Zippe-type Centrifuge - Centrifuge Uranium Enrichment

Centrifuge Uranium Enrichment

Enriching uranium is difficult because the isotopes are practically identical in chemistry and very similar in weight: U-235 is only 1.26% lighter than U-238. Separation efficiency in a centrifuge depends on weight difference. Separation of uranium isotopes requires a centrifuge that can spin at 1,500 revolutions per second (90,000 RPM). If we assume a rotor diameter of 20 cm (actual rotor diameter is likely to be less), this corresponds to a linear speed of greater than Mach 2 (Mach 1 = 340 m/s). For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second (720 - 1500 RPM) during the spin cycle, while turbines in automotive turbochargers can run up to around 2500 - 3333 revolutions per second (150,000 - 200,000 RPM).

A Zippe-type centrifuge has a hollow, cylindrical rotor filled with gaseous uranium in the form of its hexafluoride. A pulsating magnetic field at the bottom of the rotor, similar to that used in an electric motor, is able to spin it quickly enough that the U-238 is thrown towards the edge. The lighter U-235 collects near the centre. The bottom of the gaseous mix is heated, producing convection currents that move the U-238 down. The U-235 moves up, where scoops collect it. Each centrifuge has one inlet and two output lines (corresponding to the heavy and the light fractions).

At the high speed of rotation, the gas is compressed close to the wall of the rotor. The rotor can be almost a meter in length (diameter is likely to be less than 10 cm) and a temperature gradient of 300C between the top and bottom of the rotor produces a very strong convection current. In addition, very strong coriolis forces produced add to the separation efficiency.

To reduce friction, the rotor spins in a vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact is the needle-like bearing that the rotor sits on. The three gas lines must be concentric with the fixed axis as the outer rim is spinning very quickly, and the seal is very important.

After the scientists were released from Soviet captivity in 1956, Gernot Zippe was surprised to find that engineers in the West were years behind in their centrifuge technology. He was able to reproduce his design at the University of Virginia in the United States, publishing the results, even though the Soviets had confiscated his notes. Dr. Zippe left the United States when he was effectively barred from continuing his research: The Americans classified the work as secret, requiring him to become an American citizen (he refused), return to Europe, or abandon his research. He returned to Europe where, during the 1960s, he and his colleagues made the centrifuge more efficient by changing the material of the rotor from aluminum to a stronger alloy called maraging steel, which allowed higher speed. This improved centrifuge design is used by the commercial company Urenco to produce enriched uranium fuel for nuclear power stations.

The exact details of advanced Zippe-type centrifuges are closely guarded secrets, but the efficiency of the centrifuges is improved by making them longer, and increasing their speed of rotation. To do so, even stronger materials, such as carbon fiber reinforced composite materials, are used; and various techniques are used to avoid forces causing destructive vibrations, including the use of flexible "bellows" to allow controlled flexing of the rotor, as well as careful speed control to ensure that the centrifuge does not operate for very long at speeds where resonance is a problem.

The Zippe-type centrifuge is difficult to build successfully and requires carefully machined parts. To give some idea of the precision required, it was reported in 2006 that the tiny amount of material deposited in fingerprints on Iran's prototype centrifuges were enough to cause the machines to shatter. However, compared to other enrichment methods, it is much cheaper and more energy-efficient, and can be used in relative secrecy. This makes it ideal for covert nuclear-weapons programs and possibly increases the risk of nuclear proliferation. Centrifuge cascades also have much less material held in the machine at any time, unlike gaseous diffusion plants.