The following diagram shows the main options available once fuel is discharged from the reactor:
Figure 4.1 --- Flow chart of options for the back end of the nuclear fuel cycle.
In the open (once-through) cycle, the irradiated fuel is treated as waste and eventually placed in a deep geological repository. In the closed (recycle) cycle, the fuel is reprocessed to recover usable uranium and plutonium, which can be fabricated into new fuel (e.g. MOX --- mixed oxide fuel).