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Journal of Environmental Radioactivity 192 (2018) 417–425
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity
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Evaluation of ecological half-life of dose rate based on airborne radiation
monitoring following the Fukushima Dai-ichi nuclear power plant accident
Yukihisa Sanadaa,∗, Yoshimi Urabeb, Miyuki Sasakia, Kotaro Ochia, Tatsuo Toriic
Fukushima Remote Monitoring Group, Fukushima Environmental Safety Center, Japan Atomic Energy Agency, 45-169 Sukakeba, Kaihama-aza, Haramachi-ku, Minamisoma, 975-0036, Japan
NESI Inc., 38 Shinko-cho, Hitachinaka, Ibaraki, 312-0005, Japan
Collaborative Laboratories for Advanced Decommissioning Science, Japan Atomic Energy Agency, 790-1 Motooka Ohtsuka, Tomioka Town, Futaba County, Fukushima,
979-1151, Japan
Fukushima Daiichi nuclear power plant
Airborne radiation monitoring
Unmanned aerial vehicle
Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the
Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation
monitoring using manned and unmanned helicopters, the we have developed and established an analysis method
concurrently with the development of this monitoring method. In particular, because the background radiation
level differs greatly between East and West regions of Japan, we have developed a discrimination method for
natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation
monitoring data was validated through comparison with large amounts of ground measurement data. The
ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were
0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using
manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the
radiation exposure of residents.
1. Introduction
To evaluate the influence of radionuclides emitted by the accident
at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) of the Tokyo
Electric Power Company Holdings, Inc. Caused by the Great East Japan
Earthquake, various types of environmental radiation monitoring data
have been acquired by many governmental institutes and universities.
An airborne radiation monitoring technique is suitable to grasp the
overall distribution of the ambient dose equivalent (referred to hereinafter as the air dose rate) and the deposition of radionuclides because
such a technique can be used to (1) measure widely distributed radionuclides with less manpower and within short periods, (2) display maps
that are easy to understand visually, and (3) obtain measurement results at locations (e.g., forests and mountains) that are not easily accessible to humans. Analysis of the temporal and spatial changes in the
air dose rate based on multiple airborne radiation monitoring datasets
is useful for predicting and evaluating the radiation exposure to inhabitants.
The current airborne radiation monitoring technique was established in the early 2000s. Specifically, European scientists conducted
pioneering studies of airborne radiation monitoring after the accident at
the Chernobyl nuclear power plant. The method of data acquisition,
calibration, and mapping developed by Aage et al. (1999) is the basic
technique at present. Allyson and Sanderson. (1998) proposed a calibration technique by using Monte Carlo simulation. Tyler et al. (1996)
proposed field-of-view airborne radiation monitoring by comparing
airborne data with ground data. Through the European comparison
project (Eccomags project), the data acquisition method and methods
for analysis of airborne radiation monitoring data developed by each
European country were unified (Sanderson et al., 2004).
In Japan, triggered by the Three Mile Island nuclear power plant
accident in 1979, research and development of an airborne radiation
monitoring system using a manned helicopter (MRM: Manned helicopter Radiation Monitoring) were initiated primarily by researchers
from Japan Atomic Energy Research Institute (reorganized as the Japan
Atomic Energy Agency: JAEA) (Saito et al., 1988; Nagaoka and
Moriuchi, 1990). However, methods of data acquisition, data analysis,
and the MRM mapping, which correspond to measurement of wide
areas in this case, had not been established.
After the FDNPP accident, the MRM national project was started by
Corresponding author.
E-mail address: [email protected] (Y. Sanada).
Received 25 September 2017; Received in revised form 11 April 2018; Accepted 16 July 2018
0265-931X/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
(manufactured by YAMAHA Co., Ltd., Iwata, Japan), originally developed for spraying pesticides, were used for radiation measurements, as
shown in Fig. 1(d). These helicopters are operated manually for takeoff
and landing, and they have a program operator for autonomous flight
and an operator for the radiation detector. An unmanned helicopter can
conduct a programmed flight with the help of detailed self-localization
by using a Real-time Kinematic Global Positioning System (RTK-GPS),
and its flight waypoints and altitude can be set. Detailed specifications
of the unmanned helicopter are given in Sanada et al. (2016). Three
LaBr3:Ce scintillation detectors (dimensions, 1.5″ϕ × 1.5″H; total
LaBr3:Ce crystal volume, 0.13 L) are used in the URM radiation monitoring system, which was designed by the authors of the present study
and constructed by the Japan Radiation Engineering Co., Ltd., (JREC,
Hitachi, Japan). These detectors are arranged in a housing with the data
processing substrate, as shown in Fig. 1(e) and (f). The detectors record
the once-per-second readout of the spectrometers to produce a 1024channel energy spectrum rated at 3 keV per channel. The data from
these detectors are sent to a ground station via a radio channel independent of the helicopter control signal channels. The position of the
helicopter and the total count rate information of the detectors are
displayed on a map at the ground station in real time.
The airborne radiation monitoring program was conducted regularly by using these monitoring systems as a national project. In the
present study, several data sets of MRM and URM were used, as summarized in Table 1. The flight paths of each monitoring session are
shown in Fig. 2. Basically, the flight line space of the MRM was 1.8 km
(= 1 nautical mile). From MRM-2011-01 to MRM-2011-03, the flight
plan was devised considering the climatic conditions and the topography from the viewpoint of flight security. After MRM-2013-01, the
flight path was unified to evaluate detailed changes in the air dose rate.
The flight line space of the URM was 0.08 km, and the flight line was set
to be the same every year. The measurement area of URM-2012-01,
which was approximately 5 km from FDNPP, was smaller than those of
other monitoring sessions.
the Ministry of Education, Culture, Sports, Science and Technology of
Japan and the Department of Energy in the USA (Lyons and Colton,
2012; Blumenthal, 2012). Even though MRM initially monitored only
the area around the FDNPP, the areas surveyed were gradually expanded, and eventually, airborne radiation monitoring was performed
in eastern Japan, excluding Hokkaido, after October 2011 and in western Japan and Hokkaido, after May 2012 (Sanada et al., 2014a). The
distributions of air dose rate at a height of 1 m above ground level (agl.)
and of the concentration of radioactive cesium (134Cs and 137Cs) on the
ground surface were monitored in all areas in Japan under this MRM
project. This monitoring project is ongoing with periodic surveys conducted by the Nuclear Regulatory Authority of Japan (NRA) and JAEA
(NRA, 2017). While conducting the MRM project, we developed and
established data processing and calibration methods concurrently with
the development of a monitoring method. Especially, because the
background radiation level differs greatly between the eastern and
western regions of Japan, we developed a discrimination method for
natural radionuclides as the dominant background, a method for setting
the parameters for conversion to the air dose rate near the ground level,
and a mapping method (Sanada et al., 2017).
A radiation measurement technique that is more detailed than the
method using a manned helicopter is required to formulate a decontamination plan and evaluate the effect of decontamination. Radiation
measurement using an unmanned helicopter (URM: Unmanned helicopter Radiation Monitoring) is one solution because an unmanned
helicopter can be used to generate detailed air dose rate maps by flying
below 150 m, in accordance with Japanese aviation law. URM was
developed for monitoring high air dose-rate areas and riverbed areas
(Sanada et al., 2014b, 2015). Analytical methods for converting airborne detector count rates to air dose rates at 1 m agl. have been established based on the MRM method, and the validity of the MRM
method has been demonstrated thorough comparisons with large
amounts of ground measurement data (Sanada et al., 2016).
Several years after the FDNPP accident, evaluation of temporal
changes in the air dose rate is required for the evaluation of radiation
exposure to inhabitants and for post-accident response. In previous
studies, temporal changes in the air dose rate at approximately 5 km
from the FDNPP were evaluated regionally based on detailed monitoring results using URM (Sanada et al., 2016). However, temporal
changes in the air dose rate over an entire contamination area (approximately 80 km from the FDNPP) based on a comparison of the results of MRM and URM were not evaluated. Knowledge of not only the
changes in the air dose rate but also of the regional characteristics of the
changes in the air dose rate can be obtained through an analysis of
sequential MRM and URM data. In this article, we present the methods
and results of the MRM and URM missions performed around FDNPP. In
addition, we attempt to calculate the effective and ecological half-lives
considering the tendency of the temporal change and discuss the
characteristics of the changes in the air dose based on these results.
2.2. Data processing
This section describes the method used to analyze the measurement
data. The detailed methods of MRM and URM are described in our
previous articles (Sanada et al., 2016 and Sanada et al., 2017, respectively). Other techniques for analyzing radiation measurements from
aircraft are described elsewhere, for example, in International Atomic
Energy Agency (IAEA) reports (IAEA, 2003). To convert MRM and URM
data at the flight altitude to the air dose rate at a height of 1 m agl., a
straight topographically flat road with a relatively flat distribution of
the air dose rate was set as the test line. A conversion factor (CD: μSv
h−1 cps−1) was used to convert the count rate at the flight altitude in
air to the air dose rate at 1 m agl. Actually, calculation can be performed by comparing the count rate at the flight altitude to the air dose
rate measured at 30 survey points around the test line by using a NaI
survey meter. Furthermore, flights at various altitudes from 150 m to
1000 m (URM: from 10 m to 150 m) were conducted over the test line
and the effective attenuation factor in air (AF) was obtained from the
relationship between the count rate and the flight altitude.
Radiation measurement using the airborne radiation monitoring
methods is influenced by the following four sources of background
radiation: 1) cosmic rays, 2) radiation contamination by the body of the
aircraft, 3) natural nuclides in the detector crystal, and 4) radiation
from radon decay products in air. To eliminate the contributions of
these background radiation sources from the gamma ray spectra obtained, background count rate data (CBG: cps) acquired above the sea
were used. Using the air dose rate conversion factor obtained over the
test line, the count rate (Call: cps) obtained at the flight altitude was
converted into the air dose rate 1 m agl. (D1m: μSv h−1). To correct the
flight altitude, the absolute altitude with respect to the ground level
was first obtained by subtracting the 10-m cell elevation data obtained
2. Materials and methods
2.1. Airborne radiation monitoring system and data collection
The dedicated MRM radiation detection system (RSX-3, Radiation
Solution, Inc., Mississauga, Canada), which was installed on a manned
helicopter, is shown in Fig. 1(a) and (b). This system consists of six large
NaI detectors (dimensions: 2′′ × 4′′ × 1″, total NaI crystal volume:
12.6 L), a data processing unit (RS501 and RS701), and a Global Positioning System (GPS) receiver, as shown in Fig. 1(c). The system acquires a once-per-second readout of the spectrometers to produce a
1024-channel energy spectrum rated at 3 keV per channel. The readings
are synchronized with time via the GPS receiver. The spectrum and the
GPS data (date, time, latitude, longitude, and height above ellipsoid)
are recorded every second.
After the FDNPP accident, unmanned helicopters, R-MAX G1
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
Fig. 1. Photographs of (a) a manned helicopter and (d) an unmanned helicopter. Radiation monitoring systems for (b) the manned helicopter and (e) unmanned
helicopter. Block diagrams of the radiation monitoring system for (c) the manned helicopter and (f) unmanned helicopter.
Henss (2014). The air dose rate was corrected to the last date of each
campaign by using CF and the physical decay of radioactive cesium.
The detection limits of air dose rate at 1 m agl. (DDL) for MRM and
URM were 0.011 μSv h−1 and 0.04 μSv h−1, respectively. These values
were calculated based on the evaluated counting error of the background count rate (CBG) measured by the helicopter in accordance with
Currie's formulation (Currie, 1968), as follows:
using the Geospatial Information Authority of Japan digital elevation
model (DEM; GSI, 2015) from the altitude above sea level obtained
using the GPS (Hm: m). Using the difference in the actual absolute altitude from the standard altitude (Hstd = 300 m) and the AF, the reduction in radiation intensity was then corrected. The conversion
equation to obtain D1m is as follows:
D1m = (Call − CBG ) CD exp[−AF (Hm − Hstd )].
DDL = (4.65 CBG + 2.71) CD
The conversion factor for determining radioactive cesium deposition from the air dose rate (CF: in μSv h−1 Bq−1 m2) at a given day after
the FDNPP accident was defined by referring to Saito and Petoussi-
Mapping was performed by supplementing unmeasured areas via
interpolation of the measured results. Even though various methods
Table 1
Airborne radiation monitoring datasets and monitoring dates.
Measurement period
Elapsed day from the accident to end date
Start date
End date
Manned helicopter
April 6, 2011
May 31, 2011
October 22, 2011
June 22, 2012
October 31, 2012
August 27, 2013
November 2, 2013
September 1, 2014
September 12, 2015
September 14, 2016
April 29, 2011
July 2, 2011
November 5, 2011
June 28, 2012
November 16, 2012
September 28, 2013
November 19, 2013
September 20, 2014
September 29, 2015
October 15, 2016
Lyons and Colton, 2012
Sanada et al., 2017
Sanada et al., 2014a
Sanada et al., 2017
Unmanned helicopter
August 30, 2012
January 27, 2013
June 6, 2013
November 19, 2013
June 23, 2014
November 13, 2014
September 3, 2015
September 1, 2016
October 20, 2012
March 20, 2013
July 31, 2013
January 7, 2014
July 22, 2014
January 15, 2015
October 22, 2015
October 13, 2016
Sanada et al., 2016
This study
This study
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Y. Sanada et al.
Fig. 2. Flight paths of the MRM and URM. These flight paths were used to analyze the radiation measurements. Data at high altitudes agl. taken during turning of the
helicopter were not used.
Fig. 3. (A) and (b): Converted air dose rates of MRM-2013-01 and ground measurement data, which are published data from a national project (JAEA, 2017). (c) and
(d): Converted air dose rate of URM-2013-01 and ground measurement data obtained by the authors.
a variable, based on the weighted average of adjacent observed sites
within a given area. The interpolation error can be evaluated by means
of a semivariogram because kriging provides more advanced processing
than the IDW method. The spatial resolutions of the air dose rate
contour maps obtained using for MRM and URM were 250 m and 5 m,
such as kriging and spline approaches have been proposed for interpolation, the inverse distance weighted (IDW) method, which assigns
weights to the values of the measurement points linearly and in inverse
proportion to the distance, was applied to the MRM data. The IDW
method is easy to use when analyzing a large amount of data because
the parameter setting is simple (IAEA, 2003). The URM data were interpolated using kriging because this dataset was smaller than that the
MRM dataset. Kriging provides an estimate at an unobserved location of
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
in the forest and mountainous areas of Fukushima Prefecture from 2011
to 2014 (Fukushima Prefecture, 2015). To the extent possible, we selected the ground measurement data and the MRM data obtained at
nearly the same times and locations. Both datasets were corrected for
radioactive cesium decay. To validate URM measurement results, they
were compared to ground-based measurements that were performed
using an NaI survey meter (TCS-172B, Hitachi Inc., Tokyo, Japan). We
obtained air dose rate data at 500 points within a radius of 5 km from
The air dose rate of the ground data (Dg) and the airborne data (Da)
were compared by visualizing the unevenness using a scatter diagram.
The relative deviation (RD) of each measurement cell was calculated as
follows to evaluate the accuracy of the airborne radiation monitoring
Table 2
Airborne radiation monitoring and ground measurement results. The relative
deviation (in Eq. (1) of ref.) was calculated using the histogram shown in Figs. 3
and 4.
Number of
Manned helicopter
MRM-2011-01 1319
MRM-2011-02 1881
MRM-2011-03 322
MRM-2012-01 6179
MRM-2012-02 6213
MRM-2013-01 6190
MRM-2013-02 6232
MRM-2014-01 6231
MRM-2015-01 6252
MRM-2016-01 6247
Unmanned helicopter
URM-2012-01 450
URM-2013-01 798
URM-2013-02 502
URM-2013-03 515
URM-2014-01 494
URM-2014-02 501
URM-2015-01 501
URM-2016-01 500
Relative deviation (RD)
mean square
error (NMSE)
NMSE = i= 1CGi−Ki2i = 1CGi2NMSE =
2.4. Effective and ecological half-lives of air dose rate
RD = (Da − Dg )/ Dg
The calculated RDs were used to evaluate the total error and statistical uncertainty, which is shown as a histogram of frequency.
In addition, the difference between Dg and Da was quantified using
the normalized mean square error (NMSE) method for performing relative evaluation between the two datasets. The NMSE was computed as
∑ (Dgi − Dai)/ ∑ Dgi2 ;
where C is the total number of data points.
To quantitatively compare the monitoring results, the area was divided by a distinctive cell, that started from the FDNPP. The cell sizes of
MRM and URM were decided as 250 m and 5 m, respectively, considering the spatial resolution of interpolation. The air dose rate of each
cell was expressed as a relative value for standardizing the first result
(MRM-2011-01 and URM-2012-01). Time-series data of more than
1.0 μSv h−1 were selected to eliminate the influence of natural background radiation. The background air dose rate in the study area was
reported to be 0.03–0.07 μSv h−1 (Andoh et al., 2017).
The effective half-life combines both the physical decay and ecological decreasing such as migration or weathering. The ecological
decrease rate is expressed in terms of the ecological half-life. The time
2.3. Ground measurement for confirming reliability of airborne radiation
monitoring data
Confirming the reliability of airborne radiation monitoring data is
essential because studies have indicated that airborne radiation monitoring data tend to overestimate the air dose rates over complex topography (Tyler et al., 1996; Malins et al., 2015). In the present study,
we compared MRM data to a large amount of ground measurement
data. These ground measurement data were obtained multiple times at
the same positions around FDNPP (Mikami et al., 2015; JAEA, 2017). In
addition, we compared the MRM data to the ground measurement data
Fig. 4. (A) and (b): Converted air dose rates
of MRM-2015-01 and ground measurement
data in a flat area, which are published data
from a national project (JAEA, 2017). (c)
and (d): Converted air dose rate of MRM2015-01 and ground measurement data of
forest and mountainous areas obtained in
Fukushima Prefecture in 2015.
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
Fig. 5. Contour maps of air dose rates obtained by MRM. The background picture consists of DEM data obtained by GSI (2015).
Fig. 6. Contour maps of air dose rates obtained by URM. The background picture consists of DEM data obtained by GSI (2015).
dose rate at t = 0, respectively. Double exponential fitting was applied
to all data (MRM: 50,000 data per campaign; URM: 350,000 data per
campaign) in each cell within the study area. The fitting was performed
with the least-squares method by using the statistical software application R (Ihaka and Gentleman, 1996).
The relative air dose rate considering the physical decay of radioactive cesium by assuming a constant relaxation mass depth (1 g cm−2)
trend of relative air dose rates (Dr(t)), which was calculated by standardizing first survey result, approximated by a following double exponential function with effective half-lives of T'short and T'long;
′ exp(−ln2/ Tshort
′ t ) + Ilong
′ exp(−ln2/ Tlong
′ t ),
Dr (t ) = Ishort
where t is the time after the FDNPP accident, I'short and I'long are the
proportion of the short-lived and long-lived component to the total air
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
3. Results and discussion
3.1. Validation of airborne radiation monitoring data against ground
Air dose rates acquired using MRM and URM were compared to
those measured on the ground. MRM-2013-01 and URM-2013-01 are
shown in Fig. 3(a) and (c), respectively, as examples of the comparison
results. As shown in the figure, the airborne radiation monitoring data
and the ground measurement data exhibit positive correlations and
agree well. Histograms of the RD of MRM and URM air dose rate data
have shapes similar to Gaussian distribution with peaks near zero
(Fig. 3(b) and (d), respectively). However, the mean RDs and their
standard deviations were 0.25 ± 0.57 and 0.046 ± 0.38, respectively.
These indicate that the MRM data tended to be higher than the ground
measurements, whereas the URM was in relatively good agree with the
ground measurements. A comparison of the other monitoring data obtained using MRM and URM are shown in Table 2. Systematic deviations of approximately 22% and 7.6% were revealed in the MRM and
URM data, respectively.
The present analysis assumes that the measurements are performed
over an infinite plane, however the terrain was mountainous in fact. To
confirm the reliability of airborne radiation monitoring in the forest and
the mountainous areas, MRM-2015-01 was compared to the ground
data in the flat and the forest areas, as shown in Fig. 4(a) and (c), respectively. Both comparisons exhibited positive correlations, and the
slopes of the linear regression lines were close to unity. The mean RDs
in cases of the forest and the mountainous areas were smaller than that
for a flat area (Fig. 4(b) and (d)). The topography effect was not confirmed as a tendency of the data in Fig. 4(d). According to extant studies, the topography effect influences airborne radiation monitoring in
areas with complex topography (Tyler et al., 1996; Malins et al., 2015).
Efforts are underway to quantify the effect of mountainous terrain on
the analysis and to implement algorithms that would correct the data
for the topography of the ground (Ishizaki et al., 2017).
In Fig. 4(b), a few data for which the MRM result was higher than
the ground measurement were confirmed to exist. The reason for this
was that MRM may not sufficiently reproduce lower air dose rates by
means of local decontamination. This systematic deviation must be
considered when evaluating temporal changes in the air dose rate. The
NMSE of URM was lower than that of MRM. This can be described to
the fact that URM can measure the distribution of air dose rate on the
ground more precisely than MRM because the flight altitude of URM is
lower, and its flight spacing is smaller.
Fig. 7. Temporal changes in air dose rate obtained by (a) MRM and (b) URM.
The air dose rate was averaged and standardized in the oldest monitoring results (MRM-2011-01 and URM-2012-01, respectively). Confidence interval
(95%) is shown in this figure. The error bar represents the standard deviation
(σ = 1) in each measurement mesh.
was calculated for evaluation of ecological half-life. Here, the relaxation mass depth was defined as an inclination parameter of an exponential distribution function of the concentration of radioactive cesium with a depth of mass per unit area in the International
Commission on Radiation Units and Measurements' report (ICRU,
1994). For evaluation the ecological half-lives (Tshort and Tlong), double
exponential fitting was applied to relative air dose rate data which was
corrected by physical half-life to the first result date (MRM-2011-01
and URM-2012-01) such as calculated effective half-life.
3.2. Interpolated counter maps of air dose rate
Fig. 5 shows maps of the air dose rate 1 m agl. that were constructed
from MRM data. First, as seen in the air dose rate map of MRM-201101, the red-colored area with a high air dose rate (> 50 μSv h−1) was
oriented toward the northwest from the FDNPP to an area
Table 3
Calculated effective and ecological half-lives based on airborne radiation monitoring data. Numerical values smaller than the significant figures of the URM values
are shown to clarify the confidence interval.
Parameters of double exponential fitting
Not correction of radiocesium half-life
Correction of radiocesium half-life
Effective half-life (y)
Ecological half-life (y)
Relative air dose rate (t = 0)
Relative air dose rate (t = 0)
3–80 km zone from FDNPS
I′ short
I′ long
5 km zone from FDNPS
I′ short
I′ long
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
Fig. 8. Contour map of the air dose rate ratio of the
oldest and the most reliable monitoring and recent
monitoring. (a) The positions of agricultural and residential areas in the area 80 km from FDNPP. (b)
The positions of agricultural and residential areas
5 km from FDNPP. (c) Ratio of MRM-2016-01 to
MRM-2011-03. (d) Ratio of URM-2016-01 to URM2013-01.
approximately 30 km from FDNPP. For areas from 3 km to 80 km from
FDNPP, areas with 0.5–10 μSv h−1 were spread gradually in the areas
adjacent to the red-colored high air dose rate areas. The air dose rate
was relatively high south of FDNPP. The area of high air dose rate
(more than 1 μSv h−1) is decreased gradually. In a recent result (MRM2016-01), the air dose rates in more than half the areas within a range
of 3–80 km from FDNPP were less than 0.5 μSv h−1. Conversely, the air
dose rate northwest of FDNPP has remained relatively high.
Fig. 6 shows maps of the air dose rate 1 m agl. that were constructed
from the URM measured data. Four deposition patterns in which the air
dose rate is particularly high extend from the FDNPP. This indicates
there were four separate releases of radioactive material to the ground
side during the FDNPP accident. The URM-2016-01 result indicates that
the western area appears to be divided into two sections, and the high
air dose-rate area in the west originated from a location several hundreds of meters away from FDNPP boundary. The air dose rate in the
northwestern area was comparatively low. As the MRM result, the area
of high air dose rate decreased gradually.
error of the slope and intercept values in Fig. 7. Effective half-life,
which was calculated using these slope values, is listed in Table 3. The
T'short value determined using MRM and URM were 0.71 ± 0.0079
years and 0.78 ± 0.0034 years, respectively. Similarly, T'long by MRM
and URM were 5.1 ± 0.10 years and 4.3 ± 0.0012 years, respectively.
The Tshort values determined using MRM and URM were 0.61 ± 0.0061
and 0.51 ± 0.0044 years, respectively. These values agreed with the
ecological half-life of 0.50–0.62 with value in evaluated by a carborne
survey (Kinase et al., 2017). By contrast, the T long value determined
using MRM and URM were 56 ± 3.1 years and 17 ± 0.058 years,
respectively. In accordance with the previous studies (Sprung et al.,
1990), the ecological half-life of the long-term component is given: 90
years (median value; 95% confidence interval: 45–135 years). The
value obtained in this study was significantly shorter than this value
owing to human activities, including decontamination, as discussed in
the next section.
3.3. Effective and ecological half-lives in air dose rate
A counter map of the air dose rate ratio of the oldest reliable
monitoring (MRM-2011-03 and URM-2013-01) and recent monitoring
(MRM-2016-01 and URM-2016-01) was prepared to evaluate local
changes in the air dose rate, as shown in Fig. 8. In Fig. 8(a) and (c), the
positions of the agricultural and residential areas according to land use
information (GSI, 2015) are shown to investigate the geographical
features of the changes in air dose rate. Other areas were primarily
forest and mountainous areas. The Abukuma Mountains consist of approximately 30 mountains, the heights of which range from 200 m to
1200 m above sea level (Fig. 8(a)). Agricultural and residential areas
are located on the coastal side and in the Naka-dori area, which are in
an area between the Abukuma Mountains and the Ohu Mountains. In
3.4. Geographical features of changes in air dose rate
Fig. 7 shows the relative air dose rate, which was standardized
against the first monitoring results to quantitatively evaluate these
trends. The error shows the standard deviation of the ratio of the air
dose rate. The air dose rates in MRM and URM results decrease exponentially as shown in Fig. 7 (a) and 7 (b), respectively. In Fig. 7 (a),
the air dose rate 5.6 years after the FDNPP accident decreased by approximately 79%.
The double exponential fitting line and confidence interval (95%)
applied to all data is shown in Fig. 7. The confidence interval of the
effective and ecological half-life was expressed in terms of the standard
Journal of Environmental Radioactivity 192 (2018) 417–425
Y. Sanada et al.
Fig. 8(b), the areas with remarkable decreases in the air dose rate
correspond to the agricultural and residential areas. The possible reasons for this include the effects of decontamination work and normal
human activities (e.g., agricultural work, construction work, and automobile traffic). An interim storage facility for decontamination waste
is being constructed near FDNPP (MOE, 2017). Areas with remarkable
decreases in the air dose rate correspond to locations in which the interim storage facility is under construction or in which decontamination
work is being undertaken, as shown in Fig. 8(d). Based on the above
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4. Conclusion
To investigate the influence of radioactive cesium, the distributions
of air dose rate were measured using MRM and URM. In Japan, no
technical experience or sample measurements were available for evaluating the air dose rate distribution via wide-spread MRM and URM
before the FDNPP accident. Therefore, we developed and established a
data processing method while concurrently carrying out actual monitoring.
We obtained new knowledge about the tendency of the air dose rate
to decrease by analyzing past monitoring results. (1) Tshort and Tlong
were 0.61 years and 57 years, respectively, based on MRM results. (2)
The air dose rates in the agricultural and residential areas decreased
conspicuously in comparison to those in the forest and the mountainous
areas. The effective and ecological half-life is an important parameter
for predicting future air dose rate. Hereafter, it will be important to
carefully monitor and investigate the migration of radioactive cesium
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the accuracy of the analysis technique and that of the measurement
method should be increased in the future.
Special thanks are due to our colleagues of the Fukushima Remote
Monitoring Group and Research, JAEA. The present study was supported by the Nuclear Regulation Authority in Japan through a
Radiation monitoring projects using a manned helicopter around the
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