Conventional Isotope Separation Methods

Electromagnetic Isotope Separation (EMIS)
Pioneered by Alfred O. Nier, the EMIS method relies on the idea that an atom with an electrical charge travelling through a constant magnetic field will move in a circle of radius that is proportional to the mass of the atom. This phenomenon is governed by the Lorentz force equation which relates the amount of force acting on the charged particle to its velocity and to the strength of the present magnetic field. Therefore with regard to uranium isotopes in a mixed stream, the lighter 235U will travel in a tighter circle than 238U will because the electromagnetic forces will deflect the particles proportionally in the direction of the magnetic field. This technique is very versatile in the sense that it may be used with any number of combinations of isotopes. Specifically, a uranium tetrachloride (UCl4) solid is vaporized and subsequently ionized via electron bombardment to gain positive uranium ions. Typically, a device called a calutron (a type of mass spectrometer developed during the Manhattan Project) is used to accelerate those ions to create a current, which may then be acted on by strong magnets. Collection of these ions takes place as they pass through slit apertures positioned where the researcher predicts they will be deflected. One metric of comparison for isotope separation methods is the separation factor. Given a starting concentration of less than 1% for an isotope of interest, only two or three passes in this setup are necessary in order to yield samples of high purity (>80%). The drawbacks to this type of separation are manifested as logistics issues, namely the acquisition and implementation of high voltage power supplies , vacuums, magnetic power supplies, and a plethora of chemical handling systems that provide for the recovery and cleaning of process ingredients.

Gaseous Diffusion Isotope Separation (GDIS)
As World War II subsided, the Americans greatly pursued the more energy efficient - although more complicated - gaseous diffusion separation. This method uses uranium hexafluoride (UF6) gas, which passes through a porous membrane. Analogous to the previous technique, the lighter 235U will be separated using principles of molecular effusion, or the movement of gas particles through small holes. The molecular weight of a gas is inversely proportional to the square of the effusion rate of that gas through an ideal porous barrier. In order for this technique to be efficient, the diameter of the orifices must be smaller than one mean free path of a particle in motion. The difference in velocities between 235U and 238U molecules is small, hence, a single stage of gaseous diffusion will achieve a relatively small amount of separation. Consequently, cascades are necessary to enrich a sample significantly - meaning a sample of low-enriched uranium gas must pass through series of diffusion chambers. Nickel and aluminum oxide are often selected as barrier materials in these chambers, which are required to withstand years of exposure to corrosive UF6 gas. Additionally, the auxiliary facilities required for this technique are cumbersome.

Gas Centrifuge Isotope Separation (GCIS)
More recently, proliferate entities have taken advantage of the development of centrifugal technologies to produce enriched uranium. This method, like the others, takes overwhelming advantage of the ~1.5% mass difference between uranium isotopes. Uranium hexafluoride gas is injected into a centrifuge which spins at twice the speed of sound. The heaviest of these isotopes, 238U, is pushed against the outer wall of the cylinder, while 235U, a lighter isotope, remains closer to the axis of rotation.The isotope(s) of interest is then extracted and directed into another centrifuge, where the same procedure is repeated. In order to obtain weapons-grade 235U, the gaseous compound must pass through tens of thousands of centrifuges before it is suitable.These centrifuges, however, are not easy to manufacture. The rotors are fabricated with materials that allow them to rotate at speeds greater than the speed of sound. To obtain efficient separation of the two isotopes, the outer wall of the spinning cylinder must be moving at 400-500 meters per second. This produces a centripetal force significant enough to distance a fraction of the gas molecules such that heavier isotopes are excluded from an enriched stream.

In the next post I will discuss a newer, more intriguing series of separation methods.

References:
Reynolds, John H. “Alfred Otto Carl Nier 1911-1994: A Biographical Memoir.” National Academies Press. Washington D.C. p8-9.
J. J. Klein. “Motion of a Charged Particle in a Uniform Magnetic Field.” Department of Physics, The University of Calgary, Calgary, Alberta, Canada <http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.40.523>
"The Manhattan Project: Making the Atomic Bomb." Isotope Separation. Web.
"A History of US Enrichment in the 50s." Oak Ridge National Laboratory. Web.
"Electromagnetic Isotope Separation;." UN Term. United Nations. <http://unterm.un.org/dgaacs/unterm.nsf/8fa942046ff7601c85256983007ca4d8/c7b94c4d0b2f8483852569fa00003daf>
"The Separation Of The Uranium Isotopes By Gaseous Diffusion." Atomic Archive. <http://www.atomicarchive.com/Docs/SmythReport/smyth_x.shtml>
http://www.world-nuclear.org/info/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment/