Mizunami Underground Research Laboratory Project- Achievment during Phase I/II and Important Issues for Phase III Katsuhiro Hama
Tono Geoscience Center
Japan Atomic Energy Agency
1-64, Yamanouchi, Akiyo-cho, Mizunami-city, Gifu prefecture 509-6132 JAPANKey words; MIU Project, Site Characterization Technology

Abstract
The Mizunami Underground Research Laboratory (MIU) project is being pursued by the Japan Atomic Energy
Agency (JAEA) to enhance the reliability of geological disposal technologies in the crystalline host rock
(granite) at Mizunami City in Gifu Prefecture, central Japan. The project proceeds in three overlapping phases,
Surface-based investigation Phase (Phase I), Construction Phase (Phase II) and Operation Phase (Phase III). The
MIU Project has been ongoing the Phase III. During Phase I, a step-wise investigation was conducted by
iterating investigation, interpretation, and assessment, thereby understanding of geologic environment was
progressively and effectively improved with progress of investigation. An optimal procedure from investigation
to assessment was compiled as a set of geosynthesis data flow diagram for each investigation step. During Phase
II, we have evaluated adequacy of techniques for investigation, analysis and assessment of the deep geologicalenvironment established in the Phase I. For Phase III, three important issues were identified based on the latest
results.
(1) Development of countermeasure technologies for reducing groundwater inflow,(2) Development of modeling technologies for mass transport,(3) Development of drift backfilling technologies.
For the issue (1), post-grouting works have been applied to a gallery at 500 m depth. Three grouting concepts
were applied to the post-grouting works; a new grout material, a new injection system, and a new post-grouting
zone. For the issue (2), in-situ and laboratory mass transport experiments have been carried out. The
macroscopic and microscopic observations were carried out to understand the distribution of tracer (uranine)
after the laboratory diffusion experiment. For the issue (3), with a focus on hydraulic pressure andhydrochemical recovery processes around underground galleries in fractured crystalline rock, the groundwater
recovery experiment has been conducted to evaluate the natural groundwater and hydrochemical recovery of therock mass.

1. Introduction
The Mizunami Underground Research Laboratory(MIU) project is being pursued by the Japan Atomic
Energy Agency (JAEA) to enhance the reliability ofgeological disposal technologies in the crystalline
host rock (granite) at Mizunami City in Gifu
Prefecture, central Japan. The project proceeds inthree overlapping phases, Surface-based investigation
Phase (Phase I), Construction Phase (Phase II) andOperation Phase (III). The MIU Project has beenongoing the Phase III. During Phase I, a step-wise
investigation was conducted by iterating investigation,interpretation, and assessment, thereby understandingof geologic environment was progressively andeffectively improved with progress of investigation.
An optimal procedure from investigation toassessment was compiled as a set of geosynthesis
data flow diagram for each investigation step. During
Phase II, we have evaluated adequacy of techniques
for investigation, analysis and assessment of the deep
geological environment established in the Phase I.
For Phase III, three important issues were identified
based on the latest results.
(1) Development of countermeasure technologies for
reducing groundwater inflow,(2) Development of modeling technologies for masstransport,(3) Development of drift backfilling technologies.
This paper summarizes the achievment during Phase
I/II and important issues for Phase III
2. Mizunami Underground Research Laboratory
2. 1 Location

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

The MIU is located in Mizunami City, Gifu
Prefecture, in the central part of Honshu, the mainisland of Japan. The MIU facility and related surface
plant have been constructed on land in Akiyo-choleased from Mizunami City.
2. 2 Geology, Mechanical and Hydraulic

Properties of Rock Mass
The shafts and galleries have been excavatedthrough an overlying sequence of Miocene
sedimentary rock (Mizunami Group) and into the late
Cretaceous Toki Granite. The Toki Granite has beenfaulted and has undergone several episodes of upliftand subsidence from the Miocene to the Pliocene,
indicated by the presence of lacustrine and marine
sedimentary formations unconformably overlying the
granite (Fig.1).

Foundations for the shaft head frames are anchoredon bedrock of the upper part of the Mizunami Group.
Depth to bedrock at the Main and Ventilation shaftsis around GL-10m. By July 2003, the shaft entrances
had been excavated. Excavation of the upper,
approximately 50m of the shafts started in March
2004, and was followed by installation ofshaft-sinking equipment, including the mobile
scaffolding, for excavating below 50m depth. In
March 2006, shaft excavation started using all of theequipment for shaft sinking. By 2012, excavation of
the Main and Ventilation Shaft had reached 500 m
depth (Fig.2).
Current design for the MIU is as follows:• Two shafts, the Main Shaft (6.5mφ) and theVentilation Shaft (4.5m φ); and• Two experimental levels, at the 300 m level andthe 500 m level.
• Sub stages, at the 100m, 200m and 400m levels
Ventilation Shaft

Main Shaft

Water collection ring
-100m Sub Stage-200m Measurement Niche
off Ventilation Shaft-200m Sub Stage-200m Measurement Niche off
Main Shaft-300m Measurement Niche
off Ventilation Shaft-300m Sub Stage

Fig.1 Geological map and geological section
around MIU
The maximum thickness of the sedimentary
formations in this area is about 170 m. The average
P-wave velocity, unconfined compressive strengthand Young’s modulus of intact granite are 5.5 km/sec,
150 MPa and 50 GPa, respectively. Table 1 shows
hydraulic conductivities of the main rock units andseveral structural elements based on hydraulic tests indeep boreholes drilled from the surface in and aroundthe MIU site.

Table 1 Hydraulic conductivities of main
lithologies and structural elements in and aroundMIU1),2)
Stratigraphy and structural elements
Sedimentary rock

Seto Group
Upper highly fractured domain
Low angle fracture zone
Lower sparsely fractured domain

Highly fractured zone along the Tsukiyoshi fault
Fault

M ain part of Tsukiyoshi fault
Other faults

Log [k (m/s)]-5

M izunami Group
Toki granite

Hydraulic conductivity

Parallel to fault
Normal to fault-8.5 to -4.0
-6.7
-5.4
-7.7
-6.4
-11
-9.0 to -4.0
-11-300m Access/Research
Gallery-400m Sub Stage-500m Access/Research
Gallery-North-500m
Access/Research
Gallery-South-500m Sub Stage
GW pressure monitoring BH
Pilot borehole
Initial rock stress measurement BH
Solute transport investigation BH

GW chemistry monitoring BH
Rock strain monitoring BH
Geological investigation BH
Excavated by March 2014

Fig.2 Layout of MIU

3. Summary of the Phase I and Phase IIinvestigations
The investigations during Phase I followed thesequence of overall characterization of the regional
geological environment, identifying investigation
items requiring more detailed information andborehole investigations addressing the investigation
items. Investigations were scheduled so thatinteractions among excavations or borehole tests due
to simultaneous implementation of several borehole
investigations could be avoided. More specifically,
investigations were categorized into six groups asshown in Fig. 3: surveys of existing information andfive investigation steps moving from surface

2. 3 Shaft and Galleries

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanreconnaissance to crosshole tomography surveys andcrosshole hydrological tests.
Phase II

Phase I
Step 0

Step 1

Compilationof acquired
information
Geological
Mapping

Reflection
seismic
surveySedimentary
cover

Step 2

Step 3

Step 4

Deepborehole

Crosshole

Existing
borehole

Shallowborehole

Existing
borehole
investigations

Shallow
Deep borehole
borehole
investigations
investigations

Basement
graniteFault /
Joint zone

Cross-hole
tomography
Cross-hole
hydraulic
test

Undergroundfacility

Constructionofundergroundfacility

Estimated / Determined

Fig. 3 Concept of Phase I/II investigations
In the following chapter, detailed results ofhydrochemical investigations are described as an
example. Step-wise approach was applied forhydrochemical investigation in and around the MIU
construction site. Hydrochemical investigations arecarried out in step 0 (surveys of existing information),in step 2 (surveys in existing boreholes and newshallow borehole investigations) and also in step 3(new deep borehole investigations).
3.1 Phase I Investigations(1) Step 0 (Surveys of existing information)
For the local-scale area, data on groundwaterchemistry, pH and redox conditions had beenobtained mainly from borehole investigations carriedout as part of the regional hydrogeological study.
Using these data, the spatial distribution of thehydrochemical characteristics of the groundwater
between the boreholes were estimated using the
kriging method. It was indicated that the groundwaterin the granite was Na-Cl type, the concentration ofchloride ions increases with depth, the pH of thegroundwater was mildly alkaline and the redox
conditions were estimated to be weakly to strongly
reducing. Mass balance analysis and multivariate
analysis indicate that the chemistry of thegroundwater could be explained by the mixing ofseveral groundwaters with different salinities. One of
the main processes determining groundwaterchemistry was thus identified as being the mixing ofgroundwaters with different salinities(2) Step 2 (shallow borehole investigations)
One of the goals of the investigations in this step isto validate and update the hydrochemical model andto determine the geological environment prior to theconstruction of the URL. In this step, groundwaterand rock samples were collected and analyzed forhydrochemical analysis and for updating the model,
by conducting borehole investigations for the
Mizunami Group with a thickness of a few tens ofmeters to approximately 200 meters, covering theToki Granite.
The results of the investigations indicated that the
groundwater in the sedimentary rocks is
Na-Ca-HCO3 type, rich in silicon and sulphate ions inthe shallower part and Na-Cl type in the deeper partof the Mizunami Group and the upper part of theToki Granite. It was also found that the salinity of thegroundwater generally increases with increasing
depth. A low permeability formation was found at thedepth where the boundary occurs between the
Na-Ca-HCO3 type and the Na-Cl type groundwater inthe Mizunami Group, below which a significant
decrease in hydraulic head was observed. This wasconsidered to be due to the existence of the low
permeability formation and due to the fact that the
distribution of the characteristics of the groundwaterchemistry may be governed by the hydrogeologicalstructure. In the shallower part of the Mizunami
Group, the hydrogen/oxygen isotope ratio in the
groundwater varies within the range of fluctuation ofthat in the precipitation water sampled around the
URL construction site and tritium was also detected.
Thus, the origin of the groundwater in the shallower
part of the Mizunami Group is considered to be
recent precipitation, with a faster recharge ratecompared to the groundwater in the deeper part of the
Mizunami Group3).
(3) Step 3 (deep borehole investigations)
The objectives of the deep borehole investigations
include determining the hydrochemical property of
the groundwater in the granite to a depth ofapproximately 1,000 meters before construction of
the URL. A further objective is to determinehydrochemical property of the groundwater
encountered in the URL as input for planning theinvestigations in Phase II. Groundwater and rocksamples were collected and analyzed and thehydrochemical model based on the investigation
results from Step 2 and preceeding steps was
updated.
Na-Ca-Cl type groundwater with a salinity of
approximately 2,500 mg 1-1 (one tenth of the salinity
of seawater) was found in the granite at a depth ofapproximately 1,000 meters. The quantitativehydrochemical model with the 3D distribution of thegroundwater chemistry was updated by repeating themultivariate analysis, with more reliable data on thewater chemistry obtained during step 3. The pH of
the groundwater was in the range of 8 - 9 and thiswas consistent with the result predicted in Step 0.
From the thermodynamic analysis for the equilibrium
condition of the water-rock reaction and microscopic
observation of rock alteration, the water has a major
pH buffering capacity due to dissolution/precipitation
reactions with carbonate minerals3). Although there isno measured value for the redox potential in situ,
based on mineral observations and the results of

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japandissolved gas analysis, possible occurrence of redox
reactions involving iron minerals, sulphur minerals
and hydrogen sulphide gas was indicated. With
regard to the origin and residence time of the
Na-Ca-Cl type groundwater, it was concluded fromthe isotopic composition and the ratio of the
dissolved chemical components that the groundwater
was diluted from saline water that either originatedfrom fossil seawater or from long-term water-rockreactions. The hydrochemical model of the
distribution of groundwater chemistry on the site
scale that was constructed in Step 0 was updated
using the results from this step (Fig. 4) 3).
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Fig. 4 Hydrochemical conceptual model around URL
3.2 Phase II investigations
During Phase II investigations, the changes ingroundwater salinity and pH/redox conditions haveevaluated. In this section, the results concerningchanges in salinity have been summarized.
Hydrochemical monitoring was conducted in andaround the MIU during the shaft construction down
to G.L. −500 m depth (Fig. 5).

Fig. 5 Hydrochemical monitoring boreholes in andaround the MIU
Water-collection rings (WR) were placed every 25
m in the shafts to direct the water inflow and reduce
the water pressure on the concrete lining. The inflow
rate was measured at each WR in the shafts. The
highest water inflow occurred from the conglomerate

layer; this is probably the dominant groundwater flow
path in the sedimentary rock.
The groundwater chemistry in the WRs is possibly
affected by water–cement interactions caused bycontact with the concrete liner. Incidentally, Cl is not
susceptible to water–mineral interactions. The Cl
concentrations in the WRs and boreholes increased
during the initial observation period but subsequently
decreased with time. The groundwater chemistry at
depth was affected by the upconing in the initialperiod. After the excavation of the shaft had
advanced to deeper depths, the upconing point also
moved to a greater depth. In the upper depths,
upconing was reduced and the impact of shallower
groundwater infiltration increased with time 4). To
assess shallow water infiltration, stable isotopes,
tritium (3H), and chlorofluorocarbons (CFCs) as
tracers of shallow groundwater were monitored in
granitic rock for 5 years. The results showed that theshallow groundwater infiltrates into the groundwaterat depths of 200–400 m after 10 years of drainage(hundreds ton/day). The mixing ratio of surface waterin deep groundwater can be approximately estimatedusing CFC-12 data. The hydrochemical changes ingroundwater were estimated by principal components
analysis (PCA) 4). It is presumed that deep
groundwater will be replaced by shallow groundwaterin the future.
The groundwater in 07MI07, 09MI20, and 10MI26
initially had relatively high Na, Ca, and Cl and low
DIC and SO4 concentrations, which subsequently
changed to relatively low Na, Ca, and Cl and high
DIC and SO4 concentrations. This tendency was
clearly observed near the borehole collars in the
excavations (e.g., zones 5 and 6). Horizontal
monitoring boreholes were drilled sequentially in the
deepest gallery during excavation. Therefore, theobservations at each borehole were affected byupconing during the initial period. In boreholes
07MI07, 09MI20, and 10MI26 on the south side of
the Main Shaft fault, groundwater with relatively
high levels of Na, Ca, and Cl and low DIC and SO4
concentrations during early observations was
representative of the upconed-water around theVentilation Shaft. Since then, relatively shallow
groundwater has infiltrated to the area around theVentilation Shaft and is characterized by low levelsof Na, Ca, and Cl, and high DIC and SO4
concentrations. This process of groundwater
replacement is most likely occurring at a faster rateon the south side of the Main Shaft fault than on thenorth side of the fault, reflecting the extent ofhydraulic impact. However, such trends were notobserved in borehole 09MI21 on the north side of theMain Shaft fault. Borehole 09MI21 is locatedapproximately 100 m north of the Main Shaft fault
and was drilled into LSFD. Hydrochemical changes
in the groundwater here were most likely caused by

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanmixing with groundwater at 300 m depth without
having been influenced by the upconing along theshaft. In general, mixing with shallower groundwater
became the dominant process on the south side of theMain Shaft fault whereas mixing with deeper
groundwater dominated groundwater chemistry on
the north side of the fault3).
To infer the infiltration of shallow groundwater as aresult of the long-term drawdown into the shafts,
measurements of tritium and chlorofluorocarbon(CFC) concentrations were used as an index of
surface water infiltration. Tritium was detected atseveral monitoring zones even though theconcentrations dropped to zero at around 200 m depth
prior to shaft excavations3). CFC-12 was firstdetected in 2010, and the concentrations gradually
increased with time as observed in boreholes 07MI07(200 m depth), 09MI20 and 09MI21 (300 m depth),and 10MI26 (400 m depth). This implies that surface
water containing CFC-12 penetrated up to 400 m
depth within a few years.
4. Important issues for Phase III
The R&D results during Phase I/II investigations
have summarized in a synthesized research report5).
For the Phase III investigations, the following threeimportant issues were identified in the this five years(2015-2020) based on the latest results.
(1) Development of countermeasure technology fordeep
undergroundapplicationtomitigate
problems/issues
・Water-tight grouting technology(2) Development of modelling technology for masstransport
・Fracture network modelling technology forheterogeneous fracture networks
・Technology for analysis and evaluation of long-termchanges of the geological environment(3) Development of Drift Backfilling Technology
・Long-term monitoring technology
・Environment recovery technology with sealing oftunnels and the facility
Experiments into each research topic will be mainly
carried out at the 500-m depth research gallery (Fig.
6).
For the issue (1), post-grouting works have been
applied to a gallery at 500 m depth. Three grouting
concepts were applied to the post-grouting works; a
new grout material, a new injection system, and a
new post-grouting zone. For the issue (2), in-situ andlaboratory mass transport experiments have been
carried out. The macroscopic and microscopic
observations were carried out to understand the
distribution of tracer (uranine) after the laboratory
diffusion experiment. For the issue (3), we havealready started a groundwater recovery experiment as
part of the research into development of drift
backfilling technology. The main objective of thisexperiment is to observe any changes of hydraulicand hydrochemical conditions and the recovery
process during flooding of a gallery to develop
monitoring technology and to acquire the requisite
Development of Drift Backfilling Technology
Ventilation Shaft

Main Shaft

Water collection ring
-100m Sub Stage-200m Measurement Niche
off Ventilation Shaft-200m Sub Stage-200m Measurement Niche off
Main Shaft-300m Measurement Niche
off Ventilation Shaft-300m Sub Stage-300m Access/Research
Gallery-400m Sub Stage-500m Access/Research
Gallery-North-500m
Access/Research
Gallery-South-500m Sub Stage
Deep borehole
investigation to identify
origin and residence timeof deeper saline GW

Existing research galleries and boreholes• Groundwater recovery experiment• Long-term Monitoring in Drift and Facility Scale(EDZ/EdZ)• Drift Closure Experiment with Backfilling etc.

Development of Modelling Technology for
Mass Transport○Fracture network modelling for heterogeneous
fractures• Mass Transport Experiment , including-hydrogeological characterization-colloid/organics/microbe characterizationin groundwater etc.
○Technology for analysis and evaluation of longterm changes of geological environment• Investigation on mobility of analog elements• Scenario analysis on long-term changes ofgeological environment• Deep borehole investigation etc.

Development of countermeasure technology
for deep underground application
・Grouting Experiment (Post- and pre-grouting) etc.

Fig.6 Location of planned experiments
knowledge and develop the methodology for
appropriate facility closure before performing apartial backfilling experiment at the drift scale6).
Prior to construction of the experiment gallery, apilot borehole, adjacent and parallel to the gallery,
was drilled to estimate baseline hydraulic andhydrochemical conditions.
Hydraulic and hydrochemical conditions and anychanges during gallery construction have beenmonitored in the borehole. Geological mapping on
the gallery was done to determine the fracture
distribution. In additional monitoring boreholes
drilled after construction of the gallery, hydraulic andhydrochemical conditions have been monitored. And,in addition, a backfill material test was started using a
pit bored into the experiment gallery floor in order toacquire data on saturation phenomena in the backfill
material and assess its influence on thehydrochemical environment in the rock mass. A plug
will be installed at the entrance to the experimentgallery in order to understand and assess the recovery
process in terms of hydraulic pressure changes,
changes in hydrochemical conditions and in rockstress distribution around the gallery during flooding
and draining events. Applicability of monitoring
techniques and plug performance during a facility
closure phase can then be validated based on the
results.
5. Summary
This paper summarizes the results of investigations,providing an example of stepwise investigation/evaluation of the geological environment relevant togeologicaldisposalusing
surface-based
investigations and investigations during shaft
construction in the crystalline rock in the Tono area
as a case study. And know-how for the execution of
investigation/modeling can be obtained through thePhase 1/II investigation.

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

Techniques for investigations and analyses andtechnical findings established or obtained in Phase I
and II could be applied widely with appropriate
consideration of the differences in the characteristicsof the geological environment of interest and making
appropriate modifications. The R&D activities

planned for this five years (2015-2020) also haspresented. JAEA will execute the R&D plan andpromote the R&D activities in Phase III at the MIU
with the objective of building technical confidence.
References
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Kuji, M., Kuroda, H., Semba, T., Uchida, M.,
Sugihara, K. and Sakamaki, M. : “Final Report on the
Surface-based Investigation (Phase I) at the
Mizunami Underground Laboratory Project”,
JAEA-Research 2007-043 (2007).
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and Ohyama, T. : “Hydrogeological Characterization
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Takeuchi, R., Sanada, H, . Ishibashi M. and Sato, T., :“Groundwater Recovery Experiment using an
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6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan