The separation of an isotopic mixture into its component isotopes. The charged ions of isotopes are deflected in electric and magnetic fields by differing amounts, depending on their mass - an effect exploited in the mass spectrometer. The rate of diffusion of a gas of isotopic mixture depends on the isotope mass, exploited in gaseous uranium hexafluoride for the enrichment of nuclear fuel.
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.
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.
There are three types of isotope separation techniques:
Those based directly on the atomic weight of the isotope.The third type of separation is still experimental, as 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.
Enrichment cascades
All large-scale isotope separation schemes employ a number of similar stages which produce successively higher concentrations of the desired isotope. First is the separation factor (the square root of the mass ratio of the two isotopes), which is a number greater than 1.
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.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. 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.
Techniques
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 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.
Centrifugal effect
Centrifugal effect schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall.
Gas centrifuges using uranium hexafluoride have largely replaced gaseous diffusion technology for uranium enrichment.
Vortex tubes were used by South Africa in their Helikon vortex separation process.
Electromagnetic
This method is a form of mass spectrometry, and is sometimes referred to by that name. 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.
Laser
In this method a laser 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 only interact with one isotope. This method is often abbreviated as AVLIS (atomic vapor laser isotope separation). 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.
A second method of laser separation is known as MLIS, Molecular Laser Isotope Separation.
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 hexaflouride as a feedstock, and uses magnets to separate the isotopes after one isotope is preferentially ionized.
Chemical methods
Although isotopes of a single element are normally described as having the same chemical properties, this is not strictly true. Lighter isotopes tend to react or evaporate more quickly than heavy isotopes, allowing them to be separated.
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:
| k(HCO2-) = 9.54 M | k(H)/k(D) = 38 |
| k(DCO2-) = 9.54 M | k(D)/k(T) = 8.1 |
| k(TCO2-) = 9.54 M | k(H)/k(T) = 305 |
The 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, ie 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 UTAIf, 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%
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