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

Probability: the likelihood of future tectonic impacts on a repository ........................... 1

1.2

Participants in the ITM Project ..................................................................................... 3

1.3

Report content and associated reports ........................................................................ 4

2 The ITM Project and Precursor ITM activities ........................................5
2.1

ITM-1 ............................................................................................................................ 5

2.2

ITM-2 ............................................................................................................................ 5

2.3

ITM-3 ............................................................................................................................ 8

2.4

The ITM project .......................................................................................................... 11

2.4.1

ITM-4 ................................................................................................................... 11

2.4.2

ITM-5 ................................................................................................................... 12

2.4.3

ITM-6 and 7 ......................................................................................................... 15

2.5

Disseminating the Project Results ............................................................................. 15

3 The ITM Methodology Road Map ..........................................................19
3.1

Outline of the ITM Methodology ................................................................................. 19

3.2

The Steps in the ITM Methodology ............................................................................ 21

3.3

References for Section 2............................................................................................ 39

4 Assessing the Consequences of Low Probability Tectonic Event
Scenarios ...............................................................................................41
4.1

Safety Assessment Overview .................................................................................... 41

4.2

Natural-event Scenarios in Safety Assessment......................................................... 46

4.3

The Role of Probabilities in Safety Assessment, Including Scenario Probability....... 47

4.4

Event Definition for Natural-event Scenarios ............................................................. 49

4.5

Systematic Classification of Natural-event Scenario Consequences ........................ 50

4.5.1

Volcanism Scenarios........................................................................................... 50

4.5.2

Tectonic (Rock-deformation) Scenarios.............................................................. 50

4.6

Qualitative Consequences for Example Natural-event Scenarios ............................. 54

4.7

Summary .................................................................................................................... 58

4.8

References for Section 4............................................................................................ 59

1 Introduction
The potential for volcanic and rock deformation impacts on a repository site needs to beconsidered at each stage of NUMO’s siting programme. Whilst the nationwide evaluation
factors for qualification (EFQs) for PIA acceptance are designed to remove clearly unsuitablesites from consideration, they cannot guarantee that, over the next tens of thousands of years,
the risks of tectonic hazard for a chosen PIA will be acceptable. This is because large partsof Japan that are potentially suitable for siting are directly affected to varying extents by rockdeformation, the peripheral impacts of volcanic activity or the possibility of new magma
intrusion or volcanic activity. The EFQs were only intended as preliminary screening
guidleines to prevent obviously poor candidates entering the siting process.
Consequently, an integration of additional and more refined techniques is required to evaluatesites that pass the EFQ test, so that NUMO can have a clear idea of the likelihood andpotential impacts of tectonic events and processes at each PIA. The ITM project was initiatedin 2005 to provide NUMO with such a methodology, based upon state-of-the-art approaches
used internationally, developed and extended for the specific purposes of NUMO and thespecific conditions of Japan: hereafter, we refer to it as the ‘ITM Methodology’.
The ITM Project developed out of the considerations and discussions that took place at aseries of previous International Tectonics Meetings (ITM) organised by NUMO over a periodof 7 years, from 2002 to 2009. The ITM Project was completed in March 2009 and this reportprovides both an overview of the whole project and the final update of the methodology
developed, based upon progress during the final year of the project (April 2008 to March2009). In addition, a brief outline is provided of the ITM activities that preceded the ITM
Project itself.

1.1

Probability: the likelihood of future tectonic impacts on a repository

During the course of the project, both NUMO and the Japanese regulatory agencies wereconsidering how best to handle the evaluation of low probability, disruptive events (e.g.
volcanic intrusion, fault rupture) and deformation processes that are discontinuous in time andmagnitude in response to continuous regional strain, when carrying out safety assessments ofgeological repositories for radioactive wastes. Essentially, two approaches have been
adopted internationally to address this situation:
To calculate the health risk1 to people in the future by combining the probability of a
disruptive event occurring with its radiological consequences in terms of releases
from a repository: simply, risk = probability x consequence. With this approach,regulatory standards or targets can be defined in terms of risk to an individual.



To consider the impacts of a disruptive event and calculate the radiological doses2 topeople in the future and then, separately, to discuss the likelihood that this might
happen (the so-called ‘disaggregated’ approach). With this approach, separate
regulatory targets for radiation doses might be set for events (or scenarios) withdifferent degrees of likelihood (often expressed qualitatively; e.g. ‘likely’, ‘less likely’,‘highly unlikely’).

In either approach, an appreciation of probability is essential: in the first ‘risk approach’ asound quantitative estimate will provide more confident estimation of risk; in the second,some form of quantification of ‘likelihood’ is needed to decide which category to place anevent or scenario into.

1

Health risk is normally defined as the risk of death or serious genetic effects.

2

Of course, a radiological dose can also be expressed in terms of health risk, by applying accepted
dose-to-risk conversion factors.

1

The ITM methodology that has been developed in this Project is centred on a probabilistic
approach. A probabilistic approach is seen by the ITM group as the only realistic means ofquantitatively addressing the uncertainties in assessing possible hazards when there ismarked variability in the spatial distribution, the timing, the intensity and the style of the
volcanic and deformational events and processes being evaluated (for convenience, in thisreport, we frequently group these together within the general term ‘tectonic events andprocesses’). The probabilistic approach being developed is based upon and strongly
supported by deterministic models of the underlying tectonic processes that lead to magma
intrusion, volcanism and rock deformation.
NUMO is developing both the probabilistic ITM methodology and other, independent,
deterministic approaches, in parallel projects that will eventually be deployed at volunteer
sites when they arise. The weight that will be given to deterministic and probabilistic
evaluation results will depend, to some extent, on the nature and the geographical location ofthese sites. It will also be influenced by the direction in which regulatory development moveover the next few years. As noted above, however, some quantification or estimation ofprobabilities is required in any reasonable approach to safety evaluation and regulation. For
the specific tectonic circumstances of Japan, this is especially relevant and important.
The aim of the current project reported here is to use the probabilistic ITM methodology atthree important stages of NUMO’s siting programme:


SITING STAGE 1: during the literature survey (LS) stage when potential PIAs arebeing assessed. The ITM methodology will use currently available information toallow comparison of sites in terms of confidence that they are likely to prove
acceptable with respect to tectonic impacts.



SITING STAGE 2: during the planning of the PIA site investigations, to identify
geoscientific information requirements that will be needed to refine the Stage 1
analysis.



SITING STAGE 3: at the point where PIAs are being evaluated and compared inorder to select a preferred site (or sites) for detailed investigation (as DIAs).

The ITM project is mainly concerned with Stages 1 and 2. Application of the methodology in
Siting Stage 3 is several years into the future and it is expected that it will be most efficient tocarry out any necessary updates/refinements on a region-specific basis during the PIA
investigations when NUMO has narrowed down to a group of sites. The ITM project involves
methodology development and testing only and does not include actual deployment forvolunteer sites/regions. The methodology can be used to:produce regional maps of relative probability of volcanic events within 5 x 5 km areas(the approximate repository footprint, plus immediately surrounding rock volume);



produce regional maps of probability of exceeding specified strain rates (rockdeformation) within 5 x 5 km blocks;

identify possible impacts of such events and the information that needs to be fed into
safety assessment scenarios to comply with risk-based or disaggregated doseprobability regulatory requirements;



identify the information that can be obtained during PIA investigations to refine andprovide confidence in the regional probability analyses.

The way in which the methodology could be applied at any of these stages could be atdifferent levels:

simple hazard evaluation using probabilities: estimation of susceptibility to tectonic
and volcanic events and processes:oof a single site;

2

o


1.2

comparison among a group of sites.

full safety evaluation combining probabilities with consequences: safety assessments
for tectonic scenarios, evaluating in detail how an event might impact a repository at asite and what the radiological health effects would be:orisk based;odisaggregated, dose-based consequence analyses.

Participants in the ITM Project

The Project involved the close interaction of Japanese and international experts to gain thenecessary understanding to carry out the methodology development and testing. Theinternational project team would like to thank in particular all those Japanese experts who
provided information and advice and led the team on field visits, sometimes over several
weeks, to the Case Study areas and other localities where data could be gathered andobservations made on features and processes of interest.
The international experts came from the following organisations (the acronyms used in thisreport are indicated):
MCM
UBR
USF
UTX
GNS
MSL
CPE

MCM Consulting, Switzerland
University of Bristol, UK
University of South Florida, USA
University of Texas, USA
Geological and Nuclear Sciences, New Zealand
Monitor Scientific LLC, USA
Colenco Power Engineering, Switzerland3

ITM International Project Team Members
Rock Deformation

Dr Kelvin Berryman
Dr Nicola Litchfield
Dr Mark Stirling
Dr Laura Wallace
Dr Pilar Villamor
Dr John Beavan
Dr Susan Ellis
Dr Warwick Smith

GNS Sciences, New ZealandProfessor Mark Cloos

University of Texas, USA

Professor Charles Connor
Laura Connor
Koji Kiyosugi

University of South Florida, USA

Professor Steve Sparks
Susan Mahony

University of Bristol, UK

Dr Olivier Jaquet

CPE Power Engineering,
Switzerland (now at In2EarthModelling, Switzerland)Interface with PA

Dr Mick Apted

Monitor Scientific, USA

Project Co-ordination

Professor Neil Chapman

MCM Consulting, Switzerland

Volcanics

3

Work was transferred to In2Earth Modelling Ltd, Switzerland in early 2008, following movement of the
key expert.

3

1.3

Report content and associated reports

This report provides both an overview of the whole project and the final update of themethodology developed, based upon progress during the final year of the project (April 2008
to March 2009). To begin, Section 2 provides a short outline of the ITM activities thatpreceded the ITM Project itself, as well as the development of the Project over the last few
years. Section 4 reports on the conclusions of the evaluation of how to transfer the results of
the ITM methodology to NUMO’s safety assessors (Step 6 of the ITM methodology, describedin Section 3).
Two other reports comprise the main output of the Project. These describe the development,
application and testing of the methodology in two Case Study areas, in northern Honshu andKyushu:

The Tohoku Case Study Report was completed in the previous financial year andfinalised for publishing as a NUMO Technical Report in 2009 (NUMO-TR-08-03).



The Kyushu Case Study Report was completed this year and will be finalised for
publishing as a further NUMO Technical Report in mid-2009.

This report and the Case Study reports are compilations of material provided by all of theresearch groups listed above.

4

2 The ITM Project and Precursor ITM activities
The earliest work of the International Tectonics Meeting (ITM) dates back to 2002. At this time,
NUMO issued its call for volunteers and was beginning to develop its siting programme.
NUMO acknowledged at the outset that a comprehensive and scientifically defensible
approach would need to be taken to evaluating tectonic risks to a HLW repository.
The first two years of ITM were intended to familiarise key international scientists with thenature of the problem, initiate discussions between those scientists and Japanese specialists
in tectonics and to begin to identify possible approaches that could be used to assess tectonic
risks and manage them by an appropriate siting programme.

2.1

ITM-1

The initial, pilot ITM meeting took place in Tokyo in January 2002 and involved Neil Chapman,
Nagra, Switzerland (Chairman), Clarence Allen, CalTech, USA, Mick Apted, Monitor Scientific,
USA, David Jackson, UCLA, USA and Stephen Sparks, University of Bristol, UK.
The main aim of the meeting was to look at the tectonic information available to NUMO,
specifically the recently produced report by JSCE4 on geological factors in site selection, andto provide suggestions and advice on how best to deploy this in the development of tectonic
Siting Factors.
A key consideration was the need for NUMO to take geologically and tectonically informed
siting decisions that would achieve a reasonable consensus of support amongst the national
and international Earth Sciences and engineering communities. Consequently, one aim wasto advise NUMO of issues where it would potentially be exposing itself to contentious debate
if it were to adopt a particular position or approach. The meeting focussed on five main topics:the ‘predictability of stability’ of the tectonic framework of Japan;



volcanic activity;

seismic activity;

active faults;

uplift and erosion.

For each topic, a preliminary statement of a possible NUMO position was set up to catalysedebate. These statements were based on possible interpretations of the recommendations of
the JSCE report in particular (although some could be traced back to the H-12, 2nd JNC
Progress Report, or earlier, AEC guidelines, and some simply reflected the requirements of
the nuclear law). The main findings of the discussion on each topic were recorded, withpoints of agreement being highlighted. The position statements were then revised to reflect
the agreed views. These revisions were advanced for NUMO to consider as more developed
or more appropriate bases from which to develop tectonic Siting Factors.

2.2

ITM-2

The second ITM meeting took place in March 2003, again in Tokyo and the international
participants (the ITM Core Group) for this meeting were the team that eventually took the ITM
programme through to completion six years later: Neil Chapman, Switzerland (Chairman), ,4‘Geological Factors to be Considered in the Selection of Preliminary Investigation Areas for HLW
Disposal.’ Sub-Committee of the Underground Environment, Civil Engineering Committee of the Nuclear
Power Facilities, Japan Society of Civil Engineers. August 2001 (in Japanese): Draft English translation:
December 2001.

5

Mick Apted, USA, Stephen Sparks, UK, Kelvin Berryman, New Zealand, Mark Cloos, USA
and Charles Connor, USA. Bill Arnold, Sandia Laboratories, USA, also joined this meeting as
an observer and to describe the work he had been carrying out in a joint NUMO-USDOE
collaboration project on vocanism.
ITM 2 was convened with the objective of considering two issues of importance at the thencurrent stage of the NUMO siting programme:
1. How NUMO can conduct technically and scientifically defensible assessments of initial
volunteer sites on the basis of literature data only.
2. How surface-based investigations can be designed to collect the necessary information
during the PIA phase to support the future decision on DIA selection.
As noted by many ITM 2 participants, geological issues are among those that are of greatest
concern to both technical and non-technical stakeholders. It was felt that NUMO needed to be
confident that it understands the tectonic regime in a volunteer area sufficiently that it could
make a properly considered decision on whether to advance a volunteer location as a PIA.
The issues that NUMO needed to be aware of at this stage were considered to be: the likelihood that the area could be prone to disruptive tectonic processes and events
over the lifetime of a repository, which would need to be taken into account whenassessing long-term safety of a site; whether this likelihood can be adequately judged on the basis of current theory andknowledge about the time and space distribution of these disruptive mechanisms;
 whether there is enough regional and site-specific information available now to apply
current conceptual models at the volunteer sites, before it is possible for NUMO to begin
collecting its own data in the field;
 whether current conceptual models (at the time of this meeting) were sufficiently credible
and robust to withstand peer evaluation by the broader earth science community when
PIAs are being selected;
 whether gathering more data in the PIA field investigation stage can significantly reduceand quantify uncertainties in predicting tectonic impacts and their likelihood of occurrence.
Each of these issues was considered to represent a potential risk to NUMO’s programme,
and the aim of the ITM was proposed as being to help ensure that these risks are properly
discussed and understood. Hence, the information needed to explain and resolve suchconcerns is of extremely high priority to NUMO. With these objectives in mind, thepresentations by Japanese and international experts at ITM 2 and the subsequentdiscussions were focussed onto two key areas (which subsequently became the main strands
for the rest of the ITM Project):

igneous activity;rock deformation (faulting, folding and uplift).

At the end of the workshop, the participants summarised by expressing their belief that
NUMO’s mission to identify and confidently characterise a geologically stable site for longterm geological isolation of nuclear waste was tractable. There are many world-classgeological experts, expertise and facilities in Japan that are providing enormous amounts ofhigh-quality information and understanding on the dynamic character of Japanese geology. It
is not NUMO alone that is gathering and analysing such information. There is a large
resource available and NUMO needs to involve the best sources and use the best data
available.

6

ITM members noted that it would be important for NUMO to develop and publish general
methodologies showing how it plans to link these capabilities into a multi-disciplinary
programme that evolves in a continuous manner with the planned steps in its site selection
process. The importance of documenting these methodologies is not only to provide thetechnical and scientific basis for site selection, but also to communicate to Japanese policy
and public stakeholders that NUMO fully recognises and is addressing their key concerns
regarding long-term geological stability, especially over the 100 ka period of principal concern
in their safety assessment studies (at the time of this meeting: there is now interest in looking
out to 1 Ma in safety assessment work).
Both topical areas, volcanism and active rock deformation, reduced to a matter of estimating
reliable probabilities of disruption to a repository. Fully supported probabilities are not needed
until the time of justification of the choice of a repository site and its associated safety case.
Until that time, proceeding with a site where there may be uncertainty about the likelihood andimpacts of disruptive tectonics is NUMO’s own ‘developer risk’ – it could eventually prove to
be too difficult to make the safety case for such a site.
It was considered that volcanism could be addressed at that time, using available informationand it is thought unlikely that NUMO will encounter any major surprises when it evaluates
volunteer areas. The probability of proceeding with a volcanically ‘risky’ site into DIA andbeyond should be easy to establish early on. Instead, NUMO is likely to see a progressive
increase in understanding of the inevitable uncertainties involved as it moves through the PIA
and DIA stages, which will lead to increasing confidence in risk estimates. The level ofconfidence to attach to probability estimates made during the next few years, before site specific information is of sufficient quality and quantity, will be relatively low. It wasrecommended that NUMO carries out scoping PA studies of volcanic impacts to see whatlevel of likelihood of different volcanic impacts is tolerable in terms of consequent radiologicalrisk. The underlying models used to estimate probabilities and the approach to applying them
could be tested best by establishing some case studies in regions of polygenetic andmonogenetic volcanism over the coming 2-3 years, in consultation with the groups
responsible for the conceptual models and the data gathering.
For active rock deformation, the picture was not considered to be quite as simple. While it ispossible to avoid known features, there may be surprises in the DIA stage when new features,
especially at depth, are discovered. NUMO’s best approach in the meantime was to develop
an approach to identifying stable rock blocks (volumes of rock with little internal deformation,
where strain is largely taken up on well-defined boundaries) using the full range of geological,
geodetic and geophysical data that are coming available, particularly seismic, GPS and radar
imagery data. Again, this approach could usefully be tested using a case study region ofJapan.
The literature survey stage involves uncertainties for NUMO, and it was thought important that
NUMO states clearly that it recognises these uncertainties and that it has, and is continuing todevelop, approaches to address them. Initial geological analysis of specific volunteer sites will,of course, be qualitative in assigning probabilities of hazards because of limited and variable
literature data that will undoubtedly contain some gaps of information. A potential risk inreliance on literature data is in understanding the limitations, uncertainties and deficiencies
that are inherent in any set of collected data. This risk is further compounded by the fact thatinterpretations published with these data are typically not addressing repository-specificsituations and issues, and credible alternative interpretations may not be fully explored.
The participants felt that, as early as possible, NUMO should assess what types and levels of
information it is likely to have at each step of the site-selection process, and compare thisassessment to what information is needed to support a decision to proceed to the siting step.
This comparison by NUMO is necessary so that that potential information gaps andinsufficiencies can be identified and addressed well in advance of the time when siting
decisions must be made and defended. ITM members themselves expressed interest inperhaps assisting NUMO and its contractors in this effort of integrating data and information in
the areas of credible predictions of long-term igneous activity and rock deformation.

7

A major challenge facing NUMO would be its ability to integrate and interpret all of thisdiverse information, and to keep pace with the steady generation of new seismic and geodetic
data, the latest developments in techniques and evolving conceptual models. Although site specific data will always be necessary for basing siting decisions, NUMO was sensiblyplanning to supplement such site-specific perspectives with analogue information that is being
developed in two underground research laboratories in Japan. In exactly the same manner,
ITM members recommended that NUMO also consider studies from other analogue sites
outside of Japan (e.g. New Zealand and California have analogous deformation environments
and features, the circum-Pacific region is characterised by pervasive polygenetic andmonogenetic volcanism that is contemporaneous and mechanically related to similar
volcanism in Japan). Supplemental data and interpretations from such studies were thought
to be equally useful to NUMO in providing the highest level of scientific confidence toconcerned policy makers and public stakeholders regarding their future siting
recommendations. This was also an area where the international members of ITM might beable to assist NUMO.
Nevertheless, the main recommendation of ITM 2 for the next step was for NUMO to develop
volcanic and active deformation case studies in selected regions of Japan, involving theresources that NUMO already has to hand, plus other organisations that hold data andconcepts that could be useful.
Prior to the ITM 2 Meeting, NUMO arranged an extremely useful 2-day field trip to the KisoMino area of Central Japan. Of particular interest to ITM experts was examining field
relationship associated with active faults, including the Atera and Neodani Faults, as well as
accompanying subsidiary faults within several hundred metres of these principal faults. In
addition, field relationships of wider-scale deformations arising from geological and tectonic
processes were examined and discussed with respect to implications for geological disposal.
Remnants of earlier igneous activity in the Japanese islands were also inspected as
analogues of current and future igneous activity that may develop in Japan. These field trips
provided invaluable opportunities for Japanese and international ITM experts to examine theactual field relationships of Japan geology, to compare alternative interpretative models
based on such evidence, and therefore begin to establish scientific consensus on key
geological issues related to long-term safety of nuclear waste repositories. Discussions in thefield helped expand perspective and build mutual understanding among Japanese andinternational ITM experts. For future ITM workshops, it was considered useful to link fieldvisits to case study regions, if case studies were being undertaken.

2.3

ITM-3

The ITM activities during FY 2003-4 were more diverse than in previous years, as two topical‘mini-workshops’ were organised in the UK and USA as well as the full ITM-3 Workshop in
Tokyo in March 2004. The main focus remained on how NUMO would address the two key
areas of future volcanism and rock deformation (active faulting, flexuring and upliftsubsidence). Both topical areas, volcanism and active rock deformation, reduce to a matter of
estimating reliable probabilities of disruption to a repository. It was felt that fully supported
probabilities are not needed until the time of justification of the choice of a repository site andits associated safety case, but NUMO needed to have an established methodology to dealing
with these issues during each stage of its programme: PIA selection, PIA studies and DIA
selection and site characterisation. It is expected that this methodology will grow and develop
in parallel with the developing requirements of the siting programme, so that it is ‘fit-forpurpose’ at each stage.
In March 2003 it was suggested that NUMO developed and tested the underlying models
used to estimate probabilities of future volcanism and the approach to applying them by
establishing some case studies in regions of polygenetic and monogenetic volcanism overthe coming 2-3 years, in consultation with the groups responsible for the conceptual models
and the data gathering. For active rock deformation, it was suggested that an approach toidentifying stable rock blocks using the full range of geological, geodetic and geophysical datathat are coming available, particularly seismic, GPS, active faulting and folding data should be8

developed. This approach could usefully be tested using a case study region of Japan, andthe advantages of looking at the same area as the volcanism study were apparent.
The 2003-4 activities were planned against this background. Two ‘mini-workshops’ wereorganised to develop the suggested case studies for volcanism and rock deformation and toevaluate the information held on NUMO’s database for suitability. Following these workshops,
suggestions were developed for projects for the next FY to begin building the approaches toassessing the likelihood of future tectonic impacts at potential PIAs. The two outlines formed
the core of the discussions among the wider group of ITM members at the March 2004
meeting.
In addition to these workshop activities, a review was made of the report by the SubCommittee for Safety of Radioactive Waste on “For securing the basis related to the safety
regulation of HLW Disposal”, which contains many comments and suggestions on how to deal
with tectonic issues in the regulatory research programme.
The two mini-workshops were focussed on applying expert knowledge to a 'type' region ofJapan that displays volcanic and deformational characteristics similar to those that may occur
in some areas where volunteer communities might come forward. The proposed region was in
the Sengan area of north-central Honshu and was suitable for independent evaluation by boththe 'volcanics' and the 'deformation' groups, from their own perspectives. The two groups
could then integrate their evaluations and look at factors of overlapping concern. The intentionof the two mini-workshops was to begin to evaluate the data from the Sengan ‘type region’ as
a test bed to develop ideas and approaches. The data from this area were considered to be
typical (to good) and thus fairly representative of what NUMO would have to hand whenvolunteers come forward.
The first mini-workshop (on volcanism) was held in December 2003, hosted by Professor
Stephen Sparks at the University of Bristol, UK. The group heard presentations on the use ofprobabilistic and deterministic approaches to predicting future volcanism (CRIEPI-NUMO
studies and University of South Florida) and worked on-line with the NUMO GIS database toevaluate the scope and quality of data from the Sengan area. The way forward and the dataneeds identified in the discussions are summarised in the following slides.
The second mini workshop (on rock deformation) was held in January 2004 in California, USA,jointly hosted by Dr Mick Apted and Professor Mark Cloos (University of Texas at Austin) in
coordination with Dr Kelvin Berryman (GNS, New Zealand). The workshop was spanned by afield visit to tectonic accretionary rocks analogous to those in SW Japan. The field visit
examined mélange-style deformation in the Franciscan Formation, Quaternary and more
recent coastal deformation associated with plate subduction, and also strike-slip deformation(locked, as well as creeping motion) associated with the San Andreas system. The miniworkshop considered the same issues as that held the previous month in the UK, but from theviewpoint of rock deformation. The main points that came out were:

Evaluating long-term rock deformation will be a key issue for any conceivable
repository site in Japan. Slow deformation/ folding, as well as faulting, has thepotential to compromise the isolation capability of both engineered and natural barriersystems over the relevant time scale.

It was suggested developing and adopting a ‘deformation index’ or ‘strain budget’ asa fundamental framework to integrate the wide array of information that can be usedassess rock deformation. This includes aerial photography, trenching, geologicalmapping, GPS data, geodesy, multiple geophysical measurements, relevant natural
analogues from other sites both within and outside of Japan, local historical andstakeholder information, etc.

The ‘strain budget’ framework would help support NUMO's mission and decisionmaking at each stage of its step-wise siting process, starting with the literature-only
survey of volunteer sites through the final ranking and selection of a candidate site. It
would also serve as an important guide to NUMO when it must set up sitecharacterization programs, providing a basis for (1) comparing different sources of9site-specific data on rock deformation from existing different techniques, (2)identifying additional site characterization data that may be obtained to help resolve
any discrepancies, and finally (3) identifying credible alternative conceptual models
for local, long-term rock deformation.
Based on the discussions at the two mini-workshops, the two groups (‘volcanism’ and ‘rock
deformation’) went on to develop project outlines to provide NUMO with the first elements ofan approach to be used in the first (PIA selection) stage of its siting work. As the outlines
were prepared by small sub-sets of the regular ITM expert group, a key objective was to havethese suggestions presented and discussed at the March 2004 main workshop so that a
broader consensus could be reached on the best approach to each issue.
The main ITM-3 workshop was held in Tokyo from 15 – 16th March 2004 and was precededby a field visit on 13th and 14th March to examine active faults in the Kobe area and rocks of
the Cretaceous and Tertiary Shimanto Belt in Honshu. The two top-level topics that wereconsidered at this meeting were:

TOPIC 1: How to make best use of desk information during the pre-PIA definition stage(i.e. as volunteers come forward) to evaluate whether the areas would make suitable
PIAs. This means using the data to evaluate the controls on the distribution of volcanic
activity and on the nature of future rock deformation (fault and flexure behaviour).



TOPIC 2: How to define a preliminary programme of fieldwork at the PIAs selected so
as to improve the database on these issues, leading eventually to a decision onwhether the areas would make suitable DIAs.

The two suggestions being developed as a result of the two mini-workshops were introduced
for discussion in Working Group sessions within the full ITM group. The principal aspects ofthe proposals were outlined as comprising: Rock Deformation: Establishment of a probabilistic ‘deformation index’ or ‘strain budget’
as a fundamental framework to integrate the wide array of information that can be usedassess rock deformation. This includes aerial photography, trenching, geologicalmapping, GPS data, geodesy, multiple geophysical measurements, relevant natural
analogues from other sites both within and outside of Japan, local historical andstakeholder information, etc. This will help support NUMO's decision-making at eachstage of its step-wise siting process, starting with the literature survey of volunteer sites
through the final ranking and selection of a candidate site. It will also serve as animportant guide to NUMO when it must set up site characterization programmes,
providing a basis for (1) comparing different sources of site-specific data on rockdeformation from existing different techniques, (2) identifying additional sitecharacterisation data that may be obtained to help resolve any discrepancies, and finally(3) identifying credible alternative conceptual models for local, long-term rockdeformation.
 Volcanism: Development and testing in parallel of (a) deterministic, (b) empirical, (c)probabilistic and (d) Bayesian methods of establishing likelihood of volcanic activity in anarea over various future timeframes, using currently available tools and experience as abasis and the Sengan, Tohoku and Chokai-Kurikoma datasets as raw material. These
activities would be tailored to provide NUMO with the scientific and technical support thatthey will need to develop methodologies for volcanic hazard assessment in the PIA stage
that (1) provide a conservative – but not overly conservative – approach to volcanic
hazard assessment, (2) start from a generic approach that will have wide support in thescientific and technical communities, (3) explain guidelines for assessment of volcanic
hazards, (3) create consistency in the volcanic hazard assessment in the PIA stage and(4) easily transfer to the DIA stage if necessary.

10The central section of the workshop looked into detail at the current state of understanding of
the distribution of volcanoes and on approaches to active fault identification and GPS strain
data evaluation.
NUMO was keen to see the results of the ITM programme published and made more widely
available. Consequently, it was proposed that a topical article be prepared for EOS 5 (theAmerican Geophysical Union weekly publication that attracts a huge readership in the earthscience community). In addition, those involved in project work were urged to aim for
publication in the peer-reviewed literature.
The two Working Groups reached consensus on how to proceed with the proposed projects.
Each project would establish an Expert Group to provide scientific and technical input and tohelp guide the progress of the work. The concept of Case Study areas was retained. The key
features of the proposed work was as follows:

The volcanism project will test and compare a wide range of alternative approaches(within the general headings of probabilistic, empirical and deterministic) and apply
them to the same dataset. By the use of ensemble modelling, numerous realisationsof possible volcano distribution will be made and compared. This will provide an
overall likelihood and degree of confidence for the Case Study area.



For the Case Study area, the rock deformation group will develop a balanced,
probabilistic strain budget model that accounts for all seismic, GPS, fault movement
and uplift data and which can be constrained within the overall long-term tectonic(subduction) strain. The objective is to determine whether this approach can accountfor all the expected strain that has occurred within a region over a time period of
hundreds of thousands of years and can thus be extrapolated forward in time.

ITM-3 made considerable advances over the two earlier workshops. It transitioned from being
largely an information exchange meeting to being an active discussion group proposing
solutions to NUMO’s requirements in its siting programme. In the following year, it was
anticipated that ITM would have made the first steps in developing practical approaches thatcan be applied by NUMO.

2.4

The ITM project

By the end of these initial reviews and explorations, it was thus clear that a specificmethodology would need to be developed that incorporated the best available techniques intectonic hazard assessment worldwide and would be directly applicable to the Japaneseenvironment and the specific issues faced by NUMO. Several possible techniques werealready available at this time, but the techniques would need to be imported, further
developed and then tested before actual deployment by NUMO.
Exploratory work to develop a specific ‘ITM Methodology’ began in 2004-5 and the first 18
months involved the transfer of existing approaches and technologies from the scientificorganisations involved in ITM (from New Zealand, the UK and the USA) to NUMO and testing
and consideration of their applicability to the siting programme.

2.4.1

ITM-4

A milestone workshop named "Development of Strategies for Volcano Hazards Assessment"
was held at Tampa in Florida in January 2005. The main objective of the workshop was toreview the progress of work on the Case Study. One of the problems encountered in early
development of the methodology was the meaning of the "dots on the map" in the Catalogueof Quaternary Volcanoes of Japan (CQV). There were several ambiguities andinconsistencies in the CQV associated with definitions of age, interpretation of the amount ofmaterial and difference between "volcano" and "edifice" (e.g. edifices range from 0.045 to 633
5Subsequently published as: Apted, M., Berryman, K., Chapman, N., Cloos, M., Connor, C., Kitayama,
K., Sparks, S. and Tsuchi, H. (2004) Locating a Radioactive Waste Repository in the Ring of Fire. EOS,
Transactions of the American Geophysical Union, 85, No. 45, 465-471.

11km2). Some of these inconsistencies arise from lack of data, while some are probably different
qualitative interpretations by the CQV producers. One of the main activities of the volcanology
group in this period was evaluating the spatial distribution data for use in the probabilistic
analysis. This involved a range of intensive activities including field ‘ground truth’ verification
visits and, eventually, the development of a cladistic based approach to the classification of
volcanic edifices. The other principal activity was the initial development of an approach tomodelling statistical distributions of volcanoes and relating these to structural and geophysicalinformation in the Case Study area. Codes were developed for doing this, using deterministic,
empirical, probabilistic and Bayesian approaches and the first probability maps were
produced. In parallel, initial progress was made on the use of random functions and random
events modelling, using the Cox process modelling approach.
A second and equivalent milestone workshop entitled "Development of a Deformation Index
Map Methodology for Application to Siting Factors" was held at Napier, New Zealand, also in
January 2005. The rock deformation team was pursuing development of a probabilistic
methodology for tectonic evaluation of sites, using a logic tree approach to capture
uncertainties, which could be sampled using Monte Carlo analysis. Strain can be calculatedusing various parameters derived from different models. These models can be compared,
incorporating as much expert opinion as possible to produce "community models". Strainmodels are integrated using logic trees that can accommodate all models and variants ofindividual models and then weight them. New or modified models can be incorporated as new
information comes to hand. This is standard practice for NPP siting and PSHA. By the time ofthis workshop, considerable progress had been made on compiling and assessing theseismic, GPS and fault strain data for the Case Study area.
The ITM-4 meeting was held in Tokyo in March 2005 and was preceded by a field visit to
examine monogenetic volcanoes in the Yamaguchi Prefecture. The ITM-4 meeting provided
the opportunity, over a period of two days, for an intensive and lively discussion between theinternational team and Japanese experts and was the first time that definitive results of themethodology development had been presented to the Japanese geoscience community. By
this stage, there had already been considerable progress in all areas. The impressive state of
the scientific studies that had been presented at the workshop led to the proposal that alonger-term milestone should be the publication of a volume on tectonic hazard assessment
at radioactive waste repository sites. In due course, this became one of the outstanding spinoffs of the ITM project, with the completion of a 700 page edited volume in mid-2008,
scheduled for publication by Cambridge University Press in September 20096.

2.4.2

ITM-5

Work continued through 2005-6, focussed principally on the NE Japan (Tohoku) Case Study
region. The volcanism work involved the production of a derivative database for hazards
assessment, eliminating as far as possible ambiguities and inconsistencies and including thesimple definitions of volcanoes proposed at ITM-4. This database was used to prepare apaper on the applications and potential for cladistic analysis to the classification of volcanoes
which was submitted to, and subsequently published by, Bulletin of Volcanology7.
Development continued of a series of alternative datasets on the spatial and temporal
distribution of volcanoes for use to develop probabilistic hazards maps and assess the effectsof the alternative interpretations and data depictions on the exclusion zones and distributionof probability. The datasets were presented and discussed at an interim volcanics workshopthat was held in Oban, Scotland, in September 2005, to look into dynamical processesmodels to support the probabilistic studies. An associated field visit was made to examine
volcanic structures and classical sites of magma intrusions modes on the Isle of Mull.

6

Connor, C., Chapman, N. A. and Connor, L. (eds.) 2008, in press. Volcanic and Tectonic Hazard
Assessment for Nuclear Facilities. Cambridge University Press.

7

Hone, D. W. E., S. Mahony, R. S. J. Sparks and K. Martin (2007). Cladistics analysis applied to theclassification of volcanoes. Bulletin of Volcanology, 70, 203−220.

12

In parallel, work continued to use empirical models based on geological data and models,
applying probabilistic methods that rely on volcano distribution to assess hazards, andapplying Bayesian methods that combine elements of the empirical and probabilistic
approaches. Results from the Tohoku region strongly suggested that implementing thesemethodologies at the PIA scale would require a clear and defendable basis for expressing
geological observations and models in a hazard framework. For example, how thresholdvalues are established in empirical models; how geological models of structural controls on
volcanic activity are translated into probability density functions. Specific activities included
the continued analysis of available geophysical datasets (e.g., tomography, gravity,
topography, geodetic) to develop a physical model for the relationship between theseobservations and generation of new volcanic activity. Inversion techniques have beendeveloped to relate geophysical observations to the underlying properties of the mantle(density, percentage of partial melt, temperature) that give rise to the anomalies. The
geophysical model was providing insight for assessment of the physical basis for clusteringand longevity of clusters (governed by rates of heat and mass transfer in the asthenosphericwedge). The initial inversion results were presented at the American Geophysical Union(AGU) Fall Meeting, which was held in San Francisco in December 20058. The statisticalmodels of the distribution of volcanoes were further refined, along with the probability
approach, which concentrated on using the refined dataset of volcano distribution to improvemethods of estimating smoothing parameters used in these models. A systematic approach toestimating model uncertainty using Cox models with multivariate random potentials was also
developed. These results were presented at the International High Level Radioactive Waste
Meeting (May 2006, Las Vegas)9.
The rock deformation methodology developed further to integrate the broad array ofadvanced earth science techniques and observations available in Japan, again applied to theCase Study region in northern Honshu, in coordination with the volcanic hazard group. An
expert elicitation workshop on rock deformation interpretations of the northern Honshu study
area was held in August 2005 in Tokyo. The objective was to use expert elicitation techniques
to obtain opinions from the Japanese expert community to populate the Logic Trees being
developed to address alternative conceptual models of rock deformation and associateduncertainties. The logic trees of three separate datasets (faulting, seismicity and GPS) were
evaluated in successive half-day sessions. Each session began with a short introduction to
the logic tree formulation for each dataset and was followed by group discussion on the data
available and then on weightings for each of the nodes of the logic tree. The Japaneseexperts brought some database information within their field of expertise and engaged in thegroup discussions. Associated with this workshop, the international and Japanese experts
visited the Ojika Peninsula to evaluate the rock formations and tectonic structurescharacteristic of this area.
The main strand of the rock deformation work involved the development of surface strainmodels for the Honshu case study area that can be presented as strain rate maps in a 5 km x
5 km grid base. A closely related activity was the development of a substantially updated
active fault source model for the study area. The strain rate models produced by the different
techniques could then be compared, with the eventual intent of deriving a single, composite
measure of strain. All of this modelling was informed by the results of the data elicitation
workshop. The results of the work were presented and discussed at an interim workshop thattook place in New Zealand in December 2005. This workshop was associated with a shortfield visit to examine a major continental plate boundary fault and examples of flexural slip
faulting in the South Island of New Zealand.
The ITM team worked with NUMO to finalise and present a paper for the ICEM ’05 meetingthat was held in Glasgow, Scotland, in September 200510. This was the first international
8

Jaquet, O., Connor, C. and Connor, L. Bayesian and Geostatistical Methodologies for the Long-term
Estimation of Volcanic Hazard.

9

Jaquet, O., Connor, C. and Connor, L. Probabilistic Methodology for Long Term Assessment ofVolcanic Hazards.
10
Tsuchi, H., Kitayama, K. and Chapman, N. Addressing Active Tectonics in HLW Repository Siting inJapan.
13

meeting at which the total ITM approach and methodology had been exposed.
In 2006-7, ITM was converted from an information transfer to an R&D project to support thefull methodology development work. Work continued throughout this year along the lines
described above for 2005-6 and a second Case Study was added to the one that was alreadythe focus of the work in this period. It was appreciated that the geological and tectonic
situation of Kyushu and SW Honshu was considerably different to that of northern Honshu,
where the Tohoku Case Study was located. The plate tectonic situation is more complex in
the S and W of Japan and monogenetic, as opposed to polygenetic, volcanism is much morein evidence. Consequently, features such as the Quaternary volcanic front are less distinct inthis region. Work thus began on this region, with the main focus on the large island of Kyushu
itself.
The ITM-5 Meeting took place in Tokyo in July 2006 and was followed by a two day field visit
to the Sengan area of Tohoku, whose objective was to allow ‘ground truth’ verification ofsome of the assumptions being made in the ITM technique development work, as this region
forms the first ‘Case Study’ area. The two tectonic hazard evaluation methodology
development projects that lie at its core had by now made sufficient progress that they wereapproaching a useable form. This meeting thus also provided an opportunity for the ITM
members to begin to consider the key aspects of:

integrating the two main project threads (probabilities of rock deformation and of
volcanic events);

linking the geological and statistical results to potential consequences of tectonic
processes and events so as to determine what is safety-relevant and of importance insiting;

defining how NUMO can begin to utilise the methodologies in the different stages of
its siting programme.

The main vehicle for developing these aspects was to be an ‘integration workshop’ which was
to take place shortly after the ITM-5 meeting (see below). The approaches had moved
relatively quickly from a conceptual level to the point of practical realisation. As usual, theITM-5 workshop was intended as a means of presenting progress in the ITM project to a wide
range of Japanese tectonics experts and obtaining their feedback. As well as the ITM teammembers and NUMO staff, the workshop was attended by 22 Japanese experts inseismology, active fault geology, geodesy, volcanology and geodynamics. These experts
came from eleven universities and research institutes, as well as from JAEA, TEPSCO,CRIEPI and RWMC.
The ‘integration workshop’ was held in September 2006 in Utah and had three technicalobjectives, shown below in order of priority, although each was considered important in its
own right:
1. Volcano-tectonic interaction: discuss how to integrate the volcanic probability
modelling with the strain probability maps by considering how strain is accommodated
in the crust during magma intrusion in the arc environment;
2. Utilisation: prepare first outline advice on how NUMO can use (and present) the
integrated probability maps at different scales and at different stages of their siting
programme;
3. Consequences: brainstorm scenarios to place along one axis of a ‘Consequences
Matrix’ that will relate potential styles of different tectonic events to sensitive featuresof the different repository concepts being considered by NUMO, thus allowing
qualitative definition of impacts (i.e. from important to not relevant for safety);
The workshop was followed by a field visit to a number of type localities in Utah that illustrated
in particular the style and mode of volcanic intrusions into the shallow crust, which was highly
informative from the viewpoint of considering repository consequences. A short, follow-up
workshop was held in February 2007 in Tampa, USA, to discuss the content and structure of14

the monogenetic volcano database catalogue being developed for Kyushu and to check thatthe information was in suitable format for analysis for the Kyushu Case Study.

2.4.3

ITM-6 and 7

By 2007-8 the Tohoku Case study was practically completed. The final report of the study
was produced in draft form in March 2008 and finalised after editing by the end of the year.
Work was well advanced on the Kyushu Case Study, with the project throwing up a range of
new issues for the group to consider. The ITM methodology itself was, by this stage,
sufficiently developed to require no further significant modifications to complete the casestudies, although it was strongly emphasised that the methodology would need continued
refinement before being used at actual sites by NUMO.
The 6th International Tectonic Meeting in Tokyo in February 2008 was held over a period of
two days and involved 14 overseas experts from the ITM project, 24 Japanese national
experts from Universities and research organisations, and 12 NUMO staff. It followed thepattern of previous meeting, being an opportunity to update national experts on the status of
the project and obtain feedback and expert knowledge on specific issues that were being
raised by members of the international team. With increased interest in the consequence
aspect of the ITM project, there was an invited presentation by Harald Hökmark (ClayTechnology, Sweden) on the work being carried out by SKB to evaluate post-glacial
earthquake impacts on fracture displacement and their possible effect on a spent fuelrepository in basement rocks.
By now, ITM had the basis of its methodology in place, but it was emphasised at the 6th
meeting that the ITM methodology was not a simple ‘formula’ or an inflexible set of‘instructions’ involving a linear series of steps that is applicable under all circumstances. The
methodology has different avenues that can be taken in response to local conditions, so its
application must be strongly conditioned by local knowledge, especially in complex regions. It
was observed that, at the end of our project, the methodology would be complete, but notconclusive. In a subject area as dynamic and complex as tectonics, the methodology willneed frequent updating and refinement. Inevitable new developments in our understanding of
the tectonic mechanisms being studied are incorporated into the conceptual modelling ofSteps 1 and 2 (see Section 3 for a description of the ITM Steps) and the strain model
development of Step 3b. Additionally, new data from other Japanese scientific studies, as wellas international developments in probabilistic hazard analysis, will need to be incorporated in
NUMO’s database, right up to and through the DIA stage.
The final year of the ITM project involved principally the concluding work on the Kyushu Casestudy and additional refinement to the overall methodology. A central issue throughout thelast three years of the work had been the need to integrate the results of both the rockdeformation and the volcanism teams. Kyushu proved to be an ideal test-bed for doing this,owing to the evident strong structural controls on the temporal and spatial distribution of
volcanism. A field visit to Kyushu in March 2008 provided an opportunity for both teams to
examine these controls in detail. Strong links were made with local university and otherresearch groups, with the project sponsoring one post-doctoral researcher to help provide
data input to the Case Study. To conclude the integration work, a one-week integration
workshop was held in Switzerland in November 2008, involving both teams.
The ITM methodology, as delivered, clearly provides information that feeds into the NUMO
decision-making support system and the safety analysis work that will be undertaken in future.
At the end of the project, which was completed in 2008-9, the results were presented to awide Japanese audience (more than 120 participants, predominantly geoscientists) at theclosing ITM-7 meeting in Tokyo in March 2009.

2.5

Disseminating the Project Results

Since the start of the ITM work and especially since the ITM Project began to develop amethodology for evaluating tectonic hazard, one of the aims of the work has been to keep awide spectrum of the Japanese geosciences community informed about the concepts and15

progress in developing them. So far as possible, we have also tried to involve the community
in regular discussions and several experts have had active participation in aspects of the
project work itself.
Apart from the international project team and NUMO staff, the individuals identified in Table
2.1 have participated in the annual ITM workshops in Tokyo and we would like to thank them
all for their interest, challenging questions and contributions to discussion, which have all
helped to focus and improve the project and keep the team better informed.
As well as the final Case Study reports and numerous internal annual reports, the ITM project
has resulted in the publication of 22 papers and articles in the scientific literature, as well as
one book. These are shown in Table 2.2. Further publications are expected after the end of
the project.

16

Table 2.1: Participants in the annual ITM workshops in Tokyo (excluding ITM international team and
NUMO staff).

ITM-1

Daiei Inoue, CRIEPI, Japan
Kazuhiro Tanaka, Yamaguchi University, Japan
Akira Tokuyama, Fuji-Tokuha University, Japan
Shizuo Yoshida, TEPSCO, Japan

Teruyoshi Hatano, CRIEPI, Japan
Motoi Kawanishi, CRIEPI, Japan
Kazuhiko Shimizu, JNC, Japan
Kenji Terada, TEPCO, Japan

ITM-2

Dr Daiei Inoue, CRIEPI
Professor Hiroki Kamata, Kyoto University
Dr Andrew Martin, JNC
Mr Noriyuki Saito, TEPCOMr Kazuhiko Shimizu, JNC

Dr Shiro Tamanyu, Geological Survey of JapanMr Kenji Terada, TEPCO
Dr Akira Tokuyama, Fuji Tokoha University
Dr Shizuo Yoshida, TEPSCO
Dr Bill Arnold, Sandia Laboratories, USA

ITM-3

Professor Manabu Hashimoto, Kyoto University
Dr Daiei Inoue, CRIEPI
Professor Hiroki Kamata, Kyoto University
Mr Toshihiro Kayama, TEPSCO
Mr Hirofumi Kondo, CRIEPIDr Andrew Martin, JNC
Professor Yujiro Ogawa, Tsukuba University
Mr Fumikiko Ono, TEPCOMr Toshihiro Seo, JNC
Mr Kazuhiko Shimizu, JNC
Dr Shiro Tamanyu, AISTDr Yoshihiko Tamura, JAMSTEC-IFREEMr Kenji Terada, TEPCO
Dr Akira Tokuyama, Fuji Tokoha University
Dr Shizuo Yoshida, TEPSCO

ITM-4

Mr Koichi Asamori, JNC
Professor Manabu Hashimoto, Kyoto University
Mr Ryuta Hataya, CRIEPI
Professor Shintaro Hayashi, Akita University
Professor Naoshi Hirata, University of Tokyo
Dr Daiei Inoue, CRIEPI
Professor Hiroki Kamata, Kyoto University
Dr Motoi Kawanishi, CRIEPIDr Hurohumi Kondo, CRIEPIDr Hiroyuki Koyama , TEPSCO
Dr Kin-ichiro Kusunose, AISTDr Andrew Martin, Quintessa Japan
Professor Yoko Ota, Yokohama City University

Professor Takashi Nakata, Hiroshima Inst. Technol.
Dr Noriyuki Saito, TEPCO
Professor Kunihiko Shimazaki, University of Tokyo
Dr Kazuhiro Shimizu, JNC
Professor Shizuo Yoshida, TEPSCO
Dr Shiro Tamanyu, AISTDr Yoshihiko Tamura, JAMSTEC
Mr Kenji Terada, RWMC
Professor Akira Tokuyama, Fuji Tokoha University
Mr Koji Umeda, JNC
Dr Minoru Yamakawa, AESTO
Professor Hidekazu Yoshida, Nagoya Univ. Museum
Professor Yasuhisa Yusa, Fuji Tokoha University

ITM-5

Professor Akira Tokuyama, Fuji Tokoha University
Professor Shizuo Yoshida, TEPSCO
Dr Kazuhiko Shimizu, JAEAMr Kenji Terada, RWMC
Dr Daiei Inoue, CRIEPI
Professor Takashi Nakata, Hiroshima Inst.
Technol.
Professor Yoko Ota, Yokohama City University
Dr Yuichi Sugiyama, AIST
Professor Satoshi Miura, Tohoku University
Professor Kunihiko Shimazaki , University of Tokyo
Professor Naoshi Hirata, University of TokyoDr Takuya Nushimura, GSI
Dr Tsuyoshi Nohara , JAEAMr Keiichi Ueta, CRIEPI
Professor Hiroki Kamata, Kyoto University
Professor Takehiro Koyaguchi, University of Tokyo
Professor Shintaro Hayashi, Akita University
Dr Yoshihiko Tamura, JAMSTEC
Dr Hirofumi Kondo, CRIEPI
Dr Koji Umeda, JAEADr Makoto Murakami, GSI
Dr Takashi Sano, National Science Museum

ITM-6

Professor Akira Tokuyama, Fuji Tokoha University
Professor Shizuo Yoshida, TEPSCO
Dr Kazuhiko Shimizu, JAEAMr Kenji Terada, RWMC
Professor Hidekazu Yoshida, Nagoya Univ.
MuseumMr Toshihiro Seo, JAEA
Dr Daiei Inoue, CRIEPI
Professor Takashi Nakata, Hiroshima Institute ofTechnology
Professor Yoko Ota, Yokohama City University
Professor Satoshi Miura, Tohoku University

Professor Naoshi Hirata, University of TokyoDr Takuya Nishimura, GSI
Dr Tsuyoshi Nohara, JAEAMr Keiichi Ueta, CRIEPIMr Ryuta Hataya, CRIEPI
Professor Setsuya Nakada, University of Tokyo
Professor Dapeng Zhao, Tohoku University
Professor Toshiaki Hasenaka, Kumamoto University
Dr Hirofumi Kondo, CRIEPIDr Takahiro Yamamoto, JNESDr Makoto Murakami, GSI

17

Table 2.2: Books, papers and articles on the ITM project methodology and results, published by the ITM
team during the course of the project.

2004

Apted, M., Berryman, K., Chapman, N., Cloos, M., Connor, C., Kitayama, K., Sparks, S. and Tsuchi, H.
Locating a Radioactive Waste Repository in the Ring of Fire. EOS, Transactions of the American
Geophysical Union, 85, No. 45, 465-471.

2005

Jaquet O., Connor, C.B. & Connor L. Bayesian and geostatistical methodologies for the long term estimationof volcanic hazards, AGU Fall Meeting, San Francisco, California.

2005

Tsuchi, H., Kitayama, K. and Chapman, N.A. Addressing active tectonics in HLW repository siting in Japan.
th
In Proceedings ICEM’05: 10 International Conference on Environmental Remediation and Radioactive
Waste Management, Glasgow.

2006

Jaquet O., Connor, C.B. & Connor L. Probabilistic methodology for long term assessment of volcanic
hazards. In: Proceedings International High-Level Radioactive Waste Management Conference, Las Vegas,
Nevada, pp.154-161. American Nuclear Society.

2007

Hone, D. W. E., S. Mahony, R. S. J. Sparks and K. Martin. Cladistics analysis applied to the classification of
volcanoes. Bulletin of Volcanology, 70, 203−220.

2007

Chapman, N. A., J. Goto & H. Tsuchi. Likelihood of Tectonic Activity Affecting the Geological Stability of a
Repository in Japan: Development of NUMO’s ITM Methodology. OECD Nuclear Energy Agency: NEA
Geosphere Stability in Crystalline Rock; Manchester, UK

2008

Jaquet O., Lantuéjoul C. & Goto J. Estimation of long-term volcanic hazard using a Cox process with amultivariate potential. VIII International Geostatistics Congress, GEOSTATS 2008, Santiago Chile, 167-176.

2008

Tsuchi, H., Goto, J., Chapman, N. and Kawamura, H. Tectonic Hazard and the Siting of Japan’s HLW
Repository: Preliminary Results of the Tohoku Case Study in the ITM Project. In: Proceedings IHLRWM, Las
Vegas, 2008. American Nuclear Society.

2008

Jaquet, O., Connor, C. and Connor, L. Probabilistic Methodology for Long-Term Assessment of Volcanic
Hazards. Nuclear Technology, 163 (1), 180-189

2008

Goto, J., Tsuchi, H., Chapman, N. and Kawamura, H. Siting Japan’s HLW Repository 2: Addressing theTectonic Issues in a Probabilistic Approach. In: Proceedings: International Geological Congress, Oslo.

2009

Wallace, L. M., Ellis, S., Miyao, K., Miura, S., Beaven, J. and Goto, J. An enigmatic, highly active left-lateralshear zone in southwest Japan explained by aseismic ridge collision. Geology, 37, 143-146.

2009

Connor, C., Chapman, N. A. and Connor, L. (eds.). Volcanic and Tectonic Hazard Assessment for Nuclear
Facilities. Cambridge University Press

2009

Jaquet, O., and C. Lantuéjoul, Cox process models for the estimation of long-term volcanic hazard. In: C. B.
Connor, N. A. Chapman, and L. J. Connor (eds.), Volcanic and Tectonic Hazard Assessment for Nuclear
Facilities, Cambridge University Press, in press.

2009

Connor, C.B., and L. J. Connor, Estimating spatial density with kernel methods. In: C. B. Connor, N. A.
Chapman, and L. J. Connor (eds.), Volcanic and Tectonic Hazard Assessment for Nuclear Facilities,
Cambridge University Press, in press.

2009

Mahony, S. H. , R. S. J. Sparks, L. J. Connor and C. B. Connor, Exploring long-term hazards using a
Quaternary volcano database. In: C. B. Connor, N. A. Chapman, and L. J. Connor (eds.), Volcanic andTectonic Hazard Assessment for Nuclear Facilities, Cambridge University Press, in press.

2009

Connors, C., Sparks, R.S.J., Diez, M., Volentik, A.C.M.. The Nature of Volcanism. In Connor, C.B.,
Chapman, N and Connor L.J. (eds), Volcanism, tectonism and the siting of nuclear facilities. Cambridge
University Press, Chapter 3, 86-129, 2009.

2009

Mahony, S., Sparks, R.S.J., Connor, C. and Connor, L. Assessing long-term volcanic hazards using volcano
distribution databases. In Connor, C.B., Chapman, N and Connor L.J. (eds), Volcanism, tectonism and thesiting of nuclear facilities. Cambridge University Press Chapter 13, 364-386, 2009.

2009

Litchfield, N., Y. Ota, D. Merritts. Tectonic uplift and subsidence. In Connor, C.B., Chapman, N and Connor
L.J. (eds), Volcanism, tectonism and the siting of nuclear facilities. Cambridge University Press

2009

Wallace, L. M., J. Beavan, S. Miura, R. McCaffrey. Using global positioning system data to assess tectonic
Hazards. In Connor, C.B., Chapman, N and Connor L.J. (eds), Volcanism, tectonism and the siting of nuclear
facilities. Cambridge University Press

2009

Stirling, M., K. Berryman, L. Wallace, N. Litchfield, J. Beavan, W. Smith. Multidisciplinary probabilistic tectonic
hazard analysis. In Connor, C.B., Chapman, N and Connor L.J. (eds), Volcanism, tectonism and the siting of
nuclear facilities. Cambridge University Press

2009

Chapman, N. A., H. Tsuchi, K. Kitayama. Tectonic events and nuclear facilities. In Connor, C.B., Chapman,
N and Connor L.J. (eds), Volcanism, tectonism and the siting of nuclear facilities. Cambridge University
Press

2009

Kiyosugi, K., C. B. Connor, D. Zhao, L. J. Connor, K. Tanaka, Relationships between temporal-spatial
distribution of monogenetic volcanoes, crustal structure, and mantle velocity anomalies: An example from the
Abu Monogenetic Volcano Group, Southwest Japan, Bulletin of Volcanology, in review.

18

3 The ITM Methodology Road Map
This Section provides an outline of the overall concept and methodology that has beendeveloped by ITM in the form of a ‘road map’. The methodology development is essentiallycomplete and this road map represents a description and checklist for application to volunteer
sites by NUMO staff and contractors in Siting Stages 1 and 2.
As discussed in Section 1, the methodology should be regarded as a living entity, as it must
respond to new scientific knowledge and techniques that will arise over the next few years.
Consequently, it will have to be revisited from time to time. This is especially true with respect
to application in Siting Stage 3 (at the point where PIAs are being evaluated and compared inorder to select a preferred site, or sites, for detailed investigation as DIAs), as this may be fiveor more years into the future.

3.1

Outline of the ITM Methodology

The overall structure of the ITM methodology consists of:

assembling nationally available data and alternative models of the nature, causes andlocations of tectonic processes and events;using probabilistic techniques to evaluate the likelihood and scale of future tectonic
processes and events, shown as a function of their type and geographicaldistribution;feeding information on these potential likelihoods and impacts to NUMO’s
performance assessment team so that feedback can be provided on repository
performance under tectonic stress;

providing clearly justified and traceable input to decision-making on consequent site
suitability.

A probabilistic approach has been selected for the ITM methodology as it is seen as the only
realistic means of addressing the uncertainties in predicting possible hazards when there issuch variability in the spatial distribution, the timing, the intensity and the style of the volcanicand deformational events and processes being evaluated.
Naturally, the probabilistic approach being developed is based upon and strongly supported
by deterministic models of the underlying tectonic processes that lead to magma intrusion,
volcanism and rock deformation.
For convenience, the methodology for rock deformation and volcanic hazards assessment
has been applied as two parallel tasks. This recognises the fact that, although the concept ofeach approach as shown above is similar, in some parts of the methodology they differ
significantly in detail. Consequently, it was found that two teams with different specialities(structural, geophysics and tectonics specialists; volcanologists) worked efficiently in parallel.
However, it is most important that, if carried out in this way in the future, the two teams
integrate their work frequently, as there are clear overlaps in the processes being evaluated(e.g. magma intrusion impacts on rock stress regimes and vice versa). NUMO will need toensure that such integration is carried out effectively when the methodology is applied to ‘real’sites.
The broad structure presented above is shown in more detail in the top-level ‘road-map’ inFigure 3.1. It comprises a series of eight Steps, which are described in Section 3,
distinguishing between the ‘rock deformation’ approach and the ‘volcanic’ approach, where
they involve significantly different activities.

19

Figure 3.1: The Steps in the ITM methodology, shown as a top-level road-map.

There is considerable depth and detail to the application of the ITM methodology in each of
the Steps. This road-map description is intended to provide a simplified overview. The
complete detail of what is involved in each analysis is described in the two project Case Study
reports.

20

3.2

The Steps in the ITM Methodology

STEP 1: Assembling the Data
Step 1.1: Define the region of interest: The region of interest should be defined. For
deployment of the ITM methodology in Siting Stage 1 (the LS stage), we expect that theregion of interest identified around a site, or group of nearby sites, will be ~104 - 105 km2. The
larger area is more appropriate when comparing the situation of several siting options. This isbased upon our experience in the Case Study areas. Regions need to be large enough bothto contain a statistically large enough number of features that are manifestations of theprocesses being evaluated (e.g. volcanic edifices) and to contain a good spread of datapoints for modelling these processes (e.g. GPS stations). They should also be internallyconsistent, at a rough approximation, with respect to tectonic regime. For example, in the
Kyushu Case Study, it was found that the large overall region considered (approximately
100,000 km2) had to be broken down into several intermediate blocks representing
tectonically ‘coherent domains’, ranging in size from c. 20,000 to c. 50,000 km2. A region of
104 km2 is at the lowest end of the size range that is likely to be useable and the aim should
be to have a region of at least some tens of thousands of square kilometres to ensure that thestatistical approach adopted is meaningful.
For deployment at a single site scale (Stage 3: not considered in depth in the ITM project), themethodology is likely to require downscaling to 104 km2 or perhaps 103 km2, the scaledepending upon the complexity of the tectonic setting. This has not been done in the Case
Studies and would require further test application and probably some modification of themethodology. The regional scale deployment of the methodology described below is based
upon estimating probabilities within 5 x 5 km areas. For site scale deployment, a finer grid will
be required.
Step 1.2: Data gathering: Following definition of the region of interest, relevant data are
obtained from the literature to constrain possible models of magma intrusion and rockdeformation. The principal source data at Siting Stage 1 are likely to comprise a limited
number of national databases, including the following:

geological maps;



uplift and subsidence data;

topographic maps (onshore and offshore);gravity and magnetic maps;



volcanic edifices/features location, nature and age (Catalogue of Quaternary
Volcanoes);national onshore and offshore active fault map;recorded distribution of seismic events (locations, magnitudes, depths);

velocity field measurements derived from GPS for all monitoring stations.

Additional geological and geophysical information, such as geological maps, heat flow dataand interpretations of the geological, structural and tectonic histories of the region, areneeded to support the development of conceptual models of the processes of interest (inStep 3). There may also be relevant scientific publications on specific volcanoes or tectonic
features, and more generic research publications on relevant processes. This additional
information should be augmented by discussions with academic and other researchorganisations in Japan. Such discussions will be an extremely important part of ensuring boththe currency of the models used and the involvement of the national (and international)
geosciences community.
When the methodology is applied later in the siting programme, for example, to evaluate
tectonic hazard in more detail at a site already identified as a PIA, the integration of LS datafrom existing catalogues and databases with site investigation data gathered both in the PIA

21

area and in the area of interest around it, will be an important activity. This will need to bedone carefully and consistently, because the probabilistic evaluations may be being used at
this stage to support quantitative estimates of radiological impacts.
STEP 2: Sorting the Data and Identifying Alternative Conceptual Models
Step 2.1: Data Sifting and Ground-truth: The data need to be evaluated and sifted. The
databases used have not been gathered for the specific purposes of the ITM methodology, sothey are not necessarily organised in an appropriate fashion and they may not contain data in
the form in which they will be used in Steps 3 and 4. The data also need to be evaluated forreliability and consistency. In cases where inconsistent or anomalous data are identified, thisneeds to be taken into account in the assessment of uncertainty. Such issues have beenfound to be the case for the Catalogue of Quaternary Volcanoes, where mapped volcanicfeatures include a wide range of different structures with differing significance in terms ofprocesses of magma generation and intrusion. Consequently, this Step requires a close
evaluation of what the data actually represent and whether datasets are internally consistentand of uniform quality. It may be necessary to develop derivative datasets that reduce orremove anomalies and inconsistencies, but it is important that such changes are transparent
and carefully documented.
It will be essential to obtain ‘ground truth’ on observations included in some of the databases(but not all: e.g. GPS strain data). For the highest level of confidence in the statisticalmethods used in Step 4, ‘ground truthing’ is likely to involve a significant effort in a minimal
resurveying of the region of interest (e.g. visits to all volcanic structures to ensure that theircorrect classification is known). NUMO needs to take this into account in its planning: eventhough field visits to prospective PIAs may be excluded at Siting Stage 1 (LS), regional
observations by specialists (even ‘drive-by’ observations) would likely be most valuable.
An equivalent example for the rock deformation datasets is the requirement to remove thelarge-scale, elastic subduction-related interseismic overprint from the GPS velocity field. Only
the residual strain is considered to be the ‘signal’ of interest, as it is related to faulting in thecrustal rocks of the Japanese islands. It is therefore of use in developing maps that show
differential strain for ‘cells’ across the region of interest that can be linked to localised rockdeformation processes. The ITM methodology uses 5 x 5 km cells, as these 25 km2 blocks
represent a representative area to contain a repository that is likely to have an approximate
footprint of 10 km2, plus space around its margins, where there may be additional accessworks, for example. Clearly, other sizes of cell could be used in an analysis.
Step 2.2: Underlying Conceptual Models: Underlying conceptual geological models need tobe identified (or developed) to explain the distribution of features (volcanic edifices) anddifferent styles of rock deformation. These conceptual models have been developed during
the ITM work to date. In simple terms, the models are as follows:

Distribution of polygenetic volcanoes: during the Quaternary, and for the next tensof thousands of years (up to ~100 ka) this is controlled by variable magma generation
potential in the mantle and crust overlying the subducting oceanic plates. The originof this variability is not fully understood but is manifest in some structure to the
distribution of these volcanoes; for example, ‘clusters’ of volcanoes with intervening‘gaps’ in some areas. The distribution of clusters and gaps is correlated to varying
extents to gravity, basement topography and the seismic tomographic structure of thecrust and mantle wedge beneath the Japanese archipelago. There is also evidence
for temporal variations in volcanism that need to be taken into account in theassessment of future volcanism, which is also not fully understood.

Distribution of monogenetic volcanoes: this aspect of the ITM methodology wasdeveloped and tested in the Kyushu Case Study. One possibility is that locations ofmonogenetic volcanoes are influenced by local stress variations related to structure.
A volume-predictable model appears to work best, and we have developed a model
to suggest that in compressional tectonic settings with constant magma supply,
monogenetic volcanism should be generally volume-predictable. In other words, the22exact timing of future eruptions is uncertain, but in a given time interval a given
volume of magma should erupt.


Regional and Local Strain Budgets: the upper crustal rocks of the Japanesearchipelago are undergoing progressive deformation as a result of horizontal andvertical strain responses to dynamic plate tectonic forces. The amount and style ofdeformation varies from region to region depending on the geological formations
present and the location with respect to the major plate boundaries and other largescale deformation zone features, such as major (>100 km long) strike-slip fault zones(including the distance from such boundaries/features). The deformation can varyfrom compression to extension and subsidence to uplift. The resultant strain is
manifest as faulting (episodic movement along Quaternary active faults), folding anddistributed deformation. The overall strain ‘budget’ for an area is accessible using a
range of indicators, each of which characterises different strain manifestations: GPSdata on relative surface movements; coseismic movements, accessible through theseismological database and surface uplift/subsidence rate (which can be combinedwith other indicators to give an estimate of surface deformation, or ‘tilt’).

There are also more local controls on strain and stress related to geological and structural
heterogeneities. There may be local feedbacks between faulting, deeper ductile deformationand magmatism that results in local departures from regional variations related to plate scale
processes.
Following Step 2, the volcanic and rock deformation analyses take separate paths, defined
here as Steps 3a to 4a for the magma intrusion analysis and Steps 3b to 4b for the rockdeformation analysis.
It is expected that there would be some feedback to Step 2 from Step 3b. There will be arequirement to update the conceptual models after strain rates are estimated. Feedback
between modelling and data are an important aspect of ensuring that alternative models are
not overlooked.
STEP 3a: Classifying and Grouping Magma Intrusion Features
For the evaluation of possible future magma intrusion in areas that have not been affected byintrusion in the last ~2 Ma, it is first essential to classify the indicators of past intrusion(starting with the mapped volcanic features in the Quaternary catalogue).
Depending on the area being considered, different means of classification may be reasonable
and the first step is to carry out a ‘classification analysis of events’ to determine how best
to group the intrusion indicators. No internationally accepted scheme of volcano classification
exists, so the ITM methodology developed a cladistic approach for the Tohoku Case Study(Chapman et al., 2009) as well as exploring alternative ways of defining volcanic events. In
the Kyushu Case Study, where the tectonic environment and styles of intrusion were more
complex, classification needed to consider groupings of intrusions based on geochemicalaffinities of magmas and correlations with tectonic structures that control the spatial
occurrence of some groups of intrusion. In this case, there is an even stronger need to ensurethat volcanic assessment is integrated with the rock deformation and tectonic structuralassessment.
Step 3a.1: Classification Analysis:
Several approaches are possible to assign intrusion events to genetically related groups thatcan be treated in a consistent manner statistically (i.e. ‘event definition’). The use of expert
elicitation will greatly improve confidence in the classification schemes developed and,
although the method was not trialled during the ITM project, logic trees represent a wellestablished approach to depicting the results of expert elicitation.
A cladistic approach has been developed to facilitate this classification, which is based onstandard taxonomic approaches used for species classification in the biological sciences and23

for which analysis software is readily available. The methodology is described more fully in an
ITM project publication (Hone et al., 2007). In simple terms, it involves defining characteristic
properties (such as size, morphology, age, chemical composition, intrusive or eruption style)to each mapped feature and then using a software analysis package (PAUP) to assess all
possible ways of grouping the features using these characteristics. The simplest possiblegroupings that explain the most characteristics are selected (the parsimony principle, or
Ockham’s razor). The different groups are called clades, although we use the term
‘Alternative Groups’ for simplicity and because techniques other than cladistic analysis can beused to form alternative groups. In the Tohoku Case Study, the cladistic method was found towork well for the polygenetic volcanoes and to provide a deeper understanding of thestrengths and limitations of the volcano database.
Step 3a.2. Database analysis. The database of volcanic features is then analysed to identify
alternative data bases, which can be used in the probabilistic analysis. This step involves
using the clade groups and field data to verify alternative groupings of volcanoes and volcanicfeatures.
Step 3a.3: Time-series analysis: It is then necessary to carry out a time series analysis ofeach alternative database to assess whether they display different periodicity (dormancy andactivity) and whether this periodicity is structured (i.e. related to eruption history, rather thanbeing random), which is used in Step 4a.
Step 3a.4: Sensitivity analysis: It is also important to test the sensitivity of the groupings to
the size of the region considered (by extending or reducing the area the number of edifices
included is increased or reduced and the statistical groupings may change) and by adding
new ‘synthetic’ volcanoes: a large change in group characteristics could indicate instability inany model invoked to explain the distribution of volcanoes.
The sub-steps in Step 3a can thus be represented as shown in Figure 3.2.
Define discriminating
characteristics of volcanicfeatures appropriate for theregion under evaluation

Assess regional
geological model
for clustering andcorrelations uponwhich to basesensitivity test

Define each mappedfeature in term of thecharacteristics

Run PAUP on full set ofvolcanic features

Sensitivity test byadding or removing
volcanic featuresand re-running
PAUP

Alternative Groups(e.g. as clades)put members of each group intime series & send toprobability mapping in Step 4a

iterate as needed

Figure 3.2: Flowchart showing sub-steps used in Step 3a.

STEP 3b: Developing Strain Models and Estimating Strain Rates for EachFor rock deformation, the objective is to calculate strain rates across the region of interest
using independent data sources that reflect widely different time averaging: GPS derived
strain (years), surface deformation (tens of thousands of years) and seismic strains from the24seismic moment of earthquakes (centuries). These can then be compared. In Step 4b theseare presented as strain maps.
Each data source may indicate strains that are the result of one or more processes, for eachof which there may be alternative tectonic models and interpretations (e.g. boundaries ofregions that can be defined as discrete tectonic blocks, dips of major fault zones, amount,degree and depth of subduction coupling). The way that strain is calculated from the raw data
will need to account for the relative contributions of these different processes, factoring in theinherent uncertainty introduced by having alternative conceptual models. The contribution
from different processes will thus have to be estimated by expert judgement, depending on
the degree of belief in the importance of different processes/mechanisms (essentially,reflecting the alternative conceptualisations of what is driving rock deformation in the region).
Step 3b.1: Defining tectonic blocks: A first sub-step in Step 3b is to consider whether theregion of interest (or an even larger area if appropriate) can conveniently be divided into
stable rock blocks that behave internally in a relatively homogeneous way or respond in asimilar way to external, large scale tectonic driving processes. This assessment forms thebasis for the subsequent development of strain models, and there may be alternative ways ofdefining such blocks, which affects the number of models developed.
Step 3b.2: Assembling alternative conceptual models in a Logic Tree: The approach
adopted in the ITM methodology is to use Logic Trees to bring together all alternative
conceptual models identified; an example of which is shown in Figure 3.3.

A

A1
A2
A3
A4

B

C

Figure 3.3: Example of part of a Logic Tree used in Step 3b to calculate strains from GPS data for the
Tohoku Case Study area (Chapman et al., 2009).

As can be seen in Figure 3.3, the Logic Tree is constructed by asking questions of the form:How many alternative explanations (models) could describe strain in this block?’: theanswer requires a certain number of starting nodes to be established in the tree (A, B,
and C in Figure 3.3).

25



If Model A is correct, what are the alternative ways of describing its impact on
deformation?’: the answer produces branches from the node for Model A (A1, A2, A3
and A4 in Figure 3.3).

Continuing in this fashion a tree is generated that incorporates as many alternative
conceptualisations of deformation mechanisms and associated uncertainties as are deemedfeasible. Following down any one branch to the end defines how strain will need to be
calculated for that particular set of model assumptions.
Use of expert judgement: Expert judgement, elicited from a group of experts in Japanese
tectonics at a workshop, is factored into the construction of the tree to ensure that it issufficiently comprehensive of alternative models. The experts then contribute by agreeing
weightings for each branch (expressing their degree of belief in the validity of each alternative
conceptualisation).
Each strain indicator requires its own logic tree, in order to calculate strain rates. In summary,the three indicators used to date to estimate strain as follows:•

Surface deformation and active faults = mm/km/a strain. The period over which thisindicator has ‘recorded’ strain is ~10,000s years.

•

Gradients in GPS velocity = mm/km/a strain. The period over which this indicator has‘recorded’ strain is ~10s years.

•

Recorded earthquakes = seismic moment + Kostrov equation11 = strain. The period
over which this indicator has ‘recorded’ strain is ~100s years.

Despite the fact that these indicators record strain over many orders of magnitude of time, theprocesses that they are recording are widely considered to have been stable in magnitude
and direction for about 100,000 years.
Site 4 - GPS

Site 4 - SEIS
0.4

0.4

0.35
0.3

0.3

Weighted Frequency

0.25
0.2
0.15
0.1
0.05

0.25
0.2
0.15
0.1
0.05

0

89

85

81

77

73

69

65

61

57

53

49

45

41

37

33

29

25

21

9

17

5

1

1
5
9
13
17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
81
85
89

0

Nstrain/yr

13

Weighted Frequency

0.35

NStrain/yr

Site 4 - Tilt
0.4

Tohoku Site 4

0.35

1

0.25

SEIS

89

85

81

77

73

69

TILT
65

0.4

61

37

33

29

25

21

9

17

13

5

1

0

GPS

0.6
57

0.05

0.8

53

0.1

49

0.15

45

0.2

41 Exceedance
Probability of

Weighted Frequency

0.3

NStrain/yr

EQ WGTD AV

0.2
0
1

10

100

nstrain/yr

Figure 3.4: Strain rate (nanostrains/a) histograms for multiple model realisations for one of the ‘example
locations’ in the Tohoku Case Study area, calculated (clockwise, from top left) from seismic data, GPS
residuals and tilt data. The larger diagram shows the results compiled on a cumulative probability plot,
showing the probability of exceeding any given value of strain. The red line is the equally weighted
average.

11

The Kostrov equation relates the seismic strain rate to the sum of the seismic moment tensors of all
the earthquakes occurring in a given volume of the crust during a given time-interval.

26

Step 3b.3: Calculating Strain Rates: For the GPS logic tree example shown in Figure 3.3,
there were 148 different strain models (branches) for the Tohoku Case Study. The strain rates
for each of these models are calculated separately and as a weighted average. A histogram
can be produced of the frequency of calculated strains of a given magnitude at a given
location, using all the model results for a particular indicator, which can then combined withthe same results from the other indicators (see Figure 3.4). The Step 3b sub-steps in setting
up and using the logic trees are outlined in Figure 3.5.

Define distinct tectonic blocks
or sub-regions with region ofinterest and their boundaries

Overallassessment of timestability of blocks
and driving
mechanisms

Identify mechanisms drivingrock deformation in eachblock and at boundaries

Uncertainty analysis on driving
mechanism models

ExpertElicitation

Alternative Conceptual
ModelsExpertElicitation

Combine ACMs
into Logic Tree(see text for procedure)

Weight each node (alternative
conceptual model) in Logic TreeUse each branch to define strain
calculation for each alternative istrains for each model toprobability mapping in Step 4b

Figure 3.5: Flowchart showing sub-steps used in Step 3b.

STEP 4a: Magma Intrusion Probability Maps
Step 4a.1: Assessing Correlations: The first step in producing probability maps is to see
how far the spatial distribution of volcanoes can be correlated with topographical andgeophysical indicators of crustal processes. This provides evidence that distribution is not
simply random. The check can be made both for all volcanoes in the region of interest and forthe groupings derived from Step 3a. The probabilities and related uncertainties can then besuitably weighted to reflect these correlations. This sub-step may need to utilise expert
elicitation.
As an example, in the Tohoku Case Study, the broad distribution of volcanoes is correlated(although by no means perfectly) to the isostatic gravity anomaly map of the region. The
isostatic gravity anomaly map is produced by combining the Bouguer gravity anomaly map
with the topographic map, making various assumptions. In the Tohoku study it was calculated
assuming a thin elastic crustal plate 10 km thick. The isostatic anomalies reflect magma

27

generation potential, with the rate of magma accumulation at the intra-crustal Conrad
discontinuity and, possibly, the rate of magma flux at the surface (hence the likelihood of
future volcanism) being indicated by isostatic anomalies. This can be tested by plotting thehistoric magma production rate against the isostatic anomaly per unit area, but this has not so
far been tested in the ITM methodology development.
In Kyushu, the distribution of volcanism is closely tied to tectonic strain, manifest in the
distribution of active faults, GPS-derived strain and the distribution of historical seismicity. On
local scales, some volcanoes are closely associated with fault zones (Figure 3.6). Regionally,
volcanism in Kyushu closely follows regional tectonic structure. For example volcanoes of theNorthern Extensional arc are closely associated with the Shimabara-Beppu Graben. ITM
methodology calls for development and implementation of probabilistic models that reflect
these geological patterns.

Figure 3.6: Regional gravity about Aso caldera and the SW Simabara-Beppu graben. Two small-volume
Quaternary volcanoes (Akai and Omine) are found SW of Aso along graben-bounding fault. Active fault
traces (dashed lines) differ in some areas from distribution of crustal-scale gravity anomalies. Volcanismin this area provides a clear example of volcano-tectonic interaction that is common in Kyushu.

Step 4a.2: Calculating Probabilities of Magma Intrusion: For the probabilistic mapping,
three types of probability can be estimated:

P1 – the probability of a volcano edifice forming in the region of interest during theperiod of interest (e.g. a probability of 2 x 10-4 for a period of one year)P2 – the probability that a volcano will form in a specific area within the region ofinterest, such as a 5 x 5 km block, or a region extending to 15 km beyond theboundaries of a PIA (e.g. a probability of 1 x 10-4 for an area of 25 km2)

P3 – given that a volcanic event occurs in this specific area, the probability that it will
impact the repository site itself.

28

Then, the probabilistic volcanic hazard is given by P1 x P2 x P3. A variety of well-developed
statistical methods is available for estimating such probabilities, and the estimations can bedone either for all magma intrusion modes or for the various alternative groupings defined in
Step 3a. For subsequent assessment of impact scenarios in Step 6 it is important to assess
the probability of different types of event occurring, so looking at each alternative group is a
primary strategy of the ITM methodology.
The ITM methodology produces (principally) estimates of P2, in the form of regional
probability maps, as these are of most use for assessing specific locations or sites. The same5 x 5 km squares are used as for the rock deformation evaluation. Estimation of P3 will be asite-specific issue for Step 6, which will need to look at both the structural and geologicalproperties of the location, the type of intrusion being considered and the repository conceptthat would match the site.
Geostatistical and Bayesian methodologies provide a way to assess conceptual, spatial andparametric uncertainty. The methods used in the ITM methodology are:

Kernel Method: Generation of a non-homogeneous distribution map, using a
Gaussian or Epanechnikov kernel method, with an applied smoothing function (withthe effect of a range of bandwidths tested). The method is subject to uncertainties incalculated probabilities at any given point on the regional map, principally caused by
local variability of data density (or overall sparsity in terms of the number of volcanic
events over the last 2 Ma). These uncertainties vary from point to point. Figure 3.7
shows an example probability map for a small sub-region of the Tohoku Case Study
area (Chapman et al., 2009), with the two green locations selected to show how
uncertainty can vary depending on proximity to an existing cluster. Figure 3.8 shows
example results for different alternative groupings of volcanoes. It can be seen thatthe optimum bandwidth for the kernel function is different for the two groups.
As discussed under Step 3a, the introduction of additional ‘synthetic’ volcanoes canhelp quantify this uncertainty and the ITM methodology is also testing a Monte Carlosampling approach for estimating uncertainty in probability at a given site for a given
data set and a given smoothing bandwidth. An alternative method under test is to use
adaptive kernel functions, where the value of the kernel depends on the local data
density. The Monte Carlo approach is shown in the flow chart in Figure 3.9.

Figure 3.7: Spatial density of likelihood of new volcano formation in a sub-region of the area used in the
Tohoku Case Study (Chapman et al., 2009). This realisation uses a 6 km Gaussian kernel and theprobability refers to a period equivalent to that over which the mapped volcanic features were formed –about 2 Ma.

29

Figure 3.8: Probability contours (per year) for new volcanoes in part of the Tohoku Case Study area,
showing the different kernel bandwidths that are required to provide an acceptable model for two
different groups of volcano: left, explosive volcanoes, with a 33 km bandwidth; right, extrusive volcanoes,
with a 12 km bandwidth.

Input volcano distribution

Calculate a probability map (potential) for a given
volcano distribution and fixed bandwidth

Sample the potential for a new set of volcanoes (a possible
volcano distribution based on the estimated pdf)
repeat 500
times
With this new sample, recalculate the potentialof new volcano formation from new distribution
repeat foreach bandwidth
Use the range of estimates to calculate error bar(e.g., 90 percent confidence interval)
for the probability of new volcano formation at the site of interest

Output Probability with Uncertainty

Figure 3.9: Flowchart showing the Monte Carlo sampling approach that is being developed to assess
uncertainties in non-homogeneous probability mapping.

30



Cox Process Method: This multivariate approach is able to use a range of different
set of geoscientific data – for example, 3-D seismic velocity tomography of the crustand upper mantle, using a conceptual model where the tomographic features are
related to magma generation potential. This is supported by the correlation of thesestructures with specific volcanoes or with Quaternary volcano clusters.
The basis of the approach is to consider the potential for volcanism in an area to berandomly structured: even though our geological knowledge suggests there to becorrelations with geological and geophysical data, randomness is brought on by ourinherent uncertainty. Unlike Poisson distributions, which assume a constant potential
with time for new volcanoes, and the non-homogeneous approach (see above: theother method used by the ITM methodology) which assumes a deterministic potentialconditioned by selection of a kernel value, the Cox process approach assumes thepotential to be entirely stochastic. The detailed methodology used is described by
Jaquet et al. (2008a; 2008b; 2009).
The Cox process approach allows the estimation of a volcanic potential map, which isthen statistically correlated to the seismic tomography map, in order to produce aprobability map by Monte Carlo simulations. The overall approach is shown in Figure
3.10, while Figure 3.11 shows one of the results from the Tohoku Case Study.

Figure 3.10: Flowchart showing the Cox process approach used to derive volcanic probability maps.

31

Figure 3.11: (a1) and (a2): left, top and bottom: Two Cox simulations with a multivariate potential of
volcanism. The simulated events are likely to be located in zones with past activity as well as in zones
with seismic anomalies. (b): right: Probability of formation of a new volcano in the next 100,000 years
using the Cox process approach, correlated with the seismic tomography data for Tohoku region.

STEP 4b: Probabilistic Strain Maps
Step 4b.1: Constructing Different Strain Maps: For each indicator, the calculated strain
rates from Step 3b are converted to strain maps for each conceptual model branch of the
Logic Tree. The maps show calculated strains within the 5 km x 5 km areas. As noted above,
this is a reasonable resolution for the datasets being used and is also a useful size withrespect to expected repository footprint (~10 km2), but site-scale rather than regional scale
application of the methodology would require use of a finer grid. The strain rates are alsopresented together on a single map, combining them using the weightings assigned to eachbranch of the logic tree. The weighted map is thus a probabilistic representation of strain,
representing the most likely strain averaged over the time period for which the particular
indicator has been ‘recording’. For application in later Steps, only the maps of the most highly
weighted conceptual model alternatives (branches) and the combined, fully weighted
probabilistic version are likely to be useful.
Step 4b.2: Comparison and Differencing: The weighted, probabilistic maps for eachseparate indicator (i.e. GPS, seismic and tilt) are then compared. Because the different strain
indicators have variable coverage of a region, their use is complementary. The probabilistic
weighted maps for each indicator can, for instance, be differenced to assess the overallcorrelation between strain indicators. This picks out areas where the datasets are inconsistentin their strain estimates. Combined with the variability shown in the strain rate histograms for32

any selected area, such inconsistencies will identify locations where there is significant
uncertainty regarding deformation process, which may also be reflected in a wider range ofstrain rate potential (as can be seen in Figure 3.3, for example). If a potential repository site
lies within such a region, this would require special attention in Step 8, to ensure that
adequate data were gathered during the PIA investigation programme to try to reduce theuncertainty. An example of one of the strain maps generated in the Tohoku Case Study isshown in Figure 3.12, where the weighted average GPS strain data are shown together withthe location of volcanoes.

Figure 3.12: Weighted average strain map (contoured in nanostrains/a) for the Tohoku region, based on
all 148 strain maps from the GPS-based logic tree (Figure 2.3). Here, the location of volcanoes is also
shown (white triangles) as the integration of Steps 4a and 4b is a key exercise carried out in Step 5. A
weak positive correlation can be seen between the location of the volcanic front and the location ofelevated contractional strain, which is being investigated in the methodology development programme.

STEP 5: Integrated Evaluation of Each Potential Repository Site
The information from Steps 4a and 4b can be used directly in Step 6 – for example, to carryout a detailed assessment of a single site or a few alternatives. However, it is possible tocombine the data from Steps 4a and 4b to carry out an evaluation of a larger group of sites sothat they can be compared at a relatively simple level in terms of their overall susceptibility totectonic hazard. This was the objective of the two Case Studies. The text below assumes
that a large set of sites (say 5 – 10) is being compared.
Depending on the interpretations arising from the separate strain maps for each indicator, it
may be considered useful to produce a combined, higher level logic tree that weights belief in
relevance of the three strain measures and factors volcanic strain into the logic tree and theweighting process. Conversely, the likelihood of magma intrusion, as indicated on theprobability maps, needs to be interpreted in the light of deformation history and the mappedstrain rate variations around the site.
Figure 3.13 shows an example of the estimated uncertainties (1σ) in strain values for thedifferent strain indicators at four different example locations from the Tohoku Case Study.

33

Step 5 combines the data from Steps 4a and 4b to carry out an evaluation of a large group ofsites so that they can be compared at a relatively simple level in terms of their overallsusceptibility to tectonic hazard. This was the objective of the two Case Studies. The text
below assumes that a large set of sites (say 5 – 10) is being compared.
For multiple site comparisons, Step 5 produces individual site assessments that provide thefollowing information in an identical format:


description of the geological and tectonic setting of the site;

evaluation of the likelihood of each different type of magma intrusion considered
possible, in both the region around the site and at the site itself, over a period of up
to 100,000 years12;

evaluation of the uncertainties in the likelihoods;evaluation of the best estimate rock deformation potential (expressed as strainprobability histograms) and mechanisms over the same period of time – and therelated uncertainties.

Figure 3.13: estimateduncertainties (1σ) in strain
values for the different
indicators (GPS, tilt andseismic, plus weighted
average) at four different
example locations from the
Tohoku Case Study.

Depending on the interpretations arising from the separate strain maps for each indicator, it
may be useful to produce a combined, higher level logic tree that weights the three strain
measures, and factors volcanic strain into the logic tree and the weighting process.
Conversely, the likelihood of magma intrusion, as indicated on the probability maps, needs to
be interpreted in the light of deformation history and the mapped strain rate variations aroundthe site.
Figure 3.14 shows the equally weighted average strains using all indicators for all the 14
example locations in the Tohoku Case Study (Chapman et al., 2009). These might beregarded as ‘best estimate’ values for the purposes of this methodology demonstration but, ina real site inter-comparison exercise, expert judgement would be required to assign weights
to the different indicators, which would depend on confidence in both the data and theregional models. Clearly, these will vary from one area and one site to another.
12

The methodology could be developed further to provide estimates for 1 Ma, but this would require the use of muchlarger (longer duration, ~10 Ma) datasets and would involve greater uncertainties (especially concerning the timestability of the underlying tectonic processes). Knowledge of future volcanic hazard is, in any case, of diminishinginterest for safety assessment at times even after only 10,000 years, as the hazard of the waste has decreased tolevels equivalent to natural uranium ores by this stage.

34

Figure 3.14: Graph showing the probability or likelihood of exceeding strains of given magnitude, basedon the equally weighted average strains (using all three strain indicators) for all the 14 example locationsin the Tohoku Case Study (Chapman et al., 2009).

STEP 6: Interfacing with the NUMO Performance Assessment work
The methodology up to this stage is designed to deliver a set of probability maps that havetaken account of uncertainty in both conceptual models and data and which containintegrated interpretations of the sites being investigated by NUMO.
Knowledge of the likelihood of various tectonic hazards affecting a site is of limited value to
NUMO unless it can evaluate whether the impacts would be acceptable or not (in terms ofregulatory standards for radiological exposures to the public). This is a task for the NUMO
safety assessment team, who will be carrying out detailed performance assessment studiesof the long-term behaviour of the repository and its engineered barriers. However, information
on both likelihood and impacts is essential for safety assessment and both likelihood andconsequences are together directly linked through a specific ‘event definition’.
The role of the ITM methodology at this stage is thus to provide the PA team with informationon the nature of the tectonic hazards, so that it can construct scenarios upon which to basethese analyses, and to provide quantitative probability estimates of the likelihood of
occurrence of these scenarios. As discussed in Section 1 of this report, two approaches havebeen adopted internationally to utilise this information in safety assessment:13

To calculate the health risk13 to people in the future by combining the probability of a
disruptive event occurring with its radiological consequences in terms of releases

Health risk is normally defined as the risk of death or serious genetic effects.

35

from a repository: simply, risk = probability x consequence. With this approach,regulatory standards or targets can be defined in terms of risk to an individual.


To consider the impacts of a disruptive event and calculate the radiological doses14 topeople in the future and then, separately, to discuss the likelihood that this might
happen (the so-called ‘disaggregated’ approach). With this approach, separate
regulatory targets for radiation doses might be set for events (or scenarios) withdifferent degrees of likelihood (often expressed qualitatively; e.g. ‘likely’, ‘less likely’,‘highly unlikely’).

In either approach, an appreciation of probability is essential: in the first ‘risk approach’ asound quantitative estimate will provide more confident estimation of risk; in the second,some form of quantification of ‘likelihood’ is needed to decide which category to place anevent or scenario into.
The information required for either of these approaches is generated in Step 6 and comprises
the following:
1. A description of the nature of each magma intrusion event and rock deformation
process that could feasibly affect the repository (the basis for the scenario).
2. The likelihood of each magma intrusion event impacting both the repository directly
and the surrounding rock mass.
3. The variation of this probability with time over the next 100,000 years.
4. The best estimate of the magnitude and duration of rock deformation that could affectthe repository.
5. A description of how the events and processes would initiate, develop andprogressively impact the repository and the barriers.
Using this information, the PA team will be able to develop scenarios for tectonic impacts andassign probabilities to them.
The ITM methodology involves the production of a matrix that will compile the information initems (1) and (5) above. This matrix can be used to inform the scenario development work,
which could be carried out jointly by the PA team and the ITM group. Figure 3.15 shows ageneric example of part of a matrix for indirect (hydrothermal) impacts on the repository
structures (excluding the engineered barrier system – which is in a separate matrix) and thesurrounding rock. This type of matrix could be developed to be considerably more detailed,depending on the requirements of the scenario analysis.
The matrix will need to take account of the types of repository design and engineered barriers
that would be appropriate to the sites being studied, so input from NUMO repository design
work will be required. Simple factors, such as repository depth and horizontal and verticaldimensions could have a significant bearing on the level of impact of each type of rockdeformation or magma intrusion.
For the methodology development work, only one selected magmatic intrusion scenario and
one rock deformation scenario have been evaluated and presented as illustrations. For
eventual area and site-specific use of the ITM methodology, this exercise will have to becomprehensive.

14

Of course, a radiological dose can also be expressed in terms of health risk, by applying accepted dose-to-risk
conversion factors.

36

Barrier
SafetyFunction
Plugs: Shortcircuit EDZ
in tunnels
Seals: Prevent
fast transportin shafts

Temperature
Impacts

Hydrological
Impacts

MechanicalImpacts

Thermalalteration:
Cracking anddisaggregation
Thermalalteration:
Cracking anddisaggregation

Chemical
Impacts
Inflowof
magmatic brine:
Change in pH?
Inflowof
magmatic brine:
Change in pH?

Depth: Antiintrusion andradiation
protection
Hydrology: Long,
slow advective
transport

Increased
temperature:
Bouyancy
effects

Modifiedhydraulic head:
Changed flowrate and direction
Inflowof
magmatic brine:
Changes incomplexation?

Rock: Retard
radionuclides(sorption,
matrix diffusion)

No Impacts

Limited Impacts

Moderate Impacts

Extreme Impacts

Figure 3.15: Impact Matrix of the repository and surrounding rock for an illustrative indirect-volcanism
impact scenario assuming imposition of a hydrothermal convection system 10 km from a polygenetic
volcanic centre. Impacts can be categorised mainly as ‘global, slow, ‘permanent’, and ‘barrier
degradation’.

STEP 7: Utilising the Probabilistic Results
Step 7 is where the probabilistic results and any safety assessment results based upon them
are used by NUMO to assist in making siting decisions. Various decision points can beconsidered:
1. At the point where a single volunteer site comes forward in a non-excluded area. NUMO
may wish to consider the overall susceptibility of the site to tectonic impacts before it
accepts the site as a PIA, so that it can gain an initial impression of the extent to which
tectonic issues might be critical to a future safety case. This will enable them to gauge‘programme risk’ – whether they would be taking on a site with a low or high likelihood ofproving unsuitable from the tectonic viewpoint. This would use the data from the LS up to
the point of Step 4, possibly without carrying out any PA work in Step 6. The output would
inform NUMO decision makers so that can be fully aware of any tectonic contribution to
overall project risk and be able to answer questions about tectonic hazard with confidence.
2. Where there are several possible volunteer sites in the same region, so that the relative
susceptibility of the sites can be assessed – essentially at Step 5, as illustrated in the two
Case Studies and described further below. This comparative assessment could help
NUMO to focus or prioritise the site investigations and decide the order in which sites areevaluated.
3. When one or two sites are being considered for the final DIA. At this point, PIA
information would be available and the ITM methodology would have been iterated with
progressively improving data. In addition, feedback would be available from quantitative

37

safety assessments in Step 6, using Step 4 information. By this stage, there should be afull understanding of the actual tectonic hazard for any sites being considered for the DIA
and the ITM information should be at a sufficient level to feed directly into license
application level documentation.
However, each of these possible decision points is a potentially far-reaching stage of NUMO’swork and siting decisions will always involve many other factors (both technical and nontechnical). At present, it is not clear how NUMO might wish to manage this Step for any ofthese three stages of their programme. Further development is consequently anticipated here,
as the PIA siting programme begins to find potential sites, as the NUMO safety assessment
work evolves and as the national regulatory standards become more developed and the useof likelihood and risk become clearer.
No impact analyses have been carried out so far as part of NUMO’s parallel PA development
work, so it is not possible at the moment to assess radiological impacts associated with any
particular scenario. For the purposes of illustrating the methodology, Figure 3.16 shows anexample of how probability values for volcanism, derived from Step 4a might contribute tounderstanding siting confidence, as might be required at the second decision point describedabove (comparing multiple sites). It shows, for the 14 Tohoku Case Study example locations,
one example of how ‘low, medium and high’ levels of confidence might be allocated to eachsite with respect to susceptibility to a scenario of direct magmatic intrusion within the next
10,000 years. The quantitative boundaries between the levels of confidence are arbitrarily
selected here, simply to illustrate the approach.

1.0E-02
1.0E-03
1.0E-04
1.0E-05
Cox
Cox
LSCV
LSCV
SAMSE
SAMSE

1.0E-06
1.0E-07
1

2

3

4

5

6

7

8

9 10 11 12 13 14

Figure 3.16: An example of ‘low’ (red), ‘medium’ (yellow) and ‘high’ (green) confidence levels allocatedto each of the 14 Tohoku Case Study example locations, based on the probability of a direct magmatic
intrusion scenario occurring within the next 10,000 years (for different statistical methods in Step 4a.2).
The quantitative boundaries chosen for the levels are arbitrary examples, simply to illustrate theapproach.

STEP 8: Defining PIA and DIA field tectonic data requirements
The aim of Step 8 is to identify which additional data will be required from PIA site
investigations to reduce the uncertainties in the evaluations in each previous Step. In
particular, after Steps 4, 6 and 7 during the LS stage, the ITM methodology will need to bereiterated to produce more refined evaluations of probabilities and better definition of thenature of impacts for input to the decision to move from PIA to DIA (and to the progressive
development of SAs, as work on the PIAs proceeds). The following classes of information
feed into the ITM Methodology:

38

1. Regional (data coverage): filling gaps in the large regional databases that wereused in Step 3. It is clear that some classes of information will be of patchy quality
and coverage (e.g. age data for volcanic features; active fault parameters; seismicdata), which contributes to the overall uncertainty in the Step 4 probability mapping.
Whilst it cannot hope to fill all such gaps, NUMO should be prepared to gather thistype of information, so the development of an efficient programme that identifies themost important and ‘fillable’ gaps that would have maximum impact on reducing
uncertainty is important. Definition of these critical data gaps is region-specific, but
the two Case Studies have identified obvious improvements.
2. Regional (ground truth): ensuring that the key data used in Step 3 are fully
understood and being used correctly for classification and for setting up strain models.
A certain amount of this work will already have been undertaken in Step 2, but it may
be found that a more thorough evaluation is needed by the time Step 7 has beenreached. As was found for the Tohoku volcanics work (Step 3a), the results are highly
dependent on what a catalogue entry is assumed to mean and field investigation has
shown that all the data need to be checked. This is not necessarily a large operation,
but it would involve field reconnaissance visits to features (volcanic, faults, etc) by
experts across the whole of the region of interest, preferably during the LS stage, but
repeated if needed during the PIA investigations.
3. Local and Site-Scale: many topics of relevance to the ITM methodology will be
studied in any case during PIA investigations. For example, even without the need todevelop the tectonic probability models and evaluations of ITM, NUMO will already beseeking evidence of hidden active faults, characterising local active faults in detail,measuring uplift rates and looking at geothermal heat flux as part of its ‘normal’ site
investigation programme. However, the ITM methodology will provide better results if
it has access to additional information. The scope of this information will be sitespecific, but the following data needs will need to be considered (for the reasons
given):

3.3



age and eruptive style/volumes of local volcanics to constrain the intrusion
probability estimates and impact definition;

more detailed, local-scale gravity and magnetic surveys to identify hidden
volcanic structures or deformation zones that would affect the probabilistic
statistical evaluation;strain mechanism and history of faults and other large deformation features in
the neighbourhood of the site to constrain site context with respect to ‘rockblocks’ used in regional strain budget modelling;

establishment of local fixed and temporary GPS stations to localise GPS
strain within the regional picture, seeking small differences that could reflect
an overlay of site-scale deformation.

References for Section 2

Chapman, N., M. Apted, J. Beavan, K. Berryman, M. Cloos, C. Connor, L. Connor, O. Jaquet, N.
Litchfield, S. Mahony, W. Smith, S. Sparks, M. Stirling, P. Villamor and L. Wallace (2009).
Development of Methodologies for the Identification of Volcanic and Tectonic Hazards to Potential
HLW Repository Sites in Japan: The Tohoku Case Study. Nuclear Waste Management Organisationof Japan, Tokyo. Technical Report: NUMO-TR-08-03. 135 pps.
Hone, D.W.E, Mahony, S.H., Sparks, R.S.J. & Martin, K.T. (2007). Cladistics analysis applied to theclassification of volcanoes. Bulletin of Volcanology, 70, 203−220.
Jaquet O., Lantuéjoul C. and Goto, J. (2009 in press). Cox process models for the estimation of longterm volcanic hazard. In Volcanism, Tectonism, and the Siting of Nuclear Facilities, edited by C.B.
Connor, N.A. Chapman and L.J. Connor.Connor L. (2008a) Probabilistic methodology for long term
assessment of volcanic hazards, Nuclear Technology, 163, pp. 180-189.

39

Jaquet O., Lantuéjoul C. and Goto J. (2008b) Estimation of long-term volcanic hazard using a Cox
process with a multivariate potential, VIII International Geostatistics Congress, GEOSTATS 2008,
Santiago Chile, pp.167-176.
Jaquet O., Lantuéjoul C. and Goto, J. (2009 in press). Cox process models for the estimation of longterm volcanic hazard. In Volcanism, Tectonism, and the Siting of Nuclear Facilities, edited by C.B.
Connor, N.A. Chapman and L.J. Connor.

40

4 Assessing the Consequences of Low Probability Tectonic
Event Scenarios
The ITM methodology described in the previous section (and in Chapman et al., 2007) has
been successfully applied to two large study-regions in Japan; Tohoku and Kyushu. For eachregion, estimates on the probability of future disruptive tectonic and volcanic events havebeen developed based on geological, geophysical and seismological data. Such tectonic andvolcanic events are important scenarios to be considered in any credible safety assessment
for any geological repository to be located in Japan.
The purpose of this Section is to show how the probabilistic results from the ITM methodology
can be incorporated into NUMO’s safety assessment and performance assessment concerns– Step 6 of the ITM methodology. Specifically, the intent is to present a linkage between theprobabilistic and the consequence aspects of natural-event scenarios, leading to a systematicclassification of impacts on thermal, hydrological, mechanical and chemical (T-H-M-C)
conditions and the degradation and loss of safety functions of natural and engineered barriers.
In the following sections, the results and implications of the ITM studies with respect to futuresafety assessments by NUMO will be discussed. Specific topic include:

natural-event scenarios in safety assessment,

the role of probabilities in safety assessment, including scenario probability

linkage between probabilities and consequences in safety assessment of naturalevent scenarios,systematic classification of natural-event scenario consequences (impacts), and

examples of specific natural-event scenarios and their qualitative consequences onlong-term repository performance and safety.

Two key messages focus on the necessity of factoring probabilities of natural events into any
safety assessment approach that may be adopted by NUMO, and the need to organize andclassify natural-event scenarios in ways to identify R&D needs and guide developments insafety assessment methodologies.

4.1

Safety Assessment Overview

All concepts for the geological disposal of nuclear waste involve the performance analysis
and safety assessment (PA/SA) of a system of engineered and natural barriers. Thesebarriers are assigned specific ‘safety functions’, defined as either a property or process bybarrier that acts to contain or contribute to the long-term isolation of nuclear waste, preventing
unacceptable future releases of radioactivity to the biosphere and human environment.
Figure 4.1 presents an illustrative example of common barriers and basic safety functions fora vertical emplacement configuration; other geometries, dimensions, barriers and barriermaterials are of course, possible, leading to different assignments of ‘safety functions’ fordifferent concepts.
The beginning point for safety assessment is evaluating the performance of the multiplebarrier repository for the expected evolution of the repository. Safety assessment of the‘expected evolution’, or ‘base-case’, scenario, assumes initial ambient temperaturehydrological-mechanical-chemical (T-H-M-C) conditions as determined by extensive sitecharacterization. Underground construction and emplacement of heat-producing high-level
waste induces changes to the ambient T-H-M-C conditions, followed by a gradual return toambient conditions. Figure 4.2 (Kurikami, et al., 2009) presents a comprehensive illustration of
the expected evolution for a representative repository concept.

41

Figure 4.1: Safety features of the generic barrier system of a geological repository

Figure 4.3: Schematic illustration of the chain of models in a basic safety assessment

42

43

Figure 4.2 (continued on next page): ‘Storyboard’ representation of the evolution of a repository barrier system (Kurikami, et al., 2009)

44

Figure 4.3 (continued): ‘Storyboard’ representation of the evolution of a repository barrier system (Kurikami, et al., 2009)Safety assessment is conducted using an assembled chain of models that represent thecontainment and isolation processes for the different barriers of a repository. Figure 4.3
shows a simple schematic of this ‘chain-of-models’ concept, as well as the type of inputs andoutputs typically included in such safety assessments. Inputs for various values ofparameters are tabulated for the ‘expected-evolution’ scenario. While detailed modeling of
the changes in T-H-M-C conditions can be part of the ‘chain-of-models’, it is typically true thatthe expected containment time for the overpack (300 to several 1000’s of years) is longer
than significant perturbations attributable to construction and waste emplacement, so thatwhen the overpack eventually fails the repository conditions basically have returned toambient values.
Note in Figure 4.3 that the prime output of SA models is typically a calculated dose rate tosome stylized future human population. Regulators in different countries have taken different
approaches to how dose rate calculations should be conducted and evaluated, a topic that isreturned to later. Also note that SA models can also be constructed to provide information on
the flux of radionuclides at various locations within a multiple barrier repository. Such insights
can be extremely valuable to repository designers in assessing “which barriers” arecontributing “how much performance” to the long-term isolation of nuclear waste.
Figure 4.4 extends the information in Figure 4.3 by recognizing that there are inherent
uncertainties in information on parameters and processes, even for the expected-evolution
scenario. The geosphere has natural variation in its properties, often termed “aleatory
uncertainty’, that may have significant impacts on the performance of the natural barrier. In
addition, there are gaps and limitation knowledge, often called “epistemological uncertainty’that arise because of uncertainty measurement, testing, modelling and all other human-based
aspects of safety assessment. It is necessary, therefore, that estimates of the ranges ofthese uncertainties be included in any safety assessment calculation. This can be done
through logic trees, simple triangular or uniform distributions of data values vs. frequency, or
through elaborate probability density functions if there is sufficient information to constructand defend such pdf’s. The key point is that, to be fully credible, even safety assessment of
the expected, normal-evolution scenario needs to encompass uncertainties within some formof a probabilistic framework.

Figure 4.4: Schematic illustration of the treatment of probabilistic data inputs in a safety assessment

45

4.2

Natural-event Scenarios in Safety Assessment

Depending on the location and time-scale appropriate to a given repository site, the ‘expected
evolution’ scenario can also include possible perturbations arising from natural events that arecertain to occur in the future. This would include climate change (e.g., glaciation, change in
sea level), or low-level seismicity that can be expected to occur at any site within theJapanese archipelago. Repository concepts must be robustly designed to assure continued
safe isolation of nuclear waste even for typically minor perturbations arising from suchnatural-event scenarios.
Japan, however, located on the Pacific Ring-of-Fire (Apted et al., 2004) is subjected topossible re-occurring tectonic and volcanic/igneous events that could impose much more
major perturbations on a nuclear waste repository. NUMO has already acknowledged
concern for such major impacts by defining exclusionary siting criteria for a repository that
includes not siting the repository across an active fault, nor siting a repository with 15 km of aknown volcanic centre. Note, that such criteria inherently recognize location and proximity ofa natural event to the repository is a key aspect in considering safety assessment impacts.
This is a theme that is returned to later in this report.
The ITM project was initiated to aid NUMO in developing and applying methodologies forassessing the probability of either future tectonic or volcanic “natural-event” scenarios
occurring at given locations (5 km by 5 km grid block), using the extensive geological,
geophysical and seismological databases uniquely available in Japan (Chapman et al., 2007).
Such tectonic and volcanic ‘hazard maps at a regional scale have allowed NUMO to evaluate
certain siting hypotheses (e.g., persistence of ‘gaps’ in past volcanic activities in Tohoku),
limitations to geo-databases, and the complexities of extrapolating geological information toforecast future event-probabilities over different timescales (10,000 years vs. 100,000 years
vs. 1,000,000 years).
Lastly, it must be recognized that ‘natural-event scenarios’ such as for tectonics andvolcanism can occur in a wide variety of modes with a wide range in properties with a strong
dependence on site-specific details. Tectonic scenarios, for example, could include faulting(both activation of existing faults or generation of new faults), folding, or uplift, depending on
the site. Volcanism scenarios, for example, could include monogenetic, polygenetic, or largescale caldera formation.
At this relatively early stage in the development and application of the ITM methodology tolarge (106 to 105 km2) regions, certain limitations in identifying specific scenarios arerecognized. The rather coarse 5 km by 5 km grid map for the Tohoku and Kyushu regions, aswell as the absence of basic geological information such as rock lithologies and existing
structures within such 5 km by 5 km blocks, prevent confident assignment of specific naturalevents that might or might not occur in the future within such blocks. This limitation isparticularly true for the tectonic ‘strain rate’ indicator because sustained strain rate
accumulating over time can lead to a wide range of modes of stress relief, such as major
displacement along on a single large fault, distributed smaller displacement along many faults,
folding and several other stress-relief accommodations. Until more site-specific information
can be obtained, the current 5 km by 5 km resolution necessarily means the natural-events
discussed here should be considered illustrative of the advantages of the ITM methodology(Chapman et al., 2007), rather than a screening tool to assess the potential for natural event
scenarios on a site-scale basis.
This same caution applies to the consequences analyses presented in this section, which areintended to be qualitative and illustrative of the how to link natural event probabilities to
impacts on the environmental conditions and future performance of natural and engineered
barriers with a geological repository.

46

4.3

The Role of Probabilities in Safety Assessment, Including Scenario
Probability

As with probabilistic treatment of data uncertainties, there are probabilities of natural-event
scenarios that need to be accommodated within a credible safety assessment methodology.
There are several possible approaches to safety assessment, as noted below, but each
eventually requires a technical defensible and quantitative estimate of the probabilities ofnatural-event scenarios.
In the now classic definition of the ‘risk triad’, three basic questions can be raised about
natural-event scenarios:What can happen (defining the specific natural event)?

How likely is the event (defining event probability or likelihood)?

What are the consequences if the event occurs?

Colloquially, risk can then be defined as
Risk = Probability x Consequences
The risk-based approach basically allows for normalization of possible high dose
consequences of an event by multiplying by the perhaps low probability of that event. Indeed,in the US Nuclear Regulatory Commission’s standard for HLW disposal at a repository in
Yucca Mountain, Nevada, the risk-informed approach is applied in screening and eliminationof possible natural-event scenarios, stating that the repository implementer should [10 CFRPart 63.114(d) and Part 63.342]:“Consider only events that have at least one chance in 10,000 of occurring
over 10,000 years.”“DOE's performance assessments shall not include consideration of very
unlikely features, events, or processes, i.e., those that are estimated to haveless than one chance in 10,000 of occurring within 10,000 years of disposal.”This is equivalent to excluding all natural-event scenarios with a likelihood of less than 1
occurrence in 100,000,000 years, no matter what might be the speculated consequences ofthat exceptionally rare event.

Figure 4.5: Schematic of proposed regulatory targets for different scenarios for the L1 repository at
Rokkasho. Allowable dose rates for different ‘likelihoods’ of scenarios

47

There are also non-risk-based approaches to setting dose rates that nonetheless, eventually
do necessitate explicit estimate of the probability of a natural-event scenario. Specifically, a
key motivation for the ITM project on estimating probabilities of future natural events is theevolving perspectives in Japan on comparing SA calculations to specific allowable dose-rates.
Figure 4.5 presents the current proposed dose rates for natural event scenarios, as formatted
for the L1 disposal concept in Japan (disposal of LILW at depths of about 100+ m). This set ofdose rates limits could eventually be applied to NUMO’s own HLW and TRU disposal
programme.
The three key undefined terms for scenarios in Figure 4.5 are “most likely”, “unlikely” and“extremely unlikely”, each category assigned a increasingly higher allowable dose rate fordecreasing likelihood of occurrence. Setting aside possible comments on the absolute values
for these three categories, an immediate concern is that the undefined “likelihood” terms do
not recognize any specific timescale for their applicability. There is also a concern they may
have been developed for a specific site (e.g., Rokkasho). And at this site, it may be currently
believed that certain types of natural-event scenarios (e.g., volcanism, uplift, faulting) can beclearly assigned to specific “likelihood” categories, and that the associated dose rates(“consequences”) for each of the three “likelihood” categories can be reasonably met. At
other sites, however, various natural-event scenarios could have different “likelihoods” and/ or
different consequences; great care should be reserved when considering applying basically asite-specific standard to become a general, nation-wide standard.
Figure 4.6 schematically illustrates and compares the risk-dose and “likelihood” based
approaches as they might be applied in safety assessment. For both cases, different inputs,
indeed different probabilistic inputs, for parameters and processes will be required for eachscenario (i.e., “expected evolution scenario”, “possible volcanism scenarios”, “possible
tectonic scenarios”, etc.). For the risk-based approach, probabilities for scenarios will also beneeded; the “likelihood” scenario would seem not to require such scenario probabilities, but
as will be discussed below, this is only a temporary dodge of the necessity to define such
scenario probabilities.

Figure 4.6: Comparison of “risk-based” and “likelihood-based” Safety Assessments

48

The probabilistic inputs from each scenario are used to calculate dose-rate consequences
separately. In the risk-based approach, the eventual output could be calculation of theprobability-weighted, mean annual dose rate, an approach applied in the US and Swedish
regulatory licensing requirements, for example. These probability-weighted, mean-annual
dose rates for separate scenarios can, in turn, be aggregated together to present an
aggregated probability-weighted, mean annual dose rate for all of the credible future eventsand evolution of the repository.
For the “likelihood” approach, the same probabilistic inputs from each scenario are used to
calculate dose-rate consequences separately. Then, through some undefined step, eachscenario and resulting consequence analysis would be assigned to one of the three
categories in Figure 4.5. It can be conjectured that if any one scenario dose-rate results
violated the assigned peak dose rate for that scenario’s category, that the entire repository
concept would be judged to be unacceptable. It could be that two or more different naturalevent scenarios could be assigned to the same “unlikely” or “extremely unlikely” dose-ratecategory, even though there might be many order of magnitude differences in the actual
probabilities among these scenarios grouped in the same category. Furthermore, there is noformulation to aggregate all of these diverse scenarios together to get an overall assessmentof repository safety.
Ultimately, the issue with the three defined “likelihood” terms in Figure 4.5 is that it will require
some responsible authority to assign each specific natural-event scenarios into a specificcategory, and to defend those assignments. Event “probabilities” (objective values), not“likelihoods” (subjective opinions) is the only defensible basis for supporting such decisions.
Clearly, such objective probabilities can only be based on geological analysis of the sitespecific geological record and broad understanding of the plate tectonic framework of Japan.
In closing, it is clear that for either a risk-based or “likelihood” based safety standard, thatprobabilities of natural-event scenario will, and should, be required. These probability
estimates will need to be quantitative and credibly based on site-specific geologicalinformation and analysis. The ITM methodology is such a geological analysis (Chapman et al.,2007). If carefully applied, the ITM methodology can lead to specific, defensible estimates ofvarious tectonic and volcanic event probabilities that will needed by NUMO, no matter how
regulatory safety standards evolve in Japan.

4.4

Event Definition for Natural-event Scenarios“Event definition” is a key concept that directly links the development of ITM probability
estimates for natural-event scenarios (Chapman et al., 2007), and subsequent considerationof possible consequences of such natural-event scenarios. “Event definition” must include allof the relevant and distinguishing characteristics of a postulated future natural event that
could unexpectedly perturb the performance of a deep geological repository.
Some of the important aspects and factors in “event definition” include:What might be the magnitude of the event?

What might be the duration of the event?

When might the event occur?



Where might the event occur with respect to the repository barriers?

What might be the frequency of the event (i.e., can the same event occur
more than once within the repository)?

All of these aspects and factors should be identified as part of the derivation of event
probability. For example, the probability of a monogenetic event at a site is likely to be muchdifferent than the probability of a polygenetic or caldera-forming event at the same site. The

49

probability of displacement along an existing fault would increase proportionally over time.
The probability of volcanism impacts might be a function not only from the probability of afuture igneous event within the 5 km by 5 km block, but also from increased probability of afuture igneous event occurring in an adjoining 5 km by 5 km block. It will be important,
therefore, in future applications of the ITM methodology to clearly and concisely identify an“event definition” attached to the probability estimate.
The “event definition” characteristics, in turn, are the beginning of organization of a systematicconsequence safety analysis. These ‘event definition” characteristics impose importantbounds and constraints on the subsequent evaluation of (1) perturbation on T-H-M-C
processes and environmental conditions, and (2) associated degradation and loss of barrier
safety functions, up to and including actual destructions of barriers. In the following section, ageneral classification scheme is proposed by which safety assessment modellers might
usefully consider and propagate the “event definition” for various natural-event scenarios.

4.5

Systematic Classification of Natural-event Scenario Consequences

4.5.1

Volcanism Scenarios

Volcanic activity in Japan historically ranges from single-event monogenetic eruption centres,to sustained polygenetic eruption centres, to areally extensive, caldera-forming events. The
eruption energetics (power), duration, eruption volume, chemical composition, eruption
temperature, volatile (especially water) content, viscosity and many other characteristics also
range widely, as previously illustrated in ITM’s cladistic analysis (Chapman et al., 2007).
Consequences on T-H-M-C conditions, as well as the degradation if not survival of barrier
safety functions will be strongly impacted by such characteristics.
Perhaps even more importantly is consideration of where a future volcanic center might form
with respect to a repository. Several different variants to the volcanism scenario can bebroadly identified:

Direct Intersection/ Polygenetic event: As illustrated in Figure 4.7, a repository directlyintercepted by the central conduit of a polygenetic volcano will likely be completely
destroyed by the thermal and stress perturbation associated with such conduits.

Direct Intersection/ monogenetic event: In contrast, if only a single, relatively shortduration (few weeks to few years) monogenetic magmatic dike were to intercept therepository, destruction of the repository system might be localized in both time andspace.



Indirect Intersection: A third variant to the volcanism scenario might that therepository is not intercepted by any magmatic material, but could be indirectly
subjected to the impacts from the hydrothermal circulation cell developed on theflanks of a volcanic eruptive centre. Such hydrothermal circulation cells might extend
on the order of 15 km and persist for 10,000’s of years around a polygenetic eruptive
centre.

Of course sub-variants and ranges within these variant cases could also be specified. The
intent in this report is simply to stress that there is no unique “Volcanism scenario”; there areinstead a range of possible variant cases that can be readily classified according to aspect of
the “event definition” (and thereby linked to a specific probability for that specific “event
definition”).

4.5.2

Tectonic (Rock-deformation) Scenarios

As with Volcanism Scenarios, there is a wide range of variant cases to Tectonic Scenarios.
Indeed, the “strain rate” attribute used in the ITM methodology can be accommodated in an
extremely wide variety of modes. The relative probability of such modes depends strongly on
local, site-specific factors such as host-rock lithologies and existing structures that are moredetailed than the current 5 km by 5 km block size utilized to demonstrate the ITM
methodology.

50

Figure 4.7: Cross-section of a volcanic centre, showing different T-H-M-C type impacts depending on
location of repository (red rectangles).

51

A

B

D
C

Figure 4.8: Schematic illustration of different modes by which repository host rock might accommodate
sustained strain: A: all along major faults (old or new) some distance away (probably equating to a low
hazard); B: distal strain uptake from A on small (c.100 m radius) fractures in the repository (probably
equating to a medium to high hazard); C: volume deformation along many small, distributed
fractures/planes (probably equating to a little or no hazard); D: hidden/new active fault below (probably
equating to a medium hazard).

Figure 4.8 illustrates a partial set of such ”strain rate” accommodation modes:Mode A: all of the strain rate is accommodated along major faults (old or new) somedistance away from the repository, which would likely have only minor impact on
subsequent repository performance after fault movement.

Mode B: the sustained strain rate is accommodated on small (c.100 m radius)
fractures that intersect the engineered barrier system of the repository, which could
have moderate to high impacts on subsequent repository performance depending on
the amount of movement and inter-connectivity of such small, but widely distributed
fractures.

Mode C: strain rate might be accommodated by volume deformation along manysmall, distributed joints or cracks within the host rock, which might have moderate
impact on repository performance if there are associated adverse perturbation of
hydraulic conductivity of the host rock.

Mode D: strain rate might be accommodated by activation of a new fault or activationof a hidden fault below the repository, leading to possibly low to medium impact on
repository performance.

As with Volcanism Scenario, there are many possible variants to Tectonic Scenarios. Folding
might be of concern for certain types of sedimentary rock formations. It might be expected,
however, that sustained strain rates over the past would already be manifest in the foldedrock fabric of such a site discovered during detailed site characterization, allowing possible52exclusion of the site if the folding deformation is judged to be too adverse to assure long-term
waste isolation. Of even greater concern is the possibility for sustained strain rate leading to
uplift of a repository, accompanied by co-equal rates of erosion. While uplift of a repository to
the surface would obviously lead to greatly enhanced exposure pathways, it is worth notingthat adverse chemical impacts (change from reducing to oxidizing conditions) and mechanicalimpacts (formation of sub-vertical fracture zones due to mechanical unloading of the rockformation) could occur even when the repository would be uplifted to within 100 m or so to thesurface.
While detailed classification and analysis of such multiple variants to natural-event scenarios
is beyond the scope of the ITM project, it is possible to identify general sequencing of impacts
from the wide array of such variants. The sequencing of impacts from natural-event
scenarios include:Spatial factors: are the impacts local, affecting only a few waste packages or regionsof rock, or are they global impacts, affecting all waste packages and all of the host
rock?



Temporal factors: do the impacts arise slowly or instantaneously?



Permanence: are the impacts on either barrier properties or changes in T-H-M-C
conditions permanent or only temporary?

As a final part of classifying natural-event scenarios within a safety assessment framework is
consideration of the characteristics of impacts on the repository system. Two broad
categories can be envisioned:

degradation or loss of individual safety functions for one or more natural andengineered barriers,total destruction of the repository system, with loss of all barriers and barrier safetyfunctions.

The importance of classifying scenarios with respect to characteristics of impacts is that thesafety assessment experts need to understand at an early stage what assessment methods
and calculational tools may be needed to conduct safety calculations. Degradation ofindividual safety functions can be handled by appropriate time-step changes in inputs ofbarrier properties; loss of individual safety functions can often be handled by the so-called“barrier-neutralization” approach, in which a property or process by a specific barrier isomitted or set to “zero” in the chain-of-models. For natural-event scenarios leading to total
destruction of the repository system (i.e., direct intersection by a polygenetic or calderaforming volcanic event), new dose-contributing pathways (e.g., airborne transport ofradioactively contaminated tephra) may be needed.
It must be cautioned, however, that natural-event scenarios capable of destroying a multiplebarrier repository system may lead to non-radiological hazards and consequences that far
exceed those represented by the resulting radiological hazard. An explosive eruption
associated with caldera formation is an example of a regional catastrophic event of the kindthat has occurred many times in the Quaternary, albeit in human terms very infrequently.
Before initiating revised radiological safety assessments for such drastically disruptive natural
events, it would be sensible to conduct at least a bounding analysis of all of the nonradiological as well as radiological impacts to human health arising from such an event.
Figure 4.9 presents a general logic-tree schematic of how event definition, sequencing of
impacts and classification of variant cases for natural-events scenarios can be organized andclassified. Further sub-divisions of such factors are possible. Figure 4.9 should beconsidered as a beginning logic-tree structure, suitable for subsequent refinement based on
the exact type of natural-event scenarios of concern for a specific site. The purpose of such alogic-tree is to provide a visualization of the many factors that should be considered in
preparing to conduct safety assessments for variants of natural-event scenarios that go
beyond the basic “expected evolution” scenario.
In summary, early consideration of likely variant cases for natural-event scenarios andespecially classifying such variants according to expected spatial, temporal and permanence

53

factors, can be useful to safety assessment experts. In particular, such systematicclassification of variants can help to identify the types of consequence analyses that may benecessary above and beyond the “chain-of-models” initially developed for the “expected
evolution” scenario. Consideration and general classification of spatial, temporal andpermanence factors for variant cases can provide safety assessment experts with insights
into the relative severity of different types of variant cases. For example, impacts that are
global in nature are likely to have more severe consequences on repository performance thanare impacts that locally affect a few waste packages. Likewise, impacts that occur
instantaneously might have greater consequences on repository performance than otherimpacts that occur only gradually over geological time scales. Obviously, permanent changes
to barrier properties are always likely to have greater consequences on repository
performance than are changes that eventually return to ambient, expect property values.
Finally, evaluation and classification of variants using such factors would be best
accomplished with an integrated team of site geologists, repository designers, and safetyassessment experts.

Figure 4.9: Generalised logic-tree schematic of how event definition, sequencing of impacts andclassification of variant cases for natural-events scenarios can be organized and classified.

4.6

Qualitative Consequences for Example Natural-event Scenarios

Returning to Figure 4.2, NUMO has created a detailed storyboard of the “expected evolution”
of the repository and the multiple barriers within that repository system. A useful division in
the time sequencing is to consider an initial period (300 to 1000 years in Figure 4.2) ofcontainment by the overpack, followed by a period (time greater than 100 years in Figure 4.2)of slow release of radionuclides (see associated safety functions, as identified in Figure 4.1).
Of course, safety regulations are based on calculated peak dose rates, emphasizing the slowrelease performance. Containment is extremely useful to prevent water from contacting HLWduring the initial elevated thermal pulse (hence, the 300-1000 year time interval in Figure 4.2).
“Containment time” by itself, however, is not likely to significantly reduce subsequent peakdose-rate performance of a repository. This is because of the extremely long-half-lives ofmost dose-contributing radionuclides [e.g., Se-79 with a 1 x 106 year half life, Cs-135 with a
2.3 x 106 year half life, Np-237 with a 2.1 x 106 year half life, which were the key dose
contributing radionuclides in the H12 report (JNC, 2000)] compared to readily attainablecontainment times for saturated-site repositories.

54

Therefore, unless a natural event occurs within the 300-1000 year initial containment period,
the major concern arising from a natural-event scenario are impacts on the mobilization,
release and transport of radionuclides through the engineered and natural barriers.

55

56

Unreacted because nocontact by water

Present but corroding

Dry increasing tosaturated; swelling,buffering of chemistry byion-exchange/
dissolution of minor
minerals

Increase then decreasein temperature,
readjustment of
mechanical stresses,
chemical buffering
ambient conditions

No change/ ambient

No change/ ambient

Stainless steel
canister

Overpack (mildsteel)

BufferNear-field (NF) RockFar Field (FF) Rock

Biosphere

No change/ ambient;increasing, non-steadystate radionuclide release

No change/ ambient,radionuclide sorption; nonsteady-state release

No change/ ambient,radionuclide sorption;
non-steady-state release

No change/ ambient,radionuclide sorption;
non-steady-state release

Same properties as farfield rock (ambient
temperature, stress,
flow, etc.); nuclide
sorption; attain steadystate release

Saturated, buffered byalumino-silicatehydrolysis; nuclide
sorption, colloid filtration,
attain steady-state
releaseSaturated, buffered byalumino-silicate hydrolysis;
nuclide sorption, colloid
filtration, non-steady-state
releaseSame properties as far-fieldrock (ambient temperature,stress, flow, etc.); nuclide
sorption; non-steady-state
releaseAll iron oxides; no masstransfer resistance;
sorption

Multiple penetrations(decreasing masstransfer resistance)

HLW glass entirely
reacted to nuclidebearing/ solubilitylimiting precipitates

15,000 - 65,000 years

Mostly iron oxides, someiron present; nuclide
sorptionCorroding, possibly locallyfailed (mass-transfer
constraints)

Water-contact (when SS
canister fails) startdissolving, release ofradionuclides

1000 - 15,000 years

No change/ ambient,radionuclide sorption;
attain steady-state
releaseNo change/ ambient,radionuclide sorption;
attain steady-state
releaseNo change/ ambient,radionuclide sorption;
steady-state release

No change/ ambient,radionuclide sorption;
steady-state release

Same properties as farfield rock (ambient
temperature, stress,
flow, etc.); nuclide
sorption; steady-state
releaseSaturated, buffered byalumino-silicatehydrolysis; nuclide
sorption, colloid filtration;
steady-state release

Saturated, buffered byalumino-silicatehydrolysis; nuclide
sorption, colloid filtration;
steady-state release
Same properties as farfield rock (ambient
temperature, stress,
flow, etc.); nuclide
sorption; steady-state
releaseAll iron oxides; no masstransfer resistance;
sorption

Essentially all oxides, nomass-transfer resistance

HLW glass entirely
reacted to nuclidebearing/ solubilitylimiting precipitates>100,000 years

All iron oxides; no masstransfer resistance;
sorption

Multiple penetrations(decreasing masstransfer resistance)

HLW glass entirely
reacted to nuclidebearing/ solubilitylimiting precipitates

65,000 - 100,000 years

Table 4.1: Generalised evolution of a representative repository focusing on factors affecting the mobilisation, release and transport of radionuclides through multiple barriers.

Unreacted because nocontact by water

0 - 1000 years

HLW glass

BarrierTable 4.1 follows the general format of Figure 4.2 developed by NUMO, but focuses on barriers andprocesses affecting slow release of radionuclides. It is suggested, therefore, that NUMO’s primaryattention of consequence analysis for natural-event scenarios be initially placed on evaluating how
different variant cases might (1) affect the slow release of radionuclides from the EBS, and (2) affectthe plume of released radionuclides already released from the EBS to the geosphere but not yet
released from the geosphere to the biosphere (i.e., focus on the fate of released radionuclides
throughout the repository system for times longer than 1000 years after repository closure).
Table 4.2 presents a tabulation of consequences arising for three different variant cases for the
volcanism/ igneous-event scenario. For each variant, the event definition (“mode”), sequencing of
impacts and classification of impacts are provided. In addition, the qualitative impacts on T-H-M-C
conditions in the repository are also presented.

Table 4.2: Tabulation of consequences arising for three different variant cases for the volcanism/ igneous-event
scenario.
Event
Definition(Mode)

Host Rock
Dependency

Event Sequencing

Consequences

THMC Impacts
 T: <120 C

Formation ofhydrothermal
circulation systemfrom intrusion of<15 km distantmagma chamber(INDIRECT) Gradual
None Global Permanent (durationof magmatic system~1 Ma)

 InstantIntrusion of single(monogenetic)
basaltic dyke, <10m width, ~1000 C(DIRECT)

None

Intrusion ofmultiple(polygenetic)magmatic dykes(DIRECT)

None Local (initial sheetintersection followedby sustained eruption
at a few conduits) Permanent/transient(duration of intrusion~ few years)

 Instant

Inflow of warm, saline
water, possible gradualmetasomatism of clay
buffer, changed
radioelement solubilities
and sorption behaviour H: increased (buoyancy)
flow rate?

Intersection with a few
waste packages leading tosurface eruption of intact
WPs or entrained HLW;
transient and localised
high-T and faulting ofsome adjoining WPs withloss of buffer andoverpack functions; new,preferential flow path tosurface T: up to 1000 C

Loss of function of all
repository barriers Global Permanent M: None C: increased salinity,
dissolved magmatic
volatiles, neutral pH,
reducing
 H: increased (buoyancy)
flow rate?
 M: Local faulting of rock,buffer and overpack C: minor and localised
release of magmatic
volatiles, re-melting andrecrystallisation of HLW
glass?

 Extreme disruption

Table 4.3 tabulates event definition (“mode”), sequencing of impacts and characteristics of impacts forseveral different modes for variants of the tectonic, or “rock deformation” scenario. Note that impacts
may especially vary depending on whether the host rock for the repository is a “soft” (sedimentary)rock versus a “hard” (igneous or metamorphic) rock. By contrast, impacts arising from variants of the
volcanism scenario (Table 4.2) are generally the same irrespective of what type of host rock in which
the repository is located. Qualitative impacts on T-H-M-C conditions in the repository are also
identified in Table 4.3.

57

Table 4.3: Tabulation of event definition (“mode”), sequencing of impacts and characteristics of impacts forseveral different modes for variants of the tectonic, or “rock deformation” scenario.

Event
Definition(Mode)

Host Rock
Dependency

Reactivate existingfault to becomenew ‘major waterbearing feature’‘hard’ or ‘soft’rock, preexisting
structures

Microfracturing/change
in bulk rockpermeability‘soft’ rock, preexisting
structures

Event Sequencing

Consequences

THMC Impacts
 T: none Instant Global (affects
release from all WPs)

Shortening of far-field
pathway Permanent H: <100 m path in rock M: none C: none T: none Gradual Global

Increased flow rate innear-field rock Permanent H: increased flow rate
 M: none C: none T: none

Shearing of waste
package (>1 mdisplacement)‘hard’ or ‘soft’rock, preexisting
structures

Loss of buffer andoverpack functions, withhigher flow in rock for
failed WP Instant Local Permanent H: higher flow, nodiffusion control, no
colloid filtration
 M: loss of containment C: none T: none

Increased apertureof joints andfracturesintersecting waste
package‘hard’ or ‘soft’rock, preexisting
structures

Enhancedpermeability/flow in rock;erosion of buffer, loss of
buffer functions

 Gradual Local Permanent H: higher flow, nodiffusion control, no
colloid filtration
 M: none C: none T: none‘hard’ or ‘soft’rock, preexisting
structures

UpliftAt 100 m depth, change tooxidising conditions,mechanical disruption ofrock and EBS Gradual Global Permanent H: higher flow, nodiffusion control, no
colloid filtration
 M: loss of EBS integrity C: oxidising T: none

Folding‘soft’ rock, preexisting
structures

Enhancedpermeability/flow in rock;mechanical disruption of
EBS Gradual Local/Global
 Permanent H: higher flow, nodiffusion control, no
colloid filtration
 M: loss of containment C: none

4.7

Summary

Safety assessment for a geological repository containing HLW is based on calculating possible future
doses from radioactivity eventually released from the repository to the biosphere and humanpopulation. Defensible dose rate calculations require evaluation of possible doses for the “expected
evolution” scenario as well as possible doses arising from variants of credible “natural-event”
scenarios.
The determination of the probability (“likelihood”) of such natural-event scenarios must be based on
multiple lines of climatological, geological, geophysical and seismological analyses conducted at asite-specific level. The ITM project has developed and successfully tested methodologies for
obtaining such probabilities for volcanism and rock deformation (tectonic) events, albeit on a scale
somewhat larger than an actual repository site (Chapman et al., 2007).
Whether a risk-based safety standard (i.e., the consequences of a natural event are normalized by theprobability of that event) or a non-risk standard (i.e., allowable dose rates are assigned to scenarios

58having different “likelihoods”) is applied, quantitative consideration of actual event probabilities isunavoidable. Indeed, in the absence of quantitative estimates of natural-event probability, it will bedifficult to derive a bounded yet defensible safety assessment of radiological hazard for sites in Japan,
which is experiencing complex plate subduction and associated volcanic activity.
For natural-event scenarios, a logic-tree formulation is proposed to aid NUMO in classifying variants ofsuch natural-event scenarios to better plan and conduct safety assessments. Three key parts of thislogic-tree organization are proposed (Figure 4.9):
event definition

sequencing of consequences, and

classification of impacts

By considering the spatial, temporal and permanence aspect of consequences, as well as whether
impacts consequences degrade or destroy the repository system, NUMO’s safety assessment experts
can evaluate the adequacy of their current safety assessment models and codes, and identify where
possible modifications or additional models may need to be developed.
Finally, several illustrations of variants to natural-event scenarios are presented, leading to qualitative
estimates of impacts on thermal, hydrological, mechanical and chemical (T-H-M-C) conditions of arepository. Without more detailed, site characteristics, event definition, and repository conceptual
design, only such qualitative illustrations are sensible to be presented at this time. When site-specificand concept-specific information are available, however, it will be relatively easy for NUMO technicalstaff to propagate credible “event definitions” for variants of natural-event scenarios through thesuggested logic-tree approach to guide future safety assessments.

4.8

References for Section 4

Apted, M.J., K. Berryman, N. Chapman, M. Cloos, C. Connor, K. Kitayama, S. Sparks, and H. Tsuchi,
2004. Locating a Radioactive Waste Repository in the Ring of Fire, EOS, American GeophysicalUnion, Vol. 85, No. 45, 9 November 2004.
Chapman, N., M. Apted, J. Beaven, K. Berryman, M. Cloos, C. Conner, L. Connor, O. Jaquet, N.
Litchfield, S. Mahony, S. Sparks, M. Stirling, and L. Wallace, 2007. Development of Methodologies for
the Identification of Tectonic Hazards to Potential HLW Repository Sites in Japan, MCM-TR-07-03,
MCM Consulting, Baden, Switzerland.
JNC, 2000. H12: Project to establish the Scientific and Technical Basis for HLW Disposal in Japan,
Second Progress Report on Research and Development for the Geological Disposal of HLW in Japan,
Japan Nuclear Development Institute, Tokai-mura, Japan.
Kurikami, H., Suzuki, S., Wakasugi, K., Ueda, H., Ishiguro, K., Tsuchi, H., Kaku, K., Gaus, I. andVomvoris, S., 2009. Study on strategy and methodology for repository concept development for theJapanese geological disposal project, NUMO-TR-09-04, p.52-53.

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