Discussion of guidance and principles for siting, design and staging .1 International guidance

International guidance on siting has been issued by the IAEA. The IAEA broad siting guidelines for a deep waste repository issued in 1994 (IAEA 1994a) give some points of guidance with respect to site selection. With respect to repository design, there is international consensus that the safety of geological repositories should be ensured by a system of passive barriers, which is capable of functioning reliably without any supervision or maintenance measures. This does not necessarily mean that the possibility of monitoring is excluded; the disposal system must, however, be capable of providing a sufficient degree of safety without relying on institutional control and monitoring (IAEA 1989 and 2002a).

Based on Swiss and international experience, the following three broad categories of safety functions are essential for the performance and safety of a repository: (i) isolation of the wastes from the human environment, (ii) long-term confinement and radioactive decay, within the dis-posal system, of a large fraction of the activity initially present in the wastes, and (iii) attenua-tion of releases to the environment for the small fracattenua-tion of radionuclides that do not decay to insignificance within the disposal system. These safety functions are discussed in more detail in Section 2.6.2.1.

There is also broad consensus that implementation of a repository should preferably be done in a stepwise manner, see, e.g., NEA (1999a and 2002) and NRC (2001). This is also the case in Switzerland, see discussions in Chapter 1. Such a stepwise approach provides several opportu-nities for review and should allow for modifications and changes to future plans and – if this were required – should also allow the reversal of decisions in the course of the implementation process. This would include – in the most extreme case – the retrieval of emplaced wastes.

Protection Objective 1: The release of radionuclides from a sealed repository subsequent upon processes and events reasonably expected to happen shall at no time give rise to individual doses which exceed 0.1 mSv per year.

Protection Objective 2: The individual radiological risk of fatality from a sealed repository subsequent upon unlikely processes and events not taken into consideration in Protection Objective 1 shall, at no time, exceed one in a million per year.

Protection Objective 3: After a repository has been sealed, no further measures shall be necessary to ensure safety. A repository must be designed in such a way that it can be sealed within a few years.

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2.4.2 Designing the system for robustness

In Switzerland, the construction, operation and, in particular, closure of a repository for SF, HLW and ILW are likely to take place only in the relatively distant future and, to date, not all issues and uncertainties are fully resolved. Indeed, as for all technical systems, some uncer-tainties concerning their future behaviour will remain unresolved and decisions must be taken in the face of such inherent uncertainties. The approach taken in waste disposal to counter this problem is to choose the disposal system such that no open issues are likely to undermine safety and our ability to demonstrate safety, and none of the uncertainties that need to be addressed in order to demonstrate safety are likely to place undue demands on the site characterisation and research and development programmes. Such a disposal system is termed robust. The principles involved in choosing a robust disposal system are summarised in Fig. 2.4-1.

Fig. 2.4-1: The principles involved in choosing a robust disposal system

Where possible, phenomena that would be detrimental to safety, as well as sources of uncer-tainty that would hamper the evaluation of safety are avoided or reduced in magnitude, likelihood or impact. This can be achieved by choosing a system that is not prone to

unpre-A robust disposal system ensures:

- that the waste, including fissile materials, is secure and that human beings and the environment are protected from the hazardous effects of radiation, and - that security and the safety are not undermined by:

The effects of which are mitigated by:

Features of the site and specific design measures (incl. over-dimensioning of some engineered features)

Multiple phenomena contributing to safety functions

Flexibility in implementation (several options available) Which are avoided / minimized by:

Relying on effective and well understood positive phenomena

Developing a good understanding of these key phenomena for

safety assessment

Siting and design to avoid unpredict-able, detrimental phenomena (geological events, human intrusion)

Detrimental phenomena Uncertainties (that would hamper the evaluation of safety and place

undue demands on future site characterisation, etc.)

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dictable detrimental phenomena and that provides safety via processes that are (i) well under-stood, (ii) for which the necessary detailed information for safety assessment can readily be acquired, and (iii) that are likely to operate adequately, even in the event of unfavourable developments and a pessimistic interpretation of uncertainties. This includes the choice of a design in which the materials used are – in addition to providing sufficient performance – also compatible in their interactions and in which wastes that are potentially unfavourable in their interactions are segregated. Because of the limited thickness of the Opalinus Clay the design of the facility should be chosen in such a way that the disturbances of the host rock in the vertical direction are small in order not to unnecessarily reduce the vertical migration distance.

Furthermore, the site should, if possible, be amenable to characterisation, to the extent required for safety assessment, at an early stage of the project, i.e. it should be readily explorable.

Not all detrimental phenomena and uncertainties can be avoided, but their impacts can be reduced by a suitable choice of natural (geological) and engineered barriers, with multiple and to some extent independent phenomena contributing to safety. The availability of design options to address specific safety concerns also contributes to robustness.

2.4.3 Swiss regulatory guidance on siting and design in HSK-R-21 Overall system design

The requirement for a system of multiple passive barriers is laid down in the Swiss HSK-R-21 guideline (HSK & KSA 1993), which is discussed in Section 2.3.2 (Principle 4 and Protection Objective 3).

Criteria for site properties: Explorability and predictability and absence of resources HSK-R-21 gives no specific criteria for site properties that relate to long-term safety, although a general preference is expressed for "... sites at which conditions are easy to predict, both in time and space, for the purpose of the safety assessment".

The Regulatory Principle 2 discussed in Section 2.6.2 implies that sites are preferred that lack exploitable mineral resources since this helps to keep resources available to future generations and to minimise the likelihood of inadvertent intrusion.

Requirements on the engineered barrier system: The need for initial complete containment

HSK-R-21 is intended for application to disposal facilities for all types of radioactive waste and it specifies protection objectives for the repository system as a whole. This allows the system components to be adapted to specific characteristics of the waste and the geological environ-ment under consideration. HSK-R-21 states, however, that

"... in the case of HLW disposal, there is a particularly high hazard potential during the initial phase (around 1000 years). During this phase complete containment of the radionuclides within the repository should be aimed at."

This implies that it is necessary to design waste canisters with sufficient corrosion resistance to prevent groundwater reaching the wastes for at least this time.

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Safety enhancing measures HSK-R-21 states:

"Even if compliance with Protection Objectives 1 and 2 is demonstrated, the radiological conse-quences from the repository have to be reduced by appropriate measures as far as feasible and justifiable with current status of science and technology. However, owing to the uncertainties involved in determining potential radiation exposure, no quantitative optimisation procedure is required."

This implies that subjective judgement can be used in designing the individual barriers that ensure safety, provided that the overall requirements are met and that there are indeed multiple contributors to safety.

In HSK-R-21 it is further stated:

"Protection Objective 3 requires that no post-closure measures shall be necessary in order to ensure long-term safety of a repository. It is thus to be assumed in the safety analysis that future generations will not take measures to protect themselves from the exposure to radionuclides released from the repository. The applicant should, nevertheless, take measures to preserve information on the repository, including its location, design and wastes which have been emplaced. This is intended to reduce the likelihood of an unintentional intrusion into the repository."

The requirement for passive safety is critical for the choice of the system concept. Information preservation is not a critical issue at the present development stage, but will be important at licensing and especially at closure.

2.4.4 The EKRA concept of monitored long-term geological disposal

As discussed above, the draft KEG requires disposal in a geological repository, which must be monitored for a certain time before closure (KEG 2001). This requirement is consistent with the concept of "monitored long-term geological disposal" that was formulated in some detail in the report of the government advisory group, EKRA (EKRA 2000). This concept combines the need for passive safety, as ensured by geological disposal, with a cautious, stepwise approach to implementation, intended to address societal demands and also technical uncertainties. The approach involves an extended period of monitoring, during which retrieval of the waste is relatively easy, and the emplacement of a representative fraction of the waste in a pilot facility to test predictive models and to facilitate the early detection of any unexpected undesirable behaviour of the system should this occur. Thus, opportunities are provided for review and possible reversal of decisions, including the retrieval of emplaced wastes.

EKRA's concept of monitored long-term geological disposal foresees the following system elements (see Fig. 2.4-2):

• the main facility;

• the test facility;

• the pilot facility;

• a tunnel system connecting the different system components, including tunnels for the near field and environmental monitoring programmes.

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Fig. 2.4-2: Overview of the key components of the concept of monitored long-term geological disposal (taken from EKRA 2000)

Note that this is a schematic representation of a generic facility. A repository for SF, HLW and ILW in Opalinus Clay that is consistent with the concept of monitored long-term geological disposal is described in Chapter 4.

The function of the test facility is to provide the information required before the main facility can start operation, in so far as this information is not already available from site investigations.

This includes information relevant to post-closure safety as well as information for construction and operation of the main facility. The test facility is not a single facility, but is rather a series of experiments at different locations, and may be regarded as a site-specific underground rock laboratory (URL). These in-situ experiments are complemented by laboratory programmes and by generic (non-site-specific) investigations, for example in other URLs.

The aim of the pilot facility, which contains a small but representative fraction of the waste, is to provide information on the behaviour of the barrier system and to check predictive models. It also serves as a demonstration facility that provides input for decisions regarding closure of the entire facility. In addition, it should allow early detection of any unexpected and undesirable system evolution. It is foreseen that the pilot facility will be backfilled and sealed without delay, in order that it is in a state that represents as closely as possible the foreseen final state of the repository. It will, however, be monitored from a series of observation boreholes. The possibil-ity also exists for carrying out (destructive) sampling. The pilot facilpossibil-ity and its access routes are arranged in such a way that the facility can continue to be monitored for a long period after closure of the main facility. As is the case for the main facility, the pilot facility should not represent a significant risk if, during a time of crisis, it should be abandoned without the access route being closed according to plan.

The majority of the waste is emplaced in the main facility. All of the components of the main facility are designed in such a manner that, together with the geological barrier provided by the host rock, they will ensure passive safety in the post-closure phase, i.e. after closure of the whole facility. The layout of the main facility has also to ensure a high degree of robustness in the extreme event that the facility is abandoned during the extended monitoring phase (e.g.

Main facility

Tunnel-environmental monitoring

Test facility

Pilot facility Tunnel near field monitoring

Closure self-closure Surrounding rock

Surrounding rock Host rock

backfilled waste open

Operationscentre

Closure self-closure

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during a time of crisis). In particular, immediately following emplacement of the waste, back-filling of voids and sealing of the main facility has to ensure that passive long-term safety is achieved even if closure of the access routes is not carried out according to plan. In addition, provisions may be made for easy and rapid closure of the access tunnels (EKRA discusses a design that would close automatically if the repository were abandoned). Furthermore, accor-ding to EKRA the design of the repository has to allow for reasonably easy retrieval of the wastes if this were deemed necessary.

Both monitoring and retrievability are also being studied in many countries and have been (and still are) discussed in several international fora (see, e.g., IAEA 2000, EC 2000, NEA 2001a, IAEA 2001a, EC 2002). The requirement of the draft KEG for a period of monitoring before repository closure and the corresponding detailed recommendations of EKRA are considered in the proposed layout of the underground facilities for Project Entsorgungsnachweis, in particular by the inclusion in the design of a test facility and a pilot facility. This is described in Chapter 4 and in more detail in the Facilities Report (Nagra 2002b).

2.4.5 Swiss guidance on a stepwise approach to repository implementation

The existing Swiss law and also the draft KEG foresees a stepwise approach to repository implementation. Major steps are defined by the several licences that are required for implemen-ting a repository and for its eventual closure (see Section 1.2.2). Even before licensing, there are several milestones with corresponding safety reports (see Section 1.2.4). The stepwise approach is also addressed in HSK-R-21:

"The applicant has to submit a safety analysis at each stage of the licensing procedure (general, construction, operating and closure licence). Safety relevant information on the repository system obtained from preliminary investigations should be supplemented by ongoing investigations during the construction and operation of the repository. The safety analysis for the post-closure phase should be refined in accordance with the improved knowledge of the repository system."

Thus, the iterative nature of the safety assessments is very important: For each major decision-point a new safety assessment has to be prepared which will increase in detail as the project progresses and more details become available. A key issue in such a stepwise approach is how the level of confidence of all stakeholders changes through the steps. How soon does the infor-mation base become sufficiently reliable that no doubts remain about the fundamental feasibi-lity? For a stepwise procedure in which early confirmation of basic feasibility is a goal, this indicates the importance of the explorability of a system: A geological environment for which a reliable dataset can be obtained already in the early phases is advantageous.

After construction of the repository periodic re-evaluation continues. The concept of monitored geological disposal proposed by EKRA provides a good framework for such re-evaluations which will be partially based on experimental evidence from the pilot facility.

However, in such a stepwise approach with multiple decision points it is important that enough flexibility exists so that modifications and changes can be made if considered to be necessary at one of these decision-points. Thus, a project has to provide flexibility with possibilities for changes.

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2.5 Discussion of guidance and principles for safety assessment

Im Dokument TECHNICALREPORT 02-05 (Seite 82-88)