NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
[How to read the table]
The recommendations from International Review Team (IRT) to NUMO described in the OECD/NEA review report are listed in the middle column, and the observations from IRT related tothose recommendations are listed in the left column (section numbers and pages in the review report are listed together).
NUMO's response to each recommendation is listed in the right column.
2. Findings according to the remit of the review
2.1 Safety strategy
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

2.1-1

2.1
p.17

The IRT notes that, overall, NUMO relies on the most up- 2.1
to-date science and technology for methods implemented p.17
in developing safety assessments and safety cases.

The IRT recommends that NUMO continue to establish the In line with the basic strategies described in Chapters 2 and 7 of the NUMO Safety Case,
reliability of geological disposal based on the latest scientific NUMO will continue to collect the latest scientific knowledge from both Japan and abroad
knowledge and on continuous reviews.
and to develop the technology required for implementation of the project. This will befacilitated by the Coordination Council on Research and Development of Geological
Disposal (hereinafter "the Coordination Council") and the Overall Research andDevelopment Plan for the Geological Disposal Program, which will be formulated by the
Coordination Council working with relevant organisations. NUMO will develop a safetycase at project milestones and present technical confidence in geological disposal to
stakeholders. During these technology and safety case development activities, we willobtain expert advice and external reviews to confirm their reliability.

2.1-2

2.1.1
p.17

NUMO relies on regulatory requirements published by 2.1.1
various national authorities in other countries or guidance p.17
from international organisations involved in thedevelopment of geological disposal projects, pending theestablishment of dedicated regulatory requirements inJapan.
The characteristics of the waste forms are broadly definedat this stage. However, waste inventory imposes key
requirements on the waste management programme.

More details on the waste types from Japan’s waste
producers; on their physical, chemical and radiologicalproperties; and on the potential evolution of these inventories
would facilitate NUMO’s continued waste management
work. The IRT recommends carrying out sensitivity analyses
in order to bound the hypotheses and to have morecomprehensive representations of the geological repository’ssafety boundaries and safety assessment. Such a sensitivity
analysis could also help demonstrate the flexibility of thegeological repository design approach and the robustness ofthe safety case, in particular with regard to future changes in
the regulatory framework and the inventory andcharacteristics of waste to be disposed of. This is an asset thata generic safety case could provide. A clear example
concerns the inventory data, not only in terms of volumes ofwaste but also on their nature and evolution (e.g. possibility
to include MOX fuel, to consider other types of spent fuel or
TRU waste), including the durations of storage before
disposal.

1

To evaluate the effects of potential variations in the waste inventory on both design andsafety of the geological repository, NUMO has gathered information on uncertainties in thecharacteristic of vitrified HLW and TRU waste (burnup conditions of spent fuel,reprocessing conditions such as cooling time before reprocessing, storage period after
reprocessing, variability of physical and chemical properties and volume of TRU waste) incooperation with the waste producers. NUMO has also developed methodologies to assess
the characteristics of wastes. NUMO continues these technical developments as mentioned
in the “Overall Research and Development Plan for Geological Disposal Programme(FY2023–FY2027)” (hereinafter "the Overall R&D Plan").
Considering the potential impacts of these uncertainties, a sensitivity analysis using the
NUMO Safety Case as a reference is planned in order to assess the effects on the repository
design and the pre- and post-closure safety assessment. The results will be used for anassessment of the robustness of the disposal concept, repository design optimisation anddevelopment of waste acceptance criteria. Technical issues regarding assessing future waste
characteristics will be identified and required R&D initiated. The results of this work will
be reflected in the future safety case.
R&D on reprocessing of spent MOX fuel is underway. NUMO recognises that potential
impacts on the repository concept and safety assessment due to including wastes generatedby MOX fuel reprocessing should be assessed in the future. We will continue to enhance thereliability and robustness of the geological disposal programme by keeping in mind possible
changes in the waste inventory.

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

2.1-3

2.5.1
p.28

The methodology seeks completeness and a consistent 2.1.2
treatment of uncertainties. Many assumptions leading to p.18
conservatism are consistently utilised. This makes theconclusions robust as regards the potential for long-term
safety, but also reduces the ability to distinguish between
different design and host rock options. The IRT expects that
future models and datasets will be more specific once thenumber of candidate sites/host formations, with associatedrepository designs, is narrowed down.

The IRT recommends that NUMO further highlight theimportance of delaying and mitigating the migration ofradionuclides in the geosphere. The identification andevaluation of related site characteristics could be decisive inthe search for a site.

As no site has been identified at this stage, we focus on a repository design that can be
flexibly adapted to a wide range of geological environments. Thus, in the NUMO Safety
Case, we demonstrated the feasibility of a geological disposal system by considering
geosphere containment performance conservatively, due to large uncertainties in this at thepresent time. However, as the IRT points out, the nuclide migration characteristics of thegeosphere from the host rock around the repository to the biosphere could be a decisive
factor in determining the suitability of a site. Therefore, after the site is specified, especiallyin the Preliminary Investigation and later stages, NUMO will investigate and evaluate key
nuclide migration characteristics of the geosphere, including the uncertainties associatedwith them. NUMO is currently developing a method to analyze three-dimensional nuclide
migration behavior on the large scale extending from underground facilities to the biosphereand continuous refinement of this work is included in the Overall Plan.

2.1-4

2.1.2
p.18

The two key safety functions adopted by NUMO are 2.1.2
isolation and containment. These encompass the p.18
requirements for operational safety as well as for postclosure safety. From these key functions, second or evenlower-level functions are identified that have also beentaken into account to verify or ensure the proper
functioning of the disposal system over time.

In order to assess efficiency of the developed system forgeological disposal, the IRT recommends to NUMO for thefollowing stages of the project development to define safetyfunction indicators or, when relevant, minimum acceptableperformance target, also as a function of time.

In the NUMO Safety Case, dose was evaluated as an indicator to assess long-term, postclosure safety of the geological disposal system. In the future, according to therecommendations from IRT, NUMO will consider performance indicators that can more
efficiently evaluate the functions of each component contributing to the safety of the entire
geological disposal system. For example, we will study application and establishment oftarget values for indicators of rock properties that have a significant impact on thecontainment function of the host rock (e.g., permeability, chemical composition of groundwater), performance indicators associated with components of the engineered barrier system(e.g., lifetime of overpack or container of TRU wastes), and site characteristics such as travel
time from waste package to the biosphere depending on the layout of underground facilities.
These studies are described in the Overall R&D Plan.

2.1-5

2.1.4
p.19

The repository’s design is adapted to the characteristics of 2.1.4
each SDM, aiming to contain radionuclides and limit their p.19
migration. At the current stage, the design approach aims
for robust solutions, offering a sufficient margin given theinherent uncertainties both in the geological data and in theconcepts themselves.

Starting from an initial outline of design solutions, a
representation of the disposal facility architecture and its
components is proposed. As knowledge of the sites isacquired, and regulations and requirements become clearer,the choices of technological solutions are refined. The IRT
suggests keeping the design options open for as long as
possible in order to retain the ability to adapt to potential
evolutions in the project.

NUMO is developing design options such as the development of protective coating foroverpacks and retrievable PEMs for TRU waste, expanding from the design specifications
given in the H12 Report (JNC, 2000) and TRU-2 Report (JAEA and FEPC, 2007). JAEA
(Japan Atomic Energy Agency) is also continuing to study direct disposal of spent fuel andinvestigate deep borehole disposal. NUMO will retain as many design options as possible
in order to secure flexibility to respond to changing circumstances. In addition, NUMO will
study strategies for narrowing down design options and development targets as site selection
progress. We also take into account the possibility that, in the future, the regulatory bodymay require selection of the best design based on comparison of a range of design options.
Furthermore, in order to compare designs, a methodology for selecting the most appropriate
options will be developed, based on established design factors (operational safety, long term
safety, engineering feasibility, retrievability, economic rationality, environmental impact,
monitoring etc.). These technical developments are described in the Overall R&D Plan.

2.1-6

3.2
p.37

The political and social context of the development of 2.1.4
geological disposal projects also has its own requirements p.19
and dynamics that must be taken into account. Management
systems have been put in place to adequately integrate all
the information available at any one time and to reproduce
the studies and their results. NUMO has set up thesesystems to allow it to ensure good control of its projects. It
will be able to continue its studies in later phases, ensuring
adequate quality management.

Requirements
management,
includingknowledge
management, will be key to support future development of
the geological repository project. The IRT notes that theteams in charge of the project promote a systematic searchand use of the best available technology, which is highly
recommendable.

The Overall R&D Plan indicates that NUMO will further strengthen its framework for
identifying evolving requirements as the project progresses, acquiring and producing
knowledge to meet these requirements, and managing intra-generational knowledgepreservation and communication. Through active participation in international projects,
such as NEA's WP-IDKM activities, systematic mining and utilisation of the latest national
and international knowledge and technology will be facilitated. The general use of advanced
knowledge engineering technologies will be promoted, including the development of tools
for these specific purposes.

2

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.
2.1-7

Observations from IRT
2.1.4
p.19

The retrievability of waste is taken into account in the 2.1.4
facility design. The aspect of the reversibility of decisions p.19
is more a matter of management issues that will need to beinvestigated in due course, requiring strong interaction withstakeholders and future decision makers. Monitoring andsurveillance are also well taken into account.

Recommendations from IRT
Monitoring and surveillance are above all intended to ensurethat the geological repository operates under conditions thatcomply with those sought by design. They are therefore also
intended to detect as early as possible any anomaly that could
lead to additional preventive measures in relation to longterm safety. The programme should also be explicitly linked
to the issue of the reversibility of decisions, consistent with
NEA definitions (NEA, 2012a), and the retrievability of
waste packages. Monitoring and surveillance programmes
should be developed and started at an early stage in order toproduce a baseline of the sites being investigated before any
major field work.

3

NUMO’s responses to comments
Since monitoring and surveillance strategies are highly dependent on site-specificconditions, the corresponding repository design, and expected safety features, these are only
briefly outlined in the NUMO Safety Case at this time. More specific and detailed
considerations of these topics will occur after sites are identified. By the time of initiationof the Preliminary Investigation phase, a comprehensive strategy will be developed for eachphase of the project, including baseline monitoring, performance confirmation monitoring
leading to a final closure decision, and post-closure monitoring for the sites to be studied.
In doing so, in line with the Basic Policy on Final Disposal of Designated Radioactive
Wastes (hereinafter “Basic Policy on Final Disposal”) that requires to ensure thereversibility of the project and the retrievability of the waste in order to reserve options for
future generations, we will make these requirements clearly linked to the monitoring andsurveillance strategies, as IRT recommended. Development of the measurement technology
for determining baseline underground characteristics at the site (temperature, groundwaterpressure, water quality, etc.), as required for the first step of the Preliminary Investigation
phase, has already been initiated. Here we will verify the applicability and durability of themonitoring equipment and management of accumulated data through demonstration tests.
Monitoring and surveillance strategies and the development of corresponding technology
are issues that will require particular focus in the future and are clearly stated in the Overall
R&D Plan.

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
2.3 Repository Design Approach
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

2.3-1

2.3
p.24

The IRT recognises the considerable work that NUMO has 2.3.1
already produced to date. NUMO’s work also outlines the p.24
next steps to be addressed when more information on waste
inventory and on potential sites for geological disposal
becomes available.

A major input data for repository design is, as alreadyindicated, the waste inventory. The IRT stresses theimportance of improving the existing knowledge base and ofcontinuing studies on increasingly reliable data. The waste
management strategy before disposal is also decisive
information that will need to be clarified, in particular forheat-emitting waste.

As mentioned in the response to comment 2.1-2, to evaluate the effects of potential
variations in the waste inventory on both design and safety of the geological repository,
NUMO has gathered information on uncertainties in the characteristic of vitrified HLW andTRU waste (burnup conditions of spent fuel, reprocessing conditions such as cooling time
before reprocessing, storage period after reprocessing, variability of physical and chemical
properties and volume of TRU waste) in cooperation with the waste producers. NUMO has
also developed methodologies to assess the characteristics of wastes. NUMO continues
these technical developments as mentioned in the Overall R&D Plan.
Considering the potential impacts of these uncertainties, a sensitivity analysis using the
NUMO Safety Case as a reference is planned in order to assess the effects on the repository
design and the pre- and post-closure safety assessment. The results will be used for anassessment of the robustness of the disposal concept, repository design optimisation anddevelopment of waste acceptance criteria. Technical issues regarding assessing future waste
characteristics will be identified and required R&D initiated. The results of this work will
be reflected in the future safety case.
R&D on reprocessing of spent MOX fuel is underway. NUMO recognises that potential
impacts on the repository concept and safety assessment due to including wastes generatedby MOX fuel reprocessing should be assessed in the future. We will continue to enhance thereliability and robustness of the geological disposal programme by keeping in mind possible
changes in the waste inventory.

2.3-2

2.3
p.24

Tests and studies undertaken, as well as modelling, allow 2.3.1
the progressive characterisation of the phenomena to be p.25
evaluated – facilitating future tasks. Major phenomena
have at this stage been taken into account.

In developing subsequent phases of the project, the IRT
recommends considering more in-depth studies of certain
phenomena such as radiolysis, gas generation andcomprehensive couplings or interactions between barriersystem materials.

2.3.1
p.25

Knowledge of waste behaviour is expected to guide thedevelopment of the repository concepts so that the mainfunctions, particularly in the post-closure phase, aresatisfied. Much data is already available; their major
characteristics, essentially in terms of stability, are taken
into account in a satisfactory manner for the design ofdisposal facilities.

NUMO will continue to develop numerical models for analyzing processes occurring in therepository, supported by laboratory and in-situ experiments, often in cooperation withrelated organisations. This will allow us to deepen our understanding of key processes andto reflect these in developing repository designs and safety assessment scenarios. Key issues
mentioned by the IRT, especially those related to complex processes occurring in the nearfield, are shown below and included in the Overall R&D plan. NUMO will evaluate
potential effects of radiolysis for these issues.
 Improvement of RN release and transport models
 Improvement of models of interaction / coupling between near field components
and processes Further development and assessment of models of gas generation and migration
 Development of evaluation methods for colloids, organics and microorganisms
 Development of nitrate impact models.
NUMO will develop models of such processes and accumulate required model parameters
together with the technology to integrate them in order to more realistically describe theevolution of the near field.

4

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

2.3-3

2.3
p.24

Repository design follows the progressive strategy put in 2.3.2
place by NUMO, with development gradually integrating p.25
knowledge acquired on site and evaluated through safetyassessments. At this stage of NUMO’s SDM-based safetycase, the design bases stem from achievements of previous
studies presented in reports H12 (JNC, 2000a; JNC, 2000b;
AEASJ, 2019) and TRU-2 (JAEA, 2007), after having
verified that requirements and functions sought are met.
They are brought together in a catalogue of options,
including innovative solutions developed by NUMO. The
design bases are adapted to the three studied geologicalformations: Plutonic rocks, Neogene sedimentary rocks
and Pre-Neogene sedimentary rocks.

The IRT draws attention to the fact that the choice between
ramp and shaft may be impacted by the real hydrogeologic
conditions of the host site (e.g. ramp can be more challenging
to implement if permeable water-bearing formations arecrossed).

The choice between ramp (which may be spiral or straight) and shaft, and their layout, will
be designed with due attention to the hydrogeological conditions at the host site, as notedby the IRT. In addition to this, the design of access tunnels will be optimised by consideringvarious aspects such as safety and efficiency of wastes transportation, impacts on the local
environment and construction costs. To implement these, the comprehensive design
methodology based on design factors presented in the NUMO Safety Case will be further
developed.

2.3-4

2.3.4
p.26

The technical design solutions proposed by NUMO make 2.3.4
it possible to reconcile safety requirements, which must in p.26
no way be altered, with retrievability objectives.
Developments and technological tests are still planned inorder to verify the conditions of retrievability, and to
optimise them. NUMO has embarked on an encouraging
technological programme relating to retrievability.

To inform future decisions, the IRT recommends that theobservation and monitoring programme for disposal
facilities explicitly take into account the question ofreversibility, in addition to generating information on theperformance of the geological repository. Finally, thequestion of the future investigation of reversibility needs to
be clarified and proposed for social debate. NUMO should
be proactive in proposing options in this field.

Reversibility is related to the issue of decision-making in society. We will specify whichcases and what options need to be considered for our specific project boundary conditions
and implementation. As pointed out by the IRT, we will proactively develop a dialogue on
this issue with stakeholders, in parallel to the development of related retrieval technologies
and monitoring strategies, as mentioned in the response to comment 2.1-7.

5

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
2.4 Operational Safety Assessment
Observations from IRT

No.
2.4-1

Recommendations from IRT

2.4
p.27

The approach to operational safety is at a preliminary stage. 2.4
The focus of the operational safety assessment at this stage p.27
is the development of concepts and methodologies ofradiological safety assessment, and the evaluation of therobustness of the conceptual design.

p.27

The IRT notes that at the current stage of the programme,
the evaluations remain hypothetical and the realism of theoperational safety assessment would be greatly improved
with concrete site configurations.
2.4
p.27

2.4-2

With respect to the waste inventory, it will be important for
NUMO to identify the radionuclides, determining thepotential impacts during the operating phase. It will then be
possible to verify whether the provisions envisaged in theconceptual design are likely to limit the impacts sufficiently,
or even eliminate them.

NUMO’s response to comments
Waste inventories play an important role not only in repository design and post-closure
safety assessment, but also in operational technology and safety assurance, as noted by the
IRT. In the future, as mentioned in the response to comment 2.1-2, safety measures will befurther studied by examining the impact of inventory variability / uncertainties on theoperational concept, as presented in the NUMO Safety Case.
In addition, with regard to the pre-closure safety assessment methodology presented in the
NUMO Safety Case, we are continuing to develop a methodology to produce morecomprehensive operational safety assessment scenarios and quantitative evaluation
techniques. Together with such assessment development, we will promote the design ofmore robust and resilient operational system.

For future safety cases, the IRT also recommends In the design study for operations, risks during operations will be reduced through theconsidering additional design options for mitigation of stepwise refinement of the repository design and operation methods as site selection
operational risks (e.g. fire).
progresses. The effectiveness of the design and safety measures to the identified risks will
be confirmed using advanced safety assessment methodologies, as described in the responseto the comment 2.4-1. Design options that effectively contribute to reducing the identified
risks should be considered, utilising the latest knowledge at each stage, for example, for firerisks, the use of fire-resistant equipment and incombustible materials.

2.4-3

2.4
p.27

Worker exposure was not calculated as no perturbation 2.4
scenario considered at this preliminary stage resulted in a p.27
loss of containment of radionuclides, based on theassumption of a robust design with remote handlingwherever practical and adequate protection measures. At
this stage, NUMO considers that there are no potential
releases of radionuclides, and only the potential direct
exposure of the public to radiation outside the boundary
fence is analysed.

Quantitative analyses are recommended for assessing theperformance of operational safety measures.

As mentioned in the response to comment 2.4-1, the pre-closure safety assessment
methodology, presented in the NUMO Safety Case, will be continuously improved. We havequantitatively evaluated the effects of some extreme safety assessment scenarios, such as
the impact force on waste packages due to falls, and the thermal effects on waste in the eventof fire. In accordance with the Overall R&D Plan, we will continue to develop safetymeasures for the design options of facilities and equipment for a series of operational
processes, including waste reception, inspection, packaging, transportation in accesstunnels, and emplacement in disposal tunnels, focusing on operational failures withrelatively high risk of releasing radioactive materials. We will also develop a method toquantitatively evaluate the performance of these measures.

2.4-4

2.4
p.27

NUMO plans to extend this analysis once site-specific 2.4
information becomes available, by more comprehensively p.27
considering scenarios leading to a potential release ofradionuclides and their impact on the public near therepository.
The IRT notes that at the current stage of the programme,
the evaluations remain hypothetical and the realism of theoperational safety assessment would be greatly improved
with concrete site configurations.

The IRT also recommends that the safety assessment be
supplemented by including additional scenarios, such as
potential failures of the protections envisaged by theconceptual design, and by including all potential exposure
pathways to workers, to the public and to the environment.

As mentioned in the response to comment 2.4-1, and in line with the Overall R&D Plan, thesafety assessment scenarios considered will be further expanded to improve the coverage ofexternal events such as earthquakes and tsunamis, together with internal perturbing events
that are expected to occur in the repository. In addition, we will develop scenarios forcommon-factor failure modes that could cause loss of multiple safety functions.
We intend also evaluate potential impacts of radionuclide release, even in only as what-if
scenarios. Based on such a more comprehensive scenario setting, we will analyze exposure
pathways to workers and the public, based initially on examples of repository designs
presented in the NUMO Safety Case. This will allow us to study necessary safety countermeasures, as well as utilising safety assessment to plan measures to be taken in the event ofa hypothetical release of radionuclides.

6

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
2.5 Post-closure Safety Assessment
No.
2.5-1

Observations from IRT
2.5.1
p.27

The post-closure safety assessment methodology is well- 2.5.2
designed and informed by best practices and international p.28
recommendations (NEA/IAEA).

2.5.2
p.28

The analysis of the complex systems of a geologicalrepository for radioactive waste, where a multitude ofphenomena have to be taken into account on considerable
spatial and temporal scales, is, in accordance withinternational practice, done by NUMO by defining a set of
evolution scenarios. The scenarios resulting from theapplication of NUMO’s scenario developing methodology
are generally consistent and sound.

2.5-2

2.5.2
p.28

2.5-3

2.5-4

Recommendations from IRT

NUMO’s response to comments

Tools and concepts like integrated FEPs, state variables andimpact analyses are useful in the derivation of scenarios
from FEPs and the IRT encourages the further developmentof these techniques.

NUMO-FEPs are expanded and revised regularly, based on the international state-of-the-art,
and will be updated to reflect the site-specific characteristic when developing site-specificsafety cases. Additionally, the next generation of scenario development tools will improveusability and traceability. These are mentioned in the Overall R&D Plan.

In the bottom up approach, FEPs are screened from a long- 2.5.2
term safety point of view. The starting point of analysis is p.29
the NEA International FEP list (NEA, 2019; NEA, 2020).
This list is adapted on the basis of the specificities of theJapanese programme: FEPs are selected and removed
from, and newly added to, the list in function of theboundary conditions for waste and system characteristics.

All decisions to screen any FEPs out should be
systematically supported by arguments on their irrelevance.
More detailed FEP description in underlying reports should
be referenced, summarising the main arguments in thesafety case main report. This would increase thetransparency and traceability of the screening process forthe next stages.

FEPs that are irrelevant to state variables defining safety functions are screened out in the
NUMO Safety Case. The approach and result of screening are described in Supporting Report
6-5. To improve the transparency and traceability of the screening process, in the future better
descriptions, e.g., further explanation of the reasons for the exclusion of each FEP, will be
provided both in supporting reports and the scenario development section of the main safetycase reports.
The computerised information management tool, which enables tracking relevance andevidence, is under development to integrate the relationships between FEPs and statevariables with information on how these can support development of scenarios, including
associated uncertainties. The practical application of this tool will improve the transparency
and traceability of scenario development, including FEP screening.
In the future, the NUMO FEP list will be provided to the international OECD/NEA FEP
database, so that international experts will be able to view it. This will contribute to NUMO’sarguments that this reflects the latest international knowledge.

2.5.2
p.29

As mentioned above, the set of scenarios appears generally 2.5.2
to be comprehensive at this stage of NUMO’s programme. p.29

A possible additional variant scenario that could beconsidered is one where the buffer is lost due to chemical
erosion in the long term. This, as recognised by NUMO,
could occur if the buffer is exposed to low salinity
groundwater. The bounding lower limit salinities
considered by NUMO are in fact such that erosion may
occur. Future iterations of the safety case could preferably
include a variant scenario where advective rather thandiffusive conditions arise in the buffer.

NUMO is developing a quantitative evaluation method for buffer erosion, supported bylaboratory and/or in-situ experiments, considering the groundwater chemistry and hydrology
in Japan. This is supported by a natural analogue study in a Japanese bentonite mine, as notedin the Overall R&D Plan.
As well as selecting a location for the repository where the potential for buffer erosion is as
low as possible, safety assessment for advective conditions within the buffer will beconsidered in any site-specific case where such a scenario is credible.

2.5.3
p.29

The IRT appreciates the decision to present scenarios in 2.5.3
post-closure system behaviour in the form of storyboards, p.29
similar to several international examples (e.g. France andthe United States). This approach could be extended to all
scenarios and made more selfexplanatory for an external
reader. Segmentation of representations in space is likely tofacilitate the analysis of relevant physical phenomena,
supporting the development of a comprehensive
assessment.

The storyboards are a valuable tool for communication
across different disciplines involved in the description of
the evolution of the repository on a range of spatial scales.
The IRT understands that NUMO intends to further develop
the storyboard technique so that it may, in futureassessments, also contribute in a technical and scientific
way to an improved understanding of the system, and thisdevelopment is encouraged.

NUMO also recognises the storyboards are a valuable tool as the IRT noted. Following theOverall R&D Plan, the storyboard technique is being improved to describe the understandingof system temporal and spatial information in a more detailed and visually-attractive manner.
In addition, storyboards that comprehensively represent the behavior of geological disposal
systems are used to guide the development of computerised tools, described in the response
to comment 2.5-2, that link the processes of scenario development, modelling and dataset of
nuclide migration analysis, which will efficiently manage information related to theirdecision-making and evidence. NUMO will develop this tool in line with the Overall R&D
Plan.

7

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s response to comments

2.5-5

2.5.3
p.29

The IRT notes that the proper consideration of the main 2.5.3
phenomena and processes, at different space and time p.29
scales, shows a good command of analysis in the geologicalrepository project.

During later phases of development, the importance ofphenomena considered secondary at this stage mustsystematically be assessed in relation to the major
processes and phenomena taken into account in the present
assessment. Among the secondary phenomena, the IRT
notes all interactions between materials of different natures,
and also those on a large scale, such as, for example, themodifications of hydrodynamic regimes linked to climatechange. The key to the analysis is assessing whether thefunctions sought at each stage of the repository's life are
maintained. Also, a more comprehensive account of thegradual deterioration in the ability of the components tofulfil their functions would enhance the set of scenarios
analysed.

NUMO recognises that it is necessary to analyse primary and secondary processes in acomprehensive and systematic manner. Secondary phenomena which will occur in therepository are treated rather simply in the NUMO Safety Case, but these will receive more
attention in future safety cases as part of our goal to enhance assessment reliability. In
collaboration with partner organisations, NUMO is developing technologies which evaluate
evolution and degradation processes occurring in near field more realistically (see theresponse to comment 2.3-2). We are also developing a site descriptive model which can
include changes of deep subsurface environments in time and space in a realistic manner (4dimensional site descriptive model).
Generally, NUMO will develop technology for evaluation of interaction between processes
occurring in the repository as specified in the Overall R&D Plan, together with revision of
relevant FEPs (see the response to comment 2.5-1), to improve the reliability of assessmentof long-term safety function evolution.

2.5-6

2.5.4
p.30

The high level of conservatism at this stage of the 2.5.4
assessment is expected to tend towards more realism, based p.30
on less conservative assumptions. To facilitate the analysisof the geological disposal performance, NUMO relied on
the implementation of simplified models which are suitable
for the current generic phase.

In the IRT’s view, a brief summary in the main report of themathematical models and their limitations (assumptions,simplifications) vis-à-vis the conceptual process models
would improve clarity of the safety case. A chart of themodels implemented would also be useful to illustrate thelink between the complexity of the physical processes
analysed and simplifications essential for a macroscopic
representation. Both these measures would make the safetycase clearer for generalists, and also provide context for
specialists involved in detailed analyses.

Based on IRT’s recommendation, NUMO has begun to construct a flowchart which describes
all the models and codes for assessing the THMC evolution of the repository within the
NUMO Safety Case. This includes analysis of radionuclide migration for scenarios developedon the basis of such THMC evolution and resultant calculated dose rates, with explicit
illustration of input and output links between assessment models.
Models and codes developed or improved after the NUMO Safety Case are being added to
the flowchart implemented in NUMO’s information management system for post-closure
safety assessment (see the responses to comments 2.5-2 and 2.5-4). To improve thetransparency of our safety case, NUMO is planning to enhance the flowchart by adding brief
descriptions for all models and codes used to support the safety case. This will becomplemented by information on all safety case models and codes within NUMO’s
information management system.

2.5-7

2.1.1
p.17

NUMO relies on regulatory requirements published by 2.5.4
various national authorities in other countries or guidance p.30
from international organisations involved in thedevelopment of geological disposal projects, pending theestablishment of dedicated regulatory requirements inJapan.

For future assessments, the IRT recommends taking into
account the chemical toxicity aspects of the waste, as wellas the chemical risks associated with non-radioactive
elements. If these aspects are required by future regulationsin Japan, their inclusion would of course be mandatory.

At present, the specifications for some TRU wastes are limited or incomplete, in particularwith regard to chemotoxic components. We plan to extend studies of the risks of radioactive
elements to include the identification of toxic chemical substances, and their release andmigration characteristics, in cooperation with the reprocessing plant operators who produce
this waste. In addition, consideration will be given to trends in domestic and international
regulations on hazardous chemical substances and their handling within safety cases in othercountries, as well as wider consideration of chemical risk assessment methods.

2.5-8

2.5.4
p.30

For radionuclide migration, as well as for the biosphere 2.5.4
model in general, future improvements will mainly be p.30
required in the quality or representativity of data andassumptions.

It will also be appropriate to extend the analysis of transfersin the biosphere by considering the geochemicalmechanisms of reconcentration of chemical forms, forexample at an interface between soil horizons or along
redox fronts.

In order to enhance confidence of reliability biosphere assessment, as recommended,
NUMO will consider geochemical mechanisms that cause reconcentration of chemical forms
at locations such as interfaces between soil horizons or along redox fronts when developing
the current biosphere model further.

2.5-9

2.1.1
p.17

NUMO relies on regulatory requirements published by 2.5.4
various national authorities in other countries or guidance p.30
from international organisations involved in thedevelopment of geological disposal projects, pending theestablishment of dedicated regulatory requirements inJapan.

The potential impact on non-human organisms should also The need for radiological protection of the environment, including non-human species, hascomplete the analysis, in particular if an evaluation of been suggested in recommendations by international organisations such as ICRP, but is not
impact is required by future legislation.
specifically addressed in many international safety regulations and the nuclear regulations inJapan. It is thus not considered in the NUMO Safety Case. However, in the future, we will
pay close attention to international discussions and national and international regulatory
trends regarding impact analyses on non-human biota and introduce such considerations asnecessary.

8

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
2.6 Management systems
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

2.6-1

2.6
p.30

It is recognised by the IRT as being in line with good 2.6
practice, including its inherent set of iterations. It is notably p.30
aimed at ensuring adequate coupling and maintaining
flexibility. Flexibility is seen as a way to adapt the projectto site characteristics, to developments in science andtechnology, and to evolving regulations and societal
demands.
A major type of input data for this management strategy
regards the characteristics and inventory of radioactive
waste, for which knowledge will evolve over time.

The IRT recommends that a system be considered andimplemented in order to co-ordinate well with the authorities
and producers of radioactive waste the fundamental
assumptions on waste inventories on which the whole projectis based.

NUMO has cooperation agreements with the waste producers - Japan Nuclear Fuel Limited(JNFL), JAEA and electric power companies. Based on these agreements, we discuss withthem on how to share information necessary for setting up waste inventories for repository
design and safety assessment (e.g., spent fuel reprocessing conditions and quality records
during waste package production). We also consider with them how to improvemethodologies of waste characterisation and waste package production that contribute torepository safety (e.g., reducing hydrogen gas production associated with radiolysis). We
will strength such collaboration further in the future.
NUMO needs to explain in detail the rationale and processes for setting the reference waste
inventory in the safety cases. We will consider with the government what kind ofcommunication with the regulatory authorities is possible in order to understand their
expectations for this.

2.6-2

3.2
p.37

The political and social context of the development of 2.6
geological disposal projects also has its own requirements p.30
and dynamics that must be taken into account. Management
systems have been put in place to adequately integrate all
the information available at any one time and to reproduce
the studies and their results. NUMO has set up thesesystems to allow it to ensure good control of its projects. It
will be able to continue its studies in later phases, ensuring
adequate quality management.

Given the large number of cases studied within theframework of the geological disposal project, and in order tofind one's way among the different versions, each withdifferent input data and different results, the IRT
recommends that in addition to the requirements
management system, NUMO develop a configuration
management system.

A wide range of studies will need to be carried out in response to various conditions in the
future, such as changes of waste inventory, updates of the Site Describe Model to reflect the
progress of site investigations. We assess impacts on repository design and safetyassessment based on this information. To ensure that changes to the safety cases due to thesestudies are implemented consistently and transparently, and to manage the complexity ofhandling different versions of input data and resulting output from these studies, we agreewith the comment from the IRT that it is necessary to apply a configuration management
system. This will be integrated with requirements management, so that evolving case studies
can be managed in an integrated manner throughout the entire lifecycle. Therefore, we willcontinue to study the development of a system suitable for geological disposal projects,
referring to examples of the application of configuration management systems in generalindustry and related nuclear facilities.

9

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
2.7 Research and Development
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

2.7-1

2.7
p.32

In the presented documents, NUMO lists a large series of 2.7
scientific and technological topics that would deserve p.32
further R&D; however, relatively little mention on key
R&D programme priorities for the next development stage
is made.

The R&D programme calls appropriately for the study ofalternative materials for engineered barriers, in particular forthe reduction of corrosion-induced gas build-up or for costreasons. The IRT is again of the opinion that sensitivity
analyses could help NUMO focus its R&D programme.

To select potential materials for engineered barriers, which is necessary to focus R&D, asmentioned in the response to comment 2.1-4, NUMO agree to specify indicators and criteriathat express sufficiency of design requirements based on sensitivity analysis. NUMO will
progress these activities.

2.7-2

2.3
p.24

Repository design follows the progressive strategy put in 2.7
place by NUMO, with development gradually integrating p.32
knowledge acquired on site and evaluated through safetyassessments. At this stage of NUMO’s SDM-based safetycase, the design bases stem from achievements of previous
studies presented in reports H12 (JNC, 2000a; JNC, 2000b;
AEASJ, 2019) and TRU-2 (JAEA, 2007), after having
verified that requirements and functions sought are met.
They are brought together in a catalogue of options,
including innovative solutions developed by NUMO. The
design bases are adapted to the three studied geologicalformations: Plutonic rocks, Neogene sedimentary rocks
and Pre- Neogene sedimentary rocks.

The IRT also suggests that the research explore alternative
materials better suited to safe disposal in the geologicalmedium to be studied. An interesting point of the programme
concerns the packaging of TRU waste, particularly withregard to gas generation. This aspect, which had also beenraised by the IRT, must be the subject of attention for longterm safety.

As recommended, especially for the TRU waste package, NUMO recognises the importanceof future studies of alternative materials to reduce amount of gas generation and also
alternative packaging containers enhancing confinement in the case of gas pressurisation,
considering applicability of these for relevant geological environments.
For this reason, from the viewpoint of further improvement of safety arguments, NUMO
will continue to study alternative materials for primary engineered barriers, backfill, andsupplementary materials for construction and operation (such as grouting material), aiming
to balance contributions to both operational safety and long-term safety. NUMO will also
monitor international trends in this topic.

2.7-3

2.3.3
p.26

An interesting issue addressed by NUMO relates to the 2.7
arrangement of disposal structures in relation to the p.32
possible characteristics of the sites to be studied. NUMO isalso developing its capacity in this area, which it will beable to mobilise during the next phases of the geologicaldisposal project.

From the facility design point of view, the IRT recommends
integrating
thegeologicalandhydrogeologicalcharacteristics of the site into the general architecture of therepository and the design of its various components.

The features of design options tailored to the hydrogeological environment were illustrated
in the NUMO Safety Case. For example, the PEM has advantages in terms of reducing therisk of erosion of the bentonite buffer in case of water inflow during waste emplacement in
the disposal tunnel, while dead-end type disposal tunnels have more flexibility in terms of
avoiding highly permeable hydrogeological structures. NUMO will develop the facility
design methodology to tailor the various design concepts and their components to thefeatures of geological environment in a more concrete and detailed manner.

2.7-4

2.3.4
p.26

The technical provisions intended to ensure the 2.7
retrievability of disposed waste are taken into account at the p.32
design study stage. Retrievability is a practical way toimplement the reversibility of some decisions, which is a
responsibility of society, including key stakeholders, local
communities as well as NUMO. It is important to consider
what information might be needed for future decisionmaking steps, and which stakeholders should be consulted.

Retrievability is also considered; on this issue, the IRT
recommends integrating disposal monitoring and following
up with relevant parameters in terms of retrievability. The
information generated will be able to feed into decisionmaking in terms of reversibility, an issue that remains to beinvestigated.

NUMO recognises that monitoring as a means of surveying the conditions of the disposal
system can be linked to retrievability, in order to respond if an unacceptable perturbation(i.e., one which exceeds design specification) occurs in the repository before closure.
NUMO will develop a monitoring strategy in accordance with the Overall R&D Plan tomake clear what information should be monitored to assess key conditions of the repository
system. As mentioned to the response to comment 2.3-4, the process of evaluation of therepository performance based on monitoring as part of the decision-making process forprocess reversal will be discussed with stakeholders during such studies.

10

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.
2.7-5

Observations from IRT
2.5.3
p.29

The formulation and illustration of the system behaviour 2.7
through storyboards is informative and useful.
p.32
The IRT appreciates the decision to present scenarios in
post-closure system behaviour in the form of storyboards,
similar to several international examples (e.g. France andthe United States). This approach could be extended to all
scenarios and made more selfexplanatory for an external
reader. Segmentation of representations in space is likely tofacilitate the analysis of relevant physical phenomena,
supporting the development of a comprehensive
assessment.

Recommendations from IRT

NUMO’s responses to comments

Safety assessments are also the subject of the research effort
relating to improved consideration of the phenomenologicalevolution of the disposal system and its modelling, in linewith the opinions of the IRT. It also calls for the improvementof information management tools for the migration ofradionuclides and FEPs, in particular through storyboards.
The examples presented in the SDM-based safety caseillustrate that this work is underway, and the IRT strongly
encourages its continuation.

As mentioned in the response to the comment 2.3-2, NUMO will continue to develop
numerical models for analysing processes occurring in the repository, supported bylaboratory and in-situ experiments, often in cooperation with related organisations. This willallow us to deepen our understanding of key processes and to reflect these in developing
repository designs and safety assessment scenarios. Key issues mentioned by the IRT,
especially those related to complex processes occurring in the near field, are listed below
and included in the Overall R&D plan. NUMO will evaluate potential effects of radiolysis
for these issues.
 Improvement of RN release and transport models
 Improvement of models of interaction / coupling between near field components andprocesses
 Further development and assessment of models of gas generation and migration
 Development of evaluation methods for colloids, organics and microorganisms
 Development of nitrate impact models.
NUMO will develop models of such processes and accumulate required model parameters
together with the technology to integrate them in order to more realistically describe theevolution of the near field.
In addition, as mentioned in the response to comment 2.5-4, storyboards thatcomprehensively represent the behaviour of geological disposal systems are used to guidethe development of computerised tools that link the processes of scenario development,
modelling, and dataset production for radionuclide migration analysis. This also efficiently
manages information related to decision-making and the evidence used to support suchdecisions. NUMO will carry out this work in line with the Overall R&D Plan.

p . 2 9 The IRT notes that the proper consideration of the main
phenomena and processes, at different space and timescales, shows a good command of analysis in the geologicalrepository project. During later phases of development, theimportance of phenomena considered secondary at thisstage must systematically be assessed in relation to themajor processes and phenomena taken into account in thepresent assessment.

2.7-6

2.5.4
p.30

The set of calculations carried out for different disposal 2.7
configurations and conditions illustrates the capability of p.32
NUMO to adequately represent the system for various
conditions, and to carry out the required set of consequencecalculations underpinning a safety case. The numericalmodelling seems mature enough to simulate site-specificsystems, and to consider different disposal configurations
and conditions.

The IRT recommends a sustained effort be made to validate
models and codes intended to evaluate disposal behaviour
and performance. This is all the more important since thespatial and temporal scales in question are outside the usual
scope, and it is therefore essential to strengthen theconfidence of all stakeholders.

As IRT noted, validation of simulation models and codes is crucially important to build
confidence in the safety case. We are developing a systematic methodology of verificationand validation, including both testing against observational data and critical expert review.
According to the Overall R&D Plan, NUMO will continue to expand verification andvalidation of simulation models and codes in the future.

2.7-7

2.7
p.31

The IRT notes the Coordination Council on R&D of 2.7
Geological Disposal between METI, NUMO, JAEA and p.33
other R&D organisations. Additionally, the IRT
understands that NUMO’s primary role is to identify R&D
needs and integrate results in the safety case. The
expression of R&D needs should be a logical result of theSDM-based safety case.

The five-year R&D plan will terminate shortly. The
recommendations resulting from this review could
preferably be integrated in a new forthcoming R&D
programme.

With the completion of the Overall R&D Plan for FY 2018-2022, the subsequent Overall
R&D Plan for FY 2023-2027 was published on the website of the Agency for Natural
Resources and Energy (ANRE) of the Ministry of Economy, Trade and Industry (METI) in
March 2023 (Coordination Council, 2023). In the process of developing the Overall R&D
Plan (FY2023-2027) by the Coordinating Council, the recommendations of the NEA
Review Report on the NUMO Safety Case published in January 2023 were explicitly
considered and acted upon.

11

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.
2.7-8

Observations from IRT
2.7
p.31

The R&D programme for geological disposal in Japan and 2.7
its funding are established under the leadership of METI.
p.33
The IRT notes the Coordination Council on R&D of
Geological Disposal between METI, NUMO, JAEA andother R&D organisations. Additionally, the IRT
understands that NUMO’s primary role is to identify R&D
needs and integrate results in the safety case. The
expression of R&D needs should be a logical result of theSDM-based safety case. In the presented documents,
NUMO lists a large series of scientific and technological
topics that would deserve further R&D; however, relatively
little mention on key R&D programme priorities for the
next development stage is made.

Recommendations from IRT
Moving from a generic safety case to site- and waste-specificsafety case(s) would require strong steering by NUMO on
R&D priorities and their adequacy. To advise METI on the
R&D programme and associated budget, the IRT
recommends that NUMO develop an R&D proposal basedon the current safety case. Such a proposal should clearly
define and substantiate key R&D priorities to inform the next
phase of siting and the next safety case.

12

NUMO’s responses to comments
NUMO identified many important technical issues through the process of developing the
Safety Case and discussed their priority for improving confidence of future safety cases at
Coordination Council meetings, together with METI and related R&D organisations. The
Overall R&D Plan (FY 2018-2022) was revised based on these discussions and NUMO
conducted R&D activities on this basis.
While formulating the Overall R&D Plan (FY2023-FY2027), R&D achievements from theprevious Overall Plan and recent changes related to progress of the program (e.g., theinitiation of the Literature Survey and the Nuclear Regulation Authority's decision on "theConsiderations to ensure nuclear safety in the site selection phases for geological disposal")
have been taken into account. Necessary R&D topics to address recommendations from the
NEA international review have been also included. As recommended by IRT, NUMO willcontinue the process of identifying key issues within developing safety cases and reflecting
these into the Overall R&D Plan via the Coordinating Council. Thus we can appropriately
play our implementer role in steering R&D planning in Japan.

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
3. Conclusion and recommendations
Summary of key observations and recommendations
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3-1

3.1
p.35

The IRT recognises that the SDM-based safety case is an 3
adequate iterative way to describe and integrate existing p.34
knowledge at various time and space scales. The IRT notes
the “more realistic” nature of the SDMs vis-à-vis theprevious safety cases for geological disposal in Japan (JNC,
2000a; JAEA, 2007). NUMO has demonstrated its
capability and maturity in developing a safety case,including the methodologies and tools that will be used forthe assessment at specific sites, which is consistent withinternational practice.

To continue efforts to further increase its understanding of As mentioned in the response to comment 2.3-2, NUMO will continue to develop numericalthe systems in order to be able to analyse them.
models for analyzing processes occurring in the repository, supported by laboratory and insitu experiments, often in cooperation with related organisations. This will allow us todeepen our understanding of key processes and to reflect these in developing repository
designs and safety assessment scenarios. Key issues related to complex processes occurringin the near field are shown below and included in the Overall R&D plan. NUMO will
evaluate potential effects of radiolysis for these issues.
 Improvement of RN release and transport models
 Improvement of models of interaction / coupling between near field components
and processes Further development and assessment of models of gas generation and migration
 Development of evaluation methods for colloids, organics and microorganisms
 Development of nitrate impact models.
NUMO will develop models of such processes and accumulate required model parameters
together with the technology to integrate them in order to more realistically describe theevolution of the near field.
NUMO is also currently developing a method to analyze three-dimensional nuclide
migration behavior on the large scale extending from underground facilities to the biosphereand continuous refinement of this work is included in the Overall Plan (see the response tocomment 2.1-3).

3-2

2.7
p.31

The R&D programme for geological disposal in Japan and 3
its funding are established under the leadership of METI.
p.34
The IRT notes the Coordination Council on R&D of
Geological Disposal between METI, NUMO, JAEA andother R&D organisations. Additionally, the IRT
understands that NUMO’s primary role is to identify R&D
needs and integrate results in the safety case. The
expression of R&D needs should be a logical result of theSDM-based safety case. In the presented documents,
NUMO lists a large series of scientific and technological
topics that would deserve further R&D; however, relatively
little mention on key R&D programme priorities for the
next development stage is made.

To reinforce its role as specifier of R&D studies andintegrator of the results obtained with the support ofspecialised research institutions in Japan. To rely on thelessons of the SDM-based safety case to specify R&D needs,
prioritise them and submit them to the institutions in chargeof conducting studies and research.

As mentioned in the response to comment 2.7-8, NUMO identified many important
technical issues through the process of developing the Safety Case and discussed theirpriority for improving confidence of future safety cases at Coordination Council meetings,
together with METI and related R&D organisations. The Overall R&D Plan (FY 2018-2022)
was revised based on these discussions and NUMO conducted R&D activities on this basis.
While formulating the Overall R&D Plan (FY2023-FY2027), R&D achievements from theprevious Overall Plan and recent changes related to progress of the program (e.g., theinitiation of the Literature Survey and the Nuclear Regulation Authority's decision on "theConsiderations to ensure nuclear safety in the site selection phases for geological disposal")
have been taken into account. Necessary R&D topics to address recommendations from the
NEA international review have been also included. As recommended by IRT, NUMO willcontinue the process of identifying key issues within developing safety cases and reflecting
these into the Overall R&D Plan via the Coordinating Council. Thus we can appropriately
play our implementer role in steering R&D planning in Japan.

3-3

3.1
p.34

NUMO has conducted significant work to produce the 3
SDM-based safety case. This is based on the experience p.34
developed over the years from previous safety cases,
adequate consideration of foreign practices, extensive
R&D, experiments in URLs and NUMO’s sustained
activities within international bodies.

To continue its investments in international activities, bothwith the international bodies, the NEA, IAEA and the EC,
and with the organisations abroad working on geologicaldisposal projects.

International cooperation is an extremely important element in advancing Japan's geologicaldisposal program. NUMO is currently an active participant in most OECD/NEA working
groups and expert groups related to radioactive waste disposal. NUMO is also an active
member of EDRAM, participating in collaboration activities of the leading national
geological disposal implementers. We are also involved in expert meetings and international
conferences organised by the IAEA. In addition, joint research and other activities are
conducted based on bilateral Cooperation Agreements with overseas implementers andresearch institutions. This international network enables us to learn from the latest
developments in other countries, as well as to share the results of technological development
and experiences in dialogue and communication with various stakeholders. We willcertainly continue to participate in such international cooperation and activities.

13

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.
3-4

Observations from IRT
3.3
p.37

The IRT understands that the NRA is in the process ofdeveloping dedicated DGR regulations and would like toemphasise the importance of having a first set of such
regulations as early as possible. International experience
shows that regulatory requirements can and should evolve
as knowledge of disposal increases. With this approach, thefirst set of regulatory requirements would not need to cover
all detailed level requirements related to the DGR safety.
International experience shows that a pre-licensing
dialogue between the implementer and the nuclear
regulatory authority (without compromising theindependency of the regulatory authorities) provides theimplementer with an understanding of regulatory
expectations, and is an important element for successful
DGR development.

Recommendations from IRT
3p.34

NUMO’s responses to comments

To use the NUMO pre-siting SDM-based safety case to guidedialogue with the NRA. The pre-license dialogue’s objective
is to clarify expectations and steps until formal licensing
phases.

In August 2022, NRA issued “the Considerations to ensure nuclear safety in the site
selection phases for geological disposal”. In creating this NRA guide, METI and NUMO
openly exchanged opinions with the NRA, following an invitation from them. We hope thatthe dialogue between NRA and NUMO will continue and be strengthened in the future,
while ensuring that transparency is preserved. In order to enhance dialogue with the NRA,NUMO will actively provide information on how dialogue between regulatory authorities
and implementors is implemented at the international level, as well as on the efforts in thisarea in different countries. Also, NUMO will actively communicate the process of
responding to the IRT recommendations on the NUMO Safety Case, particularly in termsof related R&D plans and expected output, so that opinions on these can be obtained fromthe NRA.

翻訳
版
p.41

3-5

3.3
p.38

Regarding waste inventory and characterisation, the IRT 3
recognises that NUMO works within the boundaries of p.34
Japan’s Final Disposal Act (METI, 2000 and 2015) and
Final Disposal Plan. The IRT notes that the utilised
inventory in the safety case dates back to 2008 and isgeneric in nature.

To organise the exchange between waste producers and
NUMO in order to refine the data on the characteristics andinventory of radioactive waste to be disposed of in thegeological repository. Requested data should also ideallycover potential evolutions of waste inventories andcharacteristics.

As mentioned in the response to commnet 2.6-1, NUMO has cooperation agreements withthe waste producers - Japan Nuclear Fuel Limited (JNFL), JAEA and electric powercompanies. Based on these agreements, we discuss with them on how to share informationnecessary for setting up waste inventories for repository design and safety assessment (e.g.,spent fuel reprocessing conditions and quality records during waste package production).
We also consider with them how to improve methodologies of waste characterisation andwaste package production that contribute to repository safety (e.g., reducing hydrogen gas
production associated with radiolysis). We will strength such collaboration further in the
future.

3-6

3.3
p.38

Based on international experience, a national framework 3
including stepwise licensing and clear roles for different p.34
organisations is a prerequisite for the successful
implementation of a DGR.

To consider, for the next phase of site selection, aninternational peer review (such as an IAEA Artemis mission)
focused on the overall program and regulatory framework of
the DGR, which is useful to support the successful
development of the DGR.

We recognise the value of the suggestion of an international peer review (such as an IAEA
Artemis) focusing on the overall DGR programme. This would include R&D frameworks
covered by the Coordination Council (as described in the response to the comment 2.7-8),
regulatory frameworks and roles of related agencies. We will discuss this further with METI
in the future.

14

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
3.1 General
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.1-1

3.1
p.35

The IRT recognises that NUMO has compiled a sufficiently 3.1
comprehensive safety case for the current programme p.35
stage.

Given the large uncertainties associated with the basis for
NUMO’s SDM-based safety case, including fundamental
programme uncertainties related to the waste inventory andregulatory requirements, the IRT also considers that
NUMO’s programme development could benefit fromextending its safety case with additional sensitivity studies.
Such studies could, for example, cover a range of
inventories, different levels of requirements, data ranges ofsite properties such as rock permeability, or variation oftechnical specification like engineered barrier thicknesses.
Such an approach, used in other countries in the frameworkof pre-licensing studies, would be an effective way toillustrate the flexibility and the robustness of the proposed
geological disposal concepts and of the methodology used in
the safety case.

Based on the experience gained during the development of the NUMO Safety Case, as IRT
noted, it is recognised that future safety cases should include a range of sensitivity analyses.
These will help understand the impacts of the possible range of waste inventories, geologicalenvironmental characteristics (such as permeability of the rock that can have a significant
impact on radionuclide migration), and engineered barrier specifications on performance ofthe geological disposal system as a whole, or of individual components of this. Such studies
will demonstrate flexibility and robustness regarding the concept of geological disposal andthe methodology used in the safety case and we will continue to develop the technical basis
for this purpose.
Therefore, we will continue to study indicators that can evaluate the performance of eachcomponent in terms of the safety function of the overall system (as described in the response
to comment 2.1-4) and to develop analytical models that can more realistically evaluate
system performance without excessive conservatism (as described in the response tocomment 2.3-2).

3.1-2

3.1
p.35

The IRT recognises that the SDM-based safety case is an 3.1
adequate iterative way to describe and integrate existing p.35
knowledge at various time and space scales. The IRT notes
the “more realistic” nature of the SDMs vis-à-vis theprevious safety cases for geological disposal in Japan (JNC,
2000a; JAEA, 2007). NUMO has demonstrated its
capability and maturity in developing a safety case,including the methodologies and tools that will be used forthe assessment at specific sites, which is consistent withinternational practice.

The IRT’s recommendation to NUMO is to continue its
efforts and further increase its understanding of the systems
in order to be able to analyse repository performance to an
adequate level for future decision-making. To do this,NUMO should have access to key capabilities nationally andcontinue to draw on the potential of international
collaboration. The IRT encourages NUMO to continue its
investments in international activities, both with thereference bodies represented by the NEA and the IAEA andwith organisations abroad working with geological disposal
projects.

As mentioned in the response to comment 3-1, NUMO will continue to develop numericalmodels for analyzing processes occurring in the repository, supported by laboratory and insitu experiments, often in cooperation with related organisations. This will allow us todeepen our understanding of key processes and to reflect these in developing repository
designs and safety assessment scenarios. Key issues related to complex processes occurringin the near field are included in the Overall R&D plan.
For this, as IRT noted, international cooperation is extremely important. As mentioned in
the response to comment 3-3, NUMO is currently an active participant in most OECD/NEA
working groups and expert groups related to radioactive waste disposal. NUMO is also an
active member of EDRAM, participating in collaboration activities of the leading national
geological disposal implementers. We are also involved in expert meetings and international
conferences organised by the IAEA. In addition, joint research and other activities are
conducted based on bilateral Cooperation Agreements with overseas implementers andresearch institutions. This international network enables us to learn from the latest
developments in other countries, as well as to share the results of technological development
and experiences in dialogue and communication with various stakeholders. We willcertainly continue to participate in such international cooperation and activities.

15

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
3.3 Key points related to the safety case framework
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.3-1

3.3
p.37

The IRT understands that the NRA is in the process of 3.3
developing dedicated DGR regulations and would like to p.38
emphasise the importance of having a first set of such
regulations as early as possible. International experience
shows that regulatory requirements can and should evolve
as knowledge of disposal increases. With this approach, thefirst set of regulatory requirements would not need to cover
all detailed level requirements related to the DGR safety.
International experience shows that a pre-licensing
dialogue between the implementer and the nuclear
regulatory authority (without compromising theindependency of the regulatory authorities) provides theimplementer with an understanding of regulatory
expectations, and is an important element for successful
DGR development.

The IRT recommends that NUMO proactively engage with
the necessary organisations for the establishment of a prelicensing dialogue with the regulator based on the current
safety case. Such exchanges prior to the development of thesuccessive safety cases would reduce the risk of NUMO’s
future work deviating from the regulator's expectations.

3.3-2

3.3
p.38

Regarding waste inventory and characterisation, the IRT 3.3
recognises that NUMO works within the boundaries of p.38
Japan’s Final Disposal Act (METI, 2000 and 2015) and
Final Disposal Plan. The IRT notes that the utilised
inventory in the safety case dates back to 2008 and isgeneric in nature.

The IRT recommends that this be updated, taking into NUMO will design a DGR that can accommodate the required volumes of the specified
consideration the full spectrum of current waste inventory wastes, which are currently more than 40,000 canisters of HLW (vitrified waste) and more
and possible future waste from different streams.
than 19,000m3 of TRU waste, defined by the Designated Radioactive Waste Final Disposal
Plan (hereinafter “Final Disposal Plan”). Regarding the details of the waste inventory andits characteristics (as indicated in the responses to comments 2.1-2 and 2.3-1), NUMO
promotes information exchange with the reprocessing operators and waste producers inorder to further develop methodologies for estimation of inventories and will improve ourknowledge base by making it more realistic. NUMO will also examine the possibility ofchanges in waste inventories and specific waste characteristics that could result from plans
for the future use of nuclear power.

3.3-3

3.2
p.37

Numerous studies in support of the safety demonstration, 3.3
and the resulting models, illustrate NUMO’s ability to p.38
represent geological disposal and to describe and analyse
its possible evolutions. Here again, the limitations of theexercise are due to its generic nature. This results in highlevels of uncertainty: however, many of these will be
gradually reduced over the successive phases ofinvestigation. Phenomena considered non-determining at
this stage can be analysed during the later phases of thedevelopment of the geological disposal project. In addition,certain aspects need to be clarified independent of site data.
These are related specifically to the regulatory
requirements and the inventory data of the waste that will
have to be disposed of. As the implementer and considering
its experience, NUMO has an important role in analysing
and specifying these needs and informing the authorities
and waste generators, which will have to establish
requirements and specify waste inventories.

The IRT also recommends that NUMO carry out sensitivity
studies regarding the consequences of potential evolutions in
the inventory in terms of e.g. design, safety and disposability.
This will make it possible to identify the areas where
uncertainty needs to be reduced most and to test therobustness and the flexibility of the safety case methodology,
design options and repository capacity. Such sensitivity
studies would also foster a dialogue with the waste producers(i.e. the JNFL, JAEA and utility companies in NUMO’s case)regarding the optimisation and disposability of potential
waste types to be disposed of by NUMO. Such exploratory
analyses would also be useful in specifying the limits ofinventories to be retained for future geological disposal
studies.

16

As mentioned in the response to comment 3-4, in August 2022, NRA issued “theConsiderations to ensure nuclear safety in the site selection phases for geological disposal”.
In creating this NRA guide, METI and NUMO openly exchanged opinions with the NRA,
following an invitation from them. We hope that the dialogue between NRA and NUMO
will continue and be strengthened in the future, while ensuring that transparency ispreserved. In order to enhance dialogue with the NRA, NUMO will actively provide
information on how dialogue between regulatory authorities and implementors is
implemented at the international level, as well as on the efforts in this area in different
countries. Also, NUMO will actively communicate the process of responding to the IRT
recommendations on the NUMO Safety Case, particularly in terms of related R&D plans
and expected output, so that opinions on these can be obtained from the NRA.

As noted in the response to comment 2.1-2 and 3.1-1, and as recommended by IRT, futuresafety cases will include sensitivity analyses on the impacts of inventory changes on thedesign, safety and practicality of implementation of the repository system. This will allow
us to identify areas where uncertainty needs to be reduced the most. It would also provide
input for the robustness and flexibility of the methodologies and design options used todevelop future safety cases, as well as with respect to repository capacity.
In addition, NUMO will further promote collaboration with waste producers to develop
optimisation of future waste management and establish waste acceptance criteria based onsuch sensitivity analyses.

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.3-4

3.2
p.37

Having reviewed the various points necessary for the 3.3
realisation of geological disposal of radioactive waste, and p.38
taking into account the geological context of Japan, the IRT
considers that elements of its feasibility have been
demonstrated. The fact remains that many studies are stillnecessary for the next steps, in particular according to thecharacteristics of the sites that will be considered. NUMO
has demonstrated its ability to fulfil such a mission.

Moving from a generic safety case to site- and waste-specificsafety case(s) would require strong steering from NUMO on
R&D priorities. The IRT recommends that NUMO develop
an R&D proposal based on the current safety case in order toadvise METI on the nationwide R&D programme for DGRs.
Such a proposal should clearly define and substantiate key
R&D priorities to inform the next phase of siting and the next
safety case. According to international experience, animplementer (in this case, NUMO) should indeed have amajor responsibility in defining and steering the R&D for
geological disposal.

As mentioned in the response to comment 2.7-8, NUMO identified many important
technical issues through the process of developing the Safety Case and discussed theirpriority for improving confidence of future safety cases at Coordination Council meetings,
together with METI and related R&D organisations. The Overall R&D Plan (FY 2018-2022)
was revised based on these discussions and NUMO conducted R&D activities on this basis.
While formulating the Overall R&D Plan (FY2023-FY2027), R&D achievements from theprevious Overall Plan and recent changes related to progress of the program (e.g., theinitiation of the Literature Survey and the Nuclear Regulation Authority's decision on "theConsiderations to ensure nuclear safety in the site selection phases for geological disposal")
have been taken into account. Necessary R&D topics to address recommendations from the
NEA international review have been also included. As recommended by IRT, NUMO willcontinue the process of identifying key issues within developing safety cases and reflecting
these into the Overall R&D Plan via the Coordinating Council. Thus, we can appropriately
play our implementer role in steering R&D planning in Japan.

3.3-5

3.3
p.38

Based on international experience, a national framework 3.3
including stepwise licensing and clear roles for different p.38
organisations is a prerequisite for the successful
implementation of a DGR.

The IRT focused its peer review work on NUMO’s genericsafety case, but observed the above-mentioned elements of
the national framework that might need enhancement. To
further evaluate these aspects, the IRT also notes that aninternational peer review (such as an IAEA Artemis mission)
focused on the overall DGR programme and regulatory
framework would be useful and would support the successful
development of the DGR.

We recognise the value of the suggestion of an international peer review (such as an IAEA
Artemis) focusing on the overall DGR programme. This would include R&D frameworks
covered by the Coordination Council (as described in the response to the comment 2.7-8),
regulatory frameworks and roles of related agencies. We will discuss this further with METI
in the future.

17

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
3.4 Technical recommendations to NUMO
No.
3.4-1

Observations from IRT
3.4
p.39

Recommendations from IRTNUMO has carried out an in-depth analysis of the safety- 3.4
related functions expected for a geological repository.
p.39

p.39

NUMO’s responses to comments(Summary in the box in the page 39)
Safety-related functions expected for a geologicalrepository: for future phases, where possible, to match thesafety functions with quantitative performance indicators
and, when possible, with criteria for acceptable performance.
(In the text)
NUMO has carried out an in-depth analysis of the safetyrelated functions expected for a geological repository. For
future phases, the IRT recommends, where possible,
matching the safety functions with quantitative performanceindicators and criteria for acceptable performance.

As mentioned in the response to Comment 2.1-4, in the future, consistent with therecommendations from IRT, NUMO will consider performance indicators that can more
efficiently evaluate the functions of each component contributing to the safety of the entire
geological disposal system. For example, we will study application and establishment oftarget values for indicators of rock properties that have a significant impact on thecontainment function of the host rock (e.g., permeability, chemical composition of groundwater), performance indicators associated with components of the engineered barrier system(e.g., lifetime of overpack or container of TRU wastes), and site characteristics such as travel
time from the waste package to the biosphere as a function of the layout of undergroundfacilities. These studies are described in the Overall R&D Plan.

3.4-2

2.2
p.20

It is worth noting that one of the objectives of the 3.4
Preliminary and Detailed Investigations stages is to p.39
iteratively improve state-of-the-art knowledge for the
relevant scientific disciplines and related technologies sothat the site suitability will be ensured for operational andpost-closure safety, based on a tailored design of therepository. It is meant to reduce the residual uncertainties
through the implementation process, guaranteeing
efficiency and cost-effectiveness. NUMO’s approach tointegrating current geoscientific knowledge into a modelof the host rock and its geological environment istraceable, well-documented and well-illustrated.

The IRT encourages NUMO to continue using the SDM
approach to iteratively integrate geoscientific knowledge as
it is acquired during the different stages of the site-selection
process. Such an approach will ensure a coherent andmultidisciplinary description of the different volunteer sites.

Based on the developed generic SDMs for three rock types, which are widely distributed atrelevant depths in Japan, these SDMs will be continuously improved to reflect the latest
scientific knowledge. This provides a basis for site specific SDMs to be developed by
iterating synthesis of specific site information obtained during characterization during the
stepwise site-selection process, as describing in Chapters 2 and 7 of the NUMO Safety Case.
Such structured procedures are applicable to all volunteer sites, and so are consistent with
IRT recommendations.

3.4-3

3.2
p.36

Though appropriate at this stage of DGR development, the 3.4
SDMs remain generic in nature; the introduced p.39
conservativeness and arbitrary choices may therefore
obscure specific rock characteristics. Future challenges
include ensuring the capacity of sites located in the
territory of voluntary municipalities to provide the key
safety functions: namely, according to NUMO p.40
terminology, isolation and containment. With theknowledge already available, NUMO is able to mobilise
teams capable of pursuing increasingly detailed site
investigations and assessments. It is also able to specify
the needs for additional knowledge on the sites, inparticular through performance and safety assessments ofindividual sites. The IRT considers that, at this stage, thetools and technologies for field data acquisition and fortheir processing are available.

(Summary in the box in the page 39)
Capacity of the geological medium to provide an appropriatedelay of radionuclide transport: to be taken into account more
specifically during site evaluations. This may prove to be adiscriminating criterion in the event several volunteer
communities arise.
(In the text)
The capacity of the geological medium to provide anappropriate delay of radionuclide transport will have to betaken into account in more detail during site-specificevaluations, and this may prove to be a discriminating
criterion.

As mentioned in the response to comment 2.1-3, because no site has been identified at thisstage, we focus on a repository design that can be flexibly adapted to a wide range of
geological environments. Thus, in the NUMO Safety Case, we demonstrated the feasibilityof a geological disposal system by considering geosphere containment performance
conservatively, due to large uncertainties in this at the present time. However, as the IRT
points out, the nuclide migration characteristics of the geosphere from the host rock aroundthe repository to the biosphere could be a decisive factor in determining the suitability of asite. Therefore, after the site is specified, especially in the Preliminary Investigation andlater stages, NUMO will investigate and evaluate key nuclide migration characteristics ofthe geosphere, including the uncertainties associated with them. NUMO is currently
developing a method to analyze three-dimensional nuclide migration behavior on the large
scale extending from underground facilities to the biosphere and continuous refinement of
this work is included in the Overall Plan.

18

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.4-4

2.1.4
p.19

The repository’s design is adapted to the characteristics of 3.4
each SDM, aiming to contain radionuclides and limit their p.39
migration. At the current stage, the design approach aims
for robust solutions, offering a sufficient margin given theinherent uncertainties both in the geological data and in the p.40
concepts themselves.

(Summary in the box in the page 39)
Design options: to be kept open as long as possible in orderto keep flexibility in design as additional knowledge isacquired.
(In the text)
In terms of repository design, and given the uncertainties
inherent in the system, the IRT encourages NUMO’s plans tokeep the various options open as long as possible in order tokeep flexibility of design evolution as additional knowledge
is acquired.

As mentioned in the response to comment 2.1-5, NUMO is developing design options suchas the development of protective coating for overpacks and retrievable PEMs for TRU
waste, expanding from the design specifications given in the H12 and TRU-2 Reports. JAEA
(Japan Atomic Energy Agency) is also continuing to study direct disposal of spent fuel andinvestigate deep borehole disposal. NUMO will retain as many design options as possible
in order to secure flexibility to respond to changing circumstances. In addition, NUMO will
study strategies for narrowing down design options and development targets as site selection
progress. We also take into account the possibility that, in the future, the regulatory bodymay require selection of the best design based on comparison of a range of design options.
Furthermore, in order to compare designs, a methodology for selecting the most appropriate
options will be developed, based on established design factors (operational safety, long term
safety, engineering feasibility, retrievability, economic rationality, environmental impact,
monitoring etc.). These technical developments are described in the Overall R&D Plan.

3.4-5

3.2
p.37

The political and social context of the development of 3.4
geological disposal projects also has its own requirements p.39
and dynamics that must be taken into account.
Management systems have been put in place to adequately
integrate all the information available at any one time and p.40
to reproduce the studies and their results. NUMO has set
up these systems to allow it to ensure good control of its
projects. It will be able to continue its studies in later
phases, ensuring adequate quality management.

(Summary in the box in the page 39)
Flexibility of design options requires monitoring of changes,
which can be ensured using a configuration management
system.
(In the text)
Such flexibility also requires monitoring of changes, which
can be ensured using a configuration management system,
also covering the various interdependencies between
knowledge of the waste and the geological environment,
design of the disposal facilities, relevant phenomena, and theintegration of these aspects through safety studies.

As mentioned in the response to comment 2.6-3, a wide range of studies will need to be
carried out for various conditions in the future, such as the change of waste inventory,
updating of the Site Descriptive Model to reflect the progress of site investigations, and theimplementation of repository design and safety assessment based on this information. To
ensure that changes to the safety cases due to these studies are implemented consistently
throughout the entire process, and to manage the complexity of the series of input data andversions of results used in the studies, we agree with the comment from the IRT that it isnecessary to apply a configuration management system together with requirements
management, so that these can be managed in an integrated manner throughout the entire
lifecycle. At a later stage, we will apply a configuration management system that will
include monitoring, relating to changes of the SDM and developed design options tailored
to it, which would ensure appropriate integration of such changes into future safetyassessments.

3.4-6

3.1
p.35

The approach for reversibility (NEA, 2012a) and stepwise 3.4
decision-making regarding DGR is not reported. It could p.39
benefit from a comprehensive description of the process
leading to reverse decisions.

(Summary in the box in the page 39)
Reversibility of disposal decisions: to make proposals, inparticular by proposing decision-making processes that canmark out the life of the geological disposal, and keeping
options open for future generations.

As stated in the NUMO Safety Case, the Basic Policy for Final Disposal requires that the
project be reversible and the waste package retrievable until the repository is closed in orderto preserve options for future generations. In order to assure practicality of reversal ingeological disposal projects, it is necessary to develop a decision-making process that
determines how the project will ultimately be terminated (lifecycle decisions), while
retaining options for future generations. Based on this, it is necessary to clarify how to
implement the plan (policy of reversibility including retrievability) at each decision
milestone. We recognise the need for consensus building in the decision-making process
and associated reversibility requirements, through dialogue involving various stakeholders,
and that NUMO, as the implementer, has an important role to play in making concrete
proposals to form the basis for such dialogue. In considering the decision-making process
and the proposed approach to reversibility, various scenarios should be considered, taking
into account the long project period and acknowledging that the scientific, technological,
and social boundary conditions for the project will change with time.

19

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.4-7

2.1.4
p.19

The retrievability of waste is taken into account in the 3.4
facility design. The aspect of the reversibility of decisions p.39
is more a matter of management issues that will need to beinvestigated in due course, requiring strong interaction
p.40
with stakeholders and future decision makers. Monitoringand surveillance are also well taken into account.

(Summary in the box in the page 39)
Monitoring: to integrate retrievability into the objectives ofmonitoring,(In the text)
NUMO addresses well the retrievability of waste in disposal
and takes this into consideration in the repository design
approach. The IRT recommends integration of monitoring
aspects into the reversibility and retrievability objectives.

Information and data are necessary to determine required reversibility / retrievability in thedecision-making process for a DGR programme (see the response to comment 3.4-6). As
mentioned in the responses to comments 2.1-7 and 2.3-4, it is important to relate
reversibility and retrievability to a comprehensive monitoring strategy, which includes
determination of changes in the condition of the geological environment, the engineered
barrier systems and underground facilities. This needs to extend from the site investigations
through construction, operation and closure of the repository, and even feasibility at somepoint after repository closure. Studying such a comprehensive monitoring strategy is
explicitly described in the Overall R&D Plan and will be done in coordination with the studyof the decision-making process, as described in the response to comment 3.4-6.

3.4-8

3.2
p.37

The political and social context of the development of 3.4
geological disposal projects also has its own requirements p.40
and dynamics that must be taken into account.
Management systems have been put in place to adequately
integrate all the information available at any one time andto reproduce the studies and their results. NUMO has set
up these systems to allow it to ensure good control of its
projects. It will be able to continue its studies in later
phases, ensuring adequate quality management.

An integrated management system is in place to support theefficient implementation of activities. In order to properly
trace the numerous case studies and calculations, each with
their own sets of data and hypotheses, the IRT recommends,
as mentioned above, the implementation of a configuration
management as part of the overall management system. The
configuration management should be coupled withrequirement management and enable recording of the various
technical options studied as well as the sets of calculations
associated with each of them. The scope of configuration
management should be adequately adjusted, considering thenature of the DGR and the phase of implementation.

As mentioned in the response to comment 2.6-2, a wide range of studies will need to be
carried out in response to various conditions in the future, such as changes of waste
inventory, updates of the Site Describe Model to reflect the progress of site investigations.
We assess impacts on repository design and safety assessment based on this information. To
ensure that changes to the safety cases due to these studies are implemented consistently
and transparently, and to manage the complexity of handling different versions of input data
and resulting output from these studies, we agree with the comment from the IRT that it isnecessary to apply a configuration management system. This will be integrated withrequirements management, so that evolving case studies can be managed in an integrated
manner throughout the entire lifecycle. Therefore, we will continue to study thedevelopment of a system suitable for geological disposal projects, referring to examples of
the application of configuration management systems in general industry and related nuclear
facilities.

3.4-9

3.2
p.36

NUMO has demonstrated its ability to conduct safety 3.4
studies. NUMO conducts its safety assessments and draws p.39
up its safety cases consistent with international
recommendations and practices, in particular those
proposed by the NEA (NEA, 2005) and the IAEA (IAEA,
2012) as well as with other international practices. p.40
Numerous studies in support of the safety demonstration,and the resulting models, illustrate NUMO’s ability torepresent geological disposal and to describe and analyse
its possible evolutions. Here again, the limitations of theexercise are due to its generic nature. This results in highlevels of uncertainty: however, many of these will be
gradually reduced over the successive phases ofinvestigation.

(Summary in the box in the page 39)
Additional phenomena: to explore the phenomena
considered at this stage as non-dominant or non-determining
in order to assess how they should or should not be taken into
account in subsequent phases.
(In the text)
The design of the repository is, at this early stage,accompanied by numerous uncertainties. NUMO has
considered major phenomena likely to affect the evolution of
the repository and therefore its safety. For subsequent phases,
the IRT sees a need to evaluate also phenomena that have at
the present stage been disregarded with the justification thatthey are of secondary importance for the system evolution.

As mentioned in the response to comment 2.5-5, NUMO recognises that it is necessary toanalyse primary and secondary processes in a comprehensive and systematic manner.
Secondary phenomena which will occur in the repository are treated rather simply in the
NUMO Safety Case, but these will receive more attention in future safety cases as part ofour goal to enhance assessment reliability. In collaboration with partner organisations,
NUMO is developing technologies which evaluate evolution and degradation processes
occurring in near field more realistically (see the response to comment 2.3-2). We are also
developing a site descriptive model which can include changes of deep subsurface
environments in time and space in a realistic manner (4-dimensional site descriptive model).
Generally, NUMO will develop technology for evaluation of interaction between processes
occurring in the repository as specified in the Overall R&D Plan, together with revision of
relevant FEPs (see the response to comment 2.5-1), to improve the reliability of assessmentof long-term safety function evolution.

20

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.4-103.1
p.34

NUMO benefits from national experience, notably in 3.4
terms of radioactive waste and studies in underground p.39
laboratories. It also has access to many institutions able tosupport it in its scientific and technological developments,
whether geological, physical, chemical or digital. NUMO p.40
also has access to underground working technologies andnuclear technologies that could effectively contribute to
the development of the geological disposal project inJapan.

(Summary in the box in the page 39)
Handling and emplacement of waste packages in disposal
tunnels/vaults: to be studied, with tests in an undergroundenvironment.
(In the text)
Studies on disposal packaging should continue, in particularwith handling and emplacement tests in an undergroundenvironment.

Demonstration tests of handling and emplacement of an overpack encapsulating simulated
vitrified waste in the underground environment have been conducted by JAEA at 350 m
depth at the Horonobe URL. NUMO will determine our needs for such demonstration tests
in the underground environment when planning operational process confirmations to be
carried out by related domestic R&D organisations and reflect them in the future Overall
R&D Plans. In addition, NUMO will analyse a wide range of information about
demonstration tests of emplacement for waste similar to encapsulated vitrified waste or
TRU waste packages, which are planned in underground research laboratories in Japan andoverseas. NUMO will, to the extent possible, also participate in such collaborative research.
In the second half of the Detailed Investigation stage, NUMO will demonstrate thetechniques for handling and emplacement of vitrified waste the overpack and TRU waste
package to confirm quality control processes in the specific underground environment atselected sites. Such knowledge will feedback for improvement of handling andemplacement techniques.

3.4-11

3.2
p.36

Disposal design, while again generic by nature, also 3.4
appears to be at an advanced stage of maturity. It builds on p.39
international experience, but is also based on targeted
developments, as illustrated by the concept of PEM. The
studies carried out and the tests and models show that the p.40
needed technologies are available, and, above all, that theteams are able to develop them to meet the specific needs
that could arise during subsequent phases of the geologicaldisposal project. One of the major challenges, when realsites are studied, will consist in adapting the concepts to
the real characteristics of the geological environment inorder to ensure post-closure safety over a very long-timeframe.

(Summary in the box in the page 39)
Design developments: to be continued, particularly with aview to adapting to the characteristics of the sites that will beconsidered for future studies.
(In the text)
The IRT encourages NUMO to continue its design
developments, particularly with a view to adapting to thecharacteristics of the sites that will be considered for futurestudies.

The design methodology developed in the NUMO Safety Case will be applied to thegeological characteristics obtained from site investigations and modified to meet sitespecific challenges. Repository design tailoring to the geological environment conditions
will be performed step-by-step, with iterative feedback to the site investigation. NUMO willcontinue to develop the design system based on the methodology presented in the NUMO
Safety Case, which is stated clearly in the Overall R&D Plan.

21

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.
3.4-12

Observations from IRT
3.4
p.40

Recommendations from IRT

The approach to operational safety is also at a preliminary 3.4
stage. The IRT acknowledges NUMO’s development of a p.39
preliminary assessment at the current stage of the
programme, based on generic assumptions and an early
conceptual design.
p.40

3.4-133.4
p.40

The long-term safety assessment is based on many 3.4
assumptions that are considered very conservative at this p.39
stage. Consequently, the results of the different cases
studied are often quite similar, and do not provide moredetailed information for discriminating among design
options and/or potential host geological formations.
p.40

NUMO’s responses to comments(Summary in the box in the page 39)
Operational safety: to be supplemented by design features
allowing the reduction of operational risks, and operationalsafety assessment to be supplemented by taking into accountpossible failures of the protections envisaged by design andby analysing resultant doses to workers and to the public.
(In the text)
The approach to operational safety is also at a preliminary
stage. The IRT acknowledges NUMO’s development of apreliminary assessment at the current stage of the
programme, based on generic assumptions and an early
conceptual design. For future safety cases, the IRT
recommends considering additional design options for
mitigation of operational risks (e.g. fire). The IRT also
recommends that the safety assessment be supplemented byincluding additional scenarios, such as potential failures ofthe protections envisaged by design and by including all
potential exposure pathways to workers, to the public and to
the environment.

As mentioned in the response to comment 2.4-2, in the design study for operations, risks
during operations will be reduced through the stepwise refinement of the repository designand operational methodology as site selection progresses. The effectiveness of the design,and safety measures to counter any identified risks, will be confirmed using advanced safetyassessment methodologies, as described in the response to comment 2.4-1. Design options
that effectively contribute to reducing identified risks should be considered, utilising thelatest knowledge at each project stage. Assessment will include facilities and equipment fora series of operational processes, including waste reception, inspection, packaging,transportation in access tunnels, and emplacement in disposal tunnels. For example,
identified counter-measures would use of fire-resistant equipment and incombustible
materials to combat risks of fires.
As mentioned in our response to comment 2.4-4, and in line with the Overall R&D Plan,
the safety assessment scenarios considered will be further expanded to improve the coverageof external events such as earthquakes and tsunamis, together with internal perturbing events
that are expected to occur in the repository. In addition, we will develop scenarios forcommon-mode failures that could cause loss of multiple safety functions. We intend to also
evaluate potential impacts of radionuclide release, even if only as what-if scenarios. We will
also develop a method to quantitatively evaluate the impact based on the safety assessment
scenarios considered (see the response to comment 2.4-3).
We will analyse exposure pathways for both workers and the general public, based initiallyon examples of repository designs presented in the NUMO Safety Case. This will allow us
to study necessary safety counter-measures, as well as utilising safety assessment to planmeasures to be taken in the event of a hypothetical release of radionuclides.

(Summary in the box in the page 39)
Design and construction procedures to be adapted to theparticular characteristics of the geological formations
envisaged for the following stages in a way that allows
contrast between formations being reflected in the design.
(In the text)
The long-term safety assessment is based on many
assumptions that are considered very conservative at thisstage. Consequently, the results of the different cases studied
are often quite similar, and do not provide more detailed
information for discriminating among design options and/or
potential host geological formations. It is reasonable toimagine that the means of design and construction will be
adapted to the particular characteristics of the geologicalformations envisaged for the following stages, and hence that
contrasts between formations should be reflected in thedesign.

As mentioned in the response to comment 2.7-3, the features of design options tailored tohydrogeological structures were illustrated in the NUMO Safety Case. For example, thePEM has advantages in terms of reducing the risk of erosion of the bentonite buffer in caseof water inflow during waste emplacement in the disposal tunnel, while dead-end disposal
tunnels have more flexibility in terms of avoiding highly permeable hydrogeologicalstructures. NUMO will further develop the facility design methodology to tailor the various
design concepts and their components to features of geological environment in more
concrete and detailed manner.
The very conservative modelling and setting of parameters for the safety assessment
resulted in little difference in the results of the radionuclide migration analysis for different
geological environments and design options tailored to them (e.g., vertical emplacement
versus horizontal PEM for HLW, dead-end tunnels versus panel-type tunnels forunderground repository layout, etc.), This made it difficult to obtain information todetermine how the geological environment and design features contribute to system
performance. NUMO is currently developing a method to analyse three-dimensional nuclide
migration behaviour on a large scale extending from the repository to the biosphere,
representing design options and geological characteristics as realistically as possible, in
accordance with the Overall R&D Plan (see the response to comment 2.1-3). By applying
this method, we plan to better analyse the effects of different geological environments anddesigns on performance of the repository system.

22

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
No.

Observations from IRT

Recommendations from IRT

NUMO’s responses to comments

3.4-143.4
p.41

The presentation of cases using storyboards is relevant. It 3.4
makes it possible to better understand the different p.39
phenomena that must be taken into account at eachdisposal location, but also as a function of time. Theirdevelopment could be generalised by seeking a moreexhaustive representation of the underlying informationand the data to be processed.

(Summary in the box in the page 39)
Development of storyboards to be generalised, by seeking amore exhaustive representation of the underlying
information and the data to be processed.

As mentioned in the response to comment 2.5-4, NUMO also recognises the storyboards
are a valuable tool. Following the Overall R&D Plan, the storyboard technique is being
improved to describe the understanding of system temporal and spatial information in amore detailed and visually-attractive manner.
In addition, storyboards that comprehensively represent the behavior of geological disposal
systems are used to guide the development of computerised tools, described in the response
to comment 2.5-2, that link the processes of scenario development, modelling and datasetof nuclide migration analysis, which will efficiently manage information related to theirdecision-making and evidence. NUMO will develop this tool in line with the Overall R&D
Plan.

3.4-152.5.4
p.30

In the IRT’s view, a brief summary in the main report of 3.4
thep.39
mathematical models and their limitations (assumptions,simplifications) vis-à-vis the conceptual process models
would improve clarity of the safety case. A chart of themodels implemented would also be useful to illustrate thelink between the complexity of the physical processes
analysed and simplifications essential for a macroscopic p.41
representation. Both these measures would make the safetycase clearer for generalists, and also provide context for
specialists involved in detailed analyses.

(Summary in the box in the page 39)
Models and couplings: to establish a mapping of the various
models included in the assessment, their couplings, and theassociated codes. This would provide an overview ofmodelling efforts, make it possible to illustrate the adaptationof the various tools at the level of analysis, as well as to better
understand the simplifications of representations proposed.
(In the text)
Finally, a mapping of the various models included in theassessment, their couplings and the associated codes would
be beneficial. It would provide an overview of modelling
efforts, make it possible to illustrate the adaptation of thevarious tools at the level of analysis, as well as to better
understand the simplifications of the proposed
representations.
In this framework, the IRT recommends taking up argumentsfrom underlying reports to substantiate decisions forscreening processes to be considered at the different levels ofmodelling.

As mentioned in the response to comment 2.5-6, based on IRT’s recommendation, NUMO
has begun to construct a flowchart which describes all the models and codes for assessing
the THMC evolution of the repository within the NUMO Safety Case. This includes analysisof radionuclide migration for scenarios developed on the basis of such THMC evolution andresultant calculated dose rates, with explicit illustration of input and output links between
assessment models.
Models and codes developed or improved after the NUMO Safety Case are being added to
the flowchart implemented in NUMO’s information management system for post-closure
safety assessment (see the responses to comments 2.5-2 and 2.5-4). To improve thetransparency of our safety case, NUMO is planning to enhance the flowchart by adding brief
descriptions for all models and codes used to support the safety case. This will becomplemented by information on all safety case models and codes within NUMO’s
information management system.

3.4-162.5.4
p.30

The set of calculations carried out for different disposal 3.4
configurations and conditions illustrates the capability of p.39
NUMO to adequately represent the system for various
conditions, and to carry out the required set ofconsequence calculations underpinning a safety case. The p.41
numerical modelling seems mature enough to simulate
site-specific systems, and to consider different disposal
configurations and conditions.

(Summary in the box in the page 39)
Validation of the models and the computer codes: to increase
efforts to validate models and computing tools in the near
future.
(In the text)
To provide further confidence in the results of modelling,
there is also a need to further validate the models and thecomputer codes used in the safety case. The IRT recommends
NUMO increase its efforts on this work in the near future.

As mentioned in the response to comment 2.7-6, as IRT noted, validation of simulation
models and codes is crucially important to build confidence in the safety case. We aredeveloping a systematic methodology of verification and validation, including both testing
against observational data and critical expert review. According to the Overall R&D Plan,
NUMO will continue to expand verification and validation of simulation models and codesin the future.

23

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Annex D. Summary of IRT’s comments, findings, recommendations for each of the sections of NUMO’s pre-siting safety caseObservations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

D-1

D2.2
p.53

The basic concept for ensuring the long-term safety of D2.2
geological disposal is expressed in terms of 2 key p.53
functions, namely isolation and containment. The lattercovers the notion of containment as well as retardation,
this being clearly indicated in the report.

However, in order to avoid any risk of confusion, and also The NUMO safety case uses the term “containment” as a concept that includes bothto clearly distinguish the physical phenomena and the “confinement” and “delaying” of radionuclides, following the IAEA (2011) definition.
processes involved in order to be able to analyse them However, in some countries, “containment” and “retardation” are expressed separately, anddistinctly, it is recommended to adopt the terminology that for some stakeholders, “confinement” is associated with complete enclosure ofclearly distinguishes between containment and retardation. radionuclides, which may cause misunderstandings. Therefore, in the future, we willconsider better terminology for these functions.

D-2

D2.6
p.55

The safety case strategy consists of assessing operational D2.2
and post-closure safety based on information on the p.53
selected sites and the associated repository designs andavailable scientific and technological knowledge, in thelight of the relevant regulatory standards and therequirements of the stakeholders. In the NUMO Pre-siting
SDM-based Safety Case, generic cases are taken into
account in order to demonstrate the ability to develop
such a safety case on a real site in the near future.

The main functions identified during operation arecontainment and radiation shielding. It is also suggested toclearly analyse the risks of criticality as well as the risks
linked to the gases which could be generated by radiolysis.

The reason for the negligible criticality risk of vitrified waste, due to uranium and plutonium
being extracted during reprocessing, was given in the H12 Report, and it is believed that
this argument is still supported today. However, in the future, as we proceed with theexamination of a more realistic inventory, including TRU waste and uncertainties inradionuclide concentrations in individual packages (see the responses to Comments 2.1-2,
2.3-1, 2.4-1, 2.6-1, 3.1-1, 3.3-2 and 3.3-3), we will more comprehensively develop scenarios
for operational safety assessment for specific designs and operation methodology (see theresponses to Comments 2.4-3, 2.4-4 and 3.4.12). As part of this, with the goal of improving
the completeness of safety cases, criticality risks based on waste inventories that reflectrealistic reprocessing conditions will be included in future safety cases.
In terms of hydrogen gas generated by radiolysis during operation, as indicated in the
response to comment 2.7-2, TRU waste with a particularly high moisture content will betreated to reduce the amount of hydrogen gas generated. We also recognise that alternative
materials in the waste package and the development of a package container with enhanced
confinement in the case of gas pressurisation are issues to be studied in the future. Alongwith solving these technical issues, we will analyse risks during operation and study
countermeasures within the comprehensive safety assessment scenarios described above.

D-3

D2.6.2
p.55

The numerical values of the calculated doses can be used D2.6.2
as an indicator of the expected performance of the p.55
repository, but must be used with caution due touncertainties, in particular regarding the future biosphereand human lifestyle. To determine the safety assessment
period, in the absence of national regulations, those ofother countries were at this stage taken into account. The
main indicator used for post-closure safety assessment is
the human radiological dose.

Doses to non-human organisms and impacts ofnonradioactive hazardous substances are not included. It is
recommended for future developments to consider doses to
non-human organisms in relation to the food chain and nonradioactive hazardous substances for their chemical hazard.

As mentioned in the response to comment 2.5-9, the need for radiological protection of theenvironment, including non-human species, has been suggested in recommendations byinternational organisations such as ICRP, but is not specifically addressed in many
international safety regulations and the nuclear regulations in Japan. It is thus not consideredin the NUMO Safety Case. However, in the future, we will pay close attention tointernational discussions and national and international regulatory trends regarding impact
analyses on non-human biota and introduce such considerations as necessary.
As mentioned in the response to comment 2.5-7, we plan to extend studies of the risks ofradioactive elements to include the identification of toxic chemical substances, and theirrelease and migration characteristics, in cooperation with the reprocessing plant operators
who produce this waste. In addition, consideration will be given to trends in domestic andinternational regulations on hazardous chemical substances and their handling within safetycases in other countries, as well as wider consideration of chemical risk assessment methods.
As mentioned in the response to comment 2.5-8, in order to enhance confidence of biosphereassessment reliability, as recommended, NUMO will consider geochemical mechanismsthat may cause reconcentration of radionuclides at locations such as interfaces between soil
horizons or along redox fronts when developing the current biosphere model further.

24

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

D-4

D2.6
p.55

The safety case strategy consists of assessing operational D2.6.2
and post-closure safety based on information on the p.56
selected sites and the associated repository designs andavailable scientific and technological knowledge, in thelight of the relevant regulatory standards and therequirements of the stakeholders. In the NUMO Pre-siting
SDM-based Safety Case, generic cases are taken into
account in order to demonstrate the ability to develop
such a safety case on a real site in the near future.

It is also recommended to justify or to properly document
some of the assertions, such as for example that the upliftor the erosion could be negligible from a judicious site
selection. Since the criteria are not reported, the evaluationof this type of assertion is challenging.

NUMO agrees with this comment. We think that presentation of evidence for assertions
regarding phenomena related to the long-term stability of the geological environment, suchas uplift and erosion, need to be explicitly highlighted when constructing safety cases for ageological disposal system in Japan.
The SDM should take into account long-term evolution in topography and geologicalstructures due to uplift and erosion and the associated changes in relevant characteristics ofthe deep geological environment. These processes are, however, strongly dependent on sitespecific conditions. As shown in the NUMO Safety Case, appropriate site selection should
ensure that any repository host rock considered is sufficiently deep to be unaffected by upliftand erosion, and that favourable geological characteristics will be maintained over a
sufficiently long period of time. The SDMs were developed for such a deep host rock, which
can be assumed to change slowly, but have little significant impact on the design and safetyassessment of the repository.
In addition, uplift and erosion over 1 million years has been quantified in the area of“assumed to be favourable” in the “Nationwide Map of ‘Scientific Features’ relevant for
Geological Disposal”. This is without consideration of specific topographical and geologicalfactors that represent conditions for particular regions, or the spatial-temporal scale of theevaluation. As a result of such evaluation, the depth of the disposal site after 1 million years
should be approximately 300 m or more below the ground surface. This indicates that it canbe assumed that stable geological conditions are maintained over this time period.
Since Supporting Report 6-10 was not translated into English and not provided to the IRT,
arguments on geological stability were not sufficiently conveyed to reviewers.
In the development of site-specific safety cases, influence of long-term geological evolution
due to uplift, erosion, etc. on geological disposal system will be taken into account. These
will be based on evidence from site-specific characterization and will be documented
carefully to cover the highlighted concerns.

D-5

D2.7.2
p.56

Given the complexity of the repository system (various D2.7.2
processes [THMCR] with couplings, multiple spatial p.56
scales and characteristic time scales), it is necessary to
identify uncertainties in a systematic way to ensure
traceability
andcompletenessof
uncertainty
management.

NUMO presents the concept of management and thetreatment for each type of uncertainty, but the descriptionof the method of identification of uncertainty deserves tobe enriched. Because of its importance, it might be useful
to consider devoting a separate chapter to dealing withhigh-level uncertainties. Chapters 3, 4, 5 and 6 could
benefit from the inclusion of subsections to discuss how
uncertainty is managed for site selection, repository design
as well as operational safety assessment and post-closure.

NUMO agrees with this recommendation. In future development of the safety case, thedescription and treatment of uncertainties will be expanded by adding a section focused on
this topic in the main report. Also, we will consider describing how uncertainties aremanaged in the presentations of site selection, repository design, operational and postclosure safety assessments, and the integrated safety case. This will allow clearer
explanation of the information and data on which the treatment of uncertainty is based,
which is presently described only in the supporting reports.

D-6

D2.7.2
p.56

Given the complexity of the repository system (various D2.7.2
processes [THMCR] with couplings, multiple spatial p.56
scales and characteristic time scales), it is necessary to
identify uncertainties in a systematic way to ensure
traceability
andcompletenessof
uncertainty
management.

It might be useful to discuss in more detail the extent of"acceptable" conservatism, for example that models should
not include an excessive amount of conservatism. As an
illustration, when selecting a site, this could lead toevaluation bias. An important aspect is also how
conservatism is explained to the reader, especially the nontechnical reader who might be guided on how to understandand interpret the results.
For the clarity of the approach, it is also suggested to
identify separately what comes under variability from whatcomes under uncertainty, although the processing may
involve identical methods.

NUMO has made progress in understanding uncertainties associated with the waste
inventory and its characteristics (see responses to comments 2.1-2, 2.3-1, 2.4-1, 2.6-1, 3.11, 3.3-2 and 3.3-3) and developing analytical models that can more realistically assess
performance of repository system (see responses to comments 2.3-2 and 2.5-5). For theseevaluations, we will clarify what causes uncertainties associated with the evaluations,
including the inherent uncertainty of the fields and phenomena to which these analyticalmodels and associated data are focusing on, and the technology will be developed toquantify such information so that evaluations can be performed without over-conservatism.
NUMO aims to explain a sequence of work based on this approach to different stakeholders
in a transparent and traceable manner.

25

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

D-7

D2.7.7
p.57

The IRT noted NUMO's commitment to basing its studies D2.7.7
on the best available technologies.
p.57

However, it would seem judicious to NUMO to also
consider the scheme adopted in many large-scale projects
with the technology readiness level (TRL) Scale to assess
the evolution and maturity of the geological disposal
project.

D-8

D2.7.9p.57

The sensitivity of the various stakeholders and D2.7.9
populations to nuclear issues, and therefore in this case to p.57
the disposal of radioactive waste, means that the
information, communication and dialogue should be
proposed and presented. The societal approach is also partof the safety case, as indicated in numerous publications
under the aegis of the NEA, and also taken up by the
NUMO teams during the Global 2015 Conference.

The IRT recommends that NUMO consider this issue as NUMO recognises that promoting dialogue and communication with stakeholders using thesoon as possible and devote the necessary development to safety case is an important issue. For example, NUMO has been participating in
it in the next phases of the safety case.
OECD/NEA FSC activities for many years. In addition, the Japan Atomic Energy Society
has also established a special committee to support research on the social science aspects ofthis issue and to study methodologies for promoting communication of the safety case forgeological disposal to various stakeholders. This committee includes both experts in thesocial sciences and other relevant fields. We will continue to strengthen these activities
based on the advice of domestic and international experts.

D-9

D3
p.58

The site descriptive model (SDM) approach developed by D3
NUMO seems suitable. Models make it possible to p.58
provide design and safety assessment with fully
integrated comprehensive data rather than field data.
Models will be refined along with the progress in siting
and field survey. The generic safety case developed in the
SDM-based Safety Case is based on three typical models
covering various types of potential geological formations
encountered in Japan. Chapter 3 justifies in detail theelaboration of these models. This approach is relevant at
this stage.

For Neogene/Pre-Neogene volcanic and metamorphic
rocks, the decision was made at this stage to not elaborate
specific SDMs as three representative SDMs are
considered to cover the characteristics of these rocks. It is
however recommended that, as soon as field data become
available, the characteristics of these formations be
differentiated from those of volcanic and igneous rocks.

As recommended, if information on Neogene/Pre-Neogene volcanic and metamorphic rocks
is obtained, either from specific site investigations or from future additions to informationin the general literature, consideration will be given to developing these specific SDMs if
this is of relevance to any sites.

D-10

D3.3
p.60

For the development of SDMs for representative host D3.3
rock settings, several characteristics are compiled and p.60
then selected to provide as much representative cases as
possible. Conceptual models are proposed, but theywould gain in justification with a finer description of themethodology of their development. In particular, it will benecessary to describe how the field data will be used toarrive at the conceptual model.
For Neogene/Pre-Neogene volcanic and metamorphic
rocks, no specific SDM is elaborated. Three
representative SDMs are considered to cover thecharacteristics of these rocks and used for design andsafety assessment for the safety case. For the specificsites, respective SDMs are developed based on theliterature and characterisation.

Hydraulic conductivity is a key parameter for the different
geological formations and could be very discriminating interms of post-closure containment performance. It is thus
recommended to use the most possible representativevalues rather than having almost the same value for all.

Representative hydraulic conductivities for Neogene/Pre-Neogene volcanic andmetamorphic rocks were obtained. If further information on these rocks is obtained, either
from specific site investigations or from future additions to information in the generalliterature, we will consider refining the SDMs for Neogene/Pre-Neogene volcanic andmetamorphic rocks.
In the SDMs for specific sites, the heterogeneity of hydraulic conductivity of geologicalformations will be determined based on borehole data and directly considered whenaddressing safety-relevant issues, such as more realistic assessment of radionuclide
migration characteristics. Here we consider impacts on performance indicators for the hostrock and surrounding formations. Thus, in addition to refinement of the radionuclide
migration model in the geosphere (see responses to comments 2.1-3, 2.1-4, 3.4-1 and 3.43), we take such heterogeneity into account during tailoring designs (see responses tocomments 2.3-3 and 2.5-5), setting of parameters for sensitivity analysis, and generaltreatment of uncertainty (see comment 3.1-1).

26

As recommended, when integrating the latest scientific knowledge and past R&D on
geological disposal and preparing to advance stepwise site investigations, repository design,and safety assessment, we will consider the objective assessment of the maturity of alltechnologies using the TRL scale. This can contribute to improving the efficiency oftechnical work and also supporting arguments for safety cases. We will consider introducing
the TRL scale (or an equivalent measure) in the future.

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Observations from IRT

No.

Recommendations from IRT

NUMO’s responses to comments

D-11

D6.3
p.68

Storyboards are mostly used to describe the behaviour of D6.3
the system after the disposal facility is closed. This is a p.69
relevant way of illustrating the successive periods ofdisposal evolution.

Such storyboards would also be a good basis for analysing
the effects of uncertainties. The IRT strongly recommends
developing the work already well underway from thestoryboards for the next stages of the geological repository
project in Japan. These can also be a good way tocommunicate with stakeholders and the public, making it
clear that storyboards help to understand all of theunderlying processes and assumptions that need to beconsidered.

As mentioned in the responses to comments 2.5-4, 2.7-5 and 3.4-14, storyboards, as
knowledge management and communication tools, are recognised as valuable to promotecommon understanding of evolution of the repository system on a range of temporal andspatial scales. These can also contribute to effective integration of safety case developmentby the site investigation team, the repository design team and the safety assessment team.
Additionally, as IRT noted, the storyboard can function as a tool which promotes
communication with various stakeholders and, in particular, the general public. With a focus
on the function as a communication tool, the storyboards shown in the NUMO Safety Caseare planned to be improved to visually illustrate temporal and spatial evolution of geologicalrepository in much more detail. This can also cover the representation of uncertainties,
particularly in terms of long-term evolution.

D-12

D6.2
p.68

The role of the integrated FEPs is to group the FEPs in D6.3
blocks to structure the impact analysis. As they are p.69
general in nature, e.g. “water chemistry”, they do notseem to be used as such in the impact analysis. The roleof the integrated FEPs vs the individual FEPs in theimpact analysis should be developed. An example of a listof FEPs that are grouped into one integrated FEP would
help illustrating the approach.

NUMO applies a structured approach to the developmentof scenarios, using safety functions, state variables, factor
analysis diagrams and impact analyses, and the IRT
encourages the further development of these tools.

D6.3
p.69

The NUMO FEP list used in the SDM-based safety casewould be useful; it may be updated next on the basis of
more recent publications by the NEA.

The NUMO FEP knowledge base will be regularly expanded and updated to represent thelatest scientific knowledge, while introduction of digital tools aims to improve thetransparency and traceability of FEP screening and integration into scenarios (see responses
to comments 2.5-1 and 2.5-2). Moreover, based on this knowledge base, the methodologies
and tools related to the scenario development process, such as setting up state variables,
factor analysis, impact analysis, and clarifying the relationship with safety functions, will
be further developed.
The update of this knowledge base will also include detailed studies on FEPs which are
currently considered as secondary, as described in the responses to comments 2.5-5 and 3.49. All this will proceed in parallel to the development of tools for the integrated knowledge
management extending from the development of FEP lists to final scenario development(see responses to comments 2.5-4, 2.7-5 and 3.4-14).

D-13

D6.6
p.70

Based on the experience of the present safety case, future D6.6
development needs can be ranked in terms of their p.70
perceived importance, and also according to the overall
agenda of geological disposal in Japan. The mostsensitive phenomena and processes will be a first key for
ranking, but other criteria will also be defined. In any
case, the development programme will be built so thatmajor milestones are met.

In addition to the development needs identified by NUMO,
the IRT emphasises the need to improve the understandingof microbial processes. The impact of microbial processeson the performance of the overpack (especially corrosion),buffer and backfill and on radionuclide transport in thegeosphere will have to be evaluated.

We recognise the need to better understand the effects of microbial activity on corrosion
and, more generally, the impacts of microbial activity in buffer materials and on theunderground migration of radionuclides. This is indicated in the Overall R&D Plan. NUMO
will continue to expand its knowledge in this area, including utilising joint research with
relevant organisations, academic societies, universities and other professional agencies.

D-14

D7.1
p.72

As an alternative dose indicator, NUMO considers D7.1
transfer of radionuclides to the biosphere in a stylised p.72
release to a river, demonstrating that dilution reduces
concentrations to insignificant levels.

The IRT encourages NUMO to further develop such cases, NUMO will continue to study complementary safety / performance indicators, in particulare.g. the possibility of accumulation of radionuclides into by monitoring safety cases developed by other countries. As mentioned in the response tocomment 2.5-5, we are developing site descriptive models which can realistically describe
specific compartments of the biosphere.
changes in time and space on the large scale extending from deep underground to thebiosphere (4-dimensional site descriptive model). NUMO will improve the biosphereassessment model by considering long-term evolution of the surface environment (derived
from the 4-D SDM) as well as any geochemical mechanisms that might cause
reconcentration of radionuclides at locations such as interfaces between soil horizons oralong redox fronts (mentioned in response to comment 2.5-8). In addition, NUMO will also
study the approach to assess the cases in which radionuclides accumulate into specificcompartments in the biosphere while improving biosphere assessment model mentioned .

27

NUMO responses to the OECD/NEA international review comments on the NUMO Safety Case
5th June 2023
Observations from IRT

No.
D-15

D6.2
p.68

The role of the integrated FEPs is to group the FEPs in D7.2
blocks to structure the impact analysis. As they are p.73
general in nature, e.g. “water chemistry”, they do notseem to be used as such in the impact analysis. The roleof the integrated FEPs vs the individual FEPs in theimpact analysis should be developed. An example of a listof FEPs that are grouped into one integrated FEP would
help illustrating the approach.

D6.3
p.69

The NUMO FEP list used in the SDM-based safety casewould be useful; it may be updated next on the basis of
more recent publications by the NEA.

Recommendations from IRTNUMO has introduced the concept of state variables andfactor analysis diagrams as tools for evaluating safetyfunctions in the development of scenarios, and, as
mentioned, the IRT encourages the further development ofthese tools.

NUMO’s responses to comments
As mentioned in the response to comment D-12, NUMO will continue to develop andimprove tools for evaluating safety functions as part of scenario development. This runs inparallel with work to continuously update the FEP knowledge base, while also improving
traceability and user-friendliness by introducing modern digital technology.

References:
Coordination Council on Research and Development of Geological Disposal: Overall Research and Development Plan for the Geological Disposal Program (FY2023-FY2027) (in Japanese).
IAEA（2011）：Geological disposal facilities for radioactive waste, Specific Safety Guide, IAEA Safety Standards Series, No. SSG-14.
JAEA (Japan Atomic Energy Agency) and FEPC (The Federation of Electric Power Companies of Japan) (2007): Second progress report on research and development for TRU waste disposal in Japan, JAEA-Review 2007-010, FEPC TRUTR2-2007-01.
JNC (Japan Nuclear Cycle Development Institute) (2000): H12: Project to establish the scientific and technical basis for HLW disposal in Japan; Project overview report, JNC TN1410 2000-001.

28