This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.
1. RADIOACTIVE WASTE DISPOSAL OPTIONS:
As noted in the previous chapters that the volume of nuclear waste produced by the nuclear industry is very small compared with other wastes generated. Each year, nuclear power generation facilities worldwide produce about 200,000 m3 of low- and intermediate-level radioactive waste, and about 10,000 m3 of high-level waste including used fuel designated as waste. In the Organization for Economic Co-Operation and Development (OECD) countries, some 300 million tonnes of toxic wastes are produced each year, but conditioned radioactive wastes amount to only 81,000 m3 per year.
In the UK, for example, the total amount of radioactive waste (including radioactive waste expected to arise from existing nuclear facilities) is about 4.7 million m3, or around 5 million tonnes. A further 1 million m3 has already been disposed. Of the UK’s total radioactive waste, about 94 percent (i.e. about 4.4 million m3) falls into the low-level radioactive waste (LLW), category. About 6 percent (290,000 m3) is in the intermediate-level radioactive waste (ILW) category, and less than 0.1 percent (1000 m3) is classed as high-level waste (HLW). Although the volume of HLW is relatively small, it contains about 95 percent of the total inventory of radioactivity.
A typical 1000 MWe (Megawatt Electricity) light water reactor will generate (directly and indirectly) 200-350 m3 low- and intermediate-level waste per year. It will also discharge about 20 m3 (27 tonnes) of used fuel per year, which corresponds to a 75 m3 disposal volume following encapsulation if it is treated as waste. Where that used fuel is reprocessed, only 3 m3 of vitrified waste (glass) is produced, which is equivalent to a 28 m3 disposal volume following placement in a disposal canister.
This compares with an average 400,000 tonnes of ash produced from a coal-fired plant of the same power capacity. Today, volume reduction techniques and abatement technologies as well as continuing good practice within the work force all contribute to continuing minimization of waste produced, a key principle of waste management policy in the nuclear industry. Whilst the volumes of nuclear wastes produced are very small, the most important issue for the nuclear industry is managing their toxic nature in a way that is environmentally sound and presents no hazard to both workers and the general public.
The diversity of radioactive waste to be dealt with, as well as the range of disposal facility designs and environmental settings available, has resulted in the development of several alternative disposal concepts. Many concepts have been put into practice and radioactive waste disposal has been safely practiced around the world since the middle of the last century.Consequently, a large number of waste disposal facilities have been constructed and are being operated today for all waste categories; the only exception relates to the completion and operation of disposal facilities for high level waste and spent nuclear fuel (when declared as waste). However, the viability of disposing of this type of waste in deep geological formations has been provided through many years of research and demonstration, including experiments carried out in dedicated underground research laboratories. The selection of the design and number of disposal facilities required in a country depends on many aspects, such as:
- National waste and spent fuel management policy and strategy;
- Waste inventories;
- Plans for nuclear energy exploitation; and
- Extent of the national nuclear programme.
Landfill disposal may be suitable for very low level waste with very limited amounts of long lived activity. These facilities usually contain no complex engineered barriers or elaborated sealing. In such cases, the requirements on the waste treatment and packaging will also be less stringent. But adequate waste acceptance criteria (WAC) and quality control must ensure that the radionuclide content, especially the content of long lived activity, remains at very low levels compatible with the limited containment and isolation capabilities of this type of disposal.
Most LLW is typically sent to land-based disposal immediately following its packaging for long-term management. This means that for the majority (~90% by volume) of all of the waste types, a satisfactory disposal means has been developed and is being implemented around the world.
Concentrating on ILW and HLW, many long-term waste management options have been investigated worldwide which seek to provide publicly acceptable, safe and environmentally sound solutions to the management of radioactive waste. Some countries are at the preliminary stages of their investigations whilst others such as Finland and Sweden have made good progress in their investigations to select publicly acceptable sites for the future disposal of waste. In Carlsbad, New Mexico in the USA, the Waste Isolation Pilot Plant (WIPP) disposal facility for defence-related transuranic wastes is in operation, underground in a salt formation.
The table presented under Figure: 02, sets out the commonly accepted management options. When considering these, it should be noted that the suitability of an option or idea can be dependent on the waste form, volume and radioactivity of the waste. As such, waste management options and ideas described in this chapter are not all applicable to different types of waste.
1.1 Near-Surface Disposal Options:
The term near-surface disposal replaces the terms “Shallow Land” and “Ground Disposal”, but these older terms are still sometimes used when referring to this option.The International Atomic Energy Agency (IAEA) defined this option as the disposal of waste, with or without engineered barriers, in:
- Near-surface disposal facilities at ground level. These facilities are on or below the surface where the protective covering is of the order of a few metres thick. Waste containers are placed in constructed vaults and when full the vaults are backfilled. Eventually they will be covered and capped with an impermeable membrane and topsoil. These facilities may incorporate some form of drainage and possibly a gas venting system; and
- Near-surface disposal facilities in caverns below ground level. Unlike near-surface disposal at ground level where the excavations are conducted from the surface, shallow disposal requires underground excavation of caverns but the facility is at a depth of several tens of metres below the Earth’s surface and accessed through a drift.
These facilities will be affected by long-term climate changes (such as glaciation) and this effect must be taken into account when considering safety as such changes could cause disruption of these facilities. This type of facility is therefore typically used for LLW and ILW with a radionuclide content of short half-life (up to about 30 years).
Near-surface disposal facilities are currently in operation in:
- UK – Low Level Waste Repository at Drigg in Cumbria operated by UK Nuclear Waste Management Ltd (a consortium led by Washington Group International with Studsvik UK, Serco and Areva) on behalf of the Nuclear Decommissioning Authority;
- Spain – El Cabril low and intermediate level radioactive waste disposal facility operated by ENRESA;
- France – Centre de l’Aube operated by Andra;
- Japan – Low-Level Radioactive Waste Disposal Center at Rokkasho-Mura operated by Japan Nuclear Fuel Limited; and
- USA – three low-level waste disposal facilities at: Barnwell, South Carolina – operated by Energy Solutions; Richland, Washington – operated by American Ecology Corporation (formerly U.S. Ecology); and Clive, Utah – operated by Energy Solutions.
Near-surface disposal facilities in caverns below ground level are currently in operation in:
- Sweden – the SFR final repository for short-lived radioactive waste at Forsmark, where the depth of the facility is 50m under the Baltic seabed – operated by the Swedish Nuclear Fuel and Waste Management Company (SKB); and
- Finland – Olkiluoto and Loviisa power stations where the depth of the facilities, are each at about 100 metres.
It must be kept in mind that activities such as the decommissioning of Nuclear Power Plants (NPPs) and other nuclear facilities as well as clean-up operations may result in significant amounts of low level waste with low content of transuranic elements and/or long lived activation and fission products. For this kind of waste, disposal in near surface facilities with limited engineered barriers at/or near the site where the waste arises might be a safe, economically attractive option. Containment of the radionuclides in the waste is guaranteed by emplacing it, appropriately packaged, above the groundwater table and by limiting or avoiding rainwater percolation with a sufficiently impervious cover. Often these low level waste facilities consist of trenches, especially in remote arid areas. Assessing the suitability of the site to demonstrate appropriate radionuclide containment and waste isolation is a necessary part of the licensing procedure of all repository sites. This will require studying the geological environment of the site, especially its hydrogeology in order to evaluate the contribution of natural barriers to containing radionuclides and diluting/retarding released radionuclides so that resulting radiation exposures are kept as low as reasonably achievable and below regulatory limits.For wastes with higher radioactive content, trench disposal has been often used. A trench can be divided into individual compartments to increase radionuclide containment and flexibility of operation. After filling, a waterproofing top cover is installed. Surveillance and monitoring are required after closure during the period of institutional control. Again, the WAC will limit the type, concentration and quantity of radionuclides allowed in waste packages, reflecting the limited retention capability of this type of site.
Engineered surface repositories are equipped with surface barriers (caps), vertical barriers (cut-off vaults) and sub-horizontal barriers (floors). There are other containment technologies that may be applied, including chemical barriers that retard migration of radionuclides without impeding the water movement.
After the waste is disposed of, the void spaces in vaults are usually filled with grout or some other backfill material. The engineered barrier system may include drainage collectors to channel out infiltrating water. Underground galleries may be installed to allow the functioning of the barriers to be checked. Additional barriers might be constructed around the disposal unit to control the movement of water.
Common to all surface repositories is a period of active institutional control following repository closure. Its purpose is to prevent human intrusion and damage to the facility from, for example, burrowing animals or erosion. This active control should persist for a period of time that is sufficient to allow the radioactivity to decay to values considered no longer a hazard. Active institutional control may persist for centuries. Passive institutional control may also be applied. This may include markers, controls on land ownership, and use and archiving of records.
1.2 Geological Disposal Options:
The long timescales over which some of the waste remains radioactive led to the idea of deep geological disposal in underground repositories in stable geological formations. Isolation is provided by a combination of engineered and natural barriers (rock, salt, clay) and no obligation to actively maintain the facility is passed on to future generations. This is often termed a multi-barrier concept , with the waste packaging, the engineered repository and the geology all providing barriers to prevent the radionuclides from reaching humans and the environment.A repository is comprised of mined tunnels or caverns into which packaged waste would be placed. In some cases (e.g. wet rock) the waste containers are then surrounded by a material such as cement or clay (usually bentonite) to provide another barrier (called buffer and/or backfill). The choice of waste container materials and design and buffer/backfill material varies depending on the type of waste to be contained and the nature of the host rock-type available.
Excavation of a deep underground repository using standard mining or civil engineering technology is limited to accessible locations (e.g. under land or near shore), to rock units that are reasonably stable and without major groundwater flow, and to depths of between 250m and 1000m. At a depth greater than 1000m, excavations may become increasingly difficult and correspondingly expensive.
Deep geological disposal remains the preferred option for waste management of long-lived radioactive waste in several countries, including Argentina, Australia, Belgium, Czech Republic, Finland, Japan, Netherlands, Republic of Korea, Russia, Spain, Sweden, Switzerland and USA. Hence, there is much information available on different disposal concepts; a few examples are given here. The only purpose-built deep geological repository for long-lived ILW that is currently licensed for disposal operations is in the USA. Plans for disposal of spent fuel are well advanced in Finland, Sweden and the USA. In Canada and the UK, deep disposal has been selected and the site selection process has commenced.The Swedish proposed KBS-3 disposal concept uses a copper container with a steel insert to contain the spent fuel. After placement in the repository about 500 metres deep in the bedrock, the container would be surrounded by a bentonite clay buffer to provide a very high level of containment of the radioactivity in the wastes over a very long time period. In June 2009, the Swedish Nuclear Fuel and Waste Management Company (SKB) announced its decision to locate the repository at Östhammar (Forsmark).
Finland’s repository programme is also based on the KBS-3 concept. Spent nuclear fuel packed in copper canisters will be embedded in the Olkiluoto bedrock at a depth of around 400 metres. The country’s nuclear waste management company, Posiva Oy, expects the repository to begin disposal operations in 2020.
The deposits of native (pure) copper in the world have proven that the copper also used in the final disposal container can remain unchanged inside the bedrock for extremely long periods, if the geochemical conditions are appropriate (reducing ground waters). The findings of ancient copper tools, many thousands of years old, also demonstrate the long-term corrosion resistance of copper, making it a credible container material for long-term radioactive waste storage.
In principle, repositories in caverns provide a higher level of containment and isolation than surface repositories. Also, the likelihood of human intrusion after repository closure is much lower, since the access to a closed underground facility requires greater technical effort. Consequently, such facilities may be able to accept high concentrations of long lived radionuclides. A further advantage of deep disposal is that the need for institutional control after closure is much diminished — in most cases, the land can be put to a range of uses, including agriculture, immediately after closure.
Many different rock types could host a deep repository. Granite, salt, clay, tuff and other rocks have been considered and/or proposed, but only one site has actually been implemented, in rock salt (WIPP). A further site in a low-grade iron-bearing rocks covered widely by clay has been licensed, but is not yet implemented (Konrad). In developing a deep repository, two options are available: re-use of an existing mine or a new excavation. Of the four licensed deep geological repositories (not all of them for long lived waste), three are re-used mines (Asse, Morsleben and Konrad in Germany) and one is in a purpose-built facility (WIPP in the USA).Shafts sunk from the surface to form a silo have been used for disposal of waste contaminated with transuranic elements at the Nevada Test Site. In this case, engineered barriers largely consist of the waste package. In Novaya Zemlya, a more highly engineered variant has been proposed for permafrost soil. Compared with other engineered surface repositories, such an approach would render the frozen, immobile groundwater an additional barrier to radionuclide escape.
Appropriately implemented geological repositories render the highest possible degree of waste isolation, and can therefore accept waste with high contents of long lived radioactivity. But the effort for implementation is high, so that their construction might not be justified for disposal of limited amounts of long lived waste. In some cases, co-disposal of LILW-LL with HLW may be economically attractive and feasible.
1.3 Other Disposal Options:
1.3.1 Disposal at Sea Option:
Disposal at sea involves radioactive waste being shipped out to sea and 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 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 sea water 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, Federal Republic of Germany, Italy, Netherlands, Sweden, Switzerland and the UK, as well as Japan, South Korea, and the USA. This option has not been implemented for HLW.
1.3.2 Sub Seabed Disposal Option:
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; and
- 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 made 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 tons 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 remolded 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 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 on Disposal at sea). This method of disposal therefore provides additional containment of radionuclides when compared with the disposal of wastes directly to the seabed.
1.3.3 Disposal in Ice Sheets Option:
For 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 advantage to self-bury the wastes 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. Since 1980 there has been no significant consideration of this option.1.3.4 Disposal at a Subduction Zone Option:
Subduction zones are areas where one denser section of the Earth’s crust is moving towards and underneath another lighter section. The movement of one section of the Earth’s crust below another is marked by an offshore trench, and earthquakes 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.
1.3.5 Rock Melting Disposal Option:
The deep rock melting option involves the melting of wastes in the adjacent rock. The idea is to either 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) 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 Waste Management in the Nuclear Fuel Cycle – Appendix 1: 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 the design of weighted, heat-resistant containers of heat generating wastes such that they would continue to melt the underlying rock, and allow them to move downwards to greater depths with the molten rock solidifying above it. This alternative resembles similar self-burial methods proposed for disposal of HLW in ice sheets (see section below on Disposal 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.
1.3.6 Disposal in Outer Space Option:
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 high-level waste (HLW) or spent fuel (i.e. long-lived highly radioactive material that is relatively small in volume). The question was investigated in the United States 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, this option was abandoned.
Today only radioisotope thermal generators (TRGs) containing a few kilograms of Pu-238 are launched by NASA (see information page on Nuclear Reactors for Space).
1.3.7 Deep Boreholes:
For the deep borehole option, solid packaged wastes would be placed in deep boreholes drilled from the surface to depths of several kilometres with diameters of typically less than 1 metre. The waste containers would be separated from each other by a layer of bentonite or cement. The borehole would not be completely filled with wastes. The top two kilometres would be sealed with materials such as bentonite, asphalt or concrete.
Boreholes can be readily drilled offshore (as described in the section below on Sub seabed disposal) as well as onshore in host rocks both crystalline and sedimentary. This capability significantly expands the range of locations that can be considered for the disposal of radioactive waste.
Deep borehole concepts have been developed (but not implemented) in several countries, including Denmark, Sweden, Switzerland and USA for HLW and spent fuel. Compared with deep geological disposal in a mined underground repository, placement in deep boreholes is considered to be more expensive for large volumes of waste. This option was abandoned in countries such as Finland and USA. The feasibility of disposal of spent fuel in deep boreholes has been studied in Sweden, in order to check whether deep geological disposal remains the preferred option. The borehole concept remains an attractive proposition under investigation for the disposal of sealed radioactive sources from medical and industrial applications.
1.3.8 Disposal in Layered Salt Strata or Domes:
Geological salt environments have a very low rate (Perhaps even absence) of groundwater flow and feature gradual self-sealing of the excavations due to creep of the salt, which is plastic.
The WIPP in New Mexico for defence transuranic wastes (similar to long-lived ILW) has been operational since 1999. For this repository natural rock salt is excavated from a Permian layer several metres thick, between other types of rock, 650 metres below ground level. The wastes placed in these excavations contain large volumes of long-lived ILW, usually in steel containers. The steel containers are then placed in concrete overpacks. A backfill material is then used to surround the overpacks. The primary purpose of the backfill is to provide control of the chemical environment. Containment of the radionuclides in the waste form mostly relies on the almost complete absence of water flow in the salt. To October 2010, there had been 9000 road shipments of wastes to WIPP, and 71,000 cubic metres of ILW disposed.
Salt environments are also available in northern Germany and the Netherlands although these are salt domes rather than bedded formations. In Germany, the former salt mines at Asse and Morsleben have been used for LLW and ILW disposal though this has now been suspended. The decommissioning process is now being investigated to determine the method for backfilling and sealing the repository.
Following an exhaustive site selection process the state government of Lower Saxony in 1977 declared the salt dome at Gorleben to be the location for a German national centre for disposal of radioactive wastes. It is now considered a possible site for geological disposal of high-level waste. The site could be available as a final repository from 2025, with a decision to be made about 2019. Some €1.5 billion was spent over 1979 to 2000 researching the site. Work then stopped due to political edict, but resumption of excavation was approved following a change of government in 2009.