HKNIC - Enrichment


Natural uranium consists mainly of a mixture of two nuclides of uranium, 99.275% U-238 and 0.711% U-235. Only U-235 is "fissile", or readily able to undergo fission, or splitting atoms using neutrons, to produce energy. U-238 is “fissionable,” that is, it can only undergo fission using high energy neutrons which are not readily available.

Most of the nuclear reactors used for power generation require a slightly enriched uranium of between 3% and 5% U-235 concentration. In contrast, some research reactors may have concentrations up to 80%.

Enrichment is accomplished by feeding gaseous uranium hexafluoride, which contains both U-235 and U-238, through a large number of diffusion or centrifuge stages. The diffusion process makes use of the phenomenon that gas containing the lighter U-235 diffuses faster through a membrane than gas containing the heavier U-238. In the centrifuge process, where the gas is spun in a cylinder, the gas containing U-235 stays in the core while the gas containing U-238 is spun towards the side of the vessel. In both processes, therefore, gaseous uranium hexafluoride is separated into two streams. One is finally "enriched" to the required concentration of U-235 while the other stream is depleted of its U-235. Uranium from the second stream is known as "depleted" uranium.

The resulting enriched uranium hexafluoride is reconverted to produce an enriched uranium dioxide (UO2).

The Diffusion and Centrifuge process
Depleted uranium contains mainly U-238. It is produced during the enrichment process and can be used in applications where a high density material is required, such as for radiation shielding or the keel of a racing yacht, as well as in the production of mixed oxide fuel. It is slightly radioactive and some precautions are necessary for its storage and handling. Mixed oxide fuel or MOX is a nuclear fuel mixture containing uranium oxide and plutonium oxide.