How is enriched uranium produced?

Before I explain how enriched uranium is made, let us see
what enriched uranium is. Uranium that is found in nature consists of many isotopes of
which approximately 99.3% is U-238 and 0.7% is U-235.

When
uranium is used as a fuel for fission reactions only the atoms of U-235 undergo fission
and release energy. The U-238 does not possess this ability. Energy is produced in a
nuclear reactor when a chain reaction is started. This happens when the products of one
fission reaction can lead to one or more fission reactions. As U-235 has a very small
percentage, a chain reaction cannot be started unless its percentage is increased. This
involves the process called uranium
enrichment.

There are several ways in which
uranium enrichment is achieved. The technique used initially was called gaseous
diffusion and was based on the property that compounds containing U-235 and U-238 pass
through semi-permeable membranes at a slightly different rate. The technique is
extremely slow and inefficient and is being replaced by a method that uses a gas
centrifuge.

When molecules of U-235 and U-238 are spun in a
centrifuge, due to their higher mass U-238 molecules move to the outer portion of the
centrifuge while U-235 remains closer to the center. The U-235 is collected from here.
The process has to be repeated several times before the required increase in the U-235
concentration can be achieved, but it is around 60 times more efficient than the gaseous
diffusion method.

In addition, many new techniques using
lasers, vibrations, heat, etc., are being devised to be able to enrich uranium faster
and more efficiently.