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The Zippe-type centrifuge is a
gas centrifuge A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radiu ...
designed to enrich the rare fissile isotope
uranium-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...
(235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. The Zippe design was originally developed in the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
by a team led by 60 Austrian and German scientists and engineers captured after
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, working in detention. In the West (and now generally) the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in (and contributions to) the Soviet program, Gernot Zippe. To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known (at least at a somewhat earlier stage of development) as a Kamenev centrifuge (after Evgeni Kamenev).Oleg Bukharin, Oleg
Russia’s Gaseous Centrifuge Technology and Uranium Enrichment Complex
2004.


Background

Natural uranium Natural uranium (NU or Unat) is uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes from ura ...
consists of three
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s; the majority (99.274%) is U-238, while approximately 0.72% is
U-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...
, fissile by thermal neutrons, and the remaining 0.0055% is U-234. If natural uranium is enriched to 3%
U-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...
, it can be used as fuel for light water
nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
s. If it is enriched to 90% uranium-235, it can be used for
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
s.


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 (note this applies only to uranium metal). Centrifuges need to work with a fluid rather than a solid, and this process used gaseous uranium hexafluoride. The relative mass difference between 235UF6 and 238UF6 is less than 0.86%. Separation efficiency in a centrifuge depends on absolute mass difference. Separation of uranium isotopes requires a
centrifuge A centrifuge is a device that uses centrifugal force to subject a specimen to a specified constant force - for example, to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby ...
that can spin at 1,500 revolutions per second (90,000
rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...
). If we assume a rotor diameter of 20 cm (as in some modern centrifuges), this would correspond to a centripetal acceleration of around 900,000 x g (around 42 times the max speed of a standard, lab benchtop microcentrifuge and between 0.9 and 9 times the max speed of a standard lab ultracentrifuge) or a linear speed of greater than Mach2 in air (Mach1 = sound velocity, in air ca. 340 m/s) and much more in UF6. For comparison, automatic
washing machine A washing machine (laundry machine, clothes washer, washer, or simply wash) is a machine designed to laundry, launder clothing. The term is mostly applied to machines that use water. Other ways of doing laundry include dry cleaning (which uses ...
s operate at only about 12 to 25 revolutions per second (720–1500rpm) during the spin cycle, while turbines in automotive
turbocharger In an internal combustion engine, a turbocharger (also known as a turbo or a turbosupercharger) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake air, forcing more air into th ...
s can run up to around 2500–3333 revolutions per second (150,000–200,000rpm). A Zippe-type centrifuge has a hollow, cylindrical rotor filled with gaseous uranium hexafluoride (UF6). A
rotating magnetic field A rotating magnetic field (RMF) is the resultant magnetic field produced by a system of Electromagnetic coil, coils symmetrically placed and supplied with Polyphase system, polyphase currents. A rotating magnetic field can be produced by a poly-phas ...
at the bottom of the rotor, as used in an
electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
, is able to spin it quickly enough that the UF6 is thrown towards the outer wall, with the 238UF6 enriched in the outermost layer and the 235UF6 enriched at the inside of this layer. The centrifugal force creates a pressure gradient: On the axis of the centrifuge there is practically vacuum, so that no mechanical feedthrough or seal is needed for the gas inlet and outlets; near the wall the UF6 reaches its saturation pressure, which in turn limits the rotation speed, because condensation must be avoided. In the so-called countercurrent centrifuge, the bottom of the gaseous mix can be heated, producing convection currents. But the countercurrent is usually stimulated mechanically by the scoop collecting the enriched fraction. In such a way, the enrichment in each horizontal layer is repeated (and thus multiplied) in the next layer, similarly as in column
distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
. One scoop is behind a perforated baffle that rotates with the centrifuge; it collects the 238UF6-rich fraction. The other scoop is without baffle. It slows down the gas rotation and thus increases the pressure towards the inside, so that also the 235UF6-rich fraction can be collected without pumping. Each centrifuge has one inlet on the axis and two output lines, one collecting the gas at the bottom and one at the top. Quantitatively, the radial pressure (or density) distribution can be given byWolfang Ehrfeld, Ursula Ehrfeld, Anreicherung von Uran-235, Gmelin Handbuch der Anorganischen Chemie. 8.Aufl.System-Nr.55: U-Uran. Erg.Bd.A2: Isotope. By C. Keller. Ed.: K.-C. Buschbeck, C. Keller. Berlin, Heidelberg, New York: Springer 1980p(r)= p(R)\exp(-(M\omega^2/2kT)(R^2-r^2)) where ''p'' is the pressure, ''r'' the variable radius and ''R'' its maximum, ''M'' the molecular mass, ω the angular velocity, ''k'' the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...
and ''T'' the temperature. (This equation is similar to the barometric formula.) Writing this equation for both isotopes and dividing, gives the (''r''-dependent) isotope ratio. It only contains Δ''M'' (not the relative mass difference Δ''M/M'') in the exponent. The radial enrichment factor then results by dividing through the initial isotope ratio. To calculate the total enrichment in a countercurrent centrifuge of height ''H'', one has to add a factor of ''H''/(''R''√2) in the exponent. According to Glaser, early centrifuges had rotor diameters of 7.4 to 15 cm and lengths of 0.3 to 3.2 m, and the peripheral speed was 350 to 500 m/s. The modern centrifuge TC-21 of
Urenco The Urenco Group is a British-German-Dutch nuclear fuel consortium operating several uranium enrichment plants in Germany, the Netherlands, United States, and United Kingdom. It supplies nuclear power stations in about 15 countries, and stat ...
has a diameter of 20 cm and a length of more than 5 m, spinning with 770 m/s. Centrus (formerly Usec) plans a centrifuge with 60 cm diameter, 12 m height and 900 m/s peripheral speed. A countercurrent of the gas is stimulated either mechanically or (less preferred) by a temperature gradient between the top and bottom of the rotor. With a countercurrent-to-feed ratio of 4, Glaser calculates a separation factor of 1,74 for a TC-21 centrifuge of 5 m height. Lowering this ratio (by increasing the feed) decreases the separation factor but increases the throughput and thus the productivity. To reduce friction, the rotor spins in a
vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
. Part of the rotor with the near-by housing acts as a molecular pump, which maintains the vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact (necessary only during start-up) is the conical
jewel bearing A jewel bearing is a plain bearing in which a metal spindle (tool), spindle turns in a gemstone, jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewels are typically made ...
on which the rotor sits. Both bearings contain measures for damping vibrations. The three gas lines enter the rotor on its axis. 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 The University of Virginia (UVA) is a Public university#United States, public research university in Charlottesville, Virginia, United States. It was founded in 1819 by Thomas Jefferson and contains his The Lawn, Academical Village, a World H ...
in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, publishing the results, even though the Soviets had confiscated his notes. Zippe left the United States when he was effectively barred from continuing his research: The Americans classified the work as secret, requiring him either to become a
U.S. citizen Citizenship of the United States is a legal status that entails Americans with specific rights, duties, protections, and benefits in the United States. It serves as a foundation of fundamental rights derived from and protected by the Consti ...
(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
aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
to maraging steel, an alloy with a longer fatigue life and longer breaking length, which allowed higher speed. This improved centrifuge design was long used by the commercial company
Urenco The Urenco Group is a British-German-Dutch nuclear fuel consortium operating several uranium enrichment plants in Germany, the Netherlands, United States, and United Kingdom. It supplies nuclear power stations in about 15 countries, and stat ...
to produce enriched uranium fuel for
nuclear power station A nuclear power plant (NPP), also known as a nuclear power station (NPS), nuclear generating station (NGS) or atomic power station (APS) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power st ...
s. More recently, they use (e.g. in their model TC-21) carbon fiber reinforced walls. The exact details of advanced Zippe-type centrifuges are closely guarded secrets. For example, the efficiency of the centrifuges is improved by increasing their speed of rotation. To do so, stronger materials, such as
carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers ( Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon comp ...
-reinforced
composite material A composite or composite material (also composition material) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a ...
s, are used; but details of the material and its protection against chemical attacks are proprietary. Such are also the various techniques that are used to avoid forces causing destructive (bending) vibrations: Lengthening of a (countercurrent) centrifuge improves the enrichment exponentially. But it also decreases the vibrational frequency of mechanical resonances, which increases the danger of catastrophic failure during start-up (as happened during the
Stuxnet Stuxnet is a Malware, malicious computer worm first uncovered on June 17, 2010, and thought to have been in development since at least 2005. Stuxnet targets supervisory control and data acquisition (SCADA) systems and is believed to be responsibl ...
event in Iran). Interrupting the cylindrical rotor by flexible bellows controls the low-frequency vibrations, and careful speed control during start-up helps to ensure that the centrifuge does not operate too long at speeds where
resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
is a problem. But more (proprietary) measures seem necessary. Therefore, Russia stayed with "subcritical" centrifuges (i.e., with small lengths around 0.5–1 m), whereas those of Urenco have lengths up to 10 m. The Zippe-type centrifuge is difficult to build successfully and requires carefully machined parts. However, compared to other enrichment methods, it is much cheaper and is faster to set up, consumes much less energy and requires little area for the plant. Therefore, it can be built in relative secrecy. This makes it ideal for covert nuclear-weapons programs and increases the risk of
nuclear proliferation Nuclear proliferation is the spread of nuclear weapons to additional countries, particularly those not recognized as List of states with nuclear weapons, nuclear-weapon states by the Treaty on the Non-Proliferation of Nuclear Weapons, commonl ...
. Centrifuge cascades also have much less material held in the machine at any time than gaseous diffusion plants.


Global usage

Pakistan's atomic bomb program developed the P1 and P2 centrifuges based on early designs of Urenco; the first two centrifuges that Pakistan deployed in larger numbers but reduce it after 1981 based on estimation require for critical mass. The P1 centrifuge uses an aluminum rotor, and the P2 centrifuge uses a maraging steel rotor, which is stronger, spins faster, and enriches more uranium per machine than the P1. In Pakistan, the Zippe-type centrifuge had a local designation and was known as Centrifuge Khan (after
Abdul Qadeer Khan Abdul Qadeer Khan (1 April 1936 – 10 October 2021) was a Pakistani Nuclear physics, nuclear physicist and metallurgist, metallurgical engineer. He is colloquially known as the "father of Pakistan and weapons of mass destruction, Pakistan's ...
). Russian sources dispute the account of Soviet centrifuge development given by Gernot Zippe. They cite Max Steenbeck as the German scientist in charge of the German part of the Soviet centrifuge effort, which was started by German refugee in the 1930s. The Soviets credit Steenbeck, Isaac Kikoin and with originating different valuable aspects of the design. They state Zippe was engaged in building prototypes for the project for two years from 1953. Since the centrifuge project was top secret the Soviets did not challenge any of Zippe's claims at the time.


Zippe-type centrifuge facilities

*
Khan Research Laboratories The Dr. A. Q. Khan Research Laboratories (shortened as KRL), is a federally funded research and development laboratory located in Kahuta at a short distance from Rawalpindi in Punjab, Pakistan. Established in 1976, the laboratory is best know ...
in Pakistan * National Enrichment Facility of
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...
in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
*
Urenco Group The Urenco Group is a British-German-Dutch nuclear fuel consortium operating several uranium enrichment plants in Germany, the Netherlands, United States, and United Kingdom. It supplies nuclear power stations in about 15 countries, and stat ...
in Great Britain, the Netherlands, and Germany * Russia (where it is called Kamenev centrifuge)


See also

*
Gas centrifuge A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radiu ...
*
Ultracentrifuge An ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as (approx. ). There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Both cla ...
*
Magnetic levitation Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation (physics), suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitation ...
*
Thrust bearing A thrust bearing is a particular type of rotary bearing. Like other bearings they permanently rotate between parts, but they are designed to support a predominantly axial load. Thrust bearings come in several varieties. *''Thrust ball bearing ...
* German nuclear weapon project * Germany and weapons of mass destruction *
Pakistan and weapons of mass destruction Pakistan is one of List of states with nuclear weapons, nine states that possess nuclear weapons, and is not party to the Nuclear Non-Profileration Treaty (NPT) and any of its provisions. As of 2025, multiple unofficial sources indicate a stoc ...
* Netherlands and weapons of mass destruction * Forced labor of Germans in the Soviet Union * Russian Alsos *
Stuxnet Stuxnet is a Malware, malicious computer worm first uncovered on June 17, 2010, and thought to have been in development since at least 2005. Stuxnet targets supervisory control and data acquisition (SCADA) systems and is believed to be responsibl ...


References

{{Reflist


External links


The Zippe Type - The Poor Man's Bomb
BBC Radio 4 BBC Radio 4 is a British national radio station owned and operated by the BBC. The station replaced the BBC Home Service on 30 September 1967 and broadcasts a wide variety of spoken-word programmes from the BBC's headquarters at Broadcasti ...
, 19 May 2004
Tracking the technology
Nuclear Engineering International, 31 August 2004
Slender and Elegant, It Fuels the Bomb
The New York Times ''The New York Times'' (''NYT'') is an American daily newspaper based in New York City. ''The New York Times'' covers domestic, national, and international news, and publishes opinion pieces, investigative reports, and reviews. As one of ...
, March 23, 2004
The Gas Centrifuge and Nuclear Proliferation
Marvin Miller, October 22, 2004
The gas centrifuge and nuclear weapons proliferation
Houston G. Wood, Alexander Glaser, and R. Scott Kemp, ''
Physics Today ''Physics Today'' is the membership magazine of the American Institute of Physics. First published in May 1948, it is issued on a monthly schedule, and is provided to the members of ten physics societies, including the American Physical Society. ...
'' page 40, September 2008
Long-term Energy Security Interests of the United States: Hearing Before the Congressional Subcommittee on Economic Stabilization, December 11, 1990
page 140, John E. Gray, Vice Chairman of the
Atlantic Council The Atlantic Council is an American think tank in the field of international affairs, favoring Atlanticism, founded in 1961. It manages sixteen regional centers and functional programs related to international security and global economic prosp ...
, citing Nuclear Fuel article Here Comes the Troika whilst testifying before the Subcommittee on Economic Stabilization (United States House Committee on Banking, Finance and Urbain Affaires) Centrifuges Isotope separation Uranium Nuclear technology in the Soviet Union Soviet inventions Germany–Soviet Union relations Nuclear technology in Pakistan Netherlands–Pakistan relations K ru:Центрифуга Циппе