The Front End of the Nuclear Fuel Cycle:Enrichment

The next step in the process is the concentration,
or enrichment, of the uranium-235 isotope. Natural uranium consists primarily of a mixture
of two isotopes of uranium, and only 0.71% of natural uranium is uranium-235, which is
fissile, or capable of undergoing fission, the process by which energy is produced in
a power reactor. The remainder of the uranium is uranium-238,
and for the most common types of nuclear reactor a higher than natural concentration of uranium-235
is required. The enrichment process produces this higher
concentration, typically between 3-5% uranium-235. This enrichment process is by far and away
the most technically challenging part of the front end of the fuel cycle. We are not talking about chemical concentration
here. The two uraniums, U-235 and U-238, have identical
chemical properties. We are talking about the differences in atomic
mass, and it is less than one percent. There are two enrichment processes in large-scale
commercial use, each of which uses uranium hexafluoride as the feed. One process is gaseous diffusion, and the
newer, more efficient process is gas centrifuges. They both use the physical properties of the
molecules, specifically this one percent mass difference, to separate the isotope. The product of this stage of the nuclear fuel
cycle is enriched uranium hexafluoride, which is reconverted to produce the enriched uranium
oxide. Let’s look at how these two processes work.

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