STUDY ON PIPING AND EROSION OF BUFFER MATERIAL DURING THE
RE-SATURATION PERIOD
Takayuki ABE*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, Piping, Erosion

Abstract
The state transition of buffer material (clay) composing the engineered barrier system (EBS) for the geologicaldisposal of high-level radioactive waste is most intense during the re-saturation period after emplacement into
the disposal pit. For evaluating and predicting long-term evolution of buffer material, it is important to grasp thebehavior of the buffer material during the re-saturation period. The buffer material has to be maintained within arange of density in order to perform its long-term safety function. However, there is concern that buffer material
might flow out by piping and erosion during the re-saturation period. In this study, several laboratory tests forpiping and erosion of buffer material during the re-saturation period were conducted with various conditions, e.g.,material form, interface condition, liquid type, water flow, and water pressure. As a result, knowledge wasobtained on the outflow of buffer material, and on engineering countermeasures.

1. Introduction
2. Piping and erosion test
The method of geological disposal in Japan consistsof multiple barriers combining engineered barriers
and natural barriers. For buffer materials constituting
the engineered barrier, performance such as
suppression of migration of radioactive substances isrequired, and this performance is brought about bymaintaining a predetermined buffer material density.
However, at the time of re-saturation, there is apossibility for water channels to form due to thegroundwater flowing on the surface of the buffermaterial, and of the buffer material eroding [1]. If thedensity of the buffer material greatly decreases due tothe erosion, there is a fear that the long-term
performance will be affected. Therefore, it isnecessary to investigate the phenomenon of piping
and erosion of buffer material, and to considercountermeasures to piping and erosion.
In this study, we investigated the piping anderosion phenomenon near the surface of the buffermaterial when conditions, e.g., material, interface
condition, liquid type, water flow, water pressure, arechanged, and as a part of countermeasures to piping
and erosion of buffer material, an indoor test wasconducted to ascertain the countermeasure effect by
pre-swelling (a method of swelling the buffermaterial in advance by the water supply before thetest). In this research, as a preliminary step of thefull-scale test, a small- scale test using a small cell
was carried out in order to investigate the piping anderosion phenomenon by various parameters.

In the case of the buffer material block
emplacement construction method (hereinafterreferred to as the pellet filling method) which is thevertical emplacement of geological disposal, theexistence of clearance is expected at the boundary
between the buffer material block and thesurrounding bedrock. As a current construction
method, a method of filling pellets into the clearance
area is considered. Therefore, in this study, weinvestigated the piping and erosion phenomenon on atest system simulating the pellet filling method byindoor test.

2. 1 Test method
A schematic diagram of the test cell and the test
image are shown in Fig. 1. We simulated the pellet
filling method and tested a case filled with pellets in asmall cell and a case filled with half blocks andpellets. The test cell has an inside diameter of 110mm and a height of 50 mm, and porous disks werearranged at the top and bottom of the cell in order to
distribute water from a plane surface. The water flow
direction is from the bottom to the top, and thedrainage is through a cell provided on the top coverfrom a slit of 5 mm width provided between theupper lid of the cell and the side ring.
The test cases are shown in Table 1. Pellets

were used at 100 wt.% Kunigel V1 combined with

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

10 to 20 mm of particle size and 1 to 3 mm ofparticle size examined in a previous study [2].

Table 1 Piping and erosion test case using small cellBlocks were used at 70 wt.% Kunigel V1 and 30wt.% silica sand. The water flow rate was basically
0.1 L/min with reference to the upper limit value set
as required performance of disposal pits such as by
SKB and Posiva Company [3]. In order to check theinfluence of water flow rate, the water flow rate wasalso measured for cases of 0.01 L/min and 0.001
L/min. We used distilled water, simulated Horonobe
groundwater (0.18 M [hereinafter referred to as
simulated groundwater]), NaCl (0.5 M), CaCl 2 (0.5M) as liquid type. Water supply was carried out at aconstant flow rate from the start of the test. Simulated
groundwater
wasanalyzed
andsimulated
groundwater composition collected from 350 m
undergroundinHoronobe.
Thesimulatedgroundwater composition is shown in Table 2.

Flow rate
Specimen No.
（L/min）

Type ofLiquid

P-1
P-2
P-3

0.1
0.01
0.001

Distilled
water

P-4

0.1

P-5
P-6

0.1
0.1

BP-1

0.1

2. 2 Test Results and Discussion
Figure 2 shows the piping in the pellet case (P-1to P-3) with different flow rates using distilled water.
In the case of 0.1 L/min and 0.01 L/min, one day later,a water channel was confirmed in both cases.
However, the water channel width is thinner at a flowrate of 0.01 L/min than 0.1 L/min. In the case of0.001 L/min, no water channel was observed until
after 210 minutes, and the water pressure increased to3 MPa of the limit pressure of the cell, so the test wasterminated.
排水Outlet
試料回収

Case

上部ポーラスストーン
t=10mmPorous disk t=10mm
Cell

Case
1s

0.1
0.001
0.001

3

Table 2 Simulated groundwater composition
Reagent
ppm
Mol/L
NaCl
5,200
8.9E-02
NaHCO33,000
3.6E-02
CaCl21801.6E-03
KCl2303.1E-03
MgCl2190
2.0E-03
H3BO3410
6.6E-03

排水孔 内径20mm程度

Test images

Bentonite

Block＋BP-2
Pellet
BP-3
BP-4

1.5
Simulatedgroundwater
NaCl
CaCl2
Distilledwater
Simulated Block：1.6
groundwater Pellet：1.5
NaCl
CaCl2

P-1（0.1 L/min）P-2（0.01 L/min）P-3（0.001（After 24 hr） （After 24 hr） L/min）（AfterMax. 210 minutes）
排水孔
φ6mm
LiquidFlow rate
Initial dry density injection
Hole fordrainage
円周上にピッチ30°程度でFig. 2 Forming situationof piping (distilledFedwater,
上板 アクリルpressure(cc/min.)(Mg/m3)flow rate)(MPa)

排水キャップ アクリル
タイロッド M10 6本PelletMax
Initial dry
injection
densitypressure（Mg/m3）（MPa）

Feedingwater
Test images
50.0

2.5

110
供試体
Specimen
CellΦ100mm、高さ50mm
Φ100mm×h50mm

FeedingwaterMax.
piping injectionin the pelletLiquidcases (P-1,
Flow rate Fig. 3 shows
Initial drythedensity
1.594
Feddifferentpressure(cc/min.)(Mg/mSlot for drainage
P-4 to P-6) withthe3) same flowrate andW5mm,D15mm(MPa) water, a large
排水溝
幅5mm、深さ15mm
liquid types. In the case of distilled
試料容器
アクリル
Acrylic cell100
内径110mm t=20mm

amount of buffer material eroded out of the cell after
1.594 of simulated groundwater, one
15 days. In the case
1.595
給水
Inlet
water channel was formed one day later and the100width and position did not change much until 16 days
底部ポーラスストーン
Porous disk t=5mmt=5mm
190.0
Case3:later.
100 Even simulated
1.571 groundwater with 0.18 M was
2s
1.595
3s
Case4:thinner10
1.471
Distilledin water channel
than in３the caseof distilled
Feeding～6s
Case5: 1
1.490
water did notwater, and a large amount of buffer material
Case6: 0.1
1.465
water
Case3:erode
100 out of the cell.
1.571In the case of NaCl, the shape offormed after one Distilled
day did not
3s
Case4:the10 water channel
Pellet 1.471
part：1.074
３Pellet～6s
Case5:change
11.490
watermucheven
after 16 days, but deposition
of7s
Block
part：1.601
Case6:sediment
0.1
1.465
wasAverage：1.348
observedin the water channel. It is
Cell
100thought that this is aggregation of montmorillonite
Pellet part：1.074
part：1.531
contained inPellet
thepart：1.601
cushioning material due to the7s
Block8s
part：1.595
influence of Blockionicstrength and precipitation of silica
Block＋Pellet
Average：1.348
Average：1.563
100sand. In the case of CaCl2, one water channel formed
Cellafter one dayPellet
andpart：1.531
formed a zone like a fracture zone
Fig. 1 Schematic
diagram of small cell and test image
after 16 days.
This is because the swelling
8s
Block part：1.595
performance Average：1.563
of the buffer material was greatly
20.0

1001s2s

Bentonite

底板 アクリル

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanreduced due to the influence of Ca type in the initial
Na type bentonite in addition to the influence of theionic strength, and the erosion substance became
lumpy and easily precipitated in the water. It isinferred that the range to be scoured was expanded inorder to change the shape of the water crest by theprecipitate.
Figure 4 shows the piping in half block + pelletcase (BP - 1 to BP - 4). The flow rate of the NaCland the CaCl2 was set at 0.001 L/min at whichwater channel was not generated in the tests of theP cases with different flow rate. In all cases, thepiping was confirmed. At the initial stage of flow, aplurality of water crests were formed at the interface
between the specimen and the cell, and converged on
one water channel, and gradually grew. In all thecases, water channels were formed at the interface
between the cell at the pellet side and the cell at theboundary between the block portion and the pellet
portion, and were not formed at the interface with thecell on the block side. This is considered to be due tothe low density on the pellet side with respect to theblock side.
After 1 day

After 8 days

After 16 days

P-1

P-4

P-5

3. Suppression of piping by pre-swellingIf water is supplied while controlling so that awater channel is not formed in the buffer material andthe buffer material swells sufficiently, there will be
no gaps at the interface. In that way, even if groundwater invades afterwards, it can only infiltrate into
the buffer material by diffusion, which is considered
to be unconducive to piping. Therefore, in this study,
a test for confirming the effect of pre-swelling wasperformed on the half-block and pellet as describedabove.

3. 1 Test method
Table 3 shows the test cases of pre-swelling.
Simulated groundwater was used as the type ofpre-swelling liquid, and the flow rate was 0.1 L/min
and 0.01 L/min. In addition, distilled water was usedas the liquid type of pre-swelling, and distilled water,NaCl, and CaCl2 were used as the liquid type at thetime of the test. The water flow direction was frombottom to top. The method of pre-swelling was asfollows: (1) simulated groundwater and distilledwater were passed through the specimen after
preparation at 0.1 L/min, (2) water stopped whenwater reached the upper surface of the specimen, (3)and a viewlet was connected to the lower mouth of
the specimen. During the pre-swelling, the drain holeof the cell was closed. As a result, the interior of thecell became a diffusion field, so it could swell
without forming a water channel. After pre-swelling,
it was confirmed by visual inspection that no piping
phenomenon occurred. When shifting to piping anderosion tests, drain holes were opened to make thesame conditions as the piping and erosion test
described above.

P-6

3. 2 Test Results and Discussion

Fig. 3 Forming situation of piping (distilled water,flow rate)

Changes over time of water flow pressure and total
flow rate of cases 1 and 2 are shown in Fig. 5, and thetest situation is shown in Fig. 6. When theTable 3 Pre-Swelling Test Case
Pre-swellingNo.
Liquid type
Liquid type

BP-1

BP-2

Case1

Simulated 0.1
Simulatedgroundwater
groundwater
0.18 mol/l 0.01
Case2
Case3
Case4

BP-3
BP-4
Fig. 4 Status of piping formation (after 1 day)Max
Flow Initial dry
injection
densityratepressure(L/min) (Mg/m3）
(MPa)

Case5

Distilledwater

DistilledwaterNaCl
0.5 mol/l
CaCl20.5 mol/l

Block：1.6
Pellet：1.5

30.1

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanflow rate was 0.01 L/min, a water channel was notformed and the water pressure reached 3 MPa whichis the upper limit pressure of the acrylic cell, so thetest was terminated. In terms of the test situation,
when the flow rate was 0.1 L / min, water wasformed in the block part near the pellet part. If thepiping is determined only by the magnituderelationship between the water pressure and theswelling pressure of the buffer material, it should
occur in the pellet portion where the drying density issmall like the above test results. The water channel
was formed in the block rather than in the pellet part,
because the pellets with low dry density tended toinfiltrate during pre-swelling and it was consideredthat swelling pressure was more likely to be exerted
than in the block part.
Changes over time of water flow pressure andtotal flow rate of cases 3 to 5 are shown in Fig. 7, andthe test situation is shown in Fig. 8. In all cases
formation of water channel was not confirmed until
the water pressure reached 3 MPa, which is the upper
limit pressure of acrylic cell. It was found that pipingand erosion can be suppressed by pre-swelling thebuffer material even if the liquid used is the liquid
type with distilled water, NaCl, CaCl2 at 0.1 L/min.

Case2Fig. 6 Forming situation of piping (after one day)
3500

100000
1500
50000

1500

02000501500

送水圧力
Water pressure
総流量
Total flow

140120100

1500

80601000
Liquid type : Simulated groundwater40

Flow rate : 0.01L/min

総流量 (cc)flow（cc）
Total

160

2000

20

005

100.40.620

Flow rate : 0.1L/min

1001.00.8

15

100

3000
2500

送水圧
総流量90

液種：NaCl 0.5M
流速：0.1L/min

70

80
60

2000

50
1500

40301000
Liquid type : NaCl 0.5M

500
0.2

0.40.620
1001.00.8

3500

180

液種：模擬地下水
流速：0.01L/min

00.2

Liquid type : H2 O

経過時間(min)Elapsedtime（min）

200

500

30500

00.020経過時間(min)Elapsedtime（min）

Fig. 5 Changes over time of water flow pressure andtotal flow rate (Cases 1, 2)

Waterpressure（kPa）
送水圧
(kPa)

3500

2500

40

1000

Flow rate : 0.1L/min

経過時間(min)Elapsedtime（min）

3000

60

2000

10090

送水圧
総流量

3000

80

液種：CaCl2 0.5/3M
流速：0.1L/min

2500
2000

70
60
50

1500

4030

1000
Liquid type : CaCl2 0.5/3M
Flow rate : 0.1L/min

500
00.00.2

0.4

0.6

0.8

20

総流量 (cc)

1000

7080

経過時間
(min)
Elapsedtime（min）

500
500

液種：H2O
流速：0.1L/min

Total flow（cc）

10000pressure（kPa）
Water(kPa)
送水圧力

2500

総流量 (cc)Total
flow（cc）

150000

2000

090

3500

Liquidtype : Simulated groundwater
液種：模擬地下水
流速：0.1L/min
Flowrate : 0.1L/min

2500

100

送水圧
総流量

3000

00.0

Water
pressure（kPa）
送水圧(kPa)

3000

Test situationon the side

Case1

200000

送水圧力
Waterpressure
総流量
Totalflow(cc)
総流量flow（cc）
Total(kPa)
送水圧力pressure（kPa）
Water

3500

After watersupplyBefore the test(cc)
総流量flow（cc）
Total

breakthrough occurred, there was almost noincrease in water pressure and a water channel wasformed. On the other hand, in the case where theFrom these facts, it was found that there is ahigh possibility that piping and erosion can besuppressed by pre-swelling of buffer material.
However, it was also revealed that the inhibitory
effect is influenced by the ionic strength of the liquid
type used for the pre-swelling. From now on it will
be necessary to check the ionic strength of the liquid
type used for pre-swelling, and the presence orabsence of water channels.

pressure（kPa）
Water(kPa)
送水圧

flow rate was 0.1 L/min, the water pressure
increased to about 1.5 MPa at the beginning of waterflow,
andbreakthroughoccurred.
Once1001.0

経過時間(min)Elapsedtime（min）
Fig. 7 Changes over time of water flow pressure andtotal flow rate (Cases 3 to 5)

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

Case3

Case4

Case5

Beforethe test[3]

Technology for Engineering Components of HLW
Disposal, Advanced remote control technology
development”, RWMC (2008) (in Japanese)Åberg A., Effects of water inflow on the buffer – anexperimental study, SKB Report R-09-29 (2009)

AfterwatersupplyAfterthe test

Test
situation on
the sideFig. 8 Status of pre-swelling test (Cases 3 to 5)
4. Summary
We conducted a test using a small acrylic cellfor the purpose of grasping the formation condition ofwater channels in the vicinity of the surface of thebuffer material, and the effect of countermeasuresagainst erosion of the buffer material due topre-swelling. In the test simulating the pellet filling
method of the block stationary construction method,
formation of water channels was not confirmed if thewater flow rate was 0.001 L/min. On the other hand,in the case of NaCl and CaCl2, swelling performanceof the buffer material decreased due to ionic strengthand Ca type conversion, and phenomena such as
precipitation of silica sand and expansion of washingrange were observed.
It was also found that pre-swelling may be aneffective countermeasure for suppressing water
formation. However, in the case of pre-swelling witha liquid type with an ionic strength of 0.18, a water
channel was formed, suggesting that the ionicstrength of the liquid type used for engineered watersupply affects the pre-swelling effect. For that reason,
future study is necessary for the condition of pipingwith the ionic strength of the liquid type used forpre-swelling as a parameter.
This research is a part of “Development of Advanced
Technology for Engineering Components of HLW
Disposal” under a grant from the Japanese Ministryof Economy, Trade and Industry (METI).
References
[1]

[2]

Lennart Börgesson, Torbjörn Sandén : Piping anderosion in buffer and backfill materials Current
knowledge, SKB Report R-06-80 (2006).
Radioactive Waste Management Funding andResearch Center: “Development of Advanced

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