Study on gas migration behavior through bentonite buffer material
Tomoyuki SHIMURA*, Shinichi TAKAHASI*, Masanobu NISHIMURA*,
Kazumasa KOGA**, and Hitoshi OWADA*** Obayashi Corporation, ** Radioactive Waste Management Funding and Research Center* 2-15-2, Konan, Minato-ku, Tokyo, 108-8502, Japan.
** 6-4, Akashicho, Chuo-ku, Tokyo, 104-0044, Japan.
Key words; Radioactive waste disposal, bentonite buffer, gas migration, performance assessment

Abstract
Gasses generated due to the corrosion of metals contained in the waste packages and engineered barrier materials
will accumulate in the Engineered Barrier System (EBS) of a radioactive waste repository, and may affectlong-term stabilities of the EBS. To evaluate gas migration behavior through the bentonite buffer materials in
conjunction with a reasonable design model is one of the challenges to be solved for the performance assessmentof a radioactive waste repository.
A series of studies have been conducted for more than 10 years under the leadership of the Radioactive Waste
Management Funding and Research Center (RWMC) in Japan to better understand gas migration behavior
through bentonite. In this report, the outline of the framework of the gas migration study by RWMC as well assome of the results from laboratory gas injection tests are presented.
The laboratory test results focusing on the interfaces in bentonite materials (bentonite/bentonite interfaces)
showed that water flow through the interface was blocked (self-healed) due to the swelling of the bentonite. In
addition, the gas injection test after full saturation, indicated no significant differences in breakthrough pressurecompared to the intact (e.g. no interface) specimen. These findings suggest that the interface is unlikely to
become a dominant gas migration pathway in a saturated buffer.
It is expected that all findings acquired through this decade of study are summarized and integrated within thisfiscal year (2017) taking into consideration the parallel studies on gas scenario development and integration of
coupled modelling, and contribute to a reliable advanced repository design and performance assessment.

1. Introduction
The long-term stabilities of radioactive waste
repository are generally secured by a multibarriersystem consisting of Natural Barrier and Engineered
Barrier System (EBS). A certain amount of gas will
be generated by corrosion of metals contained in thewaste packages and EBS materials. Such gases willaccumulate in the EBS of a radioactive waste
repository and may affect the long-term stability of
the EBS. To evaluate gas migration behavior throughthe bentonite buffer materials, in conjunction with areasonable design model, is one of the challenges tobe solved for the performance assessment of aradioactive waste repository [1].
Extensive studies on gas migration behavior havebeen carried out. One of the famous international
study is the FORGE (Fate Of Repository GasEs)
project funded by EC. The following common
challenges for the understanding of gas migration
properties have been identified through the FORGE
project [2]:- Treatment of anisotropic micro phenomena withina macro model.
- Treatment of coupled phenomena considering both
EBS stability and nuclide migration over time.
- Up scaling from laboratory to realistic scale withregard to phenomena understanding.
Based on the issues identified from the findings ofthe in-situ large-scale Gas Migration Test (GMT,1997~2007) at Grimsel Test Site (GTS) [3], RWMC
has conducted a series of studies from 2008 to better
understand gas migration behavior through bentonite.
In order to find solutions to the internationallyidentified issues with regard to gas migration in EBS,
research and development focused on the following
three items: obtaining better date through laboratory
testing, advancement of analysis tools and scenario
development [4].
In this report, the outline of the framework of thegas migration study by RWMC, as well as some of
the results from laboratory saturation and gas
injection tests, are presented.

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

Fig. 1 Overview of RWMC’s study on Gas migration

2. Studies on gas migration by RWMC
The studies on gas issues, e.g. gas generation, gas
migration in EBS, and effect on long-time repository
stabilities, are being conducted by JAEA and NUMO
for Japanese HLW and Transuranic (TRU) waste
repositories [5], [6], [7]. Especially in Japanese TRU
waste repository, the following phenomena or criticalevents have been considered in migration of gases
generated in the waste packages, that could
significantly influence the performance of thedisposal system:- Gas transport and diffusion to the outside of the
EBS in the form of dissolved gas in groundwater.
- Gas pressure pushing contaminated water out fromthe waste package and disposal cavern into thegeosphere.
- Excess gas/water pressure causing stress on the
EBS components leading to mechanical failures
and preferential pathway creation, allowing
gas/water to seep out.
These phenomena have been considered in the
groundwater advection scenario as increased nuclide
migration due to the expulsion of contaminated water
resulting from gas generation in the EBS. The
performance assessment has been conducted with thebasic expulsion model. It showed that the maximum
dose calculated was not significantly different fromthe results of the reference case.
Due to the background mentioned above, the stillongoing study on gas migration in Japanese TRU
waste repository was initiated in 2008 by RWMC
with the purpose of reducing the uncertainties in theperformance assessment of a multibarrier system.
Fig. 1 shows the interrelationship of each activity of
RWMC’s gas study project.
RWMC’s study on gas has been ongoing for overten years in two phases. In the first phase(2008~2012), studies on gas migration behavior wereconducted that take into consideration the issues
identified in the TRU 2nd progress report [6],specifically laboratory gas injection tests forunderstanding gas/water permeability in bentonite
material, development of integrated modeling andanalysis method, and scenario assessment withconsideration of the transition of repository. While
substantive findings relevant to gas migration in
bentonite were acquired through the first researchphase, several issues, which arose from uncertainties
of gas behavior, remained.
In the subsequent second phase (2013~2017),
laboratory tests targeted at not only bentonite
materials but also cementitious materials haveproceeded to enhance data related to gas/water
permeability, which contribute to reduce theuncertainties of gas migration behavior. In addition,modeling approaches to apply the parameters
estimated from laboratory test results have also been
investigated to trace laboratory test sequence withhigher reliability.
Through a series of studies, it has been foreseenthat the interfaces between bentonite materials may
induce preferential pathways for gas generated in a
waste package (cf. Fig. 2). RWMC has therefore

Fig. 2 Preferential gas pathways due to interfaces

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japaninitiated a laboratory gas injection test program toevaluate the effect of interfaces (bentonite/bentonite
interfaces), which are expected to exist in bentonite
materials, on gas migration behavior. The gas
injection tests have focused on three phenomena,(re-)saturation, gas injection, and gas breakthrough(sudden flow increase), in a potential gas migration
scenario.
The latest findings through the gas injection test
will be discussed in the next chapter.

Outflow (gas/water)
CentralOutsideO-rings

Porous metals
Stainless column

Specimen

Injectionline (water)

3. Laboratory tests focusing on the existence ofinterface in bentonite(1) General idea and condition of the test
The laboratory equipment consisting of gas/water
injection system, gas/water outflow measurement
system, test column, and operation panel was set up
prior to initiation of the gas injection test. Two typesof Na bentonite (Kuni-gel V1) column specimens,with and without interface were prepared respectively.
Both specimens had a diameter of 60mm and a heightof 25mm and were placed into the column of the test
equipment as shown in Fig. 3. The specimen wascompression-molded as two layers to reach therequired dry density. The specimen without interface
was named Case-1 and the one with interface wasnamed Case-2. With respect to the initial condition ofboth specimens, the dry density of 1.36 Mg/m3 was
set as referred to in the concept of the TRU 2nd
progress report. The water content of 32.4%(saturation ratio of ca. 90%) was set to reduce theduration of saturation (cf. Tab. 1). The specimen wascompression-molded statically (compression speed of
1mm/min) by two layers to reach the required dry
density. As for the Case-2, a plate of 1mm thickness

Partition O-ring

Load cell

Injection line (gas)Fig. 3column test equipment

Tab. 1

Specification of specimen

was set at the center of the column container which
was removed after compression.
The (re-)saturation phase was initiated by water
injection from the bottom of the column. At the
beginning of water injection, no water pressure wasapplied to prevent the appearance of flow paths at theouter peripheral surface of the column specimen, andsaturation proceeded only by capillary pressure of thebentonite material. After confirmation of stable water
injection, the injection pressure was increased to 0.2

Bentonite specimen (Φ60mm)

Interface (w=1mm)

Before water injection

After saturation
Fig. 4 Bentonite column with interface

Fig. 5 Flow chart of test procedure

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

MPa which was lower than the expected swellingpressure (ca. 0.5 MPa) of the bentonite specimen.
Full saturation of the specimen was comprehensively
assessed by multiple factors such as water seepage
from the top surface, total volume of injected water,
flow balance of inflow and outflow, water
permeability, and pH. Two fully saturation specimens
were made for Case-2 so that the actual saturation
ratio could be assessed by dismantling one specimen
before proceeding to the gas injection phase. Fig. 4
shows the top view of the Case-2 specimen: before
initiating water injection (top), and after saturation(bottom). The interface of the bentonite self-healed
due to the swelling.
After confirming saturation of the bentonite
specimen, the gas injection phase was initiated byclosing the water injection valve and opening the gas
injection valve, both set at the bottom of the column
specimen. Nitrogen gas was injected under abackpressure of 0.1 MPa after which the gas injection
pressure was increased by stepwise pressurization of
0.1 MPa per two days until gas breakthrough (sudden
flow increase) occurred. The temperature oflaboratory was kept at 25 degree centigrade during all
test sequences.
Fig. 5 shows the flow chart of the test procedure.
(2) Test results of Case-1: without interface
As test result of the (re-)saturation phase of Case-1,
cumulative amount of water inflow, water injection
pressure, and swelling pressure during the water
injection are summarized in Fig. 6. In the first 3 daysof the water injection, the saturation proceeded only
by capillary pressure of the bentonite as there was nowater pressurization. After that water injectioncommenced at 0.2 MPa injection pressure.
Water seeping from the top surface was observed
after 36 days. The cumulative amount of injectedwater at that time was around 8.4mL which wasreasonable considering the volume of the water
needed for saturation of the specimen (3.49 ml) andvoid volume of the porous metal set onto the top
surface of the specimen (3.86 ml). The water
permeability determined was 6.6x10-15 m/s, which isa lower value compared to past research for the 1.36
Mg/m3 dry density of bentonite (10-11~10-13 m/s). The
pH value of the outflow was weak alkaline at around8 which suggested that the water came from thebentonite specimen. Based on a comprehensive
assessment of these results, the water injection /saturation phase was finished. As Fig. 6 indicates, theswelling pressure measured during the test was 0.47
MPa.
Following the saturation phase, the nitrogen gas
injection phase was initiated. A back pressure of 0.1
MPa was applied to the specimen in accordance withthe test procedure, then the gas injection increased
stepwise by 0.1 MPa per two days. Fig. 7 shows thechange of effective gas pressure and gas inflow
during gas injection. As result of the gas injection
increase of 0.1 MPa per two days, the gas
breakthrough with sudden flow increase occurred
after 29 days, when gas injection pressure reached 1.6
MPa. Gas injection was automatically stopped after
the gas flow rate reached 1,000 Nml/min due to theoperation of a safety valve, at which time the wholetest sequence of Case-1 was completed.
(3) Test results of Case-2: with interface
As test result of the (re-)saturation phase of Case-2,
which included an interface at the center part of thecolumn specimen, cumulative amount of water
inflow, water injection pressure, and swellingpressure during the water injection are summarized inFig. 8, as with the Case-1 results. In Fig. 8, theswelling pressure was calculated by subtracting waterpressure from the total pressure measured at thebottom. After 42 days from the start of water
injection, water seeping from the top surface wasobserved and it was confirmed that the total amountof inflow and outflow were consistent. After fillingthe outflow piping with water, the measured inflow
rate and outflow rate were balanced to determine thepermeability of the bentonite specimen. The water
permeability estimated was on the order of 10 -13 m/s
which is consistent with past research results. Basedon a comprehensive assessment of these results, thesaturation phase was finished after 100days of water
injection at which time the observed swellingpressure was 0.33 MPa. The swelling pressure waswithin the variation range of past research [8]although the pressure was slightly lower than for
Case-1 (no interface).
The gas injection phase was initiated after
confirming the saturation by the same procedure(nitrogen gas injection, and pressurization of 0.1 MPa
per two days) as for Case-1. The change of effective
gas pressure, water outflow from the top surface, andeffective stress at the column bottom during gas
injection phase are shown in Fig. 9. As the result of
the stepwise increase of gas injection, effective stressat the bottom gradually decreased accompanying the
stepwise pressurization. Gas breakthrough occurred
after 25 days when gas injection pressure reached 1.3
MPa, which is slightly lower than observed for
Case-1. The gas injection was stopped afterwards andthe test specimen was dismantled to investigate theinside of bentonite material.
Fig. 10 shows the distribution of water content
inside the Case-2 column specimen before gas
injection (left) and after gas breakthrough (right).
Both profiles were measured from sliced fragments
by dismantling as two saturated specimens wereprepared. The water content before gas injection
decreased from the bottom toward the top of thecolumn. Moreover, it was confirmed that the water

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

content around the interface is slightly higher,
compared to other areas. One of the reason for thevariation of water content from the bottom to the top
is thought to be due to the time lag of swelling andconsolidation in terms of places and time with thewater injection from the bottom to top. The time lagof swelling causes spatial differences of density andthe water content will change accordingly. Similar
phenomena are likely to occur around the gap 1mm

wide. In the profile of the after gas breakthroughsample, a decrease of water content from the top
toward the bottom is observed, but no significant
differences are seen between areas in the vicinity of
the gap and other areas. One of the reason of thisphenomena is thought to be that the gas progressed
gradually inside of the specimen from the bottom andpushed out the pore water at the same time.
Continuation of gas flow after gas breakthroughFig. 6 Saturation phase (Case-1)

Fig.7 Gas injection phase (Case-1)

Fig. 8 Saturation phase (Case-2)

Fig.9 Gas injection phase (Case-2)

Fig.10 Measurement of water content inside the bentonite
Case-2, after saturation (left), and after breakthrough (right))

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japancould also be a factor reducing the water content.
Meanwhile, the water content at the top surface of thespecimen before gas injection and after breakthroughshow almost identical values. This could be because
the gas breakthrough occurred before the gas front
reached the top surface of the specimen, although it
must be noted that both specimens were not the same.
(4) Lessons Learned from laboratory tests
The gas breakthrough pressure obtained throughgas injection test was 1.6 MPa in Case-1 (nointerface) and 1.3 MPa in Case-2 (with interface).
Both pressures were slightly different but reasonable
well within the range of variation observed in paststudies. Therefore, it is thought that the existence of
the interface has no effect on the gas breakthroughpressure although this is based on the limited resultswith a 1mm wide interface. The profile of watercontent after gas breakthrough does not show obvious
differences between interface vicinity and other areas
and this means that the interface does not function asa preferential gas pathway. In addition, it isrecognized that the interface may affect the
distribution of water content and dry density in an
extremely small range.
As 1mm width of the interface was chosen for thisstudy, the effect of different widths would be an issue
to be confirmed in further studies.

4. Conclusions
As the latest findings through the RWMC’s study
on gas migration behavior which proceeded over ten
years, the laboratory gas injection test to evaluate theeffect of interfaces expected in bentonite material
was discussed in this paper.
The major findings of this study are:
- The existence of interface has no significant effecton the gas breakthrough pressure.
- The interface does not play a role of a preferential
gas pathway.
- The interface may affect the distribution of thewater content and dry density in an extremely
small range.
- The effect of the width of interfaces would be anissue to be confirmed in future studies.
Further challenging test program focusing on thewidth of interface and the interface between different
materials are proceeding under the direction of
RWMC.
It is expected that all findings acquired through thisdecade of study are summarized and integrated
within this fiscal year (2017) taking into
consideration the parallel studies on gas scenario
development and integration of coupled modeling,
and contribute to the reliable advanced repository
design and performance assessment.

Acknowledgement
This research was initiated within a project todevelop Geological Disposal Technologies in Japan,
which was funded by the Ministry of Economy,
Trade and Industry (METI), Japan.

References[1] Namiki. K, Asano. H, Takahashi. S, Shimura. T,
Hirota. K (2014): Laboratory gas injection testsof compacted bentonite buffer material for TRU
waste disposal, Geological Society London,
Special Publications, first published May 12,
2014.
[2] Shaw. RP (2014): Overview and Key
Achievements of the FORGE Project., FORGE
Report D0.07-R.
[3] Shimura. T, Vomvoris. S, Lanyon, G. W.,
Marschall. P, Ando. K, Yamamoto. S, Asano. H(2008): Gas Migration through Bentonite/Sand:
Lessons Learned from the GMT in situ Test,
Science & Technology Series n° 334, Andra.
[4] Koga. K, Owada. H (2016): Study on coupledgas migration behavior in TRU disposal facility(1) -Outline of project-. Proceedings of 71st
JSCE Annual Meeting, CS13-040 (in Japanese)[5] JNC (Japan Nuclear Cycle Development
Institute) (2000): H12: Project to Establish the
Scientific and Technical Basis for High-Level
Radioactive Waste Disposal in Japan: Second
Progres Report on Research and Development
for the Geological Disposal of High-Level
Radioactive Waste in Japan, Supporting Report
2, Repository Design and Engineering
Technologies, JNC TN1410 2000-003.
[6] JNC (Japan Nuclear Cycle Development Institute)and FEPC (Federation of Electric Power Companies
of Japan) (2005): Second Progress Report on

Research and Development for TRU Waste
Disposal in Japan, JNC TY1400 2005-013,
FEPC TRU TR 2-2005-02.
[7] NUMO (Nuclear Waste Management Organizationof Japan) (2013): A study on improving thesafety of TRU waste disposal in Japan -Study on
disposal system design aimed at improving
disposal safety and the reliability of theassessment of TRU waste disposal for a range of
geological conditions-, NUMO TR14-03.
[8] Fujiyama. T, Tanaka. Y, Hironaga. M, Mori. A(2008): Literature survey on evaluation of gas
migration through bentonite and cementitious
materials., CRIEPI Research Report, N07021.

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