Disposal of nuclear waste
High Level Waste
This waste requires very heavy shielding as it is very radioactive. The intense radioactive decay generates a large amount of heat and this needs to be carefully considered when thinking about storage and final disposal.
High Level Waste includes spent fuel and highly radioactive liquids generated during reprocessing operations. The latter is all stored at Sellafield in high-integrity stainless steel tanks fitted with cooling coils to remove the heat generated. Management and disposal of this waste is difficult due to the high levels of radioactivity due to the short half-lives of some of the radionuclides present and the heat continually generated as a result of decay processes.
Spent fuel rods have to be moved by remote control as they are so radioactive. They are placed in cooling ponds for several months to allow short half life isotopes to decay. When they have reduced in activity enough they are reprocessed. This separates out uranium and plutonium isotopes that can be used as fuel from those that cannot.
The remaining waste can then be vitrified (encased in glass) and encased in air cooled steel drums for 50 years to allow the heat generated to reduce to manageable levels. They can then be buried deep underground in repositories (often deep mine shafts reinforced with concrete) that need to be highly shielded and carefully monitored at a geologically stable site.
For transportation purposes - any casing has to be high impact resistant and transport convoys have to be secure in case of terrorist action or any accident occurring.
Intermediate Level Waste
This waste also needs to be heavily shielded, as it can be extremely radioactive, but does not generate as much heat as High Level Waste. Some of the radioactive particles present in this waste may have very long half-lives and so require isolation for many thousands of years.
Intermediate Level Waste includes fuel element claddings removed prior to reprocessing, various sludges and ion exchange resins from fuel storage pond water treatment; concentrates of liquid waste streams; heavily contaminated scrap equipment; plutonium contaminated materials and graphite sleeves and steel components from AGR fuel assemblies. Large volumes of Intermediate Level Waste are expected from decommissioning operations are expected in the coming years.
Because of the wide range of Intermediate Level Waste sources many different forms of conditioning and packaging are required.
Low Level Waste
This waste tends to be low in radioactivity and high in bulk. It ranges from general rubbish (gloves, clothing, packaging, paper towels, over shoes, laboratory glassware, etc.) to some very low-level plutonium contaminated materials.
A lot of material classified as Low Level Waste, may in fact not be radioactive at all, but it is potentially radioactive through being in an active/contaminated area.
The low levels of radioactivity and the short-lives of the contaminants mean this waste is relatively harmless if handled properly. However, any site used for Low Level Waste disposal will need to be subject to land use restrictions for around 300 years after the site is closed. There is also always a risk of environmental problems if water leaching through the waste site finds its way into surface and ground waters.