144
Assessment of the safety and performance of a radioactive waste repository
Paul A. Smith
SAM (Safety Assessment Management) Ltd, North Berwick, Scotland, UK
6.1. Introduction
For permission to be granted to construct, operate and finally close a geological repository, the facility operations must be safe and it must also be shown to fulfil its primary role of protecting man and the environment from the long-term hazards associated with the radioactive waste that it contains. The task of analysing the performance of a repository and evaluating its safety in the long term, beyond the time when active control of the facility can be relied on, is termed long-term or post-closure SA. In this chapter, the methodologies of post-closure SA are discussed, illustrated by examples of their application in various repository programmes internationally. For brevity, the term SA is often used for assessments of safety in the post-closure period. Issues surrounding safety during construction and operation, though also of key importance, are beyond the scope of the chapter but are briefly addressed in Chapter 10. It should be noted, however, that processes occurring during repository construction and operation and in any extended open period before closure may have implications for long-term safety and may thus need to be taken into account in post-closure SA.
There has been a history of international cooperation in developing the approaches and methods for analysing the long-term safety of geological disposal. This work has been documented in numerous publications of the NEA and the IAEA since the early 1980s, where current examples of relevant publications include IAEA (1995, 2003) and NEA (1997, 1999a, 2000, 2002 and 2004a). A key conclusion, stated in various ways in many of the documents, is that absolute proof of safety is not possible for the long timescales considered, but that what is required for the implementation of a disposal system is a reasonable assurance of long-term safety. SA is the procedure by which safety is tested and this assurance developed.
The safety criteria that a repository must satisfy are defined by relevant nationally and internationally accepted safety standards. The IAEA has recommended approaches and provided standards and guidance for use by governments, regulatory bodies and
DEEP GEOLOGICAL DISPOSAL OF RADIOACTIVE WASTE |
2007 Elsevier Ltd. |
VOLUME 9 ISSN 1569-4860/DOI 10.1016/S1569-4860(06)09006-1 |
All rights reserved. |
Safety and performance assessment of repository |
145 |
implementers in formulating their own national regulations or showing compliance with them, where the radiation protection approach and standards are based on those recommended by the ICRP. National regulations set more specific legal requirements, including dose or risk targets, and in some cases the time period over which detailed quantitative evaluations should be presented. Some national regulations also set requirements with regard to protection of non-human biota or natural resources, such as groundwater. In some countries, regulations are formulated that are specific to the development of a particular disposal facility, whereas in other countries national regulations are expected to apply to a range of possible facilities.
To test compliance with regulatory safety criteria in SAs, an understanding is required of how, and under what circumstances, radionuclides might be released from a repository, how likely such releases are, and what the radiological1 consequences of such releases could be to man and the environment. Thus, scenarios for the potential evolution of the repository and its environment must be defined and their consequences evaluated using quantitative models as well as more qualitative reasoning. The development and analysis of scenarios is based on an understanding of how the geological characteristics of the site and the components of the EBS function in concert to prevent, lower the likelihood of, or attenuate such releases. This in turn involves developing a scientific understanding of relevant processes and their interactions, collating data, developing models and performing analyses related to safety (see also Chapter 8).
The use of SA to support a case for safety and to provide input to decision-making in repository planning and implementation is discussed in section 6.2. The tasks carried out in the course of SA are discussed in section 6.3, with some illustrative examples taken from recent SAs conducted by different national organisations. The issue of the timescale over which safety must be assessed is considered in section 6.4. Finally, section 6.5 addresses the construction and presentation of a safety case, based, at least in part, on the results of SA.
6.2. The role of SA and the safety case in decision-making
The process of siting, design and eventual implementation of a repository may extend over several decades and requires a commitment of considerable resources. This process typically involves a number of stages punctuated by interdependent decisions on whether and how to move to the next stage. Even at an early stage of a project, to justify a positive decision to move from one stage to the next, the organisation charged with development, as well as other stakeholders, will require some assurance that the option or options being considered have a reasonable chance of proving acceptable in terms of factors including cost, operational safety, environmental impact and, crucially, postclosure safety. SAs are therefore carried out not only at a late stage of a project, where permission to construct, operate or close a repository is being sought, but also periodically throughout the planning process and documented in safety reports. The ways in
1 There may also be potential pollutants released from the repository that are non-radioactive and these may need to be taken into account in any overall evaluation of safety. In this chapter, however, discussion is restricted to the assessment of radiological consequences.
146 |
P.A. Smith |
Fig. 6.1. An example from Sweden of the incremental process of developing a geological repository (from Fig. 1 of NEA, 1999a). SR = safety report.
which SAs and safety reports can be used as a basis for decision-making at various stages of a repository project are illustrated in Fig. 6.1.
In the early stages of a programme, several siting and design options may be kept open and the information available on any particular site or design is likely to be limited (see comments in Chapters 5 and 7). More information can be obtained over the course of a programme by site characterisation and R&D. Uncertainties can also, to some extent, be avoided or their effects mitigated by the choice of site and by design optimisation. They can never, however, be completely eliminated (and this option may not even exist in the case of a volunteer site – see Chapter 4 for discussion) and any predictions of the evolution of a repository and its environment are always subject to some uncertainty. Fortunately, there is no need to make precise predictions for SA to serve its purpose as a basis for decision-making. If a SA shows that a certain level of release of radioactivity to the environment is unlikely to be exceeded, and that this level of release satisfies all relevant criteria (such as national dose limits), then this may be enough. It may not matter, from the point of view of decision-making, if the releases that will occur in reality are actually much lower than this. SAs need only (and can only) provide predictions in the sense of bounds on the likelihood and consequences of particular adverse situations arising.
Figure 6.2 provides a schematic illustration of how SAs support decision-making. The figure shows how, within a repository programme, site selection and characterisation, repository design studies and R&D are carried out in parallel and that project milestones exist that separate the project into stages and at which progress is reviewed and decisions
assessmentBegin |
Safety and performance assessment of repository |
147 |
milestone/Project pointdecision |
|
Site selection and characterisation
Repository design studies
R&D
Review assessment basis and update
necessary
Carry out SA
Internal review of assessment and findings
No
Adequate?
Yes
(as basis for internal
Yes decision-making)
(as basis of a broader safety case for external decision-making)
Compile safety case and present to external decisionmakers
(if required for a given project milestone)
Fig. 6.2. The role of SA in decision-making within the step-wise process of repository planning and development. Note: The SA process starts at some time before the project milestone or decision point and draws on information from site characterisation (Chapter 4), repository design studies (Chapter 5) and R&D (Chapter 8).
taken regarding the following stages. Examples of typical project milestones are the point at which a single site is to be selected for detailed characterisation from the surface and the point at which a decision is to be taken on whether to begin construction of an URL at a chosen site (see also detailed comments in Chapter 7). If the decision at hand is to be supported by a SA, the first stage is to review the scientific understanding, databases, mathematical models and computer codes on which the assessment is to be based. These are termed the ‘‘assessment basis’’ in recent NEA reports (e.g., NEA, 1999a, 2002, 2004a). If some aspects of the assessment basis are judged to be inadequate, then