I. How is radioactive waste generated?
All steps in the nuclear fuel cycle generate radioactive waste. In the front end of the cycle -- i.e. uranium mining and enrichment -- the radioactivity in the waste consists only of the naturally occuring elements in the original ore body. These elements are mainly uranium and its daughter products. During reactor operation, a wide spectrum of different radionuclides are generated by nuclear reactions (mainly fission) in the fuel, as well as through neutron activation of different elements in reactor core materials and in the water circulating in the reactor vessel. The fate of the generated radionuclides is one of the following:
The terminology of low-, intermediate- and high-level waste is used only to provide a broad categorisation of radioactive waste. In contrast to high-level waste, low- and intermediate-level waste generates only negligible amounts of heat due to radiation and does not require cooling during storage. Low-level waste normally can be handled without particular shielding, while intermediate-level waste might require shielding and may contain significant amounts of long-lived radionuclides.
More than 95 per cent of the total activity will be contained in the spent fuel, or the high-level waste if the fuel is reprocessed. The rest will appear in a variety of low- and intermediate-level waste (L/ILW) generated during the day-to-day operation of nuclear reactors, storage facilities and reprocessing plants (back-end of the cycle).
This is a simplified description and the details, nuclide by nuclide, are certainly more complicated. Factors like reactor type and mode of operation of the nuclear plant, the treatment of the spent fuel and the waste handling and conditioning itself will all have an influence on determining which radionuclides are generated, the amount produced and the type of radioactive waste in which they will appear.
II. What are the types of radioactive waste?
The radioactive mill tailings from uranium mining are by far the most voluminous radioactive waste generated within the whole fuel cycle (50-100 times more by volume than all other radioactive waste). They are normally stabilised and disposed of at or close to the mine of origin. As these wastes contain natural long-lived radionuclides, they must be disposed of in a way that affords long-term protection to man and his environment. These questions are not dealt with further in this issue brief, which is limited to a discussion of the disposal of low- and medium-level waste containing artificially generated radionuclides.
During reactor operation, L/ILW is generated both as a liquid and as a solid. The liquid is contaminated water from different parts of the reactor system and from the plant. Purification or concentration of this water gives rise to slurries that are mixed with cement or asphalt to form a stable waste form.
The solid waste is any potentially radioactive material, such as filters, valves, pipes, trash, etc, from the reactor systems or the plant. Most of the solid waste is generated during maintenance and repair work. It is compacted, incinerated or simply packed in drums. Embedding and/or encapsulation in concrete are methods sometimes used to obtain stable waste packages.
During reprocessing, the spent fuel is dissolved and uranium and plutonium are separated for recycling. The main waste product is the heat-generating high-level waste solutions containing the bulk amount of fission products from the spent fuel. Some of the reprocessing waste contains substantial amounts of long-lived radionuclides, so-called alpha-waste or transuranic (TRU)-waste, and these will require the same degree of isolation from man's environment as high-level waste or spent fuel.
L/ILW is also generated at a reprocessing plant. The treatment options are the same as for reactor waste: solidification of slurries in cement or asphalt, compaction, incineration or packaging/encapsulation of solid waste.
The decommissioning and dismantling of nuclear installations will also generate radioactive waste. In addition to the same types of waste produced during plant operation, other types of waste will be generated, notably some bulky internal structures from the reactor, the reactor vessel and its surrounding concrete structure.
Other types of low- and intermediate-level waste
Nuclear research, industrial and medical uses of radionuclides also generate L/ILW. In countries with no nuclear power programme, this constitutes the main category of radioactive waste, while for countries with a nuclear programme, it represents only 5-30 per cent of the total volume of radioactive waste. In countries with nuclear weapons, significant amounts of L/ILW as well as high-level waste are generated by the military programmes.
OECD trends of radioactive wastes from nuclear power operations
The amount of radioactive waste remaining after treatment will increase in years ahead because of the continuing development of nuclear power for electricity production. Quantities shown here should be used for trend purposes only, since waste quantities can vary significantly depending upon the underlying assumptions used to determine the amounts generated per installed 1000 megawatts of electric power.
III. How is the waste handled?
L/ILW is normally conditioned and packaged in drums or other containers at the site where it is generated. In some cases, however, some types of waste are transported to a central treatment facility. In certain countries, for instance, low-level burnable waste is incinerated at a central site.
The chemical and physical properties of the waste are essential for their management. Basically two major factors must be considered in the classification of waste for its further handling, storage, transportation and disposal. They are:
Compared to the total amount of toxic waste that has to be handled by society, the volume of radioactive waste is still small. (The figure is based on data within the OECD/NEA and the OECD Environment Directorate. It only gives a rough indication of the relative order of volumes.)
The level of radiation will govern the need for additional radiation shielding during handling, storage and transportation and there are established international guidelines to be followed. The content of long-lived radionuclides will determine the type of long-term isolation required for disposal of the waste.
IV. What waste disposal methods are used?
With the exception of sea dumping, which relied largely upon dilution and dispersion in the environment, but is now suspended, all disposal concepts for L/ILW rely on isolation from the biosphere at least initially and until radioactive decay has made subsequent releases to the environment compatible with radiation protection criteria. Multi-barrier containment systems have been designed for this purpose and most countries have already defined and sometimes implemented disposal practices and policies.
The length of the isolation period required is governed by the radiotoxic properties of the waste and particularly the half-lives of the radionuclides contained. A surface or near-surface facility is usually regarded as suitable for short-lived, low-level waste, provided some form of site surveillance is maintained after closure of the site, notably to prevent intrusion by man. However, it is clearly recognised that the maintenance of institutional control (i.e. any form of surveillance by man carried out on a continuous basis under the supervision of the responsible regulatory authorities) cannot be relied upon beyond a limited time period and a maximum of 300 years is usually regarded as a prudent limit in this respect. This results in a clear requirement for surface and near-surface disposal facilities: the waste they can accept should be such that the site could be released for unrestricted use, e.g., roads, houses or even farming, at the end of the agreed institutional control period.
In contrast, deep geologic isolation as a totally passive system is considered necessary for long-lived waste. In this case, institutional control measures would not be needed in the far future to preserve the long-term integrity of a well-selected site because the probability of interference by natural events and human actions is very limited. This reasoning, however, does not exclude the possibility of deep geological disposal for short-lived waste, which would make institutional control superfluous from a strict safety viewpoint.
In practice, the main policies followed for the disposal of low- and intermediate-level waste are:
France's first surface repository, the "Centre de la Manche", has been in operation since 1969 in a 12 hectare area located at the western tip of the Cotentin Peninsula, close to the La Hague reprocessing plant. The total capacity of this centre is about 500 000 m3 of waste and up to now, it has received about 400 000 m3 of waste. The centre will be filled completely at the beginning of the 1990s. It is planned to start operation of a new disposal facility, Centre de l'Aube in north-eastern France, by early 1991. The disposal capacity will be 1 million m3 of waste.
The Swedish repository for low-level waste, SFR, is located in the bedrock below the Baltic Sea close to the Forsmark nuclear power plant, north of Stockholm. The facility, excavated rock caverns and a silo, is accessible through tunnels from the coast. The bedrock cover from the top of the caverns to the sea is 60 m. Operation of SFR began during 1988. The capacity in the first phase is 60,000 m3 of waste and in total it is planned to dispose of about 100,000 m3 of waste, which is the projected total amount of low-level waste produced until the year 2010 by the Swedish nuclear power programme.
V. How are the disposal costs financed?
The "Polluter Pays Principle" is widely applied for radioactive waste disposal, sometimes including R&D activities. This principle is incorporated in national laws, for example in Belgium, Finland, France, the Federal Republic of Germany, Spain, Sweden and the United States. On this basis, financing of radioactive waste disposal may take different forms, such as advance contributions from waste producers according to waste production and expenditure estimates, provisional or final fees at the time of waste delivery, fees on nuclear electricity production, and contribution to waste management funds. Decommissioning funds are also sometimes used to cover the disposal of decommissioning waste. Given the relatively low cost of L/ILW waste disposal (up to a maximum few per cent of the cost of nuclear electricity) and the specific financing arrangements already made at the national level, there is apparently no fundamental difficulty in this respect, even if disposal is considerably delayed.
VI. What measures are taken to ensure the safe disposal of L/ILW?
In all countries the siting, construction, operation and closure of a radioactive waste repository is subject to an extensive licensing and control procedure. There is also a control at the source of the waste production (nuclear facilities, R&D establishments, radioisotope production and application facilities, hospitals, etc.) and the radioactive substances are recorded and surveyed throughout handling, transport and storage operations. These bookkeeping procedures normally ensure that all the waste generated is actually controlled and cannot be disposed of outside the agreed system.
For disposal of waste at a repository, quantitative waste acceptance criteria have to be met. These criteria may concern:
Such criteria will, to a large extent, be based on international and national radiological protection standards but the actual quantitative criteria will also depend upon the type of site and repository in each case.
A detailed characterisation of the site is made before proceeding to final site selection and construction of the repository. It includes, for instance, measurements and modelling of the general geological characteristics of the site, the groundwater movements and the geochemical conditions. The repository design, in many cases, includes additional engineered barriers like thick concrete vaults and backfilling by dense clay. These will enhance the protection against excessive or premature groundwater intrusion to the waste and will minimise and delay radionuclide transport from the waste to the environment.
The long-term safety of L/ILW disposal can be systematically assessed through predictive modelling of gradual failure of the engineered barriers, i.e., the waste form, waste package and the backfill (if any) and the potential subsequent transport to man's environment of radionuclides by circulating groundwater. A complete safety assessment will also include an analysis of the potential effects of disruption of the repository by geological and environmental changes, e.g., faulting or glaciation, as well as human intrusive actions at the site, e.g. drilling or living at the site. During the licensing procedure, the results of the safety analysis and their inherent uncertainties will be checked and assessed by the regulatory authorities.
During construction, operation and closure of a repository, strict control will be exercised to ensure that the disposal is implemented according to the plans.
VII. What is the role of the OECD Nuclear Energy Agency?
The NEA has always been concerned with the problem of radioactive waste disposal and for the past 15 years, this has been a priority area. 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, and by joint studies of issues important for safety assessment (identification of potentially disruptive events, treatment of uncertainties). Related computer codes (in particular for probabilistic events) and data bases (used to assess the behaviour of radioactive materials in the geosphere) are developed and validated at an international level.
These activities form part of an integrated international effort to reach the level of scientific understanding needed to ensure that nuclear waste disposal systems will be able to contain and isolate the radioactive materials so that no harm will be caused to man or his environment either now or in the future. Such co-operative programmes also enhance confidence in the quality of the safety analyses upon which the acceptability of nuclear waste disposal is to be judged.
Although the NEA activities in this area are primarily focussed on deep disposal of high-level, long-lived radioactive waste, many of the results are equally valid for the disposal of low-level waste. In addition, there are regular activities directly related to questions concerning low-level waste, for example, studies of how to estimate radionuclide content in the wide range of low-level waste and a recent workshop on assessment of repositories for low-level waste.
From 1967 to 1982, sea-dumping operations for radioactive waste were carried out in the North-East Atlantic under the supervision of NEA. Up to 8 NEA countries participated in these operations. However, since 1983 there has been a non-binding moratorium on the sea-dumping of radioactive waste. A co-ordinated Research and Environmental Surveillance Programme (CRESP) was set up in 1980 and continues to operate, mainly to collect scientific information on the Atlantic disposal sites.