DEVELOPMENT OF WIRELESS MONITORING SYSTEMS FOR GEOLOGICAL DISPOSAL
Kazuhiro TSUBONO*1 Masato KOBAYASHI*1 Hiromitsu YAMAKAWA*1 Akinori HASUI*1
Kazuhiko MASUMOTO*2 Tomoaki MATSUSHITA*2
Norihisa SUGAHARA*3
Tatsuya TANAKA*4 Chiaki NAGAI*4*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*3 Sakata Denki Co., Ltd.
2-17-20, Yagisawa, Nishitokyo City, Tokyo, 202-0022, Japan*4 Obayashi Corporation
Shinagawa Intercity Tower B 2-15-2, Konan, Minato-ku, Tokyo, 108-8502, Japan
Key words: Radioactive waste, Geological disposal, Engineered barrier, Wireless monitoring systems, Wireless transmission,
Low frequency electromagnetic waves, Wireless power transfer, Magnetic resonance coupling

Abstract
In developing a monitoring program for geological disposal of radioactive waste, it is important that measuring instruments
and cables do not affect the performance of the engineered barriers. Several studies have been conducted with undergroundwireless transmission systems, wireless transmission technology for data transmission, and wireless power transfer (WPT)
technology for long-term power supply for monitoring systems. We have studied a wireless transmission technology withlow frequency electromagnetic waves and its applicability to geological disposal monitoring systems. The applicability andthe redundancy of the systems have been increased by reducing the size of both transmitter and receiver devices, anddeveloping a multi-stage relay system and transmission route changing technologies, respectively. Meanwhile, undergroundmonitoring systems have some challenges such as the limited capacity of the battery. In view of this, we have studied awireless power transfer technology using a magnetic resonance coupling method. Through design and trial fabrication oftransmitter and receiver coils, and experiments for power transfer efficiency and optimum frequency, the power transfer overbuffer materials has been confirmed to be viable, demonstrating its applicability to geological disposal facilities. In this paper,
we report on the latest development of the underground wireless transmission systems.

1. Introduction

closure. In implementing monitoring of the EBS and thenear-field, one of the technical challenges is to prevent theformation of possible preferential pathways for flow
induced by monitoring equipment installation with cables.
The Radioactive Waste Management Funding andResearch Center (hereinafter referred to as RWMC),
under its basic concept of boundary condition inmonitoring, that monitoring should not impair thefunction and performance of the barrier, has been
studying a specific way of monitoring and consider its
feasibility. We then focused on underground wirelesstransmission technology [3, 4] as an effective method toprevent the formation of possible preferential pathways
for flow due to the installation of monitoring devices withcables.
Our underground wireless transmission technology
uses low frequency electromagnetic waves withfrequencies of several kHz or less to suppress attenuationin the ground and water, instead of those in the range of

In deep geological disposal, the isolation and containmentof high-level radioactive wastes are required for long term
safety after closure of the geological repository. The longterm safety will be ensured by a passive safety systemcomposed of an engineered barrier system (EBS) and anatural barrier system (NBS) without relying on
monitoring or institutional control [1]. From the technicalviewpoint, whether monitoring is needed to understandthe state of EBS has been discussed in recent years in an
International Collaborative Research Project, MoDeRn
project (Monitoring Developments for safe Repository
operation and staged closure [2]) supported by co-funding
from Euratom.
When implementing monitoring inside or around therepository, one of its purposes is to confirm that the EBS
and NBS in the near-field will demonstrate theperformance expected to secure long-term safety after1

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2.2 Wireless relay system for a long distanceseveral MHz to GHz, which is used for general wirelesstransmission technology.
Relevant international organizations have been
paying attention to underground wireless transmission
technology. The MoDeRn has recognized theeffectiveness of underground wireless technology. It istheir policy to perform monitoring using undergroundwireless transmission in case studies on performance
confirmation monitoring. Also, the French radioactive
waste management agency (hereinafter referred to as
ANDRA) has been studying the applicability of thetechnology on monitoring in their deep geologicalrepository since 2004. From 2010 to 2014, they havecarried out collaborative research with RWMC to enhancethe efficiency of the R & D.
In underground wireless transmission technology,
the power is supplied by a chemical battery, which limits
the measurement period depending on its usage.
Therefore, in long-term operation for about 100 years
from the construction of the geological disposal site to its
closure, continuous monitoring is difficult. In order tosolve the limitation in the monitoring period, we havebeen carrying out R & D on the applicability of WirelessPower Transfer (WPT) technology to undergroundwireless transmission devices. This technology supplies
energy by electromagnetic waves or electromagnetic
fields without using cables.
This article outlines our latest development resultson the underground wireless transmission system and thestudy on the applicability of the WPT technology.

Wirelesstransmission

Relay device

Sensor~10 m
Miniaturizedtransmitter

Antennas

Control
device～500 mPC
ReceiverControl device

2.3 Multi-stage / multi-route wireless relay system
Relay device~10 m～100 m～100 m～Fig. 1 Conceptual diagram of relay systems (upper:
wireless relay system for a long distance, lower: multistage / multi-route wireless relay system).
2. 1 Development of miniaturized transmitters
We have developed a miniaturized transmitter with thepressure and heat resistance necessary for geologicaldisposal application, while limiting some functions (seeTable 1 and Fig. 2).
Table 1 Specifications for the recently developed
miniaturized transmitter.
Transmission frequency
once a week
Transmission distance*-- 12 m*Durability
About 10 years
Size (diameter×length)
Φ60 mm×200 mm
Frequency
8.5 kHz
Pressure resistance
10 MPaOperating temperature0 -- 40 ℃* Under the environmental noise of 10 mV rms.

2. Development of underground wireless transmission
system

216 mm
216mm

In order to enable monitoring in a limited space such as
inside buffer materials or near-field environment, we set
several important development goals in 2009 such as
miniaturization of transmitters, improvement of pressure
resistance, development of sensor adaptors, anddevelopment of a relay network, and then have developed
them.
As to the development of the relay network, wehave begun developing two different types ofunderground wireless relay systems. One is to transmit
data for a long distance of about 500 m by a large relay
device consuming a relatively large amount of electricity,and the other is to arrange multiple relay devices in
multiple stages, each of which can transmit data for about
100 m. A schematic diagram of the transmitter and therelay systems is shown in Fig. 1.
In this section, we report on these development
results.

約50mm

約100mm

200 mm

310mm
mm
310

60 mm

Fig. 2 Conventional transmitter (left) and miniaturizedtransmitter (right). Modified from RWMC (2010, 2012).
We have been conducting verification tests at theunderground research laboratories in the Horonobe andthe Mizunami Underground Research Laboratory(hereinafter referred to as Horonobe URL and Mizunami
URL, respectively) of the Japan Atomic Energy Agency(hereinafter referred to as JAEA) using miniaturizedtransmitters. At the Horonobe URL, we installed theminiaturized transmitters in buffer materials (see Fig. 3)
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and tunnel backfill material and have been carrying out
monitoring of soil pressure and pore water pressure in
performance confirmation tests for the engineered barrier.
Based on the analysis of wireless transmission data
received from the installed miniaturized transmitters, it isconfirmed that data are successfully transmitted andreceived at a fixed time and at a certain frequency (twice
a day).

can be relayed to a distance farther than tens of meters,
which is the transmission distance by the transmitter itself.
The reason why the two plots at the depths of 140 m
and 250 m are below the attenuation line is that theformation and the propagation of the electromagnetic
waves at these two positions are likely influenced by steel
tunnel supports. On the other hand, the value on thesurface is plotted almost on the attenuation line eventhough it is received at the farthest position from the relay
device. It may be because the attenuation decreased due tothe bypass of a steel pipe or the shaft, but the details need
to be investigated.

pore waterpressure sensor

100000Received voltage [mV rms]

pore waterpressure sensor
soil pressure
sensorFig. 3 Miniaturized wireless transmitters installed inbuffer material blocks of a simulated disposal hole inperformance confirmation tests of engineered barriers(Lower left: soil pressure sensor, upper left and right: porewater pressure sensor). Modified from RWMC (2015).

10000

1000
100

10

tunnel at the depth of 250m
tunnel at the depth of 140m
On the surface

1
0.1
1

10

100

1000

Transmission distance [m]2. 2 Development of wireless relay system for a long
distance

Fig. 4 Received voltage plotted against the transmission
distance. The dashed line denotes the theoreticalattenuation line of the relay device. Modified from
RWMC (2014).

From FY2013, we set the development of theunderground wireless relay system as a central objective.
First, we developed a wireless relay device which
includes a control device and a transmitter antenna and a
receiver antenna for a long distance. This relay device
makes it possible to transmit data to a location far awayfrom the basic transmission distance by a miniaturized
wireless transmitter alone. A large transmitter antenna
with the size of 3 m by 3 m was designed to suppress theattenuation of the electromagnetic waves by bedrock,steel tunnel supports, and electromagnetic noise at its
reception positions. The transmission frequency is set to
be 8.5 kHz.
Using the developed relay device, we performed atransmission test at the Horonobe URL. A miniaturized
wireless transmitter with a pore water pressure sensor andthe wireless relay device were placed at the depth of 250
m, and a receiver was successively placed at the depths of
250 m, 140 m, and the surface to receive the signals fromthe relay device. Fig. 4 shows the received voltage plottedagainst the transmission distance. Though the received
voltages at the depths of 140 m and 250 m are smaller
than the theoretical attenuation line of the relay device,
the signals received at all positions were confirmed to betransmitted by the relay device. This suggests thatmonitoring data from a miniaturized wireless transmitter

2. 3 Development of multi-stage / multi-route wirelessrelay system
A large internal power supply is necessary for longdistance transmission by underground wirelesstransmitters and relay devices, based on its conflicting
relation between magnetic field intensity and transmission
distance [4]. However, there is a limitation on the power
supply. To address this issue, we have proposed a multistage relay system to shorten the transmission distance
between devices and to reduce power consumption (seeFig. 5). By introducing this relay system, it is estimatedthat the power consumption can be reduced by 66% when
one transmitter is controlled, compared to the case where
only a transmitter is used to transmit data for the same
distance at the same frequency [8]. Also, we have worked
to improve redundancy by introducing a multi-route relay
system to secure transmission routes in case of
malfunction (see Fig. 5 right). The goal of the multi-stage/ multi-route relay system is that more than two
transmission routes are secured and that each relay device
receives data from more than 10 transmitters at the same
power consumption as is used by one of these transmitters.
In this section, we report on the development
results of the multi-stage / multi-route relay system.
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6th East Asia Forum on Radwaste Management Conference
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Relay device
Relay device

Transmitter

which is the target performance of the device, confirmingthat the stable transmission was secured [10].

（Multi-stage/Multi-route）
Receiver・・・
Number of rows
・・ Relay device

10000.0

15 m

Received voltage [mV rms]

ReceiverNumber of relay
Ｘ stage（Multi-stage）

・ Relay device
・Transmitter

1000.0

20 m
25 m

100.0

10.0

1.0

Fig. 5 Conceptual diagram of multi-hopping data relay(The figure on the right side shows reroute in malfunctionof the relay device.). Modified from RWMC (2015).

0.1
1

10

100

1000

Transmission distance [m]Fig. 6 Received voltage from the relay device plottedagainst the transmission distance. The dotted line and thedotted chain line denote the theoretical attenuation line of
the relay device and the transmitter, respectively. The
shaded portion denotes the area where transmission isdisabled due to the noise level of 1 mV, which waslearned from past experiments. Modified from RWMC
(2017).

2. 3. 1 Power saving by introducing multi-stage
wireless relay system
The relay system described above can reduce the power
consumption when managing one transmitter. However,the power consumption increases when the relay device
manages several transmitters due to the increase of thetime for reception standby. We analyzed the power
consumption status for each function of the relay device,and found out that the power consumption for reception
standby function consumes the most significantly (about
70 % of the total). To reduce the power consumption ofthis function, we introduced a low consumption activation
code (GOLAY code; [9]) that optimizes the receiving
circuit usage state. We developed the program andsuccessfully reduced the power consumption of thisfunction from the conventional 123 mA to 4.0 mA,
allowing the relay system to be connected to the intended
number of transmitters while saving the power.
We then designed and manufactured antennas, power
supplies, and a container to contain the whole relay device.
The specifications are summarized in Table 2.

2. 3. 2 Securing redundancy by introducing multiroute wireless relay system
Furthermore, with the aim of securing the redundancy of
the relay system in the case of malfunction, we carried out
data rerouting tests using three relay devices by thefollowing steps (see Fig. 7).
◌
◌
◌

◌

Table 2 Specifications of the relay device.
Transmission frequency
Once a week
Transmission distance--100 mDurability
About
10
years
Number of transmitters managed
10* Pressure resistance was designed to be 5 MPa, assuming
the hydrostatic pressure at the depth of 500 m.

Step 1: Transmitter 1, 2  relay device 1  relay
device2  receiver
Step 2: Malfunction of relay device 2
Step 3: Instruction signal from receiver manually sent
to relay device 3 to transmit data to thereceiverStep 4: Transmitter 1, 2  relay device 1  relay
device 3  receiver

As a result, data from the transmitter weresuccessfully sent to the receiver even when the route wasswitched from the relay device 2 to 3.
Step 1
Route 1After storing the antenna, the power supply, and theboard into the container, operation confirmation tests forthe transmission distance of 15 m, 20 m, and 25 m werecarried out at the surface, and the received voltages were
measured (see Fig. 6). All plots were in the transmission
available range, as well as the received voltage on thetheoretical attenuation line at the distance of 100 m,

Step 2
Malfunction

Step 3
Instruction ofroute change

Step 4
Monitoring
recoveryReceiverMalfunction

Relay device 2

2

1

Relay device

Transmitter

3

1

3

2

1

2

1

3

2

1

2

1

3

1

2

1

4
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Fig. 7 Conceptual diagram of route change in the case ofdevice malfunction. Modified from RWMC (2015).

We performed an application test at a laboratory tounderstand the influence on the WPT efficiency of buffermaterials, cementitious structures, and reinforcement steel.
In the past experiments, a proportional relation
between distance of coils over the diameter and theefficiency was confirmed [12]. Then, we manufactured atransmitter and a receiver coil with a smaller scale ofdiameter; 730 mm and 20 mm, respectively (the diameterof a disposal hole is assumed to be about 2220 mm). The
power transfer test for buffer material was carried out
using the transmitter and the receiver coil. This test
simulated a case where the power to run the undergroundwireless transmission devices installed in buffer materials
is sent from the disposal tunnel just above the disposal
hole. A bentonite block simulating buffer material was
installed between the transmitter and the receiver coil, and
the WPT efficiency was measured with both air andbentonite between these coils. The frequency of the power
supply was adjusted to match the resonance frequency of
the transmitter coil and the receiver coil (see Fig. 9) andwas optimized to about 120 kHz. As a result, the WPT
efficiency with air and bentonite were almost the same(see Fig. 10). Also, it was significantly improved by using
ferrite for the receiver coil.

3. Development of wireless power transfer system
The underground wireless transmission technology can
reduce the adverse effects on the barriers function. It
presents, however, a few challenges such as limited
battery capacity. To address these issues, we haveundertaken R & D that aims to apply the WPTtechnology to the underground wireless transmission
system in order to extend the possible time period formonitoring [7]. Fig. 8 is a conceptual diagram in which
the WPT technology is applied to the monitoring of anengineered barrier (buffer materials), showing a situation
where power is wirelessly supplied to wirelessmonitoring devices with sensors integrated so thatmonitoring can be carried out without batteries. To do so,
we have selected a magnetic resonance coupling method[11] (see Fig. 9) from a few WPT methods. This method
enables transferring power for a few meters and isresistant to misalignment. Also, the power transfer
efficiency through a medium such as buffer materials is
least influenced.

100
Air with ferrite insert
Air without ferrite insert
Bentonite with ferrite insert
Bentonite without ferrite insert

Power transfer efficiency [%]90

Fig. 8 Applicability of WPT technology for monitoring
technology of radioactive waste geological disposal.
Modified from RWMC (2015).

80

70
60

50
40
30
20
10
0

0

200

400

600

800

Distance between transmitter and receiver coil [mm]Fig. 10 Power transfer efficiency against the distance
between a transmitter and a receiver coil. Modified from
RWMC (2015).
Also, we manufactured a transmitter and a receivercoil with a diameter of 370 mm and conducted indoor
application tests to evaluate the influence of cementitious
structures and reinforcement steel on the power transfer
efficiency. They are used as plug materials.
In the test on cementitious structures, a concrete
block without steel was placed between the transmitterand the receiver coil, and the WPT efficiency wasmeasured as the distance between these coils was changedin the range of 100 to 600 mm. The test result shows that
the WPT efficiency at 300 mm, which corresponds to thethickness of a plug in the scale of 1/11, is only about 3%
lower than in the air (see Fig. 11).

Fig. 9 Conceptual diagram of resonance frequency
coupling method. Modified from RWMC (2015).
3. 1 Experiments of WPT
5

6th East Asia Forum on Radwaste Management Conference
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influence based on the underground environment such as
salt water and to conduct underground tests.
With all these developments described above,fundamental technologies that enable continuous
monitoring while avoiding the introduction of piping to
the barrier and near-field have progressed. It is important
to improve the practicality of underground wirelesstransmission systems through more practical experiments
using underground facilities, etc.

90
80
70

60
50
40
30
Air

20

Concrete

10
0
0

200

400

600

This research is a part of “Development of Advanced
Technology for Engineering Components of HLW
Disposal” "under a grant from the Japanese Ministry of
Economy, Trade and Industry (METI).

800

Distance between transmitter and receiver coil [mm]Fig. 11 Power transfer efficiency against the distance
between a transmitter and a receiver coil. Modified from
RWMC (2017).

References

In the test through reinforcement steel, the distanceof these coils is fixed at 300 mm, and the steel was placedin the middle. As a result, the efficiency with thereinforcement steel between these coils greatly decreased
from 82.6% to 16.6%. The efficiency reduction can be
improved by about 18% (16.6% to 34.1%) by adjusting
the resonance frequency and the optimum load resistance.
For further improvement of the efficiency, we areconsidering applying materials with less magnetic loss toreinforcement steel, and to change the shape and theposition of coils for the magnetic fluxes to bypass plugs
and to go through the bedrock.

[1]

International
AtomicEnergyAgency,SafetyRequirements, Geological Disposal of Radioactive Waste,
IAEA Safety Standards Series, No. SSR-5, IAEA, Vienna(2011).
[2] MoDeRn project website, http://www.modern-fp7.eu[3] Suyama, Y., Takamura, H., Aoki, K., Wada, R., Shimbo,
H. and Okutsu, K., Wireless transmission monitoring in ageological disposal repository (I) Concepts andAdvantages, Mater. Res. Soc. Symp. Proc., Vol. 1193, pp.
135-141, 2009.
[4] Takamura, H., Shimbo, H., Okutsu, K., Suyama, Y., Aoki,
K. and Wada, R., Wireless transmission monitoring in ageological disposal repository (II) Research andDevelopment, Mater. Res. Soc. Symp. Proc., Vol. 1193,
pp. 143-150, 2009.
[5] Radioactive Waste Management Funding and ResearchCenter: “Development of Advanced Technology forEngineering Components of HLW Disposal, Developmentof Monitoring Technology”, RWMC (2010) (in Japanese)[6] Radioactive Waste Management Funding and ResearchCenter: “Development of Advanced Technology forEngineering Components of HLW Disposal, Developmentof Monitoring Technology”, RWMC (2012), (in Japanese)[7] Radioactive Waste Management Funding and ResearchCenter: “Development of Advanced Technology forEngineering Components of HLW Disposal, Developmentof Monitoring Technology”, RWMC (2015) (in Japanese)[8] Radioactive Waste Management Funding and ResearchCenter: “Development of Advanced Technology forEngineering Components of HLW Disposal, Developmentof Monitoring Technology”, RWMC (2014) (in Japanese)[9] Nishimura, K., Data coding technique and basis of error
correction: revised edition, CQ Publisher, pp.133-134,
2010.
[10] Radioactive Waste Management Funding and ResearchCenter: “Development of Advanced Technology forEngineering Components of HLW Disposal, Developmentof Monitoring Technology”, RWMC (2017) (in Japanese)[11] Nagai, Inukai, Kobayashi, Eto, Tanaka, Ando, Imura, andHori: “Applicability study on wireless power transfer inmonitoring for geological disposal”, Journal of Nuclear
Fuel Cycle and Environment, Vol.24, No.1 (2017) (inJapanese)

4. Conclusions
We reported the latest development results of theunderground wireless transmission system and the study
on the applicability of WPT technology.
In order to enable monitoring in a limited space suchas buffer materials, we have developed miniaturizedtransmitters. Currently, demonstration tests formonitoring using these transmitters are under way at the
Horonobe URL and the Mizunami URL. We have also
developed two different types of underground wirelessrelay systems (relay system for a long distance and multistage / multi-route relay system). These systems enable
relay of data sent from miniaturized wireless transmittersto the surface. Furthermore, in the latter method,
redundancy as a relay system was improved byintroducing a function to change the transmission routeeven when some of the devices fail.
In order to enable long term monitoring independentof internal batteries, applicability of WPT technology tounderground wireless transmission devices was studied.
Based on the results of laboratory tests which showed thatthe influence on the power transfer efficiency by buffermaterials is insignificant, we concluded that theapplication of this technology to geological disposal isviable. In the future, it is necessary to confirm the6
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[12] Nagai, Inukai, Kobayashi, Tanaka, Abumi, Imura, andHori, “Scaling Law of Coupling Coefficient and Coil Sizein Wireless Power Transfer Design via Magnetic
Coupling”, IEEJ Transactions on Industry Applications,Vol.137, No.4 (2016) (in Japanese)7
6th East Asia Forum on Radwaste Management Conference
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