181

A Study on the Durability of Covering Sheets Used at
Temporary Storage Sites in Fukushima Prefecture
Yusuke TAKAHASHI1* and Masahiro KOISO2
Center for Material Cycles and Waste Management Research, National Institute for Environmental Studies
16-2 Onogawa, Tsukuba-shi, Ibaraki, 305-8506, Japan
2
Fukushima Prefectural Centre for Environmental Creation
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan*E-mail: takahashi.yusuke@nies.go.jp

1

Abstract
The durability of the covering sheets used for storage of decontamination soil and waste at temporarystorage sites (TSSs) in Fukushima Prefecture was investigated. In this study, we investigated durability fromthree viewpoints, accelerated UV exposure tests, comparisons among several municipal TSSs, and material
aging. The results indicated that the water resistance of gas-permeable/waterproof sheets (GWSs) used formore than two years at TSSs was much lower than its standard value in many cases, which means thepossibility of rainwater intrusion to storage containers holding decontamination soil. On the other hand, thetensile strength and water resistance of the geomembrane used at TSSs remained above the standard values.
The results of accelerated UV exposure tests showed no significant damage observable in the microstructureof the microporous membranes of GWSs, even after accelerated UV exposure equivalent to 10 years of
outdoor use. On the other hand, sunlight exposure for more than two years at TSSs caused significant damage
among GWSs to the microporous membrane and its water resistance. The aging of GWSs at TSSs might bestimulated not only by UV radiation and moisture but also by other factors not considered in the accelerated
UV exposure tests.
Key words :	gas-permeable/waterproof sheet, geomembrane, tensile strength, water resistance,
temporary storage site

1. Introduction
Soil removed and waste generated duringdecontamination work in Fukushima Prefecture following
the Fukushima Daiichi nuclear power plant accident on
March 11, 2011 are being kept in storage containers fordecontamination soil at temporary storage sites (TSSs)
and on-site storage locations. The two main types ofstorage containers for decontamination soil and waste are
flexible containers and large weather-resistant sandbags(MOE, 2018; FPG, 2019). The former generally consistof polyethylene or polyester woven fabric and the latterof polypropylene woven fabric which is weatherproofed
by use of light stabilizers and antioxidants. Generally, thestorage containers are covered with a sheet to avoid
deterioration from exposure to sunlight, wind and rain
(MOE, 2013). The two main types of covering sheet are gaspermeable/waterproof sheets (GWSs) and geomembrane.
GWSs are a type of geocomposite that combines nonwoven fabric and waterproof microporous membranes,
while geomembrane is a flexible waterproof sheet material
made of ethylene propylene diene monomer rubber,
Global Environmental Research
24/2020: 181-190
printed in Japan

polyvinyl chloride, polyethylene and other compounds(JIGS, 2016). In addition, a degassing pipe is required in
the case of using a geomembrane, while it is not necessary
with a GWS.
The transportation of decontamination soil and wastefrom TSSs to interim storage facilities began in 2015, andthe amount of decontamination soil and waste is estimatedto be about 14 million m3 in total while its transportation
to the interim storage facilities was about 60% complete
as of October 2020, with 551 TSSs continuing to store
decontamination soil and waste according to the Ministryof the Environment’s website with information on interim
storage facilities (MOE, 2020). As a result, there are still
many sites where decontamination soil and waste havebeen stored for longer than the originally anticipatedperiod of three years, and there are concerns aboutdamage to the above-mentioned materials used at TSSs
(MOE, 2015; MOE, 2016). There have been some cases
where the sheets covering them were damaged by sunlight
exposure at TSSs in Fukushima Prefecture (Fig. 1),
which can cause deterioration of storage containers due
to ultraviolet exposure, enabling rainwater to intrude.
©2020 AIRES

182

Y. TAKAHASHI1 and M. KOISO

Fig. 1 Fractures in waterproof sheets covering the flexible containers of decontaminationsoil and waste at a temporary strage site (TSS).

To collect knowledge for the proper management of
decontamination soil and waste and TSSs, the authors
have reported their survey results on long-term durabilityof the flexible containers and large weather-resistantsandbags used as storage containers for decontaminationsoil and waste. They found that some exposed materials
had experienced significant strength loss (Takahashi,
2018; Takahashi et al., 2019).
In this study, the authors investigated the long-term
durability of the covering sheets (i.e., GWSs andgeomembrane) which have actually been used at TSSs for
two to six years to protect the storage containers fordecontamination soil and waste.

2. Experimental Study
2.1 Collection of Sheet Materials Used at TSSs
In this study, two types of GWS and one type ofgeomembrane were collected from TSSs managed bymunicipalities in Fukushima Prefecture (Fig. 2). All thesesheet materials were used in sun-exposed conditions. The
three municipalities managing the TSSs where the sheet
materials were collected in this study are referred to here
as “Municipality a/b/c,” with the TSS in Municipality abeing called “TSS (a),” the TSSs in Municipality b being
called “TSS (b-1),” “TSS (b-2),” “TSS (b-3),” and the
TSS in Municipality c being called “TSS (c).” Thegeomembranes used in this study were made of PVC(polyvinyl chloride) with a thickness of 1.5 mm.
Photographs and low-magnification SEM images of
two types of GWS are shown in Fig. 3. The microporous
GWS membranes consists of polyethylene and a non-

woven fabric consisting of polyethylene or polyethylene
terephthalate (Nishimura et al., 2012). Type 1 uses a thin
non-woven fabric for the exposed side, and the overall
thickness of the sheet is about 5 mm (Fig. 3 (A)), while
Type 2 uses same non-woven fabric on both sides, withthe overall thickness of the sheet being about 4.5 mm(Fig. 3 (B)). Type 1’ has a different microporous membrane
structure from Type 1 (Fig. 3 (C)), but their basic structureis almost the same. Therefore, in this study, Type 1 andType 1’ are treated as similar materials with the same
weatherability and mechanical properties. The materials
collected from the TSSs in this study were GWS types 1
and 2 and the test procedures used for them are described
below in Sub-sections 2.3.2 and 2.3.3, while the test
targets of the accelerated UV exposure test described in
Sub-section 2.3.1 were GWS types 1’ and 2.
In addition, unused GWSs and geomembrane wereprepared as undamaged materials for comparison with thematerials collected from TSS.
2. 2 Method
2.2.1 Tensile Test
Tensile tests were performed using a universal tester(AGS-10kNG, Shimadzu) on the GWS and geomembranematerials prepared in Section 2.1, and the tensile strengthof the sheet body in the cross-machine direction was
calculated from the following formula:p = P / B		(1)

where p is maximum tensile strength [N/cm], P is the
maximum tensile load [N], and B is the width of the test
specimen [cm].

A Study on the durability of covering sheets used at temporary storage sites in Fukushima Prefecture

Sandbags forshielding eﬀect

183

Sandbags forshielding eﬀect

Fig. 2 Surveyed municipal TSSs (temporary strage sites) and collected sheet materials.

(A)

(B)

(C)

Fig. 3 SEM (scanning electron microscope) images of GWSs.
( A) Gas-permeable/water-proof sheet (GWS) (types 1 and 1’), (B) Gas-permeable/water-proof sheet (GWS) (Type 2),(C) Microporous membrane of GWS

The tensile strength for sheet joints was also
determined in the cross-machine direction. The
fabrication and testing conditions of the test specimens
conform to the voluntary standards of Japan Lining
Systems and Technology Association (JIS L 1908 for
GWS and JIS K 6251 for geomembrane) (JLSTA, 2014).
The number of replications was set at three.
2.2.2 Puncture Resistance Test
The conditions for preparation and testing of the test
specimens were in accordance with the method (ASTM

D 4833) specified in the voluntary standards of JLSTA.
The number of replications was set at three.
2.2.3 Water Resistance Test
The conditions for fabricating and testing thespecimens were in accordance with the method (JIS L
1092 B method (high water pressure method)) specified
by JLSTA. The number of replications was set at three.
2.2.4 Morphological Observation
The surface and cross-sectional microstructures ofthe specimens were observed using a scanning electron

184

Y. TAKAHASHI1 and M. KOISO

Table1 Covering sheet material types and test methods.
Material
condition
Material type

Accelerated UV-exposedmaterial

Unused material

Material used at municipal TSSs
a

b-1

b-2

b-3

c

GWS
GWS
GWS Geomem- GWS
GWS Geomem- GWS
GWS
GWS Geomem- GWS
GWS Geomem(Type 1) (Type 1’) (Type 2)
brane (Type 1’) (Type 2)
brane(Type 1) (Type 1) (Type 1)
brane(Type 1) (Type 2)
brane

Elapsed year0

(UV 900/1500/3000 hour)43

2.5~4.4

2

2.5~5.5

3

4

Tensile test

-

○

○

○

○

○

○

○

○

○

○

○

○

○

Water
resistance

○

○

○

○

○

○

○

○

○

○

○

○

○

○

Puncture
resistance index

-

○

○

○

○

○

○

-

-

-

-

-

-

-

microscope (SEM) (JEOL, JSM-7800F) on the GWS andthe geomembrane prepared in 2.1. The specimens werepretreated with platinum deposition.
2.2.5 Light-shielding Test
The light shielding ratios of the test specimens were
calculated from the following equation in accordancewith JIS L 1055:2009 “Testing methods for light blocking
effect of curtain materials”:
S = (1 - It / I0) × 100 (2)where S is the light shielding ratio [%], It is theilluminance with the specimen attached [lx] and I0 is theilluminance without the specimen attached [lx]. In thisstudy, I0 = 100,000 [lx]. The number of replications was
set at three.
2. 3 Test Procedure
In this study, the durability of sheet materials wasevaluated from three points of view: an accelerated UV
exposure test with UV exposure, a comparison among
several municipal TSSs, and aging of the material.
The types of prepared sheet materials andexperimental methods used are summarized in Table 1.
2.3.1 Accelerated UV Exposure Test
Unused sheet materials were exposed to UV lightusing a sunshine carbon arc lamp weathering apparatus(JISC, 2007). The tested specimens were GWS types 1’
and 2 and PVC geomembrane, shown as “accelerated UV
exposure material” in Table 1. The exposure conditions
were in accordance with the JIS L 1096 weathering test,
where the black panel temperature was 63 ± 3°C and thewater spray time was 18 minutes per cycle of 120
minutes. Under these conditions, 300 hours of accelerated
exposure is considered to be equivalent to one year of
outdoor use (JLSTA, 2014; PWRC, 2012). Three different
exposure times (900, 1500, and 3000 hours) were
examined in this study.
Tensile tests and puncture resistance tests wereconducted on the materials that were subjected to theabove accelerated UV exposure. For some of thematerials, water resistance tests were also conducted.
2.3.2 Comparisons among Municipal TSSs
The tested specimens were GWS (Type 1) andgeomembrane (PVC) shown as “material used at TSS” in

Table 1 and they were collected from the TSSs managed
by municipalities a, b and c. The same type of GWS(Type 1) was used in municipalities a and b, and the same
type of geomembrane was used in municipalities b and c.
Tensile tests and water resistance tests were conducted on
the above sheet materials. Here, the number of
replications was set at five or 10.
2.3.3 Aging of Material
The tested specimens were GWS types 1 and 2
shown as “material used at TSS” in Table 1 and they werecollected from TSS (b-2) and TSS (b-3). Tensile tests andwater resistance tests were conducted on the above sheet
materials. The tensile strength of the sheet body wasevaluated in the machine-direction and cross-machine
direction. For some of the materials, light shielding tests
were also conducted. There were fewer test specimens ofType 2 compared to Type 1. Aging of the sheet materials
was evaluated as a function of elapsed years by
calculating elapsed time from the TSS construction date.
It is important to note that the test procedures in Subsections 2.3.2 and 2.3.3 evaluated different specimens.
This is because the timing of collection and detailed
testing conditions were different, even though bothmaterials were collected from the same TSS. The test
results for Sub-sections 2.3.2 and 2.3.3 are described in
Sections 3.2 and 3.3 below, respectively.

3. Results and Discussion
3.1 Accelerated UV Exposure Test
Figure 4 summarizes the tensile curves of the sheet
materials after accelerated UV exposure.
In the GWS sheet body (Type 1’), the tensile strength
first decreased significantly due to fracturing of thesurface textile at a strain of 20 –30 mm, and then thetensile strength based on the non-woven fabric on theunexposed surface reached a maximum at a strain of
90 –100 mm. The tensile strength based on the nonwoven fabric of the unexposed surface did not change
significantly when comparing the unexposed product andthe UV exposure time of 900/1,500/3,000 hours, while
the tensile strength based on the surface textile showed aclear decrease in tensile strength depending on the UV
exposure time. This means that in the whole GWS sheet

A Study on the durability of covering sheets used at temporary storage sites in Fukushima Prefecture

185

Fractures in surface textile

Fig. 4 Tensile curve of GWS and geomembrane after accelerated UV exposure.

(Type 1’) the surface textile was susceptible to UV light
exposure. In addition, the sheet joints also showed a clear
decrease in tensile strength as a function of exposure timeto UV light, which may have been because the structureof the sheet joints peeled off when the surface textile wasfractured.
In GWS Type 2, unlike Type 1’, preferential
deterioration of the surface textile was not observed.
Comparing the unused sheet with the one exposed to UV
light for 900 hours, there was a clear decrease in strength
in both the sheet body and the joint, but the decreases instrength at 1,500 hours and 3,000 hours were less thanthat of sheet joints and surface textile of the Type 1’ sheet

body. There was no significant difference in tensile
strength between the sheet body and the joint.
Unlike in the two types of GWS, there was no clear
reduction in tensile strength of the geomembrane underunused and UV exposure conditions of 900, 1,500 or
3,000 hours. However, a decrease in maximum strain wasshown at 3,000 hours of UV exposure.
The maximum tensile strength of the sheet materials
that underwent accelerated UV-exposure is summarized
in Fig. 5. For GWS Type 1’, the tensile strength of thesurface textile was added to the tensile strength of the
sheet body. After 3,000 hours of accelerated UV
exposure, the maximum tensile strength of the surface

186

Y. TAKAHASHI1 and M. KOISO

Fig. 5 Tensile strength of GWS and geomembrane
after accelerated UV exposure.

Fig. 6 Puncture resistance of GWSs and geomembrane
after accelerated UV exposure.

textile of Type 1’ was about 53% (96.7 [N/cm]) that of theunused material, and the maximum tensile strength of theType 1’ joints was about 20% (32.8 [N/cm]) that of theunused material. On the other hand, the strength of theType 2 joints after 3,000 hours of UV exposure was about
59% (133.6 [N/cm]) that of the unused material. Thisvalue is higher than the standard value (57 [N/cm] after
1,000 hours of accelerated UV exposure) specified in thevoluntary standard of JLSTA.
The results of the puncture resistance tests of the
sheet materials that underwent accelerated UV-exposure
are shown in Fig. 6. Unlike the results of the tensile test
(Fig. 5), both GWSs Type 1’ and Type 2 were found tomeet the standard value even after 3,000 hours ofaccelerated UV exposure (the standard value for GWSs
was 500 [N], while no standard value for geomembrane
had been set) (JLSTA, 2014). This is presumably due tothe non-woven fabric on the unexposed surface retaining
its puncture resistance.
SEM images of GWSs that underwent accelerated
UV-exposure are shown in Fig. 7. The observation targets
were limited to the GWSs, with geomembrane notincluded in this study because no significant strength
reduction had been observed. Regarding both Type 1’ andType 2, the fiber structure of the surface textile wasdamaged after 3,000 hours of accelerated UV exposure.
On the other hand, the microporous membranes of the
GWSs maintained their microstructure after 3,000 hours
of accelerated UV exposure, suggesting that the water
resistance of the GWSs was maintained. We have alreadyconfirmed that after 3,000 hours of accelerated UV
exposure, GWSs types 1’ and 2 met the standard value(1,000 [mmH2O] ≈ 9.81 [kPa]) of the water resistance test
(data not shown), supporting the SEM observation result.
Therefore, GWS Type 2 and the geomembraneretained practical durability (tensile strength, puncture
resistance and water resistance) even after 3,000 hours of

Fig. 7 SEM images of GWSs (types 1’ and 2), unused material and after accelerated UV-exposure (3,000hr).

A Study on the durability of covering sheets used at temporary storage sites in Fukushima Prefecture
accelerated UV exposure equivalent to 10 years of
outdoor use. On the other hand, GWS Type 1’ showed asignificant loss of strength at the surface textile andjoints. This result confirms the fractures in GWS seen in
actual TSS environments (Fig. 1).
3.2 Comparisons among Municipal TSSs
Figure 8 shows the results of the tensile test of GWS
Type 1 and the geomembrane collected from the TSSs
managed by municipalities a, b and c. It was confirmedthat the tensile strength of GWS Type 1 collected from
TSSs (a), (b-1) and (b-2) exceeded the standard value
specified by JLSTA (57 [N/cm]). Considering that unused
Type 1’ material showed a tensile strength of 182.8[N/cm], the deterioration of the tensile strength isconsidered to be limited regarding GWS Type 1 used at
TSSs. In addition, tensile strength of the joints of thegeomembrane in the TSSs of municipalities b and c wasroughly equivalent to tensile strength of the unused sheet(184.2 [N/cm]) shown in Fig. 5, and it was confirmed thatthey maintained sufficient durability. Considering thatthe sheet body of GWS Type 1’ and geomembrane
maintained tensile strength after 3,000 hours ofaccelerated UV exposure (equivalent to 10 years of
outdoor use) as shown in Fig. 5, the results with thematerials used at TSSs are basically consistent with thetendencies of the accelerated UV exposure tests in termsof tensile strength.
Next, the results of the water resistance tests of GWS
Type 1 and geomembrane are shown in Fig. 9. Unlike thetensile tests, water resistance of GWS Type 1 was reduced
to below its standard value (9.81 [kPa]) for most samples.
Many cracks were observed on the microporous
membranes of the GWS by visual observation, which
confirmed the loss of waterproofing performance ofthe microporous membranes (no image). There were nodifferences in water resistance among the GWSs in
TSSs (a), (b-1) and (b-2). However, a few specimens

Fig. 8 Comparison among TSSs (tensile strength of the covering
sheets).

187

collected from the ponding part of the sheet maintained
their water resistance. This is presumably because theponding part of the sheet had less damage from sunlight
exposure (JAG, 2016).
The water resistance of GWS Type 1 was clearly
reduced, while the water resistance of the geomembrane
was maintained. These results indicate that GWS Type 1
used at TSSs for more than two years lost its water
resistance regardless of the location of the installation.
The possibility of fracture of the surface textile and thejoint under UV exposure was indicated in Section 3.1, butthese results show that GWS Type 1 may lose its water
resistance through sunlight exposure even if no fractures
are observed on the surface textile. Although themicroporous membranes did not show any damage under
3,000 hours of accelerated UV exposure (equivalent to 10
years of outdoor use), the water resistance deteriorated
after about two years of sunlight exposure, indicating that
degradation of the GWS in the actual TSS environment
was stimulated by factors not considered in theaccelerated UV exposure tests, such as temperaturechanges between day and night, freezing, creep stress due
to strong wind and others.
3.3 Material Aging
The results of the tensile tests of GWS types 1 and 2
collected from TSSs (b-2) and (b-3) are shown in Fig. 10.
The elapsed time of the tested sheets ranged from 2.6 to
5.5 years. As the results in Section 3.2 (Fig. 8) indicate, it
was confirmed that all specimens met the standard value(57 [N/cm]). While the tensile strength of the sheet bodywas roughly constant over time, the tensile strength of the
sheet joints had a tendency to decrease with time.
Figure 11 shows the results of the water resistancetest of the same GWS. Similar to the results in Section
3.2 (Fig. 9), these results show that GWS Type 1 lost its
water resistance after 2.5 years of use at the TSS. On theother hand, the water resistance of GWS Type 2 wasFig. 9 Comparison among TSSs (water resistance of covering
sheets).
(Results for samples below the lower measurement limit aremarked as zero.)188

Y. TAKAHASHI1 and M. KOISO

Fig. 10 
Relationship between elapsed years and tensile
strength of GWSs used at TSSs (arrow indicatesType 2; the rest are Type 1).

Fig. 11 
Relationship between elapsed years and water
resistance of GWSs used at TSSs (arrow indicatesType 2; the rest are Type 1).

found to be maintained after about three years of use.
SEM images of GWS types 1 and 2 collected from
TSSs (b-2) and (b-3) are shown in Fig. 12. Comparedwith the SEM images of the materials after accelerated
UV exposure shown in Fig. 7, the materials used at the
TSSs showed damage not only to the surface textile but
also to the internal microporous membrane. Many cracks
with width exceeding 50 μm were found on the
microporous membrane of Type 1, which clearly
indicated its structural failure. This observation confirms
the test results in Figs. 9 and 11. On the other hand, GWS
Type 2 had voids of less than 10 –50 μm in diametercompared to the unused sheet, indicating damage to themicroporous membrane structure. The damage in Type 2
was limited compared to that in Type 1, which confirms
the result of the water resistance test of GWS Type 2
shown in Fig. 11. These results show that the water
resistance of GWS Type 1 decreased after about 2.5 yearsof use, while Type 2 maintained its water resistance undersimilar conditions.
To investigate differences in weatherability between
Type 1 and Type 2, light shielding tests of the GWSs
were conducted and the results are shown in Table 2.
The results show that the light shielding ratio of the
GWSs (entire sheet) was 100%, and the ratio of thesurface textile removed from the GWSs was more than
99.9%. There was almost no difference between Type 1
and Type 2. These results suggest that the difference inweatherability between Type 1 and Type 2 does
not depend on the shading performance of the
non-woven fabric, but is influenced by other factors, suchas adiabatic performance and mechanical properties of
the non-woven fabric.

Fig. 12 SEM images of GWSs (types 1 and 2), unused or used at TSSs for three years.

A Study on the durability of covering sheets used at temporary storage sites in Fukushima Prefecture
Table 2 Light shielding ratio of GWSs (types 1 and 2) used at TSSs.

Surface textile

Entire sheet

SampleElapsed time[years]1

2

3

Average[%]

GWS (Type 1)

5.5

99.97 99.93 99.82

99.91

GWS (Type 2)

2.899.94 99.95 99.94

99.95

GWS (Type 1)

4.4

100 99.99 99.99

100

GWS (Type 2)

2.8

100

100

100

100

4. Summary and Conclusion
The durability of covering sheet materials after
accelerated UV exposure or sunlight exposure for two to
six years at TSSs for decontamination soil and waste in
Fukushima Prefecture was investigated. The GWSs
showed clear decreases in tensile strength and water
resistance, while the geomembrane did not change
significantly over time at the TSSs. In particular, thedecrease in water resistance was difficult to judge fromthe appearance of the sheet materials, and the possibilityof rainwater intrusion into the storage containers under
the sheet needs to be taken into consideration for proper
management of TSSs. Our experimental results show thatthe water resistance of GWS Type 1 was more severely
reduced by use at a TSS for 2.5 years than by accelerated
UV exposure equivalent to 10 years of outdoor use,suggesting that the deterioration rate at TSSs is
accelerated by factors not considered in the accelerated
UV exposure tests (e.g., temperature differences between
day and night, creep stress from strong winds, etc.).
Meanwhile, GWS Type 2 had a limited loss of strengthcompared to Type 1 and maintained its water resistanceeven after three years of use at TSSs.
The number of TSSs in Fukushima Prefecture has
already peaked, and the phase of TSS restoration to theoriginal state rather than managing them has begun. On
the other hand, there are still some TSSs outside of
Fukushima Prefecture where plans for transporting thewaste have not been developed. Technical issues regarding
decontamination soil and waste from decontamination
work still require a long time and continuous research toresolve before completing their final disposal.

Acknowledgement
In this study, collection of the samples tested andmeasurement of data were conducted with support fromthe municipalities maintaining TSSs (especially Nishigo
Village and Iwaki City). Also, some of the experiments inthis study were conducted with support from the
Fukushima Technology Centre. We would like to express
our gratitude to them.

189

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Y. TAKAHASHI1 and M. KOISO

Yusuke TAKAHASHI
Yusuke Takahashi is a postdoctoral fellow at the
National Institute for Environmental Studies,Japan. He specializes in analyzing the degradation
behavior of plastic materials. He is elucidating thedegradation process of civil engineering materials
such as water-proof sheets and flexible containers
used in temporary storage sites. His research has
contributed to the proper management of decontamination waste generated
in Fukushima Prefecture. He is currently working on developing methods
to predict fragmentation behavior of degraded plastics.

Masahiro KOISO
Masahiro Koiso is a researcher in the Research
Department of the Fukushima Prefectural Centre
for Environmental Creation. He specializes inmeasures for handling technical issues at
temporary storage sites for wastes generated fromdecontamination activities in Fukushima
Prefecture. The research results he has obtained
are contributing to the proper management of decontamination waste
generated in Fukushima Prefecture. Currently, he is conducting research on
how to address technical issues involved in restoring former temporarystorage sites to their original state.

(Received 9 October 2020, Accepted 27 December 2020)