Gas centrifuge
Encyclopedia
Isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
of gases. A centrifuge
Centrifuge
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
relies on the principles of centripetal force
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
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
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
from uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
. The gas centrifuge was developed to replace the gaseous diffusion
Gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238 . By use of a large cascade of many stages, high separations...
method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in cascade, that successively achieve higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.
Centrifugal process
The centrifugeCentrifuge
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
relies on the force resulting from centripetal acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
to separate molecules according to their mass, and can be applied to most fluids. The dense (heavier) molecules move towards the wall and the lighter ones remain close to the center. In addition, if one creates a thermal gradient in a perpendicular direction by keeping the top of the rotating column cool and the bottom hot, the resulting convection current carries the lighter molecules to the top while the heavier ones settle at the bottom, from which they can be continuously withdrawn.
In practice, several such centrifuges are connected in series. Each centrifuge receives one input and produces two output lines, corresponding to light and heavy fractions
Fraction (chemistry)
A fraction in chemistry is a quantity collected from a sample or batch of a substance in a fractionating separation process. In such a process, a mixture is separated into fractions, which have compositions that vary according to a gradient. A fraction can be defined as a group of chemicals that...
. The input of each centrifuge is the output (heavy) of the previous centrifuge and the output (light) of the following stage. This produces an almost pure light fraction from the output (light) of the first centrifuge and an almost pure heavy fraction from the output (heavy) of the last centrifuge.
Gas centrifugation process
Batch processing
Batch processing is execution of a series of programs on a computer without manual intervention.Batch jobs are set up so they can be run to completion without manual intervention, so all input data is preselected through scripts or command-line parameters...
, the gas centrifuge utilizes continuous processing, allowing cascading, in which multiple identical processes occur in succession.
The gas centrifuge consists of a cylindrical rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
, a casing, an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
, and three lines for material to travel. The gas centrifuge is designed with a casing that completely encloses the centrifuge. The cylindrical rotor is located inside the casing, which is evacuated
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
of all air to produce a near frictionless rotation when operating. The motor spins the rotor, creating the centripetal force on the components as they enter the cylindrical rotor. There are two output lines, one located at the top of the centrifuge and the other located at the bottom. The heavier molecules will segregate to the bottom of the centrifuge while the lighter molecules will segregate to the top of the centrifuge. The output lines take these separations to other centrifuges to continue to the centrifugation process.
Separative work units
The separative work unit (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 functionBellman equationA Bellman equation , named after its discoverer, Richard Bellman, is a necessary condition for optimality associated with the mathematical optimization method known as dynamic programming...
, defined as
Separating Uranium-235 from Uranium-238
The separation of uranium requires the material in a gaseous form; uranium hexafluorideUranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
(UF6) is used for uranium enrichment. Upon entering the centrifuge, the lighter uranium-235 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
collects towards one end of the centrifuge while the heavier uranium-238 isotope at the other. The enriched uranium is sent upstream to another centrifuging stage and the depleted uranium downstream to the previous stage. Usually, enrichments plants may include thousands of centrifuges arranged in cascades.
Separation of zinc isotopes
For some uses in nuclear technology, the content of zinc-64 in zincZinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
metal has to be lowered in order to prevent formation of radioisotopes by its neutron activation
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 often decays immediately by emitting particles such as neutrons, protons, or alpha...
. Diethyl zinc is used as the gaseous feed medium for the centrifuge cascade. An example of a resulting material is depleted zinc oxide
Depleted zinc oxide
Depleted zinc oxide is a zinc oxide depleted in the zinc isotope with the atomic mass 64, and used in corrosion prevention in nuclear pressurized water reactors....
, used as a corrosion inhibitor
Corrosion inhibitor
A corrosion inhibitor 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. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...
.
History
Suggested in 1919, the centrifugal process was first successfully performed in 1934. Jesse BeamsJesse Beams
Jesse Wakefield Beams was an American physicist at the University of Virginia.Beams completed his undergraduate B.A. in physics at Fairmount College in 1921 and his master's degree the next year at the University of Wisconsin–Madison...
and coworkers at the University of Virginia
University of Virginia
The University of Virginia is a public research university located in Charlottesville, Virginia, United States, founded by Thomas Jefferson...
developed the process by separating two chlorine isotopes
Isotopes of chlorine
Chlorine has 24 isotopes with mass numbers ranging from 28Cl to 51Cl and 2 isomers . There are two principal stable isotopes, 35Cl and 37Cl , found in the relative proportions of 37.89:12.11, not 3:1, respectively, giving chlorine a standard atomic mass of 35.453, not 35.5...
through a vacuum ultracentrifuge
Ultracentrifuge
The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 2,000,000 g . There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge...
. It was one of the initial isotopic separation means pursued during the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
, 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 (gaseous diffusion
Gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238 . By use of a large cascade of many stages, high separations...
and electromagnetic separation) had a better chance of success in the short term. However this method was successfully used in the Soviet nuclear program, making the Soviet Union the most effective supplier of enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
.
In the long term, especially with the development of the Zippe-type centrifuge
Zippe-type centrifuge
The Zippe-type centrifuge is a device designed to collect Uranium-235. It was developed in the Soviet Union by a team of 60 Austrian and German scientists captured after World War II, working in detention...
, the gas centrifuge has become a very economical mode of separation, using considerably less energy than other methods and having numerous other advantages. Enrichment by centrifuge has been used in particular by Abdul Qadeer Khan
Abdul Qadeer Khan
Abdul Qadeer Khan , also known in Pakistan as Mohsin-e-Pakistan , D.Eng, Sc.D, HI, NI , FPAS; more widely known as Dr. A. Q...
in Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
, and the method was smuggled to at least three different countries by the end of the 20th century.
See also
- Nuclear technologyNuclear technologyNuclear technology is technology that involves the reactions of atomic nuclei. Among the notable nuclear technologies are nuclear power, nuclear medicine, and nuclear weapons...
- Nuclear powerNuclear powerNuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
- Nuclear fuelNuclear fuelNuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
- Zippe-type centrifugeZippe-type centrifugeThe Zippe-type centrifuge is a device designed to collect Uranium-235. It was developed in the Soviet Union by a team of 60 Austrian and German scientists captured after World War II, working in detention...