Isotope separation is the process of centralizing certain isotopes of a chemical element by removing other isotopes. The use of the resulting nuclides varies. The largest varieties are used in research (eg in chemistry where the "marker" atom of nuclide is used to know the mechanism of the reaction). With tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, especially uranium enrichment is considered. This process is very important in the manufacture of uranium fuel for nuclear power plants, and is also required for the manufacture of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in nuclear reactors, which must be operated in such a way as to produce plutonium that is compatible with the isotopic mixture or grade . While different chemical elements can be purified by chemical processes, the isotopes of the same element have almost identical chemical properties, which makes this type of separation impractical, except for the separation of deuterium.
Video Isotope separation
Separation Technique
There are three types of isotope separation techniques:
- They are based directly on the atomic weight of the isotope.
- It's based on a small difference in the rate of chemical reactions generated by different atomic weights.
- It's based on properties that are not directly connected to atomic weights, such as nuclear resonance.
The third type of separation is still experimental; Practical separation techniques are all dependent in several ways on the atomic mass. It is therefore generally easier to separate the isotopes with larger relative mass differences. For example, deuterium has twice the usual hydrogen (light) mass and is generally easier to purify than to separate uranium-235 from the more common uranium-238. At the other extreme, the separation of the fissile plutonium from the general pollutants of plutonium-240, whilst desirable for allowing the manufacture of gun-type nuclear weapons from plutonium, is generally agreed upon as impractical.
Maps Isotope separation
Enrichment cascades
All large-scale isotope separation schemes employ a number of similar steps that produce the desired isotope concentrations higher in succession. Each stage enriches the product from the previous step further before being sent to the next stage. Similarly, tailings from each stage are returned to the previous stage for further processing. This creates a sequential enrichment system called a cascade.
There are two important factors that affect the performance of the cascade. The first is the separation factor, which is a number greater than 1. The second is the number of steps required to obtain the desired purity.
Commercial materials
Until recently, large-scale commercial isotope separation of only three elements has occurred. In each case, the more rare of the two most common isotopes of an element have been concentrated for use in nuclear technology:
- The uranium isotope has been separated to prepare enriched uranium for use as a fuel for nuclear reactors and in nuclear weapons.
- Isotopes of hydrogen are separated to prepare heavy water for use as a moderator in a nuclear reactor.
- Lithium-6 has been concentrated for use in thermonuclear weapons.
Some purified isotopic elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where refined silicon is used to improve the crystal structure and thermal conductivity, and carbon with greater purity of the isotope to make diamonds with greater thermal conductivity.
Isotope separation is an important process for peaceful nuclear technology and the military, and therefore the ability possessed by one country for isotope separation is of great interest to the intelligence community.
Alternative
The only alternative to isotope separation is to produce the isotopes needed in pure form. This can be done with appropriate target irradiation, but care is required in the selection of targets and other factors to ensure that only the required isotopes of the desired element are produced. Isotopes of other elements are not so big a problem as they can be removed by chemical means.
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 was produced after the capture of neutrons by uranium-238, but further neutron capture would result in a less fiscal and worse Pu-240, a powerful enough emitter neutron, and Pu-241 decaying to Am-241,. alpha emitter which causes self-heating and radiotoxicity problems. Therefore, the uranium target used for producing military plutonium should be irradiated for only a short time, to minimize the production of this undesirable isotope. Conversely, combining plutonium with Pu-240 makes it less suitable for nuclear weapons.
Practical separation methods
Diffusion
Often done with gas, but also with liquids, the diffusion method depends on the fact that in thermal equilibrium, two isotopes with the same energy will have different mean speeds. The lighter atoms (or molecules containing them) will move faster and are more likely to diffuse through the membrane. The speed difference is proportional to the square root of the mass ratio, so the amount of separation is small and many tiered stages are required to obtain high purity. This method is expensive because of the work required to push the gas through the membrane and many stages are needed.
The first major separation of the uranium isotope was achieved by the United States at a large gas diffusion separation plant at Oak Ridge Laboratories, which was established as part of the Manhattan Project. It used uranium hexafluoride gas as a process fluid. Nickel powder and an electroplated nickel diffusion barrier were pioneered by Edward Adler and Edward Norris. See gas diffusion.
Centrifugal
The centrifugal scheme quickly rotates the material that allows the heavier isotopes to get closer to the outer radial walls. This is also often done in the form of a gas using a Zippe-type centrifuge.
The separation of centrifugal isotopes was first proposed by Aston and Lindemann in 1919 and the first experiment reported successfully by Beams and Haynes on chlorine isotopes in 1936. But attempts to use technology during the Manhattan project were unproductive. In modern times this is the main method used worldwide to enrich uranium and as a result remains a fairly secret process, hindering the wider use of technology. In general, UF bait 6 gas is connected to a cylinder that is rotated at high speed. Near the outer rim of the heavier molecular gas cylinder containing U-238 collect, while the molecule containing U-235 concentrates at the center and then fed to another cascade stage. The use of gas centrifugal technology to enrich isotopes is desirable because power consumption is greatly reduced when compared to more conventional techniques such as diffusion plants because fewer cascade steps are required to achieve the same degree of separation. In fact, gas centrifuges using uranium hexafluoride have replaced the gas diffusion technology for uranium enrichment. As well as requiring less energy to achieve the same separation, much smaller scale plants are possible, making them an economic possibility for small states trying to produce nuclear weapons. Pakistan is believed to have used this method in developing its nuclear weapons.
Vortex tubes are used by South Africans in their Helicon vortex separation process. The gas is injected tangentially into a room with a special geometry which further increases its rotation to a very high degree, causing the isotope to separate. This method is simple because the vortex tube has no moving parts, but the energy is intensive, about 50 times greater than the gas centrifuge. A similar process, known as jet nozzle , was made in Germany, with a pilot plant built in Brazil, and they went as far as developing a site for the country's nuclear fuel.
Electromagnetic
This method is a form of mass spectrometry, and is sometimes called by that name. It uses the fact that charged particles are deflected in a magnetic field and the amount of deflection depends on the mass of the particles. This is very expensive for the quantity produced, because it has a very low throughput, but it can allow very high purity to be achieved. This method is often used to process small amounts of pure isotope for research or special use (such as isotope tracker), but not practical for industrial use.
At Oak Ridge and at the University of California, Berkeley, Ernest O. Lawrence developed an electromagnetic separation for most of the uranium used in the first US atomic bomb (see Manhattan Project). Devices that use the principle are named kalutron. After the war, this method was largely abandoned for being impractical. It is only done (along with diffusion and other technologies) to ensure there will be enough materials to use, regardless of cost. Its main contribution to the war effort is to focus more on the material of the gas diffusion crops to a higher degree of purity.
Laser
In this method the laser is tuned to a wavelength that excites only one isotope material and ionizes the atoms in a special way. Resonant light absorption for isotopes depends on the mass and certain hyperfine interactions between the electrons and the nucleus, allowing fine-tuned lasers to interact with only one isotope. Once the ionized atoms can be removed from the sample by applying an electric field. This method is often abbreviated as AVLIS (atomic isotope atom separation). This method was recently developed because laser technology has improved, and is currently not used extensively. However, this is a major concern for those who are in the field of nuclear proliferation because it may be cheaper and more easily hidden than other isotope separation methods. The tunable lasers used in AVLIS include laser dyes and laser diodes recently.
The second method of laser separation is known as molecular laser isotope separation (MLIS). In this method, infrared laser is directed to uranium gas hexafluoride, an interesting molecule containing U-235 atoms. The second laser frees the fluorine atoms, leaving uranium pentafluoride which then settles out of the gas. Cascading the MLIS stage is more difficult than with other methods because UF 5 should be refluorinated (back to UF 6 ) before being introduced to the next MLIS stage. Alternative MLIS schemes are currently being developed (using the first laser near-infrared or visible region) where enrichment of more than 95% can be obtained in one stage, but the method has not (yet) reached the industry feasibility. This method is called OP-IRMPD (Overtone Pre-Excitation - IR Multiple Photon Dissociation).
Finally, the SILEX process, developed by Silex Systems in Australia, was recently licensed to General Electric for the development of a pilot enrichment plant. This method uses uranium hexafluoride as a feedstock, and uses magnets to separate the isotopes after one isotope that is ionized exclusively. Further details of the process are not disclosed.
More recently other schemes have been proposed for the separation of deuterium using wavepack trojans in circularly polarized electromagnetic fields. The process of forming Trojan wave packets by the adiabatic fast path depends on the ultra-sensitive way on the reduced electrons and the nucleus mass with the same field frequency further causing the excitation of trojan or anti-trojan wavepacket depending on the type of isotope. They and their giants, electric dipole moment spins then -generated in phase and the rays of such atoms are divided into electric field gradients in the Stern-Gerlach experimental analogy.
Chemical method
Although the isotopes of one element are usually described as having the same chemical properties, this is not entirely true. In particular, the rate of reaction is strongly influenced by the atomic mass.
This technique of use is most effective for light atoms such as hydrogen. Lighter isotopes tend to react or evaporate faster than heavy isotopes, allowing them to be separated. This is how heavy water is produced commercially, see the Girdler sulfide process for details. The lighter isotope also breaks down faster under the electric field. This process in large cascades is used in heavy water production plants in Rjukan.
One candidate for the effect of the largest kinetic isotope ever measured at room temperature, 305, can eventually be used for the separation of tritium (T). The effect for triangular anion oxidation format for HTO is measured as:
Gravity
Carbon, oxygen, and nitrogen isotopes can be purified by cooling the gas or compound almost to their liquefaction temperature in very high columns (200 to 700 feet (61 to 213 m)). Heavier isotope sinks and lighter isotopes rise, where they are easily collected. This process was developed in the late 1960s by scientists at Los Alamos National Laboratory. This process is also called "cryogenic distillation".
SWU (unit of separation work)
The Separation Unit (SWU) is a complex unit that is a function of the amount of uranium processed and the extent to which it is enriched, ie the rate of increase in the U-235 isotope concentration relative to the rest.
This unit is strictly: the Kilogram Separation Unit , and measures the quantity of the separating job (indicating the energy used in the enrichment) when the feed and the quantity of the product are expressed in kilograms. The effort is made to separate mass F from the test ba xf into the mass P of the xp product test and mass waste W and assay xw expressed in the required number of work units, given by SWU expression WV ( xw ) PV x ) - F ( xf ), ) is a "value function," defined as V ( x ) = (1 - 2 x ) ln ((1 - )/ x ).
Separate work is expressed in SWU, kg SW, or UTA kg (from German Urantrennarbeit )
- 1 SWU = 1Ã, kg SW = 1Ã, kg UTA
- 1 kSWU = 1.0 t SW = 1 t UTA
- 1 MSWU = 1 kt SW = 1 kt UTA
If, for example, you start with 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWUs to produce 10 kilograms (22 lb) of enriched uranium in content U-235 to 4.5%
Isotope separator for research
Radioactive beams from certain isotopes are widely used in the field of experimental physics, biology and materials science. The production and formation of these radioactive atoms into ionic rays for study are all fields of research conducted in many laboratories around the world. The first isotope separator was developed in Copenhagen Cyclotron by Bohr and co-workers using the principle of electromagnetic separation. Currently, there are many laboratories around the world that supply radioactive ion beams for use. The principal of Isotope On Line (ISOL) is ISOLDE at CERN, which is a European joint facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses primarily proton spallation targets of uranium carbides to produce a variety of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), uranium carbide targets are heated to several thousand degrees so that radioactive atoms produced in nuclear reactions are released. Upon exiting the target, radioactive atomic vapor moves into the ionizing cavity. This ionizing cavity is a thin tube made of refractory metal with a high working function that allows a collision with a wall to free one electron from a free atom (surface ionization effect). After ionization, the radioactive species is accelerated by an electrostatic field and injected into an electromagnetic separator. Since the ions entering the separator have approximately the same energy, the ions with a smaller mass will be deflected by a magnetic field with a greater number of ions with a heavier mass. The different radius of curvature allows the purification of the isobaric. After being purified isobarically, the ion beam is then sent to an individual experiment. To improve the purity of the isobaric beam, laser ionization can occur within the ionizing cavity to selectively ionize a desired chain of elements. At CERN, this device is called the Resonance Ionization Resonance Source (RILIS). Currently more than 60% of all experiments choose to use RILIS to improve the purity of radioactive rays.
Beam production capability from ISOL facility
Since the production of radioactive atoms by ISOL techniques depends on the free atomic chemistry of the elements to be studied, there are certain beams that can not be produced by simple proton bombs of thick actinide targets. Refractory metals such as tungsten and rhenium do not emerge from the target even at high temperatures because of their low vapor pressure. To produce this type of block, a thin target is required. Isotope Isotope Technique On Line (IGISOL) was developed in 1981 in the cyclotron laboratory of JyvÃÆ'¤skylÃÆ'¤ University in Finland. In this technique, the thin uranium target is bombarded with protons and nuclear reaction products backing out of the target in a filled state. Recoils are stopped in the gaseous cells and then out through a small hole at the side of the cell where they are electrostatically accelerated and injected into a mass separator. This method of production and extraction takes place on shorter time scales than standard ISOL techniques and isotopes with short half-lives (sub milliseconds) can be studied using IGISOL. IGISOL has also been combined with a laser ion source in the Leuven On Line Isotope Separator (LISOL) in Belgium. Thin target sources generally provide a much lower amount of radioactive ions than a thick target source and this is their main weakness.
As experimental nuclear physics progresses, it becomes increasingly important to study the most exotic radioactive nuclei. To do so, more inventive techniques are needed to create nuclei with extreme proton/neutron ratios. One alternative to the ISOL technique described here is the fragmentation beam, in which radioactive ions are generated by fragmentation reactions in fast stable ionic beams that hit the thin targets (usually beryllium atoms). This technique is used, for example, at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University and at Radio Isotope Beam Factory (RIBF) at RIKEN, in Japan.
References
External links
- Utilization of kinetic isotope effect for tritium concentration, GM Brown, TJ Meyer et al., 2001.
- Uranium Production
- Uranium Enrichment from the World Nuclear Association
- Bibliography annotated on the electromagnetic separation of the uranium isotope forms the Alsos Digital Library
Source of the article : Wikipedia
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