United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENT 2

DEVELOPMENT OF ISODOSE MAPS REPRESENTINGANNUAL EXTERNAL EXPOSURE IN JAPAN AS A
FUNCTION OF TIMEDevelopments since the 2013 UNSCEAR Report on the levels and effects ofradiation exposure due to the nuclear accident following the great east-Japan
earthquake and tsunami
A 2015 white paper to guide the Scientific Committee’s future programme of work

Notes
The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the United Nations
concerning the legal status of any country, territory, city or area, or of its authorities, orconcerning the delimitation of its frontiers or boundaries.

©

United Nations, February 2016. All rights reserved, worldwide.

This publication has not been formally edited.

ATTACHMENT 2: DEVELOPMENT OF ISODOSE MAPS [...] 3Contents
I. INTRODUCTION .................................................................................................................. 3
II. ESTIMATES OF EFFECTIVE DOSE FROM EXTERNAL EXPOSURE
INTO THE FUTURE ............................................................................................................. 3
III. PREPARATION OF ISODOSE MAPS REPRESENTING ANNUALEFFECTIVE DOSE TO ADULTS FROM EXTERNAL EXPOSURE ................................ 6
ACKNOWLEDGEMENTS ......................................................................................................... 6
REFERENCES ............................................................................................................................. 6I.

INTRODUCTION

1.
In its 2013 Fukushima report [UNSCEAR, 2014], the Committee had madeestimates (a) of district- and prefecture-average doses and (b) of accumulated doses in thefirst year after the accident, over the first 10 years, and up to age 80 years. The Committee
has since further analysed the data and results presented in the 2013 Fukushima report toproduce an animated map of the estimates of annual adult effective dose from external
exposure into the future. The values for annual dose from external exposure per unit
deposition density of 137Cs in figure C-IX of [UNSCEAR, 2014] have been interpolated toderive estimates for each year up to 10 years after deposition, and for every tenth year up to
100 years after deposition. The data and methods used are described in this attachment.

II.

ESTIMATES OF EFFECTIVE DOSE FROM EXTERNALEXPOSURE INTO THE FUTURE

2.
The estimates of doses from external exposure in [UNSCEAR, 2014] were based
mainly on data for measured ground deposition density and ambient dose equivalent rate
provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
MEXT had compiled data on measured ambient dose equivalent rates and soil samples
collected between 6 June and 8 July 2011 for areas within 100 km of the Fukushima Daiichi
Nuclear Power Station (FDNPS). Out to 80 km from FDNPS these had been undertaken for
2 km × 2 km grids. Soil samples had been collected at five points for each location. Around
11,000 soil samples had been taken; and the deposition densities of five gamma-emitting
radionuclides (110mAg, 129mTe, 131I, 134Cs and 137Cs) had been measured using germanium
semiconductor detectors. The Committee reproduced the data it had used from the MEXT
survey of 137Cs deposition density in attachment C-2 of the 2013 Fukushima report[UNSCEAR, 2014] as an Excel® workbook with geospatial information on a 1 km grid.
3.
In [UNSCEAR, 2014], the Committee had made estimates (a) of district- andprefecture-average doses and (b) accumulated doses in the first year after the accident, overthe first 10 years, and up to age 80 years. The Committee had estimated doses from external
exposure for future years using similar dosimetric methods as for the first year, but taking

ATTACHMENT 2: DEVELOPMENT OF ISODOSE MAPS [...] 4

account of the reduction in dose rate due to radioactive decay and the implied effects ofremoval of radionuclides by natural physico-chemical processes over the period of exposure.
The detailed methodology used was presented in attachment C-12 to [UNSCEAR, 2014].
4.
Figure C-IX in [UNSCEAR, 2014], reproduced in figure I below, provides estimatesof the accumulated effective dose from external exposure, projected forward over 100 years
after the accident. The estimates of how the contributions from 134Cs and 137Cs fall with time
take account of radioactive decay and the natural movement of caesium into surfaces
assumed to be similar to that derived from data collected over the years since the Chernobyl
accident. Because detailed information was lacking on the implementation and effectivenessof remediation at specific locations, no account was taken of the possible reduction in
exposure from future remediation. The estimates of accumulated dose from external
exposure used to prepare figure I are tabulated in table 1.
Figure I. Estimated accumulated effective dose per unit deposition density from external
exposure of typical adults living in areas covered by the south trace and in the rest of Japan(figure C-IX [UNSCEAR, 2014])
The estimated doses are normalized to a deposition density of 1 MBq m−2of137

Cs

5.
The values in table 1 were interpolated to derive estimates of the annual dose fromexternal exposure, for each year up to 10 years after the initial deposition, and for every
tenth year up to 100 years after deposition. These estimates are summarized in table 2 andare derived for the locations outside of the south trace. However, after the first year andfollowing the radioactive decay of the short-lived radionuclides, these values are applicablefor locations in both the south trace and the rest of Japan.

ATTACHMENT 2: DEVELOPMENT OF ISODOSE MAPS [...] 5

Table 1. Estimated accumulated effective dose to adults from external exposure per unit
deposition density, for locations in the south trace and for the rest of Japan(derived from figure C-IX of [UNSCEAR, 2014])

137

Cs

The number of significant figures presented in the table are used for calculational purposes only; they do not
imply such a degree of accuracy in the underlying quantities

Time after deposition(years)Accumulated effective dosefrom external exposure, south trace(mSv/MBq m−2)

Accumulated effective dosefrom external exposure, rest of Japan(mSv/MBq m−2)

0.01

8.4

1.31

0.03

15.3

2.55

0.05

17.6

3.14

0.1

19.6

4.02

0.3

22.4

6.54

0.5

24.6

8.64

1

28.8

12.9

3

37.6

21.7

5

41.5

25.5

10

46.1

30.2

30

54.6

38.6

50

58.542.5

100

61.5

45.5

Table 2. Interpolated values of annual effective dose and accumulated effective dose from137
external exposure per unit Cs deposition density, derived for locations outside of the southtrace (derived from figure C-IX of [UNSCEAR, 2014])
Time after deposition(years)Annual effective dose(mSv/MBq m−2)

Accumulated effective dose(mSv/MBq m−2)

1

12.9

12.9

2

5.5

18.4

3

3.3

21.7

4

2.2

23.9

5

1.6

25.5

6

1.2

26.7

7

1

27.7

8

0.90

28.6

9

0.85

29.5

100.7530.2

200.4935.1

300.3538.6

400.2340.9

500.1642.5

1000.0345.5

ATTACHMENT 2: DEVELOPMENT OF ISODOSE MAPS [...] 6III. PREPARATION OF ISODOSE MAPS REPRESENTINGANNUAL EFFECTIVE DOSE TO ADULTS FROM EXTERNALEXPOSURE
6.
The Committee had presented the data from the MEXT survey of 137Cs deposition
density for a 1 km grid as an Excel® workbook with geospatial information in attachment
C-2 of [UNSCEAR, 2014]. This workbook provided the values for the 137Cs grounddeposition density (as at 14 June 2011) for approximately 2,200 grid cells, together with thelatitude and longitude values for the grid. This dataset had been spatially interpolated using
an Inverse Distance Weighting algorithm to estimate ground deposition density values for
all the 1 km grid cells in Fukushima Prefecture and neighbouring prefectures.
7.
For each grid cell, the adult effective dose from external exposure, as a function oftime after initial deposition, was estimated from the product of the 137Cs deposition density
on the ground and the conversion factor for annual effective dose per unit deposition density
on the ground (table 2). The annual effective dose to adults from external exposure was thus
estimated for each 1 km grid cell and for each year from years 1 to 10, then at every
10 years from years 10 to 50.
8.
The estimates of annual effective dose from external exposure for each year wereanalysed to derive an isodose line representing a dose of 1 mSv in a year. These isodose
lines were then overlaid on a portion of the World Topographical map covering Fukushima
Prefecture, or exported as shape files for use in other geospatial information systems. Maps
showing the isodose lines for years 1, 3, 10 and 50 are shown in figure II. The maps andisodose curves for each year have been combined into a single file to provide an animatedmap of the change in the isodose curve for an annual effective dose of 1 mSv from external
exposure into the future. This animation can be downloaded from http://www.unscear.org/unscear/en/publications/Fukushima_WP2015.html.

ACKNOWLEDGEMENTS
The Committee would like to express its appreciation to S. Solomon (Australia), who led
the preparation of material for this attachment.

REFERENCEUNSCEAR. Sources, Effects and Risks of Ionizing Radiation. Volume I: Report to theGeneral Assembly and Scientific Annex A. UNSCEAR 2013 Report. United Nations
Scientific Committee on the Effects of Atomic Radiation. United Nations sales
publication E.14.IX.1. United Nations, New York, 2014.

Figure II. Maps showing the isodose line for an annual effective dose of 1 mSv from external exposure to adults in years 1, 3, 10 and 50 after deposition

ATTACHMENT 2: DEVELOPMENT OF ISODOSE MAPS [...] 7