Seismic response of canister in buffer material under water invasion condition by centrifuge modeling
W.Y. Hung1, J.J. Xu2, T.A. Nguyen3, Y.C. Wu4 and M.H. Hsieh5
1

Associate Professor, Dept. of Civil Engineering, National central University, Taiwan (R.O.C)
2
Master Student, Dept. of Civil Engineering, National central University, Taiwan (R.O.C)
3
Research Assistant, Dept. of Civil Engineering, National central University, Taiwan (R.O.C)
4
Supervisor, Institute of Nuclear Energy Research, Atomic Energy Council, Executive Yuan, Taiwan (R.O.C)
5
Engineer, Institute of Nuclear Energy Research, Atomic Energy Council, Executive Yuan, Taiwan (R.O.C)
Address: No. 300, Zhongda Rd., Zhongli District, Taoyuan City 32001, Taiwan (R.O.C.),

Key words: canister, centrifuge modeling, shaking table test

Abstract
The high-level waste (HLW) disposal is isolated from Earth biosphere by multi-barrier systems including engineering and natural barriers at deep ground. The disposal holes are drilled and located in intact rock. Canisterand buffer material were then set up in the hole. The canister should be very high stability and not easily affectedby natural environment, humanity activities and tectonic movement. However, Taiwan locates at Pacific Rim ofFire and earthquake occurs frequently. As a result, the effect of earthquake-induced vibration on the canister,buffer and disposal system is impartment and needed to be investigated. This research assumes that the deposition is set up in granite deposit and the disposal hole has no deformation and cracks. Referring to KBS-3V concept and its geometry, the centrifuge model was designed with 1/10 size of prototype and tested in 10 g artificial
acceleration field. Therefore, the in situ stress between canister and buffer material was simulated. Seismic
events were then input by shaking table for pre-shaking and main shaking events. During shaking, acceleration,pore water pressure and total pressure histories were measured by sensors to realize the seismic interaction behavior between canister and buffer. Total 6 models were conducted with different initial water content conditions
to simulate the buffer after construction, after underground water invading to disposal hole and after saturation of
buffer material. After centrifuge modeling, container was disassembled to check the crack development on thesurrounding buffer materials.

1. Introduction
According to IAEA statistical data by 2017, there
are 451 nuclear reactors in operation and 60 reactors
under construction. It will produce approximately
12000 tons high-level waste (HLW) for a year. Because byproducts of nuclear power generation andspent fuel are radiotoxic to the humanity and environment, the disposal of HLW is seriously considered.
Several series of researched were conducted aroundthe word for disposal systems. For HLW disposal,
deep geological disposal, which is a multi-barriersystem, a combination of natural barrier and engineering barrier, is a widely accepted design concept.
Because of its robust defense against the external
factor of geological activities.
According to the concept of Svensk Kärnbränslehantering AB (SKB) KBS-3 for final disposal, there
are 4 points for basic concept. First, the spent nuclear
fuel is encapsulated in tight, corrosion resistant andload bearing canisters. Second, the canisters are de-posited in crystalline rock at a depth of 400–700 meters. Third, the canisters are surrounded by a buffer
which prevents the water flow and protect them.
Forth, the cavities in the rock that are required for thedeposition of canisters have been backfilled andclosed.
HLW will be stored in canister and then burry into
disposal hole with buffer surrounded. Through thehost rock and engineering barriers (canister and buffer) sealing, the system isolate the radioactive wastes
and reduce the virulence to an acceptable range due
to a long-term isolation and keep it safe from naturalenvironment, humanity and tectonic activities. Evenif, when HLW leaks out, the repository can sealleakage and dilute radio toxicity, thanks to the substantial expansion capacity of buffer and backfill material. Therefore, this design is considered as an adequate and appropriate solution.
The repository system need to have a concrete resistant against many hydrological and geological ac-

1
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japantivities in a long period. Some of these are the extreme natural disaster such as earthquake, climatechange, etc. Taiwan locates at Pacific Rim of Fire, asubduction area with earthquake frequently. Earthquake induced vibration have a certain effect to thesafety of repository system. In addition, it is considered if the initial condition of disposal system changes under long-term storage, according to Jonny
Rutqvist et al. (J. Rutqvist, 2013) the power of theradiation HLW may take more than 10000 years to
fully dissipate. Therefore, a thoughtfully investigation about the effect of environment factors on therepository components.
There are several researches carried out on thephysical and mechanical properties of bentonite
based buffer material. For instance, the physical andmechanical properties reported in SKB reports (SKB,
The Buffer and Backfill Hand book part II: Materials
and techniques 2001), (SKB, The buffer and backfill
handbook Part I: Definition, basis relationships andlaboratory method 2001). These works provide thereference values for the properties of the buffer.
High level waste repository construction is a highly complex and cost consuming process, according to
R. Pucsh (Pusch 1994) the cost for Swedish repository for radioactive waste is about 740 million SEK(about 89 million USD) in 1987. Therefore, the dataof full scale testing of repository is very limited.
Some large projects is developed by the international
cooperation for a deeper understanding on this issue.
DECOVALEX (Development of Coupled Models
and their Validation against Experiments) start in1992 is a considerable effort that sponsored and performed by an international organizations in Canada,
Finland, France, Japan, Sweden, U.K. and USA. The
main result of this project as also the numerical andtheoretical approaches between 1992 and 1995 reported in various aspects by Ove Stephansson, Lanru
Jing and Chin-Fu Fang (Stephansson, Jing and Tsang
1996). The current state of project updated on theirofficial website (DECOVALEX 2017).
The seismic performance of canister and its surrounded buffer material is critical for a reliable design. As aforementioned, the full-scale experiments
provide the reliable data for research with highly
complex and enormously expensive that usually holdby an international research project. On the otherhand,the seismic load is very difficult to generate and control in a full-scale experiment.
Therefore, this study investigates the seismic response of the canister embedded in the bentonite-based buffer material via geotechnical centrifuge
shaking table test, which is a more affordable approach to this problem.

2. Research equipment
2.1 Design of centrifuge model

Referring to final disposal concept of KBS-3V, a
1/10 model was designed. Geometry of disposal holeis show as Figure 1. Canister is installed at center ofdisposal hole surrounded by buffer, and a dead load isapplied on top of buffer to simulate the loading bybackfill of tunnel. The outer is aluminum alloy container to simulate the base rock. The aluminum alloy
can apply sufficient strength with ductile behavior
and lighten the total model weight as compared to theiron material. Before centrifuge modeling test, direct
shear tests were conducted to know the friction coefficient of interface between bentonite with granite
and bentonite with alumina alloy, respectively. Test
result shows that the difference between two interfaces is only about one degree. As result, the alumina
alloy container was then used without roughness
treatment at the inside surface.
Accelerometers are installed to top and bottom ofcontainer and canister, labeled as BTA (Box-Top
Acceleration), BBA (Box-Bottom Acceleration),
CTA (Canister-Top Acceleration) and CBA (Canister-Bottom Acceleration), respectively. Around the surface of canister, the pore water pressure and totalpressure transducers are arranged to measure the porepressure and total pressure (including water pressure,earth pressure and impact pressure) during test.

Figure 1: KBS-3V concept to centrifuge model

2.2 NCU geotechnical centrifuge
The geotechnical centrifuge of Centrifuge Modeling Laboratory of Experiment Center of Civil Engineering, National Central University (NCU) has anominal radius of 3 m as shown in Figure 2. The capacity is 100 g-ton, meaning that the centrifuge canspin to 100 g with 1-ton model. A 1-D servo-hydraulically controlled shaking table is equippedon the platform. Therefore, one-dimensional horizontal base input shaking can be applied to the tested
model during spinning of centrifuge. The shaking
table has maximum shaking force of 53.4 kN withmaximum table displacement of ±6.4 mm and raises
up to 80g centrifugal acceleration. The nominal operating frequency range is 0-250 Hz. At 80 g artificial
acceleration field, the maximum table payload is 400
kg (equivalent to a capacity of 32 g-ton).

2
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Figure 2: NCU geotechnical Centrifuge

Table 1: Initial conditions of buffer materials fortested models
Water
Bulk density
Test No.
content
Remark(kg/m3)
(%)
D-N
17
1885
No water
D-N17
1885
MX-80
MX-80
24∘water
D-W24
17
1885
42∘water
D-W42
17
1885
23∘water
W-W23
27
2008
43∘water
W-W43
27
2008

2.3 Buffer material and model preparation

(a)

(b)

(c)

Figure 3: Different shape of buffer blocks

Figure 4: Assembling of 1/10 centrifuge model

In this research, SPV 200 bentonite was usedwhich is produced from Black hills in Wyoming
U.S.A. It is sodium bentonite, which specific gravity
is 2.67; PH is 8.5-10.5; liquid limit 495%; plastic
limit 35.8% and plastic index 459.2%. According to
USCS, it is classified as CH.
Large swelling ability and extremely low permeability make bentonite suitable for using as buffer material. Bentonite compacted with high pressure toform cylinder or ring shape blocks (Figure 3) thatsimilar to the prototype disposal hole. The ring blockwith the small hole as Figure 3(a) is placed at the topof canister part (position A-1 and A-2 in Figure 1).
The small hole is for lines of sensers. The ring block
as Figure 3(b) locates aside the canister (position B-1
to B-10 in Figure 1). The solid cylinder block as
Figure 3(c) is embed at the bottom of canister in
diposal hole (position C-1 in Figure 1).
This research assumes that the deposition is set up
in granite layer at certain depth. The disposal hole isregard as an entire intact space without deformationand cracks. Referring to KBS-3V concept and geometry, the test model was built with 1/10 scale of prototype (Figure 4 and Figure 5). Through the centrifuge modeling, 10 g artificial acceleration field wasapplied to simulate in situ stress between canister andbuffer material. Seismic events were input by shaking
table with 15-cycle sinusoidal waves. During shaking,acceleration, pore water pressure and total pressure
histories were measured to realize the seismic behavior of canister and buffers in the disposal hole.

3. Test result and discussion
3.1 Test conditions

Figure 5: Setup of centrifuge model on platform

Total 6 tests were conducted as shown in Table 1
with different water content conditions. The notation
D means water content of buffer (17 %) and W represent saturated water content (27 %). The alphabet
after dash, N means no underground water invasion
and W with number means water invading with respecting water temperature. Model D-N simulates thecondition just after construction. Models D-W24 andD-W42 represent the condition that water just in-3
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanvades to disposal hole, where buffer is unsaturated
and water invaded with different temperature. Models
W-W23 and W-W43 simulate the condition after
long-term disposal, where buffer is close to saturated
and submerged in underground water.
According to SKB TR-10-10 report, final bulk
density of saturated buffer ranging from 1950 to 2050
kg/m³ is recommend. At D-series tests, bulk density
is 1885 kg/m³ with 17% water content. When water
invades into disposal hole, buffer would graduallysaturate and its density finally increases to 2008kg/m³. As results, bulk density of blocks in W-series
tests is 2008 kg/m3 with water content 27% to simulate the saturated condition. During spinning, several
seismic events including pre-shaking and main shaking events are input. A very small shaking was used
for Pre-shaking event, and it can detect basic properties of system including nature frequency. Mainshaking events are 15-cycle sinusoidal waves withdifferent amplitude in 1 Hz, 2 Hz and 3 Hz.

Figure 6: Acceleration time history (D-N test, 0.13
g-15cyc-2 Hz)

Figure 7: Acceleration time history (W-W23 test,
0.12 g-15cyc-3 Hz)

From the tests without water invasion condition,such as Model D-N test as shown in Figure 6, it isobserved that vibration response is not in synchronization. Acceleration at bottom of canister synchronize to base acceleration without phase change,
however, acceleration at top of canister does notsynchronize to base. For D-N test 0.13g-15cyc-2Hz
event, the average peak to peak delay about 0.3 second. Because of the exist gap between buffer andcanister, the rocking of canister during shaking would
lead to impact loading and the spick of acceleration isobserves. It should be noted that the container was
fixed at the shaking table at bottom, and it is a kind of
cantilever structure that is different from the real site
environment at intact rock. Therefore, the acceleration amplitude at top container is about two times ofthat at bottom. In the future, the container will be
reinforced to get higher rigidity for recreating thecloser response of rock base.
In the tests with water invasion condition, such as
Model W-W23 test as shown in Figure 7, acceleration histories for CTA, CBA and BBA are almost
synchronization without significant time delay ofshear wave propagation. It is inferring that bentonite
blocks absorb water and then its swelling leads to thedecrease of gap between canister and buffer, resulting
to more confining effect to canister. The canister cyclic moves during shaking without rocking and addition impact loading. Also, the amplitudes of baseinput motion and at bottom of canister almost thesame, indicating the shear wave entirely propagate
from rock to canister through buffer without reduction even the buffer is saturated.
The amplification of acceleration response is defined as the ratio of acceleration at different elevation(CTA and CBA) to the maxima base input acceleration at Bottom of Box (BBA) shown in Figure 8, andthe location of CTA and BBA are indicated in Figure
1. By normalization, it can be seen that the seismic
response at top of canister is larger for the models
without underground water invasion. Amplification
are over four times at top of canister. Under water
invasion condition, amplification is reduced to about
two. Tests results show that the presence of water can
reduce seismic motion because of the reducing of gap
inside disposal hole.

3.3 Contact pressure during shaking

Figure 8: Acceleration amplification at top of canister

3.2 Acceleration response

In this study, the pressure between canister andbuffer caused by swelling of bentonite after absorbing water was not measured. In order to measure thecontact pressure acting on canister and buffer, four
total pressure transducers were installed on the surface of canister (TP1-TP4 in Figure 1). The following
discussion is about the increment pressure caused bybase shaking.
The maximum increment contact pressure for all
tests subjected to different base shaking are showed

4
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanin Figure 9. Model D-N for simulating the just finished construction condition is measured the highest
contact pressure of 51 kPa, which is much smaller
than the strength of canister under the base shaking of0.20 g. The similar phenomenon as acceleration response, the increment contact pressure reduces after
water invading to disposal hole because of well confining by swelled bentonite. After the test, the watercontent of buffer close to canister was detected. The
water content is 29 % for Model D-W24, 42% forModel W-W23. With saturated water content andsubmerged into underground water, bentonite is alittle soft resulting to high amplitude and increment
contact pressure.

Figure 9: Maximum contact pressure at different baseshaking events

For seismic event of Model D-N test with baseshaking of 0.13 g, 15 cycles, 2 Hz sinusoidal waves
shown in Figure 10. From the arrangement of modeas shown in Figure 1, the right side is positive acceleration direction. Because the acceleration and inertial force are in opposite direction, it makes that whencanister suffer negative acceleration, canister is moving to positive direction and causing contact pressure.
In contrast, when canister suffer positive acceleration,
contact would decrease.
Therefore, the negative acceleration leads to inertial force to positive direction of acceleration, and thecanister moves right side and hits the buffer induces
significant increment contact pressure. The canister
quickly contact with buffer and the reaction force
push it to collide with buffer inside the gap. Therefore, input base shaking is 2 Hz and the response ofcanister is higher around 4 Hz by not symmetric asshown in Figures 6.
For the model with saturated buffer and water invasion as Model W-W42, the seismic event with baseshaking of 0.12 g, 15 cycles, 3 Hz sinusoidal waves
shown in Figure 11. Because the well confining by
swelled buffer, the responses of acceleration and increment contact pressure are synchronizing with baseinput motion. The positive total pressure was measured because the model was submerged in the waterand the separate for buffer and canister would havesuction pressure during shaking.

3.5 Cracking on bentonite blocks

Figure 10: Acceleration and contact pressure time history (Model D-N, 0.13 g-15cyc-2 Hz)

Figure 11: Acceleration and contact pressure time
history (Model W-W23, 0.12 g-15cyc-3 Hz)

Buffer is used to prevent radioactive wastes diffusing into underground water. Cracking on bentonite
blocks would provide the path for conducting wastes
and water to flow out or in, that would make bufferlose its function.
For the buffer with nature water content, Models
D-N, D-W24 and D-W42, buffer blocks set around
top potion of canister cracks after shaking as shownin Figures 12 and 13. Most of cracks are in verticaldirection and just few are in horizontal direction. For
advanced inspection, it is dry inside the most of thecracks. Therefore, the crakes may be caused byshaking, swelling of buffer close to canister or releasing confining pressure by disassembling of container. Advanced researches are needed to realize thecrack development and its process. After water invading to disposal hole, the watermark appears atapart of outer interface between container and buffer,and inner interface between buffer and canister.
For the model with saturated buffer material, Models, Models W-W23 and W-W43, there is no crack
and only watermark and swelled zone found on thesurface of blocks as shown in Figure 14. As compared to 17% water content bentonite, the buffer material with saturated water content is lower strength
but a little more ductile ability.

5
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

4. Conclusions
Total 6 dynamic centrifuge modeling tests wereconducted in the artificial acceleration field of 10 g.
The base shaking are applied to the model fromshaking table. From the test results, it was observedthat the gap between canister and buffer is filled withswelled bentonite leading to the better confining tocanister after water invading. It would lead to the
reduce of amplitude of canister and the contact pressure between canister and buffer during shaking.
Second, the maximum contact pressure is about 51
kPa which is much lower than the strength of canisterduring base shaking of 0.2 g, meaning that canisterwould not be damaged. However, the advanced studies is needed for larger base shaking.
Finally, before the buffer saturated by undergroundwater, blocks would have possible cracks during baseshaking. No matter the swelling of bentonite or artificial sealing during construction, the acceleration response and increment contact pressure could be reduced and the possible cracks could be avoided.

Figure 12: Cracks of buffer block (Model D-N)

Figure 13: Cracking blocks and watermark (Model
D-W24)

Figure 14: Watermark (Model W-W23)
References
J. Rutqvist, L. Zheng, F. Chen, H. H. Liu and J.
Birkholzer (2014), "Modeling of Coupled Thermal-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonnite-Backfilled
Repository Tunnels in Clay Formations," Rock Mechanics and Rock Engineering, Vol 47, pp. 167-186.
Pusch, R. (1994). Waste disposal in rock, Developments in Geotechnical Engineering 76, ELSEVIER,
Amsterdam.
O. Stephansson, L. Jing and C.-F. Tsang (1996),
Coupled Thermo-Hydro-Mechanical Processes ofFracture Media Mathematical and Experimental
Studies, Elsevier.
Raiko, H., Sandström, R., Rydén, H. (2010). ”Design analysis report for the canister,” TR-10-28,
Swedish Nuclear Fuel and Waste Management
Company, Sweden.
Svensk Kärnbränslehantering AB (2010a). “Design
and production of KBS-3 repository,” TR-10-12,
Swedish Nuclear Fuel and Waste Management
Company, Sweden.
SvenskKärnbränslehantering
AB(2001).
“Long-term safety for the final repository for spent
nuclear fuel at Forsmark,” TR-11-01, Swedish Nuclear Fuel and Waste Management Company, Sweden.
Svensk Kärnbränslehan-tering AB (2001), "Thebufer and backfill handbook Part 1: Definition, basisc
relationships and laboratory method," TR-02-20,
Swedish Nuclear Fuel and Waste Management
Company, Sweden.
Svensk Kärnbränslehan-tering AB (2001), "Thebufer and backfill handbook Part 2: Materials andtechniques," TR-02-12, Swedish Nuclear Fuel andWaste Management Company, Sweden.

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