ISAIA:International Symposium on Architectural Interchange in Asia 2018

Limits of decontamination of residential land in rural areas radioactively
contaminated by Tokyo Electric Power Company Fukushima nuclear power
plant accident
Koji Itonaga1, Kenji Uragami2 and Masataka Seki3
1

Professor, College of Bioresource Sciences, Nihon UniversityResearcher, College of Bioresource Sciences, Nihon University
3Researcher, College of Bioresource Sciences, Nihon University
2Abstract
This is an empirical study on the actual condition of pollution and decontamination in Iitate Village,
Fukushima Prefecture, in the affected area of radioactive contamination released by Tokyo Electric Power
Fukushima Daiichi Nuclear Power Plant caused by the Great East Japan Great Earthquake of March
2011.We have clarified the secular change of the air dose rate in residential area and housing. Although there
is a decline in spatial dose due to natural decay and decline due to decontamination, it is not an environment
suitable for residence. We investigated the housing which was released from evacuation. According to thesurvey in 2017, although the dose in the housing tends to be high in the high space, it is less than 0.6 μSv/h.
However, the room adjacent to the forest has a dose of about 1.4 times that of the central room. On thesurface of the residential land is 0.65 μSv/h on average, exceeding the regulation standard value of theradiation control area. In the forest soil surrounding the residential land after decontamination, radioactive
cesium is still 40,000 Bq/kg. Despite this contamination situation, the evacuation designation is canceled at
the end of March 2017 under unreasonable circumstances. Villagers who went back home had no choice butto produce vegetables in farmland where pollution continues even though they were decontaminated. Thisabsurdity continues to follow. Also, products of contaminated forests can’t be eaten for the foreseeable future,and rich agricultural livelihoods have been lost.
Keywords: Fukushima nuclear power plant accident, Iitate village, Limit of decontamination of residentialarea, Air dose rate, Radioactive cesium up to five words;
1. Introduction
Iitate Village in Fukushima Prefecture wasradioactively polluted in the long term due to theTokyo Electric Power Fukushima Daiichi nuclear
accident on March 11, 2011, and it became acatastrophic area. Nonetheless, evacuation wascanceled by the national policy on 31 March 2017,
except for the difficult-to-return area (Nagajima) inIidate village. Since 1994, the authors have been
carrying out support research on Iidate Village froma viewpoint of sustainable rural planning. After thenuclear accident, we have conducted resident
consciousness survey, radiation dose ratemeasurement, measurement of the availability ofradioactive cesium (Cs), dose rate measurement
inside and outside the house. We also advanced
development research on radiation reduction
materials for houses. By our comprehensive
research, decontamination limits have beenpresented. Forests accounting for about 80% of
Iidate village cannot be decontaminated and returnrisk remains high.
In this state, evacuation was canceled in Iidate
village. In an emergency, it can be said that"exceptional condition" is made by national policy
and neglected.
In this study, we report the air dose rate in thehouse in Iidate village, the Cs measurement resultin the soil in the summer of 2017 and predict futurereduction. Moreover, we reveal contradictions that
evacuation was lifted.

Contact Author:

Koji Itonaga, Professor, College of Bioresource
Sciences, Nihon Universitye-mail: itonaga@brs.nifon-u.ac.jp

Fig.1. Mountain of decontamination soil on thepaddy fieldT in Iitate village, 2018.

ISAIA:International Symposium on Architectural Interchange in Asia 2018

2. Method and Object
Following 2014, from July 23 to 24, 2017, wemeasured nine in Iidate village, one in Namie Town,
one in Kawamata Town Yamakiya district, in totalof 11 houses. The measurement items are the air
dose rate inside and outside the house and theadhesion amount of Cs to the soil depth of 30 cm inthe residential area and the mountain in the back.
Among the eleven houses, one of Namie-machi
houses was dismantled. One case of Iitate village issoil measurement of only one place in theresidential land. All housing lots have been
decontaminated, and the effects and limitations ofdecontamination are also examined.
The space dosimeter used [ALOKA PDR - 111].
In-house measurements were taken at the center of
each room, on the floor, 1 m above the floor, near
the ceiling. The residential area was measured at apoint about 1 m away from the building wall, on theground, 1 m above the ground, about 2 m above thespace. We drew a core of 30 cm in the soil with apottery. The sample soil was divided into 5 cm units
and measured with a germanium semiconductor
wave height measuring instrument (manufacturedby Canberra, RI room in Nihon University).
3. Space dose rate inside and outside atdecontaminated housing
3.1 Space dose rate in housing
The characteristics of the average value of themeasurement results are as follows. On the firstfloor of the room, the floor is the lowest, then 1 mabove the floor, the ceiling in order, the upper partof the room tends to be higher(Fig.1.). The second
floor also has the same tendency. In the 1 m above
the floor which is the living space height, theaverage is 0.34 μSv/h. Without radioactive
contamination due to a nuclear accident, the usual
air dose rate is about 0.05 μSv/h, and now it isnearly 7 times its exposure dose. Assuming thatpeople are constantly living in the room in this state,
the pollution situation exceeds 1 m Sv per a year.
According to Japanese law, the dose limit of thegeneral public is 1 m Sv per a year.
On the other hand, decontamination is effective.
Places where the air dose rate in the room exceeds
the radiation control area standard (0.6 μSv / h) arelimited to the vicinity of the ceiling.
At the floor, 1 m above the floor, near the ceiling,
the air dose rate in the room facing the mountainaround the residential land tends to be higher andthe value of the room in the middle of the housetends to be lower(Table 1.). On the floor 1 m, thecentral room is reduced to 70% of the room facing
the forest. Please note that these houses are houses
that are not renovated at the measurement stage.
The situation of indoor pollution at new housing

will be the next question to be investigated.

Floor

Above 1m from floor

Near ceiling2ndfloor

1 floorFig.1. Spatial dose in the house after decontamination (byheight from the floor) 2017
Table 1. Comparison of special dose rate by room
μSv/hTable 1. Comparison of Space Dose Rate by Room μSv/hmrooms
aboveμNear
Table1.Space dose rate comparison of1theSv/hRoom location on the floorroom facing
the forest
central room

the floor

the ceiling

0.28

0.37

0.49

0.21

0.26

0.32

other rooms

0.25

0.32

0.41

average

0.26

0.33

0.43

3.2 Spatial dose rate of residential landContamination
continues
even
after
decontamination of peeling off 5 cm of the topsoilof the residential land and then placing the soil.
Despite decontamination, the dose on the groundsurface is the highest. The average value of the air
dose rate on the ground surface is 0.65 μSv / h,exceeding the standard value of the legally requiredradiation control area. Later, I will explain in thesituation of Cs remaining in the soil, the reason isconsidered the residual influence of polluted fallen
leaves and residual Cs in the soil.
1 m above the ground, 2 m above the groundaverage 0.54 μSv / h, 0.59 μSv / h. The reason thatthe air dose rate of 2 m above the ground is higher
than 1 m above the ground can be presumed to be
the influence of radiation from surrounding forests.
The ground facing the forest is about 1.5 times theair dose rate compared to the unfaced ground.
3.3 Correlation of Space Dose Rate in andoutside the House
Dose rate at outdoor and at home is highly
correlated with a correlation coefficient of0.8(Fig.2). In the same housing investigation resultin 2014, the correlation coefficient is 0.85 and so on.
Residential housing in 2014 included houses before
decontamination. At that time, the air dose rate inISAIA:International Symposium on Architectural Interchange in Asia 2018

decontaminated houses tended to be low. Reduction
due to decontamination and natural decay is clear.
We see the shielding effect by height for both insideand outside the house. The reduction rate on thefloor surface is high and the reduction rate on theceiling surface is low. We estimate that the dosefrom surrounding forests is affecting.

Fig. 2. Comparison of indoor and outdoor air dose rates 2017residential land. The mountain was continuallycontaminated, and the maximum value of thesurface layer of 5 cm was 41394 Bq / kg under thecedar tree. The average of Cs of the surface layer ofthe residential land is 2919 Bq / kg, and the averageof the mountain is 17736 Bq / kg(Fig.3).
The surface soil of the mountain is contamination
situation of Cs six times that of the residential land.
After nuclear accident, the government formulated
the law and set the radiation contamination
countermeasure standard value at 8000 Bq / kg (itsvalue is 80 times the value before the accident).
Radioactive contaminated soil more than twice asmuch remains in the surface soil of the mountainnear the residential land.
It is very unreasonable that evacuation has been
lifted, which permits living reconstruction at suchplaces.

3.4 Estimated annual exposure in indoor living
space
Based on the average air dose rate of 1 m above
the floor of the room and the average air dose rateover 1 m of the residential land, the annual
exposure dose was estimated. We estimated at 9houses as 16 hours indoor and 8 hours outdoor stay.
The result was from 2.25 mSv per a year to 6.73 pera year. The average value is 4.58 mSv per a year,
which is more than four times the general public
exposure limit of 1 mSv per a year. In addition,
assuming only gamma ray exposure from Cs,
simply estimate the damping change of the annual
exposure dose with natural half-life of 2 years forCs 134 and 30 years for Cs 137. It can be estimatedthat 4.58 mSv per a year in 2017 will decay to 1
mSv per a year after about 50 years.
Fig.3. Comparison of the amount of Cs in the soil of theresidential land and the lands mountain 2017

4. Content of radioactive cesium in soil ofdecontamination
residentiallotandmountain of the back4.1 Residual amount of Cs of surface soil
As the cause of the high air dose rate even after
decontamination, there is residual Cs in themountain near the residential land and residentialland. Within 20 meters of the mountain adjacent to
the residential area, fallen leaves etc. of the surface
layer are decontaminated. However, the topsoil ofthe mountain is not decontaminated. We measured
the amount of Cs (Cs 134 + Cs 137) in the soil of 5
cm of the surface layer of the decontaminatedresidential area and the mountain. The ratio of Cs
134 to 137 is 1: 1 at the beginning of the disaster,roughly 1: 8 at the time of 2017 as six years later,the influence of Cs 137 is great in the future.
The depth of 5 cm of soil in the residential area
ranges from 89 to 6844 Bq/kg, which is arecontamination of the surface layer of the4.2 Characteristics of residual radioactive
cesium by depth
We analyze the remaining situation of Cs bydepth. 39% in the surface layer 5 cm and 37% in
the 5 to 10 cm layer in the residential land. In 2014,
about 90% of Cs adhered to the surface layer of 5cm, but in 2017 after decontamination, it is about
40% in the surface layer of 5 cm. It can be inferredthat the permeation and fixing of Cs into the soil aregetting deeper.
The reason for this can be inferred as follows.
① Penetrate according to the nature of the soil.
② Cs penetrated into the layer of 5 cm or less by
the decontamination operation. Prior to thedecontamination work, the wild boar had soil
disturbance to predispose earthworms, etc. As aresult, Cs of the surface layer had entered the deepsoil. After that, when decontaminating, the agitatedsoil was leveled. Then only 5 cm of the surfaceISAIA:International Symposium on Architectural Interchange in Asia 2018

layer was removed, and it can be inferred that Cs
remained at a depth of 5 cm or less.
③ Even after being covered with new soil bydecontamination, polluted leaves and contaminatedrainwater from the surrounding forests have flowed
to the new topsoil.
At the part facing the mountain at thedecontaminated residential land, decontaminationof about 10 cm is desired.
Cs in the mountain soil adjacent to the residential
area remains about 80% at the surface layer of 5 cm
and 15% even under the 5 to 10 cm layer thereunder.
There is a strong tendency to remain in the surface
layer 5 cm as compared with the residential land.
Also, Cs of about 3000 Bq/kg remains even in the 5
to 10 cm layer, so some decontamination measures
are required.

Fig.4. Cs amount in soil depth (5 cm) indecontaminated housing lot

Fig.5. Cs amount in soil depth (5 cm) in mountainbehind

5. Conclusions
In Iitate village where radioactive contamination
by nuclear power plant accident occurred,evacuation was lifted, but people are not in asituation where people can live with peace of mind.
There is still a 4 to 5 times exposure risk of 1 m per
year for the general public's dose limit. It is
irrational to release evacuation in a kind of"exceptional condition", the standard of radiation
control areas, which is the standard for avoiding
radiation before the accident. Radioactive
contamination at decontamination residential land
continues, indicating the limit of decontamination.
Especially, the continuation of living held in themountain adjacent to the residential land whereterrible pollution continues is severe.
The villagers have been deprived of symbiotic
living with mountains peculiar to rural villages, or"the right to reside in harmony with nature". Even if
it is calculated from the half life of radioactive
cesium, it is a future of more than 50 years that it
can return to the state before the nuclear accident. It
takes more than 100 years to safely live withforestry. The regeneration and resurrection oftraditional rural life culture will be the future formore than 100 years.
This situation should be strongly recognized by
the state, prefecture, village authorities and citizens.
Measures and relief for victims of radioactive
contamination (pollution victims of radioactivecontamination) that are receiving the sorrow areincreasingly needed.
Acknowledgments
This research is support research for Villagers
Iidate who are fighting a trial against Tokyo ElectricPower for damage compensation. I appreciate thecooperation of everyone involved. These are theresults of the following research grants. ①
Foundation of scientific research expenses ofMEXT (C) 15K 07655 (Representative Koji
ITONAGA), ② It is part of two studies "Nihon
University College Resources Science DepartmentStrategic Infrastructure Formation Support Project"(Shared Koji ITONAGA).
6. References
1) Itonaga Koji. (2018) Rescuing Iitate Village
from Irrationality and Abnormality-7 Years after
TEPCO’ Nuclear Accident. Research on
Environmental Disruption, 48 (1), 3-92) Itonaga Koji. (2014) Resilience Design andCommunity Support in Iitate Village in theAftermath of the Fukushima Daiichi Nuclear
Disaster 、 Planning Theory & Practice 、15(2),237-265
3) Itonaga
Koji. (2014) Contamination andcommunity support in the aftermath of theFukushima disaster 、 Bulletin of the Atomic
Scientists、June、1-8