RAPID REMOVAL OF CESIUM-137 FROM URBAN AREA AFTER THE
FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENTKazuya Yoshimura, Sector of Fukushima Research and Development
Japan Atomic Energy Agency
10-2 Fukasaku, Miharu-machi, Fukushima 963-7707, Japan
Key words; Cesium-137, urban, distribution

Abstract
Urbanized areas have a large impact on external exposure to residents. Diverse decontamination methods had
been evaluated by the model decontamination project carried out by Japan Atomic energy Agency in 2011 and2012 to reduce the external exposure. On the other hand, quantitative information of distribution of radioactive
substances, especially 137Cs, in urban area is limited, although the information is crucial for decontamination
planning. To provide quantitative information of the distribution of 137Cs in urban area, this study evaluatedrelative 137Cs inventories, which are defined as the relative values of 137Cs inventory on each urban component
such as roof and road to that on a nearby permeable plane field, for 11 buildings in the evacuation zone in 2015
and 2016. Additionally, the results were analyzed with the data obtained in the model decontamination project toevaluate temporal trend in the distribution during the initial five years after the accident. The relative inventory
obtained for paved ground were 0.2 and for other building components such as roof and wall were less than 0.04
in 2016. These values indicate the limited contamination of urban area. The relative inventories of paved ground,roof and wall in 2011 and 2012 showed similar values with those obtained in this study, suggesting that initial
run-off and the following wash-off effects during the first year after the accident largely defined the distributionof 137Cs in urban area. Some of the component showed rapid decrease in the relative inventory more than decay
during five years after the accident. The fast decrease and the obviously low relative inventories on thecomponents in urban area demonstrate the rapid removal of 137Cs from urban area.
1. Introduction
Large amounts of radionuclides were deposited on
the ground over the North Kanto and the South
Tohoku regions of Japan after the Fukushima Daiichi
Nuclear Power Plant (FDNPP) accident. The
distributions, temporal changes and migrations of theradionuclides in diverse terrestrial environments havebeen well studied so far (Saito and Onda, 2015). On
the other hand, those studies in urban area is cruciallylimited, although the area has large impact on
exposure of local residents.
A decontamination pilot project had been carriedout at residential area in the evacuation zone in 2011
and 2012 to establish guidelines of decontamination
procedures (JAEA, 2015). The project provided many
important information such as horizontal/vertical
distributions of radionuclides in the area. Studiesafter the Chernobyl nuclear power plant accident
evaluated radionuclides inventory (i.e. activity perunit area, Bq m-2) on urban components such as roofs,walls and roads, and their temporal changes (IAEA,2006 and references in there). Their studies indicatedrapid removal of the radionuclides from paved
surface more than that from undisturbed field. The

different temporal decreases in the radionuclide
inventory, especially 137Cs inventory, among theurban components largely affect to decontamination
planning such as a decision of priority of urbansurface to be decontaminated and an estimation ofefficiency of the decontamination for dose reduction.
In the case of the area affected by the FDNPP
accident, relative 137Cs inventories, which are definedas the relative values of 137Cs inventory on eachcomponent to initial 137Cs inventory on a nearby
permeable plane field, had been investigated for 11
buildings in the evacuation zone in 2015 (Yoshimuraet al., 2017), but the temporal trend in radionuclides
activity on urban surfaces have not been shown.
This study investigated the relative 137Csinventories on urban components for the 11 buildings
at evacuation zone in 2016. The objects buildings
were selected to be same with those reported byYoshimura et al. (2017). To clarify the temporal
trends in relative 137Cs inventory on urban
components, the data in 2015 (Yoshimura et al.,2017), in 2016 (this study) and obtained by thedecontamination pilot project (JAEA, 2015) wasanalyzed together.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan2. Methods
2-1. Investigation
This study evaluate the relative 137Cs inventories
on urban components for the 11 buildings in Okuma
and Tomioka towns, located in the evacuation zone,
from 12 to 22 January 2016 (figure 1). The buildings
and the adjacent fields had not been decontaminated.
Therefore, decrease in 137Cs deposited on the studied
fields has reflected only weathering effect (wash-offdue to precipitation).
The surface contamination count rate (cpm) wasmeasured for major components (roofs, walls andpaved grounds at 212, 468 and 97 points,respectively) using a Geiger-Mueller survey meter(TGS-146, Hitachi-Aloka Medical, Japan) (table 1).
Descriptions of the investigation has been reported by
Yoshimura et al. (2015).

measurement using the portable Ge gamma-ray
spectrometer showed no areal variation in the134
Cs/137Cs ratio in the studied area. However, theconversion factor have temporally increased, because
the 134Cs/137Cs ratio decreases with time due to thedifferent decay constant. Therefore, this study
retrospectively calculated the conversion factors on
the date of investigations in 2015 (conversion factorof 0.0776) and 2016 (conversion factor of 0.0993) byconsidering the physical decays of 134Cs and 137Cs.
This study also estimated the 137Cs inventory on
urban components from the count rate data obtainedby the decontamination pilot project (JAEA, 2014 a)using the Geiger-Mueller survey meter equipped witha collimator (table 2). The retrospectively estimatedconversion factor was also applied to the calculationof the 137Cs inventory.
Table 1 Measured components and number ofmeasurement points. W- and RC- indicate wood- or
reinforced concrete-structures, respectively. The
number from 1 to 3 in the “construction” column
represents stories of the object building. Initial 137Cs
inventory on permeable plane fields adjacent to thestudied buildings are also shown as INV.

Figure 1 Study site (Yoshimura et al., 2017)2-2. Evaluation of 137Cs inventory
The 137Cs inventory (Bq m-2) on object component
was calculated by multiplying the count rate byconversion factor, which is the ratio between 137Cs
inventory and the count rate obtained in this study.
To obtain the conversion factor, 137Cs inventory andthe count rate were measured at 11 paved grounds(parking areas and roads) using a portable Ge
gamma-ray spectrometer (Falcon-5000, CANBERRA,USA) equipped with a cylindrical collimator(30-mm-thick Pb, 60 mm height with 170 mmdiameter) and a Geiger-Mueller survey meter(TGS-146,Hitachi-Aloka
Medical,Japan),respectively, on 10 February 2016. The count rate
was measured at 19 points in the area with 3 m indiameter below the portable gamma-ray spectrometer.
Measurement description is mentioned in Yoshimura
et al. (2017). Figure 2 shows the relation between the137
Cs activity per unit area and count rate, and a slopeof 0.101 was obtained as the conversion factor.
This value is applicable in the case of that the134
Cs/137Cs ratio is constant. The results of in-situ

PavedINV.

ground(MBq m-2)

42－

3.63

55

103.63

39

6

3.63

38－

2.5－

46

1

2.67

34

33

7

3W-1

3230－

2.07

8RC-1－

52

9

2.05

9RC-2

32

47

18

3.7

10RC-3

34

37

39

2.54

11RC-3

949

7

2.2

212

468

97

No.

Construction

RoofWall

1
2

W-2

25

W-1

734W-1

9W-1

30

5RC-1

6W-1

7Total2-3. Relative 137Cs inventory
The relative 137Cs inventory, which is the ratio of137
Cs inventory of the component at the measured
date to the initial one on permeable plane fields
adjacent to each of the studied buildings, is useful tocompare the 137Cs distribution on urban components
among the area of which deposition amount isdifferent. To calculate the relative 137Cs inventory forthe data set obtained in this study and in Yoshimura
et al. (2017), the 137Cs inventory on the adjacent
permeable plane fields was measured using theportable Ge gamma-ray spectrometer on 4-16
February 2016.
The 137Cs deposition patterns due to FDNPP
accident was largely determined until 23 March 2011

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

(UNSCEAR, 2013). Since the decrease rate of 137Cs
inventory on permeable plane fields has beenreported to be same with its physical decay constant(Mikami et al., 2015), the initial 137Cs inventory on
the permeable plane fields at March 23, 2011 wasestimated by only physical decay correction of thedata obtained in this study. The estimated initial 137Cs
inventories on the permeable plane fields are shownin Table 1.
For the data of decontamination pilot project, the137
Cs inventory on the permeable plane fields
adjacent to the object component was estimated fromthe results of the Forth Airborne Monitoring Surveyby MEXT (2011) as the averaged value within anarea with radius 150 m from the object component.
The results of Airborne Monitoring Survey is wellcorrespondent with the inventory evaluated by soil
sampling at 2,200 points on permeable plane fields.
Since the Forth Airborne Monitoring Survey provides
the data as the value on 7 November 2011, initial137
Cs inventory was estimated by physical decay
correction.

3. Results and Discussions
3-1. Relative 137Cs inventories on urban components
The relative 137Cs inventory was summarized intable 3. The relative 137Cs inventory on the permeable
plane fields is estimated values by physical decay
correction of initial 137Cs inventory. The relative
137Cs inventory on paved ground was 0.18,
accounting for 20% of the relative 137Cs inventory on
the permeable plane fields, in 2016. Wall and roof
showed obviously low values comparing to paved
ground even when considering the standard deviationof the relative inventories. These low relative 137Cs
inventories indicates that the contamination of urban
area due to the 137Cs fallout was limited.
Table 3 Relative 137Cs inventories on urban
components and their standard deviation (SD)estimated for 2011, 2015 and 2016.
Dec.
Components

Components
Site

Roof

Tomioka

Wall75

Paved

Measured dateground
615 Jan.-3 Feb. 2012

Katsurao－－

91218 Dec. 2011

Tamura－－1224 Dec. 2011

Okuma

202

2230 Jan.-10 Apr. 2012

Kawauchi52－

9 Jan. 2012

Namie－1－

6 Dec. 2011

Jan. 2016

SDAveSDAve

SD0.98-0.92-0.90-Paved ground

0.22

0.07

0.18

0.06

0.18

0.05

Wall

0.09

0.12

0.01

0.00

0.01

0.00

Roof

0.14

0.13

0.02

0.02

0.02

0.03

plane field

Table 2 Measured components and the data number
referring in the decontamination pilot project (JAEA,2014).

Jan. 2015

2012
Ave

Permeable

Figure 2 Relation between the 137Cs activity per unit
area and count rate.

2011-Apr.

In the case of the FDNPP accident, wet deposition
was the major process of 137Cs fallout on the ground(Morino et al., 2013). Therefore, the vertical structureof wall was inferred to be little-affected by the wetdeposition, resulting in the low relative inventory. On
the other hand, paved ground and roof are horizontal
surface and directly affected by the wet deposition,
but low relative inventory was also obtained. Thiswas probably resulted by the low fixation capacity of
137
Cs by the components due to their impermeability.
The relative 137Cs inventory on paved ground, walland roof were 0.22, 0.09 and 0.14, respectively, in
2011-2012, and the values were obviously lower thanthat of permeable plane field even when considering
the uncertainty due to the difference in investigationsite and methods. This results suggest that the initial
run-off and the following wash-off effects during thefirst year after the accident largely defined thedistribution of 137Cs in urban area. The rapid removalof 137Cs from urban area during the initial period after
the deposition is well consistent with the result ofstudies in Europe after the Chernobyl Nuclear Power
Plant accident (Roed, 1987; Roed and Andersson,
2000a, 2000b).
Temporal change in the relative 137Cs inventories
are described in figure 3. The proportions of decrease
in relative 137Cs inventories on paved ground, walland roof between 2011 and 2016 were 18%, 94% and6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

85%, respectively. These proportions were larger
than that of permeable plane field (8%), of which
decrease was represented by physical decay. Thisimplies that the weathering effect one year after theaccident also facilitate the removal of 137Cs fromurban area. JAEA (2014) reported that the 137Cs
removal rate, which was defined as the ratio between
the 137Cs inventory in the catchment and discharged
amount of 137Cs from the catchment, was less thanseveral percentages during 3.5 years after theaccident. Therefore, it is suggested that the urban area
has large self-cleaning capacity more than that inother terrestrial environment.

Figure 3. Comparison of relative 137Cs inventories
among years and with that on permeable plane field.
3-2. Comparison of temporal trend of relative 137Cs
inventory on paved ground
The temporal trend of relative 137Cs inventory on
paved ground obtained this study was compared withthe results of studies in Europe after the Chernobyl
Nuclear Power Plant accident (Andersson et al.,
2002; Jacob and Meckbach, 2000) (figure 4). The
relative 137Cs inventory estimated from the data of
decontamination pilot project (about a year after theaccident) fell in the range of the relative 137Cs
inventory reported by the studies in Europe. Thissuggests that the initial behaviors (initial wash-offand weathering during the first year after theaccident) of 137Cs deposited on paved ground was
similar between this study and the case in Europe. On
the other hand, the following decrease in the relative137
Cs inventory after 2012 was faster in Europe(decreasing rate of 0.075-0.33 yr-1) than that of thisstudy(decreasing rate of 0.049 yr-1), suggesting small

weathering effect in this study more than that in thecase in Europe. This difference in decreasing rate isprobably caused by the human activities such astraffic and road cleaning. The studies in Europe wascarried out in residential area, whereas the results inthis study was obtained in the evacuation zone.
Kinase et al. (2015) reported that the decrease in dose
rate on roads in the evacuation zone is slower thanthat outside of the zone. Therefore, 137Cs on paved
ground outside of evacuation zone probably removed
rapidly more than the trend observed in this study,
although further study is necessary.

Figure 4 Temporal trend of relative 137Cs inventory
on paved ground obtained in this study and observedin Europe after the Chernobyl Nuclear Power Plant
accident.
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6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan