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When uranium is mined, it consists of heavy-weight U-238 atoms (about 99.3% of the mass), middle-weight U-235 atoms (0.7%), and light-weight U-234 atoms (< 0.01%). Each of the atoms contains 92 protons in the nucleus (which is what makes it uranium), but U-238 contains 146 neutrons, U-235 contains 143 neutrons, and U-234 has 142 neutrons. Even though the three atoms have differnt nuclear masses, they all occur in the same location in the periodic table of the elements because they are all uranium - they are isotopes (from Latin, meaning "in the same place.") U-238 is a stable isotope, but the fewer neutrons in the nucleus of U-235 makes it unstable. U-235 spontaneously undergoes nuclear fission with the release of energy.

Uranium ore does not have enough U-235 in it to keep a nuclear reaction going. However, uranium enriched in U-235 can produce a controlled nuclear reaction in commercial light water reactors to generate electricity. Normally, the amount of the U-235 isotope is enriched from 0.7% of the uranium mass to about 5%. Gaseous diffusion is the only process being used in the United States to commercially enrich uranium. Gas centrifuges can also be used to enrich uranium.

U-238 in dark blue and U-235 in light blue

The gas centrifuge uranium enrichment process uses a large number of rotating cylinders in series and parallel formations. Centrifuge machines are interconnected to form trains and cascades. In this process, UF6 gas is placed in a cylinder and rotated at a high speed. This rotation creates a strong centrifugal force so that the heavier gas molecules (containing U-238) move toward the outside of the cylinder and the lighter gas molecules (containing U-235) collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. Significantly more U-235 enrichment can be obtained from a single unit gas centrifuge than from a single unit gaseous diffusion stage.

Although no gas centrifuge commercial production plants are currently operating in the United States, both Louisiana Energy Services (LES) and USEC Inc. have recently received a license to construct and operate a commercial enrichment facility. USEC Inc. was granted a license in February 2004 for a demonstration and test gas centrifuge plant, which is currently under construction. Both LES and USEC Inc.'s commercial facilities are now under construction.

Ironically, the Bush administration claims that Iran's uranium enrichment process is intended to make nuclear weapons (requiring enrichment of U-235 to about 90% of the uranium mass) while the United States is enriching uranium itself. So far, Bush etc. have not publicized the evidence to prove their claim (hypothesis).

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Excerpted and adapted from: Uranium Enrichment, Enriched uranium