The nuclear fuel cycle can be broken down into what is generally known as the 'front end' and 'back end' operations. The front end covers the operations from the mining of uranium to the manufacture of fuel assemblies for loading into the reactors. The back end covers the operations concerned with spent fuel that leaves reactors.
Fuel used in a nuclear power plant generates electricity
for about five years. After this time it becomes less efficient and
needs to be replaced. This spent fuel still contains 96% of the
original uranium, 1% of plutonium, and also about 3% of fission
waste products. At this stage, spent fuel can either be sent for
storage pending final disposal, or reprocessed to recover the
uranium and plutonium.
The reusable uranium and plutonium can be used to produce new fuel such as Mixed Oxide (MOX) fuel. The 3% waste is transformed into a solid, insoluble glass form by a vitrification process and stored pending final disposal, for instance into a deep geological repository.