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Isotope separation

From Academic Kids

Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. This is the crucial process in the creation of a nuclear weapon.

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.
  • 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 depending 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.

Contents

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 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.

Commercial materials

To date large-scale commercial isotope separation has occurred of only three elements. In each case, the rarer of the two most common isotopes of an element has been concentrated for use in nuclear technology:

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 and in military propulsion reactors. It is not in practice possible 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 salting plutonium with Pu-241 renders it unsuitable for nuclear weapons.

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 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, see gaseous diffusion.

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 using a Zippe-type centrifuge.

Gas centrifuges using uranium hexaflouride 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 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).

Electromagnetic

This method is a form of mass spectrometry, 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 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 even higher levels of purity.

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. 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.

A second method of laser separation is known as MLIS, Molecular Laser Isotope Separation. In this method, an infrared laser is directed at uranium hexaflouride gas, exciting molecules that contain a U-235 atom. A second laser frees a flourine atom, leaving uranium pentaflouride which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be reflourinated (back to UF6) before being introduced into the next MLIS stage.

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 rates 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 commerially, 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:

k(HCO2-) = 9.54M-1s-1 k(H)/k(D) = 38
k(DCO2-) = 9.54M-1s-1 k(D)/k(T) = 8.1
k(TCO2-) = 9.54M-1s-1 k(H)/k(T) = 305

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