NUMO-TR-04-03

Development of Repository Concepts
for Volunteer Siting Environments

August 2004
Nuclear Waste Management Organization of Japan (NUMO)Inquiries about copyright and reproduction should be addressed to:
Science and Technology Department
Nuclear Waste Management Organization of Japan
Mita NN Bldg. 1-23, Shiba 4-chome, Minato-ku, Tokyo
108-0014 Japan
Phone +81-3-4513-1114 Fax +81-3-4513-1599©原子力発電環境整備機構 2004
Nuclear Waste Management Organization of Japan 2004

NUMO-TR-04-03

Preface
The primary objective of government policy, and of NUMO in implementing this policy, is toensure that a repository for Japan’s high-level radioactive waste is designed and located so as toprovide secure isolation of the waste and adequate safety for present and future generations.
This requires that the site of a repository has to be chosen carefully and an associated repository
concept is selected which is appropriate for its specific geological and environmental
conditions.
At NUMO, we have chosen to implement a volunteering approach to siting. This is constrainedby the use of “Siting Factors” which ensure that only locations which have sufficient geologicalstability are considered – an important factor in a country like Japan which lies in a tectonically
active region. The potential diversity of volunteer sites puts particular constraints on theprocess of repository concept development – requiring that we maintain a range of possible
options to ensure maximum flexibility to tailor this concept to the conditions found at aparticular site.
Our term “Repository Concept” includes not only the design and layout of the disposal systembut also the associated evaluation of operational and long-term safety and an assessment ofsocio-economic aspects. A particular challenge lies in the development of such concepts in an
open and transparent manner which allows all key stakeholders to become actively involved,
particularly the community hosting the facility and its neighbours. A logical structure allowing
systematic development of repository concepts has been established, taking into account theclose linkage between site information, repository design and safety assessment. We think it isvery important to identify such a logical structure from the outset of the siting programme,
which can be consistently applied to later site investigation stages.
This report outlines the background, principles and status of our repository concept
development work and discusses how it might progress during our staged siting andimplementation programme. A companion report (NUMO, 2004) discusses the development
and application of the previously mentioned Siting Factors.

iNUMO-TR-04-03

ii

NUMO-TR-04-03

Contents
1

INTRODUCTION........................................................................................................ 1
1.1

NUMO and its mission .................................................................................................. 1

1.2

The repository development programme and milestones .............................................. 3

1.3

Requirements for repository concept development ....................................................... 5

1.4

Timescales and long-term safety ................................................................................... 6

1.5

Overview of boundary conditions and structure of this report ...................................... 8

2BASIC PRINCIPLES................................................................................................. 12
2.1

Design philosophy ....................................................................................................... 12

2.2

Waste inventory........................................................................................................... 13

2.3

Repository infrastructure ............................................................................................. 14

3

TAILORING A REPOSITORY TO A VOLUNTEER SITE...................................... 20
3.1

Site requirements ......................................................................................................... 20

3.2

Illustration of the need for design flexibility ............................................................... 21

3.3

Tailoring of repository concepts to site characteristics – a bottom-up approach......... 23

3.4

Matching repository concepts to site characteristics – a top-down approach.............. 26

3.5

Tailoring of repository concepts - involvement of stakeholders ................................. 28

4

ENSURING SAFETY................................................................................................ 30
4.1

The importance of safety and confidence-building ..................................................... 30

4.2

Special safety concerns during construction and operation......................................... 32

4.3

Quantitative analysis methods for realistic assessment of long-term safety................ 34

4.4

Making and documenting the safety case.................................................................... 36

4.5

Monitoring and institutional control............................................................................ 37

5

DEVELOPMENT OF THE R&D PROGRAMME TO SUPPORTREPOSITORY CONCEPT DEVELOPMENT .........................................................40
5.1

Setting goals and priorities .......................................................................................... 40

5.2

R&D to provide the basis for repository concept development................................... 42

5.3

Focusing R&D for assessing volunteer sites ............................................................... 44

5.4

Long-term R&D programme priorities........................................................................ 45

6

THE FUTURE REPOSITORY CONCEPT PROGRAMME .................................. 49

7

ACKNOWLEDGEMENTS ..................................................................................... 50

8REFERENCES......................................................................................................... 51iii

NUMO-TR-04-03

APPENDIX 1REPOSITORY COMPONENT CATALOGUE ....................................... A1-1

A1-1

Introductory notes.................................................................................................... A1-1

A1-2

HLW designs ........................................................................................................... A1-1

A1-3

Safety-critical engineered barrier components ........................................................ A1-2

A1-3.1

Overpack.............................................................................................................. A1-2

A1-3.2

Buffer................................................................................................................... A1-4

A1-3.2.1

Buffer materials .......................................................................................... A1-4

A1-3.2.2

Buffer emplacement options....................................................................... A1-5

A1-3.2.3

Prefabricated EBS module (PEM) designs................................................. A1-6

A1-4

Additional engineered barriers with operational / post-closure safety roles............ A1-7

A1-4.1

Backfill ................................................................................................................. A1-7

A1-4.2

Liners .................................................................................................................... A1-7

A1-4.2.1

Liner material options................................................................................. A1-8

A1-4.2.2

Examples of liner applications.................................................................... A1-9

A1-4.3

Seals.................................................................................................................... A1-10

A1-4.4

Disposal pit cap .................................................................................................. A1-11

A1-4.5External engineered barrier................................................................................. A1-12

A1-5

Emplacement geometry ......................................................................................... A1-13

A1-5.1

Vertical .............................................................................................................. A1-13

A1-5.2

Horizontal .......................................................................................................... A1-14

A1-6

Underground layout............................................................................................... A1-15

A1-7

Disposal procedures............................................................................................... A1-17

A1-8

Surface facilities and underground access............................................................. A1-18

A1-8.1

Access routes from surface................................................................................ A1-18

A1-8.2

Surface facilities ................................................................................................ A1-19

APPENDIX 2

THE IMPORTANCE OF POST-CLOSURE SAFETY ASSESSMENT
IN REPOSITORY CONCEPT DEVELOPMENT................................... A2-1

A2-1

Decision making in stepwise development of the repository .................................. A2-1

A2-2

Definitions and general features .............................................................................. A2-2

A2-3

Making a safety case in the context of the Japanese programme ............................ A2-3

A2-3.1 Confidence in the intrinsic safety features of the disposal concept ....................... A2-4
A2-3.2 Safety assessment to illustrate robustness of the barrier functions........................ A2-5

iv

NUMO-TR-04-03

1

INTRODUCTION

1.1

NUMO and its mission

NUMO – the Nuclear Waste Management Organization of Japan – was established in October2000 on the basis of the Specified Radioactive Waste Final Disposal Act (the “Act”)
promulgated in June 2000. In this Act, we are given the clearly defined remit to develop a
project for the safe disposal of vitrified high-level radioactive waste (HLW). As indicated in
Figure 1-1, NUMO operates within a network of government and other relevant organisations
which have complementary responsibilities for the technical, legal, regulatory and funding
activities associated with HLW disposal in Japan.

Figure 1-1:

NUMO in the context of the Japanese HLW disposal programme.

Acronyms:
AEC: Atomic Energy Commission (http://aec.jst.go.jp/jicst/NC/eng/index.htm)
NSC: Nuclear Safety Commission (http://www.nsc.go.jp/english/english.htm)
METI: Ministry of Economy, Trade and Industry (http://www.meti.go.jp/english/index.html)
ANRE: Agency for Natural Resources and Energy (http://www.enecho.meti.go.jp/english/index.htm)
NISA: Nuclear and Industrial Safety Agency (http://www.nisa.meti.go.jp/english/index.htm)
MEXT: Ministry of Education, Culture, Sports, Science and Technology(http://www.mext.go.jp/english/index.htm)
NUMO: Nuclear Waste Management Organization of Japan (http://www.numo.or.jp/english/index.html)
JNC: Japan Nuclear Cycle Development Institute (http://www.jnc.go.jp/jncweb/02r-d/02index.html)
JAERI: Japan Atomic Energy Research Institute (http://www.jaeri.go.jp/english/index.cgi)
NIRS: National Institute of Radiological Sciences (http://www.nirs.go.jp/ENG/nirs.htm)
RWMC: Radioactive Waste Management Funding and Research Center (http://www.rwmc.or.jp/)
AIST: National Institute of Advanced Industrial Science and Technology (http://www.aist.go.jp/index_en.html)
CRIEPI: Central Research Institute of Electric Power Industry (http://criepi.denken.or.jp)
JNES: Japan Nuclear Energy Safety Organization (http://www.jnes.go.jp/english/index.html)

1

NUMO-TR-04-03

The Act restricts NUMO’s responsibility to the consideration of “specified” radioactive waste –which is defined to be HLW resulting from the reprocessing of spent fuel from commercial
power reactors.
The Act is also prescriptive in terms of the management option to be followed. It specifies deep
geological disposal in Japan at depths greater than 300 m below surface. Also specified are
requirements for geological stability and avoiding conflict with natural resources. The phased
process of developing a siting project, involving more detailed characterisation as options are
narrowed down, is also outlined in the legislation.
Following extensive discussions with many stakeholders, NUMO decided to take a rather
innovative approach to the siting process. Based on consideration of international experience inrepository siting, we recognised local acceptance as a critical issue which is essential to thesuccess of such projects. We thus decided on a policy of calling for volunteer communities thatwould be willing to consider hosting this facility.
We sent a call for volunteers to all 3,239 municipalities in Japan in December 2002. Of course,
potential host sites must also satisfy important geological and technical requirements.
Accordingly, the documentation accompanying the call included an explicit specification of
exclusion criteria which would be used to filter out clearly unsuitable sites1. These criteria werebased on the geological stability requirements noted in legislation and the associateddocumentation included maps showing areas that would be clearly ruled out – e.g. due to risk ofvolcanic activity or the presence of major active faults. Even if a site is not excluded by such
criteria, implementing a repository may be impractical or other volunteers may be moresuitable. A range of other selection factors (“Favourable Factors”) have thus been defined
which will allow the staged siting process to proceed in a transparent manner (described indetail in NUMO, 2004).
We have been accumulating an inventory of relevant resources of information and experienceavailable in Japan and via our international partners. This forms the starting point for
identifying and prioritising further Research and Development (R&D) which we will need toinitiate in order to establish a Japanese repository project. The present report comprises areview of the status in one particular area – the development of “Repository Concepts” (see thebox in Section 1.3) appropriate to the siting environments defined during the volunteering
process. This term “Siting Environment” includes not only the geological setting but also allother site features which influence repository implementation – such as geography, topography,socio-economic constraints and transportation infrastructure.
A companion report (NUMO, 2004) describes the Siting Factors for the staged sitecharacterisation and evaluation process and places them in the context of the geological settingof Japan. In practice, the repository design and site characterisation processes are strongly
coupled and develop in an iterative manner, with safety assessment during the various stages
leading up to repository implementation.
These reports reflect NUMO’s commitment to a policy of openness in its dealings with allinterested parties. More extensive Japanese language reports covering the same topics are
1

English translations of the documents supporting the call for volunteers, “Information Package” can be ordered
via the NUMO website (www.numo.or.jp/english/what/index.html).

2

NUMO-TR-04-03

aimed at the potential volunteer communities and relevant organisations in Japan. There is thus
a particular focus on the technical detail to support the call for volunteers. It should beemphasised that the English language documentation is not a translation, but is completely
reworked to provide the background needed to enable a productive exchange of views with aninternational technical audience.
The Japanese report on repository concepts provides an overview of the status of international
thinking on the fundamental concepts supporting development of a geological repository
programme, covering for example:
Stepwise implementation;
Components of a safety case;Reversibility and retrievability;
Ethical concerns.
Such a review is not included in this document, but all such issues have been taken into accountin the development of our programme, as described in the following sections.

1.2

The repository development programme and milestones

Japan has an active nuclear power programme which includes an installed capacity of about 46
GW(e). Plans exist to expand nuclear power generation further and to extend the life of nuclear
power plants based on careful safety review and maintenance (AEC, 2000). This strategy isdesigned to maintain the stable power production required by an industrial nation, while
complying with requirements to reduce emissions of greenhouse gases from the combustion offossil fuels. The decision to reprocess spent fuel reflects this long-term vision of nuclear power
generation, which results from Japan’s lack of significant resources of uranium and its
commitment to sustainable development, which encourages all such recycling activities.
Presently, spent fuel from Japanese reactors is reprocessed in the UK or France and vitrified
HLW returned to Japan for interim storage in a special facility at the Rokkasho site of Japan
Nuclear Fuel Ltd. (JNFL) as shown in Figure 1-2(a). On a pilot scale, fuel has also been
reprocessed and HLW produced by JNC at Tokai (Figure 1-2(b)). In the near future, a full-scale
reprocessing plant will become operational in Japan – also run by JNFL at Rokkasho.
For a continually developing programme, establishing a time plan for implementation requires
a balance to be struck between various technical and socio-economic considerations. For
example, the technical advantages of storing reprocessed waste prior to disposal for as long as
possible, in order to reduce radiogenic heat output, needs to be balanced against the ethical goalthat the generation benefiting from nuclear power should also implement, or at least initiate,
projects to ensure safe management of resulting wastes. Recently, another argument for
accelerated implementation of deep geological disposal projects has focused on national
security, due to the reduced vulnerability of underground facilities to possible terrorist threats.

3

NUMO-TR-04-03

Figure 1-2(a):JNFL Vitrified Waste Storage Center (left) and HLW storage pits (right)(photos: courtesy of JNFL).

Figure 1-2(b):JNC Tokai Reprocessing Plant (left) and HLW storage pits (right) (photos:
courtesy of JNC).

In Japan, the planned development programme for a first HLW repository is outlined in
Figure 1-3. This is based on an assumed capacity of 40,000 waste packages each containing 150
litres of vitrified waste, corresponding to reprocessing of all spent fuel expected to be produced
up to 2020. The vitrified HLW will be stored for 30-50 years before disposal (MITI2, 2000a).
According to the Final Disposal Plan (MITI, 2000b), repository operation would start in the late
2030s, with an annual emplacement of 1,000 canisters of vitrified HLW.
Following our call for volunteers, the siting process is planned to proceed in a staged fashion.
The first stage involves use of literature information to determine the basic suitability of eachvolunteered site in the light of established exclusion criteria, and to rank sites, in the event that
there are more volunteers than can go through to the second stage of field characterisation. Suchranking may also be useful to set priorities and help plan the strategy for subsequent sitecharacterisation, repository design selection and associated performance assessment.
The initial field characterisation of “Preliminary Investigation Areas” (PIAs) is restricted towork carried out from the surface. Following this stage, sites are again compared and a reduced
2

Ministry of International Trade and Industry (now METI).

4

NUMO-TR-04-03

number carried on as “Detailed Investigation Areas” (DIAs), in which an undergroundcharacterisation facility is constructed to allow the deep geological environment to be examinedin more detail. According to this plan, a site would be selected, the associated repository
concept specified and the licensing process initiated in the late 2020s, allowing construction tocommence around 2030 with first waste emplacement in the late 2030s. The time plan shown in
Figure 1-3 assumes that, to some extent, construction, waste emplacement and sealing of
emplacement tunnels proceed in parallel and emplacement operations are completed by themid- to late 2070s. As discussed later in this report, there are a range of other possible options.
We also leave open a possible period of monitoring and institutional control, with final closure
thus being sometime after 2080.

Figure 1-3:

1.3

The staged repository development programme and possible milestones. The
time plan after site selection has a model nature as it will depend, to someextent, on the repository concept selected.

Requirements for repository concept development
Box 1
Definition of a Repository Concept

We define a “Repository Concept” as a conceptual design of all surface and undergroundrepository structures tailored to a given siting environment, along with a description of how
the repository can be constructed, operated and sealed. This also includes an evaluation ofoperational and long-term safety and an assessment of socio-economic impacts. The concept
is dynamic, evolving with our programme as it moves from early generic studies through tositing and, eventually, licensing for construction and operation. Indeed, continual evolution
during the operational period is also possible, as experience is gained and technology
develops.
The key components of the Japanese HLW disposal concept are based on work carried out overtwo decades by a range of R&D organisations, led by JNC, before NUMO was established.

5

NUMO-TR-04-03

This work, most recently summarised in the H12 reports (JNC, 2000), involved a genericevaluation of the requirements for a safe repository in the types of rocks and geologicalenvironments expected to be found in Japan. The basic repository design developed by JNC
involved sealing vitrified waste in a thick steel container (the “overpack”), which is surrounded
by a highly compacted bentonite layer (the “buffer”). A number of variants of the emplacementlayout and the materials involved were examined in the H12 study, but there was no attempt at
optimisation to improve operational practicality or to match the exact conditions to be expectedin site-specific host rock environments.
Since NUMO was established, we have attempted to build on these foundations in order todevelop designs appropriate to the diversity of particular sites which may result from thevolunteering process. To do this, we have had to review the earlier designs carefully,
particularly with regard to practicality of implementation under the conditions of a working
repository and the strict operational safety and quality assurance requirements involved in any
nuclear facility. This review has been extended to cover repository concepts which have been
considered in other national programmes, as discussed in Section 2.3. Our decision to adopt a
volunteering approach to siting results in further boundary conditions being necessarily left
open – in both technical (e.g. area and volume of suitable rock) and socio-economic areas (e.g.
extent of monitoring for confidence-building, institutional control, ease of retrieval).
The repository concept will develop in a staged manner – taking a range of repository design
options (RDOs) into account and becoming more detailed as siting environments become better
defined during the volunteer siting process. As the repository concept both requires input from,and provides guidance for, the site characterisation work, these efforts will be closely
co-ordinated. This approach is presented in Figures 1-5 and 1-6 in Section 1.5.

1.4

Timescales and long-term safety

Japanese regulations specifying the long-term safety requirements for a HLW repository anddefining how compliance with these requirements will be assessed are only now in the processof being drafted. Presently, we assume that regulatory guidelines in terms of dose and risk
criteria and assessment timescales will be similar to those already established in a number ofother countries. In all cases, the regulations and the corresponding safety concepts need to take
into account the properties of the waste under consideration.
Figure 1-4(a) indicates that the period during which HLW can be considered to present a special
hazard (in terms of its total radioactivity) compared to the original ore is in the order of tens of
thousands of years. The radioactivity – and also the toxicity and heat output – of vitrified
reprocessing waste is initially dominated by relatively short-lived radionuclides and decreases
rapidly with time. After surface storage for 30 – 50 years, the most active nuclides remaining in
the waste have half-lives in the order of decades and the radioactivity of emplaced waste dropsby about 3 orders of magnitude over the first thousand years (see also Section 2.2). During thistime, it is intended to completely isolate the waste and this led to the specification of a 1,000
year minimum overpack lifetime in H12 (JNC, 2000).
After a thousand years, the rate of decrease of radioactivity and radiotoxicity is less rapid.
Nevertheless, after a few thousand years, the toxicity of the vitrified waste, based on theconcentration limits which are specified in the current regulation for existing nuclear facilities

6

NUMO-TR-04-03(STA3, 1998; amended in 2000), has decayed to about the level of the uranium ore used to
produce the initial reactor fuel (Figure 1-4(b)). This is compatible with the periods in the orderof several tens of thousands of years considered as a minimum requirement for our assessmentof site stability. Such a period can also be considered as the main focus for the demonstration ofrobust performance of the natural and engineered barriers.
Inevitably, assessments of performance become more uncertain with increasing time ofextrapolation. Our quantitative analyses will, nevertheless, be extended into the distant future
and attempt to, at least, bound the maximum consequences. Because the toxicity of the waste atsuch times has decayed to a level similar to natural geochemical anomalies, such formal
analyses can be supported by more qualitative analogue arguments.

Figure 1-4(a):3Characteristics of high-level waste from the viewpoint of evolution ofradioactivity (corresponding to 1 MTU of 4.5% enriched fuel) (N.B.: L/ILW
arisings from reprocessing are not considered.) (JNC, 2000).

Science and Technology Agency (now MEXT).

7

NUMO-TR-04-03

Figure 1-4(b):1.5

The decay in radiotoxicity of vitrified high-level waste with time after
manufacture. It can be seen that, while fission product radionuclides
dominate radiotoxicity over approximately the first 100 years, actinide
radionuclides and their daughters predominate thereafter. After about 3,000
years, the total radiotoxicity declines to the same level as that of the naturaluranium ore used to manufacture the equivalent amount of fresh fuel fromwhich the HLW was produced. The data are for one container of JNFL vitrified
waste (with 4 years spent fuel cooling before reprocessing and immediate
HLW vitrification) and the uranium ore required to produce 0.8 MTU of 4.5%
enriched UO2 fuel.

Overview of boundary conditions and structure of this report

The constraints on, and input to, the NUMO programme as presented above are summarised in
Figure 1-5. Thus, the focus on geological disposal defined in the Act means that NUMO does
not investigate any alternative management options (such as indefinite storage or partitioning
and transmutation). The Act also defines “specified waste” as HLW from reprocessing and thisis where all NUMO’s efforts are concentrated. The specification of an inventory of at least
40,000 waste packages is seen as a sensible goal (covering a major part of the wastes producedby the current generation of nuclear power plants). A 40,000 package inventory is thus used asa planning basis, but flexibility for optimised utilisation of a suitable site is explicitly
considered.
The specification by the Act of staged implementation is reflected in our process of initiallyselecting several PIAs which are then reduced to a few DIAs before a final repository site is
selected. NUMO’s decision to proceed with a volunteering approach was driven by our
8

NUMO-TR-04-03

recognition of the great importance of public acceptance. Taken together, these constraints
explain why we have adopted an approach that aims to maximise the flexibility to tailor
repository concepts to the diversity of suitable siting environments which may be found and toencourage involvement of local communities in decision-making.

Figure 1-5:

Constraints on the NUMO repository concept development programme.

The rest of this report is concerned with the more technical aspects of our preliminary
investigations to develop appropriate repository concepts, to tailor these to specific volunteer
site conditions, to assess their safety and to define the required supporting R&D programme.

9

NUMO-TR-04-03

The key features involved are illustrated in Figure 1-6. The starting point is set by overtwo decades of R&D experience, which was summarised in the H12 project (JNC, 2000). The
geology of Japan places special constraints on site selection and an explanation of how these
affect NUMO’s approach is the focus of the companion “Siting Factors” report (NUMO, 2004).

Figure 1-6:

Repository concept development during the staged site characterisation
process.

10

NUMO-TR-04-03

Chapter 2 expands on the basic principles involved in developing appropriate repository
concepts for any suitable volunteers that come forward and introduces the concept of the design
factors filter. It also provides a very general overview of the characteristics of the waste to bedisposed of and the main features of the H12 concepts.
Chapter 3 focuses on the process of tailoring repository concepts to the environments found at aparticular volunteer site and, in particular, on how to involve all relevant stakeholders in thisprocess. Chapter 4 considers the critical aspect of safety - with special concentration on thetimescales of most concern to the volunteer community. From the basis thus provided,
Chapter 5 outlines the planned supporting programme of R&D.
Finally, Chapter 6 outlines how we expect the programme to develop as volunteers arecharacterised in increasing detail by desk and field studies and the site-specific repository
concepts are iteratively optimised for the particular environments involved.
To improve the flow of text, the various repository components which are combined in order toproduce RDOs are illustrated in Appendix 1. In contrast to repository design studies in othernational programmes, this report places particular focus on operational phase safety.
Post-closure safety is, nevertheless, a key consideration which cannot be compromised and is a
special focus for Appendix 2.

11

NUMO-TR-04-03

2
2.1

BASIC PRINCIPLESDesign philosophy

At NUMO, we recognise the importance of establishing and maintaining a safety culture (seealso Chapter 4). The focus on safety – and associated quality – should permeate NUMO and all
associated partner organisations and contractors. As an implementing organisation charged
with developing an economically feasible project, we also have to carefully considerpracticality and costs (which are, ultimately, borne by the Japanese public as electricity
consumers). It should be emphasised, however, that we do not see a conflict between ensuringsafety and optimising costs.
Our design philosophy is reflected in a set of general principles and objectives for the repository
design process, which could be regarded as embodying engineering “good practice”. In ourparticular case, these are worth explicitly formulating because of our particular boundary
conditions, which include:
Lack of precedent for such a project;
Very long planning and implementation timescales;
Very long times over which passive safety must be demonstrated;
As yet undefined, in principle stepwise, licensing procedure;
Requirement for continuous public acceptance.
The foundations of this philosophy are:1) Advance planning: Pre-emptive planning is essential; safety-relevant issues are identified
as early as possible and steps taken to avoid the risk, or mitigate the consequences, of even
improbable events. Given the timescales of decades involved in this planning, a rigorous
system for requirements management (NEA, 2004) will be implemented to provide a traceof all key planning decisions.
2) Keep it simple: We aim for simplicity, with minimisation of the number of individual
processes and components, bearing in mind that the emplacement procedure has to copewith 1,000 waste packages per year for 40 years (MITI, 2000b).
3) Robust design: Complementing the aim for simplicity, we aim for robustness by focusing
on well tried and tested materials, processes and technology, including large safetymargins and ensuring that equipment and procedures “fail safe” under all expected
perturbation scenarios.
4) Integrated Safety & Quality Management: Designs of equipment and procedures include
controls and checks to ensure that specified guidelines are adhered to at all times.
5) Operational zoning: As construction activities may run in parallel to handling of highly
radioactive materials, establishment of clear zoning will act to constrain activities inparticular areas and prevent any detrimental interactions between them.
6) Minimise degradation: As the geological barrier plays a key role in the safety case (seeChapter 4), we will develop procedures to avoid (or minimise) risk of degradation ofsensitive areas (including the emplaced engineered barrier system (EBS)) during or after
construction and operation of the repository.

12

NUMO-TR-04-03

7) Human factors: The repository development and operation timescale is such that several
generations of staff will be involved, requiring special consideration of human interfaces
in safety-critical operations.
8) Stakeholder involvement: Acceptance is essential to such a project and hence we envisagean active process, involving both openness and transparency in all our work and also
working together with all stakeholders, including local communities, to develop
consensus on key issues.
9) Economic constraints: It is fundamental to the principle of sustainability that invested
effort (cost) should be appropriate to the problem and resources should not be squandered.
Thus, cost / benefit analyses are carried out throughout the planning process.
Such principles are not absolute, but form a useful checklist to ensure that all critical factors arebeing considered during the development and documentation of repository concepts for
specific siting environments.
In addition to setting out these practical principles, appropriate organisational structures will beestablished within NUMO to ensure that these principles are implemented. Such management
structures include clear allocation of responsibilities and short chains of command.

2.2

Waste inventory

The wastes considered for disposal are vitrified high-level radioactive residues from
reprocessing of spent fuel from commercial power reactors. They include wastes returned from
reprocessing overseas, by BNFL in the UK and COGEMA in France. In addition, wastes will
result from the JNFL reprocessing plant which is planned to begin operations soon at Rokkasho.
A summary of the wastes which are expected to arise from reactor operations up to about 2020
is given in Table 2-1 and an indication of the typical waste properties in Figure 2-1. It is clearthat the inventory has a model nature and that the characteristics of waste produced over the
next couple of decades will reflect variations in reactor operations (e.g. fuel burn-up) andreprocessing / vitrification technology.
Table 2-1:

Waste arisings (BNFL, COGEMA, JNFL & JNC reprocessing plants).

BNFL

COGEMA

JNC

JNFL

Number of HLW canisters already616
130
produced and placed in storage (as of(at JNFL)(at JNC Tokai)Jan. 2003)
Number of HLW canisters equivalent
to the spent fuel presently waiting to~1,600~970~13,300
be reprocessed
Additional planned arisings~23,400
Totals~2,200~1,100~36,700
N.B.: Based on the current programme, the total amount of vitrified waste is expected to reach ~30,000
canisters around 2013 and ~40,000 canisters around 2020 (MITI, 2000b).

13

NUMO-TR-04-03

Figure 2-1:

2.3

HLW properties (type: JNFL; cooling time before disposal: 50 years):Radioactivity and heat generation per canister of waste as a function of timeafter disposal.

Repository infrastructure

A repository basically consists of facilities for:Construction and maintenance of underground emplacement tunnels and / or boreholes,
including power, drainage, ventilation, etc.;
Reception, handling, temporary storage and packaging of vitrified waste containers;
Transportation and emplacement of waste underground;
Construction of other required engineered barriers;
Monitoring and assuring security of operations.
As a medium-sized industrial facility, the repository requires a significant amount of additional
infrastructure for the administration and support of the workforce involved.
In principle, there are two distinct sets of operations – those associated with construction (andclosure) which do not involve any interaction with waste and those which involve the strict
protection measures associated with the handling of radioactive material. It is generally
expected that construction, waste emplacement and sealing of disposal tunnels will run – to atleast some extent – in parallel.
Operationally, waste emplacement would involve remote handling activities in
restricted-access zones. The extent to which all subsequent backfilling / sealing can be (orneeds to be) done remotely may depend on the details of the design.
The long-term safety of a repository is assured by a combination of engineered and natural,geological barriers. Studies carried out over the last couple of decades have shown that, underthe boundary conditions set by various national programmes, many different combinations of14

NUMO-TR-04-03

waste type / engineered structures and geological settings can provide high levels of safety(Figure 2-2).

Figure 2-2:

Repository design variants considered in feasibility studies by different
national programmes (ANDRA, 2001; Gierszewski et al., 2001; Nagra, 2002;
ONDRAF/NIRAS, 2001; Posiva, 1999; SKB, 2001; U.S. DOE, 2002): These
options are not yet finalised and may be modified significantly before therepositories are implemented.

15

NUMO-TR-04-03

The key components of the H12 EBS developed by JNC are a massive steel overpack and a
thick buffer of compacted bentonite / sand (JNC, 2000). These barriers were chosen to provide
high performance, with a highly conservative design using well-known materials. Figure 2-3
illustrates the main features of this EBS and indicates the multiple processes which work
together to provide a robust isolation system.

Figure 2-3:

H12 engineered barrier system: components and potential roles (N.B.: not allthese barrier roles were analysed in the H12 study).

In a stable geological environment, such barriers can ensure that radionuclides are completely
contained for the period until the radiogenic heat has dropped to negligible levels and total
radioactivity has decayed to a small fraction of its original level. Although the other engineered
and natural barriers could efficiently retain the short-lived radionuclides decaying over thisperiod, complete containment within the overpack over the period when there are thermal,hydrological and mechanical transients (which are coupled in the near field) greatly simplifies
the resultant safety case – i.e. contributes to robustness.
Thereafter, following failure of the overpack, extremely slow dissolution of the waste will leadto eventual releases of long-lived radionuclides from the EBS, but the concentrations will be so

16

NUMO-TR-04-03

low that there is no significant radiological hazard. A large proportion of the activity will decay
completely within the EBS (and most of the rest within the geological barrier - see Figure 2-4).

Figure 2-4:

Evolution of the distribution of all radionuclides in different systemcomponents as a function of time for the H12 Reference Case (after Makino etal., 2002). The curve labelled EBS represents all EBS components (Figure 2-3)
excluding the glass matrix, which is shown separately.

The characteristics of EBS materials and alternatives (e.g. Cu-Fe or Ti-Fe composite overpacks,
various bentonite / sand buffer compositions) were extensively reported in the H12 project.
Their behaviour and evolution with time is well supported by extensive laboratory studies,mechanistic modelling and natural analogues. An area which is less well defined, however, isthe practicality of construction of such an EBS under strict quality assurance controls in an
operational repository environment (considering underground conditions of restricted space,humidity, emplacement rate, remote handling, operational safety, robustness to perturbations,etc.).
In addition to these primary engineered barriers, a number of other repository structures may
have barrier roles – e.g. tunnel liners, borehole caps, backfilling, plugs and seals for tunnels,
ramps and shafts. As yet, the performance of such structures and their possible interactions witheach other (and the primary EBS) have not been examined at a detailed level.
At NUMO, we have taken the decision to focus our efforts at present on the basic EBS
components illustrated in Figure 2-3, with emphasis on improving our understanding of theareas of uncertainty noted above, rather than looking at a wider range of variants. This
concentration of resources is justified by the extensive national and international experience
with these particular EBS components in the planning of vitrified HLW disposal concepts indifferent host rocks. In terms of exact design, layout and operational procedures, this basic

17

NUMO-TR-04-03

system also allows a wide range of implementation options, which can be tailored to specificgeological boundary conditions or socio-economic requirements.
Even with this purposely narrowed range of EBS components, there are many different ways inwhich H12 and international experience can be combined to develop repository concepts. As an
aid to developing designs, we have catalogued the various components of the repository(Appendix 1). This catalogue illustrates how, starting from the fixed point of the waste to bedisposed of, a basic design for the repository system can be assembled by choosing between
various options. Although, individually, there are no components included which arecompletely novel, specific combinations might be derived which have not been explicitly
studied before.
As previously mentioned (see Figure 1-6), we have developed a strategy for iterative
development of this basic repository concept as the process of site selection andcharacterisation progresses. Note that, although long-term safety is an essential requirement of
all designs, we also take explicit account of the principles previously discussed. In order toassemble an RDO from the various components, we examine a set of “design factors”, each ofwhich addresses an issue bearing directly on the chosen design:
Long-term safety: robustness of the post-closure safety case;Operational safety: conventional and radiological safety of construction, operation,
closure and decommissioning;
Engineering feasibility / quality assurance: fundamental feasibility of construction andoperation to defined quality levels;
Engineering reliability: practicality of implementation in view of boundary conditions(e.g. emplacement rate) and robustness with regard to operational perturbations;
Site characterisation / monitoring: effort required to satisfy technical requirements forsite characterisation and monitoring data;
Retrievability: ease of waste package retrieval after emplacement;
Environmental impact: extent of all environmental impacts associated with repository
implementation;
Socio-economic aspects: factors contributing to costs and acceptance by all key
stakeholders.
As an example, the reference H12 engineered barriers illustrated in Figure 2-3 can be examinedin terms of these factors. Referring to options listed in Appendix 1, some areas where design
improvements are possible could be:
Placing several vitrified waste packages in a single overpack. Engineering feasibility
should not be greatly influenced as designs containing 2 or 3 containers of HLW are
similar in terms of dimensions, weight and thermal loading to particular overpacks
studied for the direct disposal of spent fuel (e.g. SKB, 1999; Nagra, 2002). Reductionin the number of emplacement operations could ease engineering practicality and, if
the repository could be made more compact, site characterisation requirements andenvironmental impacts could be reduced. Such variants could reduce costs, but a more
rigorous analysis would be needed to ensure that there was no detriment to either
long-term or operational safety.

18

NUMO-TR-04-03

Prefabricating the main components of the EBS. The difficulty of quality assuring the
EBS and, in particular, handling compacted bentonite under high humidity conditions
has indicated that prefabricated EBS modules (PEMs) can improve engineering
practicality (noted as an option in H12; JNC 2000). SKB and Posiva are testing such a
concept at present (Lindgren et al., 2003) and past work in the USA indicates that suchdesigns may also be easier to retrieve (Apted, 1998).
Selecting designs and materials for secondary engineering structures / barriers (e.g.
liners, plugs, grouts) which minimise the risk of perturbations to the primary EBS(NUMO and Posiva, 2004); this contributes to the robustness of the long-term safetycase, but potential consequences for operational safety need to be carefully evaluated.
Variable layouts to make the best use of available host rock; NUMO has alreadyextended the H12 vertical emplacement concept, which considered only a single waste
package in a disposal pit, to “multiplex” options in which 2 or 3 packages are stacked
in a single pit. In principle, this can be extended further to longer boreholes containing
many waste packages or, indeed, deep boreholes drilled from the surface (e.g. SKB,
1992; Deutch et al., 2003). Similarly, horizontal emplacement panels can be stacked tomake better use of a thick host rock formation or extended as long tunnels to utilise aformation with limited access (e.g. an underwater formation accessed from land). Forvery large, multiple waste container overpacks, a cavern disposal option could beconsidered. Such layout options have clear pros and cons in terms of the design factors
above, but a detailed evaluation to allow their direct comparison can only be done on asite-specific basis.
As considered further in the next chapter, details of the design and layout of the undergroundstructures can be specified to take into account the detailed geological environment
encountered and the requirements of stakeholders. However, given that operation willcommence only in 30 years time and will extend over ~4 decades, it is not sensible to beover-specific about such details at present. It is important that options are kept open to benefit
from technical developments over this long period. Nevertheless, it is valuable to consider
likely designs appropriate to expected geological environments in order to identify R&D needs
and priorities and to initiate discussions with relevant stakeholders.

19

NUMO-TR-04-03

3 TAILORING A REPOSITORY TO A VOLUNTEER SITE
3.1

Site requirements

For all potential sites, post-closure safety is an absolute requirement which will be continuallyassessed as the characteristics of the site and the detail of the repository design become better
defined (see Chapter 4). Although such assessments are associated with great uncertainties,
especially for long times in the future, there are generally accepted methods for carrying theseout in a conservative manner. The key characteristic is, however, that levels of safety for aparticular concept at a specified site should clearly meet established standards and guidelines.
Unlike the engineered barriers, which will be designed by NUMO, the characteristics of thehost rock formations are set by the geology of the volunteer sites. A minimum requirement on
the geological environment is that it provides isolation from the dynamic surface environment
and physical stability – which together help ensure the longevity of the EBS. The H12 study(JNC, 2000) – in agreement with many studies in other countries (e.g. those shown in Figure2-2) – has shown that negligible radiological consequences can be assured for geologicalperiods of time for a wide range of different host rock formations. Characteristics of the hostrock formations which increase confidence in EBS longevity include mechanical stability, lowgroundwater fluxes and velocities and geochemical compatibility (for example reducing
conditions, near-neutral pH and low salinity).4
In addition to providing an environment that ensures good long-term performance of the EBS,
the many hundreds of metres of rock between the repository and the biosphere can provide an
extremely powerful natural barrier. In some rocks, advective groundwater flow is so low thatsolute transport is dominated by diffusion processes. In such cases, the migration of mostradionuclides from the repository to the surface may be negligibly slow. Even if groundwaterflow is more rapid, long transport paths and the interaction of dissolved radionuclides with thesolid rock matrix may greatly extend transport times to the biosphere and hence the extent ofradioactive decay en route.
In addition to processes which cause delays in release and associated decay of radionuclides,
dilution processes should also be considered. Although not commonly regarded as a barrier
function, the spreading of releases in time and the dilution of small water fluxes through deep
rocks in the larger near-surface or surface water flows tend to further reduce the concentrationof radionuclides and hence also their radiological significance.
Favourable geological features – along with other geographical, surface infrastructure andsocio-economic aspects – are discussed further as “Favourable Factors” for siting (NUMO,
2004). Already in the Act, some minimum requirements for an acceptable site are specified,with emphasis on long-term geological stability (NUMO, 2004). In terms of the practicality of
construction and operation, there are other constraints on a suitable site. In particular from theviewpoint of safety during construction and operation, there are limits to the extent to which
difficult construction conditions (e.g. low rock strength, high stress or stress anisotropy, highwater inflow, etc.) can be surmounted. Even if safe construction is possible, there may beconsiderable cost penalties.
4

N.B.: Host rocks such as salt that are virtually free of groundwater are clearly advantageous here – although they
may have their own problems (potential resource, unstable in some perturbation scenarios). In any case, no major
deposits of evaporites are found in Japan.

20

NUMO-TR-04-03

3.2

Illustration of the need for design flexibility

At the first stage of evaluating potential repository concepts at volunteer sites that satisfy thegeological stability requirements, NUMO will place particular emphasis on the design factors
relating to safety and engineering practicality (see Section 2.3). Criteria specifying minimum
levels of practicality and safety will be set for relevant geological formations and, if these areclearly not met, NUMO would reject such a site (Figure 3-1). We thus have no intention oftrying to “engineer around” problems at a location which is fundamentally unsuitable for thepresently considered disposal technology. We accept that, as technology develops in the future,
currently exotic concepts might become practical (e.g. rock melting by very hot waste
packages) which could allow such sites to be considered, but such speculative considerations
are outwith our present programme.
Given the limited amount of data which may be available at the literature study and PIA
characterisation stages, it is possible that a clear decision that a given site is unsuitable cannot
be made. If the open questions or uncertainties at a particular site are larger than in other cases,
the decision on whether to invest effort in site characterisation work or detailed evaluation of
the limits of existing repository technology may be left open until higher priority options havefirst been examined.
There are also repository design limitations which may be set by the available area of usable
rock. Apart from fundamental limitations defined by the extent of the geological formation,
there are still uncertainties about what constraints, if any, will be placed on the projected area ofthe underground repository structures at the surface – termed the “footprint”. Although thelegal situation is being investigated, there may be particular boundaries set by the geography ofthe volunteer community or by specified restricted areas (e.g. national parks, population
centres). Together with the structure of potential host formations, this may considerably limit
the available area and require consideration of high emplacement density designs.
As indicated by Figure 3-1, there is a lot of flexibility in the concepts which can be assembled
from the layouts illustrated in Appendix 1 to respond to even rather severe constraints on the
usable volume of rock. At some point, however, if the area is too small – or divided into too
many sub-units – to make repository implementation for the target inventory a practical option,
we would again reject the site.
Even at this conceptual level, it has to be recognised that design variants have consequences interms of the repository concept as a whole. For example, high density emplacement options thatminimise the repository footprint will inevitably require careful consideration of the effects ofincreased thermal loading. Again, there are several options to address potential problems –increasing surface storage time prior to disposal, accepting higher temperatures in the EBS,
keeping the bentonite dry within a PEM during the time of enhanced temperature or postponing
backfilling of the emplacement tunnels (Figure 3-2).

21

NUMO-TR-04-03

Figure 3-1:

Illustration of evaluation of constraints on a site and resultant design variants.

A key factor here is requirements management – ensuring that the justification for choosing any
approach is clearly recorded. In this example, if the requirement is specified as keeping
bentonite below the very conservative (and rather arbitrary) limit of 100°C set in H12, then theincreased surface storage or postponed backfilling options would directly limit the thermalloading and hence the temperature rise in the bentonite. However, if more recent work on thethermal stability of bentonite can be taken into account, then options with a higher maximum
temperature requirement - e.g. allowing the thermal loading to increase to 130-150°C - might
be acceptable. If the requirement is specifically focused on bentonite stability, credit for yethigher thermal stability of dry bentonite could be accepted at the cost of ensuring that the
bentonite can be ensured to remain dry (e.g. PEM shell would have sufficient longevity) under
the resultant higher temperature conditions.

22

NUMO-TR-04-03

Figure 3-2:

3.3

Illustration of options which address high thermal loading concerns forcompact repository designs.

Tailoring of repository concepts to site characteristics – abottom-up approach

As illustrated previously in Figure 1-6, NUMO is carrying out preliminary investigations to
examine how generic concepts can be combined with the expected characteristics of potential

23

NUMO-TR-04-03

sites in order to define a set of more site-specific concepts which could be used for detailed
performance assessment studies and to determine R&D requirements. In this section abottom-up approach is briefly described, while in the following section a complementary
top-down process is illustrated.
The starting point for the bottom-up approach is the catalogue of repository components(Appendix 1). The aim is to determine the constraints (if any) on any particular component set
by the characteristics of a particular site. For the initial studies, only the following geologicalcharacteristics of the host formation were considered: rock mechanics, hydrogeology,
groundwater chemistry and rock temperature (as determined by emplacement depth and thegeothermal gradient).
For each option for repository component or geometrical layout, impacts of the sitecharacteristics on the design factors were evaluated in terms of how the performance of thatcomponent in isolation would be affected by changes in these characteristics. Each of thesesub-factors (attributes) was then evaluated by a multi-disciplinary expert team with a wide
diversity of international experience in repository design, performance assessment and sitecharacterisation.
An abbreviated version of one of the resultant matrices developed by this approach is shown in
Figure 3-3. In this particular case, the option of vertical in-hole emplacement of single waste
packages (as in H12; JNC, 2000) is examined for a range of possible rock strengths (from hardrock (HR) through increasingly soft rock (SR-A, B, C, D, E), using the classification developedin H12) and hydrogeological conditions, represented by an average hydraulic conductivity(ranging from 10-12 to 10-8 m s-1).
This compartmentalised approach is somewhat simplistic, but it does force all aspects of adesign component to be judged in terms of:
Whether specific site conditions will be of any significance (if not, noted as notapplicable – “n.a.”);Whether particular parameter ranges would be favourable or unfavourable (triangularprofile which is wider for more favourable conditions) ;Whether conditions would be problematic (dotted line) or would even “kill” concepts
utilising this component (indicated by K).
Open questions are indicated by numbers which relate to a list of comments. The full matrix
also includes assessment of the significance of water chemistry (expressed as salinity ranging
from “fresh” to “brine”) and temperature (over the range 30 – 60°C).
To date, such matrices have been prepared by teams from NUMO and partner organisations,firstly for the key components of the H12 design and, in reduced form (focusing only on safety
and engineering design factors), for other components of alternative designs (e.g. various PEM
designs). Preliminary work has started on developing “overlays” for the matrices which allow
priority areas for consideration to be identified for particular sites, which are characterised byparticular combinations of these site characteristics (see Figure 5-2).

24

NUMO-TR-04-03

n.a.

Figure 3-3:

Not applicable

Example of a matrix for evaluating the impacts of site characteristics on all of
the design factors of relevance for a specific choice of a repository designcomponent (the emplacement layout) (Umeki et al., 2003),

The limitations of this approach have been recognised – in particular the risk of loss ofconsideration of interfaces and component interactions. In addition, there is a potential for
missing key site characteristics, due to the limited number of simple parameter ranges which
are used (e.g. rock stress and flow velocity could be more critical rock mechanical /hydrogeological properties in specific cases rather than rock strength and hydraulicconductivity). Further developments to address these issues are planned. Nevertheless, thedesign matrix approach has proven a very useful tool for bringing together the repository design,25

NUMO-TR-04-03

performance assessment and site characterisation teams. This is essential in a project which
requires intensive interactions of scientists and engineers with diverse areas of responsibility. Itshould also be emphasised that, even at this initial stage, environmental and socio-economic
factors are explicitly included, to ensure that these critical aspects are borne in mind by all those
involved.

3.4

Matching repository concepts to site characteristics – a
top-down approach

A complementary approach to that illustrated above analysed a) the feasibility or suitability of asimplified H12 “generic repository” in different siting environments and b) the suitability ofdifferent repository concepts for a simplified “generic site”. These two exercises serve as apreparation for the later comparison of different site-specific repository concepts.
In the first case, eight “Potential Siting Environments” (PSEs) were defined (Figure 3-4) whichwere considered reasonably representative of the types of geological situations that can befound in Japan (see also NUMO, 2004). These can be associated with typical geographicalsettings where such environments are encountered in order to develop a series of study cases.
The pros and cons of the different PSEs as potential hosts for an H12 type repository wereidentified, allowing them to be compared with each other. A multi-attribute analysis (MAA)
approach was used for this complex, multi-dimensional problem. Since the geological dataassumed were very broad, little distinction could be made amongst the expected long term
safety performance of the sites. The focus for the initial exercise was therefore on practicality
and cost of implementation and likely extent of public support and hence key attributes of thesites included topography and transportation infrastructure.
The same MAA approach was used in the second exercise to gain experience in the ranking ofdifferent repository concepts for a specific site. Two different host rocks were considered(crystalline and hard sediment) and outline repository concepts derived from key components
from the components catalogue (Appendix 1). The characteristics of these cases were:A – H12 single-level layout with vertical EBS constructed in-situ;
B – H12 single-level layout with horizontal EBS constructed in-situ;
C – H12 multi-level layout with horizontal EBS constructed in-situ;
D – Very deep borehole emplacement (3-5 km below surface) with no buffer;
E – Emplacement of PEMs in mined boreholes (300-800m below surface);F – Emplacement of PEMs in long horizontal tunnels;
G – Emplacement of casks containing 20 waste packages in caverns (with delayed
backfilling).

26

NUMO-TR-04-03

Figure 3-4:

Potential siting environments for the MAA exercise
exaggerated; repository not precisely scaled).

27(vertical

scale

NUMO-TR-04-03

This study highlighted some of the main differences between these options, but also some of the
difficulties which result from the uneven level of understanding of different concepts (tending
to lead to more favourable ranking of less well studied options). Even on the basis of rather
simple preliminary analyses, it was clear that the subjective weightings placed by different
stakeholders on the various site or concept attributes could significantly alter the resultant
ranking. Nevertheless, the increased system understanding and improved interaction between
different technical groups within NUMO has shown these exercises to be extremely valuable.
In the future, MAA could potentially also help to develop transparent and traceable
documentation for this ranking process. However, it has been seen that results from suchstudies are often hard to communicate to persons not directly involved in the discussions on
scoring and weighting attributes.

3.5

Tailoring of repository concepts - involvement of
stakeholders

As noted in the previous section, the MAA approach involves consideration of a range ofsocio-economic factors. Because of the flexibility of the MAA software used, it is relatively
easy to examine the effects of the changing weighting of different attributes (e.g. “public
acceptance” or “cost”) in order to reflect the concerns of different stakeholder groups. Ideally,
when volunteers come forward, stakeholders could take part directly in such concept and site
comparison exercises. For example, we hope to involve key stakeholders – and in particular
local communities – in the process of defining some of the attributes to be considered and theirassociated weightings.
In general terms, there are a number of decision points where external input will play a role. We
plan for a stepwise technical development of the repository concept with iterative input from,and output to, the site characterisation work. It is also intended to solicit input from the public,
with particular emphasis on the areas which are of most concern to them and where they cancontribute most. In addition, we will involve other key groups of stakeholders in the decision
process – particularly for more technical aspects (e.g. university professors, professionalsocieties such as JSCE5, AESJ6).
Although the principles are easy to list, we appreciate that some of the choices to be made inrepository concept development involve rather complex considerations. For example, manygroups consider ease of retrieval and delayed closure of the repository as a positive feature
which maximises the time available to make decisions and allows future generations to choose
the ultimate concept and disposal strategy. On the other hand, from the point of view of security
and long-term performance of the engineered barriers, it is desirable to seal the repository as
rapidly as possible and make waste retrieval as difficult as possible. Balancing these conflicting
objectives involves both technical and ethical issues and we would hope to develop consensus
on such issues with all interested parties.

5
6

JSCE: Japan Society of Civil Engineers (http://www.jsce-int.org).
AESJ: Atomic Energy Society of Japan (http://wwwsoc.nii.ac.jp/aesj/index-e.html).

28

NUMO-TR-04-03

Box 2
Reversibility and Retrievability
Reversibility: Denotes the ability to reverse one or a series of steps in repository planning anddevelopment, at any particular stage of the programme. This implies being prepared to review,
and if necessary re-evaluate, earlier decisions, as well as maintaining the technical means toreverse a step. A programme may be planned to facilitate reversibility, for example byadopting small steps and frequent reviews in the programme, as well as by implementing
specific engineering measures. Reversibility provides flexibility in the programme and canthus allow stakeholders to participate more actively in decision-making at each step. In theearly stages of a programme, reversal of a decision regarding site qualification, or the adoptionof a particular design option, may be considered. At later stages, during construction andoperation, or following emplacement of the waste, reversal may involve engineering
measures, such as the modification of one or more components, or even retrieval of waste
packages.
Retrievability: This is a special case of reversibility, being the ability to reverse the action ofwaste emplacement. Retrieval, the action of recovery of waste or waste packages, may beconsidered at various stages after emplacement. In discussing retrievability, it is important tospecify what is to be retrieved, since this affects the implementation and technical feasibility.
Retrieval could, for example, refer to recovery of individual waste packages, identified as
faulty or damaged, even as emplacement of other packages continues, recovery of some or allof the waste packages at some time after emplacement, or excavation of the waste materials in
the far future, when the packages are no longer intact. Retrievability may be facilitated by therepository design and operational strategies, e.g. by leaving underground access ways open
and emplacement / retrieval systems in place until a late stage, or through the development anduse of durable containers and easily excavated backfill.

29

NUMO-TR-04-03

4
4.1

ENSURING SAFETY
The importance of safety and confidence-building

Safety is the top priority for NUMO in its development of HLW repository concepts. Thisapplies to all the conventional activities associated with construction, operation, monitoring,closure and decommissioning of all waste management facilities, all operations involving
transportation and handling of radioactive materials and the long-term behaviour of therepository after it is finally closed and sealed. We expect rigorous overview from independent
regulatory authorities up to the point of closure and realise that we must be able to clearly
demonstrate compliance with all relevant regulations and guidelines. In order to do so, there
will be an iterative evaluation of safety throughout our development of repository concepts. In
fact, this has already started and will continue up to and throughout the implementation phase;the present chapter, together with Appendix 2, therefore lays out the basic safety-relevant issues
and explains the specialist terminology involved.
In order to promote safety consciousness, we have identified establishing a “Safety Culture” asone of our primary goals. For NUMO, the fundamental aspect of a safety culture is that all staff
accept and practice an active commitment to ensuring that safety has priority over all otherissues. This culture must permeate all activities – and we have recognised that it can be mosteasily established in a new company during the time when the basic “company culture” isevolving. Nevertheless, the safety culture needs to be actively developed and then continuously
reinforced, with special consideration of each new generation of staff.
The importance of such a safety culture has been explicitly accepted at all hierarchicalmanagement levels in NUMO (and, as with the “quality culture”, will be extended to all key
external contractors and suppliers). It is recognised to apply to all activities within a project –from first planning through implementation to completion and final decommissioning.
A specific aspect of maintaining a safety culture concerns the development of appropriate
guidelines for quality assurance (QA) or, more generally, quality management (QM). This is an
important general requirement for all nuclear facilities and is a component of the measuresneeded to build confidence in the safety of the repository (see below). QM is a formal,
systematic approach to ensuring that a project is planned and implemented to meet clearly
defined goals. As noted above, the most critical goal is safety; in the case of a repository, thisincludes not only safety during the construction and operational phases but also the lesstangible post-closure safety over geological time periods.
Demonstrating safety during this very long post-closure phase poses one of the greatest
challenges in repository programmes. The set of arguments used to address this task is oftenlabelled “the safety case” (see Box 3). Long-term safety is based on the functioning of a suite ofengineered barriers within a natural barrier system. It is essential, therefore, that the engineered
structures are always emplaced rigorously according to specifications and that previous orsubsequent repository operations do not perturb the barrier roles of the geosphere. To achievethese goals, we intend the QM system to be designed, implemented and further developed
iteratively by a multidisciplinary team including those responsible for site characterisation, fordesign and operation of the facilities and for performance assessment. In addition, further goals
for the QM system are associated with cost, timescales, environmental impact and, in particular,social acceptance.

30

NUMO-TR-04-03

The H12 project (JNC, 2000) already focused on evaluating post-closure safety and did this byusing a simple, conservative approach for generic site conditions. In order to fulfil the expanded
requirements when dealing with a range of repository concepts and with a variety of volunteer
sites, major extension of this background is required. We must be in a position to:
Evaluate safety during construction and operation of the repository (considering bothconventional and radiological hazards);
Evaluate post-closure performance more realistically, in order to identify differences
between different concepts and different sites;
Provide the documentation required for licensing procedures;
Identify requirements for, or pros / cons of, monitoring and institutional control
options;
Facilitate communication within NUMO (in particular technical groups involved insite characterisation and repository design) and with other stakeholders.
Appendix 2 provides an overview of post-closure safety assessment methodology and asummary of key results from H12. The following sections of this chapter consider each of thefirst four points above. The fifth point is a global aim and is explicitly addressed within each ofthese sections. In the following box, some of the key concepts involved are introduced and theterminology used by NUMO is defined.
Box 3
Safety in Geological Disposal: Concepts and Definitions
In geological disposal, the overriding goal is to ensure that humans and the environment areprotected at all times from harmful effects. This has resulted in the development of numerous
repository-specific safety issues and of corresponding terminology. These are summarised anddefined here.
The entire undertaking of implementing a geological repository should be governed by a strong
safety culture. This culture is maintained, in part, by having an efficient quality management
system, which includes technical quality assurance or quality control elements. Analyses ofsafety are often divided into operational phase safety and post-closure safety. Because of thelong timescales involved, most discussion has centred on the latter and a structured approach todemonstrating long-term safety has been developed. This is based upon a sound safety strategy
that allows establishment of a safety case that is robust. One important element of the safetycase is quantitative performance assessment or safety assessment. A further important elementof the safety case is a statement of confidence. For clarity the italicised terms are defined below.
Safety culture: The assembly of characteristics and attitudes in organisations and individuals
which establishes that, as an overriding priority, protection and safety issues receive theattention warranted by their significance.
Operational phase safety: Conventional and radiological safety during construction and all of
the period where the repository is open for waste emplacement or for inspection.

31

NUMO-TR-04-03

Post-closure safety: Radiological safety in the period after the facility is closed and sealed.
Safety case: A safety case is a collection of arguments in support of the safety of the repository
and is developed for both the operational and post-closure phases. A safety case comprises thefindings of a safety assessment and a statement of confidence in these findings. It should
acknowledge the existence of any unresolved issues and provide guidance for work to resolvethese issues in future development stages. For social acceptance, it is critical not only to make atechnical safety case, but also to explain it to key stakeholders.
Robust: A robust repository system is based on (1) sites with well-defined and favourable
conditions and a well designed repository; (2) sufficiently large safety factors; and (3) somedegree of redundancy. A robust performance assessment is characterised by (1) being based
either on well understood, realistic models or else on clearly conservative models and data; (2)assuring that all potentially negative processes are analysed; and (3) being relatively insensitive
to parameter and conceptual model uncertainties.
Performance assessment: An assessment of the performance of a system or subsystem and its
implications for protection and safety at a planned or an authorised facility. This differs fromsafety assessment in that it can be applied to parts of a facility, and does not necessarily requireassessment of radiological impacts.
Safety assessment: An analysis to evaluate the performance of an overall system and its impact,
where the performance measure is radiological impact or some other global measure of impact
on safety.
Confidence: Confidence results from a positive judgement by all key stakeholders that a given
set of conclusions is well supported. Confidence is based not only on the safety case, but also on
the perceived credibility of all organisations involved (especially the implementer andregulators).
These definitions are based largely on accepted international usage as recorded in the
documentation of the NEA and the IAEA (NEA, 1999a; IAEA, 2003).

4.2

Special safety concerns during construction and operation

As pointed out above, most attention in repository programmes in their early phases has been
directed towards the assessment of long-term safety. As concepts become firmer and one
moves towards realisation of facilities, the importance of the construction and operational
phases grows. Compared to evaluation of performance over geological periods of time,assessment of operational safety utilises more conventional methodology. Detailed planning of
construction and operation activities includes an assessment of the risks and consequences ofvarious incidents, accidents and perturbations. Many well-established techniques exist for suchassessment (e.g. fault trees, event trees) and their implementation can be facilitated by using
computer-aided design (CAD) tools which allow an advance “walk-through” of all suchactivities. These analyses can feed back to allow decisions to be made about design variants (e.g.
shaft vs. ramp access, horizontal vs. vertical emplacement), design details (e.g. liner material /thickness, use of rock bolts / anchors) or operational variants (e.g. extent of remote handling,
number of back-up electrical generators).

32

NUMO-TR-04-03

A particular challenge may arise when the requirements for operational safety give rise to aconflict with either post-closure safety requirements or with socio-economic wishes. A typicalexample of the former could involve use of cement / concrete for grouting, tunnel lining, etc.
(NUMO and Posiva, 2004). Construction engineers may desire extensive use of thiswell-known material to minimise difficulties (and hence risks) of construction activities. The
long-term performance assessors, on the other hand, may want to avoid or minimise thechemical complexities associated with hyperalkaline leachates from cementitious materials.
An example of the latter conflict involving socio-economic constraints might result from adesire of local communities to keep waste accessible for easy inspection for a long time,
conflicting with the engineers’ preference of sealing underground openings as quickly as
possible (see also Section 3.5).
In any case, NUMO plans iterative development of design, with active involvement of allinterested expert groups, to ensure that optimal solutions are produced. At present, including
operational safety as a design factor considered in both the bottom-up and top-down concept
development studies allows key issues to be identified (Chapter 3). In order to go further – andin particular to compare different concepts and sites – a more quantitative approach is required.
Like long-term safety assessment (Appendix 2), a starting point for quantitative analysis ofoperational safety involves scenario development. As noted previously in the design principles(Chapter 2), pre-emptive planning attempts to identify possible perturbations that may occur
during construction and operation of the repository. This is made easier by aiming forsimplicity and a robust design. Wherever possible, designs and procedures will be modified to
avoid potential hazards. However, some perturbing processes will not be able to be completely
excluded – even if they are unlikely.
Two categories of perturbation can be identified – major disturbances associated with events
which affect many aspects of construction and operation simultaneously (e.g. a major
earthquake) and minor events which affect only a single operation (e.g. failures of a single pieceof equipment). As the former are generally of more concern, they are the focus for NUMO’s
initial studies. Nevertheless, studies have already been initiated in Japan to determine therobustness of operational equipment, with emphasis on remotely operated systems (Masuda etal., 2004).
For relevant scenarios, methodologies are being developed to estimate the probability of
disturbances and then calculate their consequences. An aim is to be able to estimate
probabilities and consequences in both a conservative manner (to determine compliance withguidelines) and also as realistically as possible (in order to identify differences between specificsites and repository concepts).

33

NUMO-TR-04-03

4.3

Quantitative analysis methods for realistic assessment oflong-term safety

As discussed above for operational safety, conservative quantitative analysis to assess
repository safety is required to demonstrate compliance with regulatory guidelines (Appendix
2). A problem with such an approach is that the simplification required is often so great that theanalysis is completely insensitive to even rather major variations in site and repository concept
properties. The H12 study, for example, showed the potential for a number of different EBS
designs to meet typical regulatory guidelines, but could not quantitatively discriminate between
the safety levels for a case of vertical emplacement with a plug and large, lined overlying tunnel
and a case of horizontal emplacement in an unlined tunnel.
For any specific repository design in a particular siting environment, many different features,
events and processes (FEPs) can influence the isolation of radionuclides. These can be
represented in a set of scenarios of the type developed in the H12 project (JNC, 2000; Appendix
2). A challenge is to move on from these static, generic scenarios (Figure 4-1). The aim is tobetter represent varying slow evolution / degradation of the repository barriers for different
concepts and sites - i.e. rather than examining the different scenarios indicated in Figure 4-1 asalternative systems, considering directly the changes from the fixed starting point at time ofclosure (A and B) to produce different future conditions (e.g. A1 – A4, B1, B2). These should
also identify clearly the inherent uncertainties involved and major perturbations which candisrupt one or more barriers (due to natural or anthropogenic events).
Representations of potential evolution scenarios, even if rather unlikely, should be
distinguished from “What if?” scenarios. The latter can involve physically impossible
assumptions (e.g. disappearing canisters, direct transport of radionuclides from the EBS to thebiosphere), which can be used to improve understanding of system behaviour and help explainsome aspects of the safety case (e.g. the multi-barrier system), but do not represent any kind ofevaluation of possible system evolution. Hence, even if analysed by the same quantitative tools,
it is important to separate these in the discussion of repository performance.
In some cases, regulations in foreign countries specify separate treatment of other specialgroups of scenarios – in particular those associated with future human actions. Although such
regulations are not yet defined in Japan, special treatment of human intrusion scenarios seems
sensible and these will be considered carefully by NUMO.

34

NUMO-TR-04-03

Figure 4-1:

Development of scenarios to describe possible future evolution of a deep
geological repository.

35

NUMO-TR-04-03

In addition to improving the scenario development process, NUMO intends to further develop
the models and databases established by JNC for quantitative consequence analysis (JNC,
2000; Appendix 2). Specific aims include:
More realistically representing the geometry of all components of the engineered
barriers (essential for distinguishing between different repository design options);
Including explicit representation of all materials present in the repository engineered
structures and considering any significant interactions between them;Realistically representing the 3D geometry of the geosphere, with particular emphasison the solute transport characteristics of all relevant formations;
Developing a model of a Japan-specific biosphere which contains appropriate diet andlifestyle information and improved representation of the geosphere / biosphereinterface for inland and coastal conditions;
Incorporation of time dependency into the model chain in order to evaluate scenarios
which evolve gradually with time;
Improved assessment of uncertainties and their development in time and space;
Increased efforts to test (verify and validate) models and databases;
Development of presentation formats to make results understandable to a wider
audience.
These intended developments will, of course, be coordinated with parallel efforts in theinternational safety assessment community.

4.4

Making and documenting the safety caseIn the staged repository implementation programme, we envisage several formal licensing steps
associated with site selection and subsequent repository construction, operation and closure. At
each of these, a safety case would be presented which would become more detailed as thecharacteristics of the site and the design of the repository become better defined. As noted inBox 3, the safety case (for both operational and post-closure phases) would include a series ofmore general arguments in addition to a quantitative safety assessment. A further importantcomponent of the licensing documentation will be a quality assured documentation trail which
ensures that the assumptions, data and models supporting the safety case are fully transparent
and reproducible. The importance of QM was emphasised in Section 4.1 above and the
documentation of assumptions will be facilitated by the requirements management process,supporting repository concept development and drawing together the decision processdeveloped in the top-down and bottom-up studies (Chapter 3).
It is also to be expected that key safety documents will be subjected to peer review by
independent national and international groups. For the latter, we plan that key documents will
be produced in English. Experience has already been gained in this area during the positive
review of H12 by the NEA (NEA, 1999b). Indeed, for the production of important documents,
NUMO has already established domestic and international expert groups which serve asinternal advisory and review bodies. The deliberations of NUMO’s International Technical36

NUMO-TR-04-03

Advisory Committee (ITAC) are summarised on our English language website(http://www.numo.or.jp/english/index.html)

4.5

Monitoring and institutional control

We aim to establish a monitoring programme to ensure that all operations are carried out safely,with no detrimental impact on the environment and that the as-built repository meets the quality
requirements for ensuring long-term safety.
Box 4
Monitoring, Control, Demonstration and Performance Confirmation
Raising confidence in the safety of a repository is a continuing challenge that can lead to arange of specific activities during the repository development, operation and post-closure
phases. The behaviour of the repository system is monitored throughout its lifetime. Special
experiments or measurement programmes may be introduced as demonstration activities or
else as confirmation that the repository is performing as expected. After closure, passive andactive institutional controls reduce the probability of inadvertent intrusion. For clarity theitalicised terms are defined here (IAEA, 2001).
Monitoring: The measurement of radiological or non-radiological parameters for reasons
related to the assessment or control of exposure and environmental impacts, and to theinterpretation of such measurements. Monitoring can be continuous or sporadic.
Demonstration activities: Activities may be performed by the implementer to illustrate – inparticular to other stakeholders and the public - that the chosen repository designs andoperating modes do indeed perform as expected. Direct demonstration of long-term
post-closure safety is, of course, not feasible. Demonstration facilities may be a part of themain disposal area or may be in separate – but still representative - areas to allow intensive
monitoring without compromising the integrity of the repository.
Performance confirmation: The performance confirmation programme is a process to test andevaluate whether the repository system is working as expected and within an acceptable safetymargin. The tests and evaluations must be based on observations of changes in natural andengineered systems and components (i.e. monitoring). Monitoring provides one basis for
performance confirmation because it delivers data representing direct and indirect
observations of the natural and engineered systems that comprise the geological repository.
Scientific analysis of monitoring data provides the second basis for performance
confirmation, as it is the process by which observed behaviour will be compared with that
expected and the significance of deviations evaluated.
Institutional control: Control of a waste site by an authority or institution designated under thelaws of a country or state. This control may be active (monitoring, surveillance, remedialwork) or passive (land use control) and may be a factor in the design of a nuclear facility (e.g.
near-surface disposal facility).

37

NUMO-TR-04-03

In developing a monitoring programme, the aims to be considered include:
Assessing site perturbation due to the presence of the repository;
Assuring operational safety and performance;
Responding to unlikely perturbation scenarios during the operational phase;Collecting data for model and database confirmation or to support operational phasedecisions;Meeting regulatory requirements;
Building confidence on the part of stakeholders and the general public.
Considering the aims listed above in turn, the programme requirements for each can be outlined.
In general, the requirements are additive – going down the list involves further work, but alsoutilises the information and resources from those above.
Any major construction activity will inevitably perturb the local environment. An associatedenvironmental impact assessment (EIA) will evaluate all potential disturbances of the“environment” – e.g. air & water quality, land use and appearance, biological diversity,
economic impact, traffic. We will continuously monitor such effects during repository
construction and operation and will ensure that comprehensive “baseline” measurements of all
relevant parameters are made prior to commencing the project, so that influences due to therepository can be distinguished from natural fluctuations (see also NUMO, 2004). Suchbaseline measurements will be initiated at the start of field work (PIA stage) and hence will bedirectly considered when planning site characterisation programmes for volunteers.
The operational requirements for safety and quality monitoring in a repository are wellestablished, based on experience in the nuclear and underground construction industries. As yet,however, there is limited practical experience in combining these in a single facility and hence
we identify this as an area for future R&D.
During expected operations there would be no release of radioactivity from the waste packages
to the environment (although there will be an inevitable release of natural radioactivity due tounderground construction activities – as occurs in all such work). Nevertheless, certain
accidents (e.g. package drop, fire, explosion) or natural events (e.g. earthquake, tsunami) could
potentially give rise to a release of radioactivity. A monitoring network is needed to indicate
whether such releases have, in fact, occurred, to assess the consequences of these releases andto guide remediation activities. In principle, the requirements are similar to those for othernuclear facilities, but the monitoring network has to reflect the facilities and scenarios involved.
The safety case for the post-closure phase of the repository incorporates a logical set ofarguments supported by analyses carried out by a suite of mathematical models. As
construction and operation activities progress, there are opportunities to test the predictive
abilities of the codes and data involved using site-specific measurements. Some tests are fairly
conventional and can utilise data from some of the monitoring which would be required for the
purposes considered above. Tests of the engineered barriers (and the surrounding geosphere)
are much more difficult to devise without risking negative influences on repository
performance. Such tests may be carried out at a separate location, where performance can be

38

NUMO-TR-04-03

examined by “post-mortem” excavation. If required, a special “performance confirmation”
facility could be situated within the repository area.
Regulations for HLW disposal are not yet specified, but information needed for conventional
EIA and operational safety aspects should be provided by the systems discussed above. The
regulators may, however, desire to test the performance of the “as-built” facility; for example
by monitoring of the quality of the EBS and assessing its rate of degradation in-situ.
We consider it essential to demonstrate the safety of the repository to stakeholders and this may
require very gradual staging of decisions with extensive monitoring of emplaced waste andeven “inspectability” for an extended period. Delayed backfilling can allow such inspectability,
but is practical only for few of the repository design options presented in Appendix 1. Such
options are inevitably associated with risks to workers during the institutional control period
and of degradation of long-term performance. Hence, these options need to very carefully
consider such issues. Alternatively, a demonstration facility can be constructed elsewhere
within the site; it is possible that some technical concepts can be demonstrated without use ofreal waste.
Any design which includes keeping underground structures open for an extended period ofmonitoring / inspection will inevitably be associated with a number of issues, including:More extensive perturbation of site characteristics (hydrogeology, geochemistry, etc.)
and potential degradation of the geological barrier;Inevitable degradation of structures (e.g. cracking of liners) with possible
requirements for maintenance / repair;
Risk of accidents (e.g. loss of services leading to flooding or gas explosion) or
deliberate intrusion;
Risk of abandonment / incomplete sealing (e.g. as a result of social disruptions);
Requirement to maintain an experienced workforce over long periods.
We will openly discuss such points with local communities, and need to clearly communicate
that:
Monitoring is not required to achieve safety; the passive safety barriers and the active
measures taken during operation do this. It is simply a check that all is in order;
Monitoring can be as extensive in scope and continue for as long as future society
wishes, but it should not result in significant negative effects on safety and it is our job
to ensure this (if necessary, modifying designs to comply with societal demands).

39

NUMO-TR-04-03

5
5.1

DEVELOPMENT OF THE R&D PROGRAMME TOSUPPORT REPOSITORY CONCEPT DEVELOPMENT
Setting goals and priorities

As indicated previously in Figure 1-6, the R&D to support repository concept development will
evolve in response to progress in the site characterisation work. As discussed in Chapter 3, at
NUMO we have established a structured procedure for developing repository concepts suitablefor volunteer sites in a staged manner. This is strongly coupled to assessment of operational andpost-closure safety (Chapter 4). The interactions and data flows between these two work areas
are illustrated in Figure 5-1. In both Chapters 3 and 4, requirements for future R&D have been
identified. The challenge is to derive an overall structured R&D programme which provides the
input needed for particular milestones in an efficient and cost-effective manner.

Figure 5-1:

Coupling of information flow between repository design and performanceassessment (PA) work areas.

Box 5
Terminology: R&D and D&V
In the Japanese programme, the term research and development (R&D) is used to include all
work aimed at building fundamental scientific understanding and establishing necessary
technology. Demonstration and validation (D&V) refers to work which confirms that theresultant tools, models and engineering concepts are practical and builds the confidence
needed for implementation, licensing and public acceptance. Although these areas are often
separated, to ease discussion we will consider the latter as a sub-set of the former.
The process of defining an R&D programme is ongoing at NUMO; this is needed both forproviding the basis for repository concept development (Section 5.2) and for the assessment ofvolunteer sites (Section 5.3). It involves establishing a list of important open questions, which is40

NUMO-TR-04-03

complicated at the present programme stage when a wide range of design options is underconsideration. To help, NUMO has developed an approach in which the requirements on
individual components of repository designs are evaluated for the range of conditions expected
at potential sites (“bottom-up approach”, Section 3.3). The matrices which are used to identify
site-specific constraints (Figure 3-3) can be extended to more clearly identify and prioritise
specific open questions and the approaches to addressing these (Figure 5-2). The very specificopen questions are complemented by larger issues raised by the “top-down” approach (Section
3.4).

Figure 5-2:

Example of use of the matrix overlay approach to identify and classify open
issues (Umeki et al., 2003; Ueda et al., 2004).
The generic evaluation of a component (Matrix 1), as presented earlier in Figure 3-3,
can be extended to consider the specific conditions in a particular PSE (Figure 3-4) orvolunteer site. The resultant “overlay” (Matrix 2) highlights and classifies site-specificissues. These open issues are then expanded in Matrix 3 to identify R&D
requirements and give some indication of relative priorities.

41

NUMO-TR-04-03

With the starting point of a list derived from such work, priorities can be set for work to be
performed in the following three areas currently of relevance for NUMO's development of
repository concepts:
Preliminary work required urgently to establish the basis for developing repository
concepts from the components catalogue (Appendix 1) and assessing them in terms of
the established design factors (Figure 1-6);
Preparation for the evaluation over the next few years of volunteer sites on the basis ofliterature studies;
Longer term work which requires special consideration due to the need for:Establishing NUMO credibility;
Building up experience in NUMO teams;Long time baselines;
Preservation of expertise in Japanese expert teams;
Obtaining maximum benefit from collaborative work with partner organisations.
These are considered further in the following sections. It may be noted that there is someinevitable overlap with the R&D work that is more directly related to site selection andcharacterisation.

5.2

R&D to provide the basis for repository concept development

Until volunteers come forward, the procedure for developing site-specific repository concepts
will be advanced by further iterations of the work described in Chapters 3 and 4, using
increasingly detailed specifications of the PSE. In order to compare different RDOs, a clear
need has been identified for bringing assessment of key components to a similar level. This
involves fundamental design studies with associated scoping analysis of basic constraints (e.g.
rock mechanics, thermal analysis) and post-closure safety. A modelling toolkit for such work
exists (Figure 5-3), but improvements are planned to allow differences between RDOs / sites tobe assessed more rigorously. Development of a methodology for scoping assessments ofoperational safety is also a priority.
As illustrated in Appendix 1, the different repository concepts considered have a number ofcommon features, including:Surface facilities for waste reception, packaging, handling, administration, security,etc.;
Access tunnels or shafts;
Underground tunnels, holes or caverns for waste emplacement;
Robust overpacks for HLW;
Backfilling, sealing and plugging systems;
Service facilities for ventilation, drainage, etc.

42

NUMO-TR-04-03

Figure 5-3:

Development of scoping assessments of repository design options in
potential siting environments.

43

NUMO-TR-04-03

There are also a number of common activities, including waste packaging, transportation,
emplacement and monitoring. At the present stage of repository concept development, thefocus for NUMO R&D effort is on the waste emplacement layout, EBS design and the waste
emplacement process.

5.3

Focusing R&D for assessing volunteer sites

The general studies become more focused when volunteer sites come forward and literaturestudies are initiated to assess their suitability. Following a preliminary check on qualification ofvolunteer sites with respect to volcanoes and active faults, a literature survey will be carried out
to assess geological stability on the basis of specified Nationwide Evaluation Factors (NEFs)
and Site-Specific Evaluation Factors (SSEFs), which include potential volcanic activity, active
faults, rock deformation and seismicity, land uplift/erosion and so on (NUMO, 2004). Relevant
information for the qualified sites is also gathered to assess Favourable Factors (FFs), which
cover geological, geographical, environmental, and social aspects. After this has been done, it
is important to determine whether it would be practical and safe to develop a repository at thissite; to do this, a series of appraisal steps have been defined.
As illustrated in Figure 5-4, there is a logical structuring of the output of the literature survey tonarrow down the type of RDOs which can come into consideration. The availability ofsufficient disposal space is a particular concern that arises from the small size of many Japanesemunicipalities, the need to keep respect distances from volcanoes and active faults and thecomplicated geological structures found in many regions of Japan. A first step is thus todetermine if it is possible, at least in principle, to fit a repository into any potentially suitableformations available. This is the main emphasis of the top 4 steps in the middle column of
Figure 5-4. It is important to provide such a comprehensive and systematic logical structure fordevelopment of repository concepts from the outset of the siting process and then to apply ititeratively during later PIA and DIA stages, with appropriate modifications as required.
At this stage, site characteristics may still be rather poorly defined, but, for any site which is notclearly excluded, all available information will be drawn together to outline site-specificrepository concepts and analyse these in a quantitative performance assessment. If there are alarge number of volunteers, such an assessment can contribute, along with the defined FFs(NUMO, 2004), to ranking of sites for consideration as PIAs.
As preparation for this process, we can identify some information and R&D requirements
required to allow this work to proceed and we have already initiated some of this work.
Examples of this are:Studies of low pH concretes (at JNC and as NUMO / SKB and NUMO / CRIEPI
collaborations) to determine the extent to which potential problems caused by use of
concrete in low strength or wet host rocks can be minimised (input for steps 4 and 5 inFigure 5-4);
A review of the thermal stability of bentonite under wet and dry conditions(collaboration with Nagra) to allow the feasibility of design options based on highdensity waste package emplacement to be assessed (input for step 4 of Figure 5-4; seealso Figure 3-2).

44

NUMO-TR-04-03

Figure 5-4:

5.4

Identification of R&D requirements for volunteer sites during the stage ofliterature studies.

Long-term R&D programme priorities
45

NUMO-TR-04-03

In addition to the R&D identified above, which has priority due to the implementation time plan
(Figure 1-3), there are other R&D projects which need to be initiated soon (or are alreadyongoing). These projects include studies of the following kinds:
Important issues specific to Japanese boundary conditions and NUMO repository
concepts which are not studied elsewhere (e.g. studies of the long-term tectonic
evolution in Japan and its impact on a deep geological repository);
Areas of common interest which can be efficiently developed in collaboration with aninternational partner (e.g. NUMO / Posiva co-sponsored workshop on
bentonite-cement interaction (NUMO and Posiva, 2004));
Areas where NUMO wants to establish a leading position; in order to build up
credibility with international partners and / or the academic community and toparticipate in suitable sharing of R&D results (e.g. development of fundamental
mechanistic models of compacted bentonite);
Topics which involve top-level co-ordination / synthesis where NUMO wants to build
up in-house expertise to assist in overall programme management (e.g. developmentof a performance assessment toolbox to allow site-specific comparison of theperformance (operational and long-term) of different RDOs);
Projects which inherently require long-time studies to validate performanceassessment models (e.g. long-term corrosion / gas production tests to support future
licensing). Many of these fall under “Demonstration and Validation” as defined in Box
5);Issues which are common to all disposal concepts (e.g. studies of bentonite);
Following the progress of ongoing work in Japan and elsewhere, and trying to promote
co-ordination of efforts that can provide clear, long-term benefits (e.g. measurement offundamental chemical-thermodynamic data).
As noted in Section 1.1, there are several organisations in Japan involved in carrying out R&D
aimed directly at supporting the NUMO HLW disposal programme. In addition, there are
independent programmes for disposal of other wastes (both radioactive and stable) which may
have certain common features (e.g. environmental impact analysis aspects). These exist within
the framework of a very much larger R&D environment – which produces fundamental
information, tools and methodology which may be applicable for NUMO’s purposes.
NUMO will play an active co-ordination role, setting goals and milestones and helping to steer
and integrate individual R&D projects in the HLW disposal field. We will also encourage
sharing of information and co-operation in areas of mutual interest. Information exchange isfacilitated by the existing “nuclear infrastructure” in Japan (e.g. JAIF7, AESJ, conferences andcommunication initiatives), which will be complemented by focused events (e.g. topical
workshops, joint projects) as and when required.
The general R&D community is accessed by NUMO primarily via personal networks andindustry / academic communication initiatives. This is complemented, in particular, by
NUMO’s Domestic Technical Advisory Committee (DTAC), which comprises a group of7

JAIF: Japan Atomic Industry Forum, Inc. (http://www.jaif.or.jp/english/index.html).

46

NUMO-TR-04-03

experts in relevant technical areas. This group is a very valuable interface to the generalscientific and engineering community.
At a strategic level, we ensure that we take full advantage of international experience throughthe active support of our ITAC whose members have experience in almost all major nuclear
waste management programmes (records of ITAC meetings are available on our website).
NUMO has already established bilateral links with some of the most advanced national waste
management programmes (ANDRA, Nagra, Posiva, SKB, U.S. DOE and UK-Nirex) and has
several active collaboration projects ongoing with these partners. In addition, NUMO is amember of EDRAM (International Association for Environmentally Safe Disposal ofRadioactive Materials), which includes major nuclear waste management programmes andinitiates joint projects in areas of common interest (for details on membership and activities see
http://www.edram.org). Further, NUMO is represented in some of the important co-ordinationgroups of international organisations (e.g. IAEA, OCED/NEA) and supports / participates ininternational collaboration projects such as the ITC (International School of Underground
Waste Storage and Disposal – http://www.itc-school.org).
This wide range of international contacts ensures that NUMO benefits from collaboration andsharing of information and hence avoids unnecessarily duplicating work carried out elsewhere.
We recognise, however, that in order to be an “active partner” internationally, we need to haveareas of expertise where we are at the forefront, in order to ensure that information exchange isa two-way flow. NUMO’s structured process of repository concept development and some of
the tools / databases we are developing to support this work have already been a focus ofinterest from our partners and this seems to be one area in which we can contribute to theresearch efforts of the international community. We participate in many major international
conferences and workshops and are planning to take a more active role in hosting such events inthe future.
An example of how R&D projects fulfilling these varied general requirements can be bought
together in an integrated manner is illustrated in Figure 5-5, which shows how different
Japanese investigations on compacted bentonite are linked together.
We now plan to bring all the considerations above together in a documented R&D programme
which will provide a focus for Japanese efforts in the HLW disposal field. This will be a living
document, in that it will incorporate the flexibility to respond to the changes in technical andsocio-economic boundary conditions to be expected as our staged implementation programme
progresses.

47

NUMO-TR-04-03

Figure 5-5:

The bentonite R&D programme with an illustration of some key projects in thearea of determining fundamental properties of bentonite, bentonite / cement
interaction, practical emplacement of bentonite and design constraints set by
the requirement to ensure bentonite longevity.

48

NUMO-TR-04-03

6

THE FUTURE REPOSITORY CONCEPTPROGRAMMENUMO’s volunteering approach to site selection presents particular challenges for therepository concept development programme. As outlined in the previous chapters, our adoptionof this approach explains why we have deliberately chosen to maintain as wide a spectrum ofrepository design options as possible. This not only allows us flexibility to respond to thediversity of issues to be addressed in the different siting environments resulting from thevolunteering process, but also enables us to respond to the desires of interested parties (inparticular the local communities).
Following the process shown in Figure 1-6, Chapters 3 and 4 have outlined our progress indeveloping methodology for selecting suitable repository concepts for potential siting
environments, assessing the safety and practicality of site-specific concepts and ranking
different sites and RDOs.
As indicated in Figure 1-6, these basic procedures will be iterated further at the stages ofliterature survey of volunteers, characterisation of PIAs on the basis of surface-based studies
and then detailed characterisation of one or more concepts, for one or more DIAs. The
programme for site characterisation will also have to be tailored to environments encountered atparticular volunteer sites. Although site characterisation plans are still under development, amodular approach similar to that for the repository components is presently under consideration.
In this, a catalogue of site characterisation technologies will be prepared which can beassembled to provide a characterisation plan tailored to a specific site.
As emphasised previously, the site characterisation work provides input to, and receives
feedback from, the repository concept analyses. The engineering design, performanceassessment and site characterisation teams have already worked together – in particular in the
tailoring studies described in Sections 3.3 and 3.4. Such projects are considered essential in
preparation for the next stages when work on volunteer sites commences. There are obviously
uncertainties concerning the characteristics of volunteer sites and their number is also open. In
principle, therefore, the initial stages of characterisation of sites may have to run in parallel forseveral different locations – which could be very diverse in terms of geology, geography andsocio-economic setting.
Parallel site characterisation work may strain available resources of expert manpower,
specialist equipment and analytical facilities. This will require efficient co-ordination of these
programmes in order to follow the tight planned schedule (Figure 1-3). In addition, suchco-ordination may also be needed to ensure that equal treatment of volunteers can be clearlyshown to the public.
A concern throughout is to ensure that all critical issues can be communicated to interested
stakeholders so that key groups – including regulators, independent experts and local
communities – fully understand the resultant repository programme and, indeed, are integrated
as far as possible into its important decision processes. We thus plan to continue publishing
updates on technical developments in this report series, with English language reports on topics
which may also be of interest to international audiences.

49

NUMO-TR-04-03

7

ACKNOWLEDGEMENTS

The work reported in this document was carried out as a collaboration between NUMO and arange of partner organisations and contractors. In particular, some of the concepts andprinciples were developed together with our International and Domestic Technical Advisory
Committees (ITAC / DTAC).

50

NUMO-TR-04-03

8

REFERENCES

AEC (2000): Long-term Program for Research, Development, and Utilization of Nuclear Energy (inJapanese).
ANDRA (2001): DOSSIER 2001 ARGILE, Progress Report on Feasibility Studies & Research into
Deep Geological Disposal of High-Level, Long-Lived Waste, Synthesis Report, December
2001, ANDRA, Châtenay-Malabry Cedex France.
Apted, M.J. (1998): A Modest Proposal: A Robust, Cost-Effective Design for High-Level Waste
Packages, Materials Research Society Symp. Proc. Vol.506, pp. 589-596.
Deutch, J. (Co-Chair), Moniz, E.J. (Co-Chair), Ansolabehere, S., Driscoll, M., Gray, P.E., Holdren, J.P.,
Joskow, P.L., Lester, R.K. and Todreas., N.E. (2003): The Future of Nuclear Power, AnInterdisciplinary MIT Study, Massachusetts Institute of Technology, Cambridge MA USA.
Gierszewski, P.J., Russell, S.B., Garisto, F., Jenson, M.R., Kempe, T.F., Maak, P. and Simmons, G.R.
(2001): Deep Geologic Repository Technology Program - Annual Report 2000, Report No:
06819 - REP - 01200 - 10055 - R00, Ontario Power Generation Inc., Toronto ON Canada.
IAEA (2001): Monitoring of Geological Repositories for High Level Radioactive Waste,
IAEA-TECDOC-1208, International Atomic Energy Agency, Vienna Austria.
IAEA (2003): Radioactive Waste Management Glossary, 2003 Edition, International Atomic Energy
Agency, Vienna Austria.
Ichikawa, Y., Kawamura, K., Nakano, M., Kitayama, K. and Kawamura, H. (1999): Unified molecular
dynamics and homogenization analysis for bentonite behavior: current results and future
possibilities, Engineering Geology 54, pp. 21-31.
JNC (2000): H12 Project to Establish the Scientific and Technical Basis for HLW Disposal in Japan,
Project Overview Report, 2nd Progress Report on Research and Development for the
Geological Disposal of HLW in Japan, JNC Technical Report TN1410 2000-001, Japan
Nuclear Cycle Development Institute, Tokai-mura Japan.
Lindgren, E., Pettersson, S. and Salo, J.-P. (2003): R&D program for Horizontal Emplacement KBS-3H,
Proc. of 10th International High-Level Radioactive Waste Management Conference(IHLRWM 2003), pp.571 - 577, 30 March - 2 April 2003, Las Vegas NV USA.
Makino, H., Sawada, A., Wakasugi, K., Kato, T., Miyahara, K. and Uchida, M. (2002): Results from
Synthesis of Calculation Cases Illustrating Overall System Performance in the Safety
Assessment in H12 Report, JNC Technical Report JNC TN8450 2001-012, Japan Nuclear
Cycle Development Institute, Tokai-mura Japan (in Japanese).
Masuda, R., Asano, H., Kosuge, K., Takao, H. and Yoshida, T. (2004): Remote Handling Operation
Technique for High-level Radioactive Waste Repository - Investigation and Performance Test
for Several Different Emplacement Concepts -, Proc. WM’04 Conference, 29 February - 4
March 2004, Tucson AZ USA.
McKinley, I.G. and Umeki, H. (2004): Deep Geological Disposal of High-level Radioactive Waste-Turning up the Heat, Joint AOGS 1st Annual Meeting & 2nd APHW Conference, 5-9 July
2004, Singapore.
McKinley, I.G., Neall, F.B., Kawamura, H. and Umeki, H. (2004a): Geochemical Optimisation of a
Prefabricated Disposal System for High-level Radioactive Waste. J. Geochemical Exploration(in press).
McKinley, I.G., Alexander, W.R., Kickmaier, W. and Neall, F.B. (2004b): The cement/bentonite
interaction problem: cutting the Gordian knot, NUMO / Posiva International Workshop on
Bentonite-Cement Interaction in Repository Environments, 14-16 April 2004, Tokyo Japan.

51

NUMO-TR-04-03

MITI (2000a): Basic Policy for Specified Radioactive Waste, MITI’s Notification No.591 (in Japanese).
MITI (2000b): Final Disposal Plan for Specified Radioactive Waste, MITI’s Notification No.592 (inJapanese).
Nagra (1994): Kristallin-I, Safety Assessment Report, Nagra Technical Report NTB 93-22, National
Cooperative for the Disposal of Radioactive Waste, Wettingen Switzerland.
Nagra (2002): Project Opalinus Clay, Safety Report, Demonstration of Disposal Feasibility for Spent
Fuel, Vitrified High-level Waste and Long-lived Intermediate-level Waste(Entsorgungsnachweis), Nagra Technical Report NTB 02-05, National Cooperative for the
Disposal of Radioactive Waste, Wettingen Switzerland.
NEA (1997): Safety Assessment of Radioactive Waste Repositories - Systematic Approaches to
Scenario Development - An International Database of Features, Events and Processes. Draft
Report（24/6/1997）of the NEA Working Group on Development of a Database of Features,
Events and Processes Relevant to the Assessment of Post-Closure Safety of Radioactive
Waste Repositories, OECD/Nuclear Energy Agency, Paris France.
NEA (1999a): Confidence in the Long-term Safety of Deep Geological Repositories, Its Development
and Communication, OECD/Nuclear Energy Agency, Paris France.
NEA (1999b): OECD/NEA International Peer Review of the Main Report of JNC‘s H12 Project toEstablish the Scientific and Technical Basis for HLW Disposal in Japan, International Peer
Reviews, Report of the OECD Nuclear Energy Agency International Review Group,
NEA/RWM/PEER(99)2, OECD/Nuclear Energy Agency, Paris France.
NEA (2004): Engineered Barrier Systems (EBS): Design Requirements and Constraints, Workshop
Proceedings, Turku, Finland, 26-29 August 2003, In co-operation with the European
Commission and hosted by Posiva Oy, OECD/Nuclear Energy Agency, Paris France.
NUMO (2004): Evaluating Site Suitability for a HLW Repository, Scientific Background and Practical
Application of NUMO’s Siting Factors, NUMO Technical Report NUMO-TR-04-04, Nuclear
Waste Management Organization of Japan, Tokyo Japan.
NUMO and Posiva (2004): International Workshop on Bentonite-Cement Interaction in Repository
Environments, Summary Overview, 14-16 April 2004, Tokyo Japan (in press).
ONDRAF/NIRAS (2001): SAFIR2, Safety Assessment and Feasibility Interim Report 2, NIROND
2001-06E, Brussels Belgium.
Posiva (1999): The Final Disposal Facility for Spent Nuclear Fuel - Environmental Impact Assessment
Report, Posiva Oy, Helsinki Finland.
RWMC (1998): Radioactive Waste Data-book, Radioactive Waste Management Center, Tokyo Japan.
RWMC (2004): Development of Remote Operation Technology for HLW Repository, FY2003 study,
Radioactive Waste Management Funding and Research Center, Tokyo Japan.
SKB (1986): R&D-PROGRAMME 86, Parts I-III, Handling and Final Disposal of Nuclear Waste,
Programme for Research Development and Other Measures, Swedish Nuclear Fuel and Waste
Management Co., Stockholm Sweden.
SKB (1992): Project on alternative system study (PASS), Final report, SKB Technical Report TR 99-08,
Swedish Nuclear Fuel and Waste Management Co., Stockholm Sweden.
SKB (1999): SR 97 - Post-closure safety, SKB Technical Report TR 99-06, Swedish Nuclear Fuel andWaste Management Co., Stockholm Sweden.
SKB (2001): RD&D-Programme 2001, Programme for Research, Development and Demonstration of
Methods for the Management and Disposal of Nuclear Waste, SKB Technical Report TR
01-30, Swedish Nuclear Fuel and Waste Management Co., Stockholm Sweden.

52

NUMO-TR-04-03

STA (1988): Notification No.20, amended on 26 December 2000 (in Japanese).
Toyota, M. and McKinley, I.G. (1998): Optimization of the Engineered Barrier System, Fabrication and
Emplacement for Vitrified HLW, Proc. of 8th International Conference on High-Level
Radioactive Waste Management, 11-14 May 1998, Las Vegas NV USA.
Ueda, H., Umeki, H., Konishi, T., Naito, M. and Danda, H. (2004): Design Factors for Filtering HLW
Repository Concepts, Proceedings of Disposal Technologies and Concepts (DisTec) 2004,
26-28 April 2004, Berlin Germany.
Umeki, H., Ueda, H., Naito, M., Konishi, T. and Danda, H. (2003): The NUMO Approach forDevelopment of Repository Concepts, Proc. of 10th International High-Level Radioactive
Waste Management Conference (IHLRWM 2003), pp.1019 - 1024, 30 March - 2 April 2003,
Las Vegas NV USA.
U.S. DOE (2002): Yucca Mountain Science and Engineering Report Rev.1, Technical Information
Supporting Site Recommendation Consideration, DOE/RW-0539-1, U.S. Department ofEnergy, Office of Civilian Radioactive Waste Management, Washington DC USA.

53

NUMO-TR-04-03

54

APPENDIX 1

APPENDIX 1

NUMO-TR-04-03

REPOSITORY COMPONENT CATALOGUE

A1-1 Introductory notes
The following points should be noted when considering this catalogue of repository
components and geometric options:• The presentation is illustrative only and the examples of options are chosen to show therange available, rather than attempting to comprehensively cover all possibilities. The
only “fixed points” are the specifications of the waste shown in Section A1-2 below.
• Variants of the repository concepts described in the H12 report (e.g. options denoted as“H12” in the catalogue) remain the highest priority for NUMO; other options are being
studied in order to maintain flexibility to respond to the siting environments that may be
encountered at potential volunteer sites and to the wishes of key stakeholders.
• Comparison of options must be carried out very carefully, as the extent of past experienceand the degree of current understanding may vary considerably between particular
examples (see discussion in Chapters 2 and 3).

A1-2 HLW designs
The glass provides a low solubility matrix in which the radionuclides are homogeneously
distributed. The stainless steel canister ensures containment up to the point of encapsulation in
the overpack.
BNFL / COGEMA

JNC

JNFL

~500

~400

~500
(RWMC, 1998)

Dimensions(unit: mm)

Weight (kg)The specifications for the borosilicate glass matrix and the stainless steel fabrication canisterare given by the reprocessing organisation. Note that, for operational reasons, the canister is notcompletely filled with glass and a certain void space remains.

A1-1

NUMO-TR-04-03

APPENDIX 1

A1-3

Safety-critical engineered barrier components

A1-3.1

Overpack

The overpack has mechanical and radiation protection roles during handling and emplacement.
After emplacement, it may provide a period of complete containment during the radiogenic
thermal transient, when conditions are rather dynamic and may be strongly coupled (e.g.
thermal and water saturation profiles in the buffer); although this may not be critical for safety,
it can contribute to simplifying the presentation of the safety case. After loss of integrity, acorroded steel overpack may also serve important roles as a redox buffer (consuming radiolyticoxidants to ensure local conditions remain reducing), a physical barrier to solute transport (iffailure is localised) and a chemical barrier (sorbing radionuclides or including them irreversibly
into secondary minerals formed as corrosion products).
1. Thick steel overpack (H12 reference)
Corrosion allowance option:• Contains a single waste container;• Sufficiently thick to be mechanically stable, reduce
external dose rate and to allow for corrosion over a 1,000
year containment lifetime.

2. Composite overpack
Corrosion resistance option:• Thin layer provides corrosion resistance and steel inner
vessel provides mechnical strength;• Cu and Ti corrosion barriers considered as H12 variants;
other materials also possible (e.g. strong ceramics).

3. No overpack
Option proposed for some concepts involving highperformance geological barriers (e.g. ANDRA, 2001).

A1-2

APPENDIX 1

NUMO-TR-04-03

4. Multiple waste canister overpack
2-6 waste canister overpack:• Essentially a conventional
overpack but with more than one
waste canister;• Simple steel overpack is shown
but composite overpacks are also
feasible and could reduceweight/size of the overpack;• Void space in the cluster designs
filled with glass beads.

5. Multi-purpose, very large overpack (“cask”)
Containing up to 28 waste canisters:• Could be designed for multiple
use - transport / storage / disposal(with additional external
packaging, neutron shield, etc. as
required);• Internal void space filled withglass beads.

A1-3

NUMO-TR-04-03

A1-3.2APPENDIX 1

Buffer

The buffer plays many important roles in the H12 “robust EBS” design (see Figure 2.3). These
include:Mechanical protection from small rock movements due to its plasticity;
Hydraulic barrier, its extremely low hydraulic conductivity ensuring that solute transport
occurs predominantly by diffusion;
Colloid filtration / microbe barrier due to its microporous structure;
Chemical buffer, buffering pH into the mildly alkaline range (enhances the longevity ofoverpack and glass matrix) and providing additional redox buffer capacity (e.g. from
included pyrite);Radionuclide sorption: delaying and spreading releases of many key radionuclides.
In addition, the following functions are expected.
Thermal conduction: limiting maximum temperatures within the EBS;
Thermal stability: key properties of bentonite are preserved to above 100°C in the wet
state and significantly higher temperatures if it is kept dry.
A1-3.2.1 Buffer materials
Bentonite-based material. Major component usually Na-montmorillonite with different
amounts of minor and trace minerals such as quartz, calcite, feldspar, pyrite, etc. Exactcomposition depends on the bentonite source and pretreatment. H12 reference is aJapanese commercial product Kunigel V1.
Bentonite / sand mixtures. Significant quantities of quartz sand can be added to increase
thermal conductivity of the buffer and decrease risk of canister sinking without detrimentto other favourable bentonite properties (e.g. plasticity, low permeability, sorption,
colloid filtration). A 70:30 mixture is specified as a reference in the H12 concept.
The bentonite composition and the type and quantity of additives (e.g. sand, metal,
zeolite) can be varied to tailor properties to particular site conditions (e.g. groundwatersalinity).
Alternative buffer materials have been proposed (e.g. other clays, zeolite). However, thebuffer plays a key role in the H12 safety case and a large body of data for bentonite exists
in Japan and abroad; hence such alternatives are not being considered by NUMO atpresent.

A1-4

APPENDIX 1

NUMO-TR-04-03

A1-3.2.2 Buffer emplacement options
1. H12: in-situ buffer
emplacement for
overpacks in small
tunnels or pits
• Compacted blocks.
• Cold Isostatically
Pressed (CIP) units.
• Pellets.
• In-situ compaction.
• Combinations.

2. In-situ emplacement forlarge volume (e.g.
caverns for very largecasks)• Pellets.
• In-situ compaction.
• Compacted blocks.
• Combination.

3. In-situ ‘wet’
emplacement forboreholes (bentonite
slurry)• Displaced bentonite
mud is pumped toadjacent unfilledborehole to avoid
overflows.
4. Prefabricated buffer• Waste, overpack andbuffer contained
within a handling shell(prefabricated EBS
module (PEM); see
A1-3.2.3) for ease of
emplacement.

A1-5

NUMO-TR-04-03

APPENDIX 1

A1-3.2.3 Prefabricated EBS module (PEM) designs
1. H12-based design• Waste canister.
• Overpack.
• Bentonite / sand buffer.
• Steel handling shell.

2. Variant 1 with separated bentonite andsand• 100% bentonite buffer.
• Addition of a sand layer around theoverpack:- acts to remove the possibility ofoverpack sinking by creep;- increases radionuclide transport
resistance during the period of H2 gas
generation.
(McKinley et al., 2004a)
3. Variant 2 with separated bentonite andsand• 100% bentonite buffer.
• Addition of a second sand layer outside
the bentonite (acts to reduce thepossibility of bentonite erosion byflowing groundwater).
(Toyota and McKinley, 1998)

4. Variant with external shell serving as
overpack• Waste canister.
• Bentonite buffer.
• Outer steel layer (acts as overpack andhandling shell for the unit).
• Equivalent to the Integrated Waste
Package (IWP) proposed by Apted(1998).

A1-6

APPENDIX 1

NUMO-TR-04-03

A1-4 Additional engineered barriers with operational / post-closure
safety roles
A1-4.1

Backfill

Apart from the volume immediately around the overpack, many repository designs include void
spaces in critical areas. The material used to fill such zones is termed “backfill”. Apart fromphysically reducing the instability of openings, such backfill may have additional hydraulic,chemical or radionuclide retention barrier roles.
Various backfill compositions have been considered:
Bentonite / aggregate (sand / gravel) – mixtures containing 10-30% bentonite werestudied in the H12 project;
Crushed rock / bentonite – potentially uses rock spoil from the excavations and use of thehost rock avoids introducing additional components into the system. Performance
depends very much on the rock involved;
Crushed rock – use of the host rock avoids introducing additional components into thesystem and reduces volume of rock spoil. Performance depends very much on the rockinvolved;
Concrete-based grout – in areas where potential high pH alteration will not be
detrimental (e.g. physically or hydrologically isolated from the waste emplacement
areas), cement-based grout or concrete could be considered as a backfill material;
Bentonite or rock with other additives such as zeolites could improve specific properties(such as sorption of specific radionuclides) if this is required;
Sacrificial backfill to act as a chemical buffer between concrete tunnel liner and bentonite
buffer material (McKinley et al., 2004b).
Selection of a backfill composition and an appropriate emplacement technique will depend verymuch on details of the siting environment and the required safety roles of backfill in a specificrepository concept.
A1-4.2

LinersDepending on the properties of the host rock and underground openings, liners may be neededfor various reasons, e.g.:
Ensuring the mechanical stability of openings;
Reducing water inflow;
Providing a smooth, regular surface to aid buffer / backfill emplacement (and QA);
Providing a support structure for transportation (road, rail) or services (ventilation, power,drainage).

A1-7

NUMO-TR-04-03

APPENDIX 1

A1-4.2.1 Liner material options
Shotcrete / rock bolt combinations
Standard engineering option.
Often used as temporary measure before
more substantial liners are emplaced.
Concrete (standard OPC-based)
Standard engineering option.
Long experience with use.
Economical option.
Can be problematic in conjunction withbentonite (high pH causes alteration).
Low pH concrete
Modification to a standard engineering
option.
Little practical experience with use of thisnew material.
Maybe acceptable in conjunction withbentonite (no high pH alteration).
SteelStandard engineering option.
Long experience with use.
May allow for removal before emplacementof waste packages or backfilling of tunnels.
Gas generation (corrosion) could be a
problem in tight formations.
Plastic / resin
Non-standard option.
Limited experience with large scale use.
May be undesirable if left in place (organic
materials) after emplacement but may allow
relatively easy removal.
Geotextiles
Used in combination with other materials
such as concrete.
Novel materials
Ceramics.
(No or very little experience)

Water ingress reduction.
Some rock mechanical support e.g. where
fracturing locally reduces rock competence.
Rock mechnical support where host rock is
too weak to allow self-supporting openings or
where stability cannot be ensured oversufficiently long periods.
Water ingress prevention.
Rock mechnical support where host rock is
too weak to allow self-supporting openings or
where stability cannot be ensured oversufficiently long periods.
Water ingress prevention.

Rock mechnical support where host rock is
too weak to allow self-supporting openings or
where stability cannot be ensured oversufficiently long periods.
Water ingress reduction / prevention.

Water ingress reduction.

Drainage behind concrete liners.

Water reduction.
Mechnical support.

A1-8

APPENDIX 1

NUMO-TR-04-03

A1-4.2.2 Examples of liner applications
1. Pits
Low pH concrete.
Steel.
Plastic.
Ceramic.

2. Horizontal emplacement tunnels
In-situ emplacement and PEM:
Low pH concrete;
Steel;
Plastic.
PEM only:
Shotcrete + rock bolts;
OPC concrete.
3. Disposal tunnels (verticalemplacement)
Low pH concrete.
OPC concrete.
Steel (removable).
Shotcrete + rock bolts (only in goodrock).
4. Access tunnels
Low pH concrete.
OPC concrete.
Steel.
Shotcrete + rock bolts probably not
sufficient (operational safety).
5. Caverns / working areas
Low pH concrete.
OPC concrete.
Steel.
Shotcrete + rock bolts probably notconsidered sufficient (operationalsafety).
Use of geotextiles for drainage behind
other liner material, e.g. concrete.

A1-9

NUMO-TR-04-03

A1-4.3

APPENDIX 1

Seals

Plugs and seals to physically close tunnels and holes may be required at various locations
within a repository. A wide range of designs have been examined, depending on the host rockproperties and the requirements for the seal in terms of operational and post-closure
performance.
1. Disposal tunnel seal (H12)In situ emplacement of EBS.
Retains swelling buffer material.
Access tunnels may remain open.

2. Disposal tunnel closure (alternative)
For PEM.
For ease of operation to ensure
non-swelling backfill does not spill
into access tunnels.

3. Mined borehole seal
Example includes:
Physical seal by metal packer(“borehole divider”);“Sacrificial” bentonite barrier;Upper concrete plug (ideally low-pH
composition).
4. Major connecting tunnel seals
Isolate disposal areas.
Prevent / minimise groundwatertransport along access tunnel EDZ
from disposal areas.

5. Vertical deep borehole seals
Major safety feature of the concept toensure isolation of wastes fromsurface environment.

A1-10

APPENDIX 1

A1-4.4

NUMO-TR-04-03

Disposal pit cap

In cases where bentonite is emplaced into holes under wet conditions, a cap may be needed toprevent extrusion into the overlying tunnel and subsequent loss of density. Such a cap might
have to resist rather high swelling pressures. Practicality of various options depends on theproperties of the host rock and operational constraints (e.g. whether or not capping is done as aremotely operated procedure).
1. Rock bolted steel panel (H12 option)

2. Steel support (H12 option)

3. Granite slab – type I

4. Granite slab – type II

A1-11

NUMO-TR-04-03

A1-4.5APPENDIX 1

External engineered barrier

For the sake of completeness, options of surrounding the repository with an engineered barrier
are illustrated. Although such designs have not been studied in any detail, the principles
involved have been examined elsewhere, most notably the SKB “WP cave” concept (SKB,1986).
1. Low-permeability barrier
Surrounding low-permeability barriers
have been included in concepts fornear-surface repositories for low-level
radioactive and chemotoxic wastes.
Example considers grout injection from
boreholes drilled from a spiral accessramp to construct such a barrier.

2. Hydraulic cage
High-permeability barrier reduces
gradients over (and hence water flow
through) the repository.
Example considers sand-filledboreholes, hydrofracturing etc. from aspiral access ramp to create the highconductivity region.

A1-12

APPENDIX 1

NUMO-TR-04-03

A1-5 Emplacement geometry
Depending on the properties of the siting environment and constraints set by rock mechanics
and thermal loading, a range of different emplacement geometries can be considered.
A1-5.1

Vertical

1. Pits
Single waste package is emplacedin each pit.
Buffer may be emplaced in-situ oras PEM.

(left: H12)
2. Short boreholes (multiplex disposal)
Multiple waste packages are
emplaced in short boreholes.
EBS may be emplaced in-situ or as
PEM.

3. Mined boreholes
Boreholes some 100s of metres in
depth.
Waste packages most likely
emplaced as PEM.

4. Deep boreholes
Boreholes some 1000s of metres in
depth to ensure isolation.
Waste packages most likely to be
PEM.
Safety ensured by isolation rather
than specific host rock.

A1-13

NUMO-TR-04-03

A1-5.2APPENDIX 1

Horizontal

1. In-tunnel – linear (H12)
Waste packages emplaced centrally inthe disposal tunnel.
Buffer may be emplaced in-situ or as
PEM.
PEM may be placed end-to-end orwith backfill / buffer filling spaces
between.
For in-situ emplacement, waste
packages are separated by sections ofbuffer.
2. In-tunnel – duplex
Two waste packages emplacedside-by-side (but separated by buffer).
Has been developed for cases whereanisotropic rock stresses do not allow
the construction of circular tunnels.

3. Caverns
Waste packages arranged in large open
cavern.
May have an extended period between
waste emplacement and emplacementof buffer / backfill.

A1-14

APPENDIX 1

A1-6

NUMO-TR-04-03

Underground layout

The layout will be strongly constrained by the properties and geometry of the host rock.
1. Single level – distributed emplacement
panels (H12)
Waste emplaced in pits or tunnels,
spread over several panels (tailored tosite geology).

2. Multiple level
Waste emplaced in pits or tunnels,
spread over several panels (tailored tosite geology).
Panels stacked on several levels to
make optimum use of thick host rock.

3. Vertical mined boreholes
Extension of the multi-level layout tofurther increase utilisation of a thickformation with good isolation
properties.

4. Long horizontal tunnels
Example could be used to takeadvantage of coastal sites (emplacementzone extending off-shore).
Could be appropriate to a thin host rockformation.
Probably utilise PEM.

A1-15

NUMO-TR-04-03

APPENDIX 1

5. Caverns
Mainly considered for use with verylarge overpacks.
Dense waste disposal is possible if
caverns are left without buffer / backfill
for the thermal period.
Large stable openings allow long period
of institutional control and monitoring(and retrievability) due to delay inbackfilling.

6. Vertical deep boreholes
Great depth of disposal increases waste
isolation.
Requirements on host formation
reduced.
Minimal EBS may be sufficient (e.g.
overpack only or simple IWP).
N.B.: This option involves somefundamental changes in the basic safetyphilosophy, but is included for the sake ofcompleteness.

A1-16

APPENDIX 1

NUMO-TR-04-03

A1-7 Disposal procedures
The processes involved in waste disposal may vary, in particular depending on the extent towhich there is a desire or requirement for monitoring / inspectability and ease (or difficulty) of
retrieval. Some extreme variants are illustrated.
1. No monitoring / not inspectable
Retrieval made difficult:
Immediate backfilling & sealingof disposal tunnels/boreholes;
Example for vertical deep
boreholes.

2. Immediate emplacement of buffer
but access allows monitoringe.g. :
Vertical mined boreholes;
Short boreholes;
Pits with disposal tunnels left
open.

3. Delayed backfilling around
wastes to allow monitoring andease of retrieval
e.g.:
PEMs in long horizontal tunnels.

4. Long-term monitoring withinspectability and ease of
retrieval
Caverns.
N.B.: Ventilation, drainage andinspection facilities would beneeded during an extended open
period to maintain the waste
packages in good condition before
backfilling.

A1-17

NUMO-TR-04-03

APPENDIX 1

A1-8

Surface facilities and underground access

A1-8.1

Access routes from surface

Idealised examples are shown for illustration. For final design, the repository layout and EBS
chosen, the properties of the host rock and overlying formations and the geography /topography of the site will all have to be taken into account.
1. Vertical shafts

2. Inclined tunnels (ramps)(shafts are ventilation shafts)

3. Combination of shafts and ramp
Likely combination for many design
options if waste transport in a ramp ispreferred.
Shaft access allows more rapidtransport of personnel and other EBS/ construction materials.

A1-18

APPENDIX 1

A1-8.2

NUMO-TR-04-03

Surface facilities

There is great flexibility in the design and layout of surface facilities. Two extreme cases areillustrated – an extended design with all facilities for reception, inspection and encapsulation of
the waste and for construction and operation of the repository (including storage of rock spoil)on the surface at a single location. The second shows an example for the case where useable
land area at the repository site is limited and hence facilities are either constructed underground(e.g. waste reception, encapsulation) or at other locations (e.g. rock spoil storage).
1. Extended surface area
No limitations on surface use.
Most likely at sites with flat topography andlarge available land areas.

2. Limited surface area
Makes maximum use of undergroundstructures.
Probably appropriate to hilly and / or coastal
sites.

A1-19

NUMO-TR-04-03

APPENDIX 1

A1-20

APPENDIX 2

APPENDIX 2

NUMO-TR-04-03

THE IMPORTANCE OF POST-CLOSURE
SAFETY ASSESSMENT IN REPOSITORY
CONCEPT DEVELOPMENT

A2-1 Decision making in stepwise development of the repository
An ultimate goal of geological disposal is to isolate radioactive waste safely from humans andthe environment over timescales far beyond the normal horizon of social and technical planning.
The development of a deep geological repository takes place in several stages within a stepwise
process of planning and implementation. This process requires periods on the timescale ofdecades to reach the stage where repository operations can begin. At the end of each stage, adecision is made whether to move forward and whether the requirements for the next stage need
to be adjusted.
Significant decision points early in the disposal programme may include the choice of host rockand repository concept for the particular Preliminary Investigation Areas (PIAs) selected forfurther investigation. Once a site is selected and an initial repository concept defined, thedecisions involve planning the scope of investigation from the surface and underground,including demonstrations of the engineering feasibility of key repository elements, choices
between design variants and the optimisation of underground layout. At later stages, theprogramme will focus on the licensing procedures for construction, operation and closure.
Throughout repository development, the importance to decision-making of convincing
arguments for long-term safety is high. For many decisions, a safety case is one of the key
sources of information on which the decision is based. Generic understanding of relevant
phenomena, as well as site- and concept-specific models and data, are initially limited, withmany unresolved issues. Site characterisation, repository design and safety assessment are
therefore activities that run throughout the stepwise repository development processes or are
repeated iteratively, with the safety case being developed incrementally.
A siting and design strategy is adopted that aims to develop a predictable and robust system.
Robust systems are characterised by simple, well understood or easily characterised featuresand phenomena and an absence of, or insensitivity to, uncertain or detrimental phenomena. An
assessment strategy is adopted that provides a range of arguments and analyses for the safetycase that are well founded are supported, where possible, by multiple lines of evidence, and are
adequate in their treatment of uncertainty. The safety case may, however, emphasise a limited
number of processes or features of the repository and its environment, if these are particularly
well understood and insensitive to perturbations.
Thus, site selection, development of a suitable design for a selected site, site characterisationand other R&D activities are carried out in an iterative, stepwise manner, providing aframework for:Comprehensive scientific and technical investigations and analyses by the
implementer, including safety assessment and an evaluation of uncertainty, in thecourse of each stage of repository development;
Thorough scientific and technical review and staged development of guidance andrequirements by the regulators;

A2-1

NUMO-TR 04-03

APPENDIX 2

Political and social involvement of interested stakeholders, to take into account theirneeds.

A2-2

Definitions and general features

There are numerous special terms that are used in discussions on the safety of waste repositories.
Box 3 gives definitions of the more common ones, using text from international consensus
documents where possible. Active discussion currently concerns the definition of the safetycase and how it should be used. Although broad agreement exists on the general intention, there
is no real unanimity between countries and organisations on precise requirements, definition orapplication. Despite having an origin and use in normal industrial practice, the term does notexist in formal radioactive waste regulations in most countries and is difficult to translate into
many languages - including Japanese. At its simplest level, it is a collection of data, analyses
and arguments used to show how a repository system performs and provides safety. This
collection includes evidence of the intrinsic quality of the site and design, rigorous modelling
aimed at producing quantitative dose or risk estimates to demonstrate regulatory compliance(i.e. safety assessment) and also “softer” information to illustrate how the system or its
components work. Examples of such information are simplified descriptions of key processes,
observations of natural analogues, comparison with natural geochemical fluxes, etc.
Safety assessment is a formal method for quantifying the behaviour of each component of thedisposal system as it evolves with time and of translating this behaviour into estimates of its
impact on the overall performance of the repository. It attempts to assess the quantitative
impacts of the following factors that characterise the disposal system (also taking into accounttheir variability):
The properties of the radioactive waste to be disposed of;
The materials and structures planned to be used and the processes by which theseinteract;
The characteristics of the geological environment surrounding the repository and theprocesses taking place in it;
The behaviour of radionuclides in the near field, geosphere and biosphere, and theirradiological impact on human health.
Although there are differences between national programmes in the detailed procedures that are
followed in the course of a safety assessment, the broad tasks that are carried out in the courseof any post-closure safety assessment are essentially the same. The major elements are:
System understanding: carefully describe the appearance or state of the repository
when it has just been closed, and consider what changes the repository could undergo
in time as a consequence of both internal processes within the repository and external
forces;Scenario analysis: identify broad scenarios that illustrate the range of possibilities forthe evolution of the repository and its surrounding environment;
Consequence analysis: analyse these scenarios and evaluate their consequences forsafety, taking into account all relevant uncertainties.

A2-2

APPENDIX 2

NUMO-TR-04-03

The essence of safety assessment is the use of models of the time-dependent behaviour of thecomponents of the system, or of processes occurring within them. These produce quantitativedescription of future states of the system and resulting radionuclide behaviour. These models
should be tested as thoroughly as possible.
As a consequence of the need to project the behaviour of the disposal system into the distantfuture and to large spatial scales, safety assessment models rely heavily on information which is
extrapolated in both time and space. Part of this extrapolation includes deciding how best to
represent the geometry of the repository and the host rock in the models. As a result of spatial
and temporal variability, in both the processes modelled and the properties which they affect,
there is considerable uncertainty associated with safety assessment results. This uncertainty
increases with increasing time, particularly beyond the timescale over which geologicalstability can be confidently assured (100ka – 1Ma).
In safety assessments it is not assumed that our knowledge of the natural system or of the
behaviour of the engineered barriers is perfect or complete. However, the safety case should not
rely on properties of the geological environment or on the performance of the engineered
barriers for which knowledge is poor or lacking. In safety assessment, uncertainty about the
future evolution of the system can generally be taken into consideration by:
Selecting appropriate scenarios;
Making conservative assumptions (overestimating consequences);
Studying the influence on safety assessment results of the assumptions used, bysensitivity analyses and also by “what if? ” analyses;
Defining stylised situations in the case of the biosphere and future human actions.
A key issue is showing how much uncertainty exists in each argument and what its impact is on
confidence in overall safety. Perhaps the key definition is that a safety case comprises multiplelines of arguments to show safety (including indicating the level of confidence in the models
and data used for its evaluation), with an emphasis on frequent iterations of safety assessment
as knowledge grows. This is the approach being adopted by NUMO which sees it as part of astaged process that will need dialogue throughout, with both the public and regulators and also
peer review by a wide range of technical expert groups.

A2-3

Making a safety case in the context of the Japanese
programme

The concept of geological disposal in Japan is similar to that in other countries, being based ona multi-barrier system which combines the natural geological environment with engineered
barriers. Particular consideration is given to the long-term stability of the geologicalenvironment, taking into account the fact that Japan is located in a tectonically active zone. Due
to Japan’s complex geology, an engineered barrier system (EBS) with sufficient margins in its
isolation functions to accommodate a wide range of geological environments and their potential
future states was developed in H12 (see Section 2.3). The major contribution to overall safetybarrier performance of the disposal system is provided by the near field, while the remainder ofthe geosphere serves to reinforce and complement the performance of the EBS (see Section
2.3).
A2-3

NUMO-TR 04-03

APPENDIX 2

The safety concept assumes that major disruptive events can be excluded by site selection.
Geological environments having favourable characteristics for the disposal system provide thebasis for repository design. If the safety functions of the geological disposal system can beassured, minor amounts of radioactivity released from the EBS will further decay andconcentrations will be reduced by dilution during the long migration period in the geosphere.
Thus, the basic functions of the repository system provide intrinsic, long-term, passive safety.
To support this safety concept, R&D activities have focused on understanding natural system
attributes that favour EBS performance, including relative tectonic stability, low groundwater
flux, favourable geochemistry and a low risk of disruptive events. Safety assessment has been
conducted for a defined repository system, taking alternative future evolutions of the system
into account in order to illustrate the robustness of its intrinsic safety features (JNC, 2000).
A2-3.1 Confidence in the intrinsic safety features of the disposal concept
In order to promote confidence in the technical feasibility of the disposal concept, it is requiredto:
Select geological environments which are stable for a long time and provide
favourable conditions for EBS performance and significant radionuclide retardation
capacity;
Develop an appropriate design for containment and retardation of radionuclides in the
EBS for the selected geological environments.
Selection of geological environments suitable for a repository
Important natural phenomena which could influence the long-term stability of the geologicalenvironment include fault activity, volcanic activity, uplift and erosion, and climatic andsea-level changes. The occurrence of these natural phenomena and the extent of changes in thegeological environment caused by them were investigated over various timescales extendingout to over a hundred thousand years, based on field studies in regions where the long-term
impact of these phenomena could be observed. These studies showed that the location of
sudden localised phenomena, such as volcanic activity and major fault movement, can be well
specified and their effects can thus be avoided by selecting an appropriate disposal site. On theother hand, gradual phenomena such as uplift and denudation or climatic and sea-level changes
are more ubiquitous. It is, nevertheless, possible to estimate future trends and potential effects
by extrapolating data obtained from the field studies. The conclusion from the studies is that it
is possible to select a sufficiently stable environment for geological disposal. Nationwide
Evaluation Factors and Site-Specific Evaluation Factors for qualification of sites have beendeveloped by NUMO for selection of the PIAs (NUMO, 2004).
The characteristics of the geological environment which are important in terms of design andsafety of the repository include size and geometric configuration of host rock, groundwaterflow rates, rock permeability, the geochemical characteristics of groundwater, the thermal andmechanical properties of rock formations and solute transport properties. Favourable Factors
including these characteristics are being defined to help determine the overall practicality of arepository project (NUMO, 2004).
The datasets relevant to these characteristics will be used for subsequent studies to determine
A2-4

APPENDIX 2

NUMO-TR-04-03

appropriate repository designs and characteristics of the engineered barriers. The geological
data also provide essential input to the safety assessment, for evaluating potential doses
resulting from expected evolution scenarios.
Demonstration of repository design and engineering technology
The design requirements for the EBS and the general disposal facility were initially specified
based on utilisation of currently available technology, taking aspects such as economics andenvironmental impact into consideration. The practical feasibility of designing and emplacing
the EBS and constructing the disposal facility is then examined for a given siting environment.
The design of the EBS and disposal facility should be sufficiently flexible at early stages ofrepository development to ensure that it can be finally tailored to the specific characteristics ofa potential disposal site and respond to developments in science and technology in the decades
until it is finally implemented.
This combination of engineering design studies, taking into account the information for a given
siting environment, should show that the intrinsic safety features of the repository will perform
as expected, and are practicable from both engineering and economic viewpoints. In addition,
demonstrating that emplacement is feasible requires building confidence that the repository canbe safely constructed and that the resulting construction will be of very high quality (i.e. largely
free of defects in workmanship that would affect its barrier functions).
A2-3.2

Safety assessment to illustrate robustness of the barrier functions

While the main safety functions of the repository are intrinsic to the design and location withina deep geological system, understanding the level of robustness of the proposed system invarious geological conditions or under changes in system conditions can only be evaluatedthrough long-term repository performance assessment. The assessment method developed andapplied should be able to evaluate the safety functions and the level of robustness of the
proposed system under various conditions.
Scenario development
In order to reduce the risk of overlooking potentially important scenarios, a systematic scenario
analysis methodology has been developed and applied in H12. A comprehensive list of FEPs
(Features, Events and Processes) is firstly developed by collating the FEP lists developed inother projects (e.g. OECD/NEA, 1997; Nagra, 1994). The only scenarios modelled in detail in
H12 are “groundwater scenarios” (scenarios in which moving groundwater provides thepathways for transfer of radionuclides from the repository to the surface environment). As
shown in Figure A2-1, these include:
A Base Scenario, in which external events and processes such as natural geologicaland climatic phenomena, initial EBS defects and future human activities are excluded;
A set of perturbation scenarios, in which the potential impacts of external events andprocesses are examined.
A Reference Case is defined for the Base Scenario, incorporating a particular set of geologicalcharacteristics, design features, model assumptions and parameter values. Alternative cases arealso defined for the Base Scenario (with alternative geological settings, design features, model
assumptions or parameter values) and for perturbation scenarios.

A2-5

NUMO-TR 04-03

Figure A2-1:

APPENDIX 2

Classification of scenarios in H12.

Some FEPs could generate complete “isolation failure scenarios” (scenarios in which thehuman environment is directly affected due to the loss of physical isolation of the waste).
Examples are direct human intrusion and extreme scenarios associated with natural phenomenathat bring the wastes directly to the surface. These FEPs were screened out (e.g. on the basisthat they could be excluded by siting). Nevertheless, some “what if?” analyses have been
carried out to illustrate the magnitude of potential consequences, and thus the importance ofsiting the repository in a suitable environment.
Development of models and databases
Based on a list of feasible scenarios, models which simulate relevant phenomena in detail,together with associated databases, were established in order to quantify selected scenarios.
Models were developed to simulate the evolution of the EBS and subsequent radionuclide
migration in the rock surrounding the buffer material.
If dose or risk is to be used as an indicator of whether or not releases to the biosphere are
acceptable from the point of view of safety, some assumptions regarding the evolution of thebiosphere, including the habits of potentially exposed groups, must inevitably be made. An
approach adopted in H12 to this problem is to employ a stylised representation of conditions in
the biosphere, including the habits of potentially exposed groups, using the concept ofReference Biospheres.
In H12, it was assumed that radionuclides are transported through the host rock to a majorwater-conducting fault (retardation taken into account) which leads to a shallow aquifer (noretardation considered) near the ground surface. Radionuclides then enter the biosphere system
via a geosphere-biosphere interface (GBI). A number of GBI types (e.g. river, deep well,marine) are possible, since a wide range of geological environments throughout Japan have tobe taken into account. Among them, a river in a coastal plain is selected for the Reference Case.

A2-6

APPENDIX 2

NUMO-TR-04-03

From the standpoint of the performance assessor, data are used not only as input for models, butalso to provide support for the choice of scenarios, support for the assumed favourable
characteristics of the disposal system as well as for specific model assumptions (or theirconservatism), yardsticks for comparison with complementary performance indicators (e.g.
natural radiotoxity fluxes), etc. The collection of such data requires coordinated input from anumber of specialised disciplines.
The models mentioned above were linked together in a safety assessment model chain, whichallows the performance of the entire geological disposal system to be assessed. The Reference
Case was analysed using this model chain for a repository containing 40,000 waste packages.
The calculated results for the Reference Case indicate that sufficient containment ofradionuclides can be achieved by the EBS and the near-field host rock, provided thegroundwater flow rate is reasonably low (Figure A2-2)*.
The approach and results of the H12 safety assessment have been compared with those of othernational programmes. There are differences in the disposal concepts and assessment
methodologies adopted internationally, and an understanding of the background to, and reasons
for, these differences can contribute to confidence in the reliability of the H12 safetyassessment. The fact that different disposal concepts and assessment approaches indicate thesafety of geological disposal also suggests that there is scope for optimisation of disposal
concepts and assessment methods in the future to fit site-specific conditions.
As dose estimates inevitably include significant uncertainties from biosphere modelling,
supplementary safety indicators have been investigated. For example, it has been shown thatcomparison of radionuclide concentrations calculated at several positions in a disposal systemwith concentrations of naturally occurring radionuclides provides a useful perspective on
repository safety.
The methodology developed in the H12 project provides a basis for the performanceassessment of a geological disposal system at a site to be selected in the future. At thesite-specific stage it will be important to incorporate realistic geological features and optimised
engineered barriers without compromising the robustness in the methodology that is essential toa defendable assessment.
As discussed in Chapters 4 and 5, goals have been specified for developing a second generationof performance assessment tools, and the R&D required to support and, eventually, validatethem has been considered. Indeed this development work has already been initiated to ensurethat the capacity to compare the long-term safety of repository concepts specific to defined sites
is available for the evaluation of PIAs planned within the next decade.

*N.B.: These calculations are extended to very long times to illustrate trends – showing, for example, that very
extensively retarded actinide chains do not give rise to calculated doses above those of the more mobile fission
products (Cs + Se). As discussed in the companion report (NUMO, 2004), geological stability can be assumed ina well chosen site for at least 105 years; any model output beyond such times must be regarded as extremely
uncertain.

A2-7

NUMO-TR 04-03

Figure A2-2:

APPENDIX 2

Reference Case dose evaluation and the combined radioactivity per waste
package of all radionuclides in different system components as a function oftime (after Makino et al., 2002). The repository is assumed to contain 40,000
waste packages and all packages fail 1,000 years after disposal. The shaded
area indicates the period of increasing uncertainty in the applicability of allmodel components.

A2-8