school energy&environment

Disposal of nuclear waste

The general considerations to be made about waste classification are:

• how long will the waste keep at a dangerous level?
• what is the concentration of radioactive material in waste?
• does the waste generate heat?

The persistence of radioactivity determines for how long the waste will have to be managed. The concentration and generation of heat indicate how they will have to be handled. These considerations also offer information on the most suitable methods to dispose of the waste.
The classification varies slightly from one country to another, but usually the internationally-accepted categories are:

• extremely low radioactivity or no radioactivity;
• low radioactivity waste;
• intermediate radioactivity waste;
• high radioactivity waste.

Extremely-low radioactivity waste or not radioactive waste include neglectable quantities of radioactivity and can be treated like domestic waste.
Low-radioactivity waste includes the majority of the waste deriving from the fuel cycle. It includes paper, cloth, tools, clothes, filters and other waste that contains small quantities of radioactivity usually with a short life. It does not require any screen during handling, transport and volume reduction before the disposal. It represents 90% of the total volume, but only contain 1% of the total radioactivity.
Intermediate-radioactivity waste includes higher quantities of radioactivity and usually requires a screen. The screen can be a lead or water barrier to protect from penetrating radiations like gamma rays. Intermediate radioactivity waste essentially include resins, chemical mud, fuel metallic coating. They can be mixed with concrete or bitumen when disposed of.
High-radioactivity waste includes fission products and transuranic elements produced in the reactor, which are highly radioactive and generate heat. This waste represents more than 95% of the total radioactivity even if the quantity of material produced is modest, i.e. around 25-30 tons of extinguished fuel or three cubic metres per year of vitrified waste for a large reactor.
In order to manage high-activity waste two different strategies are used: deep disposal and extended disposal. The first occurs inside stable and deep rocks, and has undergone significant developments in the last ten years, especially with reference to knowledge, characterisation and modelling of natural safety barriers or artificial barriers. The second, instead, is considered by the community as an alternative to deep disposal. High-radioactive waste keeps radioactive for a long period of time, therefore it is necessary to keep it distant from people for thousands of years, until its radioactive level is reduced. Geological deposits are created between stable rocks in the countries that most use nuclear waste. Each country has the responsibility to dispose of its own radioactive waste, even though some countries (Russia, China), have declared they are ready to host in their own territory, under payment, the radioactive waste coming other countries. A geological deposit is normally located at 500 metres underground in a rock, clay or salt formation. The basic concept is the “multiple barrier” principle: radioactive waste, as oxide ceramics (irradiated fuel) or vitrified are then “immobilized”. Then, they are “sealed” inside corrosion-resistant containers, such as stainless steel or copper and, finally, they are “buried” inside a stable rocky formation.
Other methods to stabilize high-activity waste are now being investigated. One of the most advanced methods is called ynroc, a ceramic that comprises three titanate minerals that are geo-chemically stable and that can include radioactive waste elements in their crystal structure, immobilizing them.