Long-term above ground storage
Above ground storage is normally considered an interim measure for the management of radioactive waste (see section above onInterim waste storage). But it has, in the past, been considered as a disposal option. France investigated it for HLW within the framework of the 1991 law on research into radioactive waste management (Act No 91-1381 of 30 December 1991, also known as the 'Bataille Act' after the name of its proposer), but not as a means of final disposal. However, controlled surface storage over longer time periods (greater than a couple of hundred of years) has also been suggested as a long-term waste management option.
Long-term above ground storage involves specially constructed facilities at the Earth's surface that would be neither backfilled nor permanently sealed. Hence, this option would allow monitoring and retrieval at any time without excessive expenditure.
Suggestions for long-term above ground storage broadly fall into two categories:
- Conventional stores of the type currently used for interim storage, which would require replacement and repackaging of waste every 200 years or so.
- Permanent stores that would be expected to remain intact for tens of thousands of years. These structures are often referred to as 'Monolith' stores or 'Mausoleums'.
The latter category of store is derived from the principle of 'guardianship', where future generations continue to monitor and supervize the waste.
Both suggestions would require information to be passed onto future generations, leading to the question of whether the stability of future societies could be ensured to the extent necessary to continue the required monitoring and supervision.
No country is currently planning to implement long-term (i.e.greater than a few hundred years) above ground storage. However, France is investigating long-term interim storage, but not necessarily above ground.
Long-term above ground storage has been considered as part of the range of management concepts in Switzerland by EKRA (Expert Group on Disposal Concepts for Radioactive Waste). EKRA observed that it was unclear what additional steps would be necessary to show how the long-term above ground storage concept could be brought to the state of development comparable with that of geological disposal, and it recommended geological disposal as the preferred option.
Disposal in outer space
The objective of this option is to remove the radioactive waste from the Earth, for all time, by ejecting it into outer space. The waste would be packaged so that it would be likely to remain intact under most conceivable accident scenarios. A rocket or space shuttle would be used to launch the packaged waste into space. There are several ultimate destinations for the waste which have been considered, including directing it into the Sun.
The high cost means that such a method of waste disposal could only be appropriate for separated HLW –i.e.long-lived highly radioactive material that is relatively small in volume – rather than spent fuel. The question was investigated in the USA by NASA in the late 1970s and early 1980s. Because of the high cost of this option and the safety aspects associated with the risk of launch failure, it was abandoned.
Rock melting
The deep rock melting option involves the melting of wastes in the adjacent rock. The idea is to produce a stable, solid mass that incorporates the waste, or encases the waste in a diluted form (i.e.dispersed throughout a large volume of rock), and that cannot easily be leached and transported back to the surface. This technique has been mainly suggested for heat-generating wastes such as vitrified HLW (see information paper on Treatment and Conditioning of Nuclear Wastes) and host rocks with suitable characteristics to reduce heat loss.
The HLW in liquid or solid form could be placed in an excavated cavity or a deep borehole. The heat generated by the wastes would then accumulate resulting in temperatures great enough to melt the surrounding rock and dissolve the radionuclides in a growing sphere of molten material. As the rock cools it would crystallize and incorporate the radionuclides in the rock matrix, thus dispersing the waste throughout a larger volume of rock. There are some variations of this option in which the heat-generating waste would be placed in containers and the rock around the container melted. Alternatively, if insufficient heat is generated the waste would be immobilized in the rock matrix by conventional or nuclear explosion.
Rock melting has not been implemented anywhere for radioactive waste. There have been no practical demonstrations of the feasibility of this option, apart from laboratory studies of rock melting. In the late 1970s and early 1980s, the rock melting option at depth was taken forward to the engineering design stage. This design involved a shaft or borehole which led to an excavated cavity at a depth of 2.5 kilometres. It was estimated, but not demonstrated, that the waste would be immobilized in a volume of rock 1000 times larger than the original volume of waste.
Another early proposal was for the heat-generating wastes to be emplaced in weighted, heat-resistant containers such that they would melt the underlying rock, allowing them to move downwards to greater depths with the molten rock solidifying above. This proposed option resembles similar self-burial methods proposed for disposal of HLW in ice sheets (see section below onDisposal in ice sheets).
In the 1990s there was renewed interest in this option, particularly for the disposal of limited volumes of specialized HLW (particularly plutonium) in Russia and in the UK. A scheme was proposed in which the waste content of the container, the container composition, and the placement layout would be designed to preserve the container and prevent the wastes becoming incorporated in the molten rock. The host rock would be only partially melted and the container would not move to greater depths.
Russian scientists have proposed that HLW, particularly excess plutonium, could be placed in a deep shaft and immobilized by nuclear explosion. However, the major disturbance to the rock mass and groundwater by the use of nuclear explosions, as well as arms control considerations, has led to the general rejection of this option.
Disposal at subduction zones
Subduction zones are areas where one denser section of the Earth's crust is descending beneath another lighter, more buoyant section. The movement of one section of the Earth's crust below another is marked offshore by a trench, and earthquakes commonly occur adjacent to the inclined contact between the two plates. The edge of the overriding plate is crumpled and uplifted to form a mountain chain parallel to the trench. Deep sea sediments may be scraped off the descending slab and incorporated into the adjacent mountains. As the oceanic plate descends into the hot mantle, parts of it may begin to melt. The magma thus formed migrates upwards, some of it reaching the surface as lava erupting from volcanic vents. The idea for this option would be to dispose of wastes in the trench region such that they would be drawn deep into the Earth.
Although subduction zones are present at a number of locations across the Earth's surface, they are geographically very restricted. Not every waste-producing country would be able to consider disposal to deep-sea trenches, unless international solutions were sought. However, this option has not been implemented anywhere and, as it is a form of sea disposal, it is therefore not permitted by international agreements.
Sea disposal
Disposal at sea involves radioactive waste being dropped into the sea in packaging designed to either: implode at depth, resulting in direct release and dispersion of radioactive material into the sea; or sink to the seabed intact. Over time the physical containment of containers would fail, and remaining radionuclides would be dispersed and diluted in the sea. Further dilution would occur as the radionuclides migrated from the disposal site, carried by currents. The amount of radionuclides remaining in the seawater would be further reduced both by natural radioactive decay, and by the removal of radionuclides to seabed sediments by the process of sorption.
This method is not permitted by a number of international agreements.
The application of the sea disposal of LLW and ILW has evolved over time from being a disposal method that was actually implemented by a number of countries, to one that is now banned by international agreements. Countries that have at one time or another undertaken sea disposal using the above techniques include Belgium, France, Germany, Italy, the Netherlands, Sweden, Switzerland, and the UK, as well as Japan, South Korea, and the USA. This option has not been implemented for HLW.
Sub-seabed disposal
For the sub-seabed disposal option, radioactive waste containers would be buried in a suitable geological setting beneath the deep ocean floor. This option has been suggested for LLW, ILW, and HLW. Variations of this option include:
- A repository located beneath the seabed. The repository would be accessed from land, a small uninhabited island, or from an offshore structure.
- Burial of radioactive waste in deep ocean sediments.
Sub-seabed disposal has not been implemented anywhere and is not permitted by international agreements.
The disposal of radioactive wastes in a repository constructed below the seabed has been considered by Sweden and the UK. In comparison to disposal in deep ocean sediments, if it were desirable the repository design concept could be developed so as to ensure that future retrieval of the waste remained possible. The monitoring of wastes in such a repository would also be less problematic than for other forms of sea disposal.
Burial of radioactive waste in deep ocean sediments could be achieved by two different techniques: penetrators or drilling placement. The burial depth of waste containers below the seabed can vary between the two methods. In the case of penetrators, waste containers could be placed about 50 metres into the sediments. Penetrators weighing a few tonnes would fall through the water, gaining enough momentum to embed themselves into the sediments. A key aspect of the disposal of waste to seabed sediments is that the waste is isolated from the seabed by a thickness of sediments. In 1986, some confidence in this process was obtained from experiments undertaken at a water depth of approximately 250 metres in the Mediterranean Sea. The experiments provided evidence that the entry paths created by penetrators were closed and filled with remoulded sediments of about the same density as the surrounding undisturbed sediments.
Wastes could also be placed using drilling equipment based on the techniques in use in the deep sea for about 30 years. By this method, stacks of packaged waste would be placed in holes drilled to a depth of 800 metres below the seabed, with the uppermost container about 300 metres below the seabed.
In the 1980s, the feasibility of the disposal of HLW in deep ocean sediments was investigated and reported by the Organization for Economic Co-operation and Development (OECD). For this concept, radioactive waste would be packaged in corrosion-resistant containers or glass, which would be placed beneath at least 4000 metres of water in a stable, deep seabed geology chosen both for its slow water flow and for its ability to retard the movement of radionuclides. Radionuclides that are transported through the geological media, to emerge at the bottom of the seawater volume, would then be subjected to the same processes of dilution, dispersion, diffusion, and sorption that affect radioactive waste disposed of at sea (see section above onSea Disposal). This method of disposal therefore provides additional containment of radionuclides when compared with the disposal of wastes directly to the seabed.
Disposal in ice sheets
Since 1980 there has been no significant consideration of this option.
Containers of heat-generating waste would be placed in stable ice sheets such as those found in Greenland and Antarctica. The containers would melt the surrounding ice and be drawn deep into the ice sheet, where the ice would refreeze above the wastes creating a thick barrier. Although disposal in ice sheets could be technically considered for all types of radioactive wastes, it has only been seriously investigated for HLW, where the heat generated by the wastes could be used to achieve self-burial within the ice by melting.
The option of disposal in ice sheets has not been implemented anywhere. It has been rejected by countries that have signed the 1959 Antarctic Treaty or have committed to providing a solution to their radioactive waste management within their national boundaries.
Deep well injection (liquid)
This approach involves the injection of liquid radioactive waste directly into a layer of rock deep underground that has been chosen because of its suitable characteristics to trap the waste (i.e.minimize any further movement following injection).
In order to achieve this there are two geological prerequisites. There must be a layer of rock (injection layer) with sufficient porosity to accommodate the waste and with sufficient permeability to allow easy injection (i.e.act like a sponge). Above and below the injection layer there must be impermeable layers that act as a natural seal. Additional benefits could be provided from geological features that limit horizontal or vertical migration. For example, injection into layers of rock containing natural brine groundwater. This is because the high density of brine (salt water) would reduce the potential for upward movement.
Direct injection could in principle be used on any type of radioactive waste provided that it could be transformed into a solution or slurry (very fine particles in water). Slurries containing a cement grout that would set as a solid when underground could also be used to help minimize movement of radioactive waste.
Direct injection has been implemented in Russia and the USA.
In 1957 extensive geological investigations started in Russia for suitable injection layers for radioactive waste. Three sites were found, all in sedimentary rocks. At Krasnoyarsk-26 and Tomsk-7 injection takes place into two porous sandstone beds capped by clays at depths up to 400 metres. Whereas at Dimitrovgrad injection has now stopped, but took place into sandstone and limestone formations at a depth of 1400 metres. In total, some tens of millions of cubic metres of LLW, ILW and HLW have been injected in Russia.
In the USA, direct injection of about 7500 cubic metres of LLW as cement slurries was undertaken during the 1970s at a depth of about 300 metres over a period of 10 years at the Oak Ridge National Laboratory, Tennessee. It was abandoned because of uncertainties over the migration of the grout in the surrounding fractured rocks (shales). In addition a scheme involving HLW injection into crystalline bedrock beneath the Savannah River Site in South Carolina was abandoned before it was implemented due to public concerns.
Tenorm
Radioactive material is produced or collected as a waste product from the oil and gas industry and generally referred to as 'technologically enhanced naturally occurring radioactive material' (Tenorm)m. In oil and gas production, radium-226, radium-228 and lead-210 are deposited as scale in pipes and equipment in many parts of the world. Published data show radionuclide concentrations in scales up to 300,000 Bq/kg for Pb-210, 250,000 Bq/kg for Ra-226 and 100,000 Bq/kg for Ra-228. However, scrap steel from gas plants may be recycled if it has less than 500,000 Bq/kg (0.5 MBq/kg) radioactivity (the exemption level)n. This level however is 1000 times higher than the clearance level for recycled material (both steel and concrete) from the nuclear industry, where anything above 500 Bq/kg may not be cleared from regulatory control for recycling and must be disposed of, usually as intermediate-level waste.
The largest Tenorm waste stream is coal ash, with 280 million tonnes arising globally each year, and carrying uranium-238 and all its non-gaseous decay products, as well as thorium-232 and its progeny. This is usually just buried.
The double standard means that the same radionuclide, at the same concentration, can either be sent to deep disposal as waste (if scrap from the nuclear industry) or released for use in building materials (if contained in fly ash or recycled steel from oil/gas production).
