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" nuclide are used to figure out reaction mechanisms). By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. This process is crucial in the manufacture of uranium fuel for nuclear power plants and is also required for the creation of uranium-based nuclear weapons (unless uranium-233 is used). 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.
While 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.
Techniques There are three types of isotope separation techniques: Those based directly on the atomic weight 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 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 fission weapons from plutonium, is generally agreed to be impractical.
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 cascade. There are two important factors that characterize the performance of a cascade. The first is the separation factor, 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: Uranium isotopes have been separated to prepare enriched uranium for use as nuclear reactor fuel and in nuclear weapons. Hydrogen isotopes have been separated to prepare heavy water for use as a moderator in nuclear reactors. Tritium is both a nuisance in the coolant / moderator of water moderated reactors and a valuable product; it is thus sometimes separated from the coolant. Lithium-6 has been concentrated for use in thermonuclear weapons. Tritium is commonly produced from lithium-6 which is often enriched for this purpose.
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, and carbon with greater isotopic purity to make diamonds with greater 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 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 Pu-240 which is less fissile and worse, is a fairly strong neutron emitter, and Pu-241 which decays to Am-241, a strong alpha emitter that poses self-heating and radiotoxicity problems. 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-240 renders it less suitable for nuclear weapons.
If the desired goal is not an atom bomb but running a nuclear power plant, the alternative to enrichment of uranium for use in a light-water reactor is the use of a neutron moderator with a lower neutron absorption cross section than protium. Options include heavy water as used in CANDU type reactors or graphite as used in magnox or RBMK reactors. Obtaining heavy water however also requires isotope separation, in this case of hydrogen isotopes, which is easier due to the bigger variation in atomic weight. Both magnox and RBMK reactors had undesirable properties when run with natural uranium, which ultimately led to the replacement of this fuel with low enriched uranium, negating the advantage of foregoing enrichment. Pressurized heavy-water reactors such as the CANDU are still in active use and India which has limited domestic uranium resources and been under a partial nuclear embargo ever since it became an atom bomb state in particular relies on heavy water moderated reactors for its nuclear power. A big downside of heavy water reactors is the enormous upfront cost of the heavy water.
Methodology
Diffusion
Often done with gases, but also with liquids, the diffusion 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 through a membrane, whose pore diameters are not larger than the mean free path length (Knudsen flow). The speed ratio is equal to the inverse square root of the mass ratio, so the amount of separation is small. For example for 235UF6 versus 238UF6 it is 1.0043. Hence 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, each requiring recompression of the gas.
The first large-scale separation of uranium isotopes was achieved by the United States in large gaseous diffusion separation plants at Clinton Engineering Works, 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. Due to the high energy consumption, enrichment of uranium by diffusion was gradually replaced by more efficient methods.
The last diffusion plant closed in 2013.World Nuclear Association, US Nuclear Fuel Cycle, (2015), http://www.world-
nuclear.org/info/Country-Profiles/Countries-T-Z/USA--Nuclear-Fuel-Cycle/ The Paducah Gaseous Diffusion Plant was a US government effort to generate highly enriched uranium to power military reactors and create nuclear bombs which led to the establishment of the facility in 1952. Paducah's enrichment was initially kept to low levels, and the facility operated as a "feed facility" for other defence facilities that processed the enriched uranium at Oak Ridge National Laboratory in Oak Ridge, Tennessee, and Portsmouth Gaseous Diffusion Plant in Piketon, Ohio. The goal of Paducah and its sister facility in Piketon was adjusted in the 1960s when they started to enrich uranium for use in commercial nuclear reactors to produce energy.
Centrifugal
Centrifugal schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This is often done in gaseous form using a Zippe-type centrifuge. Centrifuging plasma can separate isotopes as well as separating ranges of elements for radioactive waste reduction, nuclear reprocessing, and other purposes. The process is called "plasma mass separation"; the devices are called "plasma mass filter" or "plasma centrifuge" (not to be confused with medical centrifuges).
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 centre 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. 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 tubes 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.
Electromagnetic
Electromagnetic separation is mass spectrometry on a large scale, so it is sometimes referred to as mass spectrometry. 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 National Laboratory and at the University of California, Berkeley, Ernest O. Lawrence developed electromagnetic separation for much of the uranium used in the first atomic bombs. Devices using his principle are named calutrons. 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 higher levels of purity.
Laser