Uranium that is found in the Earth's crust is made up of 99.289% 238U and 0.711% 235U, which are two isotopes of uranium. In order to use uranium in nuclear power plants, uranium ore must be mined, milled and then enriched to a higher percentage of 235U. Today, the Gas Centrifuge (a type of centrifuge is the main way we go about enriching uranium for nuclear fuel fabrication. In World War II, centrifuging was the first process considered for the enrichment of uranium. It was brought to the pilot plant stage before being abandoned for gaseous diffusion. This centrifuge method for the enrichment of uranium is a sophisticated version of the common method used for many years in biology and medicine for fractionating blood and other biological specimens.
Gas centrifugation, like all uranium enrichment processes, utilizes the difference in mass between the 235U isotope and the 238U isotope. Because 238U possesses three more neutrons in its nucleus compared to 235U, it has a higher mass. In gas centrifugation, the gas that is needing to be separated is pumped into the rotor. This is where the mass differential between the two isotopes is taken advantage of. The rotor spins at very high speeds, essentially creating a strong gravitational field. As seen in figure 2, the heavier gas gets pulled to the outside regions of the rotor, whereas the lighter gas stays towards the centre, each being siphoned off by a collection scoop. The amount of separation between the two isotopes depends mostly on the mass difference. The greater the difference in masses, the greater the amount of separation that is achieved. However, separation also greatly depends on the length of the rotor, and its speed of rotation. Because the mass difference between 235U and 238U is so small, a cascade is required to achieve significant amounts of separation.
The rotor is electromagnetically driven and is contained within an evacuated chamber (the casing). The UF6 gas is fed into the rotor at the axis of rotation, and the enriched and depleted gasses are withdrawn from the bottom and top of the rotor. As seen in figure 2, depicted by the arrows, an axial countercurrent is applied to the gas by means of a temperature gradient between the ends of the rotor, depicted by the shaded areas. The axial countercurrent just means that the gas within the rotor circulates from top to bottom, as shown by the arrows. As the gas moves downward near the rotor axis, the 238UF6 diffuses towards the outer wall. Consequently, the gas arriving at the upper scoop is slightly depleted in 235UF6, and the gas reaching the lower scoop is slightly enriched in 235UF6.
The gas centrifuge method is more economical than the gaseous diffusion method due to the fact that it requires 96% less electric power than a gaseous diffusion plant of the same separative work capacity. For example, a gaseous diffusion plant of 10 million SWU/yr requires 2700 MW of electrical capacity, where as the same equivalent plant using centrifuges requires 109 MW of electricity.