The radioactive fuel used to activate nuclear reactors, once used, becomes radioactive waste. Radioactive waste is also produced in other sectors (such as hospitals) and can be of different levels, depending on its degree of radioactivity. According to the level, they are treated in different ways. The waste produced by nuclear power plants is High Level Waste (HLW).
After use, the fuel is stored in cooling pools within the facility, which cool residual reactions to a standstill, reducing radioactivity to levels that can be isolated by concrete. They are then placed in dry nuclear storage containers, where the waste is held in an atmosphere of inert gas (helium), refrigerated, and insulated with various materials to protect personnel from radiation.
The waste then goes to Temporary Storage Sites (TSS), which are located underground, but close to the surface in areas with geologically stability. Their lifespan is 50 years, and they must be constantly monitored to ensure that no earthquakes have damaged their structure. TSSs are used as long as Deep Geological Storage is available. In countries with advanced nuclear technology, such as France and the UK, ATCs are located next to reprocessing plants.
Spent fuel can be reprocessed to separate uranium and plutonium; at this time, however, it is more expensive to reprocess the fuel than to extract and enrich uranium. Work is currently underway on the development of molten salt reactors, which would allow nuclear waste to be reused as an energy source until its level of radioactivity is considerably lower, thus facilitating waste management.
Finally, the high-level or semi-periodic waste goes into Deep Geological Storage (DGS) for thousands of years. Today, the DGS of reference is the Onkalo reservoir in Finland.