Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Review

Regional Case Studies: Environmental Radioactivity Levels
and Estimated Radiation Exposure Doses of Residents and Workers in
Areas Affected by the Fukushima Daiichi Nuclear Power Plant Accident
Yasuyuki Taira1, 2*, Masahiko Matsuo1, Makiko Orita1, Hitomi Matsunaga1, Yuya Kashiwazaki1,
Xu Xiao1, Shigekazu Hirao3 and Noboru Takamura1, 3
Department of Global Health, Medicine and Welfare, Atomic Bomb Disease Institute, Nagasaki University,
Nagasaki City, Nagasaki Prefecture, Japan
2
Fukushima Global Medical Science Center and Radiation Medical Science Center for the Fukushima Health Management Survey
Fukushima Medical University, Fukushima City, Fukushima Prefecture, Japan
3
Institute of Environmental Radioactivity, Fukushima University, Fukushima City, Fukushima Prefecture, Japan
1

Received 3 September 2022; revised 25 November 2022; accepted 6 December 2022

Nagasaki University established four satellite offices within a 30-km radius of the Fukushima
Daiichi Nuclear Power Plant (FDNPP) in Japanʼs Fukushima Prefecture: in Kawauchi Village,
Tomioka Town, Okuma Town, and Futaba Town. To evaluate the external and internal exposures
attributable to the FDNPP accident, environmental radioactivity levels continue to be investigatedat these sites. Our previous studies examined specific and regional case studies based on in-situ
environmental radioactivity monitoring activities. Their findings suggested that current external
and internal exposure doses of radiocesium have been controlled at the lower limit of the current“exposure situation” (1–20 mSv/y, International Commission on Radiological Protection) in theevacuation order-lifted areas of Kawauchi Village and Tomioka Town. However, conducting
long-term follow-up studies, such as environmental radioactivity monitoring, developing
countermeasures for further decontamination in the difficult-to-return zone, and implementing
restrictions on the consumption of local foods, are all needed to reduce unnecessary radiation
exposure to residents and workers, particularly since radiocesium derived from the FDNPP
accident remains in some areas around the site. These case studies have clarified scientifically the
temporal dose levels and aided in the development of environmental remediation measures thatconsider radiation exposure in the recovery and reconstruction process following the accident.
Key words: difficult-to-return zone, evacuation order-lifted areas, external exposure, Fukushima
Daiichi Nuclear Power Plant accident, internal exposure, radiocesium

1. Introduction*Yasuyuki Taira: Department of Global Health, Medicine and Welfare, Atomic
Bomb Disease Institute, Nagasaki University Graduate School of Biomedical
Sciences 1-12-4 Sakamoto, Nagasaki City, Nagasaki Prefecture 852-8523, Japan
E-mail: y-taira@nagasaki-u.ac.jp
https://doi.org/10.51083/radiatenvironmed.12.1_37
Copyright © 2023 by Hirosaki University. All rights reserved.

More than 11 years have passed since the 9.0 magnitude
Great East Japan Earthquake and subsequent tsunami
and disaster at the Fukushima Daiichi Nuclear Power
Plant (FDNPP), on March 11, 2011. As a result of thedisaster, a variety of radionuclides were released into theatmosphere and deposited in the surrounding terrestrial

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Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Fig. 1. Location of Nagasaki University satellite offices. The map
was produced by the first author (YT) using Green Map III software(Tokyo Shoseki Co., Ltd., Tokyo, Japan; http://www.tokyo-shoseki.
co.jp/company_english/philosophy.html) and reprinted under a CC BY
license with permission from Tokyo Shoseki Co., Ltd.; original copyright2003.

Fig. 2. Location of survey points in Kawauchi Village, Fukushima
Prefecture. Closed circles ( ● ) indicate measuring points for ambient
dose rates and sampling points for surface soils (December 19–20,
2011). Open circles ( ○ ) indicate measuring points for ambient dose
rates and sampling points for surface soils (December 24–25, 2012).
Crosses (×) indicate measuring points for RPLD (from October 16,
2013 through September 25, 2014). Dashed lines indicate the districtsof Kawauchi Village (Districts 1–8). Dark gray shading indicates areas
where evacuation orders were lifted as of April 1, 2012. Light grayshading indicates areas where residents were not yet permitted toreturn, as of April 1, 2012 through March 31, 2013 (available: http://
www.meti.go.jp/english/earthquake/nuclear/roadmap/; accessed
January 24, 2014). Blue areas have an estimated annual dose of 5 mSv/y,green areas have an estimated annual dose of 10 mSv/y, and yellow
areas have an estimated annual dose of 20 mSv/y, based on measuring
data (MEXT, Dec 11, 2011). The map was produced by the first author(YT) using Green Map III software (Tokyo Shoseki Co., Ltd., Tokyo,
Japan; http://www.tokyo-shoseki.co.jp/company_english/philosophy.
html) and reprinted under a CC BY license with permission from Tokyo
Shoseki Co., Ltd.; original copyright 2003.

Fig. 3. Relationships among distances from the FDNPP and external
dose rates at different sampling points. Open circles ( ○ ) indicate
sampling points for surface soils after residents returned home
(December 24 and 25, 2012). Closed circles ( ● ) indicate sampling points
for surface soils before residents returned home (December 19–20,
2011). The external effective doses from the soil samples were estimatedfrom the radioactive ﬁssion product concentrations using the formula
Hext = C·Dext·f·s where C is the activity concentration of the radiocesium[kBq/m2, estimated from the radiocesium concentration in Bq/kgdry including soil particles ( <2 mm) and collected areas of surface soil
(0.00182 m2)]; and Dext is the dose conversion coeffcient reported as the
kerma-rate in the air 1 m above the ground, per unit activity per unit
area [(μGy/h)/(kBq/m2)], with the assumption that the kerma-ratein the air and the absorbed dose rate in the air are the same value forradiocesium, with the relaxation mass per unit area (β: g/cm2) set to
3.0 due to the passage of less than two years since the FDNPP accident[3.3×10–3 (μGy/h)/(kBq/m2) for 134Cs and 1.3×10–3 (μGy/h)/(kBq/m2) for 137Cs, the International Commission on Radiation Units andMeasurements (ICRU) 1994]; f is the unit conversion coeffcient (0.7 Sv/
Gy for the effective dose rate in the body per unit of the absorbed dose
rate in air); and s is the occupancy-shielding factor (0.36 for the public;
0.2 fractional time outdoors + 0.8 fractional time indoors×0.2 building
shielding).

and marine environments1). Subsequently, international
institutes and academic institutions, as well as nationaland municipal governments, have published numerous
reports and developed databases and websites related tothe disaster. Nagasaki University, which has investigatedspecific and local cases regarding the environmental
contamination and radiation exposure doses, currently
administers four satellite offices in areas located within a
30-km radius of the FDNPP: Kawauchi Village (opened
in April 2013), Tomioka Town (September 2016),
Okuma Town (July 2020), and Futaba Town (December2021) (Fig. 1)2). Based on agreements with the fourmunicipalities, Nagasaki University has conductedenvironmental radioactivity surveys to investigate
issues related to external radiation exposure, including
residentsʼ radiation doses; internal radiation exposure,
such as exposure through the consumption of local foods
and/or the inhalation of airborne dust; and educating
the public about protection measures against ionizing
radiation3). Our previous studies evaluated the status ofenvironmental contamination in Kawauchi Village andTomioka Town, which are within 30 km and 20 km of

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the FDNPP, respectively, and estimated the radiation
exposure doses due to artificial radionuclides, such asradiocesium, among residents immediately before andafter returning to their homes.
2. Environmental radioactivity monitoring in Kawauchi
Village
2.1. Immediately before and after returning home, 2011–2013
Six months after the FDNPP accident, designatedemergency evacuation preparation zones within a
20 – 30-km radius of the FDNPP were lifted, as levels ofenvironmental radioactivity had decreased due to the
decay of short-lived radionuclides, such as iodine-131 (131I,
half-life: 8.0 d) and efforts by the national government toremediate contaminated soils4-5). In addition, by December
2011, the Japanese government announced that the
FDNPPʼs reactors had been stabilized and successfully
brought to a condition equivalent to a “cold shutdown6).”
Kawauchi Village, which is located within 30 km of theplant, was the site of several restricted areas at that time.
In January 2012, the mayor declared that residents could
safely return home, because radiation doses were lower
than those in other areas around the plant. Although
residents hoped that their return at that time would
be permanent, the environmental contamination andestimated exposure doses during the initial phase of theaccident, including radiation health risks due to returning,
had not been sufficiently reported.
First, the concentration of artificial radionuclides inenvironmental samples, such as those taken from soils,
tree needles, and local foods (e.g., wild mushrooms),
were analyzed using gamma spectrometry with a highpurity germanium detector coupled to a multi-channel
analyzer for 7,200 – 72,000 s. The purpose of theseanalyses was to evaluate the extent of environmental
contamination and radiation exposure dose rates fromthe artificial radionuclides in Kawauchi Village (Fig.
2)7). In this case, the prevalent dose-forming artiﬁcial
radionuclides from all samples were determined to beradiocesium (134+137Cs). An investigation conducted ninemonths after the accident, but before residents wereallowed to return, revealed that the estimated external
effective doses from surface soils in December 2011 were0.42 – 7.2 µSv/h (equivalent to 3.7 – 63.0 mSv/y, in thecase of staying outdoors all day long (24 h×365 d) at 1 m
above the ground under typical land conditions; the same
applies to the rest of this paragraph), and that ambient
dose rates were >2.3 µSv/h in areas within a 20-km
radius of the FDNPP. In addition, the estimated external
effective doses from surface soils in December 2011were 0.0011 – 0.38 µSv/h (0.010 – 3.3 mSv/y), while theambient dose rates were 0.18 –1.8 µSv/h in areas within

39

20 – 30 km of the plant (Fig. 3)7). Investigations conducted
to find the external effective doses from soils collected
in December 2012 found 0.045 – 0.15 µSv/h (0.39 –1.3
mSv/y) and ambient dose rates of 0.41 –1.5 µSv/hwithin a 20-km radius of the FDNPP, and 0.00020 –1.1
µSv/h (0.0017 – 9.2 mSv/y) and 0.12 – 2.9 µSv/h within a
20 – 30-km radius of the plant (Fig. 3)8). Thus, comparedto the radiocesium levels just prior to the residentsʼ
return in December 2011 (median: 0.85 (0.40 –1.4) mSv/y),radiocesium levels appeared to be decreasing, irrespectiveof the distance from the plant (median: 0.21 (0.012 – 0.56)
mSv/y) (Fig. 3)7-8).
The activity concentrations of artiﬁcial radionuclides(134+137Cs) were also analyzed in 2,654 samples of common
local food items (i.e., vegetables, edible wild plants andmushrooms, and fruits) produced and/or collected in
Kawauchi Village between May 2012 and March 2013.
Radionuclide concentrations were measured using an NaI
detector coupled with a multichannel analyzer for 1,800 s8).
Samples exceeding the radiocesium regulatory limit(100 Bq/kg for general foods) included 5 of 1,358 (0.4%)
vegetables; 307 of 737 (41.7%) wild plant and mushroom
samples (mainly mushrooms); and 4 of 256 (1.6%) fruit
samples. The radiocesium levels in the vegetables, edible
wild plants, mushrooms, and fruits in Kawauchi Village
showed a wide range in all seasons, especially in edible
wild mushrooms, which ranged from <20 – 3,472 Bq/
kg for 134Cs and <23 – 5,833 Bq/kg for 137Cs in autumn8).
The committed effective doses from the local foodsamples were sufficiently low, ranging from 18 – 48 µSv
for children and 25 – 43 µSv for adults (18 – 44 µSv formales and 20 – 48 µSv for females), compared to the
public dose limit (1 mSv/y) (International Commission
on Radiological Protection (ICRP), 1991), although thepotential for radiation exposure still exists (Fig. 4)8).
These findings suggested that residents could safely
return to their homes, provided they engaged withlong-term follow-up via environmental monitoring andcountermeasures to reduce unnecessary radiation
exposure7-8). Although the case of Kawauchi Village was
the first model for residents returning to their homes
after the accident, the external and internal exposure
doses due to residentsʼ daily activities remains a matter ofconsiderable public interest, and the timely disseminationof scientific findings to the public is considered extremely
important.
2.2. After returning home in 2013 – 2015
Despite the decontamination of areas around the FDNPP
and the homes of returning residents, radiocesium with along half-life was still present in environmental samples,such as soils, plants, and local foods (e.g., agricultural
products). In particular, residents were most concerned
about the internal radiation exposure doses associated40

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Fig. 4. Committed effective doses due to radiocesium in Kawauchi
Village, Fukushima Prefecture, May 1, 2012 through March 31, 2013. Theinternal effective doses (committed effective doses) from vegetables,
edible wild plants and mushrooms, and fruit samples were estimatedfrom the radioactive fission product concentrations using the formula
Hint = C·Dint·e where C is the median radiocesium concentration (Bq/kgfresh); Dint is the dose conversion coefficient for child intake (age: 0–19,
1.3×10-5 to 3.0×10-5 mSv/Bq for 134Cs and 9.6×10-6 to 2.2×10-5 mSv/Bq
for 137Cs) and for adult intake (age: > 20, 1.9×10-5 mSv/Bq for 134Cs and1.4 × 10-5 mSv/Bq for 137Cs), based on the ICRP Publication 67; and e isquoted from the average daily intake data (g/d) for age, sex, and season
in areas around the FDNPP issued by the Environmental Radioactivity
Monitoring Center of Fukushima, Fukushima Prefecture, Japan in
2009–2010. To calculate the committed effective doses, the undetected
data (N.D.) of the radiocesium was defined as follows:
1) In the case of “N.D. > 80%”, the value corresponding to N.D. is 5
Bq/kg (2.5 Bq/kg for 134Cs and 2.5 Bq/kg for 137Cs), one-fourth of thedetection limit (20 Bq/kg).
2) In the case of “80% ≥ N.D. >60%”, the value corresponding to N.D. is
10 Bq/kg (5 Bq/kg for 134Cs and 5 Bq/kg for 137Cs), half of the detection
limit (20 Bq/kg).
3) In case of “N.D. ≤ 60%”, the value corresponding to N.D. is 20 Bq/kg (10Bq/kg for 134Cs and 10 Bq/kg for 137Cs), the detection limit (20 Bq/kg).
Values were based on the Global Environment Monitoring SystemFood Contamination Monitoring and Assessment Programme (GEMS/
Food) administered by the World Health Organization (WHO) and the
new standard limits for radionuclides in food established by the Ministryof Health, Labour, and Welfare in Japan (MHLW). In this study,
vegetables, edible wild plants and mushrooms, and fruit samples (mainagricultural products) have been selected to calculate the committed
effective doses because these samples are frequently consumed and areassumed to be well suited for estimating potential internal exposure.
The asterisk indicates the mean value of committed internal doses forchild intake.

with ingesting local foods during their activities of dailyliving. A follow-up study was conducted to measure theradiocesium concentrations in 7,668 local foods, including
vegetables, edible wild plants, mushrooms, and fruits
produced and/or collected in Kawauchi Village between
April 2013 and December 20149). The number of samples
exceeding the regulatory limit for radiocesium (100 Bq/
kg for general foods) was 5 of 4,080 (0.1%) vegetables; 652
of 1,986 (32.8%) edible wild plants and mushrooms; and 8
of 647 (1.2%) fruits. The radiocesium concentrations of all
samples varied markedly among seasons. For example, in
the autumn, the concentrations of radiocesium in edible
wild plants and mushrooms ranged from <20 – 2,754
Bq/kg for 134Cs and <23 – 8,487 Bq/kg for 137Cs. The

Fig. 5. Committed internal doses attributable to the intake of edible
wild plants collected in Kawauchi Village, Fukushima Prefecture
between 2012 and 2015. The internal effective doses (estimatedeffective doses) from edible wild plants were estimated from theradioactive fission product concentrations using the formula H int = C·
Dint·e where C is the radiocesium concentration (the median value of 12varieties of edible wild plants) (Bq/kg-fresh); Dint is the dose conversion
coefficient for child intake (age: 0–18 years, 1.3×10-5 to 2.6×10-5 mSv/
Bq for 134Cs and 9.6×10-6 to 2.1×10-5 mSv/Bq for 137Cs) and for adultintake (age: >19 years, 1.9×10-5 mSv/Bq for 134Cs and 1.3×10-5 mSv/
Bq for 137Cs), based on ICRP Publication 72; and e is quoted from themedian value of daily intake data (g/man/d) for age and sex, as issued
by the MHLW, Japan in 2015. To calculate the committed effective
doses, the undetected data (N.D.) of the radiocesium were defined basedon GEMS/Food published by the WHO and the new standard limits forradionuclides in food by the MHLW (Fig. 4).

committed effective doses from the local food samples
were generally low, ranging from 2.5–12 µSv for malechildren and 3.0 –15 µSv for female children (24 – 43 µSv
for all males and 22 – 43 µSv for all females). Thissuggested that the internal radiation doses associatedwith ingesting these foods were acceptably low, comparedto the public dose limit (1 mSv/y). In other words, theinternal exposure doses for radiocesium were changing
at low levels, even after residents returned home toKawauchi Village.
The customs of residents, especially the satoyama(countryside) culture of harvesting sansai (edible wild
plants) and mushrooms, also need to be considered in
the reconstruction of areas affected by the disaster,such as Kawauchi Village. The range of mushrooms
collected in Kawauchi Village is relatively broad; forexample, 154 mushrooms belonging to 22 species werecollected between September and November 2013, 81
mushrooms of only one species (Sarcodon aspratus) werecollected between September and November 2014, and
159 mushrooms belonging to 23 species were collected
between September and November 2015. Radiocesium
concentrations >100 Bq/kg were detected in 125 of 154
mushrooms (81.2%) collected in 2013 and in 123 of 159
mushrooms (77.4%) collected in 2015. However, thecommitted effective doses were relatively limited, ranging
from 0.11 –1.6 mSv in 2013 and <0.001 – 0.6 mSv in 2015,
with no differences observed in the concentrations of 137Cs
between 2014 and 2015 (p = 0.45)10-11). Based on surveys of
local food consumption in Kawauchi Village (screening

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Fig. 6. Accumulated radiation dose rates in assembly halls in
Kawauchi Village, Fukushima Prefecture, October 2013 throughSeptember 2014. Values are the cumulative annual doses of the 91-day
conversion for each quarter (μGy/91 d/quarter).

against the standard limit of 100 Bq/kg radiocesiumfor general foods), the radiocesium concentrations ofseveral edible wild plant species, such as Eleutherococcus
sciadophylloides (koshiabura), Aralia elata (taranome),
and Osmunda japonica (zenmai), also exceeded standard
limits; consequently, the consumption of these plant andmushroom species should be avoided12). Moreover, it was
clear that the internal exposure doses depended on the
dietary habits of the consumers and the species of plants
consumed, with some age groups ( ≥ 50 years) ingestinglarger doses than others (<40 years)10). Nevertheless, it
was estimated that total internal doses were significantly
lower than the recommended public dose limit (1 mSv/y)(Fig. 5)12). In short, radiocesium concentrations remained
at extremely low levels between 2012 and 201512). The
monitoring system used for local foods has worked
effectively in this area, and the internal exposure risk
could be strictly controlled.
In contrast to the above, when evaluating external
exposure in areas frequented by residents, such in living
spaces (e.g., assembly halls), over the long term, passive
dosimeters, including thermoluminescent dosimeters,
optically stimulated luminescent dosimeters, and radiophotoluminescence dosimeters (RPLD), were used toestimate the dose rates in the environment and/or fora variety of wildlife under field conditions, as these
dosimeters could simply be placed in the environment.
RPLDs have also been used in field studies to determine
the exposure dose rates of soil biota in the Chernobyl
Exclusion Zone and to estimate external absorbed dose
rates in rodents and amphibians in contaminated areas
around the FDNPP13-14). The external radiation doses
in living spaces (e.g., assembly halls in Districts 1 – 8 of
Kawauchi Village) were measured using RPLDs between
October 2013 and September 2014 to evaluate a long-term
radiation dose rates during the villageʼs reconstruction(Fig. 2)15). Although radiation doses decreased temporarily
during the winter (December 2013 through March 2014)41

and the shielding coefficient varied by 0.63 – 0.84, otherthan the shielding effect of snow accumulation and onestory wooden houses, it was unclear what contributed tothe seasonal fluctuations (Fig. 6)15). Actually, in Districts
1 – 5, the cumulative annual radiation dose rates werestable, at <1 mGy/y (Fig. 6)15). Conversely, in Districts
6 – 8, the cumulative annual radiation dose rates were >1
mGy/y outdoors (Fig. 6)15). These findings suggested
that radiocesium remains present in residential areasʼ
vicinities, such as forests (Fig. 6). Following the FDNPP
accident, the residential areas, farmlands, and forests
close to residential areas ( <20 m) and roads within theevacuation order areas around the plant were extensively
decontaminated using appropriate methods. However, thedecontamination of boundary areas, including those 20 m
away from living spaces and back yards with overgrownplants, may have been limited (Fig. 2). Nevertheless, themedian individual dose was 1.4 (0.71– 3.3) mSv/y, even in
Kawauchi Villageʼs evacuation order area (i.e., within a 20km radius of the FDNPP), and the cumulative individual
doses were lower than those estimated based on ambient
doses (data not shown)16). These findings suggest thatthe discrepancy between the cumulative individual doses
and those estimated from ambient doses was mainly
due to variations in residentsʼ daily lives; the differingshield-rates of houses; and dosimeter characteristics,
such as the directional dependency of the irradiation
angle16). Furthermore, external effective dose levels
were continually decreasing due to the decay of artificial
radionuclides and the decontamination of contaminatedsoil16-17). Finally, current estimates indicated that theexternal and internal exposure doses in Kawauchi Village
due to artificial radionuclides (radiocesium) derived fromthe FDNPP accident were equivalent to the public dose
limit and/or were sufficiently lower than the upper valueof the reference level for existing exposure situations (20mSv/y), as determined by the ICRP.
3. Environmental radioactivity monitoring in
Tomioka Town
3.1. Before returning home, 2013 – 2016
In preparation for residentsʼ return to Tomioka Town,
temporary visits and residential stays were permitted
from September 17, 2016 through March 31, 2017;
however, some residents remained anxious about
radiation exposure due to the FDNPP accident. Indeed,
the size of the “difficult-to-return” zone, where theintegrated dose rates exceeded 50 mSv/y as of March
2012, accounted for approximately 12% (8.5 km 2) of
the townʼs total area (Fig. 7). As previously mentioned,
Nagasaki University established four satellite offices
in areas around the FDNPP to support the recovery
efforts in cooperation with residents and municipal

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Fig. 7. Location of survey points in Tomioka Town, Fukushima
Prefecture. Black and red dots show the measuring points of ambient
dose rates and sampling points of surface soils in the evacuation orderlifted area and difficult-to-return zone, respectively. The cross shows the
sampling site for airborne dust (i.e., Buildings G and Y) in the Yonomori
district of Tomioka Town. The map was produced by the first author(YT) using Green Map III software (Tokyo Shoseki Co., Ltd., Tokyo,
Japan; http://www.tokyo-shoseki.co.jp/company_english/philosophy.
html) and reprinted under a CC BY license with permission from Tokyo
Shoseki Co., Ltd.; original copyright 2003.

authorities. At these ofﬁces, analyses of individual doses
using dosimeters has also been conducted. During the
temporary visits, the individual doses (median) of someresidents (n=58) was 1.5 (0.59 – 3.6) mSv/y in Tomioka
Town (data not shown).
In one case, a 70-year-old man recorded the lengthsof his visits and the ambient dose rates within his homeusing an ambient dosimeter and individual dose rates
using a personal dosimeter during his visits to the town18).
The average length of his visits to prepare his home forhis return increased signiﬁcantly in April – December 2016(5.1±1.4 h), compared to his visits in April 2013 – March
2015 (3.7±0.8 h) and April 2015 – March 2016 (4.4±0.8h) (Fig. 8)18). His average daily exposure doses decreased
signiﬁcantly in April – December 2016 (4.7±1.9 µSv/d),
compared with those in April 2013 – March 2015 (12.5±
5.0 µSv/d) and April 2015 – March 2016 (8.8±3.7 µSv/d).
These fluctuations were closely reflective of changes
in ambient dose rates (Fig. 8)18). His individual dose
was estimated at 1.7 mSv/y in 201618), which suggested
that his long-term external exposure doses decreased
dramatically in his everyday Tomioka Town environment,
due to the national governmentʼs decontamination efforts
and the natural decrease in artificial radionuclides. In
addition, personal doses could be measured under various
conditions, which is extremely important for individual
risk assessments, because radiation dose rates measured
by the national and municipal governments were merely
representative environmental values.

Fig. 8. Personal records of a case study (a 70-year-old man). (a) Lengthof visits, (b) ambient dose rates, and (c) exposure doses in a resident of
Tomioka Town measured between April 2013 and March 2015, April
2015 and March 2016, and April 2016 and December 2017.

3.2. After returning home, 2017 – 2021
Tomioka Town is located within 20 km of the FDNPP.
On April 1, 2017, after the promulgation of the Act on
Special Measures Concerning Nuclear Emergency
Preparedness, the Japanese government declared that
residents in approximately 88% of Tomioka Town could
return home, because the air dose rates were sufficiently
low (i.e., <20 mSv/y) (Fig. 7). However, the external andinternal exposure risks in living spaces, such as those in
and around individual houses, was not evaluated; this was
unlike the data collected by the national and municipal
governments that were reported in the literature, collated
in databases, and published online. Therefore, the amountof environmental contamination and the external radiation
exposure doses of residents who had already returned,
or who planned to return, were evaluated by measuring
the ambient dose rates of artificial radionuclides insurface soils collected in Tomioka Townʼs residential
areas and measured via gamma spectrometry after the
restrictions in town were lifted. In addition, internal
exposure due to the intake of local foods, such as edible
wild plants, mushrooms, vegetables, and fruits that had
been produced and/or collected in Tomioka Town was
analyzed in returning residents.
Immediately after returning home in 2017, residentsʼ
ambient dose rates were 0.20±0.058 µSv/h indoors, 0.29
±0.12 µSv/h outdoors, and 0.40±0.19 µSv/h in backyards, and the additional radiation exposure dose ratein the evacuation order-lifted areas was estimated at

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43

Table 1. Ambient dose rates around residential areas and assembly halls in Tomioka Town, Fukushima Prefecture during 2017–2021
Ambient dose rate
Additional radiation Shielding
Mean in µSv/h Range in µSv/h Median in µSv/h dose rate in mSv/yfactor2017
43 0.20±0.058b
0.0086-0.37c
0.20 (0.28)d
1.5e
0.77f
Indoorsa
Outdoors
61 0.29±0.12
0.0088-0.68
0.26 (0.43)
Backyard
61 0.40±0.19
0.14-1.34
0.34 (0.63)
Indoors2018
35 0.18±0.052
0.098-0.30
0.16 (0.26)
1.3
0.68
Outdoors
59 0.26±0.084
0.088-0.48
0.25 (0.37)
Backyard
59 0.34±0.17
0.12-1.2
0.29 (0.51)
Indoors2019
10 0.16±0.055
0.11-0.27
0.14 (0.26)
1.1
0.73
Evacuation-order-lifted area Outdoors
25 0.23±0.076
0.084-0.44
0.22 (0.32)
Backyard
25 0.29±0.14
0.13-0.68
0.24 (0.53)
Indoors2020
15g 0.16±0.047
0.064-0.25
0.15 (0.21)
0.96
0.84
Outdoors
15 0.19±0.057
0.080-0.30
0.17 (0.26)
Backyard
15 0.23±0.081
0.10-0.49
0.21 (0.28)
Indoors2021
15 0.14±0.036
0.064-0.20
0.14 (0.18)
0.88
0.81
Outdoors
15 0.17±0.050
0.062-0.26
0.16 (0.23)
Backyard
15 0.21±0.060
0.076-0.36
0.20 (0.26)
Difficult-to-return zone
Outdoor2017
4
2.2±0.65
1.1-2.9
2.3 (2.7)
Backyard
4
2.1±0.23
1.8-2.4
2.1 (2.4)
Outdoor2018
5
1.0±0.89
0.24-2.4
0.37 (2.1)
Backyard5
1.2±1.1
0.21-2.8
0.53 (2.5)
Outdoor2019
5
1.6±0.65
0.32-2.2
1.8 (2.1)
Backyard5
1.5±0.62
0.27-2.0
1.6 (1.9)
Outdoor2020
40.65±0.27
0.24-0.94
0.70 (0.90)
Backyard4
1.0±0.61
0.23-1.9
0.93 (1.7)
Outdoor2021
40.61±0.27
0.22-0.94
0.63 (0.88)
Backyard4
1.0±0.57
0.20-1.6
1.2 (1.6)
aindoors mean the inside of the entrance (entrance hall) and outdoors mean the outside of the entrance (yard)b
mean±S.D.
cminimum-maximumdparentheses show 90th percentileecalculated using equation; Aext (mSv/y) = {(Cint−0.05 μSv/h)×16 h + (Cext−0.04 μSv/h)×8 h}×365 d×0.001
where Cint is the mean ambient dose rate indoors (entrance hall) (μSv/h) and Cext is the mean ambient dose rate outdoors (yard) (μSv/h). The
fixed values (0.05 μSv/h indoors and 0.04 μSv/h outdoors) in the equation are defined as the natural radiation dose rates (external terrestrial
radiation and cosmic radiation), and 16 and 8 h (24 h/d) are considered as representing the indoor and outdoor activities of daily living based on
the guideline of the Ministry of the Environment, Japan, respectivelyfambient dose ratio (indoors/outdoors) calculated from mean valuesgonly assembly halls to avoid the infection of covid-19 between 2020 and 2021
Measurement point

Year

n

1.5 mSv/y19). However, the ambient dose rate was 2.2±
0.65 µSv/h outdoors and 2.1 ± 0.23 µSv/h in back yardsof the difficult-to-return zone (Table 1)19). The doseforming artificial radionuclide (radiocesium) levels insurface soils were 0.30 mSv/y in the evacuation orderlifted areas, and 4.5 mSv/y in the difficult-to-returnzone (Table 2)19). Although 134Cs/137Cs values had been
decreasing because of the natural decay of 134Cs (half-life:
2.1 years), the contamination levels due to radiocesium
were markedly different in the two areas; they were
low in the evacuation order-lifted areas and relatively
high in the difficult-to-return zone (Table 2). In 2021, theambient dose rates were 0.14±0.036 µSv/h indoors, 0.17
±0.050 µSv/h outdoors, and 0.21±0.060 µSv/h in backyards, and the additional radiation exposure dose rate
was estimated to be 0.88 mSv/y in the evacuation orderlifted areas (Table 1). However, the ambient dose rate
was 0.61±0.27 µSv/h outdoors and 1.0±0.57 µSv/h inback yards in the difficult-to-return zone (Table 1). The
radiocesium levels in the surface soil were 0.97 mSv/yin the evacuation order-lifted areas and 6.5 mSv/y inthe difficult-to-return zone (Table 2). These findings
suggest that the estimated external exposure doses in theevacuation order-lifted areas have been decreasing due tothe effective decontamination measures of surface soils(0 – 5 cm) and the natural decay of artificial radionuclides(Tables 1 and 2, Fig. 9). Based on these findings regarding
radiocesium concentrations and effective dose rates,
decreases in external exposure doses indicated that
evacuees could safely return to Tomioka Town.
Despite the above, further environmental remediation
efforts in the difficult-to-return zone are still required.
Following the FDNPP accident, residential areas,
farmlands, forests close to residential areas, and roads
within the evacuation order areas were decontaminatedextensively by March 19, 2018;4) however, such efforts

44

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Table 2. Radiocesium distribution in surface soil (0 – 5cm) in Tomioka Town, Fukushima Prefecture during 2017–2021
Radiocesium concentration in kBq/m2
Concentration
External effective
Mean
Range
Medianratio in Bq/kg
dose rate in mSv/y
134
134
Cs/137CsCs (2.1 y) 137Cs (30 y) 134Cs (2.1 y) 137Cs (30 y) 134Cs (2.1 y) 137Cs (30 y)bcde2017 61 13±20
93±146
0.18-101
0.74-756
5.8 (37.7) 40.2 (274)0.30
0.13 (0.14)f
2018 59 15±22135±218 0.25-146
0.26-1498 8.2 (34)72 (363)
0.66
0.092 (0.10)
Evacuation-order2019 25 8.8±13114±167 0.17-55
0.99-726
2.7 (23)37 (316)0.54
0.071 (0.077)
lifted area
2020 15g 8.1±18
129±320 0.32-66
0.67-1314 2.6 (7.0)
40 (140)0.60
0.052 (0.060)
2021 15 8.2±14234±388 0.77-59
5.2-1629
3.6 (9.4)
72 (298)
0.97
0.036 (0.039)
2017
4 182±76.9 1400±585 66.8-282
515-2144 190 (258) 1471 (1975) 4.5
0.13 (0.13)
2018
5 58±42615±443 1.0-121
13-130162 (105)
650 (1109)
3.1
0.094 (0.096)
Difficult-to-return2019
5 58±62820±879 0.86-165
9.7-2328
21 (135)
307 (1917)
3.9
0.070 (0.082)
zone
2020
4 79±57
1569±1121 0.88-140
22-2791
88 (136)1732 (2694) 7.0
0.051 (0.051)
2021
4 54±291492±799 13-93
335-2559 55 (83)1538 (2309) 6.5
0.036 (0.038)
aresidences and assembly halls (same site as Table 1) (See Fig. 7)bmean±S.D.
cminimum-maximumdparentheses show 90th percentileecalculated using equation; Hext = CDextS
where C is the radiocesium (134Cs and 137Cs) concentration (mean: radiocesium inventory in kBq/m2 calculated from the radiocesium
concentration in Bq/kg, including fine particles (< 2 mm) of surface soils (0–5 cm) collected with a size of 0.00182 m2) based on the in-situ
gamma-ray spectrometry, Dext is the dose conversion coefficient reported as the ambient dose equivalent rate at 1-m above the ground per unit
activity per unit area {(μSv/h)/(kBq/m2)}, supposing the air-kerma rate and the absorbed dose rate in air were equivalent, for radiocesium withthe relaxation mass per unit area (β: g/cm2) adopted as 5.0 based on the measurement data in Fukushima Prefecture after the FDNPP accident(3.41×10−3 (μSv/h)/(kBq/m2) for 134Cs and 1.25×10−3 (μSv/h)/(kBq/m2) for 137Cs), and S is the occupancy-shielding factor (0.2 fractional time
outdoors + 0.8 fractional time indoors×0.2 building shielding = 0.36)
fmedian (90th percentile)g
only assembly halls to avoid the infection of covid-19 between 2020 and 2021
Sampling sitea

Year

n

were not conducted in the difficult-to-return area, whereentry and lodging are essentially still prohibited. Based on
the Act on Special Measures for the Reconstruction andRevitalization of Fukushima in 2017, six municipalities,
including Tomioka Town, are now making plans toconstruct a Speciﬁed Reconstruction and Revitalization
Base (SRRB) in the difficult-to-return zone. The aim of
the plan is to lift evacuation orders and allow residents
to return home4). The external exposure levels and thedecontamination effects on the landscape in the difficultto-return zone have not been evaluated thoroughly,
although data from the literature, databases, andwebsites have been published by the national and local
governments. Therefore, a detailed and high-frequency
radiation monitoring program using a car-borne survey
system to provide relatively high-density data was
implemented to evaluate the effects of decontamination
efforts on reductions in ambient and radiocesium dose
rates in the areas (i.e., the “decontaminated area” andthe “non-decontaminated area” (contaminated area:
temporary storage area for radioactive waste), withmarkedly different characteristics noted in the difficultto-return zone in Tomioka Town20). In the Yonomori
district, in the decontaminated area, the proportion oflocations with ambient dose rates >0.95 µSv/h decreased
markedly, from 59.2% in July 2018 to 0% in̶and since̶
October 2020 (Fig. 10)18). In response to the ambient dose

rates, the detection rates of radiocesium also decreased
rapidly, from 92.7% (134Cs: 35.3% and 137Cs: 57.4%) to 0%(Fig. 10)20). By contrast, the dose rates in the Oragahama
district, in the non-decontaminated area, were mainly
concentrated in the range 0.38 – 0.95 µSv/h (from 82.2% –45.2%) until March 2021 (Fig. 10)20). During this period, theradiocesium detection rates decreased gradually, from
92.7% (134Cs: 41.1% and 137Cs: 51.6%) to 15.5% (134Cs: 6.9%
and 137Cs: 8.6%) (Fig 10)20). Moreover, the physical decayof radiocesium released in areas around the FDNPP
between July 2018 and November 2021 was estimated tobe 19.9%, using dose conversion coefficients under theassumption that the depth profile of radiocesium did notchange over time and that the initial radioactivity of 134Cs
and 137Cs were 9.0 and 8.8 PBq, respectively21-22). Thissuggested that the reduction rates of radiocesium in the
Yonomori district were clearly faster than the rate ofnatural physical decay. Finally, the ambient dose rates anddetection rates of radiocesium in the decontaminated area
decreased faster than those in the non-decontaminatedarea between July 2018 and November 2021.
Significant differences in ambient dose rates wereobserved in the survey results from the decontaminatedand non-decontaminated areas (p <0.001) (Fig. 10)20). A
relatively stable downward trend in the radiocesium
frequency was observed in the decontaminated area
between 2018 and 2020 (Fig. 10). The main reason for theYasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

45

Fig. 9. Temporal changes in (a) ambient dose rates and (b) radiocesium concentrations in the evacuation order-lifted areasof Tomioka Town in 2017 and 2021.

Fig. 10. Relative frequencies of the ambient dose and detection rates of radiocesium in the (a) decontaminated area Yonomori District
and the (b) non-decontaminated area Oragahama district in the difficult-to-return zone of Tomioka Town, Fukushima Prefecture, July
2018 through November 2021. The single asterisk shows the measuring point. The double asterisk shows the dose rates for indoor (16 h)
and outdoor (8 h) activities (24 h/d), based on guidelines set by the Japanese Ministry of the Environment. The data were not normally
distributed. The Mann-Whitney U and Kruskal-Wallis H tests were used to compare differences between these areas in the same period andthe time trends within the same district. Regression lines were used to calculate the reduction rate in the average ambient dose rates.

observed decreases in the ambient dose rates and in thedetection rate of radiocesium in the decontaminated area
since July 2018 was due to decontamination efforts, suchas removing sediment and subjecting buildings to highpressure washing4). Furthermore, the estimated external
exposure doses of workers in the first year (July 2018
through July 2019) were extremely low, at 0.66 mSv/yin the decontaminated area, and 0.55 mSv/y in the nondecontaminated area 20). The case of Tomioka Town,
including its difficult-to-return zone, is expected to be thefirst reconstruction model for evaluating environmental
contamination and radiation exposure dose rates for
artificial radionuclides.
More than a decade after the FDNPP accident,
intensive decontamination and demolition ef forts
have been conducted in the SRRB of the difﬁcultto-return zone around the FDNPP. To evaluate theworking environment and internal exposure risk due toinhalation in the Yonomori district of Tomioka Townʼs
SRRB, 137Cs radioactivity levels in the airborne dust atbuilding demolition sites were measured by gamma
spectrometry using a high-volume air sampler with asingle-stage impactor (Fig. 7). The 137Cs radioactivity

levels did not increase signiﬁcantly, despite demolition
operations that used heavy machinery (Table 3)23).
Moreover, the limited resuspension factor (L-RF)
values varied by one order of magnitude, dependingon the location and whether a sampling site was insideor outside the difficult-to-return zone; the large-scale
remediation efforts may reflect a reduction rate in L-RFs
as atmospheric aerosols that were derived from a wide
area (Table 3). In this case, no substantial increases in
accident-derived 137Cs levels due to decontamination anddemolition were observed in the airborne dust samplesin Tomioka Townʼs SRRB, suggesting that the 137Cs
radioactivity in the airborne dust was primarily associatedwith particles that are resuspended by localized winds
accompanied by the transfer of construction vehicles, as
opposed to decontamination and demolition operations(Table 3 and Fig. 7)23). The internal exposure doses
in decontamination workers aspirating airborne dust
containing 137Cs were extremely low, compared tothe estimated annual effective doses or the limits
recommended by the Japanese government (Table 3)23-24).
In addition, suitable countermeasures, such as wearing
protective masks, could have helped to decrease the on-

46
T able 3.

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52
137

Measurement
point
Building G(SRRB)

Building Y(SRRB)

Cs radioactivit y in airbor ne dust at building sites in Tomioka Town, Fukushima Pref ecture during 2019–2021

Periodn

From September 4 (3)d2019 to February
2020c
From May to20 (16)
August 2020g
February 2–3h
2 (2)March 3–24i

13 (9)

Internal exposure dose rate (μSv/d)aInhaled particulates
Inhaled particulates(activity median(activity median aerodynamic L-RFs×10–8
137
Cs (mBq/m3) aerodynamic diameter ofdiameter of 5 μm)(m–1)b
1 μm)<0.11–0.73<6.8×10–6 to 4.7×10–5f <9.5×10–6 to 6.6×10–5
1.1–7.5(0.085–0.098)e

Beforedemolition<0.10–0.62(0.091–0.11)
0.35–0.36(0.091–0.098)<0.13–2.3(0.089–0.15)<0.034–0.15(0.023–0.097)

Duringdemolition
Beforedemolition
Duringdemolition
Controlbuilding<6.7×10–6 to 4.0×10–5<9.3×10–6 to 5.6×10–5

0.47–2.8

2.2×110–5 to 2.3×110–5

3.1×110–5 to 3.2×110–5

1.6<8.2×10–6 to 1.5×10–4<1.1×10–5 to 2.0×10–4

0.58–10RemarkTomioka town
From August 2019 5 (2)<1.6×10–6 to 7.1×10–6<2.2×10–6 to 1.0×10–5
0.056-0.25
office (evacuation to April 2021jorder-lifted area)acalculated using equation; Hint = C·Dint·e
where C is the average detected 137Cs radioactivity (Bq/m3) (137Cs concentration in filter dust sample), Dint is the effective dose coefficients forinhaled particulates (activity median aerodynamic diameters of 1 and 5 µm) for workers based on International Commission on RadiologicalProtection (ICRP) Publications 68 and 119 (1 µm: 4.8×10–6 mSv/Bq and 5 µm: 6.7×10–6 mSv/Bq for 137Cs) (ICRP, 1994 and ICRP, 2012), and e isquoted from the reference values of daily ventilation rates for dosimetric modeling for adult workers based on ICRP Publication 89 (13.5 m3/8h/d
for the decontamination/demolition workers and 9.6 m3/8h/d for office workers) (ICRP, 2002). In this methodology, average daily inhalation was
estimated based on internal exposure doses due to inhalation during occupational activities among heavy manual workersbcalculated using equation; K = 137Csinvent /137Cscont
where 137Csinvent is defined as the 137Cs radioactivity of airborne dust (Bq/m3) and 137Cscont is defined as the 137Cs contamination of the surface soil layer(0–0.5 cm) (Bq/m2) collected in the present study, respectively. Because decontamination in the Yonomori district of the SRRB proceeded by thesame schedule and technical methods, the surface soil contamination levels closest to airborne dust sampling points (time) were adopted as thereference values. 137Cscont (surface soil samples) were 9.7×103 Bq/m2 before demolition of Building G, 21.9×103 Bq/m2 during demolition of Building
G, and before and during demolition of Building Y, and 60.6×103 Bq/m2 for the control buildingcairborne dust was collected four times (September 5, 2019, November 21, 2019, December 5, 2019, and February 27, 2020)dsample number (137Cs detection number)edetection limit at the time of 137Cs detectionfminimum–maximum (values less than those indicated were not detected)gairborne dust was collected 20 times (from May 27 to August 6, 2020)hairborne dust was collected two times (February 2-3, 2021)
i
airborne dust was collected 13 times (March 3-24, 2021)
jairborne dust was collected five times (August 13-14, 2019, February 26, 2020, May 22, 2020, April 9, 2021 and April 12, 2021)

site inhalation of soil-derived radionuclides. The case of
Tomioka Townʼs SRRB might be the first working modelfor evaluating the environmental contamination andinternal exposure dose rates due to artificial radionuclides
derived from the FDNPP accident.
In addition to the above, the activity concentrations ofradiocesium (134+137Cs) were analyzed in 2,491 local foodsamples (i.e., vegetables, edible wild plants, mushrooms,
and fruits) produced and/or collected in Tomioka Town
between January 2018 and December 2021 using an NaI
detector coupled to a multichannel analyzer for 600 s25).
A summary of the radioactive contaminants in local
foods produced and/or collected in Tomioka Town isshown in Fig. 11. The radiocesium distribution in local
foods showed a wide range throughout the year, and thedetected rates in all local foods that exceeded the standard
limit of radiocesium (100 Bq/kg for general foods) were15.4 –17.5% between 2018 and 2021. The detected rates
exceeding the standard limit were 35.8 – 50.9% for edible
wild plants and 72.7 – 88.2% for mushrooms, while theradiocesium concentration of edible wild plants andmushrooms ranged widely, from <23,994 Bq/kg in edible
wild plants to <99,653 Bq/kg in mushrooms between
2018 and 202125). At the same time, the radiocesium
concentration in fruits and vegetables did not showa wide range, measuring <233 Bq/kg in vegetables
and <1,990 Bq/kg in fruits from 2018 – 202125). In thesescreening experiments, foodstuffs brought by residents
to the Tomioka Town Food Inspection Department of
the Tomioka Town Office were analyzed. These included
vegetables, fruits, and tubers that had been grown indecontaminated private gardens at residentsʼ homes andedible wild plants and mushrooms that had been collectedfrom non-decontaminated areas, such as boundary sites,
contaminated forests, or roadsides25). The radiocesiumin wild edible plants and mushrooms mainly contributed
to the committed effective doses, because the excess
rate against the standard limits for wild edible plants andmushrooms was relatively higher than those for fruits andvegetables (Fig. 11)25). Moreover, the committed effective

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Fig. 11. Detected rates of radiocesium contaminants in local foodsin Tomioka Town, Fukushima Prefecture, 2018–2021. The committed
effective doses in the main agricultural products (vegetables, edible wild
plants, mushrooms, and fruits) in all local food samples were estimatedfrom the radiocesium concentration using the formula H int = C·Dint·e
where C is the median radiocesium concentration (Bq/kg-fresh); Dint isthe dose conversion coefficient for child intake (age: 0–18 y, 1.3×10-5 to
2.6×10-5 mSv/Bq for 134Cs and 9.6×10-6 to 2.1×10-5 mSv/Bq for 137Cs) andfor adult intake (age: >19 y, 1.9×10-5 mSv/Bq for 134Cs and 1.3×10-5 mSv/
Bq for 137Cs), based on ICRP Publication 72; and e is quoted from theaverage daily intake survey (g/person/d) for age and sex, as issued by
Japanʼs MHLW in 2018 and 2019. To calculate the committed effective
doses, the undetected data (N.D.) of the radiocesium were defined basedon GEMS/Food published by the WHO and the new standard limits forradionuclides in food by the MHLW (Fig. 4 and 5).

doses from local foods due to the intake of agricultural
products (i.e., main food items such as vegetables, fruits,
edible wild plants, and mushrooms) in Tomioka Town
were 26 –120 µSv/y for children and 40 –152 µSv/y for
adults (19 –152 µSv/y for males and 19 –150 µSv/y forfemales) between 2018 and 202125). Despite the risk ofsevere radiation exposure, the daily consumption of edible
wild plants and mushrooms contributed only slightly toresidentsʼ committed effective doses, with the level ofradiocesium contamination in foodstuffs considerably
lower than the standard ( >100 Bq/kg for radiocesium) or
public dose limits (1 mSv/y) (Fig. 11).
Finally, current estimates indicated that the external
and internal exposure doses in Tomioka Town due
to artificial radionuclides (radiocesium) derived from the
FDNPP accident were equivalent to the public dose limit
and/or were sufficiently lower than the upper value ofthe reference level for existing exposure situations (20mSv/y), as determined by the ICRP.
3.3. Non-decontaminated forests, 2018 – 2019
Since 68% of the Fukushima Prefecture is covered byforests, and forest areas in the prefecture accumulated 72%of the total atmospheric input of 137Cs, forest ecosystems
contaminated with 137Cs may increase the local populationʼs

47

Fig. 12. (a) Path of Typhoon Hagibis in 2019, (b) precipitation after
48 h, and (c) location of Mt. Okura in Tomioka Town, Fukushima
Prefecture, Japan. (a) and (b) are reprinted and modified from theRegional Specialized Meteorological Centreʼs Best Track Data
(Graphics), published in 2019 (https://www.jma.go.jp/jma/jma-eng/
jma-center/rsmc-hp-pub-eg/bstve_2019_m.html) and a JMA Report
on Typhoon 19 (http://www.data.jma.go.jp/obd/stats/data/bosai/
report/2019/20191012/jyun_sokuji20191010-1013.pdf) under a CC BY
license, with permission; (c) map produced by the first author (YT)
using Green Map III software (Tokyo Shoseki Co., Ltd., Tokyo, Japan;
http://www.tokyo-shoseki.co.jp/company_english/philosophy.html) andreprinted under a CC BY license with permission from Tokyo ShosekiCo., Ltd.; original copyright 2003.

exposure to radiation by elevating ambient dose rates(external exposure) and through the consumption ofcontaminated forest products (internal exposure) over the
coming decades26-28). However, since Japan is susceptible
to natural disasters, such as earthquakes and heavy
rains due to tropical storms (typhoons) and localized
downpours, we decided to analyze the environmental
radiation levels in forest areas along the Mt. Okura
hiking trail (3.9 km) in Tomioka Town near the FDNPP.
Specifically, the spectra of artificial radionuclides (mainly
radiocesium) were measured using a portable survey
system and personal dosimeters. The results were used
to examine the temporal changes in the attenuation of
external exposure doses and environmental radiation
contamination due to the rainfall associated with typhoons
and heavy rains between October and December 2019
(Fig. 12)29). We confirmed: (1) the ambient dose rates of0.38 – 0.95 µSv/h in most forest areas were 79.9 – 84.7%
higher than in residential areas, (2) the number of sites
along the hiking trail where 137Cs was detected waslimited (1.1 – 4.7%), and (3) individual dose rates of 0.21 –0.34 µSv/h were lower than ambient dose rates (Fig. 13
and 14). These ﬁndings suggested that radiocesium has
remained stable in natural forests that have not been
decontaminated and that current levels are low, despite
the occurrence of heavy rainfall associated with Typhoon
Hagibis in 2019 and localized downpours29).
Is forest decontamination still necessary? The forested
area that needs to be decontaminated is vast and will
require a sizeable budget; thus, further discussions
related to forest decontamination policies are needed
f rom the perspectives of cost -ef f ectiveness and48

Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Fig. 13. Real-time map of color-scaled ambient dose rates along the
Mt. Okura hiking trail. The map data were collected using a walking
survey system, Radi-probe, in October and December 2019. This
radiation map was modified by the first author (YT) using GIS andPowerPoint software, based on the mapping data obtained by Radi-probe(GIS software: Shobunsha Publications, Inc., Tokyo, Japan. https://www.
mapple.co.jp/en/; the Radi-probe system: Chiyoda Technology Corp.,
Tokyo, Japan. http://www.c-technol.co.jp/eng). Reprinted from the map
software (Mapple, ver. 18) for the Radi-probe system under a CC BY
license with permission (No. 4-063) from Shobunsha Publications, Inc.,Tokyo, Japan; original copyright 2017 and Chiyoda Technology Corp.,
Tokyo, Japan.

prioritization. Identifying decontamination areas (zoning)
while managing external exposure and environmental
contamination may help to recover Fukushimaʼs originallandscape30). Moreover, hiking while managing exposure
to environmental contamination using a personal
dosimeter may be a model for those wishing to spend
time outdoors29).
4. Monitoring environmental radioactivity inOkuma Town and Futaba Town
The FDNPP disaster area encompasses two towns(Okuma Town and Futaba Town). These towns arelocated within a 20-km radius of the FDNPP, and bothtowns have an interim storage facility close to the FDNPP.
In the Fukushima Prefecture, large quantities of soil
and waste were collected for off-site decontamination.4)
Interim storage facilities managed and stored this soil
and waste, and specific waste ( >100,000 Bq/kg), until its
final disposal4). The soil storage facilities in the difficult-toreturn zone have been in operation since October 2017 in
Okuma Town and since December 2017 in Futaba Town4).
The soil and waste were transferred by truck from
temporary radioactive waste storage areas elsewhere in
the Fukushima Prefecture to these interim facilities4).
As a next step, following the temporary visits andstays by residents that began on December 3, 2021 inpreparation for their return to Okuma Town, evacuation
orders in Okuma Townʼs SRRB were lifted on June30, 2022. Temporary visits and stays for residents

Fig. 14. Temporal changes in ambient dose rates along the Mt. Okura
hiking trail (3.9 km). (a) All variations in ambient dose rates. The data
were obtained by a walking survey conducted between October andDecember 2019 using a Radi-probe system. The asterisk shows the
maximum µSv/h values. Maximum values were consistently measured
at the same locations. The double asterisk shows heavy rainfall events
such as typhoons or downpours and precipitation measured at a weather
station in Kawauchi Village near Mt. Okura. The 466 mm of precipitation
due to Typhoon 19, October 11–14, 2019, was equivalent to three times
the monthly average (JMA, Tokyo, Japan. Available from: http://www.
data.jma.go.jp/obd/stats/etrn/view/daily_a1.php?prec_no=36&block_no=1129&yeye=2019&month=10&day=&view=); (b) Separate variationsin ambient dose rates. Red arrows show the same location on the hiking
trail (maximum value shown). Walking speed except for breaks was in
the range 1.3–1.6 m/s.

preparing to return to Futaba Town began on January
20, 2022, and the lifting of the evacuation order in the
SRRB there was scheduled for August 30, 2022. Under
these circumstances, the continued monitoring ofenvironmental radiation in the restricted zone close toresidential areas is extremely important, so a detailed,high-frequency radiation monitoring program using carborne surveys was developed to provide generally highdensity data. These surveys have been conducted at theinterim storage facilities since October 2021. Below, theresults of this monitoring are briefly described.
At the interim storage facility in Okuma Town, theproportion of locations with ambient dose rates exceeding
1.9 µSv/h was 4.8%; most dose rate measurements ranged
between 0.38 and 1.9 µSv/h (81.4%) on October 18, 2021
(Fig. 15). The detection rates for radiocesium were20.5% for 134Cs and 29.5% for 137Cs, respectively. At theinterim storage facility in Futaba Town, the proportionof locations with ambient dose rates >1.9 µSv/h was 0%,
and most dose rates were <0.38 µSv/h (80.6%) on October
19, 2021 (Fig. 15). The detection rate of radiocesium wasonly 2.4% for 137Cs. This suggests that the distribution ofambient dose and detection rates of radiocesium differedmarkedly and were affected by factors such as trucks
transporting soil and waste to the interim storage facilities
and the decontamination and demolition activities inthe difficult-to-return zone, which are countermeasuresagainst radioactive contamination after the FDNPP

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49

dose rates due to radiocesium derived from the FDNPP
accident.
5. Conclusion

Fig. 15. Real-time map of color-scaled ambient dose rates and detected
points of 137Cs among sampling locations in the interim storage
facility sites in the difficult-to-return zone in Okuma and Futaba Towns
Fukushima Prefecture. The data were collected using a Radi-probe carborne survey system on October 18–19, 2021. The radiation map was
modified by the first author (YT) using GIS and PowerPoint software,
based on the Radi-probe mapping data (GIS software: Shobunsha
Publications, Inc., Tokyo, Japan. https://www.mapple.co.jp/en/; theRadi-probe system: Chiyoda Technology Corp., Tokyo, Japan. http://
www.c-technol.co.jp/eng). Map reprinted from the map software(Mapple, ver. 20) for the Radi-probe system under a CC BY license, withpermission (No. 4-063) from Shobunsha Publications, Inc., Tokyo, Japan;
original copyright 2019 and Chiyoda Technology Corp., Tokyo, Japan.

accident. Although the detection rates of radiocesium areshown qualitatively on the gamma-ray energy spectra anda low range of less than 0.38 µSv/h may be detected on alimited basis, long-term monitoring, in combination withvarious analytical methods and systems, such as car-borne
surveys and nuclide analyses of environmental samples,can be used to accurately evaluate the decontaminationand environmental remediation efforts in the future19).
These cases of the interim storage facility sites in Okuma
and Futaba Towns may be the first reports evaluating
environmental contamination and external exposure

In our pr e v ious s t udies, w e ma i n l y e x a m i ned
environmental contamination and the external andinternal exposure doses attributed to the FDNPP accident
in Kawauchi Village and Tomioka Town, in Japanʼs
Fukushima Prefecture. The findings suggested thatexternal doses have decreased steadily, compared tolevels detected immediately after the FDNPP accident,
and the internal doses were low compared to the publicdose limit (1 mSv/y). In particular, the current external
and internal exposure doses due to radiocesium have been
maintained at low levels in evacuation order-lifted areas,
but long-term follow-up measures, such as monitoring theradioactivity in the environment; further decontaminationof areas in the difficult-to-return zone, including SRRBs
and forested areas; and imposing restrictions on the intake
of local foods, are needed to reduce residentsʼ radiation
exposure, since radiocesium derived from the FDNPP
accident persists even in the decontaminated areas aroundthe FDNPP. The monitoring efforts and evaluation ofradiation exposure doses have been extremely effectivein Kawauchi Village and Tomioka Town; thus, long-term
follow-up and detailed evaluations of the external andinternal exposure doses and clarification of the temporal
dose levels in the recovery and reconstruction phasesof the affected areas around the plant are important for
ensuring the safety and future prosperity of residents andcommunities.
Appendix: List of equipment(1) Ambient dose rates were measured in the air ata height of 1 m above the ground using a NaI(Tl)scintillation survey meter (thallium-doped sodium iodide,
ϕ1×1 inch), which can measure gamma rays (energy
range: 50 keV – 3 MeV, ambient dose equivalent rate (H*(10)): 0 – 30.0 µSv/h at 1-cm depth; TCS-172B, HitachiAloka Medical, Ltd., Tokyo, Japan; Fig. 1)7-8, 19, 25).
(2) Radiocesium levels of the environmental samples(surface soils and airborne dust) were analyzed using ahigh-purity germanium detector (ORTEC GMX series,
Ortec International Inc., Ltd., Oak Ridge, TN, USA)
coupled to a multi-channel analyzer (MCA7600, Seiko,
EG&G Co., Ltd., Chiba, Japan) for 3,600 – 80,000 s. The
measuring time was set to the level at which the target
radionuclide (radiocesium) was detectable at low levels.
The gamma-ray peaks adopted for the measurements
were 604.66 keV for 134Cs (2.1 y) and 661.64 keV for 137Cs(30 y). Decay corrections were set to the sampling date.
Detector efficiency calibration for different measurement

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Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

geometries, including the density and thickness ofsamples, was performed using mixed activity standard
volume sources ( Japan Radioisotope Association,
Tokyo, Japan). The relative detection efficiencies of thisapparatus were 32.9 – 37.7%, with 1332.47 keV for cobalt-60(5.3 y). All environmental samples were processed andanalyzed at Nagasaki University, Nagasaki, Japan7-8, 19, 25).
(3) Radiocesium levels in local foods produced and/or collected in Kawauchi Village were analyzed using
a NaI(Tl) (thallium-doped sodium iodide, ϕ2 × 2 inch)
detector (Canberra, CAN-OSP-NAI, AREVA NC Inc.,
Meriden, CT, USA) coupled to a multi-channel analyzer(Genie 2000, Canberra Japan KK., Tokyo, Japan) covering
1,024 channels (2,000 keV) for 1,800 s. The gamma-ray
peaks used for the measurements were 604.7 keV for 134Cs(2.1 y) and 661.6 keV for 137Cs (30 y). The resolution of the
instrument was <7.5% at 661.64 keV for 137Cs. The sum
peak of 134Cs (1,401 keV) was separated from the gammaray peak of potassium-40, which is typically dominant
in natural radionuclides. To assess the geometricand detector efficiency, the LabSOCS mathematical
efficiency calibration software package (Laboratory
Sourceless Calibration Software, Canberra Japan KK.,Tokyo, Japan) was used to calibrate without the need forconventional calibration methods typically used, suchas diﬀerent measurement geometries, including thedensity and thickness of samples performed by mixed
activity standard volume sources. Local food sampling
was performed by residents of Kawauchi Village, theprocessing and measurements of radionuclides were
performed by the Kawauchi Village Office, and theanalyses of radionuclides were performed at Nagasaki
University, Nagasaki, Japan8-9, 12).
(4) Radiocesium activities in local foods produced and/or collected in Tomioka Town were analyzed using the
NaI(Tl) detector (thallium-doped sodium iodide, ϕ5 × 5inches) coupled to a multi-channel analyzer (AFT-NDA2,
Advanced Fusion Technology, Co., Ltd., Tokyo, Japan)
for 600 s. This detector covers a wide range of energy
peaks (100 keV to 3,000 keV) and can be used to analyze avariety of radionuclides in food samples. The gamma-ray
peaks adopted for measurements were 605 keV for 134Cs(2.1 y) and 662 keV for 137Cs (30 y). The resolution of thisapparatus was <7.5% at 661.64 keV for 137Cs. Calibration of
the energy and efficiency of this detector were performed
using a standard source of 134Cs and 137Cs. Local foodsampling was performed by residents of Tomioka Town,
the processing and measurements of radionuclides were
performed by the Tomioka Town Office, and the analysesof radionuclides were performed at Nagasaki University,
Nagasaki, Japan25).
(5) Long-term radiation dose rates were measured using
RPLD (FGD-201; Chiyoda Technology Corp., Tokyo,
Japan) with silver-activated phosphate glass (SC-1; AGC

Techno Glass Co., Ltd., Kawashiri, Shizuoka, Japan).
RPLDs are insensitive to ambient influences such astemperature and have low temporal and light fading. The
RPLDs measured approximately 30×40 mm, including
a holder to protect the glass elements from damage.
Regeneration treatment was performed at 400 °C for 1 h,
and annealing was performed at 70 °C for 1 h before eachmeasurment15).
(6) Surveys of the ambient dose rates and radionuclides
in Tomioka Townʼs difficult-to-return zone were carriedout using a car-borne survey system, Radi-probe (Chiyoda
Technology Corp.) and a handheld radiation detector(HDS-101GN, Mirion Technologies, Inc., Tokyo, Japan).
The Radi-probe system was installed in a vehicle andthe detector was set on the front passenger seat at
approximately 1 m above the ground. The ambient dose
rates were measured and geographic coordinates anda photograph were captured automatically every 5 s,in addition to spectrum segments every 0.2 s. Gamma
detection was performed using a large thallium-doped
cesium iodide scintillator with high sensitivity (typical
1400 cps per µSv/h for 137Cs source). The measurable
energy range of gamma-ray energy was 30 keV to 6 MeV.
Measurements were performed using a multichannel
analyzer with 512 channels. Real-time maps with colorscaled ambient dose rates and gamma-ray energy spectra
can be output using the system. The detected energy
peaks of radiocesium (134Cs and 137Cs) registered in the
nuclear library (i.e., detected net count values), and theirassociated confidence intervals, were obtained for theregion of interest (with levels 1 –10 used as reference
values). Generally, the car chassis and doors acted asshields for radiation from the outside. The shielding
factors were estimated by taking measurements insideand outside the car in flat, open areas at a height of 1
m above the ground. Since many factors can influence
the shielding factors (e.g., the type of car and number ofpassengers), the shielding effects were calculated before
each vehicle survey. The shielding factors ranged from
1.1–1.620).
(7) Surveys of ambient dose rates and radionuclides on
Mt. Okura in Tomioka Town were carried out using awalking survey system, Radi-probe (Chiyoda Technology
Corp.). The handheld radiation detector (HDS-101GN,
Mirion Technologies, Inc.) was the same as that usedin (6). For the walking survey, the shielding factor was
taken as 1.020, 29).
(8) In the case study of Kawauchi Village, individual dosesof the entire time spent conducting daily and leisureactivities were measured using a personal dosimeter(MYDOSE G2, PDM-501, Hitachi-Aloka Medical, Tokyo,
Japan) capable of measuring total dose ranges of 0.01 µSv
to 1 Sv. In the case studies of Tomioka Town, a personal
cumulative dosimeter (DOSEe;. Fuji Electric Co., Ltd.,Yasuyuki Taira et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 37–52

Tokyo, Japan) capable of measuring total cumulative
dose ranges from 0.000 mSv to 99.99 mSv (0.00 µSv/h to999.9 µSv/h) was used to measure gamma radiation, anda personal cumulative dosimeter (D-SHUTTLE; Chiyoda
Technology Corp., Hp(10)) capable of measuring total
cumulative dose ranges of 0.1 µSv to 99.9999 mSv wasused to measure gamma radiation, with doses recorded
every hour; the dosimeter incorporated a semiconductor
equipped with an error-detection prevention function anda shock sensor16-18, 29.
(9) Airborne dust samples were collected on 203×254mm quartz-fiber filters (GB100R-810A; SIBATA ScientificTechnology Ltd., Saitama, Japan) at a flow rate of 1000 L/
min using a high-volume air sampler with a single-stage
impactor (HV-1000R; SIBATA Scientific Technology Ltd.).
Cumulative flows were in the range of 349.8 – 403.9 m3 (5h 50 min to 6 h 45 min) for Building G and 250.5 – 429.9 m3(4 h 10 min to 7 h 10 min) for Building Y23).
(10) Sampling, processing, and analyses were carried out
using standard methods for radioactivity measurement,
as certified by the Ministry of Education, Culture, Sports,
Science and Technology and the Nuclear Regulation
Authority in Japan31-32).
Ethics statement
The investigations in Kawauchi Village and Tomioka
Town were approved by the ethics committee of Nagasaki
Universityʼs Graduate School of Biomedical Sciences (No.
14011079 and 17030212). Permission to measure individual
doses and ambient doses, and to collect soil samples inthe village and towns, was obtained from the relevant
municipal authorities. In addition, written informed
consent was obtained from the homeowners whereindividual and ambient doses were measured and where
samples were collected.

Conflicts of interest
The authors have no conflicts of interest to declare.
Author contributions
Yasuyuki Taira, Shigekazu Hirao, and Noboru Takamura
contributed to the research concepts and design;
Yasuyuki Taira, Masahiko Matsuo, Makiko Orita, Hitomi
Matsunaga, Yuya Kashiwazaki, and Xiao Xu contributed
to the data acquisition, analysis, and interpretation;
Yasuyuki Taira and Shigekazu Hirao contributed towriting a draft of the manuscript and revising it for
intellectual content. All authors approved the final versionof the manuscript.
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