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Uranium enrichment is the use of technology to separate the isotopes of uranium: uranium-238 and uranium-235. Uranium-235 is the only naturally occurring fissile isotope - one that will undergo fission after absorbing a neutron with zero kinetic energy. Naturally-occurring uranium consists of about 0.7% 235U and 99.3% 238U. Uranium with higher than natural content of 235U is referred to as "enriched".
Enrichment of uranium is necessary to provide nuclear fuel for the most popular nuclear reactor designs: the pressurized water reactor and the boiling water reactor. Otherwise those designs would not be able to achieve criticality, because ordinary water used as the coolant will absorb some neutrons. The typical concentration of 235U required is 3-5%. Enriched uranium is also frequently used for operational reasons in reactor designs that do not strictly require it, such as CANDU, Magnox and RBMK.
Methods of enrichment
There are two commercially used methods of uranium enrichment, both of which are based on the difference in mass between the two isotopes. The older one is gaseous diffusion, which works by forcing uranium hexafluoride gas through semipermeable membranes. Molecules of UF6 containing an atom of 235U are slightly better at permeating the membranes. The newer one is the gas centrifuge, where UF6 gas is injected into a cylindrical tall rotor spinning at extremely high speed which has a thermal gradient applied to it (it is 300°C hotter at the bottom). The enriched fraction gathers near the axis of rotation at top of the centrifuge. Centrifuge enrichment is roughly 100 times more energy efficient than gaseous diffusion, which is why all gaseous diffusion facilities are in the process of being phased out. A promising future method of enrichment is laser separation, which uses highly monochromatic laser beams to excite only the lighter molecules. Many other isotopic separation methods exist, but they are not generally used for the purpose of uranium enrichment - mostly due to their prohibitive energy requirements. One such method, vortex tube enrichment, consists of injecting uranium hexafluoride gas mixed with hydrogen tangentially into a tube, forcing the gas to travel the length of the tube in a spiral. At the end of the tube is a conical nozzle that separates the stream into a hot stream and a cool stream; centrifugal force ensures that these streams have different isotopic ratios. The hot passes through the nozzle, and the cold is forced back through the tube in a vortex of smaller diameter. To date, only the South African nuclear weapon program has used this process. This method is also notable for having no moving parts, although the plumbing involved was apparently quite complex.
After the 235U is extracted and purified from the rest of the uranium metal, the 238U that's left over is depleted uranium.
Uranium enrichment is a politically sensitive technology, because the same facilities used to enrich reactor fuel can be theoretically used to enrich uranium to weapons grade (over 85% uranium-235). While nuclear reactors, and nuclear power plants in particular, draw much more attention from the anti-weapons arm of the anti-nuclear movement than enrichment facilities, the actual proliferation potential of power reactors is almost nonexistent. It's the enrichment facilities that provide a much less expensive route to nuclear bombs. This route was used by Pakistan to build its nuclear arsenal, though no civil nuclear equipment was used.
Difficulty in clandestine production
Production of weapons-grade uranium in enrichment facilities safeguarded by IAEA would be very tough to hide, because it would require a significant reconfiguration of the plant and a dramatic reduction in the output of the legitimate products, such as low-enriched uranium.