ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

1

United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENT A-9

ATMOSPHERIC TRANSPORT, DISPERSION AND
DEPOSITION MODELLING OF AIR CONCENTRATIONOVER JAPANUNSCEAR 2020/2021 Report, Annex B, Levels and effects of radiation exposuredue to the accident at the Fukushima Daiichi Nuclear Power Station: implicationsof information published since the UNSCEAR 2013 Report
Content
This attachment contains a description of the atmospheric transport, dispersion and deposition
modelling (ATDM) of air concentration over Japan that has been used by the Committee. It
includes details of the latest source term developed by the group of researchers at the Japan
Atomic Energy Agency (JAEA) [Terada et al., 2020], the dispersion modelling and the outputs
used for the Committee’s assessments of doses and how the results of ATDM compare withmeasurements.
Notes
The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the United Nations
concerning the legal status of any country, territory, city or area, or of its authorities, orconcerning the delimitation of its frontiers or boundaries.
Information on Uniform Resource Locators (URLs) and links to Internet sites contained in thepresent publication are provided for the convenience of the reader and are correct at the time ofissue. The United Nations takes no responsibility for the continued accuracy of that informationor for the content of any external website.
© United Nations, September 2023 (corrected).1 All rights reserved, worldwide.
This publication has not been formally edited.

1

Paragraphs 4, 11 and 23, and figure A-9.V.

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

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CONTENTS
I. INTRODUCTION .................................................................................................................. 5
II. MEASUREMENT DATA ..................................................................................................... 5
III. SOURCE TERM .................................................................................................................... 7
IV. ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLINGSIMULATIONS OF AIRBORNE RADIOACTIVITY ....................................................... 10
V. ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING:
RESULTS FOR CONCENTRATIONS OF RADIONUCLIDES IN THE AIR ................. 11
VI. COMPARISON OF MODELLED AND MEASURED DEPOSITION LEVELS ............. 13VII. ESTIMATION OF CONCENTRATIONS OF RADIONUCLIDES IN AIR FOR
DOSE ASSESSMENT ......................................................................................................... 15
VIII. COMPARISON OF MODELLED AND MEASURED AIR CONCENTRATION
FOR 137CS ............................................................................................................................ 19
REFERENCES ........................................................................................................................... 25

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

I.

5

INTRODUCTION

1.
The assessment of doses from external exposure to the plume and internal exposure frominhalation of radionuclides in the plume could not be based on the few available environmental
measurements of radionuclide concentrations in air. Rather it has been based on an assumed
source term and ATDM to estimate radionuclide concentrations in air resulting from thedispersion of released radioactive material in the atmosphere and its deposition on to the ground.
2.
The concentrations of radionuclides in air were obtained from the assumed source termand ATDM in two ways: (a) directly from the ATDM; and (b) in combination with data on
measured levels of the deposition density of radionuclides. The methods used to assess doses(both from external exposure and from internal exposure due to intakes by inhalation) from theestimated concentrations of radionuclides in the air are described in attachment A-10.

II.

MEASUREMENT DATA

3.
There have been extensive measurements of the levels of radioactive material in theenvironment across Japan following the Fukushima Daiichi nuclear power station (FDNPS)
accident after 2013. JAEA has collected environmental monitoring data published by variousorganizations related to the accident, including air dose rate, and radioactive concentration(ground surface, soil, seawater, marine soil, river water, river sediment, groundwater and food),
These data sets form the Environment Monitoring Database (EMDB) which is available to the
public through an online website [JAEA, WEB]. The data sets and how they have been used by
the Committee are described in more detail in attachments A-5 to A-8.
4.
Measurements of concentrations of radionuclides in air over Japan during the release
were rather limited, in particular, in the early stages of the accident and in the areas devastatedby the tsunami. Further data have been published since the UNSCEAR 2013 Report [UNSCEAR,
2014], in particular concentrations of airborne aerosols at seven locations within the Japanese
mainland for each hour of the period from 12 March to 11 May 2011 [DTRA, 2013]. The data
were derived from measurements on samples collected by the United States Department of
Defense (DoD) and the United States Department of Energy (DOE) at or near locations where
DoD-affiliated individuals worked or lived. Aside from this new source of measurement
information, additional information about the levels of radionuclides in the air and deposited on
the ground has come from reanalysis of monitoring data collected at the time and from theapplication of some novel analysis methods. Specifically, concentrations of different
radionuclides in the air in the early stage of the FDNPS accident have been estimated at several
monitoring posts in Fukushima and Ibaraki Prefectures from pulse height distributions measured
with sodium iodide scintillation detectors [Hirayama et al., 2015; Moriizumi et al., 2019;
Terasaka et al., 2016]. In addition, concentrations of 137Cs and 129I (from which levels of 131I canbe inferred) in air at ground-level in the Fukushima and Kanto areas have been and continue tobe derived from an analysis of filter-tapes of air pollution stations (for monitoring suspended
particulate matter) [Ebihara et al., 2019; Oura et al., 2015; Tsuruta et al., 2014; Tsuruta et al.,2018], although it should be noted that only iodine in particulate form will have been collectedon these filter tapes. The latest version of the data set produced by this work includes information
on 137Cs levels for 101 locations and on 131I for 4 locations. The available measurement
information is summarized in table A-9.1.

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Table A-9.1. Summary of measurements of concentrations of radionuclides in air over Japan
Reference

Measurement type

[Hirayama et al., 2013]

LaBr3–Detector[Hirayama et al., 2015]

Automatic NaI(Tl)station[Hirayama et al., 2017]

Automatic NaI(Tl)station[KEK, 2015]

Automatic NaI(Tl)station[Terasaka et al., 2016]

Automatic NaI(Tl)station

Locations
Fukushima Prefecture, 6 measurement
points (Adatara SA, Motomiya IC,
Koriyama-higasi IC, Miharu PA, FunahikiMiharu IC, Abukuma-kogen SA)
Fukushima Prefecture, 9 monitoring stations(Hirono Town Futatunuma, Fukushima City
Momijiyama, Okuma Town Oono, Futaba
Town Koriyama, Futaba Town Yamada,Okuma Town Mukaihata, Okuma Town
Minamidai, Okuma Town Ottozawa, Naraha
Town Shoukan)
Fukushima Prefecture, 8 monitoring stations(Futaba Town Yamada, Okuma Town
Mukaihata, Okuma Town Oono, Okuma
Town Minamidai, Okuma Town Ottozawa,
Naraha Town Shoukan, Hirono Town
Futatunuma, Fukushima City Momijiyama)
Fukushima Prefecture, 8 monitoring stations(Futaba Town Yamada, Okuma Town
Mukaihata, Okuma Town Oono, Okuma
Town Minamidai, Okuma Town Ottozawa,
Naraha Town Shoukan, Hirono Town
Futatunuma, Fukushima City Momijiyama)
Ibaraki Prefecture, 6 monitoring stations(Muramatsu, Ajigaura, Araji, Ohnuki,
Ishikawa, Sugaya)
Ibaraki Prefecture, 21 monitoring stations(Toyooka, Mawatari, Oshinobe, Yokobori,Hitachinaka, Horiguchi, Hiroura, Kadobe,
Funaishikawa, Ebisawa, Muramatsu, Ohba,
Tsukuriya, Ishigami, Kuji, Isobe, Ajigaura,
Araji, Ohnuki, Ishikawa, Sugaya)

Radionuclides
132Te, 131I, 132I, 133Xe,
136Cs, 134Cs

131I

132Te, 131I, 132I, 133I,
135Xe, 133Xe, 135mXe

131I

133Xe, 132Te, 131I, 132I,
133I, 137Cs, 134Cs,
136Cs

133Xe, 132Te, 131I, 132I,[Moriizumi et al., 2019]

Automatic NaI(Tl)station[Fukushima Prefecture,
2020]

Gamma-spectrometricmeasurement with Gedetector

Fukushima Prefecture, 14 monitoring
stations (very few measurements each)[Tsuruta et al., 2018]

Filter samples

Fukushima Prefecture, 2 SPM monitoring
stations (Futaba Town, Naraha)

137Cs, 134Cs

Several prefectures, 99 SPM monitoring
stations

137Cs, 134Cs

Tokyo Metropolitan Area, 4 monitoring
stations (TIRI, TMU, RIKEN, KNZ)

129I, 131I, 137Cs[Oura et al., 2015][Ebihara et al., 2019]

Filter samples
measured with Ge
detectors
Filter samples
analysed withaccelerator mass
spectrometry

133I

132Te, 131I, 132I, 137Cs,
134Cs

140Ba, 134Cs, 136Cs,[DTRA, 2013]

Filter samples
analysed with γspectrometry systems

4 monitoring stations (Misawa Air Base,
Sendai City (Sendai Airport), Ishinomaki(City of Ishinomaki), Yokoata Air Base)

137Cs, 131I, 133I, 140La,
86Rb, 99Mo, 99m Tc,
129Te, 129mTe, 131mTe,
132Te[Amano et al., 2012]

Filter samples
analysed with γspectrometry systems

Chiba Metropolitan Area, one monitoring
station (JCAC)

131I, 132I, 133I, 132Te[Doi et al., 2013]

Filter samples
analysed with γspectrometry systems

Tsukuba, one monitoring station (NIES)

132Te, 134Cs, 136Cs,

Filter samples
analysed with γspectrometry systems

Filter samples, 3 monitoring station[JAEA, 2013]

134Cs, 136Cs, 137Cs,

131I, 133I
137Cs, 129mTe, 99Mo
131I, 133I, 132Te, 134Cs,
136Cs, 137Cs, 129mTe

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

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5.
Despite the availability of this measurement information, the number of measurementsof concentrations of radionuclides in air, particularly for 131I, was still too limited to be used toestimate doses to the public from external exposure to the radionuclides in the air or from internal
exposure from inhalation of airborne radionuclides. Instead, concentrations of radionuclides in
the air, and the resulting doses from external and internal exposure, have been estimated using
an assumed source term and ATDM to reflect the atmospheric dispersion of the released
radioactive material and the measured deposition of radionuclides on to the ground. The
measurement information has been used for validating the approach used to estimate
concentrations of radionuclides in the air. The methods used to estimate concentrations ofradionuclides in the air and the validation comparisons between the modelling results and themeasurement data are described in this attachment. Attachment A-10 describes how doses fromexternal and internal exposure to the airborne radionuclides have been calculated from theestimated air concentrations.

III. SOURCE TERM
6.
The source term and ATDM used by the Committee to estimate concentrations ofradionuclides in the air were those derived by Terada et al. [Terada et al., 2020]. An earlier
version of this source term, based on less information [Terada et al., 2012], had been used forthe same purposes by the Committee in its UNSCEAR 2013 Report [UNSCEAR, 2014]. Bothsource terms (and others in the series of refinements that have been made by the same group of
researchers) have been derived from the measurement information on concentrations ofradionuclides in the air and deposited on the ground using reverse ATDM and optimization of
the fit between measurements and modelled values.
7.
The source term derived by Terada et al. [Terada et al., 2020] represented a further
refinement of the source term estimated in a number of previous studies [Katata et al., 2015],
and the ATDM simulation was improved with an optimization method that made use of various
measurements, including air concentration, surface deposition, and more recently measured
hourly air concentrations of 137Cs derived by analysing suspended particulate matter collected at
air pollution monitoring stations [Oura et al., 2015; Tsuruta et al., 2018]. The source term
spanned the period 03:00 JST on 12 March to 09:00 JST on 1 April 2011. Details about thesource term from Terada et al. [Terada et al., 2020] are given in the following table A-9.2 (which
has been reproduced from Terada et al. [Terada et al., 2020]).

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ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Table A-9.2. Updated source term of 137Cs and 131I for the period from 05:00 JST on 12 March to
00:00 JST on 1 April 2011. Release rates are decay corrected at the shutdown time of 14:46 JST
on 11 March 2011. (Reproduced from supplementary data of Terada et al. [Terada et al., 2020])
Release period

Release rate (Bq/h)
Release height (m)Start time (JST)Duration (h)

137Cs

131I

05:00, 12 March

4.0

3.88 × 1012

4.05 × 1013

20

1.0

4.20 × 1012

4.50 × 1013

20

4.0

2.70 × 1012

2.96 × 1013

20

14:00, 12 March

1.0

7.20 × 1013

5.70 × 1015

120

15:00, 12 March

1.0

2.70 × 1014

4.20 × 1015

100 × 100 × 100a

16:00, 12 March

6.0

1.40 × 1013

1.19 × 1014

120

6.0

3.23 × 1013

2.31 × 1014

120

04:00, 13 March

5.0

2.15 × 1013

2.22 × 1014

120

09:00, 13 March

3.0

2.60 × 1013

3.04 × 1014

120

12:00, 13 March

1.0

3.80 × 1013

4.48 × 1014

120

13:00, 13 March

2.0

4.95 × 1013

6.40 × 1014

120

8.0

3.00 × 1013

3.61 × 1014

120

23:00, 13 March

3.0

8.20 × 1012

1.01 × 1014

120

02:00, 14 March

1.0

6.30 × 1012

7.81 × 1013

120

03:00, 14 March

4.0

4.40 × 1012

5.49 × 1013

120

4.0

3.50 × 1012

4.43 × 1013

120

11:00, 14 March

1.0

1.86 × 1014

2.30 × 1015

100 × 100 × 300a

12:00, 14 March

6.0

1.80 × 1012

2.33 × 1013

20

18:00, 14 March

1.0

1.10 × 1012

1.44 × 1013

20

19:00, 14 March

1.0

1.00 × 1012

1.32 × 1013

20

1.0

1.00 × 1012

1.40 × 1013

20

1.0

2.90 × 1013

3.50 × 1014

20

22:00, 14 March

1.0

1.20 × 1012

1.60 × 1013

20

23:00, 14 March

1.0

1.25 × 1012

1.80 × 1015

20

0:00, 15 March

1.0

1.30 × 1012

1.80 × 1013

20

1.0

6.60 × 1013

5.20 × 1014

20

02:00, 15 March

1.0

1.00 × 1014

3.00 × 1014

20

03:00, 15 March

1.0

1.40 × 1014

5.50 × 1014

20

04:00, 15 March

3.0

1.40 × 1013

2.00 × 1014

20

3.0

7.77 × 1013

1.23 × 1015

20

1.0

7.90 × 1013

1.30 × 1015

20

11:00, 15 March

5.0

1.06 × 1013

1.54 × 1014

20

16:00, 15 March

2.0

5.15 × 1013

5.45 × 1014

20, 120

18:00, 15 March

2.0

2.60 × 1014

1.99 × 1015

20, 120

2.0

7.20 × 1013

6.80 × 1014

20, 120

1.0

3.40 × 1014

2.10 × 1016

20, 1201.0

1.90 × 1013

2.90 × 1014

20, 120

09:00, 12 March
10:00, 12 March

22:00, 12 March

15:00, 13 March

07:00, 14 March

20:00, 14 March
21:00, 14 March

01:00, 15 March

07:00, 15 March
10:00, 15 March

20:00, 15 March
22:00, 15 March
23:00, 15 MarchANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Release period

Release rate (Bq/h)

Duration (h)00:00, 16 March

1.0

1.27 × 1013

2.80 × 1014

20, 12001:00, 16 March

5.0

6.64 × 1012

2.66 × 1014

20, 12006:00, 16 March

3.0

1.83 × 1013

3.23 × 1014

20

2.0

2.60 × 1014

2.70 × 1015

20

1.0

1.20 × 1013

2.10 × 1014

20

12:00, 16 March

1.0

1.50 × 1013

2.29 × 1014

20

13:00, 16 March

1.0

2.90 × 1013

4.44 × 1014

20

14:00, 16 March

1.0

5.00 × 1013

7.69 × 1014

20

15.0

6.20 × 1013

9.47 × 1014

20

06:00, 17 March

15.0

3.10 × 1013

5.08 × 1014

20

21:00, 17 March

3.0

3.00 × 1013

5.10 × 1014

20

00:00, 18 March

5.0

2.10 × 1013

3.60 × 1014

20

05:00, 18 March

3.0

8.15 × 1013

2.10 × 1015

20

5.0

1.98 × 1013

2.88 × 1015

20

13:00, 18 March

5.0

9.71 × 1013

2.89 × 1015

20

18:00, 18 March

11.0

5.80 × 1013

1.50 × 1015

20

05:00, 19 March

10.0

9.90 × 1013

1.91 × 1015

20

19.0

6.48 × 1012

7.69 × 1013

20

10.0

2.18 × 1013

3.37 × 1014

20

20:00, 20 March

12.0

3.70 × 1013

2.25 × 1014

20

08:00, 21 March

4.0

4.38 × 1012

1.03 × 1014

20

12:00, 21 March

4.0

4.10 × 1011

1.18 × 1014

20

5.0

2.80 × 10

12

13

20

21:00, 22 March

26.0

1.07 × 1012

2.92 × 1014

20

23:00, 22 March

25.0

2.24 × 1012

5.74 × 1014

20

00:00, 24 March

24.0

6.31 × 1011

5.13 × 1013

20

00:00, 25 March

35.0

2.20 × 1012

1.26 × 1014

20

47.0

6.91 × 1011

6.46 × 1013

20

10:00, 28 March

35.0

3.00 × 1012

1.21 × 1013

20

21:00, 29 March

14.0

5.87 × 1012

4.26 × 1013

20

11:00, 30 March

13.0

4.32 × 1013

2.66 × 1014

20

22.0

2.70 × 1012

7.95 × 1013

20

2.0

8.91 × 1011

5.30 × 1012

20

11:00, 16 March

15:00, 16 March

08:00, 18 March

15:00, 19 March
10:00, 20 March

16:00, 21 March

11:00, 26 March

00:00, 31 March
22:00, 31 March

Volume source dimension in the directions x, y, and z resulting from a hydrogen explosion.

131I

Release height (m)Start time (JST)

137Cs

09:00, 16 Marcha9

5.10 × 10

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

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IV. ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITIONMODELLING SIMULATIONS OF AIRBORNE RADIOACTIVITY
8.
Table A-9.3 provides details of the model parameters used in the recent ATDM carriedout by Terada et al. [Terada et al., 2020] and used by the Committee for its updated dose
assessment, and provides a comparison with the parameters and assumptions made in the earlier
source term and ATDM used by the Committee in the UNSCEAR 2013 Report [UNSCEAR,
2014].
9.
Unlike in the UNSCEAR 2013 Report [UNSCEAR, 2014], the Committee has used thesame ATDM of Terada et al. [Terada et al., 2020] to estimate concentrations of radionuclides in
the air as was used to derive the source term from the available measurement data using reverse
modelling. The ATDM used by Terada et al. was that in WSPEEDI, with a new meteorologicalmodel (WRF) and data assimilation method [Terada et al., 2020]. ATDM results have been
provided to the Committee for two nested grids, the finer grid covering most of Fukushima
Prefecture with a grid size of 1 × 1 km2 (local model), the coarse grid covering Fukushima
Prefecture and all neighbouring prefectures with a grid size of 3 × 3 km2 (regional model) [Terada
et al., 2020]. ATDM results have been provided for the radionuclides 131I, 132Te, 134Cs and 137Cs.
Table A-9.3. Description of atmospheric transport, dispersion and deposition modelling used in
the UNSCEAR 2013 Report and in this update
Update – this study(regional model)

UNSCEAR 2013 Report[UNSCEAR, 2014]

Source term

[Terada et al., 2012][Terada et al., 2020][Terada et al., 2020]

ATDM-meteorological
model

NOAA(HYSPLIT) GDASaWSPEEDI-WRF

WSPEEDI-WRF

Area

approx.
3 000 × 2 000 km

36.310° N – 38.024° N
139.384° E – 141.506° E

34.416° N – 39.553° N
137.805° E – 143.989° E

Height range

0–>10 km

0–9 306 m

0–9 306 m# of grid cells

601 × 401

187 × 187

187 × 187

# of height levels

56 layers;
10 within the lowest 1 km

28

28

Grid size

5 × 5 km2

1 × 1 km2

3 × 3 km2

Time period (JST)03:00, 12 March 2011 to
06:00, 4 April 2011

16:00, 12 March 2011 to
15:00, 31 March 2011

16:00, 11 March 2011 to
15:00, 31 March 2011

Time resolution

3 hours

1 hour

1 hourRadionuclides

137Cs, 136Cs, 134Cs, 131I,

137Cs, 134Cs, 132Te, 131I (total +137Cs, 134Cs, 132Te, 131I (total +132I, 133I (for all iodines:

separately for 3 iodine forms(elemental, organic,
particulate))

separately for 3 iodine forms(elemental, organic,
particulate))- Precipitation intensity- Wind speed (U-/V-direction)- Air concentration- Total deposition- Dry deposition- Wet deposition- Fog deposition- Precipitation intensity- Wind speed (U-/V-direction)- Air concentration- Total deposition- Dry deposition- Wet deposition- Fog deposition

separately for gaseous(elemental and organic
combined) and particulate
forms), 132Te, 133Xe
Resultsa

Update – this study(local model)Modelparameter/assumption- Air concentration- Total deposition

United States National Oceanic and Atmospheric Administration (NOAA) ATDM (Hybrid Single-Particle Lagrangian Integrated Trajectory
Model (HYSPLIT)) combined with Global Data Assimilation System (GDAS) meteorological model.

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

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V. ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITIONMODELLING: RESULTS FOR CONCENTRATIONS OF
RADIONUCLIDES IN THE AIR
10.
Figures A-9.I to A-9.IV (left side: local model, right side: regional model) show thecalculated time-integrated concentrations of 131I (total of all three forms), 132Te, 134Cs and 137Cs
in air for the period from 11 March to 31 March 2011, as derived from the time-dependent air
concentrations calculated by the ATDM of [Terada et al., 2020]. These maps demonstrate that asignificant proportion of the releases of radionuclides to atmosphere were dispersed over theocean.
Figure A-9.I. Time-integrated concentration of total 131I in air for the period from 11 March to
31 March 2011, derived from the atmospheric transport, dispersion and deposition modelling
results of [Terada et al., 2020]

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ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Figure A-9.II. Time-integrated concentration of 132Te in air for the period from 11 March to
31 March 2011, derived from the atmospheric transport, dispersion and deposition modelling
results of [Terada et al., 2020]

Figure A-9.III. Time-integrated concentration of 134Cs in air for the period from 11 March to
31 March 2011, derived from the atmospheric transport, dispersion and deposition modelling
results of [Terada et al., 2020]

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

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Figure A-9.IV. Time-integrated concentration of 137Cs in air for the period from 11 March to
31 March 2011, derived from the atmospheric transport, dispersion and deposition modelling
results of [Terada et al., 2020]

VI. COMPARISON OF MODELLED AND MEASURED DEPOSITIONLEVELS
11.
The ATDM results can be compared with measured levels of radionuclides in theenvironment to validate the model. Figures A-9.V and A-9.VI show a comparison of the ATDM
results for the deposition densities of 137Cs and 131I with the measurements from the JAEAEMDB for the reference date of 14 June 2011 [Saito and Onda, 2015]. On average, the ATDM
model better reproduces the deposition density of 137Cs than that of 131I. Perfect matching ofmodelled and measured deposition levels would result in a diagonal line as shown infigure A-9.V. For 137Cs, the data in figure A-9.V (left side) are evenly distributed around such aline, with 49% of the data points below and 51% above. For 131I, many more (83%) data points
are below the line indicating that the ATDM has underestimated the measured deposition levels.
This finding is also supported by the median value of the ratio of modelled and measured
deposition densities, which is 1.05 for 137Cs and only 0.33 for 131I (geometric mean is 0.86 for137
Cs and 0.64 for 131I). For 137Cs, about 34% of the modelled deposition densities are within afactor of 2 of the measured values (30% for 131I), about 68% within a factor of 5 (65% for 131I)
and about 84% within a factor of 10 (84% for 131I). It is difficult to assess the uncertainty of themodel predictions of air concentration from this comparison of deposition data, as the deposition
model in the ATDM and the uncertainty in the meteorological data regarding precipitation bringin significant additional uncertainty.
12.
It can be seen from figure A-9.VI that there are significant differences in the performanceof the ATDM depending on the location: while the modelled deposition densities match themeasurements quite well along the coast (indicated by green colours; especially visible for 131I),
the model underestimate deposition densities to the north-west of FDNPS, in the area of

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ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Fukushima City, in the Nakadori valley and in the western parts of Fukushima Prefecture (bluecolours; in all these areas the underestimation is larger for 131I than for 137Cs). Especially for137
Cs, the model tends to overestimate the deposition densities in the mountainous regions westof FDNPS and also south of FDNPS (orange colours).
Figure A-9.V. Comparison of modelled and measured deposition levels of 137Cs and 131I

Figure A-9.VI. Ratio of modelled and measured deposition levels for 137Cs (left side) and 131I
(right side)ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

15

VII. ESTIMATION OF CONCENTRATIONS OF RADIONUCLIDES IN
AIR FOR DOSE ASSESSMENT
13.
The Committee has used two different methods to estimate concentrations ofradionuclides in air for the purposes of assessing doses to the public. The first method was based
solely on the results of the ATDM provided by Terada et al. [Terada et al., 2020]. This approach
was used for estimating air concentrations in areas of Fukushima Prefecture that were evacuated,because, when estimating doses to those members of the public who were evacuated, information
was needed on the concentration of radionuclides in the air as a function of time.
14.
The second method was based on the estimated time-integrated concentrations in air
derived from the measured deposition densities of radionuclides by dividing by the ratio (the“bulk deposition velocity”) of the deposition density to the time-integrated concentration in air
estimated from the ATDM results as a function of location. While the estimates of radionuclide
concentrations in air and deposition densities of radionuclides provided by the assumed source
term and ATDM simulations at any specific location are associated with significant uncertainties,
the ratios of these two estimates are much less so. In particular, the ratios are relatively insensitive
to the absolute magnitude and temporal pattern of the estimated release of the radioactive
material, which is associated with much uncertainty. The main uncertainties in these ratios result
from uncertainties in the parameters that describe wet and dry deposition. The Committee usedsuch location-dependent ratios, derived from the ATDM analyses, to infer time-integrated
radionuclide concentrations in air from the measured deposition densities of radionuclides in all
regions of Japan except in the evacuated areas.
15.
The “bulk deposition velocities” were estimated from the ATDM results for theradionuclides in the plume and deposited on the ground as a function of time and location, using
the local model of Terada et al. (with a spatial resolution of 1 × 1 km) within Fukushima
Prefecture; in other prefectures, average values for the “bulk deposition velocities” using theregional model (with a spatial resolution of 3 × 3 km) were used. The “bulk deposition velocities”
applied were based on the ratios of the estimated time-integrated activity concentrations in air tothe deposition densities of the radionuclides. This approach was applied to non-evacuatedsettlements of Fukushima Prefecture and the rest of Japan.
16.
The ATDM results provided by Terada et al. include time-dependent concentrations in
air and deposition densities for the radionuclides 132Te, 131I, 134Cs and 137Cs. For 131I, threechemical forms have been considered: inorganic particulate, elemental vapour and organic, using
the constant ratios, based on experimental and other evidence, of 0.5 for 131Iparticulate/131Itotal,
0.2 for 131Ielemental/131Itotal and 0.3 for 131Iorganic/131Itotal. Concentrations in air and deposition
densities of 132I were derived by assuming it to be in secular equilibrium with its parent, 132Te(a reasonable assumptions given its much shorter half-life of 2.3 hours compared to the half-lifeof 132Te of 3.2 days). The short-lived radionuclide 133I was also not considered in the ATDM
model from Terada et al. [Terada et al., 2020], and thus the activity concentration in air of 133I
had to be estimated from the calculated concentrations of 131I in air by applying a time-dependent(to consider the different half-lives of these iodine isotopes) radionuclide ratio based on the
133 131
I: I ratio at 14:46 on 11 March 2011 from Nishihara et al. [Nishihara et al., 2012]. The
fractions of 132I and 133I in different physico-chemical forms (particulate, elemental and organic
iodine) were assumed to be the same as those for 131I. Other shorter lived radionuclides (e.g.,
129
Te, 129mTe, 131mTe, 135I, 136Cs) have not been included; their contribution to the total dose frominhalation would be small in comparison to the radionuclides that have been included (see e.g.,[Ohba et al., 2017]). It was not possible to estimate the air concentrations of noble gases,

16

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

specifically of 133Xe, from the ATDM results; the implications of this for the assessment of doses
is addressed in attachment A-10.
17.
The results of the estimation of concentration of radionuclides in air using the deposition
scaling approach are illustrated in the following four figures. Concentrations of radionuclides in
air – as shown in figures A-9.VII and A-9.VIII for 137Cs and 131I, respectively – were derived
from the deposition measurement data from the JAEA EMDB data sets [Saito and Onda, 2015],
in particular, the 2,200-location soil deposition data set for 137Cs (see attachments A-6 to A-8).
Bulk deposition velocities – as shown in figures A-9.IX and A-9.X – were derived from theresults of the ATDM local model by calculating the ratio of deposition density to time-integrated
concentration in air.
Figure A-9.VII. Concentration in air for 137Cs (time-integrated) estimated from deposition
measurements using the deposition scaling approachANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Figure A-9.VIII. Concentration in air for 131I (time-integrated) estimated from deposition
measurements using the deposition scaling approach

17

18

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Figure A-9.IX. Bulk deposition velocity for 137Cs estimated from atmospheric transport,
dispersion and deposition modelling (i.e., ratio of the modelled deposition density to themodelled time-integrated concentration in air)

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

19

Figure A-9.X. Bulk deposition velocity for 131I estimated from atmospheric transport, dispersion
and deposition modelling (i.e., ratio of the modelled deposition density to the modelled timeintegrated activity concentration in air)VIII. COMPARISON OF MODELLED AND MEASURED AIR
CONCENTRATION FOR 137CS
18.
A comparison is made in this section between modelled and measured time-integrated
concentrations of 137Cs in air over the period when the main releases from the FDNPS accident
occurred (12–22 March 2011). The modelled air concentrations used in the comparison havebeen derived using two different approaches (directly from ATDM or indirectly using thedeposition scaling approach).

20

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

19.
Time dependent, concentrations of 137Cs (and also of 129I, from which levels of 131I canbe inferred) in air at ground-level in Fukushima Prefecture and several other prefectures (Chiba,
Ibaraki, Kanagawa, Miyagi, Niigata, Saitama and Yamagata), and in the larger Tokyo
Metropolitan Area have been measured (and are still being measured in ongoing research
projects) from analyses of filter-tapes of air pollution stations for monitoring suspended
particulate matter [Ebihara et al., 2019; Oura et al., 2015; Tsuruta et al., 2014; Tsuruta et al.,2018]; it should be noted, however, that only iodine in particulate form would have beencollected on the filter tapes. The latest version of the data set includes information on 137Cs levels
for 101 locations, with 23 located within, or close to the border of, Fukushima Prefecture (seefigure A-9.XI). Only measurements at these 23 locations have been considered in thecomparison. No other measurements of 137Cs in air within the Fukushima Prefecture are known
to the Committee that provide a continuous time series over the full period where the main
releases of the FDNPS accident occurred (12–22 March 2011). Only with such “complete” timeseries of measurements can meaningful comparisons be made between modelled and measured
air concentrations. There were also two locations within Fukushima Prefecture where
measurements of 131I in air are available, but these did not cover the time periods of interest forthe dose assessment.
Figure A-9.XI. Locations within or close to the border of Fukushima Prefecture where 137Cs air
concentrations have been measured

20.
A comparison of measured and modelled 137Cs time-integrated concentrations in air ateach of the 23 locations within or close to Fukushima Prefecture is provided in table A-9.4. The
time-integrated air concentrations estimated from ATDM using deposition scaling are within afactor of two of the measured time-integrated air concentration at six out of the 23 monitoring
locations, while this is the case for only three of the monitoring locations if the time-integrated
air concentrations estimated directly using ATDM are used. Similarly, time-integrated air
concentrations estimated using deposition scaling are within factors of five and ten of themeasured air concentrations at 13 and 17 monitoring locations, respectively, whereas theANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

21numbers are 6 and 16, respectively, for time-integrated air concentrations estimated directly
using ATDM. The time-integrated concentrations estimated directly by ATDM are higher atsome locations than the measured concentrations and lower at others, but are generally lower bylarge factors (an order of magnitude and more) at the lower concentrations; at higher
concentrations (3 × 106 Bq s/m3 and above), the ATDM estimates are more likely to be higher
than the measured concentrations by factors of a few. The concentrations estimated using ATDM
and deposition scaling also tend to be lower than the measured concentrations at the lower
concentrations, but provide a much closer match to the measurements at higher concentrations.
This comparison confirms that the deposition scaling approach provides a generally better
estimate of the time-integrated air concentration of 137Cs. As the measurements are not uniformly
distributed across Fukushima Prefecture and their number is still relatively limited, they cannot
be considered as being fully representative for the whole prefecture and thus no general
conclusion can be drawn about the extent of any possible over- or underestimation of the timeintegrated air concentration derived using ATDM.
Table A-9.4. Comparison of modelled and measured concentrations of 137Cs in air (timeintegrated)

No.

Location

137Cs measurement

137Cs concentrations(Bq s/m3)

derived directly fromATDM (Bq s/m3)

137Cs concentration
derived from ATDM anddeposition scaling(Bq s/m3)

1

Aizuwakamatsu

1E+05

8E+02

8E+03

2

Asahi

2E+06

2E+05

2E+05

3

Daishin

2E+06

2E+05

8E+05

4

Furukawa

2E+06

3E+05

5E+06

5

Futaba Town

2E+08

1E+09

2E+08

6

Haramachi

2E+07

4E+07

3E+07

7

Kitakata

1E+05

4E+02

9E+03

8

Minami-aizu

4E+05

2E+04

2E+04

9

Minamimachi

2E+06

2E+05

5E+06

10

Moriai

2E+06

2E+05

5E+06

11

Muikamachi

1E+03

8E+03

2E+04

12

Naraha

7E+07

2E+08

1E+07

13

Nihonmatsu

3E+06

1E+05

2E+06

14

Shibata

3E+05

2E+06

2E+06

15

Shinchi

3E+06

2E+07

1E+07

16

Shirakawa

2E+06

2E+05

1E+06

17

Shiroishi

1E+06

7E+05

3E+05

18

Soma

1E+07

2E+07

7E+06

19

Sugitsumacho

2E+06

3E+05

5E+06

20

Sukagawa

3E+06

4E+05

2E+05

21

Tanakura

1E+06

3E+05

2E+06

22

Yabuki

2E+06

4E+05

3E+05

23

Yonezawa-kanaike

5E+05

4E+03

8E+04

22

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

21.
A comparison of modelled and measured air concentrations is also shown in figure A-9.XII;
modelled concentrations based directly on ATDM are shown as red triangles and those based on
deposition scaling as blue circles. The figure shows the generally better agreement withmeasurements obtained using deposition scaling, with estimates distributed both above andbelow the corresponding measured values. On the other hand, using ATDM directly to estimate
time-integrated air concentrations, results in estimates that are less close to the measurements
and, at the higher air concentrations, generally higher than the measured values. At lower air
concentrations neither method provides a good fit with the measurements, with estimated values
being generally below the measurements.
Figure A-9.XII. Comparison of modelled and measured concentration of 137Cs in air (timeintegrated) (red triangles – estimates based directly on atmospheric transport, dispersion anddeposition modelling; blue circles – estimates based on atmospheric transport, dispersion anddeposition modelling and deposition scaling)

22.
The source term and ATDM results have been used directly to provide air concentrations
as a function of time for estimating doses to evacuees. A more relevant comparison in this casemay be provided by considering the air concentration integrated over a shorter time period andmeasurements made at monitoring stations in or near to the evacuated locations, specificallyFutaba, Haramachi and Naraha. The ratios of the modelled air concentration to the measurements
at these three stations integrated over the time period of 12–15 March 2011 (when most of theANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

23evacuations took place) are 1.5, 2.9 and 3.0, respectively. This again shows the overestimationof the modelled air concentrations compared with the measurements at the higher concentrationsin these locations. However, it also indicates somewhat better agreement (less than or equal to afactor of three) between the modelled air concentrations and the measurements in the evacuatedareas, if consideration is limited to the time periods and locations of greater relevance to theevacuees.
23.
Figures A-9.XIII and A-9.XIV show further comparisons of measured and modelled
137
Cs concentrations in air (the former of concentrations estimated from deposition scaling andthe latter of modelled concentrations based directly on ATDM). The two modelling approaches
perform differently (assuming performance is to be assessed by the level of agreement with the
air concentration measurements) in different areas of Fukushima Prefecture. Both modelling
approaches reproduce the measured (time-integrated) air concentrations of 137Cs quite well along
the coast of Fukushima Prefecture. The direct ATDM approach, however, significantly
underestimates the concentrations in the central (Nakadori valley) and western parts of the
Fukushima Prefecture, sometimes by orders of magnitude; the deposition scaling approach
performs better in these areas but differences between the modelled and measured concentrations
are evident in the western part of the prefecture.
Figure A-9.XIII. Comparison of modelled and measured concentrations in air for 137Cs(time-integrated) – using the deposition scaling approach

24

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

Figure A-9.XIV. Comparison of modelled and measured concentrations in air for 137Cs (time-integrated)using atmospheric transport, dispersion and deposition modelling results directly

ANNEX B, ATTACHMENT A-9: ATMOSPHERIC TRANSPORT, DISPERSION AND DEPOSITION MODELLING …

25

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