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Isotope separation

Isotope separation is the process of purifying specific isotopes of a chemical element by removing unwanted impurities. While in general atomic elements are purified through chemical processes, the fact that isotopes of the same element have the same chemical properties precludes them from this type of separation. Instead separation techniques must be used which isolate the different isotopes based on their unique atomic weights. Most methods rely on the difference of masses of the isotope species involved and therefore it is easier to separate isotopes with a larger mass difference. For example deuterium has twice the mass as hydrogen and is generally easier to purify than Uranium-235 and Uranium-238.

Most separation schemes employ a number of stages which have gradually higher purity. Each stage enriches the product of the previous step further before being sent to the next stage. This creates a sequential enriching system called a cascade.

There are two important factors that affect the performance of a cascade. First is the separation factor (the square root of the mass ratio of the two isotopes), which is a number greater than 1. Second the number of required stages to get the desired purity.

Isotope separation is an important process for the preparation of fuel for nuclear fission devices, such as nuclear reactors and nuclear weapons, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community.

Table of contents

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 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 United States uses large diffusion separation plants at Oak Ridge Laboratories, which were established as part of the Manhattan Project for uranium enrichment.

Centrifugal Force Centrifugal force 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. Centrifuges have a smaller throughput than diffusion plants and therefore may require many centrifuges operating in parallel to process large amounts of material. They are more economical than diffusion schemes, however, and this may be of relevance to a small nation attempting to produce a nuclear weapon. In particular, Pakistan is believed to have used this method in developing its nuclear weapons.

Electromagnetic This method uses the fact that charged particles are deflected in a magnetic field and the amount of deflection depends upon the particle's mass. This method is very expensive, and has an extremely low throughput but 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 undesirable 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 that an isotope absorbs is dependent upon its mass, allowing finely tuned lasers to only interact with 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 been developing recently as laser technology has improved, and is currently not used extensively. However, it is major concern to those in the field of nuclear proliferation in that it may be cheaper and more easily hidden than other methods of isotope separation.

Chemical methods Other methods use chemical reactions which vary with mass, and are most effective for light atoms such as hydrogen. Lighter isotopes may react or evaporate more quickly than heavy isotopes, allowing them to be separated. This is how heavy water is produced.

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