GROUNDWATER FLOW ANALYSIS FOR EVALUATING FACTORS ON WATER
INFLOW TO THE FACILITY DURING THE OPERATION PERIOD
Masataka IMAI*1, Minoru EMORI*1, Tomoko ISHII*1, Masahiro KAWAKUBO*1, Kazuhiro TSUBONO*1,
Masayoshi YAMAURA*2, Thuneyuki MAEMURA*2 and Eiko SUGIMOTO*2*1 Radioactive Waste Management Funding and Research Center
Nichirei Akashicho Bldg. 12
F, 6-4, Akashicho, Chuo-ku, Tokyo 104-0044, Japan*2 Dia Consultants Co., Ltd.
2-272-3 Yoshino-cho, Kita-ku, Saitama-shi, Saitama 331-0811, Japan
Key words; Geological disposal, Disposal system, Groundwater flow analysis, Density-dependent flow
Abstract
In order to ensure the long-term safety of geological disposal system for high level radioactive waste, it isnecessary that each component of the disposal system should maintain its initial performance as required by thelong-term safety assessment, taking into consideration disturbances caused by the construction and operation of
the disposal facility. This research aims at developing an evaluation method to predict evolution of the state
and/or performance of the engineered barrier system (EBS), focusing on disturbances during the construction
and operation. For this evaluation, unsteady groundwater analysis is necessary for considering the hydrauliccharacteristics of the site and the staged shape transition of its disposal facility during the construction andoperation. For this development purpose, a coastal site was investigated as a test case. First, numericalsimulations of density-dependent groundwater flow were performed using a simple two-dimensional verticalmodel, taking the operation process into consideration, and major factors that may affect the undergroundhydraulic environment were summarized. Then, three-dimensional groundwater analysis on different regional
scales was conducted to evaluate the influence of the operation procedure on water inflow to the facility. As aresult, it was confirmed that the hydrostatic pressure of sea water was a dominant factor on water inflow to thefacility. It was also possible to understand the trend of changes in water inflow to the disposal tunnel according
to the operation process.

1. Introduction
We consider satisfying the following two
requirements necessary to show the feasibility of thedisposal system in the geological disposal of highlevel radioactive waste (Figure 1):(a) Long-term safety after closure(b) Prerequisite for long-term safety assessment
after closure (initial performance of EBS at thetime of facility closure).
Start of theoperation

CloserFunctional level of EBS

Duringoperation(a) Long term safety after closure

Change of function level
during the operation0

Elapsed time(b) Prerequisite for long-term
safety assessment after closure(closed required performance)

1000 years
after closer

Figure 1 Feasibility of the disposal system handledwithin the engineering framework
The (a) has been researched as an approach tolong-term safety assessment after closure, by the H12report (H12 Project to Establish Technical Basis for
HLW Disposal), etc., in Japan1). But at the current
stage, when a candidate site has not yet beenmaterialized, (b) has not been researched sufficiently,
including changes in EBS function due todisturbances during the construction and operation of
the disposal facility.
Therefore, in this research we look at methods
related to (b) regarding the feasibility of the disposal
system under limited conditions. Through thisresearch, we believe that it will be possible topromptly present its feasibility at when candidate
sites and disposal concepts are materialized in the
future.
In this research, we are advancing the developmentof methods, especially focusing on disturbance of
hydraulic fields during the construction and operation
periods, in conjunction with other research work2). It
is necessary to consider the hydraulic characteristicsof the site and tunnel deployment of the disposal
facility during the disturbances accompanying theconstruction and operation. Therefore, we will
construct an analysis method for investigation,
focusing on the water inflow at the disposal facility
by unsteady groundwater flow analysis, with the

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japancoastal area as the test case, for the purpose ofevaluating hydraulic fields during disturbances. The
following three are set as concrete implementation
items:(i) To understand the specific influence of thecoastal area on the underground hydraulicenvironment.
(ii) To grasp the influence of the operation
procedure (disposal tunnel excavation) on thewater inflow of the disposal tunnel.
(iii) To present engineering measures to mitigate
influence (countermeasure against water inflow,etc.).
This paper is the result of studies in the first year of
3-year plan, and it is part of the result of making ananalysis model and summarizing examination of (i)and (ii) above.
The analysis code used for this series of studies are
the seepage flow/advection dispersion analysis
program Dtransu 2D・EL and Dtransu 3D・EL basedon the finite element method.
2. Specific influence of the coastal area on theunderground hydraulic environment
We summarized the main factors that may affect the
groundwater environment in the coastal area, tounderstand the influence on the disposal facility fromthe environment generally assumed for the coastal
area of Japan. Based on the results, we analyzed thegroundwater flow considering each factor, andstudied the influence of each factor on the
groundwater flow field around the disposal facility.
2.1 Influence of the operation procedure on thewater inflow of the disposal tunnel
The study group on technical issues of geologicaldisposal at the coastal seabed, etc. (hereinafterreferred to as "Coastal Study Group"), reported that
items to be considered in the coastal geologicalenvironment
are
sea
level
variation,
seawater/freshwater interface, hydraulic fields andchemical fields3). When selecting what seems to havea great influence on the water inflow of the disposal
facility
duringtheoperation
period,seawater/freshwater interface and hydraulic fields arenotable.
The seawater/freshwater interface is supposed tohave a shape protruding toward the sea side fromnear the shoreline because of the density difference
between salt water and fresh water. The
seawater/freshwater interface clearly separates thehydraulic field of fresh water and salt water, and aunique hydraulic field is formed near theseawater/freshwater interface. For this reason, thepositional relationship between the disposal facility
and the seawater/freshwater interface is an importantfactor in considering the influence of the hydraulicfield around the disposal facility.

A small hydraulic gradient and the hydrostaticpressure of sea water in the hydraulic field can beconsidered as the unique environment of the coastal
area. In Japan, due to large precipitation rate, thegroundwater surface generally takes a shape
conforming to the topography. The coastal area tends
to have gentle slope, leading to a small hydraulicgradient and a slow groundwater flow velocity. In
addition, the hydraulic gradient becomes small
because the sea side has a constant water level. Thus,
the hydraulic gradient is small in the coastal area bothon the land side and the sea side, and the groundwaterflow velocity in the area can be considered to beslow.
In addition, hydrostatic pressure equivalent toseawater (salt water density × seawater depth) always
acts on the coastal seabed. The depth of the disposal
facility is based on the depth from the seabed, and ingeneral the depth of the seabed becomes greater
going from the shoreline out to sea. In other words, it
is considered that the water pressure on the facility
increases as the location of the disposal facility goes
toward the sea side.
These three items were considered as importantfactors that may affect the underground hydraulicenvironment, and their influences are evaluated bygroundwater flow analysis as follows.
2.2 Groundwater flow analysis
Assuming a period from the construction of thedisposal facility to its closure, we conducted
groundwater flow analysis for multiple cases
combining each factor, and evaluated the influence ofeach factor on the water inflow volume of thedisposal facility.
(a) Setting-analysis-case
The specific factors (the seawater/freshwaterinterface, the small hydraulic gradient, and thehydrostatic pressure of the seawater) in the coastal
area shown in 2.1 are inseparable from thegeographical position of the disposal facility. This
makes it necessary to consider the location of thedisposal facility for each analysis case.
The land side being within 20 km from the coastline
from a transportation point of view and the sea sidearea being within 15 km from the coastline from theviewpoint of engineering were used as guides for
examination. Base on this, we set the location of thedisposal facility as being 2.5 km/15 km away fromthe shoreline to both the land side and the sea side.
To consider the influence of salt density, we also
investigated the influence by presence/absence ofdensity-dependent flow. Moreover, since theinfluence of the local heterogeneity of the hydraulicgeological structure is assumed around the disposal
facility, a case where caprock was provided for eachdisposal facility position was also examined. Table 1
shows the list of analysis cases.

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

Table 1 List of Analysis cases
Influence factor

disposal facility. The boundary conditions set are
shown in Figure 4.

Case name

Geographical location of thedisposal facility

Land (U)Sea (S)

Horizontal distance fromshoreline (km)

15.0

2.5

Scale of the initial dynamic
water gradient near the facility

Large

Small

2.5

15.0
Very small

Salt density

Caprock

Consider

None

U-2-1

U-1-1

S-1-1

S-2-1

Not consider

None

U-2-2

U-1-2

S-1-2

S-2-2

Consider

Exist

U-2-3

U-1-3

S-1-3

S-2-3

(b) Hydraulic geological structure model
The hydrogeological structure was set to exclude
influences peculiar to any particular area, due to thecurrent situation in which a disposal candidate site is
not yet specified in Japan. Specifically, the geologicaldivision was basically uniform, and a simple vertical
two-dimensional model used. The terrain is roughly
divided into terrestrial and oceanic areas, the sea side
is divided into the continental slope of the coastal
shelf and offshore areas, and the land side is divided
into lowlands and plateaus, mountains and hills. The
slope of the terrain was set using the median of thestatistical data for each classification4). The caprock
was set at 200m above the disposal facility, with awidth of 3,000m and a thickness of 100m in order tomake it large enough for blocking the groundwaterflow towards the facility. Figure 2 shows the analysis
mesh that was set up. The permeability coefficient set
for the analytical model was set to 2.3E-8 (m/s) for
host rock, and one of the caprock was set to 1/100
thereof.

Figure 2 Analysis mesh diagram(c) Analysis conditions
In case of not considering salt density, the initialconditions before the construction of the facility were
set to the result of the seepage flow analysis in steadystate. In case of considering salt density, the initialconditions were set to the results of the washout
analysis, in which density-dependent flow analysis
were used changing the sea level from the state where
the top of the land to the current shoreline. The
washout analyses were conducted for building theseawater/freshwater interface (salt distribution) for
the initial conditions. Figure 3 shows the salt
concentration distribution in case of considering salt
density.
Next, we conducted groundwater flow analysis inunsteady state that maintained the open condition of
the tunnel until 100 years after constructing the

Figure 3 Salinity concentration distribution(initial condition)(a) Considering density-dependent flow(b) Not considering density-dependent flow
Figure 4 Boundary conditions
2.3 Results and discussion
Factors considered to influence the hydraulicenvironment around the facility in the coastal area
were organized and the effects of each factor on thewater inflow volume of the disposal facility werearranged based on groundwater flow analysis. Figure
5 compares the total amount of water inflow at thedisposal facility in each analysis case, and Figure 6
shows the time course of the amount of water inflow
with/without consideration of caprock, from the startof excavation of the disposal tunnel to 100 years later.
Based on these results, the following conclusions
were obtained.
(i) Hydraulic gradient (initial groundwater flow
velocity)
In cases where disposal facilities were installed at
15 km on the sea side with a small hydraulic gradient,
the amount of water inflow at the disposal facility
was larger than in other cases. Therefore, it isconsidered that the influence of the hydraulicgradient is relatively small as compared with thehydrostatic pressure of the seawater. (Figure 5).
(ii) Seawater/freshwater interface
The difference in the amount of water inflow aftertunnel excavation due to the positional relationshipwith the seawater/freshwater interface did not appear
clearly. This is due to the stronger influence of otherfactors, and it
is consideredthat theseawater/freshwater interface has little influence onwater inflow volume in those cases where distance
between the facility and the seawater/freshwaterinterface (shoreline) is about 2.5km (Figure 5).
(iii) Pressure of the seawater
If the disposal facility is set up in the sea side, theamount of the tunnel water inflow will be larger thanthat installed in the land side. This is considered to be
due to the fact that the depth of the seabed becomes
greater (water pressure becomes higher) as it goes

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanfa ci lity water i nflow volume (㎥)

3000
2500

Land
15km

Sea
2.5km

2.5km

15km

2000
1500
1000
500
0
U-2-1 U-2-2 U-2-3 U-1-1 U-1-2 U-1-3 S-1-1 S-1-2 S-1-3 S-2-1 S-2-2 S-2-3
1 year later
10 years later〇-〇-1：Considering density-dependent flow〇-〇-2：Not considering density-dependent flow
100 years later〇-〇-3：Considering density-dependent flow and caprock

4.0
3.5
facility water inflow volume （mL/min/m）

further offshore. From the results of (i) and (ii), it isconsidered that the influence of seawater hydrostaticpressure on water inflow volume is relatively large
compared with the influence of other factors.
(iv) Density-dependent flow
In the initial stage after excavation (about 1 to 10
years after excavation), the amount of water inflow atthe disposal facility when not considering thedensity-dependent flow is slightly larger than that ofconsidering the density-dependent flow. (Figure 5).
This trend is the same regardless of the geographicalposition of the disposal facility. It is assumed that thedifference in the amount of water inflow is due to theinitial hydraulic gradient near the disposal facility
and the difference in density between salt water andfresh water. In addition, the effect of the initial stateon the amount of water inflow in the facility
gradually decreases as the open condition of thetunnel maintained for a long time (about 10 to 100
years after excavation) after excavation, and it
reaches to the value defined by the boundary
condition of the top surface of the model. Especially,in the land side, almost the same amount of water
inflow is obtained regardless of whether thedensity-dependent flow is considered or not, since theupper boundary condition is the rain penetration
boundary. In the sea side, the upper boundary is thewater pressure of the seabed, so in the caseconsidering the density-dependent flow, the water
inflow tends to increase slightly due to the differencein salt density.
(v) Caprock
When there is caprock above the facility, it will beless affected by the boundary conditions of the uppersurface of the model than if it does not exist, so thetendency of the tunnel water inflow and its change
does not depend on the geographical position of thedisposal facility. (Figure 5,Figure 6). Also, as timegoes by, the influence of flow inhibition by caprockappears and the amount of water inflow decreases.
Afterwards, when the unsaturated zone is formed
above the facility, the water inflow tends to decrease
markedly (Figure 6).

3.0
2.5
2.0
1.5
U-2-1:Land side 15km, considering caprock

1.0

U-2-3:Land side 15km, not considering caprock
0.5

U-1-1:Land side 2.5km, considering caprockU-1-3:Land side 2.5km, not considering caprock

0.0
0

20

40
60
80
Elapsed time since the end of excavation （year）

100

Figure 6 Change over time of the amount of water
inflow at the disposal facility due to the caprockconsidered/not considered (in land)
3. Influence of the operation procedure on thewater inflow of the disposal tunnel
To grasp the influence on the water inflow conditionof the disposal tunnel considering the difference inthe operation procedure, a method of modeling thetunnel deployment according to the operation
procedure was investigated, using three-dimensional
groundwater flow analysis. From the results of theresearch in 2 above, the hydrostatic pressure of sea
water which is shown to have a large influence as afactor specific to the coastal area was considered inthis analysis.
3.1 Hydraulic geological structure model
As a prerequisite in this research, the target hostrock is Neogene sedimentary rocks, the shape of thedisposal pit is panel type, and the disposal concept isthe vertical disposal method. Regarding the hydraulicgeological structure model for the same rock, NUMO
has already developed a geological environment
model that considers the current generic stage5).
Therefore, three-dimensional models in threedifferent scale; the wide scale, the disposal site scale,and the near field scale respectively, were constructedin this study based on the hydraulic geologicalstructure model developed by NUMO. We aimed toconstruct a hydrological analysis system based on thenesting method that enables evaluating groundwaterflow field consistently in different scales.
The wide scale model and the disposal site scalemodel have been constructed so far. We modeled
the hydraulic geological structure fractures and faults
by substituting equivalent permeability coefficients
considering their permeability. The disposal tunnel
was not modeled in the wide scale model, but it wasmodeled in the disposal site scale model as a group of
nodes given the pressure head of 0 m. The hydraulicgeological structure model of each scale made isshown in Figure 7.

Figure 5 Comparison of the amount of water inflow
at the disposal facility in each case

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

(a) Wide scale model

15 km offshore from the shoreline. In this model, theseabed located 15 km offshore from the shoreline is
E.L.-135m, and the disposal facility is installed at a
depth of 500m (E.L.-635m) from the seabed. Also, inthe land side the terrain has a slope of 1% of theterrain up to the 15 km from the shoreline. In thethree-dimensional analysis, these settings arefollowed, and the boundary condition shown in
Figure 8 is given to the wide scale model. Based on
the result of the analysis in the wide scale model, theboundary conditions of the disposal site scale model
were set to the outer peripheral surface of the model
as fixed pressure heads.
Top of the land side:
Shoreline
Specified headTop of the sea side:with a hydrodynamic gradient of 0.01
Specified head EL 0

Wide scale modelSea bed: EL -135m
Disposal facility: EL -635m
The dotted line shows the assumed
topographic surface

Disposal site scale model

Disposal facility

Figure 8 Boundary condition of the wide scale model(b) Disposal site scale model
Figure 7 Hydraulic geological structure model ofeach scale (bird's eye view)
3.2 Groundwater flow analysis
Using both the wide scale and the disposal site scale
described in 3.1, we analyzed groundwater flow
considering the excavation order of the main tunneland disposal tunnel in one panel, and evaluated theinfluence of the disposal facility on the water inflow
volume. Steady flow analysis was carried out for thewide scale model, and unsteady flow analysis was
carried out for the disposal site scale model, and as
mentioned above, the latter boundary condition was
handed over from the former analysis result. A list ofanalysis cases is shown in Table 2.
Table 2 List of analysis cases
Case setCase 1

Excavation procedure
Excavate the entire main tunnel and disposal
tunnel at once.
① Excavate the main tunnel at once.

Case 2 ② Excavate the disposal tunnels at the same time
4 years after ①.
① Excavate the main tunnel at once
Case 3 ② Excavate the disposal tunnel in three phases 4
years after ①.

3.3 Analysis condition
From the engineering point of view, the location of
the disposal facility under the coastal seabed is said
to be about 15 km from the shoreline. The location of
the disposal facility has been obtained from theresults of the research in 2 above, that the influenceof the hydrostatic pressure of the seawater is greater
toward the ocean. Therefore, the location of thedisposal facility was set as a conservative setting at

3.4 Results and Discussion
To grasp the influence of the operation procedure on
the water inflow condition of the disposal tunnel,
groundwater flow analysis was carried out inconsideration of the excavation order of the main
tunnel and disposal tunnel. The analysis results areshown in Figure 9 to Figure 12. From the results, thefollowing conclusions were obtained on the influenceof the disposal tunnel on water inflow volume.
・Assuming the emplacement of buffer material starts
about 10 years after the main tunnel excavation,
the amount of water inflow in the main tunnel anddisposal tunnel at that time will be nearly the samevalue in any case, and it is not different regardlessof the excavation process (Figure 9, Figure 10).
・Considering the excavation process, it is possible toevaluate a sudden change in the amount of water
inflow immediately after excavation (Figure 9,
Figure 10). However, this study is the result of
excavation of multiple disposal tunnels at once,
and we are to consider more detailed excavation
process as a future subject.
・Comparing the total amount of water inflow fromthe early days of excavation, Cases 2 and 3 arefewer than Case 1. This is due to changes in therange in which water inflow is drawn as the
excavation progresses. Accordingly, it isconsidered that more realistic water inflow can beevaluated by considering the excavation process indetail (Figure 11).
・The amount of water inflow at the disposal tunnel
showed a tendency to increase as it was closer tothe edge (rock) of the disposal facility (Figure 12).
It is considered that this is because the rock sidekeeps higher water pressure than the tunnel side.

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

Water inflow (L/min/m)

0.30

case1
case2
case3

0.25
0.20
0.15
0.10
0.05
0.00
0

1

2

3

4

5

6

7

8

9

10

Elapsed time (year)

Figure 9 Changes in the amount of water inflow atthe disposal tunnels
Water inflow (L/min/m)

0.30

case1
case2
case3

0.25
0.20
0.15
0.10
0.05
0.00
0

1

2

3

4

5

6

7

8

9

10

Elapsed time (year)

Cumulative amount of water inflow (m3 )Figure 10 Changes in the amount of water inflow at
the main tunnel
4,000,000
case1
case2
case3

3,500,000
3,000,000

facility, to evaluate the functional change of the EBSof the disposal system. This paper reported the resultsof the first year of the 3-year plan.
First, we clarified the influence of environment
unique to the coastal area on the groundwater
environment around the facility. As a result, it was
concluded that the influence of the hydrostaticpressure of the seawater is relatively large as a factor
affecting the amount of facility water inflow locatedin the coastal area. Next, taking that result into
consideration, we have grasped the influence of theoperation procedure on the water inflow at thedisposal tunnel, and the change in the amount ofwater inflow at the disposal tunnel by tunnel
excavation. In addition, through this series of studies,
we advanced improvement of the analysis model andanalysis method.
In the future, to evaluate the vicinity of the disposal
tunnel, we will make a model (near field model) thatprecisely models the area around the disposal tunnel.
Using the model, we will examine engineering
methods (countermeasures against water leakage,etc.) and evaluate the behavior of the EBS in
re-saturation conditions. We are to continue
improving the analysis system through this study.

2,500,000
2,000,000

1,500,000
1,000,000

500,000
0
0

1

2

3

4

5

6

7

8

9

10

Elapsed time (year)

Figure 11 Changes in the total amount of water
inflow at the disposal facility since the start of theconstruction(a) Case 1(b) Case 2(c) Case 3

Color scale

Figure 12 Comparison of disposal facilities water
inflow volume due to differences in excavation
process (5th year from excavation start)
4. Conclusion
In this research, we have been examining theamount of water inflow to the disposal facility
focusing on the disturbance of the hydraulic field dueto the construction and operation of the disposal

Acknowledgments
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) Japan Nuclear Cycle Development Institute:“Second Progress Report on Research andDevelopment for the Geological Disposal of
HLW in Japan, H12: Project to Establish the
Scientific and Technical Basis for HLW
Disposal in Japan, Supporting Report 2
Repository
DesignandEngineering
Technology”, JNC TN1410 2000-003, 2000.
2) Radioactive Waste Management Funding andResearch Center: “Development of Advanced
Technology for Engineering Components of
HLW Disposal, Development of Coastal
Disposal System”, 2016, (in Japanese).
3) The Japanese Ministry of Economy, Trade andIndustry: “Study Group on Technical Issues
Associated with Geological Disposal in Coastal
Areas Compiled a Report”, 2016, (in Japanese).
4) Kazue Wakamatu et al.:” Topography andground digital map of Japan”, Tokyo University
Press, 2005, (in Japanese).
5) Nuclear Waste Management Organization ofJapan, “External expert workshop on NUMO
safety case”, 2016, (in Japanese).

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