Centrifuge model test to gain reliability of the future predictionin terms of long term THM processes in deep geological repository
Soshi Nishimoto
Central Research Institute of Electric Power Industry (CRIEPI)
1646 Abiko, Abiko-shi, Chiba, 270-1194 Japan
Key words; centrifuge, scaling law, near-field, THM processes, geomechanical interaction

Abstract
The long-term behavior in the near-field of deep geological repository is governed by coupled
thermal-hydraulic-mechanical (THM) processes. THM processes will continue for hundreds of years in theinitial stage of disposal. These processes influence the overpack displacement, the swelling behavior of thebuffer, the deformation of the disposal hole, and so on. The overpack displacement is influenced by the swelling
behavior of buffer. The swelling pressure of the buffer generated is influenced by the depth of the repository andthe stiffness of the surrounding bedrock. To clarify the long term behavior in the near-field, the researches by thefull-scale tests and the numerical analyses have been carried out. However, the former are difficult due tolocation, time, and economic restraints, and the latter are necessary to verify the applicability of the numericalmodel. Centrifuge model test can be used to replicate an event, similar to what can be done with a prototype, andin fact is a reduced-scale version of a prototype. Based on scaling laws, any two investigations of the same
conditions using a centrifuge model test and a prototype are similar and related. However, the centrifuge model
test has the advantage that it can greatly shorten the long time needed to see behavior resulting from the typicallyslow flow of groundwater that satisfies Darcy’s law. If the time acceleration test using the reduced-scale modelof the near-field is available on the basis of the centrifugal scaling law, then the long-term reliability of thegeological repository can be improved by empirical laboratory data. Our aim is; to conduct the time acceleration
test using the centrifugal equipment; to measure the equivalent data of long-term behavior of the overpack,buffer and bedrock; and to evaluate the long-term THM behavior of the near-field in a geological repository bylaboratory measurements.

1. Introduction
The high level radioactive waste (HLW) disposal
repository in Japan will be built in deeper than 300m
in the underground1). The vitrified waste is enclosed
in a metallic overpack. It is then packed, surroundedby buffer material made of clay bentonite, into adisposal hole drilled deep into the bedrock. The
near-field is composed of such a heterogeneouscomposite. The phenomena of the near-field is
expected to be very complex. And, it keeps occurring
for a very long term. Probably, within current year,
the government will announce the scientific suitablearea for the disposal. Then, discussion may start at
candidate areas.
In the near-field, various processes will occur
under the effect of heat from the waste, rock stressand underground water. This THM transition will
dominate and continue for hundreds of years in theinitial stage, construction, operation, and closure
stage. These state are transition period of artificial

disturbance and resaturation, and phenomena
dominates THM, mechanically unstable (Fig. 1).
In contrast to the initial stage, after resaturation,
nuclide gradually leaks and migrates. Chemical
phenomena, diffusion and permeation will occur in along term of more than thousands of years. This
period is mechanically stable and slow alternation,
permeation, and diffusion keep occurring (Fig. 1). In
safety assessment, chemical phenomena, diffusionand permeation after resaturation are the target andevaluated finally. On the other hand, the initial state
until resaturation significantly affects the state after
resaturation. Therefore, it is important to accurately
evaluate the THM transition as the initial state, in
hundreds of years in order to improve reliability ofsafety assessment.
Many researchers have been studying methods ofprediction of behavior in HLW disposal repository,
prototype test, physical modeling experiment,
numerical simulation. Prototype test has a high

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

Fig. 1 Schematic of predictable transition in HLW geological disposal repository

demonstration. Approximately actual behavior if
using real size and design can be observed. But, it isinevitably a few test and several year of evaluation
time because of restrictions of place and economy.
On the other hand, numerical simulation can analyze
many case and any evaluation time, but if only
simulation, the demonstration is low. Physicalmodeling experiment is in middle. Demonstration,
test quantities and evaluation time depend on themodel size and the scaling law. These methods havemerits and demerits. We think in HLW disposal
research, it is necessary to combine and thencomprehensive evaluation (Fig. 2).
Here, we focus on the centrifuge physical model
testing. On the basis of centrifugal scaling law, future
THM behavior is able to predict by time-acceleration
test2). Conducting time acceleration test using thiscentrifuge, we think we can observe the equivalentdata of THM long-term behavior of near-field by thelaboratory measurements. And then, these results
contribute to the verification and validation (V&V) of
the numerical simulation in the evaluation of THM
long-term behavior of the near-field.
The aim of the study is; to conduct the time
acceleration test using the centrifugal equipment; tomeasure the equivalent data of long-term behavior of
the overpack, buffer and bedrock; and to evaluate thelong-term THM behavior of the near-field in a HLW
disposal repository by laboratory measurements. Thisapplication is aiming to enhance the reliability of theprediction of the future of the near-field, hence thesafety of the geological disposal.
2. Centrifuge model test
2.1 Our concept of evaluation of THM long term
behavior in near-field
We think two phases in the evaluation of THM
long term behavior in near-field. One is future
prediction by numerical analysis and the other isunderstanding phenomena by experiment and test. By
numerical analysis, we can estimate the future stateof target disposal repository. By experiment and test
using the similar materials and conditions of thetarget repository, we can understand THM
phenomena. After that, we comparatively discuss
using
both
results,
andthenevaluatecomprehensively evaluate the validity of thepredicted phenomenon based on the limit of thenumerical simulation.
Here, as points, we think ONLY simulating thefuture by a numerical analysis is not equal toevaluating long-term behavior in the near-field. And
long-term
behavior
shouldbeevaluatedcomprehensively with accompanying verification of
the simulation by tests (Fig. 2).
Understanding phenomena and contributing data toverify and validate the numerical simulation.
Centrifuge model test is small and simplified model,
but time scale is equivalent the actual andFig. 2 Improvement of reliability of future predictionof HLW disposal repository

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanparameterable test. Parameter are stress, elasticity,
gap/joint, density of buffer, temperature, and so on...
The present study firstly targets stress and elasticity.
2.2 Centrifugal scaling law in the static field
The behaviors expected in the near-field can be
attributed to two-phase mixtures that consist of therock/soil and pore fluid if the centrifugal model is thesame material as the prototype. In a centrifugal model
test, by subjecting 1/N scale model to N times theearth's gravitational/ centrifugal acceleration (G)based on a centrifugal scaling law in a static field(where N is the scale factor and gravity /centrifuge
acceleration level), the following are true: (1) the
stress-strain behavior of a specimen in the model willbe identical to that in the prototype, and (2) theequivalent elapsed time for the migration ofunderground water that satisfies Darcy’s law, thestress due to consolidation and swelling, and the
distribution of elastic strain can be shortened to 1/N2,
compared with the full-scale elapsed time (Table 1).
For instance, the behaviors for 100 yr can be
simulated in about 6 weeks using a 30 G centrifugal
force field. This study treat the behaviors of thenear-field as occurring in a static field.
2.3 Centrifugal model of near-field and centrifugal
equipment
The near-field model in this study (Fig. 3a)3–5),
which is 1/30 the size proposed in the provious
report6) (Fig. 3b), consists of a single model-overpack,ring- and cylinder-shaped buffers, and a cylindricalsedimenrary rock mass. The overpack and buffers areplaced in the bore hole drilled in the rock mass. The
overpack is 27-mm diameter by 62 mm height, and6160 kg/m3 density. The buffer is made bycompacting a Na-bentonite powder (Kunigel-V1),
after it was oven-dried at 110 °C for 24h. Overall
volume of the bentonite is about 54 mm in diameter

Table 1. Scaling law of the centrifugal model test.

by about 127 mm in height, and the initial dry density
is 1740 kg/m3 (the dry density after swelling is 1550
kg/m3, including the swelling-filled gap between the
overpack, the rock mass and the bentonite).
The rock mass is a cylinder with a diameter andlength of 180 mm. The disposal borehole of 56.7 mm
in diameter by 127 mm in height is then created at thecenter of the cylinder. we used Tage tuff, assuming
the repository composed of sedimentary soft rock.
The physical properties of Tage tuff in the laboratory
tests are: about 2.9 GPa (dry) and 1.2 GPa (wet) in
the Young’s modulus of the uniaxial compressionaltest, about 1720 kg/m3 (dry) and 2000 kg/m3 (wet) inthe density, and about 10-11 m/s in the permeability5).
Nishimoto et al.7) developed a geotechnicalcentrifuge, "CENTURY5000-THM" (Centrifugal
Test equipment for Ultra-long time Range of 5000
Years: simulating coupled Thermal-HydraulicMechanical processes) that enables us to perform along-term operation of up to six months. The mainfeatures of the centrifuge are shown in Fig. 4.
We adopted the simple model shown in Fig. 3a inthe present centrifugal test. This model does notconsider the entire disposal repository from thesurface to deep underground, but focuses on the area

Fig. 3. (a) Near-field model of the centrifugal model test, and (b) the target HLW disposal repository.

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

Fig. 4. Schematics of "CENTURY5000-THM".

surrounding the overpack. Therefore, the stresscorresponding to the lithostatic pressure is supportedby the pressure vessel. As for the scaling law of theself-weight stress, we considered only thesurroundings of the overpack, including the bentonite
buffer and bedrock.
2.4 Test condition
We conducted the normal temperature- 3, 5) andheating-test 4) (Table 2) as the parameter of theconfining pressure (the STRESS-constraint condition,that is, surrounding of the bentonite DEFORM). And,
we referred the centrifugal data of Nakamura andTanaka8) under the STRAIN- constraint and normal
temperature condition. Their study used a modelcomposed of bentonite and an overpack with a steel
container, and did not include the bedrock under thestrain-constraint condition, that is, surrounding of thebentonite DOES NOT DEFORM.
The temperature conditions of the overpack areconstantly 25 °C and 95 °C (normal temperature- andheating-test, respectively). We measure the change inthe strain of the rock mass and the bentonite, the soilpressure of the bentonite (called the "bentonite
pressure"), and the displacement of the overpack. In
addition, we attach the thermocouples in theheating-test at the same place as a part of the strain
gauges. Centrifuge model tests are conducted with acentrifugal force field of 30 G, along withpredetermined confining pressures and injection porepressures. Confining pressure (Pc) is applied at 6 to
10 MPa as a result of the isotropic hydraulic pressure(Table 2). The predetermined pore pressure (Pp) of
the injected distilled-water is constantly controlled at

the bottom end of the model. The back pressure isalso constantly controlled under the consolidationdrainage (CD) condition of 0.5 MPa at the top end of
the model. In contrast, the test condition of Nakamura
and Tanaka 5) is the consolidation-undrainage (CU).
The model uses initially dry conditions to measure
the saturating and swelling processes of the rockmass and bentonite. Test procedures are, (1) the
predetermined confining pressure and the ambient
temperature are loaded to the model, (2) the
centrifugal acceleration of 30 G is applied after all
measuring sensor values are steady, and then, (3)pore water injection and heating of the overpack arestarted. The injection pressure of pore water isconstantly controlled after it approaches thepredetermined value.

3. Result and discussion
In the centrifugal model tests, the displacement of
the overpack, the bentonite pressure, the strain of therock mass, and temperature (only the heating-test)
were measured under the confining pressures. As forthe example of results obtained by the tests, we show
the temporal change of bentonite pressure in thenormal temperature- and heating-test under the
stress-constraint conditions (Fig. 5). We define thebentonite pressure as the value obtained when theback pressure is subtracted from the measured soilpressure. The data plotted in the figure are the values
over time starting from the injection of the water (andthe heating of the overpack). And, we describe themusing the test elapsed time (lower horizontal axis) in
the following paragraph.
3.1 Stress- and time-dependency
The bentonite buffer placed into the disposal holeof the rock mass swells by the injected water thatpermeates the bottom and lower half-side of the rockmass, and then fills the gaps and joints in the holefrom the lower part sequentially. The bentonite
pressure is measured at the top of the model in thepresent tests (Fig. 3). Therefore, the soil pressuretransducer showed little change from the start of the
20 to about 200 h, because the bentonite pressure
could only be measured at the soil pressure

Table 2. Test conditions of the centrifugal model tests.
Sa mple N o.
Confining pressure, MPa
Injection (pore) pressure,MPa
Ba ck pressure, MPa
Overpa ck, °CAmbience, °CCentrifug a l a ccelera tion, G
Test time, dEquiva lent time, yr

N orma l tempera ture-test
TG-11 *
TG-12 *
7.0
9.0

TG-03*
5.0

TG-10 *
6.0

2.5

3.0

3.5

0.5

0.5

0.5
25
25
30
39
96.2

34
83.8

24
59.2

2

H ea ting -test
3
3
TG-17*
TG-19*
6.0
10.0

TG-05 *
10.0

(N T* )
-

4.5

5.0

4.9

3.0

0.5

0.5

25
25
30
39
96.1

0.5

67
165.2

40
98.6

5.0

0.5
95

31

35
30

80
197.3

52
128.2

*Nishimoto et al. 3, 5); *2Nakamura and Tanaka 5); *3Nishimoto et al. 4)

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japantransducer after the gaps and joints were filled. Then,the bentonite pressure increased rapidly by theswelling of the bentonite and was measured the local
maximum. In the normal temperature test, beyond the
local maximum values of the soil pressure ofbentonite were measured, the values changed togradual decrease and were not convergent throughthe tests. In contrast, the values in the heating-tests
were measured the additional second local maximum,
and then, changed to gradual decrease.
The general trend of the bentonite pressure weresimilar between our data and NT 8), but the variations
in our data are larger than their results. In addition,
our data clearly had a confining pressure-dependency
in terms of the local maximum value, as well as atime-dependency after the local maximum values
because the measured values were not convergent.
Similar trend were obtain in the results of thedisplacement of the overpack.
The long-term behaviors of the near-field are
qualitatively considered due to the following factors:(1) the disposal hole drilled in the deep undergroundshrinks due to the lithostatic pressure; (2) the
bentonite buffer generates swelling pressure due tothe imbibition caused by the reflood of groundwater;and (3) geomechanical interactions occur between the
bedrock around the disposal hole and the bentonite
buffer. In terms of the change from the local
maximum to the test-end value of the bentonite

Fig. 5. Temporal change of bentonite pressure.
(Upper) normal temperature-test, (lower)
heating-test.

pressure, the test results of NT 8) were convergent
when about 40 equivalent years passed (Fig. 5). By
contrast, our data were not convergent with thegradual change. No observation of the convergence inthe test results of TG-12 (normal temperature-test)
and TG-17 (heating-test) for the longest period of
about two and three months which correspond to 165
and 197 equivalent years, respectively, that is, they
have the time-dependency obviously.
The deformation of the disposal hole (host rock)yields to the convergent swelling pressure of thebentonite by the geomechanical interaction between
the bentonite and the surrounding bedrock5). In
addition, the disposal hole/bedrock surrounding
bentonite deforms in our tests under stress-constraint
conditions, and it is considered that the stiffness inthe bedrock decreases and the suction disappears
gradually while the condition changes from dry to
wet by the pore water, and then the bedrock softens5).
Consequently, we interpret that the bentonite
pressure keeps decreasing gradually until thegeomechanical interaction between the stress of thedeformation of the disposal hole/bedrock andswelling of the bentonite balanced. Due to thegradual and continuous deformation of the disposal
hole/bedrock surrounding the bentonite, we infer thatthe convergent time of the displacement of theoverpack and the bentonite pressure becomes longer
than that of Nakamura and Tanaka 5).
3.2 Effect of heating
Fig. 6 shows cross-sectional images of the model
made by X-ray CT after the centrifugal tests. In thenormal temperature-test, the density both the bedrockand the bentonite reaches the supposed saturated
density (about 1900-2000 kg/m3). In the heating-test,
the density of the bedrock reaches the supposed
saturated density, in contrast, that of bentonite dosenote reaches the supposed saturated density, and theCT value (equivalent density) decreases toward theoverpack (1700-1800 kg/m3).
It is considered that the a little evaporation of porewater is generated in the heating-test because thetemperature of the overpack is set to 95°C. It ishighly unlikely to the boiling of water due to 0.5 MPaof the back pressure. However, it is a possible thatwater vapor is relatively generated more in the
bentonite around the overpack with high temperaturecompared with the bedrock, and then, the differential
pressure is formed. As a result, it is inferred that porewater is pushed away from the bentonite to the
bedrock, and the zone of a low density in the
bentonite is formed.
In other word, it is suggested in the heating-test
that (1) the density of the bentonite decreases because
the unsaturated zone in the bentonite is formed [Fig.
6(right)], (2) the bentonite pressure and the stiffness
in the disposal hole decreased, and then, (3) the trend

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

Fig. 6. Cross sectional image of the model by X-ray CT in the prost-test, (left) normal temperature-test, and(right) heating- test. Be: bentonite, OP: overpack.

in the displacement of the overpack and the strain of
the bedrock [expansion to compression, Fig. 5(lower)] change because of a stress constantcondition. Moreover, it is inferred that theunsaturation in the bentonite moves on to thesaturation eventually, however, the disappearance of
the unsaturated zone is necessary in order that thegeomechanical interaction between the stress by the
deformation of bedrock and the swelling pressure of
the bentonite reach equilibrium under the hightemperature condition.
4. Conclusion
We carried out the centrifugal model tests toevaluate a long term behavior in the near-field of ageological repository for HLW disposal. The results
and conclusions are as follows.
(1) In results of the local maximum value of thebentonite pressure in the present tests, these values
were obviously different depending on confiningpressure (‘stress- dependency’). In addition, thesevalues after the local maximum values were notconverged in test-time (‘time-dependency’). Thesebehaviors are distinctly different from the results ofthe previous centrifugal model test in astrain-constraint condition.
(2) X-ray CT scanning of the model after thecentrifugal tests show that the CT value (equivalentdensity) of the bedrock in the heating-test reaches the
supposed saturated value, in contrast, that ofbentonite dose note reaches the supposed saturated
value, and the density decreases toward the overpack.
These results contribute to the validation ofnumerical analysis.
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6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan