Uranium Enrichment Techniques
A state selecting uranium for its weapons must obtain a supply of uranium ore and construct an enrichment plant because the U-235 content in natural uranium is over two orders of magnitude lower than that found in weapons grade uranium (>90 percent U-235 U). Ordinary natural uranium contains only 0.72 percent 235 U, the highly fissionable isotope, the rest of the material being largely the much less fissionable isotope U-238 (which cannot sustain a chain reaction). The fissile material must be separated from the rest of the uranium by a process known as enrichment. Uranium enriched to 20 percent or more U-235 is called highly enriched (HEU). Uranium enriched above the natural U-235 abundance but to less than 20 percent is called low-enriched (LEU). Several enrichment techniques have been used.
Uranium-235 is of particular interest because it is the only fissile material that occurs in nature in significant quantity, and it can be used to construct a nuclear explosive device if a sufficient quantity can be acquired. In a typical sample of natural uranium, only 0.72 percent of the atoms are 235 U atoms, and it can be assumed that all of the remaining atoms are 238 U atoms. 8 Natural uranium typically has a composition of 0.0055 atom % 234 U, 0.7205 atom % 235 U, and 99.274 atom % 238 U. For most purposes, the tiny fraction of 234 U can be neglected. Higher concentrations of 235 U are required for many applications, and the use of uranium isotope separation processes to increase the assay of 235 U above its natural value of 0.72 percent is called uranium enrichment.
While low-enriched uranium (LEU) could technically mean uranium with an assay anywhere between slightly greater than natural (0.72 percent) and 20 percent 235 U, it most commonly is used to denote uranium with an assay suitable for use in a light-water nuclear reactor (i.e., an assay of <5 percent). Similarly, the term "highly enriched" uranium (HEU) could be used to describe uranium with an assay >20 percent, but it is commonly used to refer to uranium enriched to 90 percent 235 U or higher (i.e., weapons-grade uranium). The term "oralloy" was used during World War II as a contraction of "Oak Ridge alloy," and it denoted uranium enriched to 93.5 percent 235 U.
Manhattan Project scientists and engineers explored several uranium-enrichment technologies, and production plants employing three uranium-enrichment processes -- electromagnetic isotope separation (EMIS), liquid thermal diffusion, and gaseous diffusion -- were constructed at Oak Ridge, Tennessee, during the period from 1943 to 1945. Centrifugation was tried, but the technology needed to spin a rotor at an appropriate speed was not then practical on an industrial scale. The aerodynamic separation processes developed in Germany and South Africa did not exist during World War II; neither, of course did laser isotope separation or plasma separation. The World War II Japanese nuclear program made some attempts to find a purely chemical process.
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