the speed of revolution as well as with the height of the centrifuge while in a cascade each centrifuge also builds on the enrichment achieved in the previous stages.
Despite having a larger operative power in each stage compared to the gaseous diffusion process, the amount of uranium that can pass through each centrifuge stage in a given time is typically much smaller. Typical modern centrifuges can achieve approximately 2 to 4 SWU annually, and therefore in order to enrich enough HEU in one year to manufacture a nuclear weapon like that dropped on Hiroshima would require between three and seven thousand centrifuges. Such a facility would consume 580 to 816 thousand kWh of electricity, which could be supplied by less than a 100 kilowatt power plant. The use of modern weapon designs would reduce those numbers to just one to three thousand stages and 193 to 340 thousand kWh. More advanced centrifuge designs are expected to achieve up to ten times the enrichment per stage as current models which would further cut down on the number necessary for the clandestine production of HEU. The reported sale of older European based centrifuge technology to countries like Libya, Iran, and North Korea from the network run by A.Q. Khan, the former head of the Pakistani nuclear weapons program, highlights the concerns over the smaller size and power needs of the centrifuge enrichment process from a proliferation standpoint.
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