United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENT A-1

THE ESTIMATION OF EXTERNAL DOSES FROMDEPOSITED RADIONUCLIDES FOLLOWING THE
ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR
POWER STATION AND THEIR VALIDATIONUNSCEAR 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 describes the model used to estimate external doses from depositedradionuclides in the Committee’s update of the doses estimated in the UNSCEAR 2013 Report.
The model is based on that used in the UNSCEAR 2013 Report but has been improved to takeaccount of extensive monitoring data collected in the years following the accident from car-borne
surveys and those carried out on foot. Comparisons have been made with the results of numerous
personal dosimetry campaigns, carried out at various times and locations within Fukushima
Prefecture, and the model has been validated for wider use within areas of Japan affected by theaccident at the Fukushima Daiichi Nuclear Power Station (FDNPS).
Notes
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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, February 2023 (corrected). 1 All rights reserved, worldwide.
This attachment has not been formally edited.

1

Tables A-1.13, A-1.15 and the title of table A-1.16.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

3

CONTENTS
I. EXTERNAL DOSE ASSESSMENT IN THE UNSCEAR 2013 REPORT .......................... 5
II. RADIATION MONITORING OF AMBIENT DOSE RATES IN JAPAN SINCE
THE ACCIDENT ................................................................................................................... 6
III. REVISION OF THE DOSE ASSESSMENT METHODOLOGY FOR M2020 ................... 7
A. Motivation ........................................................................................................................ 7
B. The computational schema ............................................................................................... 7
C. Radionuclide composition of the deposited radioactive material .................................... 7
D. Dose rate coefficients for external exposure from radioactive material
deposited on the ground ................................................................................................. 11
E. Dynamics of ambient dose rates due to downward migration and weathering .............. 13
F. Dynamics of ambient dose rates in populated areas....................................................... 15
G. Shielding factors for houses ........................................................................................... 17
H. Fractions of time spent in various locations (occupancy factors) .................................. 17
I. Summary of M2020........................................................................................................ 19
IV. VALIDATION OF M2020 FOR DOSES FROM EXTERNAL EXPOSURE .................... 20
A. Scenario A: The Fukushima Health Management Survey ............................................. 20
B. Personal dosimetry studies ............................................................................................. 22
C. Outcomes of the personal dosimetry surveys in Minamisoma City andNaraha Town for 2014–2019 ......................................................................................... 28
D. Summary ........................................................................................................................ 35
E. Variability of individual external doses in Fukushima Prefecture ................................. 36
V. RESULTS OF THE MODEL CALCULATIONS ............................................................... 37
A. Cumulative doses of external exposure per unit 137Cs deposition density ..................... 37
B. Doses from external exposure in the first year following the Fukushima Daiichi
Nuclear Power Station accident ..................................................................................... 39
C. Cumulative doses from external exposure for the population of Japan ......................... 40
D. Post-remediation doses ................................................................................................... 41
VI. SUMMARY ......................................................................................................................... 44
REFERENCES ........................................................................................................................... 47

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

I.

5EXTERNAL DOSE ASSESSMENT IN THE UNSCEAR 2013 REPORT1.
The external dose assessment methodology applied in the UNSCEAR 2013 Report[UNSCEAR, 2014] was largely based on post-Chernobyl experience gathered in territories of
the Republic of Belarus, the Russian Federation and Ukraine contaminated by radioactive
material [Golikov et al., 2002; Jacob, 1996; Likhtarev et al., 2002]. The model applied in the
UNSCEAR 2013 Report had used pragmatic approximations and parameters and is briefly
summarized below.
2.
The effective dose rate 𝐸𝐸̇𝑖𝑖 (𝑡𝑡) for a person from age/social group i at time t after thedeposition of radionuclides was calculated as follows:𝐸𝐸̇𝑖𝑖 (𝑡𝑡) = 𝑟𝑟(𝑡𝑡)𝐴𝐴Cs-137 �
𝑚𝑚

𝐴𝐴𝑚𝑚

𝐴𝐴Cs-137eff
𝑒𝑒 −𝜆𝜆𝑚𝑚𝑡𝑡 𝑑𝑑̇𝑚𝑚,𝑖𝑖
� 𝑓𝑓𝑗𝑗 (𝑡𝑡)𝑝𝑝𝑖𝑖𝑖𝑖(A-1.1)𝑗𝑗

where 𝐴𝐴𝑚𝑚 is the initial deposition density of the radionuclide m at 15 March 2011 (Bq/m2); 𝜆𝜆𝑚𝑚eff
is its radioactive decay rate (s−1); 𝑑𝑑̇𝑚𝑚,𝑖𝑖
is the effective dose rate coefficient for the ith age groupattributed to the initial source configuration immediately after deposition of the radionuclide m(Sv/s)/(Bq/m2); 𝑟𝑟(𝑡𝑡) is a phenomenological function representing the reduction with time of thedose rate in air above undisturbed soil due to migration of the radionuclide into the soil depth,
weathering and runoff (unitless); 𝑓𝑓𝑗𝑗 (𝑡𝑡) is the time-dependent location factor, i.e., a ratio of theambient dose rate in air in a specific location to that above open ground undisturbed from themoment of deposition (unitless); 𝑝𝑝𝑖𝑖𝑖𝑖 (𝑡𝑡) is the fraction of time spent in various locations, termed
here as occupancy factors, representing behaviour of an individual belonging to the age/socialgroup i of the population of interest (unitless).
3.
In the UNSCEAR 2013 Report [UNSCEAR, 2014], the dose rate coefficients used in themodel for calculation of external exposures, hereafter termed as M2013, were essentially basedon the effective dose per air kerma ratios for idealized irradiation geometries from theInternational Commission on Radiological Protection (ICRP) Publication 74 [ICRP, 1996] andthe dose rate coefficients for the environmental sources in soil [Jacob et al., 1986; Petoussi-Hensset al., 2012; Saito and Petoussi-Henss, 2014]. Correspondingly, the effective dose rate
coefficients were taken as constant within each of the three considered age groups: children ofage 1, 10 years and adults. For organ doses, only absorbed dose to the thyroid was inferred fromthe voxel phantom data [Petoussi-Henss et al., 2012] for the same three age groups.
4.
Location factors, used in M2013, represented the effects of external dose reduction invarious anthropogenic or semi-natural environments: paved and unpaved surfaces, various types
of houses or dwellings, etc. The location factors in M2013 were largely based on the postChernobyl experience and the values of their parameters were determined for conditions specificto areas in the Russian Federation and Ukraine contaminated by radioactive material. The
distinctive features of the location factors in M2013 are their dependence on time and asignificant reduction of dose within populated areas in comparison to those undisturbed.
5.
The UNSCEAR 2013 Report [UNSCEAR, 2014] focused on providing dose estimates
for the three representative age groups: 1-year-old infants representing babies and preschool
children, 10-year-old children representing school children and 20-year-old person as an adult.
Based on the housing conditions, the population was assumed to inhabit wooden, wooden
fireproof and concrete houses and buildings. Daily activities outside homes (e.g., nursing, study

6

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

or occupation) were assumed to take place indoors in concrete buildings for preschool and school
children and for indoor (office) workers.
6.
Fractions of time assumed to be spent by different population groups in various locations
were termed occupancy factors in M2013 and these were established based on the information
provided by the Japanese authorities and national statistics [MIC, 2011].
7.
The cumulative effective doses for members of the public were calculated by integrating
the effective dose rate (equation A-1.1), taking into account age-dependent changes of the dose
rate coefficients and the occupancy factors. The integral absorbed dose in the thyroid wascomputed similarly, using appropriate thyroid absorbed dose rate coefficients. The integration
periods were selected to represent the effect of exposure during 1 and 10 years after the accident
as well as during a lifetime, which was defined as the time from the beginning of the exposure
to age 80. That is, for the three selected age groups, i.e., 1 and 10-year-old children, and 20-yearold adults, the lifetime integration periods were 79, 70 and 60 years, correspondingly.

II.

RADIATION MONITORING OF AMBIENT DOSE RATES IN JAPANSINCE THE ACCIDENT

8.
The earlier assessments of external exposures after the accident at FDNPS [UNSCEAR,
2014; WHO, 2012; WHO, 2013] were largely based on previous experience gathered fromradioecological and dosimetric studies after global fallout and the Chernobyl accident. Climate,landscape, soil and vegetation types, population density, land use, agricultural practice, diet andlifestyle in Japan differ significantly from those in the previous studies. Different physicochemical properties of the released and deposited radioactive material, especially different
particulate forms affecting mobility or solubility, also contributed to the specificity of the postaccidental situation in Japan.
9.
Following the FDNPS accident, extensive monitoring had been undertaken in Japantargeting systematic characterization of the concentrations of radionuclides in the natural, seminatural and anthropogenic environments. A summary of this monitoring can be found in the
recent review [Saito et al., 2019b] as well as in section II.B of appendix A of this UNSCEAR
report. The extensive databases gathered due to these monitoring programmes provided substantial
empirical evidence and allowed checks to be made of the plausibility and validity of the methods
used for external dose assessment. At the same time these data were used to establish and justify
alternative assessment methods better suited for characterization of the Japan-specific situation.
10.
The methods applied for monitoring of the ambient dose rates in Japan varied greatly,from stationary monitoring, sampling and in-situ measurements to surveys conducted withmobile (air- and car-borne surveys) or portable (walk survey) instruments. All methods wereapplied systematically and repeatedly thus allowing the creation of a detailed time- and spatialdistribution of ambient dose rate and deposition densities of radionuclides released during theaccident.
11.
Two types of data were especially useful: firstly, data on deposition density and isotopic
composition of radionuclides at specified fixed locations; and, secondly, the observed timedependence of ambient dose rates in various environments including populated areas. The latter
were used to check model assumptions for those parameters governing the environmental
behaviour of radionuclides on the ground; these undergo radioactive decay, redistribution due toprocesses of migration in soil, weathering, runoff and human activities, including agriculturaland industrial practices as well as decontamination and remediation activities.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

7

III. REVISION OF THE DOSE ASSESSMENT METHODOLOGY FOR
M2020
A.

Motivation

12.
Following the UNSCEAR 2013 Report [UNSCEAR, 2014], substantial changes occurredin Japan with regard to the accumulation of data and understanding the impact of the FDNPS
accident on the population and the environment. New data have been obtained and analyses madeof: (a) radionuclide concentrations in the environment; (b) the dynamics of ambient dose rates
and radionuclide behaviour in the environment; (c) active and extensive monitoring andremediation activities, complemented by improvement of the radiological situation; and(d) return of people to previously evacuated places. The studies undertaken in Japan since 2011
have shown a high degree of compatibility with previous scientific findings from decades ofresearch undertaken globally; but also significant differences in some Japan-specificcircumstances. Additionally, new dosimetric data and models, recommended by ICRP [ICRP,
2020], have become available for the assessment of external doses following an accident.
13.
An improved, Japanese-specific model for estimating external doses from depositedradionuclides, hereafter referred to as M2020, has been developed to take account of these newdata and related developments and is described below.
B.

The computational schema

14.
The computational schema adopted for M2020 remains essentially the same as that in the
UNSCEAR 2013 Report [UNSCEAR, 2014] and can be described by the following equation
similar to equation A-1.1 above:𝐷𝐷̇ (𝑡𝑡|𝑎𝑎) = 𝑟𝑟(𝑡𝑡)𝐴𝐴Cs-137 � 𝜌𝜌𝑚𝑚 𝑒𝑒 −𝜆𝜆𝑚𝑚𝑡𝑡 𝑑𝑑̇𝑚𝑚 (𝑎𝑎 + 𝑡𝑡) � 𝑓𝑓𝑗𝑗 (𝑡𝑡)𝑝𝑝𝑗𝑗 (𝑎𝑎 + 𝑡𝑡)
𝑚𝑚(A-1.2)𝑗𝑗

where 𝐷𝐷̇(𝑡𝑡|𝑎𝑎) is either effective or organ equivalent dose rate for an individual of age a at the𝐴𝐴
beginning of exposure; 𝜌𝜌𝑚𝑚 = 𝐴𝐴 𝑚𝑚 is the ratio of deposition density of the radionuclide m to
Cs-137that of 137Cs on 15 March 2011 (unitless); 𝑑𝑑̇𝑚𝑚 (𝑎𝑎 + 𝑡𝑡) is the corresponding age-dependent dose
rate coefficient for radionuclide m including also the effect of its radioactive progeny; 𝑝𝑝𝑗𝑗 (𝑎𝑎 + 𝑡𝑡)
is the fraction of time spent by the individual of age 𝑎𝑎 + 𝑡𝑡 in the location j; the other parameters
remain unchanged and have the same meanings as in equation A-1.1.
C.

Radionuclide composition of the deposited radioactive material

15.
The calculation of external doses was performed using population-weighted averagedeposition densities of radionuclides. The data were based on environmental monitoring andwere structured in a similar manner to that in the UNSCEAR 2013 Report [UNSCEAR, 2014].
The detailed information can be found in section II.B of appendix A of this report. The population
data used for averaging were taken from the results of the Japan 2010 census [MIC, 2011].
16.
The data for 2,148 locations aligned and distributed over a 2-km spatial grid were
provided by the Japan Atomic Energy Agency [Saito et al., 2019a]. The data consisted of themeasured deposition densities of 137Cs, 134Cs, 131I, 110mAg, 129mTe, 89Sr, 90Sr, 238Pu, 239Pu and8

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

240

Pu. Such detailed data were available for the 59 municipalities of Fukushima Prefecture andfor neighbouring prefectures: 23 municipalities of Miyagi Prefecture, 5 municipalities of
Yamagata Prefecture, 5 municipalities of Tochigi Prefecture and 6 municipalities of Ibaraki
Prefecture. All 2,148 locations had measurements of 137Cs deposition density. However,measured deposition densities for other radionuclides were not available for all locations.
17.
The data set of deposited radionuclides for each municipality was further modified. First,
for 799 data points the concentration of 131I was reconstructed from measurements of 129I in theenvironment [Muramatsu et al., 2015]. The total number of locations with 131I deposition density
levels was 1,297, of which 799 were based on the 129I measurements. When both measured andreconstructed estimates were available at the same location, the reconstructed value was adopted
as potentially more accurate.
18.
As in the UNSCEAR 2013 Report [UNSCEAR, 2014], the measured deposition densities
of shorter-lived radionuclides were not available in all locations. Where measurements werelacking, the deposition densities were estimated using observed radionuclide ratios [UNSCEAR,
2014] in two geographical areas: the so-called South trace, which was formed on 15 March 2011
mainly due to dry deposition, and the rest of the country [IAEA, 2015d]. The ratios as of
15 March 2011 assumed for deposition densities of radionuclides are shown in table A-1.1 andare, essentially, the same values as used in the UNSCEAR 2013 Report [UNSCEAR, 2014].
Table A-1.1. Initial ratios of deposition density at 15 March 2011 used to calculate values for
shorter-lived radionuclides from that of 137Cs
Deposition density relative to that of 137Cs
Area

137

134

136

All of Japan
excluding the South trace b

1.0

1.0

0.17

8.3–37c

The South trace

1.0

1.0

0.17

25–250c

Cs

Cs

Cs

131

I

129m

Te (132I)a110m

1.1–1.9c

7.6–13c

0.0028

1.7–28c

12–190c

0.0028

Te

132

Ag

a Deposition density of the daughter 132I was assumed equal to that of the parent 132Te at the time of deposition.
b The towns of Naraha, Hirono, Yamatsuri, Iwaki City of Fukushima Prefecture, the towns of Kitaibaraki, Takahagi of Ibaraki

Prefecture.
c Non-linear relationships were applied for deposition density ratios. See text, equation A-1.2 and table A-1.2.

19.
In M2013, the initial ratios of deposition densities for all radionuclides were assumed tobe constant across the selected territories and assessed by simply multiplying the initialdeposition density of 137Cs by the respective ratio (see equation A-1.1) [UNSCEAR, 2014]. Thisapproach was also applied in M2020 for 134,136Cs and 110mAg. For 131,132I and 129m,132Te, adifferent approach was used. Based on measured deposition densities of 131I and 129mTe,
phenomenological non-linear approximations were used to estimate deposition densities of theseradionuclides for locations without measured data, both in the South trace and in the rest ofJapan. These measured and approximated values were used for calculation of external doses in
populated areas.
20.
The initial deposition densities of 131I and 129mTe plotted versus the initial deposition
density of 137Cs are shown in figures A-1.I and A-1.II; the red dots and lines represent data for
the South trace and the blue dots and lines represent data for the rest of Japan. Despite their highvariability, the data points for the two areas seem to systematically differ from each other;
especially, when plotted on log-log scale. For 129mTe, this effect is more pronounced. A simple
linear approximation of the log-transformed data resulted in the following phenomenologicalnon-linear approximation for deposition density ratios of either 131I or 129mTe to that of 137Cs:

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

𝜌𝜌𝑥𝑥 = 𝑎𝑎 𝐴𝐴𝑏𝑏Cs-137

9(A-1.3)where x stands for either 131I or 129mTe, 𝐴𝐴Cs-137 is the initial deposition density of 137Cs (kBq/m2),
and the coefficients a and b (unitless), obtained via a non-linear model fit, are shown intable A-1.2. Shaded areas in figures A-1.I and A-1.II indicate the 95% confidence interval of thefit, defined by the uncertainty of the fitted parameters, not the prediction confidence interval that
represents the sample variance. The results of the fit suggest different regression slopes for thedata for the South trace and for the rest of Japan. Consequently, a straightforward extrapolationof the approximation (equation A-1.3) beyond the range supported by the data may result in anunderestimation, for the South trace for high values of 137Cs deposition density. Therefore, forpractical application, it has been assumed that the maximum of the two values of the deposition
density ratio calculated for the South trace and for the rest of Japan should be used for locations
within the South trace. The applicability ranges of equation A-1.3 for different radionuclides andgeographical areas are also shown in table A-1.2.
Table A-1.2. Parameters of non-linear relationships between deposition densities of 131I, 129mTeand 137Cs (see equation A-1.3)
Parameters in equation A-1.3
Radionuclide
131

I

129m

Te

The South trace

232

339.6

−0.473

Applicable in range
𝐴𝐴Cs-137 (kBq/m2)

Rest of Japan

1 064

37.31

−0.163

1–10 000The South trace

304

22.02

−0.441

0.6–320

Rest of Japan

898

1.865

−0.059

1–10 000

Area

N

𝑎𝑎

𝑏𝑏

2–250

Figure A-1.I. Relationship between deposition densities of 137Cs and 131I for the sampling locations in
the South trace (red dots) and in the rest of the country (blue dots). Corresponding phenomenologicalapproximations fitted using equation A-1.3 are shown as lines and their 95% confidence intervals areindicated by the shaded areas

10

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Figure A-1.II. Relationship between 137Cs and 129mTe deposition densities for sampling locationsin the South trace (red dots) and the rest of the country (blue dots). Corresponding phenomenologicalapproximations fitted using equation A-1.3 are shown as lines and their 95% confidence intervals
are indicated by the shaded areas

21.
The radionuclides shown in table A-1.3 were considered as contributors to external
exposure from radioactive material deposited on the ground. For these radionuclides, thecontribution of radioactive progeny has been either assumed to be in secular equilibrium or
ignored due to their low decay energy yield or high mobility and removal from the environment(noble gases).
22.
External doses for remote prefectures were estimated from the deposition density of 137Cs
and the deposition density ratios of each radionuclide at a representative location in eachprefecture.
Table A-1.3. Properties of the radionuclides deposited on the ground and their radioactive
progeny [ICRP, 2008]
Parent radionuclide
Name
110m

Ag

129m

Te

132

Te

Radioactive progeny

Half-life (d)

f (%)

Name

Half-life (d)Equilibrium
applicable?

249.8

1.36

110

2.847×10−4

yes

64

129

0.048

yes

36

129

5.734×109

no

3.204

100

132

I

0.096

yes

1.18

131m

Xe

11.84

no

Yes b

33.6

131

I

8.021

132

I

0.096

133

I

0.867

Ag
Te
I

Negligible?

Yes a

none
2.89

133m

Xe

2.19

no

Yes b

97.11

133

Xe

5.243

no

Yes b

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Parent radionuclide
Name

Half-life (d)Radioactive progeny
f (%)

Name

134

Cs

754.2

none

136

Cs

13.16

none

137

Cs

1.102×104

137m

94.40

Ba

Half-life (d)Equilibrium
applicable?

1.772×10−3

yes

11

Negligible?

a Contribution to dose is negligible due to long half-life and low energy yield [ICRP, 2008].
b Noble gas with fast removal.

D.

Dose rate coefficients for external exposure from radioactive material
deposited on the ground23.
The recent ICRP Publication 144 [ICRP, 2020] provided a comprehensive set of dose
rate coefficients for external exposure to environmental sources. It presented dose rate
coefficients for air kerma, ambient dose equivalent and effective dose as well as equivalent organ
doses for six ages and both sexes (only adults). For ground sources, the radionuclides wereconsidered to be in the form of planar sources located at various depths in soil and volume
sources exponentially distributed in soil depth with relaxation mass per unit area ranging from
0.5 to 100 g/cm2.
24.
Similar to the approach used in M2013, the “effective” planar source at mass depth
0.5 g/cm2, combined with the phenomenological function 𝑟𝑟(𝑡𝑡) (see equations A-1.1 and A-1.4),
was taken as the basis for calculating external doses after the accident.

25.
Values of the free-in-air kerma rate 𝐾𝐾̇𝑎𝑎𝑎𝑎𝑎𝑎 coefficients (µGy/h)/(MBq/m2) and the ambient
dose equivalent rate 𝐻𝐻̇ ∗ (10) coefficients (µSv/h)/(MBq/m2) at a height of 1 metre above theground from an infinite planar radionuclide source located in soil at a mass depth of 0.5 g/cm2
for the list of radionuclides in table A-1.3 are shown in table A-1.4, where the dose rate
coefficients are for an equilibrium mixture of the parent and the daughter radionuclides.
Table A-1.4. Free-in-air kerma rate coefficients and ambient dose equivalent rate coefficients for
an infinite planar radionuclide source at a depth of 0.5 g/cm2 in soil [ICRP, 2020]
𝐾𝐾̇𝑎𝑎𝑎𝑎𝑎𝑎(µGy/h)/(MBq/m2)

𝐻𝐻̇ ∗ (10)(µSv/h)/(MBq/m2)

129m

Te+

0.24

0.30

132

Te+

7.56

9.43

131

I

1.17

1.57

132

I

6.87

8.45

133

I

1.87

2.37

134

Cs

4.78

5.92

136

Cs

6.28

7.70

137

Cs+

1.75

2.17

Radionuclide a
110m

Ag+

8.25

10.0

a The “+” sign indicates that the dose coefficient includes a contribution from radioactive progeny under equilibrium conditions.

26.
The ICRP Publication 144 [ICRP, 2020] is the first to present ICRP-recommended
effective dose rate coefficients for radionuclide sources in soil. Following the general ICRP
methodology [ICRP, 1975; ICRP, 1996; ICRP, 2002; ICRP, 2010] and the tissue weighting

12

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

factors [ICRP, 2007], the new ICRP effective dose rate coefficients are given for six reference
ages for an infinite planar radionuclide source at a mass depth of 0.5 g/cm2 in table A-1.5.
Table A-1.5. Effective dose rate coefficients for an infinite planar source at a mass depth of
0.5 g/cm2 in soil [ICRP, 2020]
Effective dose rate 𝐸𝐸̇ (µSv/h)/(MBq/m2) forRadionuclide a
Newborn

1 year

5 years

10 years

15 years

Adult

8.79

7.88

7.28

6.72

6.30

6.10

129m

Te+

0.23

0.21

0.19

0.17

0.16

0.15

132

Te+

7.97

7.14

6.60

6.05

5.66

5.47

131

I

1.23

1.10

1.02

0.93

0.86

0.83

132

I

7.29

6.53

6.03

5.54

5.19

5.03

133

I

1.97

1.77

1.63

1.49

1.39

1.35

134

Cs

5.04

4.51

4.16

3.82

3.58

3.47

136

Cs

6.70

6.01

5.55

5.13

4.82

4.64

Cs+

1.83

1.63

1.51

1.38

1.29

1.26

110m

Ag+

137

a The “+” sign indicates that the effective dose rate coefficient includes a contribution from radioactive progeny underequilibrium conditions.

27.
ICRP Publication 144 also presents sex-specific organ equivalent dose rate coefficients(µSv/h)/(MBq/m2), which for low-LET photon and electron sources can be regarded as
numerically equal to the absorbed dose rate coefficients. In the present report, these dose rate
coefficients have been used to calculate doses for the following organs: colon, red bone marrow,thyroid and female breast. The age-dependent thyroid equivalent dose rate coefficients are shownin table A-1.6 (male) and in table A-1.7 (female).
Table A-1.6. Thyroid equivalent dose rate coefficients for males for an infinite planar source at amass depth of 0.5 g/cm2 in soil [ICRP, 2020]
Thyroid equivalent dose rate (µSv/h)/(MBq/m2) for maleRadionuclide a
Newborn

1 year

5 years

10 years

15 years

Adult

9.05

7.62

7.19

6.57

6.41

6.36

129m

Te+

0.23

0.19

0.19

0.17

0.16

0.16

132

110m

Ag+

Te+

8.27

6.83

6.48

5.95

5.75

5.71

131

I

1.30

1.02

1.00

0.90

0.86

0.86

132

I

7.54

6.25

5.90

5.44

5.28

5.23

133

I

2.05

1.66

1.59

1.46

1.42

1.40

134

Cs

5.23

4.29

4.05

3.76

3.64

3.60

136

Cs

6.89

5.83

5.54

5.02

4.88

4.83

137

Cs+

1.91

1.55

1.46

1.37

1.32

1.30

a The “+” sign indicates that the dose rate coefficient includes a contribution from radioactive progeny under equilibrium
conditions.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

13

Table A-1.7. Thyroid equivalent dose rate coefficients for females for an infinite planar source at
a depth of 0.5 g/cm2 in soil [ICRP, 2020]
Thyroid equivalent dose rate (µSv/h)/(MBq/m2) for female

Radionuclide a
Newborn

1 year

5 years

10 years

15 years

Adult

9.18

7.82

7.32

6.64

6.51

6.21

129m

Te+

0.24

0.19

0.19

0.17

0.17

0.16

132

110m

Ag+

Te+

8.38

7.00

6.62

5.97

5.89

5.63

131

I

1.33

1.04

1.02

0.90

0.89

0.88

132

I

7.64

6.42

6.03

5.47

5.39

5.14

133

I

2.09

1.70

1.62

1.47

1.44

1.41

134

Cs

5.30

4.41

4.15

3.77

3.73

3.56

136

Cs

6.99

5.96

5.62

5.07

4.97

4.74

Cs+

1.93

1.59

1.50

1.37

1.35

1.29

137

a The “+” sign indicates that the dose rate coefficient includes a contribution from radioactive progeny under equilibrium
conditions.

E.

Dynamics of ambient dose rates due to downward migration andweathering

28.
The effect of natural processes, resulting in the redistribution of radionuclides inundisturbed areas and the corresponding reduction of the ambient dose rates in air, was accounted
for in a similar manner to that in M2013, i.e., via an empirical reduction function 𝑟𝑟(𝑡𝑡) applied tothe dose rates from a planar source in soil at a mass depth of 0.5 g/cm2, which effectively
approximates dose rates from freshly deposited radionuclides [Jacob et al., 1986; Jacob andParetzke, 2017; Petoussi-Henss et al., 2012]. Conventionally, the reduction function 𝑟𝑟(𝑡𝑡) isapproximated by a pragmatic two-exponential expression with empirically defined parameters:𝑟𝑟(𝑡𝑡) = 𝐴𝐴𝑒𝑒

−

ln 2
ln 2
𝑡𝑡
−
𝑡𝑡
𝑇𝑇𝐴𝐴 + 𝐵𝐵𝑒𝑒 𝑇𝑇𝐵𝐵

(A-1.4)

where 𝑡𝑡 is the time since the accident (year); typically, the coefficients 𝐴𝐴 and 𝐵𝐵 add up to 1 andrepresent relative weights of the two, fast and slow, processes characterized by their respective
half-lives 𝑇𝑇𝐴𝐴 and 𝑇𝑇𝐵𝐵 (year). In M2013 [UNSCEAR, 2014], derived from the Chernobyl-based
studies [Golikov et al., 2002; Jacob, 1996; Likhtarev et al., 2002], the parameters 𝐴𝐴 = 𝐵𝐵 = 0.5
were selected and the half-lives for the fast and the slow components were 1.5 and 50 years,
respectively.
29.
Since the publication of the UNSCEAR 2013 Report [UNSCEAR, 2014], further
extensive monitoring has been undertaken in Fukushima Prefecture and the neighbouring
prefectures. This has provided a means for checking the assumed phenomenologicalrelationships over a period of several years of observations. Specifically, for the reduction
function 𝑟𝑟(𝑡𝑡), the data reported by Mikami et al. [Mikami et al., 2019] for fixed undisturbed
locations, mostly in populated areas, have been used in combination with the earlier data fromstudies of global fallout in Europe and North America [Miller et al., 1990; Schimmack et al.,1998]. These data, corrected for the ratios of caesium isotopes and their decay to express a net
effect of downward migration, weathering and runoff, were fit by the two-exponential function(equation A-1.4), resulting in the following parameters (see equation A-1.5):

14

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

𝐴𝐴 = 0.37, 𝑇𝑇𝐴𝐴 = 2.8 year, 𝐵𝐵 = 0.63, 𝑇𝑇𝐵𝐵 = 20.7 year

(A-1.5)30.
The data and the model estimates using the M2013 and M2020 parameters are shown infigure A-1.III. The 95% confidence interval of the fit is indicated by a blue shaded area.
31.
Also plotted in figure A-1.III is the reduction function computed using the dose rate
coefficients [ICRP, 2020] for a more realistic radionuclide source exponentially distributed in
the soil depth, whose relaxation mass per unit area gradually changes with time (see e.g.,[Minenko et al., 2006]). Mikami et al. [Mikami et al., 2019] reported values of the “effective”
relaxation mass per unit area 𝛽𝛽eff 2 based on the successive observations made in the undisturbed
sites, mostly in populated areas of the 80-km zone around FDNPS, covering a period from late2011 to 2017; their data suggest a linear approximation for the relaxation mass per unit area:𝛽𝛽(𝑡𝑡) = 𝛽𝛽0 + 𝑣𝑣𝛽𝛽 𝑡𝑡(A-1.6)

with the following parameters: 𝛽𝛽0 = 0.8 g/cm2 and 𝑣𝑣𝛽𝛽 = 0.415 (g/cm2)/year.

32.
Comparison of the empirical reduction function (equations A-1.4 and A-1.5) and the dose
reduction assessed for an exponential source in soil using the time-dependent relaxation mass
per unit area (equation A-1.6) shows good agreement, especially, in the late period where no
Japan-specific data exist yet (see figure A-1.III). Based on this observation, the empiricalfunction (equation A-1.4) with the parameters (equation A-1.5) had been regarded as a plausible
approximation of the trend in the future dose reduction and was, therefore, applied in the external
dose calculations to approximate dose reduction due to natural processes on undisturbed land in
populated areas.
33.
As seen from figure A-1.III, the variant of the empirical reduction function (equation
A-1.4) used in the UNSCEAR 2013 Report [UNSCEAR, 2014] results in a faster dose reduction
for 15 years following the accident. The phenomenon of slower dose reduction due to natural
redistribution of radionuclides in Japan following the FDNPS accident was explicitly addressedby Saito et al. [Saito et al., 2019b]. It was identified as one of the specific features of the postaccident situation in Japan that required critical evaluation with implications for the external
dose assessment methodology.

2“Effective” relaxation mass per unit area 𝛽𝛽eff is defined [Mikami et al., 2019; Saito et al., 2019a] as theparameter of an exponential distribution in soil which results in the same absorbed dose rate at 1 metre above theground as the real source of the same inventory (integral deposit).

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

15

Figure A-1.III. Reduction of the dose rate in air above undisturbed ground due to downward
migration in soil, weathering and runoff
Points represent results derived from environmental measurements of Mikami et al. [Mikami et al., 2019] (solid circles),Schimmack et al. [Schimmack et al., 1998] (solid square) and Miller et al. [Miller et al., 1990] (solid triangle). Thelines show phenomenological functions used in the UNSCEAR 2013 Report [UNSCEAR, 2014] (red solid line) and
fitted to the data (blue solid line) as well as the dose rate reduction for time-dependent exponential source in soil withweathering and runoff (dashed blue line). The blue shaded area indicates 95% confidence interval for the fitted curve

F.

Dynamics of ambient dose rates in populated areas

34.
Dose rates in air above contaminated ground in populated and, especially, in urban areas
are known to be less than those observed at the same time in virgin or undisturbed environments[Eged et al., 2006; Golikov et al., 2002; Jacob and Meckbach, 1987; Jacob, 1996; Likhtarev etal., 2002; Meckbach and Jacob, 1988; UNSCEAR, 2014]. Extensive and systematic monitoring
activities were undertaken in Japan after the accident using airborne and car-borne radiation
monitoring surveys as well as those on foot [Andoh et al., 2018a; Andoh et al., 2018b; Kinase etal., 2017; Mikami et al., 2019; Saito et al., 2019a]. The comprehensive analysis by Saito et al.
[Saito et al., 2019b] revealed systematic differences between the various monitoring methods,
probably reflecting the fact that: (a) airborne observations were providing dose response
averaged over larger areas and different landscapes; (b) car-borne surveys were largely
constrained to roads and paved surfaces; and (c) surveys on foot included paved roads and streets,
residential areas with more unpaved areas, etc. Correspondingly, as shown in Saito et al. [Saito
et al., 2019b], the airborne (helicopter-based) measurements demonstrated the slowest reduction
in dose rates with time, similar or even slower than observed at fixed location (mostly
undisturbed) sites. The car-borne surveys showed the fastest dose rate reduction, with the datafrom surveys carried out on foot intermediate between the car-borne and the fixed location
measurements.

16

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

35.
M2013 [UNSCEAR, 2014] was largely based on Chernobyl experience in environments
significantly different from those in the affected areas of Japan. Consequently, its use in Japan
may have resulted in some systematic over- or underestimation of doses from external exposure.
36.
Mountainous forest landscape of the affected parts of Fukushima Prefecture, higher
population density and almost all flatland being used or involved in anthropogenic activities aswell as urbanized populated areas determine specificities of the post-accidental situation in
Japan. To evaluate the implications of these specificities, the data from the car-borne surveys[Andoh et al., 2018a] were analysed and compared to location factors from M2013 for paved
and unpaved surfaces in outdoor locations.
37.
Data for the reduction in the external dose rate (solid symbols) derived from the carborne surveys are shown in figure A-1.IV [Andoh et al., 2018a; Kinase et al., 2017], with theeffect of radioactive decay removed. The lines in figure A-1.IV represent model calculations
based on dose rate reduction due to downward migration into the soil (see equation A-1.4) withvarious parameters and the additional location factors for non-natural surfaces. M2013 estimates
are for paved and unpaved surfaces and are shown as red solid and red dashed lines, respectively.
The blue solid line in the figure represents the dose rate reduction obtained from a fit to the carborne data and uses the following location factors for road and streets:
𝑓𝑓roads (𝑡𝑡) = 0.30 exp �−

ln 2𝑡𝑡� + 0.65
0.95

(A-1.7)

where time t is in years. The blue shaded area represents the 95% confidence interval of the fitted
approximation (equation A-1.7) due to uncertainty in the parameters.
38.
For residential areas, the location factor was derived using the observations of Andoh etal. [Andoh et al., 2018b] who demonstrated that the data from surveys carried out on foot,collected in the period 2.3–5.7 years after the accident, showed higher dose rates than observedin the car-borne surveys in the same areas by 20% on average, ranging from +13% at higher dose
rates (>1 µSv/h) to +30% at lower dose rates (<1 µSv/h). The following approximation was
selected for M2020 to represent such behaviour in residential areas:
𝑓𝑓residential (𝑡𝑡) = 0.22 exp �−

ln 2𝑡𝑡� + 0.78
0.95

(A-1.8)where t is in years and the resulting dose rate reduction factor is shown in figure A-1.IV as theblue dashed line.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

17

Figure A-1.IV. The external dose rate reduction factors for paved areas in populated places as:(a) derived from the car-borne surveys [Andoh et al., 2018a; Kinase et al., 2017] and shown as
solid symbols; (b) calculated using M2013 (red lines); and (c) fitted to the data and implemented
in M2020 (blue lines). The blue shaded area indicates 95% confidence interval for the fitted curve

G. Shielding factors for houses
39.
Unlike in M2013, where the indoor location factors had their own time dependence, in
M2020 the location factors 𝑓𝑓𝑗𝑗 (𝑡𝑡) for indoor locations have been obtained as a product of thelocation factor for residential areas (equation A-1.8) and a constant shielding factor, representing
the effect of reduction of the anthropogenic dose inside houses. The same types of house wereconsidered as in the UNSCEA R 2013 Report: a wooden house, a wooden fireproof house and aconcrete house or building. For these house types, the shielding factors were assumed to be thesame as in M2013: 0.4 for wooden houses, 0.2 for wooden fireproof houses, and 0.1 for concretebuildings. These values are based on the earlier experience and studies [Golikov et al., 2002;
Likhtarev et al., 2002; Meckbach and Jacob, 1988; UNSCEAR, 2014] and also consistent withthe recent observations made by the Japanese authors; e.g., Matsuda et al. [Matsuda et al., 2017]
reported shielding factors for one- and two-storey wooden or light-steel frame houses to be inrange from 0.38 to 0.49, while their data also suggested that shielding factors for the concretebuildings would not exceed 0.15.
H.

Fractions of time spent in various locations (occupancy factors)

40.
Papers published since 2013 in peer-reviewed journals have presented deterministic
estimates of annual occupancy factors for four age-social groups of Japanese people; some ofthem were derived from the official sources of the national population statistics [MIC, 2017].

18

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

The summary in table A-1.8 contains arithmetic means (AM) of fractions of time 𝑝𝑝𝑗𝑗 (𝑎𝑎 + 𝑡𝑡) spentby various social groups of adult Japanese people and children of various ages in different
exposure conditions.
Table A-1.8. Fractions of time (occupancy factors) spent in various locations by different
population groups
Fraction of time for the population groupChildrenType of location

Adult workers

Retired a

1 year10 yearOutdoorIndoor

0.9

0.9

0.7

0.9

0.9

At home and others

0.7

0.7

0.7

0.6

0.9

At work, school, kindergarten, etc.

0.2

0.2

Outdoors, including:

0.1

0.1

0.3

0.1

0.1

Residential areas

0.1

0.1

0.2

0.1

0.1

Indoors, including:Unpaved surfaces

0.3

0.1

a Exposure conditions and external doses for retired people are similar to those for indoor workers.

41.
The data collected mostly in Fukushima Prefecture do not contradict the national
statistical data provided by the Statistics Bureau of Japan. 3 Neither summarized papers nor data
presented by the Statistics Bureau of Japan contain explicit values for time spent by various
groups of people indoors and outdoors. However, these have been estimated indirectly from thedata presented above.
42.
Deterministic estimates of external doses have been made for the following four social/
age groups:− A 1-year-old infant as a representative for the group of preschool children (those aged
0–5 years in March 2011);− A 10-year-old child as a representative for the group of school children (those aged
6–15 years in March 2011);− A 20-year-old adult as a representative for the group of adults (those aged 16 and older
in March 2011) who work mostly indoors, including students and pensioners;− A 20-year-old adult as a representative for the group of adults (those aged 16 and older
in March 2011) who work mostly outdoors.
43.
Explicit consideration has not been given to retired people as a group; their doses will bebroadly comparable to those for indoor workers.
44.
According to Japanese national statistics [MIC, 2015], the majority of the population of
Fukushima Prefecture and neighbouring prefectures reside in wooden or wooden fireproof oneto-two-storey houses. Therefore, in M2020 it is assumed for all age-social groups of Japanesepeople that they live (at home and others) in wooden one-to-two-storey houses and work or study(at work, school, kindergarten, etc.) in concrete buildings.

3

See website: [MIC, 2017].

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

I.

19

Summary of M2020

45.
Doses to members of public from exposure to radioactive material deposited in theterrestrial environment following the accident at FDNPS were calculated as follows.
46.
Based on the computational schema described above, the integrated doses for various age
and social population groups, for evacuees and those returning subsequently to their homes were
estimated using the following equation:
𝑡𝑡2

𝐷𝐷(𝑡𝑡1 , 𝑡𝑡2 |𝑎𝑎0 ) = 𝑐𝑐 � 𝐷𝐷̇(𝑡𝑡|𝑎𝑎0 )d𝑡𝑡

(A-1.9)𝑡𝑡1

where 𝐷𝐷̇(𝑡𝑡|𝑎𝑎0 ) is the rate of the dose quantity of interest, effective or organ equivalent dose; 𝑎𝑎0
is the age when accidental radiation exposure started for the considered population group; times
𝑡𝑡1 and 𝑡𝑡2 bound the exposure period; and the unit conversion coefficient 𝑐𝑐 depends on theselected dimensions of the quantities used (time, deposition density, dose rate coefficients, decay
data, half-lives).
47.

The dose rate was evaluated as:𝐷𝐷̇(𝑡𝑡|𝑎𝑎0 ) = 𝑟𝑟(𝑡𝑡)𝐴𝐴Cs-137 � 𝜌𝜌𝑚𝑚 𝑒𝑒 −𝜆𝜆𝑚𝑚𝑡𝑡 𝑑𝑑̇𝑚𝑚 (𝑎𝑎 + 𝑡𝑡, 𝑠𝑠) � 𝑓𝑓𝑗𝑗 (𝑡𝑡)𝑝𝑝𝑗𝑗 (𝑎𝑎 + 𝑡𝑡)
𝑚𝑚(A-1.10)𝑗𝑗

where 𝐴𝐴Cs-137 is the deposition density of 137Cs on 15 March 2011 (Bq/m2); 𝑟𝑟(𝑡𝑡) is the empirical
two-exponential reduction function (see equation A-1.4) describing dose reduction due to naturalprocesses of redistribution (downward migration, weathering, runoff):
ln 2

ln 2

𝑟𝑟(𝑡𝑡) = 0.37 𝑒𝑒 − 2.8 𝑡𝑡 + 0.63 𝑒𝑒 −20.7𝑡𝑡(A-1.11)

where 𝑡𝑡 is time since 15 March 2011 (year); 𝜌𝜌𝑚𝑚 is the initial ratio of the radionuclide 𝑚𝑚
deposition density to deposition density of 137Cs (unitless); 𝜆𝜆𝑚𝑚 is the radioactive decay rate of
the radionuclide 𝑚𝑚 (1/year); 𝑑𝑑̇𝑚𝑚 (𝑎𝑎 + 𝑡𝑡, 𝑠𝑠) is the dose rate coefficient for an “effective” planarsource at a mass depth of 0.5 g/cm2 for the radionuclide 𝑚𝑚 and its progeny in equilibrium
conditions for a person of age 𝑎𝑎 + 𝑡𝑡 and sex 𝑠𝑠 (mSv/year)/(MBq/m2); 𝑓𝑓𝑗𝑗 (𝑡𝑡) are the locations
factors (unitless) defined as follows in equation A-1.12:
1,outdoor, undisturbed areas
1.0, outdoor, populated areasln20.4, indoor, wooden house
𝑓𝑓𝑖𝑖 (𝑡𝑡) =−𝑡𝑡⎨�0.22 𝑒𝑒 0.95 + 0.78� × �0.2, indoor, wooden fireproof house⎪⎩
0.1, indoor, concrete house⎧⎪(A-1.12)

𝑝𝑝𝑗𝑗 (𝑎𝑎 + 𝑡𝑡) is the occupancy factor, i.e., fraction of lifetime spent in the location 𝑗𝑗, for a person ofage (𝑎𝑎 + 𝑡𝑡) and of a specific occupancy type. The occupancy types considered are those forpreschool and school children, outdoor workers, indoor workers and pensioners. The occupancy
factors adopted in M2020 have been simplified, compared with those in M2013, and are shownin table A-1.8.
48.
The uncertainties in the model estimates were assessed using a Monte Carlo simulation
based on equations A-1.9 and A-1.10 and using estimated or implied uncertainties in the area-

20

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

averaged values of 137Cs deposition density and other model parameters. Particularly, areaaveraged values of 137Cs deposition density were taken as log-normally distributed with anestimated geometric standard deviation (GSD) of 1.5. The dose rate coefficients [ICRP, 2020]
were assumed to follow normal distributions with the 95% confidence interval specified by arelative error of 20%. The empirical dose rate reduction function (equation A-1.11) was assumed
to be log-normally distributed with a GSD of 1.2 for integration periods of 1 and 10 years after
the accident and with a GSD of 1.3 for longer integration periods to address higher uncertainty
due to extrapolation of the fitted function beyond the data-supported domain. Deposition density
ratios 𝜌𝜌𝑚𝑚 (see table A-1.1 and equation A-1.3) were also assumed to be log-normally distributedwith a GSD of 1.1 for all radionuclides except for 131I, for which the uncertainty was expressedby a GSD of 1.5 to reflect the higher variability observed in the measured data (see figure A-1.I).
Uncertainty specific to location and occupancy factors was evaluated as corresponding to lognormally distributed quantities with a GSD of 1.2, correspondingly, the uncertainty of the
combined factor (see inner sum in equation A-1.2) was taken as represented by a GSD of 1.3,
assuming statistical independence of both factors. Sample size in the stochastic simulations waschosen equal to 10,000.
49.
This Monte Carlo technique was applied to generate samples for different ages, types ofoccupancy and periods of exposure. The main statistical properties of the generated samples are
summarized in table A-1.9. Specifically, the ratios of the estimated geometric mean (GM) and5th and 95th percentiles to the corresponding AM of a generated distribution were calculated.
These ratios for different population groups were found to be very similar, so only the groupaveraged values are shown in table A-1.9.
Table A-1.9. Statistical characteristics of uncertainty associated with M2020 estimates ofcumulative external doses
Exposure period

5th percentileGM

AM

95th percentile

1 year

0.54

0.94

1

1.66

10 years

0.54

0.94

1

1.66

Lifetime

0.49

0.93

1

1.76

IV. VALIDATION OF M2020 FOR DOSES FROM EXTERNAL EXPOSURE
50.
The validity of external dose estimates made with M2020 was tested by applying themodels to the results of the several personal dosimetry surveys performed in Fukushima Prefecture
and published in the scientific literature. Based on published data, several validation scenarios weredeveloped and used for comparing the models’ predictions and the reported measurements.
A.

The Fukushima Health Management Survey (Scenario A)

51.
The Fukushima Health Management Survey (FHMS) was undertaken to obtain
individualized estimates of population exposure following the FDNPS accident and involved
residents of seven areas in Fukushima Prefecture [Akahane et al., 2013; Ishikawa et al., 2015].
The study was essentially a model-based dose reconstruction produced by integration of ambient
dose rates, model-simulated and interpolated from measured values, taking into account details
of individual behaviour acquired through interviews or questionnaires. The effective doses forthe residents of Fukushima Prefecture were reconstructed for the four-month period from
15 March 2011 to 11 July 2011. The model used in the study for external dose calculations waslargely compatible to the methodology presented in the UNSCEAR 2013 Report [UNSCEAR,ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

21

2014], excluding outdoor location factors, which were unnecessary as the ambient dose rates
were estimated for living places, while the indoor location factors were conservatively taken as
time independent shielding factors.
52.
For comparison with dose estimates made using the M2013 and M2020 models, the threeareas with non-evacuated populations have been selected: Aizu, Kennan and Minami-Aizu[Ishikawa et al., 2015].
53.
The external doses were calculated for exposures to radioactive sources in air (plume)and on the ground for the period from 15 March 2011 to 11 July 2011, accounting for the ageand social population structure as reported by Ishikawa et al. [Ishikawa et al., 2015]. The results
were obtained using both the M2013 and M2020 models. Comparison of the area-averaged
inferred doses with the model-calculated doses is shown in figure A-1.V.
54.
The estimates using the M2013 and M2020 models agree well with the values of external
doses inferred in FHMS [Akahane et al., 2013; Ishikawa et al., 2015] for the area Aizu
(figure A-1.V, circles). Both models underestimate the FHMS values for Minami-Aizu(figure A-1.V, triangles) by a factor of 1.3–2. However, the absolute difference is small,
0.03–0.05 mSv; in particular, it is less than the assumed natural background dose of 0.086 mSv
during the study period, so associated uncertainties are relatively high and the model estimates
can be regarded as comparable with the inferred FHMS values. The FHMS values for the Kennan
area are larger, by approximately 0.25 mSv for M2013 and 0.16 mSv for M2020; thesedifferences may be attributed to several factors, e.g., differences in the model assumptions andparameters, inherent uncertainties of the atmospheric dispersion models used in FHMS,
assumptions on the population structure, lifestyle and dwelling types, underestimation of thecontribution from shorter-lived radionuclides.
Figure A-1.V. Comparison of the external effective doses for the period 15 March 2011 to 11 July 2011
inferred from individual questionnaires [Akahane et al., 2013; Ishikawa et al., 2015] and the dose
estimates obtained using M2013 [UNSCEAR, 2014] and M2020 models (this UNSCEAR report)

Aizu (M2013)

0.6

Kennan (M2013)
Minami-Aizu (M2013)
Aizu (M2020)
Kennan (M2020)

0.5

Minami-Aizu (M2020)

Model estimate (mSv)

0.4

0.3

0.2

0.1

0
0

0.1

0.2

0.3

Inferred dose (mSv)

0.4

0.5

0.6

22

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

55.
For example, the Kennan area includes the Yamatsuri municipality which, in both M2013
and M2020, is assigned to the South trace with a higher contribution of shorter-lived
radionuclides, resulting in a net anthropogenic external dose of 0.6 mSv. The municipalities ofSamegawa, Hanawa and Tanagura, also belonging to the Kennan area and adjacent to Yamatsuri,
are assumed to have radiological conditions typical of the rest of Japan. An alternative
assumption, that these municipalities should have been attributed to areas with conditions typicalof the South trace, would have increased the M2020 estimates for the whole Kennan area from
0.44 to 0.49 mSv, which is only 20% less than that reported in the FHMS results.
56.
The comparison of the FHMS estimates with those based on M2020 cannot be judged asa fully independent objective benchmark, because the estimates are inferred based on various
methods and assumptions. More informative comparisons can be made with results of personal
dosimetry campaigns conducted in the affected areas.
B.

Personal dosimetry studies

57.
Following the FDNPS accident, several personal dosimetry campaigns have been andcontinue to be conducted in Japan and the results of some have been reported in the peerreviewed literature. These campaigns targeted various areas and were performed over a range of
periods of time, seasons and or different population groups.
58.
Typically, luminescence dosimeters were distributed among population groups in the
study areas. At the end of the study period, the dosimeters were collected and processed. The
total, background and anthropogenic, accumulated doses, in terms of the personal dose
equivalent, 𝐻𝐻𝑝𝑝 (10), were determined in dosimetry laboratories and the same assumed value ofthe background dose was subtracted. The net measured dose was often assumed as a proxy for
effective dose and, correspondingly, no further conversion was applied to the measured values.
In such cases, for comparison with M2013 and M2020 estimates, the reported measured
anthropogenic personal equivalent doses were converted to effective dose values for therespective age groups using the conversion coefficients derived by Satoh et al. [Satoh et al.,2017]. The model calculations were performed for specially defined population groups andbehavioural patterns selected to match, as closely and reasonably as possible, the study
population groups and their living and working conditions.
1.

Scenario B: Kawauchi, Tamano, Haramachi areas in 2012

59.
Scenario B was derived from the personal dosimetry study undertaken in Kawauchi
Village, Tamano area (a part of Soma City) and Haramachi area (a part of Minamisoma City) in2012 [Harada et al., 2014]. The dosimeters were worn for two months from 1 August 2012 to
30 September 2012. The study group consisted of 483 adults. The background dose during thestudy period was assumed to be 0.1 mSv deduced from the reported pre-accident annual
background dose in the range of 0.61–0.63 mSv.
60.
The results of the study are compared to the M2013 and M2020 estimates in figure A-1.VI,from which it is seen that the model-calculated values significantly underestimate the values
reported by Harada et al. [Harada et al., 2014]. The reason for the systematic differences apparent
in figure A-1.VI is unclear and may have several origins. Notably, comparisons with results fromother studies for the same areas and time periods (see the following subsections) demonstratesignificantly better agreement between the measured and the model-calculated values, suggestingANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

23

that there may be an unknown systematic factor influencing the measured values reported byHarada et al. [Harada et al., 2014].
Figure A-1.VI. Comparison of the external effective doses from personal dosimetry data [Haradaet al., 2014] and the dose estimates obtained using M2013 [UNSCEAR, 2014] and M2020 models(this UNSCEAR report)

Haramachi (M2013)

0.4
Kawauchi (M2013)
Tamano (M2013)
Haramachi (M2020)

0.35

Kawauchi (M2020)
Tamano (M2020)

0.3

Model estimate (mSv)

0.25

0.2

0.15

0.1

0.05

0
0

0.1

0.2

0.3

0.4

Measured dose (mSv)

2.

Scenario C: Minamisoma City school children in 201261.
The study selected for this validation scenario was conducted in Minamisoma City
among 520 school children during three months in September–November 2012 [Nomura et al.,2015; Nomura et al., 2016]. The children represented various school grades (primary, secondaryand high), thus making it possible to compare their doses with the model-estimated doses for
10-year-old children and adults.
62.
The reported measured anthropogenic doses were in the range of 0.17–0.22 mSv and aresmall and close to the assumed background dose 0.14 mSv for the study period, whichcorresponds to an annual background dose of 0.54 mSv. Therefore, variations in the backgroundabout the single value assumed in the study would result in additional unknown uncertainties in
the measured values. Indeed, Nomura et al. [Nomura et al., 2015; Nomura et al., 2016] reportedpart of the measured data to have zero values after subtraction of the assumed background dose.
63.
Comparison of the measured and model-calculated external doses is shown infigure A-1.VII, where the error bars indicate ranges of the estimates derived for various ages andhousing types. M2013 underestimates the measured values while M2020 is in much better
agreement with the measured doses.

24

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Figure A-1.VII. Comparison of the external effective doses from personal dosimetry data for
Minamisoma City school children in September–November 2012 [Nomura et al., 2015; Nomura etal., 2016] and the corresponding dose estimates obtained using M2013 [UNSCEAR, 2014] andM2020 models (this UNSCEAR report)

0.25

0.2

Model estimate (mSv)

0.15

0.1

High-school (M2013)

0.05

Primary and secondary (M2013)
High-school (M2020)
Primary and secondary (M2020)

0
0

0.05

0.1

0.15

0.2

0.25

Measured dose (mSv)

3.

Scenario D: Adults in Fukushima City and in the neighbourhood

64.
The study was performed during one month in the period February–April 2012 [Takahara
et al., 2014] and included 499 adult participants from Fukushima City and neighbouringmunicipalities comprising: the city office employees, retired persons, members of the contractor
association and agriculture cooperatives. The natural background dose was assumed to be
0.05 mSv during the month of the study and was inferred from the reported pre-accident annual
background dose of 0.63 mSv.
65.
The measured and model-calculated values are compared in figure A-1.VIII. Bothmodels predicted anthropogenic external doses close to those obtained from personal dosimeters.
In general, M2020 provides a better fit to the measured doses while M2013 tends tounderestimate them.
66.
The model calculations for the members of the contractor association and agricultural
workers were conservative and based on an assumption that they spent 100% of their working
time outdoors. These estimates are shown in figure A-1.VIII as solid upward pointing triangles
and empty downward pointing triangles, respectively. M2020 (blue symbols) clearly
overestimates the measured doses for these groups. Less conservative M2020 estimates for
agricultural workers, assuming their working time is equally shared between indoor and outdoor
activities, is shown as solid downward pointing triangle and is in much better agreement withthe measurements. This difference demonstrates the effect of uncertainty in the occupancy
factors when trying to make valid comparisons between modelled and measured doses.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

25

Figure A-1.VIII. Comparison of the external effective doses from personal dosimetry data for
adults in Fukushima City and neighbouring municipalities in February–April 2012 [Takahara etal., 2014] and the corresponding dose estimates obtained using M2013 [UNSCEAR, 2014] andM2020 models (this UNSCEAR report)

0.25
City office (M2013)
Pensioners (M2013)
Contr. assoc. (M2013)
Agr. coop. (wgt.) (M2013)
Agr. coop. (outd.) (M2013)

0.2

City office (M2020)
Pensioners (M2020)
Cont. assoc. (M2020)
Agr.coop. (wgt.) (M2020)
Agr.coop. (outd.) (M2020)

Model estimate (mSv)

0.15

0.1

0.05

0
0

0.05

0.1

0.15

0.2

0.25

Measured dose (mSv)

4.

Scenario E: Minamisoma City children

67.
The study selected for this validation scenario was conducted from June 2012 to February
2013 in three cycles, each of three months duration, and included 881 children from Minamisoma
City [Tsubokura et al., 2015]. Similar to other studies, the natural background dose was assumed
to be the same for all children and equal to 0.14 mSv for each three-month period of the study(deduced from the annual background dose of 0.54 mSv). The measured anthropogenic effective
doses, 0.13–0.15 mSv, were comparable with this assumed background dose.
68.
The average effective doses for the three study periods predicted by M2013 and M2020
are compared in figure A-1.IX with the measured doses reported by Tsubokura et al. [Tsubokura
et al., 2015]. Both models predict doses comparable with those measured, with M2013 slightly
underestimating and M2020 slightly overestimating them, typically by of the order of about ten
or up to a few tens of per cent.

26

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Figure A-1.IX. Comparison of the external effective doses from personal dosimetry data for
Minamisoma City school children in 2012–2013 [Tsubokura et al., 2015] and the corresponding
dose estimates obtained using M2013 [UNSCEAR, 2014] and M2020 models (this UNSCEAR report)

5.

Scenario F: Soma City children

69.
The study selected for this validation scenario presented results of the personal dosimetry
campaigns conducted in Soma City among preschool and school children from October 2011 toNovember 2015 [Tsubokura et al., 2017]. The study included five 3-month-long measurement
periods in each calendar year: October–December 2011, July–September 2012, May–July 2013,
September–November 2014 and September–November 2015.
70.
In total, the study involved 14,405 participants among school and preschool children:
3,812 in 2011, 3,824 in 2012, 2,979 in 2013, 1,937 in 2014 and 1,853 in 2015. The children wereresidents of four areas within Soma City with elevated deposition density (Akagi, Tamano,
Yamakami and Yawata) as well as in the rest of the city. The background dose was assumed to be
0.14 mSv for each three-month period of the study, deduced from the annual natural backgrounddose of 0.54 mSv. From 2013 onwards, the reported average anthropogenic doses obtained fromthe personal dosimeters did not exceed the background dose. Tsubokura et al. [Tsubokura et al.,2017] found that the individual dose distribution was log-normal with a GSD in the range of
1.35–1.75.
71.
M2013 and M2020 predicted doses are compared in figure A-1.X with the measurements
reported by Tsubokura et al. [Tsubokura et al., 2017]. The data points for each of the five areas
within the city are shown for the period 2011–2015 and demonstrate a reduction in dose withtime. The model predictions are broadly comparable with the measurements and their reduction
over time. M2020 provides a better overall fit to the measured doses with M2013 tending tounderestimate them. The spread of the data in figure A-1.X around the diagonal (dashed line)

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

27

reflects the uncertainty of the model estimates due to uncertainties in model parameters, in thedeposition density levels, and assumptions regarding the people’s behaviour and housing;typically the uncertainty can be characterized as not exceeding a factor of two.
Figure A-1.X. Comparison of the external effective doses from personal dosimetry data for Soma
City children in 2011–2015 [Tsubokura et al., 2017] and the dose estimates obtained using M2013[UNSCEAR, 2014] and M2020 models (this UNSCEAR report)

6.

Scenario G: Adults in Minamisoma City

72.
This scenario was derived from a study [Tsubokura et al., 2018] conducted in
Minamisoma City in 2017 among 25 adult indoor workers. The duration of the study was only
two weeks and the measured total (anthropogenic plus natural background) doses were used tocompare with the similar values obtained in other prefectures distant from Fukushima Prefecture.
The measured average total dose was 27 ± 6 µSv for the two-week period which corresponds toan annual total effective dose of 0.7 mSv. Assuming the annual background dose to fall within
the range of 0.54 mSv (typical of Minamisoma City) to 0.63 mSv (typical of Fukushima City),
the measured annual anthropogenic dose would be in the range of 0.07 to 0.16 mSv with verylarge uncertainty. Comparison with model estimates (approximately equal to 0.07 mSv) would
not be informative in this case.

28

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

C.

Outcomes of the personal dosimetry surveys in Minamisoma City andNaraha Town for 2014–201973.
The municipalities of Minamisoma City and Naraha Town provided the Committee withanonymous data on personal doses measured using luminescence dosimeters. The data included
information on the town or municipality of residence, personal age and sex as well as the net
measured dose (Minamisoma City) or the total measured dose (Naraha Town). The data provided
were grouped in three age ranges, preschool children represented by a 1-year-old infant, school
children represented by a 10-year-old child, and adults. No significant differences were foundbetween male and female doses, so sex-averaged doses were used in comparisons with the model
estimates.
74.
Data for those who wore a dosimeter for 90 days or longer, and whose dosimeters were
processed within 80 days after collection, were selected for further analysis. The data set from
Minamisoma City contained only net doses for which many were reported blank, presumably,
due to being below detection level. The reported data for Naraha Town were total measured
doses from which the background dose was subtracted assuming an annual background dose of
0.54 mSv (i.e., the value assumed in other studies in Minamisoma City [Nomura et al., 2015;
Nomura et al., 2016; Tsubokura et al., 2015; Tsubokura et al., 2017; Tsubokura et al., 2018]). If
the subtraction resulted in a negative value, then this value was replaced by zero in the analysed
data set.
75.
Dose distributions for the three age groups in each location were analysed. A substantial
part of the measured data comprised zero doses. Cumulative distributions for each data set andage group were analysed and empirical percentiles were evaluated for the following cumulative
probabilities: 0.25 (if non-zero), 0.5, 0.75, 0.9, 0.95 and 0.975. From these percentiles, assuming
log-normality of the distributions, GSD was estimated, which, in combination with the median(assumed GM), was used to compute AM and other statistics, including minimum and maximum
values, confidence intervals and the fraction of non-zero measurements. Most of the dose
distributions were reasonably close to log-normal, sometimes showing indications ofmultimodality (Odaka area of Minamisoma City).
76.
The data provided by the local authorities of Minamisoma City and Naraha Town enabled
direct comparison between the measured and the modelled doses. In the figures A-1.XI–XIII
below, both the M2020 (solid line) and M2013 (dashed line) doses are compared with thosemeasured (circle – 50th percentile, star – AM, error bars – 68% confidence interval, the number
is the sample size) for three age groups. Based on the information provided by Minamisoma
City: (a) 92% of the population was considered to be living in wooden houses and the remaining
8% in brick or concrete buildings; (b) occupancy factors were modified to account for thereported working hours of kindergartens, schools and offices; and (c) all kindergartens wereassumed to be in wooden buildings.
1.

Haramachi area of Minamisoma City77.
The residents of the Haramachi area in Minamisoma City were the largest group among
those measured and, as such, are the best source of drawing statistical inferences from the data.
The measured and modelled dose rates, in terms of mSv/year, are shown in figure A-1.XI below,
where circles indicate GM, stars – AM, error bars show the 1-sigma (𝑃𝑃 = 0.683) confidence
intervals and numbers indicate the number of people measured.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

29

78.
M2020 significantly outperforms M2013 and falls within the 68% confidence interval
for individual distributions for adults and school children; it does, however, overestimate doses
for preschool children by a factor of about two, approaching the upper boundary of theconfidence interval. Two factors may have contributed to this difference: firstly, the assumptionthat all kindergartens were in wooden buildings; and, secondly, the deposition density data used
for the modelled doses may not have been representative for the group measured.
79.
The modelled and measured doses in the figures demonstrate compatible timedependence of the 3-month-average doses in the studied period.
80.
Median values of the measured anthropogenic doses were all small (i.e., lower than thenatural background) and are, therefore, themselves associated with significant uncertainty.
Substantial variability in the natural background dose between individuals in the studied groups
was a likely cause of the reported doses of zero for about 30–50% of preschool children, 17–40%
of school children and 18–30% of adults.
2.

Kashima area of Minamisoma City

81.
Results for a smaller group of residents of the Kashima area in Minamisoma City are
shown in figure A-1.XII. The M2020 dose estimates are broadly comparable with those measured
and with their variation over time. The agreement is best for the preschool children, with moderate
underestimation of doses for school children and adults. The model estimates, however, fallwithin the 1-sigma (𝑃𝑃 = 0.683) confidence interval. The M2020 doses are in better agreementwith the measurements than those from M2013 which systematically underestimates them.
82.
The measured data for the Kashima area show zero values for about 14–55% of preschool
children, 25–60% of school children and 22–37% of adults.

30

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Figure A-1.XI. Three-month-averaged effective dose rates for population groups in Haramachi
area of Minamisoma City measured by personal dosimeters (symbols, error bars) and calculatedwith M2013 (dashed line) and M2020 (solid line) for: (a) preschool children; (b) school children;and (c) adults. The numbers indicate the sample size for each data pointANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

31

Figure A-1.XII. Three-month-averaged effective dose rates for population groups in Kashima
area of Minamisoma City measured by personal dosimeters (symbols, error bars) and calculatedwith M2013 (dashed line) and M2020 (solid line) for: (a) preschool children; (b) school children;and (c) adults. The numbers indicate the sample size for each data point

32

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

3.

Odaka area of Minamisoma City

83.
Results for an even smaller group of residents of the Odaka area in Minamisoma City aresummarized in table A-1.10. The fraction of non-zero doses in the measured groups was often
less than 50%, making it impossible to determine median and related statistics of the distributionof doses. For such cases, only the span of the measured doses, bounded by the minimum and the
maximum, and 90% confidence intervals are tabulated to enable comparison with the modelled
doses. Due to limited sample size and incomplete statistical parameters, the results for the Odaka
area are not shown in a figure.
84.
Despite the rather low statistical power of these data, M2020 provides a better fit to themeasured doses and clearly outperforms M2013, which tends to underestimate them.
Table A-1.10. Statistical properties of the personal dosimetry monitoring data for residents of
the Odaka area of Minamisoma City in the period 2015–2019 and comparison with the M2013 andM2020 estimates for a hypothetical matching population
Effective dose for 3 months (mSv)Time

Number ofpeopleNon-zero(%)

Measured
Min.

Max.

AM

GM

Modelled
GSD

q(5%)

q(95%)

M2013

M2020

Preschool children
2015, Q2

38

32

0

0.18

0

0.13

0.08

0.14

2015, Q3

37

38

0

0.15

0

0.13

0.07

0.13

2015, Q4

34

29

0

0.15

0

0.12

0.07

0.13

2016, Q1

25

32

0

0.11

0

0.11

0.06

0.12

2016, Q2

24

46

0

0.12

0

0.11

0.06

0.11

2016, Q3

26

38

0

0.12

0

0.11

0.06

0.11

2016, Q4

24

46

0

0.11

0

0.11

0.05

0.10

2017, Q1

20

35

0

0.10

0

0.10

0.05

0.10

2017, Q2

17

41

0

0.10

0

0.10

0.05

0.09

2017, Q3

14

43

0

0.12

0

0.12

0.05

0.09

2017, Q4

14

36

0

0.09

0

0.09

0.04

0.09

2018, Q1

12

42

0

0.12

0

0.12

0.04

0.08

2018, Q2

9

56

0

0.11

0.06

0.05

1.8

0

0.11

0.04

0.08

2018, Q3

7

57

0

0.14

0.07

0.06

1.8

0

0.14

0.04

0.08

2018, Q4

9

44

0

0.13

0

0.13

0.04

0.07

2019, Q1

5

60

0

0.13

0

0.13

0.04

0.07

0.09

0.08

1.3

School children
2015, Q2

70

41

0

0.22

0

0.10

0.05

0.09

2015, Q3

71

39

0

0.22

0

0.13

0.05

0.08

2015, Q4

74

39

0

0.15

0

0.11

0.04

0.08

2016, Q1

78

27

0

0.12

0

0.08

0.04

0.08

2016, Q2

75

27

0

0.13

0

0.09

0.04

0.07

2016, Q3

81

32

0

0.15

0

0.11

0.04

0.07

2016, Q4

74

24

0

0.12

0

0.10

0.03

0.07

2017, Q1

74

27

0

0.12

0

0.10

0.03

0.06

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

33

Effective dose for 3 months (mSv)
Number ofpeopleTime

Non-zero(%)

Measured
Min.

Max.

AM

GM

Modelled
GSD

q(5%)

q(95%)

M2013

M2020

2017, Q2

72

29

0

0.12

0

0.10

0.03

0.06

2017, Q3

66

32

0

0.12

0

0.11

0.03

0.06

2017, Q4

67

33

0

0.13

0

0.10

0.03

0.05

2018, Q1

57

40

0

0.12

0

0.09

0.03

0.05

2018, Q2

46

33

0

0.15

0

0.11

0.03

0.05

2018, Q3

49

35

0

0.13

0

0.10

0.03

0.05

2018, Q4

50

34

0

0.15

0

0.13

0.03

0.05

2019, Q1

37

38

0

0.13

0

0.10

0.02

0.05

Adults2015, Q2

726

63

0

1.41

0.09

0.05

2.9

0

0.34

0.05

0.10

2015, Q3

755

63

0

1.16

0.09

0.05

2.8

0

0.34

0.04

0.10

2015, Q4

767

64

0

1.15

0.08

0.05

2.7

0

0.35

0.04

0.09

2016, Q1

752

49

0

0.86

0

0.30

0.04

0.09

2016, Q2

725

64

0

0.90

0.08

0.05

2.4

0

0.26

0.04

0.08

2016, Q3

718

66

0

0.79

0.08

0.05

2.4

0

0.28

0.03

0.08

2016, Q4

718

62

0

0.70

0.07

0.05

2.3

0

0.25

0.03

0.08

2017, Q1

706

61

0

1.31

0.07

0.05

2.3

0

0.24

0.03

0.07

2017, Q2

624

63

0

1.45

0.08

0.05

2.5

0

0.26

0.03

0.07

2017, Q3

645

62

0

0.81

0.08

0.05

2.5

0

0.27

0.03

0.07

2017, Q4

639

57

0

0.79

0.07

0.05

2.2

0

0.24

0.03

0.06

2018, Q1

589

59

0

0.84

0.07

0.05

2.2

0

0.24

0.03

0.06

2018, Q2

579

55

0

1.94

0.07

0.04

2.6

0

0.22

0.02

0.06

2018, Q3

599

56

0

0.52

0.07

0.04

2.6

0

0.22

0.02

0.06

2018, Q4

573

56

0

0.52

0.07

0.05

2.2

0

0.23

0.02

0.05

2019, Q1

533

60

0

0.65

0.07

0.05

2.3

0

0.21

0.02

0.05

4.

Naraha Town (Iwaki)

85.
Measured doses for residents of Naraha Town, currently living in Naraha or in Iwaki, arecompared with model estimates in figure A-1.XIII. The M2020 estimates demonstrate good
agreement with the measured data, being somewhat low for the adults from Naraha currently
living in Iwaki. The higher measured doses may be a result of deposition density of radiocaesium
at their home or place of work differing from those assumed in the model estimates.

34

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Figure A-1.XIII. Three-month-averaged effective dose rates for population groups in Naraha
Town (blue) and Iwaki City (orange) measured by personal dosimeters (symbols, error bars) andcalculated with M2013 (dashed line) and M2020 (solid line) for: (a) preschool children; (b) school
children; and (c) adults. The numbers indicate the sample size for each data pointANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

D.

35

Summary86.
Predictions of doses estimated by M2013 and M2020 have been compared with thosemeasured in personal dosimetry campaigns carried out in various areas of Fukushima Prefecture.
In most cases, M2020 outperforms M2013 and fits better the measured doses. The only exception
is scenario B [Harada et al., 2014] where the estimates of both models are systematically lessthan the reported measured values.
87.
The ratio of M2020 model-calculated external doses and the estimates obtained from thevalidation scenarios derived from the published results of personal dosimetry studies, as well asbased on the external doses measured by personal dosimeters as reported by the authorities of
Minamisoma City and Naraha Town is shown in figure A-1.XIV. The plotted ratios span the
period from 2011 to 2019 and fall within a range of 0.25 to 2. The mean ratio over the whole
period is 0.965 and the 90% confidence interval is from 0.53 to 1.67.
Figure A-1.XIV. Ratio of model-calculated and measured group-averaged values of external doses
from the validation scenarios (violet and orange symbols) and data from Minamisoma City andNaraha Town (blue symbols)

88.
M2020 was derived using the extensive monitoring data collected after the accident and,due to this, it adequately characterizes the observed dynamics of ambient dose rate over the first
decade after the accident in semi-natural and anthropogenic environments of Fukushima
Prefecture. Due to the decreasing trend of the ambient dose rate, uncertainty in the long-term
component (which describes the dose rate dynamics decades into the future), would notcontribute significantly to the time integral of dose.
89.
Most of the measured doses were small and close to the assumed background levels. This
results in additional uncertainty or bias if the real background values were higher than assumed.

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ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

The results reported by Nomura et al. [Nomura et al., 2019] for the study in Fukushima Prefecture
showed that, after subtraction of the background dose, the measured anthropogenic doses were
zero for: 62.5% of the measured population outside Minamisoma City; 33% of the residents ofHaramachi and Kashima areas; and 10.5% of those returned to the previously evacuated Odaka
area. These results clearly indicate that the unknown spatial and temporal variability in thenatural background radiation challenges accurate determination of relatively small
anthropogenic doses. Future studies are needed to better characterize variability of the
background doses and to quantify the uncertainties in those measured.
E.

Variability of individual external doses in Fukushima Prefecture

90.
Along with the uncertainty of the external doses for certain groups of residents ofmunicipalities in Fukushima Prefecture, described in section III.I, it is of interest to estimate
variability among individuals around the average values. Within one municipality, an individual
dose may depend on non-uniform distribution of radionuclides across its territory, site
development, the type of residential building, occupancy, lifestyle and household habits, etc.
91.
The Committee evaluated the variability in individual doses by statistical processing ofindividual monitoring data from several municipalities. Databases of large-scale monitoring
programmes, conducted by the municipal authorities of Minamisoma City (2015–2019,
64,996 measurements, 3,400–4,900 per quarter), Iwaki City and Naraha Town (2014–2018,
7,062 measurements), were provided by the municipality authorities [Minamisoma, 2019] andwere analysed. For each data set for a monitoring period from one to three months, statisticalcharacteristics were calculated for three population groups (adults, school children, andpreschool children) in each municipality. Published data on statistical properties of individual
dose distributions obtained from personal dosimetry studies were also used, including
499 measurements among adults of Fukushima City in 2012 [Takahara et al., 2014] and520 measurements of school children in Minamisoma City in 2012 [Nomura et al., 2016]. These
characteristics of the distributions normalized to the corresponding average dose and averaged
for all studies are shown in table A-1.11.
Table A-1.11. Variability of measured individual external doses in Fukushima Prefecture (ratio ofpercentiles to average values, unitless)Parameters of effective dose distributions (relative units)
Population group
5%50%

Average

95%

Preschool children

0.38

0.88

1.00

2.03

School children

0.35

0.86

1.00

2.12

Adults

0.28

0.81

1.00

2.37

92.
Statistical parameters presented in table A-1.11 demonstrate moderate variability of
individual external doses. The measured external doses for 5% of the monitored persons do not
exceed the value which is by 2.6–3.6 times less than the average dose and the 5% received
highest doses which exceed the average value by a factor of 2 to 2.4. The dose variability isslightly more in adults than in children presumably because of greater variability in theiroccupancy (e.g., outdoor and indoor workers, etc.). The Committee has judged that the variability
parameters summarized in table A-1.11, can be considered applicable to the whole of Fukushima
Prefecture and for other exposure periods.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

37

V. RESULTS OF THE MODEL CALCULATIONSA.

Cumulative doses of external exposure per unit 137Cs deposition
density93.
M2020 differs from M2013 [UNSCEAR, 2014] in several respects, including non-linear
approximations for initial radionuclide activity concentration ratios in deposited material. The
latter results in different time integrals of external dose per unit initial deposition density of 137Cs,
depending on different contributions to the dose from shorter-lived radionuclides and whether
the deposition is in the South trace or elsewhere and on the 137Cs deposition density. Similar tothat shown in [UNSCEAR, 2014], figure A-1.XV presents the time dependence of the
cumulative effective dose per unit deposition density of 137Cs for areas located in the South trace
and the rest of the country and for three different values of the 137Cs deposition density: 10 and
100 kBq/m2 and 1 MBq/m2. The calculated doses are for an adult living in a wooden house andworking indoors in a concrete building (assumed to be representative of a typical adult).
Figure A-1.XV. Accumulation of effective dose from external exposure per unit 137Cs deposition
density with time after the accident for areas with different initial 137Cs deposition density on the
territories of the South trace (red symbols and lines) and the rest of Japan (blue symbols andlines). The cumulative effective doses are shown for the initial deposition densities of 137Cs: 0.01
(upward triangles, dashed lines), 0.1 (circle, solid lines) and 1 (downward triangles, dashed lines)
MBq/m2

38

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

94.
For 137Cs deposition density 100 kBq/m2, M2020 predicts cumulative effective doses
higher than those estimated using M2013 [UNSCEAR, 2014] by 18% for the South trace andabout 50% for the rest of Japan.
95.
Unlike M2013, the cumulative effective dose in M2020 per unit initial deposition densityof 137Cs depends on the latter due to use of non-linear approximations (see equation A-1.3 andtable A-1.2) for radionuclide activity concentration ratios. Correspondingly, for lower 137Cs
deposition density the cumulative effective dose per unit 137Cs deposition density increases,
especially for the South trace (see dashed curves and upward pointing triangles in figure A-1.XV
for 10 kBq/m2 of 137Cs); the relative contribution of shorter-lived radionuclides is greater for theareas with lower radionuclide deposition densities. On the other hand, due to use of non-linear
approximations the difference between cumulative doses for areas in the South trace and in therest of Japan decreases when deposition density 137Cs increases and at 137Cs deposition densityof 1 MBq/m2 becomes almost indistinguishable (see dashed curves and downward pointing
triangles in figure A-1.XV).
96.
Cumulative effective doses, estimated using M2020 and M2013 for initial 137Cs
deposition density 100 kBq/m2, are compared in table A-1.12 for representatives of various age
and social groups for the period 1 year, and 10 years after the accident, as well as for the period
up to age 80, for preschool children, school children and adult workers at the time of the accident.
The selected value of 100 kBq/m2 for 137Cs initial deposition density is representative of nonevacuated areas in Japan with higher deposition densities of radiocaesium.
Table A-1.12. Cumulative effective dose (mSv) from external exposure, estimated for initialdeposition density 100 kBq/m2 of 137Cs as of June 2011, for various members of the public fromtime of the accident onwards
Cumulative effective dose (mSv) for members of the public as of 2011
for initial 137Cs deposition density 100 kBq/m2 in June 2011
Exposure
duration

1-year-old(preschool children)

10-year-old(school children)

M2013

M2013

M2020

M2020

Adult outdoor
worker
M2013

Entire Japan except the South tracea

Adult indoor
worker

M2020

M2013

M2020

1 year

2.1

2.3

1.8

1.9

1.8

2.6

1.6

1.6

10 years

4.9

6.3

4.2

5.4

3.9

7.6

3.8

4.7

Lifetime b

6.7

8.5

6.0

7.6

5.6

10.8

5.6

6.7

The South tracea

1 year

4.9

2.9

4.1

2.4

4.0

3.2

3.7

2.0

10 years

7.6

6.9

6.5

5.9

6.2

8.2

5.9

5.1

Lifetime b

9.4

9.1

8.3

8.1

7.9

11.5

7.7

7.1

a The municipalities of Naraha, Hirono, Yamatsuri and Iwaki City of Fukushima Prefecture, and the towns of Kitaibaraki and

Takahagi of Ibaraki Prefecture.
b Time up to age 80, assuming 20-year-old adult at the time of the accident.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

B.

39

Doses from external exposure in the first year following the
Fukushima Daiichi Nuclear Power Station accident

97.
Doses from external exposure in the first year following the accident have been estimatedfor non-evacuated municipalities of Fukushima Prefecture (Group 2), for some neighbouring
prefectures (Group 3) and for all other prefectures in the rest of Japan (Group 4). Prefectureaverage doses for Groups 2, 3 and 4 prefectures are shown in table A-1.13. For Group 3
prefectures and for Fukushima Prefecture, the ranges of municipality-averaged doses are given.
For Chiba, Gunma and Iwate (close to Fukushima Prefecture and with enhanced deposition
densities in comparison to other Group 4 prefectures) the prefecture-average doses are tabulated.
For the remaining Group 4 prefectures, the range of prefecture-averaged values is indicated.
Table A-1.13. Estimated municipality- or prefecture-average effective doses from external
exposure in the first year following the accident for those residing in the areas indicated
The first-year effective dose (mSv)
Municipality or prefecture
Adult

10-year-old

1-year-old

Group 2 – Fukushima Prefecture a
Municipalities not evacuated b

0.04–3.6

0.05–4.2

0.06–5.0

Group 3 – the neighbouring prefectures c
Ibaraki Prefecture

0.13–0.73

0.16–0.87

0.19–1.03

Miyagi Prefecture

0.09–0.85

0.11–1.01

0.13–1.20

Tochigi Prefecture

0.28–0.90

0.33–1.07

0.39–1.26

Yamagata Prefecture

0.08–0.11

0.09–0.13

0.11–0.15

Group 4 – the rest of Japand

Chiba Prefecture

0.35

0.42

0.50

Gunma Prefecture

0.23

0.28

0.33

Iwate Prefecture

0.20

0.24

0.29

0.00–0.15

0.0–0.18

0.0–0.21

39 remaining prefectures

a Group 2: members of the public living in the non-evacuated municipalities of Fukushima Prefecture.
b Excluding evacuated areas within Fukushima Prefecture.
c Group 3: members of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.
d Group 4: members of the public living in Chiba, Gunma and Iwate prefectures and the remaining 39 prefectures.

98.
The doses in table A-1.13 are average values within a designated municipality or
prefecture; where ranges of dose are indicated these are the ranges of average values between
different municipalities within one or other prefecture, or between prefectures within a given
Group. They do not represent the variability in individual doses within one or other municipality
or prefecture.
99.
M2020 takes into account age- and social group-specific occupancy factors and different
shielding properties of the typical houses in Japan. Ratios of the average first-year dose for the fourmain population groups considered (see table A-1.8) to that for a “typical adult”, assumed to be an
adult indoor worker living in a wooden one-to-three storey house, are shown in table A-1.14. The
variation in the average first-year dose between the groups ranges from 0.5–1.6.

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Table A-1.14. Ratios of the effective dose from external exposure in the first year to each of thevarious age/population groups of the Japanese population to that of an adult living in a wooden
house and working indoors in a concrete building
Ratio of the first-year effective doses (dimensionless)
Adult worker

Dwelling type10-year-old

1-year-old

1.6

1.2

1.4

0.7

1.2

0.8

0.9

0.5

1.0

0.6

0.7

IndoorOutdoorWooden, one-to-three-storey house

1.0

Wooden fireproof, one-to-threestorey house
Concrete, multi-storey apartment

C.

Cumulative doses from external exposure for the population of Japan

100. The cumulative doses of external exposure for different groups of the population of Japan
were calculated for various areas of the whole country. Conditional on degree of detail of theavailable data on deposition density of radionuclides and on the value of anticipated dose, thecalculated data represent various spatial scales, ranging from municipality-averaged doses inFukushima Prefecture and some proximal prefectures (Groups 2 and 3) to prefecture-averaged
values for the territories in the rest of the country (Group 4). The ranges of the computed values
for the Group 2, 3 and 4 territories are shown in table A-1.15.
Table A-1.15. Estimated municipality- or prefecture-averaged effective doses from external
exposure for adults, 10-year-old children and 1-year-old infants at the time of the accident overthe first year and the first 10 years and to age 80 years
Municipality- or prefecture-averaged effective dose a (mSv)
Age group in
March 2011

Group 2
Fukushima Prefecture b

Group 3Neighbouring prefectures c

Group 4
The rest of Japan d

1-year exposure
Adult

0.04–3.6

0.09–0.74

0.0–0.35

10-year-old

0.05–4.2

0.10–0.88

0.0–0.42

1-year-old

0.06–5.0

0.12–1.0

0.0–0.50

10-year exposure
Adult

0.12–10.6

0.22–2.1

0.0–1.010-year-old

0.13–12.0

0.25–2.3

0.0–1.1

1-year-old

0.16–14.1

0.29–2.7

0.0–1.3

Lifetime exposure to age 80 yearse0.17–15.1

0.30–2.9

0.0–1.4

10-year-old

0.18–16.7

0.34–3.2

0.0–1.6

1-year-old

0.21–19.0

0.38–3.7

0.0–1.78

Adulta The reported doses are ranges of municipality-average doses for the Group 2 and Group 3 prefectures and ranges of prefectureaverage doses for the Group 4 prefectures. The ranges of dose tabulated reflect the variation in average doses between different
municipalities within one or other prefecture, or between prefectures within a given Group; they do not reflect the variability ofdoses received by individuals within one or other designated municipality or prefecture.
b Non-evacuated municipalities of Fukushima Prefecture.
c The prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.
d The prefectures of Chiba, Gunma and Iwate and the remaining 39 prefectures.
e Assumed to be a 20-year old.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

D.

41

Post-remediation doses

101. Since 2013, extensive remediation work has been underway in the regions of Japan withthe higher deposition densities to reduce the dose rate and concentrations of radionuclides inareas from which people were evacuated (special decontamination area (SDA)) or continue tolive and grow food (intensive contamination survey area (ICSA)). This work includes the use oftechnologies for decontamination of inhabited areas, and of countermeasures in agriculture (suchas top soil removal or reverse tillage or ploughing, additional fertilization, etc.) and in forestry(such as the removal of fallen leaves and other plant material). The experimental studies andtests in Fukushima Prefecture were basically completed in 2012. A large-scale environmental
remediation programme was then launched in the affected areas of Fukushima Prefecture andsome neighbouring prefectures.
102. In some ICSA areas, local authorities initiated early remediation activities focused mostly
on public areas and, especially, in children’s and public facilities (kindergartens, schools,
hospitals and so on). Similar work, although in the temperate European environment, wasintensively conducted in the Chernobyl-affected areas three decades ago, and the conclusions
and recommendations from this work were summarized by the Chernobyl Forum [IAEA, 2006]
and UNSCEAR [UNSCEAR, 2011].
103. The most intensive remediation in Japan was conducted in 2013–2017, in 11 SDA
municipalities of Fukushima Prefecture. This work included implementation of various tested
clean-up technologies in public facilities, residential areas, farmland, forest areas and roads[IAEA, 2015a]. Remediation of the SDA was completed at the end of March 2017. Postremediation monitoring, implemented six months to a year after the remediation work wascompleted, demonstrated a further reduction in dose rates in all remediated environments.
104. The effectiveness of remediation when assessing public doses is normally quantified using
the dose reduction factor (DRF) 4 which subtracts the effects of radioactive decay and migration ofradionuclides in the soil [Balonov et al., 1992; IAEA, 2006; IAEA, 2015c; Ulanovsky et al., 2011].
The Japanese Ministry of the Environment (MOE) made an estimate of the external dose reduction
in 2015 [MOE, 2020a], based on the results of 330,000 air dose rate measurements taken before
and after remediation and conducted in five types of areas 5 (public facilities, residential areas,
roads, farmland and forests), between March 2012 and October 2013, in 10 SDA municipalities inFukushima Prefecture and 90 ICSA municipalities in 8 prefectures. To estimate the annual dose
for residents and, separately, for schoolchildren, a simple model was applied which multiplied thedose rate by the duration of a person’s stay in each relevant location. The effects of decay ofradionuclides and their migration were subtracted from the total decrease in the dose for the period
under consideration (August 2011 to August 2013); the model used for the analysis was notdescribed and the parameters of migration were not reported. The results of the MOE model
assessment of the efficiency of remediation are shown in table А-1.16 [MOE, 2020a].
105. According to MOE, the effectiveness of large-scale remediation was to reduce the dosefrom external exposure by about 27% for residents of the SDA and somewhat less, about 22%,for residents of the ICSA. The effectiveness of remediation was similar for children. The
percentage reductions shown in table A-1.16 correspond to DRFs of about 1.4 for the SDA and
1.3 for the ICSA.

4

DRF is the ratio of dose before remediation to dose after remediation assessed with account for radioactive
decay and radionuclide migration.
5
No measurements conducted in dwelling houses.

42

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

Table A-1.16. Average reduction (in %) of effective dose from external exposure from theaccident at the Fukushima Daiichi Nuclear Power Station to residents of remediatedmunicipalities over two years (August 2011–August 2013) [MOE, 2020a]
Population

Area

Total dose
reduction (%)

Contribution ofnatural processes (%)

Contribution ofremediation (%)

General public

SDA

67

40

27

ICSA

62

SDA

66

ICSA

64

Schoolchildren

22
40

26
24

106. Tsubokura et al. [Tsubokura et al., 2019] assessed the effectiveness of remediation at asite in the ICSA (Minamisoma City) based on measurements of individual external dose on
residents. In the earliest period (2013–2014), the reduction in the annual dose in areas that had
been remediated was greater than that in areas that had not been remediated by 18% in adults
and 14% in children. 6 In the following year (2014–2015), this difference was 9% in adults and
10% in children, and in 2015–2016, no reliable difference was found. In the same municipality,
Minamisoma City, Murakami et al. [Murakami et al., 2019] estimated the reduction in annual
dose due to remediation to be 11%, based on individual dose measurements. These
measurements on people suggest slightly lower DRFs of 1.1–1.2 in the ICSA.
107. Based on the MOE model calculations [MOE, 2020a; MOE, 2020b] and the partial
validation of these calculations by measurements of individual external dose [Murakami et al.,2019; Tsubokura et al., 2019], the Committee has judged that remediation has resulted in a dose
reduction factor of about 1.3 in the SDA and of about 1.1 to 1.2 (depending on initial dose rateand decontamination timing) in the ICSA. While these dose reduction factors can be used toprovide an indication of the benefits of the remediation work, more reliable estimates of thedoses to the public taking account of remediation require much more extensive information frompost-remediation measurements of individual doses on people. The results of such postremediation monitoring have not yet been published or provided to the Committee.
108. The dose reduction factors are in broad agreement with remediation carried out in 1989
in the Bryansk region, an area with some of the highest levels of radionuclide deposition
following the Chernobyl accident, where a DRF of around 1.3 was estimated [Balonov et al.,1992]. Ulanovsky et al. [Ulanovsky et al., 2011] estimated a DRF of 1.5 for rural areas ofBelarus, the Russian Federation and Ukraine affected by the Chernobyl accident.
109. The completion of the extensive remediation work has enabled the earlier lifting of
evacuation orders and the gradual return of people. The return of people has not yet been
permitted to the “Areas where Returning is Difficult” in six municipalities (entire Futaba andOkuma Towns and parts of Namie and Tomioka Towns, Iitate and Katsurao Villages). In thesesix municipalities, further remediation is continuing with the aim of lifting the remaining
evacuation orders within the following four to five years.
110. The Committee’s estimates of the external doses that were or would be received in
2019–2021 and up to the age of 80 years by those who were evacuated if they returned to theirhomes and regular lifestyles are shown in tables A-1.17 and A-1.18 for adults and for 1-year-old
infants at the time of evacuation in 2011, respectively. They are based on M2020 and a DRF of
1.3 for the SDA to take account of remediation.
6

Samples with less than 40 measurements were excluded.

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

43

Table A-1.17. Annual and lifetime effective doses from external exposure of adults (indoor workers)
who were evacuated from municipalities in Fukushima Prefecture if they returned to their homes
Effective external dose (mSv) in the period
Municipality
2019

2020

2021

Lifetime a

Futaba Town

2.6

2.4

2.1

38

Hirono Town

0.090

0.080

0.080

1.3

Iitate Village

0.94

0.83

0.75

13

Katsurao Village

0.58

0.52

0.46

7.7

Kawamata Town

0.12

0.11

0.090

1.7

Kawauchi Village

0.12

0.10

0.090

1.5

Minamisoma City (Odaka)

0.15

0.13

0.12

2.0

Namie Town

1.2

1.1

0.95

16

Naraha Town

0.15

0.14

0.12

2.2

Okuma Town

2.3

2.1

1.9

32

Tamura City

0.050

0.040

0.04

0.69

Tomioka Town

1.2

1.1

0.99

18

a The period for adults aged 20 at the time of the accident from their return in 2019 to age 80.

Table A-1.18. Annual and lifetime effective doses from external exposure of 1-year-old infants atthe time of the accident who were evacuated from municipalities in Fukushima Prefecture if they
returned to their homes
Effective external dose (mSv) in the period
Municipality
2019

2020

2021

Lifetime a

Futaba Town

3.3

2.9

2.6

40

Hirono Town

0.12

0.10

0.090

1.4

Iitate Village

1.2

1.0

0.93

14

Katsurao Village

0.72

0.64

0.57

8.5

Kawamata Town

0.15

0.13

0.12

1.8

Kawauchi Village

0.14

0.12

0.11

1.8

Minamisoma City (Odaka)

0.18

0.16

0.15

2.2

Namie Town

1.5

1.3

1.2

18

Naraha Town

0.19

0.17

0.15

2.4

Okuma Town

2.9

2.5

2.3

35

Tamura City

0.050

0.050

0.050

0.69

Tomioka Town

1.5

1.4

1.2

19

a The period for 1-year-old infants at the time of the accident from their return in 2019 to age 80.

111. The external doses calculated for adults using M2020 in table A-1.17 have been
compared with recent individual measurements of 239 adults, mostly office workers, living in
10 former evacuated locations, which are listed in table A-1.17, excluding Okuma and Futaba
towns [Nomura et al., 2020]. Measurements of external doses were made with personal
dosimeters worn by the study participants during 14 days in February 2019. The area-averaged

44

ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

measured values correspond to annual dose in 10 locations of 0.93 mSv, including dose frombackground radiation assessed to be 0.54 mSv per year for Fukushima Prefecture. The variability
in the estimated mean annual doses in different locations was small, ranging from 0.7–1.1 mSv.
Deduction of background dose results in a measured area-averaged anthropogenic annual dose
due to residual deposited caesium radionuclides of 0.39 mSv. The M2020 model estimate (seetable A-1.17, excluding Okuma and Futaba Towns) including a DRF of 1.3 was 0.46 mSv, whichis in good agreement with the measured data [Nomura et al., 2020].
112. In their earlier paper, Nomura et al. [Nomura et al., 2019] estimated, inter alia, annual
external doses to 112 people who returned to Odaka Ward (in the SDA) of Minamisoma City
following completion of remediation. The median annual dose to a mixed age group of returnees
in 2017 was estimated as 0.4 mSv from the three-month individual dose measurements. The
Committee’s assessed annual dose in 2017 for indoor workers residing in Odaka Ward using
M2020 is 0.20 mSv for indoor workers including a DRF of 1.3. However, the estimates of
Nomura et al. are for group members who spent more time outdoors than the nominal time
assumed for indoor workers in M2020. With the relevant correction, the modelled annual dosein 2017 becomes about 0.25 mSv, which is in reasonable agreement with the individual dose
estimated by Nomura et al. from measurements.

VI. SUMMARY
113. The follow-up study of population exposures in Japan after the FDNPS accident was
supported by the review and revision of the dose assessment methodology for external exposures
to deposited radioactive materials in human habitats and in the environment. Results of thecritical review of the new findings and data acquired in Japan after the accident suggested somemodifications of the external dose assessment methodology were needed and the careful analysis
undertaken has resulted in formulation of the modified model, termed M2020, for calculation of
the population doses due to external exposure from radionuclides deposited on the ground. The
definition of parameters of M2020 has been based on a comprehensive set of data obtained fromextensive and systematic monitoring of affected areas of Japan. M2020 is, therefore, tailored toJapan-specific environmental and social conditions and the post-accidental reality. It inherits
many of the features of the model used in the UNSCEAR 2013 Report, M2013, that was largely
based on data from global fallout and experience following the accident at the Chernobyl Nuclear
Power Station; but, in addition, it has benefited from new observations specific to Japan.
114. M2020 also benefits from the new age- and sex-dependent dose rate coefficients forexternal environmental radiation sources. Although the changes due to introduction of the newdose rate coefficients are not large, nor do they compromise the dose estimates in the UNSCEAR
2013 Report, they provide the most up-to-date values currently available. Their use enables
organ-specific dose estimates for both sexes and explicitly demonstrates the small range ofvariability associated to mean sex- and organ-specific dose rate coefficients for external exposure
to environmental sources.
115. One of the most important changes in M2020 is related to the observed dynamics of the
reduction in external dose rate with time due to radioactive decay and various natural processes,of which migration in soil is the most influential. The slower dose rate reduction observed within
5–6 years following the accident was not anticipated based on experience elsewhere and theprocesses contributing to this slower decrease in dose have yet to be fully understood. Further
studies of the physical and chemical properties of the deposited radioactive material, its
composition and environmental behaviour may provide necessary information to explain theseobservations.

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45

116. The environmental conditions in the affected areas of Japan differ substantially from thepreviously studied areas in Europe and North America, due to presence of mountainous areas,high anthropogenic use of available flatlands and relatively high population density in Japan. It
is likely that these differences have contributed to the differences observed relative to experienceelsewhere, which required to reconsider the location factors and their dynamics and to use the
new approximations to reproduce as observations of ambient dose rate dynamics results of thepersonal dosimetry studies among the population of Fukushima Prefecture.
117. It was already noticed [IAEA, 2015b; UNSCEAR, 2014] that radionuclide compositionof the deposited radioactive material varied considerably across Fukushima Prefecture and inother parts of Japan. M2020 introduces new phenomenologically-based non-linear
approximations that better address ratios of deposition densities of radionuclides 131I, 132I, 129mTeand 132Te to that of 137Cs, which were obtained from the environmental measurements, including
recently updated values for 131I based on measurements of the longer-living anthropogenic iodine
isotope 129I. Use of the new non-linear approximations enabled account to be taken of systematicdifferences of radionuclide composition of deposited material between locations with high andlow deposition density of 137Cs (see figure A-1.XV).
118. M2020 was validated using: independent results from the FHMS; a number of personal
dosimetry studies reported in the peer-reviewed literature; and monitoring data provided by themunicipalities of Minamisoma City and Naraha Town. The M2020 predictions were within afactor of about two of the measured doses and provided a better fit to the measurements thanM2013, the model used in the UNSCEAR 2013 Report (see figure A-1.XV).
119. Population-weighted average external doses from deposited radioactive material havebeen calculated using M2020 for the following reference age/social groups of the public (as of2011) permanently residing in a prefecture or municipality: 1-year-old infants, 10-year-old
children, 20-year-old adults (indoor workers) living in wooden houses. For people living in othertype of houses (wooden fireproof or concrete multi-storey apartments), ratios of the effective
dose from external exposure in the first year for each of the various age/social groups of theJapanese population to that of an indoor worker living in a wooden house have been provided.
Both average effective and organ absorbed doses to the three age/social groups of the public
were assessed for the following time periods: 1 year and 10 years after the accident, and up to
age 80. Additionally, area-averaged in-utero doses to red bone marrow of children born inMarch–December 2011 and fetal thyroid doses of children born in March–October 2011 inFukushima Prefecture (excluding evacuated areas) were calculated. Both average effective andorgan absorbed doses from external exposure to deposited radionuclides were assessed forevacuees up to the end of the first year after the accident. Average annual effective doses to thethree age/social groups of the public in 2021 were also estimated. Collective effective and organ
absorbed doses from external exposure received by population of Japan over 1, 10 and 80 years
were assessed. External dose estimates are tabulated in attachments A-13 to A-20. In theseestimates, no account has been taken of the effects of remediation; the available information
indicates that, as a result, doses may have been overestimated by at most 10–20%, which is small
compared with the inherent uncertainties in the dose assessment.
120. Estimates have been made of the uncertainty in external doses predicted using M2020
taking account of uncertainties in the input data and model parameters. The 90% confidence
interval, averaged over locations and population groups and expressed in terms of the respective
AM, was 0.54–1.67 for exposures during the first decade and 0.49–1.76 for lifetime exposures.
Given the magnitude of these uncertainties, the small differences observed between measured
and modelled doses have little statistical significance.

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ANNEX B, ATTACHMENT A-1: THE ESTIMATION OF EXTERNAL DOSES FROM DEPOSITED RADIONUCLIDES …

121. Analysis of anonymous personal dosimetry data provided by the municipalities of
Minamisoma City and Naraha Town enabled individual dose distributions to be derived. For the
majority of the data sets, the individual doses were log-normally distributed, mostly left-censoredand sometimes showing signs of multimodality (e.g., when individual doses for 5–10% of thosein the studied group were higher than could be inferred from properties of distribution for the
rest of the group). Under the assumption of log-normality, the estimated statistical parameters
corresponding to normalized 90% confidence intervals range from 0.3–0.4 to 2–2.4, expressing
the degree of individual variability of the measured external doses.
122. Estimates have also been made of external doses from deposited radioactive material for
evacuees were they to return to their homes. These were based on M2020 with a dose reduction
factor to take account of remediation of 1.3.

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47

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