OVERVIEW OF THE NUMO SATETY CASE AT PRE-SITING STAGE
T. Fujiyama, S. Suzuki, A. Deguchi, H. Umeki
Nuclear Waste Management Organization of Japan
Mita NN Bldg. 1-23, Shiba 4-Chome, Minato-ku, Tokyo 108-0014 Japan
Key words; Geological disposal, Safety case, Vitrified waste, TRU wastes

Abstract
NUMO has developed a safety case for geological disposal at pre-siting stage in Japan. This safety case provides
the multiple lines of arguments and evidence to demonstrate the feasibility and safety of geological disposal,
which will encourage stakeholder confidence in the safe implementation of geological disposal and will provide
a basic structure for a safety case which will be applicable to any potential site. This paper will provide a brief
overview and approach of this safety case, emphasizing practical approaches and methodology which will beapplicable for the conditions/constraints during an actual siting process. The geological environments of various
potential host rocks are developed as site descriptive models, which are, as realistically as possible, based on thecurrent best understanding of Japanese geology. A rigorous and flexible methodology for tailoring repository
design to site descriptive models has been developed. Safety assessment models and data also have beendeveloped and applied for demonstration of the technical basis of the safety assessment.

1. Introduction
1.1 Background
The “H12 Report” (JNC, 1999)1 published in 1999
by the Japan Nuclear Cycle Development Institute(now the Japan Atomic Energy Agency, “JAEA”)
demonstrated the feasibility of safe and technicallyreliable geological disposal of high level waste(HLW), based on a generic study. On the basis of the
H12 Report, the Act on Final Disposal of DesignatedRadioactive Waste (the Final Disposal Act) came into
force and NUMO was established as theimplementing body in 2000. Intermediate-level waste
generated from reprocessing of spent nuclear fuel andmixed-oxide fuel fabrication (termed “TRU waste” inJapan) was also included in the inventory for disposal
in 2007, based on the feasibility demonstrated by the“TRU-2 Report” (JAEA and FEAC, 2007)2. These
wastes are to be disposed of in stable rock at depths
greater than 300 m. NUMO has been developing key
technologies required for the safe implementation of
the geological disposal project since its establishment
and initiated the siting process by open solicitation ofvolunteer municipalities in 2002. So far, however, no
volunteer municipality has appeared and no candidate
host rock type has been specified.
The Fukushima-Daiichi NPP accident in 2011
increased nationwide concerns about the feasibility
and reliability of safe geological disposal in Japan.
After re-thinking the implementation process byadvisory expert groups organized by Ministry of
Economy, Trade and Industry (METI), “the Basic
Policy on the Final Disposal of Specified Radioactive
Waste” was amended in 2015, so that theGovernment leads the search for volunteer sites. This
procedure involves nominating more suitable areasfrom a geo-scientific point of view to initiate
discussions and cooperation with local municipalities,
finally leading to acceptance of a site investigation,
which will be carried out by NUMO.
Under the new Basic Policy, METI and NUMO
have enhanced to communicate with the public inorder to promote understanding of the HLWmanagement policy, focusing on geological disposal.
In parallel, requirements and criteria for nationwide
scientific screening has been discussed at theadvisory expert groups. The groups have examined
the requirements and criteria for geological disposal
from the viewpoints of geological environment andits long-term stability, pre-closure safety, safety of
the waste transportation, in order to define scientificfeatures with which all areas are classified in anationwide map. Based on the discussion for about
two years, METI published the “Nationwide Map ofScientific Features for Geological Disposal” in July2017.
In the Map, areas are classified into four categories
defined by scientific features with associated criteria3.
The map does not intend to indicate deterministicallyspecific areas as a candidate site for a geologicalrepository. Stepwise investigations and assessments
should be conducted by NUMO, with the acceptanceof local communities.
1.2 Purpose of developing the NUMO Safety Case
Publication of the ‘Nationwide Scientific Map ofJapan’ would bring a new stage to the siting process

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

and need more convincing explanation of the safetyof geological disposal. Taking such changes inboundary conditions into account, it is important at
this time for NUMO to integrate required
technologies, including the latest R&D output, inorder to confirm the feasibility and safety ofgeological disposal in Japan since the H12 Report,
which could be recognized as the first generic safetycase in Japan. Thus NUMO has developed the safetycase at the pre-siting stage, which, with the site
descriptive models (SDMs) recently developed,
provides a more advanced site-specific basis than thegeneric safety case in the H12 Report. This has beendeveloped in a manner that will allow it to provide
the basic structure of future safety cases that would
be applicable to any site that might arise from the site
selection process.
This paper provides a brief overview and theapproach for development of the NUMO Safety Case.
More detailed discussions are presented in a series ofassociated papers4-8 in this conference.
2. Premises of developing the NUMO Safety Case
2.1 Boundary conditions and basic approach
NUMO has developed the Safety Case considering
the current constraints that include legal requirements
and the current situation of Japanese disposal program,i.e. candidate sites have not been specified yet, safety
regulations are still under development, as well as thebasic policies mentioned above.
Despite the fact that there has not been a site andspecific host rock identified, the NUMO Safety Casehasdeveloped
detailedgeologicalandhydrogeological models for potential host rockenvironments, which are based on field data
HLW
(Vitrified waste)

TRU waste
Group

(mm)

accumulated since the H12 Report and current best
understanding of geological environments in Japan.
Repository design and safety assessment have beenthus performed for these geological models, thereby
providing underpinning evidence to demonstrate thetechnical feasibility and the safety for the various
types of Japanese geological environments.
2.2 Features of radioactive waste
The highly radioactive liquid residue separated in
the reprocessing of spent fuel is mixed with glass in a
glass melter, poured into stainless steel canisters andcooled to form vitrified HLW. Vitrified waste
produced by Japan Nuclear Fuel Limited (JNFL) will
be responsible for most of the HLW inventory. In thissafety case, the specification of the reference waste
form shown in Fig.1.
TRU waste includes spent silver absorbent,
compressed hull and end pieces, etc. generated as aresult of the reprocessing of spent fuel at JAEA and
JNFL. They are grouped according to their features
as shown in Fig.1. Group 1 includes highly soluble,
little sorptive I-129, group 2 also includes highly
soluble, little sorptive C-14, group 2 and some ofgroup 4 (group 4H) have significant heat output andgroup 3 contains nitrate, which may need special
consideration in terms of potential chemical
perturbations. Specifications of some waste are still
being studied in order to improve safety, andtherefore have not been fixed yet.
It is currently difficult to estimate the total
generation of radioactive waste to be disposed of bygeological disposal, because such numbers depend on
future nuclear power policies. Therefore, in the
Safety Case at this stage, a repository with capacity
4

1

2

3

Iodine adsorbent

Hull & endpieces

Concentrated solution, etc.

Low

High

Organic waste

Nitric acid solution waste
Mortar, etc.
Rubber gloves（incineration / compression）

Effluent
PelletIntakeSectioning&
compression

（mm）

430

620

430
• Weight ：500 kg• Thickness of canister：6 mm• Number of canisters ：2448 (as of Apr 2017)• Calorific power：350 W• Surface dose：160 Sv/h
(50 years af ter production)

（mm）

890

890

1335

620

Waste
package
image(e.g.)

Tools

Dried pellet（mm）Drum (200Liter)

Drum (200Liter)

Canister

Features• Radioactive iodine(I-129) is included• Cemented waste
form• Relatively high heatpower
• Radioactive carbon(C-14) is included• Nitrate is contained• Solidified waste withe.g., mother or bitumen

Volume319 [m3]

5,792 [m3]5,228 [m3]Calorific
power (at timeof production)

1600
1200

Adsorbent to remove
radioactive iodine

Non-combustible waste

Metal pipes
（mm）

1600

低レベル放射性廃棄物

Container

620
890

1340

Description

Hull

Silveradsorbent

Drum (200Liter)• Incineration ash or noncombustibles• Solidified waste withcement, etc.
5,436 [m3]1,309[m3]

1

90

1

16

210

[W/package][W/package][W/package][W/package][W/package]

Fig. 1. HLW and TRU waste groups and their characteristics

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

for the following volume of waste was assumed,
based on the latest “Disposal Plan for Final Disposalof Specified Radioactive Waste” by METI in 2008:
 HLW: 40,000 canisters of vitrified waste(disposal of 1,000 canister per year) TRU waste: 19,000 m3
In the NUMO Safety Case, co-disposal of HLW andTRU waste is considered.
2.3 Safety functions of repository
In the pre-closure phase, the repository has toensure the safety of both the local population andworkers against both radiation risks and otherindustrial hazards. The safety functions related toradiation protection during the pre-closure phase are
the main focus here. With regard to industrial safetyof workers, the repository needs to have safetyfunctions to prevent accidents and establish a healthy
working environment during the period up to final
closure of the facility.
After closure, the repository will perform its
isolation and containment roles based on the safetyfunctions. In terms of isolation, the assurance ofprotection from geological perturbations isparticularly important in Japan due to its location
relative to active tectonic plates, which results insignificant potential for volcanism, fault movement
and uplift/erosion at some locations. In addition, thegeological environment should contribute to
reduction of the risk of anthropogenic perturbations –predominantly by excluding areas containing mineral

Survey andevaluation ofgeologicalenvironment

resources that might be exploited in the future. These
safety functions related to isolation are a role of thegeological environment, which will be verified
through investigation and evaluation during sitecharacterization.
3. Safety strategy and approach for developing the
NUMO Safety Case at pre-siting stage
3.1 Site characterization and synthesis into SDM(1) Site selection concept
After nominating investigation sites withacceptance of local municipalities, the stepwise site
selection process (Fig.2) will be conducted aiming at
acquiring information on geological environments inand around the proposed sites in order to develop
repository designs and associated safety assessmentsthat verify the proposed sites possess the required
safety functions.
Geological disposal has to consider that key
properties are determined by attributes of a site that
extend over large areas and depths and which are
inherently
heterogeneousonallscales.
Characterization of such environments needs toconsider all relevant scales, although the level ofdetail required is different, being greatest for thevolume of rock containing the repository and themost safety-critical parts of the geological barrier.
The key features of the geosphere and their spatial
extent are determined from the output of theassociated safety assessment.

Literature
Investigation Stage

Preliminary
Investigation Stage

Detailed
Investigation stage

Collection of literature(at national & local levels)

Surf ace based investigations(seismics, boreholes etc.)

Characterisation in underground researchlab (URL), surf ace based surveys

Exclusion of
ineligible sites
based on geologicalphenomena ·quaternary
unconsolidated
sediments

Estimation ofgeologicalenvironment
characteristics

Development ofregional scalegeological model

Repository
designInitialengineered
barrier
specif ication

SafetyAssessment

Outline of saf ety assessment
based on repository design

Initial disposal
f acility concept

Selection of PI areas sites andplanning of PI stageExclusion of
ineligible sites
based on geologicalphenomena andtunnel excavation

Understanding
geologicalenvironment
characteristics

Conf irmation of
avoidance ofremarkable
inf luence of natural
phenomenon

Improvement of regional
scale geological models,development of repository
scale geological model

Conceptual
design ofengineered
barriers

Conceptual
design ofdisposalf acility

Detailed
understanding ofgeologicalenvironment
characteristics

Improvement of regional &repository scale models,development of panel scalegeological models

Basic designofEngineered
Barriers

Disposal f acility
designSaf ety assessment basedon repository design

Saf ety assessment
based on repository design

Selection of DI areas andplanning of DI stage

Selection of disposal facility
construction site

Fig. 2. Site selection stages on the way to implementation

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, JapanLS: literature survey
Regional scale(several tens km)

Climatic change
PI: preliminary investigation

PI: supplementary

Repository scale(several km)
Igneousactivity

DI: detailed
investigation

Uplift/erosionSea-levelchangePanel scale ～ Nearfield scale(several hundreds m)suitable and can be used for repository design andsafety assessment; for each candidate host rock, defining a
representative geological setting that includes all
key features (e.g. faults, fractures and sedimentary
structures, etc.) that are required for realisticallyassessing its advantages and disadvantages as arepository site.

Earthquake
Fault
movement

Fig. 3. Components of the three stages of site
investigation and scales of SDMs
The information is synthesized into a site
descriptive model (SDM). The SDM is the basis of
the design of the repository and the safety assessment.
Fig.3 shows the relationship between the ranges of
the three investigation phases and scales of SDM.
SDMs are developed in three spatial scales; regional
scale (tens of km x tens of km), repository scale(several km x several km), panel and near-field scale(several hundred meters x several hundred meters).
These nested models assure consistency of THMC
conditions within and between them, with a level ofdetail of geological information appropriate to theinvestigation phases considered, the investigation
technology availability, the features of the analyticalmodels, the capabilities of calculation codes, etc.
(2) Approach to site assessment in the NUMO
Safety Case
Generic repository design and safety assessment
were performed in the H12 and TRU-2 Report for
two illustrative geological settings, namely crystalline
rock and sedimentary rock. However, geoscientificknowledge has expanded significantly since then due
in particular to multidisciplinary research programs;
for example, JAEA’s underground researchlaboratory projects. It is thus important to update theprevious repository design and safety assessment on
the basis of the current best understanding ofJapanese geological environments.
In the absence of candidate sites, establishing thebasis for demonstration of the capacity to model
relevant geological environments at a levelappropriate for site selection is based on:
 presentation of a concept for stepwise sitecharacterization along with demonstration of thetechnology for interpretation and integration of
expected output to form the basis for design andsafety assessment; preparation for the literature study, based on acompilation of geological information on anationwide scale, involving selection ofgeological environments that are potentiallyFollowing the categorization of all potential hostrock environments in Japan, rock types are groupedby identifying key characteristics/properties relevant
to geological disposal. As a result, three types ofpotential host rock environments, ‘Igneous rocks’,‘Neogene sedimentary rocks’, and ‘Pre-Neogene
sedimentary rocks’ are examined in the NUMO
Safety Case4.
3.2 Repository design and engineering(1) Repository design strategy
In a repository design, the specifications of therepository (engineered barriers, surface facilities,
underground layout, etc.) are determined so that thesafety functions of the repository are assured for thedefined SDM. The repository specifications are
established based on geological environment
information obtained in each investigation phase
together with the technological knowledge baseavailable at that time, subject to constraints such as
assuring engineering feasibility. The repository willthus be designed in detail and optimized in a stepwisemanner, based on the progress of site investigationand input from associated safety assessment.
Therefore, NUMO has developed the methodology
shown below so that the repository can be designedwith required flexibility:
 incorporating multiple requirements as design
factors so that the repository can be designed in aconsistent manner, responding to policy
developed in the stepwise site selection process; repository design options will emphasise
flexibility in terms of ability to be tailored to thevariety of geological environments expected andto respond to the progress of science andtechnology during the long implementation
period.
The term design factors denotes the features andcapability that the repository design may be required
to have, e.g. long-term post-closure safety,
pre-closure
safety,engineering
feasibility,
retrievability and economic efficiency.
(2) Approach adopted in the NUMO Safety Casefor repository design and engineering feasibility
As the conditions of the geological environment
cannot be specified in detail before candidate sites

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

come forward, demonstration of repository design
involves the following5:
 Starting from the repository concept developed in
the H12 and TRU-2 Report, repository design
specifications tailored to the SDMs of three types
potential host rock environments are illustrated,which meet pre- and post-closure safety
requirements and demonstrate engineering
feasibility;
 Specific features of SDMs, such as major fault
zones, are specifically assessed to determine theirimpacts on different disposal concepts; Based on
this, determine whether specific SDMs arecompatible with developing practical designs withrequired safety functions; Assess the specifications for specific repository
designs to determine if implementation is feasibleusing existing engineering technology or that isreasonably expected to be developed in the future.
3.3 Pre-closure Safety Assessment(1) Pre-closure safety strategy
Evaluation of radiological safety is generally basedon regulations that govern reactors and other nuclear
facilities. Since the accident at Fukushima Dai-ichi
NPP, such regulations have been under review andthis has resulted in new waste management
requirements that will apply to a geologicalrepository, although these have not been formulated
as yet.
Pre-closure safety assessment considers not only
the expected operations, but also the impact of
perturbations due either to operational disturbances(e.g. equipment failure) or external events(earthquakes, tsunami, flooding, etc.).
Crediblescenarios for such perturbations must be developed,
their potential consequences assessed and whereappropriate, potential counter-measures introduced.
Many of the perturbations depend on the geologicaland geographical setting of the site and cannot be
assessed in detail in a generic manner. Nevertheless,the potential for reducing impacts via engineeringcounter-measures can be illustrated. For otherperturbations, counter-measures developed for othernuclear facilities can be referred. Even though thescenarios are not described in detail, for illustrative
purposes, the impacts of some “worst case” accidents
are assessed in order to determine safety levels anddevelop approaches to reduce risks.
(2) Approach adopted in the NUMO Safety Casefor pre-closure safety evaluation
At present, criteria for geological disposal facilities
in relation to the program licensing rule have notbeen formulated. Further, in the absence of a specificdisposal site, the extent to which the surface
environmental conditions in which the facility will be
constructed is very limited. This in turn constrains

sensible evaluation of the impact of site-specificexternal perturbations such as earthquakes or tsunami.
On the other hand, evaluation of safety is possible for
maximum design base accidents. Therefore, the
NUMO Safety Case at this stage will6:
 illustrate the concepts and methodology forradiation safety assessment for both workers andlocal residents around the facility, focusing onvulnerable transportation and operational
processes and with reference to the safety
regulations for other relevant nuclear facilities;
 present results of the illustrative pre-closure
safetyassessment,
while
verifyingthecompatibility of potential counter-measures toreduce risks with long-term post closure safety of
the repository.
3.4 Post-closure Safety Assessment(1) Post-closure safety strategy
For long-term post-closure safety assessment, themethodology used internationally has been evaluated.
For the repository designs tailored to geologicalenvironments at selected sites, such assessments willdefine representative scenarios to evaluate theradiological impact on the surrounding populace,with full consideration of all uncertainties involved.
Such analysis should confirm that the repository
meets safety standards and will not have a significant
impact on the living environment at any time in the
future.
NUMO has been involved in technology
development related to fundamental components ofthe safety assessment: scenario development,
modeling and development of a dataset. With a view
to subsequent site selection, the preparation of thissafety assessment methodology and confirmation of
its applicability is necessary. Preparation of an
appropriate safety assessment analysis toolkit isespecially important to allow objective comparisonand evaluation of the performance of alternative
disposal concepts, where the methodology must also
handle different uncertainties in terms of site
conditions and repository specifications.
(2) Approach adopted in the NUMO Safety Casefor post-closure safety assessment
The framework of safety assessment (scenarios tobe considered, time frame of the safety assessment,
radiological standards, etc.) is to be presented by theregulatory bodies. Until then, the safety assessment
will be conducted based on a temporary frameworkof dose standards defined in relation to internationalguidelines, such as the ICRP recommendations9.
Scenarios that include events that lead to the lossof the isolation function or a significant reduction of
the containment function may not need to beconsidered if an appropriate site is selected, while thescenarios leading to radiation exposure via

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japangroundwater flow are the main focus of theassessment. In handling scenarios with significantly
different occurrence probabilities, the risk-informed
approach explicitly takes into account not only theconsequence but also the likelihood of occurrence.
Based on the above, the Safety Case at this stage
assesses post-closure safety involving7:
 a framework for safety assessment set on the basisofinternationalguidelines
and
other
considerations; a risk-informed approach, introduced in order toappropriately deal with various kinds of
uncertainties;
 the safety assessment analysis method isdeveloped in order to allow the objective
comparison and evaluation of performance ofdifferent repositories, reflecting differences ingeologicalenvironmental
conditions
andrepository designs;
 the post-closure long-term safety will be assessed
for the SDMs developed and the repository
designs tailored to them;
It should be noted that, in the absence of a specificsite, there are constraints on the extent to which
site-specific natural perturbations can be taken into
account in the safety assessment. Therefore, suchsite-specific issues will be dealt with conservatively,
based on expert opinion.
4. Conclusions and a look to the futureThis paper describes the overview, safety strategy
and approach for development of the NUMO Safety
Case considering the current boundary conditions,
including general purposes and context. In the
NUMO Safety Case, the geological environments ofvarious types of potential host rock are being
developed as SDMs, which have been developed asrealistically as possible based on the current best
understanding of Japanese geological environments.
A practical methodology for tailoring repository
design to volunteer siting environments and advanced
safety assessment models and data have beendeveloped and applied for demonstration of technicalbasis. The preliminary results of the design and safetyassessment would underpin the feasibility and safetyof geological disposal in Japan. NUMO and relevant
R&D organizations have started discussion on thenext mid-term R&D plan for geological disposal in
Japan, based on the technological issues extracted
through the development of this Safety Case. The
NUMO Safety Case will form a significant step on
the pathway towards repository implementation.

REFERENCES
1. JNC, “H12: Project to establish the scientific andtechnical basis for HLW disposal in Japan”,
JNC-TN1410-2000-001～004 (2000).
2. JAEA and FEPC, “Second progress report on
research and development for TRU waste disposal
in Japan -repository design, safety assessment andmeans of implementation in the generic phase-”,
JAEA-Review 2007-010 (2007).
3. METI, Advisory Committee for Natural
Resources and Energy, Electricity and Gas
Industry Committee, Geological Disposal
Technical WG, “Summary of Requirement andCriteria for Nationwide Map of ScientificFeatures for Geological Disposal” (2017). (inJapanese)
4. K. Ota, H. Saegusa, T. Kunimaru, S. Yamada,“Geosynthesis of a state-of-the-art knowledge
base into SDMs in the NUMO Safety Case”,
Proceedings of EAFORM 2017, Osaka, Japan(2017). (submitted)
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to SDM in the NUMO Safety Case”, Proceedingsof EAFORM 2017, Osaka, Japan (2017).
(submitted)
6. K. Yamashina, S. Suzuki, S. Kubota,“Preliminary study of pre-closure safetyassessment in the NUMO Safety Case”,
Proceedings of EAFORM 2017, Osaka, Japan(2017). (submitted)
7. K. Ishida, T. Hamamoto, S. Shibutani, K. Fujisaki,
M. Yamada, “Preliminary study of post-closure
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Proceedings of EAFORM 2017, Osaka, Japan(2017). (submitted)
8. T. Hamamoto, S. Shibutani, K. Ishida, K. Fujisaki,
M. Yamada, Y. Tachi, “Assessment of sorption
and diffusion in the rock matrix in the NUMO
Safety Case”, Proceedings of EAFORM 2017,
Osaka, Japan (2017). (submitted)
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Publication 122, annuals of the ICRP, Vol.42,
No.3 (2013).

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan