Isotope separation
Encyclopedia
Isotope separation is the process of concentrating specific isotope
s 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 also required for the creation of uranium based nuclear weapons. Plutonium based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade. This theory was first recognized by Charles H. Townes.
While in general chemical elements can be purified through chemical processes
, isotopes of the same element have nearly identical chemical properties, which makes this type of separation impractical, except for separation of deuterium
.
The third type of separation is still experimental; practical separation techniques all depend in some way on the atomic mass. It is therefore generally easier to separate isotopes with a larger relative mass difference. For example deuterium
has twice the mass of ordinary (light) hydrogen
and it is generally easier to purify it than to separate uranium-235
from the more common uranium-238
. On the other extreme, separation of fissile plutonium-239
from the common impurity plutonium-240, while desirable in that it would allow the creation of gun-type nuclear weapons from plutonium, is generally agreed to be impractical.
.
There are two important factors that affect the performance of a cascade. The first is the separation factor (the square root of the mass ratio of the two isotopes), which is a number greater than 1. The second is the number of required stages to get the desired purity.
Some isotopically purified elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where purified Silicon
is used to improve crystal structure and thermal conductivity.
Isotope separation is an important process for both peaceful and military nuclear technology, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community.
This is particularly relevant in the preparation of high-grade plutonium-239
for use in weapons. It is not practical to separate Pu-239 from Pu-240 or Pu-241. Fissile
Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce non-fissile Pu-240 and worse, then Pu-241 which is a fairly strong neutron emitter. Therefore, the uranium targets used to produce military plutonium must be irradiated for only a short time, to minimise the production of these unwanted isotopes. Conversely, blending plutonium with Pu-241 renders it unsuitable for nuclear weapons.
method relies on the fact that in thermal equilibrium, two isotopes with the same energy will have different average velocities. The lighter atoms (or the molecules containing them) will travel more quickly and be more likely to diffuse through a membrane. The difference in speeds is proportional to the square root of the mass ratio, so the amount of separation is small and many cascaded stages are needed to obtain high purity. This method is expensive due to the work needed to push gas through a membrane and the many stages necessary.
The first large-scale separation of uranium isotopes was achieved by the United States
in large gaseous diffusion separation plants at Oak Ridge Laboratories
, which were established as part of the Manhattan Project
. These used uranium hexafluoride
gas as the process fluid. Nickel powder and electro-deposited nickel mesh diffusion barriers were pioneered by Edward Adler and Edward Norris. See gaseous diffusion
.
Centrifugal effect
schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This too is often done in gaseous form using a Zippe-type centrifuge
.
The centrifugal separation of isotopes was first suggested by Aston and Lindemann in 1919 and the first successful experiments were reported by Beams and Haynes on isotopes of chlorine in 1936. However attempts to use the technology during the Manhattan project
were unproductive. In modern times it is the main method used throughout the world to enrich uranium and as a result remains a fairly secretive process, hindering a more widespread uptake of the technology. In general a feed of UF6 gas is connected to a cylinder that is rotated at high speed. Near the outer edge of the cylinder heavier gas molecules containing U-238 collect, while molecules containing U-235 concentrate at the center and are then fed to another cascade stage. Use of gaseous centrifugal technology to enrich isotopes is desirable as power consumption is greatly reduced when compared to more conventional techniques such as diffusion plants since fewer cascade steps are required to reach similar degrees of separation. In fact, Gas centrifuge
s using uranium hexafluoride have largely replaced gaseous diffusion technology for uranium enrichment. As well as requiring less energy to achieve the same separation, far smaller scale plants are possible, making them an economic possibility for a small nation attempting to produce a nuclear weapon
. Pakistan is believed to have used this method in developing its nuclear weapons.
Vortex tube
s were used by South Africa
in their Helikon vortex separation process
. The gas is injected tangentially into a chamber with special geometry that further increases its rotation to a very high rate, causing the isotopes to separate. The method is simple because vortex tubes have no moving parts, but energy intensive, about 50 times greater than gas centrifuges. A similar process, known as jet nozzle, was created in Germany, with a demonstration plant built in Brazil, and they went as far as developing a site to fuel the country's nuclear plants.
, and is sometimes referred to by that name. It uses the fact that charged particles are deflected in a magnetic field
and the amount of deflection depends upon the particle's mass. It is very expensive for the quantity produced, as it has an extremely low throughput, but it can allow very high purities to be achieved. This method is often used for processing small amounts of pure isotopes for research or specific use (such as isotopic tracers), but is impractical for industrial use.
At Oak Ridge
and at the University of California, Berkeley
, Ernest O. Lawrence developed electromagnetic separation for much of the uranium used in the first United States atomic bomb (see Manhattan Project
). Devices using his principle are named calutron
s. After the war the method was largely abandoned as impractical. It had only been undertaken (along with diffusion and other technologies) to guarantee there would be enough material for use, whatever the cost. Its main eventual contribution to the war effort was to further concentrate material from the gaseous diffusion plants to even higher levels of purity.
is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially. The resonant absorption of light for an isotope is dependent upon its mass and certain hyperfine
interactions between electrons and the nucleus, allowing finely tuned lasers to interact with only one isotope. After the atom is ionized it can be removed from the sample by applying an electric field
. This method is often abbreviated as AVLIS
(atomic vapor laser isotope separation). This method has only recently been developed as laser technology has improved, and is currently not used extensively. However, it is a major concern to those in the field of nuclear proliferation
because it may be cheaper and more easily hidden than other methods of isotope separation. Tunable laser
s used in AVLIS include the dye laser
and more recently diode lasers.
A second method of laser separation is known as MLIS
, Molecular Laser Isotope Separation. In this method, an infrared laser is directed at uranium hexafluoride
gas, exciting molecules that contain a U-235
atom. A second laser frees a fluorine
atom, leaving uranium pentafluoride
which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be refluorinated (back to UF6) before being introduced into the next MLIS stage. Alternative MLIS schemes are currently being developed (using a first laser in the near-infrared or visible region) where an enrichment of over 95% can be obtained in a single stage, but the methods have not (yet) reached industrial feasibility. This method is called OP-IRMPD (Overtone Pre-excitation - IR Multiple Photon Dissociation
).
Finally, the SILEX
process, developed by Silex Systems in Australia, has recently been licensed to General Electric for the development of a pilot enrichment plant. The method uses uranium hexafluoride as a feedstock, and uses magnets to separate the isotopes after one isotope is preferentially ionized. Further details of the process are not disclosed.
s are very slightly affected by atomic mass.
Techniques using this are most effective for light atoms such as hydrogen. Lighter isotopes tend to react or evaporate
more quickly than heavy isotopes, allowing them to be separated. This is how heavy water
is produced commercially, see Girdler sulfide process
for details. Lighter isotopes also disassociate more rapidly under an electric field. This process in a large cascade
was used at the heavy water production plant at Rjukan
.
One candidate for the largest kinetic isotopic effect
ever measured at room temperature, 305, may eventually be used for the separation of tritium
(T). The effects for the oxidation of triated formate
anions to HTO were measured as:
The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass F of feed of assay xf into a mass P of product assay xp and waste of mass W and assay xw is expressed in terms of the number of separative work units needed, given by the expression SWU = WV(xw) + PV(xp) - FV(xf), where V(x) is the "value function," defined as V(x) = (1 - 2x) ln ((1 - x) /x).
Separative work is expressed in SWUs, kg SW, or kg UTA (from the German Urantrennarbeit )
If, for example, you begin with 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWU to produce 10 kilograms (22 pounds) of uranium enriched in U-235 content to 4.5%
Isotope Separators for Research=
Radioactive beams of specific isotopes are widely used in the fields of experimental physics, biology and materials science. The production and formation of these radioactive atoms into an ionic beam for study is an entire field of research carried out at many laboratories throughout the world. The first isotope separator was developed at the Copenhagen Cyclotron by Bohr and co-workers using the principle of electromagnetic separation. Today, there are many laboratories around the world which supply beams of radioactive ions for use. Arguably the principal Isotope Separator On-Line (ISOL) is ISOLDE at CERN, http://isolde.web.cern.ch/ISOLDE/ which is a joint European facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses mainly proton spallation of uranium carbide targets to produce a wide range of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), a uranium carbide target is heated to several thousand degrees so that radioactive atoms produced in the nuclear reaction are released. Once out of the target, the vapour of radioactive atoms travels to an ionizer cavity. This ionizer cavity is a thin tube made of a low work function metal allowing for collisions with the walls to liberate a single electron from a free atom. Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approxiamtely equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.
metals such as tungsten and rhenium do not emerge from the target even at high temperatures due to their low vapour pressure. In order to produce these types of beams, a thin target is required. The Ion Guide Isotope Separator On Line (IGISOL) technique was developed in 1981 at the University of Jyvaskyla cyclotron
laboratory in Finland
http://www.jyu.fi/science/laitokset/fysiikka/en/research/accelerator/igisol. In this technique, a thin uranium target is bombarded with protons and nuclear reaction products recoil out of the target in a charged state. The recoils are stopped in a gas cell and then exit through a small hole in the side of the cell where they are accelerated electrostatically and injected into a mass separator. This method of production and extraction takes place on a shorter timescale compared to the standard ISOL technique and isotopes with short half-lives (sub millisecond) can be studied using an IGISOL. An IGISOL has also been combined with a laser ion source at the Leuven Isotope Separator On Line (LISOL) in Belgium http://fys.kuleuven.be/iks/lisol/equipment/equipment.htm. Thin target sources generally provide significantly lower quantities of radioactive ions than thick target sources and this is their main drawback.
As experimental nuclear physics progresses, it is becoming more and more important to study the most exotic of radioactive nuclei. In order to do so, more inventive techniques are required to create nuclei with extreme proton/neutron ratios. The most promising technique to date is by using multiple targets. By first producing a radioactive beam by an ISOL method and then reaccellerating it to make it hit a secondary thin target, very exotic nuclei can be produced. The National Superconducting Cyclotron Laboratory
(NSCL) at Michigan State University is a good example of such a facility. The higher the energy of interaction, generally the more exotic the nucleus produced. It then becomes necessary to be able to slow these nuclei down once they have been produced. Pioneers at the Japanese facility RIKEN
were the first to use a giant gas catcher and novel electric fields to do this, which is becoming the standard technique.
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...
s of a chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
by removing other isotopes, for example separating natural uranium
Natural uranium
Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...
into 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...
and depleted uranium
Depleted uranium
Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...
. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is also required for the creation of uranium based nuclear weapons. Plutonium based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade. This theory was first recognized by Charles H. Townes.
While in general chemical elements can be purified through chemical processes
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
, isotopes of the same element have nearly identical chemical properties, which makes this type of separation impractical, except for separation of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
.
Separation techniques
There are three types of isotope separation techniques:- Those based directly on the atomic weightAtomic weightAtomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
of the isotope. - Those based on the small differences in chemical reaction rates produced by different atomic weights.
- Those based on properties not directly connected to atomic weight, such as nuclear resonances.
The third type of separation is still experimental; practical separation techniques all depend in some way on the atomic mass. It is therefore generally easier to separate isotopes with a larger relative mass difference. For example deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
has twice the mass of ordinary (light) hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and it is generally easier to purify it than to separate 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 the more common 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...
. On the other extreme, separation of fissile plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
from the common impurity plutonium-240, while desirable in that it would allow the creation of gun-type nuclear weapons from plutonium, is generally agreed to be impractical.
- See also: Enriched uraniumEnriched uraniumEnriched 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...
Enrichment cascades
All large-scale isotope separation schemes employ a number of similar stages which produce successively higher concentrations of the desired isotope. Each stage enriches the product of the previous step further before being sent to the next stage. Similarly, the tailings from each stage are returned to the previous stage for further processing. This creates a sequential enriching system called a cascadeCascade (chemical engineering)
In chemical engineering, a cascade is a plant consisting of several similar stages with each processing the output from the previous stage. Cascades are most commonly used in isotope separation, distillation and other separation or purification processes....
.
There are two important factors that affect the performance of a cascade. The first is the separation factor (the square root of the mass ratio of the two isotopes), which is a number greater than 1. The second is the number of required stages to get the desired purity.
Commercial materials
To date, large-scale commercial isotope separation of only three elements has occurred. In each case, the rarer of the two most common isotopes of an element has been concentrated for use in nuclear technology:- UraniumUraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
isotopes have been separated to prepare enriched uraniumEnriched uraniumEnriched 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...
for use as nuclear reactorNuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
fuel and in nuclear weapons. - HydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
isotopes have been separated to prepare heavy waterHeavy waterHeavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
for use as a moderator in nuclear reactors. - LithiumLithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
-6 has been concentrated for use in thermonuclear weaponsNuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
.
Some isotopically purified elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where purified Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
is used to improve crystal structure and thermal conductivity.
Isotope separation is an important process for both peaceful and military nuclear technology, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community.
Alternatives
The only alternative to isotope separation is to manufacture the required isotope in its pure form. This may be done by irradiation of a suitable target, but care is needed in target selection and other factors to ensure that only the required isotope of the element of interest is produced. Isotopes of other elements are not so great a problem as they can be removed by chemical means.This is particularly relevant in the preparation of high-grade plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
for use in weapons. It is not practical to separate Pu-239 from Pu-240 or Pu-241. Fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...
Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce non-fissile Pu-240 and worse, then Pu-241 which is a fairly strong neutron emitter. Therefore, the uranium targets used to produce military plutonium must be irradiated for only a short time, to minimise the production of these unwanted isotopes. Conversely, blending plutonium with Pu-241 renders it unsuitable for nuclear weapons.
Diffusion
Often done with gases, but also with liquids, the diffusionDiffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
method relies on the fact that in thermal equilibrium, two isotopes with the same energy will have different average velocities. The lighter atoms (or the molecules containing them) will travel more quickly and be more likely to diffuse through a membrane. The difference in speeds is proportional to the square root of the mass ratio, so the amount of separation is small and many cascaded stages are needed to obtain high purity. This method is expensive due to the work needed to push gas through a membrane and the many stages necessary.
The first large-scale separation of uranium isotopes was achieved by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in large gaseous diffusion separation plants at Oak Ridge Laboratories
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
, which were established as part of 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...
. These used uranium hexafluoride
Uranium 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...
gas as the process fluid. Nickel powder and electro-deposited nickel mesh diffusion barriers were pioneered by Edward Adler and Edward Norris. See 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...
.
Centrifugal effect
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This too is often done in gaseous form using a 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 centrifugal separation of isotopes was first suggested by Aston and Lindemann in 1919 and the first successful experiments were reported by Beams and Haynes on isotopes of chlorine in 1936. However attempts to use the technology 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...
were unproductive. In modern times it is the main method used throughout the world to enrich uranium and as a result remains a fairly secretive process, hindering a more widespread uptake of the technology. In general a feed of UF6 gas is connected to a cylinder that is rotated at high speed. Near the outer edge of the cylinder heavier gas molecules containing U-238 collect, while molecules containing U-235 concentrate at the center and are then fed to another cascade stage. Use of gaseous centrifugal technology to enrich isotopes is desirable as power consumption is greatly reduced when compared to more conventional techniques such as diffusion plants since fewer cascade steps are required to reach similar degrees of separation. In fact, Gas centrifuge
Gas centrifuge
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centripetal force 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...
s using uranium hexafluoride have largely replaced gaseous diffusion technology for uranium enrichment. As well as requiring less energy to achieve the same separation, far smaller scale plants are possible, making them an economic possibility for a small nation attempting to produce a nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
. Pakistan is believed to have used this method in developing its nuclear weapons.
Vortex tube
Vortex tube
The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gas into hot and cold streams. It has no moving parts....
s were used by South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
in their Helikon vortex separation process
Helikon vortex separation process
The Helikon vortex separation process is an aerodynamic uranium enrichment process designed around a device called a vortex tube. This method was designed and used in South Africa for producing reactor fuel with a uranium-235 content of around 3–5%, and 80–93% enriched uranium for use in nuclear...
. The gas is injected tangentially into a chamber with special geometry that further increases its rotation to a very high rate, causing the isotopes to separate. The method is simple because vortex tubes have no moving parts, but energy intensive, about 50 times greater than gas centrifuges. A similar process, known as jet nozzle, was created in Germany, with a demonstration plant built in Brazil, and they went as far as developing a site to fuel the country's nuclear plants.
Electromagnetic
This method is a form of mass spectrometryMass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
, and is sometimes referred to by that name. It uses the fact that charged particles are deflected in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
and the amount of deflection depends upon the particle's mass. It is very expensive for the quantity produced, as it has an extremely low throughput, but it can allow very high purities to be achieved. This method is often used for processing small amounts of pure isotopes for research or specific use (such as isotopic tracers), but is impractical for industrial use.
At Oak Ridge
Oak Ridge, Tennessee
Oak Ridge is a city in Anderson and Roane counties in the eastern part of the U.S. state of Tennessee, about west of Knoxville. Oak Ridge's population was 27,387 at the 2000 census...
and at the University of California, Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
, Ernest O. Lawrence developed electromagnetic separation for much of the uranium used in the first United States atomic bomb (see 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...
). Devices using his principle are named calutron
Calutron
A calutron is a mass spectrometer used for separating the isotopes of uranium. It was developed by Ernest O. Lawrence during the Manhattan Project and was similar to the cyclotron invented by Lawrence. Its name is a concatenation of Cal. U.-tron, in tribute to the University of California,...
s. After the war the method was largely abandoned as impractical. It had only been undertaken (along with diffusion and other technologies) to guarantee there would be enough material for use, whatever the cost. Its main eventual contribution to the war effort was to further concentrate material from the gaseous diffusion plants to even higher levels of purity.
Laser
In this method a laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially. The resonant absorption of light for an isotope is dependent upon its mass and certain hyperfine
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...
interactions between electrons and the nucleus, allowing finely tuned lasers to interact with only one isotope. After the atom is ionized it can be removed from the sample by applying an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. This method is often abbreviated as AVLIS
AVLIS
AVLIS Is an acronym which stands for atomic vapor laser isotope separation and is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions....
(atomic vapor laser isotope separation). This method has only recently been developed as laser technology has improved, and is currently not used extensively. However, it is a major concern to those in the field of nuclear proliferation
Nuclear proliferation
Nuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "Nuclear Weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the...
because it may be cheaper and more easily hidden than other methods of isotope separation. Tunable laser
Tunable laser
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range....
s used in AVLIS include the dye laser
Dye laser
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...
and more recently diode lasers.
A second method of laser separation is known as MLIS
Molecular laser isotope separation
Molecular laser isotope separation is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS...
, Molecular Laser Isotope Separation. In this method, an infrared laser is directed at uranium hexafluoride
Uranium 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...
gas, exciting molecules that contain a U-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...
atom. A second laser frees a fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
atom, leaving uranium pentafluoride
Uranium pentafluoride
Uranium pentafluoride an inorganic chemical consisting of uranium and fluorine. The compound exists both as individual UF5 molecular entities and with extended order in several crystal-lattice forms. The monomer, formed by photolysis of uranium hexafluoride, has a square pyramidal geometry...
which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be refluorinated (back to UF6) before being introduced into the next MLIS stage. Alternative MLIS schemes are currently being developed (using a first laser in the near-infrared or visible region) where an enrichment of over 95% can be obtained in a single stage, but the methods have not (yet) reached industrial feasibility. This method is called OP-IRMPD (Overtone Pre-excitation - IR Multiple Photon Dissociation
Infrared multiphoton dissociation
Infrared multiphoton dissociation is a technique used in mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original molecule. -How it works:...
).
Finally, the SILEX
Silex
Silex is any of various forms of ground stone. In modern contexts the word refers to a finely ground, nearly pure form of silica or silicate....
process, developed by Silex Systems in Australia, has recently been licensed to General Electric for the development of a pilot enrichment plant. The method uses uranium hexafluoride as a feedstock, and uses magnets to separate the isotopes after one isotope is preferentially ionized. Further details of the process are not disclosed.
Chemical methods
Although isotopes of a single element are normally described as having the same chemical properties, this is not strictly true. In particular, reaction rateReaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
s are very slightly affected by atomic mass.
Techniques using this are most effective for light atoms such as hydrogen. Lighter isotopes tend to react or evaporate
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
more quickly than heavy isotopes, allowing them to be separated. This is how heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
is produced commercially, see Girdler sulfide process
Girdler sulfide process
The Girdler sulfide process, also known as the Geib–Spevack process, is an industrial production method for making heavy water , an important component of many nuclear reactors because it acts as a neutron moderator. It takes its name from Karl-Hermann Geib and Jerome S...
for details. Lighter isotopes also disassociate more rapidly under an electric field. This process in a large cascade
Cascade (chemical engineering)
In chemical engineering, a cascade is a plant consisting of several similar stages with each processing the output from the previous stage. Cascades are most commonly used in isotope separation, distillation and other separation or purification processes....
was used at the heavy water production plant at Rjukan
Rjukan
Rjukan is a town and the administrative center of Tinn municipality in Telemark . It is situated in Vestfjorddalen, between Møsvatn and Tinnsjå, and got its name after Rjukanfossen west of the town. The Tinn municipality council granted township status for Rjukan in 1996. The town has 3 386...
.
One candidate for the largest kinetic isotopic effect
Kinetic isotope effect
The kinetic isotope effect is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called "isotope fractionation," although this term is somewhat broader in meaning...
ever measured at room temperature, 305, may eventually be used for the separation of tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(T). The effects for the oxidation of triated formate
Formate
Formate or methanoate is the ion CHOO− or HCOO− . It is the simplest carboxylate anion. It is produced in large amounts in the hepatic mitochondria of embryonic cells and in cancer cells by the folate cycle Formate or methanoate is the ion CHOO− or HCOO− (formic acid minus one hydrogen ion). It...
anions to HTO were measured as:
| k(HCO2-) = 9.54 M−1s−1 | k(H)/k(D) = 38 |
| k(DCO2-) = 9.54 M−1s−1 | k(D)/k(T) = 8.1 |
| k(TCO2-) = 9.54 M−1s−1 | k(H)/k(T) = 305 |
Gravity
Isotopes of Carbon, Oxygen, and Nitrogen can be purified by chilling these gases or compounds nearly to their liquification temperature in very tall columns (200 to 700 feet tall - 70 to 200 meters). The heavier isotopes sink and the lighter isotopes rise, where they are easily collected. The process was developed in the late 1960s by scientists at Los Alamos National Laboratory.http://www.lanl.gov/quarterly/q_w03/spotlight.shtml This process is also called "cryogenic distillation".http://www.lanl.gov/orgs/pa/News/080801.htmlThe SWU (separative work unit)
Separative Work Unit (SWU) is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, i.e. the extent of increase in the concentration of the U-235 isotope relative to the remainder.The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass F of feed of assay xf into a mass P of product assay xp and waste of mass W and assay xw is expressed in terms of the number of separative work units needed, given by the expression SWU = WV(xw) + PV(xp) - FV(xf), where V(x) is the "value function," defined as V(x) = (1 - 2x) ln ((1 - x) /x).
Separative work is expressed in SWUs, kg SW, or kg UTA (from the German Urantrennarbeit )
- 1 SWU = 1 kg SW = 1 kg UTA
- 1 kSWU = 1.0 t SW = 1 t UTA
- 1 MSWU = 1 kt SW = 1 kt UTA
If, for example, you begin with 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWU to produce 10 kilograms (22 pounds) of uranium enriched in U-235 content to 4.5%
Isotope Separators for Research=
Radioactive beams of specific isotopes are widely used in the fields of experimental physics, biology and materials science. The production and formation of these radioactive atoms into an ionic beam for study is an entire field of research carried out at many laboratories throughout the world. The first isotope separator was developed at the Copenhagen Cyclotron by Bohr and co-workers using the principle of electromagnetic separation. Today, there are many laboratories around the world which supply beams of radioactive ions for use. Arguably the principal Isotope Separator On-Line (ISOL) is ISOLDE at CERN, http://isolde.web.cern.ch/ISOLDE/ which is a joint European facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses mainly proton spallation of uranium carbide targets to produce a wide range of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), a uranium carbide target is heated to several thousand degrees so that radioactive atoms produced in the nuclear reaction are released. Once out of the target, the vapour of radioactive atoms travels to an ionizer cavity. This ionizer cavity is a thin tube made of a low work function metal allowing for collisions with the walls to liberate a single electron from a free atom. Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approxiamtely equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.
Beam Production Capability of ISOL Facilities
As the production of radioactive atoms by the ISOL technique depends on the free atom chemistry of the element to be studied, there are certain beams which cannot be produced by simple proton bombardment of thick actinide targets. RefractoryRefractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
metals such as tungsten and rhenium do not emerge from the target even at high temperatures due to their low vapour pressure. In order to produce these types of beams, a thin target is required. The Ion Guide Isotope Separator On Line (IGISOL) technique was developed in 1981 at the University of Jyvaskyla cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
laboratory in Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
http://www.jyu.fi/science/laitokset/fysiikka/en/research/accelerator/igisol. In this technique, a thin uranium target is bombarded with protons and nuclear reaction products recoil out of the target in a charged state. The recoils are stopped in a gas cell and then exit through a small hole in the side of the cell where they are accelerated electrostatically and injected into a mass separator. This method of production and extraction takes place on a shorter timescale compared to the standard ISOL technique and isotopes with short half-lives (sub millisecond) can be studied using an IGISOL. An IGISOL has also been combined with a laser ion source at the Leuven Isotope Separator On Line (LISOL) in Belgium http://fys.kuleuven.be/iks/lisol/equipment/equipment.htm. Thin target sources generally provide significantly lower quantities of radioactive ions than thick target sources and this is their main drawback.
As experimental nuclear physics progresses, it is becoming more and more important to study the most exotic of radioactive nuclei. In order to do so, more inventive techniques are required to create nuclei with extreme proton/neutron ratios. The most promising technique to date is by using multiple targets. By first producing a radioactive beam by an ISOL method and then reaccellerating it to make it hit a secondary thin target, very exotic nuclei can be produced. The National Superconducting Cyclotron Laboratory
National Superconducting Cyclotron Laboratory
National Superconducting Cyclotron Laboratory is located on the campus of Michigan State University and is the leading rare isotope research facility in the United States...
(NSCL) at Michigan State University is a good example of such a facility. The higher the energy of interaction, generally the more exotic the nucleus produced. It then becomes necessary to be able to slow these nuclei down once they have been produced. Pioneers at the Japanese facility RIKEN
RIKEN
is a large natural sciences research institute in Japan. Founded in 1917, it now has approximately 3000 scientists on seven campuses across Japan, the main one in Wako, just outside Tokyo...
were the first to use a giant gas catcher and novel electric fields to do this, which is becoming the standard technique.
External links
- Utilization of kinetic isotope effects for the concentration of tritium, GM Brown, TJ Meyer et al., 2001.
- Uranium Production
- Uranium Enrichment from the World Nuclear Association
- Annotated bibliography on electromagnetic separation of uranium isotopes form the Alsos Digital Library