NUMERICAL ANALYSIS OF INFLOW CONTROL FOR QUALITY MANAGEMENTOF BUFFER MATERIAL USING DISCRETE FRACTURE NETWORK MODEL
Kazuhiko MASUMOTO*1 Makoto NAKAJIMA*1 Hayato NONAKA*1
Tomoko ISHII *2 Mayumi JO*2 Masataka IMAI*2 Hiroyuki ATSUMI*1*1 Kajima Corporation
2-19-1, Tobitakyu, Chofu city, Tokyo, 182-0036, Japan*2 Radioactive Waste Management Funding and Research Center
Pacific Marks Tsukishima, 1-15-7, Tsukishima, Chuo-ku, Tokyo, 104-0052, Japan
Key words; Numerical analysis, Groundwater inflow analysis, Discrete fracture network model, Geologicaldisposal, Engineering barrier, Buffer material, Grouting technique

Abstract
In the vertical disposal concept of high level radioactive waste, it is important to evaluate the influence ofgroundwater flow into disposal pits on the performance of buffer materials as an engineering barrier because theinflow might cause the erosion of the buffer materials. The purpose of this study was to evaluate the inflow
control technique (e.g. grouting) around disposal pits using numerical analysis. The model for the simulation
should be a discrete fracture network model when the groundwater mainly flows along fractures in a rock mass,
therefore, the discrete fracture network model was selected to estimate the inflow control technique in this study.
The numerical flow analysis was conducted using LT-Flow, which is a groundwater simulation program using
a pipe network structure, into which the fracture network was converted. Firstly, a single disposal tunnel wasmodelled for the simulation, and then five disposal tunnels were modelled by assuming disposal panels. The
results of numerical flow analysis showed that the effectiveness of grouting for the disposal pits to control the
inflow and the impact of grouting on other disposal pits or surrounding disposal tunnels could be evaluatedquantitatively.

1. Introduction
In the repository for high-level radioactive waste,
the groundwater inflow to the disposal pit impedes
the emplacement of the buffer material at theoperation phase. In addition, during the re-saturation
period after the buffer has been emplaced, thegroundwater inflow to the disposal pit may lead topiping and erosion, which may interrupt the
self-repairing functions of the bentonite and affectlong-term performance1),2). Grouting and otherengineering measures could be considered to reduce
the flow of groundwater into disposal pits, but thehydrological properties of the surrounding rock may
affect the effectiveness of these measures. In the caseof a rock mass in which fractures control
permeability, it is particularly necessary to evaluate
effects using a fracture model that has been turned
into the hydrogeological structure model. In thisresearch, an analytical approach was conducted bycreating a fracture network model based on actual
investigation data of underground researchlaboratories, and variance in the volume ofgroundwater flowing into disposal pits due to the
distribution of the fractures, and the influence of theengineering measures on the hydrological fieldaround the disposal tunnel were evaluated. First, a
study was made in a single disposal tunnel model,
and then a study was made in multiple disposal
tunnel models that assumed disposal panels. The
following is a report of these studies.
2. Study of groundwater flowing into disposal pits
based on the fracture network model
2-1 Creation of a hydrogeological structure
model
Geological and hydrological investigation datafrom the Horonobe Underground Research Center3)
were used to construct the hydrogeological modelthat was used in this research. The fracture data were
the results of an investigation in the Horonobe URL
350m-level tunnel of two connection tunnels ofroughly the same length, East and West, which
orthogonally intersected each other. The hydrological
data were derived from the results of a hydrologicaltest at the boreholes conducted near a ventilation
shaft, and the excavation damaged zone was from theresults of a hydrological test conducted in the
350m-level tunnel. Table 1 shows the geometricalparameter set of these fractures, while Table 2 shows

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

the parameter set used to create the hydrologicalmodel.
Based on the statistical data of these fractures, a
100m×100m×100m block scale geological structure
model4) was created using a probabilistic method.
The number of realizations of the model was set at 10
based on the results of fracture network analysis4)
conducted at the Mizunami URL. At that time, thehost rock of the Horonobe URL was sedimentaryrock, so it was decided to create the model byconsidering matrix structure. Specifically, fractureswith a minimum 1.25m to the set 3.82m radii weregenerated to model the matrix structure. The total
number of fractures that were created was 77,813 as

the average of 10 realizations, which werecoordinated with the set intensity of the 3D fractures.
The hydrogeological structure model was constructedby extracting 22% of the fractures from thisgeological structure model, which was the proportionof permeable fractures at the Horonobe URL. These
fractures were given a transmissivity by thesimulation analysis of hydrological test. Figure 1 isan example of realization. In this research, a model inwhich the fracture network structure was converted
into a pipe network5) was used for flow analysis.

Table 1 Geometrical parameter set of the fracture

Fracture orientation

Fracture size distribution

Fracture intensity

Set offracture

Dip Strike

FishercoefficientPower lowminimum
radius (m)P32(m2/m3)

P32adj*(m2/m3)NW

N69W 52N

16.9

3.5

3.82

0.54

0.94

NE

N51E 51N

13.6

3.5

3.82

0.25

0.44

OthersN78E 43S

5.2

3.5

3.82

0.20

0.34

total

‐

‐

‐

‐

0.98

1.71

*convert to 1.25m of minimum radius
Table 2 Parameter set for hydrogeological modelParameter

Investigation data

Proportion of permeable fractures

22.2%

Permeability coefficient of interval
including permeable fractures

6.0×10‐9m/sec(test interval 65m)

Permeability coefficient of EDZ

1.0×10‐6m/sec(width 65m)

Permeability coefficient of matrix

1.8×10‐11m/sec

5m

disposal
坑道
坑道
tunnelExcavation damaged zone (EDZ)

Blue: permeable fracture
Brown: matrix structure
Figure 1 Hydrogeological structure model of
discrete fracture network

Excavation damaged zone
with 0.5m width
Figure 2 Modeling of Excavation Damaged Zone

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japannon-permeable boundary of the plane which
orthogonally intersects with the axis of the disposal
tunnel, with the other planes having a total head of
350m. In addition, the analysis was conducted in 3
steps: 1) at the time of disposal tunnel excavation, 2)
at the time of disposal pit excavation, and 3) at thetime grouting measures were implemented.

2-2 Modeling the disposal tunnels and pits
One disposal tunnel and 10 vertical disposal pits
were modeled in the model constructed in 2-1, andthe flow analysis was conducted in the model. The
disposal tunnel was a circular cavern having a lengthof 100m and diameter of 5m in the center of themodel region. The excavation damaged zone was set
with a thickness of 0.5m on the outer periphery of thedisposal tunnel (Fig. 2). The boundary of the tunnel
was set as the atmospheric pressure boundary. The
disposal pits were turned into a model by setting pit
intervals of 6.6m, radius of 1.5m, and 5.0m from thefloor as the atmospheric pressure boundary. The
conditions of the model boundary were a

2-3 Estimations of amount of groundwaterflowing into disposal tunnels and pits
Table 3 shows the results of calculating the volumeof groundwater flowing into disposal tunnels for thetime of disposal tunnel excavation. The tunnel inflow
volume was the 10 realization average, which

Table 3 Results of flow analyses during disposal tunnel excavation

HR01
HR02
HR03
HR04
HR05
HR06
HR07
HR08
HR09
HR10

Single‐tunnel
inflow（L/min）
7.10
10.35
2.84
3.76
3.20
8.15
10.78
48.25
2.06
10.77

Avg.

10.73

RZN

Multiple‐tunnel inflow (L/min）
G1

G2

G3

G4

G5

0.03
0.98
0.65
1.43
15.57
5.36
5.84
2.501
117.60
9.93
15.99
‐

2.49
1.38
0.56
1.63
6.04
3.60
1.95
4.91
24.44
2.50
4.95

3.43
4.52
1.05
1.06
1.74
4.63
7.37
30.77
1.01
6.83
6.24
5.25

4.49
8.14
4.11
1.11
0.09
0.61
4.08
20.51
0.77
1.69
4.56

6.02
27.83
4.09
3.70
7.89
14.25
5.78
11.65
6.79
3.74
9.17
‐

Table 4 Results of flow analyses during disposal pit excavation
RLZ
HR01
HR02
HR03
HR04
HR05
HR06
HR07
HR08
HR09
HR10

7.10
10.35
2.84
3.76
3.20
8.15
10.78
48.25
2.06
10.77

4.65
8.82
2.04
3.21
2.13
6.55
7.70
38.19
1.57
9.22

avg.

10.73

8.41

No.1

No.2

No.3

No.4

No.5

No.6

No.7

No.8

No.9

No.10

tunnel anddisposal pit
inflow（L/min）

0.12
0.12
0.05
0.21
0.06
0.09
0.01
5.54
0.02
0.07

0.35
0.19
0.07
0.02
0.11
0.23
0.01
0.48
0.00
0.19

0.17
0.04
0.08
0.05
0.02
0.11
0.04
1.04
0.06
0.97

0.04
0.02
0.20
0.00
0.20
0.42
0.95
0.71
0.09
0.03

0.46
0.16
0.01
0.01
0.82
0.16
0.11
0.40
0.00
0.14

0.04
0.25
0.00
0.12
0.00
0.64
0.35
1.47
0.00
0.07

0.74
0.28
0.32
0.04
0.00
0.01
0.32
0.78
0.17
0.05

0.45
0.09
0.29
0.09
0.56
0.14
1.15
0.34
0.10
0.04

0.53
0.23
0.00
0.02
0.00
0.02
0.88
0.05
0.09
0.01

0.00
0.27
0.00
0.02
0.00
0.06
0.17
0.01
0.47
0.04

7.55
10.48
3.05
3.79
3.90
8.43
11.69
49.01
2.57
10.83

2
0
0
0
2
1
3
5
0
1

11.13

total 14

disposal pit inflow （L/min）tunnel inflow tunnel inflow
after pit
before pit
excavation
excavation

0.27

number of
disposal pit of>0.5L/min

Fracture with
>0.01L/min

Inflow volume
1 10‐1 10‐2 10‐3 10‐4 10‐5 10‐6 (L/min)

Disposal tunnel

Disposal pit

No.1

2

3

4

5

6

7

8

Figure 3 Distribution of disposal pit inflow

9

10(HR10)

Figure 4 Distribution of fractures intersectingwith disposal pit

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

2-4 Changes in inflow volume due to grouting
measures
Grouting measures as an inflow control technique
for the disposal pits were turned into a model by
reducing the permeability within a 1.5m area aroundthe disposal pits that showed an inflow volume of atleast 0.1L/min. When creating the model, thepermeability coefficient of pipes that go through thearea of grout improvement shown in Figure 5, wasreduced to 13% for the initial model based on the
improvement target value of pre-grouting. As a resultof this calculation, the inflow volume to the disposalpits subjected to grouting was reduced by an averageof 33%, and the number of disposal pits having
inflow volume of 0.1L/min or less was increased
from 51 to 76. Figure 6 shows an example of theanalytical results. In the No. 2, 3 and 5 pits where
grouting was taken, the inflow volume wassignificantly reduced. However, at No. 4, which was
sandwiched between these grouting pits, it can beseen that the inflow volume had increased slightly
due to changing groundwater flow paths.
3. Study of groundwater flowing into the disposalpits in the multiple tunnel model
3-1 Modeling the multiple tunnels
Assuming actual disposal panels, the presence orabsence of a nearby disposal tunnel, the continuationof fractures into the tunnel, etc., may affect the
groundwater inflow to the disposal pits. Therefore,
the volume of groundwater flowing into disposal pits
was analytically estimated by creating a model ofmultiple tunnels. When constructing this model, the10 hydrogeological structure models created in 2-1
were each given 5 parallel disposal tunnels having a

1.5m

3.0m

5.0m

6.5m

：permeability‐reduced pipe
Figure 5 Modeling of grouting around disposal pit

disposal pit inflow (L/min)indicated 10.73L/min. This roughly correspondedwith the actual measured value of inflow for 100m of
the 350m-level tunnel of Horonobe URL.
Table 4 shows the results of calculations made forthe time of disposal pit excavation. The average
inflow volume of the disposal pits was 0.27L/min,
which varied depending on the realization anddisposal pit. This can be said to be the result of
inconsistencies in the fractures. For example, Figure
3 shows the distribution of disposal pit inflow
locations, and Figure 4 shows the distribution of
fractures having a flow volume of 0.01L/min or morethat intersects with the disposal pits, for one
realization (HR10). In addition to the inflow locations
seen from the excavation damaged zone adjacent to
the tunnel, groundwater inflow can be seen in places
where fractures intersect with disposal pits. The
number of disposal pits having an inflow volume perpit of at least 0.5L/min was a maximum of 5 pits /10
pits per realization. A total of 14 of the 100 disposalpits had a value of at least 0.5L/min.

HR10

1.0

before grouting

0.8

after grouting

0.6
0.4
0.2
0.0
1

2

3

4

5

6

7

8

9

10

dipsosal pit No.

Figure 6 Results of flow analyses
before and after grouting

100m

100m

100m

Figure 7 Multiple-tunnel model of
discrete fracture network
Table 5 Summary of number of disposal pits
with >0.5L/min inflow
Multiple‐tunnel

RZN

Single‐tunnel

G2

G3

G4

Total

HR01
HR02
HR03
HR04
HR05
HR06
HR07
HR08
HR09
HR10

2/10
0/10
0/10
0/10
2/10
1/10
3/10
5/10
0/10
1/10

0/10
0/10
0/10
0/10
1/10
0/10
0/10
0/10
1/10
0/10

0/10
0/10
0/10
0/10
1/10
0/10
3/10
6/10
0/10
0/10

1/10
1/10
1/10
0/10
1/10
0/10
2/10
7/10
0/10
0/10

1/30
1/30
1/30
0/30
3/30
0/30
5/30
13/30
1/30
0/30

Avg. per100 pits

14.0/100

8.3/100

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japandiameter of 5.0m and spaced at 12m intervals. The
floors of each of these disposal tunnels were given 10
disposal pits having a height of 5m and radius of
1.5m (Figure 7). As was the case with the single
tunnel, the plane which orthogonally intersected withthe axis of the disposal tunnel became thenon-permeable boundary, while other model
boundaries had a total head of 350m. It should be
noted that the groundwater volume was evaluatedusing data from the middle 3 of the 5 disposal tunnels(G2 to G4 in Figure 7) in order to evaluate theinfluence of adjacent disposal tunnels.

z
x
G2

y
G3

⑥

②

1.19
Initial inflow
0.08 0.52
0.02 0.02（L/min）⑤③

0.88
0.26 0.54④①

0.64 4.36

01
04 03 02
07 06 05
Disposal Pit 10 09 08
Order of grouting ①～⑥
Inflow >0.5L/min

Figure 8 Grouting pits and order of grouting (HR08)

3-3 Changes in inflow volume due to grouting
measures in multiple tunnel models
Using the multiple-tunnel model of HR08, which
was the realization having the greatest volume of
inflow during disposal pit excavation, a flow analysis
was conducted when grouting measures were
implemented. Figure 8 shows 6 disposal pits where
the inflow value exceeded 0.5L/min and where
grouting as an inflow control technique wasconducted in the order of initial inflow volume ((1) to(6) in the figure). As with 2-4, grouting was modeledby reducing the permeability within an area of 1.5m
around the target disposal pit, and the flow analysis
was conducted. Figure 9 shows the results of the
inflow volume reduction by the grouting, and theamount of inflow increase in disposal pits of aroundthe grouting pits, arranged in the order of (1) to (6).
Inflow volume decreased in all of the grouting pits,
but in the disposal pits of adjacent tunnels, inflow
volume increased slightly due to change of8
inflow change （L/min）

3-2 Estimations of amount of groundwaterflowing into disposal tunnels and pits in themultiple tunnel models
Flow analyses were conducted at the time of thedisposal tunnel excavation, and comparisons weremade between the analytical results of the single- andmultiple-tunnel models (Table 3). Although variance
was seen in each realization, it could be concluded
that inflow locations were dispersed when there wasan adjacent disposal tunnel. In the multiple-tunnel
model, the average inflow volume per disposal tunnel
was 5.25L/min, which was only about half the value
in the single-tunnel model.
Next, flow analyses were conducted at the time ofthe disposal pit excavation. Table 5 shows thenumber of disposal pits with an inflow volume of atleast 0.5L/min per pit. Here it can be seen that only
25 disposal pits had a value of at least 0.5L/min of
the 300 pits in the multiple-tunnel model, that is, 8.3
pits per 100 pits, about 60% of the number of pits inthe single-tunnel model. From these results, we could
confirm that the existence of adjacent disposal
tunnels or pits helped to greatly reduce the amount ofgroundwater flowing into disposal tunnels and pits.

increase around grouting pit
decrease of grouting pit

6
4
2

0.34

0

‐0.96

‐2
‐4

0.15

0.37

0.12

0.08

0.17

‐0.53

‐0.59

‐0.38

‐0.40

1.23

‐3.59

‐6
‐8

‐6.45

Figure 9 Results of inflow change of disposal pits
in the order of grouting

groundwater flow paths. However, overall theamount of inflow reduction in grouting pits was
6.45L/min, while the amount of inflow increase indisposal pits of adjacent tunnels was limited to
1.23L/min, indicating that the grouting measures
were effective in reducing the amount of inflow.
4. Concluding remarks
In a rock mass where fractures control
permeability, variance in disposal pit inflow due tothe distribution of fractures, and the influence ofengineering measures on the hydrological fields
around disposal tunnels could be quantitatively
evaluated with analyses using discrete fracture
network models. In addition, by creating a model ofmultiple disposal tunnels, it was possible toquantitatively evaluate the reduction of inflow
volume by the dispersion of inflow locations due tothe influence of adjacent tunnels, and the increase ininflow volume resulting from changing groundwaterflow paths.
This research was conducted as part of the“Development of Advanced Technology forEngineering Components of HLW Disposal” project
under a grant from the Agency for Natural Resources
and Energy of the Japanese Ministry of Economy,
Trade and Industry (METI).

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

3)

4)

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Börgesson, L. and Sandén, T. : Piping anderosion in buffer and backfill materials Current
knowledge, SKB Report R-06-80, 2006.
Iwatani, T. et al. : Study on erosion of buffermaterial in vertical disposal concept of high level
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Aoyagi, K. and Kawate, S. : Collection of
measurement data in 2013 fiscal year at the
Horonobe Underground Research Laboratory
Project, JAEA-Data/Code2015-017, 2015.
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6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan