Report of the Special Review Committeeon
the NUMO safety case

December, 2019

Special Review Committee on the NUMO Safety Case
Atomic Energy Society of Japan-1-©原子力発電環境整備機構: Nuclear Waste Management Organization of Japan, 2019
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〒108-0014 東京都港区芝4丁目1番地23号 三田NNビル12階
原子力発電環境整備機構 技術部
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Executive Summary

The Nuclear Waste Management Organization of Japan (NUMO) has prepared the NUMO Safety Case,
entitled “Realization of safe geological disposal of high-level radioactive waste and TRU waste in Japan development of a safety case for the selection of an appropriate site” (hereafter the Safety Case). The
purpose of the Safety Case is to explain how to realize safe geological disposal of high-level radioactive
waste and TRU waste, reflecting the latest scientific knowledge and technical developments.
NUMO requested the Atomic Energy Society of Japan (AESJ) to review the Safety Case, with theobjective of checking the feasibility of such a Safety Case at a stage where no site has been specified,
within the context of the latest scientific and technological knowledge and developments in Japan andoverseas, and also from the viewpoint of objectivity, scientific and technical validity, and technicalreliability.
The NUMO Safety Case, with readers assumed to be experts with some technical understanding ofgeological disposal, is a report of a highly specialized nature. The reports making up the Safety Case havebeen developed as a hierarchical group of documents that vary in detail from the upper-level main reportto the lower-level supporting reports, with a traceability that allows experts in various fields to select the
information and the level of detail by themselves. The documents consist of an overview report, a main
report and supporting reports. Referring to the role and basic structure of a safety case as outlined byinternational organizations, the descriptions in the reports provide the framework for a safety case that will
be updated in the future, along with the information bases. This basic form of safety case will be preparedat each stage when potential sites for geological disposal are being narrowed down and finally the site forthe geological repository is approved.
In collaboration with the Division of the Nuclear Fuel Cycle and the Environment, the AESJ establisheda review group - the Special Review Committee on the NUMO Safety Case (hereafter the Review
Committee) – consisting of 14 members. The Review Committee engaged in comprehensive discussions
from December 2018 to December 2019. These discussions related to issues of scientific and technicalreliability in the context of a generic safety case at a stage where no site has been specified, as well as thetechnical capabilities and methodologies required by NUMO for site characterization, engineering design,and safety assessment to allow appropriate steps to be taken for future site selection.
The overall findings of the Review Committee on the NUMO Safety Case are as follows:

Compiling a structured NUMO Safety CaseThe NUMO Safety Case describes the information bases underpinning the safety case framework as
the starting-point for a safety case to be updated at each stage of the disposal project; these will thenprovide the basic framework for the safety case after a site has been identified.
-i-

The Review Committee agrees to the compilation of the NUMO Safety Case in line with the structure
proposed internationally by the OECD/NEA and IAEA. On the other hand, since it may be difficult
to understand the concept of a safety case for readers other than experts in geological disposal, theReview Committee suggested explaining clearly why such a structure is desirable.



The position, objectives, and assumed readership of the NUMO Safety Case is appropriate.

Basic concept for ensuring safety

For the basic concept for ensuring safety, the approaches for selection and modeling of the geologicalenvironment, for repository design and engineering technology and for safety assessment have been
identified as key elements for the upper level of the safety case. It is important to ensure consistency
in the context in the NUMO Safety Case.

In an interdisciplinary field such as geological disposal, collaboration is essential for managing theentire spectrum of technical aspects. For the NUMO Safety Case, a basic management philosophy
has

been

establishedtoensureclose

collaboration

betweenthefieldsof

sitecharacterization/evaluation, repository design and safety assessment.


The Review Committee places a high value on ensuring that the basic concept for ensuring safety isappropriate in terms of consistency with international approaches for safety case development, as well
as being designed according to the specific situation in Japan. In particular, the NUMO Safety Case
discusses the characteristics of the geological environment of the Japanese archipelago, which islocated in a tectonically active zone, from the viewpoint of the basic concept for geological disposal
and safety functions, and, on this basis, develops the basic concept for ensuring overall safety. The
Review Committee considers it highly important that this study should provide the motivation formoving the geological disposal project one step forward, with the timely presentation by theGovernment of the “Nationwide map of scientific features for geological disposal”.

Scientific-technological basis underpinning the Safety Case
① Selection and modeling of geological environments with characteristics suitable for geologicaldisposal

Based on the results of compiling a wide range of data on the geological environment, the various
geological environments in Japan were categorized specifically from the viewpoint of geologicaldisposal. In addition, site descriptive models of the three types of host rocks to be studied weredeveloped, and the procedure for constructing these models was outlined. The Review
Committee can thus confirm that progress has been made since the H12 Report when no site wasspecified.

-ii-



Based on the latest scientific knowledge and the results of categorizing the geologicalenvironments of Japan, a basic approach for site selection and investigation/evaluation
technologies was systematically developed. An approach that allows the information on thegeological environment to be interpreted stepwise in the future and to be integrated into sitecharacterization models of the potential host rocks was shown to be sufficient for therequirements for repository design and safety assessment. The Review Committee places a highvalue on providing the technical basis for supporting site selection in the future.



The Review Committee agrees that the NUMO Safety Case addresses the need for future datacompilation for sedimentary rocks that contain carbonate / organic material, particularly PreNeogene sedimentary rocks (accretionary sedimentary rocks) for which there is currently little
information.

② Design of the disposal facility and engineering technology

Based on the site descriptive model, the methods and procedures for designing, constructing,operating and closing the repository (engineered barriers, surface facilities, undergroundfacilities) which satisfy the design factors are described concretely and in sufficient detail.



Although uncertainties are unavoidable in engineering design, the Review Committee suggests
that, in addition to the uncertainties associated with information/data on the material properties
for the geological environment characteristics and engineered barriers depicted by the site
descriptive model, uncertainties associated with the boundary conditions and the resulting design
specifications should be considered.

During repository operation, there is a high level of concern for safety of the nuclear facility.
Therefore, the NUMO Safety Case addresses safety before repository closure (operational phase)in an independent chapter (Chapter 5). On the other hand, because the considerations are largely
site-dependent, key scenarios were developed only for internal initiating events without
considering external initiating events. To ensure the completeness of the safety assessment
scenarios, the Review Committee recommends also considering externally initiated events.

③ Long-term post-closure safety assessment


The approach for long-term post-closure safety assessment is consistent with internationalguidelines and methodologies applied in other countries and is reasonable as an approach thatquantitatively demonstrates safety in a situation where no specific site has been identified. The
Review Committee considers it particularly important that a site-specific evaluation based on
detailed site-specific information should be underpinned by a generic evaluation demonstrating
the technical feasibility of geological disposal, in preparation for developing a safety assessment
technology based on a more realistic consideration of the geological environment and disposal
system characteristics in Japan.
-iii-



From the viewpoint of improving the reliability of safety assessment, there is still room for
improvement regarding the following points:

Clarification of the positioning, role, and relationships of the series of tools and analysis
methods (e.g. storyboards, FEPs, attribute analyses, etc.) used for scenario development.



Reinforcement of quality assurance for mathematical models, codes, data and analyses
related to nuclide migration processes.

Demonstration that uncertainties to be evaluated in the safety assessment have been
considered sufficiently comprehensively.

Specification of the relevance of evaluating possible perturbations of the undergroundenvironment before closure, retrievability, and abnormal events during construction andoperation.

Discussion of differences by comparing safety assessment results with previous results.

Progress since the H12 and Second TRU Reports


The Japanese archipelago is located in a tectonically active region, and various geologicalenvironments with faults and fractures are expected. Against this background, the geologicalenvironment was categorized based on the results of compiling information on a wide range of
geological data. The technology for developing typical site descriptive models that would provide
the basis for assessing the design and safety assessment of the repository was presented. The
Review Committee recognizes that these are significant developments since the H12 Report at astage where no site had been identified.

A horizontal prefabricated emplacement module (PEM) emplacement method for vitrified waste
was introduced as an alternative to the vertical bentonite block method. In addition, enhanced “Btype” packages were introduced for wastes such as TRU. Alternatives such as optimization of
overpack thickness were also presented. The Review Committee therefore agrees to an approachthat considers alternatives in parallel while leaving flexibility for future decisions.



The Review Committee finds that significant progress has been made regarding the following
aspects of long-term post-closure safety assessment.


Adoption of a risk-based approach based on a disaggregated dose/probability approach. This
includes assessment of the probability of scenario occurrence and, in developing scenarios,
adopting an integrated method that combines a bottom-up approach using FEPs and a topdown approach based on safety functions.

-iv-



Development of a safety assessment system considering co-disposal of vitrified waste andTRU waste.

Development of a nuclide migration analysis model that can consider the effects of multiple
faults and fractures in the site descriptive model and the layout of the disposal facility in amore realistic manner.



Assessment of the biosphere using the latest data and dose conversion factors.

Reliability of the Safety Case

The required quality of a basic safety case that will be updated as the site selection processprogresses was confirmed from the following viewpoints:


The NUMO Safety Case is consistent in context and structured in such a way as to properly
guide the general conclusions of the Safety Case.

The NUMO Safety Case considers the geological characteristics and social conditions ofJapan and considers all aspects that are important for a safety case as required at the present
stage.



The NUMO Safety Case is consistent with basic Japanese policy on final disposal, referringto international standards and similar experience in foreign countries.



The NUMO Safety Case uses technical terms in Japanese which are consistent withinternational definitions and can be understood by geological disposal experts.



The main conclusions of the NUMO Safety Case are presented clearly and, regarding themultifaceted reasoning that led to the conclusions, traceability and objectivity are generally
ensured based on the supporting reports.

The NUMO Safety Case makes a clear distinction between generic issues that do not dependon the site and site-specific considerations, and more site-specific research and development
to be carried out in the future.



After confirming the above, the Review Committee agrees that the NUMO Safety Caseconsiders various site environmental conditions and is reliable as a safety case at the current
stage, and consequently is valid as a basic structure for a safety case after identification of asite. In addition, and considering the gradual improvement in reliability as the safety casedevelops, the Review Committee acknowledges that the topics for research and development
up to the next stage are adequately described from the perspective of long-term R&D activities.

-v-

The Review Committee expects that their review will contribute to improving the scientific and technicalreliability of the NUMO Safety Case and will promote confidence in the reliability of NUMO in
discussions on geological disposal.

-vi-Content

1. Introduction ····································································································· 1
1.1 Background to the review ················································································ 1
1.2 The NUMO Safety Case ················································································· 1
1.3 Scope, objectives and review process ··································································· 3
1.4 Review report ······························································································ 7

2. Concepts and methods for developing the NUMO Safety Case ··········································· 8
2.1 Positioning, objectives and assumed audience for the NUMO Safety Case ··························· 8
2.2 Significance of preparing the NUMO Safety Case with an established safety case structure ······· 8
2.3 Basic concept for ensuring safety ·········································································· 9

3. Scientific-technological basis for the NUMO Safety Case ················································15
3.1 Selection and modeling of a geological environment suitable for geological disposal ············15
3.1.1

Outline of review ·················································································15

3.1.2

Basis concept for site selection and status of preparation of investigation / assessment
methods ····························································································173.1.3

Status of development of site descriptive models integrating information on the geologicalenvironment ·······················································································18

3.2 Design of the repository and engineering technology ················································23
3.2.1

Outline of review ·················································································23

3.2.2

Design workflow ··················································································24

3.2.3

Design factors and requirements ································································25

3.2.4

Concept for ensuring safety in designing surface facilities ···································25

3.2.5

Material properties for the design ·······························································26-vii-

3.2.6

TRU and other waste characteristics and their disposal concepts ····························26

3.2.7

Design of the underground facility layout ······················································27

3.2.8

Retrievability ······················································································27

3.3 Safety assessment for the operational phase ···························································29
3.3.1

Outline of review ·················································································29

3.4 Assessment of long-term post-closure safety ··························································30
3.4.1

Outline of review ·················································································30

3.4.2

Concept for safety assessment ···································································32

3.4.3

Scenarios ···························································································33

3.4.4

Models ·····························································································36

3.4.5

Data ································································································38

3.4.6

Assessment based on analysis of nuclide migration ···········································39

4. Reliability as a Safety Case ··················································································42
4.1 Outline of review ···························································································42
4.2 Reliability as a safety case at the present stage ··························································43
4.3 Validity as a basic form for a safety case after site identification ·····································45
4.4 Issues to be solved in the next stage and future research and development ··························46

5. Concluding statement ·························································································48

References ········································································································51

Appendix: Members of the Special Review Committee on the NUMO Safety Case ·························· 53-viii-

1. Introduction
1.1 Background to the review
On November 21st 2018, The Nuclear Waste Management Organization of Japan (NUMO) announced the
publication of its Safety Case entitled “Realization of safe geological disposal in Japan - development ofa safety case for the selection of an appropriate site” (hereafter the NUMO Safety Case). The Safety Casedemonstrates how to realize safe geological disposal of high-level radioactive waste and TRU waste,
reflecting the latest scientific knowledge and technical development results.
For the smooth promotion of the geological disposal project by NUMO, the Safety Case aims to show
comprehensively how the investigation of a site would proceed after acceptance by a local community ofthe start of a literature survey, how the design, construction, and operation of a safe repository would be
implemented, and how long-term safety after closure would be assured, in order to demonstrate thattechnical preparations are in place to respond to the results of the literature survey.
To achieve this goal, it is important that the safety case report is reviewed by an independent academic
third-party. Therefore, NUMO requested the Atomic Energy Society of Japan (AESJ) to review the
documentation. The objective is to check feasibility as a safety case at a stage where no site has been
specified, from the viewpoint of the latest scientific and technological developments in Japan and overseas,
and from viewpoint of objectivity, scientific and technical validity, and technical reliability. With thesupport of the Division of the Nuclear Fuel Cycle and the Environment, AESJ established a review group- the Special Review Committee on the NUMO Safety Case (hereafter the Review Committee) –consisting of 14 members. The names and affiliations of the committee members can be found in the
Appendix. The members consist of five experts in geology, four experts in engineering, and four experts
in safety assessment. The chairman is an expert familiar with research and development in the field ofgeological disposal. The Nuclear Safety Research Association (NSRA) provided the secretariat at therequest of AESJ.

1.2 The NUMO Safety Case
In Japan, the “Specified Radioactive Waste Final Disposal Act” (the “Final Disposal Act”) was enacted in
2000, based on the proposal for implementation of geological disposal presented in the report "Basicconcept for disposal of high-level radioactive waste", formulated in 1998 by the Special Committee on
High-Level Radioactive Waste Disposal, the Japan Atomic Energy Commission and also in the report,“Technical reliability of geological disposal of high-level radioactive waste in Japan - the second progress
report on research and development on geological disposal”, prepared in 1999 by the Japan Nuclear Cycle
Development Institute (hereafter the H12 Report).
Based on the Final Disposal Act, NUMO was established in 2000 as the implementation body for thegeological disposal project. The Final Disposal Act stipulates that the repository site should be selected in-1-

a three-stage process consisting of literature surveys, preliminary investigations, and detailed
investigations. From 2002, NUMO started an open solicitation procedure for municipalities nationwide
with a view to investigating the feasibility of sites for construction of geological disposal facilities, but nolocal government has applied to date. A literature survey, the first step in site selection, has not yet been
initiated.
For this reason, in 2013 discussions were carried out with a view to restructuring geological disposal policyin order to revise the direction that should be followed for promoting the geological disposal project. In
May 2015, the “Basic Policy on Final Disposal of Specified Radioactive Waste” was approved by the
Cabinet. In the revised basic policy, the following matters were clarified:・Not passing on the burden of waste disposal to future generations.
・Ensuring reversibility of the project and retrievability of the waste.
・Periodically evaluating the technical reliability of final disposal with collaboration of the national
government, NUMO, and related research institutes.
・The national government should request local governments to cooperate in the literature survey, while
indicating areas that are considered scientifically relevant (scientifically favorable areas).
In 2017, to increase public interest and understanding of geological disposal, the government presented a“Nationwide map of scientific features for geological disposal” based on nationwide data. This map aims
to provide an overview of the characteristics that need to be considered when selecting a site for geologicaldisposal and how these features are distributed throughout Japan.
In promoting the activities for selecting sites for geological disposal based on the “Nationwide map ofscientific features for geological disposal”, it is important for NUMO to show the public how safe
geological disposal can be realized and to gain the confidence of the public in geological disposal projects,
based on the latest scientific and technological knowledge, in addition to knowledge accumulated to date.
Accordingly, NUMO, as the implementing organization for geological disposal, has compiled the NUMO
Safety Case aimed at showing the latest scientific knowledge and technological developments andexplaining methods for realizing safe geological disposal for the specific geological environment of Japan.
Referring to the role and basic structure of safety cases presented by international organizations, theframework of the NUMO Safety Case will be described; this will be updated in the future when locations
for geological disposal have been narrowed down and, finally, the site for the repository is approved. The
objective of the NUMO Safety Case is to present a generic safety case for the Japanese situation (norepository site identified), based on current knowledge, addressing the following matters:-2-

① The technical basis exists for the investigations and assessments aimed at selecting suitablegeological environments for geological disposal. Also, site descriptive models which reflect thecharacteristics of the deep geological environment of Japan are presented.
② The technical basis exists for designing a repository which satisfies the necessary requirements(safety, engineering feasibility, retrievability, environmental conservation, economic feasibility,etc.). Engineering technology also exists for constructing, operating and closing the repository.
③ The technical basis exists for evaluating operational safety prior to closure of the repository as well
as long-term safety after closure. Moreover, the repository safety assessment should confirm that
there is no significant radiological impact on humans.
④ Through a series of studies applying the latest technologies in the areas of investigating andevaluating the geological environment, repository design and safety assessment, technical measures
are identified for further improving the reliability and future action plans for these areas.
The NUMO Safety Case, which assumes a readership of experts with some technical knowledge about
geological disposal, is a highly specialized technical report. It has been developed as a hierarchical groupof documents with the level of detail increasing from the main report/overview report to the supporting
reports, with traceability that allows experts in various fields to select the information and the level ofdetail by themselves. The overview report summarizes the key points of the main report. The main reportis the central document of the NUMO Safety Case and gives an overview of how safe geological disposal
can be realized. The supporting reports provide the detailed rationale as well as the analysis methods andresults supporting the content of the main report.
It should be noted that "review version" is included in the title of the released NUMO Safety Case reportbecause it is intended for review in terms of technical reliability by domestic and independent overseasorganizations. NUMO will then revise the report as required depending on the review comments.

1.3 Scope, objectives and review process
The scope of the review covered only the “main report” of the NUMO Safety Case. It should be noted,
however, that the reviewers also referred to the supporting reports where necessary. It should also be notedthat not only the supporting reports published at the time of the review but also supporting reports to be
published later should be reviewed.
The objective of this review is to check the technical reliability of the NUMO Safety Case for a stage inthe program when no site has been specified and to determine whether it reflects the latest technicalknowledge in Japan and overseas focusing on the safety case concept presented by the OECD/NEA andthe IAEA, which will support the basis for site selection for geological disposal in Japan.
The review process is as follows: Each member of the Review Committee reads through the NUMO Safety
Case and expresses their views from the following provisional viewpoints, considering the characteristics
and the situation of Japan and referring to the TOR (Terms Of Reference) [1, 2, 3] for reviews of safety-3-cases in other countries and guidelines for international peer reviews [4]. Based on this, the issues of theCommittee members are compiled and reviewed to formulate the overall Committee review. The
viewpoints for review are as follows:

Viewpoint 1: Validity of the entire Safety CaseIs the structure and logic of the “basic concept for ensuring safety (Chapter 2)” applied to achievesafe disposal in the NUMO Safety Case valid as a higher-level approach? Based on this, adetailed discussion is developed on the scientific and technological bases underpinning the
Safety Case in separate chapters (Chapter 3 to Chapter 6); these are summarized in Chapter 7 asgeneral conclusions and future tasks. Based on this, can the reliability of the present NUMO
Safety Case be confirmed?



Are there any omissions in the NUMO Safety Case, considering the items that are required for ageneric safety case?

Is the intention behind preparing the NUMO Safety Case consistent with the national basic
policy on final disposal?



Are the multifaceted arguments leading to the main conclusions of the NUMO Safety Case
adequately presented in the main report? Are they sufficiently traceable, transparent, andobjective?



Are the remaining major uncertainties clearly stated, as well as how they will be handled infuture research and development?



Is consistency maintained with international standards and good practices of other countries?

Is the technical terminology consistent with international definitions and are the Japanesedescriptions such that their meaning can be correctly understood?



Are the progress and differences from the H12 Report and the Second TRU Report evaluatedand clearly stated for the stage where no site has been identified?

Viewpoint 2: Validity of the scientific-technological basis underpinning the safety case
(Selection of a suitable geological environment)

Are the selection and modeling of suitable geological environments conducted according to theapproach defined in the basic concept for ensuring safety?

-4-



Have the site selection procedure and policy been identified? Has the applicability of thegeological environment investigation and evaluation technologies been confirmed? Are theytechnically feasible?

Is the selection and modeling of the geological environment applicable to a range of geologicalenvironments in Japan?

Does the developed site descriptive model provide the information necessary for the design andsafety assessment of the repository?

(Repository design and engineering technology)

Has the repository design and engineering technology been formulated according to the approachdefined in the basic concept for ensuring safety?

Is the designed disposal system technically feasible considering the site descriptive model andthe characteristics of the waste?



Are the methods for design, construction, operation and closure of the repository technically
proven or an extension of proven technology, and are they technically feasible?

Has waste retrieval technology been properly examined and its feasibility demonstrated? Have
research and development activities for improving feasibility been properly presented?

(Safety assessment)Is the safety assessment conducted according to the approach defined in the basic concept for
ensuring safety?

Is the basis for setting the post-closure safety assessment framework (scenario classification,
evaluation period, human intrusion, etc.) consistent with international standards?



Are the post-closure safety assessment models developed appropriately according to the site
descriptive model?



Are the safety functions of the individual barriers of the disposal system described appropriately
considering the timescales involved? Are the priorities expected for each barrier considered?



Are the scenarios and analysis cases assumed in the safety assessment sufficiently
comprehensive?



Are the safety assessment methodology, models and codes appropriate? Is there a scientific basis
to support them?



Are the features, events and processes (FEPs) affecting the evolution of the disposal system
clearly documented? Is this sufficiently comprehensive compared to the international FEP
database?
-5-



Does the operational safety assessment consider the safety regulations applied to other nuclear
facilities and similar underground facilities (mines, tunnels, etc.)?

Is the influence of retrievability on operational and post-closure safety being assessed?

The review process was as follows:① The 1st Review Committee meeting was held on December 27, 2018 to confirm how to proceed withthe review. Following an explanation by NUMO of the structure of the NUMO Safety Case, theReview Committee discussed priority issues and the main aspects of the review and assigned a review
scope to each member.
② Each member prepared comments and questions for each chapter of the NUMO Safety Case. The
comments amounted to about 700 sheets in all, and the secretariat compiled these comments andquestions as a table and classified the comments for each chapter of the NUMO Safety Case.
③ The responses of NUMO to the members’ questions were fed back to the Review Committee. The
Review Committee asked NUMO for supplementary explanations if needed. In the review, related
attached documents and references were referred to as appropriate. It should be noted that, since the
NUMO Safety Case is a highly technical document, aspects of social acceptability, such as whether
the report is acceptable to municipalities or how the content should be communicated to the generalpublic, were excluded from the review.
④ For compiling the consolidated opinion of the Review Committee, issues were extracted from thecomments presented by each member, supplementary explanations of NUMO, and discussions in the
Review Committee, and a draft overview of the aggregated opinions was prepared for each issue.
These approaches and draft overviews were also presented and discussed in the Review Committee
as appropriate. NUMO was asked to participate in the discussion only to correct misunderstandingsof the Committee, in a way that did not compromise the neutrality of the review.
⑤ Regarding Chapters 3 to 7 of the NUMO Safety Case with highly technical content, a working group
was established for each area, separately from the Review Committee, to examine the technicalmaterial in detail. In addition, because there are relationships between each technical field, task
meetings were held by each working group chairman as appropriate in order to exchange opinions.
⑥ The review results were synthesized according to the collective opinions approved by the Review
Committee. The Committee confirmed such synthesis with all members at a review meeting or by email.
⑦ The Review Committee submitted a report on the interim status at the General Session of the AESJ
2019 Fall Meeting as “Current Status of Peer Review of the NUMO Safety Case”-6-⑧ The review results were compiled in the form of the “Report of the Special Review Committee on
the NUMO Safety Case” (hereafter the review report). At the same time, an English version was
prepared.
⑨ The secretariat was responsible for checking the traceability of cited references and typographicalerrors in the NUMO Safety Case.

1.4 Review report
Chapter 2 of this review report presents the Review Committee's collective opinions on the descriptionsin Chapters 1 and 2 of the NUMO Safety Case. In Chapter 3, summaries are presented of the more detailed
review results on the scientific and technical basis for each technical area described in Chapters 3 to 6 of
the Safety Case. In Chapter 4, the evaluation results are presented for Chapter 7 “Reliability of the Safety
Case” and, finally, concluding statement is provided in Chapter 5.
The review report summarizes the results of the Review Committee and will help NUMO to assess its
achievements and make revisions to the NUMO Safety Case draft version in order to fulfill the
responsibilities assigned to it by the government. Any factual misconception in the content of the review
report is the responsibility of the Review Committee.

-7-2. Concepts and methods for developing the NUMO Safety Case
2.1 Positioning, objectives and assumed audience for the NUMO Safety Case
According to the history and background of the geological disposal program in Japan, NUMO presented
the objectives and significance of its Safety Case in terms of how it aims to achieve safe geological disposal,
with recognition of NUMO being an organization trusted by society and the importance of dialogue withlocal communities and the public.
The readers are assumed to be experts with some technical knowledge of geological disposal. The NUMO
Safety Case is a highly specialized report and was developed as hierarchical group of documents ranging
in detail from the upper-level main report to the lower-level supporting reports, with traceability that allows
experts in various fields to select the information and the level of detail by themselves. The documents
consist of an overview report, a main report and supporting reports. For more general readers, a separate
document will be prepared to explain the Safety Case transparently using simple expressions and as few
technical terms as possible.
NUMO defined the objectives and significance of its Safety Case according to the background of thenational geological disposal program; NUMO is an organization that should be trusted by a society as animplementing organization, and should realize the importance of dialogue with local communities and the
public. The Review Committee places a high value on this recognition. It should be noted that, regarding
a safety case to be used as material for dialogue with stakeholders, which was one of the motivations forcompiling the NUMO Safety Case, the Review Committee recommends that this point should be stated as
an objective of the Safety Case because this is a step further than previous technical reports. The Review
Committee also suggests noting that the NUMO Safety Case aims at promoting trust in order to allow the
project to move forward to the next stage.
The Review Committee also places a high value on the structure, including overall content and supporting
reports containing technical details, being appropriate for the NUMO Safety Case. The Review Committee
also agrees that the division of the Safety Case into a hierarchical group of documents according to theassumed readership is appropriate. The report was written on the premise that the content of H12 Report
was known to potential readers, but the Review Committee recommended that the basis for the technicalfeasibility of geological disposal and the concepts for the disposal system in the H12 Report should be
briefly explained again at the beginning of the NUMO Safety Case.

2.2 Significance of preparing the NUMO Safety Case with an established safety case structure
The NUMO Safety Case states that information is presented in a safety case framework that can act as atemplate for future updates after sites are identified at future milestones of the stepwise implementation of
the disposal project. The Review Committee agrees that the NUMO Safety Case follows the establishedform of safety case as shown by OECD/NEA and the IAEA and followed internationally. On the other-8-

hand, for readers other than experts in geological disposal, it seems that the understanding of the safetycase concept is not necessarily sufficient. The Review Committee suggests explaining in more detail why
the form of the safety case is appropriate.

2.3 Basic concept for ensuring safety
As requirements to be considered in geological disposal in Japan, NUMO states that “the waste”, “thesafety functions of the repository”, and “the key elements for implementing the disposal project as
stipulated in the national plan (i.e. stepwise site selection, reversibility and retrievability, and prospects forsafety regulations)” should be defined. After defining these requirements, NUMO established upper-levelapproaches to ensuring safety at each stage of site selection, repository design, safety assessment, andmanagement of geological disposal; this is termed the “Basic concept for ensuring safety”.
In the OECD/NEA guidelines on the safety case, an upper-level approach for ensuring safety is defined asa “safety strategy”. NUMO replaced this term in its Safety Case with an easy-to-understand and familiar
term: “Basic concept for ensuring safety”. The Review Committee agrees that NUMO should use the term
“Basic concept for ensuring safety”, taking into consideration Japanese impressions of the word “strategy”.
NUMO presented the basic concept for site selection, repository design and safety assessment as well as
management, assuming close cooperation between the different technical fields. In addition, NUMO
outlined specific methods for managing an integrated safety case to be updated at each stage.
In addition to the basic concepts for each technical field, NUMO described not only “long-term safetyfunctions after closure” but also basic concepts for ensuring comprehensive safety as requirements to beconsidered when implementing geological disposal projects. These were “consideration of uncertainty ofstate setting during system evolution”, “basic concept of NUMO for reversibility and retrievability”,“efforts for environmental impact assessment and environmental conservation”, “necessity for geologicaldisposal-specific monitoring and clarification of the monitoring plan”, “study on the relationship between
ensuring operational phase safety and site selection”, and “consideration of methods for effective use of
stable underground areas and design options”.
The Review Committee considers it highly important that such a basic concept for ensuring safety is
consistent with international safety case development methods and is planned appropriately according to
the situation in Japan. The Committee also places a high value on the characteristics of the geologicalenvironment of the Japanese archipelago, located in a tectonically active region, being discussed from theviewpoints of the basic concept for geological disposal and the safety functions, and the basic concept for
ensuring overall safety was prepared based on these discussions. The Review Committee also considers it
important that these discussions should move the geological disposal project one step forward, andrecognizes that this provided the motivation for the timely presentation of the “Nationwide map ofscientific features for geological disposal” by the government.
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It is necessary to set a spatial scale and a timescale when considering the long-term safety functions
required for the repository. NUMO outlined a policy in which it considers these scales, including
uncertainties in prediction; the site descriptive model, repository design, and safety assessment will be
updated to revise the Safety Case in response to the progress of the stepwise site selection process andevaluate the suitability of sites as part of site selection. The Review Committee acknowledges that thisapproach is extremely important in the development of a safety case.
The H12 Report generally discussed the technical feasibility and reliability of geological disposal in Japan.
On the other hand, the NUMO Safety Case is based on the policy of implementing the repository designand safety assessment after developing a site descriptive model based on the classification of rock types
which generally describe the site conditions assumed to be selected in Japan; this is a significant
development since the H12 Report. The Review Committee places a high value on this point andrecognizes that it should be emphasized in the NUMO Safety Case. The advantage of preparing such a site
descriptive model is that, when a candidate site is identified, the repository design and safety assessment
can be tailored to a model that matches the characteristics of the host rock at that location, providing astarting-point for developing a site-specific model more efficiently.
After the accident at TEPCO's Fukushima Daiichi Nuclear Power Station, new standards were establishedin safety regulations, which were also implemented for vitrified waste storage facilities. Considering thatthe geological disposal facility is similar to a vitrified waste storage facility during the operational period
before repository closure, NUMO decided to present a concept and method for assessment of theradiological safety of the surrounding public and workers during transportation and operational processes.
Furthermore, NUMO clearly stated that not only radiological consequences but also general occupational
accidents caused by natural disasters and by operations in disposal facilities were to be considered.
The safety cases in other countries that have been published so far mainly discuss post-closure safety.
However, NUMO addressed operational safety as an independent chapter in its Safety Case, considering
the strong concerns about safety during the operational phase of a nuclear facility. The Review Committee
agrees with this policy.

Dealing with uncertainties related to the post-closure safety of a repository


The Review Committee attaches high importance to the basic idea that uncertainties can be reduced
by stepwise incremental data expansion. However, considering the importance of uncertainties, adescription of how to deal with uncertainties should not be included in the “Basic Management
Concept” section of the NUMO Safety Case. The Review Committee suggests that the structure
should be reviewed and the description of uncertainties should be contained in an independent section
on “Addressing uncertainties related to safety”.

-10-



The Review Committee suggests showing more clearly that uncertainties can be reduced by assessing
performance repeatedly using detailed data obtained prior to repository closure and that theuncertainties in the site descriptive model will be reduced by the gradual progress of site selection.



The uncertainties associated with models and parameters and the uncertainties in setting scenarios
have different implications. The Review Committee suggests explaining the differences in these
implications and also specifying the concept for selecting and setting scenarios relating to unlikely
events and human intrusion.

Considering that uncertainties in the fields of site investigation, repository design and safetyassessment are propagated to other fields, establishing how to deal with these mutual influences isimportant as an interdisciplinary approach to dealing with uncertainty. The Review Committee
suggests adding an additional explanation on this point.

Implications of stylization in safety assessment
In the evaluation of the biosphere in the long-term post-closure safety assessment, information on
future human life forms cannot be based on scientific evidence. Therefore, the NUMO Safety Case
calculates the exposure dose based on the assumption that future humans will live in the same manneras today and evaluates the safety performance of the repository using this as an indicator. The Review
Committee suggests that this concept should be described further in the report.

Safety functions and timescales

The NUMO Safety Case describes safety functions over a timescale exceeding the period after
repository closure when the containment functions are expected to gradually decrease due to physicaland chemical alteration of the engineered barriers. However, if favorable geological environment
characteristics were maintained, the changes to the repository system would be very slow and it isassumed that there would be no significant loss of the nuclide migration retardation function. At afirst glance, this could be read as a contradictory statement that the function is not lost even though
the performance has degraded. Therefore, the Review Committee recognizes that this needs careful
explanation and suggests adding the following explanation: “Although the containment function is
expected to decrease, this would be local and, even if some safety functions decline, other safetyfunctions work complementarily to ensure the overall containment performance (multiple safetyfunctions)”.



At appropriately selected sites, from current understanding of the drivers of natural perturbation
phenomena, NUMO justifies the isolation and containment functions of the geological environment
based on the premise that the trends of natural phenomena such as volcanic activity and fault activity
would likely continue for about 100,000 years in the future.
-11-



Regarding the period of expected predictability of crustal deformation, the Review Committee
suggests clarifying the meaning of continuity of safety functions, reviewing key papers in more detailto justify that these continue over the order of 100,000 years, thus making the basis for the period of
extrapolation of crustal deformation more certain.

Argument on the stability of the geological environment in a geological disposal facility
Considering that there is a strong interest in the fact that the Japanese archipelago is located in atectonically active region, the stability of the geological environment should be described accurately
to ensure that discussions of general trends related to crustal deformation and discussions of thegeological environment features that are important for the feasibility and long-term safety of arepository are not confusing. The meaning of the geological environment that is relevant in thediscussion of geological disposal is defined in the glossary as follows: “The geological environment,
i.e. the underground environment from the viewpoint of geological disposal, is composed of the rockmass and the groundwater contained therein. Geological environment characteristics is a general term,
meaning the following characteristics: geology and geological structure, thermal and mechanicalproperties of the rock mass, geochemical properties of groundwater, properties of groundwater flow
and mass transfer.” The Review Committee suggests that the geological environment characteristics
should be described not only in the glossary but also separately in the NUMO Safety Case report andexplained in the text.

Approach to selection and modeling of the geological environment

NUMO’s basic concept for site selection is as follows: in response to the stepwise site investigation
process proceeding from literature surveys to detailed investigations, the site descriptive model isrefined from a regional scale to a repository scale, and then to a panel scale; repository design andsafety assessment are repeated using information on the characteristics of the geological environmentthat will become more detailed with the progress of investigations. Finally, a site will be selected
where a repository with the required safety functions can be designed and constructed. The Review
Committee places a high value on NUMO’s site selection process being an appropriate and effective
approach for site selection in Japan. In particular, the stepwise site investigations reduce theuncertainty associated with site descriptive models and increase the reliability of design and safetyassessment. This also contributes to building trust in NUMO. The Review Committee recognizes thatthese are important viewpoints to be described.



In 2017, the Geological Disposal Technology Working Group of the national government compiled
a document with the title “Summary of Requirement and Criteria for Nationwide Map of ScientificFeatures for Geological Disposal”. In this document, eight unfavorable conditions were presented. It
is difficult to understand how these are handled as preconditions for the construction of a site
-12-

descriptive model in the NUMO Safety Case. The Review Committee suggests that NUMO should
explain carefully how to assess boundary conditions for setting the constraints on model
representation of events / features and their distribution, together with the process for selecting thegeological environment information to be used in the model, to assure avoidance of such unfavorable
conditions.

Approaches to repository design and engineering technologyThe basic concept for repository design is outlined as follows: the requirements for the repository aredefined in the form of design factors and safety functions; then, a process for obtaining design
examples (“specifications” in the NUMO Safety Case) that satisfies these is shown. Thus, thedesigned component is assessed in terms of “required performance” within the engineering design
process. The Review Committee recognizes that this is consistent with a performance-based design
approach as defined in the “Agreement on Technical Barriers to Trade (TBT Agreement) of the World
Trade Organization (WTO) [5]. The Review Committee places a high value on presenting a design
framework which meets international standards.



The NUMO Safety Case shows that the repository design is performed to current specifications, andthis is described more concretely in Chapter 4 as “Design performed based on design requirements
and basic specifications (materials, shape, dimensions, etc.)”. However, the “specifications” and the“scope that establishes the specifications” in the NUMO Safety Case might be different from thoseused in general structural design. The Review Committee suggests that the definition of each term
should be clarified. Also, it should be clarified that the design of the repository is performance-based
rather than specification-based, and NUMO should show an example of designs that would satisfy
the design factors.



The Review Committee suggests that the method for monitoring during the period while retrievability
is assured should also be reviewed with the possibility of reducing uncertainties.

Approach to safety assessment
The NUMO Safety Case refers to the risk-based approach as an approach to long-term safetyassessment after closure; this assumes that it is considered reasonable to evaluate the safety of therepository by the magnitude of risk (= probability × magnitude of impact). However, as this can beread as if a risk assessment were performed, it needs to be stressed that only in the scenario
categorization is the probability of occurrence considered. A dose/probability disaggregated approach
is applied in the NUMO Safety Case, which assesses the impact for specific scenarios when theprobability of occurrence is 1. The Review Committee suggests describing that there are two methods-13-

for risk-based approaches: “integrated approaches” and “dose/probability disaggregated approaches”,
and that the latter is used in the NUMO Safety Case.

Basic concept of management


The Review Committee considers it important that, in addition to dealing with uncertainty relating tosafety, NUMO should address the attitude of management towards reducing management risks
caused by uncertainties regarding policy, socio-environmental change, etc.



NUMO refers to the basic concept of management for close collaboration between the areas of sitecharacterization and evaluation, repository design and safety assessment. In projects that consist ofinterdisciplinary areas such as geological disposal, collaboration in managing the entire project isextremely important. Therefore, the Review Committee places a high value on NUMO establishinga basic management concept. It should be noted that the Review Committee suggests adding textual
explanations to the figure showing the interrelationships between each field in the report in order tounderline specific actions and ensure their effectiveness.

The validity of the process of developing a safety case by integrating information obtained from theinvestigation and evaluation of the geological environment and from repository design into the safetyassessment is extremely important from the viewpoint of demonstrating the reliability of the safetycase. Therefore, the Review Committee suggests adding a description of the information integration
methodology.



NUMO operates various management systems. These are specific management tools for ensuring
collaboration and integration between different fields. If these systems are positioned as a“management basis”, it will be easier to understand the effectiveness of such cooperative management.
In the NUMO Safety Case, the figure explaining the linkages between each field and the figure
explaining the relationship between the Safety Case and each chapter quoted in the summary section
are shown. The Review Committee suggests adding an explanation of the figures in the management
section.

-14-

3. Scientific-technological basis for the NUMO Safety Case
3.1 Selection and modeling of a geological environment suitable for geological disposal
3.1.1 Outline of review
In addition to presenting investigation and evaluation technologies for geological environment
characteristics and the basis for design and safety assessment, Chapter 3 of the NUMO Safety Casedescribes approaches for developing a site descriptive model, and the current level according to the“Basic concept for ensuring safety of geological disposal” described in Chapter 2. The two specificgoals are

Demonstrating that the basic concepts for investigation/evaluation technologies for site selection
are developed systematically by showing the latest scientific knowledge and technologicaldevelopments related to Japan's geological environment.



Demonstrating that NUMO possesses the technology for interpreting and integrating the geologicalenvironment information acquired in the different stages into a site descriptive model of the hostrock to be studied, which will provide the basis for the design and safety assessment of therepository.

The Review Committee considered in detail the technical content that has been compiled in line with these
two goals, from the specific viewpoint of “validity of the scientific-technological basis underpinning the
Safety Case” shown in Chapter 1 of this report. The specific viewpoints are as follows:・ Are the requirements and conditions specified for the geological environment as being suitable forgeological disposal specified sufficiently and appropriately according to the basic concept for
ensuring safety and the preconditions at the present stage shown in Chapter 2 of the NUMO Safety
Case?
・ Can all processes from data acquisition to analysis/evaluation and development of a site descriptivemodel be traced in response to the requirements and conditions specified for the geologicalenvironment?
・ Is the site descriptive model fully functional as a basis for repository design or safety assessment in
the light of the objectives of the NUMO Safety Case?
・ Have the developed technologies reached a necessary and sufficient level based on the latest scientificand technical knowledge considering the objectives of the NUMO Safety Case? Can it be said thatthe level is sufficient considering domestic and foreign technological trends? Does the report mention
continuous improvement and introduction of technologies that are considered necessary in the futurewith reference to overseas precedents?

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・ Has knowledge about the geological environment characteristics of Japan located in a tectonically
active region been updated appropriately and utilized based on the latest research results since the
H12 Report.
・ Is information collected and analyzed from a broad viewpoint and results derived in an unbiased
manner?
In the working group, the opinions submitted by each committee member were compiled, opinions werediscussed, and issues considered to be particularly important for the technical review were extracted. The
Review Committee's opinions were then consolidated and agreed to as the review results. The overallfindings of the review results are as follows.


Based on the results of collecting and compiling a wide range of geological environment data, thevarious geological environments in Japan were categorized specifically from the viewpoint ofgeological disposal. Typical site descriptive models were developed for the three types of hostrock. In addition, the construction process for the site descriptive model was specifically shown.
The Review Committee places a high value on this steady progress since the H12 Report.



The Review Committee acknowledges that the two individual goals shown in Chapter 3 of the
NUMO Safety Case - to show that the basic concepts for investigation/evaluation technologies forsite selection are systematically developed based on the latest scientific knowledge andtechnological developments related to Japan's geological environment and to show that NUMO
possesses the technology for interpreting and integrating the geological environment information
into the site descriptive model of the host rock to be studied - have reached a sufficient level inview of the above review points. The Review Committee sets a high value on this as a technicalbasis in future site selection.



The NUMO Safety Case covers a wide range of information related to Japan's natural phenomena,
geological environment characteristics, geological investigation and evaluation technologies, etc.
The Review Committee recognizes that there are many points where further improvement isdesirable from the viewpoint of the logic, objectivity and understandability of the described
content.

For the above general findings, the following sections are described in detail from the viewpoints outlined
below, set considering the two goals presented in Chapter 3 of NUMO Safety Case.


Basic concept for judgments required in site selection and the status of preparation of
investigation/evaluation technologies
Integration of information on the geological environment for the development of a site
descriptive modelProbabilistic assessment technique for assessment of natural phenomena and associatedevolution of the geological environment
-16-



Development status of the site descriptive model by integrating information on the geologicalenvironmentLong-term evolution and timescales of geological environment characteristics

Response to uncertainties relating to geological environment characteristics

Radionuclide leaching/retardation functions expected in the geological environmentHow to treat the Darcy flow velocity in the hydrogeological model of fault and groundwaterflow analysis

Influence of high carbonate concentration groundwater

3.1.2 Basic concept for site selection and status of preparation of investigation/assessment methods
NUMO plans to select a site appropriate for constructing a repository by comparing the results ofgeological environment investigations and evaluations with statutory requirements and considerations
stipulated by NUMO, and has prepared a workflow (flowchart) for literature surveys and preliminary
investigations, as well as a list of investigation and evaluation technologies used at each stage.
The Review Committee places high importance on these technical systems, developed based on existing
documents and covering a variety of geological environment conditions that are expected in Japan, being
compiled as a practical system with a sufficient basis for making judgments in promoting the literature
surveys and preliminary investigations. The Review Committee acknowledges that the technologies
applied for investigation/evaluation are appropriate based on currently available technologies. However,
the Committee suggests clarifying to what stage of the site selection these technologies should be applied
for the indicated issues, as well as their scope, by reviewing the latest research results.

Integration of geological environment information for the development of a site descriptive model
NUMO will integrate geological environment information into a site descriptive model that expresses thespatial distribution of the geological environment characteristics in order to put the information obtained
from the geological investigations into a form that can be used for the design and safety assessment of therepository. Repeating the loop of investigations and evaluations, repository design and safety assessment,
NUMO then checks whether the potential site has sufficient safety functions.
The Review Committee places a high value on the process of integrating geological environment
information into the site descriptive model as a very important effort to ensure the reliability of the safetycase, particularly the traceability of the information. However, there is no specific description of what kindof feedback was obtained on the site descriptive models from the areas of repository design and safetyassessment. The Review Committee suggests that additional explanations will be necessary.
-17-

Probabilistic evaluation technique for assessment of natural phenomena and associated evolution of
the geological environment
NUMO has developed a method called ITM (International Tectonics Meeting)-TOPAZ (Tectonics ofPotential Assessment Zone) for long-term safety assessment for a period exceeding about 100,000 years
in the future. Using this method, NUMO addressed a probabilistic evaluation of the possibility of natural
phenomena such as volcanic/igneous activity, earthquake/fault activity, etc. that could significantly affectthe safety functions of the repository.
The Review Committee places a high value on the ITM-TOPAZ method as a new attempt to objectively
clarify long-term uncertainties relating to natural phenomena that will be important in the tectonically
active geological environment of Japan. This method is an advanced approach that makes full use of thelatest geoscientific knowledge and mathematical methods obtained through the cooperation of foreign
experts. It also serves as a platform that unifies information on natural phenomena and expert opinions in
an integrated manner, taking time and space into account. The Review Committee suggests that domestic
experts familiar with the geological environment of Japan should participate in future efforts. The
Committee also suggests specifically addressing the policy and method for collaboration with persons witha modeling background for long-term evolution of the geological environment characteristics and safetyassessment of geological disposal systems in Chapter 6 of the NUMO Safety Case. In addition to volcanic
and igneous activity that has been reported so far, the Review Committee hopes that more examples of theapplication of this method will be accumulated for earthquake/fault activity and uplift/erosion.
It should be noted that, in the NUMO Safety Case, the title is “Probabilistic evaluation method forevolution of the state of the geological environment due to the occurrence of natural phenomena”.
However, the Review Committee thinks this title needs to be corrected because, from the original title, themethod may be misunderstood to apply to evolution in the state of the geological environment rather than
to the occurrence of natural phenomena.

3.1.3 Status of development of site descriptive models integrating information on the geologicalenvironment
NUMO has been developing a methodology for comprehensively evaluating the suitability of a sitethrough the design of the repository and the safety assessment mainly by developing a site descriptivemodel with various geological features such as faults and fractures in order to investigate, evaluate, andselect sites with the geological conditions necessary for isolating and confining radioactive waste on thelong term.
According to this methodology, the geological environment is investigated in each stage through literature
surveys, preliminary investigations and detailed investigations. The geological environment data areinterpreted/integrated into a site characterization model describing the geological environment (geologicalstructure model, hydrogeological structure model, hydraulic model, rock mechanics model) so that the-18-

geological structures that define the geological environment and the thermal, hydraulic, mechanical, andchemical conditions are consistent with each other. The siting environment information obtained fromthese models is used to design the repository while evaluating the engineering feasibility and the safetyassessment is then performed based on the geological information on the siting environment and theprovisional design. Repeating this series of studies, the level of detail of investigations and of evaluation
and reliability will be improved by reducing uncertainties.
In order to show that such a site descriptive model can be specifically developed for various geologicalenvironments in Japan, NUMO selected the examples of plutonic rocks, Neogene sedimentary rocks and
Pre-Neogene sedimentary rocks as representative geologies that should be considered in the repository site
selection. The potential host rocks were set for these geologies. NUMO developed a geological model anda hydrogeological model considering geological information observed for each rock type, such as thescales of rock bodies and strata, the dominant orientation, length and distribution density of faults andfractures in the rock. In addition, it considered parameters such as the hydraulic conductivity of the matrix
and the faults and fractures in the bedrock. The level of detail of the models was set according tohierarchical spatial scales (i.e. regional scale, repository scale, panel scale).
Considering that the geological environment of Japan was summarized in the H12 Report, the Review
Committee places a high value on the three types of potential rocks that were selected as representative in
the NUMO Safety Case and appropriate consideration being given to the geological characteristics ofJapan. The Review Committee believes that, for these three types of host rocks, a series of processes should
be integrated as a site descriptive model that will serve as the basis for repository design and safetyassessment, and the investigation and evaluation technologies used in each stage of site selection should
be systematically compiled as universally as possible.
The NUMO Safety Case compiles and analyzes a wide range of relevant scientific knowledge, establishes
the standards for quality assurance and categorizes the potential rock types into plutonic rocks, Neogene
sedimentary rocks, and Pre-Neogene sedimentary rocks under current conditions when no site has been
identified. Compared with the H12 Report, this is considered to have advanced to a more realistic approachbased on the geological conditions of Japan. In particular, the site descriptive model of Pre-Neogene
sedimentary rock, which is estimated to occupy nearly half of Japan's surface, had been established in theabsence of a pre-existing synthesis. This was an important result for examining the practicality of thetechnologies to be applied to the complex geological structures caused by thrust faulting in accretionary
prisms and fold structures in accretionary complexes. On the other hand, no actual field survey wasdescribed to test modeling methods based on the regularity of fault and fracture distribution. The Review
Committee suggests considering other cases, such as Sweden and Finland, that take a similar approach inorder to explain the validity of the method more clearly.
The Review Committee considers it highly important that the system presented in the NUMO Safety Caseshould be an effective approach for selecting a suitable site for geological disposal, even under conditions
where the rock physical properties assumed in Japan are heterogeneous and the spatial geological data are-19-

limited. In addition, many of the technologies applied to modeling are consistent with those used in foreigncountries such as Finland and Sweden and the Review Committee places a high value on the technicalsystem being the state-of-the-art.

Long-term evolution and the timescales of the geological environment characteristics
In Japan, the location of the repository is not clear. Therefore, NUMO selected geological formations androck bodies as “potential host rocks for consideration” in which safety functions are expected to be
maintained for a long period of time in order to construct a site descriptive model. The potential host rocks
under consideration were evaluated to show that long-term evolution of the geological environment would
not impair the durability of the safety functions of the repository.
The Review Committee places a high value on current knowledge being compiled and the direction of
future efforts shown regarding the long-term evolution of the characteristics of the geological environment.
In addition, to evaluate the geological environment of Japan more realistically, appropriate issues havebeen identified for the next stage. However, the situation that “long-term evolution of topography andgeological structure including uplift and subsidence, climate and sea-level changes were not specified” inthe description of the “approach” in Chapter 2 of NUMO Safety Case was treated as obvious in Chapter 3of the report. This may be misleading to readers who refer only to Chapter 3. The Review Committee
suggests that NUMO should clarify the precondition again, including the rationale.
NUMO uses the terms “geological timescale”, “timescale”, and “time change” in the description of thetemporal characteristics of the geological environment and the properties of long-term evolution. However,
it is not clearly explained what periods these “times” indicate. Improvements are needed to allow a better
understanding of the readers. The Review Committee suggests that the time range should be as specific as
possible, considering its relevance to the timescale used in the H12 Report.

Handling uncertainty on the geological environment characteristics
For reducing uncertainty using an iterative approach, NUMO confirmed that the effectiveness of this waschecked through activities from geological investigation to development of a site descriptive model as partof the research in the deep underground research facilities in Mizunami and Horonobe by the Japan Atomic
Energy Agency (JAEA). In addition, in future efforts related to long-term evolution of the geologicalenvironment, NUMO plans to develop methodologies such as systematization that allow optimization ofmodeling and analysis technologies according to temporal and spatial scales and features.
In this way, NUMO explained the concept of uncertainties being caused by the restricted amount of
information at each spatial scale shown in the NUMO Safety Case and how to handle this based on
examples from Mizunami and Horonobe. NUMO also mentioned the need for such research as part of thesystematization of investigation technologies in future efforts. The Review Committee agrees with these-20-considerations. In the NUMO Safety Case, however, there are no considerations of important issues in theassessment of uncertainty at each stage using the iterative approach, and no example of a foreign country
where investigations at an actual disposal (candidate) site have been conducted. The Review Committee
considers that supplementary explanations will be necessary regarding the status in Japan and foreigncountries.

Radionuclide leaching/retardation function expected for the geological environment
For the requirements of long-term post-closure safety, NUMO described that the geological characteristicsof the deep underground will change slowly over the long term, and the possibility of sudden or rapidchanges will be small. Considering this range of change, if favorable geological environment
characteristics from the viewpoint of geological disposal were maintained, it could be said that thegeological environment would be “stable over a long period”. The expected period for reduction of therisk presented by the waste due to radioactive decay would be about 100,000 years. Therefore, thepossibility of reducing or losing the leaching/retardation function expected in the geological environment
would be extremely small even for a long period of time exceeding 100,000 years.
The Review Committee agrees with NUMO that this is very important information in the context of
explaining the safety of geological disposal systems. However, to explain this point more clearly, theReview Committee suggests supplementing the knowledge with other information currently cited in the
OECD/NEA report [6]. In addition, in Chapter 2 of the NUMO Safety Case, there is a description that, “inthe long time period beyond about 100,000 years in the future, the uncertainty of future predictions related
to the evolution of the geological environment and the occurrence of natural phenomena increases.” It is
important that “evolution of the characteristics of the geological environment in a very limited area within
the crust” should not be confused with large-scale “evolution in the crust and tectonic plates associatedwith changes in the location of volcanoes and faults, as well as uplift and erosion”. For this reason, theReview Committee suggests improving the description so that the expression is more consistentthroughout the report.

How to treat the Darcy flow velocity in the hydrogeological model of fault and groundwater flow
analysis
For faults (fault zones) with a scale of several kilometers or more, NUMO has established ahydrogeological structure model with a core zone and a damaged zone around it, based on knowledge offault morphology and structure, hydraulic characteristics, etc. reported in fault surveys in Japan andoverseas.
Recognizing that this hydrogeological model properly reflects the generally accepted hydraulic structureof faults, the Review Committee agrees that NUMO should treat this model as a representative conceptual-21-

model that describes the internal structure of large faults. On the other hand, since asymmetrical fault zones
that contain a significant number of fractures have been found in the Houraikyou, Mt. Rokko and the San
Andreas faults, from the viewpoint of expanding the usability of this model the Review Committee
suggests describing a wider range also referring to this research.
In addition, NUMO calculated Darcy flow velocity as one of the results obtained from groundwater flow
analysis using a hydrogeological structure model, and used this for subsequent discussions.
The Review Committee agrees to using the Darcy flow velocity as a measure of groundwater movement
at the scale to be analyzed when considering the position and layout of the repository. However, there isno description of the technical meaning of Darcy flow velocity in the NUMO Safety Case. The Committee
recognizes that it is necessary to explain the technical meaning in detail. In addition, the Committee
suggests that the relationship between the actual flow velocity used in safety assessment and the Darcy
flow velocity should also be accurately described in Chapter 3 or Chapter 6 of the NUMO Safety Case.

Influence of high bicarbonate concentration groundwater
The bicarbonate concentration and pH of the groundwater greatly affect complexation for some metal ionsand carbon steel corrosion and it can be a very important technical element for ensuring the quality(reliability) of these data for safety assessment. The groundwater sampled from a borehole, however, may
be different to the original due to degassing, etc. To solve these problems, NUMO screens water quality
data and estimates the concentration of bicarbonate based on charge balance. The Review Committee
agrees to NUMO adopting this approach. For groundwater with high sodium ion and chloride ion
concentrations, however, it is necessary to be aware that the error of measurement values becomes large
when estimating the concentration of bicarbonate from the difference in ion concentration. The Review
Committee also agrees that NUMO Safety Case addressed the need for data preparation in future work for
sedimentary rocks that contain carbonate / organic matter, especially Pre-Neogene sedimentary rocks(accretionary sedimentary rocks) for which there is little information.

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3.2 Design of the repository and engineering technology
3.2.1 Outline of review
In Chapter 4 of the NUMO Safety Case, based on the basic concept for ensuring the safety of geologicaldisposal described in Chapter 2, a design concept for the repository (engineered barriers, surface facilities,
underground facilities) system and examples were presented using the results of the site descriptive model
examined in Chapter 3. The detailed review of the Review Committee was carried out from the following
specific viewpoints, considering the aspects of the "validity of the scientific and engineering basis to which
the Safety Case refers" shown in Chapter 1 of this report.
Is the design of the repository based on the approach specified in the basic safety concept for
ensuring the NUMO Safety Case (Chapter 2)?



Are the performance requirements of the repository established appropriately as a basic
framework for selecting a design, and is the design process for satisfying the required
performance shown clearly?



Are the values of material properties for the design and their rationale shown clearly? Are thevalues appropriate for the stage when no site is selected?

Does the examination leave room for alternative selections, such as examining two or more
activities in parallel, considering future technology developments?

Are the safety measures for abnormal conditions included, as experienced by the 2011 Great
East Japan Earthquake, in addition to normal conditions?



Has retrievability been examined rationally?

The opinions submitted by each member were collected and compiled based on the above-mentioned
viewpoints. The general results of the review are as follows.


For the three site descriptive models for candidate host rocks (crystalline rock, Neogene
sedimentary rock and Pre-Neogene sedimentary rock) discussed in Chapter 3 in the NUMO Safety
Case, the methodology and procedures for design, construction, operation and closure of therepository which satisfy the five design factors (operational safety, long-term post-closure safety,
retrievability, engineering feasibility and economic viability) were described in detail in Chapter 4of the NUMO Safety Case. Although there are many indefinite elements at present, the pertinent
information needed for the engineering design of the repository was compiled and the contents
shown as realistically as possible, so that feasibility of implementation can be reasonably assured.



The Review Committee places a high value on the current approach, which leaves room for future
selection allowing a flexible approach to future uncertainties, for examining alternative approaches-23-

in parallel, such as introducing horizontal emplacement of a PEM system instead of verticalemplacement for vitrified waste, introduction of B-type packages for TRU waste, andrationalization of overpack thickness.
Although uncertainty is unavoidable in engineering design, the Review Committee suggests that,in addition to considering the uncertainties associated with information/data on the material
properties used for the geological characteristics and engineered barriers depicted by the site
descriptive model, uncertainties associated with the boundary conditions assumed for the design or
the resulting range of design specifications should be considered. Since it is very challenging tomaintain the safety of the repository over a very long time period, the implementing organizationand the regulatory body are required to ensure safety by assuming as many scenarios as possible
after the repository is closed. The Review Committee considers it highly important that issues tobe considered in the future are described comprehensively in the NUMO Safety Case. In the actual
design, however, technologies related to design, construction, operation and closure are expected
to continue to evolve. The Review Committee suggests describing that the main issues must be
updated with the latest reliable knowledge.

3.2.2 Design workflow
In Chapter 4 of the NUMO Safety Case, the requirements for design were specified by defining therepository properties and performance required as “design factors”. In addition, design requirements were
set to ensure the performance indicated by the defined requirements. The repository was designed basedon these requirements to determine the basic specifications (materials, shape, dimensions, etc.). The
specific “repository design workflow” was shown as a figure, which includes a sequence of design
workflow with feedback during the design and layout of the engineered barriers, surface facilities andunderground facilities.
However, regarding the figures shown as "Design workflow based on design factors and design
requirements" and " Design workflow of the repository", it was not clear whether the object of the impact
assessment and subsequent feedback to determine the validity of the defined specifications was only therepository with engineering design (engineered barriers, surface facilities and underground facilities), theentire geological disposal system including the natural barrier, or whether it includes a review of design
factors and design requirements. In addition, the relationship between “impact assessment” in Chapter 4
and “safety assessment” in Chapters 5 and 6 of the NUMO Safety Case was also difficult to understand.
It was described in design validation and feedback that there are two processes: a review of the design
requirements for the engineering design of the repository and a safety assessment of the entire geologicaldisposal system. An explanation of the relevant figure should be provided in the text. At that time, in thedesign study of the repository, there were small feedback loops that were handled in engineering-24-technology and large feedback loops including safety assessment. These should be used properly
according to the objectives.
In addition, because the repository is designed to ensure very long-term safety, there is a possibility that
trade-offs will occur on the time axis between operational safety and long-term post-closure safety (forexample, drainage function of the permeable layer in contact with the mechanical plug and long-term
stability of a drained mechanical plug). It is important that the repository design allows for feedback of theresults from assessment of the evolution of the repository to allow required changes to repository
components and their specifications.
Based on the above, the Review Committee suggests that the description of the design framework should
be clearly understandable by describing the feedback concepts and preconditions.

3.2.3 Design factors and requirements
The characteristics and functions required of the repository as an entire geological disposal system were
specified as design factors under the assumed geological environment. The basis for this is, however,unclear. Since these settings are also an important design process, the Review Committee suggests adding
more detailed explanations.
The current description explaining the design factors is difficult to understand; it explains the content of
the requirements for each component of the repository. Together with a figure showing the design
workflow, the Review Committee suggests adding an explanation with a description of the requirements
at the beginning of Chapter 4 of the NUMO Safety Case.

3.2.4 Concept for ensuring safety in designing the surface facilities
The Review Committee places a high value on considering safety measures for abnormal conditions inaddition to normal conditions, which is the specified situation for the development of nuclear facility
design in Japan based on the experience of the 2011 Great East Japan Earthquake. On the other hand,
specific studies on seismic activity and tsunamis were not conducted on the ground that the conditions of
the site geological environment had not been specified. The Review Committee suggests, however, adding
evaluation results for scenarios that assume, for example, earthquakes and tsunamis such as caused by the2011 Great East Japan Earthquake. In addition, the Review Committee suggests ensuring consistency with
Chapter 5 “Assessment of operational safety” of the NUMO Safety Case.

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3.2.5 Material properties for the design
Establishing geological environment characteristics
In this study, information required for the design in terms of thermal characteristics, mechanicalcharacteristics, hydraulic characteristics and chemical characteristics of the groundwater in theassumed geological environment was derived from the site descriptive models for plutonic rocks,
Neogene sedimentary rocks and Pre-Neogene sedimentary rocks specified in Chapter 3 of the NUMO
Safety Case. The geological and environmental characteristics in the actual design should utilize insitu or laboratory tests using rock samples as well as physical tests etc. after site selection. The Review
Committee suggests describing that the geological and environmental characteristics for design wereselected tentatively considering variations in the three potential host rocks, and that the characteristicsof the geological environment used in the actual design will have to be determined with sufficientaccuracy through future field investigations and tests.

Establishing material properties for the engineered barriers
The values for the material properties used in the design of the overpack, waste package, and buffermaterial that make up the engineered barriers are taken from the literature and test results, in addition
to the site descriptive model in Chapter 3 of the NUMO Safety Case. For example, the pH andchemical composition of the pore water in the buffer material in contact with the overpack were
calculated based on the water quality model in Chapter 3. The Review Committee suggests describingthat the values used will have to be updated with the latest reliable knowledge during the actual design,
although they are meaningful as representative values to be used for the design studies at the present
time. Moreover, the Review Committee suggests describing the need to carefully consider whether
the assumed boundary conditions will be maintained over the long term, since some of the estimatednumerical values are based on these.

3.2.6 TRU and other waste characteristics and their disposal concepts
In discussing the characteristics of the waste package as a design basis, there are some remaining problems,
especially for the TRU waste. The percentage distribution of the nuclides in each waste package will be
determined by reprocessing in the future. Regarding the inventory of individual radionuclides containedin each TRU waste package, the relevant supporting report shows the values selected, the basis for theselection and their validity. However, since these are important for safety assessment, the Review
Committee suggests describing these values in the main report. The waste properties include chemicalcomposition, leaching properties, heat resistance, etc. As the characteristics of waste such as TRU are very-26-

variable, the Review Committee suggests clarifying and describing the characteristics of the waste
packages as a design condition.
The disposal of vitrified waste, one of the wastes for co-disposal with waste such as TRU, and the startingpoint for examining disposal is already well documented in the H12 Report, including repository concept
catalogs and design options. A future task will be to proceed with the same design options for TRU waste
disposal.
In the design of the engineered barriers for a TRU waste repository, the description of the specifications
for waste package type A was omitted based on the premise of the Second TRU Report. The Review
Committee suggests adding an explanation of the difference between the specifications of waste package
types A and B. In addition, regarding removal of pore water to suppress hydrogen gas generation due toradiolysis in filled mortar, the Review Committee suggests describing the effects of mortar shrinkage due
to drying together with the differences in the functions required of fillers in waste package types A and B.

3.2.7 Design of the underground facility layout
The layout-determining features (LDF) are defined as the characteristics that determine the layout of theunderground facilities as given by the site descriptive model at the repository scale. The emplacement
position determining features (EDF) are defined as properties that determine the emplacement position of
the vitrified waste packages (hydraulic properties of faults and fractures less than 1 km in length) as givenby the site descriptive model at the panel scale. The Review Committee places a high value on clearly
describing the approach for defining these features. The explanation of LDF and EDF, however, is limited
to an explanation of the abbreviations in footnotes. The Review Committee suggests properly explaining
the concept in the main report as an important design issue.

3.2.8 Retrievability
The NUMO Safety Case discusses the impact of maintaining retrievability before closing the repository
to investigate the engineering feasibility of waste retrieval technology. In the NUMO Safety Case, three
conditions were specified according to the difficulty of retrieval. Impacts on the geological environmentof the near-field, the engineered barriers, and required maintenance of tunnels were considered for the casewhen ease of retrieval is maintained. For these three conditions, the applicability of the retrieval technology
was considered only for conditions where the impact was intermediate.
In the report [7] on “Reversibility and retrievability (R & R) for deep geological disposal of high-level
radioactive waste and spent fuel” published by the OECD/NEA in 2011, it is described that retrievability
is not part of the basic concept for the long-term safety of the repository, but retrievability has to be-27-

considered in each stage of the waste life cycle in terms of ease of retrieval, passive safety and active
management, because retrievability facilitates safety arguments and could be desirable for reasons otherthan safety. Following the “Internationally agreed staged classification of retrievability (R scale)”proposed by the OECD/NEA, the Review Committee suggests describing the impact not only for theintermediate case but also for the other cases in the NUMO Safety Case.

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3.3 Safety assessment for the operational phase
3.3.1 Outline of review
In Chapter 5 of the NUMO Safety Case, and based on the basic concept for ensuring the safety ofgeological disposal described in Chapter 2, scenario development, impact assessment based on abnormal
scenarios and examples of responses after accidents, etc. were presented for a site descriptive model andrepository design based on it in a pre-closure safety assessment. The Review Committee has conducted adetailed technical review from the following specific viewpoints related to “relevance of scientific andtechnological foundations relied on by the Safety Case”, as described in Chapter 1 of this review report.
·

Is the safety assessment performed according to the approach defined in the basic concept for ensuringsafety in the NUMO Safety Case (Chapter 2)?

·

Is the basis for setting the scenarios for operational safety assessment consistent with international
standards?

·

Are the scenarios and analysis cases assumed in the safety assessment sufficiently comprehensive?

·

Is the scientific basis supporting the safety assessment methodology, models and codes adequately
outlined?

Based on the above viewpoints, the opinions submitted by each member were collected and compiled as
review results. The general findings of the review are as follows.


In Chapter 5 of the NUMO Safety Case, safety assessment prior to closure was discussed from theviewpoint of radiological protection and general occupational safety. In accordance with the safetycase and safety assessment guidelines issued by the IAEA, normal-state scenarios and abnormal-state(accident) scenarios were developed and studied. As accident scenarios, dropping a waste package,fire within the facility, loss of external power supply and other equipment failure were assumed. The
possibility of mechanical and/or thermal damage to the overpacks and waste packages in which
vitrified materials are encapsulated that could cause a release of radioactive material was also
evaluated. The Review Committee concludes that the overall evaluation framework and basic concept
including abnormal condition (accident) scenarios are appropriate.

The events in the accident scenarios, on the other hand, are not necessarily sufficient. Because of thelarge degree of site dependence, important scenarios were set only for internal initiating events
without considering external initiating events. The Review Committee suggests establishing andevaluating scenarios for external initiating events that could be assumed even at the generic stage ofthe project.

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3.4 Assessment of long-term post-closure safety
3.4.1 Outline of review
In Chapter 6 of the NUMO Safety Case, based on the basic concept for ensuring the safety of geologicaldisposal described as a long-term post-closure safety assessment technology in Chapter 2 of the report,
scenario development, nuclide migration analysis models and the development of datasets, a series ofmethodologies up to dose evaluation and analysis techniques were shown for a site descriptive model andthe design of repository. The Review Committee conducted a detailed technical review based on thespecific viewpoints described below, related to “relevance of scientific and technological foundations
relied on by the Safety Case”, as described in Chapter 1 of the report.
·

Is the safety assessment performed according to the approach defined in the basic concept for ensuringsafety in the NUMO Safety Case (Chapter 2)?

·

Is the basis for specifying the post-closure safety assessment framework (scenario classification,
evaluation period, application of stylization, etc.) consistent with international standards?

·

Are the disturbances expected during construction/operation and maintenance of retrievability
properly taken into account in the safety assessment?

·

Is the post-closure safety assessment model properly constructed in line with the site descriptivemodel and the design of the repository?

·

Are the safety functions of the individual barriers in the geological disposal system properly described
considering the timescale, and is safety assured by multiple safety functions over the long term
considered?

·

Are the scenarios and analysis cases assumed in the safety assessment sufficiently comprehensive?

·

Are scientific bases for supporting safety assessment methodologies, models and codes shown?

·

Are the features, events and processes affecting the evolution of the disposal system clearly
documented and sufficiently comprehensive compared to the international FEP database?

Based on the above specific viewpoints, the opinions submitted by each member were collected andcompiled as review results. The general findings of the review are as follows:

The system for long-term post-closure safety assessment presented in Chapter 6 of the NUMO Safety
Case is consistent with international guidelines and methodologies in other countries, and isreasonable as an approach that quantitatively demonstrates the safety of geological disposal in asituation where no specific site has been identified. The Review Committee places a high value on,in particular, efforts in the NUMO Safety Case to conduct an evaluation using site-specificinformation rather than a generic evaluation that demonstrates the technical feasibility of geological-30-

disposal and the preparations for the safety assessment technology being established to consider more
realistically the geological environment and disposal system characteristics in Japan.


The Review Committee acknowledges that significant progress has been made in the following
aspects based on the results since the H12 Report and the Second TRU Report:


Adopting a risk-based approach and a disaggregated dose/probability approach considering the
likelihood of scenario occurrence

Utilizing integrated methods and storyboards that combine a bottom-up approach using FEPs
and a top-down approach based on safety functions in developing scenariosIntroducing (more) practical methods to handle the characteristics of the specific site descriptivemodel constructed in Chapter 3 and the geological disposal system developed and designed in
Chapter 4 of the NUMO Safety Case (including remaining artifacts introduced during
construction and operation)

Utilization of the latest database for data selection for nuclide migration

Establishing a safety assessment system considering the co-disposal of vitrified waste and TRU
waste, and

Evaluation of biosphere incorporating the latest knowledge.

From the viewpoint of improving the reliability of long-term post-closure safety assessment, there is
still room for improvement regarding the following points;

Clarification of the positions, roles, and relationships of a series of tools and analysis methods(storyboards, FEPs, attribute analysis, etc.) used for scenario development

Reinforcement of information on quality assurance for mathematical models, codes, data andanalysis of nuclide migration processes

Clarification that the uncertainties to be included in the safety assessment are considered
comprehensively

Clarification of relevance to the evaluation of possible perturbations of the undergroundenvironment before closure, retrievability, accidents during construction and operation, and

Discussion of differences by comparing the safety assessment results with previous results.

In the following sections, the results of the Review Committee are compiled on issues raised by thecomments submitted in the Committee regarding safety assessment concepts, scenarios, models, data andassessment based on nuclide migration analysis.
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3.4.2 Concept for safety assessment
Evaluation period
A logical approach is to classify the state of the disposal system after closure into four timeframes (T1T4) and to define the FEPs related to the safety functions for each time section. At the current stage
when no site is specified, however, it is considered difficult to define the FEPs, including the possibilityof changes in the geological environment over a long timescale. In this sense, care needs to be taken in
handling T4. For example, although T3 is defined as “the period when it is considered that thecharacteristics of the current geological environment will not change significantly after the start ofradionuclide migration”, based on this definition it was necessary for T4 to consider changes in thegeological environment. On the other hand, in the evaluation of the reference scenario, it was assumed
that the characteristics of the geological environment would not change significantly during theevaluation period up to the occurrence of the maximum dose. The Review Committee suggests that,after clarifying the concept of T4 in the reference scenario and handling of changes in the geologicalenvironment, from the viewpoint of consistency with the long-term evolution of the geologicalenvironment in Chapter 3 of the NUMO Safety Case, the description should be changed for ease ofunderstanding of the relationship between the evaluation timeframes and the scenario categories.

Relationships between uncertainties
The uncertainties in scenarios, models and data described in Chapter 2 of the NUMO Safety Case areconsistent with the classification and definition of uncertainties in domestic and international safetyassessments. In the long-term post-closure safety assessment, however, since it is necessary toappropriately evaluate a range of uncertainties, a careful description is required of the policy for dealing
with uncertainties and specific approaches. In particular, as it was difficult to understand the relationshipbetween the uncertainties handled in each scenario classification derived from the reference scenario
and the uncertainties in scenarios/model/data, it was difficult to appreciate that the uncertainties to beconsidered in the safety assessment are comprehensively evaluated. For example, the uncertainties indata for the reference scenario were handled in the form of scenario uncertainties with alternative
scenarios. Regarding the relationship between the uncertainties in scenarios/models/data and thescenario categories, the Review Committee suggests revising the description so that readers can easily
understand that the impacts of uncertainties are comprehensively and rationally evaluated.

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3.4.3 Scenarios
Storyboard
A method for depicting the evolution of the safety functions and the state of the disposal system using
storyboards that describe it from the period from closure in several temporal and spatial frames wasconsistent and appropriate with the latest scenario development methodology, while the basic procedure
for safety assessment was illustrated. Since the storyboard appears to be positioned at the upper level inscenario development, readers may be led to misunderstand that scenarios are determined a priori. The
evolution of the system state presented in the storyboard was in fact to be clarified through attributeanalysis and impact analysis, and the content of the storyboard should have been updated based on theseanalyses. This concern is related to the fact that the storyboard development method is not clearly
described. There are also some descriptions in the main report to the effect that a storyboard wasdeveloped, and scenarios were derived based on this storyboard. Therefore, the Review Committee
suggests that explanations should be provided to avoid the above misunderstandings.
Based on the above, the Review Committee suggests clarifying the objectives of introducing thestoryboard concept in safety assessment, and the methods for its development and application, as wellas the position of the storyboard in the assessment. The validity of the figure showing the flow of basic
procedures for safety assessment (especially the relationship between the storyboard and attributeanalysis/impact analysis) should also be reconsidered.

Development of scenarios
The Review Committee places a high value on scenarios being developed using scenario development
technologies based on the method used in the H12 Report and according to latest scenario development
technology in Japan, and the FEP list necessary for scenario development being structured to improvethe comprehensiveness of the scenarios. On the other hand, it is difficult to understand, as describedlater, how to develop a scenario based on attribute analysis, impact analysis and storyboards in the
explanation of the basic procedure for safety assessment shown in the figure. The Review Committee
suggests adding an explanation to improve the transparency of the process related to scenario
development.
In addition, the Review Committee considers it important that the introduction of the new integrated
FEP list, which is a reduced version of the previous FEP list (NUMO FEP list) for scenario development,
should be seen as an attempt to improve the complexity of presenting correlations using many FEP lists.
For example, there are some merits because using an integrated FEP list in the attribute analysis diagram
is simpler than using the NUMO FEP list. On the other hand, the Review Committee points out that
there is still reluctance to use the integrated FEP list in the subsequent scenario development from the-33-

viewpoint of comprehensiveness. The NUMO FEP list and the integrated FEP list should be mutuallycomplementary from the viewpoint of comprehensiveness and practicality. In this regard, the Review
Committee suggests supplementing the description, including examples of application, of how the
newly introduced FEP list will be effective.

Attribute analysis/Impact analysis
The Review Committee considers it appropriate that attribute analysis and impact analysis are
introduced as methods for analyzing the impact on safety functions, which are key in scenario
development, and comprehensive evaluation of the factors that may impact safety functions throughstate variables. On the other hand, since it is difficult to understand how scenarios are developed basedon attribute analysis and impact analysis, the Review Committee suggests adding an explanation. While
the safety functions and state variables were explained using buffer material as an example in theattribute analysis, the Review Committee suggests explaining them with a list rather than an example,
since the attribute analysis is important for deriving scenarios. Furthermore, the Review Committee
suggests clarifying the basis for selecting the phenomena in the impact analysis and the process forderiving scenarios through associated evaluation. Regarding the perturbation of the geologicalenvironment due to construction and operation, the effect of maintaining retrievability, and the impactof accident conditions during construction and operation on long-term safety, as discussed in Chapter 5,
only a scenario in which the original state is recovered is considered although, in reality, they may affectthe initial conditions of long-term safety assessment. The Review Committee suggests adding a more
careful explanation of this as well as referring to the relationships with Chapter 5. It should be noted
that, although not directly covered in this review, the Committee considers it extremely important toexpand the information on various FEP lists in order to improve the comprehensiveness of attributeanalysis and impact analysis, and, in particular, to continue research and analysis for FEPs considered
to have a significant impact on safety functions.

Scenario categories
The Review Committee acknowledges that it is appropriate to classify scenario categories according to
the likelihood of occurrence of events based on a risk-based assessment method, and this scenario
classification is recognized as one of approaches to responding to uncertainties in the scenarios. In
general, models and data are set based on a scenario, and uncertainties in models and data in thereference scenario are evaluated within the reference scenario category. Uncertainties are handled in the
different scenario categories (alternative scenarios) in the NUMO Safety Case, which describes that “analternative scenario includes variants, reflecting the uncertainties that should be considered based on
scientific rationality with respect to the state of evolution of the repository and the evaluation models-34-and data set in the reference scenario”. The Review Committee agrees with the concept in the NUMO
Safety Case, however it is difficult to understand what uncertainty is handled in which scenario category.
As a result, it is difficult to confirm whether the scenario categories developed in the NUMO Safety
Case sufficiently cover the uncertainties to be considered. The Review Committee suggests restructuring
the relationship between scenario classification and handling of various uncertainties from the viewpointof consistency and adding explanations to the main report as necessary.

Reference scenario
The Review Committee places a high value on the reference scenario being set by combining
conservative assumptions based on technological progress after the H12 and Second TRU Reports,
aiming to represent the phenomena in the disposal system more reliably in a highly probable scenario.
The Review Committee suggests, however, adding supplementary explanations and a rational basis forthe following points related to the setting of the reference scenario.
Assumption that perturbations considered to be caused by construction/operation andmaintaining retrievability will not significantly affect the initial state for the long-term safetyassessment



Assumption that the effects of cement leaching of the mechanical plug together with that in
the associated permeable layer can be ignored, although the NUMO Safety Case describedthat the cement leaching of the mechanical plug will occur and the cement component is likely
to diminish

Assumption that hydrogen gas generated due to corrosion of overpacks and PEMs will notsignificantly affect the safety functions



Assumption that the initial state of the EDZ would be maintained over a long time period,
while the state of EDZ is thought to evolve due to various processes such as cement interaction
with tunnel support, Fe interaction with the PEM, and stresses resulting from buffer material
swelling and overpack corrosion, andHandling of uplift and erosion in the reference scenario (in particular, the effect of uplift anderosion was not considered in T4).

Alternative scenarios
In the derivation of alternative scenarios, uncertainty factors, which were defined for each componentof the disposal system, were difficult to understand in terms of their effect on the safety functions. The-35-

Review Committee suggests adding an explanation as to why this structure has been selected. In addition,
the NUMO Safety Case describes that a case combined alternative cases that was not considered because
the probability of occurrence was very small; however, there are cases where the relevance cannot becompletely ruled out depending on the process (for example, the effect of overpack corrosion products
on glass dissolution rate and on sorption performance of the buffer material). The Review Committee
suggests confirming again that these scenarios that should not be considered in the reference scenario
and should be considered as alternative scenarios are sufficiently comprehensive in the NUMO Safety
Case and expanding the explanation. In addition, while handling of analysis cases corresponding to thealternative scenarios is shown in the table, the Review Committee suggests, from the viewpoint oftraceability/transparency, referring to the supporting reports that are the basis for the uncertainties suchas “not considered” or “included in the base case” in the table.

Unlikely event scenarios
Although there is an explanation that unlikely event scenarios consider events that are very unlikely inassessing system robustness, the Review Committee suggests clarifying the screening criteria for the
events in this scenario category because these are ambiguous. In addition, in the discussion of the latest
regulatory standards for intermediate depth disposal, the unlikely event scenario category was not
introduced and all scenarios other than the reference scenario were treated as alternative scenarios,
which may have some influence on geological disposal. Given that a longer evaluation timescale isrequired for geological disposal, it is obvious that some kind of evaluation is necessary for unlikely
external events. Therefore, the Review Committee suggests clarifying the difference from the alternative
scenario category and adding an explanation to avoid misunderstanding.

3.4.4 Models
Nuclide migration analysis model
The nuclide migration analysis model in the NUMO Safety Case considers the effects of multiple faultsand fractures in the site descriptive model and the geometry of the repository more realistically, which
is significant progress from the previous generic safety assessment model. The Review Committee
recognizes, however, that more detailed explanations are necessary regarding the differences from themodel in the H12 Report and the Second TRU Report. In particular, the Review Committee suggests
describing clearly the concept of selecting specific processes for handling flow field heterogeneity, theuse of fundamental solutions, simplification with one-dimensional models and the connection between
different scales, etc., as well as the entire workflow of safety assessment analysis.

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Since nuclide migration in the engineered barriers and in the near-field (the host rock around theengineered barriers) was modeled on the same near-field scale, the amount of nuclides confined in theengineered barrier region (waste, container, buffer material) could not be distinguished from the amountof nuclides migrating within the near-field geological environment or panel-scale area. In order to showthat most of the relatively short-lived radionuclides decay within the engineered barriers, the Review
Committee suggests using the engineered barrier region as the minimum unit of spatial scale, andindicating the transfer rate from the engineered barrier region to the near-field scale as the “release term”.
Regarding the nuclide migration analysis model using the particle tracking analysis method and multichannel model, the Review Committee suggests adding explanations of the following items in order toimprove reliability.
Process of conducting nuclide migration analysis based on the results of particle tracking
analysis (including processes for optimizing hydraulic transmissivities)

Verification cases for the phenomenological model of mass transport (nuclide migration
between fractures and matrix, mutual diffusion between media) set for the Neogene
sedimentary rocks



Objective of simplifying the nuclide migration model using the multi-channel model (reducing
computational load, ensuring conservativeness, etc.)

Explanation why the calculated flow velocity (transmissivity coefficient) is smaller than thatobtained only by the fracture network model when the pressure head information obtainedusing the equivalent porous model is converted to the fracture network model.

The Review Committee suggests adding an explanation for the following aspects related to the nuclide
migration analysis model.
Definition of “release rate” in the dissolution of glassNuclide migration process within the EDZ



Relationship with the assumption that parallel plate models are used for all rocks and themicro-permeability conceptual model types constructed for each of the potential host rocks
plutonic rocks, Neogene sedimentary rocks and Pre-Neogene sedimentary rocks shown in
Chapter 3.

In the regional-scale model, it was described that ignoring fault migration resistance gave a robust safetyassessment for long-term uncertainties related to the depth of the underground facility and changes in-37-

fault/fracture length due to uplift and erosion. However, assuming that the repository approaches theground surface due to uplift and erosion, the dose obviously becomes higher than that based on theabove setting. The Review Committee suggests re-examining the dose, because there will be a change
in the chemical environment around the repository, acceleration of nuclide migration in the engineered
barriers due to increased groundwater flow and increased exposure due to stripping of contaminated soil.

Biosphere evaluation model
Regarding the evaluation of biosphere, the Review Committee acknowledges the appropriateness ofconducting the evaluation based on global standards such as the IAEA coordinated research project
BIOMASS. In addition, the latest dose conversion factors and migration parameters were used todetermine the conversion factors to dose. The Review Committee places a high value on properly
considering progress since the H12 Report and the Second TRU report. However, the Committee
suggests describing this in the main report, because the basic conditions for biosphere evaluation
described in the supporting reports (evaluation objectives, evaluation indicators, evaluation concept,
disposal system, site conditions, source term and GBI, evaluation period, society, etc.), the concept ofsetting compartments and the process for deriving the conversion factors to dose represent core
information that underpins the biosphere assessment.

3.4.5 Data
Nuclides for evaluation and radioactivity inventory
When selecting the nuclides for evaluation, the selections made in other countries are used as the basis.
Fundamentally, however, it is necessary to provide a reasonable explanation based on the waste streams
in Japan, and it is necessary to describe the calculation conditions in a traceable form when calculating
the inventories. The inventories are calculated by a nuclide generation/decay calculation code, but it isnecessary to show into which streams and to what extent they are distributed during the reprocessing
process. Although these are described in the supporting reports, the Review Committee suggests thatthey be described in the main report because these parameters contribute significantly to the evaluation
results, in particular for those in which significant conservative assumptions are made about the
distribution of I-129 and Cl-36.

Nuclide migration data
The nuclide migration data adopted in the NUMO Safety Case are selected appropriately using the latest
scientific knowledge and databases since the H12 Report. It should be noted that large values for both-38-

the permeability and the diffusion coefficient (of some elements) of the buffer mass are used in thereference scenario because they are conservative. Since conservativeness of the parameters depends on
the scenario, however, it is necessary to carefully explain how to handle this in the alternative scenarios
for parameters where conservative values have already been set in the reference scenario.

Multi-channel model data
In the nuclide migration analysis in the NUMO Safety Case, an approximation method with a onedimensional multi-channel model is used based on the results of mass transport analysis using threedimensional particle tracking. The calculation method for the transmissivity assigned to each channel
and the transmissivity data, however, are not described in the main report, nor in the supporting reports.
The Review Committee suggests adding this information from the viewpoint of traceability of thenuclide migration analysis.

3.4.6 Assessment based on analysis of nuclide migration
Analysis results
As described in Section 3.4.4, in order to quantitatively demonstrate the performance of the engineered
barrier system, the Review Committee recommends adding the migration rate (Bq/y) through theengineered barrier (buffer material) (or a value derived using the conversion coefficient to dose in thebiosphere) in the dose rate evaluation results.
In this analysis, although likely processes and data were used based on the risk-based theory, moreconservative values were set considering their uncertainty. There is a possibility that the peak of thetotal dose appears earlier than the period expected from the intrinsic performance of the geologicaldisposal system. At the current stage when no site is specified, the Review Committee agrees to adopting
such a method for dealing with uncertainty, however, it would like to point out also that the conservative
setting of one scenario does not necessarily mean that it is conservative in other scenarios. This isparticularly important for the case considering uplift and erosion, which was not covered in the NUMO
Safety Case.
While the NUMO Safety Case resulted in U-233, a Np-237 daughter nuclide, giving a relatively highdose due to complexation because of the high bicarbonate concentration in sedimentary rock pore water,
the Review Committee suggests explaining that the results were based on the assumption of highbicarbonate concentrations maintained in all of the migration paths, and that even a geologicalenvironment with such bicarbonate concentration yields a negligible dose no larger than 10 μSv/y.

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The dose evaluation results for the alternative cases show quantitatively that they are sufficiently low
compared to the dose guideline set in the NUMO Safety Case. However, it is necessary to avoid
misunderstandings in interpretation. For example, fluctuation of the effective diffusion coefficient of
the buffer material in the alternative cases has a small impact on the dose evaluation results. This
indicates the robustness of the geological disposal system but does not mean that the low diffusivity ofthe buffer material is unnecessary.
It was explained that there is robustness in ensuring safety in the NUMO Safety Case since the analysis
results for unlikely scenarios gave 1-20 mSv or less, which may be misleading for the reader. For
example, it could be interpreted that safety would be ensured even if a volcano directly affected therepository, which may even raise doubts about the reliability of the entire analysis. The Review
Committee suggests, in addition to clarifying the meaning of robustness, explaining carefully that theanalyses were made very conservatively for events that have a very low likelihood of occurrence. Also,
although it was a major premise that such events should be avoided in the site selection process, thereference dose value used for comparison with the goal of the implementation body was weighted by
the probability of occurrence, and it should be noted that future efforts will be important for building arobust system that minimizes the impact of unlikely events over the long term. The same applies to thesummary of the human intrusion results.

Comparison with previous safety assessments
The Review Committee places a high value on updating the nuclide migration analysis and dose
evaluation based on the H12 Report and the Second TRU report as the technological basis for the longterm post-closure safety assessment in the NUMO Safety Case, referring to the latest research anddevelopment results and the progress of science and technology. On the other hand, the dose evaluation
results presented in the NUMO Safety Case are clearly different from the previous results. In particular,
in the dose assessment for the reference scenario for vitrified waste, there is a notable difference in that
the NUMO Safety Case shows that Se-79 (plutonic rock), I-129 (Neogene sedimentary rock with low
Cl- concentration groundwater) and U-233 (Neogene sedimentary rock with high Cl- concentration
groundwater) are dominant, while the dominant nuclide in the H12 Report was Cs-135. There was alsoa difference in the time when the maximum dose would occur in the reference scenario in both cases.
The Review Committee suggests analyzing the reasons for the differences from previous safetyassessments and clarifying changes compared to previous assessments and related uncertainties.

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Safety assessment towards stepwise site selection
The NUMO Safety Case discusses the investigation and evaluation of the geological environment,
repository design and prospects for safety assessment in the current situation where no site is specified.
In the future, once a site has been identified, the safety assessment will be performed using the basic
information and evaluation technologies indicated in the NUMO Safety Case. However, further
considerations will be necessary in the future regarding how the safety assessment will be performed
using the information refined through stepwise investigations, to what extent the safety assessment will
be achieved based on the quality, quantity and uncertainty of the information obtained at each
investigation step, and how to optimize safety in cooperation with geological investigations andengineering design.
In addition, the dose evaluation in the NUMO Safety Case resulted in values below the dose criteria inall scenarios. From this, it was considered that existing processes and natural phenomena have an
extremely low possibility of compromising the safety of geological disposal. However, thedemonstration of the safety of geological disposal is not established based on this evaluation. Rather, toimprove the reliability of future NUMO safety cases, it is necessary to deepen the understanding of thesafety functions that should be further strengthened, to identify processes and parameters that contributeto safety, and to clarify issues for reducing and minimizing uncertainty.
The Review Committee suggests that the above points should be considered and that the core messagesof the safety assessment should be described more clearly based on the role of safety assessment in
future site selection.

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4. Reliability as a safety case
4.1 Outline of review
In the Review Committee, the detailed technical review of " reliability as a safety case" presented in
Chapter 7 of the NUMO Safety Case, from the specific viewpoints described below, related to "the validity
as an entire safety case" presented in Chapter 1 of this report:
・ Consistency of the context: Is each chapter consistently developed based on the basic concept for
ensuring safety (Chapter 2) specified for the Safety Case, and are the general conclusions of the Safety
Case properly derived in Chapter 7 based on the conclusions of each chapter?
・ Validity of the descriptions: Are the progress and differences from the H12 Report and the Second
TRU Report evaluated and is the technical basis specified? Are there any omissions of importantelements required for a safety case at the present stage, considering the characteristics and situationof Japan? Furthermore, is the applicability as a basic form of the safety case after site selection
demonstrated?
・ Recognition of future tasks: Is there a clear distinction between site-independent universal issues andsite-specific issues, and is the research and development to be implemented in the future described?
・ Compatibility within Japan and abroad: Is the intent behind preparing the NUMO Safety Caseconsistent with the government's basic policy on final disposal, is it consistent with international
standards and similar precedents in other countries, is the technical terminology consistent withinternational definitions, and is the report written in Japanese that can be understood correctly?
・ Integration as a report: Are the main conclusions of the NUMO Safety Case presented in the main
report clear and intelligible, is the basis for the conclusions traceable and objective?

Based on these viewpoints, the Review Committee considered the opinions submitted by each member
regarding reliability as a safety case. The results of the review are compiled as “reliability as a safety caseat the current stage” and “relevance as a basic form of safety case after site identification1”. Furthermore,
the tasks until site identification and the long-term R&D up to and after site identification are compiled as“Future tasks and R&D for the next stage”.
The general view of the Review Committee is as follows:

The Review Committee confirms the following: the NUMO Safety Case is structured to properly
lead to the general conclusions reached, and is consistent with the defined context: considering
the geological characteristics and social conditions of Japan, there were no omissions of1“After site identification” refers to the stage after a specific site including the literature survey area has
been identified.
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important issues required for a safety case to be developed at the present time. The report isconsistent with Japan's basic policy on final disposal and refers to international standards andsimilar precedents in other countries. The technical terminology is consistent with international
definitions, and the Japanese can be understood correctly. In addition, the main conclusions of
the report are presented clearly and, considering the supporting reports, there is traceability andobjectivity in the basis for the conclusions. There is also a clear distinction between siteindependent and site-specific aspects, and the research and development to be conducted in the
future is described.


Following the above confirmation, the Review Committee agrees that the NUMO Safety Caseis reliable as a safety case at the current stage and relevant as a basic form for a safety case aftersite identification, with consideration of various siting environment conditions. Furthermore, toassure gradual improvement of the reliability of future safety cases, required R&D from a longterm perspective is covered in “future tasks and R&D for the next stage”.

4.2 Reliability as a safety case at the present stage·

The NUMO Safety Case describes the technical reliability as a safety case at the present stage fromthe following five viewpoints;
1)

Demonstrating technologies for constructing a repository that satisfy the required safetyfunctions

2)

Ensuring the safety of geological disposal in Japan's geological environment

3)

Ensuring reliability as a safety case4)

Making clear future efforts to improve reliability, and5)Showing the possibility of using the NUMO Safety Case as a basic form of safety case to
be updated after a site has been specified.

·

The descriptions are in accordance with “basic concept for ensuring safety” presented in Chapter 2 of
the NUMO Safety Case, which outlined the assessment basis for the safety case. The site descriptivemodel, design/construction/operation technologies for the repository, and safety assessment before
and after closure are summarized in Chapters 3 to 6 of the NUMO Safety Case. Also described are
the progress since the H12 Report and the Second TRU Report, ensuring retrievability, and efforts toimprove reliability in the future. These were described here again.

·

In the NUMO Safety Case, the basic concept for site selection in Japan is shown as a three-stage
siting process, and requirements for the geological environment were described for each investigation
stage. In addition, the basic policy for site selection requirements and standards and factors affecting
the safety functions are presented. In other words, the basic concept of NUMO is presented-43-

concerning selection requirements (statutory requirements) for the three stages of investigation
stipulated in the Final Disposal Act, as well as the requirements for clarifying unfavorable andpreferable areas shown in the “Nationwide map of scientific features for geological disposal”.
·

In addition, the NUMO Safety Case describes current efforts to ensure reliability as a safety case,
how to ensure quality and how to deal with uncertainty by applying NUMO’s quality management
system. Furthermore, in the safety assessment of disposal systems, in addition to evaluation using
dose as an indicator, multi-faceted studies are also conducted using natural analogues andcomplementary indicators such as concentration, time and flux. The importance of natural analogues
and complementary indicators is agreed internationally. The evaluation based on these various
indicators in the development of the safety case could be used as objective and user-friendly
information for decision-making to advance to the next stage in a long-term geological disposal
project. The Review Committee acknowledges that it is also the preferred approach in terms of
ensuring the safety of geological disposal.

·

The Review Committee acknowledges that the NUMO Safety Case is structured appropriately to leadto the general conclusions of the Safety Case with contextual consistency and that, considering thegeological characteristics and social conditions of Japan, there is no omission of important issues
required for a safety case at the present time.

·

In addition, the Review Committee acknowledges that the NUMO Safety Case is consistent withJapan's basic policy on final disposal and refers to international standards and similar precedents inother countries. The Review Committee confirms that the technical terminology is consistent withinternational definitions, and that the Safety Case is written in understandable Japanese. In addition,
the main conclusions are shown clearly, and the supporting reports provide traceability and objectivity
as the basis for the conclusions.

·

On the other hand, the term “long-term” is an issue that is interpreted differently by experts in thecontext of geological disposal. In the report, there is a description that makes it easy to understand atimescale of “long-term over 100,000 years in the future”; however, there are many descriptions inwhich the timescale is vague, such as “long term after closure”, “over a long term”, “long-term
evolution of the geological environment”. The Review Committee suggests rewriting the descriptions
to respond to the question of how long does “long-term” mean so that readers can understand theconcept, while maintaining logical consistency between each chapter in the NUMO Safety Case.

·

The NUMO Safety Case also describes the development of technology for integrating information
into the site descriptive model. It is described that, by repeating the investigation and evaluation
processes in the geological investigations, the uncertainty associated with understanding the
distribution of spatially inhomogeneous characteristics such as faults and fractures is gradually
reduced. Therefore, the Review Committee suggests providing a careful description from a higherlevel viewpoint since stepwise reduction of such uncertainties is important for improving thereliability of the NUMO Safety Case.
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·

For dealing with uncertainties associated with the site descriptive model, it was shown that the data
used for setting the site descriptive model were derived from an open database in such a way as toensure the quality of the input data for design and safety assessment. The Review Committee
acknowledges that management to ensure the traceability is important for the quality assurance ofhandled data, covering the original data referred to in the development of the database, informationsuch as specific conditions regarding the process for developing the database, and the data after
processing.

·In order to demonstrate the reliability of the Safety Case, with reference to legal requirements andrequirements/criteria related to scientific characteristics as criteria for determining site suitability,
NUMO provided “criteria for judging suitability” in selecting preliminary investigation areas. The
Review Committee recognizes that this basic concept is important and suggests describing it moreclearly.

·

The Review Committee considers it desirable to describe the concept of retrievability clearly fromthe viewpoint of the reliability of the Safety Case.

·

The Review Committee considers it desirable to confirm the meaning and effectiveness of
supplementary indicators such as concentration, time, and flux which complement dose evaluationand to explain carefully the relationship with related parameters such as biosphere concentrations andflux.

4.3 Validity as a basic form for a safety case after site identification·

In the NUMO Safety Case, the basic concept for site selection is shown as a three-stage site
investigation process. The report provides the proposed content to be included in the safety case foreach stage by clarifying which investigation stage each requirement for the geological environment
corresponds to. Therefore, the Review Committee agrees that the NUMO Safety Case, which
considers various site environmental conditions, could be used as a basic form of safety casefollowing site identification.

·

The NUMO Safety Case raises the issue of developing technologies for modeling geologicalenvironment characteristics because the geological environment of Japan is located in a tectonically
active zone which evolves over time. It describes that, in the future, it is planned to improve thereliability of modeling technology for long-term evolution of geological environment characteristics.
Furthermore, it was pointed out that there is not much information about Pre-Neogene sedimentaryrocks. Accordingly, the Review Committee suggests describing these issues in the summary with theaim of being able to say “basic technology is in place”.

·

NUMO proposed issues to be addressed in the future to improve reliability. The Review Committee
considers that, in order to clarify the objectives and goals, it is important to indicate by which site-45-

investigation stage each problem and issue should be solved. This is also linked to when the safetycase will need to be updated. The Review Committee also points out that there was an unsupported
step to the current statement that “the items to be included in the NUMO Safety Case are clear” in thesummary.
·

The Review Committee suggests that the relationship between the basic form of the safety case andthe development of a safety case at each stage of site selection should be described more clearly. For
example, describing how the "rough safety assessment" in the literature survey stage will be
performed by developing the basic form of the safety case is important because it explains how tocustomize the basic form according to the quality and quantity of information obtained at that stage.
Such a viewpoint is considered to contribute to clarifying the relationship between the basic form of
the safety case shown in the NUMO Safety Case and the safety case at each stage of site selection.

4.4 Issues to be solved in the next stage and future research and development·

The Review Committee acknowledges that, in the NUMO Safety Case, a distinction is made between
site-independent issues and site-specific issues, and the research and development to be conducted in
the future is outlined.

·

Improving the reliability of the safety case should continue in the future, and NUMO is in a position
to lead research and development to be carried out in this respect.

In terms of objectives, it isimportant, together with clarifying the distinction between generic and site-specific aspects, toidentify the research and development activities to be addressed in the future and to prepare theinfrastructure for conducting the research. In addition, after a site has been identified, the NUMO
Safety Case will need to be continuously updated based on scientific knowledge from appropriate site
investigations and evaluations. The Review Committee confirms that the NUMO Safety Case wasprepared on the basis of the current technical foundation and with future efforts in mind.
·

As a future issue, it is also important to identify what is currently lacking in the safety case for thestage where there will be narrowing down to actual sites. From this point of view, the NUMO Safety
Case showed that plutonic rocks and Neogene sedimentary rocks had been systematically investigatedin URLs (Mizunami Underground Research Laboratory and Horonobe Underground ResearchCenter), but that the information on Pre-Neogene sedimentary rocks that make up accretionary
complexes was limited. According to the “Nationwide map of scientific features for geologicaldisposal”, Pre-Neogene sedimentary rocks may have favorable characteristics and there is apossibility that they may be distributed in coastal areas which are considered preferable in terms oftransportation. For this reason, the NUMO Safety Case recognizes the need to acquire data for model
construction in the future. The Review Committee agrees that this recognition is an urgent issue for
improving the reliability of future safety cases. It should be noted that, since it is thought that there
are specific problems associated with the case of constructing the repository beneath the seabed near-46-

the coast, the Review Committee suggests adding this case from the viewpoint of repository designand engineering technology.
·

The NUMO Safety Case did not consider that safety is a site-dependent issue as a basic stance.
However, concerning operational safety before the repository is closed, there is safety independent of
the site and that dependent on the site. For example, securing the distance from the facility to the siteboundary is site-dependent, but accident scenarios such as equipment failure and fire are events
independent of the site. Such a viewpoint contributes to understanding the uncertainties even at thepresent stage when a site is not specified. Considering that the operational period of the geologicaldisposal project will be longer than normal experience of developing tunnels and mines, it will be
more important in the future to improve the reliability of the safety case by considering the siteindependent and site-dependent issues from the above viewpoint.

·

Generally, in dealing with uncertainty in the development of the site descriptive model, theuncertainty in investigation and assessment of a geological environment is dependent on thegeological environment itself and on measurement techniques, etc. At the present stage, it is difficult
to deal with the mutual uncertainties uniformly and representative characteristic values wereconservatively set for each potential host rock. In order to improve the reliability of the safety case,
the Review Committee suggests examining how to deal with the uncertainty in site characterizationand evaluation methods. In addition, to improve the reliability of models and evaluations, it will benecessary to continue R&D according to the type of uncertainty.

·

A site descriptive model developed by probabilistically generating faults and fractures is consideredapplicable for evaluating the feasibility of geological disposal or to proceed to the next stage of thesite selection investigations. For the final safety assessment, however, the location and size of large
faults and fractures must be determined by investigations up to that point. In addition, when using amodel based on probabilistically generated faults and fractures in the final safety assessment, it isnecessary to confirm the sufficiency of the data for developing the model and the effectiveness of themodel for safety assessment. Such handling of faults and fractures will be an important issue in the
future when site selection is conducted based on the results of safety assessment using a site
descriptive model developed for the site. The Review Committee agrees that the NUMO Safety Caseconsiders these approaches. However, the Review Committee suggests adding a description of whatkind of future investigations will reduce uncertainty regarding the distribution of faults and fracturesin different stages.

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5. Concluding statement
The Review Committee reviewed the technical reliability of the descriptions in the NUMO Safety Casecompiled as a safety case at a time when no site has been specified, and summarizes the results in thisreview report.
In the NUMO Safety Case, the comprehensive technical results are compiled based on informationacquired from R&D in the underground research laboratories, etc. since the H12 Report and the Second
TRU Report.
In its Safety Case, NUMO describes the following: the technical basis for acquiring geologicalenvironment information required for site selection has been established; the site descriptive model
reflecting the characteristics of the deep underground environment in Japan is presented; the technicalbasis for designing a repository that meets requirements is established, and the technical basis for
conducting safety assessments before and after repository closure is established.
Overall, the NUMO Safety Case is presented with a sufficient level of scientific and technical reliability
as a safety case for the generic stage of site selection, and is consistent with the international framework.
The Review Committee agrees that NUMO has appropriate and sufficient technical capabilities andmethodologies for investigating site characteristics, engineering design and safety assessment to be
conducted in each stage of the site selection process in the future, and is an organization with technicalreliability.
In order to select a site with the geological conditions required to isolate and confine radioactive waste
over long times, NUMO has developed a site descriptive model unique to Japan, reflecting thecharacteristics of the geological structures of the Japanese archipelago which is located at the convergenceboundary of multiple tectonic plates. In the current stage, before selecting sites by integrating thegeological environment information obtained from investigations in the future, and the prospect ofdesign/construction and ensuring safety based on this information, the establishment of the abovementioned site descriptive model unique to Japan by NUMO is an important achievement worthy of
special mention.
The Review Committee hopes that the results of this review will contribute considerably to improving thescientific and technical reliability of the NUMO Safety Case. At the same time, it is hoped that the NUMO
Safety Case will promote trust in NUMO among the public, and will encourage discussions on geologicaldisposal.
For future developments, the Review Committee is concerned with how to make the general public awareof the NUMO Safety Case. In terms of public awareness, it is considered that the descriptions in the report
should be communicated by NUMO to experts in geological disposal, then to general scientists and thento the general public through the media.

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In considering such communication, it is necessary to recognize that the NUMO Safety Case is has been
prepared for experts with some knowledge of geological disposal. For traceability, the report is structured
as an overview, main report and supporting reports, and all references are easily searchable and linked tosubordinate information. It can be said that a very effective approach has been taken in the documentationof the Safety Case, which has enormous amount of information. Regarding transparency, the descriptionsin the report are structured in the form of safety case and are logically clear. Therefore, for experts ingeological disposal, the NUMO Safety Case is sufficiently transparent and traceable.
However, it should be remembered that a document that is transparent to experts in geological disposal
does not necessarily become a document that is transparent to general scientists and engineers. This is thefate of professional technical reports.
Transparency in a safety case refers to attributes such as being written clearly and being understandable tothe intended reader. From this perspective, the NUMO Safety Case can be considered sufficient for thegeological disposal community, but will not necessarily be appropriate for the general scientific andtechnical community. Even in the Review Committee, which included experts considered to be involvedin geological disposal, there were often discussions where there was no consensus.
One reason for this is that geological disposal is a technical undertaking that requires the integration of awide variety of technical fields and the underlying methodology and terminology are different in thesefields. The second reason is that geological disposal is based on unique concepts.
For example, in the disposal project, the concept is that, after the waste is released from active humanmanagement by “disposal”, the status becomes passive and the conditions are controlled by the naturalenvironment and the waste can be isolated and confined for a long time. This seems unfamiliar to many
people other than disposal experts.
The concept of safety assessment after the repository is closed is unfamiliar to the general public.
Predictive methods are the only way to understand the future status of geological disposal systems andradionuclide migration behavior. Thus, there is always uncertainty here, and a complete description isessentially impossible. Recognizing this uncertainty, a scenario that assumes the future state of the systemis established, and an analytical model is applied along with this scenario to evaluate the exposure dose.
This evaluation method is sufficiently effective for safety assessment, and it is considered that safety isreasonably demonstrated. This way of thinking is unique to the safety assessment of geological disposal.
The detail and reliability of the models will improve as the investigations of the geological environment
progress stepwise. The degree of improvement will be determined iteratively by the effectiveness of therepository design and the safety assessment to which the site descriptive model is applied. In addition,
while considering the feasibility of the entire disposal system under the assumed geological environment
represented in a site descriptive model updated according to the progress of site characterization, theperformance required for the repository and the feasibility of design and construction are evaluated.
Furthermore, in the safety assessment, the uncertainty in the prospect of ensuring safety with respect to-49-

the geological environment and repository design is reduced at each stage. Such a stepwise approach basedon cooperation among the areas of geological investigations, repository design, and safety assessment is
unique to geological disposal.
In the future, when using the NUMO Safety Case to promote communication with the scientific andtechnical community (other than geological disposal experts) and with other people, explaining that theabove-mentioned concepts are unique to geological disposal is important.

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References[1] OECD/NEA (2003), SAFIR 2: Belgian R&D Programme on the Deep Disposal of High-level andLong-lived Radioactive Waste, An International Peer Review, ISBN 92-64-18499-6.
[2] OECD/NEA (2004), Safety of Disposal of Spent Fuel, HLW and Long-lived ILW in Switzerland, AnInternational Peer Review of the Post-closure Radioactive Safety Assessment for Disposal in the
Opalinus Clay of the Zürcher Weinland, ISBN 92-64-02063-2.
[3] OECD/NEA (2006), Safety of Geological Disposal of High-level and Long-lived Radioactive Waste
in France, An International Peer Review of the “Dossier 2005 Argile” Concerning Disposal in the
Callovo-Oxfordian Formation, ISBN 92-64-02299-6.
[4] OECD/NEA (2005), International Peer Reviews for Radioactive Waste Management, General
Information and Guidelines, ISBN 92-64-01077-7.
[5] WTO (World Trade Organization) (1995), Agreement on Technical Barriers to Trade.
[6] OECD/NEA (2009), Considering Timescales in the Post-closure Safety of Geological Disposal ofRadioactive Waste, OECD/NEA.
[7] OECD/NEA (2012), Reversibility of Decision and Retrievability of Radioactive Waste –Considerations for National Geological Disposal Programmes, ISBN 978-92-64-99169-9.

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-52-

Appendix

1Members of the Special Review Committee on the NUMO Safety Case

Osamu TOCHIYAMA (Chair)
Nuclear Safety Research Association
Selection and Modeling of Geological Environment
Kenji AMANO

Japan Atomic Energy Agency

Tetsuro HIRONO

Osaka University

Seiichiro IOKA

Hirosaki University

Michito SHIMO

Fukada Geological InstituteKen-ichi YASUE

University of Toyama

Repository Design and Engineering Technology
Takumi HARUNA

Kansai University

Yasuaki ICHIKAWA

Okayama University

Atsushi IIZUKA

Kobe University

Kenichiro NAKARAI

Hiroshima University

Post-closure Safety Assessment
Taishi KOBAYASHI

Kyoto University

Yuichi NIIBORI

Tohoku University

Seiji TAKEDA

Japan Atomic Energy Agency

Keiichiro WAKASUGITokai University

Secretariat
Aki HARADA

Nuclear Safety Research Association

Hirohisa ISHIKAWA

Nuclear Safety Research Association

Sumio MASUDA

Nuclear Safety Research Association

Hirokazu TACHIKAWA Nuclear Safety Research Association-53-