![Countercurrent Gas Centrifuge](https://upload.wikimedia.org/wikipedia/commons/b/b4/Countercurrent_Gas_Centrifuge.svg)
A gas centrifuge is a device that performs
isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
of gases. A
centrifuge
A centrifuge is a device that uses centrifugal force to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities (e.g. cream from milk) or ...
relies on the principles of
centrifugal force
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container.
A prominent use of gas centrifuges is for the separation of
uranium-235
Uranium-235 (235U 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 exis ...
(
235U) from
uranium-238
Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it ...
(
238U). The gas centrifuge was developed to replace the
gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through semipermeable membranes. This produces a slight separation between the molecules containing uranium-235 (235U) and uranium-2 ...
method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in series, that achieve successively higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.
Centrifugal process
The
centrifuge
A centrifuge is a device that uses centrifugal force to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities (e.g. cream from milk) or ...
relies on the force resulting from centrifugal
acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
to separate molecules according to their mass, and can be applied to most fluids.
[Basics of Centrifuge - Cole Parmer](_blank)
/ref> The dense (heavier) molecules move towards the wall and the lighter ones remain close to the center. The centrifuge consists of a rigid body rotor rotating at full period at high speed. Concentric gas tubes located on the axis of the rotor are used to introduce feed gas into the rotor and extract the heavier and lighter separated streams. For 235U production, the heavier stream is the waste stream and the lighter stream is the product stream. Modern Zippe-type centrifuge
The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in ...
s are tall cylinders spinning on a vertical axis, with a vertical temperature gradient applied to create a convective circulation rising in the center and descending at the periphery of the centrifuge. Diffusion between these opposing flows increases the separation by the principle of countercurrent multiplication
A countercurrent mechanism system is a mechanism that expends energy to create a concentration gradient.
It is found widely in nature and especially in mammalian organs. For example, it can refer to the process that is underlying the process of ...
.
In practice, since there are limits to how tall a single centrifuge can be made, several such centrifuges are connected in series. Each centrifuge receives one input and produces two output lines, corresponding to light and heavy fractions
A fraction (from la, fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight ...
. The input of each centrifuge is the output (light) of the previous centrifuge and the output (heavy) of the following stage. This produces an almost pure light fraction from the output (light) of the last centrifuge and an almost pure heavy fraction from the output (heavy) of the first centrifuge.
Gas centrifugation process
The gas centrifugation process uses a unique design that allows gas to constantly flow in and out of the centrifuge. Unlike most centrifuges which rely on batch processing
Computerized batch processing is a method of running software programs called jobs in batches automatically. While users are required to submit the jobs, no other interaction by the user is required to process the batch. Batches may automatically ...
, the gas centrifuge uses continuous processing, allowing cascading, in which multiple identical processes occur in succession.
The gas centrifuge consists of a cylindrical rotor
Rotor may refer to:
Science and technology
Engineering
*Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
* Helicopter rotor, the rotary wing(s) of a rotorcraft ...
, a casing, an electric motor
An electric motor is an Electric machine, electrical 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 Electromagneti ...
, and three lines for material to travel. The gas centrifuge is designed with a casing that completely encloses the centrifuge.[Gas Centrifuge Uranium Enrichment](_blank)
/ref> The cylindrical rotor is located inside the casing, which is evacuated of all air to produce a near frictionless rotation when operating. The motor spins the rotor, creating the centrifugal force on the components as they enter the cylindrical rotor. This force acts to separate the molecules of the gas, with heavier molecules moving towards the wall of the rotor, and the lighter molecules towards the central axis. There are two output lines, one for the fraction enriched in the desired isotope (in uranium separation, this is U-235), and one depleted in it. The output lines take these separations to other centrifuges to continue the centrifugation process.[What is a Gas Centrifuge?](_blank)
The process begins when the rotor is balanced in three stages. Most of the technical details on gas centrifuges are difficult to obtain because they are shrouded in "nuclear secrecy".
The early centrifuges used in the UK used an alloy body wrapped in epoxy impregnated glass fibre. Dynamic balancing of the assembly was accomplished by adding small traces of epoxy at the locations indicated by the balancing test unit. The motor was usually a pancake type located at the bottom of the cylinder. The early units were typically around 2 metres long (approx.), but subsequent developments gradually increased the length. The present generation are over 4 metres in length. The bearings are gas-based devices, as mechanical bearings would not survive at the normal operating speeds of these centrifuges.
A section of centrifuges would be fed with variable-frequency AC from an electronic (bulk) inverter, which would slowly ramp them up to the required speed, generally in excess of 50,000 rpm. One precaution was to quickly get past frequencies at which the cylinder was known to suffer resonance problems. The inverter is a high frequency unit capable of operating at frequencies around 1 kilohertz. The whole process is normally silent; if a noise is heard coming from a centrifuge, it is a warning of failure (which normally occurs very quickly). The design of the cascade normally allows for the failure of at least one centrifuge unit without compromising the operation of the cascade. The units are normally very reliable, with early models having operated continuously for over 30 years.
Later models have steadily increased the rotation speed of the centrifuges, as it is the velocity of the centrifuge wall that has the most effect on the separation efficiency.
A feature of the cascade system of centrifuges is that it is possible to increase plant throughput incrementally, by adding cascade "blocks" to the existing installation at suitable locations, rather than having to install a completely new line of centrifuges.
Concurrent and countercurrent centrifuges
The simplest gas centrifuge is the concurrent centrifuge, where separative effect is produced by the centrifugal effects of the rotor's rotation. In these centrifuges, the heavy fraction is collected at the periphery of the rotor, and the light fraction from nearer the axis of rotation.
Inducing a countercurrent flow uses countercurrent multiplication
A countercurrent mechanism system is a mechanism that expends energy to create a concentration gradient.
It is found widely in nature and especially in mammalian organs. For example, it can refer to the process that is underlying the process of ...
to enhance the separative effect. A vertical circulating current is set up, with the gas flowing axially along the rotor walls in one direction, and a return flow closer to the center of the rotor. The centrifugal separation continues as before (heavier molecules preferentially moving outwards), which means that the heavier molecules are collected by the wall flow, and the lighter fraction collects at the other end. In a centrifuge with downward wall flow, the heavier molecules collect at the bottom. The outlet scoops are then placed at the ends of the rotor cavity, with the feed mixture injected along the axis of the cavity (ideally, the injection point is at the point where the mixture in the rotor is equal to the feed).
This countercurrent flow can be induced mechanically or thermally, or a combination. In mechanically induced countercurrent flow, the arrangement of the (stationary) scoops and internal rotor structures are used to generate the flow. A scoop interacts with the gas by slowing it, which tends to draw it into the centre of the rotor. The scoops at each end induce opposing currents, so one scoop is protected from the flow by a "baffle": a perforated disc within the rotor which rotates along with the gas—at this end of the rotor, the flow will be outwards, towards the rotor wall. Thus, in a centrifuge with a baffled top scoop, the wall flow is downwards, and heavier molecules are collected at the bottom.
Thermally induced convection currents
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
can be created by heating the bottom of the centrifuge and/or cooling the top end.
Separative work units
The separative work unit
Separative work – the amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units which are so calculated as to be proport ...
(SWU) is a measure of the amount of work done by the centrifuge and has units of mass (typically ''kilogram separative work unit''). The work necessary to separate a mass of feed of assay into a mass of product assay , and tails of mass and assay is expressed in terms of the number of separative work units needed, given by the expression
:
:where is the value function The value function of an optimization problem gives the value attained by the objective function at a solution, while only depending on the parameters of the problem. In a controlled dynamical system, the value function represents the optimal payof ...
, defined as
:
Practical application of centrifugation
Separating uranium-235 from uranium-238
The separation of uranium requires the material in a gaseous form; uranium hexafluoride
Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
(UF6) is used for uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
. Upon entering the centrifuge cylinder, the UF6 gas is rotated at a high speed. The rotation creates a strong centrifugal force that draws more of the heavier gas molecules (containing the U-238) toward the wall of the cylinder, while the lighter gas molecules (containing the U-235) tend to collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage.
Separation of zinc isotopes
For some uses in nuclear technology, the content of zinc-64
Naturally occurring zinc (30Zn) is composed of the 5 stable isotopes 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn with 64Zn being the most abundant (48.6% natural abundance). Twenty-five radioisotopes have been characterised with the most abundant and stabl ...
in zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
metal has to be lowered in order to prevent formation of radioisotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
s by its neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emittin ...
. Diethyl zinc
Diethylzinc (C2H5)2Zn, or DEZ, is a highly pyrophoric and reactive organozinc compound consisting of a zinc center bound to two ethyl groups. This colourless liquid is an important reagent in organic chemistry. It is available commercially as a sol ...
is used as the gaseous feed medium for the centrifuge cascade. An example of a resulting material is depleted zinc oxide Depleted zinc oxide (DZO) is a zinc oxide depleted in the zinc isotope with the atomic mass 64, and used as a corrosion inhibitor in nuclear pressurized water reactors.
The depletion of 64Zn is necessary, because this isotope is transformed into ...
, used as a corrosion inhibitor
In chemistry, a corrosion inhibitor or anti-corrosive is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness ...
.
History
Suggested in 1919, the centrifugal process was first successfully performed in 1934. American scientist Jesse Beams
Jesse Wakefield Beams (December 25, 1898 in Belle Plaine, Kansas – July 23, 1977) was an American physicist at the University of Virginia.
Biography
Beams completed his undergraduate B.A. in physics at Fairmount College in 1921 and his mas ...
and his team at the University of Virginia
The University of Virginia (UVA) is a Public university#United States, public research university in Charlottesville, Virginia. Founded in 1819 by Thomas Jefferson, the university is ranked among the top academic institutions in the United S ...
developed the process by separating two chlorine isotopes through a vacuum 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 ...
. It was one of the initial isotopic separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
means pursued during the Manhattan Project
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
, more particularly by Harold Urey
Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in the d ...
and Karl P. Cohen, but research was discontinued in 1944 as it was felt that the method would not produce results by the end of the war, and that other means of uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
(gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through semipermeable membranes. This produces a slight separation between the molecules containing uranium-235 (235U) and uranium-2 ...
and electromagnetic separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
) had a better chance of success in the short term. This method was successfully used in the Soviet nuclear program
The Soviet atomic bomb project was the classified research and development program that was authorized by Joseph Stalin in the Soviet Union to develop nuclear weapons during and after World War II.
Although the Soviet scientific community disc ...
, making the Soviet Union the most effective supplier of enriched uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...
.
In the long term, especially with the development of the Zippe-type centrifuge
The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in ...
, the gas centrifuge has become a very economical mode of separation, using considerably less energy than other methods and having numerous other advantages.
Research in the physical performance of centrifuges was carried out by the Pakistani scientist Abdul Qadeer Khan
Abdul Qadeer Khan, (; ur, ; 1 April 1936 – 10 October 2021), known as A. Q. Khan, was a Pakistani nuclear physicist and metallurgical engineer. He was a key figure in Pakistan’s nuclear weapons program and is colloquially known as the ...
in the 1970s–80s, using vacuum methods for advancing the role of centrifuges in the development of nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
for Pakistan's atomic bomb. Many of the theorists working with Khan were unsure that either gaseous and enriched uranium would be feasible on time. One scientist recalled: "No one in the world has used the ascentrifuge method to produce military-grade uranium.... This was not going to work. He was simply wasting time." In spite of skepticism, the program was quickly proven to be feasible. Enrichment via centrifuge has been used in experimental physics, and the method was smuggled to at least three different countries by the end of the 20th century.
See also
* Nuclear technology
Nuclear technology is technology that involves the nuclear reactions of atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in smoke detectors an ...
* Nuclear power
Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...
* Nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
Footnotes
References
"Basics of Centrifugation."
Cole-Parmer Technical Lab. 14 Mar. 2008
Global Security.Org. 27 Apr. 2005. 13 Mar. 2008
2003. Institute for Science and International Security. 10 Oct. 2013
External links
Annotated bibliography on the gas centrifuge from the Alsos Digital Library
History of the Centrifuge
Agreement between the Government of the United States of America and the Four Governments of the French Republic, the United Kingdom of Great Britain and Northern Ireland, the Kingdom of the Netherlands, and the Federal Republic of Germany Regarding the Establishment, Construction and Operation of Uranium Enriching Installations Using Gas Centrifuge Technology in the United States of America
United States Department of State
The United States Department of State (DOS), or State Department, is an executive department of the U.S. federal government responsible for the country's foreign policy and relations. Equivalent to the ministry of foreign affairs of other n ...
{{DEFAULTSORT:Gas Centrifuge
Centrifuges
Isotope separation
Nuclear chemistry
Nuclear proliferation
Uranium
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