The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 1,000,000 g (9,800 km/s²). There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Both classes of instruments find important uses in molecular biology, biochemistry and polymer science. Theodor Svedberg invented the analytical ultracentrifuge in 1923, and won the Nobel Prize in Chemistry in 1926 for his research on colloids and proteins using the ultracentrifuge.
The vacuum ultracentrifuge was invented by Edward Greydon Pickels. It was his contribution of the vacuum which allowed a reduction in friction generated at high speeds. Vacuum systems also enabled the maintenance of constant temperature.
In 1946, Pickels cofounded Spinco (Specialized Instruments Corp.) and marketed an ultracentrifuge based on his design. Pickels, however, considered his design to be complicated and developed a more “foolproof” version. But even with the enhanced design, sales of the technology remained low, and Spinco almost went bankrupt. The company survived and was the first to commercially manufacture ultracentrifuges, in 1947. In 1949, Spinco introduced the Model L, the first preparative ultracentrifuge to reach a maximum speed of 40,000 rpm. In 1954, Beckman Instruments (now Beckman Coulter) purchased the company, forming the basis of its Spinco centrifuge division.
Preparative ultracentrifuge
Preparative ultracentrifuges are available with a wide variety of rotors suitable for a great range of experiments. Most rotors are designed to hold tubes that contain the samples. Swinging bucket rotors allow the tubes to hang on hinges so the tubes reorient to the horizontal as the rotor initially accelerates. Fixed angle rotors are made of a single block of metal and hold the tubes in cavities bored at a predetermined angle. Zonal rotors are designed to contain a large volume of sample in a single central cavity rather than in tubes. Some zonal rotors are capable of dynamic loading and unloading of samples while the rotor is spinning at high speed.
Preparative rotors are used in biology for pelleting of fine particulate fractions, such as cellular organelles (mitochondria, microsomes, ribosomes) and viruses. They can also be used for gradient separations, in which the tubes are filled from top to bottom with an increasing concentration of a dense substance in solution. Sucrose gradients are typically used for separation of cellular organelles. Gradients of caesium salts are used for separation of nucleic acids. After the sample has spun at high speed for sufficient time to produce the separation, the rotor is allowed to come to a smooth stop and the gradient is gently pumped out of each tube to isolate the separated components.
Zippe-type centrifuge
The Zippe-type centrifuge is a particular design of gas centrifuge. It was developed in the Soviet Union by a team of 60 German scientists working in detention, captured after World War II. The centrifuge is named after the team’s lead experimenter, Gernot Zippe.
Natural uranium consists of three isotopes; the majority (99.274 percent) is U-238, while approximately 0.72 percent is U-235 and the remaining 0.0055 percent is U-234. If natural uranium is enriched to contain 3 percent U-235, it can be used as fuel for light water nuclear reactors. If it is enriched to contain 90 percent Uranium-235 , it can be used for nuclear weapons.
Enriching uranium is difficult because the isotopes are very similar in weight: U-235 is only 1.26% lighter than u-238. It requires a centrifuge that can spin at 1,500 revolutions per second (90,000 RPM). For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second during the spin cycle.
The device 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 in the center. 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.
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.
After the captured scientists were released 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 or return to Europe. 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 it to spin even faster. 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 very careful control of the rotation speed to ensure that the centrifuge does not operate for very long at speeds where resonance is a problem.
The Zippe-type is difficult to build successfully, and it requires very 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[1]. However, compared to other enrichment methods, it is much cheaper 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 up in the machine at any time, unlike gaseous diffusion plants.
In 2004 Abdul Qadeer Khan, a Pakistani engineer, admitted to operating a smuggling ring responsible for supplying at least three countries with Zippe-type centrifuges.
Pakistan’s nuclear program developed the P1 and P2 centrifuges — the first two centrifuges that Pakistan deployed in large numbers. The P1 centrifuge uses an aluminum rotor, and the P2 centrifuge uses a maraging steel rotor, which is stronger, spins faster, and therefore enriches more uranium per machine than the P1 centrifuge’s aluminum rotor.
