NUMO-TR-09-04

Study on strategy and methodology
for repository concept development
for the Japanese geological disposal
project

H. Kurikami, S. Suzuki, K. Wakasugi,
H. Ueda, K. Ishiguro, H. Tsuchi,
Nuclear Waste Management Organization of JapanK. Kaku, I. Gaus, S. Vomvoris
Nagra

September 2009
Nuclear Waste Management Organization of Japan (NUMO)

2009 年 9月
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NUMO-TR-09-04

Study on strategy and methodology
for repository concept development
for the Japanese geological disposal
project

H. Kurikami, S. Suzuki, K. Wakasugi,
H. Ueda, K. Ishiguro, H. Tsuchi,
Nuclear Waste Management Organization of JapanK. Kaku, I. Gaus, S. Vomvoris
Nagra

September 2009
Nuclear Waste Management Organization of Japan

Table of contents
1

2

3

Introduction ................................................................................................................................. 1
1.1

Background of this study ...................................................................................................... 1

1.2

Overview of the collaborative study on repository concept development ............................. 1

1.3

Structure of this report .......................................................................................................... 3

Development of NUMO’s PA/RD roadmap at the PI stage ......................................................... 4
2.1

Premises for the roadmap .................................................................................................... 4

2.2

Interaction with site characterisation activities ...................................................................... 4

2.3

PI roadmap........................................................................................................................... 7

2.3.1

Task 0: Available information from the LS stage............................................................. 7

2.3.2

PI – introductory and geophysical investigation phase................................................... 9

2.3.3

PI – borehole investigation phase 1 ............................................................................. 17

2.3.4

PI - borehole investigation phase 2 .............................................................................. 24

2.3.5

Assessment of PI findings and developing input to the DI stage .................................. 30

Scenario development .............................................................................................................. 34
3.1

Background ........................................................................................................................ 34

3.2

Scenario development in H12 ............................................................................................ 35

3.2.1

Classification of scenarios in H12................................................................................. 35

3.2.2

Scenario development procedure in H12 ..................................................................... 37

3.2.3

Limitations of the H12 methodology ............................................................................. 38

3.3

3.3.1

Recognition that scenario development is a “top-down” process ................................. 38

3.3.2

Identification of individual scenarios ............................................................................. 39

3.3.3

The issue of completeness........................................................................................... 44

3.4

4

Scenario development in more recent PAs......................................................................... 38

Development of NUMO’s methodology for scenario analysis............................................. 48

3.4.1

Introduction .................................................................................................................. 48

3.4.2

Requirements defined for the scenario development methodology.............................. 49

3.4.3

The scenario development logic and a first dry run ...................................................... 49

3.4.4

Discussion and further developments .......................................................................... 55

Development of a Requirements Management System (RMS) ................................................ 57
4.1

Introduction......................................................................................................................... 57

4.2

RMS development in overseas organizations .................................................................... 57

4.2.1

Information exchange meeting in 2007 ........................................................................ 57

4.2.2

The SKB case .............................................................................................................. 58

4.2.3

The Nagra case............................................................................................................ 64

4.3

Overview of the current N-RMS being developed by NUMO.............................................. 67

4.3.1

Background of N-RMS development............................................................................ 67

4.3.2

Development process of the N-RMS............................................................................ 68

4.3.3

Overall structure of the N-RMS .................................................................................... 69i5Concluding remarks.................................................................................................................. 73

Acknowledgements......................................................................................................................... 74
References...................................................................................................................................... 74
Appendix
A. Survey of focused PA issues in European countries............................................................. A-1
A.1 Issues addressed.............................................................................................................. A-1
A.2 PA issues arising from engineering considerations ........................................................... A-1
A.2.1 The need for interaction between PA analysis and engineers in identifying key issues................................................................................................................................. A-1
A.2.2 Examples of issues of engineering practice of potential significance of PA ................. A-4
A.2.3 Significance of the issues from PA perspective – example of issues affecting buffer
density .................................................................................................................... A-13
References................................................................................................................................... A-17

ii

List of figures
Figure 1.1 Schedule of geological disposal program in Japan ....................................................... 1
Figure 2.1 Interactions between SC, RD and PA............................................................................ 5
Figure 2.2 Outline of the proposed PA/RD roadmap for the PI stage ............................................. 6
Figure 2.3 Planning of the site characterization activities at the PI stage - Flow chart(Koike, 2008) ................................................................................................................. 8
Figure 2.4 Schematic illustration showing how the legally defined Evaluation Factors for
Qualification affect the identification of the location and geometry of aPreliminary Investigation Area (after NUMO, 2004b)..................................................... 9
Figure 3.1 Classes of scenarios in H12 ........................................................................................ 36
Figure 3.2 H12 scenario development procedure......................................................................... 37
Figure 3.3 Alternative view of the H12 scenario development procedure..................................... 39
Figure 3.4 Schematic illustration of the structuring of assessment cases, with each caseconsisting of a scenario, a model set and a parameter set.......................................... 41
Figure 3.5 Range of influence of possible deviations from the expected evolution of the
Project Opalinus Clay disposal system (after Figure 5.7-1 in Nagra, 2002a) .............. 42
Figure 3.6 Safety function indicators and criteria for the SR-Can canister and buffer (after
SKB, 2006).................................................................................................................. 44
Figure 3.7 FEP management tasks (in grey) in the wider context of the Project Opalinus
Clay assessment methodology (after Figure 3.7-2 of Nagra, 2002a) .......................... 48
Figure 3.8 Hybrid scenario analysis workframe............................................................................ 50
Figure 3.9 T-1 Storyboard - Overview of total system evolution based on the H12
repository concept ....................................................................................................... 52
Figure 3.10 T-2 Detailed storyboard of EBS module (THMC analysis)........................................... 53
Figure 3.11 Relationship between the FEP knowledge base and tasks T-1, T-2 and T-3............... 54
Figure 3.12 Safety functions and safety function indicators of EBS module................................... 54
Figure 3.13 Sequences of more probable safety-relevant phenomena derived from T-2 ............... 55
Figure 4.1 SKB’s concept of different types of requirements for a radwaste disposal
project ......................................................................................................................... 60
Figure 4.2 SKB’s hierarchal structure of the RMS ........................................................................ 60
Figure 4.3 Organization of database system for the SKB RMS.................................................... 62
Figure 4.4 An example of a decision-making process by SKB ..................................................... 62
Figure 4.5 Screen shot of Nagra’s QM system............................................................................. 64
Figure 4.6 Flow diagram of the development process of the N-RMS ........................................... 69
Figure 4.7 Procedure for development of the trial RMS ............................................................... 69
Figure 4.8 Overall structure of the N-RMS and expected interactions between the
sub-projects................................................................................................................. 70
Figure 4.9 Overall system concept of the N-RMS with two functions - decision-making
support function and RMS database management function........................................ 71iii

Figure 4.10 Initial screen shot of the N-RMS.................................................................................. 72
Figure 4.11 Typical screen shot of the N-RMS ............................................................................... 72
Figure A.1 The need for interaction between PA practitioners and engineers in defining
the repository “initial state” and its subsequent evolution ........................................... A-2
Figure A.2 Illustration of how long-term safety requirements influence the engineering
measures employed and the overall planning of repository construction andoperation .................................................................................................................... A-3
Figure A.3 Example layout adaptation of a KBS-3H repository at Olkiluoto (see Gribi et al.,2008; Smith et al., 2008) .......................................................................................... A-14
Figure A.4 Relationship between buffer swelling pressure, dry density and ionic strength,
expressed as molar NaCl concentrations (after Figure 4-7 in SKB, 2006) ............... A-16
Figure A.5 Deriving a range of buffer density that, if maintained, should ensure that all thesafety functions of the buffer are provided (typical PA assumptions hold) ................ A-17

List of tables
Table 3.1

Tasks of the bias audit group in Project Opalinus Clay................................................ 47

Table 4.1

Main players involved in developing and using SKB’s RMS........................................ 63

Table A.1

Examples of experience to date on engineered barrier emplacement ....................... A-3

Table A.2

Examples of elements or issues of engineering practice that may give rise touncertainties and potential deviation from design values of key PA parameters
and the corresponding issues/processes for long-term safety.................................... A-4

Table A.3

Examples of PA-relevant issues, their potential causes, the PA assumptions or
parameters potentially affected, the potential effects on system performanceand the information requirements to quantify these effects ........................................ A-7

Table A.4

Safety function indicator criteria related to buffer density (based on SKB, 2006)................................................................................................................................. A-15

iv

1

Introduction

1.1

Background of this study

NUMO has selected an open solicitation approach for the site selection process in Japan, bothfor the high-level waste repository (open call in 2002) and the TRU repository (open call in2008). Such an approach, albeit flexible, poses a special challenge for NUMO's repository
design.
A detailed repository design must be tailored to the given geological and surface environment,
not least to take advantage of the potential for optimization. The repository design must fulfill
a broad range of requirements resulting, for example, from the long-term and operational safety
goals, engineering practicality and socio-economic issues. The special challenge for NUMO'sdesign team is to be able to specify a number of different designs for different geological andgeographical sites and, moreover, to be able to finalize specific designs within a very limited
time period (see Figure 1.1. NUMO plans to select site through the literature survey (LS) stage,
the preliminary investigation (PI) stage and the detailed investigation (DI) stage; the site should
be selected by around 2028).
For the Swiss radioactive waste disposal program, Nagra has experience drawn from looking at
a wide range of geological environments for both the HLW and L/ILW projects. It was thus
agreed to set up a collaborative study aimed at transferring relevant knowledge from the Swissgeological disposal program to the Japanese one.
Literature Survey(LS)

Preliminary
Investigation (PI)

Detailed
Investigation (DI)

Repository
construction

Selection of Detailed
Investigation Area (DIA)

Selection of
repository site

Initiation ofoperation

Mid of H20’s(Around 2013)

Around H40(Around 2028)

End of H40’s(Up to 2037)

Figure 1.1 Schedule of geological disposal program in Japan.

1.2

Overview of the collaborative study on repository concept
development

The repository concept (RC) collaborative study was initiated in 2001, mainly as a technicalknowledge transfer between the implementing organizations of Japan and Switzerland. The
focus was on developing a methodology for tailoring repository concepts to a broad range of1

site environments. The outcome of the study between 2001 and 2003 (phase 1) is summarized
in NUMO (2004a).
The RC collaborative study was further continued and expanded to cover a wide range of topics
from the strategic to the technical level. Every year, two main meetings were organized, one in
Switzerland and one in Japan. These meetings were supplemented by a number of working
meetings both in Switzerland and Japan. These working meetings allowed the members of the
NUMO-Nagra team to exchange discuss and further develop ideas and concepts applicable to
the Japanese environment. The main meetings also discussed work performed by contractors
and guided future developments.
The 2nd phase of the RC study was completed in FY 2007. The goal of this report is tosummarize the main results of the study between FY 2004 and FY 2007, as a complement to
NUMO (2004a).
For each of the fiscal years covered in this report, the main topics investigated as part of the RC
collaborative study are as outlined below.
FY 2004

Multiple attribute analysis (MAA) of different repository designs
Preliminary ideas for application of a prototype requirements management system (RMS) toan integrated repository project
Identifying future requirements for PA models and databases.

FY 2005
Structured approach for tailoring repository concepts to specific sites



Further development and implementation of a requirements management system (RMS)

Establishing a performance assessment code and database development program.

FY 2006Structured approach for tailoring repository concepts to specific sites, to include:Completion of the NUMO Structured Approach (NSA) policy report

Development of ideas for NSA implementation

Further development and implementation of a requirements management system, including
review of the RM structure of NUMO's operational safety study
Establishing performance assessment strategies and methods, including:2Development of the PA strategy in the three investigation stages

A study of scenario development



Identification of topics related to the PA methodologies.

FY 2007Interaction between repository concept (RC) development and site characterization



Study on structure and contents of the RMSStudy on development and evaluation of performance assessment (PA) scenariosSurvey on specific issues for RC development

Summary report on RC collaboration (draft).

1.3

Structure of this report

From the extensive list of topics covered under the collaboration, three topics are selected here
and described in more detail.
Chapter 2 summarizes the roadmap for the preliminary investigation (PI) stage. The main focus
in developing the PI roadmap was to identify potential interactions among different disciplines,
e.g. site characterization, repository design and safety assessment.
Chapter 3 covers the methodology for scenario development for safety assessment. The
scenario development method used in the H12 report is reviewed and more recent
methodologies used in European programs are presented. NUMO’s strategy for scenario
development is also described.
Chapter 4 is dedicated to the development of NUMO's Requirements Management System(RMS). As case studies, the ongoing developments at SKB and Nagra are briefly summarized.
NUMO's idea for RMS development is also summarized in Chapter 4.
Chapter 5 contains the conclusions of the collaborative study.

3

2
2.1

Development of NUMO’s PA/RD roadmap at the PI stage
Premises for the roadmap

The aim of the PA/RD (performance assessment/repository design) roadmap is to establish a
guideline for implementing site characterization, repository design and safety assessment
activities at the preliminary investigation (PI) stage. Because such efforts are multidisciplinary,
one of the focal points of the PA/RD roadmap was to identify the interactions among thedisciplines throughout activities during the PI stage.
The time horizon assumed is the one considered in the current framework for the PI stage, i.e.
that the activities for the PI stage must be implemented within 5 years. This time constraint was
one of the boundary conditions in setting up the overall site investigation program for theroadmap. As is shown below, within this period the project team assumed that two, more or lesssequential field investigation campaigns can be organized, executed and completed.
As noted in Chapter 1, the site and, consequently, the site conditions have not been specified inJapan to date. Thus, the roadmap was developed in a generic manner without going into
detailed technical specifications. Once the site conditions are better defined, the processes in theroadmap can be further optimized.

2.2Interaction with site characterisation activities

Site characterization (SC), repository design (RD) and performance assessment (PA) activities
are closely interlinked. This is particularly true if the time available is limited and the activities
within these disciplines need to be optimized. The interaction and integration of these
disciplines is illustrated in Figure 2.1. Typically, PA and RD provide input to SC on what sitedata should be collected during the PI, based on an idealized preliminary conceptualization(from literature data for example) of the site from SC.
It is expected that the results of the SC will result in refinement (or, if necessary, revision) of theinitial thoughts on the proposed repository concept (or concepts) for a site and, consequently, on
the demonstration of the safety of the disposal system. This in turn may make it necessary torevise the initial PI SC plan and proceed, in a phased approach, with investigations aimed at
answering the newly posed questions.
For this reason, NUMO has developed a staged roadmap for planning the PI (that mainly
focuses on site characterization) [Preliminary Investigation Phase Planning Manual (PIPM).
Koike, 2008]. This plan foresees the need for input from the RD/PA group on identifying andprioritizing the issues and focusing of the SC at different steps of the PI stage.
4

A similar roadmap was also developed for the RD/PA work, showing the links and feedbacks
between RD/PA and SC. Thus, the ultimate goal is to consolidate the PI SC and RD/PA
roadmaps into one comprehensive roadmap covering the entire scientific and technical work
processes of PI stage.
The current version focuses on RD/PA during the PI stage; the interactions with SC activities
are included and highlighted. A basic structure for such a roadmap has been developed and is
illustrated in Figure 2.2. In the subsequent sections, each step of the roadmap is outlined in
more detail. A common structure was adopted for each task, which includes the following
information:


Task of SC, RD or PA

Issues to be considered in the task

Required input from RD/PA group

Main outcome from the task.

Site
Characterisation(SC)

What are the critical PA
parameters for the naturalbarrier?

What are theengineering criteria
for site feasibility?
What are the criticalengineering parameters
for the natural barrier?

What are the safety
criteria for site
feasibility?

What are the design criteria
for safety?

Repository Design(RD)

Performance
Assessment (PA)What are the critical PA
parameters for theengineered barrier?

Figure 2.1 Interactions between SC, RD and PA.

5

SC Group

RD Group

PA Group

0. Available information from LS stage

1.1 Define boundary of PIA (and SIA)
1.2 Surface exploration

1.3 Potential areas for surface facility
1.4 Target for geophysical investigation
1.5 Geophysical investigation
1.6 Compilation of obtained
outcomes

1.7 PA
conceptualization

2.1 Target for borehole investigation phase-1
2.2 Borehole investigation (Phase1: screening)
2.3 Construction of SDM ver.0.5

2.4 Preliminary selection of host formations
2.5 Check feasibility ofrepository construction

2.6 Check feasibility oflong-term safety

3.1 Target for borehole investigation phase-2
3.2 Borehole investigation (Phase2: host formations)
3.3 Construction of SDM ver.1.0

3.4 Selection of host formation(s)3.5 Refined repository design

4.1 Full safety assessment

4.2 Compiling RD/PA implication for DI phase
4.3 Develop a preliminary safety case
4.4 Identify required R&D topics for DI

Figure 2.2 Outline of the proposed PA/RD roadmap for the PI stage.

6

2.3

Roadmap at the PI stage

2.3.1 Task 0: Available information from the LS stage
The detailed planning of activities during the PI stage rests on the findings of the assessments
made during the literature survey (LS) stage. The roadmap outlined here assumes that thefollowing information has been compiled and decided upon:
Boundary of volunteered area
This is expected to be defined by the volunteer municipality, but NUMO may need to negotiate
to ensure that it will be large enough for the expected footprint. This may be the caseparticularly if the potential repository host rock is in an offshore area.
Confirmation of non-violation of the exclusion criteria
Using the available information, NUMO will check whether there are areas that could violate
the exclusion criteria. Such areas will be excluded from the proposed PIA.
Site Descriptive Model (SDM) version 0
Based on literature data, a version 0 conceptual and Site Descriptive Model (SDM) is developed
by NUMO. Among other things, this model identifies (hypotheses of) the main geologicalfeatures such as topography, main rock type boundaries and regional faults and puts the area
into its regional geological setting based on an overall understanding of Japan's geology. Itshould be noted that the SDM version 0 contains a high level of uncertainty and clear
identification of uncertainties is critical for PI planning.
Provisional selection of host rock(s)Using the SDM version 0, potential host rock(s) in the area are identified by NUMO, e.g. byapplying the procedures described in the PIPM (see Figure 2.3).

7

(fStep1: Preparation

Step2: Conceptualmodel and site
description model

Step6: Evaluationof modeling

Step3: Initialrepository
concepts selection

Step7:Development ofIntegrated PI
program

Step4:
Establishment of
main PI targets

Validation

PI planning

Step5: selection of
Investigation methods

Site specific PI
programFigure 2.3

Planning of the site characterization activities at the PI stage – Flow chart (Koike,2008).

Range of RCs suitable for the potential host rocks
Repository concepts judged to be suitable for the identified potential host rocks are proposed,
e.g. by applying the procedures discussed in NUMO(2004a).
Regional groundwater flow model
A regional groundwater flow model will be developed by NUMO based on the SDM version 0.
This model will establish groundwater flow in the regional setting and will, at least, provide agood indication of groundwater divides and regional recharge and discharge areas. Such agroundwater flow model will be developed by the SC group.
Draft PI SC plan
A first PI site characterization plan needs to be developed. It should identify the PI targets andpriorities, the steps in the PI, plans for the investigations and plans for further site descriptivemodeling, e.g. by applying the PI roadmap (Koike, 2008).
Draft PA plan for the PI stage with safety assessment version 0
A PA plan needs to be developed for the PI stage. Such a plan should identify the main activities8

in future PAs, identify potential site- and RC-specific critical issues and outline the structure of
the site-specific "safety case" – to indicate how it is conceived that safety will eventually beargued. The plan should also confirm the feasibility of constructing a safe repository at the site
(safety assessment version 0) – but there is no need for formal compliance (further aspects of
the PA plan are discussed in the RC report for FY 2006, Kitayama et al., 2007). One of theobvious exercises at this stage is to compare the expected performance of the disposal systemwith H12 or preliminary PA studies by NUMO.
2.3.2 PI - introductory and geophysical investigation phase
It is foreseen that the PI will start with surface-based mapping and geophysical investigations.
The results of these investigations will be important for the detailed planning of borehole
investigations and may also result in a need to revise some of the targets for these.
(1) Task 1.1: Define the boundary of the PIA (and SIA)
According to the Final Disposal Act, a PIA must not contain any of the features specified by thenational exclusion factors and should be large enough to provide sufficient understanding of thehost rock. However, as is pointed out in NUMO(2004b), there could be cases where
supplementary investigation areas (SIA), i.e. areas outside the PIA, would be needed forproviding additional information as shown in Figure 2.4. In selecting the PIA, both the PIA
itself and the SIA(s) should be identified (if needed).

Literature
survey area

Volunteer area

Terrace surface where uplift
can be quantified outside ofPreliminary Investigation
Area

Preliminary
Investigation
Area
Thermalimpact area

Supplementary
Investigation Areas
Active fault (nationwide
literature survey)

Figure 2.4

15km radius from centreof Quaternary volcano(nationwide literature
survey)Active fault (site-specificliterature survey)

Schematic illustration showing how the legally defined Evaluation Factors for
Qualification affect the identification of the location and geometry of a Preliminary
Investigation Area (after NUMO, 2004b).

9

Responsibility for the task
The definition of the PIA (and SIA) is primarily the task of the SC group.
Issues to be considered for this task
The following issues needs to be considered for this task:












Main features defining the regional geological setting should be available from SDM version
0 in Section 2.3.1 (Task 0). Examples of such features include topography, main rock typeboundaries and regional faults.
Location of features to be excluded from a PIA as found during the LS stage
Potential repository footprint and potential depth range considering the provisional host rockand range of potential repository concepts
In the case of a coastal site, how extensive an area of sea will be included in the PIA or SIA.
Potential discharge area of regional groundwater to at least be included in the SIA (based on
the regional groundwater flow model based on SDM version 0)
Since there is no flexibility for expanding the PIA at a later stage, the PIA should be defined
as extensively as possible
Socio-economic factors have to be considered in defining PIAs (and SIAs). Current andfuture land use in the relevant region needs to be considered. Political boundaries will also be
carefully analyzed and considered in defining the PIA.

General guidelines for PIA selection are included in NUMO(2004c), but it is worth expanding
these to include more comprehensive issues.
Required input from the RD/PA group for this task
The RD/PA group needs to provide information on the potential repository footprintconsidering the provisional host rock(s) and range of potential repository concepts identified
during the LS stage. It is important to carefully investigate existing tunnels or undergroundfacilities constructed in the potential host formation and to include such areas in PIAs or SIAs.
Main outcome
The outcome of this step is a clear boundary for the PIA (and SIA if needed).
(2) Task 1.2: Surface explorations
Surface explorations are carried out to provide a two-dimensional SDM and - early on - input tothe three-dimensional SDM. Features included in the SDM version 0 can be checked in cases
where they are visible at the surface. Surface explorations also provide input for identifying
10

suitable locations for geophysical explorations and borehole campaigns.
Responsibility for the task
Surface explorations are carried out by the SC group supported by contractors, e.g. field
geologists.
Issues to be considered for this task
The surface exploration task focuses on key features of the SDM version 0 that can be detectedusing surface-based methods. Examples of such features are:

Thickness and characteristics of the rock formations (mainly at outcrops)Characteristics and properties of the overburden

Estimation of uplift and erosion rates or subsidence/sedimentation rates (e.g. terrace
surfaces)Presence of faults or lineaments



Monitoring of seismic activityPresence of natural resources that could attract future human intrusion (confirmation of LS
results)
Early assessment of surface hydrology – streams, springs.

Required input from the RD/PA group for this task
The RD/PA group should help in identifying key issues in the site characterization plan.
However, this should already have been done in the LS stage (task 0).
Main outcome
The main output from this step are the results of the surface explorations. This information canbe used for modifying the location and size of potentially usable host rock formations and theirmain geometric constraints, later to be used for updating the SDM version 0. If the potential
host rock is exposed at the surface (outcrop), the properties of rock can be provisionallyanalyzed.
(3) Task 1.3: Identification of potential areas for the surface facilities
From a land use planning viewpoint and in the interactions with the local municipality, there is
generally a need at an early stage to establish potential locations for the surface facilities.
However, the optimum location is not only a function of the surface conditions; it also has to besuitable for the underground facilities. This means that the location of the surface facilities may
11not always be clearly defined in the early stages of the PI – but potential locations could be
identified. In reality, socio-economic factors for locating the surface facilities may be adominant factor and the siting of the underground facilities may need to accommodate suchconsiderations.
Responsibility for the task
Identification of potential areas for the surface facilities needs to be done jointly by the
SC/RD/PA groups as well as with the groups within NUMO that deal directly with interactions
with the municipalities.
Issues to be considered for this task
The following issues should be considered:Socio-economic factors (e.g. land use, land prices, surrounding environment)Routes for material transportation at the surface (logistics)



Topographic conditions



Whether all of the facilities will be located at the surface or partly subsurfacePotential discharge point (e.g. river) of the groundwater pumped up during construction andoperation
Potential discharge area of deep (potential repository level) groundwater
RD/PA input regarding suitable or less suitable locations, with regard to the subsurface
conditions (see below).

Required input from the RD/PA group for this task
The RD/PA input at this early (i.e. SDM version 0) stage can only be relatively generic. Thismeans that the likely input from RD/PA would be aimed at maintaining sufficient flexibility inthe surface location until a better understanding of the volume is achieved. This means that thelocation of the surface facilities should preferably be reconsidered both after SDM version 0.5
and after SDM version 1.0. Making a decision too early could lead to non-optimized solutions.
Typical input from RD/PA (at any stage) is given below.


Any constraints in location given the potential host rock and repository concept. Such
constraints could include complex geotechnical conditions (e.g. very weak rock, highly
transmissive and difficult to grout, etc.) in some areas of the PIA, being too far away fromthe potential host rock or a limited volume of host rock that should not be disturbed byaccess routes from the surface
Potentially suitable areas for the surface facilities and where to locate subsurface access, i.e.
12

those locations that would not unduly disturb the host rock.
Main outcome
The main outcome of this step is identification of potential area(s) for the surface facilities.
However, the suitability of these areas needs to be further explored by the RD/PA team once
they have more information from the subsurface.
(4) Task 1.4: Define targets for geophysical investigations
Targets and priorities for the PI could be developed using the methodology outlined in Step 2 of
the PI roadmap (Koike, 2008). The targets for the geophysical investigations are generally asubset of the overall targets of the PI. Based on the general targets, specific targets for thegeophysical campaign are identified. The geophysical investigations are generally suitable for
identifying different geological formations and gently dipping features such as faults.
It should be noted that geophysical investigations usually need to be followed up by a borehole
campaign. This means that the planning of the geophysical campaign also needs to consider the
potential borehole campaign – even if the outcome of the geophysics will be used to modify theborehole campaign. Furthermore, complementary geophysical investigations may be consideredthroughout the PI and even during the DI stage.
Responsibility for the task
Defining the targets is a joint task of the SC/RD/PA groups.
Issues to be considered for this task
The investigation issues will be site-specific and should be based on hypotheses made during
LS and formulated in the SDM version 0. Clear targets will help in detailed planning of thegeophysical investigations (e.g. required resolution and investigation area). Typical issues
would include:








Extent (vertical and horizontal) of the potential host formations, locations of large faults anddisplacements
Additional information obtained from surface explorations, e.g. revised ideas about
formation boundaries or potential faults
Suitable locations for boreholes (i.e. considering surface constraints) for following up thegeophysical campaign. This may affect the exact location of seismic profiles
Identifying the key volume of interest, although at this early stage the investigations should,
in principle, cover the entire PIA.

13

Required input from the RD/PA group for this task
The RD/PA group should help to identify the key issues in the site characterization plan,although this should already have been done in the LS stage (task 0). Examples of important
input include:

Key volume of interest (i.e. potential host rock and potential depth range)
Feedback on priorities for the planned investigations. For the given site and concept is therea potential volume problem – or are hidden active faults the main issue – or both?
Safety constraints regarding the density and location of boreholes.

Main outcome
The main outcome of this stage are revised and refined targets for the PI and a revised PI plan.
(5) Task 1.5: Geophysical investigations
This step concerns the detailed planning and execution of the geophysical investigations, basedon the targets defined in the previous step.
Responsibility for the task
Performing the geophysical investigations should be the responsibility of the SC group, with
contractors for carrying out the actual field investigations.
Issues to be considered for this task
Detailed planning is required prior to the execution of the geophysical investigations, e.g.
selection of applied methods, horizontal extent of the investigation area, density of seismic
sources and receivers. Depending on the depth of the host formation and required investigationarea, the investigation area may extend beyond the PIA (but it must be within SIA). Specificissues to be considered include:Meeting appropriate targets according to plan

Handling unexpected findings.

Required input from the RD/PA group for this task
No, or very little, input from the RD/PA group is required for this step. A possible exception
would be if the SC group needs additional feedback on priorities during the actual
investigations in the case where there are problems completing the original plan or if
unexpected findings are made. The SC team may then need some feedback from RD/PA on the
RD/PA significance of these findings or deviations.
14

Main outcome
The findings from the investigations are used for revising the SDM version 0 (see next step –task 1.6). It is not expected, nor desirable, for the RD/PA team to interpret the geophysical data
themselves.
(6) Task 1.6: Compilation of obtained results
A full update of the SDM is not considered necessary for this step (unless there are significant
unexpected findings). However, the geophysical data need to be interpreted and put into thecontext of SDM version 0, such that the data can be used as input for the borehole campaign.
This interpretation, as well as the interpretation of the surface exploration data, could also beseen as the early stages of development of SDM version 0.5, but the results are intermediate andshould not be formally published. It therefore needs to be considered how the results can becommunicated among the disciplines and how they will be utilized in the subsequent phases.
Responsibility for the task
Compilation of obtained results is the task of the SC group supported by contractors.
Issues to be considered for this task
The following issues are to be considered for this task:







Have the surface and geophysical data addressed key uncertainties in SDM version 0 andhave new uncertainties been identified, specifically regarding the dimensions of the hostformation and potentially layout-determining features?
Is there any reason to seriously question the conceptual model of SDM version 0?
Have any critical risks been identified which could potentially cause significant problems for
RD or PA?
Are the proposed borehole locations identified in the planning of the geophysicalinvestigations still appropriate?

Required input from the RD/PA group for this task
There is essentially no new input required from the RD/PA group for this task. However, thecompilation/evaluation should be done considering the RD/PA input to the PI program, i.e. on
what issues are of relevance.
Main outcome
The main outcome of this task is a summary of the results from surface and geophysicalinvestigations. This summary should include:

15





Updated information on the extent (vertical and horizontal) of the potential host formations
and the locations of large faults and displacements. However, it would be too early to makeany extensive assessments of the implications of this information. This can wait until SDM
version 0.5
Geometric issues/hypotheses to be tested in the borehole campaign.

It has to be considered in advance how (and in what form) the complied results can becommunicated to the PA/RD group without formal updating of the SDM.
(7) Task 1.7: PA conceptualization
In order to allow an efficient assessment of site suitability after the first borehole campaign, it isimportant for the PA groups to assess the information obtained during the surface andgeophysical investigations.
Responsibility for the task
PA conceptualization is the task of the PA group.
Issues to be considered for this task
The following issues are to be considered for this task:Is the previous conceptualization for PA use still valid?

Updating of safety assessment modeling needs (based on the plan prepared during LS)Scoping safety calculationsPlanning numerical groundwater flow modeling addressing a selection of scales, features andrequired resolution.

It should be remembered that migration and flow are not the only safety-relevant site issues.
The need for extensive groundwater flow modeling depends on the host rock and repository
concept. There may be other issues, e.g. geochemical or geomechanical ones, that could also be
important and where planning of the modeling needs and approaches would be necessary.
Required input from the SC group for this task
Required input from the SC group to the PA conceptualization includes:

Geometry of the main features suggested to be of hydraulic significance and information
about their characteristics (e.g. porous or fractured medium)Help in selection of the modeling domain for the groundwater flow numerical modelFeatures/characteristics that may have an impact on long-term safety (potential for
16

aggressive groundwater, potential rock stability problems).
Main outcome
The main outcome of the PA conceptualization is a modified perspective of thefeatures/characteristics that may have an impact on long-term safety (if relevant), hypotheses
that could impact the further characterization and an updated safety assessment modeling plan.
Such a modified perspective and identified potentially critical uncertainties should beaccommodated in the borehole campaign planning.
2.3.3 PI - borehole investigation phase 1
It is expected that the borehole investigations will be divided into two phases, with anassessment of targets and objectives after the first stage. However, the actual PI plan may
indicate that further phasing of the investigations is more appropriate. Nevertheless, theplanning, execution and assessment of the findings would essentially follow the same pattern
and a need for integration between the SC/RD/PA groups, regardless of the details of the PI
phases.
(1) Task 2.1: Targets for borehole investigation phase 1
Targets and priorities for the PI could be developed using the methodology outlined in Step 2 of
the PI roadmap (Koike, 2008; see Figure 2.3). The targets for the borehole investigations are
generally a subset of the overall targets of the PI. Based on the general targets and the
additional hypotheses formed during the surface and geophysical investigations, as well as on
additional feedback from the RD/PA group, specific borehole targets are defined.
Responsibility for the task
Defining the targets is a joint task of the SC/RD/PA groups.
Issues to be considered for this task
The main aim of the borehole investigation phase 1 is to determine the general geologicalconditions of the site and to guide the further characterization work during borehole
investigation phase 2. In defining targets and priorities, the following issues should beconsidered:Critical features for which early information is needed
Deciding whether there is a need (i.e. if the PIA is large) to obtain information to aid infocusing on a promising volume for more detailed investigations during borehole
investigation phase 2

17






Checking relevant hypotheses by assessing the surface and geophysical information
Obtaining basic properties of the potential host rock formations (e.g. hydraulic, mechanical,
geochemical properties)
If the borehole campaign is performed under time pressure or under tight budget constraints,
it is important to set clear priorities for the targets.

Required input from the RD/PA group for this task
The RD/PA input for this task would be a reassessment of the RD/PA significance of the key
issues already identified in the site characterization plan developed at the end of the LS. Morespecifically, the RD/PA groups should suggest what properties are of key interest given the typeof formations considered. The answer may be quite different depending on the type of hostformation and what is already known about it. They could concern:









Dimensions (or thickness) of the rock formations
Properties of the host formation (e.g. permeability, thermal properties, rock mechanicalproperties)
Groundwater composition (key factors such as salinity, Eh, pH or "aggressive water" could
have a large impact on what are potentially suitable RCs)
Role of water-conducting features (WCFs) in mass transport (modeled as porous or fractured
media)
More accurate determination of the host rock volume, i.e. confirm potential faults, layer
thickness, etc.

Considering this, the RD/PA group should provide feedback on what hypotheses suggested by
the geophysical data interpretation need to be explored.
Main outcome
The main outcome of this task are detailed prioritized targets for borehole phase 1.
(2) Task 2.2: Borehole investigation (phase 1 - screening)This task involves the detailed planning and execution of the phase 1 borehole investigations.
Responsibility for the task
The detailed planning and execution of the borehole investigations is the task of the SC groupsupported by contractors for borehole drilling and investigation.

18

Issues to be considered for this task
A detailed plan of borehole drilling and investigations has to be prepared considering the targets
identified in task 2.1. It will be necessary to consider constraints and wishes regarding borehole
locations, including:Optimum location for meeting targets, e.g. to ensure encountering of a typical section of thehost rock volume, to drill through as many potential formations as possible or located suchthat it will hit other features of interest
Location of the borehole could be outside the volunteer area but must be within the PIA –surface conditions need to be suitable
Consideration of whether boreholes can be drilled through the potential repository area(based on feedback from the RD/PA group).

Although this is a single step in this roadmap, it will represent major work for the SC group to
select contractors and specify drilling and investigation plans and to implement the borehole
investigations.
Required input from the RD/PA group for this task
The key input to this task from the RD/PA group is set by the targets of task 2.1. In addition, thefollowing input is needed:Whether boreholes can be drilled through the potential repository area. The answer will
depend on the repository concept, the size of footprint in relation to the potentially available
host rock volume and on perceived problems in later sealing of boreholes
Whether there is a possibility that the borehole might be used for other purpose, e.g.
monitoring or additional testing
Feedback during the borehole investigations in case there is a need to modify theinvestigations.

Main outcome
The main outcome are the data from the investigations with assessment of measurement
uncertainty. These data, which form the input to SDM version 0.5 (see next section) would
potentially include information on:Confirmation of the geological settingTypical hydraulic, thermal and mechanical properties of potential host rock formations

Hydraulic properties of WCFs in the host rock and surrounding rock

Mineralogy of the rock matrix, small-scale structure of fracturesmineralogy/thickness/extent of fracture coatings, infill, etc. from borecores
19

and

the



Typical groundwater composition in the host rockInitial stress (e.g. orientation and magnitude).

(3) Task 2.3: Construction of SDM version 0.5
The investigations during the PI provide primary data (measurement values and directly
calculated values), which are compiled in a database. In order to make use of the compiled(measured) information, it must be interpreted and presented in a geoscientific Site Descriptive
Model (SDM); see step 5 of the PI roadmap (Koike, 2008). After borehole investigation phase 1,
there are sufficient data to warrant an update of SDM version 0 into SDM version 0.5. ThisSDM in turn is key component for planning the completion of the PI stage. More specifically, it
provides a geo-dataset for use in preliminary PA and design.
Responsibility for the task
Site descriptive modeling is the task of the SC group, but it has to address the needs of the
RD/PA groups.
Issues to be considered
Issues to be considered in the construction of SDM version 0.5 include:

Identification of layout-determining features (large faults)Dimensions of the host formation

Typical properties of the host formation (and other formations if they are judged to be ofimportance)Expected variability of properties with time (particularly groundwater flow and composition)

Identification of key uncertainties related to PI targets



Surface hydrology - dilution and evolution.

Required input from the RD/PA group
The RD/PA group should already have provided their main input to the targets and plans forsurface and geophysical investigations and to the borehole investigation plan. However, the
RD/PA groups also need to provide feedback on the derivation of the geo-dataset for use in PA
and design.
Main outcome
The main outcome of this task is the SDM version 0.5 with its description of:

Identification of layout-determining features (large faults)Dimensions of the host formation20



Typical properties of the host formation and surrounding formations: hydraulic, thermal andmechanical, groundwater composition, mineralogy and initial stress (e.g. orientation andmagnitude)

Identification and assessment of key uncertainties related to PI targets



A geo-dataset for use in PA and design.

(4) Task 2.4: Preliminary selection of host formations
SDM version 0.5 will allow a much more informed, although still preliminary, selection ofpotential host formations for the repository. This selection in turn affects selection of repository
concepts.
Responsibility for the task
The preliminary selection of host formation(s) is a joint task of the SC/RD/PA groups.
Issues to be considered for this task
The selection should be based on NUMO's "Guideline for host rock selection". Issues to beconsidered include:

Identification of options – are there any – or is the selection evident?



Volume and properties of the different formations



Assessment of the potential for exploring different options.

Required input from the RD/PA group for this task
The RD/PA group needs to provide the following input:

RCs suitable for the potential host rock(s) considered

Footprint needed for different RCs in different host rocksAn overall, qualitative assessment of the potential for success of different options.

Main outcome
The main outcome of this task is the preliminary selection of host formations.
(5) Task 2.5: Checking the feasibility of repository construction
A key element for the further planning of the PI is to check the feasibility of repository
construction for the proposed RC in the suggested host rock(s). This assessment, together withthe long-term safety feasibility check (see next step) is essential for guiding further work anddecisions in the PI.
21

Responsibility for the task
Checking the feasibility of repository construction is the task of the RD group.
Issues to be considered for this task
The following issues are to be considered for this task:Extent of the host formations within the volunteer area
If one formation cannot accommodate the planned waste volume (40,000 canisters), consider
the possibility of using two or more rock formations
If the potential rock formation has a small horizontal extent but is very thick, a multi-layer
repository layout can also be consideredStress conditions and strength of the rock formation



Thermal properties of the host formation (from core sample tests)Potential risks and measures for mitigating them (through design modification), e.g. smaller
host rock extent, formation at much greater depth, higher permeability, etc. andconsequences of such risks will be analyzed
Complex groundwater composition (salinity, corrosive, redox – affects potential for selectionof the EBS and its longevity).

Input needed from the SC group
The main input would be the SDM version 0.5 and the selection of the host rock made in theprevious task. In addition, close discussion with the SC group will be required to identify thelevel of uncertainty involved in SDM version 0.5.
Main outcome
The main outcome of this task is:

An outline repository layout identifying areas for repository panels and accesses
An assessment of whether there are constructability issues and, if so, how they would affectRC selection
Identification of further PI (and DI) targets related to constructability and more detailed
design work.

(6) Task 2.6: Checking the feasibility of long-term safety
Based on SDM version 0.5 and the preliminary repository design assessments of task 2.5, it ispossible to update the assessment of the feasibility of assuring long-term safety. This feasibility
check is essential for deciding whether to continue the PI and for planning how they should be22completed. Additional aspects of the performance of this task are discussed in the RC report for
FY 2006 and in Kitayama et al. (2007).
Responsibility for the task
Checking the feasibility of long-term safety is the task of the PA group.
Issues to be considered for this task
Checking the feasibility of assuring long-term safety will involve a systematic evaluation of all(potential) critical issues identified during the LS on the basis of the understanding obtained in
SDM version 0.5. At this stage, the evaluation would go further and may also involve (simple)
quantitative analyses; it should also assess whether there are additional critical issues. The
following can be considered:







Updating the groundwater flow model (based on the hydraulic description in SDM version
0.5)
Assessing conditions that could impair the functioning of the EBS for the intended RC(s).
Retention properties of the host rock and the "near" far-field (hydraulic conductivity, flow
distribution, potential for sorption) and variation with time.
Exploitable resources and other issues related to human intrusion.

One possibility for judging the feasibility of long-term safety would be to compare the site
conditions as described in SDM version 0.5 with the conditions assumed for H12 (if relevant).
It should also be noted that regional migration, or the length of the radionuclide transport path,is generally of very little significance for assuring the long-term safety of a RC. Generally, mostRCs depend on containment or excellent retention in a relatively restricted (e.g. 100s of meters)
volume of rock, while migration in the far-field adds little to safety. These aspects need only beconsidered in the later preparation of the safety case.
Main outcome
The main outcome of this task would be:Confirmation of the feasibility of long-term safety and a judgment whether there is goodreason to continue to borehole investigation phase 2
Feedback on what uncertainties in the SDM version 0.5 appear to be most important toresolve – as well as whether there are additional critical issues (will provide a revised list ofissues and targets for borehole investigation phase 2)
Evaluation of the PI program: are the critical issues properly addressed – or is there a needfor re-focusing?
23





Key aspects of groundwater flow (e.g. overall groundwater flow regime, dominant flow path,residence time (indication for groundwater dating), prediction of dominant discharge area)
Feedback on potential repository concepts – if a selection is to be made.

2.3.4 PI - borehole investigation phase 2
Borehole investigation phase 2 is an extension of phase 1 and should generally follow theoverall targets of the PI. However, selection of host formation(s) and the RD/PA assessment
made using SDM version 0.5 should allow this phase to be focused on the potential hostformation and on issues/targets judged to be particularly important after assessing the results ofphase 1. It should also be noted that borehole investigation phase 2 need not necessarily be
restricted to boreholes – depending on targets and site conditions there may be a justification forcarrying out additional complementary geophysical investigations (particularly if borehole
investigations call into question hypotheses made on the basis of geophysical investigations or
are in apparent contradiction to them).
(1) Task 3.1: Define PI targets for borehole investigation phase 2
The hypotheses and uncertainties identified in SDM version 0.5, the repository construction
issues identified in task 2.5 and the issues relating to long-term safety (task 2.6) are used todefine detailed targets for the completion of the PI. These targets are usually resolved bydrilling additional boreholes, but complementary geophysical investigations may also be
important.
Responsibility for the task
Defining PI targets for borehole investigation phase 2 is a joint task of the SC/RD/PA groups.
Issues to be considered for this task:
The main objective of the phase 2 investigations is to obtain information on selected hostformations.
Refined target and priorities will be selected based on SDM version 0.5 andthe assessments made by RD and PA.
Required input from the RD/PA group for this task
Refined targets and priorities are based on the outcome of task 2.4 "Pre-selection of hostformations", task 2.5 "Checking the feasibility of repository construction" and task 2.6"Checking the feasibility of long-term safety". The RD/PA groups should address the need for
additional refined information from the site considering:

Reassessment of RD/PA significance of the key issues already identified in the sitecharacterization plan
24







Potentially interesting host formations where investigations can be focused, considering boththe properties of the host formations and the required footprint of the RC in these formations.
Identification of further PI (and DI) targets related to constructability and more detailed
design work (output from "Checking the feasibility of repository construction")
Feedback on what uncertainties in the SDM version 0.5 appear to be most important toresolve – as well as whether there are additional critical issues (output from "Checking thefeasibility of long-term safety" and "Checking the feasibility of repository construction").

Main outcome
The main outcome is an agreed set of prioritized targets and an outline of borehole investigation
program phase 2.
(2) Task 3.2: Borehole investigation phase 2 – characterization of host formations
This task includes the detailed planning and execution of the phase 2 investigations.
Responsibility for the task
Investigation phase 2 is the task of the SC group supported by contractors for borehole drillingand investigations.
Issues to be considered for this task
The main issue to be considered for this task is the detailed planning of the phase 2
investigations and executing this program. Among others, the following issues should beconsidered:








Optimum locations of boreholes for meeting targets, e.g. crossing crucial features andpenetrating as many rock formations of interest as possible
In the case where the critical features are sub-vertical, inclined or controlled boring will beconsidered
Constraints regarding borehole locations, including legal and practical restrictions, andreconsideration of whether boreholes can be drilled through the potential repository area(based on feedback from the RD/PA group)
Investigations need not necessarily be restricted to boreholes – depending on targets and site
conditions there may be a justification for carrying out complementary geophysicalinvestigations.

When performing the program, it is also necessary to be prepared to redirect efforts in the eventthat investigations are not fulfilling the specified goals.

25Required input from the RD/PA group for this task
The key input for this task from the RD/PA group is determined by the targets of task 3.1 (seeSection 2.5.1). In addition, the following input is needed:

Whether boreholes can be drilled through the potential repository area. The answer will
depend on the repository concept, the size of footprint in relation to potentially available hostrock volume and on perceived problems in later sealing of boreholes
Feedback during the borehole investigations in case there is a need to modify theinvestigations.

Main outcome
The main results are the data from the investigations with assessed measurement uncertainty.
These data, which form the input to SDM version 1.0 (see next section) would potentially
include:









Data on the geometry of the host formation and on the location of layout-determining
features potentially affecting the size of the host formation
Additional data on key host formation properties (mechanical, thermal, hydraulic)Detailed hydrogeochemical data sufficient for understanding hydrogeochemical processes ofimportance for the long-term evolution of the EBS (a reasonable set of data on current
groundwater composition and on fracture and rock mineralogy along flow paths where
groundwater composition changes)
Data of importance for assessing migration properties (flow distribution, potential forsorption and diffusion, etc.) of the potential host formations and surrounding rock judged to
be important for radionuclide retention
Data on rock engineering properties – issues of concern for access (e.g. depth of overburden,
occurrence of weak rock, water problems).

(3) Task 3.3: Construction of SDM version 1.0
After completion of the PI phase, SDM version 0.5 is updated into SDM version 1.0. This SDM
is important for concluding the PI stage. More specifically, it provides a geo-dataset for use indesign, PA, a preliminary safety case and in planning the DI stage.
Responsibility for the task
Site descriptive modeling is the task of the SC group, but it has to address the needs of the
RD/PA groups.

26

Issues to be considered
Issues to be considered in constructing SDM version 1.0 include:Addressing the targets set out for the PI phase (see task 3.1)

More focused information on the potential host formations will be includedCompiling the input needed for RD and PA

Addressing overall confidence and uncertainty.

Required input from the RD/PA group
The RD/PA group should already have provided their main input to the targets and plans forsurface and geophysical investigations and to the borehole investigation plan. However, the
RD/PA group also needs to provide feedback on the derivation of the geo-dataset for use in PA
and design.
Main outcome
The main outcome of this task is the SDM version 1.0. Its focus will need to be related to thetargets set out by RD and PA, but would likely include:















Refined geometry of the host formation – confirmed location of layout-determining featurespotentially affecting the size of the host formation
At least a statistical description of the spatial variability of key host formation properties(mechanical, thermal, hydraulic)
Current distribution of groundwater composition and a reasonable understanding of
geochemical processes of importance for long-term evolution of the EBS
At least a statistical description of the spatial variability of the migration properties of thepotential host formations and surrounding rock considered important for radionuclide
retention
Engineering properties of host rock and potential access routes
Likely evolution of the site (particularly groundwater flow and composition) and associateduncertainties
Information of relevance for assessing dilution in aquifers/surface waters and evolution
thereof
Quantification (as well as alternative hypotheses if needed) of key uncertainties related to PI
targets.

27

(4) Task 3.4: Preliminary selection of host formation(s)Generally, this task will be similar to the pre-selection made after SDM version 0.5, but with theadded knowledge provided by SDM version 1.0. An overriding question is whether SDM
version 1.0 has altered any of the facts considered when selecting formations after SDM version
0.5.
Responsibility for the task
The preliminary selection of host formations is a joint task of the SC/RD/PA groups.
Issues to be considered for this task
The selection should be based on NUMO's "Guideline for host rock selection". Issues would
be similar to those for the preliminary selection made after SDM version 0.5, i.e.:

Relation to potential surface facilities



Identification of options – are there any – or is the selection evident?



Volume and properties of the different formations



Assessment of the potential for exploring different options.

Required input from the RD/PA group for this task
Input would be similar to that provided after SDM version 0.5, but considering the new
information, and would generally be a qualitative assessment of the potential success ofdifferent RCs in the different potential host formations. This assessment would include:Footprint neededPotential strength of the safety case to be made

Engineering challenges.

Main outcome
The main outcome of this task are the selected host formation(s) – as well as the arguments
made for their selection (important part of the safety case). Given the selection of the hostformation, it also has to be considered whether it is necessary to renegotiate the location of thesurface facilities.
(5) Task 3.5: Refined repository design
Even if a repository will only be constructed after the detailed investigation stage, a refined
repository design in three dimensions is needed both for assessing the potential for constructing
a safe repository at the site and to provide input for planning of the DI. Evidently, the DI need to28

be focused in the repository volume and it is also likely that the underground access routes used
for the DI will be part of the access routes to the final repository.
Responsibility for the task
Clearly it is the task of the RD group to refine the repository design.
Issues to be considered
There are several issues to be considered in developing a refined repository design, including:

Safety-related requirements (restrictions on materials to be used, thermal requirements,
respect distances, acceptance criteria, etc.)
Tunnel/cavern stability requirements (e.g. disposal and access tunnels, cavern for working
area)



Environmental issues

Minimization of footprint (consistent with other requirements)



Operational safety (both related to underground construction work and radiation protection
during operation)Efficiency and practicability of construction and operation



Cost (probably not a key issue at this stage, but still an element of engineering feasibility).

Input needed from SC and PA
Refining design obviously requires SDM version 1.0 as input. However, the PA group also
needs to provide input on the basis for setting requirements on the host rock and the EBS. Suchrequirements may include:

Thermal (e.g. bentonite temperature < 100 degrees C) and mechanical and chemical loads
that the design should be able to handle

Respect distances to layout-determining features



Avoidance criteria for disposal tunnels if judged necessary.

These requirements would need to be developed from the PA work based on SDM version 0.5.
The requirements may be modified after the full safety assessment (see below). In practice, this
input tends to be developed as an iterative exercise between RD and PA, since requirements
will change depending on the details of the design (e.g. depending on the thickness of the buffer,
it may be acceptable for some of the buffer to experience temperatures above 100 degrees C).

29

Main outcome
The main outcome of this task includes:








A refined repository layout including access routes and the location of the undergroundexcavations needed for the DI
A first version of "production line reports", defining the initial state for PA and identifying
issues of concern for further assessment
Feedback to DI on key uncertainties that would require additional site data to be sufficiently
resolved
Feedback to PA on requirements that pose design problems – can they be modified?

2.3.5 Assessment of PI findings and developing input to the DI stage
The main results of the PI are the SDM version 1.0 and the refined repository layout. This
information is then to be analyzed in a full safety assessment made to support a decision tocontinue with a DI stage as well as for planning the DI stage if this is decided.
(1) Task 4.1: Full safety assessment
Given the mass of information obtained in the PI and the substantial commitment to continuing
to the DI stage, it is essential to assess the long-term safety of proposed repository concepts atthe site.
Responsibility for the task
Conducting a safety assessment is the task of the PA group.
Issues to be considered
The full safety assessment should apply a state-of-the-art PA methodology covering all aspects
expected to be part of a full PA (see Kitayama et al., 2007). However, since this is not part of alicense application a formal comparison with regulatory criteria may still be unnecessary. The
assessment should thus include:Repository evolution (groundwater flow, hydrogeochemistry, mechanical, thermal)



Resulting performance (dose/risk, etc.) for assessed RCs at the studied sites



Identification of remaining outstanding issues of key importance for safety (i.e. affecting
long-term stability of the EBS and/or radionuclide migration) and assessment of theirimportance

Identification of the main safety features (safety arguments)

Assessment of the long-term safety implications of the remaining uncertainties regarding the30site as identified and quantified in SDM version 1.0

Implications for repository EBS design at the different sites (e.g. need for special containers,
buffer material, dimensions, etc.). Assessment whether all requirements set out for thedetailed design are necessary or if they could be relaxed
Implications for repository excavation design and layout at the sites – rock excavation: Are
there site-specific design constraints that need to be observed in order to ensure safety
Assessment of whether additional requirements may be necessary.

Main outcome
The main outcome of this task is the safety assessment report version 1.0.
(2) Task 4.2: Compiling RD/PA implications for the DI stage
The uncertainties and key remaining issues identified in SDM version 1.0, in the refined layout
and in the safety assessment report version 1.0 form input for planning the RD/PA activities of
the DI phase.
Responsibility for the task
This is a joint task of the RD/PA/SC groups
Issues to be considered
The following issues should be considered:







Key remaining issues as identified in the full safety assessment (see task 4.1)
Safety-related restrictions on underground access (location of access, groundwater control,chemicals that can be used, etc.)
Plans for verification of assumptions made on rock conditions and engineering feasibility (tobe executed during the DI phase)
Characterization targets for the DI phase.

Main outcome
The main outcome of this task is a report or memo compiling the RD/PA implications for the
DI stage.
(3) Task 4.3: Developing a preliminary safety caseEven if not formally required for proceeding to the DI stage, it would be good to develop a
preliminary safety case based on the findings during the PI stage. This preliminary safety case31

would be based primarily on the safety assessment version 1.0, but would also consider otheraspects that are part of a safety case. Furthermore, an outline for the preliminary safety caseshould be developed at an early stage (first version already at the LS stage), since this will
provide considerable guidance on what to consider during the PI.
Responsibility for the task
Developing the safety case is a joint task of the PA, RD and SC groups.
Issues to be considered
The preliminary safety case should at least consider the following:The full safety assessment and its list of safety arguments and remaining issues is a key
component of the safety case
How the host rock, repository depth and repository design were arrived at (information to befound in the detailed design work)Summarize what makes the repository design at the site safeAddress engineering feasibility (as further described in "production line reports")








Summarize confidence in SDM version 1.0 – is there anything else to consider in addition to
the input given to PA (i.e. is the degree of understanding robust)?
List remaining issues and address whether they concern optimization. If remaining issues
fundamentally concern feasibility, a decision to go underground is probably premature
Outline plan for resolving remaining issues
Make it reasonable for the remaining issues to be resolved by the data obtained during the DIor from other planned R&D



List complementary safety arguments

Make a final statement on safety.

Main outcome
The main outcome of this task is a documented preliminary safety case.
(4) Task 4.4: Identify R&D topics required for DI
The preliminary safety case and its supporting documentation is the main source for identifying
R&D topics that need to be addressed during the DI stage. It should be noted that such R&D
need not necessarily be restricted to underground investigations. There may be many issues thatwould require further technical development (i.e. in the laboratory) and it may also be
worthwhile to continue with some surface-based borehole investigations.
32

Responsibility for the task
Identifying R&D topics required for DI is a joint task of the SC/RD/PA groups.
Issues to be considered
The main issues to be considered are those remaining from long-term safety, engineering designand site understanding as identified when developing the preliminary safety case (see above). It
should be assessed whether further resolution of these issues is needed (input from thepreliminary safety case).
Input needed from RD/PA
The main input needed from RD/PA is a list of remaining critical issues from the design work,
the safety assessment version 1.0 and from the preliminary safety case.
Main outcome
The main outcome of this stage will be an R&D program for the DI stage.

33

3
3.1

Scenario development
Background

Scenario development is a key stage in any PA. Scenarios define the sequences of events andprocesses to be considered in determining, as a function of time, the possible ways in which ageological repository will perform (or fail to perform) its fundamental roles of isolating thewaste from the human environment and containing its radionuclides.
The IAEA's Radioactive Waste Management Glossary (IAEA, 2003) defines a scenario as:"A postulated or assumed set of conditions and/or events. They are most commonly used inanalysis or assessment to represent possible future conditions and/or events to be modelled,
such as possible accidents at a nuclear facility, or the possible future evolution of a repository
and its surroundings".
As explained, for example, in SKI's SITE-94 PA (SKI, 1997):"Scenarios are not predictive devices, but are means of stimulating and disciplining theimagination so as to provide an organised way of illustrating possible future behaviour of thesystem and defining how such behaviour might arise".
A specific set of objectives for scenario development was identified at an OECD/NEA
workshop on scenario development in 1999 (NEA, 2001):(1) "To demonstrate or try to ensure completeness or sufficiency in the scope of a PA".
It should, for example, be ensured in scenario development that all relevant scientificknowledge is taken into account.
(2) "To decide which FEPs to include in a PA and how to treat them".
A clear rationale should be provided for excluding FEPs, e.g. on the basis of irrelevance toassessment scope, low probability, etc.
(3) "To provide traceability from data and information to the PA scenarios, models, parametervalues, and calculation cases".
Scenario development should provide a documented link between scientific understanding(including uncertainties) and the assessment cases chosen for analysis.
(4) "To promote transparency and improve comprehensibility of the PA and the PA results".
Thus, the above link should be clear and understandable.
(5) "To guide decisions concerning future work".

34

Scenario development should provide a systematic way of identifying (and prioritizing) criticaluncertainties and unresolved issues.
Since H12, progress has been made in developing methodologies that satisfy these various
objectives. The aim of this Chapter is to indicate some particular areas of progress that NUMO
could consider when evaluating the applicability of the H12 methodology (or revised
methodologies provided by JAEA) in future Japanese PAs.
Scenario development in H12 is summarized and reviewed in Section 3.2. Some aspects of
scenario development in more recent PAs are discussed in Section 3.3, focusing on themethodologies applied in the Swiss Project Opalinus Clay (Nagra, 2002a) and the Swedish SR
Can project (SKB, 2006). The focus is on: (i) the link between scientific understanding(including uncertainties) and the assessment cases chosen for analysis – including therecognition that this is a "top down" rather than "bottom up" process and (ii) the issue ofcompleteness (i.e. objectives 1 and 3 above), since these are identified as areas where the H12
methodology could particularly be enhanced.
The use of some aspects of scenario development to guide decisions concerning future work
(objective 5 above), which may be of more immediate relevance to NUMO, is discussed in
Section 3.2.3.

3.2

Scenario development in H123.2.1 Classification of scenarios in H12
As illustrated in Figure 3.1, two groups of scenarios were defined in H12 (JAEA, 2000), as
required by the Atomic Energy Commission of Japan (AEC) guidelines:

Groundwater scenarios

Isolation failure scenarios.

35Groundwater scenarios

Base scenario

Perturbation scenarios- Stable geology- EBS performs as designed- Unchaning biosphere

Account for:- natural phenomena- future human actions- initial defects- ...

AEC guidelines

Isolation failure scenarios- Direct magma intrusion- Direct human intrusion- meteorite impact- ...

(negligible) likelihood of occurrence of “initiating FEPs” / “what if?” calculations of consequences

Figure 3.1 Classes of scenarios in H12.

Groundwater scenarios are those in which groundwater provides the principal pathways bywhich radionuclides could reach the human environment. These were further subdivided into:

A base scenario, in which the repository was assumed to be implemented according todesign specifications, the EBS was assumed to evolve broadly as expected, the geologicalenvironment was assumed to be stable and the biosphere was hypothetically assumed toremain unchanged over the entire timeframe addressed in the PA
Perturbation scenarios, in which one or more of the above assumptions was changed as aresult, for example, of some postulated natural phenomena or future human actions.

The groundwater scenarios were considered to have a non-negligible likelihood of occurrence
and were analyzed in detail in the main part of the PA. The isolation failure scenarios, on theother hand, which included scenarios such as direct magma intrusion into the repository, could
be excluded from the main part of the PA, for example on the grounds that siting criteria would
render them impossible or of negligible likelihood. Their consequences were nonethelessevaluated in additional "what-if?" calculations in order to illustrate the robustness of the H12concept.

36

3.2.2 Scenario development procedure in H12
The scenario development procedure adopted in H12 is illustrated in Figure 3.2.

NUMO’s concerns• Requirements of scenario analysis in the each stage• FEP management in each the stage• Management of site information in the scenario
analysis• Roadmap to the license application

Safety concept of the disposal system
Safety functions of the disposal system
System components

Scenario Development

Evaluation of variability of the system andunde rstanding of its behavior

Inte
International
rnationalFEP
FEPlistlist(refer(referto
toforeignforeignstudies)studies)Develop
Developcompre
comprehensive
hensiveFEP
FEPlistlist
SelectSelectFEPs
FEPstotobe
beconside
considered
red
ininthethesafety
safetyassessment
assessmentcalculations
calculations

Classify
ClassifyFEPs
FEPs

・FEPs avoided by suitable site selection
・FEPs avoided by suitable design
・FEPs with extre mely low probability

Identify
IdentifyFEPs
FEPstotobe
beconside
considered
red
ininthethesafety
safetyassessment
assessmentcalculations
calculations

・FEPs with insignificant consequences
Develop
Developaagroundwater
groundwaterscenario
scenario
Exclusion from safety assessment
calculations *Model development
Quantitative assessment
Safety criteria proposedby regulatory bodies overseas= evaluate likelihood of occurrence and impact… but considered in “what-if?” calculations
procedure not fully specified!!

Figure 3.2 H12 scenario development procedure.

The procedure consists of the following steps:
Step 1: On the basis of the available scientific understanding of the characteristics andevolution of the repository and its environment, a comprehensive list ofconcept-specific features, events and processes (FEPs) is developed and checked forcompleteness against international FEP lists.
Step 2: The FEPs are classified according to their likelihood of occurrence and possible impact
on system evolution.
Step 3: On the basis of this classification, a decision is made as to which FEPs to carryforward in the main part of the PA (either as part of the base scenario or as part of one
or more perturbation scenarios) and which FEPs to exclude (though possibly
considering them in "what-if?" calculations).
Step 4: Groundwater scenarios are developed from the FEPs carried forward, but theprocedure used is not fully specified (Section 3.2.3, below).
37

3.2.3 Limitations of the H12 methodology
In H12, in common with most descriptions of PA methodologies at the time, it is stated or
implied that PA scenarios are synthesized from collections of individual FEPs. The procedureby which this is achieved is, however, not clearly specified.
The base scenario is said to emerge "bottom-up" from the constituent FEPs and this basescenario is used to generate an influence diagram. The influence diagram shows which FEPs
influence (or are influenced by) other FEPs in this scenario. It is then stated that the basescenario influence diagram is used to evaluate the impact of events and processes on safetyfunctions. Again, how these events and processes are selected and how the influence of thesafety functions is assessed is unclear in the H12 documentation.
More recent PAs have sought to address these limitations, as described in the following
sections.

3.3

Scenario development in more recent PAs

3.3.1 Recognition that scenario development is a “top-down” process
Although several PAs at the time of H12, including H12 itself, state or imply that scenarios aresynthesized in a "bottom-up" fashion from individual FEPs, none outline an explicit proceduredescribing how this synthesis is achieved. Nagra recognized at the time of Project Opalinus
Clay that the actual procedure by which assessment cases (scenarios and the models anddatasets used to assess them) are identified is rather a series of judgements based on a
description of the characteristics and evolution of the system that is as full and accurate as
possible, including a description of uncertainties and resulting possible deviations fromexpected evolution.
According to the safety report of Project Opalinus Clay (Nagra, 2002a):"The identification of individual assessment cases is a matter of expert judgement, guided by:Understanding of the system and its evolution …; andUnderstanding of the fate of radionuclides in the Reference Case, and sensitivity to various
conceptual assumptions and parameter variations …"
The process is thus a "top-down" one, in which the starting-point is not a fragmented set of
FEPs, but rather a comprehensive system description. This is almost certainly the process thatwas, in reality, also adopted in H12, in which case the H12 scenario development methodology
would perhaps more accurately be represented as shown in Figure 3.3, in which there is a direct
link from the "evaluation of variability of the system and understanding of its behavior" to38

"developing a groundwater scenario". In this description of the approach, the role of FEP lists is
to support "completeness checking", i.e. to ensure that no important phenomena have been
overlooked in the scenarios and assessment cases, as described in Section 3.3.3.

NUMO’s concerns• Requirements of scenario analysis in the each stage• FEP management inCompletenesseach the stage
checking• Management of site information in the scenario
analysis• Roadmap to the license application

Safety concept of the disposal system
Safety functions of the disposal system
System components

Scenario Development

Evaluation of variability of the system andunde rstanding of its behavior

Inte
International
rnationalFEP
FEPlistlist(refer(referto
toforeignforeignstudies)studies)Develop
Developcompre
comprehensive
hensiveFEP
FEPlistlist
SelectSelectFEPs
FEPstotobe
beconside
considered
red
ininthethesafety
safetyassessment
assessmentcalculations
calculations

Classify
ClassifyFEPs
FEPs

・FEPs avoided by suitable site selection
・FEPs avoided by suitable design
・FEPs with extre mely low probability

Identify
IdentifyFEPs
FEPstotobe
beconside
considered
red
ininthethesafety
safetyassessment
assessmentcalculations
calculations

・FEPs with insignificant consequences
Develop
Developaagroundwater
groundwaterscenario
scenario
Exclusion from safety assessment
calculations *Model development
Quantitative assessment
Safety criteria proposedby regulatory bodies overseas

Figure 3.3 Alternative view of the H12 scenario development procedure.

3.3.2 Identification of individual scenarios
The identification of scenarios in PA is part of the wider task of uncertainty analysis. Safetyassessments treat uncertainties primarily by defining and analyzing a wide range of assessment
cases – i.e. specific model realizations of different possibilities or illustrations of how a system
might evolve and perform. Each of the cases, which may be analyzed deterministically,
probabilistically or by some combination of these approaches, addresses the impact of someparticular uncertainty or combination of uncertainties.
Most PAs define a base, or reference, scenario, generally representing the expected or more
likely evolutionary paths of the disposal system, and a set of alternative scenarios representing
substantially different and generally less likely evolutions compared to the base or reference
scenario. They also generally define a number of individual assessment cases representing moreminor variants within each scenario.

39

Organization of assessment cases
The organization of assessment cases within scenarios is an operational matter that can vary
substantially between different PAs. One approach is to categorize uncertainties as, for example,
scenario uncertainties, model uncertainties and parameter uncertainties (see Box 1); it should
however be noted that, in any such scheme, categorization is always somewhat subjective anddependent on the chosen models and assessment approach. A scenario is then represented by agroup of individually analyzed cases. Within each scenario group, sub-groups of cases address
alternative possibilities arising from conceptual model uncertainties. Finally, individual cases
within each sub-group address alternative possibilities arising from parameter uncertainties.
This is the approach used in the Swiss Project Opalinus Clay. The resulting hierarchy ofassessment case groupings is illustrated schematically in Figure 3.4.
Box 1:

Example of a categorization scheme for different classes of uncertainty(after NEA, 1997)

A scenario represents a set of FEPs and interactions. Scenario uncertainty results fromdifficulties in identification of a complete set of scenarios, a complete set of FEPs foreach scenario and correctly identifying which interactions between significant FEPs
must be considered in a PA.
Conceptual model uncertainty refers to uncertainty about the model used to represent agiven set of FEPs and interactions, or choice of models. Simplifications introduced, forexample by applying one- or two- dimensional PA-models is part of the conceptualmodel uncertainty, as is the uncertainty introduced by selecting a scale ofspatial/temporal representation.
Parameter uncertainty refers to uncertainty in the parameter values to be used in a(given) conceptual model.

40

Parameter set 1.1.1
Parameter set 1.1.2
Model set 1.1
Parameter set ...

Normal evolution scenario

Model set 1.2

Model set ...

Model set 2.1

Altered evolution scenario A

Model set 2.2

Model set ...

Altered evolution scenario B

Figure 3.4

Schematic illustration of the structuring of assessment cases, with each caseconsisting of a scenario, a model set and a parameter set.

In Project Opalinus Clay, alternative scenarios and assessment cases were selected byconsidering the range of influence of the possible deviations from the expected evolution of thedisposal system, as illustrated in Figure 3.5. The expected evolution scenario is represented by
the green box across the top of the diagram. In this scenario, the clay barrier (i.e. the bentonite
and Opalinus Clay together) is largely undisturbed by the phenomena occurring and provides
effective isolation of the waste by limiting mass transport of radionuclides over very long
periods of time. The possible deviations from the expected evolution for all significant
phenomena that influence repository performance are listed in the column on the left. For eachphenomenon, the bar represents the domain over which there may be an influence. For example,
alternative climates influence only the biosphere, thus the bar extends through this domain only.
The color coding represents transport through the host rock (black) and the tunnel and seal
system (grey), with red bars representing "what if?" cases.

41

Figure 3.5

Range of influence of possible deviations from the expected evolution of the Project
Opalinus Clay disposal system (after Figure 5.7-1 in Nagra, 2002a).

42

Safety functions and pillars of safety
The NEA review of Project Opalinus found that there was clear linkage from uncertainties to
PA assessment cases (NEA, 2004). It also noted, however, that concepts such as safetyfunctions and pillars of safety, which are central to the safety case, were not linked to scenario
development. This linkage has received much attention in the recent Swedish SR-Can PA (SKB,
2006).

The methodology used to identify variant scenarios is based on that developed for SR-Can. It
consists of the following steps:










Step 1: Consider the main safety functions of each of the main components of thedisposal system
These main components are essentially the overpack, the buffer and the host rock. The safetyfunctions of the host rock include, for example, the protection of the EBS and providing atransport barrier to any radionuclides released following overpack failure.
Step 2: For each safety function, identify one or more "safety function indicators"
A safety function indicator is a measurable or calculable property of the system that is
critical to a safety function being fulfilled. For example, an adequate buffer density is criticalto colloid filtration, which is (whether explicitly stated or not) one of the buffer safetyfunctions in all repository concepts employing a clay buffer.
Step 3: For each safety function indicator, derive "safety function indicator criteria"
Safety function indicator criteria specify the range of values that the safety function
indicators should have if it is to be assumed in PA that the corresponding safety functions are
fulfilled. Examples of safety function indicators and criteria for the SR-Can canister andbuffer are given in Figure 3.6. It should be noted that overall system safety does not dependon all criteria being met at all times – i.e. they are not "safety requirements". Rather, if asituation (scenario) can be envisaged in which a criterion is not fulfilled, then theconsequences of the loss of the corresponding safety function need to be assessed in the PA.
Step 4: Build a model of the system and assess its evolution – with focus on the fate of
the safety functions
A major effort is needed in building a model of how the system, and especially the safetyfunctions, would evolve. This system model is used as input to the next step.
Step 5: Identify the failure modes that could occur in the course of system evolution
Use the system model to consider what would have to happen for a particular component tocease to fulfill its various safety function indicators – and hence to fulfill its various safetyfunctions. For example, processes leading to a loss or redistribution of buffer mass could
lead to a violation of the buffer density criterion and hence to a loss in the capacity of the43buffer to filter colloids (as well as other consequences, such as advective solute transport andmicrobial activity in the buffer).


Step 6: Consider if and when the occurrence of such failure modes is plausible
For example, based on system understanding, could groundwater flows or changes incomposition arise over the course of time that could lead to significant buffer erosion?
Step 7: Consider the implications of loss of one safety function on the others
For example, would a loss of buffer mass affect not only the safety functions of the buffer,
but also canister integrity (via an increased transfer of corrosive agents to the canister
surface).
Step 8: Identify plausible descriptions of the evolution of safety functions over time
These would be defined in many programs as "scenarios", although in SR-Can, because of
the way in which regulatory guidance is formulated, all those that are considered "probable"
are classified as variants within the "main scenario". Scenarios outside the scope of the main
scenario are those that are judged less likely to occur (see Section 11.3 of SKB, 2006).
Canister
C1. Provide corrosion barrier
Copper thickness > 0

C2. Withstand isostatic load
Strength > isostatic load

C3. Withstand shear load
Rupture limit > shear stress

Buffer
Bu1. Limit advective transport−12a) Hydraulic conductivity < 10 m/sb) Swelling pressure > 1 MPa
Bu4. Damp rock shear
3
Density < 2,050 kg/m

Bu2. Filter colloids3
Density > 1,650 kg/m

Bu5. Resist transformation
Temperature < 100 °C

Bu3. Eliminate microbes
Swelling pressure > 2 MPa

Bu6. Prevent canister sinking
Swelling pressure > 0.2 MPa

Bu 7. Limit pressure on canister and rock
Temperature > −5 °C

Figure 3.6

Safety function indicators and criteria for the SR-Can canister and buffer (after SKB,
2006).

3.3.3 The issue of completenessIn some recent PAs (e.g. the Swiss Project Opalinus Clay – Nagra 2002a), a distinction is madebetween (i) phenomenological uncertainty in how FEPs influence the evolution andperformance of a disposal system and (ii) what is called in the Swiss case "completenessuncertainty", which is uncertainty as to whether all relevant features, events and processes(FEPs) and all relevant scientific understanding of these FEPs have been taken into account in44

the assessment. Scenario, model and parameter uncertainties, as defined in Box 1, are all
phenomenological uncertainties and are the focus of the uncertainty analysis within PA. In
addition, however, whether or not it is classified as a type of uncertainty, all PAs must address
the issue of completeness.
It is generally recognized that completeness uncertainty cannot be eliminated. It can, however,be reduced by certain practices within PA and by certain principles in siting and design (e.g.
designing for simplicity). The insensitivity of system evolution and performance to some typesof completeness uncertainty can also be tested by means of "what-if?" assessment cases (see e.g.
Figure 3.5).
Specific measures to reduce completeness uncertainties in recent PAs include:The use of international FEP lists, tailored to the system under consideration, as checklists
against which to compare the assessment basis, assessment cases and the models used fortheir evaluation
The use of international FEP lists as checklists is a feature of all recent PAs, including H12,
Project Opalinus Clay and SR-Can. In Project Opalinus Clay, it formed part of a wider
process of FEP management or "bias audit". In carrying out the project, a bias audit group
was established that was responsible for ensuring that all relevant scientific understanding
was taken into account in the PA and that the information and computer codes used in the
PA were appropriate. The FEP management tasks of the bias audit group are shown inrelation to assessment methodology as a whole in Figure 3.7 and are also summarized inTable 3.1.
The issuing of appropriate guidelines to experts involved in the project
In Project Opalinus Clay, for example, the experts chosen were asked (i) to take account of
the views of the scientific and technical community as a whole and not simply to present
their own personal opinions and (ii) to interact with others in their own field and in otherrelevant fields. Experts were also made aware of all available information (and its
limitations) that was relevant to the judgment they were being asked to make, by giving themaccess, for example, to the documentation structure and centralized catalogue of all relevant
documents.
The use of structured documentation with prescribed formats
In SR-Can, for example, the scientific understanding of each process was documented
according to the structure shown in Box 2. Through the structuring of the process documentsin this way, the responsible experts are obliged to consider, for example, any natural
analogue studies that may be relevant to a particular process.

45

Box 2:

Process documentation in SR-Can

For each process, the process reports give:Overview / general descriptionBoundary conditions

Model studies / experimental studies

Natural analogues/observations in natureTime perspectiveHandling in the safety assessment SR-Can

Uncertainties

Iterative development of the system description
In all PAs, the system description used as a basis for scenario development and analysis is
likely to undergo iterative development and repeated internal and external reviews (including
reviews of earlier PAs) over many years, gradually building confidence in the completenessof the description.

46

Table 3.1 Tasks of the bias audit group in Project Opalinus Clay.
Task

Description

Products

Develop a

The project-specific FEP (feature, event and process)

The Opalinus Clay (OPA) FEP

project-specific FEP

database is a structured, comprehensive list of FEPs

database for Project Opalinus

database and check forthat are potentially relevant to the disposal system

Clay is given in Appendix 2 ofcompleteness against

under consideration, with accompanying FEP

Nagra (2002b). The audit

international FEP

descriptions, based on international experience andagainst international FEP lists

databases

the scientific understanding of the disposal system.

is covered in Appendix 3 of

The scope of the list is constrained by a set of

Nagra (2002b).

assessment bounding rules. These define the boundsof the SA and exclude classes of conditions andevents that are not relevant to the aims of the SA or
ruled out by regulatory guidance (e.g. considerationof deliberate human intrusion). The FEP database is
audited (checked for completeness) againstinternational FEP databases.
Identify key

The key safety-relevant phenomena within the

Key safety-relevant

safety-relevant

system concept and safety concept, together withphenomena are given in Table

phenomena and checktheir associated uncertainties, are identified and6.8-1 of Nagra (2002a) and infor completenessaudited against the FEP database by checking that

Appendix 2 of the present

against theeach FEP is included.

document. The audit against

project-specific FEP

the OPA FEP database isdatabases

described in Appendix 6 ofNagra (2002b).

Identify the various

The super-FEPs (broad groupings of related FEPs)

The super-FEPs are given in

realizations ofthat are represented (sometimes with alternative

Table 6.8-1 of Nagra (2002a).

different phenomena

realizations) within the assessment cases are audited

The audit against the OPA("super-FEPs") within

against the OPA FEP database. Provided the

FEP database is described in

the assessment cases

assessment cases have been properly derived andAppendix 6 of Nagra (2002b).

and check forfully capture the key safety-relevant phenomena andcompleteness against

their uncertainties, this audit should reveal no

the project-specificomissions.

FEP databases
Check that there are

Available codes are audited against the super-FEPs

The qualification of the codes

codes available thatthat are to be represented. This may lead to theused in the project is covered

are suitable

recognition of a need to develop a new code or makein Appendix 8 of Nagra("qualified") tomodifications to existing codes. Once the ability of a(2002b).

analyze thecode to represent a given set of super-FEPs is

assessment cases.

established, the code is said to be qualified.

47

Figure 3.7

FEP management tasks (in grey) in the wider context of the Project Opalinus Clayassessment methodology (after Figure 3.7-2 of Nagra, 2002a).

Section 3.4 summarises NUMO's methodology on scenario analysis as developed at the time ofwriting this report.

3.4

Development of NUMO’s methodology for scenario analysis

3.4.1 Introduction
The study of the scenario analysis methodologies presented in the previous sections covers thetwo major approaches followed to date, namely the FEP-based ("bottom-up") and the("top-down") one based on the analysis of how the safety functions of disposal systems may beaffected by relevant events and processes. Although this FEP-based "bottom-up" method cansummarize and check completeness of system understanding, selecting key safety-relevant
FEPs and combining them in an appropriate manner requires complex screening procedures anddecision-making based on extensive multidisciplinary knowledge. In reality the complexity of
the work was reduced on the basis of expert judgment, which was really the key factor on
moving from large FEP lists to the limited number of scenarios considered for consequence48

analysis.
The alternative ("top-down") method also requires extensive system understanding, especiallyto define the evolution of safety functions with time and to integrate safety-relevantcharacteristics into assessment cases in a transparent, traceable and comprehensive manner.
By combining the bottom-up and top-down approaches, a "hybrid" scenario analysis method
has been developed. This aims to provide a traceable procedure of proceeding from a"storyboard", which depicts repository evolution with time on a range of spatial scales, to a setof scenarios that are associated with an assessment of their likelihood of occurrence. Based on a
dry run of such hybrid scenario analysis, we confirmed that the storyboards could provide aplatform to allow a wide range of experts to share system understanding. In addition,
storyboards can form a useful interface for communication of this difficult topic with various
stakeholders. The methodology is described in Wakasugi et al (2009).
3.4.2 Requirements defined for the scenario development methodology
The following requirements were specified for developing the new approach:Ensuring transparency and traceability

Maintaining complete and comprehensive records of decisions

Facilitating understanding of the associated arguments supporting decisions

Compatibility with a staged approach to site selectionEase of integration of interdisciplinary knowledge

Explicit consideration of various time frames (important to account for geological evolutionof sites)User-friendliness

Ability to serve as an interface with various stakeholder groups.

3.4.3 The scenario development logic and a first dry run
The general logic of the scenario development procedure ("work frame") is illustrated in Figure
3.8. This consists of a number of "Tasks" (T-1 to T-6), forming a work flow that is linked to an
associated FEP Knowledge base. A provisional safety concept is the starting point of safetyassessment, based on legal requirements and regulatory guidelines. This safety concept isclosely linked to specific geological settings and repository concepts that are, in turn, derived
from preliminary site characterization, safety assessment and repository design (NUMO, 2007).
Thereafter, a key feature of the process is development of a "storyboard". Conventionally, astoryboard is a scene-by-scene description of the progress of a film which focuses ondeveloping the story in terms of text plus images. In the present context, it refers to a time
sequence of images indicating the evolution of a repository system, which includes a "zoom"
49representation of such evolution at different spatial scales and in different locations. The images
are complemented by explanatory text, which is related to the FEPs that are relevant at any
particular time for a specific repository area. Clearly this needs input from generalists, withwide experience in not only safety, but also site characterisation and repository design /engineering.
International andregulatory FEP list

Legal Requirements:
Regulatory Guidelines

Provisional Safety Concept
Geological Setting

Repository Concept

T-1:Development / presentation of system
understanding:Storyboards
FEP KnowledgeBase

T-2: Detailed analysis:
Repository system modules

T-3: Safety Function
indicators

T-4: likelihood assessment:
A set of most probable cases with variants
T-5 Assessment of
perturbations
T-6 Scenario specification- Set assessment Case
- Consequence analysis- Scenario classification

Figure 3.8 Hybrid scenario analysis workframe.

T-1 is the initial task, aimed at providing a synthesis of understanding of the technical basis for
subsequent scenario development tasks. The goal is to illustrate understanding of the evolutionof the key safety-relevant components of the entire repository system on the basis of the tacitknowledge of experts. A first outline must thus be produced by experienced staff with extensive
system understanding (generalists). Thereafter, however, details can be refined by iteration withappropriate specialists.
As illustrated in Figure 3.9, working over storyboards can be a valuable aid to facilitating
communication between geologists, design engineers and safety assessors and is an essential
part of developing a safety concept. T-1 also provides the required transparency and traceability,
in a manner that is easy to understand, while integrating interdisciplinary knowledge on a rangeof time frames and assessment scales (engineered barrier system, repository, natural barrier,surface conditions - see Figure 3.9. Assessment time frames are selected on the basis of50

expected periods over which key safety functions operate, which are defined in the provisionalsafety concepts.
T-2 provides more detailed analysis of repository system modules (e.g. Figure 3.10, for the EBS
module), developing more detailed storyboards for repository system modules on the basis ofsafety function/indicators and simple scoping calculations (in T3). This task assesses
understanding compiled within the FEP knowledge base; in some cases gaps in knowledge may
be identified, which lead to requirements for future R&D work. This process acknowledges thelimitations in the extent to which performance can be assessed and is, in effect, a distillation by
the PA team of the extensive expert knowledge resulting from the interactions to develop the
T-1 storyboards. Complex evolution can be simplified by considering thermal, hydrological,
mechanical, and chemical (THMC) components separately although, in real life, these may bestrongly coupled. EBS components examined are the vitrified waste, steel overpack, bentonite
buffer, concrete lining and backfilling (based on the H-12 repository concept – JAEA, 2000).
Time frames are as for the T-1 storyboard. 300 years after repository closure corresponds to theestimated re-saturation period of the EBS and end of significant thermal transients. 1000 years
is the complete containment lifetime of the overpack as established in the reference design.
After re-saturation, preferential flow will occurred in the excavated damaged zone (EDZ) which
could be a key radionuclide migration pathway. The T-2 storyboard in Figure 3.10 was
developed in a 1st dry run using Japanese site-specific conditions.
The relationship between the FEP knowledge base and the storyboard development tasks is
illustrated in Figure 3.11. The FEP knowledge base contains not only a list of disposal system
specific FEPs, but also detailed background information in terms of safety functions andinter-relationships with other FEPs. The T2 storyboards are thus developed in an iterative
manner, as understanding of the site-specific evolution of specific modules is developed andcompared against the required safety functions. Through this iteration, uncertainties in specificspace and time frames can also be identified and documented within the T-2 storyboard.

51

Figure 3.9

T-1 Storyboard – Overview of total system evolution based on the H-12 repository
concept.

52

Figure 3.10 T-2 Detailed storyboard of EBS module (THMC analysis).

The red line indicates the starting point of radionuclide migration
following failure of the overpack.

53

FEP
Knowledge
Base

T-1:Development / presentation of system
understanding: Storyboards
T-2: Detailed analysis:
Repository system modules

T-3: Safety Function
indicators

Figure 3.11 Relationship between the FEP knowledge base and tasks T-1, T-2 and T-3.

T-3 provides safety factors and indicators derived from the provisional safety concept, which isa conceptual outline of how safety will be demonstrated. Safety functions can be conveniently
subdivided in terms of the key functions of Containment, Retardation, Dilution (dispersion) and
Isolation. In Figure 3.12, safety functions and function indicators are outlined for the EBS
modules and, for the specific case of the buffer, key phenomena that influence these functions
are identified.
ModuleVitrified waste

Overpack

Safety function

Safety function indicators

Containment

Preventing RN release

Dissolution rate: glass material, surface area, fracture distribution,
temperature and water chemistry (esp. pH)

Retention

Precipitation of RN

Formation of secondary phases: porewater chemistry (esp. Eh)Containment

Maintaining tightness

Corrosion rate and mode: material, thickness, chemistry,
corrosion rate, corrosion mode, solute flux

Retardation

RN sorption

Containment

Preventing overpack sinking

Mechanical characteristics: density, homogeneity

Stress and chemical buffering

Deformability: composition, density, thickness
Chemical buffering: mineralogy, density, thickness

Maintaining diffusion

Low permeability: density; Self sealing: composition, density

Colloid filtration

Density, homogeneity; Self sealing: composition, density

Sorption

Retardation factor: density, mineralogy, porewater chemistry

Mechanical properties: strength, thickness, stress field

BufferRetardation

Eh buffering, chemistry of porewater, corrosion products

Buffer: Density, mineralogy, homogeneity and geometry (thickness) are identified as key safety function indicators
allowing phenomena that influence them (these are safety relevant Super FEPs) to be identified from T-2.
Safety function indicators

Significant phenomena

Mineralogy

Chemical alteration by high pH plume

Homogeneity

Alteration  cementation / fracturing

Density

Dissolution, erosion  density reduction

Geometry

Density reduction  deformation

Figure 3.12 Safety functions and safety function indicators of EBS module.

A synthesis of the output from T-2 and T-3, results into the expected periods of safety function
operation for EBS components and it highlights the safety-relevant characteristics of each EBS
component; the latter facilitates development of realistic assessment cases, instead of theover-simplified cases considered in the past.
T-4 identifies the key safety functions of T-2 system modules for specific time frames, using
safety function indicators together with likelihood estimates from the FEP knowledge base(complemented by simple scoping calculations). Using the T-1 storyboard overview, these54characteristics are arranged into a series, representing evolution in a base case scenario. In thisprocess, likelihoods and uncertainties are assessed, which produces variations of this scenario(Figure 3.13).
T-5 identifies perturbations of more probable assessment cases based on the geological setting,considering site specific natural events (illustrated in T-1), their likelihood and their potential
effects on relevant safety functions.
Finally, T-6 involves classification of assessments cases into various scenario classes, as definedby regulator, based on T-5 (perturbations), T-4 (more probable cases) and scoping calculationsof effects. In the first step, the most likely case(s) is assessed, together with its uncertainties inthe various space and time frames. The base case conceptual model can then be defined as abase case scenario with expected safety functions. Some variations of the conceptual model canbe handled by simplified mathematical models and codes. The FEP knowledge base can also
provide required system geometry, chemical, hydrological, mechanical, biological and physicalbehaviour characteristics, allowing T-6 to be carried out in an iterative manner.

Figure 3.13 Sequences of more probable safety-relevant phenomena derived from T-2.

3.4.4 Discussion and further developments
The work frame approach adopted here is readily amenable to the treatment of uncertainties. T
1 and T-2 storyboards can document uncertainties identified within the compilation ofmultidisciplinary system understanding. In particular, long time disposal system evolution is55

influenced by natural processes and events with intrinsic uncertainties. During the dry run,several Japan specific uncertainties have been identified, for instance:Possible volcanic activities beyond 100ka

Change of near field conditions due to contact with oxidized groundwater beyond 1Ma

Change of surface conditions beyond the next ice age

Interaction between steel corrosion products and bentonite buffer beyond 10 kaChemical and mechanical interactions of cementitious material and bentonite bufferPossible influence on safety functions due to gas and microbial activity.

Handling uncertainties needs to be developed further, however, with a more practical method ofrecord keeping within storyboards. In the next stage of development, the method will be applied
to a wider range of repository concepts and siting environments to check that it has theflexibility required to respond to the challenges of NUMO's volunteering siting program.

56

4Development of a Requirements Management System
(RMS)

4.1

Introduction

The concept of requirements management has been in use in the engineering industry for sometime but is only just starting to infiltrate the world of radioactive waste disposal. The key
strength of requirements management (RM) is that it documents the decisions involved andtheir justification, in terms of both hard information (data, models, system understanding, etc.)
and softer, more qualitative grounds for any given project. Thus, it also forms a memory ofimportant historical information about a project.
This is increasingly becoming an issue as radioactive waste programs around the world head
into their third and fourth decades with new staff who are not familiar with decisions mademore than 20 years ago. As the older staff responsible for these decisions leave, the danger
always exists that the transfer of knowledge about fundamental program decisions will be lost,leaving new staff to "reinvent the wheel".
NUMO has thus decided to include an RMS as part of its radioactive waste management
program. Various RM approaches and systems were examined as part of this collaboration. At
the end, because NUMO would like to expand the definition of the RMS to also include arecord of the decisions taken (and the rationale behind them), NUMO decided to develop its
own RMS in order to implement the Japanese geological disposal project in an efficient,sustainable and consistent manner. This chapter introduces some of the highlights derived fromthe NUMO-Nagra collaborative study on RMS development.

4.2

RMS development in oversea organizations

4.2.1 Information exchange meeting in 2007
Within the period of this collaborative project on RC development, an information exchange
meeting on RMS development was organized in 2007. The meeting took place at the SKB
offices, with participants from NUMO, Nagra, SKB and, as support to NUMO, Obayashi. All
the participants in the information exchange meeting were involved in RMS development for
their own national program. The aims of the meeting were specified in advance as follows:
Overall aim
Exchange of information on RM/RMS among the lead organizations (SKB-NUMO-Nagra).
57

More specific aims


Understanding the background and boundary conditions of RM/RMS development in eachorganization



Understanding similarities and differences in RM/RMS development among organizations



Identifying common issues for RM/RMS development among the programs

Identifying areas / topics of mutual benefitEach organization to provide updated information on RMS development.

In order to exchange the basic information in a structured manner, the following template wasused for the presentation of each organization:


Aims of requirements management (RM)Structure of the requirementsWho defines the input to the RMS and who uses the RMS

Experience to date

Current open issues.

Detailed presentations and discussions, as well as the main points brought up by eachorganization during the meeting, are summarized in an internal report: Record of theInformation Exchange Meeting on Requirements Management System (RMS) between NUMO,
SKB and Nagra, Stockholm/Sweden, 10 September 2007".
4.2.2 The SKB case
SKB has defined three aims for RMS development, namely:

Aim 1: Facilitate the license applicationFor SKB

For reviewing authorities

Aim 2: Facilitate communication among repository designers / constructors


Settle design premises

Identify couplings and dependencies

Aim 3: Make decisions and the development of the design traceable.

The requirements should be used for:
Communication
Definition of problem and scope
58







Understanding the context

Design

Making the right "things"



Optimization



Risk management

Change control

Quality assurance

Qualification – controlling – testingTraceability.

It is recognized that there are different types of requirements as shown in Figure 4.1. SKB
defined two domains in the RMS structure - the problem domain and the solution domain. In
the problem domain, the problem is clearly defined considering the stakeholder requirements(top-level requirements). In the solution domain, system requirements are derived fromstakeholder requirements and design requirements are further derived from system requirements.
For system requirements and design requirements, system concept and specific solutions are
identified respectively.
SKB then put the requirements into a hierarchical structure as shown in Figure 4.2. The basic
concept was taken over from the V-model used for the DOORS system (see Reference list) andmodified to suit SKB's needs. The requirements structure was defined at different levels,
namely stakeholder, system requirements, sub-system requirements, design requirements andspecifications. The top-level requirements (stakeholder requirements) are given conditions for
SKB, while system requirements and sub-system requirements should be defined by SKB.
Furthermore, detailed requirements such as design requirements and specifications could be
defined by designers, who could be supporting organizations specialized in facility design.

59Figure 4.1 SKB's concept of different types of requirements for a radwaste disposal project.

Figure 4.2 SKB's hierarchal structure of the RMS.

The different types of requirements shown in Figure 4.1 are defined more precisely as follows:

Stakeholder requirement: A requirement expressing SKB's objectives for a system and theprinciples which will form the basis for the system design
System requirement: A requirement expressing functions, characteristics or qualities asystem must have to meet the stakeholder requirements
Sub-system requirement: A requirement expressing the functions, characteristics or qualities
a sub-system must have to meet the system requirements
60





Design requirement: A requirement expressing how the product and/or the process of
installation, manufacturing, building, sealing or testing must, given the constraints, bedesigned to meet the sub-system requirements
Specification: Document which specifies requirements on a product and/or process of
installation, manufacturing, building, sealing or testing.

In addition to the requirements, SKB introduced the concept of "constraints" as shown in Figure
4.2. The constraints are given conditions for designers that have to be accounted for. In thefollowing, examples of such constraints are provided:

Spent fuel

Data from the nuclear power plants and CLAB

Cost calculations (plan reports)

Site characteristics
Processes in repository evolutionSite description
Background reports to safety assessment

The system itself – results from previous design stages
Engineered barriers

ActivitiesTechnical systems



Facility

Mishaps during operationMishaps defined in the operational safety assessment.

SKB has also introduced the basic principle of a "link" between higher levels of requirements
and lower levels of requirements. The links are to be stated with specific expressions, forexample, the link from higher levels of requirements to lower levels of requirements should be
described with the statement "this implies that …" or "this leads to that …", whereas the linkfrom lower levels to higher levels should be qualified with the statement "because the …" or "tomeet the requirement that …".
Based on the conceptual development of the RMS, the structure of the database system has
been established by SKB as shown in Figure 4.3. All the contents related to the RMS are
included in the database system and links between the contents are established in the system.
Figure 4.4 shows an impression of the SKB decision-making process. The process starts withwriting requirements by responsible experts and then suggestions will also be made by61

responsible experts. The suggestions are reviewed by "external" reviewers (experts in the areaof interest) and then passed on to the decision-maker to look at the issue with a higher or wider
perspective. Finally, the decision should be made by decision-makers considering all of the
input given by responsible experts and reviewers.

Figure 4.3 Organization of database system for the SKB RMS.

Figure 4.4 An example of a decision-making process by SKB.

Table 4.1 shows, as an example, the staff who should be involved in developing and using
SKB's RMS.

62

Table 4.1 Main players involved in developing and using SKB's RMS.
Higher level requirements

Lower level requirements, design requirements,-stakeholder, system andspecifications and constraints

sub-system requirements
Responsible

Requirements and database

Experts on the sub-system or component to beexperts (writers)

managers

designed
Requirements and database managers

Reviewers

Decision-makers

Experts on radioactive waste

Persons responsible for the sub-systems andmanagement

components the requirement is linked to Experts

Experts on applications Lawyers

invited by the authors

Project leaders of final repository

 With vital importance for nuclear safety andradiation protection: project leaders of final

project

repository project, council of requirements

Council of requirements

 Regarding facility design: project leaders offacility design projects
 Regarding engineered barriers: managers oftechnique development unit Regarding parts of the facility or technicalequipment: designers

The experience with the RMS up to the date of the workshop and open issues identified by SKB
for further development were summarized in the workshop as follows:












Introducing an RMS brings about new ways of organizing work, particularly for designers
and constructors
The RMS and management systems must be integrated

RMS – quality assurance of productManagement system – quality assurance of work and activities

Introducing an RMS is a slow processIt requires communication

It is initially hard to see the benefits

It is hard to separate the requirements from the solution – strict rules for how to write
requirements must be applied
Requirements indirectly affecting the design, e.g. the use of best available, robust andreliable, are hard to handle
Many open issues on design requirements and constraints (reference design).
63

4.2.3 The Nagra case
Nagra emphasized that its RMS is part of the Quality Management System (QMS). Nagra's QM
system is run by Intranet within Nagra in order to ensure that all the users have easy access tothe most updated QM system. Figure 4.5 shows a screen image of Nagra's QM system. It waspointed out that the RMS is part of the strategic planning (formal process, shown as part of 1.1
in the figure) with periodic check-points, but it has direct links to the projects (input todevelopment of requirements, boundary conditions for projects, etc).

Figure 4.5 Screen shot of Nagra’s QM system.

The broad aims of the RMS being developed by Nagra can be summarized as:

Aims at a systematic and traceable approach to developing, eliciting and treating/respecting
requirements
Ensures that no important requirements are "forgotten" (comprehensiveness in development
and treatment of requirements)Allows requirements (requirements, preferences, issues) to be prioritizedProvides an overview of and handles contradictory requirements
64






Provides an organizational memory of important issues (requirements)
Provides an adequate structure for their application in projects / activities ( betteroverview: what, where, when, why, how)
Reflects on various requirements (categories) and how they relate to each other(interdependencies)

Documents (all) the relevant requirements obeyed by a projectFacilitates communication between different disciplines within the organization / project

Facilitates interaction with different groups of interests (project, management, waste
producers, authorities, public, etc.)
Helps to develop an adequate "work culture" (team, interdependence).

Considering the above aims, Nagra has been developing an RMS with a relatively simple
structure to gain initial practical experience in implementing an RMS for the radioactive waste
disposal program. The following points were introduced to summarize the observations andexperience with the RMS up to the date of the workshop:

Requirements can be related to hardware, reports, procedures, etc.

Requirements are sometimes related to a specific concept, design, etc.








Need to link to "configuration data"( data clearance)Some requirements are conditional (distinguish from generic)It may be useful to keep track of "flexibility left" due to alternatives

Requirements are often the result of a line of reasoning  need to link with the report that
contains the reasoning (still the main emphasis of Nagra work)
Treatment / application of requirements may require specific procedures
Decision-making and corresponding reports are not replaced by "ticking-off" allrequirements, but "managing" requirements may be essential in preparing reports/arguments.

The text below documents the requirements management activities at Nagra. It should be notedthat Nagra initiated identification of the requirements based mainly on the Sectoral Plan for
Deep Geological Repositories prior to the system development. A large number of requirements
are defined both explicitly and implicitly in the Sectoral Plan; for the latter, Nagra is required tointerpret and translate all implicit requirements into a more practical form for implementation of
the disposal program in Switzerland.


Development of requirementsTop down: hierarchical structure of requirements  develop structure and deriverequirements (e.g. safety concept  safety functions …)
65





Application of requirements (boundary conditions)








Bottom up: at the start of a major project, evaluate explicit and implicit requirements thatare essential for, or contribute effectively to, successful completion of the project(adequate design, site, etc.)
At the start of each important project / activity: check for relevant requirements (ear-markcritical requirements)
Then: translate requirements into boundary conditions and work processes for conducting
the project ( measures to be taken)
When completing the project / activity: check that requirements have been fulfilled

Project manager: ensure that, for a given issue, the same requirements ( specifications /assumptions) are used by all projects / activities (data clearance process  part of QMS(through IT-tool)).

The basic principles for developing a database for the requirements management system are:

Development of requirements underway (specific projects)

Waste management program

Site selection process

Repository designGeneral license application



As soon as (preliminary) results are available feed into database (significant backlog)

Application of requirements (procedures): underway

Database content to date: high-level (legal) requirementsTechnical-scientific requirements are currently being entered (in parallel with the productionof reports)

Data entry is slow / few users as yet!

Database: FileMaker Pro.

Finally, "challenges" in developing the requirements management system are shown below,grouped into two categories:

Challenges related to RM
Links between different (e.g. higher-level to lower-level) requirements

Distinguishing between "hard" requirements as opposed to "nice to have" features

Large number of high-level (legal) requirements

Missing requirements, ambiguous and unclear requirements
66





Coping with uncertainties in scientific understanding

Difficulties related to RM as a process

Many high-level (legal) requirements, but only few lower-level requirements are recorded(although many of them are described in documents)
Requirements as part of projects are fully recognized, but their management within aformal system is still under development.

At the time of the workshop, experience and "lessons learned" had already been obtained andwere summarized as follows:

Two possible approaches to deriving requirements







Develop requirements through dedicated projects (often top-down)
Collect / elicit continuously (hidden) requirements based on scientific understanding,experience, etc. (bottom-up)
But: need for iteration (screening for rubbish, conflicts, misunderstandings, poor
documentation, nomenclature, etc.)

Requirements should be clearly linked to their origin or basis (stakeholder requirement /experimental data / technical constraints, etc.) and their line of argument (incl. how to apply)The need to consider uncertainties  requirements may change

Low-level requirements should be explicitly linked to higher-level requirementsStart with a simple software tool: the intended purposes of the system should be further
defined (based on company-wide experience) before selecting another tool or upgrading theexisting tool
Do not make it too clever / too detailed.

4.3

Overview of the current N-RMS being developed by NUMO

4.3.1 Background of N-RMS development
As mentioned many times to date, NUMO intends to proceed with its program following theprinciple of the NUMO Structured Approach (NSA). The aims of NSA can be summarized as
below:


To consistently implement the stepwise approach of the geological disposal program in thestages of site selection, licensing, construction, operation and closure
To satisfy the engineering, social and legal requirements for the disposal program with anappropriate degree of flexibility
To maintain the traceability of the decision-making processes of NUMO.
67

In this context, the main aim of the N-RMS development is:

The comprehensive information management of the requirements and decision-making
processes in a structured manner for the geological disposal program.

The following key functions are expected for the N-RMS:


Record-keeping of all relevant information for the implementation of the disposal systemApproval system for the decision-making

Change management

Project management.

4.3.2 Development process of the N-RMS
As shown in Figure 4.6, the N-RMS will be developed in 4 stages - the NSA development stage,
the trial RMS stage, the practical RMS stage and, finally, application to the implementation
process. The current N-RMS is being developed within the framework of the trial RMS stage
and it is expected that this stage will continue into another phase from FY'08 onwards. It is,however, not known whether NUMO will continue with the same contractor group and willdevelop the next phase of the N-RMS fully utilizing the current version of the trial N-RMS.
Figure 4.7 shows the detailed development process of the N-RMS. The process starts with the"conceptual design based on the NSA" and continues with "analysis of the structure of thetechnical work". Based on this, there is a very important step, namely structuring the N-RMS.
The structuring includes establishing NUMO's entire work structure and structuring of NUMO'stechnical work. The next step is to "design the trial RMS". During this step, it is expected thatthe system components and database structure based on the DCRA (decisions, considerations,requirements, arguments) model will be developed. The next step is "preliminary description of
the DCRA database" and NUMO decided to use the information from the H12 report and fromother reports to supplement H12 (should there be more recent work since H12). Finally adry-run will be performed to check the feasibility of the developed trial N-RMS.

68

Figure 4.6 Flow diagram of the development process of the N-RMS.

Figure 4.7 Procedure for development of the trial RMS.

4.3.3 Overall structure of the N-RMS
Figure 4.8 shows the overall structure of the N-RMS, consisting of the following three
sub-projects:Sub-project 1:

Repository concept and safety assessment69Sub-project 2:

Management of technical workSub-project 3:

Site characterization

As shown in Figure 4.8, the function of sub-project 2 plays a central role in the N-RMS. It
defines the high-level requirements for the sub-projects 1 and 3 in order to optimize the overall
technical project scheme in a top-down manner. Sub-projects 1 and 3 will then define further
detailed requirements and the decisions will be made within the N-RMS.

Figure 4.8 Overall structure of the N-RMS and expected interactions between the sub-projects.

Figure 4.9 shows the overall system concept of the N-RMS. The N-RMS is expected to have
two functions, namely a "decision-making support function" and an "RMS databasemanagement function". The decision-making support function consists of the RMS browser,
management of "decisions" and change management. On the other hand, the RMS databasemanagement function consists of the database corresponding to the management of the DCRA
and the one corresponding to the support in constructing the DCRA.

70

Figure 4.9

Overall system concept of the N-RMS with two functions – decision-making support
function and RMS database management function.

Figure 4.10 shows screen shots of the N-RMS. The trial system was developed as a bilingual
system and language can be changed between Japanese and English. Since it is on an open
webpage, the system is protected by user name and password. Figure 4.11 shows an example of
the N-RMS screen. The left window in the figure shows the hierarchical structure of therequirements, which provides a good overview of the N-RMS structure. The right window has 6
tags consisting of "main information (1)", "main information (2)", "requirements", "related
information", "auxiliary information" and "master information". Each tab has its own
description and files can be attached in the attachment box. Under the requirements tab, therequirement is stated with a short text. In the requirements tab, many requirements can be
included and each requirement has its own description called "argument of requirement" and"background knowledge". Since the requirement itself is described with a short text, a detailed
explanation of the requirement can be written under the "argument of requirement" tab.

71

Figure 4.10 Initial screen shot of the N-RMS.

The language can be changed between Japanese and English.

Figure 4.11 Typical screen shot of the N-RMS.

The left window shows the structure of the N-RMS andthe right windows are for further detailed information.

72

5Concluding remarks

A broad range of issues has been studied by the NUMO-Nagra team as part of the collaborative
studies on repository concept development. A summary of the first part of these studies can befound in NUMO (2004a).
Some of the main topics studied over several years include:Strategy and methodology development for tailoring repository concepts (e.g. NUMO
Structured Approach, PI roadmap, uncertainty treatment, engineering constraints andgeosynthesis)Requirements Management System and

Improvement of performance assessment (scenario development methodology).

In addition, topical issues were also studied to review the latest developments in overseas
programs (e.g. iron/bentonite interactions, high temperature effects on bentonite, CO2
sequestration, SKB production line reports).
This report summarizes three of the specific areas studied and advances made through thiscollaboration in the period FY 2003 to FY 2007, namely:

Development of NUMO's PA/RD roadmap

Scenario development



Development of a requirements management system.

The project has established a platform for a compact, efficient and quick information exchange
and know-how transfer both at strategic and program level, as well as at the technical level.
Consistent with NUMO's mission, the emphasis was on developing the strategies and tools thatwill allow successful management of the program for a geological repository for specified
radioactive waste in Japan.
Developments in some areas will continue to keep abreast of the best available technology at
the international level, but instruments and technologies will also be tailored for application
within the Japanese environment. The project team remains confident that the results from thisstudy have contributed, either explicitly or implicitly, to advancing the Japanese geologicaldisposal program.

73

Acknowledgements
The authors value the contributions from a wide range of RC Collaboration Team members. In
particular, we would like to express our gratitude to Dr. H. Umeki (JAEA), Mr. Y. Sakabe(Hokkaido Electric) and Mr. Y. Sugita (JAEA) for their contributions to both project
coordination and technical development. It should also be noted that the work was performed
with extensive contributions from the Nagra Technical Support Team, including Dr. P. Smith(SAM), Dr. I. McKinley (McKinley Consulting), Dr. J. Andersson (JA Streamflow AB), Dr. S.
Mishra (Intera) and Dr. H. Kawamura (Obayashi).

References
DOORS: A requirements Management Tool for Systems and Advanced IT Applications;
http://www.telelogic.com/Products/doors/doors/index.cfm.
IAEA (2003): Radioactive waste management glossary – Radioactive waste disposal –Dictionaries; IAEA Library Catalgouing in Publication Data, IAEAL 03-00320,
International Atomic Energy Agency, Vienna, Austria.
JAEA (2000): H12 Project to establish technical basis for HLW disposal in Japan, Project
overview report, 2nd progress report on research and development for the geologicaldisposal of HLW in Japan. JNC TN1410 2000-001. JAEA (formerly JNC), Tokai, Japan.
Kitayama K., Ishiguro K., Takeuchi M., Tsuchi H., Kato T., Sakabe Y. and Wakasugi K.
(2007): Strategy for Safety Case Development – Impact of Volunteering Approach for
Siting a Japanese HLW Repository; Proceedings OECD Nulcear Energy Agendy
International Symposium: Safety Cases for the Deep Disposal of Radioactive Waste:
Where do we stand?, 23-25 January 2007, Paris, France.
Koike A. (2008): The results of research and development – (2) Development of Investigation
Technologies and Assessment Methods of Geological Environment, Proc. of NUMOTechnical Workshop, pp.25-36, Tokyo (in Japanese).
Nagra (2002a): Project Opalinus Clay – Safety Report – Demonstration of disposal feasiblity
for spent fuel, vitrified high-level waste and long-lived intermediate-level waste(Entsorgungsnachweis); Nagra Technical Report series, NTB 02-05, Nagra, Wettingen,
Switzerland.
Nagra (2002b): Project Opalinus Clay – FEP Management for Safety Assessment –Demonstration of disposal feasiblity for spent fuel, vitrified high-level waste andlong-lived intermediate-level waste (Entsorgungsnachweis); Nagra Technical Reportseries, NTB 02-23, Nagra, Wettingen, Switzerland.
74

NEA (1997): The Probabilistic System Assessment Group – History and Achievements 1985 –
1994; OECD Nuclear Energy Agency, Issy-les-Moulineaux, France.
NEA (2001):Scenario Development Methods and Practices – An Evaluation based on the NEA
Workshop on Scenario Development, Madrid, Spain, May 1999; OECD Nuclear Energy
Agency, Issy-les-Moulineaux, France.
NEA (2004): Safety of Disposal of Spent Fuel, HLW and Long-lived ILW in Switzerland – AnInternational Peer Review of the Post-closure Radiological Safety Assessment for
Disposal in the Opalinus Clay of the Zurcher Weinland; OECD Nuclear Energy Agency,
Issy-les-Moulineaux, France.
NUMO (2004a): Development of Repository Concepts for Volunteer Siting Environments;
NUMO Technical report series, TR 04-03, NUMO, Tokyo, Japan.
NUMO (2004b): Evaluating Site Suitability for a HLW Repository – Scientific Background andPractical Application of NUMO's Siting Factors; NUMO Technical report series,
TR 04-04, NUMO, Tokyo, Japan.
NUMO (2004c): Background and technical evidence of Siting Factors for the Selection ofPreliminaly Investigation Areas, NUMO Technical report series, TR 04-02, NUMO,Tokyo, Japan (in Japanese).
NUMO (2007): The NUMO Structured Approach to HLW Disposal in Japan; NUMO
TR 07-02 (Japanese version TR 07-01), Tokyo, Japan.
SKB (2006): Long-term Safety for KBS-3 Repositories at Forsmark and Laxemar – A First
Evaluation – Main Report of the SR-Can Project; SKB Technical Report series TR-06-09,
Svensk Kärnbränslehantering AB, Stockholm, Sweden.
SKI (1997): SKI SITE-94 Deep Repository Performance Assessment Project – Summary; SKI
Report 97:5, Swedish Nuclear Power Inspectorate, Stockholm, Sweden.
Wakasugi K., K. Ishiguro, T. Koyama, H. Shiratsuchi, S. Yashio and H. Kawamura (2009):
Developing a methodology for scenario analysis based on understanding of long-term
evolution of geological disposal systems; Proceedings of the MRS'09 Symposium – 33rd
Int. Symposium of the Scientific Basis for Nuclear Waste Management, Saint Petersburgh,
Russia, 24-29 May 2009.

75

Appendix A: Survey of focused PA issues in European
countries
A.1

Issues addressed

Among the various issues addressed in recent PAs in European countries, the issue of how
specific elements of engineering practice affect the initial state and subsequent evolution of
the repository is receiving increasing attention. The general issue of how to assess thesignificance of possible deviations from the planned initial state is considered in Section A.2,
focusing on deviations from planned buffer density. In addition, following the recent
experience from the earthquake near the Niigata-Kariwa nuclear power plant in Japan, an
issue of considerable interest to NUMO is the detection and evaluation of consequences for arepository of earthquakes occurring on active faults. Although active faults will be avoided
when designing a repository layout, earthquakes on such faults could lead to secondary shear
movements along fractures that intersect the repository tunnel system and could potentiallydamage the barrier system. Work on this topic has been carried out recently by SKB and
Posiva in the context of post-glacial earthquakes and is summarized in Section A.3. Finally, in
Section A.4, the issue of the treatment of the near-field/geosphere interface is discussed.
Potential gains in calculated system performance compared with H12 may be obtained byadopting the model assumptions employed by SKB and Posiva in their safety assessments.
The conditions under which these model assumptions may be valid are discussed.

A.2
A.2.1

PA issues arising from engineering considerations
The need for interaction between PA analysts and engineers in
identifying key issues

Typically, a repository for radioactive waste will evolve from its initial state, when the firstwaste packages are emplaced, through an early, transient phase in which, for example, thebuffer saturates and heat output from the waste declines to low levels, towards a quasi-steadystate (the "target state", see Figure A.1). In this quasi-steady, target state, the key
safety-relevant physical and chemical characteristics of the barriers (e.g. temperature, buffer
density and swelling pressure) are subject to much slower changes than in the transient phaseand the main safety functions of barriers are expected to be provided.

A- 1

Engineering practiceEngineers need to know: are there issues ofengineering practice that may affectevolution to "target state" such that longterm safety functions not provided?

"Initial state"
Early evolution(saturation, "thermal phase", etc)

PA practitioners need to know:what is the initial state, including
uncertainties, potential deviations,that could affect evolution?

"Target state"(long-term safety functions provided)Longer-term

Figure A.1

The need for interaction between PA practitioners and engineers in defining therepository "initial state" and its subsequent evolution.

Describing the evolution of the repository towards the target state and beyond, taking into
account all relevant uncertainties, is the responsibility of PA practitioners. The way in which
the system evolves, however, depends on its initial state and this initial state will depend on
decisions taken by engineers on how to implement the chosen repository design. In PAs
carried out at the early stages of a repository program, an idealized view is often taken of theinitial state. It may be assumed, for example, that the buffer is perfectly homogeneous, that
there are no stray materials present that could interfere with the repository safety functions
and that mishaps or accidents during manufacture and emplacement of the engineered barriers
can be avoided, or the consequences rectified. Table A.1, however, shows how engineeringexperience to date indicates that obtaining an "ideal" initial state can be difficult, or at least
subject to some significant uncertainty. Thus, as a program matures, the validity of theseassumptions regarding the initial state must be evaluated and alternative, more realistic
assumptions considered if necessary. In order to define a realistic repository initial state, PA
practitioners need information from engineers on operational procedures and other elementsof engineering practice that may give rise to uncertainties and potential deviation from design
values of key parameters, such as buffer density. Engineers, on the other hand, need to knowwhether or not their engineering decisions could have a potential adverse effect on thecapacity of the repository to perform its safety functions as it evolves over time. Figure A.2
illustrates how the long-term safety functions and corresponding requirements on each barriercomponent affect the required conditions after the initial, transient period (the "target state" inFigure A.1). These in turn affect the engineering measures employed and the overall planningA- 2

of repository construction and operation. Thus, overall, repository design and PA both requireinteraction between PA practitioners and engineers.
Table A.1 Examples of experience to date on engineered barrier emplacement.
Examples from the Grimsel Test Site, Switzerland
FEBEX

More effort than originally thought was required to emplace bentonite blocks

GMT

Buffer was emplaced by in-situ compaction - good quality control was demonstrated,but operation is difficult if no contact handling is allowed (e.g. in the HLW case)
Buffer was emplaced by combination of blocks (lower part) and pellets (upper part) -EB

auger emplacement system was demonstrated, but significant effort was required toachieve high buffer density and homogeneity (partly due to too many obstacles, e.g.
instrumentation)
Demonstration and optimisation of different emplacement measures

ESDRED

Examples of tests of KBS-3 emplacement methodologies
Canister emplacement machine built and tested. Further modification of the machine

DEMO TEST

or repository design is required
Prototype repository

Engineered barrier emplacement was achieved in a quality assured manner, but
plastic sheets had to be used to avoid contact between bentonite blocks and inflowing
water in deposition holes

TotalsystemOverall
Overallplanningplanning
e.g.
e.g.logistics,
logistics,
cost,
cost,time
time

Total
TotalsystemsystemPerformance
Performance

Repository
Repository
concept
concept

Component

Control
Controlparameter
Engineeringparameter
Engineeringformeasuresforconst
const&&
measuresoperation
operation

Condition
Conditionafter
after
transienttransientperiod
period(stable(stablecondition)
condition)e.g. Saturated buffer

e.g. Buffer as emplaced

Initial state

Figure A.2

SafetySafetyrequirements
requirementsofofeacheachbarrier
barriercomponent
component
e.g. Altered buffer

Post transient state

LT future(e.g. after 100s years)(e.g. after 100 000 years)

Illustration of how long-term safety requirements influence the engineering
measures employed and the overall planning of repository construction andoperation.

A- 3

A.2.2

Examples of issues of engineering practice of potential significance toPA

Table A.2 gives some examples of elements or issues of engineering practice that may giverise to uncertainties and potential deviation from design values of key PA parameters. The
corresponding PA issues (issues/processes for long-term safety) are also shown. Since NUMO
has not, as yet, fixed its reference repository concept, the list of issues is not comprehensive
and the descriptions are, of necessity, rather general.
Table A.2

Examples of elements or issues of engineering practice that may give rise touncertainties and potential deviation from design values of key PA parameters andthe corresponding issues/processes for long-term safety.

Engineering practice

Issues / processes for long-term safety

CanisterDamagetocanister duringtransportation or emplacement (e.g.
by dropping, derailing, fire,
flooding)
Erroneous emplacement of canister Loss of mechanical integrity
 Cause of pitting corrosion
 What are the criteria for determining if canister to be retrieved or not Loss of buffer thickness
 Further displacement (rotation) of canister due to gravity and unusual

stress condition
 What is the minimum buffer thickness to maintain safety function?

Bentonite
Early swelling of bentonite invertical (pit) emplacement

 Loss of swelling pressure Lower density – higher hydraulic conductivity
 Difficulty in canister emplacement
 Bentonite swelling out of pits

Heterogeneity in bentonite density
 Density difference between block
section and pellet section

 Heterogeneity in swelling pressure Heterogeneity in hydraulic properties Heterogeneous stress condition for canister Density differences derived from Heterogeneity in swelling pressure

gaps between bentonite blocks
and components
 Heterogeneous density due topellet emplacement

 Heterogeneity in hydraulic properties Heterogeneous stress condition for canister Heterogeneity in swelling pressure Heterogeneity in hydraulic properties Heterogeneous stress condition for canister

Emplaced buffer density too low Low swelling pressure High hydraulic conductivity
 Bentonite flushing out

Emplaced buffer density too high

 Extreme swelling pressure

Backfill
Heterogeneous emplaced density
Gaps at top of tunnel
Low emplaced density

 Preferential GW flow path
 Risk that materials will be flushed out

Rock
EDZ

A- 4

Engineering practice

Issues / processes for long-term safety Initial thickness of EDZ

 Depending on excavation method Development of EDZ with time Depending on the rock type – more critical for soft rock than hardrock
 Also depending on the rock support applied

Thermally induced rock spalling
Tunnel roughness and irregular
diameter size

 Depending on the rock properties and thermal load

Seals and plugs
Large amount of cementitious
materials
Bentonite-concrete
composite
design for hydraulic plug

 High pH plume around seals and plugs
 Interaction between bentonite and cement

Engineering materials potentially remaining post-closure
Transportation: rails, concrete floor Mostly remain in tunnel
 Relatively large amount of material
 High pH issue for concrete Corrosion expansion and gas generation for metals (e.g. rails)Rockstability:
shotcrete, liner

rockbolts,

 Remain in rock / tunnel
 Large amount of material
 High pH issue for concrete Corrosion expansion and gas generation for metals (e.g. rails)

Drainage: drain sheets, sumps,
drainage pipes Remain in rock / tunnel
 Large amount of material
 Organic issues for drain sheet
 High pH issues for sumps

Utilities: lights, cables
Ventilation: ducts, fans
Handling: steel handling(PEM) Will be removed after operation?
 Will be removed after operation?

shell Remain in tunnel
 Fe-bentonite interaction

Some potentially important PA issues may be summarized as follows:
Buffer density may deviate from the "target" value
A low average density could be caused, for example, by the presence of gaps or by piping(transient water flows) and erosion during saturation. A high average density could be causedby corrosion expansion of a metallic, prefabricated emplacement module (PEM), if this isused to emplace the engineered barrier system, or by tunnel convergence. Heterogeneous
buffer density also is caused by the above-mentioned affects, as well as by the combined useof bentonite blocks and pellets.
Buffer-rock interface may be perturbed
Physical disturbance to the buffer/rock interface could be caused, for example, by thepresence of an excavation damaged zone (EDZ), by rock spalling, which is the brittle
fracturing of the rock surface into splinters, chips or fragments (spalling could occur as aresult of the relief of high initial rock stresses upon rock excavation or as a result ofA- 5thermal-mechanical effects following waste emplacement) or by the physical degradation oftunnel support (liner), if such support is used. Chemical disturbance could be caused, forexample, by cement/bentonite and cement/rock interactions, or by iron/bentonite interactions.
Engineered barrier system may generate gas
Gas may be generated, for example by corrosion of the overpacks, the metallic shell of thePEMs, if these are used, transportation rails and other metallic structures.
Engineered barrier system may interact with stray materials
Stray materials, if present in sufficient amounts, could damage the canister surfaces orotherwise adversely affect the repository safety functions. Organic substances or theirdegradation products could, for example, form complexes with radionuclides that would
lower radionuclide sorption and thus increase radionuclide release and transport rates in theevent of canister failure.

Table A.3 shows the potential causes of each of these factors, the PA assumptions or
parameters potentially affected, the potential effects on system performance and theinformation requirements to quantify these effects.

How to evaluate whether potential deviations from target values are significant from a PA
perspective is discussed further in Section A.2.3, using the example of deviations from thetarget value of saturated buffer density.

A- 6Table A.3

Examples of PA-relevant issues, their potential causes, the PA assumptions or
parameters potentially affected, the potential effects on system performance andIssue

the information requirements to quantify these effects.
Potential
PA assumptions orPotential
causes inparameters
effects onengineering
potentiallysystempractice
affected
performance

Information required toquantify effects
GeneralEngineering

Buffer density may deviate from "target" value
Low average
buffer density(with respect
to designvalue)

 Early swellingof buffer invertical (pit)emplacement
 Emplaced
buffer density
too low (large
gaps forblocks, lowbulk density ofpellets,
insufficientin-situcompaction,etc.) Piping anderosion during
operational
phase

Swelling pressure
provides a tight seal
between the buffer
and the drift wall

 Increased

 Minimum

mass transfer
acrossbuffer/rockinterface Potential forflow andradionuclidetransport along
this interface

swellingpressure toachieve atightbuffer/rockinterface Relationshi
p betweenswellingpressure andbufferdensity (willbe afunction of
GW
composition)

Canister sinking is
negligible

Sinking ofcanister throughlow-density
buffer could
lead to reduced
physicalprotection ofcanister by
buffer andlower buffertransport
distances forreleasedradionuclides

Minimum
buffer densitythat still
prevents
significant
canistersinking
Potential
measures: Long-term
experiment Modeling

Buffer provides aneffective colloid
filter; radionuclides
transported throughbuffer only as
solutes or gas

Colloidtransport
through buffer
gives reduced
limitation ofradionuclide
releases bybuffer

Minimum
buffer densitythat stillachieves
effective
colloidfiltration

A- 7

Range ofpossiblebuffer density
reduction due
to identified
potential
causes, e.g.
engineering
emplacement
experiment
under
realistic
undergroundconditions

IssueHigh average
buffer density(with respect
to designvalue)

Potential
causes inengineering
practice Emplaced

buffer density
too high
 Accumulation
due topiping/erosion
elsewhere Corrosion
expansion of
metals (e.g.

PA assumptions or
parameters
potentiallyaffected

Potential
effects on
systemperformance

Microbial activity is
low within thesaturated buffer

Reducedcanister lifetime

Minimum
buffer densitythat still
suppresses
microbial
activity

Transport in thebuffer is diffusiondominatedAdvective
transport gives
reducedattenuation ofradionuclide
releases bybuffer

Minimum
bufferhydraulicconductivity
that still
ensures
diffusion-do
minatedtransport;
hydraulicconductivity
as a functionof bufferdensity

Radionuclide
diffusion
parameters forbuffer based on
design buffer
density

Increasedradionuclide
diffusion rates

Diffusion
parameters as
a function of
buffer density

Buffer swellingpressure insufficientto perturb host rockby opening existing
fractures or creating
new fractures

Reducedattenuation ofradionuclide
releases during
geospheretransport Stress

A- 8

Information required toquantify effects
GeneralEngineering

distribution
in host rock Rockstrength Fracture
behavior
under highstress (byin-situ test)

Range ofpossiblebuffer density
increase due
to identified
potential
causes by e.g.
LTexperiment orIssuePotential
causes inengineering
practice

PA assumptions or
parameters
potentiallyaffected

Potential
effects on
systemperformance

PEM, canister)

Buffer swellingpressure insufficient
to lead to isostatic
collapse of canister

Reducedcanister lifetime(isostatic
collapse or
stress induced
corrosion)
N.B. canister
thickness isdetermined byradiolysis of thewater and somemargin exists
for stress failurein H12 case

Buffer sufficiently
plastic to protect
canisters against
small rock shear
movements

Reducedcanister lifetime

Relatively fast
saturation leading tohigh/moderate
thermalconductivity of
buffer

Low thermal
conductivity of
buffer resulting
in heatdegradation of
bufferproperties

Saturation
period ofhigh density
buffermaterials

 Gaps between Buffer transport

properties
homogeneous (orat least diffusiondominated in any
location) No regions oflocally reduced
density giving
advective
transport paths

Advective
transport paths
through buffer
give
reducedlimitationofradionuclide
releasesbybuffer Degree andbentonite
blocks Gaps at top oftunnel Localized
piping anderosion Heterogeneous
filling withpellets
 Difference indensity
betweenblock-filledandpellet-filledparts of buffer

Reducedcanister lifetime(collapse due tonon-uniform
loading)

Degree andrate ofhomogenizati
on of buffer Volume

reduction due
to tunnel
convergence(e.g. OPA
case)

Heterogeneous
buffer density(design value
may beachieved on
average)

Pressure on canister
is uniform(isostatic) following
buffer saturation

Buffer-rock interface may be perturbed

A- 9

Information required toquantify effects
GeneralEngineering Minimum

modeling

swellingpressurerequired for
isostaticcollapse Swellingpressure asa functionof bufferdensityrate ofhomogeniza
tion ofbuffer Possibilityofcontinuous
low density
and low
hydraulicconductivity
transport
paths

Range ofpossible local
buffer density
variations
due toidentified
potential
causes by,e.g. in-situ
emplacement
testIssuePotential
causes inengineering
practice

Physicaldisturbance ofbuffer/rockinterface(EDZ, rockspalling)

 Stress relief

Degradation oftunnel support(e.g. shotcrete,
liner)

 Tunnel

Cement/benton
ite interactions
andcement/rockinteractions

High-pHleachates from
cement in plugs
and seals,
concrete
flooring,
shotcreting, etc.

duringexcavation
 Long-term
developmentof EDZ due tohigh stress andcreep behavior
 Thermal-mechanicalimpact ofheat-emitting
waste

supportrequired undersomeconditions(e.g. soft rock)PA assumptions or
parameters
potentiallyaffected

Potential
effects on
systemperformance

Buffer/rockinterface treated ashighly conductive"mixing tank" (H12assumption may beover-conservative if
disturbance issmall; see Section
4.3.2)

Extent ofphysicaldisturbanceaffects rate ofradionuclidetransport acrossbuffer/rockinterface

Tunnel support will
not be a preferential
flow path

Due to thedegradation oftunnel support,
concretebecomes porous
and acts as aradionuclidetransport
pathway along
the tunnel

Physical andchemical propertiesof buffer and hostrock unperturbed bycement/rockinteractions

Similar toiron/bentonite
interactions(below) but notethat high-pH
conditions, if
they were toreach thecanister surface,
could befavorable interms ofcorrosion rate

A- 10

Information required toquantify effects
GeneralEngineering Rock stress Driftdistribution
 Thermal-mechanicalpropertiesof rock Local rateof buffersaturationandswellingpressurebuild-up
 Self-sealing
capacity ofrock Long-term
degradationofcementitiou
s materials
 Propertiesof degradedconcreteseparationand canisterpitch Use of rock
support Physicalextent andhydraulicpropertiesof disturbedzone General Reference

scientificunderstandi
ng of rates
andproducts of
cement/bentonite
interactions
andcement/rockinteractions
 Masstransport
properties
andlong-term
stability oftheinteraction
products

design(including
quantities
anddistributionofcementitiouscomponents)
 Likelihoodof removalofcementitiouscomponents

 Type andamount ofconcreteused fortunnelsupportIssueIron/bentonite
interactions

Potential
causes inengineering
practice

PA assumptions or
parameters
potentiallyaffected

Potential
effects on
systemperformance

Corrosion ofsteel handlingshell (PEM),
transportation
rails, liner, etc.

Physical andchemical propertiesof buffer and hostrock unperturbed
from interactions
with cement

Loss of swellingpressure could
give increased
mass transfer
acrossbuffer/rockinterface;potential forflow andradionuclidetransport along
this interface

Information required toquantify effects
GeneralEngineering General Reference

scientificunderstandi
ng of rates
andproducts ofiron/bentoniteinteractions
 Masstransport
properties
andlong-term
stability oftheinteraction
products

design(including
quantities
anddistributionof iron/steel
components)
 Likelihoodof removalof iron/steel
components

 Gas transport

 Gas

 Reference

pathways
could (if notresealed)
providepreferential
pathways forradionuclidetransport
 Delayed
repository
saturation
where hostrock relatively
tight could

generation
rate andmigration
behavior
 Generalscientificunderstandi
ng of gastransport in
the buffer(including
breakthroughpressures),design(including
quantities
anddistributionof iron/steel
components)
 Likelihoodof removalof iron/steel
components

Change to a lessplastic material
could
compromise theability of thebuffer to protect
the canisterfrom rockmovements
Change to a lessplastic material
with reduced
self-sealing
capacity could
lead to buffer
fracturing andreducedattenuation ofradionuclide
releases byaffected parts ofbufferEngineered barrier system may generate gas
Impact of
repository-gen
erated gas

Corrosion ofsteel handlingshell (PEM),
transportation
rails, rock bolts,
liner, etc.

No accumulation ofgas; physical andchemical propertiesof buffer and hostrock unperturbed bygas pressurizationand transport

A- 11

IssuePotential
causes inengineering
practice

PA assumptions or
parameters
potentiallyaffected

Potential
effects on
systemperformancedelay/limit
radionuclide
releases

Information required toquantify effects
GeneralEngineering
EDZ andhost rock Self-sealing
capacity ofthesecomponents
followinggas
breakthroughEngineered barrier system may interact with stray materials
Interactions
involvingother straymaterials

Stray materials
of interest to PA
include thosecontaining
nitrogencompounds,
such asammonium
nitrates andNOx species
injected into
rock during
blasting, andorganic
materials

No significant
interactions
involving otherstray materials

Organic
materials(including
materials suchas plastics,
cellulose,
hydraulic oil
and surfactants,
and also cement
additives
released as
cementdegrades) will
decompose andadd reducing
capacity to therepository nearfield
Organic
materials could
form complexes
withradionuclides
that would
lower
radionuclide
sorption andthus increase
radionuclide
release andtransport rates
Nitrogencompounds
could
potentiallydamage thecanister surfaces

A- 12Rate ofdecompositio
n of nitrogencompounds;
potential
complexes
and their
sorption
properties

Range ofpossiblequantities andcompositionsof straymaterials

A.2.3Significance of the issues from a PA perspective - example of issues
affecting buffer density

In order to evaluate whether potential deviations from the target value for saturated buffer
density are significant from a PA perspective, two questions need to be addressed:
 What is the potential range of deviation?
What is the range of values for buffer density consistent with the buffer performing its
safety functions?

The potential range of deviation can be assessed using detailed process modeling, simplified
scoping calculations or qualitative arguments, depending on the understanding that isavailable for the different potential processes leading to the loss or redistribution of buffer
mass.
Several scoping calculations have been performed for such processes in the course of thesafety assessment for a KBS-3H repository at the Olkiluoto site, Finland (Gribi et al., 2007;
Smith et al., 2007).
Figure A.3 shows, as an example, the results of scoping calculations of variations in buffer
density caused by piping and erosion during the repository operational period and subsequent
period of buffer saturation.

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Deposition of eroded material  2007 kg m-3??

Erosion by piping 
1995 kg m-3 if only one unit affected;
1990 kg m-3 if 2 units affected;
1985 kg m-3 if 2 units affected;…
1898 kg m-3 if 20 units affected

Figure A.3

KBS-3H: current reference design

Example layout adaptation of a KBS-3H repository at Olkiluoto (see Gribi et al.,2008; Smith et al., 2008).

In the current KBS-3H reference design, as analyzed in Gribi et al. (2007) and Smith et al.
(2007), each copper canister, with a surrounding layer of bentonite clay, is placed inperforated steel cylinders prior to emplacement in horizontal deposition drifts. The entire
canister/buffer/cylinder assembly is called the supercontainer. There is an initial gap between
each supercontainer and the drift wall, which becomes filled with bentonite as the buffer
swells. The supercontainers are separated one from another by relatively tightly fitting
bentonite distance blocks.
During operation of the drift and subsequent saturation, localized water inflow, which could
vary significantly along the drift, may potentially give rise to large hydraulic pressure
differences between the void spaces around neighboring supercontainers. This is because
water will enter these void spaces at different rates, depending on the local characteristics ofintersecting water-conducting fractures. The concern is that, if high pressure differencesdevelop too rapidly, this could result in transient water flows ("piping") along the interface,which could in turn lead to erosion and loss of bentonite density in some supercontainer
sections and possibly an increase in density in others.
The scoping calculations show how the saturated buffer density of the buffer in the drift
section where piping originates varies according to the (uncertain) number of downstream
supercontainer units affected. The results indicate that the average saturated density is:
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

1995 kg m-3, if only 1 supercontainer unit is filled with a water / bentonite suspension

1990 kg m-3, if 2 supercontainer units are filled1985 kg m-3, if 3 supercontainer units are filled1898 kg m-3, if 20 supercontainer units are filled.

In the downstream supercontainer units that become filled with the water / bentonite
suspension due to piping and erosion, the average saturated density of the buffer is increased
to 2007 kg m 3, irrespective of the number of filled units.
The issue of what is the range of values of buffer density consistent with the buffer
performing its safety functions has been addressed by SKB in the SR-Can safety assessment(SKB, 2006). SR-Can introduced the concepts of safety function indicators and safetyfunction indicator criteria. A safety function indicator is a measurable or calculable propertyof the system that is critical to a safety function being fulfilled. If the safety function
indicators fulfill certain criteria, then the safety functions can be assumed to be provided. If,
however, plausible situations can be identified where the criteria for one or more safetyfunction indicators are not fulfilled, then the consequences of loss or degraded performance ofthe corresponding safety function should be evaluated in PA. Safety function indicators for the
buffer include buffer density and also other properties that are related to density, namely
swelling pressure and hydraulic conductivity. The associated safety function indicator criteria
developed by SKB are shown in Table A.4.
Table A.4 Safety function indicator criteria related to buffer density (based on SKB, 2006).

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In PA, the buffer safety functions can be assumed to be provided as long as these various
criteria are satisfied. However, in order to evaluate whether or not processes such as piping
and erosion that give rise to buffer mass loss or redistribution can potentially lead to a failure
to meet the criteria on swelling pressure and hydraulic conductivity, the particular relationshipbetween these indicators and buffer density needs to be considered. This is complicated by thefact that swelling pressure and hydraulic conductivity are functions not only of buffer density,
but also of the ionic strength of the buffer porewater (see, e.g. Figure A.4) and so therelationships are both site-specific and potentially time-dependent.

Figure A.4

Relationship between buffer swelling pressure, dry density and ionic strength,
expressed as molar NaCl concentrations (after Figure 4-7 in SKB, 2006).

In the safety assessment of a KBS-3H repository at Olkiluoto, using these relationships andtaking into account the expected site-specific evolution of ionic strength over time, a range ofsaturated buffer densities was derived - 1890 to 2050 kg m-3 - that, if maintained, should
ensure that the buffer safety functions are provided, taking into account the evolution ofgroundwater and buffer porewater salinity until any future major climate change (Figure A.5;
see Section 5.3.2 of Smith et al. 2007 for details).
Based on the above-mentioned scoping calculations, it was concluded in the safetyassessment of a KBS-3H repository at Olkiluoto that the redistribution of bentonite by piping/ erosion in a KBS 3H deposition drift has no significant effect on the density of bentonite (i.e.
buffer density remains within the range 1890 to 2050 kg m-3), provided that only a few
supercontainer units are filled and provided local density changes are quickly homogenizedby the plasticity of bentonite.
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Swelling pressure1

If engineering or other issues are identified that could give rise to densities outside this range
and PA analyses show that the consequent loss of buffer safety functions may have an
unacceptable detrimental impact on safety, modified engineering designs may need to beconsidered. This is an example of feedback from PA to engineering design (Figure A.1).

10-12 ms-1

Hydraulic conductivity1

1750 kg m-3

2MPa
1 MPa

0.2 MPa
1650 kg m-3

1890 kg m-3

2050 kg m-3

Ensures protection of canister against rock shear
Prevents colloid-facilitated radionuclide transport
Prevents significant microbial activity
Avoids advective transport in buffer
Ensures tightness / self-sealing
Prevents significant canister sinking

Range of buffer densities
ensuring that all
PA assumption hold
Figure A.5

Deriving a range of buffer density that, if maintained, should ensure that all thesafety functions of the buffer are provided (typical PA assumptions hold).

Example from the safety assessment of a KBS-3H repository at Olkiluoto (seeSection 5.3.2 of Smith et al., 2007 for details).

References
Gribi P., Johnson L., Suter D., Smith P., Pastina B. and Snellman M. (2007): Safety
Assessment for a KBS-3H Spent Nuclear Fuel Repository at Olkiluoto – Process
Report; POSIVA 2007-09 and SKB R-08-36. Posiva Oy, Olkiluoto, Finland andSwedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, Sweden.
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SKB (2006): Long-term Safety for KBS-3 Repositories at Forsmark and Laxemar – A First
Evaluation – Main Report of the SR-Can Project; SKB Technical Report series
TR-06-09, Svensk Kärnbränslehantering AB, Stockholm, Sweden.
Smith P.A., Johnson L.H., Snellman M., Pastina B. & Gribi P. (2007): Safety assessment for aKBS-3H spent nuclear fuel repository at Olkiluoto - Evolution report. POSIVA 2007-08
and SKB R-08-37. Posiva Oy, Olkiluoto, Finland and Swedish Nuclear Fuel and Waste
Management Co (SKB), Stockholm, Sweden.

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原子力発電環境整備機構（略称：原環機構）
Nuclear Waste Management Organization of Japan (NUMO)