The most important radioactive component of uranium mill tailings is radium, which decays to produce radon. Other potentially hazardous substances in the tailings are selenium, molybdenum, uranium, and thorium.
Uranium mill tailings can adversely affect public health. There are four principal ways (or exposure pathways) that the public can be exposed to the hazards from this waste. The first is the diffusion of radon gas directly into indoor air if tailings are misused as a construction material or for backfill around buildings. When people breathe air containing radon, it increases their risk of developing lung cancer. Second, radon gas can diffuse from the piles into the atmosphere where it can be inhaled and small particles can be blown from the piles where they can be inhaled or ingested. Third, many of the radioactive decay products in tailings produce gamma radiation, which poses a health hazard to people in the immediate vicinity of tailings. Finally, the dispersal of tailings by wind or water, or by leaching, can carry radioactive and other toxic materials to surface or ground water that may be used for drinking water.
(4) Low-Level Radioactive Waste
Low-level radioactive waste (LLW) is radioactively contaminated industrial or research waste such as paper, rags, plastic bags, protective clothing, cardboard, packaging material, organic fluids, and water-treatment residues.
LLW is generated by government facilities, utilities, industries, and institutional facilities. LLW generators include approximately 100 operating nuclear power reactors, associated fuel fabrication facilities, and uranium fuel conversion plants, which together are known as nuclear fuel-cycle facilities. Hospitals, medical schools, universities, radiochemical and radio-pharmaceutical manufacturers and research laboratories are other users of radioactive materials which produce LLW. The clean-up of contaminated buildings and sites will generate more LLW in the future.
Disposal Management
1982, the NRC improved its regulatory requirements. That year, the NRC established disposal site performance objectives for land disposal of LLW technical requirements for the siting, design, operation, and closure for near-surface disposal facilities; technical requirements concerning waste packaging for land disposal; classification of waste; institutional requirements; and administrative and procedural requirements for licensing a disposal facility. Though the 1982 NRC regulations exempted existing NRC disposal site licensees, the NRC and the states are working to incorporate such requirements into those licenses.
In 1988, the DOE, which is self-regulating, issued its own orders governing the DOE disposal sites.
The general regulatory framework for the disposal of LLW has changed to account for new technology, what we have learned from past disposal practices, and current wisdom about environmental protection. As a result of increasing costs of LLW disposal at existing sites, pre-disposal waste processing (e.g., volume reduction) is a more common practice. The waste is processed by separating radioactive from nonradioactive components and by compacting bulk waste before packaging for disposal. Consequently, while the volume of waste to be disposed of is reduced, the concentration of radioactivity is greater. This waste requires more stringent safeguards for its disposal.
Site Selection for Disposal
Since then, four of the commercial sites have closed, mainly because of problems with site instability. These problems included the collapse of the earth covering the waste and difficulties in managing surface- and ground-water contamination. Since then the technology and requirements governing disposal sites have been upgraded. New disposal facilities must be designed to avoid two kinds of failures: those caused by long-term processes such as subsidence and those caused by more unpredictable events such as human intrusion (either intentional or unintentional) and natural disaster.
(5) Disposal of Naturally Occurring and Accelerator-Produced Wastes
Accelerator-Produced Materials
Accelerator-produced radioactive waste is produced during the operation of atomic particle accelerators for medical, research, or industrial purposes. The accelerators use magnetic fields to move atomic particles at higher and higher speeds before crashing into a preselected target. This reaction produces desired radioactive materials in metallic targets or kills cancer cells where a cancer tumor is the target. The radioactivity contained in the waste from accelerators is generally short-lived, less than one year. The waste may be stored at laboratories or production facilities until it is no longer radioactive. An extremely small fraction of the waste may retain some longer-lived radioactivity with half lives greater than one year. There are no firm estimates of the amount of this type of radioactive waste; however, it is generally accepted that the volume is extremely small compared to the other wastes discussed.
Naturally Occurring Radioactive Materials (NORM)
Naturally occurring radioactive materials (NORM) generally contain radionuclides found in nature. Once NORM becomes concentrated through human activity, such as mineral extraction, it can become a radioactive waste. There are two types of naturally occurring radioactive waste: discrete and diffuse. The first, discrete NORM, has a relatively high radioactivity concentration in a very small volume, such as a radium source used in medical procedures. Estimates of the volumes of discrete NORM waste are imprecise, and the EPA is conducting studies to provide a more accurate assessment of how much of this waste requires attention. Because of its relatively high concentration of radioactivity, this type of waste poses a direct radiation exposure hazard.
The second type, diffuse NORM, has a much lower concentration of radioactivity, but a high volume of waste. This type of waste poses a different type of disposal problem because of its high volume. The following are six sources of such naturally occurring radioactive materials.
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From residents of jerusalem burning their waste in the vale of Gehema to the dumping or burying of waste in shallow oceans the problem of waste disposal has always been present. With the increase over time of the population the amount of waste produced per person has also increased.
Proper disposal is essential to ensure protection of the health and safety of the public and quality of the environment including air, soil, and water supplies.
Secure landfill, chemical, biological and physical treatment, land treatment/solar evaporation, incinerator, resource recovery, secure landfill for chemical treatment waste. Some hazardous material are just left at open dumps or released into open waterways as the cost of converting them into reusable or recyclable products is too high. Abandoning the waste is just alot cheaper.
Some methods for disposal involve reducing the concentration ot the toxic materials to lower levels whereby others involve stabilising or solidifying a detoxification process involving using molten sodium metal reactions has been developed.
Radioactive waste disposal practices have changed substantially over the last twenty years. Evolving environmental protection considerations have provided the impetus to improve disposal technologies, and, in some cases, clean up facilities that are no longer in use. Designs for new disposal facilities and disposal methods must meet environmental protection and pollution prevention standards that are more strict than were foreseen at the beginning of the atomic age.
Disposal of radioactive waste is a complex issue, not only because of the nature of the waste, but also because of the complicated regulatory structure for dealing with radioactive waste. There are a variety of stakeholders affected, and there are a number of regulatory entities involved. Federal government agencies involved in radioactive waste management include: the Environmental Protection Agency (EPA), the Nuclear Regulatory Commission (NRC), the US Department of Energy (DOE), and the Department of Transportation. In addition, the states and affected Indian Tribes play a prominent role in protecting the public against the hazards of radioactive waste.
The federal government (the EPA, the DOE, and the NRC) has overall responsibility for the safe disposal of HLW and spent fuel. The EPA is responsible for developing environmental standards that apply to both DOE-operated and NRC-licensed facilities. Currently, the NRC is responsible for licensing such facilities and ensuring their compliance with the EPA standards. DOE is responsible for developing the deep geologic repository which has been authorized by Congress for disposing of spent fuel and high level waste. Both the NRC and the Department of Transportation are responsible for regulating the transportation of these wastes to storage and disposal sites.
Uso of landfill or incineration has problems as although the incineration of different pollutant reduces the volume of waste originally generated but also produces gas or liquid phase pollutants which can be quite toxic.
A large percentage of hazardous waste generated and treated are stored in landfill which becomes dangerous if they have no lining or no groundwater monitoring system to detect contamination. The landfills also release a small amount of waste into ground the amount depending o n the type buried, the weather conditions and the landfill design. The vulnerabiltity to rainfall esp playing a part enhacned by construction failures. Incorrect slop gradients at the sides of the landfill leading to surface cracks that aid lead to large scale productiong of leachates
Limestone should be avoided as it is so easy to pollute. Monitoring and covering surface cracks reduces the amount of leachate produced .
This contamination affect the food chain of man via bioaccumulation and progressive eco-system food chain concentration. The chemical have adverse health effect and can increase risk of cancer
Suggestions for waste disposal seabed disposal, geologic disposal, very deep hole disposal, ice sheet disposal, deep well injection, space disposal and chemical resynthesis, waste partitioning and transmutation. If reprocessing becomes politcally acceptable then waste can be processed with an acid base to remove the uranium and plutonium for recycling.
Geologic envirnonments such as salt beds, basalts and granite which after filling would be surrounded with absorbent backfill packaging such as bentonite.
International agreements may prevent seabed disposal but would be packaged into chemically stable canisters and inserted into stable floor sof deep ocean. LLW disposed of in landfills, dig trenches and fill with packaged LLW then cover a thicker than usual layer of the soil and geologic materials removed.
Like all industries, the thermal generation of electricity produces wastes. Whatever fuel is used, these wastes must be managed in ways which safeguard human health and minimise their impact on the environment.
Nuclear power is the only energy industry which takes full responsibility for all its wastes, and costs this into the product. Nuclear power is characterised by the very large amount of energy available from a very small amount of fuel. The amount of waste is also relatively small. However, much of the waste is radioactive and therefore must be carefully managed as hazardous waste. Radioactivity arises naturally from the decay of particular forms of some elements, called isotopes. Some isotopes are radioactive, most are not, though in this publication we concentrate on the former.
There are three kinds of radiation to consider: alpha, beta and gamma. A fourth kind, neutron radiation, generally only occurs inside a nuclear reactor. Different types of radiation require different forms of protection:
* Alpha radiation cannot penetrate the skin and can be blocked out by a sheet of paper, but is dangerous in the lung.
* Beta radiation can penetrate into the body but can be blocked out by a sheet of aluminium foil.
* Gamma radiation can go right through the body and requires several centimetres of lead or concrete, or a metre or so of water, to block it.
ll of these kinds of radiation are, at low levels, naturally part of our environment. Any or all of them may be present in any classification of waste. Radioactive wastes comprise a variety of materials requiring different types of management to protect people and the environment. They are normally classified as low-level, medium-level or high-level wastes, according to the amount and types of radioactivity in them. Another factor in managing wastes is the time that they are likely to remain hazardous. This depends on the kinds of radioactive isotopes in them, and particularly the half lives characteristic of each of those isotopes. The half life is the time it takes for a given radioactive isotope to lose half of its radioactivity. After four half lives the level of radioactivity is 1/16th of the original and after eight half lives 1/256th. The various radioactive isotopes have half lives ranging from fractions of a second to minutes, hours or days, through to billions of years. Radioactivity decreases with time as these isotopes decay into stable, non-radioactive ones.The rate of decay of an isotope is inversely proportional to its half life; a short half life means that it decays rapidly. Hence, for each kind of radiation, the higher the intensity of radioactivity in a given amount of material, the shorter the half lives involved. Three general principles are employed in the management of radioactive wastes:
* concentrate-and-contain
* dilute-and-disperse
* delay-and-decay.
The first two are also used in the management of non-radioactive wastes. The waste is either concentrated and then isolated, or it is diluted to acceptable levels and then discharged to the environment. Delay-and-decay however is unique to radioactive waste management; it means that the waste is stored and its radioactivity is allowed to decrease naturally through decay of the radioisotopes in it. Can you identify the application of these principles in the rest of this publication?
Types of radioactive waste (radwaste)
Low-level Waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc. which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed of more carefully than normal garbage. Usually it is buried in shallow landfill sites. To reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Worldwide it comprises 90% of the volume but only 1% of the radioactivity of all radwaste.
Intermediate-level Wastecontains higher amounts of radioactivity and may require special shielding. It typically comprises resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning. Worldwide it makes up 7% of the volume and has 4% of the radioactivity of all radwaste. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried, but long-lived waste (from reprocessing nuclear fuel) will be disposed of deep underground.
High-level Waste may be the spent fuel itself, or the principal waste from reprocessing this. While only 3% of the volume of all radwaste, it holds 95% of the radioactivity. It contains the highly-radioactive fission products and some heavy elements with long-lived radioactivity. It generates a considerable amount of heat and requires cooling, as well as special shielding during handling and transport. If the spent fuel is reprocessed, the separated waste is vitrified by incorporating it into borosilicate (Pyrex) glass which is sealed inside stainless steel canisters for eventual disposal deep underground.
On the other hand, if spent reactor fuel is not reprocessed, all the highly-radioactive isotopes remain in it, and so the whole fuel assemblies are treated as high-level waste. This spent fuel takes up about nine times the volume of equivalent vitrified high-level waste which results from reprocessing and which is encapsulated ready for disposal.
Both high-level waste and spent fuel are very radioactive and people handling them must be shielded from their radiation. Such materials are shipped in special containers which prevent the radiation leaking out and which will not rupture in an accident.
Whether reprocessed or not, the volume of high-level waste is modest, - about 3 cubic metres per year of vitrified waste or 25-30 tonnes of spent fuel for a typical large nuclear reactor. The relatively small amount involved allows it to be effectively and economically isolated.
Radioactive materials in the natural environment
Naturally-occurring radioactive materials are widespread throughout the environment, although concentrations are very low and they are not normally harmful.
Soil naturally contains a variety of radioactive materials - uranium, thorium, radium and the radioactive gas radon which is continually escaping to the atmosphere. Many parts of the earth's crust are more radioactive than the low-level waste described above. Radiation is not something which arises just from using uranium to produce electricity, although the mining and milling of uranium and some other ores brings these radioactive materials into closer contact with people, and in the case of radon and its daughter products, speeds up their release to the atmosphere. (See also Radiation and Life in this series.)
Wastes from the nuclear fuel cycle
Radioactive wastes occur at all stages of the nuclear fuel cycle, the process of producing electricity from nuclear materials. The cycle comprises the mining and milling of the uranium ore, its processing and fabrication into nuclear fuel, its use in the reactor, the treatment of the spent fuel taken from the reactor after use and finally, disposal of the wastes.
The fuel cycle is often split into two parts - the "front end" which stretches from mining through to the use of uranium in the reactor - and the "back end" which covers the removal of spent fuel from the reactor and its subsequent treatment and disposal. This is where radioactive wastes are a major issue.
Residual materials from the "front end" of the fuel cycle
The annual fuel requirement for a l000 MWe light water reactor is about 25 tonnes of enriched uranium oxide. This requires the mining and milling of some 50,000 tonnes of ore to provide 200 tonnes of uranium oxide concentrate (U3O8) from the mine.
At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while radon gas concentrations are kept to a minimum by good ventilation and dispersion in large volumes of air. At the mill, dust is collected and fed back into the process, while radon gas is diluted and dispersed to the atmosphere in large volumes of air.
Residual wastes from the milling operation contain the remaining radioactive materials from the ore, such as radium. These wastes are discharged into tailings dams designed to retain the remaining solids and prevent any seepage of the liquid. Eventually the tailings may be put back into the mine or they may be covered with rock and clay, then revegetated.
The tailings are around ten times more radioactive than typical granites, such as used on city buildings. If someone were to live continuously on top of the Ranger tailings they would receive about double their normal radiation dose from the actual tailings (ie they would triple their received dose).
With in situ leach (ISL) mining, dissolved materials other than uranium are simply returned underground from where they came.
Uranium oxide (U3O8) produced from the mining and milling of uranium ore is only mildly radioactive - most of the radioactivity in the original ore remains at the mine site in the tailings.
Turning uranium oxide concentrate into a useable fuel has no effect on levels of radioactivity and does not produce significant waste. First, the uranium oxide is converted into a gas, uranium hexafluoride (UF6), as feedstock for the enrichment process.
Then, during enrichment, every tonne of uranium hexafluoride becomes separated into about 130 kg of enriched UF6 (about 3.5% U-235) and 870 kg of 'depleted' UF6 (mostly U-238). The enriched UF6 is finally converted into uranium dioxide (UO2) powder and pressed into fuel pellets which are encased in zirconium alloy tubes to form fuel rods.
Depleted uranium has few uses, though with a high density (specific gravity of 18.7) it has found uses in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and radiation shielding. It is also a potential energy source for particular (fast neutron) reactors.
Wastes from the "back end" of the fuel cycle
It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. More than 99% of the radioactivity produced during the fission reaction is retained in the fuel rods. The balance is within the reactor structure.
About 25 tonnes of spent fuel is taken each year from the core of a l000 MWe nuclear reactor. The spent fuel can be regarded entirely as waste (as, for 40% of the world�s output, in USA and Canada), or it can be reprocessed (as in Europe). Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay.
Storage pond for spent fuel at UK reprocessing plant
The costs of dealing with this high-level waste are built into electricity tariffs. For instance, in the USA, consumers pay 0.1 cents per kilowatt-hour, which utilities pay into a special fund. So far more than US$ 18 billion has been collected thus.
Reprocessing?
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 first large-scale French and UK plants started up in 1995 and 2001 respectively. Across Europe, over 35 reactors are licensed to load 20-50% of their cores with MOX fuel.
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.
Major commercial reprocessing plants are operating in France and UK, with capacity of over 5000 tonnes of spent fuel per year, - equivalent to at least one third of the world's annual output. A total of over 55,000 tonnes of spent fuel has been reprocessed at these over 35 years.
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
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.
Waste disposal
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.
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 nation�s 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.
Layers of protection
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:
* Immobilise waste in an insoluble matrix, eg borosilicate glass, Synroc (or leave them as uranium oxide fuel pellets - a ceramic)
* Seal inside a corrosion-resistant container, eg stainless steel
* In wet rock: surround containers with bentonite clay to inhibit groundwater movement
* Locate deep underground in a stable rock structure
* Site the repository in a remote location.
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.
What happens in USA and Europe?
In USA high-level civil wastes all remain as spent fuel stored at the reactor sites. It is planned to encapsulate these fuel assemblies and dispose of them in an underground engineered repository about 2010, at Yucca Mountain, Nevada. (Suggest search Newsletters for Yucca Mountain.) This is the program which has been funded by electricity consumers to US$ 18 billion (ie @ 0.1 cent per kWh), of which about US$ 6 billion has been spent.
In Europe some spent fuel is stored at reactor sites, similarly awaiting disposal. However, much of the European spent fuel is sent for reprocessing at either Sellafield in UK or La Hague in France. The recovered U and Pu is then returned to the owners (the Pu via a MOX fuel fabrication plant) and the separated wastes (c3% of the spent fuel) are vitrified, sealed into stainless steel canisters, and either stored or returned. Eventually they too will go to geological disposal.
Sweden represents the main difference, it has centralised spent fuel storage, CLAB, near Oskarshamn, and will encapsulate spent fuel there for geological disposal by about 2015. Finland is establishing a final repository for spent fuel at Olkiluoto. European funding is at similar level to the USA per kWh.
A natural precedent
We have an example in nature to suggest that final disposal of high-level wastes underground is safe. Two billion years ago at Oklo in Gabon, West Africa, chain reactions started spontaneously in concentrated deposits of uranium ore. The reactions continued for hundreds of thousands of years forming plutonium and all the highly radioactive waste products created today in a nuclear power reactor. Despite the existence at the time of large quantities of water in the area, these materials stayed where they were formed and eventually decayed into non-radioactive elements. The evidence is there.
Alternatives to nuclear electricity
No technology is absolutely safe or without environmental effects. We should therefore compare the production of electricity from nuclear energy with the other options available to us. (see also Energy for the World: Why Uranium? in this series) Burning coal in power stations is still the major source of electricity worldwide, followed by hydro, uranium and gas.
A 1000 MWe light water reactor uses about 25 tonnes of enriched uranium a year, requiring the mining of some 50,000 tonnes of uranium ore. By comparison, a 1000 MWe coal-fired power station requires the mining, transportation, storage and burning of about 3.2 million tonnes of black coal per year. This creates around 7 million tonnes of carbon dioxide not to mention sulfur dioxide, depending on the particular coal. Solid wastes from a coal-fired power station can be substantial and cause environmental and health damage. (see also Sustainable Energy - Uranium, Electricity and Greenhouse in this series)
Many people are concerned about the possible warming of the earth through enhancement of the greenhouse effect. About half of this is due to steadily increasing carbon dioxide in the atmosphere over the past 150 years, largely from the burning of fossil fuels, particularly coal.
To investigate or consider:
* Why are nuclear wastes sometimes said to be a problem which is too difficult to solve?
* What are the advantages and disadvantages of the two ways of dealing with high-level waste (reprocessing and vitrification, or treating whole fuel assemblies as waste)?
* How do nuclear wastes compare with other industrial wastes? (Look at their hazard, the care which is taken with them and the funding involved.)
* What other industrial wastes decay over time so that their hazard steadily diminishes?
* How are the wastes from coal-fired electricity generation disposed of?
I. What is high-level radioactive waste?
The term high-level radioactive waste (HLW) generally refers to the highly radioactive wastes requiring permanent isolation from man's environment that arise as a byproduct of nuclear power generation. In countries where the spent nuclear fuel arising from reactor operations is chemically reprocessed, the radioactive wastes include highly concentrated liquid solutions of nuclear fission products. These are later solidified, generally in a glass matrix in a process known as vitrification, although other solidification processes are possible. Both the liquid solutions and the vitrified solids are considered HLW. If the spent nuclear fuel is not reprocessed, it, too, is considered as HLW to be disposed of by appropriate means (see reference 5). Because HLW contains relatively high concentrations of both highly radioactive and extremely long-lived radionuclides, special disposal practices are needed. Although the relative amount of HLW is small with respect to the total volume of radioactive waste produced in nuclear power programmes, it contains 99% of the radioactivity in this volume. Furthermore, it takes about 10,000 years for the radioactivity of such wastes to decay to the level which would have been generated by the original ore from which the nuclear fuel was produced, should this ore never have been mined.
Although certain reprocessing wastes and spent fuel are almost invariably considered the only sources of HLW, there are other waste types that, because of their level of radioactivity, may require a similar degree of isolation from man's environment, and therefore should be borne in mind when discussing radioactive waste disposal options. By far the most important of these other waste types is generally referred to as alpha-bearing wastes (also called transuranic (TRU) waste) because of its relatively high concentration of long-lived radionuclides that emit alpha particles as they decay. Indeed, these wastes are produced in volumes greater by a factor of 5-10 than HLW. A main difference between such wastes and HLW, however, is that TRU waste does not generate intense levels of radioactivity and heat.
II. When will disposal of HLW be necessary?
HLW, whether spent nuclear fuel or vitrified reprocessing waste, generates such intense levels of both radioactivity and heat that heavy shielding and cooling is required during its handling and temporary storage. The wastes are therefore best stored in specially engineered cooling pools or vaults for several decades prior to disposal. While stored, both the temperature and radioactivity of the wastes gradually decrease, simplifying their handling and disposal considerably.
Storage cannot be relied upon in the long-term to provide the necessary permanent isolation of the wastes from man's environment, and future generations should not have to bear the burden of managing wastes produced today. Seen from this perspective, while disposal of HLWis not an urgent technical priority, it is nevertheless an urgent public policy issue. These political aspects have led to the need for the nuclear industry in recent years to demonstrate the feasibility and safety of HLW disposal and, in some countries, laws have been implemented that require operational HLW disposal capability in the next 15-50 years. In particular, the Federal Republic of Germany and the United States plan to begin disposing of HLW in the early 2000s, France by about 2010, Belgium, Canada, Finland, Japan, Spain, Sweden, and Switzerland by about 2020, and the United Kingdom somewhat later. All HLW produced so far is currently being stored; no permanent disposal has yet occurred.
III. What options are available for HLW disposal?
Among the options discussed for disposing of HLW, an international consensus has emerged that deep geological disposal on land is the most appropriate means for isolating such wastes permanently from man's environment (see references 1-3). However, the full range of options also includes disposal in geological formations under the deep ocean floor, disposal on the ocean floor, disposal in glaciated areas, extraterrestrial disposal, and destruction by nuclear transmutation. In addition, extended storage, whether at production sites or in a centralised store, may, in principle, be considered an acceptable waste management strategy, provided it is not supposed to be perpetuated for longer than feasible and safe and is to be replaced by a more permanent solution at a later date.
A. Disposal of HLW in deep geological formations on land
The basic requirement for any geological formation is its ability to contain and isolate the radioactive wastes from man's environment until the radiotoxicity of the wastes has decayed to non-hazardous levels. In order to increase the safety of geological disposal, most such disposal concepts rely on a system of independent and often redundant barriers to the movement of radionuclides in an effort to provide a high degree of assurance that exposures to man will remain at acceptably low levels. These barriers generally include (1) the leach-resistant waste form itself, (2) corrosion-resistant containers into which the wastes are encapsulated, (3) special radionuclides- and groundwater- retarding material placed around the waste containers, commonly referred to as backfill, and (4) the geological formation itself -- the principal barrier -- which should both retard the transport of radionuclides in circulating groundwater, and isolate the waste from man's environment.
There are five important reasons why deep geological disposal on land has evolved into the disposal method of choice for virtually every country with a nuclear power programme.
1. It is an entirely passive disposal system with no requirement for continuing human involvement to ensure its safety.
2. Radioactive wastes present no hazard while they remain in a deep underground repository. Because of their depth of burial (several hundreds of metres or more), the possibility of intentional human intrusion is virtually eliminated, and, with a suitable choice of location, the likelihood of inadvertent human intrusion can be made minimal.
3. Flexibility and convenience are provided by the large variety of geological environments suitable for disposal. Geological units under consideration are rock salt, argillaceous formations (clays), and a range of crystalline rock formations including granite, welded tuff, basalt, and various metamorphic rock types.
4. The disposal option is demonstrably practical and feasible with currently existing technology used in other mining and civil engineering practices.
5. Although waste disposal implies the lack of intention to retrieve the waste, the repository can be designed so that the waste can be recovered, while the repository is in operation or even after closure.
B. Other options for the disposal of HLW
Disposal in geological formations under the stable, deep ocean floor, also called subseabed disposal, is conceptually similar to deep geological disposal on land, but there are a few notable differences. Whether the waste is emplaced in the relatively soft near-seabed unconsolidated sediments, or in the underlying consolidated sediments or even deeper basalt, the emplacement technology is not entirely defined. A major difference, however, would be the enormous dilution capacity provided by the ocean, should the containment system prematurely fail and allow substantial releases of radionuclides to the ocean floor. Another significant difference is that this disposal would be ideally suited for the establishment of international cooperative activities, although using the high sea, which is common property, represents a major political complication. Nonetheless, subseabed disposal is currently the only other disposal option under serious consideration as an alternative to deep geologic disposal on land.
With regard to other disposal options that have been discussed in OECD countries, disposal of HLW on the ocean floor in some kind of highly engineered containment would not be internationally acceptable at this stage. Disposal in glaciated areas, in Antarctica for example, would require substantial changes to international legal and political agreements. Disposal into space would provide the greatest degree of isolation from man's environment, but its practicality, cost, technological complexity, and potential risks all argue against it at the moment. Finally, nuclear transmutation, the conversion of long-lived radionuclides into shorter-lived or even stable nuclides, is not considered feasible in the near future.
The remainder of this paper will address only deep geological disposal on land, as this is currently the preferred disposal option throughout the OECD countries and, indeed, worldwide.
IV. What happens to HLW during disposal?
Disposal of HLW is preceded by some period of interim storage, either on site or at a centralised location, during which time the temperature and radioactivity of the HLW decrease systematically. Movement of the wastes to the disposal site will be necessary, and this may be accomplished using specially constructed collision- and fire- resistant shipping casks, transported via designated ship, train, or truck, according to national circumstances. Finally, special waste packaging is envisaged in most disposal concepts, either at the disposal site or at some interim site. It should be noted that liquid HLW must be solidified prior to its transport, packaging, or disposal.
During disposal, the individual waste packages will be lowered down shafts or transported into the repository through sloping tunnels. Once at the repository, the waste packages will be emplaced into holes predrilled into the sides or floor of the repository using equipment developed for this purpose. In most concepts, these holes will then be backfilled with suitable material. Filling the repository may require anywhere from 10 to 50 years or more, depending on the individual nuclear programme. Finally, the repository itself will be backfilled and sealed, including all shafts, boreholes, and tunnels which may have been drilled during repository construction. With a suitable choice of waste packaging, backfill, and geological environment, the radioactive materials should remain isolated from man's environment for many tens of thousands of years at least.
V. How can the safety of HLW disposal be assessed?
The long-term safety of HLW disposal can be systematically assessed through predictive modelling of the gradual failure of the engineered barriers (i.e., the waste form, waste package, and backfill) and the subsequent transport to man's environment of radionuclides by circulating groundwater. Such safety assessments must be based on a good physical understanding of the processes involved in the release and transport of radionuclides, as well as those acting on, or likely to act on, the repository and the geological formation. In addition, the potential interplay between these processes must be understood (see reference 4). Finally, substantial site investigation efforts will be needed, involving the collection of data at the surface as well as in situ, at the proposed repository location.
As a preliminary step to in situ studies, several OECD countries have developed Underground Research Laboratories in representative geological environments to demonstrate the safety of the geological disposal option. These laboratories are being used to provide data in support of generic safety assessments, to evaluate engineering feasibility, and to develop and refine techniques for site investigation. Laboratories in Belgium (Mol), Canada (Lac du Bonnet), the Federal Republic of Germany (Asse), Sweden (Stripa), and Switzerland (Grimsel) have been the focus of major international or bilateral cooperative research programmes (see Reference 7).
VI. How much does disposal of HLW cost and how can these costs be financed?
There are many parameters which affect the cost of HLW disposal, most importantly the size of the nuclear programme. Other parameters of concern that may vary from one estimate to the next are the depth of burial, the length of time the HLW cools prior to emplacement, the type of waste package used, the need to design for waste retrievability, and whether the disposal involves spent fuel or vitrified reprocessing wastes. Despite variations in these parameters, most estimates conclude that the cost of deep geological disposal will represent only a few per cent of current electricity generating costs.
According to the "polluter-pays" principle, the cost of HLW disposal is properly financed by the nuclear utilities. In many countries, a special waste fund has been established to cover the cost of HLW disposal according to which the utility may pay either an annual fee or an amount that in some way corresponds to the relative amount of HLW produced. The establishment of such a fund is important because disposal of HLW will not occur until many years - several decades in most cases - after its production. Whatever the system of financing, a general principle is that future generations should not have to pay for disposal of the wastes generated today.
VII. What is the role of the OECD Nuclear Energy Agency?
The NEA has been concerned with the problem of high-level waste disposal for more than a decade and this topic has developed in recent years into a priority area of its programme. Its principal role is to assist its Member countries in the further development of methodologies to assess the long-term safety of radioactive waste disposal systems and to increase confidence in their application and results. This is done through the exchange of information and experience among national experts, joint studies of issues important for safety assessment (identification of potentially disruptive events, treatment of uncertainties), the development of related computer models (in particular for probabilistic events) and data bases (used to assess the behaviour of radioactive materials in the geosphere), and their validation at an international level.
The NEA also sponsors international research and development projects (the Stripa Mine Project, in Sweden, and the Alligator Rivers Project, in Australia) and co- ordinates activities of its Member countries involving in situ research and site investigations (see reference 6). It ensures that working links are maintained at the international level between performance assessment projects, field projects and underground research laboratories, through specialised working groups operating in the Agency's framework. The ultimate purpose of this international effort is to reach the level of scientific understanding required to ensure that nuclear waste disposal systems will be able to contain and isolate the radioactive materials so that they will not cause any harm to man or his environment either now or in the future. Such co-operative programmes are also aimed at enhancing confidence in the quality of the safety analyses upon which the acceptability of nuclear waste disposal is to be judqed.
Radioactive waste disposal should isolate the waste from humans and the environment for necessary times as to ensure no potential future releases of radioactive substances to the environment which would constitute an unacceptable risk. Builit at or near surface for low level or short lived wastes or deep underground in geological formations for gigh lkevel and long lived wastes
The conditions prevailing at Yucca Mountain are significantly different to those considered in other national repository programmes in that Yucca Mountain is in a closed basin and the repository is in an oxidising environment above the water table. The IRT has taken due account of these differences in conducting the review. Geological disposal was defined in a 1995 Collective Opinion of the Nuclear Energy Agency (NEA) Radioactive Waste Management Committee titled “The Environmental and Ethical Basis of Geological Disposal.” According to page 16 of that document, geological disposal is provided by a system that will:
(a) “isolate the wastes from the biosphere for extremely long periods of time,” and (b) “ensure that residual radioactive substances reaching the biosphere will be at concentrations that are insignificant compared, for example, with the natural background levels of radioactivity.” Geological disposal hould also “provide reasonable assurance that any risk from inadvertent human intrusion would be very small.”
A theory that if the harmful chemical were adequately diluted its potential for harm would decrease was challenged when there was a search for the threshhold of exposure to xrays below which oone whould get cancer
