This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.
Almost all major nuclear operations produce residues containing radioactive materials; in reactor operations, for example, the fission products, new elements (such as plutonium) produced by neutron irradiation and the unfissioned nuclear fuel are all radioactive. Some of this material can be put to further use: useful radioisotopes, for instance, can be extracted from the fission products and plutonium can be extracted for subsequent use as concentrated nuclear fuel. Theoretically, it should be possible to find some application for most of the radioactive substances that are produced in the course of atomic operations.In practice, however, it is not possible to utilize all these substances; and not all of those which may have some practical value which can be easily extracted in usable form. As a result, even after some usable materials have been extracted, the atomic energy industry is left with considerable quantities of radioactive substances.
Something has to be done to these unusable by-products- the so-called radioactive wastes. Because they are radioactive, steps must be taken to ensure that the radiations they emit do not pose any threat to human health and safety. From this arises the problem of safe disposal of radioactive wastes – a problem that has been growing in size and complexity with the development of nuclear operations.
Disposal is the final stage of the radioactive waste management process. The term “Disposal” refers to the emplacement of radioactive waste into a facility or a location with no intention of retrieving the waste. Disposal options are designed to contain the waste by means of passive engineered and natural features and to isolate it from the accessible biosphere to the extent necessitated by the associated hazard. The term disposal implies that retrieval is not intended; it does not mean that retrieval is not possible.
By contrast, the term “Storage” refers to the retention of radioactive waste in a facility or a location with the intention of retrieving the waste. Both options, disposal and storage, are designed to contain waste and to isolate it from the accessible biosphere to the extent necessary. The important difference is that storage is a temporary measure following which some future action is planned. This may include further conditioning or packaging of the waste and, ultimately.
A number of design options for disposal facilities have been developed and various types of disposal facility have been constructed in many States and are in operation. These design options have different degrees of containment and isolation capability appropriate to the radioactive waste that they will receive.The specific aims of disposal are:
- To contain the waste;
- To isolate the waste from the accessible biosphere and to reduce substantially the likelihood of, and all possible consequences of, inadvertent human intrusion into the waste;
- To inhibit, reduce and delay the migration of radionuclides at any time from the waste to the accessible biosphere; and
- To ensure that the amounts of radionuclides reaching the accessible biosphere due to any migration from the disposal facility are such that possible radiological consequences are acceptably low at all times.
The balance between the importance of each of the above mentioned aims and the extent to which and the way in which they are accomplished will vary, depending on the characteristics of the waste and the type of disposal facility.Disposal facilities are not expected to provide complete containment and isolation of waste over all time; this is neither practicable nor necessitated by the hazard associated with waste, which declines with time.1. TYPES OF DISPOSAL FACILITY FOR RADIOACTIVE WASTE:
A number of design options for disposal facilities have been developed and various types of disposal facility have been constructed and are in operation around the world.Within any State or region, a number of disposal facilities of different designs may be required in order to accommodate radioactive waste of various types. The following disposal options have been adopted in one or more States, corresponding to recognized classes of radioactive waste:
- Specific Landfill Disposal Facility: Disposal in a facility similar to a conventional landfill facility for industrial refuse but which may incorporate measures to cover the waste. Such a facility may be designated as a disposal facility for very low level radioactive waste (VLLW) with low concentrations or quantities of radioactive content. Typical waste disposed of in a facility of this type may include soil and rubble arising from decommissioning activities;
- Near Surface Disposal Facility: Disposal in a facility consisting of engineered trenches or vaults constructed on the ground surface or up to a few tens of metres below ground level. Such a facility may be designated as a disposal facility for low level radioactive waste (LLW);
- Disposal of Intermediate Level Waste Facility: Depending on its characteristics, intermediate level radioactive waste (ILW) can be disposed of in different types of facility. Disposal could be by emplacement in a facility constructed in caverns, vaults or silos at least a few tens of metres below ground level and up to a few hundred metres below ground level. It could include purpose built facilities and facilities developed in or from existing mines. It could also include facilities developed by drift mining into mountainsides or hillsides, in which case the overlying cover could be more than 100 m deep;
- Geological Disposal Facility: Disposal in a facility constructed in tunnels, vaults or silos in a particular geological formation (e.g. in terms of its long term stability and its hydrogeological properties) at least a few hundred metres below ground level. Such a facility could be designed to receive high level radioactive waste (HLW), including spent fuel if it is to be treated as waste. However, with appropriate design, a geological disposal facility could receive all types of radioactive waste;
- Borehole Disposal Facility: This is a disposal in a facility consisting of an array of boreholes, or a single borehole, which may be between a few tens of metres up to a few hundreds of metres deep. Such a borehole disposal facility is designed for the disposal of only relatively small volumes of waste, in particular disused sealed radioactive sources. A design option for very deep boreholes, several kilometres deep, has been examined for the disposal of solid high level waste and spent fuel, but this option has not been adopted for a disposal facility by any State; and
- Disposal of Mining and Mineral Processing Waste Facility: Disposal usually on or near the ground surface, but the manner and the large volumes in which the waste arises, its physicochemical form and its content of long lived radionuclides of natural origin distinguish it from other radioactive waste. The waste is generally stabilized in situ and covered with various layers of rock and soil.
2. DISPOSAL FACILITY LIFE CYCLE:
The development (i.e. site selection and evaluation, and facility design and construction) of most types of disposal facility is likely to take place over extended periods of time. The period over which disposal facilities will be operated prior to closure will, in most cases, also extend over decades. Different activities will be conducted in this period of development, such as site selection and evaluation, and facility design and construction, with decisions being made to proceed to the next set of activities or the next step in the development of the facility.
Such a step by step approach enables:
- The ordered accumulation and assessment of the necessary scientific and technical data;
- The evaluation of possible sites;
- The development of disposal concepts;
- Iterative studies for design development and safety assessment with progressively improving data;
- Technical and regulatory reviews; and
- Public consultation and political decisions.
However, the level of study and the process will depend on the facility and on national practices.
The step by step approach, together with the consideration of a range of options for the design and operational management of a disposal facility, is expected to provide flexibility for responding to new technical information and advances in waste management and material technologies. It also enables social, economic and political aspects of the disposal facility to be addressed, to ensure that all reasonable measures have been taken to further prevent, inhibit or delay releases to the environment.
This approach may include options for reversing a given step or even, for most types of facility, for retrieving waste after its emplacement, if this were considered to be appropriate.
The following three periods are associated with the life cycle:
- The Pre-Operational Period: This period includes concept definition, site evaluation (selection, verification and confirmation), safety assessment, and design studies. It also includes the development of those aspects of the safety case for safety in operation and after closure that are required in order to set the conditions of authorization, obtain the authorization and proceed with the construction of the disposal facility and the initial operational activities.
The monitoring and testing programmes that are needed to inform operational management decisions are put in place;
- The Operational Period: This period begins when waste is first received at the facility. From this time, radiation exposures may occur as a result of waste management activities, and these are subject to control in accordance with the requirements for protection and safety. Monitoring, surveillance and testing programmes continue to inform operational management decisions and to provide the basis for decisions concerning the closure of the facility or parts of it. Safety assessments for the period of operation and the period after closure and the safety case are updated as necessary to reflect actual experience and increasing knowledge. In the operational period, construction activities may take place at the same time as waste emplacement in, and closure of, other parts of the facility. This period may include activities for waste retrieval, if considered necessary, prior to closure, activities following the completion of waste emplacement and the final closure and sealing of the facility; and
- The Post-Closure Period: This post-closure period begins at the time when all the engineered containment and isolation features have been put in place, operational buildings and supporting services have been decommissioned and the facility is in its final configuration. After its closure, the safety of the disposal facility is provided for by means of passive features inherent in the characteristics of the site and the facility and the characteristics of the waste packages, together with certain institutional controls, particularly for near surface facilities. Such institutional controls are put in place to prevent intrusion into facilities and to confirm that the disposal system is performing as expected by means of monitoring and surveillance. Monitoring may also be carried out to provide public assurance. The licence will be terminated after the period of active institutional control; the necessary technical, legal and financial requirements have been fulfilled.
The disposal system (i.e. the disposal facility and the environment in which it is sited) is developed in a series of steps in which the scientific understanding of the disposal system and of the design of the disposal facility is progressively advanced. Safety assessment is an important tool for guiding site selection and evaluation and for assisting with the design of the facility. It is also used for evaluating the prevailing level of understanding of the disposal system and for assessing the associated uncertainties through the various steps in the development of the facility. The extent and complexity of such an assessment will vary with the type of facility and will be related to the hazard potential of the waste.
Moreover, the development of disposal facilities that incorporate provisions in design or operation to facilitate reversibility, including retrievability, is considered in several national programmes for waste management. In some States, post-closure retrievability is a legal requirement and constitutes a boundary condition on the options available, which must always satisfy the safety requirements for disposal. No relaxation of safety standards or requirements could be allowed on the grounds that waste retrieval may be possible or may be facilitated by a particular provision. It would have to be ensured that any such provision would not have an unacceptable adverse effect on safety or on the performance of the disposal system. This subject is not extensively dealt with in this Safety Requirements publication.
3. DESCRIPTION OF THE DISPOSAL SYSTEM:
The description of the disposal system should record all of the information and knowledge about the disposal system and should provide the basis on which all safety assessment is carried out. Information will be obtained and knowledge about the disposal system will evolve and mature as the project progresses and assessment is carried out in an iterative manner. As knowledge is developed, it should be used to determine future needs for system characterization and facility design. The system description should contain, depending on the type of disposal facility, information on the following:
- The near field, including:
- The types of waste (e.g. the origin, nature, quantities and properties of the waste and the radionuclide inventory);
- System engineering, e.g. waste conditioning and packaging, disposal units, engineered barriers, cap or cover of the disposal facility, drainage features); and
- The extent and properties of the zone disturbed by any excavation or construction work.
- The far field, e.g. geology, hydrogeology, hydrology, geochemistry, tectonic and seismic conditions, erosion rates; and
- The biosphere, e.g. climate and atmosphere, water bodies, the local population, human activities, biota, soils, topography and the geographical extent and location of the disposal facility.
Depending on the type of disposal facility, the description of the disposal system should include the following:
- A clear specification and description of the components of the system and their interfaces and associated uncertainties;
- A description of the overall safety concept and the safety functions;
- A description of how the components of the system will continue to be able to fulfill their assigned safety functions, both for the expected evolution of the system and for less likely events;
- A discussion of how regulatory or other requirements on system components have been addressed in the facility design;
- A description of the radiological, thermal, hydraulic, mechanical, chemical and biological processes that may affect the disposal system;
- A description of the interactions that may occur between system components;
- A description of how spatial heterogeneity of the waste has been taken into account, including associated uncertainties;
- A description of possible time dependent changes in the properties and behaviour of the system components and their interfaces, including how components may degrade or fail, and associated uncertainties;
- A description of possible environmental changes and their impacts on the components of the disposal system; and
- A description of possible radionuclide migration pathways both for the expected evolution of the system and for less likely events.
The description of the disposal system should provide information on the data supporting the safety assessments, including the following:
- An outline of how the management system will ensure the quality of all safety related data that have been used;
- The sources of all data used (e.g. by reference to measurements and reports);
- The rationale behind the site characterization programme (e.g. sample selection, sample location) — data acquisition programmes should reflect the conclusions from any previous safety assessment on the need for information for the subsequent iteration of the assessment;
- A description of the techniques that have been used to characterize the site and collect monitoring data, and the uncertainties associated with these techniques and data;
- A description of how the radionuclide inventory has been estimated, and the uncertainties associated with the inventory; and
- Any information used to support an understanding of possible future human behaviour in the region of the disposal facility (e.g. current human practices in the area, records of mineral exploration).
The depth and extent of information provided in the description will be influenced by the disposal facility type and will be more extensive and complex for facilities designed for the disposal of larger quantities and more long lived or higher activity waste. The description for a facility designed for the disposal of very low level waste will be less extensive and complex than one for high level waste. The actual extent and complexity for any particular facility will depend on a number of factors, including the amount of waste, its particular radioactive characteristics, the nature and complexity of the host site and the associated meteorological and hydrological characteristics. A justification for the extent and complexity of the description should be provided as part of the safety case and this should be agreed with the regulatory body as part of the discussions that should take place at the conceptualization of the disposal facility and throughout its development and operation.
4. SAFETY ASSESSMENT:
Safety assessment was defined as the overall process of performing quantitative assessments of the radiological impact of the facility for the period after closure. This included development of the context for the assessment and description of the disposal system and its environment, as well as interpretation of the results. Safety assessment in the IAEA Safety Guide also relates to aspects relevant for safety beyond the quantitative assessment of radiation risks, such as operational safety and the management system.
The following sections provide an overview of the key elements of the safety assessment:
4.1 Management System:
Application of a suitable management system will contribute to confidence in the safety case and an assessment should be carried out as to the adequacy of the management system governing all safety related work, including provision of the necessary financial and human resources.
The requirements on the management system influence the development of the safety case in two ways. First, the description of the management system applying to the various stages of facility development should represent an important element of the safety case, contributing to the confidence that the relevant requirements and criteria for site selection, design, construction, operation, closure and post-closure safety are met. Second, programmes should be set up to ensure the quality of all activities associated with the safety case and safety assessment, such as data collection and modeling;
4.2 Non-Radiological Environment Impact:
Radioactive waste may contain potentially hazardous non-radioactive components (e.g. heavy metals, pathogens). In particular, waste from uranium mining usually contains many non-radioactive toxic and/or carcinogenic substances in significant concentrations. The site selection and design development for the disposal system should provide adequate protection of people and the environment against such non-radiological hazards.
The assessment of non-radiological impacts arising from the disposal facility will be required and governed by environmental protection legislation. Environmental protection legislation and its associated regulations will result in several requirements on the construction, operation and closure of the disposal facility. Examples are restrictions in terms of traffic or noise pollution, which may limit the construction and operation of the facility. Other examples are limits, controls and conditions required for water management at the facility in construction and operation, as well as provisions made for post-closure control of water discharges. Such requirements arising from environmental protection legislation should be properly considered in the facility design. Thus, the integration of safety arguments should also take into account non-radiological impacts and should demonstrate the overall safety of the disposal facility and its overall compliance with all relevant legislative and regulatory requirements;
4.3 Radiological Impact Assessment for the Period after Closure:
Assessment of the post-closure radiological impact forms the core of the safety case for a disposal facility. In addition to qualitative assessments, this involves a comprehensive quantitative analysis of the evolution of the disposal system and its environment, possible challenges to the safety functions and the resulting potential radiological impacts. In a systematic approach to radiological impact assessment for the period after closure, scenarios are used to describe possible evolutions of the disposal system and its environment. The potential migration of radioactive substances from the disposal facility, their movement in the environment and resulting radiation risks are quantitatively analyzed by means of conceptual and mathematical models;
4.4 Operational Safety:
In the assessment of safety in the operational stage, similar approaches are applied to those applied for predisposal management of radioactive waste. Also relevant for the operation of disposal facilities are Safety Requirements and Safety Guides for the operation of nuclear power plants. In addition, other issues, such as mining safety, may need to be considered in the assessment of safety in the operational stage of a disposal facility (e.g. in the case of deep geological disposal). Applicable requirements for non-radiological aspects (e.g. those for occupational health and safety) should be applied in an integrated manner with those for radiological aspects; how this is achieved will depend on the type of facility, the legal and regulatory framework, and the stage of facility development; and
4.5 Site and Engineering Aspects:
Quantitative assessment of the evolution of the disposal system as part of the radiological impact assessment for the period after closure should result in conclusions on the adequacy of the chosen or proposed site as well as of the intended design of the disposal facilities. The conclusions drawn from the quantitative assessment should be supplemented by qualitative arguments and assessments. The integrated set of results of the qualitative and quantitative assessments should provide:
- Sufficient demonstration of the adequacy of the site and engineering;
- Reasonable assurance of compliance with the relevant safety requirements; and
- Assurance that the safety strategy set out for the facility is fulfilled.
The safety of any disposal facility depends primarily on the favourable characteristics or properties of natural barriers and the engineered barriers. Important characteristics of the natural and engineered barriers include their robustness and reliability over prolonged periods. Aspects that favour the robustness and reliability of a disposal facility and its environment and arguments for the quality of a specific site and facility design should be made on the basis of the provisions made in respect of these aspects.