Thus, to ensure that no significant environmental releases occur over periods of tens of thousands of years after disposal, a 'multiple barrier' disposal concept is used to immobilise the radioactive elements in high-level (and some intermediate-level) wastes and to isolate them from the biosphere. The principal barriers are:
For any of the radioactivity to reach human populations or the environment, all of these barriers would need to be breached before the radioactivity decayed.
If the spent fuel is later reprocessed, it is dissolved and separated chemically into uranium, plutonium and high-level waste solutions. About 97% of the spent fuel can be recycled leaving only 3% as high-level waste. The recyclable portion is mostly uranium depleted to less than 1% U-235, with some plutonium, which is most valuable.
Arising from a year's operation of a typical l000 MWe nuclear reactor, about 230 kilograms of plutonium (1% of the spent fuel) is separated in reprocessing. This can be used in fresh mixed oxide (MOX) fuel (but not weapons, due its composition). MOX fuel fabrication occurs at 5 facilities in Europe, with some twenty years of operating experience.
The 3% of the spent fuel which is separated high-level wastes amounts to 700 kg per year and it needs to be isolated from the environment for a very long time. These liquid wastes are stored in stainless steel tanks inside concrete cells until they are solidified.
1- Immobilising high-level waste
Solidification processes have been developed in France, UK, US and Germany over the past 35 years. Liquid high-level wastes are evaporated, mixed with glass-forming materials, melted and poured into robust stainless steel canisters which are then sealed by welding.
Borosilicate glass from the first waste vitrification plant in UK in the 1960s. This block contains material chemically identical to high-level waste from reprocessing. A piece this size would contain the total high-level waste arising from nuclear electricity generation for one person throughout a normal lifetime.
The vitrified waste from the operation of a 1000 MWe reactor for one year would fill about twelve canisters, each 1.3m high and 0.4m diameter and holding 400 kg of glass. Commercial vitrification plants in France, UK and Belgium produce about 1000 tonnes per year of such vitrified waste (2500 canisters) and some have been operating for more than 16 years.
Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters. Negatively pressurized compaction room, where dismantled pieces are compacted and placed in Specialized containers, as shown in the figure.
A more sophisticated method of immobilising high-level radioactive wastes has been developed in Australia. Called 'SYNROC' (synthetic rock), the radioactive wastes are incorporated in the crystal lattices of the naturally-stable minerals in a synthetic rock. In other words, copying what happens in nature. This process is now being tested in USA.
2- Final disposal of high-level waste is delayed to allow its radioactivity to decay. Forty years after removal from the reactor less than one thousandth of its initial radioactivity remains, and it is much easier to handle. Hence canisters of vitrified waste, or spent fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks for at least this length of time. (see chapter 8)
The ultimate disposal of vitrified wastes, or of spent fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada for all the nation¹s spent fuel.
One purpose-built deep geological repository for long-lived nuclear waste is in operation in New Mexico, though this only takes defence wastes.
After being buried for about 1,000 years most of the radioactivity will have decayed. The amount of radioactivity then remaining would be similar to that of the naturally-occurring uranium ore from which the fuel originated, though it would be more concentrated.
Sunday, December 27, 2009
Disposal stages of high level waste / Waste management
Publié par Dr.taki eddine à l'adresse 8:28 AM