The Long-Term Safety Strategy for the Geological Disposal of Radioactive Waste

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Geological Disposal Programme

The Long-Term Safety Strategy for the

Geological Disposal of Radioactive Waste

SFC1

level 4 report: second full draft

P. Smith, SAM Ltd

B. Cornélis, freelance technical writer M. Capouet, ONDRAF/NIRAS

M. Van Geet, ONDRAF/NIRAS Belgian agency for radioactive waste and enriched fissile materials

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ONDRAF/NIRAS

NIROND-TR REPORT 2009-12 E Geological Disposal Programme

The Long-Term Safety Strategy for the

Geological Disposal of Radioactive Waste

SFC1

level 4 report: second full draft

P. Smith, SAM Ltd

B. Cornélis, freelance technical writer M. Capouet, ONDRAF/NIRAS

M. Van Geet, ONDRAF/NIRAS

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This report was written by: P. Smith (SAM Ltd), B. Cornélis (freelance technical writer), M. Capouet (ONDRAF/NIRAS) and M. Van Geet (ONDRAF/NIRAS).

It was edited by: the SFC1 coordination team of ONDRAF/NIRAS (M. Van Geet, M. Capouet, X. Sillen and H. Van Humbeeck).

It was reviewed by: the members of the SFC1 steering committee (ONDRAF/NIRAS: M. Van Geet, M. Capouet, P. De Preter, X. Sillen and H. Van Humbeeck; SCK•CEN: J. Marivoet and G. Volckaert) and by W. Cool (ONDRAF/NIRAS) and P. Lalieux (ONDRAF/NIRAS).

It was approved by: P. Lalieux (ONDRAF/NIRAS).

Contact persons at ONDRAF/NIRAS: M. Van Geet: m.vangeet@nirond.be M. Capouet: m.capouet@nirond.be

ONDRAF/NIRAS

Avenue des Arts 14 1210 BRUSSELS BELGIUM

www.nirond.be

The data, results, conclusions and recommendations contained in this report are the property of ONDRAF/NIRAS. The present report may be quoted provided acknowledgement of the source. It is made available on the basis that it will not be used for commercial purposes. All commercial uses, including copying and re-publication, require prior written authorisation of ONDRAF/NIRAS.

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Document Datasheet

Title

The Long-Term Safety Strategy for the Geological Disposal of Radioactive Waste

Subtitle

Author(s) of the document

P. Smith (SAM Ltd),

B. Cornélis (freelance technical writer),

M. Capouet (ONDRAF/NIRAS),

M. Van Geet (ONDRAF/NIRAS)

Reviewer(s) of the document

M. Capouet (ONDRAF/NIRAS),

W. Cool (ONDRAF/NIRAS)

P. De Preter (ONDRAF/NIRAS),

P. Lalieux (ONDRAF/NIRAS),

J. Marivoet (SCK•CEN),

X. Sillen (ONDRAF/NIRAS),

M. Van Geet (ONDRAF/NIRAS),

H. Van Humbeeck (ONDRAF/NIRAS),

G. Volckaert (SCK•CEN)

Series Geological

Disposal Programme

Publication date 2009 - 06

Document type NIROND-TR Review status Working Document

Status Open Revision number 1

ONDRAF/NIRAS number of report

NIROND-TR 2009-12 E Subcontractor reference number

NA

ISBN NA Total number of

pages

62

Approver(s) of the document

P. Lalieux (ONDRAF/NIRAS)

This report, NIROND-TR 2009-12 E, presents a refined view of the safety strategy first described in the report NIROND-TR 2006-04 E and intended to guide the stepwise development of a geo-logical disposal system for high-level waste and low- and intermediate-level waste, long-lived (category B&C wastes) in Belgium. This second full draft report incorporates adaptations based on the application of the safety strategy in the framework of the preparation of the Safety and Feasibility Case 1 (SFC1).

This report cannot be considered as the final version of a supporting document to SFC1. It re-mains a living document that will be discussed with the Belgian safety authorities in the period 2009–2011. It also provides a basis for continuing discussions and contributions from

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Executive summary

For over 25 years, ONDRAF/NIRAS has been studying geological disposal in poorly indurated clays as a solution for the long-term management of high-level waste and low- and intermedi-ate-level waste, long-lived (HLW/LILW-LL or category B&C wastes), geological disposal in Boom Clay becoming progressively the reference solution, Ypresian Clays being considered as an alternative solution. In line with international practice, ONDRAF/NIRAS plans its geological repository for category B&C wastes and its implementation in a cautious, stepwise process, punctuated by the submission to the government of key documents, marking the end of succes-sive stages in its B&C programme. These documents, called “safety and feasibility cases”, are intended to support decisions to be requested.

A need to formalise the iterative approach to developing a geological repository adopted in the past and to refine and develop it further into a formal safety strategy was identified in the light of the findings of the international peer review of the Safety Assessment and Feasibility In-terim Report 2 (the second major synthesis report of the B&C programme, published in 2001) and in the light of the need to organise the B&C Programme in a more structured, better founded way, towards the production of the successive safety and feasibility cases.

To this end, ONDRAF/NIRAS set out to formalise its safety strategy, which it defines as “the it-erative process guiding, firstly, the stepwise development of plans for a safe and feasible geo-logical repository and its implementation procedures and, secondly, the successive licence ap-plications, this based on a concentrate and confine approach and taking account of all relevant boundary conditions”. More specifically, it is the iterative process for developing the safety concept and the design of a geological repository and for acquiring evidence, developing ar-guments and carrying out analyses to show that these are both safe and feasible to implement as planned. This process was initiated in 2006 and led, in 2007, to a first draft report structur-ing and documentstructur-ing the safety strategy in a formal manner, of which the present report is an update. Many of the elements of this strategy were already implicitly contained in the existing approach, but some new elements were also incorporated.

The safety strategy has been developed on the basis of the experience, knowledge and under-standing acquired in the context of the studies on geological disposal in Boom Clay, with the intention that it remains valid during the whole process of developing and implementing a safe geological repository. It is expected to be applicable to poorly indurated clays in general, thus also to Ypresian Clays, though this has still to be confirmed. It will have to be completed, re-fined and possibly adapted for the programme stages beyond SFC1, in particular to encompass aspects that it does not currently take into account, such as waste acceptance and siting. It will also have to take due account of aspects of safeguards.

As in the earlier report on the safety strategy, the present report is first and foremost a report presenting a working methodology, whose application is then illustrated. It defines the aims of the safety strategy and describes its main elements, in terms of inputs, successive steps and outputs, including how these are linked together. The present report is a living document, that will be discussed, in particular, with the Belgian safety authorities in the period 2009–2011. A

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final documented view on safety strategy, which will integrate new experience in the applica-tion of the safety strategy and further refinements, together with the still-to-be-formalised fea-sibility assessment methodology, which includes operational safety, would then be available by 2013.

* * *

In the safety strategy (see also the figure below), system development, that is, development of the system comprising the geological repository and the host formation in which it is built, and assessment of its safety and feasibility are constrained by boundary conditions. Boundary con-ditions include, in particular, the relevant international and national regulatory framework and, because the type of solution to be implemented for the long-term management of category B&C wastes has not yet been decided at the institutional level, the working hypotheses ONDRAF/NIRAS has had to make so as to manage its geological disposal programme in a fo-cused way.

High-level strategic choices made by ONDRAF/NIRAS on the basis of existing knowledge and understanding, while being constrained by the boundary conditions, aim to provide a general orientation for disposal system development and to establish the broad features that the system should have in order to meet its safety and feasibility objectives.

Strategic choices of the B&C Programme

1. The repository shall be constructed at depth within the Boom Clay formation, considered as reference host formation, with the overlying sedimentary formations providing the geological coverage. Ypresian Clays are considered as an alternative host formation.

2. The materials and implementation procedures shall not unduly perturb the safety functions of the host formation, or of any other component.

3. In the case of heat-generating waste, the engineered barriers shall be designed to provide complete containment of the wastes and associated contaminants at least through the thermal phase. 4. Waste types shall be divided into groups to be emplaced in separate sections of the repository.

5. Repository construction and operation shall proceed as soon as possible, but taking due account of scientific, technological, societal and economic considerations.

6. The different disposal galleries and repository sections, and the repository as a whole, shall be closed (access routes backfilled and sealed) as soon as practically possible following emplacement of the wastes.

7. There are preferences for permanent shielding of the wastes and for minimisation of operations in the underground.

8. There are preferences for materials and implementation procedures for which broad experience and knowledge already exists.

9. Repository planning shall assume that post-closure surveillance and monitoring will continue for as long as reasonably possible.

Boundary conditions and strategic choices are translated into requirements related to the dis-posal system as a whole, to subsystems or to individual components of the system. These re-quirements can be either general or specific, and cover a wide range of domains, such as

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long-term safety, feasibility, which is taken to include operational safety and costs, knowledge of the waste to be disposed, knowledge of uncertainties and aspects of quality assurance.

System development starts with the development of the so-called “safety concept”, defined as the integrated description of the elements on which the passive long-term safety of the pro-posed disposal system rests. This development is carried out on the basis of existing knowledge and understanding and of the requirements on the system and some of its subsystems. The de-scription of the safety concept is based primarily on the safety functions fulfilled by the main components of the system and on the features of the system and its implementation that pro-vide robustness. Together with the remaining requirements, the safety concept is translated into a structured set of so-called “safety and feasibility statements”, forming the cornerstone of the application of the safety strategy. Safety and feasibility statements, and the assessment of the level of support that is available for them, provide a useful tool to steer research and develop-ment activities, to guide the application of quality assurance procedures (such as completeness checks of the assessment basis and of safety and feasibility assessments, via lists of features, events and processes), and to structure the documentation of safety and feasibility cases. With the safety concept as a basis, the development of a repository design (and, as the pro-gramme matures, the development of implementation procedures), is carried out iteratively, in parallel with the development of the remainder of the assessment basis, comprising the existing knowledge and understanding and the analysis tools, and with so-called “preparatory safety and feasibility assessments”. Preparatory assessments, which are seen as the first step in assess-ing the safety and feasibility of the proposed disposal system, are partial assessments, generally taking the form of exploratory calculations (sensitivity analyses and/or model calculations of the evolution of the system or part(s) thereof). These aim to assess the impact of uncertainties on long-term safety and feasibility, qualitatively and, to the extent possible, quantitatively. To-gether with the development of the assessment basis, preparatory assessments generate multi-ple lines of evidence, arguments and analyses to support safety and feasibility statements. They also aim to identify any significant deficiencies in current knowledge and understanding and in the plans to address these in the research and development programme. Modifications can then be made in the programme and, should this appear necessary, in the strategic choices.

Formal assessments of the proposed disposal system are carried out provided preparatory as-sessments indicate good prospects that these will confirm the safety and feasibility of the re-pository to the extent needed for the programme stage at hand. The formal assessments should provide a finer understanding of the remaining uncertainties and their safety relevance.

If, as expected, the results of formal assessments, together with the other lines of evidence, arguments and analyses available, confirm the safety and feasibility of the proposed disposal system to a level that is judged adequate in view of the targeted decision, the main output of applying the safety strategy — that is, the safety concept and a repository design with possible variants, the implementation procedures (depending on the current programme stage), and the evidence, arguments and analyses substantiating the structured set of safety and feasibility statements — are used to compile a safety and feasibility case aimed to support the decision at hand. Should formal assessments not confirm the safety and/or the feasibility of the disposal

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system, then the research and development programme, and possibly also the strategic choices, again need to be adjusted.

After a decision has been taken, the safety strategy is again applied iteratively, with a view to developing the necessary support for the decision to be requested at the end of the next pro-gramme stage. The application of the safety strategy will take account of any new boundary conditions that may have emerged. At least some of the requirements are thus expected to have to be adapted accordingly, and hence the structured set of safety and feasibility statements. The whole process of system development and assessment of its safety and feasibility will be re-peated as necessary throughout the period of repository planning and licence applications.

Boundary conditions

Strategic choices

(based on the hypothesis of geological disposal in a poorly indurated clay)

Requirements

System development + assessment of safety and feasibility Development: concept

Development: design, in parallel with remainder of assessment basis

Safety concept

Assessment of safety and feasibility

Guidance for adjusting the RD&D programme

yes

Compile SFCi (i = 1 …n)

and move to next programme stage Structured set of safety and feasibility statements

no no

yes

Requirements on system and some subsystems

Requirements on subsystems and components

Assessment basis

Input to SFCi (i = 1 …n)

Repository design + im-plementation procedures Phenomenological description of disposal system evolution + uncertainties Methods, models,

codes and datasets Formal assessment of S&F

conclusive? Preparatory assessment of S&F

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1 Introduction

According to the International Atomic Energy Agency (IAEA), the fundamental safety objective of all radioactive waste management activities is “to protect people and the environment from harmful effects of ionizing radiation” [1]. Waste disposal, that is, emplacement of waste in an appropriate facility without the intention of retrieving it, is meant to provide that protection for present and future generations in such a way that the need for further action is minimised. As stated by the International Commission on Radiological Protection (ICRP) [2]: “The principal objective of disposal of solid radioactive waste is the protection of current and future genera-tions from the radiological consequences of waste produced by the current generation.” Since the wastes and their packaging may contain toxic chemical substances, in some nations, includ-ing Belgium, the definition of the objective of disposal is extended to include protection from exposure to the chemically toxic, as well as radiologically toxic, constituents of the wastes, collectively designated as “contaminants”. Furthermore, some wastes have the potential to be misapplied (for example, diverted for terrorist or other unauthorised purposes) if adequate pro-vision is not made for their security (safeguards).

Geological disposal, that is, disposal in a repository constructed in a stable geological forma-tion, is the solution currently recommended at the international level to protect man and the environment from the risks associated with high-level radioactive waste (HLW or category C waste, which generates heat and includes vitrified high-level waste and spent fuel declared as waste by its owner) and low- and intermediate-level waste, long-lived (LILW-LL or category B waste) (in particular, [1, 3, 4, 5, 6, 7]). This solution is based on a strategy of concentration and confinement of the radionuclides and other contaminants present in the wastes and their pack-aging, instead of a strategy of dilution and dispersion 1. It employs a system of engineered and natural barriers between the wastes and the surface environment in order to prevent these ra-dionuclides and the other contaminants ever reaching it in such concentrations that they could present an unacceptable risk, for man and the environment.

The protection provided by a geological disposal system, that is, a repository together with the host formation in which it is built, is achieved by passive means, meaning that it is not reliant on human society to provide, for example, supervising and monitoring structures, financial resources and human specialist knowledge. Passive systems have the advantage that their ca-pacity to protect human beings is unlikely to be affected by the possibility of future instabilities in human society. Furthermore, they avoid imposing the burden of managing the disposed wastes on future generations that have not benefited from the activities that have given rise to these wastes.

Consequently, in planning for geological disposal, the proposed disposal system must be shown to provide passive long-term safety if implemented according to design specifications and

1

In the case of radioactive waste, the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matters (commonly called the “London Convention”), which Belgium has ratified, prohibits disposal at sea, which is an example of a “dilute and disperse” approach.

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to be feasible to implement according to these specifications, with due regard to engineering practicality, operational safety and financial costs.

The planning of a geological repository is a lengthy process, involving a range of detailed stud-ies. In Belgium, research, development and demonstration (RD&D) activities related to the safety and feasibility of geological disposal as solution for the long-term management of cate-gory B&C wastes have been ongoing for many years. The R&D programme initiated as early as 1974 by the Belgian Nuclear Research Centre (SCK•CEN) at Mol was pursued from the early 1980s under ONDRAF/NIRAS’ responsibility. These studies quickly focused on the Boom Clay formation at Mol–Dessel, in northeastern Belgium, as a potential host formation for a geologi-cal repository, benefiting significantly from the existence of the HADES underground research laboratory at Mol, which was constructed at the beginning of the 1980s soon after the inception of the programme.

The state of scientific and technical research on the possible disposal of B&C wastes in clay layers carried out in the period 1974–1989 was presented in SAFIR (Safety Assessment and Fea-sibility 2 Interim Report) in 1989 [8] and the state of research carried out in the period 1990– 2001 was presented in SAFIR 2 (Safety Assessment and Feasibility Interim Report 2) in 2001 [9, 10]. Over time, geological disposal in Boom Clay had progressively become the reference solu-tion of ONDRAF/NIRAS for the long-term management of category B&C wastes, geological dis-posal in Ypresian Clays being considered as an alternative solution. These solutions had not, however, been discussed at the societal level and had not been formally confirmed at the politi-cal (federal) level. SAFIR 2 and its national (Annex 5 in [9]) and international [11] peer reviews, the latter under the auspices of the Nuclear Energy Agency (NEA) of the Organisation for Eco-nomic Cooperation and Development (OECD), confirmed that the proposed solution of disposal in Boom Clay is promising: Boom Clay appears exempt from major flaws in terms of safety and feasibility for the wastes that had been studied most, namely vitrified high-level waste from re-processing and, to a lesser extent, spent fuel. The conclusions of the NEA peer review mention, in particular, that the acquired knowledge and expertise enable the programme to move on to the process of choosing a site for implementing the disposal solution, while pursuing the RD&D nec-essary to reduce the remaining uncertainties. They mention, however, just as ONDRAF/NIRAS does in the context document [12] of SAFIR 2, that the conditions for implementing such a solu-tion are not so far met. On the one hand, the solusolu-tion should benefit from adequate societal port and its development should fit in a decision-making process, yet to be established, sup-ported by the stakeholders. On the other hand, the legal and regulatory framework applicable to disposal should be made more specific and be supplemented.

After the SAFIR 2 peer reviews, ONDRAF/NIRAS reassessed its B&C Programme to take account of recommendations, carried out further the RD&D and started to prepare its so-called “Waste Plan”, a general plan on the long-term management of radioactive waste in Belgium, focused on B&C wastes. This plan will be submitted to the government by the end of 2010 and should lead to a decision-in-principle regarding the type of solution to be implemented for the

2

In the B&C Programme, the term “safety assessment” refers to the assessment of long-term or post-closure safety. It excludes operational safety assessment, which is taken to fall within the scope of feasibility assessment.

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term management of category B&C wastes, currently assumed to be geological disposal in a poorly indurated clay. Recently, ONDRAF/NIRAS also launched a societal dialogue around the choice of that solution and developed an outline of a stepwise decision-making process in-tended to help the programme converge towards the implementation of the solution that will be chosen through the decision-in-principle. The B&C Programme thus now comprises both an RD&D programme and societal aspects.

In line with international practice, the geological repository for B&C wastes and its implemen-tation are planned in a cautious, stepwise process, punctuated by the submission to the gov-ernment of key documents marking the end of successive periods in the B&C Programme, called “programme stages”. The ends of the first and second stages of the B&C Programme were marked by SAFIR and SAFIR 2; the ends of forthcoming stages in the B&C Programme will be marked by so-called “safety and feasibility cases” [13], which will be submitted in support of decisions to be requested. According to the stepwise process, the proposed disposal system, and the means to assess its safety and feasibility, are developed iteratively and in parallel over a number of years and the system is repeatedly evaluated, refined or more fundamentally re-vised in the light of increasing knowledge, becoming progressively more firmly established. Safety and feasibility cases are defined by ONDRAF/NIRAS as follows in the context of the long-term management of category B&C wastes (HLW/LILW-LL).

Safety and feasibility case

An integration of scientific and technological arguments and evidence that describe, substantiate and, if possible, quantify the safety and feasibility of, and the level of confidence in, the proposed long-term management solution for HLW/LILW-LL, namely geological disposal, at a given stage of development. It consists of a series of documents supporting in particular the statements that

the proposed disposal system will provide passive long-term safety if implemented according to design specifications;

the proposed disposal system can be constructed, operated and progressively closed taking into account operational safety issues, and its costs can be covered with the current funding mechanism (in other words, it is feasible).

These statements need to be supported at least to the degree necessary for the decision at hand. Uncertainties and open issues may remain as long as the safety and feasibility case discusses their significance in the context of the decision at hand and provides guidance for work to resolve those that have been found to be both relevant and significant to future development stages.

SFC1, the first safety and feasibility case, planned for 2013, will be devoted to assessing the safety and feasibility of a disposal system in, on the one hand, one or several zones delineated in Boom Clay (SFC1BC) and, on the other hand, one or several zones delineated in Ypresian

Clays (SFC1YC), with a view to supporting a decision of the type “go for siting”. The part

de-voted to Ypresian Clays will however be much less detailed than that dede-voted to Boom Clay, since it will focus on a study of the possibility to transfer the achievements related to geological disposal in Boom Clay to Ypresian Clays.

If SFC1 leads to a decision of the type “go for siting”, which is likely to also be based on non-technical elements, such as input from the recently launched societal dialogue, ONDRAF/NIRAS

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will prepare an SFC2, intended to provide the competent authorities with the scientific and tech-nical elements needed for enabling them to choose, with full knowledge of the facts, the site of the future repository, together with an integrated preliminary project of disposal system tai-lored to this site and that takes due account of local socio-economic conditions. Based on SFC2, a go-ahead for launching the detailed engineering studies needed to prepare licensing applica-tions could be given. The decision of the type “go for licensing” would give the green light to the preparation of the licence application files and would thus mark the transition towards the project phase, that is, the phase aiming to bring the chosen integrated project from its current status to the status of a project that is ready to be implemented.

1.1 Need for a safety strategy

A need to formalise the iterative approach to developing a geological disposal system adopted in the past and to refine and develop it further into a formal safety strategy was identified in the light of the findings of the NEA peer review of SAFIR 2 and of the necessity to organise the B&C Programme towards the production of decision-oriented documents — the safety and feasibility cases that will be documented and presented to the competent authorities. The aim was to organise the approach in a more structured, better founded way so as to guide the RD&D activities required for generating the necessary input to build safety and feasibility cases, con-centrating in particular on any major uncertainties or open questions that could call into ques-tion either the safety or feasibility of the proposed disposal system.

To this end, ONDRAF/NIRAS set out to formalise the safety strategy for guiding, firstly, the stepwise development of a geological disposal system and its implementation procedures and, secondly, the licence applications required for repository construction, operation and closure 3. This process was initiated in 2006, in close collaboration with members of the two main part-ner organisations of ONDRAF/NIRAS involved in research and development, assessments, dem-onstrations and other activities aimed at establishing a safe and feasible geological disposal system: SCK•CEN and the economic interest grouping EURIDICE, which brings together SCK•CEN and ONDRAF/NIRAS. The collaboration led, in 2007, to a first draft report structuring and docu-menting the safety strategy in a formal manner [14], of which the present report is an update (Section 1.3). Many of the elements of this strategy were already implicitly contained in the existing approach, but some new elements were also incorporated.

The expression “safety strategy” is used rather than “long-term safety and feasibility strategy” since long-term safety is the principal objective of disposal and is often simply referred to as safety. This objective will, however, only be achieved if the proposed disposal system is feasi-ble to implement, and so considerations of feasibility as well as safety enter into the safety strategy.

3

According to current Belgian regulations, a safety and feasibility case is not a legally required document and does not replace the legally required licensing documents needed before constructing, operating and closing a disposal facility. A safety and feasibility case is, however, likely to support the licensing documents.

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1.2 Current scope of the safety strategy

The safety strategy has been developed on the basis of the experience, knowledge and under-standing acquired in the context of the studies on geological disposal in Boom Clay, with the intention that it remains valid during the whole process of developing and implementing a safe geological repository. It is expected to be applicable to poorly indurated clays in general, thus also to Ypresian Clays, though this has still to be confirmed. It will have to be completed, re-fined and possibly adapted for the programme stages beyond SFC1, in particular to encompass aspects that it does not currently take into account, such as waste acceptance and siting. It will also have to take due account of aspects of safeguards.

1.3 Purpose and structure of the report

This report presents a refinement of the first iteration of the safety strategy, as described in [14], based on the practical experience acquired in applying the strategy and on the further concepts developed in the framework of two other important reports for the future SFC1: the plan for the Safety and Feasibility Case 1 [13] and the long-term safety assessment methodology for the geological disposal of radioactive waste [15]. This refined strategy does not contradict the first description.

As in the earlier report on the safety strategy, the present report is first and foremost a report presenting a working methodology, whose application is then illustrated. It defines the aims of the safety strategy and describes its main elements, in terms of inputs, successive steps and (intermediate) outputs, including how these are linked together (Figure 2 in Chapter 2). These outputs include a description of the disposal system currently under consideration, as well as the conceptual thinking behind associated choices.

The present report remains a living document, that will be discussed with the Belgian safety authorities in the period 2009–2011. It also provides a basis for continuing discussions and contributions from ONDRAF/NIRAS staff, partner organisations and subcontractors. A final documented view on safety strategy, that will integrate the still-to-be-formalised feasibility assessment methodology, which includes operational safety, would then be available by 2013. The remainder of this report is structured as follows.

Chapter 2 defines the safety strategy and describes its inputs, major steps and outputs. Chapter 3 gives examples of the application of the safety strategy at the current stage of the B&C Programme, with a focus on vitrified high-level waste and spent fuel disposal. Much of the discussion relates to choices in the design of the engineered components of the repository. (However, ONDRAF/NIRAS is, at the same time, continuing RD&D activities to improve its knowledge of the Boom Clay and the surrounding geological environment.) Chapter 4 describes how the safety strategy will be further developed and applied, and the present report further developed, in the coming years.

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2 The safety strategy: inputs, steps and outputs

The safety strategy is defined by the B&C Programme as follows:

Safety strategy

The iterative process guiding, firstly, the stepwise development of plans for a safe and feasible geological repository and its implementation procedures and, secondly, the successive licence applications, this based on a concentrate and confine approach and taking account of all relevant boundary conditions (Figure 1).

More specifically, the safety strategy is the iterative process for developing the safety concept and the design of a geological repository and for acquiring evidence, developing arguments and carrying out analyses to show that these are both safe and feasible to implement as planned. The process is designed to be flexible, allowing, for example, for the incorporation of new sci-entific information as it becomes available. The safety strategy can also be viewed as the whole of coordinated actions that connect together the various elements contributing to developing a geological repository and that integrate them through an iterative process.

Figure 1 – Iterative process of repository planning leading to eventual licence applications, showing the relationship between the safety strategy and the successive safety and feasibility cases that will be presented by ONDRAF/NIRAS to its supervising authority at key decision points in the B&C Programme. The figure is based on the assumption that the requested decisions are taken as planned. Boundary conditions Safety strategy to develop safety concept + repository design SFCi (i = 1 to n) Does case support a 1st licence application? Programme stage i : RD&D

no Safety strategy to support further applications SFCj (j = 1 to m) Programme stage j : implementation Does case support a closure licence

application? no towards repository closure yes yes towards implementation

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The term “safety concept” is defined by the B&C Programme as follows:

Safety concept

The integrated description of the elements on which the passive long-term safety of the proposed system for the disposal of category B&C wastes rests, where this description, which is given at a level of detail appropriate to the stage of disposal system development, includes (1), the safety functions provided by the main components of the system and its geological coverage and (2), features of the system and its implementation providing robustness by ensuring that each of the safety functions will, in reality, be fulfilled over at least the assigned time frames, irrespective of any remaining uncertainties.

In the safety strategy (Figure 2), system development and assessment of its safety and feasibil-ity are constrained both by boundary conditions (Section 2.1.1) and by a number of strategic choices (Section 2.2.1) made by ONDRAF/NIRAS, which are themselves constrained by the boundary conditions. These strategic choices and the boundary conditions are translated into requirements (Section 2.2.2) related to the disposal system as a whole, to subsystems or to in-dividual components of the system with a view to meeting the objectives of passive long-term safety and feasibility.

System development and assessment of its safety and feasibility are carried out largely in parallel, iteratively. System development (Section 2.2.3) starts with the development of the safety concept, on the basis of existing knowledge and understanding and of the requirements on the system and some of its subsystems. The safety concept, together with the remaining requirements, is trans-lated into a structured set of statements, collectively designated as safety and feasibility state-ments, used as a guiding tool throughout further system development and safety and feasibility assessments. With the safety concept as a basis, the development of a repository design (and, as the programme matures, the development of implementation procedures) is carried out iteratively, in parallel with the development of the remainder of the assessment basis, comprising the existing knowledge and understanding and the analysis tools, and with preparatory safety and feasibility assessments (Section 2.1.2). Preparatory assessments, which are seen as the first step in assessing the safety and feasibility of the proposed disposal system (Section 2.2.4), are partial assessments, generally taking the form of exploratory calculations, aimed to assess the impact of uncertainties on long-term safety and feasibility, qualitatively and, to the extent possible, quantitatively. To-gether with the development of the assessment basis, they generate multiple lines of evidence, arguments and analyses to support safety and feasibility statements. They also aim to identify any significant deficiencies in current knowledge and understanding and in the plans to address these in the RD&D programme. Modifications can then be made in the programme and, should this appear necessary, in the strategic choices (Section 2.3.2).

Formal assessments of the proposed disposal system are carried out provided preparatory as-sessments indicate good prospects that these will confirm the safety and feasibility of the re-pository to the extent needed for the programme stage at hand. The formal assessments should provide a finer understanding of the remaining uncertainties and their safety relevance (Sec-tion 2.2.4). Together with the other lines of evidence, arguments and analyses available, they then provide input to the safety and feasibility case, in the form of the safety concept and a repository design with possible variants, implementation procedures (depending on the current programme

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stage), and evidence, arguments and analyses (Section 2.3.1). Should formal assessments not confirm the safety and/or the feasibility of the disposal system, then the RD&D programme, and possibly also the strategic choices, again need to be adjusted (Section 2.3.2).

Figure 2 – Overview of the safety strategy, which is the iterative process guiding, firstly, the stepwise development of plans for a safe and feasible geological repository and its implementation procedures and, secondly, the successive licence applications, this based on a concentrate and confine ap-proach and taking account of all relevant boundary conditions. The iterative nature of system devel-opment at the design level (and, to a lesser degree, at the level of the safety concept) and the role of assessments in system development are indicated by the feedback loops.

Boundary conditions (Section 2.1.1)

Strategic choices (Section 2.2.1)

(based on the hypothesis of geological disposal in a poorly indurated clay)

Requirements (Section 2.2.2)

System development + assessment of safety and feasibility Development: concept

(Section 2.2.3)

Development: design, in parallel with remainder of assessment basis (Section 2.2.3) Safety concept Develop repository design (Re)consider implementation options Develop knowledge and understanding Assessment of safety and feasibility (Section 2.2.4) Build assessment toolbox Phenomenological description of disposal system evolution + uncertainties Methods, models, codes and datasets Design (materials,

layout, etc.)

Guidance for adjusting the RD&D programme (Section 2.3.2)

yes

Compile SFCi (i = 1 …n)

and move to next programme stage Structured set of safety and feasibility statements

Assess support for S&F statements through pre-paratory assessment Formal assessment of S&F conclusive? Perform formal assessment of S&F Enough support to go to formal assessment? no no yes Requirements on system and some subsystems

Requirements on subsystems and components

Assessment basis (Section 2.1.2) Implementation procedures Input to SFCi (i = 1 …n) (Section 2.3.1)

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Several iterations may be needed within every programme stage in order to develop a design that satisfies all relevant requirements, an assessment basis that is adequate for the purposes of safety and feasibility assessments, and the evidence, arguments and analyses to support a safety and feasibility case that is adequate for the purposes of the next programme stage. Some of these iterations may consist, for example, of a designer working alone and assessing in qualita-tive terms the advantages and disadvantages of different options. Other iterations may require for example a multidisciplinary team to perform a detailed evaluation of the potential impact of a specific feature on long-term safety.

The safety strategy is not expected to be particularly prone to change, in terms of the elements and sequence of steps foreseen, as the programme progresses from one stage to the next. The safety strategy will, however, need to be completed and refined (Section 1.2). Should signifi-cant changes need to be brought to it, their impact on past work would then need to be exam-ined.

Although the safety strategy is expected to remain relatively stable, its outputs themselves will progressively develop as the safety strategy is repeatedly applied. At early stages of the B&C Programme, those of SAFIR and SAFIR 2, the decision to proceed from one stage to the next was based on a demonstration of the broad viability of disposal. For this purpose, a broad-brush description of at least some aspects of the disposal system — the main components, what functions they performed and how they were to be implemented — was found to be sufficient, and quantitative assessments could be limited to long-term safety. At later stages, more formal analyses of additional factors such as operational safety will also be made. It will then be nec-essary not only to show that the safety concept and the repository design are “fit for purpose” in that they can be shown to provide passive long-term safety and are feasible to implement, but also to select between different potentially suitable design variants, given that more than one design may conform to the safety concept and satisfy safety and feasibility requirements. For choosing between design options, more detailed descriptions of the safety concept and pos-sible designs will have to be developed and assessed in terms of all relevant factors.

Thus, although the same strategy will be applied repeatedly at successive programme stages, the outcome in terms of the description of safety concept and repository design, and in terms of the evidence, arguments, and analyses to support a safety and feasibility case, will vary as the programme develops, decisions are made, knowledge is acquired and the assessment basis is refined. This support includes not only work carried out within the B&C Programme, but also within other disposal programmes internationally, and advances in science and technology in general. The need for wide-ranging evidence and arguments to support the safety and feasibil-ity case is likely to grow as the programme proceeds through successive stages. Some less es-sential issues that, for example, were deferred at early stages, will have to be addressed at later stages.

The inputs, major steps and outputs of the safety strategy, which are major foundations of safety and feasibility cases, are detailed in Sections 2.1, 2.2 and 2.3, respectively.

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2.1 Input for applying the safety strategy

At every programme stage, the application of the safety strategy is constrained by the so-called “boundary conditions” and uses the existing knowledge base and analysis tools of the assess-ment basis.

2.1.1 Boundary conditions

The application of the safety strategy must take account of boundary conditions, which the B&C Programme groups into the following five categories:

ONDRAF/NIRAS working hypotheses, international framework,

Belgian legal and regulatory framework, institutional policy,

other stakeholder conditions.

The relevant boundary conditions must be identified exhaustively in order to ensure complete-ness of the strategic choices and of the requirements to which they lead. Except for the working hypotheses, all of them originate from actors that are external to ONDRAF/NIRAS.

Many of the existing boundary conditions (Section 3.1) are likely to be modified during reposi-tory planning. In addition, new boundary conditions will emerge as, for example, the safety authorities (Federal Agency for Nuclear Control or FANC) develop further the existing Belgian legal and regulatory framework and as the safety concept, the repository design and possible variants are discussed with various stakeholders. The process defined by the safety strategy must be repeated to account for any such changes when appropriate.

2.1.1.1 ONDRAF/NIRAS working hypotheses

The ONDRAF/NIRAS working hypotheses are hypotheses that ONDRAF/NIRAS has had to make in order to be able to plan and carry out its B&C Programme in a focused way. They currently amount to saying that ONDRAF/NIRAS’ reference solution for the long-term management of category B&C wastes is geological disposal in Boom Clay, that this solution should be imple-mented as soon as possible, and that geological disposal in Ypresian Clays, also as soon as possible, is an alternative solution. During the course of the programme, these hypotheses will be considered by third parties outside ONDRAF/NIRAS, such as the Federal Government, the supervising authority, FANC and local partners, and they will be confirmed, elaborated, modi-fied or rejected as a result of their deliberations.

2.1.1.2 International framework

The international framework has been divided into three groups of items respectively called conventions and directives, general texts on safety and radiological protection, and texts and practice regarding radioactive waste management and disposal, by international organisations

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that are active in nuclear-related matters (for instance, the IAEA, the ICRP, the NEA/OECD and the European Union), and also, where relevant, guidance observed in other industries. The interna-tional framework is not compulsory, except for

the European directives, which must be implemented through promulgation of Belgian legislation;

the conventions and treaties to which Belgium is a signatory, for example, the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Convention, 1972) and its Protocol (1996), the Joint Convention on the Safety of Spent Nuclear Fuel and the Safety of Radioactive Waste Management [16], the Treaty establish-ing the European Atomic Energy Community (Euratom, 1957) and the Treaty on the Non-Proliferation of Nuclear Weapons (1968).

ONDRAF/NIRAS considers it appropriate, however, to take all relevant aspects of the interna-tional framework into consideration for all its activities.

Conventions and directives include treaties and directives at the European level and conventions and treaties at the wider international level.

The general texts on safety and radiological protection include the basic principles of ra-diological protection of the ICRP [17, 18], the basic rara-diological standards defined through the so-called international Basic Safety Standards of the IAEA [19], which have been in-corporated into European legislation [20] and Belgian legislation [21 and its subsequent amendments], and the Fundamental Safety Principles of the IAEA [1, 3].

The texts on radioactive waste management and disposal include recommendations of the ICRP on the application of radiological protection principles to radioactive waste disposal [22, 2], and standards and principles of the IAEA [1, 3, 7, 23]. Relevant general practice at the international level includes in particular the two essential principles of robustness and demonstrability, where robustness is closely related to the demonstrability of long-term safety.

► Robustness refers to the ability of a disposal system to provide safety in all reasonably foreseeable circumstances despite the remaining uncertainties as regards its evolution and functioning, which entails that its components fulfil their assigned long-term safety functions in those circumstances despite the remaining uncertainties as regards their exact behaviour. Long-term safety functions are defined as the functions that a disposal system should fulfil so as to achieve its fundamental objective of providing long-term safety through a concentration and confinement strategy, while limiting the burden placed on future generations (for example, [1, 3, 7, 23]).

► Demonstrability is achieved by adopting methods of system development that make it possible to demonstrate (in the sense of providing convincing arguments) that safety will be provided no matter what reasonably foreseeable disturbances may impact on system evolution.

Robustness and demonstrability favour concepts and designs that incorporate features such as defence in depth and flexibility, are simple in terms of their features and evolution, and use well-understood materials and techniques.

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► Defence in depth implies siting and designing for multiple safety functions. The exis-tence of multiple safety functions, provided via a range of physical and chemical phe-nomena with no undue reliance on any single barrier or phenomenon, is recognised by all programmes as contributing to robustness by mitigating the effects of uncertainties on the overall performance of a disposal system, and reducing the possibility that any single perturbing phenomenon or uncertainty can undermine all of the functions (see for example [7], paragraph 3.25).

► Maintaining a degree of flexibility in repository design (including layout) allows, for example, for the possibility of unexpected geological features being encountered at a site. An example of flexibility is a “design-as-you-go” approach to the design of lay-out.

In addition, good practice involves, for instance, seeking multiple lines of evidence, argu-ments and analyses where possible for key eleargu-ments of system understanding, using best available techniques and implementing safety and quality management systems, which, for example, covers planning pre-emptively for all reasonably foreseeable accidents.

2.1.1.3 Belgian legal and regulatory framework

The Belgian legal and regulatory framework is essentially composed of all the legal and regula-tory items, at the federal and regional levels, relevant to the planning, licensing, construction, operation, closure and post-closure surveillance and monitoring of a repository. Two major texts for the radiological aspects are the royal decree of 20 July 2001 [21] on the protection of the public, workers and the environment against the dangers of ionizing radiation, and the law of 2 August 2002 [5], which enforces the Belgian commitment to the IAEA Joint Convention on the Safety of Spent Nuclear Fuel and the Safety of Radioactive Waste Management, whereby Belgium has, in particular, committed itself to give due regard to IAEA standards and recom-mendations on radioactive waste management. The legal and regulatory framework that is ap-plicable to a repository is being further developed as regards radiological aspects, and aspects regarding the non-nuclear legislation that is applicable to a repository are being clarified. Ra-diological aspects to be further developed are, for instance, the dose constraint that is applica-ble to a disposal facility, the safety indicators complementary to the dose to be used, the time frames that safety assessment calculations and the safety and feasibility case must cover, and the way to treat altered-evolution scenarios and scenarios of human intrusion in these assess-ments. ONDRAF/NIRAS follows international guidance and practices for those aspects that still need to be included in the legal and regulatory framework. In practice, FANC has recently is-sued what is currently a guidance note on the management policy for licence applications, con-taining safety principles that will apply to any final disposal facility for radioactive wastes in Belgium [24].

Another important text constraining the B&C Programme is the law of 31 January 2003 on the progressive nuclear phase-out, which foresees in the shutdown, after 40 years of operation, of each of the seven Belgian commercial nuclear reactors [25].

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2.1.1.4 Institutional policy

Institutional policy items are policy-oriented decisions or recommendations made by compe-tent authorities, but without being incorporated in the legal and regulatory framework (al-though they may be incorporated at a later time).

Institutional policy items are currently limited to the resolution of the Chamber of 22 Decem-ber 1993 regarding the use of MOX fuel in Belgian nuclear power plants and spent fuel reproc-essing [26], confirmed by the Council of Ministers the same year and confirmed again on 4 December 1998. This resolution entrusts the government with the task to [translation] “give the priority to research and development, including in an international framework, with a view to implement, eventually, the direct disposal of spent fuel, this without reducing the current research programme in the field of disposal of reprocessing waste in deep geological layers.” This resolution lead ONDRAF/NIRAS to place the study of geological disposal of spent fuel at the same level as that of reprocessing waste.

In the future, policy-oriented decisions or recommendations could, for example, relate to the reprocessing moratorium or to monitoring.

2.1.1.5 Other stakeholder conditions

Other stakeholder conditions are those associated with the implementation of a repository that may be set either by a Belgian non-institutional stakeholder or by a foreign institutional stake-holder. Examples could be conditions set by a local municipality for it to accept the implemen-tation of the repository on its territory or conditions imposed by a neighbouring country regard-ing the location of the repository with respect to a shared border, as long as these conditions do not undermine safety or feasibility.

2.1.2 Assessment basis

The assessment basis is defined by ONDRAF/NIRAS as the knowledge base and analysis tools required for safety and feasibility assessments. More specifically, it includes the following (Figure 2):

the description of the design of the proposed disposal system and, depending on the stage of planning and development, implementation procedures;

the empirical and theoretical scientific knowledge and understanding that are relevant to the assessment of the safety and feasibility of the disposal system under consideration, in-cluding relevant process models and computer codes, a description of the expected phe-nomenological evolution of the system, and the scientific knowledge and understanding underlying this description, including associated uncertainties;

the various methods, models, computer codes and datasets needed for assessing safety and feasibility, including the safety assessment methodology and the feasibility assessment methodology.

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This definition is based on that of the NEA [27], extended to include aspects of feasibility as well as long-term safety.

At every programme stage, the assessment basis comprises the existing knowledge and under-standing, including relevant knowledge from foreign disposal programmes. It supports the car-rying out of both preparatory and formal assessments of the safety and the feasibility of the disposal system, and its further development is guided by the findings of these assessments (Section 2.2.4.3). This development, through an iterative and stepwise process, is a major ob-jective of the RD&D programme. Completeness checks are carried out periodically to ensure, as far as possible, that the assessment basis is internally consistent and that it is “complete” or, in other words, that no feature, event or process (FEP) potentially important for the long-term evo-lution of the system has been overlooked. These checks use of the disposal-system-specific catalogue of FEPs compiled on the basis of an international FEP list and follow the structured set of safety and feasibility statements as a guiding tool (Section 2.2.4.1).

2.2 Major steps within the safety strategy

ONDRAF/NIRAS distinguishes four major steps in the application of the safety strategy:

the making of strategic choices, which are high-level in nature,

the definition of the requirements ensuing from the boundary conditions and the strategic choices,

the development of a safety concept and repository design on the basis of those require-ments,

the assessment of the safety and feasibility of the disposal system, where system develop-ment and assessdevelop-ment of its safety and feasibility are conducted largely in parallel and itera-tively.

2.2.1 Making of strategic choices

Given existing knowledge and understanding and the constraints imposed by the boundary conditions, strategic choices are made regarding repository planning and the broad features that the disposal system should have in order to meet its safety and feasibility objectives (Figure 2). These choices are high-level in nature, and many are expected not to have to be modified with respect to most reasonably foreseeable scientific and technological developments and modifications to the boundary conditions, such as changes in dose or risk criteria specified in regulation. They provide a framework for the development of the safety concept and of the repository design (Section 2.2.3), with some of the choices translating directly into requirements on the system as a whole, on subsystems or on components of the system.

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2.2.2 Definition of the requirements

The boundary conditions and strategic choices lead to requirements that are generally identified in a top-down manner, starting with requirements that apply to the disposal system as a whole, and progressing to more specific requirements on subsystems and on the individual system components as these are identified and defined in the course of system development (Figure 2). Requirements are translated into a structured set of safety and feasibility statements (Sec-tion 2.2.4.1).

Requirements regarding long-term safety, for instance those concerning how different safety functions should be fulfilled within different time frames, tend to be the principal considera-tions guiding system development at early stages of the development process, with require-ments relating to feasibility of implementation becoming increasingly important (though with-out compromising long-term safety) as development proceeds.

Requirements on the system as a whole are expected to be stable during a given programme stage and from one programme stage to the next. By contrast, requirements on subsystems and components will, at least for some of them, become more stringent as the programme pro-gresses from one stage to the next, in particular as a result of the need to reduce safety-relevant uncertainties. In addition, these requirements may need to be modified to take account of changes in boundary conditions or of specific design and implementation choices.

2.2.3 System development

System development is carried out on two levels (Figure 2). At the higher, more general level is the development of the safety concept. At the lower, more detailed level is the iterative de-velopment of a repository design (and, as the programme matures, the making, or re-evaluation, of implementation choices), carried out in parallel with the further development of the knowl-edge base and the analysis tools of the assessment basis and with so-called “preparatory as-sessments” of the safety and the feasibility of the disposal system.

Safety concept development implies the development of an integrated description of the elements on which the passive long-term safety of the proposed system for the disposal of category B&C wastes rests, based on the requirements set on the system and relevant sub-systems. This description, which is given at a level of detail appropriate to the stage of disposal system development, includes (1), the safety functions provided by the main components of the system and its geological coverage and (2), features of the system and its implementation providing robustness by ensuring that each of the safety functions will, in reality, be fulfilled over at least the assigned time frames, irrespective of any remaining uncertainties. The safety concept is translated, together with the remaining requirements, into a structured set of safety and feasibility statements, forming the cornerstone of the ap-plication of the safety strategy (Figure 2 and Section 2.2.4.1).

Design development (including specific material choices, dimensions, layout of the under-ground openings, implementation procedures, etc.) is guided by the more detailed re-quirements placed on subsystems and components, reflected in the set of safety and

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feasibility statements. The choice between design options and the way the system is as-sessed may be affected by boundary conditions.

System development has an iterative nature at the design level and, to a lesser degree, at the level of the safety concept, which constitutes thus an element of the framework guiding the RD&D programme. The design may be modified as the programme progresses through succes-sive stages, in order to better adapt the disposal system to relevant requirements and to take advantage of advances in science and engineering. Changes to the safety concept are expected only if strategic choices on which it is based need to be modified. Such changes should be as-sessed in terms of their impact on past work.

2.2.4 Assessment of the safety and the feasibility

The assessment of the disposal system in terms of both passive long-term safety and feasibility is carried out largely in parallel with system development. It uses the structured set of safety and feasibility statements (Section 2.2.4.1) as a guiding tool, and involves the evaluation of the importance of each statement taken individually (Section 2.2.4.2) and the assessment of the level of support available for each of those statements, given current knowledge and under-standing and the programme stage at hand (Section 2.2.4.3). More specifically,

safety assessment can be seen as aiming to assess whether or not the statement claiming that the proposed disposal system will provide passive long-term safety, in both radiologi-cal and non-radiologiradiologi-cal terms, if implemented according to design specifications is well substantiated by various lines of evidence, arguments and analyses;

feasibility assessment aims to assess whether or not the statement claiming that the pro-posed disposal system is feasible, with due regard to engineering practicality, operational safety and financial costs, is well substantiated by various lines of evidence, arguments and analyses.

The full set of safety and feasibility statements must be assessed and judged adequately well substantiated, given the decision at hand, prior to the compilation of the corresponding safety and feasibility case.

In practice, the various steps around the development and use of safety and feasibility state-ments are carried out to some extent in parallel, but they are described sequentially hereafter.

2.2.4.1 Development of safety and feasibility statements

Given the objective of compiling a decision-oriented set of reports — a safety and feasibility case — towards the end of every future programme stage, ONDRAF/NIRAS has chosen to organ-ise the assessment of the safety and feasibility of the proposed geological disposal system around a structured set of claims, the so-called “statements”, about what the system does and the properties that it has. The approach is one of “theorem-demonstration”, and is only possible when there is pre-existing knowledge and understanding available on which to base the state-ments or “theorems”. This type of approach is more appropriate than a deductive approach or

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an encyclopeadic compilation of knowledge and understanding, since statements can be spe-cifically formulated to address the needs of decision makers. Furthermore, the “theorem-demonstration” approach is also a powerful tool to guide current and future RD&D activities (Section 2.3.2).

Safety and feasibility statements are derived from the requirements on the disposal system as a whole, the various subsystems and the individual system components, often having a one-to-one correspondence with requirements, and from knowledge and understanding from the as-sessment basis. They can be very general in nature, an example being the statement that the proposed disposal system will provide an adequate level of passive long-term safety if imple-mented according to design specifications. They can also be more specific, such as the state-ment that a metallic overpack for category C waste of a specific design and in a specific physi-cal and chemiphysi-cal environment will remain intact for a given minimum period of time.

Some statements, particularly those that are more general in nature and are, for example, a direct translation of the safety concept, can be formulated early in the programme, while other, more detailed statements gradually emerge as the programme progresses, the safety concept and the design become better defined and more firmly established, and the assessment basis in general is further developed. Safety and feasibility statements are thus developed and structured in a top-down manner, starting with the most general (high-level) statements and progressing to increas-ingly specific (lower-level) statements (for example, statements regarding the properties of the host formation or the required performance of specific system components) (Figure 3).

Figure 3 – The top-down development of the structured set of safety and feasibility statements.

According to the current view, the statements on which SFC1 will be built will be structured in four branches: safety statements, feasibility statements, a few statements regarding the defini-tion of the disposal system and the way it was developed and statements regarding residual uncertainties (Section 3.5). It is the combination of the level of support that is available for safety, feasibility and definition statements, given the decision at hand, with the level of sup-port that is available for statements concerning confidence in the possibility to manage residual uncertainties that will enable ONDRAF/NIRAS to state that it has confidence that the safety

con-There is confidence that the safety concept and the design of the proposed disposal system show sufficient promise to proceed to the next programme stage

Residual uncertainties