PRELIMINARY STUDY OF POST-CLOSURE SAFETY ASSESSMENT IN THE
NUMO SAFETY CASE
K. Ishida, T. Hamamoto, S. Shibutani, K. Fujisaki, M. Yamada
Nuclear Waste Management Organization of Japan
Mita NN Bldg. 1-23, Shiba 4-Chome, Minato-ku, Tokyo 108-0014 Japan
Key words; post-closure safety assessment, risk informed approach, radionuclide migration

Abstract
NUMO has developed a generic safety case for the geological disposal in Japan. This safety case provides themultiple lines of arguments and evidences to demonstrate the feasibility and the safety of the 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. The key distinguishing featuresof the safety assessment are “application of the risk-informed approach”, “scenario development withcomprehensiveness, traceability and transparency”, and “realistic radionuclide migration model reflecting therepository design”. For the purpose of optimization of protection, the disaggregated approach, as one ofrisk-informed approaches, which makes it possible to evaluate a potential impact of a future event and its
probability separately, is applied as a frame of the safety assessment. Developed scenarios are classified into 4categories on the basis of their probabilities; “Likely scenario”, “Less likely scenario”, “Very unlikely scenario”
and “Human intrusion scenario”. The methodology of the scenario development results from a desire to combine
a bottom-up what is called FEP-based approach and a top-down method based on the safety functions,appropriate to risk-informed assessment. The scenarios considered to be “likely” are developed to be as realistic
as possible, representing comprehensive current understanding of relevant FEPs in terms of extent and rate of
impact on radionuclide containment and eventual release and transport. The methodology consisting of overall
procedure and associated toolkits is aiming to increase traceability and transparency. For a rational repository
design, it is important that the assessment model can show the difference of the performance among candidate
designs, such as the panel layout and the disposal galleries. Then NUMO’s model is based on the 3D particle
tracking method which can effectively describes the radionuclide migration behavior reflecting the repository
design.
1. Introduction
The results from R&D carried out over 20 years
was integrated into a generic safety case for thegeological disposal in Japan and published in the H12report1 in 1999 in order to demonstrate the genericfeasibility of geological disposal of high-level
radioactive waste (HLW) in Japan. This provided thetechnical basis to formulate the Final Disposal Act,
which established the Nuclear Waste Management
Organization in Japan (NUMO) as the implementerof geological disposal of specified waste. A generic
study
on
safe
geologicaldisposalforintermediate-level waste generated from reprocessingof spent nuclear fuel and mixed–oxide fuelfabrication (termed TRU waste in Japan) was also
issued in 2007 (TRU-2 report2) and the Act wasamended to include TRU waste in the NUMO
geological disposal program.
NUMO has been promoting a stepwise siting
process based on a volunteering approach3 and, inparallel, carrying out an intensive R&D program
focusing on technical issues identified from H12 andTRU-2 in order to increase both the technicalreliability and also public confidence in the safety ofgeological disposal. As no volunteer site has yet
appeared, such R&D has been conducted without
reference to any specific candidate site or host rocktype. Taking into account the progress in scientificand technical knowledge resulting from this R&D,
NUMO is now developing an updated generic safetycase (NUMO-SC) that builds on H12 and TRU-2 todemonstrate technical feasibility and safety ofgeological disposal in Japan on the basis ofstate-of-the-art scientific knowledge and identify
important future R&D issues.
In line with the international manner (e.g. IAEA4
and OECD/NEA5), the general purpose and context
of the NUMO safety case is explicitly specified interms of the background outlined above and theprogress to date in defining required post-closure
performance goals. The potential settings for a
volunteer geological disposal project in Japan also set
key boundary conditions, requiring a safety strategy
that includes realistic assessment of pre- andpost-closure safety to allow both comparison of sites

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

and also possible repository concepts that could betailored to them6,7,8.
The geological environments of a range ofpotential candidate sites are represented as site
descriptive models (SDMs), which have beendeveloped as realistically as possible based on recent
progress in geosciences and site characterization
technology7. Appropriate repository concepts are thendeveloped by tailoring them to such siting
environments on the basis of a comprehensive set of
Design Factors, as described in Goto et al8.
This paper focuses on the approach andmethodology for scenario development andradionuclide migration modelling in the NUMO-SC.
2. Risk informed approach
Safety regulations for geological disposal have yet
to be formulated in Japan. Safety criteria and otherrelevant requirements for safety assessment havetherefore been assumed based on international
standards.
Although it is impossible to precisely predict the
future, estimates of doses and/or risks over very longtime periods can be derived for bounding future
evolution scenarios within a performance assessment
and compared with appropriate criteria to support the
post-closure safety. Referring to international
standards and guidelines, it has been recommended
that the likelihood of such scenarios should bedetermined (qualitatively and/or quantitatively) andincorporated into the assessment by either anaggregated or disaggregated approach4,9. These
recommendations have already been reflected insafety regulations in some countries (e.g. Germany10,
the UK11 and Sweden12, 13).
Referring to “the Commission considers that
although a similar level of protection can be achievedby these two approaches, more information may beobtained to reach risk-informed decisions fromseparate consideration of the probability ofoccurrence of a particular situation giving rise to adose, and the resulting dose.” in ICRP Pub.1229,
which recommends the regulatory framework of thegeological disposal, the disaggregated approach isapplied to the safety assessment of NUMO-SC.
To define scenario categories and target doses forgeological disposal, the following assumptions were
made:
• The depth for geological disposal largely
mitigates the effects of natural processes andhuman activities relevant to near surface andsub-surface disposal;• Perturbing natural events (extreme events) which
could cause significant degradation on the safetyfunctions of repository system should be
effectively excluded by the stepwise siting
process;•

The residual probability of extreme events thatcannot be precluded by siting is regarded as very
low, even over the long-term.
The scenario categories and target doses are defined
as follows:• “likely scenario”: 10 μSv/y• “less-likely scenario”: 300 μSv/y• “very unlikely scenario”: 20 - 100 mSv for the
first year, 1 -20 mSv/y after the first year.
• “human intrusion scenario”: 20 - 100 mSv forthe first year, 1 -20 mSv/y after the first year.
3. Scenario development methodology
In the late nineties, scenario development was often
described as a bottom-up process14, whereby
scenarios were developed in essence from FEP
databases. Nowadays, it is recognized that, in practice,
the approaches actually adopted are better describedas top-down or "hybrid", taking as their starting-point
an integrated (top-down) understanding of the system
under consideration, including uncertainties in theinitial state5. A bottom-up element remains to theextent that FEP databases or FEP catalogues are still
used, but the focus is generally on checking
completeness, which occurs parallel to the main
assessment process. Based on this perspective,
NUMO has developed a hybrid methodology,
combining top-down (approach using safetyfunctions) and bottom-up (FEP-based approach)approaches in a complementary manner (Fig. 1).
Geological disposal system
To define safety functions

SDM

DesignNUMO- FEP list

System understanding• THMC Evolution• External events (e.g. uplift-erosion)• “Storyborad”

To identify key safety-relevant FEPs
• To Identify factors affecting safety functions with “fishbone”

To evaluate effects key safety-relevant FEPs
on safety functions• Reserve FEPs
• Uncertainty analysis• “Tabular summary”

Scenario description

Figure 1 Overview of scenario development
The evolution of safety functions and the interaction
between them is developed based on the system
understanding which is closely linked to the site andthe design. It is described with “storyboard” in
NUMO-SC to understand the evolution of thegeological disposal system. The safety functions havethe role of identifying the key safety-relevant
processes/phenomena.
In addition, in parallel with the top-down approach,a comprehensive and NUMO FEP catalogue isderived from referencing generic FEP lists provided
by OECD/NEA15, taking account of other FEP
activities such as Japan Atomic Energy Agency FEPs

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

(JAEA16, 17) and geological disposal repository in the
NUMO-SC.
The relationship between the safety functions andkey relevant FEPs which could perturb suchproperties of the safety functions can be presented
thought “state variable”, which defines the state of
the safety function on “fishbone diagram” (Fig. 2).
As a result, FEPs that need to be considered can
explicitly be selected for classifying and checking
completeness of scenarios and extracting the key
safety-relevant uncertainties.

* Example of engineered
barrier / bufferState variable

State variable

Mineralcomposition

thickness
Corrosion (repository)

pH conditions

Creep (repository)・・・Concentration of disturbing
substance (repository)・・・

Retardation of R.N. due tosorption, matrix diffusion

REDOX condition

Safety function

Pore watercomposition
State variable

・・・

FEP Database

Figure 2 Structure of fishbone diagram

Each fishbone diagram is associated with asummary of knowledge-base consisting ofdescriptions of phenomenological understanding of
the FEPs that could drive system evolution anduncertainties together with handling them through thescenario analysis. An example for a part of bufferedsafety function (“Retardation of nuclide migration by
sorption”) is shown in Table 1 which is called“tabular summary”. The uncertainty and probabilityof scenarios are also discussed in the analysis toclassify scenarios into defined categories based on
the best current understanding of the FEPs anduncertainties summarized as in Table 1. The likely
scenario consists of most probable evolutions ofsafety functions, leading to be realistic.
In addition to the hybrid approach which ensures thecompleteness and comprehensiveness, the recording
materials, such as fishbone diagram and tabularsummary, make it possible to ensure traceability andtransparency.

Table 1 Tabular summary of the issues associated with buffer performance
ReferenceState variable
FEP

pH condition(repository)

Redox
condition(repository)

Knowledge about the effects
When the pH rises, secondary
minerals such as iron hydroxide
and CSH gel increase.
The pH and secondary minerals
react commutatively.

Under the oxidizing atmospherereducing agents such as pyrite
oxidizes and the mineralcomposition changes.

For the co-disposal, the high pH
plume originated from the TRU
disposal facility reaches to likely
Mineralization perturbing
species’cause a deterioration by
concentration dissolution or secondary-mineral(repository)
precipitation.
(Refer to FEP sheet "3.2.4.1-H
pH condition (repository)")

corrosion(repository)

Definition of handling through thescenario analysis
 Although there is an uncertainty in the change ofmineral composition, it is shown by the analysis that
For the likely and less-likely
the gap clogging by the secondary mineral localizes
scenarios, parameters are set in
the affected area to the boundary.
consideration of the impact
 It is shown by the analysis that, even if the gap
variation range.
clogging is not caused, the change of most buffer
Uncertainties to be consideredmaterials requires 10 thousand years or more.
 It is shown by the analysis that migration of theoxidation-reduction front by the alpha radiolysis issuppressed by the reaction with the reducing agent,such as pyrite in the buffer material.
 The time of migration is limited after the overpack
opening even if the occurrence is assumed.

The impact can be disregarded by sufficient
separation of the HLW and TRU facilities in the caseof co-disposal.

The change of redox condition
resulting from the waste bodyradiation and the oxidizing
groundwater penetration from theoutside are not dealt with by the
likely and less-likely scenarios.

It is not dealt with by the likely andless-likely scenarios.

The Fe ion produced by
 The analysis shows that the gap clogging at thereceptacle corrosion causes
iron-bentonite boundary limits the affecting area
phenomena, such as ion
and that the change from the initial mineralexchange of the bentonite,composition in the region of most buffer materials is
For the likely and less-likely
dissolution of the montmorillonite
small.
scenarios, parameters are set inand secondary-mineral The time when the possibility of a tychopotamic loss
consideration of the impact
precipitation, to change thedue to the buffer-material change by the iron impact
variation range.
mineral composition. But, thecannot be denied thoroughly is after tens ofcomposition change is localized
thousands years (based on the quantitative ratio ofby clogging of the gap between
the iron cumulative supply by the receptacle
the bentonite and overpack.
corrosion and the buffer material).

4. Realistic radionuclide migration modelingreflecting the repository
It has been assumed to be important for dose
calculation that groundwater is media forradionuclide migration because of its relatively
higher probability. Therefore, its assessment, whichis required more realistically than any other, isfocused on in this report.

smaller scale is provided boundary conditions fromthe larger scale and gives calculated radionuclide flux
at its boundary to the larger. In case of regional scale
and biosphere, their boundary conditions are based onnationwide information in Japan.
As the near-field scale targeting more safetyfunctions than others requires more realistic
assessment, it is focused on in this report.

4.1. Approach of safety assessment
Taking account of both geometry and size ofcomponents of the geological repository, themigration regions from the waste to biosphere are
divided into 4 regions; near-field scale, repository
scale, regional scale and biosphere (Fig. 3). The

4.2. Radionuclide migration model
NUMO has developed a three-dimensional“PARTRIDGE” (Particle tracking in deep geologicalenvironment)18 that allows random-walk simulationof nuclide transport in a spatially-heterogeneous flow
field. This is intended to evaluate differences among

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanrepository design options and host rockcharacteristics.
PARTRIDGE consists of a groundwater flow
module and a mass transport module. The
groundwater flow module can deal with adeterministically-defined high permeability zone suchas a fault, a statistical discrete fracture network andalso engineered structures, including access tunnels
and the EBS (Fig. 4). These are expressed as anequivalent heterogeneous hydraulic conductivity
tensor, using the Crack Tensor method andnumerically solved by using FEMWATER code18.
The Darcy velocity obtained from FEMWATER is
distributed among the fractures and rock matrix (incase of hard rock, flow in the matrix is assumed to bezero). Thereafter, the pore velocity in fractures isevaluated and used in the nuclide transport analysis.
Based on the groundwater flow analysis, 3-D masstransport from the individual waste packages isanalyzed by the random-walk method considering
advection, dispersion and matrix diffusion in themass transport analysis module. Transport of nuclides
is simulated by an instantaneous pulse source ofparticles (Fig. 4).
PARTRIDGE is a powerful tool to realisticallydescribe the radionuclide migration behavior.
However, it is unsuitable for a large number ofradionuclide migration calculations at present,
because the calculation cost is too huge andimprovements are required to handle chemical
reactions and radioactive decay.
Therefore, 1D multi-channel migration model isadopted for dose calculations as well as H12 andTRU-2. However, unlike fracture features observed at

the bore hole surface were directly used fordeveloping channels in those previous reports,
fracture connectivity and geometries of artificial
components are reflected on model using 3D particle
tracking with non-sorbing particle as follows;• Calculation of particle flux（ϕ3D(t) ) at 100 m
downstream from the gallery• Approximation of ϕ3D(t) by transmissibility
coefficient (TNFi(t)) and weighting factor Wi ofeach channel the channels of the host rock.

Figure 3 Relationship between 4 regions

Figure 4 Framework of development of migration modeling4.2. Migration parameter value setting
Several reactive transport analyses and thermal
conductivity analyses have been conducted at the
scenario development stage to understand the systemevolutions, such as bentonite alteration, concrete
structure alteration and thermal environment changearound the wastes (Fig.5). “Solubilities”, “Kds” and6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

“Des”, are defined also using results from thoseanalyses.
“Solubilities” are derived from pore water chemistryin HLW buffer and TRU mortar infilling material
from thermodynamic calculations. To define the porewater of the HLW system, ion exchange reaction ofmontmorillonite, acid-base reactions of hydroxyl
groups on montmorillonite surface, dissolution andprecipitation reaction of accompanying minerals andreactions of iron corrosion products are taken into
account. The Na-bentonite will persist for a long timeon the basis of mass balance arguments, leading tochemical condition of the pore water can be definedas constant through the assessment time. In contrast,in the TRU system, the chemical condition is definedto evolve from Region I to Region II as alkali
components are leached from the filler (mortar).

Figure 5 Bentonite alteration due to iron-bentonite
interaction.

Assuming
thermodynamic
equilibriumandsolubility established by pure solid phases, thesolubility of relevant elements can be calculated.
Important uncertainties associated with such
solubilities include lack of empirical data of thermal
effects and kinetics. For thermal effects, solublities
using data-set at 25°C are adopted at thermal
conditions (30°C or 45°C) at each repository depth
because almost thermodynamic data have empirically
been obtained at the room temperature (25°C) and thedifference between 25°C and 45°C can be assumed
not to be significant19. For kinetics, solubilities areconservatively assessed assuming control byamorphous phases for many elements in the likely
scenario; in real life these would gradually age tomore crystalline forms with associated reduction insolubility.
“Kds” for bentonite are defined by approximation of
results from diffusion experiment using of compacted
bentonite by the modified Fickian approach becausebatch experiments can’t realistically describe theradionuclide sorption behavior in the compacted
bentonite. While “Kds” for host rocks are logarithmic
mean values of data around the groundwaterconditions in JAEA-SDB(Sorption Database)20.
Uncertainty relevant to thermal condition is expected
to be significant because of reasons similar to“solubilities”.
“Des” of nuclides measured in the buffer varydepend on their electric charge and the bentonite
density. The charge of each element is set based on
the dominant chemical species obtained as a result of
thermodynamic equilibrium calculation with the porewater. Neutral ion, cationic ion and anionic ion havebeen reported to be dependent on the ionic strengthand montmorillonite density21.“De” for host rock has
also been reported to depend on electric charged
states of elements and porosity of host rock. “Des” forspecified conditions are derived from empiricalequations based on measured data in the JAEA-DDB(Diffusion Database)22. Since “De” is well known tohave temperature dependency, correction is madeaccording to the expected temperature at each
repository depth (30°C or 45°C).
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November 27-29, 2017, Osaka, Japan

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