Downstream fuel cycle
The Nuclear Fuel Cycle : Back End and Generic Issues

Once loaded into the reactor, the fuel does what it was manufactured to do: produce energy through fission. This process comes to an end when the fuel reaches the set limit in terms of burnup (maximum amount of energy the fuel can deliver per unit mass) and reactivity (the neutron multiplication coefficient in the fuel cannot fall below a minimum value acceptable in terms of the overall neutron balance of the reactor core). The fuel is then said to be "exhausted" or "spent". It must therefore be discharged from the reactor and replaced with fresh fuel.

During irradiation in the reactor, some of the heavy nuclei initially contained in the uranium fuel ( ²³⁵ U, ^...