MIGRATION EXPERIMENT OF VOID AIR IN BUFFER MATERIAL DURINGSEEPAGE
Naohiko TAKAMOTO*1, Ichizo KOBAYASHI*2, Masahiro KAWAKUBO*1,Masataka IMAI*1, Tomoko ISHII*1*1 Radioactive Waste Management Funding and Research Center
Nichirei Akashicho Bldg. 12F, 6-4, Akashicho, Chuo-Ku, Tokyo, 104-0044, Japan*2 Kajima Corporation
2-19-1, Tobitakyu, Chofu City, Tokyo, 182-0036, Japan
Key words: Geological disposal, Engineered barrier, Buffer material, Seepage, Void airAbstract
The buffer material composing the engineered barrier system for the geological disposal of high-levelradioactive waste in Japan is expected to control migration of groundwater and radionuclides. The Radioactive
Waste Management Funding and Research Center has been focusing on the re-saturation process of the buffermaterial with groundwater after emplacement, and researching mainly the behavior of buffer material based onlong-term laboratory tests. The state transition of the buffer material is the most significant during this period,
and the state after the period will affect the continual long-term behavior. Therefore, it is important to predict thestate after the re-saturation process, for assessing the long-term performance of the buffer material. In this study,
we conducted a one-dimensional long-term laboratory model test focusing on the migration of void air due to theseepage and swelling of buffer material during the re-saturation process, and obtained data for analyticallyevaluating the seepage process of the buffer material. The result showed the void air pressure increased very
gradually. This implied that the pressure increase of void air was mitigated by dissolution of the void air into
water in parallel with water seepage into the buffer material. Moreover, it was also noted that the seepage rate ofthe buffer material was slowed by trapped void air. In the future, it is necessary to consider the influence of heatfrom the waste; it is possible to predict the state of saturated buffer material and evaluate the breakthroughoccurrence of the buffer material due to the pressure increase of the void air, by constructing an analyticalmethod to reproduce the results of this model test and extrapolating the influence of heat and seepage behavior.

1. Introduction
There is a broad international consensus that
passive safety of geological disposal of radioactivewaste will be ensured by multiple safety functions
such as combination of a natural barrier and anengineered barrier system (EBS). In the Japanese
underground environment, a large amount ofgroundwater is expected in general, so the buffermaterial composing the EBS for geological disposal
is required to ensure several functions, e.g.,mechanical protection, hydraulic barrier, chemical
buffer, and controlling migration of radionuclides.
It is assumed that long-term maintenanceavailability of functions expected of buffer materials
is greatly affected by the initial state change in there-saturation period after emplacement. Thus, it isimportant to predict the state of buffer materials after
the re-saturation process, for assessing the long-term
performance of the EBS and long-term safetyassessment of the geological disposal system. The
Radioactive Waste Management Funding andResearch Center has been acquiring mainly long-term
data on swelling behavior, chemical alteration,mechanical performance change, groundwater

seepage, and gas migration, in order to understand thebuffer material behaviors during the re-saturation
process by groundwater after the installation of thebuffer material. In this study, we focused on themigration of void air in the buffer material during there-saturation process, and the void air pressure in asystem in which void air is confined was measured
by test equipment simulating one dimension. We also
obtained data to analytically evaluate the possibility
for breakthrough occurrence of buffer material due tothe increase in void air pressure and the seepage /swelling behavior of buffer material up to saturation[1].

2. Methods for migration of void air experiment
Using test equipment simulating one dimension
with 50 mm diameter and 1000 mm height as shownin Fig. 1, the swelling process associated with thegroundwater seepage after setting the buffer material
was simulated, and a long-term experimentallaboratory scale test was conducted, to measure thebehavior of pressure increase due to compression ofvoid air during the seepage process. In the actual

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanrepository, groundwater seepage due to spring waterfrom the bedrock occurs from around the buffermaterial toward the inside, so that void air may be
trapped inside the buffer material, and uniform
saturation of the buffer material may be inhibited. Asa result, void air pressure may increase during
re-saturation. Therefore, in this test, it is assumed thatthe void air was trapped inside the buffer material,and seepage water was fed from top and bottom inthe test equipment. The void air pressure wasmeasured at the center of the cell. Because migrationof the void air indicates a change in degree ofsaturation, the saturation change of the buffermaterial was measured. Resistivity measurement wasused as a method of measuring the saturation change
in the buffer material without disturbing the specimen.
In this resistivity measurement, a current is passed
between electrodes in contact with the buffer material,
and the degree of saturation is calculated from theresistance value between the electrodes. The
correlation between the resistivity and the degree ofsaturation for buffer material was previously obtainedby a preliminary laboratory test (Fig. 2) [2]. In the
one-dimensional test equipment, 39 electrodes wereinstalled at intervals of 25 mm, the resistivity value
was measured, and the change in the degree ofsaturation for the buffer material was investigated. Amixture of bentonite (Kunigel V1) and sand wasemployed as a buffer material, with silica sand of 30
wt% against total weight. The buffer material wascompacted and filled in an acrylic resin cell to avoid
the occurrence of a gap at the interface between theacrylic resin cell and the specimen. The dry densityof the specimens was 1.6 Mg/m3. The volume of void
air in the buffer material at this density will beapproximately 20% to 40% under the pressure of one
atmosphere. Table 1 shows the test conditions andmeasurement items.
To measure void air pressure, as conditioning of
the state in the cell, two water feeding methods offlow rate control and pressure control were used. Inorder to avoid breakthrough occurrence of the buffermaterial due to the initial sudden feeding of water
under pressure control, distilled water was fed byflow rate control at the initial stage of the test until
the buffer material swelled to a certain extent.
Subsequently, the pressure control was switchedwhen it was confirmed from the resistivity
distribution that the seepage region was generated (50days later). The resistivity distribution, the amount offeeding water, the void air pressure, and the feedingpressure were measured at regular intervals. The
pressure was measured with an absolute pressuregauge to avoid the influence of atmospheric pressure.
The feeding water pressure was gradually increased
to avoid breakthrough occurrence of buffer material
due to sudden feeding of water; the behavior of void
air pressure was measured at 876 kPa (corresponding
to 1 MPa of a general gauge pressure gauge). As forthe seepage rate in the situation where the void airwas not trapped, the test was separately carried out
under the water feeding condition from the lower partusing the same shaped test equipment (the setting forwhich the air escapes to the top part) [1, 3].
Syringe pump
inlet
Flow/Pressure
control

Bentonite spesimen

tierod（50φ×4）

filterPorous metal

0.03

Air compressor

Acrylic cell

1.0

Pressure gauge

Electrode for specific resistance measurement@25mm×39ch

0.03

0.3
0.1

inlet

Syringe pump

Flow/Pressure
control
unit：mFig. 1 Test equipment for migration of void air in the saturating buffer material

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

Fig. 2 Relationships between resistivity and degree of saturation of buffer material with various dry densities [2]Table 1 Test conditions and measurement items
Initial water content

10 %

1000

800

was formed. Therefore, it was switched to pressure
control on the 53rd day after starting the test. Fig. 3
shows the temporal change in the void air pressure
and inflow rate after switching to pressure control. To
avoid breakthrough due to water feeding through the160
160

800

1.0
1.00.8流速（下部）
流速（上部）

876kPa

送水圧 (ｋPa)

Distilledwater

Resistivity
Amount of feeding water
Void air pressure
Feeding pressure
Swelling pressure

Voidair pressure at center of specimen
供試体中央部圧力
Feeding
給水圧pressure
供試体中央部圧力
Inflowrate (bottom of specimen)
流速（下部）
給水圧
Inflowrate (top of specimen)
流速（上部）

1000

Feedingpressure(kPa)
送水圧(ｋPa)

Measurement items

1550.8

155

600

0.6

150 0.6600

400150

400

145

200
145
200

0.4

流量（mL/Day）

3. Results and Discussion
After 50 days from the start of the test, regionswith decreased resistivity were formed at the upperand lower ends of the specimen. This means that acertain degree of seepage area due to feeding water

Liquid

供試体中央部圧力 (kPa)
流量（mL/Day）
Inflowrate (mL/day)

1.6 Mg/m3(mixture of bentoniteand silica sand)Kunigel V1: 70 wt%
Silica sand: 30 wt%

Void air pressure (kPa)
供試体中央部圧力(kPa)

Bentonite dry density

0.4

0.2

0.2

0000500

1000

試験時間 (Day)500 550th1000

1400.01500
1400.01500

試験時間
Elapsedtime(Day)(day)
Fig. 3 Temporal change in the void air pressure and inflow rate

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanbuffer material, the feeding pressure was increased
step by step with the initial pressure of 10 kPa.
Finally, the feeding pressure was increased to 876
kPa on the 550th day. At the stage where the feedingpressure was low, the void air pressure decreased.
Because this implied that the seepage rate due to thecapillary phenomenon of the pore water was fasterthan the feeding rate, negative pressure seems to havebeen reached in this state. In addition to the above,
there is a possibility that the void air pressure
decreased due to dissolution of void air into porewater. However, at this time, the surface area andamount of pore water were small, so the influence forincrease of void air pressure due to dissolution ofvoid air in pore water was presumed to be small.
When the feeding pressure was set to 876 kPa, thevoid air pressure was increased. At this time, it wasimplied that a saturated region confining void air wasformed. Subsequently, as the seepage of the central
unsaturated region progressed, the void air pressure

increased with a constant gradient: the increase wasabout 3 kPa in about 2 years and was a very slow rate.
After setting the feeding pressure to 876 kPa, thefeeding rate stabilized and gradually decreased
compared to the period before the feeding pressure
was set to 876 kPa. This implied that irregular void
air migration had not occurred and void air pressurechanged by compression of the void air volume due
to water seepage. At this time, the balance of masstransfer to the specimen seems to have been
dominated by feeding and dissolution of the void air
into the pore water. Therefore, the pressure increasesof the void air caused by the groundwater seepage
from the surroundings of the buffer material wereexpected to be mitigated by the dissolution of thevoid air into the pore water, and it was consideredthat breakthrough of the buffer material due topressure increase in void air did not occur.

Fig. 4 Temporal transition of the saturation distribution converted from the resistivity (pressure control process)Fig. 4 shows the temporal transition of thesaturation distribution converted from the resistivity
during the pressure control process. Under thecondition that the initial water content was 10%, thedegree of saturation of the buffer material was about
40%. As shown in Fig. 4, the degree of saturation
gradually changed from the top and the bottom
toward the center of the specimen, and the degree ofsaturation reached to about 90% at the top andbottom. However, from the 400th day, the sresistivity of the upper and lower parts tended todecrease. These decreases in resistivity mean

decreases in the degree of saturation when the dry
density is constant. On the other hand, when thedensity changes, it does not necessarily mean achange in the degree of saturation. As the swelling
proceeds, the upper and lower end faces of thespecimen may be compressed by the specimen
swelling and the dry density may increase. In thiscase, assuming that the degree of saturation isconstant at 100% and calculating the density from thechange in resistivity by Fig.2, the dry density has
increased from 1.76 to 1.79 Mg/m3. From thecomplete saturation line shown in Kobayashi et al.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan(2007) [4], the swelling pressure of Kunigel V1 alone
with a dry density of 1.8 Mg/m3 is about 1.42 MPa.
In this test, because the pressure acting on the upperand lower end faces was 1.6 MPa, an increase in dry
density due to consolidation may have occurred. On
the other hand, as compared with the temporalchange of the saturation distribution under thecondition that the air escapes to the upper part tested
separately (Fig. 5) [1,3], for example, the increase
range in saturation after about 1100 days reachedabout 400 mm from the upper and lower end faces ofthis test result, whereas in the test result of the(mm)specimenbottomfromfrom
Distance(mm)
theofbottom
Distance

1000

condition that air escapes to the upper part, it reached
the increase range of saturation of about 600 mm ormore (blue bold line in Fig. 4 and Fig. 5). Therefore,
it was found that the seepage rate of the buffermaterial was slowed by the void air trapped inside thebuffer material. Also, as shown in Fig. 5, there wasno decrease in resistivity on the lower face side asshown in Fig. 4. Therefore, it is evident that thepropagation of pressure is different in the state wherethe void air has escaped, and the density difference ofthe buffer material hardly occurs.

Elapsed time (day)
018
108
200
292
382
612
738
977
1117
1488
1651
1805
2029
2217
2385
2474

H2 O

800

600

400

200

030

40

5060

70

80

90

100

110120Degree of saturationfrom(%)
resistivity (%)
Degree convertedof saturation

Fig.5 Temporal transition of the saturation distribution under the condition
that the air escapes to the top part [1, 3]

4. Conclusion
In this study, in order to evaluate the seepage
process of buffer material during the re-saturation
process, focusing on the migration of void air
accompanying swelling, we conducted a long-termlaboratory experimental model test simulating one
dimension in a system in which void air is confined.
As a result of this model test, the pressure of void air
increased very gradually; it was learned that thepressure increasing of void air was mitigated bydissolution of the void air to water in parallel withwater seepage into the buffer material. As the void air
was trapped, it was revealed that the seepage rate of
the buffer material slowed down.
In the actual disposal pit, because the void airpressure increases by the heat from the waste and thesolubility of air decreases due to the temperature
increase of the pore water, it is expected that the void
air pressure increases. Therefore, it is necessary toconsider the influence of heat generated during theseepage period. This model test was a test at room
temperature; in the future, based on the separately
obtained seepage rate [1, 3], it is expected that thebehavior of the void air pressure at the repository canbe evaluated by simulating in consideration of theinfluence of heat and dissolved amount of air. It is
assumed that the behavior of the void air pressure at
the repository can be evaluated by simulating theinfluence of the dissolved amount of heat and air.
Furthermore, it is expected that a method showing thestate after saturation of the buffer material can beconstructed by studying a three-dimensional
mechanical simulation which combines various
behaviors during the re-saturation process. Indicating
the state of the buffer material after saturation leads
to the initial state of safety assessment, which isimportant information for the evaluation of thelong-term safety of the repository.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, JapanIn this test, it was suggested that the seepage rate isslowed by trapped void air and there is a possibility
that density nonuniformity may be promoted. As acountermeasure for keeping the performance of thebuffer material against such a phenomenon, it ispossible to assume, for example, a constructionalcountermeasure which allows escape of the void air
in the buffer material.

This research is a part of “Development ofAdvanced Technology for Engineering Componentsof HLW Disposal” under a grant from the Japanese
Ministry of Economy, Trade and Industry (METI).

References[1] Radioactive Waste Management Funding andResearch Center (2017): Development ofAdvancedTechnologyforEngineering
Components of HLW Disposal. Development of
Monitoring Technology. (In Japanese)[2] Radioactive Waste Management Funding andResearch Center (2014): Development ofAdvanced
Technologyfor
Engineering
Components of HLW Disposal. Development of
Monitoring Technology. (In Japanese)[3] Tomoko ISHII, Masahiro KAWAKUBO,
Hidekazu ASANO, Ichizo KOBAYASHI, Patrik
SELLIN,David
LUTERKORT,Peter
ERIKSSON (2017)：Experimental evaluation todetermine rates of groundwater seepage intobuffer materials. Clay Conference 2017, 93.
[4] Ichizo KOBAYASHI, Masaru TOIDA, Takeshi
SASAKURA and Hideki OHTA (2007):
INTERPRETATIONS OF COMPRESSION/SWELLING BEHAVIOR OF COMPACTED
BENTONITEUSINGCONSTANTWATER-CONTENT LINE AND CONSTANT
DEGREE-OF-SATURATION LINE. Journal ofJapan Society of Civil Engineers Ser. C, Vol. 63,
No.4, pp. 1065-1078. (In Japanese)

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