[U]ranium enrichment is the process that separates U-235 from U-238 in order to increase the proportion of the former isotope. Separation is measured by the kilogram separative work unit (SWU), representing the amount of uranium processed and the degree to which it is enriched. The gas centrifuge exploits the mass difference between these two isotopes (three neutrons) by spinning uranium hexafluoride gas (UF6) at extraordinarily high speeds (twice the speed of sound), forcing the lighter U-235 to the center, where it can be “scooped off” at the top. These centrifuges must be arranged in cascades, or groups of centrifuges, as each cascade enriches the material only slightly before feeding it into the next. Although this process may sound fairly simple, the specialized materials and precision engineering necessary are very difficult to achieve.
The necessary ingredient for the enrichment process, UF6, must be free of any impurities, as impurities may condense and trigger blockages in the valves and piping of the cascades, causing the centrifuges to crash. Once this gas is produced with the highest degree of purity, it is then ready to be fed into the centrifuge, a machine made of many complex parts. The main components are (1) rotor and end caps; (2) bearing and suspension systems; (3) electric motor and power supplies; (4) center post, scoops, and baffles; (5) the vacuum system; and (6) the casing. The first challenge is to acquire the specialized materials for these parts. High-strength, corrosion-resistant materials, such as maraging steel, aluminium alloys, titanium, glass-fiber resins, or carbon fiber, are essential for most of the aforementioned components. Maraging steel specifically provides not only protection but also the capacity for faster rotor speed.
The second challenge is to construct a perfectly balanced centrifuge rotor (an almost impossible task) that can rotate at supercritical speeds (about 100,000 rpm). In addition to the complex engineering necessary for the construction of other centrifuge parts, a method must be devised to control the temperature and convection in the vacuum. Now imagine replicating this precision engineering in cascades of about three thousand centrifuges.
Khan, Feroz Hassan. Eating Grass: The Making of the Pakistani Bomb. Stanford University Press; Stanford. 2012. p. 142