Electromagnetic Isotope Separation Uranium Enrichment
One of the earliest successful enrichment technique was electromagnetic isotope separation (EMIS), in which large magnets are used to separate ions of the two isotopes. The first large-scale uranium enrichment facility, the Y-12 plant at Oak Ridge, Tennessee, used EMIS in devices called "calutrons." The process was abandoned in the United States because of its high consumption of electricity, but was adopted by the Iraqis because of its relative simplicity and their ability to procure the magnet material without encountering technology transfer obstacles. Both gaseous diffusion and EMIS require enormous amounts of electricity.
The EMIS process is based on the same physical principle as that of a simple mass spectrometer -- that a charged particle will follow a circular trajectory when passing through a uniform magnetic field. Two ions with the same kinetic energy and electrical charge, but different masses (i.e., 235 U + and 238 U + ), will have different trajectories, with the heavier 238 U + ion having the larger diameter. The different diameters of the trajectories of the two uranium ions allow for the separation and collection of the material in receivers or "collector pockets." EMIS is a batch process that can produce weapons-grade material from natural uranium in only two stages. However, hundreds to thousands of units would be required to produce large quantities of HEU because of the process's relatively low product collection rate and the long cycle time required to recover material between runs.
In the uranium EMIS process, uranium ions are generated within an evacuated enclosure (called a "tank") that is located in a strong magnetic field. For the EMIS ion source, solid uranium tetrachloride (UCl 4 ) is electrically heated to produce UCl 4 vapor. The UCl 4 molecules are bombarded with electrons, producing U + ions. The ions are accelerated by an electrical potential to high speed and follow a circular trajectory in the plane perpendicular to the magnetic field. In the U.S. EMIS separators, the ion beam traverses a 180-deg arc before the ions pass through slit apertures at the collector. A major problem with the EMIS process is that less than half of the UCl 4 feed is typically converted to the desired U + ions, and less than half of the desired U + ions are actually collected. Recovery of unused material deposited on the interior surfaces of the tanks is a laborious, time-consuming process that reduces the effective output of an EMIS facility and requires a large material recycle operation.
In the U.S. EMIS program, production of weapons-grade uranium took place in two enrichment stages, referred to as the a and b stages. The first (a) stage used natural or slightly enriched uranium as feed and enriched it to 12-20% 235 U. The second (b) stage used the product of the (a) stage as feed and further enriched it to weapons-grade uranium. To allow more efficient use of magnets and floor space, the individual stages were arranged in continuous oval or rectangular arrays (called "race-tracks" or, simply, "tracks") with separator tanks alternated with electromagnetic units. The U.S. EMIS separators are referred to as "calutrons" because the development work was carried out at the University of California (Berkeley) during the early 1940's using cyclotrons.
Although most applications of the EMIS process have been applied to the commercial production of both stable and radioactive isotopes, all five recognized weapons states have tested or used the EMIS process for uranium enrichment. Even with the problems associated with using the process, an EMIS facility could be attractive for a country desiring a limited weapons-grade uranium enrichment program. The process might be especially appealing as a method for further enriching partially enriched material. It has been well documented that EMIS was the principal process pursued by the Iraqi uranium enrichment program. This occurred at a time when EMIS had been discarded and largely forgotten as a method for uranium enrichment because it is both energy intensive and labor intensive, and it is not economically competitive with other enrichment technologies.
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