United Nations Scientific Committee on the Effects of Atomic Radiation

S O U R C E S , E F F E C T S A N D R I S K S O F I O N I Z I N G R A D I AT I O N

UNSCEAR 2020/2021 Report

Volume II
SCIENTIFIC ANNEX B:
Levels and effects of radiation exposure due to the accident at
the Fukushima Daiichi Nuclear Power Station: implications of
information published since the UNSCEAR 2013 Report

SOURCES, EFFECTS AND RISKSOF IONIZING RADIATION
United Nations Scientific Committee on the
Effects of Atomic Radiation
UNSCEAR 2020/2021
Report to the General Assemblywith Scientific Annexes

VOLUME II
Scientific Annex B

UNITED NATIONSNew York, 2022

NOTE
The report of the Committee without its annexes appears as Official Records of the General
Assembly, Seventy-sixth Session, Supplement No. 46 (A/76/46).
The designations employed and the presentation of material in this publication do not imply
the expression 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.
The country names used in this document are, in most cases, those that were in use at the time thedata were collected or the text prepared. In other cases, however, the names have been updated,
where this was possible and appropriate, to reflect political changes.
The attachments cited in this annex are electronically available for download fromhttp://www.unscear.org/unscear/en/publications/2020_2021_2_Attachments.html

UNITED NATIONS PUBLICATION
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This publication has not been formally edited.
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ANNEX BLEVELS AND EFFECTS OF RADIATION EXPOSURE
DUE TO THE ACCIDENT AT THE FUKUSHIMADAIICHI NUCLEAR POWER STATION:IMPLICATIONS OF INFORMATION PUBLISHED
SINCE THE UNSCEAR 2013 REPORTCONTENTS
LIST OF ABBREVIATIONS......................................................................................................................................... 3
I.

INTRODUCTION................................................................................................................................................. 5

II.

RADIONUCLIDE RELEASES TO ATMOSPHERE, DISPERSION AND DEPOSITION............................ 10
A. Introduction............................................................................................................................................ 10
B.

Overview of current understanding ................................................................................................ 11

C. Summary ................................................................................................................................................. 19
III. RADIONUCLIDE RELEASES TO, AND DISPERSION AND DEPOSITION IN, THE MARINEENVIRONMENT ....................................................................................................................................................... 20
A. Introduction............................................................................................................................................ 20
B.

Overview of current understanding ................................................................................................ 21

C. Summary ................................................................................................................................................. 32
IV. TRANSFER OF RADIONUCLIDES IN TERRESTRIAL AND FRESHWATER ENVIRONMENTS ............ 33
A. Introduction............................................................................................................................................ 33
B.

Overview of current understanding ................................................................................................ 34

C. Remediation ........................................................................................................................................... 43
D. Summary ................................................................................................................................................. 48

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UNSCEAR 2020/2021 REPORT

V. ASSESSMENT OF DOSES TO THE PUBLIC ................................................................................................ 49
A. Introduction ............................................................................................................................................ 49
B.

Overview of dose assessment in the UNSCEAR 2013 Report.................................................... 51

C. Information for dose assessment...................................................................................................... 52
D. Overview of methodology for assessing public exposures....................................................... 53
E.

Results of dose estimation .................................................................................................................. 57

F.

Summary.................................................................................................................................................. 72

VI. ASSESSMENT OF DOSES TO WORKERS .................................................................................................... 74
A. Introduction ............................................................................................................................................ 74
B.

Overview of current understanding ................................................................................................ 75

C. Summary.................................................................................................................................................. 83
VII. HEALTH IMPLICATIONS ................................................................................................................................ 84
A. Introduction ............................................................................................................................................ 84
B.

Overview of current understanding ................................................................................................ 85

C. Summary.................................................................................................................................................. 96
VIII. ASSESSMENT OF DOSES AND EFFECTS FOR NON-HUMAN BIOTA .................................................. 97
A. Introduction ............................................................................................................................................ 97
B.

Overview of current understanding ................................................................................................ 98

C. Summary................................................................................................................................................ 102
IX. CONCLUSIONS .............................................................................................................................................. 102
ACKNOWLEDGEMENTS...................................................................................................................................... 107
APPENDIX A. ASSESSMENT OF DOSES TO THE PUBLIC............................................................................ 109
APPENDIX B. COMPARISON BETWEEN VARIOUS ATTRIBUTES AND CONSEQUENCES OF THE
ACCIDENTS AT CHERNOBYL AND FUKUSHIMA DAIICHI NUCLEAR POWER STATIONS ................... 189
SUPPLEMENTARY INFORMATION ................................................................................................................... 199
REFERENCES .......................................................................................................................................................... 201
GLOSSARY.............................................................................................................................................................. 237

The attachments cited in this annex are electronically available for download fromhttp://www.unscear.org/unscear/en/publications/2020_2021_2_Attachments.html

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

LIST OF ABBREVIATIONS
AMAD

Activity median aerodynamic diameter

ARS

Acute radiation syndrome

ATDM

Atmospheric transport, dispersion and deposition modelling

CNPS

Chernobyl Nuclear Power Station

CR

Concentration ratio

DoD

Department of Defense

DOE

Department of Energy

DRF

Dose reduction factor

EMDB

Environment Monitoring Database

FDNPS

Fukushima Daiichi Nuclear Power Station

FM

Fresh mass

FHMS

Fukushima Health Management Survey

IAEA

International Atomic Energy Agency

ICRP

International Commission on Radiological Protection

ICSA

Intensive contamination survey area

JAEA

Japan Atomic Energy Agency

JST

Japan standard time

M2013

Model used in the UNSCEAR 2013 Report

M2020

Model for estimating external doses from deposited radionuclides. An
update of the model used in the UNSCEAR 2013 Report for estimatingexternal doses from deposited radionuclides

MEXT

Ministry of Education, Culture, Sports, Science and Technology

MHLW

Ministry of Health, Labour and Welfare

MOE

Ministry of Environment

MRI

Magnetic resonance imaging

NIRS

National Institute of Radiological Sciences

OOP

Out of prefecture

RET

Rearranged during transfection

RIP

Radiocaesium interception potential

SDA

Special decontamination area

TEPCO

Tokyo Electric Power Company

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UNSCEAR 2020/2021 REPORTUNSCEAR

United Nations Scientific Committee on the Effects of Atomic Radiation

WBC

Whole-body counting

WSPEEDI

Worldwide version of system for prediction of environmental
emergency dose information

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

5

I. INTRODUCTION
1.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
assessed radiation exposures of the public, workers and non-human biota that resulted from the 11 March2011 accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) of the Tokyo Electric Power
Company (TEPCO) in Japan and reported its findings, including a discussion of the associated healthrisks and health effects, to the United Nations General Assembly in October 2013 [U10]. A full reportwith scientific annexes was published by the United Nations on 2 April 2014 [U10].
2.
Most of the scientific information used in the report of the Committee (referred to as the“UNSCEAR 2013 Report” [U10] hereafter) was limited to that published or provided officially (by theJapanese Government, TEPCO and international organizations) by the end of October 2012.
Subsequently, a significant amount of relevant information has been published and, with time, questions
were anticipated from delegations, government authorities, scientists, civil society and the media about
the continuing validity of the UNSCEAR 2013 Report. Accordingly, the Committee implemented a plan
to maintain awareness of new scientific developments in the follow-up to the accident and published threewhite papers [U11, U13, U14] setting out evaluations of the implications of such developments for theperiod up to the end of 2016.
3.
The results of these reviews of new information generally confirmed the broad findings andconclusions of the UNSCEAR 2013 Report [U10] within the inherent uncertainties [U11, U13, U14].
However, the Committee became aware of increasing evidence that some of the doses to the public were
overestimates, with those from ingestion significantly so [U10, U11, U13, U14]. In addition, there was aconsiderable amount of new information becoming available on the levels of radionuclides in theenvironment, and in particular on concentrations of released radionuclides in the air as a function of timeand on their physico-chemical forms. This information could enable better and less uncertain estimatesof doses to the public, particularly from inhalation. The Committee recognized that carrying out somemore detailed analyses of this information would contribute to an improved understanding andcommunication of differences in the estimates of doses to the public and of the underlying science andits implications. It would also help address inevitable questions within various constituencies and enable
more authoritative statements (with reduced uncertainty) to be made on a number of issues.
4.
Against this background, the Committee decided at its sixty-fifth session (11–14 June 2018) toprepare a report summing up all of the information available (up to the end of 2019) 1 on the levels andeffects of radiation exposure due to the FDNPS accident and on the implications of this information. It
was agreed that the report should be based on reviews of the peer reviewed literature supplemented bylimited, but more detailed, analyses. These analyses were to focus on doses to members of the public
where more recent information, specifically from measurements on people and from measurements of
the levels of radionuclides in the environment, would support estimates appreciably different from thosepresented in the UNSCEAR 2013 Report [U10], and on understanding the reasons for, and implicationsof, the differences. The aim was to provide an authoritative and updated assessment of the levels andeffects of radiation exposure due to the FDNPS accident.

1 The Committee has, exceptionally, taken account of information that became available after this date, where it would affect thefindings of the report.

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UNSCEAR 2020/2021 REPORT5.
The reviews of the literature reported in the three published white papers and carried out
subsequently have been conducted by a group of scientific experts (the Expert Group), supported by agroup of scientific experts from Japan (the Japanese Working Group), and under the direction of twosenior technical advisers, and a project manager. The detailed analyses were conducted by expert task
groups set up for the purpose under the leadership of relevant members of the Expert Group and withinvolvement of relevant members of the Japanese Working Group. Some of the experts were assisted intheir work by supplementary staff in their national institutes. An expert supported by Japan assisted thesecretariat in Vienna. All involved were required to declare any potential conflicts of interest, which werereviewed by the secretariat to confirm that there were no conflicts of interest for the work in which theexperts were involved. The work was carried out in accordance with a quality plan for the project.
6.
The secretariat provided support to the technical work, inter alia, by arranging a visit by the Expert
Group to Japan to discuss research work being carried out in Japan on the levels and effects of radiation
exposure due to the FDNPS accident with Japanese researchers, by convening meetings in Vienna of thetask groups carrying out the detailed analyses to facilitate the planning and implementation of this work,by providing a platform for online meetings and an online workspace for sharing and managing data andinformation, and by liaising with governments and other international organizations. The Governmentsof Australia, France, Germany, Japan, Norway and the United Kingdom made in kind contributions
through expertise in this project.
7.
The aim of this scientific annex is to provide a summary of all of the scientific information available(up to the end of 2019) relating to the levels and effects of radiation exposure due to the FDNPS accident
and an appraisal of the implications of this information for the UNSCEAR 2013 Report [U10]. Specificobjectives are to:(a) Summarize all of the information available and assess its implications for the findings andconclusions presented in the UNSCEAR 2013 Report;(b) Validate and, where necessary, revise estimates of doses to the public, based on more detailed
analyses of the available information, and update the commentary on the health implications;(c) Set out an improved appraisal of the uncertainties and variabilities in the estimates of doses tothe public;(d) Where possible, better address issues and objectives not fully addressed in the UNSCEAR 2013
Report.
While self-standing, the annex is intended to be read in conjunction with the UNSCEAR 2013 Report
and the subsequent white papers and does not repeat all of the information available in these publications.
In particular, it does not repeat detailed background or contextual information set out in the UNSCEAR
2013 Report or in the subsequent International Atomic Energy Agency (IAEA) report on the accident[I5], for example, about the chronology of the accident or the actions taken in response, where
understanding has not significantly changed. Where relevant, the reader is, instead, referred to thesereports, where further information can be found.
8.
The annex does not address policy issues with respect to human rights, public health protection,
environmental protection, radiation protection, emergency preparedness and response, accident
management, nuclear safety, radioactive waste management, prospective releases and related issues; itdoes not intend to provide advice to local governments, the Government of Japan or to national andinternational bodies. The annex also does not address other effects (not associated with exposure toradiation) that can arise as a result of accidents such as that at FDNPS, including distress and anxiety
from, among other things, disruption of life, loss of homes and livelihoods, and social stigma, which canANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

7

have major impacts on mental and social well-being. Evaluating such effects is not part of theCommittee’s mandate; however, they are important for understanding the broader health implications of
the accident.
9.
The annex comprises a main text with nine chapters and two appendices, supported by
23 attachments. Chapter I introduces the aim, background, scope and method of working. Chapters II to
VIII set out summaries of all of the information available (up to the end of 2019) in each of the following
areas relevant to the understanding of the levels and effects of the FDNPS accident:−

Chapter II: Radionuclide releases to atmosphere, dispersion and deposition;−

Chapter III: Radionuclide releases to, and dispersion and deposition in, the marine environment;−

Chapter IV: Transfer of radionuclides in terrestrial and freshwater environments;−

Chapter V: Assessment of doses to the public;−

Chapter VI: Assessment of doses to workers;−

Chapter VII: Health implications;−

Chapter VIII: Assessment of doses and effects for non-human biota;−

Finally, chapter IX sets out conclusions.

10. Appendix A describes the Committee’s updated dose assessment and presents revised estimates ofthe doses to members of the public. Appendix B contains a comparison between the accident at FDNPS
with the accident at Chernobyl to provide an overview of the main differences and similarities between
them. Supplementary information lists the supporting attachments. Where numerical values have been
estimated by the Committee, they are generally quoted to two significant figures (although, for estimateddoses of the order of a few microsieverts (µSv) or less, only one significant figure has generally beenquoted); where numerical values are quoted directly from other publications, the number of significant
figures used in the original publication have been given. This enables better comparison between values,
however the values themselves are normally associated with considerable uncertainty, and this degree ofprecision should not be inferred.
11. FDNPS lies in Fukushima Prefecture (see figures I(a) and (b)) of the Tōhoku region in Japan. It is
located about 230 km north-east of Tokyo on the east coast of Japan. On 11 March 2011, an earthquakeof magnitude 9.0 occurred along the Japan Trench. The earthquake and the following tsunami triggereda severe nuclear accident at FDNPS. In the UNSCEAR 2013 Report [U10], the Committee provided an
overview of the main events that took place at the FDNPS site that led to releases of radioactive material
into the environment and the measures taken by the Japanese authorities to protect the public and workers.
In its subsequent report, IAEA [I5] has provided a more detailed description of the accident, including
its chronology and context, and the actions taken in response.

UNSCEAR 2020/2021 REPORTThe boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by the United Nations.

8

Figure I(a). Map of Fukushima Prefecture showing its municipalities, the location of the Fukushima Daiichi Nuclear Power Station, the 20 km restricted area
and the 30 km evacuation prepared area

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

9

Figure I(b). Map showing nearby prefectures [U10]

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations.

12. The measures taken by the Japanese authorities included immediate and late (“deliberate”)
evacuation, sheltering in homes, iodine prophylaxis, and restricting distribution and consumption ofcontaminated foodstuffs (milk, vegetables, grains, meat, fish, etc.) and water. These actions were
supported by radiation surveys of people and places. People within 3 km of FDNPS were ordered to be
evacuated late on 11 March 2011, and those within 20 km were ordered to be evacuated on 12 March
2011. On 15 March 2011, people living between 20 km and 30 km from FDNPS were ordered to shelterindoors, and on 25 March 2011 were advised to begin voluntary evacuation and be prepared to evacuate
depending on future developments at FDNPS. In addition, environmental monitoring revealed that there
were areas where radioactive material had been deposited at high levels 2 outside of the 20-km evacuation
zone. “Deliberate evacuation areas” were established for specific areas where the effective dose might
exceed 20 mSv within a year, and most residents of these areas were then evacuated between April andJune 2011. More localized areas (“specific spots recommended for evacuation”) were designated from

2 Average values of deposition density for 131I ranged from 0.2 to 25 MBq/m2 and for 137Cs from 0.02 to 3.7 MBq/m2 [U10].

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UNSCEAR 2020/2021 REPORT

June 2011 onwards, and the local authorities provided advice to potentially affected residents on theiroptions for relocating or remaining and information on how to mitigate future radiation exposures. The
localized areas included areas within Date City, Minamisoma City, and Kawauchi Village. Decisions
were made and instructions issued on taking stable iodine, but were not fully implemented. On 16 March
2011, an instruction was issued that anyone still remaining within 20 km of FDNPS should take stableiodine, but this instruction was not implemented, because the area was considered to have already been
evacuated. On 14 March 2011, the Fukushima Prefectural government decided to distribute two stableiodine tablets to each resident younger than 40 years of age between 20 km and approximately 50 km of
FDNPS, distributing approximately 1 million stable iodine tablets by 20 March 2011, but administration
was not implemented uniformly. The UNSCEAR 2013 Report [U10] and the IAEA report [I5] provide
further details.

II. RADIONUCLIDE RELEASES TO ATMOSPHERE, DISPERSION
AND DEPOSITION

A. Introduction
13. In the UNSCEAR 2013 Report [U10], the Committee presented a summary of the main events inthe progression of the accident at FDNPS that followed the earthquake and tsunami [U10]. Several ofthese events led to releases of radioactive material to the environment. Estimates of the amounts andtemporal pattern of the releases to the atmosphere, and understanding of the transport and dispersion of
the released material in the air and its deposition on to the ground are summarized in this chapter;
estimates of the releases to the marine environment are summarized in chapter III.
14. Radioactive material was released to the atmosphere from FDNPS over an extended period. The
pattern of release was complex, both temporally and spatially. Releases began on 12 March 2011 and therate of release varied considerably in magnitude over the following week, with marked increases
associated with particular events at each reactor unit (e.g., hydrogen explosions, venting, and leakage
from the reactors and their containment systems). After the first week, the rates of release gradually
declined, albeit with some fluctuations over more limited periods. By the beginning of April 2011, therelease rates were thousands of times less than the release rates that occurred during the first week of theaccident, although these much lower release rates persisted for many weeks [U10].
15. During the preparation of the UNSCEAR 2013 Report [U10], the Committee was provided withaccess to the results of extensive measurements that had been made in Japan, particularly of the levels of
various radionuclides deposited on the ground, and based its assessment of doses to the public largely onthese measurement data. There were gaps in the measurement data, however, and there was little
information available on concentrations of released radionuclides in the air and how they varied withtime, so the Committee had to make use of modelling to supplement the measurement information.
A considerable amount of measurement data has become available since the publication of the
UNSCEAR 2013 Report and all of the measurement information available is summarized in this chapter.
This information has provided an important input into the detailed analyses described in appendix A,
aimed at validating and, where necessary, revising the approaches used to estimate doses to the public in
the UNSCEAR 2013 Report.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

11

B. Overview of current understanding
1.

Measurement data

16. Dose-rate measurements from automatic stations within Japan were the most abundant data available
over the course of the accident, although in Fukushima Prefecture most automatic monitoring posts were
inoperative due to loss of communication and power, and measurements there came mostly from portabledose-rate monitors, such as hand-held type survey-meters and the so-called car-borne survey system. In
addition, extensive surveys were made of radionuclides deposited on the ground and in soils following theaccident, and also of dose rates due to deposited material. Measurements of concentrations of radionuclides
in air over Japan while the release was happening were much more limited, especially in the early stages of
the accident and in the areas devastated by the tsunami. Measurements of radionuclides in Japan werelargely focused on 131I, 134Cs and 137Cs. Limited data were also available for other radionuclides, such as132
Te, 129mTe, 132I and 133I, including both measurements of concentrations in the air and measurements ofdeposition density on the ground. The measurement information available at the time of the UNSCEAR
2013 Report [U10] was described in detail in that report.
17. At the time of the FDNPS accident, there were about 70,000 United States Department of Defense(DoD) affiliated individuals based in Japan. In response to the accident, DoD provided humanitarianassistance and disaster relief and developed radiation dose assessments for the DoD-affiliated individuals.
To support the dose assessments, measurements were made on samples collected by DoD and the UnitedStates Department of Energy (DOE) at or near locations where DoD-affiliated individuals worked or
lived. This measurement information has been made available and includes new data on concentrationsof several radionuclides in air for five locations on the Japanese mainland for each hour in the period
from 12 March to 11 May 2011 [D7].
18. Aside from this new source of measurement information, additional information about the levels ofradionuclides in the air and deposited on the ground has come from reanalysis of monitoring datacollected at the time and from the application of some novel analysis methods. Specifically,
concentrations of different radionuclides in the air in the early stage of the FDNPS accident have been
estimated at several monitoring posts in Fukushima Prefecture from results of gamma spectrometry withsodium iodide (NaI(Tl)) scintillation detectors [H17, M44, T29]. In addition, concentrations of 137Cs and
129
I (from which levels of 131I can be inferred) in air at ground-level in the Fukushima and Kanto 3 areas
are being derived from an analysis of filter-tapes of air pollution stations (for monitoring suspended
particulate matter) [E1, O25, T50, T51], although it should be noted that only iodine in particulate form
will have been collected on the filter-tapes. The latest version of the data set produced by this work, which
includes information on 137Cs (and 134Cs) concentrations in air for 101 locations and on 131I concentrations
in air for 4 locations, has provided an important input into some of the models and assumptions
underpinning the update to the estimation of doses to the public described in appendix A (and particularly
attachments A-9 and A-12). The locations within and close to Fukushima Prefecture where concentration
measurements of 137Cs (and 134Cs) in air are available from this work are shown in figure II.

3 Kanto area contains six prefectures (Chiba, Ibaraki, Gunma, Kanagawa, Saitama and Tochigi) and Tokyo Metropolis.

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UNSCEAR 2020/2021 REPORT

Figure II. Locations of continuous measurements of radiocaesium concentrations in air fromfilter-tapes in or close to Fukushima Prefecture) [O25, T51]

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations.

19. Another major advance has been the retrospective reconstruction of deposition densities of 131I fromthe measured deposition densities of 129I [F13, M50], which has been completed for nearly 1,000 surface
soil samples from across Fukushima Prefecture and in the neighbouring prefectures of Miyagi and Ibaraki[M18]. The reconstructed data for 131I deposition have the potential to further improve estimations of the131
I source term and also to improve assessments of doses to members of the public, including evacuees,
from inhalation. This data set has also provided an important input into the Committee’s updated publicdose assessment reported in appendix A.
20. Ohba et al. [O4] measured the short-lived radionuclides, 132I, 132Te, 133I and 135I, on the clothing of
two individuals using gamma spectrometry, and found average ratios (relative to 131I, and decay corrected
to 12:00 on 12 March 2011) for 132Te (and its daughter 132I), 133I and 135I of 2.3, 1.1 and 0.3, respectively.
Recent results of measured 137Cs/129m,132Te ratios in the environment are consistent with 132Te beingtransported, dispersed and deposited in the environment as a particulate species [D2].

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13

21. Measurements of 90Sr [R4, Z2, Z3], 238Pu and 239+240Pu [K13] have been reported for a small numberof sampling points, generally in locations within Fukushima Prefecture. The deposition densities of 90Sr
[R4, Z2] and 110mAg [M29] were significantly (many orders of magnitude) lower than those of 137Cs. The
deposition densities of 238Pu and 239+240Pu were very low and often below detection limits, but indicatedthat trace amounts of uranium from the fuel cores were released together with plutonium isotopes [D10,I23, K13, S4]. Bu et al. [B14] have developed and applied mass spectrometric methods to samples toassess deposition densities of actinides and fission products.
22. Several studies have found radiocaesium associated with water-insoluble “glassy spherules”,
several micrometres in diameter [A1, A3, Y1]. Ikehara et al. [I28], using a novel method based on
autoradiography, detected radiocaesium associated with microparticles, with a typical size of a few
micrometres, and with an extremely high level of specific radioactivity (about 1011 Bq/g) in theenvironmental samples. These glassy microparticles do not adsorb to the mineral phase of soil and arequite persistent in the environment [S37]. Such particles have been collected from various materials,
including aerosol filters (e.g., [A1]), soil [F24, I28, S14], plant leaves and other tissues [Y1, Y4],
breathing protection [H13], and particulate matter in river water [M33]. Satou et al. [S15] isolated
considerably larger particles associated with radiocaesium from environmental samples within a small
region close to FDNPS. These particles were up to 400 micrometres in diameter, and some wereassociated with more than 20 kBq of radiocaesium per particle, but they had much lower radiocaesium
concentrations than the particles with diameters of a few micrometres. It is not clear to date what
proportion of the radioisotopes of caesium released to the atmosphere from the FDNPS accident may
have been associated with such particles. Ikehara et al. [I29] studied radiocaesium (134Cs and 137Cs) in
soil and reported that the percentage of radiocaesium associated with such particles in the studied samples
ranged from 2% to 80%. Further information on the distribution of these particles and their possible
influence on the levels of exposure of the public is given in appendix A and attachment A-3.

2.

Releases to the atmosphere(a) Estimates of the total releases of radionuclides
23. In the UNSCEAR 2013 Report [U10], the Committee summarized the numerous estimates that had
been published of the magnitude of the release of radionuclides from FDNPS. The Committee noted thatthe majority of the estimates were made using either reverse or inverse modelling from measurements inthe environment. The estimates were not always directly comparable: some were estimates of the total
quantities released; others were releases over a limited period of time, or only included that fraction of
the release partly or wholly dispersed over the Japanese land mass (i.e., did not include the fraction of therelease dispersed directly over the ocean). The Committee concluded that the total release of 131I fellwithin the range of about 100 to about 500 PBq, and that of 137Cs fell generally in the range of 6 to 20 PBq.
In the report by IAEA on the accident [I5], the releases to atmosphere were estimated to lie in the rangesof 100 to 400 PBq for 131I and 7 to 20 PBq for 137Cs, when the very first estimates based on limited
information were excluded. Estimates of the total release published since the UNSCEAR 2013 Report
are summarized in table 1 for comparison with the estimates in that report and other relevant publications.

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UNSCEAR 2020/2021 REPORT

Table 1. Comparison of range of estimates given in the UNSCEAR 2013 Report [U10] with estimates
made since of the total amounts of 131I and 137Cs released to the atmosphere as a result of the
Fukushima Daiichi Nuclear Power Station accident
Date of publication

UNSCEAR 2013 Report [U10]2014

100–500

6–20

2014

400

11

[A2]

a

131

I

137

Cs

[W17]a

2014

a

2015

151

15

[I5]b

2015

100–400

7–20

[K6]

12–19

[Y28]a

2016

8

a

2016

8

[K19]c

2017

754

29

d

2020

120

10

[S16]

[T28]
a

Total release (PBq)

Reference

Estimate based on inverse or reverse modelling.

b

Based on a summary of 20 published estimates made in 2011–2013 by different organizations.

c

Estimate based on forward projection using severe accident progression code.

d

Estimate using optimization based on Bayesian inference.

24. The estimates made since the UNSCEAR 2013 Report [U10] generally confirm the ranges
estimated by the Committee and IAEA, with most of the estimates at the lower end of the ranges [I5, K6,
K19, S16, S19, T28, W17, Y28]. The exceptions are the estimates of Kim et al. [K19], which were
somewhat higher. These estimates were based on a severe accident progression code and were therefore
reliant on assumptions about the exact conditions and the timing of various accident-related events inside
the reactors. As noted in the UNSCEAR 2013 Report, while all of the estimates are associated withuncertainty, those based on inverse or reverse modelling using measurements of radioactive material in
the environment are preferable in the context of assessing levels and effects of exposure as a result of thereleases. Excluding the estimates of Kim et al. [K19], the ranges correspond to about 2% to 8% of thetotal inventory of 131I and about 1% to 3% of the total inventory of 137Cs in the three operating reactors(Units 1–3) from which the releases occurred at the time of the accident. For perspective, the estimatedreleases (based on the averages of published estimates) of these radionuclides from the FDNPS accident
were about 10% and 20% for 131I and 137Cs, respectively, of the releases of these radionuclides estimatedfor the Chernobyl accident (see appendix B).
25. The two radionuclides 131I and 137Cs, together with 134Cs, made by far the largest contribution to the
exposure of the public. A large number of other radioisotopes of iodine, caesium and other elements was
also released, with the relative amounts released determined by their volatility. For example, essentiallythe entire inventory of 133Xe, 85Kr and other noble gases would have been released [B7], and the estimatesin table 1 suggest that a few per cent of the inventory of volatile elements, including iodine, caesium andtellurium was released; on the other hand, the volatilities of strontium, barium and plutonium are much
lower and their releases were correspondingly also much lower. This has been confirmed bymeasurements of their levels in the environment [D10, H21, H29, I25, S18, T30, Z4]. This contrasts
markedly with the Chernobyl accident, where much larger fractions of the less volatile elements (e.g.,strontium and plutonium) were released directly to the atmosphere (see appendix B).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

15(b) Estimates of the source term
26. Numerous estimates have also been made of the temporal pattern of the rate of material released(commonly referred to as the “source term”), in particular for 131I and 137Cs. In the UNSCEAR 2013
Report [U10], the Committee noted that, although there was broad agreement on the total amounts ofradioactive material released, there were large differences in the temporal patterns of release rates and in
the extent to which they correlated with events on site. For the purposes of providing a sound basis for
estimating levels of radioactive material in the terrestrial environment where no measurements existed
and the subsequent estimation of doses to the public, the Committee chose, in the UNSCEAR 2013
Report, to use the source term estimated by Terada et al. [T27]. The Committee has subsequently [U11,U13, U14] recommended for future assessments the use of the latest in a series of estimates of the source
term for releases to atmosphere by the same group of researchers at the Japan Atomic Energy Agency(JAEA). These estimates have been made through progressive refinements to the previous estimate,taking account of all of the measurement information as it has become available. In the latest source term
produced by this group, Terada et al. [T28] improved the atmospheric transport, dispersion and deposition
modelling (ATDM) simulation with an optimization method based on Bayesian inference, and usedvarious measurements such as air concentration, surface deposition, fallout, and newly released estimatesof hourly air concentrations of 137Cs derived by analysing the filter-tapes of air pollution stations. This
optimization improved not only the source term but also the wind field in the meteorological calculation,
which led to a reduction in the discrepancies (between the calculations and the measurements) in theplume passage time at monitoring points to less than three hours. The Committee has used this source
term in the revised estimates of doses to members of the public (see chapter V and appendix A). The
pattern of the release for this source term [T28] is illustrated in figure III for the releases of 131I and 137Cs.
Figure III. Release rate as a function of timea for (a) 131I and (b) 137Cs

16

a

UNSCEAR 2020/2021 REPORT

Decay corrected at the shutdown time of 14:46 Japanese Standard Time (JST) on 11 March 2011, according to [T28].

27. Since the UNSCEAR 2013 Report [U10], several researchers have investigated which events during
the course of the accident were the dominant sources of atmospheric releases at different times during
the major release period of 12 to 21 March 2011. They have been able to identify which reactor unit was
the source of releases that resulted in deposition on to specific areas of the Japanese mainland, mainly
based on measured small differences in the ratios of different radioisotopes of caesium [C4, J1, S34, T51,
T52, Z4]. Other researchers [N5, T50] have used environmental monitoring data to identify at least ninemajor, separately identifiable releases from FDNPS.
28. There remains particular uncertainty about the chemical form in which iodine was released fromFDNPS: Lebel et al. [L1] estimated that, overall, about one half of the iodine had been released as aparticulate, with the other half released in a volatile or gaseous form (which comprised both elemental
and organic forms, although the relative amounts released of each remains uncertain). Amano et al. [A10]reported 131Igas/131Itotal ratios as: 0.7 from 15 to 16 March 2011; 0.52 from 20 to 21 March 2011; and 0.68
from 22 to 23 March 2011. Tsuruta et al. [T52] summarized 131Igas/131Itotal ratios ranging from 0.44 to 0.72
for the releases that occurred at different times. The Committee has recommended [U13] that explicit
account should be taken of releases of iodine in organic forms in any future update of the UNSCEAR
2013 Report [U10]. The source term that has been used in the revised dose assessment described inappendix A includes estimates of the amount of iodine released in all three chemical forms (elemental,
organic and particulate). It assumes constant ratios (over time) of 0.2 for 131Ielemental/131Itotal, 0.3 for
131
Iorganic/131Itotal and 0.5 for 131Iparticulate/131Itotal [K6, T28], but the Committee has also given explicit
consideration to the uncertainty in these ratios (see appendix A and attachment A-12).
29. Hirose [H21] has suggested that the particle size of the 131I-bearing particles may have differed fromthose bearing radiocaesium, and therefore that the dispersion and deposition behaviour of 131I-bearing
particles may have differed from those bearing 134Cs and 137Cs. The Committee considers that the effects
of any differences in particle size suggested by Hirose are likely to be small (in terms of exposure of the
public) compared with those due to assumptions about the chemical form of radioiodine and therefore

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

17that it is reasonable to assume that particulate iodine and caesium behave in a similar way during
atmospheric transport and deposition.
30. Steinhauser et al. [S36] have pointed out that continuing releases of radioactive materials from the
FDNPS site to the atmosphere are possible as a result of decommissioning and dismantling activities on
the site. Steinhauser et al. further noted, as have Igarashi et al. [I24], Ochiai et al. [O2] and Akimoto [A7],that resuspension of deposited 137Cs can lead to continuing concentrations of 137Cs in the air and its
redeposition. The process by which any resuspension predominately occurs is still not fully understood[K21]. Nevertheless, all of these studies have confirmed that resuspension did not significantly contributeto the long-term exposure of the public.

3.

Dispersion in the atmosphere

31. The fate of radioactive material released to the atmosphere during the accident at FDNPS wasdetermined by the meteorological conditions pertaining at the time and the physical characteristics of eachrelease, such as its height and whether it was in gaseous or particulate form. These conditions, which varied
considerably during the period of the releases, determined where the material was dispersed and the rate at
which it was diluted in and deposited from the atmosphere. The releases that largely determined the levels
and patterns of radionuclides deposited on the Japanese land mass occurred on 12, 14–16, and 20–23 March
2011. Oura et al. [O25], Mathieu et al. [M9], Sato et al. [S13], Tsuruta et al. [T52], and Moriizumi et al.
[M44] have carried out comprehensive analyses of the main events and have generally agreed that themeteorological features that determined the fate of these releases were probably as follows:(a) Material initially released on 12 March 2011 went towards the Pacific Ocean, but the release in
the afternoon of 12 March 2011, in particular that resulting from the venting operation and alsohydrogen explosion in Unit 1, initially spread northwards along the eastern coast of the main islandwith significant dry deposition (particulate matter that settles on the ground), and later shifted to anorth-north-easterly direction, over the coastal area of Miyagi. This has been verified by relatively
low 134Cs/137Cs ratios in this area, indicating Unit 1 as the source of this deposition [K29];(b) Material released from late at night on 14 March until 16 March 2011 (first from Unit 2, laterfrom both Units 2 and 3 [C4]) moved towards the south, depositing along the southern coastal areaof Fukushima Prefecture and the north-eastern area of Ibaraki Prefecture on the morning of 15 March
2011. Then, the direction of plume gradually changed clockwise from towards the south to towards
the north-west and encountered rainfall on the evening and night of 15 March 2011. The plume
moved towards the south again before daybreak on 16 March 2011 and released material was
deposited by wet deposition. In this region the 131I/137Cs ratio (of the deposited material) was
significantly elevated [H29], and it has been noted that the 135Cs/137Cs ratio was also different in thisarea [Z4]. The released material was further dispersed and resulted also in dry deposition ofradionuclides in the prefectures of Tokyo, Saitama and Kanagawa, albeit at reduced levels;(c) A major discharge of radioactive material from Unit 2 on 15 March 2011 resulted in a releasethat was firstly transported south-west and later gradually transported north-north-west by theprevailing wind. Precipitation scavenging (rainfall and possibly snowfall washing radioactive
material out of the air and depositing it on to the ground) occurred during 15 March 2011 andcontinued until the morning of 16 March 2011. These events are acknowledged to be the cause of
the relatively high 137Cs deposition densities monitored to the north-west of FDNPS [D10];

18

UNSCEAR 2020/2021 REPORT(d) Material released during the period 20 to 23 March 2011 (from Units 2 and 3 [C4]) was dispersed
over parts of the Japanese territory encountering rainfall on occasions and resulting in wet and dry
deposition, for example in areas of the prefectures of Iwate, Miyagi, Ibaraki and Chiba. The episodeof 20 to 21 March 2011, in particular, involved the release of several plumes, and light winds
favoured stagnation of the released material. Releases during this period are the main source of
deposition in the Kanto plain region to the south, and in the zone to the north between the north of
the Miyagi Prefecture and the south of the Iwate Prefecture [M9]. During this time (mainly during
20 March 2011), the plume of released material was monitored along the coast, then to the north andthen the north-west (e.g., [T29]).
The fate of these releases relative to the measured deposition density levels of 137Cs is illustrated infigure IV.
Figure IV. The regions impacted by the deposition from the main release episodes (indicated inoutline with the corresponding periods of releases) superimposed on a map of measured 137Cs
deposition [M9].

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

19

32. The dispersion and deposition of released material has been modelled by many groups (e.g., [K6,
N1, S13, T27, T28, W18]). There is general agreement that most of the radioactive material released tothe atmosphere was blown eastward by the prevailing winds, depositing on to and dispersing within the
North Pacific Ocean. The Science Council of Japan [S20] evaluated and compared several models used
to analyse the transportation and deposition of radioactive materials released during the FDNPS accident.
One of the findings was that deposition of 137Cs on to the mainland of Japan, as assessed with different
regional-scale models, was about 27 ± 10% of the total emission. A review by Hirose [H21] based on
many papers published between 2011 and 2016 concluded that around 80% of the total atmosphericrelease of 137Cs was deposited on to the North Pacific Ocean, and this has been confirmed by Terada etal. [T28]. Estimates of the amounts released into the marine environment via this route are consideredfurther in chapter III. All the above-mentioned dispersion model predictions were able to replicate thebroad pattern of measured deposition density of 137Cs over the Japanese land mass. At specific locations,the model estimates were generally within a factor of 10 (higher or lower) of the measured depositionlevels, but sometimes better.
33. The Committee has used the results of the source term and ATDM carried out by Terada et al. [T28]
in its revised dose assessment described in chapter V and appendix A. Time-dependent concentrations in
air and deposition densities of the radionuclides 4 132Te (and its daughter radionuclide, 132I, with which it
is assumed to be in equilibrium), 131I, 134Cs, and 137Cs have been provided to the Committee for two
nested grids:(a) The finer grid covering most of Fukushima Prefecture with a horizontal resolution of 1 km;(b) The coarse grid covering Fukushima Prefecture and all neighbouring prefectures with ahorizontal resolution of 3 km [T28].
Terada et al. did not include the short-lived radionuclide 133I and the Committee has therefore estimatedthe concentration of 133I in air from the calculated concentrations of 131I in air. The three different
chemical forms of radioiodine have been considered separately. Further details of the ATDM results
provided to the Committee and how they have been used in the dose assessment are provided inappendix A and attachments A-9 and A-10.

C. Summary
34. There was a considerable amount of measurement data available at the time of the UNSCEAR 2013
Report [U10] on the concentrations in the terrestrial environment of the radionuclides released as a resultof the accident that contributed most to the exposure of the public. This extensive information provided
the basis for the majority of estimates of the quantities of radionuclides released and of the temporal
pattern of the release, and for the assessment of exposures of the public. However, there were some gaps
in the measurement data with little information available on concentrations of radionuclides in the air andtheir variation with time. Some additional measurement information has become available since the
UNSCEAR 2013 Report. In addition, reanalysis of, and the application of techniques not previously
applied to, the results of monitoring and sampling carried out earlier has provided additional data,

4 The ATDM results of Terada et al. did not include 133Xe, a radionuclide also released in significant quantities. This radionuclide

is not deposited on the ground from the atmosphere but would have contributed to the dose from external exposure to airborne
radioactive material. Because it is soluble in body tissue, it would also have made a minor contribution to internal exposure frominhalation of airborne material. However, its overall contribution to doses to the public would have been small (less than 1%, seeattachment A-10) and it was accordingly not included in the revised dose assessment.

20

UNSCEAR 2020/2021 REPORT

particularly on concentrations of radionuclides in the air and how these varied with time. Thesedevelopments have led to refinements in estimates of the source term and to an improved basis forassessing exposures of the public.
35. In the revised assessments of doses to the public described in chapter V and in more detail inappendix A, the Committee has made use of the latest in a series of estimates of the temporal pattern of
release (source term) for releases to atmosphere by the same group of researchers at JAEA that made theestimate of the source term used in the UNSCEAR 2013 Report [U10]. In this source term, the total
release of 131I was estimated to be 120 PBq and of 137Cs was estimated to be 10 PBq. The Committee has
also made use of the modelling carried out by this group of the transport, dispersion and deposition of
the source term in the atmosphere. As in the UNSCEAR 2013 Report, this modelling has been used toinfer the concentrations of radionuclides in the air or on the ground where measurement information was
not available or where measurements could no longer be made.
36. The Committee continues to consider that the total release to the atmosphere of 131I was within therange of about 100 to about 500 PBq, and that of 137Cs was generally in the range of 6 to 20 PBq. The
ranges correspond to about 2% to 8% of the total inventory of 131I and about 1% to 3% of the total
inventory of 137Cs in the three operating reactors (Units 1 to 3) from which the releases occurred at thetime of the accident. For perspective, the estimated releases (based on the averages of publishedestimates) of these radionuclides from the FDNPS accident were about 10% and 20% for 131I and 137Cs,
respectively, of the releases of these radionuclides estimated for the Chernobyl accident.
37. The releases that largely determined the levels and patterns of radionuclides deposited on theJapanese land mass occurred on 12, 14–16, and 20–23 March 2011. About 20% of the total atmosphericrelease of 137Cs was dispersed over, and a substantial fraction of this was deposited on to, the Japaneseland mass.

III. RADIONUCLIDE RELEASES TO, AND DISPERSION AND
DEPOSITION IN, THE MARINE ENVIRONMENT

A. Introduction
38. The accident at FDNPS was, because of its coastal location, the first major nuclear power station
accident in which significant amounts of radioactive material were released directly and indirectly into
the marine environment. Firstly, as noted in chapter II, a large proportion of the releases of radioactive
material to the atmosphere were dispersed over, and deposited on to, the surface of the Pacific Ocean,
as a result of the westerly/south-westerly winds that blew part of the time during the release periods
and during the initial dispersion phase. Secondly, radioactive material was released directly into theocean, initially as a result of leakage of highly contaminated water from a trench outside Unit 2.
Compared to the Chernobyl accident, in which dispersion of radioactive material released toatmosphere and its deposition over sea surfaces (particularly in the Baltic and Black Seas) resulted ina maximum concentration of 137Cs in the sea of 2,400 Bq/m 3, direct releases into the ocean as a resultof the FDNPS accident led to peak concentrations three to four orders of magnitude higher. For

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

21

comparison, prior to the FDNPS accident, background concentrations of 137Cs, released fromatmospheric weapons tests conducted in the late 1950s and early 1960s, ranged from 1 to 2 Bq/m3 in
North Pacific surface waters [B17].
39. During the course of the accident, following the initial leakage from the trench, direct releases tothe marine environment occurred as a result of the deliberate discharge of weakly-contaminated waterfrom storage tanks, emptied to create capacity for the storage of highly-contaminated water from thetrench. These releases and the earlier leakage were localized, occurring at the power station’s outlet
channels and through the harbour. The releases occurred mainly between the end of March and the firstweek of April 2011. On 6 April 2011, the direct release of highly contaminated water to the ocean fromthe trench outside Unit 2 was brought to an end by the transfer of the water to the storage tanks.
40. After this first phase, releases decreased considerably, but a number of sources of continuing
releases to sea have been identified. Inside the FDNPS site, groundwater was contaminated by the highly
radioactive reactor cooling water contained in the basements of the reactor buildings, and there had beenleakage from the storage facilities. These releases had been progressively reduced by remedial actions,such as the establishment of sealing barriers and pump wells and the replacement of damaged storage
tanks. Outside the FDNPS site, rivers draining catchment areas on to which radioactive material released
to the atmosphere had been deposited transported some of this material to the ocean. These inputs fromrivers increased during periods of heavy rain and floods that transported high suspended sediment loads
with adsorbed radiocaesium to the ocean.
41. Monitoring of some radionuclides (131I, 134Cs and 137Cs) started on 21 March 2011, after detectionof radioactive material in seawater, at a few points in the coastal area around FDNPS, firstly in seawaterand then in the sediment and marine organisms. The number of measurement points gradually increased.
These measurements were carried out at the time using high detection limits, but numerous samples havesubsequently been reanalysed using more precise techniques. In the months following the accident,
scientific cruises and various sampling operations were carried out allowing measurements to be acquired
throughout the North Pacific Ocean.

B. Overview of current understanding
1.

Releases to the marine environment(a) Releases in the initial phase
42. Estimates that have been made of the total releases to the marine environment in the initial phase(March–April 2011) for a number of the more radiologically-significant radionuclides are summarizedin table 2. These estimates include some reviewed in the UNSCEAR 2013 Report [U10] and some more
recent estimates.

22

UNSCEAR 2020/2021 REPORT

Table 2. Summary of estimates of total releases to the marine environment in the initial phase(March–April 2011)
Radionuclide

Estimated release in the initial phase

References

DIRECT RELEASES FROM THE FDNPS SITE
137

Cs (andCs)

3.5–5.6 PBq[E6, K11, K30, M35, T48, T49]

131

I

9–13 PBq[K11, K30, T48]

129

I

7–8 GBq[G7, H31]

Sr

0.04–1 PBq[C1, P3]H

0.3–0.7 PBq[K1]

Pu and 240Pu

Negligible (concentrations close to thebackground level)

90
3

239

134

[B13, M19]

DEPOSITION FROM THE ATMOSPHERE137

Cs (and 134Cs)

5–11 PBq[E6, K11, K30, T44]

I

57–100 PBq[K11, K30]

131

43. Different methods have been used to estimate direct releases of 137Cs to the ocean. An inverse model
approach based on concentration measurements made by TEPCO at the outlet of the FDNPS discharge
channels has led to estimates of the release of 137Cs ranging from 3.5 to 5.6 PBq [E6, K11, K30, M35,
T48, T49]. Other methods have been based on estimates of inventories in seawater. For example, Charetteet al. [C2] estimated a release of 134Cs of 11 PBq using a 134Cs inventory based on:(a) Samples collected in June 2011 at 50 stations spanning the area extending from 50 up to 600 km
off Japan;(b) A water mass age (the time since the water parcel left the coast) derived from radium isotopes.
As noted by the Committee in the UNSCEAR 2013 Report [U10], estimates of releases from backward
extrapolation of inventories are likely to be more uncertain and to overestimate the source term. Less
weight has been given to these estimates, and they are not included in table 2.
44. The direct releases of other radionuclides have, in many cases, been made using measured ratios of
the concentration of these radionuclides to the concentration of 137Cs.
45. Numerical modelling of dispersion in the atmosphere and deposition onto the ocean surface has
been used to estimate indirect releases to the marine environment. This method has led to estimates of
the indirect release of 137Cs (and of 134Cs, the two radioisotopes being found in the reactors of the affectedunits, and released to the environment, in approximately equal amounts) to the whole Pacific Ocean in
the range of 5 to 11 PBq [E6, K11, K30, T44]. Of this total, 1.14 PBq has been estimated to have been
deposited over a 300,000 km2 area of high deposition to the north-east and south-east of FDNPS [T49].
46. There is little information on the deposition of other radionuclides on to the ocean surface, apartfrom estimates of 57 and 99 PBq for 131I [K11, K30].
47. The percentage of the total release of 137Cs to atmosphere that was deposited on to the surface of the
Pacific Ocean has been estimated at around 80% [H21]. For comparison, for the Chernobyl accident, of theorder of 7% of the release of 137Cs to atmosphere (which was larger than the release from FDNPS) was
estimated to have been deposited on to the surface of the Baltic and Black Seas combined (see appendix B).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

23(b) Estimation of the long-term releases
48. On 6 April 2011, the major leakage of contaminated water to the ocean was brought to an end by thetransfer of the water in the trench to the storage tanks. This resulted in a reduction in the 137Cs concentration
measured at the outlets of FDNPS by about a factor of 100 in one month. However, over the longer term
(2012–2013), the 137Cs concentration has not decreased as rapidly, falling by a factor of about three between
October 2011 and 2013 at the northern discharge outlet [H20]. Discharge of groundwater contaminated by
the leakage of radioactive reactor cooling water was found to be responsible for most of these residual
concentrations. Monitoring also showed sporadic increases in the concentration of 137Cs at the outlets [H20],
some of which corresponded to heavy rainfall events. TEPCO, the FDNPS operator, has suggested thatthese increases may be due to contaminated water pooled on the roof of the buildings of the damaged
reactors being washed by rainfall into coastal waters via the site drainage.
49. Based on measured concentrations of 137Cs in the FDNPS harbour and the exchange rate between
the harbour and the ocean, Kanda [K3] estimated the release of 137Cs between June 2011 and September
2012 to be 17 TBq, less than 1% of the total release before June 2011. On the basis of modelling andusing the concentration of 137Cs at the outlet of the FDNPS discharge channels, Tsumune et al. [T49]
estimated the release at 40 TBq for the period June 2011 to February 2012, and a simple calculation basedon this estimate would indicate a release of about 19 TBq for the period from February 2012 to mid-2015.
The construction of an impermeable wall to reduce the outflow of groundwater to the sea was completedin October 2015 and has resulted in a significant decrease in the concentrations of 137Cs in the harbour.
Based on Tsumune et al. [T49], annual releases to the ocean from groundwater on the FDNPS site
subsequently (i.e., after October 2015) can be estimated to be about 0.5 TBq.
50. Rivers draining catchment areas onto which radionuclides that were released to the atmospherehave been deposited are a further source of releases into the Pacific Ocean. Yamashiki et al. [Y9]estimated that about 5 TBq of 137Cs was washed from the Abukuma river basin into the ocean between
August 2011 and May 2012, but much of this release (61%) occurred during an 8-day period
corresponding to the passage of typhoon Roke. Other authors have estimated the annual discharge of137
Cs from rivers to be between 5 and 10 TBq [A4, K22], equating to about 1% to 2% of the initialdeposition on to the catchment area of the rivers most affected by the FDNPS accident. These
catchments would be expected to be a continuing source of radiocaesium releases to the Pacific Ocean
in the future.
51. Finally, groundwater beneath sand beaches has been found to be a reservoir of 137Cs, probably
accumulated in the days and weeks after the accident [S6]. Annual releases from this reservoir have been
estimated at 0.6 TBq.

(c) Summary of releases to the marine environment
52. Table 3 summarizes the estimated amounts of 137Cs released into the marine environment via thedifferent routes identified and over the short and longer time periods. By far the majority of the material thathas been released into the ocean entered the marine environment in the first one to two months after theaccident, with the amounts entering during the first month from deposition of radioactive material released
to atmosphere (deposited over a very large area of ocean) and the amounts entering coastal water fromleakages and deliberate releases from the FDNPS site in the first two months being broadly comparable.
Direct releases from the FDNPS site thereafter, following the various countermeasures put in place, were
two to three orders of magnitude lower. Releases to the marine environment from rivers draining catchments
on to which radioactive material released to the atmosphere was deposited may now be larger than the direct

24

UNSCEAR 2020/2021 REPORT

releases from the FDNPS site. In addition, these inputs occur mostly over short timescales, during rare
events such as typhoons, and from localized sources, particularly river mouths, and are likely to continue
for many years. There may also be some continuing releases from groundwater.
Table 3. Summary of estimates of releases of 137Cs to the marine environment
SourcePeriodAmount of 137Cs released

March 2011

5–11 PBq

March–May 2011

3–6 PBq

June 2011–February 2012

40 TBqFebruary 2012–October 2015

19 TBqa

After October 2015

0.5 TBq/yb

Input from rivers

Years

5–10 TBq/yGroundwater under beaches

Years

0.6 TBq/y

Deposition from the atmosphere

Direct releases from the FDNPS sitea

Estimated from [T49] using a mean concentration of 1,000 Bq/m3 at the outlets of the discharge channels for the period.

b

Estimated from [T49] using a concentration of 100 Bq/m3 at the outlets of the discharge channels.

2.

Dispersion of released material in the marine environment

53. A large number of the radiocaesium measurements made in the coastal area near FDNPS were
initially recorded as below the detection limit, which was elevated in the immediate post-accident period(at around 10 kBq/m3) because the priority was to carry out a large number of simple, rapid
measurements. Many of the samples collected have subsequently been reanalysed [K49, O17] with lower
detection limits (0.1 to 1 Bq/m3). The measurements have been used to interpret the transport ofradiocaesium at different temporal and spatial scales and show:(a) Rapid dispersion of released radiocaesium out of the coastal zone (on a time scale of about one
month);(b) Rapid transport of radiocaesium in surface water, mainly directed to the east and towards theNorth American coast (on a 1 to 4 year time scale);(c) Downward transport of radiocaesium in the water column to depths of a few hundred metres ofthe ocean during March and early April 2011.

(a) Concentrations in the coastal zone of the Fukushima Daiichi Nuclear

Power Station

54. In the first few months after the accident, a rapid decline in radiocaesium concentrations was observed
around FDNPS. At the outlet channel of FDNPS, a peak value of 68 MBq/m3 of 137Cs was recorded on
6 April 2011. By the end of April 2011, after the major leakage was stopped, the radiocaesium concentration
had reduced to about 200 kBq/m3; it then reached about 1 kBq/m3 in May 2012 and remained around thisstabilized average value until mid-2015 [B16], after which it decreased to 0.1 kBq/m3. The rapid decreasein the first few months after the accident at this point, very close to FDNPS, mainly reflects the decrease in
the source term. In the first 10 km off the coast of FDNPS, Fukuda et al. [F16] showed that the dissolved
137
Cs concentrations in seawater measured during May and October each year between 2013 to 2015,
remained one to two orders of magnitude higher than those before the accident. Concentrations were highest

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

25

within the first 5 km from the coast (20 to 220 Bq/m3), and showed higher temporal variability, probably
related to heavy rainfall increasing the input from rivers and/or increased discharges from the FDNPS
harbour. Concentrations reached 2 to 4 Bq/m3 at 30 km from the coast.
55. As described in more detail in the UNSCEAR 2013 Report [U10] and the IAEA Report on theaccident [I5], models of the dispersion of radionuclides released directly into the ocean have indicatedthat, during the first month after the accident, radioactive material was initially transported in a southerly
direction along the coast by the wind blowing mainly towards the south at the time. The released material
was then subsequently transported eastwards by the Kuroshio current and (following the separation of
the current from the coast) the Kuroshio Extension, which transport warm, saline waters northwards along
the south coast of Japan and then eastward. After mid-April 2011, the wind blew mainly northwards
inducing a northward and eastward surface current and dispersion of radionuclides in the entire coastal
area together with interaction with the Oyashio current, which transports cold, less saline water
southwards along the north-eastern coast of Japan.
56. Modelling has similarly indicated that radionuclides released to atmosphere were deposited on tothe ocean over a wide region. The contribution to concentrations of radiocaesium in surface water due tothese deposits from the atmosphere was much lower than that due to the direct releases into the ocean,
except perhaps during the first days, when direct releases were not detected, and at large distances during
the first month.

(b) Dispersion at the scale of the Pacific Ocean
57. The various measurements taken in the months and years following the accident have shown thedispersion of radiocaesium in surface waters across the Pacific Ocean and its migration down the watercolumn. Radiocaesium was first transported eastward in surface water by the Kuroshio Extension at
speeds of 3 to 7 km/d [A13]. Concentrations of 137Cs reached a peak of a few thousand Bq/m3 in 2011 in
the surface water of the western Pacific (and a few hundred Bq/m3 further east and north), then rapidly
decreased to a few tens of Bq/m3 in 2012 and generally less than 3 Bq/m3 in 2015 [A14]. By June 2013,137
Cs attributable to the FDNPS accident had spread on to the Canadian continental shelf, and, by
February 2014, had increased the concentration of 137Cs throughout the upper 150 m of the water column
to 2 Bq/m3 [S31], about double the levels of 137Cs found prior to the accident. The first observation on
the shoreline of British Columbia was on February 2015 with a concentration of 6 Bq/m3 [S32]. At thesame time, there was also dispersion in a north-south direction (radiocaesium was detected between 25°N
and 63°N during summer 2012) [K49]. In addition, as a result of the cooling and sinking of surface water
during March and early April 2011, radiocaesium moved downward in the first few hundred metres of
the water column north and south of the Kuroshio current and Kuroshio Extension, in the intermediatewater masses known as central mode water and subtropical mode water. Peaks of 134Cs concentration of
10 to 20 Bq/m3 (corrected to the accident date) were observed at depths of around 300 to 400 m [K50].
Based on a synthesis of observations from August to December 2012, Inomata et al. [I33] found thatabout 56% of the 134Cs released into the North Pacific Ocean had been transported eastward in the surface
layer, with 28% and 16% transported in intermediate waters, respectively, south and north of the Kuroshio
current and Kuroshio Extension.
58. Radiocaesium released from FDNPS has also been detected at low concentrations in other areas:
the concentration of 134Cs and 137Cs in the surface waters of the Sea of Japan gradually increased during
the 2013–2016 period from 1.5 to 2.5 Bq/m3 [I34, T19]; and the 134Cs concentration in the Bering Sea
increased to about 1 Bq/m3 (decay corrected) in 2017 [K51]. On the other hand, the transport ofradiocaesium westward from Japan appears to have been limited, as no increase in concentration has been
detected at different locations in Indonesia [S44, S45] or in the northern South China Sea [Z5].

26

UNSCEAR 2020/2021 REPORT(c) Radionuclides in sediment and suspended matter
59. Many interacting processes influence the distribution of 137Cs in sediments and suspended matter.
Radiocaesium is extremely soluble in seawater with less than 1% of 137Cs in the open ocean found on
marine particles that may accumulate on the ocean floor. On the other hand, radiocaesium inputs fromrivers and groundwater are generally bound to the clay minerals of sediments, although some degree ofdesorption would be expected once the sediments enter seawater. Sediments accumulated on the coastal
shelf can be resuspended by strong currents and waves and then transported laterally. Bioturbation of
sediments can mix radionuclides sorbed on to sediment particles deeper into the sediment layer, but canalso return deeper material to the sediment surface. Finally, bioirrigation can increase the rate of exchange
between sediment pore water and bottom seawater.
60. Numerous samples of seabed surface sediment (with initial monitoring coordinated by the Ministryof Education, Culture, Sports, Science and Technology (MEXT) of Japan [K59]) were analysed from theend of April 2011 off Fukushima Prefecture and surrounding prefectures. In the coastal zone of
Fukushima Prefecture, 137Cs concentrations generally lay between 10 Bq/kg and 1,000 Bq/kg of dry
sediment, compared with values of 1 Bq/kg before the accident [A11]. In general, the concentrations ofradiocaesium in the uppermost sediment layer were highest off the FDNPS site, and higher south of
FDNPS than north of it. However, high resolution measurements have revealed strong horizontal
heterogeneities in 137Cs concentrations in the sediment at the small scale (1 m to 100 m), with isolated
peak values larger than 5,000 Bq/kg [T32].
61. The inventory of 137Cs in ocean sediments has been estimated at between 38 and 230 TBq for thecoastal region (up to 30 km from the coast) around FDNPS [B5, K59, K60, O22], corresponding to about
1% to 5% of the total direct release to the ocean. However, 137Cs concentrations in the water column havedecreased since the accident, and, by the end of 2012, the sediment inventory was estimated to be 5 to
10 times larger than the inventory in the overlying water [B15, B16].
62. The concentration of 137Cs in the surface sediment and its vertically integrated amount have also
been observed to decrease with time [K60, O23] (although the authors disagree over the importance ofdownward migration of 137Cs in the sediment), and the same has been observed for 129I [O24]. Kusakabe
et al. [K60] estimated that 76% of the 137Cs deposited in the surface sediment had been transported outof the FDNPS coastal area within about 5 years. Matsumoto et al. [M14] postulated that the small
particles with which 137Cs is more associated moved due to wave action from shallow areas to deeper
areas, where they accumulated. The significant inverse correlation observed between the concentrationof 137Cs in bulk suspended matter and distance from FDNPS [K42] also suggests an input of radiocaesium
from the land to the sediment and its gradual transportation to offshore regions.
63. Studies of sediments in Tokyo Bay have illustrated the transfer by rivers of radiocaesium released
to atmosphere, deposited on to the area located in the north-eastern part of the Tokyo Metropolitan Area,
and then washed out into the bay. In 2016, the sediments with the highest concentrations of radiocaesium
were found in the areas of the river estuaries where the currents were weakest, with very much lower
levels in the deeper central bay [Y11], suggesting accumulation of sediments containing radiocaesium inthe river estuaries. However, in 2017, Kubo et al. [K43] noted an increase in the radiocaesium inventory
in the central bay by a factor of three compared to the previous inventory assessed over the 2011–2016
period, and suggested that the radiocaesium present in the sediment at the river estuaries will graduallyenter the bay, particularly after heavy rains. In addition, Kubo et al. estimated that only about 9% of the137
Cs which had been deposited over the basin draining into Tokyo Bay had already flowed into the bay.
64. Ikenoue et al. [I30] used autoradiography to detect 137Cs-enriched particles in sediment samples,and found that a small number of these particles contributed a relatively large proportion (between 9%

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

27

and 64%) of the 137Cs concentration in sediment samples. The heterogeneous distribution of theseparticles is therefore likely to be one of the main factors responsible for the temporal and spatial variationsof 137Cs concentrations in sediment samples. Kubo et al. [K42] found the contribution of highly
radioactive particles of a few microns in size to the radiocaesium concentration of samples of suspended
matter ranged from 13% to 54%.
65. A more detailed, comprehensive and up-to-date review of the transfer of radionuclides released asa result of the FDNPS accident in the marine environment around Japan, including the distribution ofradionuclides in sediments and in marine organisms has recently been published by IAEA [I9] and should
be consulted for further information. 5(d) Summary of dispersion of released material in the marine environment
66. Figure V summarizes the ranges of concentrations of 137Cs observed in different compartments andareas of the Pacific Ocean 6 and how these have changed over time. This figure shows two major effects:−

Firstly, in 2011, there is a strong decrease in concentrations of 137Cs in seawater with distance fromFDNPS: in the coastal strip (between 15 and 30 km offshore), concentrations are systematicallyseveral orders of magnitude lower than at the outlet of the power station. This is probably due tothe energetic and variable currents in the region, and these energetic dynamics are also likely to be
the cause of a very rapid decrease in the concentrations of radiocaesium close to FDNPS in 2012
and thereafter, following the cessation of the main leakage in April 2011;−

Secondly, it is clear that concentrations in sediment have not declined over time as rapidly as
concentrations in seawater, indicating that the sediments could provide an important long-termsource of 137Cs in seawater.

5 The IAEA report became available after the end of 2019 and was accordingly not included among the information reviewed by
the Committee and summarized in this report. Further, the information included in the IAEA report, while a valuable compendium
in its own right, does not affect the findings of this Committee’s report and therefore did not warrant being reviewed as an exception
by the Committee.
6 The maximum values observed in the surface of the Pacific Ocean have been progressively moving from west to east as thereleased material has been transported eastward.

28

a“FDNPS outlet” refers to the outlets to the ocean from the FDNPS site; “Coastal (15–30 km)” refers to water between 15 and 30 km from the FDNPS site; “Surface Pacific” refers
to water at distances greater than 200 km from the FDNPS site; and “Coastal sediment” refers to sediments within 30 km from the FDNPS site.

UNSCEAR 2020/2021 REPORT

Figure V. Range of concentrations of 137Cs in different regions and compartments of the marine environmenta and at various time scales (the yellow zone
represents the range of background values found in seawater before the accident)

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

3.

29

Radionuclide transfers to and concentrations in marine foods

67. As part of the response to the FDNPS accident on 17 March 2011, the Japanese Government
established “provisional regulation values” for radioactive material in foodstuffs above which foodstuffs
were prohibited from distribution and consumption. For fish products, the provisional regulation values
were 2,000 Bq/kg for radioiodine and 500 Bq/kg for radiocaesium. From 1 April 2012, a lower “standard
limit” 7 of 100 Bq/kg was introduced by the Japanese Government for radiocaesium. The Japanese andprefectural governments began regular monitoring of foodstuffs in March 2011 and have made the results
available on websites (see, for example, [M4, M5, M6]). TEPCO also sampled marine products fromwithin a 20 km radius of FDNPS and has made the results publicly available (see supplementary material
to [W2, W3]).
68. Wada et al. [W3] analysed all of the data relevant to the coastal waters off Fukushima Prefecture topresent a detailed description of 131I, 134Cs and 137Cs levels in marine foods after the FDNPS accident.
Measurements were focused on the concentrations of radionuclides in the muscles of different species ofpelagic and demersal fish, as well as different invertebrates (cephalopods, bivalves, gastropods, andcrustaceans), and seaweeds of various kinds. In total, nearly 40,000 samples were analysed.
69. Regarding 131I, all samples collected after August 2011 showed levels that were below the detection
limit (16.2 Bq/kg-wet mass 8 on average), reflecting the short, 8-day, half-life of this radionuclide. Only
three samples (two from larvae of sand lance and one from a seaweed), from April and May 2011, were
found which exceeded the provisional regulation value of 2,000 Bq/kg [W2].
70. The trend for radiocaesium is shown in figure VI. This presents the total number of samples
analysed every three-month period off the coast of Fukushima Prefecture, and the number of samples
which exceeded the limit for radiocaesium of 100 Bq/kg established by the Japanese Government to apply
from 1 April 2012. In 2011, 41% of samples exceeded 100 Bq/kg. The highest concentrations frequently
corresponded to products from the shallow coastal waters south of FDNPS [W2, W3]. In 2012, thepercentage exceeding 100 Bq/kg decreased to 17% and in 2015 to 0.05%.

7 The terms “provisional regulation value” and “standard limit” are those used in the English version of handbooks providing
information on effects of the FDNPS accident published by the Radiation Health Management Division, Ministry of theEnvironment, Government of Japan and the Japanese National Institute for Quantum and Radiological Science and Technology.
The terms used in Japan may not correspond exactly with the Japanese translation of these terms. The equivalent guideline levels
for the purposes of international trade, as recommended by the Codex Alimentarius Commission, are 100 Bq/kg for radioiodine
and 1,000 Bq/kg for radiocaesium [F1].
8 Much of the radioecology literature and most of the source publications quoted give concentration values in Bq/kg-wet weight or
Bq/kg-dry weight. The correct quantity for the amount of matter, measured in kilograms, is mass, and not weight. The Committee
has used “mass” in this report, while recognizing that the source publications have often used “weight”.

30

UNSCEAR 2020/2021 REPORT

Figure VI. Number of fishery product samples caught each quarter off the coast of Fukushima
Prefecture, showing the number of samples with a concentration of radiocaesium above
100 Bq/kg [M6]71. The corresponding trend for prefectures other than Fukushima Prefecture is shown in figure VII.
The percentages of samples with radiocaesium concentrations exceeding 100 Bq/kg were significantly
lower than in Fukushima Prefecture at 2.6% in 2011 and 1% in 2012, and have decreased further withtime. Takata et al. [T20] reports that in the Sea of Japan and the southern coast influenced by the Kuroshio
current, the maximum concentration of 137Cs in fish after the accident was 1.1 Bq/kg-wet mass, while thefollowing year it was less than 0.32 Bq/kg-wet mass, about two orders of magnitude lower than in the
coastal waters off Fukushima Prefecture.
72. In terms of the guideline level recommended by the Codex Alimentarius Commission for the
purposes of international trade [F1], 76 samples of marine fish caught off Fukushima Prefecture (about
2.5% of the total measured) were found with radiocaesium concentrations exceeding the guideline level(of 1,000 Bq/kg) in the first year after the accident. In the second year, this had reduced to 17 samples(about 0.3% of the total) and in the third year to one sample (about 0.01%) (see supplementary material
to [W3]).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

31

Figure VII. Number of fishery product samples caught each quarter off the coast of prefectures otherthan Fukushima Prefecture, showing the number of samples with a concentration of radiocaesium
above 100 Bq/kg [M6]73. Wada et al. [W2] showed that some demersal fish (i.e., those living and feeding near the sea floor)
frequently had radiocaesium concentrations above the limit established by the Japanese Government, andthat the decreasing trend of radiocaesium concentrations in these fish was more gradual than those foundfor pelagic fish species (i.e., those living far from the sea floor), invertebrates and seaweeds. Tateda et al.
[T23] used a dynamic biological compartment model to show that the rapid increase of radiocaesium
concentrations observed in bivalves, gastropods, seaweeds and plankton feeding fish was due to the direct
uptake of water with elevated radiocaesium concentrations immediately after the FDNPS accident. The
subsequent rapid decrease in radiocaesium concentrations in seawater after the accident was then reflected
in a decline in radiocaesium concentrations in these marine species at a rate governed by biologicalprocesses. In contrast, the more gradual decreasing trend in the concentrations found in demersal fish areindicative of possible sources of radiocaesium other than seawater. Tateda et al. [T23] and Wang et al.
[W10] have explained this more gradual rate of decrease as due to the ingestion of sediment-based prey
(benthic invertebrates with radiocaesium concentrations reflecting levels in the sediment rather than levels
in seawater [S35]) as well as of detritus and sediments [T23, T24]. They postulate that the continuingelevated levels of radiocaesium in sediment, particularly of the coastal zone, may be the reason for the few
specimens still found to be exceeding the limit established by the Japanese Government in 2015. On theother hand, Fievet et al. [F8] used a model of the transfer of radiocaesium to fish from seawater and feeding
to explain the higher and more persistent concentrations of radiocaesium in demersal fish compared withpelagic fish as due to their more confined habitat along a narrow coastal strip close to the source of thecontinuing inputs of radioactive material into the marine environment.
74. No concentration of 137Cs higher than 50 Bq/kg-wet mass has been detected in fish that migrate
over a wide area in the ocean. Such migratory fish moving between the western and eastern North Pacific
Ocean could be biological vectors of radionuclides between these two distant regions. This has been
confirmed by measurements in the muscle of 15 Pacific bluefin tuna sampled in August 2011 in the
California region, which showed a 137Cs concentration of 6.3 ± 1.5 Bq/kg-dry mass [M1] and the presenceof 134Cs indicating that the source was releases from the FDNPS accident.

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UNSCEAR 2020/2021 REPORT

C. Summary
75. In the UNSCEAR 2013 Report [U10], the Committee concluded that the FDNPS accident
resulted in direct releases to the ocean of about 10 to 20 PBq of 131I and 3 to 6 PBq of 137Cs and 134Cs.
In addition, it concluded that 60 to 100 PBq of 131 I and 5 to 11 PBq of 137Cs and 134Cs entered the ocean
indirectly following release to atmosphere and dispersion over and deposition on to the ocean surface.
Information and analyses that have since become available have generally confirmed these ranges of
releases in the first one to three months after the accident. Over a longer timescale, about a further
60 TBq of 137Cs was released from the FDNPS site, largely from releases of site groundwater. Oncemeasures to reduce these releases were completed in October 2015, releases of 137Cs from the site
declined to about 0.5 TBq per year. A larger source of continuing direct releases of 137Cs to the marine
environment is rivers draining land on to which radioactive material released to atmosphere wasdeposited. Inputs of 137Cs to the marine environment from rivers have been estimated at 5 to 10 TBq
per year. In addition, these inputs occur mostly over short timescales, during rare events such as
typhoons, and from localized sources such as river mouths.
76. Measurements and analysis of 137Cs in seawater around the FDNPS site, across the Pacific Ocean
and in neighbouring seas have confirmed the rapid dispersion and dilution of the released material inseawater and its general movement eastwards. By 2013, the concentrations of 137Cs even in the coastal
waters (15 to 30 km) off the FDNPS site were of the order of the levels prevailing before the accident(1 to 2 Bq/m3). Concentrations of 137Cs in coastal sediments have not declined as rapidly. These are more
likely to be influenced by inputs of 137Cs bound to clay minerals in sediments. The inventory of 137Cs incoastal sediments is now thought to exceed the inventory in the overlying water column, and thesediments could provide a long-term source of 137Cs in seawater.
77. Concentrations of 137Cs in marine foods have also generally declined rapidly following the accident:
while 41% of samples caught off the coast of Fukushima Prefecture in 2011 had radiocaesium
concentrations exceeding 100 Bq/kg, the limit established by the Japanese Government to apply from
1 April 2012, this had declined to 17% in 2012 and, 0.05% in 2015.
78. The regular monitoring that began just after the accident in the vicinity of FDNPS was graduallyextended to more water sampling stations and also to the monitoring of sediment and edible marine
products. The monitoring was primarily performed for decision-making and in response to the concernsof civil society. However, the results have also improved the scientific understanding of theradionuclide transfer pathways in the different compartments of the marine environment. The
monitoring carried out was complemented by numerical modelling. Within a period of about a month,
it provided initial estimates of the dispersion of the released radioactive material. In addition, thecombination of observations and modelling made it possible to quantify the amount of radiocaesium(134Cs and 137Cs) released directly into the ocean.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

33

IV. TRANSFER OF RADIONUCLIDES IN TERRESTRIAL AND
FRESHWATER ENVIRONMENTS

A. Introduction
79. Radionuclides released to the atmosphere from the FDNPS accident were transported and dispersedin the air as described in chapter II. Most of this radioactive material was dispersed over the ocean anddeposited on to the ocean surface, entering the marine environment, as described in chapter III.
Radioactive material dispersed over the Japanese land area was deposited on to surfaces in the urban,
agricultural and natural environments, including tree canopies, other plant surfaces, soil, lakes, rivers andstreams. This chapter summarizes current understanding of the subsequent movement of this material
through different components of the terrestrial and freshwater environments. A good understanding of
the transfer of radionuclides in terrestrial and freshwater environments is important in supporting theestimates of exposures from ingested radionuclides described in chapter V and appendix A. It is also
important in developing appropriate environmental remediation programmes.
80. In the UNSCEAR 2013 Report [U10], the Committee based its estimates of doses from ingestionin the first year after the accident on measurements of concentrations of radionuclides in foodstuffs.
Thereafter, modelling was used to predict the transfer of radionuclides through terrestrial food chains.
The model [B12] had been developed for use in Europe and was modified for use in Japan by taking
account of Japanese agricultural practices, crop yields and data on the transfer of radionuclides to specificfoods (e.g., rice). New information that has since become available on the transfer of radionuclides
through both terrestrial and freshwater environments in Japan is summarized in this chapter and theimplications of this new information for the findings of the UNSCEAR 2013 Report are discussed.
A more detailed, comprehensive and up-to-date review of the transfer of radionuclides released as a resultof the FDNPS accident in the terrestrial and freshwater environments in Japan has recently been publishedby IAEA [I9] and should be consulted for further information.
81. The focus of this chapter is generally on transfers of 134Cs and 137Cs in the terrestrial and freshwater
environments. Other radionuclides released in significant quantities (e.g., 132Te, 132I, 133I and 133Xe) either
have short radioactive half-lives (less than a few days) or do not deposit on the ground, and would nothave contributed significantly to exposure of the public as a result of transfer through terrestrial andfreshwater environments. Iodine-131 (with a longer half-life of 8 days) was measured in foodstuffs in themonitoring that began several days after the accident, in some cases above the regulation value that wasapplied. However, information on its transfer in terrestrial and freshwater environments is sparse. In
addition, measurements made of radionuclide concentrations (including of 131I) in food and drinking
water have been used directly to estimate the exposure of the public in the first year after the accident
from ingestion of foodstuffs (see, in particular, [M47]). A detailed understanding of the transfer of 131I
through terrestrial and freshwater environments is therefore much less relevant to the Committee’s
updated estimates of doses to the public than that of the two longer-lived caesium radionuclides (withhalf-lives of more than 2 years).

34

UNSCEAR 2020/2021 REPORT

B. Overview of current understanding
1.

Radiocaesium migration and fixation in soil

82. Rapid penetration of radiocaesium was reported to depths of 20 cm in some soils within one totwo months after the FDNPS accident [T1] and in paddy fields before cultivation [S28]. However, a numberof sources subsequently reported that most radiocaesium 9 remains in the top 5 cm soil layer of undisturbed
soils indicating that strong fixation occurred in the upper soil layers [F2, L4, M10, M15, N16].
Radiocaesium has been found to have migrated down sampled soil in Japan at variable rates that were
influenced by factors such as the rate of precipitation, the amount of organic matter present, bioturbationand soil temperatures in Japan [K37, K38]. In the catchment of Okuma Town, with high levels of depositedradionuclides, vertical migration in undisturbed soils was reported to be relatively fast with small amountsof 134Cs and 137Cs detected in some soils at soil depths of more than 20 cm [K37, K38].
83. Radiocaesium deposited onto agricultural land soil surfaces in March 2011 was subsequently mixedby ploughing, commonly to a depth of around 15 cm for rice paddies and 25 cm for other agricultural
products and upland fields in Japan. Radiocaesium concentrations in soil around plant roots will,
therefore, have been diluted considerably by this routine practice.
84. The extent of binding of radiocaesium in soils is measured using the distribution (partition)coefficient (Kd), which relates the concentration associated with soil to that in the soil solution. A high
Kd indicates strong sorption and low amounts of radiocaesium in the soil solution, leading to low
bioavailability of radiocaesium for transfer to plants and animals, although this also depends on thepotassium concentration in the soil solution. The radiocaesium interception potential (RIP) is an intrinsic
soil property, normally experimentally determined under laboratory conditions, and quantifies the
specific radiocaesium fixation capacity of a soil. 10 Bioavailability of radiocaesium is higher when there
is a low potassium concentration in the soil solution. Three separate extensive studies of many different
soil samples in Fukushima Prefecture and surrounding regions [T21, U3, Y3] have found RIP values tobe lower for Andosols 11 compared with other soil types in Japan. Surveys of paddy fields [N6] and of
farmland soil samples [Y3] have found widely varying values of RIP even among samples with identicalsoil types and geological features. For the farmland soils, the measured RIP values were positively
correlated with the clay, silt, and exchangeable potassium and calcium content. Many soils and sediments
in areas affected by releases from the FDNPS accident contain relatively high amounts of micaceous clay
minerals and Yamaguchi et al. [Y3] has postulated that this has contributed to strong binding ofradiocaesium in soil in Japan. Soil RIP values in most areas of Fukushima Prefecture are generally higher
than those of the radioecologically sensitive regions, with Histosols and podzols, 12 around the Chernobyl
Nuclear Power Station (see appendix B).

9 Some studies reported in this section give data specifically for 137Cs only as it has a longer radioactive half-life than 134Cs. Whenthe term radiocaesium is used in this section it refers to both 134Cs and 137Cs.
10 K can be derived from RIP by taking account of the concentrations of potassium and ammonium in soil solution. Decreasingdpotassium and ammonium concentrations in soil solution increase Kd.
11 Andosols are soils found in volcanic areas and are usually defined as soils containing high proportions of glass and amorphous
colloidal materials.
12 A Histosol is a soil consisting primarily of organic materials; podzols are the typical soils of coniferous or boreal forests.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

35

2. Radionuclide transfers to, and concentrations in, crops and animal
products
85. The deposition of radionuclides released to atmosphere from the FDNPS accident occurred at atime of year when there were only a few crops present that might enter the food chain, such as leafy
vegetables, winter wheat, green tea and evergreen fruit [Y2]. The extent of interception of depositedradionuclides by plants depends on many factors such as the area and characteristics of the plant surfaces.
Radionuclide concentrations on these surfaces then decline due to processes such as wash-off during
precipitation and dilution due to plant growth. Fesenko et al. [F4] found that the time for 131I andradiocaesium concentrations to decline to half that initially present in weed leaves in areas affected by
the FDNPS accident was 4 and 8 days respectively when samples were taken from one week to twomonths after the major deposition period. A portion of the radionuclides retained on plant surfaces has
been found to be partially absorbed and translocated into edible parts such as green tea leaves and fruits[S9, T9, T15, T18].
86. In general, however, root uptake was the major pathway of 137Cs transfer to most crops after the
FDNPS accident occurred. Since the FDNPS accident, a number of site-specific studies have reported
concentration ratio 13 (CR) values from soil to brown rice derived from both pot and field experiments(e.g., [E3, E4, F10, F11, I27, K7, K36, O14, O15, S3, T47, W6, Y13]). The site-specific CR values variedwith soil type, exchangeable potassium status, management approaches and time after the accident and,for the period 2011–2013, were mostly within an order of magnitude of that assumed in the UNSCEAR
2013 Report [U10] (5.0×10−3). Irrigation water may influence rice concentrations [U4], but radiocaesium
entering paddy fields via irrigation water was reported to be a minor contributor to the concentrations inrice [T47].
87. Uematsu et al. [U5] showed that the radiocaesium concentration in rice plants and ryegrass was
significantly correlated with the ratio of radiocaesium to potassium concentrations in soil water.
A detailed analysis of the factors influencing the transfer of 137Cs to rice was also carried out using
extensive data from a large-scale study on paddy fields [Y8]. Higher CR values were observed in paddy
ﬁelds in which the exchangeable potassium contents were low. The results enabled the CR from soil tobrown rice to be predicted from the exchangeable potassium content. Furthermore, they also allowed arecommended value of exchangeable potassium to be established that would ensure that the radiocaesium
content of brown rice remained below the level at which the food would be restricted.
88. The results of recent large-scale studies that provide information on how CR for the transfer of 137Cs
to brown rice changes with time are summarized in table 4. The studies show a general decline in CR
with time, thought to be due to binding of radiocaesium with soil components. Yamamura et al. [Y8]derived a modelled rate of decline in CR from soil to rice of 0.17 ± 0.02/y and an eﬀective half-life 14 of
4.0 years from 2012 to 2015. This is consistent with the conclusions of Tagami et al. [T7], and the studies
suggest that the mean soil-to-rice CR for 137Cs in Japan was approaching that prevailing before theaccident (3.4×10−3) after less than three years, although analysis over a longer time period would provide
further confidence in these observations. Figure VIII shows how the concentration of 137Cs in rice has
declined rapidly while its deposition density in soil has remained elevated.

13 The transfer of radionuclides to crops is normally quantified as the concentration ratio (defined, in this chapter, as the ratio of theconcentration of a radionuclide in the food product to its concentration in soil to 20 cm depth on a dry mass basis). The term transfer
factor is also used for this ratio.
14 The effective half-life is specific to each radionuclide, medium and compartment of interest and is defined as the time required
for the radionuclide concentration in a specific compartment to be reduced to half of the initial value as a result of all environmental
factors, including radioactive decay.

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UNSCEAR 2020/2021 REPORT

Table 4. Time dependence of 137Cs concentration ratio for transfer to brown rice
CR measure

2012

2013

2014

2015

Median4.7×10−3

2.3×10−3

1.9×10−3

2.0×10−3

Maximum

1.8×10−1

1.4×10−1

1.9×10−1

2.6×10−2

Minimum

5.3×10−4

2.5×10−4

2.7×10−4

2.2×10−4

5.6×10−3

3.5×10−3

2.3×10−3

Mean
a

Year
2011

1.2×10−2

Reference

[Y8]

[T7]a

Data from Tagami et al. [T7] are geometric means.

Figure VIII. Comparison of 137Cs concentration in brown rice and the 137Cs deposition density in soilat the same site over time,a showing the effect of releases from the Fukushima Daiichi Nuclear
Power Station accidentaNuclear Regulation Authority “Environmental Radiation Database” [N16].

89. The transfer of radiocaesium to soybean [H27, L6, T21] and buckwheat [K44] seems to have been
higher than that to other agricultural crops [Y2], although detailed scientific analysis of all relevant data
has not yet been undertaken to confirm these findings. A large sample study by Takeda et al. [T21]reported CR values for soybean in 2011 of 5.2×10−3 to 5.3×10−1 from 46 sample sites. The vast majorityof monitored samples of soybean and buckwheat had radiocaesium concentrations below the level atwhich food restrictions were applied (see table 5); the small number of exceedances were associated withlack of potassium fertilization in fields with low contents of micaceous minerals [Y2].

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

37

90. Concentration ratio values have been reported for crops such as adzuki seeds, rye, Amaranthus,
Kenaf, brassica crops, leafy green vegetables, green tea and root crops [A17, A18, D3, H19, H27, H28,
K44, W16]. The average CR values reported for such crops were generally similar to those assumed in
the model used in the UNSCEAR 2013 Report [U10] of 1.0×10−2 for cereals and leafy green vegetables,and 7.0×10−3 for root crops.
91. Studies of these and other crops have noted reductions with time in the CR for 137Cs since the
FDNPS accident similar to those noted above for rice. In many cases the rate of reduction of 137Cs uptake
into crops has been rapid, with an effective half-life of 2 to 3 years (e.g., butterbur, soybean, green tea
and buckwheat [T2, T4, T9]). However, for three to four years after the accident, CR values for somegrasses have remained relatively high compared with other crops in Japan [S42]. The radiocaesium
concentration of Italian rye grass was linearly correlated with the amount of adhered soil, and plants witha higher yield had less adhered soil [S43]. Adhered soil accounted for about half of the total radiocaesium
measured in the plant samples.
92. Relatively high radiocaesium CR values (up to a factor of 20 higher than for other areas) were also
reported for a range of plants, including eggplant, pumpkin, soybean and cabbage, grown in soil withhigh radiocaesium deposition densities and low potassium concentrations in Okuma Town, near FDNPS,
in 2012 [O15].
93. Fruit is an important crop in Fukushima Prefecture. Initial radiocaesium concentrations among
fruits were highest for evergreen fruits and for Japanese cherry, which had partially emerged at the timeof the accident. Persimmon is often dried in Fukushima Prefecture to form an astringent persimmon(Hoshi-gaki) and semi-dried astringent persimmon (Ampo-gaki). The drying process leads to higher
radiocaesium concentrations. Some studies on green tea plants [T9], orchard trees [T13, T14, T16, T17]
and other trees have shown, for the first time, that bark, branches, leaf buds and old leaves can constitutea significant store of radiocaesium for several years after deposition. The upper soil layer contains few
roots of fruit trees, which are mostly located lower down the soil profile, so root uptake of radiocaesiumby fruit trees was probably not important after the FDNPS accident [S11]. Therefore, radiocaesium
adhered to bark surfaces was a more important source of radiocaesium in fruit than radiocaesium retainedin the upper 3 cm soil layers, above the rooting zone of fruit trees [K57, K58, S7, S9, T15, T18]. Longterm changes in 137Cs concentrations in mature fruit after the FDNPS accident have most often been
described by two decreasing components: an initial rapid component with a half-life of 100–200 days,
followed by a slower component with a half-life of several years [S10, T4].
94. The transfer of radioiodine and radiocaesium to milk, and radiocaesium to other animal tissues, hasbeen measured in controlled conditions with a range of different animals [H6, K28, O13, O18, O19, T11].
Reported transfer coefficient 15 values were similar to those in the international literature [H33, I3] and to
those assumed in the model used in the UNSCEAR 2013 Report [U10] to assess doses to members of the
public from ingestion of food after the first year. However, in that model, based on experience in Europe
and experience following the Chernobyl accident, it was assumed that cows grazed on pasture from midApril to November and that sheep were fed outdoors all year, whereas, sheep production is rare in Japan,
and few cattle graze pasture. Of the 17,100 dairy cattle in Fukushima Prefecture, only 720 were reported
to have been grazing outdoors during February 2011 [M3]. Most farm animals are kept in closed barns
in Japan [M7] and were fed on uncontaminated stored feed after the FDNPS accident [H33].
Consequently, in early spring 2011, milk from housed dairy cows had lower concentrations of 131I, 134Cs
and 137Cs compared with those few animals that were outdoors on pasture [K28]. In contrast, after the

15 Feed transfer coefficient is the radionuclide activity concentration in the animal product (milk, meat, eggs) (Bq/kg wet mass or
Bq/L) divided by the daily radionuclide intake of the animal (Bq/d).

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UNSCEAR 2020/2021 REPORT

Chernobyl accident, most livestock were grazing pasture when deposition occurred and intake ofradionuclides from ingestion of meat and milk was significant (see appendix B). The model assumptions
used in the UNSCEAR 2013 Report therefore overestimated the radiocaesium concentrations in milk andmeat from cattle, as has been confirmed by the low 137Cs and 134Cs concentrations reported in monitoringdata for agricultural animal products.
95. There were some exceptions to this general finding. In summer 2011, elevated radiocaesium
concentrations were measured in beef from cattle fed on rice straw which had intercepted depositedradiocaesium. A few ruminants grazing on upland slopes, with low fertility stony soils, were found tohave radiocaesium concentrations exceeding the limits established by the Japanese Government, despite
remediation using inverse tillage [M7].

3.

Radiocaesium transfer to, and concentrations in, wild food products

96. The estimates of ingestion doses to the public made in the UNSCEAR 2013 Report [U10] did notinclude edible plants from forests, wild game animals or fish and other species from freshwater systems.
After the Chernobyl accident, the transfer of radiocaesium to berries, mushrooms and hunted wild animals
in forests, and to grazed plants in extensively managed uplands, was much higher than that for agricultural
crops. The collection and consumption of produce from forests, hunting and the use of uncultivated, poor
nutrient status land for animal grazing provided important exposure pathways. The transfer ofradiocaesium to freshwater fish was also relatively high. A similarly enhanced transfer of radiocaesium
to these food groups has been observed in Japan.
97. A wide range of edible wild plants are consumed by Japanese people, especially those communities
living in mountainous areas. The new sprouts growing from rhizomes of bamboo shoots, butterbur andudo are widely consumed, and also the new buds on the top of branches of Koshiabura, which has had
the highest reported radiocaesium concentrations of wild plants, and fatsia [N10]. The relatively high andsustained transfer of radiocaesium to these plants [H14, K25], is reported to be associated with:(a) The low potassium status of associated soils and relatively low clay mineral content [N10];(b) The plants’ need for potassium in the early stages of growth [K26];(c) Mycorrhizal fungi (in Koshiabura) [S41]. Annual mean radiocaesium concentrations in bamboo
shoots have gradually decreased with time, but the rate of reduction was slower than that ofagricultural produce [F21].
98. The transfer of radiocaesium to forest mushrooms in Japan was higher in many different species
than that for agricultural crops. Species with both high and low accumulation of both radiocaesium
released from the FDNPS accident (e.g., [K35, N4, N7, T6] and radiocaesium from historic atmosphericnuclear weapons tests have been identified in Japan [T5]. There is also widespread cultivation of thewidely consumed shiitake mushrooms grown on logs.
99. Particularly high radiocaesium concentrations have been reported for wild boar in many forested
areas (e.g., [F20]). Tagami et al. [T3] reported high aggregated transfer factors 16 of radiocaesium tomuscle of a range of wild animal species (particularly in bear, wild boar, sika deer and copper pheasant)in five prefectures between 2011 and 2015. The decrease in radiocaesium concentrations with time was

16 Aggregated transfer factor (m2/kg), is the ratio of the radiocaesium concentration in the sample of the animal (e.g., muscle) (Bq/kg

wet mass) divided by the deposition density (Bq/m2).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

39

initially fast, but the longer-term decline has been comparatively slower, with an effective half-life of
0.6 to 1.4 years, although confidence in these values is low due to the limited time period of
measurements. Aggregated transfer factor values of 137Cs for muscle of bear and wild boar from
Fukushima Prefecture were lower than those observed after the Chernobyl accident [N9].
100. An extensive monitoring programme by Fukushima Prefecture showed that concentration ratios
were higher for freshwater fish than coastal marine fish [W5]. A higher proportion of freshwater fish
than marine fish had exceeded the limit established by the Japanese Government, especially in the firsttwo years after the accident [W4]. Cultured fish, which are more commonly eaten in Japan than “wild”
fish, are fed with commercial pellets and had much lower radiocaesium concentrations than wild fish
[W4, Y7].
101. Radiocaesium concentrations in freshwater fish have been found to be highly variable, as they areaffected by the characteristics of the surrounding catchment, the water, and their food source. The
radiocaesium concentrations found in both river and lake fish were reported to be correlated with that of
the surface soil of the catchment [A15, M10, T37]. A trophic level effect was also reported, whereby
radiocaesium concentrations were generally in the order: carnivorous (e.g., salmonids) > omnivorous >herbivorous > planktivorous species [A15, I26, W4]. Radiocaesium concentrations in freshwater fish andamphibians were affected more strongly by their feeding preferences than by water chemistry [I36, I37].
For many, but not all fish species, larger fish in both rivers and lakes had higher radiocaesium
concentrations than smaller fish [I36, I37, W5].
102. There was an initial rapid reduction with time since 2011 in both the radiocaesium concentrations
and CR 17 values in freshwater fish species [A15, I26, W4], which are influenced by radiocaesium
concentrations in both water and the food web constituents [W4]. The 137Cs concentration reduced by
one half within 1 to 2.5 years in lake fish [S49, S50, W4], and, with a few exceptions, there was little
further decline in the CR values for freshwater fish with time in both lakes and rivers after 2015.
Similarly, CR values for 137Cs in edible species other than fish, such as crabs, shrimps and frogs,
continued (up to 2018) to be 1 to 2 orders of magnitude higher than those observed before the FDNPS
accident [T8].

4.

Food monitoring

103. As part of the response to the FDNPS accident, the Japanese Government and prefectural authorities
began monitoring food and drinking water on 16 March 2011. Provisional regulation values were
established by the Japanese Government for radioactive material above which foodstuffs were prohibitedfrom distribution and consumption. From 1 April 2012, lower limits were introduced by the Japanese
Government for radiocaesium (134Cs and 137Cs). The provisional regulation values and limits forradiocaesium are summarized in table 5.

17 The concentration ratio for aquatic food is the ratio of the concentration of a radionuclide in the aquatic food product (Bq/kg wetmass) to its concentration in water (Bq/kg).

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UNSCEAR 2020/2021 REPORT

Table 5. Concentrations of radiocaesium (134Cs and 137Cs) in foodstuffs and drinking water above
which restrictions on supplies were introduced in Japan from March 2011a
Provisional regulation values established by theJapanese Government for radiocaesium fromMarch 2011
Food and water
category

New limits established by the Japanese Government
for radiocaesium from 1 April 2012
Food and water
category

Regulation value (Bq/kg)

Limit (Bq/kg)

Drinking water

200

Drinking water

10

Milk, dairy products

200

Milk

50

500

General foods

100

Food for infants

50

Vegetables
GrainsMeat, eggs, fisha.

The equivalent guideline level for the purposes of international trade, as recommended by the Codex Alimentarius Commission,is 1,000 Bq/kg for radiocaesium in all of the food products listed [F1].

104. Radiocaesium (134Cs and 137Cs) concentrations exceeding the provisional regulation value of
500 Bq/kg were initially detected in crops from the areas to the north and west of FDNPS. A
comprehensive monitoring scheme was then put in place for agricultural products (rice, cereals,
vegetables, and orchard fruit), forest products (e.g., mushrooms and edible wild plants), agricultural
animal products (e.g., beef, pork, milk) and freshwater products, which were measured for radioactive
material content before sale. Many thousands of measurements have been carried out each year since theaccident. The percentage of measured samples in Fukushima Prefecture in which the radiocaesium
concentration exceeded 100 Bq/kg between March and June 2011 was 18% in crops, 3% in livestock
products, 49% in forest products, and 52% in ﬁshery products [S27] (although the provisional regulation
values that applied at that time were higher than 100 Bq/kg). After June 2011, measured radiocaesium
concentrations in all monitored agricultural foods reduced quickly and were mostly below the new limitof 100 Bq/kg established by the Japanese Government for general food in most agricultural crop andlivestock products (see figure IX). Radiocaesium concentrations in wild food and freshwater ﬁsheryproducts have declined more slowly, with only a small number of samples exceeding the new limit of
100 Bq/kg for general food after 2014 (see figures X and XI).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Figure IX. Changes with time in the percentage of monitored terrestrial agricultural foodstuffs[M4, M5] that exceeded 100 Bq/kg of radiocaesium (134Cs and 137Cs)

41

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UNSCEAR 2020/2021 REPORT

Figure X. Changes with time in the percentage of monitored wild foodstuffs [M4, M5] that exceeded
100 Bq/kg of radiocaesium (134Cs and 137Cs)

Figure XI. Changes with time in the percentage of monitored freshwater fish samples caught eachquarter [M6] that exceeded 100 Bq/kg of radiocaesium (134Cs and 137Cs)

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

43

105. Some agricultural foodstuffs monitored in 2011 exceeded the Codex Alimentarius guideline levels
recommended for the purposes of international trade [F1]. Four hundred and forty-two samples ofvegetables (3.6% of the total measured) and 43 samples of milk (2.2% of the total measured) were foundto have 131I concentrations exceeding the guideline level of 100 Bq/kg. In the same year, radiocaesium
concentrations exceeded the guideline level of 1,000 Bq/kg in 89 meat samples (0.70% of the total
measured), three fruit samples (0.11% of the total measured), and 39 vegetable samples (0.04% of thetotal measured). No agricultural samples exceeded the guideline levels from 2012 onwards [H1, M4,
M5]. For freshwater fish, 19 samples of fish (2.2% of the total measured) exceeded the guideline level
for radiocaesium in the first year after the accident, and one sample (0.03% of the total measured) was
above the guideline level in the second year after the accident [M6].
106. The vast majority of food consumed in Japan after the accident had radionuclide concentrations thatwere lower than the limits established by the Japanese Government. This was due to (a) prohibition of
the collection and consumption of wild foods in affected areas, (b) rapid reduction in radiocaesiumtransfer over the first three years after the FDNPS accident for crops grown on agricultural land due tostrong binding of radiocaesium in the prevailing soil types in affected areas, and (c) application ofremediation options for crops, notably the application of potassium to agricultural land where necessary.
There was also little production of food in areas with high levels of radiocaesium deposition due torestrictions and the evacuation of residents from these areas.

C. Remediation
107. The natural processes by which radiocaesium was transferred through terrestrial and freshwater
environments have been discussed in section IV.B. These processes, as well as radioactive decay, havegenerally reduced the concentrations of caesium radionuclides in the compartments of the environment towhich the public were most exposed. The Japanese Government decided to implement large-scale
remediation projects to further reduce these concentrations and, therefore, the exposures of members of the
public, and to enable people to return to the areas evacuated following the accident, by establishing an
acceptable basis for this to occur. Details of the projects and their outcome have been published by the
Ministry of the Environment (MOE) [M39]. These projects were carried out based on the “Act on Special
Measures concerning the Handling of Environment Pollution by Radioactive Materials”, enacted in August
2011 [M40]. The MOE established the structures and institutional arrangements necessary for this work,
including the related laws and decontamination guidelines. The total cost of the work has been estimated at
29 trillion yen (as of September 2017), with about 15 trillion yen contributed by the Government and about
14 trillion yen by municipalities [M39]. About 77,000 workers were involved in these activities in the period
1 January 2012 to 31 December 2016.
108. The areas to be remediated were designated into two categories based on the estimated additional
annual effective dose that would be received by people living in these areas as assessed in autumn 2011:(a) Special decontamination area (SDA). This included the eleven municipalities in the areas thatwere within a 20-km radius of FDNPS and the “Deliberate evacuation areas”, situated beyond the
20-km radius, where the additional annual effective dose for individuals might exceed 20 mSv. The
Japan Self-Defense Force remediated key locations, such as municipal offices in this area, startingin December 2011, and, from January 2012, remediation was carried out in affected municipalities
by construction companies under contract to MOE. Most of the remediation work was completed in
March 2017, although agricultural remediation continues. A total of 23,000 residential lots,
8,700 hectares of farmland, 7,800 hectares of forest, and 1,500 hectares of roadways have been

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UNSCEAR 2020/2021 REPORT

remediated. As a result of the work, evacuation orders were lifted for an area of 780 km2, about 70%of the total area affected by evacuation orders, and it was confirmed that the additional annual
effective dose for residents who returned to this area was about 1 mSv [M39];(b) Intensive contamination survey area (ICSA). This included 92 municipalities in eight prefectures
where the additional annual effective dose for individuals was estimated to be between 1 and 20 mSv
in some parts of the municipality. The work was carried out by the municipalities supported bycentral Government, starting in around April 2011, when requests from residents triggered theremediation of schools, kindergartens, nursery schools and parks, etc. Subsequently, under the “Acton Special Measures” [M40], construction companies were contracted by each municipality to carryout work based on its specific remediation implementation plan. This work started in January 2012
and was completed in March 2018. As a result of the work in the ICSA, it was confirmed that theadditional annual effective dose for residents living there was 1 mSv or less in 2016, and that thelong-term goal set in the basic policy of the “Act on Special Measures” [M40] was almost fully
achieved [M39].
109. Remediation efforts addressed the two pathways of internal exposure from ingestion of food anddrinking water containing radionuclides released as a result of the accident and external exposure toradionuclides deposited on to surfaces. The focus was on the caesium isotopes, 134Cs and 137Cs.

1.

Remediation measures addressing internal exposure

110. Internal exposure from ingestion was greatly reduced by the imposition of widespread restrictions
on the sale and distribution of foodstuffs containing radionuclides above the limits established by theJapanese Government, supported by extensive monitoring of food products from affected areas.
Demonstration pilot studies into remedial measures focused on soil removal and the application of
additional potassium and of a caesium binder, zeolite. A large number of other studies provided cropspecific and soil type-specific information [S26] that assisted the farming community to produce food inwhich concentrations of radionuclides were below the limits established by the Japanese Government. In
contrast to the Chernobyl accident, where uptake of radiocaesium by crops from some of the prevailingsoil types was high and persisted for many years in some areas [F5], in the SDA and ICSA the uptake ofradiocaesium by crops was relatively low, in part due to the presence of micaceous minerals binding theradiocaesium, the high fertility and exchangeable potassium status of most farmland soils in Japan, andintensive farming methods employed in Japanese agriculture [H32].
111. Because the FDNPS accident happened before the growing season, there was limited interceptionof deposited radionuclides by early crops such as leafy vegetables, evergreen fruit and wheat seedlings.
To address subsequent radiocaesium uptake into crops, crop production was only permitted on farmland
where concentrations of radiocaesium in soil were below specified levels dependent on the CR value foreach crop. For example, for growing rice, the permitted radiocaesium concentration in soil was initiallyless than 5,000 Bq/kg-dry mass, based on an assumed maximum CR value of 0.1 (for the initial limit
established by the Japanese Government of 500 Bq/kg). For the remediation of areas with higher
radiocaesium concentrations in soil, removal of top soil was the most effective action, although top soil
removal not only produced large amounts of waste but also enhanced soil erosion, encouraged weeds andreduced the fertility of the soil. Other measures included reverse tillage to bury radiocaesium, soil
suspension in water and removal in paddies, ploughing soil to dilute the radiocaesium concentrations in
the rooting zone, and the application of additional fertilization.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

45

112. From 1 April 2012, when the limit established by the Japanese Government for general foods was
reduced to 100 Bq/kg, addition of potassium fertilizer to the soil became a more widely used action, basedon the strong correlation between the uptake of radiocaesium and the exchangeable potassium status of
the soil. The application of additional potassium to farmland soils was carried out to maintain an
exchangeable potassium concentration above a specified threshold during the growing season. An initialthreshold of 25 mg K2O/100 g soil was subsequently reduced to 20 mg K2O/100 g soil to take into accountreductions in radiocaesium concentrations due to radioactive decay and fixation of radiocaesium by soil,
and concerns regarding the environmental impact of enhanced potassium inputs. In Tomioka Town, in
2016, the exchangeable potassium content in more than 80% of soils which had top soil removed was
below the threshold level of 20 mg K2O/100 g soil [K56]. The application of potassium fertilizer was,therefore, particularly important for agricultural fields where excessive top soil removal may have caused
a considerable decrease in exchangeable potassium content.
113. It is difficult to estimate the reduction in internal exposure due to these remediation measurescompared with the effects of natural decay, migration and food monitoring. After 2011, only a very small
number of farm products exceeded the limits established by the Japanese Government, and there was a
rapid reduction with time in the uptake of radiocaesium in crops and agricultural animal products in boththe SDA and the ICSA. Research studies on farmland to evaluate potential remediation options continue
in the so-called “Areas where Returning is Difficult”. 18
114. Remediation activities to address continued elevated radiocaesium concentrations in wild plants,
game and fish are more difficult to develop than for farmland, emphasizing the importance of local
monitoring of all such products. Some remediation efforts in contaminated catchments may affectradiocaesium concentrations in aquatic foodstuffs. Ongoing research efforts provide information on
processes such as the removal of radiocaesium from the water column by adhesion to sediments.

2.

Remediation measures addressing external exposure

115. To address external exposure to radionuclides deposited on to surfaces, including natural surfaces
such as soil, sediment or trees, and man-made surfaces such as asphalt, concrete, walls, roofs, and floors,a range of different remediation techniques was developed. The application of the different techniques
depended on the ambient dose rate. For residential areas in the SDA and for relatively higher dose areasin the ICSA, remediation included comprehensive removal of all contaminated surfaces (including top
soil removal and power washing). For farmland, vegetation and the top 5 cm of topsoil were removed. In
some higher dose SDA areas, the soil was covered with crushed granite, and the entire soil profile was
mixed to dilute the residual radiocaesium present. For other farmland in the SDA and ICSA, the soil was
ploughed and potassium and zeolite were added. In more contaminated areas, the topsoil and subsoil werealso exchanged. For roads, remediation involved shot blasting the surface in the SDA, and cleaning ofroad surfaces and ditches in both the SDA and ICSA. For forests, remediation activities were focused onforest boundaries close to housing or areas where farmers worked and involved removal of fallen leaves
and other plant material.
116. An indication of the effectiveness of remediation can be provided by comparing the ambient dose
rate before and after remediation. Such a simple comparison will also include the effects of radioactive
decay and vertical and horizontal migration as well as the effects of remediation. Table 6 presents thefractional ambient dose rate reduction (the ratio of the reduction in the ambient dose rate at 1 m above

18 Areas designated in the context of the review of evacuation areas where the annual cumulative dose exceeds 50 mSv and where

the annual cumulative dose may not fall below 20 mSv in the subsequent five years.

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UNSCEAR 2020/2021 REPORT

the target surface after remediation had been implemented at each measurement location 19 to the ambient
dose rate before remediation) for a range of remediation activities carried out in the SDA and ICSA. The
fractional reductions measured varied widely for the different target areas and depended on the method
applied and the ambient dose rate prior to remediation (see table 6).
Table 6. Fractional ambient dose rate reductions in the special decontamination area and theintensive contamination survey area [E8], adapted from [Y23] (based on Fukushima Prefecture data
[F19, K31, M37])
Target area

Remediation

Fractional ambient dose rate reductiona

Ambient dose rate prior to remediation(µSv/h)

≤1

1–3

3–10

>10

AGRICULTURAL LAND
SDA

Cut weeds, remove 5 cm
topsoil and cover withclean soil

0.34

0.49

0.47

0.80

SDA/ICSAInterchange topsoil andsubsoil, add zeolite andpotassium

0.34

0.49

0.47

0.80

SDA/ICSA

Ploughing with zeolite andpotassium

0.21

0.31

0.29

0.50

FOREST
SDA

Remove fallen leaves andorganic matter

0.19

0.27

0.39

0.59

ICSA

Remove fallen leaves

0.10

0.14

0.20

0.30

ROADS
SDA

Shot blasting and cleaning
ditches

0.15

0.30

0.20

0.33

ICSA

Cleaning roads and ditches

0.08

0.15

0.20

0.33

RESIDENTIAL AREASa

SDA/ICSA

Full remediation

0.29

0.35

0.49

0.70

ICSA

Localized remediation (inhigher dose areas)

0.15

0.18

0.25

0.35

The ratio of the reduction in the ambient dose rate at 1 m above the target surface after remediation at each measurement location

to the ambient dose rate before remediation.

117. The fractional reductions given in table 6 include all factors contributing to a reduction in ambient
dose rates, namely the effect of remediation, radioactive decay (which would be similar for all target
areas), and vertical and lateral migration. The contribution of remediation to these reductions tended to
be greater for areas with higher ambient dose rates before remediation.
118. For the SDA, the reductions in the ambient dose rates following remediation were measured
immediately after completion of the work, and then again about 6 months to a year afterwards to verify
the reduction (see figure XII). The first set of measurements showed average fractional reductions in

19 The background dose rate was deducted in each case.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

47

mean ambient dose rate of 0.60 for residential areas (including schools, parks, cemeteries and large
facilities), 0.58 for farmland (including fruit orchards), 0.42 for roads and 0.27 for forests [M39]. The
second set of measurements showed the ambient dose rate to have reduced on average by 0.73, 0.68 and0.61 in residential areas, farmland and roads respectively, and by 0.46 in forested areas, which coverabout 70% of Fukushima Prefecture [M39]. The greater reductions in the second set of measurements
reflect the influence of other contributing factors (e.g., radioactive decay, migration). In forests, surface
material is continually accumulated due to the loss of tree components and attached radiocaesium fromtree surfaces that is affected by rainfall intensity [A19]. According to Thiry et al. [T31], remediation inforests was likely to be more effective when conducted after the peak of transfer to the organic layer,
which typically occurs around three to five years after the initial deposition, although the time periodvaries with tree and soil organic layer characteristics.
119. The fractional reductions in the ambient dose rates in the SDA reported by MOE [M39], and thevariation with the ambient dose rate before remediation are illustrated in figure XII. No information
was provided by MOE [M39] about the uncertainties in these ambient dose rate reductions or theirstatistical significance.
Figure XII. Fractional ambient dose rate reductionsa for different target areas and initial ambient
dose rates in the special decontamination area (based on data from [M39])a

The ratio of the reduction in the ambient dose rate at 1 m above the target surface after remediation at each measurement location

to the ambient dose rate before remediation.

120. In municipalities within Fukushima Prefecture that were within the ICSA, the average fractional
ambient dose rate reduction after remediation was 0.42 in residential areas, 0.55 in schools and parks and0.21 in forests [M39]. Measurements were made between June 2011 and February 2016.
121. These reductions for the SDA and ICSA include not only the reduction in the ambient dose rateachieved by remediation, but also those due to natural decay and weathering. Using monitoring datacollected before remediation, MOE [M39] estimated the decline over time in the ambient dose rate
without remediation for comparison with the reduction in the estimated average ambient dose rateachieved after remediation, to assess the effect of remediation alone. In the SDA, the average ambient

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UNSCEAR 2020/2021 REPORT

dose rate was estimated to be 59% lower in March 2018 than it would have been without remediation. In
the ICSA, the average ambient dose rate was estimated to be 38% lower in March 2016 than it would
have been otherwise. The detailed methodology used to derive these estimates has not, however, been
provided in the MOE report.
122. Detailed analysis of the ambient dose rate reduction with time was reported by Saito et al. [S2] who
compared the contribution of different factors to the rate of reduction in the ambient dose rate over the
first five years within an 80-km zone around FDNPS. The average ambient dose rate measured inundisturbed fields in the 80-km zone was estimated to be reduced as a result of remediation alone byapproximately 20%.

3.

Production of radioactive waste

123. An important consequence of the remediation efforts was the production of large amounts ofradioactive waste (about 2.68 million tonnes by the end of May 2020 [M41]) stored at temporary storagesites. The total volume of removed soil generated is approximately 22 million m3. From 2016 onwards,
the temporary storage sites have been gradually dismantled and restored. Some of the wastes generated
have been reduced in volume using incinerators, and less contaminated waste has been recycled and used
for civil engineering projects. Work is ongoing on the treatment and the transport of radioactive waste to
the interim storage facilities that are being built near FDNPS.

D. Summary
124. The extensive investigations carried out following the FDNPS accident have enhanced
understanding of the transfer of radionuclides through the terrestrial and freshwater environments inJapan. In terms of the model used in the UNSCEAR 2013 Report [U10] to estimate transfers ofradionuclides to foodstuffs from the first year after the accident onwards, a number of observations canbe made:(a) The concentration ratios assumed in the 2013 model for radiocaesium in crops (e.g., cereals, leafy
green vegetables, root crops) were similar to the ranges of values subsequently reported for Japanesecrops, resulting in broadly consistent estimated concentrations of radiocaesium in these foodstuffs;(b) The concentrations of radiocaesium in milk and meat were likely overestimated, owing to theassumption that cattle grazed pasture outdoors between mid-April and November whereas, inpractice, the majority of cattle (≈95%) in Fukushima Prefecture were housed in closed barns and fed(uncontaminated) stored feed following the accident;(c) The concentrations of radiocaesium in fruit were likely to have been underestimated as it was
assumed that soil was the only source of radiocaesium and no allowance was made for its transferfrom bark, branches, leaf buds and old leaves of trees;(d) Individuals consuming significant amounts of wild produce and freshwater products may havereceived doses from ingestion appreciably higher than those set out in the UNSCEAR 2013 Report.
This model has not, however, been used in the revised dose assessment described in chapter V.

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125. Radiocaesium appears to be strongly bound to many soils in Japan, and there were no soils that had
particularly high rates of radiocaesium uptake into plants, as occurred after the Chernobyl accident. The
concentration ratios for the transfer of radiocaesium from soil into agricultural food have generally
declined rapidly from elevated levels found immediately after the FDNPS accident to values that were
not much higher than those reported prior to the accident within a few years, although this needs
confirmation by further analysis over longer periods of time.
126. Monitoring programmes that began immediately after the accident enabled timely restrictions to be
applied to prevent foodstuffs being marketed, where the radionuclide concentration exceeded regulation
values and limits established by the Japanese Government. The radionuclide concentrations in mostmonitored foodstuffs have declined rapidly following the accident. Since 2015, no samples of livestockand crop products and only a few samples of monitored wild food and freshwater fish have been found
to exceed the limits. In addition, a small number of monitored agricultural food samples (less than a few
per cent) exceeded the Codex Alimentarius guideline levels for international trade in 2011, and nosamples have done so from 2012 onwards.
127. The large-scale remediation projects implemented by the Japanese Government and themunicipalities have been successful in further reducing the concentrations of radiocaesium in terrestrial
and freshwater environments, in addition to the effects of natural processes and radioactive decay andallowing the lifting of evacuation orders. In the SDA, reductions in the ambient dose rates (including
radioactive decay and migration of radiocaesium, as well as remediation) of between about 20% and 80%
have been found, depending on the surface remediated and the initial dose rate. In the ICSA, reductionsin the ambient dose rates of between about 20% and 55% were observed. The implications of these
reductions in ambient dose rates for the assessment of doses to the public are considered further inchapter V and attachment A-1.
128. The assessment of levels of exposure following accidental releases of radioactive material is greatly
facilitated by the availability of a validated model or models for reliably predicting the time-dependenttransfer of radionuclides through terrestrial and freshwater environments. Such models need to take into
account country-specific consumption and agricultural habits and the potentially important influence ofsoil types on radionuclide bioavailability.

V. ASSESSMENT OF DOSES TO THE PUBLIC

A. Introduction
129. This chapter sets out the basis for the Committee’s revised estimates of doses to the public in Japanand presents a summary of the doses estimated. 20 More details on the information available to theCommittee, the methodology it has used, and its revised dose estimates can be found in appendix A. As
in the UNSCEAR 2013 Report [U10], the Committee’s aim has been to make realistic, rather than

20 Throughout this report, the estimates of doses to the public presented are those from exposure to radioactive material released asa result of the FDNPS accident, i.e., the doses are those in addition to doses from exposure to natural sources of radiation and anyother sources of exposure. The average annual effective dose to the Japanese population from natural sources has been estimatedat 2.2 mSv [O21].

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conservative, estimates of doses. For ease of comparison with the UNSCEAR 2013 Report, estimates
were made of municipality- and prefecture-average doses for the same population groups (those people
who were evacuated, residents of municipalities in Fukushima Prefecture that were not evacuated,
municipalities in some neighbouring prefectures, and prefectures in the rest of Japan). While the same
population groups have been used as in the UNSCEAR 2013 Report, the allocation of some prefectures
to these groups is slightly different (see table 7). This is due to differences in the spatial coverage of themost recent radionuclide deposition density information used in the dose assessment.
Table 7. Population groups considered
Population
group

Geographical areas

Spatial resolution

1

Locations where people were evacuated in
the days to months after the accident

Representative areas were used for each location
identified in 40 evacuation scenarios

2

Municipalitiesa and parts of municipalities
of Fukushima Prefectureb not evacuatedMunicipality level for external and inhalation
pathways, based on the estimates for each of the
1-km grid points, averaged over the municipality
Prefecture level for ingestion pathway

3

4

Selected prefectures (Miyagi, Tochigi,
Ibaraki and Yamagata) in eastern Japanthat are neighbouring to Fukushima
Prefecture

Municipality level for external and inhalation
pathways, based on the estimates for each of the
1-km grid points, averaged over the municipality

All remaining prefectures of Japan

Prefecture level for external and inhalation pathways

Average for the four prefectures (Miyagi, Tochigi,
Ibaraki and Yamagata) for ingestion pathway

Average of the rest of Japan (i.e., the 42 prefectures,
excluding Fukushima, Miyagi, Tochigi, Ibaraki andYamagata) for ingestion pathwayaEach prefecture of Japan is divided into municipalities. A municipality is a local administrative unit; the municipalities are usedprimarily in the Japanese addressing system to identify the relevant geographical areas and collections of nearby towns and villages.
The term “municipality” has been used in this report in place of the term “district” used in the UNSCEAR 2013 Report [U10].
b

Japan comprises 47 prefectures. In Japanese the word “prefecture” is used for translating references to “ken” in Japanese.

130. Average doses in the first year after the accident have been estimated for the same age groups(represented by the 20-year-old adult, the 10-year-old child and the 1-year-old infant) and the same
dosimetric endpoints (the absorbed dose to selected organs – the thyroid, red bone marrow, colon andfemale breast – and the effective dose) as in the UNSCEAR 2013 Report [U10]. In addition, estimateshave been made of the average absorbed doses to the fetal thyroid over the 30-week development periodof the fetus and of the average absorbed dose in utero to the red bone marrow over the 40-week term ofpregnancy. Projections were also made for the three age groups of effective dose and absorbed dose to
the thyroid over the first 10 years after the accident and until an attained age of exposed individuals of
80 years. In this report the Committee has also estimated the distributions of individual doses in eachmunicipality or prefecture taking account of uncertainties (e.g., arising from modelling assumptions) andall major sources of variability among the population, including the age distribution, varying levels of
exposure within a municipality or prefecture, behaviours (such as the amount of time spent indoors or
outdoors), types of diet, and the filtering of the air and shielding provided by different types of buildings.
The 5th and 95th percentiles of the distributions have been presented as well as some example
distributions (see section V.E.2).

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Exposure pathways
131. For the radioactive material released to the atmosphere, the principal pathways by which membersof the public can be exposed are:(a) External exposure to radionuclides in the air;(b) External exposure to radionuclides deposited from the air on to the ground surface by either wetor dry deposition;(c) Internal exposure from inhalation of radionuclides in the air;(d) Internal exposure from ingestion of radionuclides in food and drinking water.
These pathways were considered in the UNSCEAR 2013 Report [U10] and have again been consideredin the dose assessment reported here. Members of the public could also have been exposed to radioactive
material released to the marine environment, through ingestion of fish and other seafood and exposure toradionuclides in the sea or on sediments at the seashore. The former pathway has been included in theestimation of internal exposure from ingestion of radionuclides in food; the latter pathway has not been
included because it was not expected to be a significant contributor in view of the 20-km evacuation zone
established around the FDNPS site.

B. Overview of dose assessment in the UNSCEAR 2013 Report132. In general, measurements made on people, either of external exposures or of radionuclides in thebody, provide a direct source of information for estimating doses. At the time of the UNSCEAR 2013
Report [U10], information on such measurements covered only a limited number of people and locations
and was insufficient to make comprehensive estimates of doses to the public; the measurements were
only used as one means of checking the validity of the Committee’s dose assessment.
133. The dose assessment in the UNSCEAR 2013 Report [U10] was accordingly largely based on
measurements of radioactive material in the environment, combined with models describing how people
were exposed to this material. Further details of the methodology used can be found in the UNSCEAR
2013 Report. Data were available regarding the radiation levels and deposition densities of radioactive
material in every prefecture in Japan, the concentrations in foodstuffs, and public and worker exposure.
Many of these data were provided by official government agencies in Japan; and many were publishedin peer-reviewed scientific journals. Twenty-five Member States of the United Nations other than Japan
officially provided information in response to the Committee’s request for data to support its assessment.
Additional data were made available by other international organizations and the Committee also
considered data made available by several non-governmental organizations. All data were evaluated todetermine their suitability for the assessment. Most of the scientific information used in the UNSCEAR
2013 Report was limited to that published or disclosed by the end of October 2012. 21

21 In finalizing the report, the Committee took into account where appropriate and practicable any significant new scientificinformation that became available after this date up until the end of 2013.

52

UNSCEAR 2020/2021 REPORT134. Models were used, inter alia, for estimating:(a) The levels of radionuclides deposited on the ground where insufficient measurement
information was available (generally only in the evacuated areas in the weeks following theaccident), where the Committee’s chosen source term was used with ATDM;(b) Concentrations of radionuclides in the air, where the source term and ATDM were used, either
directly for the period of evacuation and for radionuclides which were not deposited on the ground(e.g., noble gases), or to derive air concentrations from measured deposition densities;(c) The variation, as a function of time, of the external dose rate from deposited radionuclides;(d) Levels of radionuclides in food beyond the first year after the accident, for which measurements
were not available, where the Committee used a modified version of the FARMLAND model [B12]for estimating the transfer of radionuclides through terrestrial food chains.
135. The Committee took account of the measures implemented by the Japanese authorities to protect
the public, including the precautionary evacuation of approximately 78,000 residents within the 20-km
area around the FDNPS site and some nearby areas, which took place between 11 and 15 March 2011,
and the deliberate evacuation, based on environmental measurements, of about 10,000 residents of several
municipalities to the north-west of the FDNPS site, which took place between April and June 2011. Abouta further 30,000 people were also included among those evacuated: these included residents of other partsof municipalities that were partly within the 20-km area, some of the people living within the “evacuation
prepared area” between 20 and 30 km of the FDNPS site, who were advised to begin voluntary evacuation
on 25 March 2011, as well as some living outside the 30-km radius. In addition, foodstuffs and drinking
water containing more than prescribed concentrations of radioactive material were prohibited from saleor consumption. The Committee did not take account of other protective measures including directives
to shelter in place during the main releases and to take stable iodine, as sufficiently precise information
was not available. Nor was the Committee able to take account of the large land remediation programmesthat were initiated in some of the more affected parts of Fukushima Prefecture and neighbouring
prefectures due to the lack of detailed information at the time of the UNSCEAR 2013 Report [U10] about
the scale and efficacy of the implemented remediation actions.

C. Information for dose assessment
136. The Committee’s revised estimates of doses to the public from internal and external exposures inthis chapter use information collected for the UNSCEAR 2013 Report [U10], together with new
information that has since become available (up to the end of December 2019) 22 . This includes
measurement data on people (in particular, personal dosimeters, whole-body counting (WBC) and thyroid
measurements), and new information on concentrations of radionuclides in the air during the release andon the ground, on food and drinking water consumption, on occupancy factors, on location factors andon protective measures, including evacuation scenarios.
137. Chapters II, III and IV have provided summaries of the information now available on releases tothe atmosphere and to the ocean, on the distribution of the released radioactive material in the air, on the
ground and in the ocean and on transfers through the terrestrial, freshwater and marine environments to

22 New information that has become available after December 2019 has been taken into account exceptionally where it may have asignificant impact on the estimates of doses.

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foodstuffs. More measurement-based information is available on the levels of released radionuclides
deposited on to the ground surface and on concentrations of radionuclides in air. More information is alsoavailable on the transfer of radionuclides through the terrestrial environment in Japanese conditions, andmore measurements are available, over a longer period of time, on the levels of radionuclides in foods.
138. In addition, there is further information available relevant to the dynamics of ambient dose rates,
including about natural migration and weathering processes in Japanese conditions, and on dose rates above
different types of surface from car-borne surveys and those carried out on foot. Numerous measurement
campaigns have been conducted to assess individual doses from external exposure through surveys of daily
activity patterns of residents, and individual measurements using personal dosimeters. The most significant
of such measurement campaigns (some thousands of measurements per year) were carried out bymunicipalities, including, for example, Minamisoma City, Date City, Fukushima City, Naraha Town andsome others [F18, M31]. The results of these large-scale measurement campaigns have been made widely
available to the public and additional external exposure measurement data has been provided to theCommittee by the municipalities of Minamisoma City and Naraha Town. The Committee has made use of
the survey data and the scientific results published in peer-reviewed journals to develop a revised model toapply to the wider population, and used the results of the measurement campaigns to provide validation of
the estimates of dose from external exposure made using this model.
139. Thyroid monitoring was conducted from as early as 15 March 2011 on evacuees and permanent
residents of Fukushima Prefecture, covering around 1,200 persons of various ages and both sexes. Wholebody monitoring campaigns have been carried out by national institutes, such as JAEA and the Japanese
National Institute of Radiological Sciences (NIRS), and by universities, hospitals and municipalities.
These large-scale measurements began in July 2011 for evacuees and in October 2011 for residents. The
Committee has used publicly available information from these monitoring campaigns to validate its
estimates of doses from internal exposure, as well as data provided by Japan and the Russian Federation.
140. New sources of information that are available and relevant to estimation of the dose from ingestion
include measurements of the radiocaesium content in the whole daily diet sampled by the duplicate-dietor market-basket methods. 23 These methods are based on measurements made on the food and drinking
water as consumed by people, and some studies (e.g., [T53]) have taken account of factors such as
removal of radionuclides during food preparation and cooking. In addition, studies have been publishedsetting out well-founded estimates of doses from ingestion of food and drinking water which take account,for example, of specific food distribution systems in Japan, or specific circumstances for those who were
evacuated, based on surveys (e.g., [M47]).
141. Further details about the information available and the data that the Committee has used to makeits updated estimates of doses to members of the public can be found in appendix A.

D. Overview of methodology for assessing public exposures
142. There is considerably more, good quality, measurement information available than wasavailable to the Committee at the time of preparation of the UNSCEAR 2013 Report [U10]. In
addition, Japanese researchers have carried out many detailed assessments of doses and publishedthe results in the peer-reviewed literature. The Committee’s use of this information and its

23 In the duplicate-diet method, subjects weigh and put aside a duplicate portion of all the foods they have eaten for analysis; in themarket-basket method, the analysis is carried out on a basket of food, based on per capita food consumption data, as purchased at
a food market.

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UNSCEAR 2020/2021 REPORT

assessment results are described in detail in appendix A and summarized in the paragraphs below.
In its revised assessment of doses to members of the public, the Committee has placed greater
reliance on the results of studies published in the peer-reviewed literature rather than carrying out
its own modelling. However, it still needed to make use of some models for a full dose assessment
comparable with that in the UNSCEAR 2013 Report. The approaches used by the Committee,
including the models used, are described in appendix A. The Committee has used the additional
information that has become available to refine its models, reducing, and providing a better
estimation of, uncertainties and assessing the distribution of doses in the population taking accountof variabilities. The measurements made on people, in particular, have proved valuable in validating
the models that had to be used for estimating doses in the wider population. Some of the maindifferences between the approaches used in the UNSCEAR 2013 Report and those the Committee
applied to update its dose assessment are summarized in the following paragraphs.
143. While there is more information available on the concentrations of radionuclides in the air anddeposited on to the ground and other surfaces, all of the information now available is still not sufficient
to allow estimates of doses based solely on measurements and some modelling has been necessary. In
particular, in order to estimate doses from radioactive material in the air, both from external exposure to
this material and from internal exposure from inhalation of this material, modelling was needed to obtain
estimates of concentrations of radionuclides in air at all locations of interest and for particular time
periods, as measurements remain sparse. The approach used in the UNSCEAR 2013 Report has again
been used. Nonetheless, the greater amount of measurement information now available has been used totest this approach and provide greater assurance about its validity.
144. Accordingly, exposures from radioactive material in the air at specific times and locations have been
estimated from the assumed time sequence of the release of the more significant radionuclides and theirtransport through the atmosphere using ATDM. In this report, the Committee has used the latest source term
developed by the group of researchers at JAEA [T28], derived from measurement information on levels ofradionuclides in the air and deposited on the ground using reverse modelling and ATDM (Worldwide
version of System for Prediction of Environmental Emergency Dose Information – WSPEEDI), and better
correlated with events on the FDNPS site. Terada et al. [T28] have then used the same ATDM with thissource term to estimate the time-dependent concentrations in air and deposition densities of theradionuclides 132Te (and its daughter radionuclide, 132I, with which it is assumed to be in equilibrium), 131I,
134
Cs and 137Cs. For 131I, three chemical forms have been considered: elemental vapour, inorganic
particulate, and organic, using the constant ratios, based on experimental and other evidence, of 0.2 for131
Ielemental/131Itotal, 0.5 for 131Iparticulate/131Itotal, and 0.3 for 131Iorganic/131Itotal. These ATDM results have been
provided to the Committee for two nested grids:(a) The finer grid covering most of Fukushima Prefecture with a horizontal resolution of 1 km;(b) The coarse grid covering Fukushima Prefecture and all neighbouring prefectures with ahorizontal resolution of 3 km [T28].
145. Terada et al. [T28] did not include the short-lived radionuclide 133I and the Committee has therefore
estimated the concentration of 133I in air from the calculated concentrations of 131I in air by applying atime-dependent ratio of the two radioisotopes based on the ratio at 14:46 on 11 March 2011 from
Nishihara et al. [N12], adjusted for their radioactive half-lives. Terada et al. [T28] also did not include133
Xe which would have contributed significantly to the dose from external exposure to airborne
radioactive material, adding about 30% to the estimated effective doses from this pathway. However, theestimated effective dose from this pathway is typically only about 1–3% of the estimated effective dosefrom internal exposure from inhalation of airborne radioactive material, so the contribution of 133Xe to
the estimated total effective doses from airborne material would have been less than 1%, which is small

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55

compared with uncertainties in these estimates. Further details of the ATDM results provided to theCommittee and how they have been used in the dose assessment are provided in appendix A andattachments A-9 and A-10.
146. As in the UNSCEAR 2013 Report [U10], the ATDM estimates of concentrations of radionuclides
in the air as a function of time have been used directly to estimate doses to evacuees from external
exposure to the airborne radionuclides and doses from internal exposure from inhaled radionuclides. At
locations where members of the public were not evacuated, the ATDM estimates of deposition density
and of time integrated concentration of each radionuclide in air have been used to calculate ratios bywhich measured deposition densities have been scaled to infer a time-integrated air concentration. Fromthe latter, external exposure to airborne radionuclides and internal exposure from inhalation have been
derived. New measurement information on time-integrated concentrations of 137Cs in air at a number oflocations has been compared with estimates based on ATDM and estimates derived using the ratio
method (see appendix A). The comparisons provide support to the method being used by the Committee
to estimate time-integrated air concentrations of radionuclides and the resulting public doses fromexposure to radionuclides in the air.
147. Since the UNSCEAR 2013 Report [U10], further information has become available relating to
the deposition densities of 131I and to the dynamics of ambient dose rates, in particular on dose rates
above different types of surface in the environment specific to Japanese conditions. Extensive
measurements of external exposure made directly on people have also become available. In addition,
the International Commission on Radiological Protection (ICRP) has developed new dosimetric data
and models relevant for external dose assessment in a post-accident situation [I22]. The Committee
has taken account of these developments to derive an improved model (M2020) for assessing doses
from external exposure to deposited radionuclides and made comparisons with the measurements madeon people to provide more robust testing and validation of this model. Using this model, external dose
rates are higher in residential areas and decline more gradually over time than predicted by the model
used in the UNSCEAR 2013 Report (see appendix A).
148. The Japanese population traditionally has an iodine-rich diet, containing up to tens of thousands ofmicrograms of stable iodine a day, that is greater than the worldwide average by about two orders ofmagnitude [K5, L3, N2, Z6, Z7]. As a result, the fractional uptake of radioiodine into the thyroid
following intake via ingestion or inhalation among the Japanese population can be expected to be lower
than the reference ICRP value used in the UNSCEAR 2013 Report [U10]. The new ICRP model of iodine
biokinetics in adults [I21] has been used to calculate dose coefficients for inhalation and for ingestion of
131 132 133
I, I, I and 132Te, for three types of diet that may be more appropriate for the Japanese population:a typical Japanese diet; a kelp-rich diet; and a Western pattern diet, that is popular among some groupsof the Japanese population. The dose coefficients were developed for dose to the thyroid, red bonemarrow, female breast, and colon, as well as for effective dose. The indicative dose coefficients for intake
by inhalation and ingestion have been calculated for an adult female, an adult male, and for a 10-yearold child, a 1-year-old infant, and the 35-week fetus. The resulting dose coefficients for those with atypical Japanese diet are about a factor of two lower than those used in the UNSCEAR 2013 Report (andrecommended by ICRP for general application worldwide). Further details can be found in appendix A
and attachments A-2 and A-4.
149. In the UNSCEAR 2013 Report [U10], the Committee made no allowance for any reduction in thedose from inhalation for people being indoors during the passage of the plumes of released material, in
the absence of more specific information. Based on new information about the factor by which doses
from inhalation may be reduced while indoors, which was experimentally derived for Japanese houses,
the Committee has used a reduction factor varying between 0.1 and 0.95, with an average of 0.5, for

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UNSCEAR 2020/2021 REPORT

assessing doses from inhalation of radionuclides when people were indoors. Account has been taken of
the amount of time different groups of people spent indoors. Further details can be found in appendix A
and attachment A-10.
150. In the UNSCEAR 2013 Report [U10], the Committee estimated doses in the first year from ingestion
from measurements of 131I, 134Cs and 137Cs made in a wide variety of foods and in drinking water in Japan.
From emerging information from whole-body monitoring, the Committee was aware, at the time of
preparation of the UNSCEAR 2013 Report, that these estimates were likely to be substantial overestimates.
Possible reasons for this included that the food monitoring measurements used, particularly in the first year,
were not based on random sampling, but on samples aimed at identifying foods containing highconcentrations of radionuclides that should be subject to restrictions. In addition, where measurement results
were below the minimum detection level, a constant concentration of 10 Bq/kg was assumed.
151. Further information has since become available that has enabled the Committee to make more
realistic and robust estimates of doses from ingestion. For radiocaesium, this includes WBC
measurements made on people; measurements made on food actually consumed by people, either frommarket-basket or from duplicate-diet surveys; and detailed assessments based on measured levels in foodin Japan. For radioiodine, this includes: measurements of the radioiodine content of thyroids; and detailed
assessments based on measured levels in food and drinking water in Japan. Appendix A provides further
details about these sources of information. Making use of these different sources of information toestimate doses is complicated by factors such as the timing of the measurements, and the different routes
of intake and the different radionuclides included in each case.
152. In light of the strengths and weaknesses of these sources of information and the dose estimates thatcan be derived from them, the Committee has based its estimates of doses from ingestion in the first year
to both adult and child residents on the estimates of Murakami and Oki [M47] derived by modellingintakes of 131I, 134Cs and 137Cs using food monitoring data. While this is a similar approach to that usedin the UNSCEAR 2013 Report [U10], Murakami and Oki used data from a wider range of sources than
was available to the Committee when preparing that report, and, unlike that report, took account of theregional trade in foods (specifically, of the proportion of foods from different parts of Japan on sale atfood markets, where most Japanese citizens purchase food). Murakami and Oki compared their estimatesof the effective dose from ingestion of 134Cs and 137Cs in food and drinking water with the estimatesderived from market-basket and food-duplicate surveys and found good agreement. The Committee has
adjusted the estimates of Murakami and Oki to take account of the specific dose coefficients it has derived
for radioiodine intakes for Japanese people. The Committee has compared the dose estimates for the firstyear with the results of the whole-body monitoring studies and the thyroid monitoring studies (seeappendix A and attachments A-2 and A-3).
153. For time periods after the first year, the Committee has used estimates of doses to adults andchildren from ingestion of radiocaesium derived from the market-basket and duplicate-diet studies as its
starting point, and then used the model developed by Smith et al. [S33] to estimate how intakes ofradiocaesium from food and drinking water change over the longer term. This model was developed froman analysis of long-term measurement data collected annually from 1963 to 2008 on the 137Cs content offood products and in the whole diet in Japan resulting from fallout of radiocaesium from atmosphericnuclear weapons testing. Further details of the approach used by the Committee in updating its assessmentof doses from ingestion can be found in appendix A.
154. As part of the response of the Japanese authorities to the FDNPS accident, there was widespread
evacuation of residents living nearby at different times to reduce radiation exposures. In a recent study,
Ohba et al. [O5] have refined the 18 evacuation scenarios used to assess doses to evacuees in the
UNSCEAR 2013 Report [U10], resulting in 37 new representative evacuation scenarios. As these 37 newANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

57

evacuation scenarios did not include those evacuated from Hirono Town and Katsurao Village, theCommittee has also included three scenarios, considered in the UNSCEAR 2013 Report, which represent
those evacuated from these locations, giving 40 evacuation scenarios in total. Within these 40 scenarios,
the Committee has considered four types of human activities: (a) normal living conditions; (b) preparing
for evacuation; (c) evacuation; and (d) sheltering, and used a method to estimate doses consistent withthat used for residents, although no allowance was made for any reduction in dose from inhalation that
may have occurred while in buildings or vehicles during evacuation. Doses from ingestion of food before
and during evacuation have been assumed to be negligible, based on survey results [H15, K10], and thedoses from drinking water estimated by Miyatake et al. [M34] have been used, again with adjustment forthe specific dose coefficients for iodine intakes for Japanese people. The assessment of doses to thoseevacuated also included doses received at their evacuation destinations, mostly in less affected areas of
Fukushima Prefecture but also in other prefectures; it was also assumed, as in the UNSCEAR 2013
Report, that evacuees remained at these destinations subsequently. Projected doses from depositedradionuclides have also been assessed for the evacuated locations, in order to determine the dose averted
by evacuation.
155. Experimental studies and tests of environmental remediation technologies started in the affectedareas of Japan in summer 2011 and continued in Fukushima Prefecture until 2013. Large scale
remediation of the evacuated municipalities started in 2013 in the SDA, with the support of theGovernment of Japan, and local authorities initiated remediation activities in the ICSA. The information
available about the technologies used, the scale of the implemented environmental remediation actions,and the resulting reductions in ambient dose rates have been summarized in chapter IV. However,information about the reductions achieved by remediation in the dose actually received by the public
remains quite limited. As a result, an assessment of the effects of environmental remediation on theestimates of doses has been limited to the doses received by evacuees following return to their homes.

E. Results of dose estimation
156. As in the UNSCEAR 2013 Report [U10], the Committee has produced an extensive set of estimatesof effective doses and absorbed doses to particular organs for the public in Japan. This is presented in
more detail in appendix A. The Committee has firstly made estimates of municipality- and prefectureaverage doses to defined groups of the population for ease of comparison with the equivalent estimates
made in the UNSCEAR 2013 Report.
157. In this report, the Committee has, in addition, made more quantitative and comprehensive estimatesof the uncertainties and variabilities about these municipality- or prefecture-average doses. The results
include the 5th and 95th percentiles of the distribution of individual doses in the different population groups,
taking account of major sources of uncertainty and variability among the population, including data andmodelling uncertainties, the age distribution of the population, varying levels of exposure within amunicipality or prefecture, lifestyle (such as the amount of time spent indoors or outdoors), types of diet,and the filtering of the air and shielding afforded by different types of buildings. The estimated 5th and95th percentiles are presented with the estimated average doses in section V.E.1; and some examples of the
distributions of individual doses in different population groups are presented in section V.E.2. Further
details on how uncertainties and variabilities have been addressed can be found in attachment A-12.

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1.

Average doses to different population groups in Japan(a) Average doses to residents (i.e., those not evacuated) in the first year
158. Table 8 summarizes the estimated municipality- or prefecture-average effective doses and absorbed
doses to the thyroid for the first year following the accident for adults, 24 children and infants living inareas of Japan that were not evacuated. Compared with the equivalent table in the UNSCEAR 2013
Report [U10] (see table 5 in that report), the Committee’s updated estimates of the average effective
doses at the upper end of the ranges are generally a few tens of per cent lower than in the UNSCEAR
2013 Report and more than ten times lower at the lower end of the ranges; average absorbed doses to thethyroid are around half those in the UNSCEAR 2013 Report at the upper end of the ranges and more thanten times lower at the lower end of the ranges. These differences are largely due to the more realistic
estimates made in this report of doses from ingestion and the use of dose coefficients for intakes ofradioiodine that are specific to the Japanese population. These differences and their effects on the dose
estimates are explored in more detail in appendix A.
Table 8. Ranges of estimated municipality- or prefecture-average effective doses and absorbed doses
to the thyroid for the first year following the accident for typical residents of Japan that were notevacuated
The doses are in addition to the background doses due to natural sources of radiation. 25 The values are the rangesof the average doses in the municipalities of the Group 2 and Group 3 prefectures and the ranges of the average
doses in the prefectures of the Group 4 prefectures. These estimates were intended to be characteristic of theaverage dose received by people living in a given municipality or prefecture and do not reflect the range of doses
received by individuals within the respective populations

Geographical area
Age groupa

Ranges of absorbed dose to thethyroid (mGy)

Adult10-year-old

1-year-old

Adult

10-year-old

1-year-old

Group 2 – Fukushima
Prefecture

0.079–3.8

0.10–4.5

0.12–5.3

0.48–11

1.0–17

1.2–21

Group 3 – neighbouring
prefecturesc

0.10–0.92

0.13–1.1

0.15–1.3

0.31–3.3

0.52–5.2

0.62–6.3

Group 4d – rest of Japan

0.004–0.36

0.005–0.43

0.005–0.51

0.034–0.48

0.073–0.63

0.087–0.74baRanges of effective dose (mSv)

Detailed estimates are not tabulated here for doses to the fetus but can be found in attachment A-14. Ranges of average fetal absorbed

doses to the thyroid over the 30-week development period of the fetus are about 70% to 80% of the tabulated adult thyroid doses.
bGroup 2 – Members of the public resident in the municipalities or parts of municipalities in Fukushima Prefecture that were not evacuated.

cGroup 3 – Members of the public resident in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata. The radionuclide

deposition density information in parts of these prefectures was sufficient for estimates of doses to be made from inhalation andexternal exposure pathways at the municipality-average level on a 1-km square basis. As a result, prefectures making up Group 3
are different from those considered in the UNSCEAR 2013 Report [U10].
dGroup 4 – Members of the public resident in the remaining 42 prefectures of Japan, including the previous Group 3 prefecturesof Gunma, Chiba and Iwate.

24 Average doses to adults who were indoor workers have been chosen to be representative of adults as a whole. The variability ofthe effective doses and absorbed doses to the thyroid among the population, including whether adults were indoor or outdoor
workers, or retired has been considered in section V.E.2 below.
25 The average annual effective dose to the Japanese population from natural sources has been estimated at 2.2 mSv [O21].

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59

159. Figure XIII shows a map illustrating municipality-average effective doses in the first year to infants
living in municipalities of Fukushima Prefecture that were not evacuated and in parts of the neighbouring
prefectures where there was sufficient spatial coverage of the deposition density information. Absorbed
doses to the thyroid for all ages and effective doses to children and adults show a similar geographicalpattern that broadly reflects the deposition density of radionuclides in the different areas (seeappendix A). In addition, figure XIV shows the municipality-average effective dose in the first year to
infants residing in each of the municipalities of Fukushima Prefecture that were not evacuated (Group 2).
Figure XIII. Estimated municipality-average effective doses in the first year following the accident toinfants living in Group 2 municipalities of Fukushima Prefecture and some municipalities of Group 3
prefectures

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).

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UNSCEAR 2020/2021 REPORT160. External exposure to deposited radionuclides generally makes the largest contribution to theeffective dose. Internal exposure from intakes of radionuclides (in particular, radioiodine) 26 by inhalation
generally makes the largest contribution to the absorbed dose to the thyroid, although external exposure
to deposited radionuclides also makes an important contribution. This is in contrast to the doses estimatedin the UNSCEAR 2013 Report [U10] where ingestion of radionuclides in food and water was acomparatively more important exposure pathway 27 for both effective dose and absorbed dose to thethyroid. The Committee had acknowledged in the UNSCEAR 2013 Report that its estimates of doses
from ingestion had been likely overestimates. The Committee has made more realistic estimates of doses
from the ingestion of food and drinking water in this report, in the light of the information that has become
available from a wide range of sources about intakes of radionuclides by members of the public (seeappendix A for further information).
161. Within Fukushima Prefecture, the municipalities with high ground deposition density (ofFukushima City, Nihonmatsu City, Date City, Koriyama City, Koori Town and Otama Village, see
figure XIV) had the highest estimated average effective doses to individuals who were not evacuated,with the municipality-average effective doses to infants in the range of 3.6 to 5.3 mSv in the first year.
Average effective doses in the first year for adults were estimated to be about 70% of those for infants.
The 5th and 95th percentiles of the distribution of individual doses in each municipality are up to about
two times lower and higher, respectively, than the estimated average doses.
162. For the municipalities of the Group 3 prefectures (Ibaraki, Miyagi, Tochigi and Yamagata), themunicipality-average effective doses to adults were in the range of 0.1 to 0.9 mSv for the first year, with thedominant contribution again being from external exposure to deposited radionuclides. The prefectureaverage effective doses to adults for the prefectures in the remainder of Japan were in the range of 0.004 to
0.4 mSv, with external exposure to deposited radionuclides being the dominant pathway.
163. Figure XV shows a map of the estimated prefecture-average effective dose in the first year for infantsin Japan, with municipality-average effective dose also shown for Fukushima Prefecture as an inset.
Prefecture-average doses for other prefectures were lower than those for Fukushima Prefecture and areconsiderably lower for the more distant prefectures where the effective dose estimates were less than thenormal variations in background doses due to natural sources of radiation.

26 The radioiodines included in the dose estimates comprise 131I, 132Te/132I and 133I. Of the total absorbed doses to the thyroid frominhalation of radioiodines, 131I has been estimated to contribute about two-thirds or more (see attachment A-10 for further details).
27 It is also in contrast to the Chernobyl accident where doses from ingestion made a more significant contribution. This is due tomany differences between the Chernobyl accident and that at FDNPS, including the time of year the accidental releases occurred(early spring for Fukushima; late spring for Chernobyl), differences in soil composition, the food restrictions imposed and the levelof dependence of the local population on local food.

Figure XIV. Estimated municipality-average effective doses in the first year following the accident to infants living in each municipality of Fukushima
Prefecture apart from those evacuated
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Figure XV. Estimated prefecture-average effective doses in the first year following the accident
to infants
The main map shows the prefecture-average effective doses. The average dose for Fukushima Prefecture
includes only locations that were not evacuated. The inset map shows the municipality-average doses for theGroup 2a municipalities of Fukushima Prefecture

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).
aGroup 2 includes all municipalities or parts of municipalities of Fukushima Prefecture that were not evacuated.

Figure XVI. Estimated municipality-average absorbed doses to the thyroid in the first year following the accident to infants living in each municipality ofFukushima Prefecture apart from those evacuated
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64

UNSCEAR 2020/2021 REPORT164. Figure XVI shows the estimated municipality-average absorbed dose to the thyroid in the first year
for infants residing in the municipalities of Fukushima Prefecture that were not evacuated (Group 2). The
highest absorbed doses to the thyroid in the first year were to individuals living in the municipalities of
Minamisoma City, Date City, Fukushima City, Soma City and Koori Town. The highest municipalityaverage absorbed dose to the thyroid in the first year was estimated to have been about 21 mGy for infants
living in the municipality of Minamisoma City. More than 80% of this dose was due to inhalation, morethan 10% was due to external exposure to deposited radionuclides, and about 5% was due to ingestion.
The estimated absorbed doses to the thyroid in the first year for adults and children living in themunicipality of Minamisoma City were about 60% and 85%, respectively, of those for infants. Estimatesof the average absorbed doses to other organs can be found in appendix A. The 5th and 95th percentilesof the distribution of individual absorbed doses to the thyroid are generally about 2 to 3 times lower and
2 to 3 times higher than the estimated average doses, respectively, although they can be more than fourtimes lower and higher respectively, in municipalities where internal exposure from inhalation ofradionuclides has been estimated to make the largest contribution to the total dose.
165. For Group 3 prefectures (Ibaraki, Miyagi, Tochigi and Yamagata), the municipality-averageabsorbed doses to the thyroid of infants in the first year were estimated to be in the range of 1 to 6 mGy,
with the dominant exposure pathway again being inhalation. For the remainder of the 42 prefectures ofJapan, the prefecture-average absorbed doses in the thyroid of infants were estimated up to about 1 mGy,
with the dose from ingestion and from external exposure to deposited radionuclides making
comparatively larger contributions (about 40% each) to the total than that from inhalation.

(b) Average doses to evacuees in the first year
166. Doses in the first year to people evacuated from Group 1 municipalities (Futaba Town, Hirono
Town, Namie Town, Naraha Town, Okuma Town, Tomioka Town, Iitate Village, Kawamata Town,
Minamisoma City, Tamura City, Kawauchi Town and Katsurao Village) were estimated as the sum ofdoses received before and during evacuation, and doses received during the remainder of the year at thelocation to which they were evacuated. The estimated ranges of average effective doses and absorbed
doses to the thyroid for different groups of evacuees are summarized in table 9.
167. The estimated average effective doses to adults in the first year for the different evacuation groups
ranged from less than 0.05 mSv to about 6 mSv. The 5th and 95th percentiles of the distributions ofindividual effective doses are generally two to three times lower and higher, respectively, than the average
effective doses. The corresponding average absorbed doses to the thyroid in the first year ranged from lessthan 1 mGy to about 15 mGy for adults and between about 2 mGy to about 30 mGy for infants. The 5th and95th percentiles of the distributions of absorbed doses to the thyroid are between about two and four times
lower and about two to three times higher, respectively, than the average doses. The Committee estimatedthat the evacuation of municipalities averted effective doses to adults of up to about 40 mSv and absorbed
doses to the thyroid of infants of up to about 500 mGy (see tables A12 and A13 of appendix A).

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65

Table 9. Estimated ranges of average effective doses and absorbed doses to the thyroid for groupsof evacuees for the first year following the accident
The doses are in addition to the background doses due to natural sources of radiation. The values were the rangesof average doses for groups of evacuees represented by the different evacuation scenarios. These estimates wereintended to be characteristic of the average dose received by people evacuated from each municipality and do
not reflect the range of doses received by individuals among the population of the evacuated municipality

Age groupa

Before and during
evacuation

At the evacuation
destination

First year total

RANGE OF EFFECTIVE DOSE (mSv)
Adult0.027–3.6

0.005–2.7

0.046–5.5

10-year-old

0.058–4.3

0.006–3.2

0.10–6.5

1-year-old

0.079–5.2

0.005–3.8

0.15–7.8

RANGE OF ABSORBED DOSE TO THE THYROID (mGy)

a

Adult

0.39–150.034–3.2

0.79–15

10-year-old

0.65–210.072–4.1

1.6–221-year-old

0.78–30

0.087–4.7

2.2–30Detailed estimates are not tabulated for doses to the fetus, but can be found in attachment A-18. Ranges of the average fetal

absorbed dose to the thyroid over the 30-week development period of the fetus are about 70% to 80% of the tabulated total adultthyroid doses.

(c) Doses over longer time periods
168. The Committee also estimated municipality-average and prefecture-average doses accumulated
over the first 10 years after the accident and accumulated up to the age of 80 years. These are presented
in table 10 for residents. Compared with the equivalent table 7 in the UNSCEAR 2013 Report [U10], theaverage effective doses accumulated over the first 10 years and up to the age of 80 years at the upper endof the ranges are generally within a few tens of per cent of those estimated in the UNSCEAR 2013 Report(although the differences for the Group 4 prefectures are larger due to the different prefecture groupings
used in this report). At the lower end of the ranges, they are up to ten or twenty times lower. The
95th percentiles of the distributions of effective doses over a lifetime are generally less than a factor of
two greater than the average doses. The more realistic estimates of doses from ingestion made in thisreport and the different model used to estimate doses from external exposure to deposited radionuclides
are largely responsible for these differences. The differences between the estimates made in this report
and those made in the UNSCEAR 2013 Report are explored further in appendix A. External exposure
from deposited radionuclides is generally by far the most important exposure pathway for effective doseover the longer-term.

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Table 10. Estimated ranges of municipality- or prefecture-average effective doses to adults, children
and infants (as of 2011) over the first year, first 10 years and to age 80 years
Ranges of municipality- or prefecture-average effective dosea (mSv)
Age group in
March 2011

Group 2b –Fukushima PrefectureGroup 3c –neighbouring
prefectures

Group 4d – rest of Japan

1-YEAR EXPOSURE
Adult

0.079–3.8

0.10–0.92

0.004–0.36

10-year-old

0.10–4.5

0.13–1.1

0.005–0.43

1-year-old

0.12–5.3

0.15–1.3

0.005–0.51

10-YEAR EXPOSURE
Adult

0.16–11

0.25–2.5

0.009–1.0

10-year-old

0.19–120.30–2.9

0.008–1.2

1-year-old

0.22–14

0.34–3.4

0.007–1.3

LIFETIME EXPOSURE TO AGE 80 YEARSe

a

Adult

0.22–15

0.32–3.6

0.010–1.4

10-year-old

0.24–17

0.38–4.0

0.009–1.6

1-year-old

0.27–19

0.43–4.5

0.008–1.8

The reported doses are ranges of the municipality-averaged doses for the Group 2 and Group 3 prefectures and the prefecture-

average doses for the Group 4 prefectures. These estimates of dose are representative of the average doses received by people living
at different locations and do not reflect the range of doses received by individuals within the population at these locations.
bGroup 2 includes all municipalities or parts of municipalities of Fukushima Prefecture that were not evacuated municipalities.

cMembers of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.

dMembers of the public living in the remaining 42 prefectures of Japan. This group now includes the prefectures of Chiba, Gunma,and Iwate, which were included in Group 3 in the UNSCEAR 2013 Report [U10].
e

For adults, this is the dose from age 20 years at the time of the accident up to age 80 years.

169. Generally, the municipality-average or prefecture-average effective doses that would be incurred
over the first 10 years were estimated to be between about two and three times the doses in the first year,
and the lifetime doses were up to about a factor of four times higher. These increases over time are larger
than estimated in the UNSCEAR 2013 Report [U10] and are the result of the dose rate from depositedmaterial observed in Japanese conditions declining more slowly over time than in the model used in the
UNSCEAR 2013 Report (M2013). Most of the absorbed dose to the thyroids of residents of Japan wasreceived during the first year from inhalation of radioiodine. The absorbed dose to the thyroid up to age
80 years is estimated to be about twice that received in the first year, largely due to continued exposure
from the longer-lived radioisotopes of caesium (from external radiation from deposited radionuclides andintake by ingestion). These dose estimates do not take account of the effects of remediation work carriedout in the ICSA; this remediation work may have reduced doses in the ICSA from external exposure todeposited radionuclides by about an additional 10–20% following completion of the work (seeappendix A and attachment A-1), and the overall effect on the estimates of doses in table 10 (and in
table 11) would have been small in comparison with the uncertainties in these dose estimates.

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67(d) Estimates of current levels of exposure170. To provide an indication of the current levels of exposure resulting from the accident, theCommittee has estimated annual effective doses in 2021 in the non-evacuated municipalities ofFukushima Prefecture (Group 2), the Group 3 prefectures and the remaining prefectures in Japan(Group 4) and these are summarized in table 11. The estimated average annual effective doses are all lessthan 0.5 mSv in Fukushima Prefecture (Group 2) and below 0.1 mSv elsewhere in Japan. The 5th and95th percentiles of the distributions of individual doses are about two to three times lower and two times
higher than the average doses, respectively. External exposure to deposited radionuclides generally
accounts for more than 95% of the total dose. Figure XVII shows the estimated average annual effective
dose to infants in 2021 by municipality for Fukushima Prefecture and parts of the Group 3 neighbouring
prefectures. In contrast to table 11 (which considers only the municipalities in Fukushima Prefecture thatwere not evacuated), the figure also includes the municipalities in Fukushima Prefecture that were
evacuated (apart from the “Areas where Returning is Difficult”), assuming that residents have returned,
and takes account of remediation, but only in the SDA (see below).
Table 11. Ranges of estimated municipality- or prefecture-average effective doses to adults, children
and infants in the year 2021
Geographical area

Adult

10-year-old

1-year-old

Group 2 – Fukushima
Prefecture

0.004–0.31

0.004–0.36

0.004–0.42

Group 3 c – neighbouring
prefectures

0.005–0.070

0.006–0.082

0.006–0.097

Group 4d – rest of Japan<0.001–0.028<0.001–0.033<0.001–0.039baRanges of municipality- or prefecture-average annual effective dosea (mSv)

The reported doses are ranges of the municipality-averaged doses for the Group 2 and Group 3 prefectures and the prefecture-

average doses for the Group 4 prefectures. These estimates of dose are representative of the average doses received by people living
at different locations and do not reflect the range of doses received by individuals within the population at these locations. The
doses are in addition to doses from natural background radiation and other sources of exposure and are for the people in the specified
age groups in 2021. The adult doses are for indoor workers as representative of adults as a whole.
bMunicipalities of Fukushima Prefecture that were not evacuated.

cMembers of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.

dMembers of the public living in the remaining prefectures of Japan.

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Figure XVII. The average annual effective dose in 2021 for infants living in municipalities ofFukushima Prefecturea and some municipalities of Group 3 prefectures

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).
aIncludes municipalities that were evacuated (with the exception of the “Areas where Returning is Difficult”), assuming people

have returned to their homes, and assuming a dose reduction factor of 1.3 for remediation in the SDA.

171. Estimates of the effective doses due to external exposure that would be received by evacuees if they
were to return to their homes are summarized in table 12 and discussed in more detail in appendix A. The
dose estimates include a dose reduction factor of 1.3 to take account of the remedial actions that havebeen completed in the SDA (see appendix A and attachment A-1 for further details).

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69

Table 12. Estimated ranges of average effective doses from external exposure to adults and infants(as of 2011) annually up to 2021 and up to age 80 years if they were to return to the municipalities
from which they were evacuatedAge groupaRanges of annual effective dose and effective dose up to age 80 years
from external exposure (mSv)a
2019

2020

2021

Lifetime

Adult

0.050–1.2

0.040–1.1

0.040–0.99

0.69–18

1-year-old

0.050–1.5

0.050–1.4

0.050–1.2

0.69–19

Excluding municipalities containing “Areas where Returning is Difficult”.

(e) Collective doses
172. To provide some perspective on the overall exposure of the Japanese population from the accident,
the Committee also estimated collective effective doses and collective absorbed doses to the thyroid for
the Japanese public. 28 The resulting collective effective dose and collective absorbed dose to the thyroidfor the first year, for the first 10 years and over a lifetime 29 are given in table 13. These time periods werechosen for comparison with the UNSCEAR 2013 Report [U10] and to indicate the annual collective dose
and to illustrate the temporal accumulation of the collective dose commitment. The main contributor tothe collective effective dose was external exposure from radionuclides deposited on the ground. The main
contributor to the collective absorbed dose to the thyroid in the first year was internal exposure due toinhalation and ingestion of radioiodine. Compared to the Committee’s estimate in 2013 (table 8 of the
UNSCEAR 2013 Report) the collective effective dose is slightly lower, while the collective absorbed
dose to the thyroid is reduced by a factor of two. The estimates of the collective doses to the populationof Japan can be compared with estimates for populations of European countries exposed to radiation
following the 1986 Chernobyl accident (see appendix B). The collective effective dose to the populationof Japan due to a lifetime exposure following the FDNPS accident is approximately 10–15% of thecorresponding value for European populations exposed to radiation following the Chernobyl accident,
and the collective absorbed dose to the thyroid is approximately 3% of that due to the Chernobyl accident.
For further perspective, the annual collective effective dose to the population of Japan from natural
background radiation can be estimated to be about 280,000 man Sv 30 (about 20 times the collective
effective dose in the first year after the FDNPS accident).

28 The Committee has used the quantity, collective (effective) dose, for many years to compare the radiation exposures of
populations from different sources of ionizing radiation, or following different protection measures. The collective (effective) doseis always estimated for a defined population over a specified period of time. It is the product of the mean effective dose to a specified
population from a particular source, and the number of people in that population, integrated over a defined period of time.
Importantly, calculated doses are recommended only for comparative purposes and not for estimations related to health effects.
Collective dose is not intended as a tool for epidemiological risk assessment. Moreover, the aggregation of very low individual
doses over extended time periods is inappropriate for use in risk projections and, in particular, the calculation of numbers of cancer
deaths from collective doses based on individual doses that are well within the variation in background exposure should be avoided.
29 In this report, the Committee has used the terms “collective effective dose” and “collective absorbed dose to the thyroid” anddescribed them as being for different time periods. This follows the approach used in the UNSCEAR 2013 Report. Strictly, thecorrect terms are “collective effective dose commitment” and “collective absorbed dose commitment to the thyroid” (for the timeintegrated quantity), and the descriptions should make clear that the dose commitment (time-integrated) has been truncated at thedifferent time periods. The Committee has retained the same terms as used in the UNSCEAR 2013 Report for ease of comparison
with that report and for simplicity, while recognizing that it is not strictly correct.
30 Based on a population of 128 million and an average annual effective dose from natural radiation of 2.2 mSv [O21].

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Table 13. Collective effective doses and absorbed doses to the thyroid for the population of Japan(128 million in 2010)
Exposure duration
Dose categoryaFirst year

10 years

Lifetimea(up to age 80 years)

Collective effective dose(thousand man Sv)

12

32

44

Collective absorbed dose to thethyroid (thousand man Gy)

24

44

57

The collective dose over a lifetime (up to age 80 years) represents more than 97% of the collective dose commitment in each case.

(f) Estimation of doses in countries neighbouring, or in close proximity to,Japan

173. The Committee’s assessment of doses to the public in countries neighbouring, or in relatively close
proximity, to Japan has been based on the assessment in the UNSCEAR 2013 Report [U10] and a review
of more recent estimates published in the peer reviewed literature. The Committee confirmed its conclusionthat the effective doses and the equivalent doses to the thyroid for people living outside of Japan were lessthan 0.01 mSv in the first year following the accident (see appendix A, chapter IV, section E).

2.

Distributions of doses to individuals

174. The Committee’s estimates of municipality- and prefecture-average doses set out above areintended to be characteristic of the average doses received by people living in each municipality or
prefecture. The ranges of doses indicated reflect how the average doses for a particular age at exposure
vary between municipalities or between prefectures rather than the range of doses received by individuals
within a given municipality or prefecture.
175. The Committee has, in addition, made estimates of the distributions of individual doses in eachmunicipality and prefecture taking account of numerous factors that may influence how doses may vary
between individuals (e.g., concentrations of radionuclides in air and on the ground where they live andwork, their diet, lifestyle (e.g., fraction of the time spent indoors/outdoors), shielding provided bybuildings where they live and work, etc.). These distributions are presented in attachment A-21, andattachment A-12 provides further details of how these distributions were derived. The Committee has
provided estimates of the 5th and 95th percentiles of these distributions of doses in the relevant populationto accompany the estimates of the average doses.
176. Distributions of individual doses (both effective dose and absorbed dose to the thyroid) in the firstyear and up to age 80 for some example municipalities within Fukushima Prefecture and for the prefecture
as a whole are provided in appendix A, section IV.F. The distribution for the effective dose in the populationof Fukushima Prefecture (excluding evacuees) in the first year is shown in figure XVIII, as an example.
This shows that most individuals received doses within a range from about 10 times lower to 3 times higher
than the prefecture-average dose (of about 2 mSv). The distribution shows a double peak, which is likely to
be a consequence of the dominance of the pathway of external exposure to deposited material and thedifferent deposition densities of released radionuclides in areas where most people in Fukushima Prefecture
live. As a further example, the distribution of the absorbed dose to the thyroid in the population of

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71

Fukushima City in the first year is shown in figure XIX. This shows a similar pattern, but with a narrower
range, with most individuals receiving absorbed doses to the thyroid within a range of about three times
lower than the average and two times higher than the average dose of about 9 mGy.
Figure XVIII. Distribution of effective dose in the first year in the population of Fukushima Prefecture(of 2 million people)

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UNSCEAR 2020/2021 REPORT

Figure XIX. Distribution of absorbed dose to the thyroid in the first year in the population of
Fukushima City (of 296,000 people)

F. Summary
177. In the UNSCEAR 2013 Report [U10], the Committee’s aim had been to make realistic estimates ofdoses. However, because of the lack of information in some areas, conservative assumptions weresometimes made, most, if not all, of which were recognized at the time and acknowledged in the report.
The assumptions made in the estimation of doses from ingestion of foodstuffs in the first year were among
the more significant of these.
178. In this report, the Committee has been able to make more realistic assumptions because of theavailability of much more information since the UNSCEAR 2013 Report [U10]. This information
included: (a) improved estimates of the temporal pattern of release of radionuclides from FDNPS and of
their dispersion in the atmosphere and deposition on to the ground or over the ocean; (b) more extensive

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measurements of the radionuclides in the environment, for example, in air, on the ground, in foodstuffs
and drinking water; (c) measurements of radionuclides in people, in particular, their thyroids and wholebody; (d) extensive personal dosimetry campaigns in a number of municipalities to measure external
doses for people with different habits; and (e) various assessments, published in the peer reviewed
literature by Japanese and other researchers, of doses to people from one or other exposure pathway.
179. In updating its dose assessment, the Committee has chosen to rely, to the extent possible, on
measurements of radiation or radioactive material in people and/or the environment. In some cases, suchmeasurements have been used, almost directly, as the basis of the dose estimates in this report; in others,
the measurements have been used to validate models developed for the purposes of estimating doses to thewider population. The use of models (e.g., M2020) validated by local radiation measurements, including
human measurements, has improved the reliability and reduced the uncertainty of the dose estimates.
180. The main changes and/or improvements in the approach adopted by the Committee and theirimplications are:(a) An improved source term (based on the totality of measurements in the environment, correlatedwith the main events on the FDNPS site and taking account of the three chemical forms in whichradioiodine was released) was used, together with improved ATDM, to estimate the concentrationsof radionuclides in air for which only limited measurements were available; this resulted in adifferent spatial and temporal pattern of concentrations of radionuclides in air, with increases in air
concentrations (and doses) at some location and decreases at others;(b) A new empirical and validated model was developed to estimate external doses fromradionuclides deposited on the ground based on extensive measurements of the variation of dose ratewith time in Japanese conditions (e.g., soil types, climate); this generally resulted in an increase inestimated external doses, typically by several tens of per cent, compared with the UNSCEAR 2013
Report [U10], and a slower decrease in the dose rates with time;(c) A biokinetic model was developed, specific to the Japanese population, whose diet is generally
iodine-rich, to make more realistic estimates of doses from intakes of radioiodine by inhalation oringestion; this resulted in a decrease in the estimated thyroid doses by a factor of about two comparedwith the UNSCEAR 2013 Report;(d) Greater realism was incorporated into the modelling of various factors used in estimating doses(e.g., air filtration when inside different types of buildings, habits and behaviours, etc.) to takeaccount of Japanese specific information. By far the most significant change, compared with the
UNSCEAR 2013 Report, was making an allowance for the filtration of air afforded by buildings; asa result, estimates of doses from the inhalation of radionuclides decreased by a factor of about two;(e) Much more realistic estimates of doses from the ingestion of food and drinking water were madebased on better information about what members of the public actually bought and consumed,
including from duplicate-diet and market-basket studies. Over the longer-term, an empirical model
was used, based on measurements over 45 years of radiocaesium in food products and the whole dietin Japan from fallout from atmospheric nuclear weapons testing. These changes have reduced theestimates of doses from ingestion of food and drinking water by at least a factor of ten comparedwith the UNSCEAR 2013 Report.
181. Taken together, the effect of these changes has been revised estimates of municipality- andprefecture-average doses at the upper end of the ranges that are up to a few tens of per cent lower
compared with those presented in the UNSCEAR 2013 Report [U10] for effective doses in the first year,

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UNSCEAR 2020/2021 REPORT

and up to about a factor of two lower for thyroid doses in the first year. The effect of the changes on
estimated effective doses over a lifetime is more complex because of several competing factors:(a) For estimated average doses at the upper end of the ranges (where external exposure to depositedmaterial is the dominant pathway), the differences are generally less than a few tens of per cent;(b) Estimated average doses at the lower end of the ranges (where ingestion is a more importantcomponent) are up to an order of magnitude or more lower than in the UNSCEAR 2013 Report.
182. The much greater amount of information available has enabled the Committee to assess not only
the municipality- and prefecture-average doses to members of the public, but also the distributions ofdoses among individuals within a municipality or prefecture (and 5th and 95th percentile bounds), taking
account of uncertainties (such as measurement and modelling uncertainties) and major sources ofvariability. This includes the age distribution, varying levels of exposure within a municipality or
prefecture, lifestyle (such as the amount of time spent indoors or outdoors), types of diet, and the filteringof the air and shielding afforded by different types of buildings. These distributions typically show thatmost individuals were estimated to have received doses within a range of about 10 times lower to a few
times higher than the average dose, with many more people estimated to have received doses less than
the average than doses greater than the average.

VI. ASSESSMENT OF DOSES TO WORKERS

A. Introduction
183. A few thousand occupationally-exposed workers were employed at the FDNPS site prior to theaccident. Many more (in excess of 21,000 up to the end of 2012) were employed following the accident
in recovery and related operations; the majority of these (almost 18,000) were employed by contractorsof TEPCO [M22]. In addition, a few hundred workers from the emergency services, including
firefighters, police, Japan Coast Guard and personnel of the Japan Self-Defense Force, were deployed on
the site, and tens of thousands were engaged in emergency response activities off-site. Further tens of
thousands of municipal workers and United States military personnel carried out environmental radiation
measurements and other support activities in the areas evacuated.
184. Following the accident, TEPCO and other organizations worked in and around the FDNPS site tobring the nuclear reactors under control and to reduce the release of radioactive material. The “upper
radiation exposure dose limit” 31 of 100 mSv was immediately applied for workers engaged in emergency
works at FDNPS. This was increased to 250 mSv on 15 March 2011, and returned to its pre-existingvalue of 100 mSv, on 1 November 2011 for workers commencing work after that date, on 16 December
2011 for most other workers, and on 30 April 2012 for all remaining workers [M22, W1]. Initialcapabilities for monitoring radiological conditions effectively were severely hampered [I35]. Few on-sitemonitoring systems remained following the tsunami. Most electronic personal dosimeters, computer
systems for activating and recording dose from these devices and many portable survey instruments werelost in the flooding. Installed radiation monitors were also lost when the tsunami flooded the electrical
31 The English translation of the term used in the relevant Japanese legislation [M21].

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distribution equipment [I35]. Emergency responders needed to share electronic personal dosimeters andto log their individual doses manually until the end of March 2011. For teams where personnel were
thought to be working under similar conditions, only one worker in the team wore a dosimeter for many
missions [I4].
185. TEPCO gradually improved on-site radiological monitoring [Y15]. From 1 April 2011, personal
dosimeters were provided to every worker. Comprehensive radiation maps based on measured dose rates
were used to optimize the protection of workers. A coordination centre was established 20 km to thesouth of FDNPS to manage and oversee radiation protection of all personnel entering the restricted area
and the facility, including the provision of full-face respirators and protective clothing. Potassium iodide
tablets were prescribed to selected workers from 13 March 2011 to 21 November 2011 in order to blockradioiodine uptake to the thyroid. During this period, approximately 17,500 tablets were prescribed to2,000 workers. Further information on the selection criteria for workers and the distribution of tablets issummarized by IAEA [I7].
186. To assess doses from internal exposure, on 22 March 2011, JAEA, at the request of TEPCO, started
individual in vivo monitoring of workers who were responding to the emergency. These initial
measurements were made with a simple mobile whole-body counter, which was subject to relatively highenvironmental background levels because of its location. Where an assessed effective dose due toradionuclide intake was over 20 mSv, the worker was additionally monitored at JAEA facilities, and,
where the assessed dose from internal exposure was in excess of 250 mSv, the worker was also monitored
by NIRS. For most workers, the only radionuclides detected were 131I, 134Cs and 137Cs and reported doses
due to internal exposure only took account of intakes of these radionuclides. For some workers withhigher effective doses, 136Cs and 129mTe were also detected, but the contribution to the assessed dose frominternal exposure due to intakes of these radionuclides was small (less than 0.5%). Data on exposure toother short-lived radionuclides such as 132Te, 132I and 133I were lacking. Wherever possible, effective
doses and thyroid doses resulting from internal exposure were assessed from the results of in vivo
measurements. Assessments of internal exposure for TEPCO workers were performed either by TEPCO
or by NIRS, using quality-assured software tools.

B. Overview of current understanding
1.

Doses to the Fukushima Daiichi Nuclear Power Station workers

187. The information available to the Committee on FDNPS worker exposures is drawn from the dose
distribution data regularly published by the Japanese Ministry of Health, Labour and Welfare (MHLW)[M22]. These data provide information on the numbers of workers with assessed doses within specified
ranges, in defined time periods. Distributions of effective dose from external and internal exposure were
presented separately until April 2012. Subsequently, distributions were published for total effective dose.
Distributions of thyroid equivalent doses among FDNPS workers for the period March–December 2011were published separately by TEPCO [T25]. Additional information on doses due to internal and external
exposure up to April 2012 for 21,776 workers was provided to the Committee. Only a limited amount ofdata for individual workers has been provided to the Committee, and this has always been anonymized.

76

UNSCEAR 2020/2021 REPORT188. The UNSCEAR 2013 Report [U10] summarized the doses received by occupationally-exposed
workers in the period up to the end of October 2012. In this period, a total of 24,832 workers were reported
to have been involved in mitigation and other activities on the site and were occupationally exposed toradiation. Of these, about 15% were employed by TEPCO, with the remainder employed by contractors
and subcontractors.
189. In the UNSCEAR 2013 Report [U10], the Committee carried out an assessment of the extent to which
the individual doses to workers reported in Japan provided a reliable and accurate measure of the doses
actually incurred, by reviewing the methodologies used in Japan and by making independent assessmentsof doses for small numbers of workers within defined groups. 32 TEPCO provided the Committee withdetailed information on the types of personal dosimeter, technical standards and calibration methods used,
and of the system used for allocating personal dosimeters to individuals, for monitoring external exposure.
TEPCO, JAEA and NIRS provided the Committee with detailed information on the instruments,
measurement systems, calibration phantoms and methodologies used for in vivo monitoring and assessing
doses from internal exposure, together with monitoring data for selected workers. Further details on theapproaches and methods used by the Committee can be found in the UNSCEAR 2013 Report. In that report,
the Committee judged that the instrumentation, technical standards, calibration methods and othermethodologies used to monitor and assess external exposures in Japan met generally-accepted requirements
for individual monitoring during a radiation emergency. The major factor potentially affecting the reliabilityof external exposure assessments had been the sharing of electronic personal dosimeters during March 2011.
The Committee judged the measurement systems, calibration phantoms, calibration and quality control dataand software packages used to assess internal exposures to be appropriate for the types of dose assessments
performed. Through its independent assessments of selected workers, the Committee confirmed thereliability of the assessments of internal exposure reported by TEPCO for those of its workers where 131I in
the body had been detected, but not for those of its workers for whom 131I had not been detected in the body
because of the delays in initiating monitoring. Due to discrepancies between the Committee’s estimates ofdose due to internal exposure in the UNSCEAR 2013 Report and those reported by contractors, theCommittee could also not confirm the reliability of the internal exposure assessments reported bycontractors for their workers [U10]. The reasons for these discrepancies have been discussed in detail in the
UNSCEAR 2013 Report and subsequent publications [U10, Y16, Y19].

(a) Effective doses
190. As the Committee noted in its 2015 White Paper [U11], doses for a significant number of workers
have been re-evaluated and revised since the UNSCEAR 2013 Report [U10, Y16, Y19]. The Committee
considers the two re-evaluations to be well-designed and thorough and the revised dose estimates to bereliable given the shortcomings (outlined above) of some of the monitoring data that underlie them.
191. As well as reporting re-evaluated doses, TEPCO has continued to report regularly on distributionsof annual effective doses among occupationally-exposed FDNPS workers, including those employed by
TEPCO and by contractors. 33 Dose distributions up to 31 March 2020 are shown in figure XX withsummary information in table 14 [M22]. These dose distributions reflect the dose re-evaluations carriedout in 2013 and 2014. They are presented in years following the accident (to the end of March each year)[M22], although updates are available on a monthly basis.

32 In the UNSCEAR 2013 Report [U10], the Committee carried out independent assessments of the doses due to internal exposurefor 12 of the 13 workers with the highest reported internal exposures, for 42 randomly selected workers (21 TEPCO workers and
21 contractor workers) and for 13 workers from the emergency services.
33 Including subcontractors.

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192. The highest reported effective dose was 679 mSv 34 (see table 14) for the TEPCO worker who also
received the highest reported committed effective dose due to internal exposure (590 mSv). Six workers(all employed by TEPCO) received effective doses from internal and external exposure greater than
250 mSv, and a further 168 workers (TEPCO and contractors) received effective doses from internal andexternal exposure in the range of 100 to 250 mSv [M22]. Of the six workers receiving total doses greaterthan 250 mSv, five were assessed to have received effective doses from internal exposure alone in excess
of 250 mSv. Also, nine workers were assessed to have received effective doses from internal exposure
alone between 100 and 250 mSv [Y16, Y19]. These doses are all cumulative doses for the period up to
the end of March 2012.
193. Since April 2013, no individual annual effective dose has exceeded 50 mSv, and since April 2018,
no individual annual effective dose has exceeded 20 mSv. The average effective dose (from both internaland external exposure) has fallen from about 13 mSv, in the period after the accident up to the end of
March 2012, to an annual dose of 2.5 mSv in the year to the end of March 2020 [M22].
Figure XX. Dose distributions for Fukushima Daiichi Nuclear Power Station workersa since theaccidentaIncludes both TEPCO and contractors’ workers.

34 Doses, numbers of workers and other numerical values quoted from source publications are reported in this chapter to the numberof significant figures given in the source of this information (e.g., [M22]) to avoid any confusion that may result from rounding to
two significant figures. The number of significant figures used does not indicate the precision or uncertainty in the quoted figure.

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Table 14. Maximum and average effective doses to Fukushima Daiichi Nuclear Power Station
workers [M22]aaTime period

Maximum effective dose(mSv)

Average effective dose(mSv)

Number of workers

March 2011–March 2012

679

13

21 135

April 2012–March 2013

54

5.7

13 742

April 2013–March 2014

42

5.3

14 746

April 2014–March 2015

40

5.0

20 730April 2015–March 2016

43

4.3

18 196

April 2016–March 2017

39

2.9

15 835

April 2017–March 2018

33

2.7

13 943

April 2018–March 2019

20

2.4

11 306

April 2019–March 2020

20

2.5

10 708

Includes both TEPCO and contractors’ workers.

(b) Thyroid doses
194. Wherever possible, internal exposures of FDNPS workers were assessed from the results of in vivo
measurements of radionuclide amounts in the body. Where 131I was measurable in the thyroid, absorbed
doses to the thyroid were assessed from the measurements; and where 131I was not measurable in the
thyroid, more indirect methods had to be used to estimate absorbed doses to the thyroid. 35 As reported
by TEPCO for the period March–December 2011 [T25], 1,757 workers were assessed (by TEPCO, NIRS
and/or contractors) to have received committed absorbed doses to the thyroid greater than 100 mGy.
Thirteen TEPCO workers were estimated to have received committed absorbed doses to the thyroid of
2 Gy or more. In the UNSCEAR 2013 Report [U10], the Committee carried out independent dose
assessments for 12 of these 13 workers and confirmed the dose assessments made by TEPCO.
195. A worker epidemiology study, the Nuclear Emergency Workers Study [J4, K24, M23, Y18], hasbeen established in Japan to examine adverse health effects in a target group of approximately
20,000 FDNPS emergency workers. As part of this study, more sophisticated evaluations of individual
doses of FDNPS workers are being carried out. The doses reported by TEPCO, and the standard methods
defined by MHLW for assessing these doses, are based on a Reference Worker. 36 For the epidemiology
study, doses to the participating individuals are being assessed, taking into account information andparameter values specific to each individual. In particular, for six of the workers with the highest thyroid
doses, Kunishima et al. [K54] have created realistic mathematical models (known as voxel phantoms) of
35 In estimating doses to the public, account has been taken of dose coefficients for intakes of radioiodine specific to the Japanese
population to reflect the iodine-rich diet more typical in Japan (see chapter V). For many workers, absorbed doses to the thyroid
have been estimated directly from measured levels of 131I in the thyroid (and therefore already reflect actual uptake of radioiodine
into the thyroid). For those workers where 131I was not measurable (about 40% of the total, but those more likely to have received
lower doses), one of two indirect methods had to be used to estimate absorbed doses to the thyroid. Where the “environmental
ratio” method was used, the use of dose coefficients specific to the Japanese population could have led to some differences in theestimated doses. However, these differences would have been small compared with the large uncertainties associated with the useof the indirect methods.
36 An idealized male or female worker with characteristics defined by ICRP for the purpose of radiological protection with defined
anatomical and physiological characteristics [I15].

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79

the neck and the thyroid gland from magnetic resonance imaging (MRI) and ultrasound scans carried out
on the workers. Numerical simulations have then been carried out to determine:(a) In vivo measurement calibration factors for 131I in the thyroid;(b) Committed 37 absorbed doses to the thyroid corresponding to a measurement of unit 131I activity
in the thyroid at a specified time after the intake.
The committed absorbed doses to the thyroid for the six workers resulting from this assessment (which
is described as preliminary) are compared in table 15 with the assessments reported in the UNSCEAR
2013 Report [U10]. The absorbed doses to the thyroid assessed by Kunishima et al. [K54] for thesesix individuals show some significant differences from their reported doses: with one exception, there-evaluated doses are higher, by between a few per cent and almost a factor of three. The differences arelargely due to differences between the thyroid volumes of these individuals and the ICRP Publication 30
reference value of 19.9 cm3 [C5], with all but one of the six workers having lower thyroid volumes, andproportionately lower thyroid masses compared with the reference value.
Table 15. Thyroid volumes, original thyroid dose estimates and re-evaluated thyroid dose estimatesfor six workers with the highest thyroid doses
Subject code

Thyroid absorbed dosea (Gy)

Kunishimaa

UNSCEAR 2013
Reportb

Thyroidvolumea,c
(cm3)

A

A

28.2

12

8.4

0.7

B

B

6.5

11

32

2.9

C

F

18.7

4.6

4.8

1.0

D

C

10.4

7.5

14

1.9

E

D

12.3

5.8

9.0

1.6

F

E

12.9

5.3

7.9

1.5

14.8

7.7

12.7

1.6

Arithmetic means

Original values(A)dRe-evaluatedvalues (B)Ratio (B/A)

a

From [K54], tables 1, 2 and 3.

b

From the UNSCEAR 2013 Report [U10], inferred from information presented by Kunishima et al. [K54].

c

To be compared with the ICRP Publication 30 reference value of 19.9 cm3 [C5].

dDifferences between these values and the NIRS values presented in table D12 of the UNSCEAR 2013 Report [U10] are minor.

196. The Committee considers that the reassessment of thyroid doses carried out by Kunishima et al.
[K54] to be of high quality and that, where they are to be used for the purposes of epidemiology or healthrisk assessment, the assessed committed absorbed doses to the thyroid for these six workers should be
revised accordingly. The largest assessed committed absorbed dose to the thyroid due to internal exposure
from inhalation of 131I is now 32 Gy.
197. The results of this reassessment raise the question of whether other workers are also likely to havegenerally lower thyroid volumes than assumed for a Reference Worker and have received
correspondingly higher thyroid doses. There is, however, evidence that mean thyroid masses for definedgroups of adults in Japan do not differ significantly from the standard reference values used in dosimetry.
37 The committed dose is the dose received during a specified period after intake, usually 50 years for workers.

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Kudo et al. [K47], for example, examined data from Japanese autopsy subjects in the age range of 19 to
52 years obtained between 2012 and 2016, and found a mean thyroid mass of 21.9 g, with standard
deviation ± 11.3 g, for 2,204 males, and a mean of 17.8 g, with standard deviation ± 11.5 g, for
698 females. Although there is considerable variation, these mean values compare well with the mostrecent recommended reference values of 20 g for males and 17 g for females [I17]. Clearly, usingparameter values, including thyroid mass or volume, that are specific to the individual, provides the bestbasis for assessing absorbed dose to the thyroid for any particular individual, but there is no evidence tosuggest that using standard reference values for thyroid mass or volume is not appropriate for the Japanese
population in general. The use of standard reference values in the assessment of absorbed doses to thethyroid for FDNPS workers as a whole therefore remains valid. Nevertheless, for workers whose
individual thyroid absorbed dose is to be reassessed for the purposes of epidemiology or health risk
assessment, measurement of thyroid volume, either by MRI or ultrasound scan, subject to appropriate
ethical approval, would be beneficial, with the reassessment of absorbed dose to the thyroid then making
use of the corresponding individual thyroid mass.
198. To assess the health implications of the estimated exposures of workers, the Committee has madeestimates of the mean and the upper bound (or limit) of the 95% interval of the absorbed dose to thethyroid for the workers with assessed dose greater than 100 mGy. It has based the estimates on the
published dose distributions of thyroid equivalent doses among FDNPS workers for the period March–
December 2011 [T25]. The mean absorbed dose for these workers was approximately 370 mGy, with an
upper bound of about 1 Gy. These values for mean and upper bound take into account the re-evaluatedthyroid dose estimates presented in table 15.

(c) Assessment of doses from short-lived radionuclides
199. In vivo monitoring of FDNPS workers was started on 22 March 2011 and the results were used in
the assessment of doses from internal exposure. However, the delay from the start of the accident meantthat the shorter-lived radioisotopes that were likely to have been present, including 132Te, 132I, 133I and136
Cs, would not have been detectable. In its UNSCEAR 2013 Report [U10], the Committee had
estimated the contribution from such short-lived radionuclides to be in the order of 20% relative to thatfrom 131I. In its review of the methods used in Japan, MHLW considered that the standard methods it
subsequently prescribed were sufficient to account adequately for the contribution of the short-lived
radionuclides 132I and 132Te [Y16]. It estimated the contribution from 132Te to the committed effective
dose to be approximately 10% of the contribution from 131I, which is broadly consistent with theCommittee’s estimate, given the uncertainties in both.

(d) Biodosimetry assessments of Fukushima Daiichi Nuclear Power Station

workers200. Dicentric chromosome assays were performed for 12 workers, for the purposes of medical triage andplanning for the selected workers [S46, S47]. Estimated whole body absorbed doses of 10 of the individuals
who had provided samples for dicentric chromosome assay were all less than 300 mGy (95% upper limit).
Reasonable agreement was found with the results of personal dosimeter measurements for the eightindividuals for whom these measurements were available, particularly at the higher assessed doses.

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81(e) Assessment of doses to the eye lens of Fukushima Daiichi Nuclear Power

Station workers

201. In the UNSCEAR 2013 Report [U10], the Committee found insufficient information on the external
exposure of workers to beta radiation to make an informed assessment of dose to the eye lens. In 2017,
Yokoyama et al. [Y24] proposed that eye lens dose in the early stages of the accident could be equatedto measured values of Hp(10) (the personal dose equivalent at a depth of 10 mm in soft tissue) arisingfrom exposure to gamma radiation. Hp(10) is an operational quantity that generally provides aconservative estimate of effective dose from external exposure. Yokoyama et al. [Y25] subsequently
reported annual distributions of eye lens doses of FDNPS workers determined by TEPCO using Hp(0.07),
the personal dose equivalent at a depth of 0.07 mm. For the time period March 2011 to the end of March
2012, a total of 77 workers were estimated to have received an equivalent dose to the eye lens of morethan 100 mSv and 10 of these were estimated to have received more than 150 mSv, although none
received more than 200 mSv [T26, Y25].
202. The highest doses would have been received in the early days after the accident in March 2011.
Yokoyama et al. [Y24] state that, at that time, radiation exposure was mainly to gamma radiation and thatthe use of full-face masks was obligatory for almost all areas. In those circumstances, the dose to the eyelens from beta radiation would be a small fraction of total dose (from beta and gamma radiation) due tothe shielding effect of the mask, and Hp(10) would provide an adequate measure. However, it remains tobe established whether full-face masks, which would have adequately limited beta irradiation of the eyelens, were actually used by workers who were exposed to high air concentrations of iodine radioisotopes
emitting beta and gamma radiation during the early stages of the accident. If full-face masks were notworn, measurements of dose due to exposure to gamma radiation could underestimate the true dose to
the lens of the eye. As pointed out by Kunishima et al. [K54], workers with the highest absorbed doses
to the thyroid from inhalation of 131I “… were in charge of operations at places with high radiation levels
during the early phase of the accident, such as the main control rooms of the reactors, outside near thedamaged reactor buildings. Some of the workers were not provided with masks having charcoal filters at
the beginning and had to eat and drink at the main control rooms when air with high concentrations ofradioiodine entered from outside.” The magnitude of the assessed absorbed doses to the thyroid of thoseworkers who received significant intakes of radioiodine during these early stages suggests that adequate
respiratory protection (such as would have been provided by a full-face mask) was not always used, or
not completely effective, during that period.
203. The worker epidemiology study [J4, K24, M23, Y18] includes plans for an evaluation of doses
to the eye lens, but no results have yet been published. The Committee suggests that a possible
approach to the assessment of the dose to the eye lens for the most exposed workers may be to estimate
the dose to the eye lens (using Hp(3) if feasible) that would result from immersion in airborne 131I andassociated shorter-lived radionuclides at concentrations that would give rise to the measured thyroid131
I contents of these workers. Such an assessment would require information on material-specificfactors including the physico-chemical form of the airborne material and individual-specificparameters including breathing rate, which ideally would be derived from information on exposure
conditions in the workplace at the time.

2.

Assessment of doses to other workers

204. The UNSCEAR 2013 Report [U10] also summarized reported doses to workers other than FDNPS
workers. For the 260 firefighters who were deployed on-site, the maximum reported effective dose fromexternal exposure was 29.8 mSv. The maximum committed effective dose from internal exposure for the

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firefighters was less than 1 mSv. Of the Self-Defense Force personnel who were deployed on-site,
15 were reported to have received effective doses from external exposures in the range of 10 to 100 mSv,
and the remainder (132 people) were reported to have received less than 10 mSv. For those deployed offsite, five were reported to have received effective doses from external exposures in the range of 10 to
20 mSv, with the remainder (8,453) reported to have received less than 10 mSv. A limited number ofdoses from internal exposure were reported; the maximum committed effective dose was 3.8 mSv for one
on-site worker. For the 13 police officers who were deployed on-site, reported effective doses fromexternal exposure and committed effective doses from internal exposure were less than 10 mSv and lessthan 0.1 mSv, respectively.
205. United States military personnel provided humanitarian assistance and disaster relief support toJapan. The United States Defense Threat Reduction Agency has reported doses for DoD personnel andaffiliated people located at or near United States military facilities in Japan (comprising service members,
civilian employees, family members of service members and civilian employees, and contractor
employees). Doses were assessed for about half of 58,000 individuals who were shore-based [D9], theremainder being located away from the Japanese mainland in the Okinawa Prefecture. Exposures wereassumed to have occurred during the 60-day period starting on 12 March 2011. Reported total effective
doses to adults from external and internal exposure lay in the range of 0.01 to 1.2 mSv, while reported
equivalent doses to the thyroid lay in the range of 0.07 to 12 mSv [D5]. These dose estimates wereintended to be conservative, and a later probabilistic analysis showed that this was indeed the case, withthe reported values lying above the 95th percentile of the probability distributions determined using
realistic model parameter values [D6]. Most of the measurements of radionuclides in the body were below
the minimum detectable level; for the remainder, the reported committed effective doses lay in the rangeof 0.02 to 0.25 mSv, while the reported committed equivalent doses to the thyroid lay in the range of 0.29
to 4.2 mSv [D8]. A separate assessment was made for the approximately 17,000 fleet-based (i.e., naval)
personnel [D9]. Dose estimates were again intentionally conservative. Maximum values of effective doseand equivalent dose to the thyroid were estimated to be 0.35 and 3.4 mSv, respectively.
206. A number of studies have addressed the topic of exposures of the large number of workers (up to
40,000 at the peak in 2015) involved in off-site environmental remediation activities [K2, T12, T39].
Yasui [Y17] reported on the establishment in December 2013 of a central dose registration system, andpresented results for the years 2012, 2013 and 2014, while Asano and Ito [A16] updated these results forthe years 2015, 2016 and 2017. An MOE report [M39] utilized the same database to report dose statistics
for the 76,951 workers who were involved in remediation activities during the period 1 January 2012 to
31 December 2016. Of these workers, 67% received cumulative doses in the 5-year period of 1 mSv or
less, while 31%, 1.9%, 0.2% and 0.01% received doses in the ranges of 1 to 5 mSv, 5 to 10 mSv, 10 to
15 mSv and 15 to 20 mSv, respectively. No workers received a cumulative dose greater than 20 mSv.
The average cumulative dose received was 1.0 mSv. These results confirm that doses to remediation
workers were small.
207. Various studies have addressed the exposure of other types of workers employed in areas aroundthe FDNPS site after the accident. Sakumi et al. [S5] found that, during 2013, 64 workers employedwithin the deliberate evacuation area received annual effective doses from occupational exposure in therange of 0.9 to 3.6 mSv. Matsuda et al. [M12] found that five members of a radiation emergency medical
assistance team received effective doses in the range of 0.05 to 0.13 mSv during a 6-day period in March
2011, while Kodama et al. [K32] found that 101 employees at Minamisoma General Hospital had
committed effective doses from radiocaesium intakes received up to August 2011 that were less than
1 mSv in all cases. Kim et al. [K18] assessed the exposure of workers involved in transportation ofradioactive remediation waste and found that effective doses lay in the range of 0.27 to 1.1 mSv for a
specified number of journeys made.

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83

C. Summary
208. Although the reported doses to workers as a result of the FDNPS accident have been subject tosome revision since the UNSCEAR 2013 Report [U10], the general findings of that report remain valid:
the average effective dose of the 21,135 workers involved in mitigation and other activities at the FDNPS
site from March 2011 to the end of March 2012 was about 13 mSv. About 36% of the workforce received
total effective doses of more than 10 mSv over that period, while 0.8% received doses of more than
100 mSv. Annual effective doses have been considerably lower since April 2012, with average annual
effective doses declining from about 6 mSv in the year to end of March 2013 to 2.5 mSv in the year toend of March 2020, and no individual receiving an annual effective dose of more than 50 mSv since theyear to end of March 2013.
209. A recent re-evaluation of the absorbed doses to the thyroid of six workers who received thehighest doses has revealed that their absorbed doses to the thyroid, estimated using individual-specificmeasurements of thyroid size, are with one exception higher than previously reported, in one case byalmost a factor of three. The largest assessed committed absorbed dose to the thyroid due to internal
exposure from inhalation of 131I is now 32 Gy. However, there is no evidence to suggest that using
standard reference values for thyroid mass or volume is not appropriate for the Japanese population in
general, and the Committee believes that the absorbed doses to the thyroid reported in the UNSCEAR
2013 Report [U10] for the workers as a whole remain valid. For workers whose individual absorbed
dose to the thyroid is to be reassessed for the purposes of epidemiology or health risk assessment,
measurement of thyroid volume, either by MRI or ultrasound scan, subject to appropriate ethicalapproval, would be beneficial, with the reassessment then making use of the corresponding individual
thyroid mass.
210. Some estimates of dose to the lens of the eye have become available, but they rely on assumptions
about the wearing of full-face masks, when there is evidence that those workers who would have received
the highest exposures of the eye lens may not have been wearing such equipment during the early stagesof the accident. The Committee has suggested a possible approach to the assessment of the dose to theeye lens for these workers, based on measured 131I content of the thyroid.
211. The delay in commencing monitoring of radioiodine in the thyroid of workers is potentially thesource of the largest errors in the assessed doses to workers from internal exposure. Timely monitoring(e.g., WBC, thyroid measurements, personal dosimetry, etc.) of representative groups of workers at theearliest opportunity in the event of a similar incident in the future would greatly enhance the quality andinformativeness of any assessment of doses made for the purpose of evaluating health risks to workers.
If appropriate human measurements are not made in the immediate aftermath of an accident, doses topeople can only be assessed using models together with other measurements that may be available.
Experience has shown that the use of such models often leads to conservative estimates of doses.

84

UNSCEAR 2020/2021 REPORTVII. HEALTH IMPLICATIONS

A. Introduction
212. The UNSCEAR 2013 Report [U10] set out the Committee’s assessment of the health implicationsof the FDNPS accident based on the estimates of the doses to the public and to workers made in thatreport, the health risk assessment carried out by the World Health Organization [W14] and theCommittee’s knowledge and understanding of the health effects of radiation exposure. The Committee
summarized the state of understanding about the health effects associated with radiation exposure,including that health effects have traditionally been classified in two categories:(a) Deterministic effects or tissue reactions, which occur after high doses of radiation normallydelivered over a short period of time, and which occur above specific dose thresholds and, in mostcases, shortly after exposure;(b) Stochastic effects, which manifest as an increased incidence of disease in a population, with theincidence after irradiation tending to increase with increasing dose, but where it is not possible todistinguish by observation or testing whether or not the disease of a specific patient has been causedby the radiation exposure.
213. The Committee explained that, in estimating values of the risk of stochastic effects due to exposurefor members of various exposed groups, it has used the term “discernible” for cases where the estimatedrisk of the disease was sufficiently large in a large enough population to be detectable, compared to thenormal statistical variability in the baseline incidence of the disease in that population. Conversely, whenrisks may be inferred from existing knowledge (i.e., using models), but the level of the inferred risk is
low and/or the number of people exposed is small, the Committee has used the phrase “no discernible
increase” 38 to express the idea that currently available methods would most likely not be able todemonstrate an increased incidence in the future disease statistics due to irradiation (that is, theattributable risk is too small compared to the baseline levels of risk to be detected). The Committeeemphasized that its use of the term “no discernible increase” did not equate to an absence of risk or ruleout the possibility of excess cases of disease due to irradiation, nor the possibility of detection of abiomarker for certain types of cancer in certain subgroups being identified in the future that could be
associated with radiation exposure. Nor was it intended to disregard the suffering associated with any
such cases should they occur.
214. In the UNSCEAR 2013 Report [U10], the Committee also noted that nuclear accidents of themagnitude of the FDNPS accident, and the associated protective measures, tend to lead to distress andanxiety from, among other things, disruption of life, loss of homes and livelihoods, and social stigma,
which can have major impacts on psychological and social well-being [U10]. The Committee pointed outthat evaluating such effects is not part of its mandate, although they are important for understanding thebroader health implications of the accident. It also noted that the evacuation following the accident caused
immediate aggravation of the condition of already vulnerable groups: for example, more than
50 hospitalized patients were reported to have died either during or soon after the evacuation [T22] and38 The Committee further elaborated that it had used the phrase “no discernible increase” where, although a disease risk in the longer
term can be theoretically inferred on the basis of existing risk models, an increased incidence of effects is unlikely in practice to beobserved in future disease statistics using currently available methods, because of a combination of the limited size of population
exposed and low exposures, i.e., consequences that are small relative to the baseline risk and their uncertainties.

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85

upwards of 100 elderly people may have died in subsequent months because of a variety of conditions
linked to the evacuation [Y21].

B. Overview of current understanding
1.

Health implications for the public215. Based on its understanding of the exposures of the public as estimated in the UNSCEAR 2013
Report [U10], which fell well below the thresholds for deterministic effects, the Committee expected nodeterministic tissue reactions, and noted that no acute health effects that could have been attributed toradiation exposure had been reported. No information has become available since the publication of the
UNSCEAR 2013 Report, either on the understanding of the exposures of the public or on any reported
acute health effects, that would alter the Committee’s conclusions.
216. Based on its understanding of the exposure levels of the public as estimated in the UNSCEAR
2013 Report [U10], the Committee also estimated in that report that, compared to the baseline risk in
the general Japanese population of total solid cancer, and cancers of nearly all specific anatomicalsites, a general radiation-related increase in the incidence of such cancers would not be expected to bediscernible [U18, W14]. However, the Committee noted, “While the lifetime cancer risks due toradiation exposure may not result in a discernible increase in disease incidence for the whole of thegeneral population, the risks for some cancers and age groups in principle might.” (paragraph 172 of
the UNSCEAR 2013 Report).
217. To address the discernibility of risks from its revised estimates of exposures of the public resulting
from the FDNPS accident, the Committee has assessed the statistical power to detect excess risks of total
solid cancer (excluding thyroid cancer and non-melanoma skin cancer) and of several cancer types thatare notably radiation-induced (leukaemia, breast cancer and thyroid cancer). The approach used in therisk estimation was the same as used in the World Health Organization calculations of risk [W9, W14]
and the calculations for the UNSCEAR 2013 Report [U10] (described in [U11]), but with updates ofbackground cancer incidence rates in Japan and risk coefficients [B10, B11, F23, G6, I10, L5, U17, U18,
W7, Z1], as well as the updated dose estimates (see chapter V). The statistical power was estimated forlifetime risk and for early risks up to ages 30 or 40 for males, females and both together. Risks were
calculated for exposure at age 1 (based on the number of residents of Fukushima Prefecture exposed atages from in utero through to 5 years), age 10 (based on the number of residents of ages 6 to 19 years),
and age 20 (based on the number of residents of ages 20 to 35). In addition, the detectability of excess
thyroid cancer risk in the cohort of children who have been screened for thyroid cancer incidence in the
Fukushima Health Management Survey (FHMS) was estimated. The analyses incorporated several
conservative assumptions so as not to underestimate statistical power. The analyses used the lifetime (or,as relevant, the 30 or 40 years) estimated mean absorbed dose in the relevant organ and the corresponding
95th percentile upper bound on the mean dose for each of the non-evacuated municipalities and theevacuated municipalities in Fukushima Prefecture. The 95th percentile upper bounds have been assessedusing Monte Carlo analysis (see attachment A-12 for further details). To concentrate on the areas withhigher exposures, the analyses were also repeated including only the municipalities that had lifetime
average effective doses of over 5 mSv. In the case of thyroid cancer, first-year absorbed doses in the
thyroid over 5 mGy were used to define the higher dose group. The statistical power to detect excess riskin those who were in utero at the time of the FDNPS accident was also evaluated. The discernibility of86

UNSCEAR 2020/2021 REPORT

risks for the various groups and tumour outcomes is briefly summarized in the substantive sections that
follow. Further details can be found in attachment A-23.
218. The Committee’s revised estimates are that municipality-average effective doses to adult evacuees
were less than 6 mSv in the first year after the accident. In the most affected municipalities that were notevacuated, average effective doses to adults in the first year were estimated to be less than 4 mSv, andlifetime effective doses to adults were up to about 15 mSv. The uncertainties are such that the 95% upperbounds of the average doses were less than about twice these values. The Committee has again focused
its attention on the organs, age groups and time periods where an increased risk is more likely to become
discernible as an increase in the incidence of the disease. Considering both evacuated and non-evacuatedmunicipalities, estimated mean lifetime absorbed doses pertinent to the risks of all solid cancers (colon
dose), leukaemia (red bone marrow dose) and breast (breast dose) were all under 15 mGy for those
exposed in early (ages 0 to 5 years) or later childhood (ages 6 to 19 years), or in early adulthood (ages 20
to 35). These low doses are in contrast to the dose distributions of most studies where excesses of thesecancers have been observed, which have larger proportions of higher doses [B10, B11, F23, G6, H34,
L12, P6, R2, R3, W7, Z1].

(a) Thyroid cancer among children and adolescentsInferred increases in incidence of thyroid cancer
219. In the UNSCEAR 2013 Report [U10], the Committee concluded that the occurrence of a large
number of radiation-induced thyroid cancers, as were observed after the Chernobyl accident, could be
discounted because doses were substantially lower. However, it considered that doses towards the upperbounds of the ranges of doses estimated in the UNSCEAR 2013 Report could imply an increased risk forindividuals that, among sufficiently large population groups, might lead to discernible increases in theincidence of thyroid cancer due to the radiation exposure.
220. From all of the information now available, the Committee estimates the municipality-averageabsorbed doses to the thyroid of infants in the first year to be up to about 30 mGy, for those who were
evacuated, and up to about 20 mGy, for those who remained in the non-evacuated areas (see chapter V
and appendix A). The uncertainties in these estimates of average doses have been assessed using Monte
Carlo analysis (see attachment A-12 for further details). The uncertainties are such that the 95% upperbounds on the average absorbed doses to the thyroid were around twice these values. Across evacuatedand non-evacuated municipalities, the estimated mean thyroid dose in the first year for infants was about
13 mGy. There were five non-evacuated municipalities and 16 groups of evacuees with estimated mean
thyroid doses in the first year for infants of over 10 mGy, and one non-evacuated municipality and twogroups of evacuees with mean thyroid doses of over 20 mGy. Estimated mean absorbed doses to thethyroid for children and for adults (ages 10 and 20 years at exposure, respectively) were about 15% and
40% lower, respectively, than those in infancy.
221. The likelihood of discernibility of excess thyroid cancer risk among children and young adults inFukushima Prefecture exposed as a result of the FDNPS accident was evaluated in three ways. First,
assessments were made of the lifetime risk of thyroid cancer among those exposed from in utero to
5 years of age, 6 to 19 years of age, and 20 to 35 years of age using recent sex- and age-specific thyroid
cancer rates from four unexposed Japan prefectures. These groups were analysed as if they were ages 1,
10 and 20, respectively. Estimated thyroid doses and radiation risk coefficients for thyroid cancer fromthe Hiroshima-Nagasaki Life Span Study, from Chernobyl studies and from a recent pooled analysis
of nine studies with low-dose data were applied to evaluate whether an excess would likely bedetectable [B10, F23, L12, Z1]. Second, since the radiation risk of thyroid cancer is thought to be

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87highest at younger attained ages, similar analyses were conducted up to ages 30 and 40 years. Third,
the rates of incident thyroid cancer detected in the FHMS (see below) ultrasound examination
programme were modelled and extended proportionally to ages 30 or 40 years, with estimates of the
distribution of participants by age at exposure, municipality of residence, and participation rates in
FHMS in adulthood.
222. All these statistical power analyses suggest that excess thyroid cancer risk attributable to radiation
exposure was most likely not discernible in any of the age groups (see attachment A-23). For example,
females of ages in utero to five years at initial exposure comprise the most susceptible subgroup. For thissubgroup, about 16 to 50 cases of thyroid cancer attributable to radiation could be inferred from theestimated exposure, depending on the risk model assumed. By comparison, an average of about 650 incident
thyroid cancers (with a 95% confidence interval of approximately 600 to 700 cancers) would be observedin Fukushima Prefecture over the lifetime in the absence of systematic population screening for thyroid
cancer. In addition, the extensive variation in thyroid cancer rates (over 60%) seen among unexposed
prefectures would likely be even larger for smaller geographic units such as municipalities, and would create
considerable statistical “noise” that would limit the ability to discern radiation risk. A statistical power
analysis showed that an excess of 50 cases or less would be undetectable among the much larger, uncertain
baseline number of thyroid cancers. For thyroid cancer incidence in FHMS up to either age 30 or 40 years,
analyses of children for the age groups of in utero to 5-years old, or 6 to 18-years old, at the time of the
FDNPS accident, did not indicate that excess thyroid cancer would likely be discernible up to either age 30
or 40 (see attachment A-23).
223. An additional possible concern is thyroid cancer risk after antenatal (in utero) exposure. Studies ofthyroid cancer prevalence among those exposed antenatally by the atomic bombings in Japan [I31] andby the Chernobyl accident [H7, H8] found too few cancers to provide informative quantitative estimates,
so the thyroid cancer risk after in utero exposure is unknown to date. Antenatal thyroid doses fromreleases from the Mayak facility averaged about 10 mGy [E5] but thyroid cancer risk has not yet beenreported for the antenatal group. No studies have been reported on thyroid cancer occurrence among those
exposed in utero to radiation from the FDNPS accident. The Committee has estimated absorbed doses tothe thyroid of those exposed in utero and found that the statistical power was too low to discern an excess
risk of thyroid cancer because of the combination of relatively low thyroid doses and a small sample size:
about 16,000 in utero individuals at the time of the accident (see attachment A-23).

Observed incidence of thyroid cancer
224. The Fukushima Health Management Survey launched in 2011 to “evaluate radiation doses ofcitizens and [record] their health conditions”, and included thyroid ultrasound examinations of children,
as well as general health examinations, reproductive health assistance, and mental health counselling inview of radiation risk concerns. Between October 2011 and April 2015, FHMS conducted the first roundof thyroid screening of 300,472 individuals who were aged 18 years or younger at the time of the FDNPS
accident to detect thyroid cancer prevalence [F7, S24]. The screening used highly sensitive ultrasoundequipment, and fine needle aspiration cytology of cysts larger than 20 mm and of nodules larger than
5 mm in diameter was performed as necessary. Those suspected to be malignant were surgically removed
after informed consent; 116 were cytologically diagnosed as malignancy or suspected malignancy.
Additional rounds of screening were conducted to detect thyroid cancer incidence. The second round of
ultrasound screening of 270,540 individuals in 2014–2015 diagnosed 71 suspected or confirmed thyroid
cancers [F7]. Those who were in utero at the time of the FDNPS accident were added to the second andsubsequent rounds of thyroid ultrasound screening. The third round of screening included 217,921
individuals and 31 cases were cytologically diagnosed as malignancy or suspected malignancy [F7].
A fourth round of thyroid ultrasound screening is underway, with cytological confirmation and surgery
as needed [F7].

88

UNSCEAR 2020/2021 REPORT225. In the first round of screening, the rates of diagnosed, suspected or confirmed thyroid cancer among
the approximately 300,000 individuals who were children or adolescents (ages 0–18) at the time of the
FDNPS accident were found to be much higher than those documented in the cancer registries of otherprefectures of Japan. One author group has argued [T45, T46] that the higher rates provide evidence ofan increased risk of thyroid cancer due to radiation exposure. However, most other authors attribute thedifference in rates to the ultrasensitive thyroid screening used in FHMS (e.g., [W8]), noting that rates forJapan as a whole are based primarily on clinically-indicated thyroid examinations, not on universal childpopulation screening. When the Republic of Korea introduced widespread population thyroid screeningof adults, unrelated to any radiation exposure, thyroid cancer incidence rates increased greatly [A5, P1],
although thyroid cancer mortality rates did not increase appreciably. Other countries have reported similarfindings [V1]. These observations suggest that the increased incidence rates may be due to over-diagnosis(i.e., detection of thyroid cancer that would not have been detected without the screening and would nothave caused symptoms or death during a person’s lifespan) [A5, I11, M28, V1].
226. Several additional pieces of evidence add weight to the suggestion that the apparent detected excessof thyroid cancers is probably unrelated to radiation exposure:(a) At the absorbed doses to the thyroid estimated by the Committee, a large excess of thyroid
cancer, as seen in the FHMS screening programme, would not be expected [I38, K15];(b) In studies following the Chernobyl accident, an increased incidence of thyroid cancers was notobserved during the first four years after the Chernobyl accident (when a radiation-related excess
would not be expected because the minimum latency period after exposure had not yet been reached)[T35, W12]. During the first four years after the FDNPS accident, 116 suspicious or malignant
thyroid cancers were found in the first round of the FHMS screening programme [F6, F7, O16]. Thisis suggestive of a screening effect rather than the result of exposure to radiation [W15];(c) Both in studies following the Chernobyl accident [T35] and following external radiation
exposure [L12], a strong increase in thyroid cancer incidence was seen in those exposed in earlychildhood (aged less than 5-years old). In the Japanese Life Span Study of atomic bombing survivors,
the greatest risk of thyroid cancer was seen among those exposed in early childhood [F23]. Beyond
the minimum latency period after the Chernobyl and FDNPS accidents, there was a remarkabledifference in the distribution of thyroid cancers by age at exposure: numerous cases associated withradiation occurred in the Chernobyl children exposed at ages 0 to 4 years [T35, U15], while only one
case occurred in children exposed at ages 0 to 4 years in Fukushima in the first four rounds ofscreening (see figure XXI) [F7]. Since early childhood is a time of high susceptibility to thyroid
cancer from radiation exposure, the comparatively low numbers of early childhood cancers imply
that the Fukushima cancers are primarily related to attained age rather than to radiation exposure;(d) A study conducted in three Japanese prefectures that had no radiation exposure resulting fromthe FDNPS accident, using the equivalent ultrasound equipment and diagnostic protocol as that used
by FHMS [H11], found prevalences of thyroid cysts and nodules as high as those found in the firstround of FHMS screening, albeit for a smaller study size and absolute numbers of cases. Other
thyroid cancer screening results of young people in Japan without radiation exposure were alsosimilar to those in the FHMS cohort [N3], as were estimated background (unexposed) rates in ascreening programme of children from the Chernobyl area [W8];(e) The molecular biology of the thyroid cancers found in the FHMS screening programme has notconfirmed a radiation aetiology [M32]. Some studies had reported a higher than expected frequency

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89

of rearrangements of the RET 39 gene in post-Chernobyl thyroid cancer, as did studies of the more
highly exposed Japanese atomic bombing survivors [H2, K27, N11], suggesting that some RET
rearrangements might be regarded as a marker for radiation exposure. In contrast, the FHMSdetected thyroid cancers had a low frequency of RET rearrangements and a predominance of BRAF 40
mutations. BRAF mutations are characteristic of sporadic (spontaneous) thyroid cancers in older
patients. However, other studies that used age-matched unirradiated control groups have suggested
that RET rearrangements are characteristic of thyroid cancer occurring at early ages, but are notindicative of radiation exposure [F3, P5]. The low frequency of RET rearrangements andpredominance of BRAF mutations in Fukushima children thus may reflect that the cancers werelargely diagnosed in adolescence and early adulthood [S51], rather than in early childhood. In any
event, the pathological findings provide no indication of a radiation aetiology in Fukushima thyroid
cancer cases.
Figure XXI. Percentage of the total number of thyroid cancers observed in childrena (percentage
distribution) by age at exposure to radioactive material: in Belarus, the Russian Federation andUkraine in the first 10 years after the Chernobyl accident,b and in Fukushimac within about
8 years after the Fukushima accidenta

The total numbers of thyroid cancers observed in children in these time periods are 1,567 for Chernobyl and 233 for Fukushima.

b

Data were provided for this tabulation by representatives of the respective countries [U16]. The figure includes data from boththe initial “prevalence” screening and any subsequent “incidence” screenings.
c

In the 17 to 18 age group in Fukushima, the percentage may be an underestimate of the actual thyroid cancer frequency because of asignificant decrease in the percentage of individuals who received thyroid screening once they reached an attained age of about 18 years.

39 RET is an abbreviation for “rearranged during transfection”. The human RET gene provides instructions for producing a proteinthat is involved in signalling within cells.
40 The human BRAF gene encodes a protein called B-Raf which is involved in sending signals inside cells directing cell growth.

90

UNSCEAR 2020/2021 REPORT227. Within the FHMS screening programme (designed with a uniform screening protocol), several
investigators have compared thyroid cancer rates across Fukushima regions or municipalities withdifferent estimated external exposure levels to look for evidence of a relationship between exposure andthyroid cancer incidence. The results are summarized in table 16. Most investigators have found nostatistically significant increases in thyroid cancer rates with increased exposure level [O7, O9, O10, S52,
W8]. Nakaya et al. [N8] employed a spatial analysis of FHMS-detected thyroid cancer by municipalityof residence that avoided arbitrary divisions of exposure levels, and also addressed uncertainties
associated with detected nodules that did not undergo confirmatory testing. They found no statisticalassociation of geographic location with thyroid cancer prevalence.
228. Kato [K8], on the other hand, reported a marginally significant trend for combined thyroid cancer
prevalence and incidence by estimated municipality external exposure levels (see table 16), though their
methodology has been criticized [O8, O11]. Yamamoto et al. [Y6] also reported a statistically significant
positive radiation exposure-related trend for thyroid cancer prevalence or incidence in the first two roundsof FHMS screening, based on estimated average external exposure levels and numbers of screened
individuals and thyroid cancer cases for each of the 59 municipalities in Fukushima Prefecture. The study
has several weaknesses: for example:(a) It was based on only municipality radiocaesium deposition densities and failed to take into
account evacuations or variations in radioiodine deposition;(b) There was dose-related confounding across municipalities in the fractions who received fineneedle aspiration cytology and confirmation of malignancy [O12];(c) It failed to take into account the substantial variability in personal doses, so that the regression
analysis intervals were too narrow, and the “significant” results should be viewed circumspectly.
A recent study found no association across municipalities of external radiation measurements and thyroid
cancer prevalence in the first round of screening, soon after the accident, but reported a positive
association for the later second round. However, there was no association of soil measurements of 131I
levels and thyroid cancer rates for either round of thyroid screening [T33].
Table 16. Summary of studies of the relative risk of thyroid cancer found in the Fukushima HealthManagement Survey screening programme in relation to estimated external radiation exposurelevelsOdds ratio estimates of relative risk (95% CI)a
Reference
Next higher
exposure level(lowest exposure)

Next higher
exposure level

Next higher
exposure level

1.00b

1.7 (0.81, 4.1)

1.5 (0.63, 4.0)

[T45]

1.00b

1.21 (0.80, 1.82)

1.08 (0.60, 1.96)

[W8]

1.00c

1.00 (0.67, 1.50)

1.49 (0.36, 6.23)

[O7]

d

0.76 (0.43, 1.35)

0.24 (0.03, 1.74)

[O7]

1.00e

1.19 (0.58, 2.43)

1.21 (0.64, 2.3)

1.22 (0.55, 2.70)

1.00f (a)1.08 (0.58, 2.01)

1.05 (0.53, 2.09)

1.44 (0.75, 2.75)

0.95 (0.48, 1.88)

1.00 (b)0.55 (0.24, 1.26)

0.93 (0.45, 1.93)

0.73 (0.33, 1.60)

0.59 (0.24, 1.47)

1.00g,h1.03 (0.58, 1.83)

1.23 (0.68, 2.23)

1.31 (0.77, 2.23)

1.00i (a)1.02 (0.36, 2.86)

2.20 (0.82, 5.93)

2.32 (0.86, 6.24)

Reference group

1.00

Next higher
exposure level

[S52]
[O9]
[K8]
1.62 (0.59, 4.46)[O10]

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91

Odds ratio estimates of relative risk (95% CI)a(lowest exposure)

Next higher
exposure level

Next higher
exposure level

Next higher
exposure level

Reference
Next higher
exposure level1.00 (b)1.19 (0.32, 4.41)

2.24 (0.63, 8.03)

1.30 (0.32, 5.19)

0.91 (0.22, 3.80)

1.00 (c)0.81 (0.27, 2.46)

1.68 (0.57, 4.94)

1.92 (0.65, 5.67)

1.27 (0.42, 3.85)

j1.00 (a)1.22 (0.43, 3.49)

1.12 (0.43, 2.95)

1.60 (0.59, 4.33)

1.00 (b)1.66 (0.47, 5.86)

0.54 (0.12, 2.45)

0.26 (0.03, 2.42)

Reference group

a

[O12]

Except as noted, the analyses in the various studies have been adjusted for sex and age at screening. The odds ratio shows therelative risk in each exposure level group compared with the risk in the lowest exposure (reference) group. The exposure levels of
the groups in the table were selected by the authors using various metrics and cutpoints for surrogates of external radiation dose, so
the numbers in the respective groups are not identical across studies. Furthermore, the data included in the various studies differed,
so the studies are not strictly comparable and caution should be used in the interpretation. Definitions of the exposure levels in thevarious studies (reference group and successively higher groups) are given below for each study.
b

Tsuda et al. [T45] did not report estimated exposure levels, but compared their “middle” and “nearest” areas to one segment of

the “least contaminated” area. Wakeford et al. [W8] used the exposure-level breakdown from [T45], but calculated odds ratios
based on the entire “least contaminated” area. Neither the Tsuda et al. nor Wakeford et al. analyses were adjusted for sex and age.
c

Ohira et al. [O7] used a breakdown of estimated external doses <1 mSv, 1 to <2 mSv, and ≥2 mSv, respectively.

d

Ohira et al. [O7] used a breakdown of estimated personal external doses <1 mSv, 1 to <2 mSv, and ≥2 mSv, respectively for

those for whom they were available.
eSuzuki et al. [S52] used a breakdown by Fukushima areas, namely Aizu, Iwaki and Soma, Nakadori, and the evacuation zone,

respectively.
f

For reference groups marked with (a), Ohira et al. [O9] used a breakdown of municipalities by estimated per cent of people

screened with external doses ≥1 mSv: <0.67%, 0.67% to <5.7%, 5.7% to <55.4%, 55.4% to <66%, ≥66%, respectively, based on
115 cases among 300,473 in the first round of screening (P for trend = 0.69). For reference groups marked with (b), they tabulated
by quintiles of estimated personal external doses, with 56 cases among 129,181 screened; the quintiles had median values of 0.2,
0.3, 0.7, 1.3 and 1.9 mSv, respectively (P for trend = 0.52).
gKato [K8] used a breakdown of municipalities by per cent of people screened with estimated external doses ≥1 mSv (estimatedmean doses given in parentheses): <0.80% (0.20 mSv), 0.80% to <5.7% (0.50 mSv), 5.7% to <55.4% (0.74 mSv), and ≥55.4%(1.37 mSv), respectively.
h

Kato [K8] claimed that the Ohira et al. [O9] numbers for screening in the first round were incorrect, made an alternate “corrected”

tabulation, and further added municipality results for screening in the second round to extend follow-up to 6 years. There was no
indication of adjustment for age and sex. For the first and second rounds of screening combined, Kato showed a marginal association(P for trend = 0.07). The result was interpreted to indicate that the marginally significant trend was attributable to the second roundscreening data. However, the correctness of the disparate, dose-based municipality groupings by Kato versus Ohira et al. (seefootnote f) is unclear.
i

Ohira et al. [O10] analysed 70 thyroid cancers among 241,832 people screened in the second round of FHMS screening. The quintiles

of municipalities based on proportions of residents with estimated external doses ≥1 mSv were <0.67%, 0.67% to <5.7%, 5.7% to<55.4%, 55.4% to <66% and ≥66%, respectively. Reference groups marked with (a) are for all thyroid cancers >5 mm; reference groups
marked with (b) are for only the thyroid cancers >10 mm in diameter (n = 34); and reference groups marked with (c) are for all thyroid
cancers >5 mm, where quintiles are based on the proportions of residents with estimated external doses ≥2 mSv. Proportions with
≥2 mSv were <0.05%, 0.05% to <0.25%, 0.25% to <8.8%, 8.8% to <12.2%, and ≥12.2%, respectively.
j

Ohira et al. [O12] divided the 59 municipalities into quartiles according to the first year combined internal and external thyroid

doses according to the UNSCEAR 2013 Report [U10]. They evaluated thyroid cancer risk in the second round of FHMS thyroid
screening, examining those screened at ages 6–14 (reference group marked (a)) and ≥15 (reference group marked (b)) separately.
The UNSCEAR 2013 Report upper estimates of dose were used for each municipality. Relative risks were adjusted for sex, age
and year of examination. Neither test for trend by dose category was statistically significant.

229. Two caveats to the above studies are of note. First, the FHMS screening programme has been
conducted primarily as a health-supportive programme rather than a rigorous scientific study that aimed
to control for all sources of data bias. Thus, it was observed in the second round of screening that, in the

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less-exposed municipalities, comparatively lower percentages of suspect nodules/cysts tended to haveconfirmatory examinations and fine needle aspiration cytology; comparison of the rates of confirmed
thyroid cancer in the more-exposed to the less-exposed municipalities may therefore be upwardly biased[O10, O11]. Second, patterns of external exposure do not necessarily reflect total absorbed doses to thethyroid. All except the last study reported in table 16 [O12] used external doses to define the dose
categories; only the last study used estimated absorbed doses to the thyroid from both internal andexternal exposure. Few personal measurements of internal thyroid exposures were made, so comparisons
between regional/municipality levels of internal thyroid exposure and thyroid cancer rates are complex
and uncertain. For the second round of screening, Ohira et al. [O10] analysed risk according to theestimates of absorbed dose to the thyroid from internal exposure developed by Kim et al. [K15]. For
municipalities for which the estimated 90th percentiles of absorbed doses to the thyroid from internal
exposure were 10 to 20 mGy and >20 mGy, the relative risks compared to the baseline group (<10 mGy)
were 1.50 (95% CI: 0.71, 3.16) and 0.63 (95% CI: 0.31, 1.27), respectively, thereby providing no
indication of a progressive trend in thyroid cancer risk by internal dose level. Similarly, Toki et al. [T33]reported no significant association between municipality measured soil levels of 131I and thyroid cancer
incidence. In any case, the limited information on absorbed doses to the thyroid from internal exposure
should temper the interpretation of the studies in table 16.

(b) Other health conditions among Fukushima residents
230. To date, there are no reports of an excess in the incidence of childhood or adult leukaemia among
Fukushima residents. This is in accord with the view expressed in the UNSCEAR 2013 Report [U10]that any increase in leukaemia was not expected to be discernible. Since the UNSCEAR 2013 Report,
additional information has become available regarding excess leukaemia risk after childhood radiation
exposure. Recent high-quality studies [B4, L11, M20] have been reviewed by the Committee and theCommittee has concluded [U17] that the studies support risk estimates for leukaemia consistent withthose used in the UNSCEAR 2013 Report. Statistical power analyses of lifetime doses and risks wereconducted for the cancer endpoints of leukaemia and breast cancer that are known to be especiallysusceptible to radiation exposure, plus all solid cancer (see attachment A-23 for further details). The
Committee found that, for all those evacuated and for residents of municipalities in Fukushima Prefecturethat were not evacuated, as well as for the subset with effective doses over 5 mSv, excess risks of breast
cancer would not be discernible (see attachment A-23).
231. The expected lifetime excess risk of leukaemia in the Fukushima Prefecture population is also
small. For instance, in the subgroup from in utero up to 5 years of age at the time of the FDNPS accident,
who are expected to be the most sensitive group for leukaemia, about 10 to 50 excess incident cases ofleukaemia during their lifetime could be inferred, depending on the model assumed, from the estimateddose levels to the red bone marrow. The baseline number (in the absence of radiation exposure from the
FDNPS accident) is about 640 cases with a 95% confidence interval of 590 to 690. Furthermore, thisrange does not include possible variations in background rates among geographic localities, changes inrates over calendar time, etc. Even 50 potential excess cases would probably not be discernible among
640 expected cases, given the estimated range of uncertainty in the baseline risk. Since leukaemia risk
tends to be expressed at early ages after childhood exposure, statistical power analyses were also
conducted for up to either 30 or 40 years of age; these indicated that any excess in the incidence ofleukaemia would probably not be discernible for most subpopulations. The greatest statistical power wasfor those initially exposed at ages in utero up to 5 years of age and observed up to 40 years of age.
Including both sexes, and depending on the risk model assumed, about 10 to 40 cases could be inferredfrom the estimated radiation exposure, compared to about 160 baseline cases in the absence of radiation
exposure, with an estimated range of uncertainty in the baseline estimate of about 50 cases. In summary,because of the low estimated doses to the child and young adult populations, excess cases of leukaemia

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attributable to radiation exposure are generally not discernible, although a possibility exists of discernible
risk among those aged 5 years and under at the time of the accident.
232. When rates of various adverse pregnancy outcomes were compared in the months before and after
the FDNPS accident, the post-accident incidences were not found to be elevated for stillbirth or preterm
delivery [F9]. There also was no reported increase in congenital anomalies overall, or for specific typesof anomalies, such as cryptorchidism (undescended testes) [F9, K34]. Rates of preterm deliveries, lowbirthweight and congenital anomalies were uninfluenced by the nuclear accident and were similar to thoseobserved throughout Japan [K48, Y14]. In contrast, both Scherb et al. [S17] and Körblein and Küchenhoff[K39], conducted spatiotemporal regression analyses of perinatal mortality data that included Fukushima
Prefecture and four or five neighbouring prefectures in comparison to the remainder of Japan, andreported increased rates in perinatal mortality of 15% and 10%, respectively, in Fukushima andneighbouring prefectures after the accident. However, Körblein and Küchenhoff [K39] acknowledgedthat their data unexpectedly showed no excess perinatal mortality in those conceived 1 to 3 months after
the accident, and that the highly aggregated results also had appreciable temporal variability and might
be due to other non-radiation factors. The large 10% to 15% estimated increases in perinatal mortality
reported for the groups of prefectures are not considered plausible, given the estimated effective doses of
only 0.2 to 4.3 mSv [K39, S17].
233. In the UNSCEAR 2013 Report [U10], the Committee could not exclude that a small number ofpregnant women may have received absorbed doses to the uterus of about 20 mGy, but concluded that
any increase of the risk would not lead to a discernible increase in the incidence of childhood leukaemia
or other childhood cancers, because of the relatively small number of pregnant women involved and therarity of childhood cancer. The study of Japanese atomic bombing survivors found that the risk of solidcancers among the prenatally exposed group was nominally smaller than, but statistically compatible
with, the risk among those exposed at postnatal ages 0 to 5 years [P6]. The Southern Urals studies ofchildren with in utero irradiation when mothers worked at the Mayak Production Association (medianprenatal dose of about 19 mGy) [T36] or were exposed to radioactive material released to the Techa River(mean prenatal dose about 4 mGy) did not show a risk of solid cancer due to prenatal exposure [A9, K40].
There was mixed evidence regarding an increased risk of leukaemia in that cohort (mean prenatal doseabout 30 mGy) [K40, S19]. The Committee’s revised estimates of absorbed doses to those exposed inutero (red bone marrow dose) are that average doses were less than about 2 mGy, with 95% upper boundon the average dose of about 3.5 mGy. The Committee has assessed the resulting excess risk of childhood
leukaemia up to age 20 and found that the excess risk would not be discernible (see attachment A-23).
The Committee therefore still does not expect any discernible increase in the incidence of childhood
leukaemia or other childhood cancers as a result of prenatal exposure.
234. A variety of reports have indicated an increased prevalence of obesity, dyslipidemia, glucose
intolerance, diabetes mellitus, liver and renal dysfunction, high blood pressure, atrial fibrillation andpolycythaemia among evacuees after the FDNPS accident [E2, H5, K48, K53, S48, Y10]. Prevalences ofhypertension, dyslipidemia and diabetes mellitus increased after the accident among evacuees, but did
not increase among non-evacuees, suggesting the effects were associated with evacuation rather thanradiation exposure [O6]. There was little evidence of an increase in acute myocardial infarction after theaccident [Y5]. The observed differences appear to reflect changes in lifestyle and psychological stresses
following the disaster and evacuation, rather than effects of radiation exposure. There is little likelihoodof a discernible excess incidence of cardiovascular diseases attributable to radiation at the levels of
exposure estimated for members of the public. Likewise, no excess incidence of cataracts is expected tobe discernible among the public.
235. Various adverse effects on mental health in Fukushima Prefecture were associated with the disasterrelated stresses and radiation risk perceptions [F15, M2]. Psychological effects were especially evident

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among those who were evacuated, and included excess anxiety, depression, problem drinking, and adverse
psychological health effects among children and their mothers. However, several studies have reported noassociation of these effects with radiation exposure levels [F15, G5, M8, O3]. The frequency of reported
psychological health effects has also decreased with time [K48]. In addition, further studies have provided
evidence that evacuation may have increased mortality among those in elderly care [N14, Y22].

2.

Health implications for workers engaged in emergency work

236. No deterministic health effects or deaths have been observed among workers engaged in emergency
work that could be attributed to radiation exposure. Studies suggest little risk for hypothyroidism below
thyroid doses of one or more gray [D1, I32, I39, S22]. Given that the 13 workers with the highest
estimated absorbed doses to the thyroid are now estimated to have received doses in the range of 2 to
32 Gy (see chapter VI), the Committee cannot preclude the possibility of hypothyroidism in these more
exposed workers, although it continues to consider the likelihood of such effects to be low.
237. Whether there is a discernible risk of cataracts is uncertain; some estimates of dose to the lens of
the eye have become available, but they rely on assumptions about the wearing of full-face masks, when
there is evidence that those workers who would have received the highest exposures of the eye lens may
not have been wearing such equipment. Another issue is uncertainty in the level of a dose-effect thresholdfor cataracts, if any. ICRP [I20] has indicated there may be a dose threshold of 0.5 Gy, but with substantial
uncertainty. Several [A20, L9, W19], though not all [L7], large studies among radiation workers suggest
a dose-related risk of detectable lens opacities (cataracts), although two large studies of radiation workers
did not find a dose-related risk of clinically significant cataracts requiring surgery [A21, L9]. No
associations of radiation exposure with glaucoma or macular degeneration have been observed in otheroccupational studies [B9, L10].
238. For most workers, the effective doses in the first year received as a result of the accident were low,with an average dose of about 13 mSv, 41 and with 64% receiving an effective dose within the first yearof less than 10 mSv [S25]. The Committee therefore expects no discernible increase in health effects
among the overall group of emergency workers that could be attributed to their radiation exposure.
A small proportion of workers (174 workers, about 0.8%) received effective doses within the first yearof 100 mSv or more, with an average of about 140 mSv [S25]. Among this group, a small increased riskof cancer could be inferred over the lifetime: risk estimates for this subgroup correspond to about two to
three additional cases of cancer in addition to about 70 cases that would occur spontaneously [E7]. The
Committee therefore considers that it is unlikely that an increased incidence of cancer due to irradiation
would be discernible, because the normal variability in baseline rates of cancer incidence is much larger
than the inferred radiation-associated cancer rates.
239. In the UNSCEAR 2013 Report [U10], the Committee acknowledged that there was potentially anenhanced risk of thyroid cancer for the group of 13 workers who received the largest absorbed doses tothe thyroid (now estimated to be in the range of 2 to 32 Gy). However, the Committee continues toconsider that the number of workers exposed at these high absorbed doses to the thyroid is likely too
small to discern an increased incidence in thyroid cancer. Similarly, it does not consider that the dose
distribution and size of the group of 1,757 emergency workers thought to have had absorbed thyroid
doses exceeding 100 mGy to be sufficient to discern an increased incidence. This is because of the low
41 Doses to workers have generally been reported in terms of effective dose. The two main contributors are the broadly uniform
exposure of the whole body from external radiation and internal exposure of the thyroid from intakes of radioiodine. The averageabsorbed doses in most organs of the body other than the thyroid in the first year was 11 mGy [I5].

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susceptibility of adults to radiation-induced thyroid cancer [K23, L2], the relatively small number ofworkers with higher thyroid doses, and the normal statistical fluctuations in background incidence.
240. IAEA reported [I6] the interim results of a survey involving ultrasound examinations of the thyroidof 627 emergency workers engaged at FDNPS with absorbed doses to the thyroid of more than 100 mGy
and 1,437 with lower thyroid doses. No significant differences between the incidence of thyroid diseases
in the two groups were found, consistent with the relatively low doses received by these adults and theshort time interval between exposure and examination. The negative finding is also in accord with threelarge studies of thyroid cancer in over 350,000 radiation workers that did not find a radiation doseassociated excess after several decades of follow-up [H12, K23, L2].
241. Regarding leukaemia risk, the Committee estimated absorbed doses to the red bone marrow for thesmall group of 37 most highly exposed workers to be up to about 200 mGy, but again did not expect any
increase in incidence of leukaemia to be discernible because of the small number of workers in this group.
The magnitude of leukaemia risk seen in other radiation workers is compatible with this expectation [S29].
242. MHLW is sponsoring a study of the FDNPS emergency worker cohort with a comprehensive healthassessment, including cancer incidence and mortality, a clinical study of general health parameters,laboratory blood and urine tests, and special substudies of thyroid cancer, cataract, psychological status,
radiobiological mechanistic markers, and physical and biological reconstruction of personal doses [J4,K24, M23, Y18]. Statistical power analysis conducted as part of this study confirmed the Committee’sfindings (as set out in more detail in attachment A-23) that the inferred 60-year incidence of all solidcancer among the workers would not achieve adequate statistical power to be discernible [J4]. The study
expands upon the health monitoring now required of nuclear operators in Japan [Y20]. Questionnaire
information is being obtained on potential confounding factors, such as, medical history, medical
radiation exposures, lifestyle and sociodemographic data, psychological stresses, and exposure to othertoxic agents. Blood samples will be stored, and linkage with mortality and cancer databases will be
implemented. To date, the clinical infrastructure of the study has been established, 7,270 workers haveagreed to participate and clinical examinations have been conducted on 5,748 (by December 2019) [K24,
M23]. This study has led to the reassessment of the absorbed doses to the thyroid of six of the workers
with the highest doses [K54].
243. Up to the end of 2018, the Japanese Government has awarded compensation to a total of six
occupationally-exposed nuclear power station workers who were employed at FDNPS following theaccident and accumulated part of their exposure at the FDNPS site. Three of these workers werediagnosed with leukaemia [M24, M25, M26], two with thyroid cancer [M24] and one with lung cancer[M24]. The Committee has informed on the elements of scientific attribution of malignancies to radiation
exposure (see annex A of the UNSCEAR 2012 Report [U12]). Scientific attribution is not equivalent tolegal imputation in an occupational context. Therefore, the granting of these awards (and, likewise, others
granted previously or in the future to occupationally-exposed workers) does not imply a scientifically
proven cause-effect relationship between radiation exposure and any particular case of cancer. Rather, itis the result of the application of a scheme, 42 developed several decades ago by the Japanese Government,
42 In 1976, the Japanese Government established the basis for the award of compensation to occupationally-exposed workers
through the Industrial Accident Compensation Insurance scheme [M36]. The initial focus of the scheme was leukaemia. According
to this scheme, leukaemia may be deemed eligible for medical compensation by a medical review panel if the dose was at least
5 mSv times the number of years between (first) exposure and diagnosis of the malignancy. A similar scheme applies for certain
other lympho-haematopoietic malignancies with modification based on their comparative radiosensitivity. Up to the end of 2018,some 16 nuclear power station workers in Japan had been granted compensation under this scheme for lympho-haematopoietic
malignancies, of which about half were leukaemia. Three of these 16 workers worked at FDNPS. Solid cancer may be deemed
eligible for medical compensation by a medical review panel if the effective dose was at least 100 mSv, the time between (first)
exposure and diagnosis of the malignancy was five years or more, and there was no other aetiology than radiation. These criteria
are kept under continuing review and may be revised in light of new scientific evidence.

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for industrial accident compensation insurance [M36]; knowledge and understanding of the effects ofradiation has since improved considerably.

C. Summary
244. In the years since the UNSCEAR 2013 Report [U10] no adverse health effects among Fukushima
Prefecture residents have been documented that are directly attributable to radiation exposure from the
FDNPS accident. There have been some modest changes in the Committee’s updated estimates of doses
to members of the public compared with the Committee’s previous estimates (see chapter V). In addition,
estimated baseline cancer risks have undergone a modest amount of change, and new data have become
available to update estimates of radiation risk for various cancer endpoints. The Committee has therefore
carried out reanalyses of lifetime risks for particularly radiosensitive malignancies: leukaemia, thyroid
cancer and female breast cancer, as well as all solid cancers (excluding thyroid cancer and nonmelanoma
skin cancer).
245. The Committee’s updated statistical power analyses suggest that excess thyroid cancer risk that
could be inferred from radiation exposure was most likely not discernible in any of the age groups
considered. For thyroid cancer incidence taking account of the thyroid screening being carried out in
FHMS, analyses of children did not indicate that excess thyroid cancer would likely be discernible up toeither age 30 or 40. For those exposed in utero, the Committee similarly found that the statistical power
was too low to discern an excess risk of thyroid cancer.
246. A substantial number of thyroid cancers have been detected among exposed children. However, theexcess does not appear to be associated with radiation exposure, but rather a result of the application ofhighly sensitive ultrasound screening procedures. Other features of the thyroid cancer occurrence do not
fit well with a radiation aetiology: (a) no excess of thyroid cancer has been observed in those exposed
before age 5, in contrast to the large excess observed in the same age group exposed as a result of theChernobyl accident; and (b) thyroid cancers were observed within 1 to 3 years after exposure following
the FDNPS accident rather than beginning 4 to 5 years after exposure as in Chernobyl and other radiation
studies. Nevertheless, because the natural progression of thyroid cancer based on the possible causes inyoung patients remains unknown, there is a need for further research into the biological mechanisms (ingeneral, and not necessarily related to radiation exposure) that lead to thyroid cancer at young ages.
247. While the updated estimated doses to the red bone marrow have not increased, the Committee’s
estimate of leukaemia risk per mGy has increased somewhat compared to the UNSCEAR 2013 Report[U10]. However, for risk models based on the experience of the Japanese atomic bombing survivors or on
the broadest, multi-study risk data, any increased incidence of leukaemia is still unlikely to be discernible
among Fukushima Prefecture residents of any ages, although there would be some possibility of detecting
risk if the true average doses were actually at the estimated upper bound of uncertainty. The Committee has
assessed the excess risk of childhood leukaemia up to age 20 resulting from its revised estimates of absorbed
doses to the red bone marrow for those exposed in utero and found that the excess risk would not be
discernible. Likewise, the levels of exposure of members of the public have been too low for the Committee
to expect discernible increases in the incidence of breast cancer or other solid cancers.
248. There has been no credible evidence of excess congenital anomalies, stillbirths, preterm deliveries
or low birthweights related to radiation exposure. Increases in the incidence of cardiovascular andmetabolic conditions have been observed among those evacuated following the accident but are probably
associated with concomitant social and lifestyle changes and are not attributable to radiation exposure.

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Excess psychological distress also occurred in the aftermath of the combined earthquake, tsunami and
FDNPS accident.
249. The health of the FDNPS emergency workers is being monitored. Because the majority of workers
had low exposures with effective doses within the first year of less than 10 mSv, and only a small fractionof workers received effective doses within the first year of 100 mSv or more, an increase in the incidenceof cancers is unlikely to be discernible for leukaemia, total solid cancers or thyroid cancer. The
Committee has insufficient information to reach an informed judgement on the risk of cataracts among
workers. The Japanese Government recently updated its plan for health monitoring of nuclear power
station workers [Y20]. In addition, a detailed investigation of health parameters of the FDNPS emergency
workers is underway.
250. Care is needed in interpreting the results from sensitive ultrasound thyroid screening following
radiation exposure after a major radiological accident. There is compelling evidence that sensitive
ultrasound screening detects many more cases of thyroid cancer than would be detected following thepresentation of clinical symptoms. The consequential over-diagnosis of thyroid cancers has the potential
to cause anxiety among those screened and might lead to unnecessary treatment.

VIII. ASSESSMENT OF DOSES AND EFFECTS FOR
NON-HUMAN BIOTA
A. Introduction
251. The Committee had assessed exposures of non-human biota and associated effects in general in its
scientific annexes to the UNSCEAR 1996 Report and UNSCEAR 2008 Report [U6, U8]. The
benchmarks 43 set out and used in those annexes, and in the UNSCEAR 2013 Report [U10], were basedon a large synthesis of information derived primarily from radiobiological literature spanning many
decades of experimental work and (to a limited extent) including analyses of field observations fromearlier accidents. The benchmarks were that chronic dose rates of less than 100 µGy/h to the most highlyexposed individual organisms would be unlikely to have significant effects for population integrity of
most terrestrial communities, and that maximum dose rates of 400 µGy/h to any individual in aquatic
populations of organisms would be unlikely to have any detrimental effects at the population level. The
Committee has examined the impact of the FDNPS accident on non-human biota inhabiting terrestrial,
freshwater and marine ecosystems. It has considered estimated radiation exposures, including
contributions from short-lived radionuclides such as 131I, in terms of the intermediate phase after theaccident (approximately the first two months) and the late phase (months to years). The methodology,
including detailed descriptions of the geographical areas and the timescales considered, and the basis forthe benchmarks applied are provided in the UNSCEAR 2013 Report.

43 Benchmarks in this context are dose rates below which non-human biota populations are unlikely to be significantly harmed
based on current knowledge; they have been used to assess the likelihood that adverse effects occurred as a result of exposures.
Benchmarks have often been established using information from chronic, stationary or quasi-stationary, exposures and, as such,
were not fully compatible with a quickly changing, post-accidental situation. The interpretation of the dose rates reported for earlyand late periods after the accident was thus not straightforward and a balance was required between maintaining consistency withearlier UNSCEAR publications and relating effects of radiation exposure to the time-integrated dose, correcting for dose rate
effects, where appropriate.

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B. Overview of current understanding
1.

Estimates of levels of radionuclides in various biota

252. In the UNSCEAR 2013 Report [U10], the Committee used measurements of radionuclides inorganisms and in the environment and applied suitable models to estimate levels of radionuclides inselected reference organisms. Numerous studies published since the UNSCEAR 2013 Report provide
updated information on levels of radionuclides in non-human biota and processes influencing
radionuclide transfer within ecosystems [U11, U13, U14]. Fuma et al. [F22] reported that fungi, ferns,
and mosses accumulated high amounts of radiocaesium, from areas in close proximity to FDNPS, with134
Cs and 137Cs concentrations of 104 to 106 Bq/kg fresh mass (FM). Earthworms, amphibians, and thesoft tissue of snails also had levels as high as 104 to 105 Bq/kg FM of 134Cs and 137Cs. Small mammals
with maximum 134Cs and 137Cs concentrations in excess of 105 Bq/kg FM have also been captured in the
exclusion zone [K46]. The highest reported concentration of 134Cs and 137Cs in marine fish muscle,
sampled in the FDNPS port and reported by Johansen et al. [J5], was 740,000 Bq/kg FM in an
Hexagrammos otakii (greenling; sampled in February 2013). Concentrations in fish are generally ordersof magnitude lower than this level tens of kilometres offshore [S23].
253. These levels are generally consistent with the data used in the UNSCEAR 2013 Report [U10], butsome reported radionuclide levels warranted more detailed consideration. The study of Qiu et al. [Q1]indicated that substantial bioaccumulation of 110mAg in marine crustaceans may have occurred in somelocations near to FDNPS: a 110mAg/137Cs ratio in excess of one (one or two orders of magnitude higher
than the ratio found in coastal sediments) was reported for a Fukushima coastal location (althoughradionuclide levels were generally low, rarely exceeding 100 Bq/kg FM). Silver has a relatively highconcentration ratio for fish, so it is plausible that 110mAg was more important than assumed in the
UNSCEAR 2013 Report. 44 Horiguchi et al. [H26] similarly measured relatively elevated levels of 110mAg
in shellfish, although the 110mAg/137Cs ratios were considerably higher in this study.

2.

Estimates of doses for reference organisms

254. In the UNSCEAR 2013 Report [U10], the Committee estimated that total (internal and external)
absorbed dose rates, including contributions from very short-lived 132Te and 132I, may have been as highas 1,000 µGy/h for some terrestrial organisms over short periods (hours to days). In the intermediatephase (i.e., the first two months), dose rates of 300 µGy/h may have been possible for soil-dwellingorganisms in areas of high deposition density (such as Okuma Town, where the maximum deposition
density of 137Cs was about 15 MBq/m2). For aquatic ecosystems, maximum exposures were calculated
from estimated concentrations in seawater to be more than 20,000 µGy/h for aquatic macroalgae forlocations closest to the discharge point (Fukushima North Channel), with the contribution from 131I being
dominant, and 140 µGy/h for marine fish. Macroalgae dose rates were estimated to have fallen rapidly,
in line with the 8-day half-life of 131I. From measured radionuclide concentrations in biological samples,
corresponding to the late phase of the accident, terrestrial mammals and birds were estimated to havebeen exposed to total dose rates between 1.2 µGy/h and 2.2 µGy/h in areas encompassing an indicative
range of 137Cs deposition densities (between 50 and 500 kBq/m2), and dose rates to marine organisms
were estimated to be in the range of 0.10 to 0.25 µGy/h.

44 In terms of levels and effects of radiation exposure for non-human biota.

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255. The IAEA report [I6] similarly included estimates of total absorbed dose rates for non-human biota.
Estimates were derived:(a) From time-integrated concentrations of radionuclides in soil, freshwater and seawater for“early” (first 30 days), “intermediate” (31–90 days), and “late” (91 days to 1 year) periods after
the accident;(b) From directly measured radionuclide concentrations in biota, generally for the “late” period.
Estimates of the doses and dose rates based on measurements of radionuclide levels in biota were ofa similar order of magnitude to those presented in the UNSCEAR 2013 Report [U10], with maximum
dose rates for terrestrial systems of around 80 µGy/h for wild grass and around 8 µGy/h for alarge mammal.
256. Fuma et al. [F22] estimated total absorbed dose rates, mainly for the period May 2011 to March 2012,
to herbaceous plants (up to 44 µGy/h), amphibians (up to a maximum of 76 µGy/h), insects (up to a
maximum of 23 µGy/h), and earthworms (up to a maximum of 40 µGy/h) for areas close to FDNPS. Kubota
et al. [K45] estimated the average total absorbed dose rate to small mammals (rodents), from an area withrelatively high deposition levels around 4 km west of FDNPS (Ottozawa, Okuma Town, where deposition
densities of 137Cs exceeded 1 MBq/m2), at approximately 52 µGy/h (in December 2013). These and otherexposure estimates [F14], for periods within approximately the first two years of the accident, are similar to
the estimates made earlier in the UNSCEAR 2013 Report [U10], providing support for the assessment
approach used in that report and the validity of the associated exposure calculations.
257. For the marine environment, evidence of the persistence of elevated concentrations in benthic
marine fish [S23] indicates that longer term (>1 year post accident) exposures may have been higher thanmight be inferred from the estimates made in the UNSCEAR 2013 Report [U10]. Johansen et al. [J5]estimated a maximum dose rate of about 130 µGy/h in early 2013, similar to the Committee’s estimate
for marine fish in 2011. This discrepancy may have numerous causes, including not taking sufficientaccount of accumulation of radionuclides in sediment and sediment-associated food chains in the
UNSCEAR 2013 Report, and/or the continued (and unforeseen) importance of diffuse sources of releases
to the marine environment (see chapter III). Further work has continued to refine marine biota transfer
and dose modelling (see, for example, [K41, V2, V3]).

3.

Predicted and observed effects

258. In the UNSCEAR 2013 Report [U10], the Committee had inferred the effects due to the estimatedradiation exposures by applying its generic evaluations of dose‐effects relationships. It had concluded that
exposures of both marine and terrestrial non-human biota following the accident had been, in general, too
low for acute effects to be observed, although some exceptions had been considered possible because of
local variability. It had further concluded that, in general, population-relevant effects on non-human biota
in the marine environment would have been confined to areas close to where highly radioactive water wasdischarged and released into the ocean. 45 Although the Committee had been unable to exclude a risk ofeffects to individuals of certain terrestrial species, in particular mammals, it had considered observable
45 Predicted exposures for macroalgae at locations very close to the discharge point of around 20 mGy/h are within the range of

10–100 mGy/d considered to cause potential effects on reproduction and growth rate, with 3-month integrated doses of 6.8 Gy
signalling potential acute effects [I18], at least in theory. Therefore, the calculated exposures for macroalgae could have been
at a level where acute impact could have occurred; however, these exposures were transient and the Committee had concludedin the UNSCEAR 2013 Report that there would not be lasting population level effects.

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effects at the population level to be unlikely. Any radiation effects would have been constrained to a limitedarea, in the terrestrial environment, where the deposition density of radioactive material was greatest, and,beyond this area, the potential for effects on biota was considered insignificant.
259. Since the UNSCEAR 2013 Report [U10], deleterious effects in non-human biota have been
observed in the field, although not all of these would be expected to impact on populations. There havebeen numerous studies in which cytogenetic, physiological and morphological effects have been observedin areas of enhanced radiation levels following the FDNPS accident. These include: chromosomal
aberrations in splenic lymphocytes of two species of Japanese field mice (Apodemus speciosus) [K9,K46]; DNA (single- and double-strand) breaks in earthworms [F12]; lowered blood neutrophil, monocyte
and lymphocyte counts in fish [S53]; haematological changes in monkeys [O1]; and morphologicalabnormalities in gall-forming aphids [A8]. Significant increases in morphological abnormalities havebeen observed in coniferous trees, such as Japanese red pine (Pinus densiflora) [Y26] and Japanese fir(Abies firma) [W11] in areas with elevated radionuclide deposition densities following the FDNPS
accident. Although Watanabe et al. [W11] considered that there was no decisive evidence that any single
factor was causally related to these increased morphological effects, the authors also concluded that thepositive correlation between ambient dose rates and the frequencies of such negative changes wasindicative of ionizing radiation being a potentially important factor. Indeed, accumulated doses invegetation in areas with relatively high levels of deposited radionuclides were estimated in the
UNSCEAR 2013 Report to be similar to those at which disturbances in growth, reproduction andmorphology of conifers had been observed following the Chernobyl accident [S39].
260. There have also been studies in which no significantly elevated effects have been observed despite
there being relatively enhanced exposure levels. Such studies include: no clear abnormalities, from
histological analysis, in the reproductive tissues of frogs (where internal concentrations of 137Cs exceeded
47 kBq/kg FM) [M16]; no increases in the frequencies of morphologically abnormal sperm in Japanesefield mouse (Apodemus speciosus) [O20]; no increase in epigenetic markers (methylation of DNA) in
Brassicaceae plants within the exclusion zone with increasing radiation levels [H25]; no significant
effects on the blood cell counts in cattle [S8]; no impact on chromosomal inversion frequency on fruit
flies (Drosophila melanogaster) [I40]; no significant effects of the exposure to ionizing radiation on
Japanese tree frog (Hyla japonica) body condition or tissue carotenoid concentration [G3]; and an
absence of higher levels of genetic damage in bird nestlings, despite the conclusion, in the same paper,that lower survival and reproduction and/or fledging rate prevailed [B6].
261. The observations of sublethal effects (including cytogenetic damage and morphological effects) are
not inconsistent with the findings of the UNSCEAR 2013 Report [U10]. In contrast, several studies havealso suggested that there have been substantial (non-human biota) population impacts. The abundance offorest and grassland avian species has been observed to be lower in areas with enhanced exposure levels(ambient dose rate equivalents around 40 µSv/h) [M42], although concerns have been expressed
regarding some of the conclusions drawn within these studies [B2, B3], e.g., problems with field
dosimetry, low power statistics, and implying causation from correlation in the presence of confounding
factors. Some of these concerns have been addressed in subsequent rigorous dose reconstructions of someof the underpinning data sets [B1, G2], albeit that this dose reconstruction work itself has been subject tocriticism [S30]. Mousseau and Møller [M46] reported that butterflies and cicadas exhibited lower
abundances with increasing radionuclide deposition densities, despite such declines not being observed
for bumblebees, dragonflies or grasshoppers. Arachnids were stated to exhibit a statistically significant
increase in abundance at sites with high levels of radionuclide deposition, attributed to the decrease in
the number of (insect-eating) birds reported within these areas. The Committee has reviewed numerous,
mainly field-based, studies on the impact of the FDNPS accidental releases upon the pale grass blue
butterfly (Zizeeria maha) [U9, U11, U13]. The studies have generally concluded that artificial
radionuclides from FDNPS were the cause of genetic and physiological damage to this species of insect

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and that the cumulative effects of exposures could have caused a deterioration in the population [H23,
H24]. In follow-up studies from the same group [G8], involving the feeding of butterfly larvae withartificial diets incorporating relatively high levels of 137Cs, no effects on endpoints including pupation,
eclosion and survival rates were observed. This led these authors to the conclusion that biological effects
in the field may be mediated through ecological systems and cannot be estimated solely based on radiation
doses. In contrast to the above-mentioned studies inferring population-level impacts on biota, Lyons et al.
[L13] concluded, based on data analysed from a network of remote cameras in the Fukushima exclusionzone, that effects of radiological exposure on mid- to large-sized mammals did not appear to manifest in
a population-level response. Furthermore, confounding factors, including the removal of humanmanagement activities, can have substantial negative impacts on populations of some insect species suchas bees [Y27], and need to be accounted for in assessments of stressor impacts.
262. An assessment by IAEA [I6] considered impacts for all ecosystems. This assessment used asomewhat different methodology to that adopted by the Committee in the UNSCEAR 2013 Report [U10](which incorporated dynamic transfer models, enabling the estimation of maximum dose rates occurring
over shorter time periods). But the overall results were largely consistent, given the substantial
uncertainties associated with assessment models. IAEA concluded that, based on current knowledge, no
impacts on populations and the ecosystems (both terrestrial and marine environments) were expected andthat long-term effects were not expected. IAEA also noted that knowledge of the impacts of radiation
exposure on non-human biota needed to be strengthened by improving the assessment methodology andunderstanding of radiation induced effects on biota populations and ecosystems.
263. The Committee reviewed over 20 years of research on radiation impacts in the Chernobyl exclusionzone [U8] and found no adverse radiation induced effects reported in plants and animals exposed to a
cumulative dose of less than 0.3 Gy during the first month after the Chernobyl accident (i.e., <10 mGy/d,on average). For the most contaminated areas, a few years were needed for recovery from the majorradiation-induced adverse effects on plants and animals [U8]. Studies in the Chernobyl exclusion zonethat are still ongoing provide tentative evidence that organisms in their natural environment may be moresensitive to radiation than those tested under laboratory conditions [G1]. Furthermore, studies on highlevels of biological organization (e.g., population, ecosystem function) are still scarce and very few ofthem consider the impact on ecology and evolution of species.
264. The Committee continues to consider it unlikely that there would be severe, regional population-level
impacts on non-human biota as a result of the FDNPS accident, although it acknowledges the results of thestudies in which substantial population impacts have been observed for some types of organisms. The causeof these results remains to be identified definitively and the observations need to be followed up.

4.

Uncertainties

265. Overall uncertainties associated with the types of models applied in the Committee’s assessment,
particularly when the approach involves a biological transfer component, are large: estimations of
exposures to non-human biota using different models often differ from one another by more than one
order of magnitude (see, e.g., [V4]). A further substantial uncertainty is associated with exposures fromshort-lived radionuclides in the initial phases of releases, such as doses to the mammalian thyroid from131
I, as discussed in [S38].
266. The benchmarks used by the Committee to assess effects on non-human biota constituted the best
insight that could be achieved into the doses at which biological effects were likely. However, thebenchmarks were largely based on exposures of small groups of individuals, maintained in isolation and102

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under controlled laboratory conditions. 46 There will clearly be some limitations to the applicability ofthis information when extrapolating to infer effects for ecosystems. Disturbances induced by stressors
cannot be entirely understood from knowledge of the stressor’s effects on individual organisms,
considering that such effects may act as triggers of perturbation, which propagate through higher levelsof biological organization within ecosystems [B8]. This view appears to be supported by recent metaanalysis of effects data, partly involving findings from Chernobyl exclusion zone, and suggesting thatorganisms in their natural environment may be more sensitive to radiation than those tested underlaboratory conditions [B1, G1]. Nonetheless this contention has been questioned [S30] and requires
further testing and verification. Confounding factors render interpretation of environmental impacts fromradiation exposure challenging. For example, IAEA [I6] contended that the severe earthquake andtsunami caused significant environmental stress to the terrestrial and marine environments along thenorth-eastern coast of Honshu, that was considered to be far in excess of that caused by radiation
exposure.

C. Summary
267. The Committee continues to consider that regional impacts on wildlife populations, with a clear
causal link to radiation exposure resulting from the FDNPS accident, would have been unlikely althoughdetrimental effects on individual organisms might have been possible. Indeed, various cytogenetic,
physiological and morphological (sublethal, individual level) effects in some plants and animals havebeen observed in areas of enhanced radiation levels following the FDNPS accident, in the absence of any
reported wide-scale group impacts. However, such studies remain scarce and current conclusions strongly
depend on the studied organism. A few studies have indicated substantial population impacts on selectedwildlife groups, but, given the wealth of underpinning radiobiological evidence to the contrary, thefindings of these studies and their robustness, regarding reproducibility and adequate account being takenof confounding factors, remain subject to some doubt. However, assessed uncertainties are large andunderstanding is far from complete, and it is possible that current assessment methodologies are
inadequate. Further analysis would be useful of the impacts of radiation exposure on non-human biota
under field conditions, such as those present in the exclusion zone of FDNPS, as was done in the
Chernobyl exclusion zone. Moreover, further research would be useful to investigate the dose responseat the population level, and higher levels of biological organization, within natural environments, taking
account of elements of ecosystem function and structure.

IX. CONCLUSIONS
268. The Committee has assessed and summarized the information available at the time of the
UNSCEAR 2013 Report [U10] and the new information that has become available to the Committee
since the completion of that report. The significant amount of new information that has become available
has enabled the Committee to carry out a higher quality and more robust assessment of the levels and
46 Furthermore, the benchmarks commonly used pertain to chronic exposures under current and existing exposure situations whereas
values dedicated to acute exposures, which are scarce in the literature, may be more appropriately applied. In this respect, an acutethreshold value proposed at the ecosystem level for the marine environment (see [S38]), also considered by the Committee [U7](i.e., value of 4.84 Gy), may be applicable.

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effects of radiation exposure due to the FDNPS accident than set out in the UNSCEAR 2013 Report.
From its assessment, the Committee has concluded the following:(a) The total releases to the atmosphere of 131I and 137Cs ranged generally between about 100 toabout 500 PBq and 6 to 20 PBq, respectively. The ranges correspond to about 2% to 8% of the total
inventory of 131I and about 1% to 3% of the total inventory of 137Cs in the three operating units(Units 1–3) from which the releases occurred at the time of the accident. The releases of less volatile
radionuclides (e.g., 90Sr and 239Pu) were negligible. About 80% of the total amount released wasdispersed over and deposited on to the Pacific Ocean;(b) For the purposes of estimating concentrations of radionuclides in the environment where
measurements do not exist or can no longer be made, the Committee recommends the use of thelatest in a series of estimates of the temporal pattern of release (or the so called “source term”)
developed at JAEA. The most recent estimate is that of [T28] and the Committee has used it inrevising its estimates of doses to the public from airborne radionuclides. The total releases of 131I and137
Cs to the atmosphere in this source term are 120 and 10 PBq, respectively;(c) Releases of radionuclides into the Pacific Ocean occurred directly and indirectly and still
continue, albeit at decreasing rates. They comprise: (i) direct releases in the first three months (inleakage and deliberate release of water containing radionuclides), amounting to about 10 to 20 PBq
of 131I and about 3 to 6 PBq of 137Cs; (ii) deposition of radionuclides on to the ocean surface following
their release to, and dispersion in, the atmosphere, amounting to about 60 to 100 PBq of 131I and 5 to
8 PBq of 137Cs; (iii) direct release of about 60 TBq of 137Cs in ground water draining from the site
up to October 2015, when measures were taken to reduce these releases, and about 0.5 TBq per year
thereafter; and (iv) continuing indirect releases of about 5 to 10 TBq of 137Cs per year via rivers
draining catchment areas on to which radioactive material released to the atmosphere was deposited;(d) The material released to the Pacific Ocean was rapidly dispersed and diluted in seawater: by
2012, the concentrations of 137Cs, even in the coastal waters off the FDNPS site, were little above
the levels prevailing before the accident. Concentrations of 137Cs in marine foods have declined
rapidly: 41% of samples caught off the coast of Fukushima Prefecture in 2011 exceeded the longterm limit established by the Japanese Government, decreasing to 17% in 2012, and, from the
beginning of 2015, to just four samples out of 9,000;(e) A large body of Japan-specific information has been accumulated on the levels and transfer ofradionuclides released to the atmosphere through terrestrial and freshwater environments.
Radiocaesium has been found to be more strongly bound to soils in Japan than in many soils affectedby the Chernobyl accident, resulting in its reduced transfer to crops and its reduced rate of downward
migration into soil, with implications for doses via ingestion and from external radiation from
deposited radionuclides. Concentrations of radionuclides in most monitored foodstuffs have declined
rapidly following the accident. Since 2015, no samples of livestock and crop products, and less thana few per cent of most monitored wild food and freshwater fish products, have exceeded the limit
established by the Japanese Government;(f) The levels of radiation exposure and concentrations of radionuclides in the environment
following the accident at FDNPS have been extensively characterized through measurements andmonitoring campaigns. They provide a broadly sufficient basis for making realistic estimates ofdoses to the Japanese population for most pathways of exposure. The exception is exposure from theinhalation of airborne radionuclides for which measurements are relatively sparse. The Committee
had, therefore, to rely on a model to estimate concentrations of radionuclides in air over the Japaneseland mass and used the source term and associated ATDM by Terada et al. [T28] for this purpose;

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UNSCEAR 2020/2021 REPORT(g) The Committee has used information available to it on measurements made on people largely
for the purposes of validating the approaches and models it has used to estimate doses. These
measurements comprised WBC, levels of radioiodine in the thyroid and personal dosimetry
campaigns. The Committee had access to measurements made on people reported in the publishedliterature and requested further information from national, regional and municipal authorities inJapan. Notwithstanding the large amount of measurements that have been and continue to be madein Japan that were not yet publicly available, in particular personal dosimetry campaigns and WBC,the only information made available to the Committee was data from personal dosimetry campaigns
undertaken by Minamisoma City and Naraha Town. Access to the wider body of measurements inJapan would have enabled the Committee to further refine and/or enhance the validation of its
models. Broad agreement was found between the Committee’s dose estimates and those derived fromthe available measurements made on people. This has provided support for the validity of the
improved models used for estimating doses from intakes into the body of radioiodine andradiocaesium and external doses from radionuclides deposited on the ground. Of particular note, inthis respect, is the improved and validated approach for estimating doses from ingestion ofradionuclides that has resulted in doses from this exposure pathway that are lower by at least ten
times than those estimated in the UNSCEAR 2013 Report;(h) The estimated municipality- and prefecture-average effective doses to infants in the first year
are in the range of about 0.2 to 8 mSv for municipalities that were evacuated, in the range of about
0.1 to 5 mSv in other municipalities of Fukushima Prefecture, in the range of about 0.1 to 1 mSv inneighbouring prefectures, and from 0.005 to 0.5 mSv for prefectures in the rest of Japan. The average
effective doses to children and adults were estimated to be about 80% and 70%, respectively, of thedoses to infants. In the first 10 years after the accident, these effective dose estimates would increaseby between two and three times, and over a lifetime (up to age 80 years) by about four times. In
2021, average annual effective doses have been estimated to be less than 0.5 mSv in all nonevacuated municipalities in Fukushima Prefecture and below 0.1 mSv elsewhere in Japan. In theevacuated communities where evacuation orders have been lifted, average annual effective doses in
2021, taking account of remediation work completed in these areas, are generally less than 1 mSv;(i) The estimated municipality- and prefecture-average absorbed doses to the thyroid for infants in thefirst year are in the range of about 2 to 30 mGy for municipalities that were evacuated, in the range ofabout 1 to 20 mGy for other municipalities in Fukushima Prefecture, in the range of about 0.6 to 6 mGyin neighbouring prefectures, and in the range of about 0.09 to 0.7 mGy for prefectures in the rest ofJapan. The average absorbed doses to the thyroid for children and adults are estimated to be about 80%
and 50%, respectively, of the doses to infants. The estimated absorbed dose to the thyroid over a lifetime
for infants at the time of the FDNPS accident is about twice the dose in the first year;(j) The greater amount and better quality of information now available has enabled the Committee
to make quantitative, and less subjective, estimates of the distributions of individual doses in eachmunicipality of Fukushima Prefecture, for the prefecture as a whole, and for other prefectures. The
distributions take account of uncertainties in the estimated doses and variability between individuals
resulting from where they live and work, their lifestyle, and other habits. The estimated doses havebeen presented in terms not only of the average, but also of the 5th and 95th percentiles of the
distribution. In general, 90% of the individuals in each population group were estimated to havereceived doses within a range from about three times lower than the average dose to about threetimes higher. Individuals within each population group are estimated to be more likely to havereceived effective doses lower than the average than higher than the average. The distribution ofindividual effective doses in the first year within Fukushima Prefecture, for example, shows thatnearly all individuals were estimated to receive an effective dose between about ten times lower andthree times higher than the average for the prefecture;ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

105(k) Compared with the estimates in the UNSCEAR 2013 Report, the Committee’s revised estimatesof average effective doses in the first year are considerably lower (by more than an order ofmagnitude) for municipalities or prefectures with lower doses and up to about 30% lower for thosewith higher doses. For absorbed doses to the thyroid, the Committee’s revised estimates of average
doses in the first year are similarly much lower for municipalities or prefectures with lower doses
and up to about two times lower for those with higher doses. The reductions in the doses are
attributable to: (i) more realistic assessment of intakes of radionuclides by ingestion; (ii) accountbeing taken of lower concentrations of radionuclides inside buildings; and (iii) the use of Japanspecific dose coefficients for the intake of radioiodine. These reductions have been counterbalancedby modest increases in doses from external exposure resulting from the use of an improved andvalidated model. The use of a different source term and ATDM for estimating doses from theinhalation of radionuclides has led to increases in doses at some locations and decreases in others;(l) The Committee’s revised estimates of doses to the public and their associated uncertainties provide
a more realistic assessment of the exposure of the public resulting from the accident at FDNPS
compared with the UNSCEAR 2013 Report. While the uncertainties in the estimated doses remain
large, the Committee is of the view that further research is unlikely to reduce them significantly, orchange the central estimates, other than in specific circumstances (e.g., to take account of further
information on the efficacy of remediation). The measurements made in the environment and on people
in the immediate aftermath of the accident cannot be repeated, and estimates of doses using thesemeasurements cannot go beyond the limitations of this information, however much research is
conducted subsequently;(m) The Committee has taken account of the impact of remediation activities on its estimates ofdoses to evacuees if they return to their homes. It has not taken account of the impact of remediation
activities completed across wider areas of Japan, although the effect of the resulting overestimate ofdoses after the completion of this work is likely to be small in comparison with other uncertainties in
its dose estimates. Further information is needed about the effect of the remediation work in reducing
individual doses as measured on people to make more precise estimates of doses following remediation;(n) The average effective dose to the more than 20,000 emergency workers involved in mitigationand other activities at the FDNPS site from March 2011 to the end of March 2012 was about 13 mSv.
About 36% of the workforce received an effective dose of more than 10 mSv and about 0.8%(174 workers) have been assessed to have received more than 100 mSv over that period. Annual
effective doses have been considerably lower since April 2012, with average annual effective doses
declining from about 6 mSv in the year to the end of March 2013 to about 2.5 mSv in the year to theend of March 2020. No worker has received an annual effective dose of more than 50 mSv sinceApril 2013;(o) Absorbed doses to the thyroid of six emergency workers who received the highest doses haverecently been reassessed, in particular using individual-specific measurements of thyroid size. The
largest assessed committed absorbed dose to the thyroid due to internal exposure from inhalation of131
I is now 32 Gy. There is no evidence to suggest that using standard reference values for thyroid
mass or volume is not appropriate for the Japanese population in general; but the Committee has
noted that further individual-specific reassessments of thyroid dose are foreseen. For workers whose
individual absorbed dose to the thyroid is to be reassessed for the purposes of epidemiology or healthrisk assessment, measurement of thyroid volume, subject to ethical approval, would be beneficial aspart of the reassessment. Beta dose to the lens of the eye of workers was identified as a research needin the UNSCEAR 2013 Report but an assessment of these doses has yet to be reported;(p) Timely monitoring (e.g., WBC, thyroid measurements, personal dosimetry) of representative
groups of workers and the public at the earliest opportunity after an accident would greatly enhance

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the quality and informativeness of any assessment of doses to workers and the public following aradiological or nuclear accident. If appropriate human measurements are not made in the immediate
aftermath of an accident, doses to people can only be assessed using models together with othermeasurements that may be available, for example, in the facility where the accident occurred and/orin the wider environment. Experience has shown that the use of such models often leads toconservative estimates of doses;(q) No adverse health effects among Fukushima residents have been documented that are directly
attributable to radiation exposure from the FDNPS accident. The Committee’s revised estimates ofdose are such that future radiation-associated health effects are unlikely to be discernible. The
Committee believes that, on the balance of available evidence, the large increase, relative to that
expected, in the number of thyroid cancers detected among exposed children is not the result ofradiation exposure. Rather, they are the result of ultrasensitive screening procedures that haverevealed the prevalence of thyroid abnormalities in the population not previously recognized. An
increase in the incidence of cancers is unlikely to be discernible in workers for leukaemia, total solidcancers or thyroid cancer. The Committee has insufficient information to reach an informed
judgement on the risk of cataracts;(r) Care is needed over the widespread use and interpretation of sensitive ultrasound thyroid
screening following radiation exposure as a result of events such as the FDNPS accident. There iscompelling evidence that sensitive ultrasound screening detects many more cases of abnormalities
and cancer in the thyroid than would be detected following the presentation of clinical symptoms.
The consequential over-diagnosis of thyroid cancers, many of which may never result in clinicalsymptoms, has the potential to cause considerable anxiety among some of those screened and to leadto unnecessary treatment, the detrimental effects of which may outweigh those of the radiation
exposure itself, especially if the thyroid doses are relatively low. If such screening is to be carriedout, perhaps including even further advances in thyroid assessment equipment and techniques, aswell as determining evidence of a dose-response, an unexposed control group needs to be studied at
the same time and using an equivalent protocol and technical equipment. These safeguards will
reduce the risk that cancers diagnosed following any future accident are attributed wrongly to
exposure to radiation;(s) The Committee continues to consider that regional impacts on wildlife populations with a clear
causal link to radiation exposure resulting from the FDNPS accident would have been unlikely,
although detrimental effects on individual organisms might have been possible, and some effects
have been observed in plants and animals in the absence of any wide-scale group impacts. A few
studies have indicated substantial population impacts on selected wildlife groups, but these findings
remain subject to some doubt. It is possible, however, that current assessment methodologies are notadequate and further analysis of impacts under field conditions could be valuable. Further research
could also be useful on the impacts of radiation exposure on non-human biota under field conditions
that are able to take account of higher levels of biological organization, within natural environments,and elements of ecosystem function and structure.

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ACKNOWLEDGEMENTS
The Committee acknowledges with gratitude the work of the experts directly involved in conducting theevaluation, all the support staff in their organizations who assisted them, as well the critical reviewers fortheir valuable independent expertise. Further, the Committee would like to commend the spirit ofcollaboration with the countries that contributed with data which greatly enhanced the outcome of thisstudy. The views expressed in the annex remain those of the Committee and do not necessarily represent
the views of the United Nations or its Member States.

Coordination expert group
Project manager: N. Kelly (United Kingdom); Senior technical advisors: W. Weiss (Germany) andM. Akashi (Japan).

Expert group
Coordinating lead writer: N. Kelly (United Kingdom); Members and lead writers: S. Solomon(Australia), C. Estournel (France), F. Gering (Germany), P. Strand (Norway), M. Balonov (RussianFederation), G. Etherington (United Kingdom), B. Howard (United Kingdom) and R. Shore(United States).

Task group public exposure
Leader: M. Balonov (Russian Federation); Members: D. Broggio (France), L. Chipiga (RussianFederation), V. Berkovskyy (Ukraine) and V. Drozdovitch (United States); Observers: S. Kinase * (Japan)and A. Ulanowski (IAEA).

Task group atmospheric dispersion
Leader: F. Gering (Germany); Members: A. Mathieu (France), D. Quelo (France), T. Aono (Japan),
M. Chino (Japan), Y. Moriguchi (Japan), H. Nagai (Japan), P. Bedwell (United Kingdom) andS. Leadbetter (United Kingdom).

Japanese Working Group
Members: K. Akahane (Japan), M. Akashi (Japan), T. Aono (Japan), M. Chino (Japan) and K. Ozasa(Japan).

Critical reviewers
G. Hirth (Australia), H. Vandenhove (Belgium), J. Chen (Canada), S. Charmasson (France), J-R. Jourdain(France), R. Michel (Germany), S. Shinkarev (Russian Federation), A. Wojcik (Sweden), J. Brown **(United Kingdom), R. Wakeford (United Kingdom), N. Harley (United States), B. Napier (United States)
and D. Pawel (United States).

* Until May 2019 (IAEA observer) and later (Japan).
** As of 2019 (IAEA observer).

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Other contributing experts
M. Cook (Australia), B. Orr (Australia), T. Hamburger (Germany), J. Brown (Norway), C. Robinson(Norway) and G. Ratia (Ukraine).

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APPENDIX A. ASSESSMENT OF DOSES TO THE PUBLIC

I. INTRODUCTION
A1.
This appendix summarizes the information available (as at the end of 2019) 1 of relevance to theassessment of the doses received by members of the public as a result of the Fukushima Daiichi Nuclear
Power Station (FDNPS) accident. It describes the analyses that have been carried out using thisinformation to develop improved methodologies for assessing doses to the public and presents the resultsof their application, i.e., updated estimates of doses.
A2.
In the UNSCEAR 2013 Report [U10], the Committee’s aim was to make realistic estimates ofdoses. However, information that has become available since the completion of that report demonstrated
that some of the doses presented in the report were overestimates, in a few cases by a considerable margin.
In particular, as identified in its subsequent reviews of information published since the UNSCEAR 2013
Report [U11, U13, U14], the Committee considered that doses to the public from ingestion were likely
to have been much lower than those estimated from the limited information then available in the
UNSCEAR 2013 Report. This possibility had even been recognized in the UNSCEAR 2013 Report itself
from the emerging information from measurements made directly on people. As the Committee
acknowledged in the UNSCEAR 2013 Report, the tendency to make cautious assumptions is “aninevitable consequence” when the information available is incomplete.
A3.
In addition, much more information has since become available to enable more quantitative andless subjective assessments to be made of the uncertainties and individual variability in the estimates ofdoses. For example, because of a lack of measurement information about concentrations of radionuclides
in the air, estimates of doses from inhalation of radionuclides in the air were largely based on
concentrations derived from an assumed source term for the release of radionuclides into the atmosphereand atmospheric transport, dispersion and deposition modelling (ATDM). At any particular location, such
estimates are associated with considerable uncertainty. In addition, the Committee had to make numerous
assumptions, for example, about the chemical forms in which iodine was released and was present in theenvironment. Only very broad and subjective estimates of some of the uncertainties were presented in
the UNSCEAR 2013 Report, and some may have been underestimates. Much more measurement
information is now available, particularly on concentrations of radiocaesium and radioiodine in the air,on the chemical form of iodine, on radionuclides consumed in foodstuffs, etc. These have enabled better
estimates of the release of radionuclides into the atmosphere and of how the released material wastransported, dispersed and deposited in the environment as a function of time, and of doses from theingestion of food. As a result, the Committee has been able to make not only more realistic estimates ofdoses, but also more quantitative and less subjective estimates of their uncertainties and the range ofvariability among individuals within the population.

1 The Committee has, exceptionally, taken account of information that became available after this date, where it would affect thefindings of the report.

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UNSCEAR 2020/2021 REPORTA4.
The Committee considers that sufficient new information is now available to support revised
estimates of doses to the public that avoid, to the extent practicable, the conservative assumptions madein its previous assessment and provide more quantitative and less subjective assessments of uncertainties
and variabilities. To the extent practicable, the Committee has aimed to base its revised dose estimateson measurements, firstly those made on people directly and secondly those made in the environment, andto make greater use of these measurements in validating its updated dose estimates. It has also placed
greater reliance on dose assessments made by other researchers and published in the peer-reviewed
literature, rather than developing its own models and making its own predictions. For ease of comparison
with the UNSCEAR 2013 Report [U10], dose estimates have been made for the same age groups(20-year-old adult, 10-year-old child and 1-year-old infant) and the same dosimetric endpoints (theabsorbed dose to selected organs – the thyroid, red bone marrow, colon and female breast – and theeffective dose). In addition, estimates have been made of the average absorbed doses to the fetal thyroid
over the 30-week development period of the fetus and of the average absorbed dose in utero to the redbone marrow over the 40-week term of pregnancy. Estimates have also been made of doses in the firstyear after the accident, over the first 10 years and until an attained age of exposed individuals of 80 years,
and of annual doses in the year 2021.

A. Exposure pathways
A5.
For the radioactive material released to the atmosphere, the principal pathways by which
members of the public can be exposed are:(a) External exposure to radionuclides in the air;(b) External exposure to radionuclides deposited from the air on to the ground surface by either wetor dry deposition;(c) Internal exposure from inhalation of radionuclides in the air;(d) Internal exposure from ingestion of radionuclides in food and drinking water.
These were the pathways considered in the UNSCEAR 2013 Report [U10] and have again been
considered in the dose assessment reported here. Members of the public could also have been exposed toradioactive material released to the marine environment, through ingestion of fish and other seafood andexposure to radionuclides in the ocean or on sediments. The former exposure pathway has been includedin the estimation of internal exposure from ingestion of radionuclides in food; the latter pathway has notbeen included because it was not expected to be a significant contributor in view of the inaccessibility ofthe ocean and sediments within the 20-km evacuation zone established around the FDNPS site.

B. Overview of dose assessment in UNSCEAR 2013 Report
A6.
In general, measurements made on people, either of exposures or of radionuclides in the body,provide a more direct source of information for estimating doses. At the time of the UNSCEAR 2013
Report [U10], information on such measurements covered only a limited number of people and locations
and was insufficient to make comprehensive estimates of doses to the public and was only used as one
means of checking the validity of the Committee’s dose assessment.

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111

A7.
The dose assessment in the UNSCEAR 2013 Report [U10] was accordingly largely based on
measurements of radioactive material in the environment, combined with models describing how people
were exposed to this material. Data were available regarding the radiation levels and deposition densities
of radioactive material in every prefecture in Japan, concentrations in foodstuffs, and, to a more limited
extent, public exposure. Many of these data were provided by official government agencies in Japan; andmany were published in peer-reviewed scientific journals. Twenty-five Member States of the United
Nations other than Japan officially provided information in response to the Committee’s request for data
to support its assessment. Additional data were made available by other international organizations andthe Committee also considered data made available by several non-governmental organizations. All data
were evaluated to determine their suitability for the assessment. Most of the scientific information usedin the UNSCEAR 2013 Report was limited to that published or disclosed by the end of October 2012.
A8.

In the UNSCEAR 2013 Report [U10], models were used, inter alia, for estimating:(a) The levels of radionuclides deposited on the ground where insufficient measurement
information was available (generally only in the evacuated areas during the periods of theevacuations, where time dependent information is needed), where the Committee’s chosen source
term was used with ATDM;(b) Concentrations of radionuclides in the air, where the source term and ATDM were used, either
directly for the period of evacuation and for radionuclides which were not deposited on the ground,or to derive air concentrations from measured deposition densities;(c) The variation, as a function of time, of the external dose rate from deposited radionuclides;(d) Levels of radionuclides in food beyond the first year after the accident, for which measurements
were not available, where the Committee used a modified version of the FARMLAND model [B12]for estimating the transfer of radionuclides through terrestrial food chains.

A9.
The Committee took account of the measures implemented by the Japanese authorities to protect
the public, including the precautionary evacuation of approximately 78,000 residents within the 20-km
area around the FDNPS site and some nearby areas, which took place between 11 and 15 March 2011,
and the deliberate evacuation, based on environmental measurements, of about 10,000 residents of several
municipalities to the north-west of the FDNPS site, which took place between March and June 2011.
About a further 30,000 people were also included among those evacuated: these included some of thepeople living within the “evacuation prepared area” between 20 km and 30 km from the FDNPS site,
who were advised to begin voluntary evacuation on 25 March 2011, as well as some living outside the
30-km radius. In addition, foodstuffs and drinking water containing more than prescribed concentrationsof radioactive material were prohibited from sale. The Committee did not take account of other protective
measures including directives to shelter in place during the main releases and to take stable iodine, assufficiently precise information had not been available. Nor was the Committee able to take account of
the large land remediation programmes that were being implemented in some of the more affected partsof Fukushima Prefecture due to the lack of detailed information at the time of the UNSCEAR 2013 Report[U10] about the scale and efficacy of the implemented remediation actions.
A10.
The Committee’s updated estimates of public doses have been compared with the estimates
made in the UNSCEAR 2013 Report [U10] in section IV.H of this appendix.

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UNSCEAR 2020/2021 REPORT

II. INFORMATION FOR DOSE ASSESSMENT
A11.
The Committee’s revised estimates of doses to the public from internal and external exposure
described in this appendix make use of information collected for the UNSCEAR 2013 Report [U10],
together with new information that has become available since (up to the end of December 2019). 2 This
includes measurement data on people 3 (in particular, personal dosimeters, whole-body counting (WBC)and thyroid measurements), and new measurements of concentrations of radionuclides in the air during
the release and on the ground, as well as new information on radionuclides in foodstuff as consumed,
occupancy factors, dose reduction factors (location factors) and information on protective measures,
including evacuation scenarios.
A12.
Chapters II, III and IV have provided summaries of the information now available on releases
to the atmosphere and to the ocean, on the distribution of the released radioactive material in the air, on
the ground and in the sea and on transfers through the terrestrial, freshwater and marine environments tofoodstuffs. More measurement-based information is available on the levels of released radionuclides
deposited on to the ground surface and on concentrations of radionuclides in air, more information isavailable on the transfer of radionuclides through the terrestrial environment in Japanese conditions, andmore measurements are available, over a longer period of time, on the levels of radionuclides in foods.

A. Information from measurements made on people within
Japan
1.

Measurements of individual doses from external exposure

A13.
Numerous measurement campaigns have been carried out to assess individual doses fromexternal exposure through surveys of daily activity patterns of residents, measurements of ambient dose
rates and individual measurements using personal dosimeters. The most significant of such measurement
campaigns (some thousands of measurements per year) were carried out by municipalities, including, forexample, Minamisoma City, Date City, Fukushima City, Naraha Town and some others [F18, M31]. The
results of these large-scale measurement campaigns have been made widely available to the public andprofessionals. In addition, the municipalities of Minamisoma and Naraha provided the Committee withanonymized data on personal external doses measured between 2014 and 2019. The large-scalemeasurements of individual doses [F18, M31, N13, N15, T40, T41] were usually carried out using glass
dosimeter badges worn for a period of months, which were subsequently read in laboratories. In smallerscale scientific studies, either electronic dosimeters with silicon detectors, that provided data on both
personal doses and dose rates, or optically stimulated luminescence dosimeters made from aluminium
oxide and worn on a strap around the neck were used [H4]. Annual natural background radiation dosesof between 0.54 and 0.63 mSv have been subtracted from the measurements as appropriate.

2 New information that has become available after December 2019 has been taken into account exceptionally, where it may have asignificant impact on the estimates of doses.
3 These measurement data are described further in section II.A and are examined in detail in attachments A-1, A-2 and A-3. The
measurement data can be described in terms of log-normal distributions, giving the Committee confidence in the quality of thesedata (see the International Atomic Energy Agency (IAEA) report [I5] for a more detailed discussion of the useful role played byanalysis of measurement data in terms of log-normal distributions).

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113

A14.
Several authors (for example [T40, T41]) have analysed sample data from the large-scalemeasurement campaigns to study the patterns of external exposure of Japanese people of different ages
and different occupations and to test dosimetric models.
A15.
Other authors (for example [H4, N13, N15, T10, T42]) carried out measurements of theindividual doses on smaller numbers of participants (some tens to some hundreds of people), often
accompanied by behaviour surveys and measurements of dose rates in air, with the aim of developing
dosimetric models.
A16.
The Committee has made use of some of these data and other scientific results published in peerreviewed journals to validate its estimates of doses from external exposure and in the development of arevised model (M2020) to apply to the wider population. Further details can be found in attachment A-1.

2.

Thyroid measurements

A17.
Thyroid monitoring to measure directly the 131I content in the thyroid of members of the public
provides the most direct source of information on thyroid doses from internal exposure, althoughestimating doses to the thyroid from measurements requires some assumptions and modelling and issubject to both measurement and modelling uncertainties. Thyroid monitoring began in March–April
2011, on evacuees, permanent residents of Fukushima Prefecture, and residents of Tokyo Metropolitan
Area, covering in total more than 1,500 persons of various ages and both sexes. The studies and theirresults are summarized in table A1. In general, personal interviews were not carried out with those on
whom the measurements were made, so assumptions were needed about where those monitored resided,
about their consumption of foodstuffs and whether they had been administered stable iodine.
Table A1. Summary of thyroid monitoring studies

Reference[H30, K14,
K17]

[T34]

Measurement
dates

Location

Number ofpeoplemonitored

Method26–30 March2011

Iwaki City,
Kawamata
Town, Iitate
Village

1 080 (children
aged 1–15)

Hand-held
NaI(TI)
scintillator
survey12–16 April 2011

Evacuees from
Namie Town
(Tsushima),
coastal area of
MinamisomaCity

62 (aged
0–83 years)

NaI(TI)
scintillation
spectrometer

Results
598 individuals withmeasurements ator below measured
background;
858 individuals
with net exposure
rate ≤0.1 µSv/h

I detected in
46 people

131

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UNSCEAR 2020/2021 REPORT

Reference

Measurement
dates

Location

Number ofpeoplemonitored

MethodResults

Whole-bodycountercalibratedusing bottle
manikinabsorption
phantom

I detected in
55 people (withminimum
detection level(MDL) 30 Bq); andin further
49 people incontinued
measurements

Spectrometricmeasurements

3 adults exceeded
MDL of 100 Bq of
131
I, with thehighestmeasurement
being 130 Bq. Each
person wasinterviewed at thetime ofmeasurement

131

[M11,
M45]

[U20]

15 March–
April 2011

8–20 April 2011

Evacuees andshort-term
visitors to
Fukushima

173

Tokyo andsuburbs

268 Russian
embassy staff,
members of
their families,
and other
Russiancitizens (mainly
adults with afew children)

A18.
The Committee has used the results of most of these thyroid measurements to validate its
estimates of doses to the thyroid from internal exposure. Further details of the methods used and theestimates of doses derived from the thyroid measurement information can be found in sections III.D and
IV.G below and in attachments A-2 and A-4.

3.

Whole-body monitoring data

A19.
Whole-body monitoring campaigns have been carried out by national institutes, such as theJapan Atomic Energy Agency (JAEA) and the National Institute of Radiological Sciences (NIRS), andby universities, hospitals and municipalities. In most cases, subjects have been monitored with fixedwhole-body counters equipped with large NaI(Tl) detectors scanning the body for a few minutes, therecorded spectrum enabled either to measure the radiocaesium body content through proper calibration,or to put an upper limit on the content in the case of measurements below MDL.
A20.
The results of these campaigns that are publicly available are summarized in table A2. The largescale measurements began in July 2011 for evacuees and in October 2011 for residents, i.e., four andseven months after the accident, respectively, and long after the first intakes may have occurred. The
measured levels of radiocaesium in the body can be used to estimate doses from its intake by inhalationand ingestion; but such estimates depend on knowledge or assumptions about the temporal pattern ofintake before the measurements were made and the rate at which radiocaesium is excreted from the body.
Further information about the data and the estimates of doses from internal exposure due to intakes ofradiocaesium that can be derived from the whole-body monitoring data is set out in attachment A-3.
A21.
There have also been whole-body measurement campaigns carried out where the results havenot been made publicly available. For example, the Fukushima prefectural government began carrying
out whole-body measurements on residents in June 2011 and had screened more than 100,000 residents
by 2013 [H9]. The Committee has not had access to information from these campaigns.

Table A2. Summary of whole-body monitoring campaigns carried out following the Fukushima Daiichi Nuclear Power Station accident[M11, M45]

Measurement
programme

Measurement
dates

Target population

March–April 2011

Adult evacuees andfirst responders

WBC

Evacuees from
11 municipalities
of Fukushima
Prefecture

JAEA fixed WBC in
Tokai-Mura
JAEA mobile
units inFukushima
Prefecture

Age group

AdultsNumber ofpeoplemonitored
173

Findings– 39% of measurements of 134Cs above MDL (33 Bq)– 32% of measurements of 137Cs above MDL (33 Bq)– 80% of measurements below MDL = 300 to 370 Bq[M43]

11 July 2011–31 January 2012

– 137Cs/134Cs ratio is between 1.12 and 1.26
Adults3 128Children(<17 years)

6 799– No correlation between committed effective dose ofchildren and parents– Maximum body burden: 2.7 kBq (<8-years old),
14 kBq (adults)– Dose calculation based on acute inhalation– Detection rate: 47% (134Cs, MDL = 320 Bq),
26% (137Cs, MDL = 570 Bq)– 137Cs/134Cs = 1.1[K16]

27 June–28 July 2011

Evacuees fromvariousmunicipalities ofFukushima
Prefecture

NIRS fixed WBC

Adults125Children(<15 years)

49– Committed effective dose based on acute intake on
15 March 2011
– Data compare well with JAEA estimates– 134Cs body burden adult: ~550 Bq/body (mean),
300 Bq/body (median), 7 kBq/body (max)– Committed effective dose based on the mostconservative scenario– No significant difference in committed effective dose
for adults and children

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Reference

115

[H9]

11 July–29 July
2011

October 2011–
February 2012

Target population

Measurement
programme

Age group

Evacuees from
Minamisoma City
who hadsubsequently
returnedAnzai chair-type
WBC

Adults(≥16 years)

Residents ofFukushima
Prefecture (73%),
Ibaraki Prefecture(23%)

Number ofpeoplemonitored
566

Findings– Complex and indirect corrections had to be applied tothe measurements for shielding by the body
– Average body burden of 134Cs in July 2011 was
825 ± 360 ± 110 Bq– Some measurements might have been affected byexternal contamination

Fixed WBC at
Hirata Central
Hospital

Adults

4 716

Children(<15 years)

6 310– 137Cs detection rate maximum in November–
December 2011 (15% for all population), thendecreases below 5% after February 2012(MDL = 300 Bq)– 137Cs body burden, whole population: ~12 Bq/kg(mean), ~9.5 Bq/kg (median), 77 kBq/kg (max)– Children detection rate = 16.4%, median bodycontent = 590 Bq (for positive measurements)[T38]

October 2011–
March 2012

Residents of
Minamisoma City

Fixed WBC at
Minamisoma
Municipal
General Hospital

Adults

8 066

Children(<15 years)

1 432– Adults detection rate = 38%, median bodycontent = 744 Bq (134Cs and137Cs, for positive
measurements)– MDL = 210 Bq for 134Cs, 250 Bq for 137Cs in adults– Difference in exposure between adult and children is
statistically significant

UNSCEAR 2020/2021 REPORT[H10]Measurement
dates

116

ReferenceReference[T38]

October 2011–
March 2012

April 2012–
March 2013

Target population

Residents of
Minamisoma City

Residents of
Minamisoma City

Measurement
programme

Fixed WBC at
Minamisoma
Municipal
General Hospital

Fixed WBCs at
Minamisoma
Municipal
General Hospitaland Hirata
Central HospitalAge group

Number ofpeoplemonitored

Findings– 134Cs or 137Cs detected in 40% of adults (MDL = 210 Bq
for 134Cs, 250 Bq for 137Cs) and 9% of children

Adults7 214

Children(<15 years)

1 067

Median age
14 years
Age range
2–97 years– For positive measurement of 134Cs and 137Cs medianbody burden was 11 Bq/kg for adults and 8.5 Bq/kg
for children– Contains an analysis of risk factors (food consumption
and habits) related to Cs detection, not detailed here
– 137Cs detected in 612 participants (6.1%) from
Minamisoma Hospital (MDL = 210 Bq for 134Cs,
250 Bq for 137Cs) and 144 participants (0.7%) from
Hirata Hospital (MDL = 300 Bq for 134Cs and 137Cs)

30 622– >50 Bq/kg 137Cs detected in 9 participants– Median 137Cs content in body of these 9 participants
4 830 Bq (69.6 Bq/kg); range 2 130 to 15 918 Bq(50.7 to 216.3 Bq/kg)– All 9 participants consumed homegrown produce andwild mushrooms

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[S40]

Measurement
dates

117

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UNSCEAR 2020/2021 REPORT

B. Environmental monitoring data
A22.
There have been extensive measurements of the levels of radioactive material in the environment
across Japan following the FDNPS accident. JAEA has collected environmental monitoring data
published by various organizations related to the accident, including dose rate in air (from airborne anddeposited radionuclides), and radionuclide concentrations (ground surface, soil, seawater, marine
sediments, river water, river sediment, groundwater, and food). These data sets form the Environment
Monitoring Database (EMDB) which is available to the public through an online website [J3].
A23.
A total of 62 data sets related to the measurement of dose rate in air, radionuclide grounddeposition density, and radionuclide concentrations in air and in food and drinking water were
downloaded from EMDB by the Committee over the period between August and October 2018. These
data sets reported the results of over 200 radiation surveys carried out in Japan between March 2011 andMarch 2018 by government bodies and other organizations. Only a small subset of these EMDB data sets
were used for this dose reassessment. These are listed in table A3.
A24.
The Committee systematically catalogued these downloaded EMDB data sets, with the files
renamed to reflect the source and type of data in the data files. The full collection comprised over 16 GBof measurement data and over a thousand files. A full list of the downloaded EMDB data sets is provided
in attachment A-5. The Committee undertook a process of quality assurance, including extensive checks
to determine whether the measurements were scientifically sound and fit for purpose, consistent with theprocesses undertaken in the preparation of the UNSCEAR 2013 Report [U10].
A25.
The downloaded EMDB data sets and any derived data sets were categorized into three data
set types: “original”, “primary” and “derived”, reflecting the nature of the data analysis and the quality
assurance processes. Original data sets were as downloaded from a trusted website at a specific timeor provided by a trusted source as per an official request or similar. Aside from a possible renaming of
the file, the content in the original downloaded data set files was un-modified. Primary data sets were
produced from the original data sets by adding additional quality assurance, metadata and workflow
worksheets to the spreadsheet. Finally, derived data sets were produced from one or more primary data
sets, through a combination of information at the same location or the addition of additional
information such as demographic data. These were subjected to independent checking and testing
before use by the Committee.
A26.
Consistent with the approach in the UNSCEAR 2013 Report [U10], the recorded latitude andlongitude of the sampling locations for the EMDB radionuclide ground deposition data sets were used toallocate the updated deposition measurements to the corresponding cell in the Japan 1 km grid to allow
combination with the population data. The Japanese Government has divided Japan into a grid for the
purposes of reporting relevant geospatial information, including population data. The dimensions of thesegrid cells were approximately 10 by 10 km. A third-order grid was obtained by equally dividing thesecond-order grid cells into further 10 by 10 cells. The horizontal and vertical distances of the third-order
grid cells were approximately 1 by 1 km.
A27.
Two data sets, in particular, the 2,200-location soil deposition data set and the data set with 131I
deposition densities derived from analysis of measurements of 129I [M50] formed the basis for theassessment of external exposure from deposition in municipalities within Fukushima Prefecture, and thederived tables are provided in attachments A-6 and A-7. The two data sets were combined with otherpopulation and demographic data to produce a derived data set provided in attachment A-8. In the data

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

119

set used for the UNSCEAR 2013 Report [U10], 131I was detected at approximately 400 locations out of
the more than 2,200 locations at which soil was sampled; the updated data set in attachment A-8 now
includes 131I deposition densities at approximately 800 locations.
Table A3. Japan Atomic Energy Agency Environment Monitoring Database data sets used in the
update of doses to the public
Data set title

Website data set description

Organization

Period covered

Results of theradionuclide
analysis of soil
sampling at
2 200 locations inFukushima
Prefecture andneighbouring
prefectures

Ambient dose equivalent rates were
measured, and soil samples collectedfrom 6 June to 8 July 2011 for areas
within 100 km from FDNPS. From FDNPS
out to 80 km these were undertaken on
2 km × 2 km grids. Between 80 km and100 km from FDNPS they were divided
into 10 km × 10 km grids. Ambient dose
equivalent rates were measured at aheight of 1 m above the ground surface
at one location in each of these divided
grids (nearly 2 200 locations in total), andsoil samples were collected at five points
at each location. Around 11 000 soil
samples were taken; deposition densities
of five gamma-emitting radionuclides(110mAg, 129mTe, 131I, 134Cs and 137Cs) were
measured using Germanium
Semiconductor Detectors. JAEA
conducted the survey with cooperationof various universities and research
institutes. Ministry of Education, Culture,
Sports, Science and Technology (MEXT)
was responsible for the coordination of
the measurement data and assessment ofvalidity [S1]MEXT

Decay correction:
14 June 2011

Radioactivity
concentration
analysis of iodine in
the distribution
survey of radioactive
substancesThis series of data was created to derive
deposition density of 131I using thedeposition density of 129I. This data set
was prepared from published results of129
I deposition densities obtained in thesoil samples where 131I was measured in
the first distribution survey. A part of thesoil sample was used to measure 127I
(stable iodine) by Inductively Coupled
Plasma Mass Spectrometry. The ratio of
129
I atomicity to 127I atomicity wasmeasured by accelerator mass
spectrometry. The concentration (Bq/kg)
of 129I in the soil sample was derived using
concentration of 127I and the (129I)/(127I)
ratio. The 129I deposition density (Bq/m2)
was derived from the mass and volume of
the relevant soil sample [M17, M49]

MEXT, and theSecretariat of the
Nuclear
Regulation
Authority

Fiscal year 2011–
Fiscal year 2013

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UNSCEAR 2020/2021 REPORT

Table A4. Other environmental monitoring data sets used in the assessment of doses to the public
Data set title

Measuredconcentrations in
soils

Website data set description

Organization

Period covered

These data were officially provided by theGovernment of Japan for the UNSCEAR
2013 Report [U10]. There are
3 422measurements of 134Cs and 137Cs in
cultivated soils in 15 prefectures ineastern Japan (Fukushima, Iwate, Miyagi,
Yamagata, Ibaraki, Tochigi, Gunma,
Saitama, Chiba, Tokyo, Kanagawa,Niigata, Yamanashi, Nagano and
Shizuoka), based on samples collectedfrom 26 April 2011 to 3 February 2012.
The cultivated soil activities were
adjusted to a reporting date of 5
November 2011. The original data sets
were supplied as pdf files, data wereconverted and cross-checked to produce
data in spreadsheet format. The
Committee reviewed this data set andconsidered it acceptable and fit for
purposeJapanese
Ministry ofAgriculture,
Forestry and
Fisheries

26 April 2011 to
3 February 2012
Decay corrected –5 November 2011

A28.
In the UNSCEAR 2013 Report [U10], the Committee’s focus was on estimating exposures ofindividuals considered to be representative of different subsets of the Japanese population. Dose estimates
were presented as population-weighted averages for different geographical areas, either municipalities,
or prefectures, depending broadly on distance from FDNPS. In the updated data sets, the spatial coveragein prefectures neighbouring and nearby Fukushima Prefecture was different and this has led to somechanges in the groupings of prefectures compared with the UNSCEAR 2013 Report.
A29.
Measurements of concentrations of radionuclides in air over Japan while the release was
happening were rather limited, in particular, in the early stages of the accident and in the areas devastatedby the tsunami. New data on the concentrations of radionuclides in airborne aerosols have become
available for seven locations within the Japanese mainland for each hour of the period from 12 March to
11 May 2011 [D7]. The data were derived from measurements on samples collected by the United States
Department of Defense (DoD) and the United States Department of Energy (DOE) at or near locations
where DoD-affiliated individuals worked or lived. Further information about the levels of radionuclides
in the air and deposited on the ground has come from reanalysis of monitoring data collected at the timeof the accident by the application of some novel analysis methods. Specifically, concentrations ofdifferent radionuclides in the air in the early stage of the FDNPS accident have been estimated at several
monitoring posts in Fukushima Prefecture from gamma spectrometry using sodium iodide scintillation
detectors [H17, M44, T29]. In addition, concentrations of 137Cs and 129I (from which levels of 131I can beinferred) in air at ground-level in the Fukushima and Kanto areas are being derived from an analysis offilter-tapes of air pollution stations (for monitoring suspended particulate matter) [E1, O25, T50, T51],
although it should be noted that only iodine in particulate form will have been collected on the filtertapes. The latest version of the data set produced by this work, which includes information on 137Cs levels
for 101 locations and on 131I for 4 locations, has provided an important input into the detailed analyses
described in this appendix. These new sources of information on concentrations of radionuclides in air
are summarized in table A5.

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121

Table A5. Summary of measurements of concentrations of radionuclides in air over Japan
Reference

[H16]

[H17]

[H18]

Measurement type

LaBr3 scintillation
detector

NaI(Tl) scintillation
spectrometer in
automatedmonitoring station

NaI(Tl) scintillation
spectrometer in
automatedmonitoring station

Location
Fukushima Prefecture, 6 measurement
points (Abukuma-kogen service area,
Adatara service area, Funehiki-Miharu
interchange, Koriyama-higasi
interchange, Miharu parking area,
Motomiya interchange)

Radionuclides

132

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

Fukushima Prefecture, 9 monitoring
stations (Fukushima City Momijiyama,
Futaba Town Koriyama, Futaba Town
Yamada, Hirono Town Futatsunuma,
Naraha Town Matsudate, Okuma Town
Minamidai, Okuma Town Mukaihata,
Okuma Town Oono, Okuma Town
Ottozawa)
Fukushima Prefecture, 8 monitoring
stations (Fukushima City Momijiyama,
Futaba Town Yamada, Hirono Town
Futatsunuma, Naraha Town Matsudate,Okuma Town Minamidai, Okuma Town
Mukaihata, Okuma Town Oono, Okuma
Town Ottozawa)

131

132

Te, 131I, 132I, 133I, 135Xe,
133
Xe, 135mXe[K12]

NaI(Tl) scintillation
spectrometer in
automatedmonitoring station

Fukushima Prefecture, 8 monitoring
stations (Fukushima City Momijiyama,
Futaba Town Yamada, Hirono Town
Futatsunuma, Naraha Town Matsudate,Okuma Town Minamidai, Okuma Town
Mukaihata, Okuma Town Oono, Okuma
Town Ottozawa)[T29]

NaI(Tl) scintillation
spectrometer in
automatedmonitoring station

Ibaraki Prefecture, 6 monitoring stations(Ajigaura, Araji, Ishikawa, Muramatsu,
Ohnuki, Sugaya)[M44]

NaI(Tl) scintillation
spectrometer in
automatedmonitoring station

Ibaraki Prefecture, 21 monitoring
stations (Ajigaura, Araji, Ebisawa,
Funaishikawa, Hiroura, Hitachinaka,
Horiguchi, Ishigami, Ishikawa, Isobe,
Kadobe, Kuji, Mawatari, Muramatsu,
Ohba, Ohnuki, Oshinobe, Sugaya,
Toyooka, Tsukuriya, Yokobori)

133

[F17]

Gammaspectrometricmeasurement withGermanium
detectors

Fukushima Prefecture, 14 monitoring
stations (very few measurements each)

132

[T51]

Filter samples

Fukushima Prefecture, 2 suspended
particulate matter monitoring stations(Futaba Town, Naraha Town)

131

133

I

I

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

Xe, 132Te, 131I, 132I, 133I

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

137

Cs, 134Cs

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Reference

Measurement type

LocationRadionuclides[O25]

Filter samples
measured withGermanium
detectors

Several prefectures, 99 suspended
particulate matter monitoring stations

137

[E1]

Filter samples
analysed withaccelerator mass
spectrometry

Tokyo Metropolitan Area, 4 monitoring
stations (Institute of Physical andChemical Research, Kakinoki-zaka,
Tokyo Metropolitan Industrial
Technology Research Institute, Tokyo
Metropolitan University)

129

[D7]

Filter samples
analysed withgammaspectrometry
systems

4 monitoring stations (Ishinomaki (Cityof Ishinomaki), Misawa Air Base, Sendai(Sendai Airport), Yokota Air Base)[A10]

Filter samples
analysed withgammaspectrometry
systems

Chiba Metropolitan Area, 1 monitoring
station (JCAC)[D4]

Filter samples
analysed withgammaspectrometry
systems

Tsukuba, 1 monitoring station (NIES)[J2]

Filter samples
analysed withgammaspectrometry
systems

Filter samples, 3 monitoring stations

140

134

Cs, 134Cs

I, 131I, 137Cs

Ba, 134Cs, 136Cs, 137Cs,
131 133 140
I, I, La, 86Rb,
99
Mo, 99mTc, 129Te,
129m
Te, 131mTe, 132Te

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

131
132

131

I, 133I

Te, 134Cs, 136Cs, 137Cs,
129m
Te, 99Mo

I, 133I, 132Te, 134Cs, 136Cs,
137
Cs, 129mTe

C. Monitoring of food and drinking water
A30.
In the UNSCEAR 2013 Report [U10], the Committee estimated doses from ingestion using theFood and Agriculture Organization/IAEA food database, which included measurement data for over
500 types of foodstuffs sampled in all 47 prefectures in Japan, and data on concentrations of radionuclides
in drinking water provided by the Japanese Ministry of Health, Labour and Welfare. The foodmeasurement data used were for foodstuffs as marketed and information about how much of particular
foodstuffs were consumed per capita of the population was based on surveys carried out in Japan.
A31.
In addition to the food and drinking water monitoring data in the JAEA EMDB data sets listed
in attachment A-5, new sources of information available that are relevant to estimating the dose fromingestion include measurements of the radiocaesium content in the whole daily diet sampled by the
duplicate-diet or market-basket methods. These methods generally take account of intakes from both foodand drinking water. In the available literature, six publications contained data of this type collected in
2011–2013 in Fukushima Prefecture, four neighbouring prefectures (Ibaraki, Iwate, Miyagi and Tochigi)
and eight distant prefectures (Hokkaido, Kanagawa, Kochi, Nagasaki, Niigata, Osaka, Saitama andTokyo) located in various parts of Japan. These studies are summarized in table A6.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

123

Table A6. Summary of sources of information for estimating intakes of radiocaesium in the daily diet
Reference[K33]

Measurement
dates

July 2011

Location

Average daily intakes of eachof 134Cs and 137Cs: 3 Bq/d(Soma City) to 0.4 Bq/d(Fukushima City)

Market-basket(grocerystores)14 foodsamplegroups in
three
study
areas

Market-basket(local foodmarkets)

Daily intake rate of 137Cs
estimated at 0.4 Bq/d
(Tokyo), 3 Bq/d (Miyagi
Prefecture), 3.3 Bq/d(Fukushima City)

200

Duplicate-diet(100 families)

Mean concentration in daily
meals of 134Cs
0.27 ± 0.45 Bq/kg, and of
137Cs 0.37 ± 0.61 Bq/kg

Duplicate-diet

Mean dietary intake of each of
134Cs and 137Cs:
4.5 ± 2.6 Bq/d (Fukushima
Prefecture)

Duplicate-diet

Average daily intake of 134Cs
and 137Cs combined:
1.1 Bq/d (Kawauchi Village)
to 3.5 Bq/d (Tamano area,
Soma City)

September 2011

[S12]

November 2011–
March 2012 andJune–
September
2012

Kenpoku andKenchu areasof Fukushima
Prefecture

4 December 2011

Fukushima
Prefecture andneighbouring
prefectures

August 2012

Three
municipalities(Kawauchi
Village, Soma
City andMinamisoma
City) of
Fukushima
Prefecture

Winter 2012,
autumn 2012,
winter 2013

Three regions(Hamadori,
Nakadori andAizu) of
Fukushima
Prefecture, fourneighbouring
prefecturesand eight
distant
prefectures[U2]

Results

10–25 daily
food sets
in each
city

[T53]

[H4]

MethodIwaki, Soma,
Nihonmatsu
and Fukushima
citiesTokyo, Miyagi
Prefecture,
Fukushima City

[H3]

Numberof samples

53

125

939

Market-basket
and duplicatediet

Average daily intake for
Fukushima Prefecture(55 samples) 1.1 ± 1.5 Bq/dDaily intake of 134Cs and 137Cs
combined (market-basket
method): 0.17–1.7 Bq/d(winter, 2012), 0.15–
0.68 Bq/d (autumn, 2012),
0.15–1.3 Bq/d (winter, 2013)
Intake rate was higher inFukushima Prefecture andneighbouring prefectures
than in distant areas anddecreased with time in most
areas

A32.
Intakes from food and drinking water have been assessed in a number of studies by Japanese
researchers [H15, K10, M34, M47]. Murakami and Oki [M47] made estimates of the doses from ingestionof food and drinking water containing 131I, 134Cs and 137Cs, based on food monitoring data. They useddata from a wider range of sources than was available to the Committee when preparing the UNSCEAR
2013 Report [U10] and took account of the regional trade in foods (specifically, of the shares of foods

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UNSCEAR 2020/2021 REPORT

from different parts of Japan arriving at food markets, where most Japanese citizens purchase food). The
studies by Hirakawa et al., Kawai et al. and Miyatake et al. [H15, K10, M34] have focused on the early
period after the FDNPS accident. They have provided evidence based on surveys that, because of theafter-effects of the earthquake and tsunami and the resulting collapse in supply chains, very little, if any,
food potentially containing elevated levels of radioactive material released as a result of the accident was
consumed, particularly in the municipalities that were evacuated, even before food restrictions were put
in place. Most of the food that evacuees consumed immediately after the accident was sourced from either
stockpiles prepared before the accident or relief supplies from outside the affected area. As a result, themajority of intakes from ingestion in this early period, certainly for evacuees, would have been fromdrinking water. Measurements of radionuclides in drinking water began shortly after the accident andwere used in the UNSCEAR 2013 Report to estimate doses from ingestion of drinking water. However,measurements of the shorter-lived radionuclides, specifically 131I, were relatively limited and estimatesof doses from ingestion of 131I in tap water have therefore generally made use of models, complemented
by measurements where available.

D. Radiation measurements outside of Japan
A33.
In the UNSCEAR 2013 Report [U10], the assessment of doses to the public in countries
neighbouring Japan and in the rest of the world was based on a review of estimates published in theliterature, supported by the extensive measurements and dose assessments carried out by Member States.
The Committee has made a further review of the literature published since the UNSCEAR 2013 Report.
These are summarized in section IV.E of this appendix together with the Committee’s latest assessmentof doses in countries neighbouring, or in close proximity to, Japan.

III. METHODOLOGY FOR ASSESSING DOSES TO THE PUBLIC
A34.
There is considerably more good quality measurement information available than was available
to the Committee at the time of preparation of the UNSCEAR 2013 Report [U10], including
measurements made on people, in the environment and in foodstuffs as consumed by members of the
public. As well as this additional measurement information, Japanese researchers have carried out many
detailed assessments of doses and published the results in the literature. The Committee has made use of
all of this information in its revised assessment of doses to members of the public. Where possible, theCommittee has placed greater reliance on the results of studies published in the literature rather thancarrying out its own modelling. However, it still needed to make use of some models for a full dose
assessment comparable with that in the UNSCEAR 2013 Report. The approaches used by the Committee,
including the models applied, are described briefly in this section, with further detail provided inattachments. The Committee has also used the additional information that has become available since the
UNSCEAR 2013 Report to test and refine its models, reducing and providing a better estimation of
uncertainties and variabilities; the measurements made on people, in particular, have proved valuable invalidating the models that had to be used for estimating doses in the wider population.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

125

A. Regions considered for dose estimation
A35.
In order to estimate doses to members of the public in Japan, the Committee again focused onfour groups of geographical areas (see table A7):−Group 1 included locations in Fukushima Prefecture from which members of the public were
evacuated in the days to months after the accident according to 40 evacuation scenarios;−

Group 2 included all municipalities and parts of municipalities in Fukushima Prefecture thatwere not evacuated;−

Group 3 included selected prefectures (Ibaraki, Miyagi, Tochigi and Yamagata) in eastern Japanthat were neighbouring Fukushima Prefecture;−

Group 4 included all the remaining 42 prefectures of Japan.

Table A7. Population groups considered
Population
group

Geographical areas

1

Locations where people were
evacuated in the days to months
after the accident

2

Municipalitiesa and parts ofmunicipalities of Fukushima
Prefectureb not evacuated3

Selected prefectures (Ibaraki, Miyagi,
Tochigi and Yamagata) in easternJapan that are neighbouring toFukushima Prefecture

Spatial resolution
Representative areas were used for each location
identified in 40 evacuation scenarios
Municipality level for external and inhalation pathways,
based on the estimates for each of the
1-km grid points, averaged over the municipality
Prefecture level for ingestion pathway
Municipality level for external and inhalation pathways,
based on the estimates for each of the
1-km grid points, averaged over the municipality
Average for the four prefectures (Ibaraki, Miyagi, Tochigi
and Yamagata) for ingestion pathway
Prefecture level for external and inhalation pathways

4aAll remaining prefectures of Japan

Average of the rest of Japan (i.e., the 42 prefectures,
excluding Fukushima, Ibaraki Miyagi, Tochigi andYamagata) for ingestion pathway

Each prefecture of Japan (see below) is divided into municipalities. A municipality is a local administrative unit; themunicipalities are used primarily in the Japanese addressing system to identify the relevant geographical areas and collections of
nearby towns and villages. The term “municipality” has been used in this report in place of the term “district” in the UNSCEAR
2013 Report [U10].
b

Japan comprises of 47 prefectures. In Japanese the word “prefecture” is used for translating references to “ken” in Japanese.

126

UNSCEAR 2020/2021 REPORTA36.
The spatial resolution adopted for assessing the doses in each of these groups was dependent on
the available data:−

Estimates for Group 2 were made at the municipality level for the external exposure andinhalation pathways, and at the prefecture level for the ingestion pathway;−

The selection of the four prefectures in Group 3 was based on the spatial coverage of theavailable measurement information on deposition densities. Estimates of dose for Group 3 weremade at the municipality level for a number of municipalities (where the spatial resolution
allowed) for the external exposure and inhalation pathways. The dose from ingestion for all fourof these prefectures in Group 3 (Ibaraki, Miyagi, Tochigi and Yamagata) was based on the
average for these four prefectures;−

Doses for all Group 4 prefectures were assessed at prefectural level for the external exposure
and inhalation pathways, and on the basis of an average for the remaining 42 prefectures together
for the ingestion pathway.

B. Source term and atmospheric transport, dispersion anddeposition modelling for Japan
A37.
While there is more information available on the concentrations of radionuclides in the air
and deposited on to the ground and other surfaces than was available for the UNSCEAR 2013 Report[U10], all of the information now available is not sufficient to allow estimates of doses based solely
on measurements and some modelling has been necessary. In particular, in order to estimate doses
from radioactive material in the air, both from external exposure to this material and from internal
exposure from its inhalation, modelling was needed to obtain estimates of concentrations ofradionuclides in air at all locations of interest, as measurements remain sparse. The approach usedin the UNSCEAR 2013 Report has again been used. Nonetheless, the greater amount of measurement
information now available has been used to test this approach and provide greater assurance about
its validity.
A38.
Accordingly, exposures from radioactive material in the air at specific times and locations
have been estimated from the assumed time sequence of the release of the more significant
radionuclides and their transport through the atmosphere using ATDM. In this report, the Committee
has used the latest source term developed by the group of researchers at JAEA [T28]. This source term
[T28] has been developed from all of the measurement information on levels of radionuclides in the
air and deposited on the ground using improved ATDM simulation with an optimization method basedon Bayesian inference, as well as information on events occurring on-site as the accident progressed.
It represents a further refinement of the series of source terms estimated by the same group of
researchers (e.g., [C3, K6, K30, T27]) and has been optimized using various measurements, such as
air concentration, surface deposition, and newly released hourly air concentrations of 137Cs derived byanalysing suspended particulate matter collected at air pollution monitoring stations [O25, T51].
Further details about this source term and ATDM and how its results compare with measurement
information are provided in attachment A-9.
A39.
Terada et al. [T28] have used the same ATDM (Worldwide version of System for Prediction ofEnvironmental Emergency Dose Information – WSPEEDI) with this source term to estimate the time-ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

127

dependent concentrations in air and deposition densities of the radionuclides 132Te (and therefore of its
daughter radionuclide, 132I, with which it is assumed to be in equilibrium), 131I, 134Cs and 137Cs. For 131I
(and the other radioiodines), three chemical forms have been considered: inorganic particulate, elemental
vapour, and organic, using the constant ratios, based on experimental and other evidence, of 0.5 for131
Iparticulate/131Itotal, 0.2 for 131Ielemental/131Itotal and 0.3 for 131Iorganic/131Itotal. This ATDM includes a new
meteorological model (Weather Research and Forecasting Model), better modelling of deposition, andan improved data assimilation method [T28]. The ATDM results have been provided by JAEA to theCommittee for two nested grids:−

The finer grid covering most of Fukushima Prefecture with a horizontal resolution of 1 km;−

The coarse grid covering Fukushima Prefecture and all neighbouring prefectures with ahorizontal resolution of 3 km [T28].

A40.
Terada et al. did not include the short-lived radionuclide 133I and the Committee has therefore
estimated the concentration of 133I in air from the calculated concentrations of 131I in air by applying atime-dependent ratio of the two radioisotopes based on the ratio at 14:46 JST on 11 March 2011 [N12],
adjusted for their half-lives. Terada et al. also did not include 133Xe, and the Committee has not included
this radionuclide in its estimates of doses. Xenon-133 would have contributed significantly to the dosefrom external exposure to airborne radioactive material, adding about 30% to the estimated effective doses
from this pathway. However, the estimated effective dose from this pathway is typically only about 1% to
3% of the estimated effective dose from internal exposure from inhalation of airborne radioactive material,so the contribution of 133Xe to the estimated total effective doses from airborne material would have beenless than 1%, which is small compared with uncertainties in these estimates (see attachment A-10 for further
details). The details of the ATDM model and the outputs used for the Committee’s assessments aredescribed in attachment A-9.
A41.
As in the UNSCEAR 2013 Report [U10], the ATDM estimates of concentrations of radionuclides
in the air as a function of time have been used directly to estimate doses to evacuees from external exposure
to the airborne radionuclides and doses from internal exposure from inhaled radionuclides. At locations
where members of the public were not evacuated, the ATDM estimates of deposition density and of time
integrated concentration of each radionuclide in air have been used to calculate ratios by which measured
deposition densities have been scaled to infer a time integrated air concentration from which external
exposure to airborne radionuclides and internal exposure from inhalation have been derived. As theCommittee stated in the UNSCEAR 2013 Report, while estimates of radionuclide concentrations in air anddeposition densities provided by the assumed source term and ATDM simulations at any specific location
are uncertain, the ratios of these two estimates are, in general, likely to be much less so. For the revised
source term and ATDM adopted by the Committee in this report, the time-integrated air concentrations
estimated directly from the ATDM and those derived by scaling the measured deposition density by theratio method have been compared with the available measurements of time integrated air concentration infigure A-I. The figure shows that, using the ratio method around 50% of the estimates are within a factor of
about 2 of the measurements and almost all estimates are within a factor of about 10; and that estimates are
distributed both above and below the corresponding measured values. On the other hand, using the source
term and ATDM to estimate time integrated air concentrations, results in estimates that are less close to themeasurements and, at the higher air concentrations, generally higher than the measured values. At lower air
concentrations neither method provides a good fit with the measurements, with estimated values being
generally below the measurements.

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UNSCEAR 2020/2021 REPORT

Figure A-I. Comparisona of the measured time integrated concentration of 137Cs in air with estimatedvaluesa

The straight line shown in the figure indicates where the modelled air concentration and the measured air concentration would beequal. Data points above this line indicate measured air concentrations where the modelled air concentrations overestimate themeasurements; data points below this line indicate where the modelled air concentrations underestimate the measurements.

A42.
These comparisons provide support to the method being used by the Committee to estimate time
integrated air concentrations of radionuclides and the resulting doses to residents (i.e., those not evacuated).

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129

C. Assessment of external exposure
1.

External exposure from deposited material(a) Doses for residents
A43.
In the UNSCEAR 2013 Report [U10], the Committee assessed doses from external exposure toradionuclides deposited on the ground and other surfaces from the measurement data on deposition
densities of radionuclides using models for:(a) The estimation of the air kerma rate in free-in-air condition at a reference site;(b) Time-dependent location factors that take account of where people were located and their sizeand of shielding provided by buildings, as well as radionuclide migration in the environment owing
to weathering, cleaning and other factors;(c) Occupancy factors that take account of the amounts of time spent by different population groups
in different types of location;(d) Coefficients to convert air kerma rate to absorbed dose rate in a particular organ or effective
dose rate.
Location factors made use of information on house types typical of Japan but were largely based on postChernobyl experience and parameters determined for areas in the Russian Federation and Ukraine.
Occupancy factors were derived based on available data from Japanese national surveys at that time. The
dose rate coefficients used were based on the computational phantom for adults specified by theInternational Commission on Radiological Protection (ICRP) [I19] and other voxel phantoms fordifferent age groups with account taken of growth [G4, P2].
A44.
Since the UNSCEAR 2013 Report [U10], further information has become available relating to
the deposition densities of 131I and to the dynamics of ambient dose rates in the environment specific toJapanese conditions. Extensive measurements of individual dose from external exposure made directlyon people have also become available. In addition, ICRP has developed new dosimetric data relevant forassessing dose from external exposure in a post-accident situation [I22]. The Committee has taken
account of these developments to derive an improved model (M2020) for assessing doses from external
exposure to deposited radionuclides and made comparisons with the measurements made on people toprovide more robust testing and validation of this model. Details of the improved model and its validation
can be found in attachment A-1.
A45.
In the UNSCEAR 2013 Report [U10], to derive deposition densities where measurements forsome radionuclides were lacking (e.g., 131I), the measured deposition density of 137Cs (which was measured
in all samples) was scaled by the average ratio of the deposition density of the radionuclide to that of 137Cs
for samples where both were measured. Different ratios were used for the South trace (Hirono Town, Iwaki
City, Naraha Town and Tomioka Town in Fukushima Prefecture, and Kitaibaraki City and Takahagi Cityin Ibaraki Prefecture) because the ratios for 129mTe and 131I were significantly elevated there in comparison
with elsewhere. In its revised assessment, the Committee has used a similar approach with the same ratios,
apart from those for 131I, 132Te (and its daughter 132I) and 129mTe. For these radionuclides, the available
measurement data are more consistent with ratios based on non-linear relationships with the deposition
densities of 137Cs rather than constant values, and approximate empirical relationships have been derived
from the data. Table A8 provides a summary of the ratios applied.

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UNSCEAR 2020/2021 REPORT

Table A8. Initial ratios at 15 March 2011 used, where measurements were lacking, to estimate
deposition densities of the shorter-lived radionuclides from the initial deposition density of 137Cs
Areaa

Deposition density ratios for radionuclide (dimensionless)
137

Cs

134

Cs

136

Cs

131

I

Te

129m

132

Te (132I)a

110m

Ag

All of Japan
excluding the South
trace

1.0

1.0

0.17

8.3–37b1.1–1.9b

7.6–13b0.0028The South tracec

1.0

1.0

0.17

25–250b1.7–28b

12–190b0.0028

Activity of the daughter 132I was assumed equal to activity of the parent 132Te at the time of deposition.

b

Non-linear relationships were applied for isotopic activity ratios. See attachment A-1 for more details.

cHirono Town, Iwaki City, Naraha Town and Tomioka Town of Fukushima Prefecture, Kitaibaraki City and Takahagi City of

Ibaraki Prefecture.

A46.
As indicated above, ICRP has developed revised dose rate coefficients for air kerma and ambient
dose equivalent at 1 m above an infinite planar source at a mass depth of 0.5 g/cm2 in soil [I22]. It has
also presented dose rate coefficients for effective dose and sex-specific organ dose equivalents for
six reference ages. While these revisions do not greatly affect the estimates of doses, the Committee has
used these ICRP dose rate coefficients rather than the dose rate coefficients used in the UNSCEAR 2013
Report [U10], because they represent the current state of knowledge and are consistent with its aim to
make realistic estimates of doses.
A47.
Extensive monitoring studies in Fukushima and neighbouring prefectures have been publishedsince the UNSCEAR 2013 Report [U10] on the effects of natural migration in soil, weathering andrun-off processes [M30]. These studies have enabled the development of a model for the time-dependent
location factor (i.e., the factor by which the air kerma rate in free-in-air condition is reduced according
to where those exposed are located) that is more representative of undisturbed locations in Japan. In
particular, the revised model predicts a slower reduction in dose over the first 15 years due to natural
redistribution of radionuclides in undisturbed soil than the model used in the UNSCEAR 2013 Report, in
better agreement with observations in Japan.
A48.
Additional studies [S2] have examined the different changes with time in dose ratemeasurements in populated and, especially, urbanized areas made in airborne and car-borne surveys andthose carried out on foot. The Committee has used the results to develop a dose rate reduction model thattakes account of the fact that Fukushima Prefecture has a high population density with almost all flatland
being used or involved in anthropogenic activities and provides a better fit with the most representative
survey data (car-borne for roads and streets; surveys carried out on foot for residential areas including
unpaved areas). The results comparing the car-borne survey data and the data from the surveys carriedout on foot with the revised model and the model used in the UNSCEAR 2013 Report [U10] are illustrated
in figure A-II. They demonstrate the improved estimates provided by the revised model for both “roads
and streets” and “residential” areas, again showing the slower reduction in the dose rate with timecompared with the model used in the UNSCEAR 2013 Report.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

131

Figure A-II. The external dose rate reduction factors for populated places as (a) derived from the carborne surveys [A12, K20] and shown as solid symbols; (b) calculated for paved and unpaved areasusing the model used in the UNSCEAR 2013 Report [U10] (red lines); and (c) fitted to the data andimplemented in the revised model (blue lines)

A49.
The Committee has used the same house types and shielding factors as used in the UNSCEAR
2013 Report [U10] as they are consistent with recent observations made by Japanese authors(e.g., Matsuda et al. [M13]). Similar occupancy factors representing the time spent by different
population groups in different locations have also been assumed. Further details can be found in
attachment A-4.
A50.
Several personal dosimetry surveys have been carried out to measure doses from external exposure
directly, typically using optically stimulated luminescence dosimeters or radio-photoluminescence
dosimeters, as described in section II.A.1 of this appendix. These measurements have been compared withthe predictions from the new model and that used in the UNSCEAR 2013 Report [U10]. Full details areprovided in attachment A-1. A summary of the comparisons is provided in section IV.G of this appendix.

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UNSCEAR 2020/2021 REPORT(b) Doses for evacuees
A51.
As part of the response of the Japanese authorities to the FDNPS accident, there was widespread
evacuation of residents living nearby at different times to reduce radiation exposures. Most of theresidents of the municipalities of Futaba Town, Hirono Town, Kawauchi Village, Naraha Town, Okuma
Town and Tomioka Town, as well as those residents of the municipalities of Katsurao Village,
Minamisoma City, Namie Town and Tamura City living within a 20-km radius of FDNPS, were
evacuated as a precaution between 12 March and 15 March 2011. The residents of the whole of themunicipality of Iitate Village, as well as parts of the municipalities of Katsurao Village, Kawamata Town
and Namie Town, to the north-west of the FDNPS site, were subsequently evacuated between April andJune 2011, based on environmental measurements.
A52.
In the UNSCEAR 2013 Report [U10], the assessment of doses to residents of these evacuatedcommunities for the period before and during the evacuation was based on the results from aquestionnaire survey to ascertain the locations and movements of these people. NIRS used the results ofthis survey to define 18 scenarios representative of the movements of residents local to FDNPS, following
the accident [A6].
A53.
In a recent study, Ohba et al. [O5] have refined these evacuation scenarios by conducting ahierarchical clustering analyses of 100 to 300 randomly sampled behavioural questionnaire responses
from each of the seven municipalities in the evacuation area. This analysis resulted in 37 new
representative evacuation scenarios. These 37 evacuation scenarios did not include those evacuated fromthe municipalities of Hirono Town and Katsurao Village, and the Committee has additionally included
three of the original 18 evacuation scenarios which represent those evacuated from these locations. The
resulting 40 evacuation scenarios which the Committee has used to assess doses to evacuees are
summarized in table A9. Ohba et al. used their new scenarios for the purposes of comparing doses
estimated using models with doses reconstructed from thyroid measurements carried out in March 2011.
As a result, these scenarios only consider the movements of the evacuees up to midnight on 25 March
2011. Where people were evacuated after this date (e.g., some residents of the municipality of Iitate
Village), their evacuation date and final destination are not shown in the table, but their evacuation at alater date and their final destination have been taken into account in accordance with scenarios used in
the UNSCEAR 2013 Report [U10]. Information on the numbers of evacuees by evacuation scenario isprovided in attachment A-11.
A54.
Within these 40 evacuation scenarios, the Committee has considered four types of humanactivities: normal living conditions; preparing for evacuation; evacuation; and sheltering. For normal
living conditions, the Committee has made the same assumptions about human behaviour as were madefor residents, who were not evacuated. For the evacuation preparation, evacuation, and sheltering
activities, the Committee assumed occupancy factors that reflected the nature of activities being
undertaken. Because of a lack of measurement data for this early period after the accident, the Committee
assessed external doses during evacuation using estimated concentrations of radionuclides deposited on
the ground derived from the assumed source term and ATDM. Ohba et al. [O5] used the same source
term and ATDM in their dose assessment for 37 scenarios, although the Committee’s assessment is basedon a finer spatial and temporal resolution.

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133

Table A9. Evacuation scenarios considered
Ohba et al. [O5] (scenarios 01–37) and NIRS survey [A6] (scenarios 38–40)

Scenario

Location at 11 March 2011

Start > Route > Destination

Timinga,b01(FT1)

Futaba Town

Futaba > Kawamata > OOPc

AM2 on 12 March

02(FT2)

Futaba Town

Futaba > Iwaki > OOP

PM1 on 12 MarchFutaba Town

Futaba > Odaka > Fukushima >OOP

AM2 on 12 MarchFutaba Town

Futaba > Haramachi> Koriyama

PM2 on 12 MarchFutaba Town

Futaba > Namie > Kawamata >OOP

PM1 on 12 March

06(TM1)

Tomioka Town and
Kawauchi Villaged

Kawauchi > OOP

PM1 on 15 March

07(TM2)

Tomioka Town

Tomioka > Ono > OOP

PM1 on 12 March

Tomioka Town

Tomioka > Kawauchi > Koriyama >OOP

AM2 on 12 March

09(TM4)

Tomioka Town

Tomioka > Iwaki

PM1 on 12 March

10(NR1)

Naraha Town

Naraha > Iwaki > OOP

AM2 on 12 March

11(NR2)

Naraha Town

Naraha > Iwaki > OOP

AM2 on 12 March

12(NR3)

Naraha Town

Naraha > Iwaki

AM2 on 12 March

13(NR4)

Naraha Town

Naraha > Hirono > Aizu > OOP

AM2 on 12 March

14(NR5)

Naraha Town

Naraha > Iwaki > OOP > Iwaki

AM2 on 12 March

15(OK1)

Okuma Town

Okuma > Tamura > Aizu

PM1 on 12 March

16(OK2)

Okuma Town

Okuma > Tamura

AM2 on 12 March

17(OK3)

Okuma Town andFutaba Townd

Futaba > Kawamata > Iwaki > OOP

AM2 on 12 March

18(OK4)

Okuma Town and Tamura Cityd

Tamura(no further evacuation)

Okuma Town

Okuma Town > Odaka > Haramachi> Sukagawa > OOP

AM1 on 12 March

20(NM1)

Namie Town

Namie > Haramachi > OOP

AM2 on 12 March

21(NM2)

Namie Town

Namie > Soma

PM1 on 12 March

22(NM3)

Namie Town

Namie > Tsushima > Koriyama

AM2 on 12 March

23(NM4)

Namie Town and Tsushimad

Tsushima > Nihonmatsu

AM2 on 16 March

24(NM5)

Namie Town

Namie > Kawamata > OOP

AM2 on 13 March

25(IT1)

Iitate Village

Iitate > Koriyama

AM2 on 16 March

Iitate Village

Iitate > Kawamata > Fukushima >Aizu

AM2 on 15 March

27(IT3)

Iitate Village

Iitate > OOP

AM2 on 19 March

28(IT4)

Iitate Village

Iitate > Fukushima City

22 June03(FT3)04(FT4)05(FT5)08(TM3)19(OK5)26(IT2)

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Scenario

Location at 11 March 2011

Start > Route > Destination

Timinga,b29(OD1)

Odaka ward of
Minamisoma City

Odaka > Haramachi > Iwaki > OOP

PM2 on 12 March

30(OD2)

Odaka ward of
Minamisoma City

Odaka > Kawamata > Aizu > OOP

PM1 on 12 March

31(OD3)Haramachi ward of
Minamisoma City

Haramachi > Date > Haramachi >OOPAM2 on 12 March32(OD4)

Odaka ward of
Minamisoma City

Odaka > Haramachi > Fukushima >OOP

PM2 on 12 March

33(OD5)

Odaka ward of
Minamisoma City

Odaka > Haramachi > Soma > OOP

PM1 on 12 March

34(HK1)

Haramachi ward of
Minamisoma City

Haramachi > Fukushima > OOP

PM1 on 17 March

35(HK2)

Iitate Village

Iitate > Koriyama > OOP

AM1 on 12 March

36(HK3)

Kashima ward of
Minamisoma City

Kashima > Haramachi > Iitate >OOP

AM2 on 12 March

37(HK4)

Haramachi ward of
Minamisoma City

Haramachi > Soma

PM1 on 18 March

38 (10 in[U10])

Hirono Town

Hirono > Ono Town Office

12 March

39 (12 in[U10])

Katsurao Village

Katsurao > Azuma General
Gymnasium

14 March

40 (14 in[U10])

Katsurao Village Office

Katsurao > Azuma General
Gymnasium

21 MarchaAM1, AM2, PM1, PM2 refer to early morning, late morning, early afternoon and late afternoon, respectively.

b

The focus of Ohba et al. [O5] was on evacuation before 26 March 2011. The timings of later evacuations (in particular, of themunicipalities of Iitate Village and parts of Tamura City in May and June 2011, are as set out in the UNSCEAR 2013 Report [U10].
c

OOP denotes out of prefecture and indicates that the destination was a prefecture other than Fukushima Prefecture.

d

In these scenarios, people were evacuated from the first named location to the second named location on the day of the earthquake(11 March 2011).

A55.
The assessment of doses to those evacuated also included doses received at their evacuation
destinations, either in less affected areas of Fukushima Prefecture or in other prefectures, and it was
assumed, as in the UNSCEAR 2013 Report [U10], that evacuees remained at these destinations
subsequently. Projected external doses from deposited radionuclides have also been assessed for theevacuated locations, as described above for residents, in order to determine the dose averted by evacuation.

(c) Effects of environmental remediation and doses following population

returnA56.
Experimental studies and tests of environmental remediation technologies started in the affectedareas of Japan in summer 2011. They included technologies for remediation of inhabited areas,
countermeasures in agriculture (e.g., top soil removal and additional fertilization) and in forestry. Thesestudies and tests in Fukushima Prefecture continued until 2013. Since 2013, large scale remediation of
the evacuated municipalities started in a special decontamination area (SDA), with the support of theGovernment of Japan. In some affected areas beyond the SDA (the intensive contamination survey area

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

135(ICSA)), local authorities initiated remediation activities, mostly focused on public areas and especiallyon children’s facilities (kindergartens, schools, hospitals, etc.), but also including household plots.
A57.
There is detailed information available about the technologies used and the scale of theenvironmental remediation actions that have been carried out, and about the resulting reductions inambient dose rates. This has been summarized in chapter IV. The measured reductions included theeffects of radioactive decay and other natural processes, such as migration of radionuclides in the soil.
Based on a large number of ambient dose rate measurements taken before and after remediation, theMinistry of the Environment (MOE) used a model to estimate the average reduction in the effective doseto the public as a result of remediation in the SDA and ICSA (i.e., having subtracted the contributions
from natural processes) [M38]. The average reductions estimated by the MOE correspond to dose
reduction factors (DRFs – the ratio of the dose before remediation to the dose after remediation assessedwith the contribution of radioactive decay and radionuclide migration in the soil subtracted) of about 1.4
for the SDA and 1.3 for the ICSA. Comparison with some measurements of individual external dose on
people in a municipality in the ICSA suggest slightly lower DRFs of 1.1–1.2 in the ICSA [M48, T43].
The Committee has accordingly judged that remediation has resulted in a DRF of about 1.3 in the SDA
and of about 1.1 to 1.2 (depending on initial dose rate and remediation timing) in the ICSA. Further
details are provided in attachment A-1. While these dose reduction factors can be used to provide anindication of the benefits of the remediation work, more reliable estimates of the doses to the public
taking account of remediation require much more extensive information from post-remediation
measurements of individual doses on people. The results of such post-remediation monitoring have notyet been published or provided to the Committee.

2.

External exposure from radionuclides in the air

A58.
There were insufficient measurements of gamma dose rate and of the concentration ofradionuclides in air during the passage of the plumes of released material for an assessment to be made ofexternal exposure from these radionuclides in air based solely on environmental measurements. External
exposures due to radionuclides in air were therefore calculated from estimated concentrations ofradionuclides in air based on the assumption that the plume could be represented by a semi-infinite cloud,an appropriate assumption where the distribution of radionuclides in air can be considered uniform overhundreds of metres. Estimates of radionuclide concentrations in air were averages for the approximately
1 km squares and were derived from the assumed source term and ATDM, as described in section III.B ofthis appendix.
A59.
In estimating doses from radionuclides in air for residents, the same assumptions as insection III.C.1(a) of this appendix (i.e., for external doses from deposited radionuclides) were adopted
for the fraction of the time spent indoors and the shielding provided by buildings. Full details of themethods of calculation of external exposures from radionuclides in air and related parameter values using
the ATDM results are given in attachment A-10. For evacuees, external exposures from radionuclides in
the air were estimated according to the evacuation scenarios and the four types of human activity
described in section III.C.1(b) of this appendix.

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D. Assessment of internal exposure
1.

Dose coefficients for radioiodine intakes

A60.
The Japanese population traditionally has an iodine-rich diet containing up to tens of thousands ofmicrograms of stable iodine a day [K5, L3, N2, Z6, Z7]. Stable iodine comes mainly from seafood, andparticularly algae, which is a traditional component of the Japanese diet: typically, kelp (algae seaweed)
contributes up to 90% of the total iodine intake in Japan [K4]. In contrast, the reference ICRP biokinetic
model [I12, I13] assumes a moderate level of dietary intake of stable iodine at about 200 μg/d for adults.
A61.
As a result of this iodine-rich diet, the fractional uptake of radioiodine into the thyroid following
intake via ingestion or inhalation among the Japanese population can be expected to be lower than thereference ICRP value of 30%. For example, Kusuhara and Maeda [K61] obtained a mean value of thefractional uptake of 16.1% ± 5.4% for 15 Japanese male subjects from Nagasaki with normallyfunctioning thyroid glands, and a similar study in Tokyo resulted in the value of 12.8% ± 5.7% [K52].
A62.
In the UNSCEAR 2013 Report [U10], assessments of doses from intakes of radioiodine werebased on the current set of ICRP reference dose coefficients for members of the public [I14]. These agedependent dose coefficients were derived by ICRP with the use of a reference three-compartmental iodine
biokinetic model [I13] and reference dosimetric models [I14]. In 2017, ICRP adopted a new more detailed
model of iodine biokinetics in adults [I21] that is based on the model of Leggett [L3]. This new model
has been used to calculate dose coefficients for inhalation of various physico-chemical forms ofradioiodine (particulate aerosols with Type F absorption and activity median aerodynamic diameter(AMAD) = 1 µm, methyl iodide vapour and elemental iodine vapour) and for ingestion of 131I, 132I, 133I
and 132Te, for three types of diet that may be more appropriate for the Japanese population:−

A typical Japanese diet (with fractional uptake of radioiodine (U) of 15%);−

A kelp-rich diet (U = 5%);−

A Western pattern diet (U of about 30%), that is popular among some groups of the Japanese
population.

The dose coefficients were developed for dose to the thyroid, red bone marrow, female breast and colon,
as well as for effective dose.
A63.
The indicative dose coefficients for intake by inhalation and ingestion have been calculated foran adult female, an adult male, and for a 10-year-old child and a 1-year-old infant. Additionally, the dosecoefficients for in utero exposure have been assessed for acute intake at the 35th week of pregnancy interms of the fetus absorbed thyroid dose and fetus effective dose per maternal exposure.
A64.
Further details of the model and its parameter values can be found in attachment A-2 and of theresulting dose coefficients can be found in attachment A-4. The Committee has used the dose coefficientsfor a typical Japanese diet in estimating doses from intakes of radioiodine; these dose coefficients resultin doses from intakes of radioiodine lower by about a factor of two than the dose coefficients used in the
UNSCEAR 2013 Report [U10].

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

2.

137

Assessment of internal doses from inhalation of radionuclides

A65.
The assessment of internal exposures from the inhalation of radionuclides required informationon the concentrations of radionuclides in air, the age-dependent breathing rates and the dose coefficientsfor intake via inhalation. As described in section III.B above, concentrations of radionuclides in air havebeen estimated from the results of the assumed source term and ATDM derived by Terada et al. [T28].
Full details of the methods of calculation of internal exposures from inhalation of radionuclides in air andrelated parameter values using the ATDM results are given in attachment A-10.
A66.
For residents, the time-integrated concentrations of radionuclides in air have been derived frommeasured deposition densities of radionuclides, by scaling according to the ratio of the modelled time
integrated concentration in air of each radionuclide to its modelled deposition density.
A67.
The Committee used reference (i.e., ICRP) values of the age-dependent breathing rates anddose coefficients for 134Cs and 137 Cs for absorbed doses to the thyroid, red bone marrow, female
breast and colon, and for effective dose [I16, I17]. These dose coefficients for intake by inhalation
were derived using a default particle size (AMAD of 1 μm) and assuming Type F absorption forradiocaesium. Thyroid dose coefficients specific to the Japanese population for each of the threephysico-chemical forms of iodine considered have been used as described in section III.D.1. The
inhalation rates applied were the average rates over a day from the ICRP model of the respiratory
tract [I16].
A68.
In the UNSCEAR 2013 Report [U10], the Committee made no allowance for any reduction in thedose from inhalation (as a result of the filtering effects of buildings) for people being indoors during thepassage of the plumes of released material, in the absence of more specific information. Hirouchi et al.
[H22] has presented new information about the factor by which doses from inhalation may be reduced
through being indoors, which was experimentally derived for Japanese houses. The reduction factors ranged
from less than 0.1 to approximately 1. The Committee has accordingly used a reduction factor of 0.5 (withan uncertainty described by a triangular distribution with minimum = 0.1, peak = 0.5 and maximum = 0.95,
as proposed by Ohba et al. [O5]) for assessing doses from inhalation of radionuclides when people wereindoors. For residents, the same occupancy factors as described in section III.C.1(a) of this appendix havebeen applied to account for the amount of time different groups of people spent indoors.
A69.
For evacuees, the concentrations of radionuclides in air to which they were exposed before andduring evacuation were determined from the estimates of the time dependent concentrations of thereleased radionuclides in air as assessed by Terada et al. [T28] using the assumed source term and ATDM.
Time integrals of air concentration were estimated for each of the 40 evacuation scenarios listed in
section III.C.1(b) of this appendix. Doses were estimated from these air concentrations using the same
dose coefficients as for residents. No allowance, however, was made for any reduction in dose that may
have occurred while in buildings or vehicles during evacuation (i.e., owing to the filtering effects ofbuildings/vehicles). This may have resulted in an overestimate of inhalation doses to evacuees that, in theabsence of more detailed information on how the evacuations were conducted, is difficult to quantify;
but any overestimate is unlikely to be large.

3.

Assessment of internal doses from ingestion

A70.
In the UNSCEAR 2013 Report [U10], the Committee estimated doses from ingestion frommeasurements of 131I, 134Cs and 137Cs in a wide variety of foods and in drinking water in Japan. Fromemerging information from whole-body monitoring, the Committee was aware, at the time of preparation

138

UNSCEAR 2020/2021 REPORTof the report, that these estimates were likely to be substantial overestimates. Possible reasons for thisidentified in the UNSCEAR 2013 Report included that the food monitoring measurements used,particularly in the first year, were not based on random sampling, but on samples aimed at identifying
foods containing high concentrations of radionuclides. In addition, where measurement results were
below the limit of detection, a constant concentration of 10 Bq/kg, the nominal limit of detection, wasassumed for 131I, 134Cs and 137Cs, although for 131I this was only for the first four months.
A71.

Further relevant information has since become available:−

For radiocaesium, this includes: WBC measurements made on people; measurements made on
food actually consumed by people, either from market-basket or from duplicate-diet surveys;
and detailed assessments carried out by Japanese researchers based on measured levels in food;−For radioiodine, this includes: measurements of the radioiodine content of thyroids; anddetailed assessments carried out by Japanese researchers based on measured levels in foodand drinking water.

Each source of information, in particular its strengths, weaknesses and limitations in respect of estimatingor validating doses to the public, is considered in more detail below.
A72.
Making use of these different sources of information to estimate doses is complicated by factors
such as the timing of the measurements, and the different routes of intake and the different radionuclides
included:−

Estimates of exposures made from measurements on people, either whole-body or thyroid
monitoring, reflect combined intakes from ingestion and inhalation, are based on one
measurement made at a specific time, and focus on intakes of specific radionuclides(radiocaesium or radioiodine, respectively);−

Estimates made from market-basket or duplicate-diet survey measurements are not informative
for intakes that occurred before the survey samples were taken and generally reflect intakes fromingestion of caesium radionuclides only (because iodine radionuclides were generally notmeasurable by the time the surveys started);−

Estimates made using models separately consider each intake route (inhalation, ingestion of foodand ingestion of drinking water) and can include all significant radionuclides but need to rely on
many modelling assumptions.

The resulting approach used by the Committee in updating its assessment of doses from ingestion is
described in section III.D.3(c) of this appendix. Further details of the measurements and studies
underpinning the Committee’s approach can be found in attachments A-2 and A-3.

(a) Intakes of caesium radionuclides
Whole-body counting data
A73.
Data available from WBC campaigns, summarized in table A2, provide information on thewhole-body content of radiocaesium from as early as June 2011. Some of these campaigns, for example,
the campaigns of measurements at the Municipal General Hospital of Minamisoma City and at Hirata
Central Hospital, begun in October 2011, have continued into the longer term. In the campaign at theANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

139

Municipal General Hospital of Minamisoma, radiocaesium was detected in the body in 40% of the adults
and 9% of the children in autumn 2011, but these proportions had fallen to around 15% of the adults and
1% of the children in spring 2012. By spring 2013, radiocaesium was no longer detected in children, andwas detected in only around 3% of the adults, this proportion falling to just 1% in spring 2014.
A74.
The strength of the whole-body monitoring data for estimating doses is that they are fromdirect measurements on people. However, the estimation of realistic doses from such measurements is
not straightforward and relies on knowledge about the temporal pattern of intake, not only for intakes
before the measurement, but also for any intakes that may occur after the measurement up to the endof the period of interest (e.g., the end of the first year). In all of the whole-body monitoring studies,
there was generally insufficient knowledge about this temporal pattern of intake. There were also
uncertainties associated with the measurement methods used and other factors, such as possible
external contamination of clothing, and how representative those measured were of the wider
population. A further weakness and a potential source of uncertainty, was the low, and rapidly
declining, fraction of measurements found above MDL, which required assumptions to be made about
how to treat the larger, and rapidly increasing, fraction of measurements below the MDL. As a result,much of these data (particularly measurements carried out at longer times after the accident) are only
informative in putting an upper limit on the magnitude of the doses. There are also the additional
complications that the doses estimated are from total intakes, and therefore that some apportionment
has to be made between intakes from inhalation and intakes from ingestion in order to estimate orvalidate doses from ingestion, with further assumptions likely to be needed for estimating doses to thewider population.
A75.
In light of these qualifications to dose estimates derived from the WBC studies, the Committee
has concluded that these estimates are best used to provide validation of doses estimated by othermethods. Further details of the whole-body monitoring studies, estimates of the doses from intakes ofradiocaesium to various population groups that can be derived from these studies, and comparisons
between these estimates and those derived using other approaches are provided in attachment A-3.
A summary of the comparison between doses derived from whole-body monitoring studies and thoseestimated using the methods adopted by the Committee in this report is given in section IV.G.3 below.
Market-basket and duplicate-diet studies
A76.
Data on daily intakes of radionuclides from market-basket and duplicate-diet samples of foodand drinking water have been collected between July 2011 and winter 2013. The studies are summarizedin table A6. The studies indicate that, during the first year after the accident, daily intakes of 134Cs and137
Cs were each of the order of 1–5 Bq/d in Fukushima Prefecture, around 3 Bq/d in neighbouring
prefectures, and around 0.4 Bq/d in distant prefectures. These intake estimates provide a further indicationof the likely overestimate of doses from ingestion made in the UNSCEAR 2013 Report [U10]: based on
the per capita food intakes used in the UNSCEAR 2013 Report, the minimum intakes of each of 134Cs
and 137Cs from food alone in the first year would have been more than 10 Bq/d because of the assumptionthat the concentration of 134Cs and 137Cs in food should be set to 10 Bq/kg if the measured concentration
was below the MDL.
A77.
The results of these market-basket and duplicate-diet studies enable more realistic estimates tobe made of doses to both adults and children from ingestion of 134Cs and 137Cs in food and drinking waterover the first few years after the accident. They are based on measurements made on the food as consumed
by people, and, in some cases, take account of factors such as losses during food preparation and cooking.
They therefore represent a more realistic basis for estimating doses from ingestion than the approach usedin the UNSCEAR 2013 Report [U10].

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A78.
The strength of these studies is that they provide more direct estimates of doses from ingestion,
the only additional knowledge required being (in the case of the duplicate-diet studies) the dosecoefficient (i.e., the dose per unit intake) of the radionuclide(s) ingested, for which ICRP reference values
are available, and also (in the case of the market-basket studies) information about food consumption
rates. Weaknesses include that they are not informative of doses from ingestion before the first studies
were carried out (July 2011) and that they may not be representative of the wider region (e.g., a whole
prefecture) to which the estimated doses may need to be applied.
Dose estimates using food monitoring data and other survey results
A79.
Murakami and Oki [M47] made estimates of the doses in the first year from ingestion of food anddrinking water containing 131I, 134Cs and 137Cs based on food monitoring data, a similar approach to thatused in the UNSCEAR 2013 Report [U10]. However, Murakami and Oki used data from a wider range ofsources than was available to the Committee when preparing the UNSCEAR 2013 Report and took accountof the regional trade in foods (specifically, of the shares of foods from different parts of Japan arriving at
food markets, where most Japanese citizens purchase food). They have provided estimates, withuncertainties, by age and sex, of equivalent dose to the thyroid and effective dose from 131I intakes and ofeffective dose from 134Cs and 137Cs intakes for residents of Fukushima City, Tokyo and Osaka. They
estimated average doses in these three locations as well as doses to agricultural workers (about 4% of the
population) in Fukushima City who may have preferentially consumed locally produced vegetables andevaluated the effect of countermeasures to restrict the distribution of food, voluntarily withhold rice, andprovide bottled water instead of tap water for infants. They compared their estimates of the effective dosefrom ingestion of 134Cs and 137Cs in food and drinking water with the estimates of Koizumi et al. [K33] andHarada et al. [H3], which were derived from market-basket and food-duplicate surveys in five periods from
July 2011 to March 2012, and found general agreement within a factor of about two.

(b) Intakes of iodine radionuclides
Thyroid monitoring
A80.
Measurements were made between 26 and 30 March 2011 of 131I in the thyroid glands of
1,080 children in three locations in Fukushima Prefecture (Iitate Village, Iwaki City and Kawamata Town)
and the results have been most recently reported by Kim et al. [K18]. Additional measurements of 131I in
the thyroid are available from other studies on smaller numbers of people, including adults [K55, T34, U1,
U20]. The Committee has analysed these measurements and made its own estimates of absorbed doses tothe thyroid from internal exposure to 131I at several locations in Fukushima Prefecture. Further detail on themethodology used by the Committee, including the corrections made for background radiation and thetreatment of the large proportion of measurements of dose rate that were equal to or less than the backgrounddose rate, is given in attachment A-2. Assumptions were also needed about where the individuals who were
measured were, and their habits, during the passage of radioactive plumes, and where they subsequently
lived, as this information was generally lacking. While dose estimates derived from these measurement data
have the advantage of being based on direct measurements on people, they have similar weaknesses to thosedescribed above for the whole-body measurements and are not informative about thyroid doses to the wider
population. The resulting reconstructed doses to the thyroid have therefore been used for validation of theestimates of absorbed dose to the thyroid from intakes of 131I made using models for intakes via inhalationand intakes via ingestion of food and drinking water (see section IV.G. below).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

141

Estimates of radioiodine intakes from drinking water
A81.
Miyatake et al. [M34] used models to supplement the measurement data on drinking water andmake estimates of the doses from 131I intakes from ingestion of tap water to people in the 12 municipalitiesof the evacuation area. The authors based the estimates on the results of earlier survey work by Hirakawa
et al. [H15] that identified that consumption of drinking water was likely to be the only important sourceof radionuclide intakes by ingestion by evacuees. Miyatake et al. [M34] used the same assumed source
term and ATDM [T28] as used by the Committee in this report. However, the authors used a simpler setof evacuation scenarios and the ICRP dose coefficients for intakes of radioiodine. Miyatake et al. foundgood agreement between their estimates of 131I concentration in tap water and measurements. They
accounted for the different sources of tap water at different locations along the evacuation routes, andpresented estimates of equivalent doses to the thyroid for adults, 10-year-old children and 1-year-old
infants for 12 municipalities for median, mean and 95th percentile intakes of drinking water. They gave
separate, detailed consideration to the water supply at Iitate Village, including the relative amounts
supplied by three water purification plants and by well or spring water.

(c) Approach used by the Committee to estimate doses from ingestion
A82.
In view of the quality and detailed nature of the study by Murakami and Oki [M47], the validation of
its results against the market-basket and duplicate-diet studies described above, and its inclusion of estimatesof doses from 134Cs, 137Cs and 131I, the Committee has based its estimates of doses from ingestion in the firstyear to both adults and children on the estimates of Murakami and Oki. Murakami and Oki’s dose estimateshave been adjusted to account for the dose coefficients for radioiodine intakes specific to Japanese people. The
dose estimates derived from this study have been compared with the results of the thyroid monitoring studies
and the whole-body monitoring studies as described in detail in attachments A-2 and A-3 and summarized in
section IV.G. The estimated doses for Fukushima City have been considered representative of doses fromingestion of food and drinking water to residents of Fukushima Prefecture as a whole; this is a plausible
assumption given food distribution and purchasing practice in Japan. Doses in other prefectures in the first year
have been estimated from doses estimated by Murakami and Oki for Fukushima City by scaling according to
the ratios of the average radiocaesium concentrations monitored in foods for neighbouring prefectures and forthe rest of Japan to that in Fukushima Prefecture. The resulting doses from ingestion in neighbouring
prefectures and in the rest of Japan estimated by the Committee are consistent with the estimates made by
Murakami and Oki for the Tokyo and Osaka Metropolitan Areas [M47].
A83.
For evacuees, doses from ingestion of food before and during evacuation have been assumed tobe negligible, based on survey results [H15, K10]. Doses from drinking water estimated by Miyatake etal. [M34] for 12 evacuated municipalities have been used, with adjustment for the specific dosecoefficients for iodine intakes for Japanese people (see section III.D.1 of this appendix). For doses after
evacuation, the same approach as used for residents has been adopted.
A84.
For time periods after the first year, the Committee has used as its starting point estimates of doses toadults and children from ingestion of radiocaesium derived from the market-basket and duplicate-diet studies
for Fukushima and other prefectures, and then used the model developed by Smith et al. [S33] to estimate how
intakes of radiocaesium from food and drinking water change over the longer term. This model was developed
from an analysis of long-term measurement data collected annually from 1963 to 2008 on the 137Cs content offood products and in the whole diet in Japan resulting from fallout of radiocaesium from atmospheric nuclear
weapons testing. The observed annual concentrations of 137Cs in food products and in the whole diet were
described by Smith et al. in terms of coefficients representing a fast decline in concentrations after deposition,a slow decline as a result of soil fixation processes, and the very long-term component which declines due tovertical migration, erosion and further slow reductions in bioavailability. The initial fast decline in the model

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is not relevant for intakes more than a year after deposition, but the slow and long-term declines from the model
have been applied to intakes derived from market-basket and duplicate-diet measurements in various
prefectures of Japan from March 2012 onwards. The model takes no account of food restrictions, but the
proportion of monitored food found to be above the limit (of 100 Bq/kg) established by the Japanese
Government was low (generally below 1%) by the second quarter of 2012. Further details of the model ofSmith et al. [S33] and its implementation can be found in attachment A-3.

E. Microparticles
A85.
As indicated in chapter II, several studies have identified radiocaesium in the environment
associated with water-insoluble “glassy spherules”. The fraction of the radiocaesium deposited on to theground in the form of such microparticles varies from a few per cent at distances of up to 30 km fromFDNPS to up to about 80% at large distances (more than 200 km) [I29, U19]. Radiocaesium in the formof such microparticles would be expected to migrate more slowly into the soil and have lower transfer toplants and animals. The effect of such microparticles on estimated doses is complex, resulting in higher
doses from one exposure pathway and lower doses from another, than as assumed in the models used inthis report. While the presence and behaviour of radiocaesium in the environment in the form of
microparticles is worthy of further scientific investigation, the Committee judges that it will have nomaterial impact on the overall levels of dose to the public estimated in this report.

F. Assessment of variabilities and uncertainties
A86.
The Committee’s aim throughout has been to make realistic estimates of the average radiation
exposures of members of the public in defined population groups (see table A7). Among the individual
members in each group, there will be some variability between the radiation exposures of each person,
and this variability may be significant. In addition, any method of estimating doses will be associatedwith uncertainty, whether the estimated doses have been derived from direct measurements on people orfrom indirect measurements in the environment coupled with modelling. In light of the additional
information that has become available since the UNSCEAR 2013 Report [U10], the Committee has mademore quantitative and less subjective assessments of the variabilities and uncertainties in its estimates ofpublic doses, enabling it to report distributions of individual doses in municipalities and prefectures, andthe 5th and 95th percentiles of the distributions, as well as municipality- and prefecture-averages. The
Committee’s approach to its assessment of the variabilities and uncertainties in its estimates of doses is
set out in further detail in attachment A-12.

G. Assessment of collective doses
A87.
The collective dose to the general public is a quantity intended primarily for the optimization ofprotection, comparing technologies or protection measures. Comparisons have been made of thecollective dose integrated over a defined time period with those from other events associated withradionuclide releases to the environment (such as the Chernobyl accident). As in the UNSCEAR 2013
Report [U10], the Committee has estimated the collective effective dose and the collective absorbed doseto the thyroid over the first year, the first 10 years and up to age 80 years, for the population of Japan.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

143

These time periods were chosen, as in the UNSCEAR 2013 Report, to indicate the annual collective dosein the first year and to illustrate the temporal accumulation of the collective dose commitment.
A88.
The collective dose due to external exposure was reassessed using the same assumptions about
population sizes, age and social composition, dwelling types and occupation as in the UNSCEAR 2013
Report [U10]. However, for residents of Fukushima Prefecture, the Committee has used different values
for the proportions of people living in different dwelling types: 37% of the population were assumed tolive in wooden one-to-three storey houses, 42% in wooden fireproof houses, and 21% in multi-storey
concrete buildings [M27]. The values used in the UNSCEAR 2013 Report were the average values forthe whole of Japan (30%, 30% and 40%, respectively) and have again been assumed for other prefectures.
A89.
The collective dose from internal exposure includes doses from inhalation and ingestion. The
collective inhalation dose (both effective and thyroid dose) has been assessed by summation of inhalation
doses over the first year of exposure for all localities of Fukushima Prefecture and other prefectures ofJapan. The collective ingestion dose (both effective and thyroid dose) in the first year has been assessedfrom the estimated doses from ingestion for the different population groups in the first year integrated
over the population of Japan, taking account of the age distribution. For subsequent years the ingestion
dose has been estimated by means of time integrals over 1–10 years and 11–80 years of the radiocaesium
intake in food and drinking water from the market-basket and duplicate-diet studies with decreases overtime from the model of Smith et al. [S33], and with subsequent integration over the population of Japan(see section IV.C. below).

IV. RESULTS
A90.
The methodologies described in section III of this appendix have been used to derive detailed
estimates of the doses by age group (20-year-old adult, 10-year-old child and 1-year-old infant) for:(a) Municipalities in Fukushima Prefecture that were evacuated or partially evacuated according to
the 40 evacuation scenarios (Group 1);(b) The municipalities and parts of municipalities within Fukushima Prefecture that were notevacuated (Group 2);(c) The selected four prefectures in eastern Japan neighbouring Fukushima Prefecture (Group 3);(d) The rest of Japan (Group 4).
A detailed dose assessment was conducted for all municipalities in Fukushima Prefecture and somemunicipalities of the Group 3 prefectures 4 of Ibaraki, Miyagi, Tochigi and Yamagata. Additional
estimates of dose were made for those municipalities that were in the evacuation zones and for thosemunicipalities that were partially evacuated. For ease of comparison, the estimated doses are presented
in a similar way to that used in the UNSCEAR 2013 Report [U10]. As well as estimates of the average
doses in each municipality or prefecture for the population groups considered in this report, estimates arealso provided of the uncertainties in these values and of the distribution of individual doses within eachof the population groups (see section IV.F of this appendix).

4 The Committee has used a slightly different reporting of results for prefectures in its revised assessment because of changes inthe spatial coverage of the deposition density data. The previous Group 3 prefectures of Gunma, Chiba and Iwate were not analysedat the municipality level and are now included in Group 4.

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UNSCEAR 2020/2021 REPORT

A91.
Following a review of more recent information on estimates of doses to members of the public
in countries neighbouring, or in close proximity to, Japan, the Committee has revisited its estimates ofsuch doses in the UNSCEAR 2013 Report [U10] (see section IV.E).

A. Estimates of doses in Japan in the first year
1.

Effective dose

A92.
Table A10 summarizes the ranges of municipality- or prefecture-average effective doses
estimated for the first year following the accident, for the representative 20-year-old adult, 5 10-year-old
child and 1-year-old infant residing in the non-evacuated municipalities of Fukushima Prefecture(Group 2), the selected neighbouring prefectures (Group 3) and the remaining prefectures in Japan(Group 4). The doses are summed over the main exposure pathways and are intended to be characteristicof the municipality- or prefecture-average doses received by people living in each municipality or
prefecture. The estimates in table A10 reflect the range of average doses across the municipalities within
prefectures, not the ranges of doses received by individuals within the populations at these locations (i.e.,the variability in the doses between individuals). The relative contribution of each main pathway to thetotal estimated doses varied between municipalities and prefectures reflecting the levels of radionuclides
in the environment and exposure conditions. Detailed results for each municipality and age group
including the 5th and 95th percentiles of the distributions of individual effective doses are provided inattachment A-13.
A93.
Comparison with the equivalent table C6 in the UNSCEAR 2013 Report [U10] indicates that
the Committee’s revised estimates of the ranges of effective dose from external exposure and inhalation
pathways taken together are broadly comparable (within a few tens of per cent) with those in that report.
This apparent similarity is, however, masking much larger differences in the average dose estimates foreach municipality and in the average doses from these two pathways when considered separately (seesection IV.H below and attachment A-13). The revised estimated doses from ingestion are much lower,
however, by about 10 to 30 times, reflecting the more realistic approach adopted that is supported by thewhole-body measurements made on people (see section IV.G) and the market-basket and duplicate-dietstudies that measured intakes from the food actually bought or consumed.
A94.
Figure A-III shows the estimated average effective dose for infants for each municipality ofFukushima Prefecture that was not evacuated; municipalities that were partly evacuated are also includedin the figure. The highest estimated doses were to individuals in the municipalities with high levels of
deposition density (Date City, Fukushima City, Koriyama City, Nihonmatsu City, Koori Town andOtama Village). The municipality-average total effective doses to infants in these municipalities were in
the range of 3.6 to 5.3 mSv in the first year. The contribution of external exposure from depositedradionuclides to the total effective dose was dominant. Adults were estimated to have received average
effective doses in the first year, about 70% of those received by infants.

5 Average doses to adults who were indoor workers have been chosen to be representative of adults as a whole. The variability ofthe effective doses and absorbed doses to the thyroid among the population, including whether adults were indoor or outdoor
workers, or retired has been considered in section IV.F of appendix A.

Table A10. Ranges of estimated municipality- or prefecture-average effective doses in the first year following the accident for residents of Japan for locations
that were not evacuatedRanges of average effective dose by pathwaya (mSv)
External +inhalation

Ingestionc

10-year-old
External +inhalation

Total

Ingestionc

1-year-old
Total

External +inhalation

Ingestionc

Total

0.10–4.5

0.061–5.3

0.055

0.12–5.3

GROUP 2d – FUKUSHIMA PREFECTURE
Municipalities notevacuated

0.043–3.7

0.036

0.079–3.8

0.051–4.5

0.051

GROUP 3e – NEIGHBOURING PREFECTURES
Ibaraki Prefecture

0.14–0.81

0.009

0.15–0.81

0.17–0.98

0.013

0.18–0.99

0.20–1.1

0.015

0.21–1.2

Miyagi Prefecture

0.18–0.91

0.009

0.19–0.91

0.22–1.1

0.013

0.23–1.1

0.26–1.3

0.015

0.27–1.3

Tochigi Prefecture

0.28–0.92

0.009

0.29–0.92

0.33–1.1

0.013

0.35–1.1

0.39–1.3

0.015

0.41–1.3

Yamagata Prefecture

0.093–0.13

0.009

0.10–0.14

0.11–0.16

0.013

0.13–0.18

0.13–0.19

0.015

0.15–0.21

0.005–0.43

0.0g–0.50

0.005

0.005–0.51

GROUP 4 – REST OF JAPANf

42 remaining
prefecturesa0.0g–0.36

0.004

0.004–0.36

0.0g–0.43

0.005

The reported doses are the ranges of the municipality-average doses for the Group 2 and Group 3 prefectures and the prefecture-average doses for the Group 4 prefectures. These estimates of dose areintended to be characteristic of the average doses received by people living at different locations and do not reflect the ranges of doses received by individuals within the population at these locations.
b

Adult indoor workers have been considered to be representative of adults.

c

Doses to a subgroup of the population (agricultural workers) who preferentially consumed local vegetables may be larger by a factor of about 4 to 5.

dGroup 2: Members of the public living in the non-evacuated municipalities of Fukushima Prefecture.

eGroup 3: Members of the public living in the neighbouring prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.

fGroup 4: Members of the public living in the remaining prefectures of Japan, including the previous Group 3 prefectures of Chiba, Gunma and Iwate.

g

Estimated doses that are very much less than 1 µSv have been assigned a value of 0.0.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

AdultbGeographical area

145

146

Figure A-III. Estimated average effective dose in the first year to infants in each municipality of Fukushima Prefecture apart from those that were evacuated

UNSCEAR 2020/2021 REPORTANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

147

A95.
Figure A-IV shows the municipality-average effective doses in the first year following theaccident for infants living in municipalities of Fukushima Prefecture that were not evacuated and somemunicipalities of the Group 3 prefectures. The spatial distribution in the estimated doses shown in thisfigure reflects the pattern of the releases and depositions of radionuclides in the different municipalities
in the area.
Figure A-IV. The average effective dose in the first year following the accident for infants living inmunicipalities of Fukushima Prefecture that were not evacuated and some municipalities of Group 3
prefectures

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).

A96.
For municipalities of the Group 3 prefectures (Ibaraki, Miyagi, Tochigi and Yamagata), themunicipality-average effective doses to infants were in the range of 0.15 to 1.3 mSv in the first year, withthe dominant contribution again being generally from external exposure to deposited radionuclides. The
prefecture-average effective doses to infants for the prefectures in the remainder of Japan were in therange 0.005 to 0.51 mSv, with external exposure from deposited radionuclides making the dominant
contribution to the higher average doses in this range.

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A97.
Figure A-V shows the prefecture-average effective doses in the first year to infants in the rest ofJapan. Prefecture-average doses in other prefectures were lower than those in Fukushima Prefecture andwere considerably lower in the more distant prefectures, where the dose estimates were less than thevariation in background doses from natural sources of radiation [O21].
Figure A-V. Estimated average effective doses to infants in the first year following the accident
The main map shows the prefecture-average effective dose. Fukushima Prefecture average only includes
municipalities that were not evacuated. The inset map shows the municipality-average doses for municipalities
in Fukushima Prefecture that were not evacuated

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

149

A98.
All of the estimated doses are representative of the average doses to the population in therespective municipalities for Group 2 and Group 3 prefectures and in the respective prefectures forGroup 4. These average doses are subject to uncertainties in the measurement data and in the models used
to estimate the doses, and the doses to individuals in the population group will vary about the averagedepending on location within the municipality or prefecture and individual habits, such as the amount oftime spent indoors. The Committee has estimated the 5th and 95th percentiles of the distributions ofeffective doses to individuals in the first year in each population group, taking account of all the major
sources of individual variability as well as the uncertainties, and these are presented in attachment A-13.
The 5th percentiles are around half the average effective dose and the 95th percentiles are around twice
the average. Example distributions of individual effective doses are presented and discussed in
section IV.F.
A99.
In the Committee’s revised dose estimates, external exposure to deposited radionuclides isgenerally the dominant contributor to effective dose in the first year (except in areas largely unaffectedby dispersion of the released radioactive material, where the doses are comparatively low and ingestionis the dominant pathway). This contrasts with the effective doses estimated in the UNSCEAR 2013
Report [U10], where the relative contribution from ingestion of food was much larger. The Committee’s
revised estimates of effective doses from ingestion are based on a more realistic assessment of thispathway and are 10–30 times lower than assessed in the UNSCEAR 2013 Report.

2.

Estimates of organ doses in non-evacuated areas in the first year

A100. Ranges of first-year average absorbed doses to the thyroid of various age groups for all of themunicipalities of Fukushima Prefecture that were not wholly evacuated, most municipalities of theGroup 3 prefectures and for the rest of Japan (Group 4) are presented in detail in attachment A-14 andsummarized in table A11. Most of the absorbed dose to the thyroid was estimated to have been received
over the first months after the accident. Comparison with the equivalent table C10 in the UNSCEAR
2013 Report [U10] again shows that revised estimates of doses from ingestion are significantly lower
than estimated in the UNSCEAR 2013 Report. The revised ranges of the average absorbed doses to thethyroid from the external exposure plus inhalation pathways are very similar to those estimated in the
UNSCEAR 2013 Report. The detailed breakdown in attachment A-14 shows, however, that this apparent
similarity is the net result of many competing factors. These factors and their effects on the Committee’s
revised dose estimates in comparison with the dose estimates in the UNSCEAR 2013 Report areconsidered in further detail in section IV.G below.
A101. Figure A-VI shows the estimated average absorbed dose to the thyroid of infants in eachmunicipality of Fukushima Prefecture that was not evacuated. The highest municipality-average doses tothe thyroid were in the municipalities of Minamisoma City, Date City, Fukushima City, Soma City andKoori Town. The highest municipality-average absorbed dose to the thyroid was estimated to have been
21 mGy for infants living in the municipality of Minamisoma City, with more than 80% of this due toinhalation, more than 10% due to external exposure to deposited radionuclides, and about 5% to ingestion.
The estimated absorbed doses to the thyroid for adults and children in the first year were about 60% and85%, respectively, of those for infants.

Geographical area

AdultcExternal +inhalation

10-year-old

Ingestiond

External +inhalation

Total

Ingestiond

1-year-old
Total

External +inhalation

Ingestiond

Total

1.0–170.070–20

1.1

1.2–21GROUP 2e – FUKUSHIMA PREFECTURE
Municipalities not evacuated

0.051–10

0.43

0.48–110.061–16

0.95

GROUP 3 – NEIGHBOURING PREFECTURES
f

Ibaraki Prefecture

0.22–2.0

0.11

0.33–2.2

0.30–3.0

0.25

0.55–3.2

0.35–3.5

0.31

0.66–3.9

Miyagi Prefecture

0.39–3.2

0.11

0.50–3.3

0.55–4.9

0.25

0.80–5.2

0.64–6.0

0.31

1.0–6.3

Tochigi Prefecture

0.30–1.2

0.11

0.41–1.3

0.35–1.4

0.25

0.60–1.7

0.40–1.7

0.31

0.72–2.0

Yamagata Prefecture

0.20–0.90

0.11

0.31–1.0

0.26–1.4

0.25

0.52–1.7

0.31–1.6

0.31

0.62–1.9

0.073

0.073–0.63

0.0h–0.65

0.087

0.087–0.74

GROUP 4g – REST OF JAPAN
42 remaining prefecturesa0.0h–0.45

0.034

0.034–0.48

0.0h–0.56

The reported doses are the ranges of the municipality-average doses for the Group 2 and Group 3 prefectures and the prefecture-average doses for the Group 4 prefectures. These estimates of dose areintended to be characteristic of the average doses received by people living at different locations and do not reflect the ranges of doses received by individuals within the population at these locations.
b

Detailed estimates are not tabulated here for doses to the fetus but can be found in attachment A-14. Ranges of average fetal absorbed doses to the thyroid over the 30-week development period of the

fetus are about 70% to 80% of the tabulated adult thyroid doses.
cAdult indoor workers have been considered to be representative of adults.

dDoses to a subgroup of the population (agricultural workers) who preferentially consumed local vegetables may be larger by a factor of about 3.

eGroup 2: Members of the public living in the non-evacuated municipalities of Fukushima Prefecture.

fGroup 3: Members of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata. These prefectures were grouped together to calculate the dose from ingestion in these prefectures.

gGroup 4: Members of the public living in the remaining prefectures of Japan, including the previous Group 3 prefectures of Chiba, Gunma and Iwate.

h

Estimated doses that are less than 1 µGy have been assigned a value of 0.0.

UNSCEAR 2020/2021 REPORT

Ranges of absorbed dose to thyroida,b (mGy)150

Table A11. Ranges of estimated municipality- or prefecture-average absorbed doses to the thyroid in the first year following the accident for residents ofJapan for locations that were not evacuatedFigure A-VI. Estimated average absorbed dose to the thyroid in the first year to infants in each municipality of Fukushima Prefecture apart from those thatwere evacuated
ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...
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UNSCEAR 2020/2021 REPORT

A102. Figure A-VII shows the estimated average absorbed doses to the thyroid of infants in the firstyear by municipality for Fukushima Prefecture and parts of the Group 3 neighbouring prefectures. For
the Group 3 prefectures (Ibaraki, Miyagi, Tochigi and Yamagata), the average absorbed doses in the
thyroid of infants were estimated to be in the range of 0.62 to 6.3 mGy, with the dominating exposure
pathway again being inhalation. In the other prefectures of Japan, the prefecture-average absorbed dosesin the thyroid of infants were estimated to have been up to about 0.7 mGy, with the dose from ingestion
being the dominant exposure pathway in most prefectures, but with external exposure to depositedradionuclides and inhalation pathways making significant contributions (up to about 60% and 20%,
respectively) to the total in some prefectures (specifically, Chiba, Gunma and Iwate).
Figure A-VII. The average absorbed dose to the thyroid in the first year following the accident for
infants living in municipalities of Fukushima Prefecture that were not evacuated and somemunicipalities of Group 3 prefectures

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).

A103. The Committee has estimated the distributions of individual absorbed doses to the thyroid ineach municipality and prefecture and the 5th and 95th percentiles of the distributions are presented inattachment A-14. The 5th and 95th percentiles are generally up to around 2–3 times lower and 2–3 times

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153

higher than the average absorbed doses to the thyroid, although they can be more than 4 times lower andhigher, respectively, in municipalities where internal exposure from inhalation of radionuclides makes
the largest contribution to the total dose. Example distributions of individual absorbed doses to the thyroid
are presented and discussed in section IV.F below.
A104. Estimates have also been made of the absorbed doses to the fetal thyroid. For the municipalities
of Fukushima Prefectures that were not evacuated, the municipality-average fetal absorbed dose to thethyroid over the 30-week development period of the fetus ranged from about 0.4 to 8 mGy. Generally,
the municipality-average fetal absorbed doses to the thyroid were estimated to be about 70% to 80% of
the corresponding adult thyroid dose in the first year.
A105. For the municipalities and parts of municipalities within Fukushima Prefecture that were notevacuated, the municipality-average absorbed doses to the different organs: (a) red bone marrow;(b) colon; and (c) female breast in the first year were all estimated to be in the range of 0.06 to 3.9 mGy
for adults, 0.06 to 4.5 mGy for children, and 0.06 to 5.2 mGy for infants. The municipality-average inutero absorbed doses to the red bone marrow over the 40-week term of pregnancy range from about
0.05 to 2.1 mGy (i.e., about half the adult red bone marrow dose in the first year). For municipalities ofGroup 3 prefectures (Ibaraki, Miyagi, Tochigi and Yamagata), the municipality-average absorbed doses
to the red bone marrow, colon and the female breast in the first year were all estimated to be in the rangeof: (a) 0.08 to 0.98 mGy for adults; (b) 0.09 mGy to 1.1 mGy for children; (c) and 0.10 mGy to 1.3 mGyfor infants. In the remaining 42 prefectures of Japan, the prefecture-average absorbed doses in the redbone marrow, the colon and the female breast in the first year were estimated to have been less than about
0.5 mGy for adults, children and infants. The estimated absorbed doses to the red bone marrow, the colonand the female breast for the different age groups in the first year following the accident for all of themunicipalities of Fukushima Prefecture, most municipalities of the Group 3 prefectures and the rest ofJapan (Group 4) are presented in detail in attachments A-15, A-16 and A-17, respectively.

3.

Effective doses and organ doses for evacuees

A106. The evacuations undertaken to protect the public from the releases from FDNPS reduced theradiation exposures that would otherwise have been received. Doses were estimated for 40 groups ofpeople who were evacuated at different times and moved to different locations (see table A9). The doses
were assessed for the period before and during the evacuation. The last evacuation occurred on 22 June
2011 from Iitate Village. The dose estimates for the period before and during the evacuation 6 were basedon the ATDM results for radionuclide concentration in air and deposition density in the days following
the accident, and estimates from Miyatake et al. [M34] of doses from ingestion of drinking water. The
estimated average effective doses to adults in these groups are summarized in table A12. The estimatesof average effective dose to adults over the periods of these evacuations were generally less than around
1 mSv, apart from the last evacuees from Iitate Village, who were estimated to have received an average
effective dose before evacuation of 3.6 mSv.
A107. The estimated average effective doses in the first year to those who were evacuated are the sumof the doses received before and during evacuation and the doses received during the remainder of theyear at the location to which they were evacuated. These doses are also summarized in table A12. The
average effective doses to adults were estimated to be generally less than about 4 mSv in the first year.
The one exception is again the last evacuees from Iitate Village who were estimated to have received an
6 For most scenarios. For scenario IT4, where people were not evacuated until 22 June, these assumptions applied up to midnighton 25 March, after which doses were estimated based on the same assumptions as for residents up to 22 June 2011.

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UNSCEAR 2020/2021 REPORT

average effective dose in the first year of 5.5 mSv. Children and infants were estimated to have received
average effective doses in the first year about 20% to 30% and 40% to 60% higher, respectively, thanthose for adults. Table A12 also shows the 95th percentiles of the distribution of individual doses among
those evacuated according to each scenario. The 5th and 95th percentiles are generally 2–3 times lowerand higher, respectively, than the average effective doses. Detailed results of the estimates of effective
doses to the evacuees and parameters of the distributions of individual doses are provided inattachment A-18.

Table A12. Estimated effective doses to adults evacuated from municipalities of Fukushima Prefecture

Effective dose to adults (mSv)
Locality

Scenario No.

Destination

Evacuationa

Destinationb

Total first yearc

Mean

Mean

Mean

95th percentile

Projectedd

Avertede01(FT1)

Saitama City

0.27

0.084

0.35

1.1

43

43

Futaba02(FT2)

Ibaraki Prefecture

0.50

0.29

0.79

2.0

43

43

Futaba03(FT3)

Ibaraki Prefecture

0.30

0.29

0.58

1.4

43

43

Futaba04(FT4)

Koriyama City

0.56

1.9

2.5

4.3

43

41

Futaba05(FT5)

Tochigi Prefecture

0.078

0.53

0.61

1.0

43

43

Kawauchi06(TM1)

Niigata City

0.041

0.005

0.046

0.072

1.6

1.5

Tomioka07(TM2)

Chiba City

0.25

0.25

0.50

1.1

18

17

Tomioka

08(TM3)Chiba City

0.13

0.25

0.38

0.67

18

17

Tomioka09(TM4)

Iwaki City

0.39

0.39

0.77

1.8

18

17

Naraha10(NR1)

Nasushiobara City

0.14

0.64

0.78

1.3

2.7

1.9

Naraha11(NR2)

Chiba City

0.22

0.25

0.47

0.97

2.7

2.2

Naraha12(NR3)

Iwaki City

0.39

0.39

0.77

1.8

2.7

1.9

Naraha13(NR4)

Tochigi Prefecture

0.069

0.53

0.60

1.0

2.7

2.1

Naraha14(NR5)

Iwaki City

0.27

0.39

0.66

1.4

2.7

2.0

Okuma

15(OK1)

Aizuwakamatsu City

0.32

0.33

0.65

1.5

37

37

Okuma

16(OK2)

Tamura City

0.34

0.46

0.80

1.7

37

37

155

FutabaANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

The doses calculated are effective doses for each evacuation scenario, before and during evacuation, and for the first year following the accident. The dose estimates areintended to be characteristic of the average effective doses received by groups of people evacuated from each locality. The 95th percentile indicates the upper bound of the
distribution of individual doses

Scenario No.

Destination

Evacuation

Destination

Total first yearc

Mean

Mean

Mean

95th percentile

Projectedd

Avertede

Okuma

17(OK3)

Shinjuku Ward

0.23

0.10

0.33

0.86

43

43

Okuma

18(OK4)

Tamura City

0.71

0.46

1.2

3.2

0.64

–

Odaka

19(OK5)

Nasushiobara City

0.30

0.64

0.94

1.7

2.2

1.3

Namie

20(NM1)

Shinjuku Ward

0.15

0.10

0.25

0.57

16

16

Namie

21(NM2)

Soma City

0.44

0.69

1.1

2.3

16

15

Namie

22(NM3)

Koriyama City

0.34

1.9

2.3

3.8

16

14

Tsushima

23(NM4)

Nihonmatsu City

0.85

2.3

3.1

5.7

16

13

Namie

24(NM5)

Yonezawa City

0.31

0.058

0.37

1.3

16

16

Iitate25(IT1)

Koriyama City

0.64

1.9

2.6

4.5

12

10

Iitate26(IT2)

Aizu Region

0.083

0.33

0.41

0.68

12

12

Iitate27(IT3)

Saitama City

0.28

0.084

0.36

0.99

12

12

Iitate28(IT4)

Iitate Village

3.6

1.9

5.5

9.1

12

6.6

Odaka

29(OD1)

Shinjuku Ward

1.0

0.10

1.1

3.9

2.2

1.1

Odaka

30(OD2)

Tsuruoka City

0.027

0.078

0.11

0.18

2.2

2.1

Haramachi31(OD3)

Yokohama City

0.17

0.054

0.22

0.58

2.3

2.1

Odaka

32(OD4)

Shinjuku Ward

0.61

0.10

0.72

2.3

2.2

1.5

Odaka

33(OD5)

Saitama City

0.43

0.084

0.52

1.6

2.2

1.7

Haramachi34(HK1)

Yokohama City

0.26

0.054

0.31

0.91

2.3

2.0

Iitate35(HK2)

Yamagata City

0.13

0.078

0.21

0.44

12

12

Kashima36(HK3)

Yokohama City

0.54

0.52

1.1

2.4

1.6

0.55

UNSCEAR 2020/2021 REPORT

Localityb

156

Effective dose to adults (mSv)aEffective dose to adults (mSv)
Locality

Scenario No.

Destination

Destination

Total first yearc

Mean

Mean

Mean

95th percentileb

Projectedd

Avertede

Haramachi

37(HK4)

Soma City

0.55

0.69

1.2

2.7

2.3

1.1

Hirono Town

10 (old)

Ono Town Office

0.46

0.30

0.75

2.1

1.9

1.2

Katsurao Village

12 (old)

Azuma Gymnasium

0.17

2.7

2.9

4.8

7.4

4.5

Katsurao Village

14 (old)

Azuma Gymnasium

1.2

2.7

3.9

7.4

7.4

3.5

a

The effective dose for the evacuation is an estimate of the dose that people received before and during evacuation.

b

The effective dose at destination is an estimate of the dose that people received for the remainder of the first year following evacuation.

c

The total first-year effective dose is an estimate of the dose in the first year that people received before and during evacuation and at destination for the remainder of the year.

d

The effective dose that is projected is an estimate of the dose that people would have received in the first year if they had not been evacuated.

e

The effective dose that is averted is an estimate of the dose that people avoided by being evacuated. In some cases, this can be estimated to be a small negative value, because of the assumption that people

were outdoors during the passage of the plume of radioactive material during evacuation, but would have been indoors if not evacuated. These cases are indicated by “–”.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Evacuation

a

157

158

Table A13. Estimated absorbed doses to the thyroid of infants evacuated from municipalities of Fukushima Prefecture

Absorbed dose to the thyroid of infants (mGy)
Original location

Scenario No.

Destination

Evacuation

Destinationb

Mean

Mean

Mean

95th percentileaTotal first yearc

Projectedd

Avertede

Futaba01(FT1)

Saitama City

3.7

0.19

3.9

7.8

490

490

Futaba02(FT2)

Ibaraki Prefecture

14

0.68

15

47

490

480

Futaba03(FT3)

Ibaraki Prefecture

10

0.68

11

33

490

480

Futaba04(FT4)

Koriyama City

12

3.7

15

43

490

480

Futaba05(FT5)

Tochigi Prefecture

2.8

1.0

3.8

6.4

490

490

Kawauchi06(TM1)

Niigata City

3.5

0.087

3.5

5.9

7.4

3.9

Tomioka07(TM2)

Chiba City

5.3

0.43

5.7

11

99

94

Tomioka

08(TM3)Chiba City

4.7

0.43

5.1

9.3

99

94

Tomioka09(TM4)

Iwaki City

10

1.6

12

33

99

87

Naraha10(NR1)

Nasushiobara City

3.5

1.2

4.6

8.2

12

7.6

Naraha11(NR2)

Chiba City

6.1

0.43

6.5

16

12

5.7

Naraha12(NR3)

Iwaki City

9.5

1.6

11

32

12

1.1

Naraha13(NR4)

Tochigi Prefecture

2.7

1.0

3.7

6.1

12

8.4

Naraha14(NR5)

Iwaki City

6.3

1.6

7.9

20

12

4.2

Okuma

15(OK1)

Aizuwakamatsu City

5.4

1.5

6.9

17

320

310

Okuma

16(OK2)

Tamura City

5.9

1.7

7.6

19

320

310

UNSCEAR 2020/2021 REPORTThe doses calculated are absorbed doses to the thyroid for each evacuation scenario, before and during evacuation, and for the first year following the accident. The dose
estimates are intended to be characteristic of the average absorbed doses to the thyroid received by groups of people evacuated from each locality. The 95th percentile
indicates the upper bound of the distribution of individual doses

Absorbed dose to the thyroid of infants (mGy)
Original location

Scenario No.

Destination

Destinationb

Total first yearc

Mean

Mean

Mean

95th percentile

Projectedd

Avertede

Okuma

17(OK3)

Shinjuku Ward

5.3

0.23

5.5

13

490

490

Okuma

18(OK4)

Tamura City

3.7

1.7

5.4

10

2.8

–

Odaka

19(OK5)

Nasushiobara City

7.4

1.2

8.5

21

21

12

Namie

20(NM1)

Shinjuku Ward

5.7

0.23

5.9

15

120

120

Namie

21(NM2)

Soma City

12

2.0

14

45

120

110

Namie

22(NM3)

Koriyama City

2.6

3.7

6.3

11

120

120

Tsushima

23(NM4)

Nihonmatsu City

9.0

4.2

13

34

120

110

Namie

24(NM5)

Yonezawa City

12

0.37

13

44

120

110

Iitate25(IT1)

Koriyama City

12

3.7

16

39

57

41

Iitate26(IT2)

Aizu Region

4.2

1.5

5.7

9.6

57

51

Iitate27(IT3)

Saitama City

8.3

0.19

8.5

18

57

48

Iitate28(IT4)

Iitate Village

14

2.5

16

30

57

41

Odaka

29(OD1)

Shinjuku Ward

30

0.23

30

100

21

–

Odaka

30(OD2)

Tsuruoka City

1.8

0.38

2.2

3.6

21

19

Haramachi31(OD3)

Yokohama City

3.9

0.15

4.0

6.6

21

17

Odaka

32(OD4)

Shinjuku Ward

23

0.23

23

76

21

–

Odaka

33(OD5)

Saitama City

14

0.19

15

44

21

6.2

Haramachi34(HK1)

Yokohama City

7.0

0.15

7.1

17

21

14

Iitate35(HK2)

Yamagata City

5.3

0.41

5.7

9.7

57

51

Kashima36(HK3)

Yokohama City

12

0.78

12

34

20

7.6

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Evacuationa159

Scenario No.

Destination

Evacuation

Destinationb

Total first yearc

Mean

Mean

Mean

95th percentile

Projectedd

Avertede

Haramachi

37(HK4)

Soma City

16

2.0

18

56

21

2.4

Hirono Town

10 (old)

Ono Town Office

3.2

1.5

4.7

9.8

10

5.1

Katsurao Village

12 (old)

Azuma Gymnasium

0.77

4.7

5.5

9.2

30

25

Katsurao Village

14 (old)

Azuma Gymnasium

8.8

4.7

14

36

30

17

a

The absorbed dose to thyroid for the evacuation is an estimate of the dose that people received before and during evacuation.

b

The absorbed dose to thyroid at destination is an estimate of the dose that people received for the remainder of the first year following evacuation.

c

The total first-year absorbed dose to the thyroid is an estimate of the dose in the first year that people received before and during evacuation and at destination for the remainder of the year.

d

The absorbed dose to thyroid that is projected is an estimate of the average dose that people would have received in the first year if they had not been evacuated.

e

The absorbed dose to thyroid that is averted is an estimate of the average dose that people avoided by being evacuated. In some cases, this can be estimated to be a small negative value, because of theassumption that people were outdoors during the passage of the plume of radioactive material during evacuation, but would have been indoors if not evacuated. These cases are indicated by “–”.

UNSCEAR 2020/2021 REPORT

Original location

160

Absorbed dose to the thyroid of infants (mGy)aANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

161

A108. The average effective dose in the first year to infants for each evacuation scenario is shown infigure A-VIII. The figure shows the contribution of the effective dose over the period of the evacuation andof the effective dose at the destination. For several scenarios, effective doses received at the destination
exceed those received before and during evacuation. The largest average annual effective dose to infants isagain for the late evacuees from Iitate Village. Comparison with figure A-III shows that (apart from this one
scenario) the doses to evacuees were generally comparable with those for municipalities of Fukushima
Prefecture that were not evacuated.
Figure A-VIII. Average effective dose in the first year to infants for each evacuation scenario

A109. The estimates of average absorbed doses to the thyroid for those evacuated are shown intable A13, where the doses presented are those to infants. The average absorbed doses to the thyroid of
infants for the period before and during the evacuations were estimated to be up to about 30 mGy. These
values are consistent with those obtained in a recent assessment of absorbed doses to the thyroid of infantevacuees that used a similar methodology [O5] (see attachment A-22). The doses were principally frominhalation during the passage of the airborne radioactive material through the affected areas in the earlydays of the accident, with a smaller contribution from ingestion of drinking water. The average absorbed
doses to the thyroid for the first year for infants who were evacuated (including doses before and during
the evacuation and doses during the remainder of the year at the destination) also ranged up to about
30 mGy. The absorbed doses to the thyroid of children and adults are about 80% and 40%, respectively,of those to infants. The Committee has estimated the distribution of doses to individuals within each
population group and the 95th percentiles of the distribution for each scenario are also shown intable A13. The 5th and 95th percentile absorbed doses to the thyroid are between about two and fourtimes lower and about two to three times higher, respectively than the average dose. Detailed results ofthe estimates of average absorbed doses to the thyroid for those who were evacuated, including estimates
for adults and children, are provided in attachment A-18.
A110. Estimates have also been made of the absorbed doses to the fetal thyroid. Generally, the fetal
absorbed doses to the thyroid over the 30-week development period of the fetus were estimated to beabout 70% to 80% of the adult thyroid dose in the first year reported in attachment A-18.

162

UNSCEAR 2020/2021 REPORT

Figure A-IX. Average absorbed dose to the thyroid in the first year to infants for each evacuation
scenario

A111. Figure A-IX shows the average absorbed dose to the thyroid for infants in the first year for eachof the evacuation scenarios, with the contribution of the dose for the period of the evacuation and thedose at the destination separately indicated. For the absorbed dose to the thyroid, much more of the total
in the first year was contributed by the period before and during the evacuation than was the case for theeffective dose. Detailed results of the estimates of average absorbed dose to the thyroid of the evacuees
are provided in attachment A-18. The protective effect of iodine blocking possibly implemented by someresidents was not taken into account in the assessment. However, the general iodine-rich diet of theJapanese people has been reflected in the dose coefficients used.
A112. Comparisons with the doses to evacuees estimated in the UNSCEAR 2013 Report [U10]indicate that the Committee’s revised estimates of average effective doses in the first year are about a
few tens of per cent lower, but that the revised estimates of absorbed dose to the thyroid in the first year
are between three and four times lower. This reduction is largely a reflection of the lower Japan-specificdose coefficients for intakes of radioiodine and the much lower destination doses from ingestion.
A113. The evacuation of municipalities was estimated, on average, to have averted effective doses toadults of up to about 40 mSv and absorbed doses to the thyroid of infants of up to about 500 mGy. In
several scenarios, the average doses estimated to have been received by the evacuees were similar to
those estimated to have been received had they stayed in place.
A114. For the small number of hospital and nursing-home patients, residents and other individuals in
the 20-km zone for whom the 40 evacuation scenarios were not applicable, higher doses could not be
ruled out. The doses that were averted, when added to the estimates of dose received before and during
the evacuation, can be used as estimates of the doses to people who might have stayed in the evacuation
zone, and as an upper bound for any individual who might have gained long term access to the zone.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

163

B. Estimation of doses in Japan over longer time periods
A115. Estimates have also been made of municipality- and prefecture-average doses accumulated overthe first 10 years after the accident and up to age 80 years. The doses from external exposure wereassessed with similar methods as for the first year but taking account of the reduction in dose rate due toradioactive decay and the model described in section III.C.1 and attachment A-1. Table A14 shows thedependence of doses from external exposure on the exposure duration and location for various age groupsof the Japanese population. Compared with the equivalent table C13 in the UNSCEAR 2013 Report[U10], the doses from external exposure are generally higher, by factors varying by population group,
exposure duration and location between a few per cent and a factor of about two. The differences reflect
the different model used (see attachment A-1 for further details). No account was taken of the possible
reduction in exposure as a consequence of remediation in these estimates of doses. The effect of theresulting overestimation of doses for time periods after the completion of remediation work in areas thathave been remediated on the doses presented in tables A14, A15 and A16 would generally have beensmall in comparison with the uncertainties in these dose estimates.
Table A14. Dependence of the effective dose from external exposure normalized by deposition
density of 137Cs on the exposure duration
Effective dose from external exposure per unit deposition densitya(mSv per 0.1 MBq/m2 as of June 2011)
Exposure duration

Age/population group (as of 2011)
1-year-old

10-year-old

Adult outdoor worker

Adult indoor worker

ENTIRE JAPAN EXCEPT FOR SOUTH TRACEb
1 year10 years
Lifetime

c

2.3

1.9

2.6

1.6

6.3

5.4

7.6

4.7

8.5

7.6

10.8

6.7

SOUTH TRACEb

1 year

2.9

2.4

3.2

2.0

10 years

6.9

5.9

8.2

5.1

Lifetimec

9.1

8.1

11.5

7.1

a

No account was taken of the reduction in exposure as a consequence of remediation in some areas.

b

Hirono Town, Iwaki City, Naraha Town and Tomioka Town of Fukushima Prefecture, and Kitaibaraki City and Takahagi Cityof Ibaraki Prefecture.
cTime up to age 80, assuming 20-year-old adult at time of the accident.

A116. For doses from the ingestion of radionuclides for time periods after the first year, the Committee
has used estimates of doses to adults and children from ingestion of caesium radionuclides derived fromthe market-basket and duplicate-diet studies as its starting point, and then used the model developed bySmith et al. [S33] to estimate how intakes of radiocaesium from food and drinking water change over the
longer term. The estimated municipality-average or prefecture-average effective doses for adults,
children and infants, integrated over 10 years after the accident and up to age 80 years are provided inattachment A-19 together with the parameters of the distributions of doses to individuals in each
population group. Ranges of the average total effective doses within different municipalities orprefectures from external and internal exposures are summarized in table A15. Compared with theequivalent table C14 in the UNSCEAR 2013 Report [U10], the estimated average effective doses

164

UNSCEAR 2020/2021 REPORT

accumulated over the first 10 years and up to the age of 80 years at the upper end of the ranges are
generally within a few tens of per cent of those estimated in the UNSCEAR 2013 Report (although thedifferences for the Group 4 prefectures are larger due to the different prefecture groupings used in thisreport); at the lower end of the ranges, they are up to 10 or 20 times lower. The 95th percentile of the
distribution of effective doses over a lifetime to individuals in each population group due to uncertainties
and variabilities is generally less than a factor of two greater than the average dose.
Table A15. Ranges of estimated municipality- or prefecture-average effective doses to adults,
children and infants (as of 2011) over the first year and first 10 years and to age 80 years
Age group in March2011

Ranges of municipality- or prefecture-average effective dosea (mSv)
Group 2bGroup 3cGroup 4d

1-YEAR EXPOSURE
Adult

0.079–3.8

0.10–0.92

0.004–0.36

10-year-old

0.10–4.5

0.13–1.1

0.005–0.43

1-year-old

0.12–5.3

0.15–1.3

0.005–0.51

10-YEAR EXPOSURE
Adult

0.16–11

0.25–2.5

0.009–1.0

10-year-old

0.19–120.30–2.9

0.008–1.2

1-year-old

0.22–14

0.34–3.4

0.007–1.3

LIFETIME EXPOSURE TO AGE 80 YEARS

a

Adult

0.22–15

0.32–3.6

0.010–1.4

10-year-old

0.24–17

0.38–4.0

0.009–1.6

1-year-old

0.27–19

0.43–4.5

0.008–1.8

The reported doses are ranges of the municipality-averaged doses for the Group 2 and Group 3 prefectures and the prefecture-

average doses for the Group 4 prefectures. These estimates of dose are representative of the average doses received by people living
at different locations and do not reflect the range of doses received by individuals within the population at these locations.
bMunicipalities of Fukushima Prefecture that were not wholly evacuated.

cMembers of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.

dMembers of the public living in the remaining prefectures of Japan, including the previous Group 3 prefectures of Gunma, Chiba

and Iwate.

A117. Figure A-X shows the estimated average effective dose as a function of age group for thedifferent integration times for people living in Fukushima City. The estimated average effective doses
over the 10-year-exposure period and the average effective doses up to age 80 years are larger by factorsof up to 2.5 and 4, respectively, than those received in the first year. External exposure to depositedradionuclides is by far the dominant pathway.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

165

Figure A-X. Estimated average effective dose as a function of age group (as of 2011) for the different
integration times for people living in Fukushima City

A118. Most of the absorbed dose to the thyroid of residents of Japan was received during the first year
from inhalation of radioiodine. Continued exposure from the longer-lived radioisotopes of caesium (fromexternal radiation from deposited radionuclides and intake by ingestion) is estimated to result in anabsorbed dose to the thyroid up to age 80 years about twice that received in the first year.

C. Estimation of current levels of exposure
A119. To provide an indication of current levels of exposure resulting from the accident, table A16
sets out the ranges in the estimated municipality- or prefecture-average annual effective doses in 2021 in
the non-evacuated municipalities of Fukushima Prefecture (Group 2), the Group 3 prefectures and theremaining prefectures in Japan (Group 4). The estimated average annual effective doses are all less than
0.5 mSv in Fukushima Prefecture (Group 2) and below 0.1 mSv elsewhere in Japan. The estimatedaverage dose in each municipality or prefecture and the parameters of the distribution of individual doses,
as well as the contributions from the pathways of external exposure to deposited radionuclides andinternal exposure from ingestion are provided in attachment A-19. The 5th and 95th percentiles of the
distributions of individual doses are about two to three times lower and two times higher than the average
doses, respectively. External exposure to deposited radionuclides generally accounts for more than 95%

166

UNSCEAR 2020/2021 REPORTof the total dose. Figure A-XI shows the estimated average annual effective dose to infants in 2021 bymunicipality for Fukushima Prefecture and parts of the Group 3 neighbouring prefectures. It includes themunicipalities in Fukushima Prefecture that were evacuated (apart from the “Areas where Returning is
Difficult”), assuming that residents have returned, and takes account of remediation in the SDA (seeparagraph A120 below).
Table A16. Ranges of estimated municipality- or prefecture-average effective doses to adults,
children and infants in the year 2021
Geographical area

Ranges of municipality- or prefecture-average annual effective dosea (mSv)
Adult

10-year-old

1-year-old

GROUP 2b – FUKUSHIMA PREFECTURE
Municipalities notevacuated

0.004–0.31

0.004–0.36

0.004–0.42

GROUP 3c – NEIGHBOURING PREFECTURES
Ibaraki Prefecture

0.009–0.037

0.010–0.043

0.012–0.051

Miyagi Prefecture

0.010–0.067

0.011–0.079

0.013–0.093

Tochigi Prefecture

0.010–0.070

0.011–0.082

0.013–0.097

Yamagata Prefecture

0.005–0.008

0.006–0.009

0.006–0.011

GROUP 4d – REST OF JAPAN
42 remaining
prefecturesa<0.001–0.028<0.001–0.033<0.001–0.039

The reported doses are ranges of the municipality-averaged doses for the Group 2 and Group 3 prefectures and the prefecture-

average doses for the Group 4 prefectures. These estimates of dose are representative of the average doses received by people living
at different locations and do not reflect the range of doses received by individuals within the population at these locations. The
doses are in addition to doses from natural background radiation and other sources of exposure and are for the people in the specified
age groups in 2021. The adult doses are for indoor workers as representative of adults as a whole.
bMunicipalities of Fukushima Prefecture that were not evacuated.

cMembers of the public living in the prefectures of Ibaraki, Miyagi, Tochigi and Yamagata.

dMembers of the public living in the remaining prefectures of Japan.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

167

Figure A-XI. The estimated average annual effective dose in 2021 for infants living in municipalities
of Fukushima Prefecture and some municipalities of Group 3 prefectures

The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by
the United Nations (Source: Adapted from Japan administrative level 0-2 boundaries, Japanese prefecture governments).

A120. The completion of remediation work in the SDA has allowed the removal of most evacuation
orders and the gradual return of people to remediated areas. Estimates of the annual external doses
received in the period 2019–2021 and up to the age of 80 years by those who were evacuated if they
returned to their homes and regular lifestyles are shown in table A17 for those who were 20-year-old
adults or 1-year-old infants in 2011. These estimates include a DRF of 1.3 to take account of the effectof remediation in the SDA.

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UNSCEAR 2020/2021 REPORT

Table A17. Estimated annual average and lifetime effective doses from external exposure for adults
and infants at the time of the accident if they were to return to municipalities from which they were
evacuated
Effective dose from external exposure (mSv)
Annual doses and doses up to age 80
years for those who were adults at thetime of the accident

Annual dose and dose up to age 80 years
for those who were infants at the time of
the accident

2019

2020

2021

Lifetime

2019

2020

2021

Lifetime

Futaba Towna

2.6

2.4

2.1

38

3.3

2.9

2.6

40

Hirono Town

0.090

0.080

0.080

1.3

0.12

0.10

0.090

1.4

Iitate Village

0.94

0.83

0.75

13

1.2

1.0

0.93

14

Katsurao Village

0.58

0.52

0.46

7.7

0.72

0.64

0.57

8.5

Kawamata Town

0.12

0.11

0.090

1.7

0.15

0.13

0.12

1.8

Kawauchi Village

0.12

0.10

0.090

1.5

0.14

0.12

0.11

1.8

Minamisoma City
(Odaka)

0.15

0.13

0.12

2.0

0.18

0.16

0.15

2.2

Namie Town

1.2

1.1

0.95

16

1.5

1.3

1.2

18

0.15

0.14

0.12

2.2

0.19

0.17

0.15

2.4

Okuma Town

2.3

2.1

1.9

32

2.9

2.5

2.3

35

Tamura City

0.050

0.040

0.04

0.69

0.050

0.050

0.050

0.69

Tomioka Town

1.2

1.1

0.99

18

1.5

1.4

1.2

19

Municipality

Naraha TownaaMunicipality entirely within “Areas where Returning is Difficult” and not included in figure A-XI (Iitate Village, Katsurao Village,

Namie Town and Tomioka Town, also contain some “Areas where Returning is Difficult” but are included in the figure).

D. Estimation of collective doses
A121. The collective effective doses 7 and the collective absorbed doses to the thyroid from the major
exposure pathways (external exposure, inhalation and ingestion) were estimated for the first year,
10 years and to age 80 years following the accident 8 for Fukushima Prefecture, neighbouring prefectures
and the rest of Japan, and are summarized in table A18. Further details can be found in attachment A-20.
7 The Committee has used the quantity, collective (effective) dose, for many years to compare the radiation exposures of populations

from different sources of ionizing radiation, or following different protection measures. The collective (effective) dose is always
estimated for a defined population over a specified period of time. It is the product of the mean effective dose to a specified
population from a particular source, and the number of people in that population, integrated over a defined period of time.
Importantly, calculated doses are recommended only for comparative purposes and not for estimations related to health effects.
Collective dose is not intended as a tool for epidemiological risk assessment. Moreover, the aggregation of very low individual
doses over extended time periods is inappropriate for use in risk projections and, in particular, the calculation of numbers of cancer
deaths from collective doses based on individual doses that are well within the variation in background exposure should be avoided.
8 In this report, the Committee has used the terms “collective effective dose” and “collective absorbed dose to the thyroid” anddescribed them as being for different time periods. This follows the approach used in the UNSCEAR 2013 Report. Strictly, thecorrect terms are “collective effective dose commitment” and “collective absorbed dose commitment to the thyroid” (for the timeintegrated quantity), and the descriptions should make clear that the dose commitment (time-integrated) has been truncated at thedifferent time periods. The Committee has retained the same terms as used in the UNSCEAR 2013 Report for ease of comparison
with that report and for simplicity.

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169

External exposure from deposited radionuclides makes the largest contribution to the collective effective
dose for all exposure durations. For exposure durations of more than a few years, external exposure from
deposited radionuclides also makes the largest contribution to the collective absorbed dose to the thyroid,
although ingestion of radionuclides in food makes a greater contribution to the collective absorbed doseto the thyroid in the first few years. The estimates of the collective doses to the population of Japan canbe compared with estimates for populations of European countries exposed to radiation following the1986 Chernobyl accident (see appendix B). The collective effective dose to the population of Japan due
to a lifetime exposure following the FDNPS accident is approximately 10% to 15% of the corresponding
value for European populations exposed to radiation following the Chernobyl accident, and the collective
absorbed dose to the thyroid is approximately 3% of that due to the Chernobyl accident. For further
perspective, the annual collective effective dose to the population of Japan from natural backgroundradiation can be estimated to be about 280,000 man Sv (about 20 times the collective effective dose in
the first year after the FDNPS accident).
Table A18. Collective effective doses and collective absorbed doses to the thyroid for the populationof Japan (128 million in 2010)
Exposure duration
Exposure pathway

First year

10 years

Lifetime(up to age 80 years)a

COLLECTIVE EFFECTIVE DOSE (thousand man Sv)
Inhalation

0.27

0.27

0.27

External

11

30

42

Ingestion

0.64

1.2

1.4

Total

12

32

44

COLLECTIVE ABSORBED DOSE TO THE THYROID (thousand man Gy)aInhalation

5.7

5.7

5.7

External

11

31

43

Ingestion

7.1

7.6

7.8

Total

24

44

57

The collective dose over a lifetime (up to age 80 years) represents more than 97% of the collective dose commitment in each case.

E. Radiation exposure outside of Japan
A122. In the UNSCEAR 2013 Report [U10] the Committee concluded that the effective doses andequivalent doses to the thyroid for individuals living outside of Japan were less than 0.01 mSv in thefirst year following the accident. This was based on an analysis of estimates in the published peerreviewed literature, the results of World Health Organization preliminary dose estimation [W13] anddose assessments carried out by Member States of the United Nations (see table C15 in the UNSCEAR
2013 Report).
A123. The Committee has reviewed literature, published since the UNSCEAR 2013 Report [U10], on
assessments made of doses in countries neighbouring, or in relatively close proximity to, Japan including
the Republic of Korea, the Russian Federation, the People’s Republic of China, and Taiwan Province of170

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China. The estimated doses in these more recent publications are summarized in table A19 and mostly
relate to doses from ingestion of food either grown in the countries themselves or imported from Japan.
In light of the studies summarized in the UNSCEAR 2013 Report and in table A19, the Committee sees
no reason to revise its conclusion that the effective doses and equivalent doses to the thyroid for people
living outside of Japan were less than 0.01 mSv in the first year following the accident.
Table A19. Estimated doses reported for countries neighbouring, or in close proximity to, Japan

Location[Reference]

Committed effective
dose in the first year
from intakes ofradiocaesium (mSv)

Comments

Asia-Pacific [P4]7×10−3

Realistic estimate based on consumption of fish caught in the
open North West Pacific Ocean (includes a conservative
estimate of the contribution from 90Sr of about 10% of thetotal)

Taiwan Provinceof China [L8]

5.7×10−3

Based on consumption of contaminated imported food. Doses
were estimated for a “worst-case scenario” frommeasurements

People’s Republicof China [S21]3×10−5

Based on environmental monitoring where activity
concentrations in vegetables, milk and drinking water were
below detection limits

People’s Republicof China [W20]9×10−6

Based on environmental monitoring and atmospheric modelling

People’s Republicof China [Y12]

1.4×10−5–2.2×10−5

Based on measurements of seafood and algae; measured levels
not significantly different from those before the accident

Russian Federation[R1]

2.7×10−3

Based on measurements and environmental monitoring oflevels in milk

F. Distributions of doses in the population
A124. The Committee’s dose estimates for residents and for evacuees set out in this appendix are
intended to be characteristic of the average doses received by people living in each municipality or
prefecture, or represented by each evacuation scenario. The ranges of doses indicated in the tables abovereflect how the average doses vary between municipalities or between prefectures rather than the rangeof doses received by individuals within a given municipality or prefecture.
A125. The Committee has, in addition, made estimates of the distributions of individual doses in eachmunicipality and prefecture taking account of uncertainties and of numerous factors that may influence
how doses may vary between individuals (e.g., concentrations of radionuclides in air and on the groundwhere they live and work, their diet, lifestyle (e.g., fraction of the time spent indoors/outdoors), shielding
provided by buildings). The dose distributions were estimated using a Monte Carlo approach, where theuncertainties and variabilities were described by assigning probability distributions to the input
parameters. These probability distributions were either estimated based on available data or by expert
judgement. See attachment A-12 for further details.

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171

A126. The 5th and 95th percentiles of the distributions of individual doses have been provided withestimates of the average doses in attachments A-13 to A-19. Distributions of individual doses (botheffective and absorbed dose to the thyroid) in the first year and over a lifetime are provided inattachment A-21 for municipalities in Fukushima Prefecture that were not evacuated, for subgroups ofthe population that were evacuated, and for the non-evacuated municipalities of Fukushima Prefecture asa whole and evacuees as a whole. Examples of these distributions are presented in figures A-XII to
A-XVII for:(a) Effective doses in the first year in Fukushima Prefecture as a whole;(b) Absorbed doses to the thyroid in the first year in Fukushima City;(c) Effective doses in the first year among evacuees;(d) Absorbed doses to the thyroid in the first year among evacuees;(e) Absorbed doses to the thyroid in the first year in Fukushima Prefecture as a whole;(f) Effective doses over a lifetime in Fukushima Prefecture as a whole.
The average dose and the median dose are shown for each distribution.
A127. The first example in figure A-XII (the distribution of effective doses in the first year in
Fukushima Prefecture) shows that most individuals were estimated to have received an effective dose
within about ten times lower and three times higher than the average dose in the prefecture (around2 mSv). A small proportion (around a thousand) of the population of about 2 million people were
estimated to have received effective doses less than 0.1 mSv and an even smaller proportion were
estimated to have received doses more than 10 mSv. The distribution shows one peak centred around anannual effective dose of about 1 mSv and a second centred around an annual effective dose of 2 to 3 mSv.
This is likely to be due to the dominance of the contribution from external exposure to depositedradionuclides to the total dose and differences in the deposition densities of radionuclides in the main
population centres in Fukushima Prefecture.
A128. The second example in figure A-XIII (the distribution of absorbed doses to the thyroid in thefirst year in Fukushima City) shows a similar pattern, although the distribution is a little narrower: mostindividuals in Fukushima City received an absorbed dose to the thyroid within the range of three times
lower and three times higher than the average dose for Fukushima City (around 9 mGy), with moreindividuals receiving a dose lower than the average than higher than the average.
A129. The third example in figure A-XIV (distribution of effective doses in the first year for evacuees)
shows that most evacuees were estimated to have received effective doses between about 0.1 and 5 mSv,
and more were estimated to have received doses less than the average than above the average effective
dose (of around 0.9 mSv). Compared with the previous distributions in figures A-XII and A-XIII, thedistribution shows a larger difference (of about a factor of nearly two) between the average dose and the
median dose (P50), reflecting greater asymmetry.
A130. The fourth example in figure A-XV (distribution of absorbed doses to the thyroid in the firstyear for evacuees) shows that nearly all of the evacuees were estimated to have received doses between
five times lower and about three times higher than the average of about 4.5 mGy. There is again acomparatively large difference between the median dose and average dose. About 15% of the evacuees
were estimated to have received absorbed doses to the thyroid of about 1 mGy or less, and around 0.2%
were estimated to have received more than 100 mGy.

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A131. The fifth example in figure A-XVI (distribution of absorbed dose to the thyroid in the first year
in Fukushima Prefecture) shows the distribution of absorbed dose to the thyroid in the first year in the
non-evacuated municipalities of Fukushima Prefecture. Most individuals in Fukushima Prefecture were
estimated to have received absorbed doses to the thyroid in the first year between about six times lower
and three times higher than the average for the prefecture as a whole of about 5 mGy. Only about 1% of
the population were estimated to have received doses lower than 0.5 mGy or greater than 20 mGy.
A132. The final example in figure A-XVII (distribution of effective doses over a lifetime for
Fukushima Prefecture) shows the doses ranging over about two orders of magnitude, with nearly all of
the individuals in Fukushima Prefecture estimated to have received effective doses over a lifetime
between ten times lower and four times higher than the average dose for the prefecture (of about 7 mSv).
This distribution shows a clear double peak (similar to, but more pronounced than, that apparent infigure A-XII), with a large proportion of the population (about 40%) estimated to have received doses ina distribution centred around an effective dose of about 2 mSv, and a second large proportion (just over40%) estimated to have received doses in a distribution centred around an effective dose of about 10 mSv.
This pattern is again likely to be a consequence of the dominance of the pathway of external exposure todeposited material and the different deposition densities of released radionuclides in areas where mostpeople in Fukushima Prefecture live.
A133. Finally, figure A-XVIII (cumulative distribution of doses in the first year in Fukushima
Prefecture) provides examples of an alternative presentation of the distributions of dose in the form of
cumulative distributions. The figure shows the fraction of the population of Fukushima Prefecture
estimated to have received an effective dose in the first year and an absorbed dose to the thyroid in thefirst year above the level indicated.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Figure A-XII. Distribution of effective dose in the first year in Fukushima PrefectureaaIncludes all age groups.

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Figure A-XIII. Distribution of absorbed dose to the thyroid in the first year in the municipality ofFukushima Cityaa

Includes all age groups.

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Figure A-XIV. Distribution of effective dose in the first year for evacueesaaIncludes all age groups.

175

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UNSCEAR 2020/2021 REPORT

Figure A-XV. Distribution of absorbed dose to the thyroid in the first year for evacueesaaIncludes all age groups.

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177

Figure A-XVI. Distribution of absorbed dose to the thyroid in the first year in Fukushima PrefectureaaIncludes all age groups.

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UNSCEAR 2020/2021 REPORT

Figure A-XVII. Distribution of effective doses over a lifetime in Fukushima PrefectureaaIncludes all age groups.

aIncludes all age groups.

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Figure A-XVIII. Reverse cumulative distribution of doses in the first year in Fukushima Prefecture (the fraction of the population estimated to have received adose above the indicated level)a

179

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G. Comparison of modelled and measured doses
1.

Effective dose from external exposure

A134. Several personal dosimetry surveys have been carried out to measure doses from external
exposure directly, typically using luminescence dosimeters. The surveys included those carried out in
Kawauchi Village, the Tamano area of Soma City and the Haramachi area of Minamisoma City [H4], on
school children in Minamisoma City [N13, N15], on adults from Fukushima City and neighbouringmunicipalities [T10], on children from Minamisoma City [T40], children in Soma City [T41], and adults
in Minamisoma City [T42]. In addition, the municipalities of Minamisoma and Naraha provided theCommittee with anonymized data on personal external doses measured between 2014 and 2019 using
luminescence dosimeters. These measurements have been compared with the predictions from the new(M2020) model and that used in the UNSCEAR 2013 Report [U10]. The new model provides a better fit
to the measurements for all these surveys, apart from that of Harada et al. [H4], where both models givedose estimates systematically lower than the reported measured values. Taken together, themeasurements span a time period of eight years after the FDNPS accident. For this whole period, the newmodel provides dose estimates that are within a factor of two of the measured doses, indicating that the
new model adequately addresses the observed dynamics of the ambient dose rate over an extended period.
Full details are provided in attachment A-1.

2.

Absorbed doses to the thyroid

A135. The measurements made on the thyroids of 1,143 individuals at various locations in Fukushima
Prefecture between 26 March and 16 April 2011 have been analysed and the Committee has estimatedabsorbed doses to the thyroid from them. The majority of the measurements (1,080) were made between
26 and 30 March 2011 on children at locations in Iitate Village (299), Iwaki City (134) and Kawamata
Town (647). Background measurements were made at the time of the thyroid measurements on these
1,080 children, but in 598 cases (55%), the measured exposure rate was less than or equal to background.
A lower limit of exposure rate was derived for each of these cases. Age-dependent calibration coefficientsfor the measuring device derived by Kim et al. [K18] have been used. Further details of the dosereconstruction methods used can be found in attachment A-2.
A136. Some of the measurements were carried out in locations that had been evacuated, and there is alack of published information about where those monitored were in the period before the measurements
were made. Information was also not available about other factors such as whether the individuals
consumed locally produced foodstuffs and whether they changed clothes before the measurement. Suchinformation does exist, but it was not available to the Committee. Nevertheless, some, albeit lessinformative, comparisons can be made on the assumption that those measured at a particular location
were representative of those who had been evacuated from or had remained at that location in accordancewith the evacuation scenarios relevant to that location as defined by Ohba et al. [O5] (seesection III.C.1(b) of this appendix). The Committee has accordingly estimated absorbed doses to thethyroid for comparison with the doses derived from thyroid measurements (“measured”), and the results
are presented in table A20; more detailed information can be found in attachment A-2. The comparison
shows very good agreement between the estimated and “measured” doses (the ratio of modelled toANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

181“measured” varies from about 0.4 to 1.3), providing some confidence in the Committee’s updated
estimates of thyroid doses and, more generally, in the approach used by the Committee to estimate doses
from intakes by inhalation and ingestion. This comparison is, however, based on a small number ofmeasurements in a limited geographical area, and both modelled and “measured” doses are associatedwith significant uncertainty (with 5% and 95% confidence intervals spanning wide ranges – seeattachment A-2).
Table A20. Comparison of modelled and “measured” thyroid doses
The number of people measured in each group is given in parentheses

Median absorbed dose to the thyroid (mGy)
Locality

Adult
Modelled“Measured”

1-year-old

Modelled“Measured”

Modelled“Measured”

Iwaki City

1.2

2.2

1.8 (79)

2.6

4.6 (55)

Kawamata
Town

0.95

1.8

2.6 (361)

2.1

4.5 (286)

1.4

2.3

3.6 (220)

2.8

7.1 (79)

Iitate Villagea10-year-oldaNamie Town

22

21 (6)

36

41

Minamisoma
Citya

5.8

6.5 (15)

9.9

12

Tamura City

0.50

1.2 (1)

1.1

1.2

10 (1)

Excluding persons evacuated shortly after the accident.

A137. Ohba et al. [O5] also found good agreement between their modelled doses and measurementsof people known to have been evacuated. Their comparison is more informative than that of theCommittee in that it was limited to evacuees whose whereabouts before and after the accident were
known to the authors (based on behaviour questionnaire survey sheets – information that was notavailable either publicly or to the Committee). The comparison of Ohba et al. provides added confidence(albeit by proxy) in the validity of the approach used by the Committee to estimate doses to the thyroid(and, by association, doses from intakes by inhalation and ingestion, more generally). The approach usedby Ohba et al. to estimate doses by inhalation was essentially the same as that of the Committee(i.e., based on the Terada et al. [T28] source term, the related ATDM data base and the evacuation
scenarios listed in table A9). The only difference was in the use of different spatial and temporal
resolutions in the respective computations. The doses estimated by the Committee are compared withthose for Ohba et al. in attachment A-22 and are broadly comparable within their respective uncertainties.

3.

Effective doses from intakes of radiocaesium

A138. In the UNSCEAR 2013 Report [U10], the Committee referred to estimates of effective dose
derived from whole-body measurements made on evacuees and residents beginning in July 2011 [H9,M43]. Including further information now available from these and other WBC measurements (seetable A2), the Committee has made estimates of the effective dose from intakes of 134Cs and 137Cs forboth evacuees and non-evacuees (residents) of Fukushima Prefecture for comparison with doses
estimated by other methods.

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A139. To assess annual doses from these whole-body measurements, assumptions needed to be madeabout when and how the intake of the radionuclides measured in the body took place and whether further
intakes occurred before the end of the year in question. In three of the studies [K16, M43], the authors
assumed that the intake was by inhalation over a short period. For the other two studies [H9, T38], theCommittee has assumed that the intakes were by inhalation (over a few hours to a few days) and ingestion(over a few weeks). The Committee has also adopted a simplifying assumption that all measurements
below the MDL were equal to half of that MDL. The Committee has only reconstructed doses from themeasurements made on adults because of the lower detection rate and greater uncertainties associatedwith the measurements made on children. The resulting committed effective doses from intakes of 134Cs
and 137Cs for each of the groups measured, as reconstructed from the whole-body measurements are
summarized in table A21. These estimates of committed effective dose are compared in table A21 withcommitted effective doses estimated for the same population groups of the different studies using themodels adopted by the Committee in this study.
Table A21. Committed effective doses to adults from intakes of 134Cs and 137Cs in the first year
estimated from whole-body measurements compared with corresponding doses estimated using
the models adopted by the Committee in this report

Population measured
using WBC

Minamisoma Citya [T38]bWBC(all intakes)

Using models adopted in this report

Committed effective dose up to
time of measurement (μSv)

Average committed effective dose fromintakes within first year (μSv)

Average

Inhalation

Ingestion

All intakes

~50f

~42k

~19p

~61

g

l

p

~32

~5.1m

~19p

~24

n

~19

p

~27

~4.8o

~19p

~24

Minamisoma City [H10]~80

Hirata Central Hospitalc [H9]

~50h

d

Median13i

Evacuated municipalities [M43]~25

Evacuated municipalitiese [K16]~30–70j

~7.8

~19

a

Adult residents of Minamisoma City.

b

Hayano et al. [H10] reported only on measurements made of those who lived in “the most contaminated zone”, about 85% ofwhom had been evacuated at different times during March 2011, but had returned in July 2011.
c

Seventy-three per cent from Fukushima Prefecture, 23% from Ibaraki Prefecture with the remainder from Tochigi and Miyagi

Prefectures. No information available on where, within the prefectures, those measured were at the time of the accident or where
they lived subsequently within the respective prefectures.
dAdult evacuees from Futaba Town, Hirono Town, Iitate Village, Katsurao Village, Kawamata Town, Kawauchi Village,

Minamisoma City, Naraha Town, Namie Town, Okuma Town and Tomioka Town.
e

Adult evacuees from the same municipalities as in footnote d above apart from Minamisoma City and inclusion of Tamura City.

Median doses only estimated for Futaba Town, Iitate Village and Namie Town.
f

Estimated by the Committee assuming short term intake with all measurements less than the MDL assumed to be equal to half

the MDL.
g

Estimated by [H10] assuming acute inhalation of radiocaesium in March 2011.

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h183

Estimated by the Committee assuming short-term intake with all measurements less than the MDL assumed to be equal to half the

MDL. The Committee has used the measurement data in the first year after the accident at FDNPS, although the authors restricted theiranalyses to measurements made after March 2012 because of possible surface contamination of clothes before this date.
i

Estimated by the authors by extrapolating to the 50th percentile assuming a log-normal distribution and assuming acute inhalationof radiocaesium in March 2011. The median dose takes no account of potential bias consequent upon a fraction of the measurements
being affected by contamination on clothes. The authors also presented dose distributions for each of six evacuated municipalities,
with the doses varying from about a factor of two lower, to a few tens of percent higher, than those for all municipalities.
j

Estimated by the authors assuming a log-normal distribution and an acute intake by inhalation on 12 March. The quoted values

are the range of median doses of those measured in Namie Town (26 μSv), Iitate Village (27 μSv) and Futaba Town (68 μSv),
which are not necessarily the median dose in the whole population of the evacuated municipalities.
k

Estimated by the Committee as the average dose for residents of Minamisoma City from air concentrations predicted by scalingmeasured deposition densities by ratios derived from ATDM.
l

Estimated by the Committee as the average dose for evacuees from Kashima and Haramachi wards of Minamisoma City (whocomprised 82% of those measured) based on predicted air concentrations using source term and ATDM.
m

Estimated by the Committee as the average dose to residents in Fukushima Prefecture based on predicted air concentrations

derived by scaling measured deposition densities by ratios derived using source term and ATDM.
n

Estimated by the Committee as the average dose for all evacuees from Fukushima Prefecture based on predicted air

concentrations using source term and ATDM.
o

Estimated by the Committee as the average dose for evacuees, with account taken of where those measured were evacuated from,based on predicted air concentrations using source term and ATDM.
p

Estimated by Murakami and Oki [M47] from measured concentrations in food and drinking water in Fukushima City. The

Committee has assumed this estimate to be applicable to the whole of Fukushima Prefecture while recognizing that there will besome variation with location.

A140. The values presented in table A21 are subject to large uncertainties and are, in many cases, notdirectly comparable. Estimates of the uncertainties in the values presented and discussion of the limitations
on the comparisons are set out in detail in attachment A-3. Nevertheless, the sums of the doses frominhalation and ingestion intakes estimated using the models the Committee has adopted in this report arebroadly in agreement with the committed effective doses estimated from the WBC measurements. This
indicates that the modelling approaches used by the Committee are able to provide estimates of doses
comparable with the measurements made on people. The estimates given in table A21 also indicate that
average effective doses from intakes of radiocaesium are low (both in absolute terms and relative to the total
doses from all radionuclides and all exposure pathways), ranging from a few tens of µSv to about 100 µSv
for those population groups for which whole-body measurements were made. While intakes of radiocaesiumby inhalation and ingestion may make only a small contribution to the overall doses, the comparison
provides some assurance that the methodology used by the Committee for estimating doses via inhalationand ingestion more generally are broadly valid. Further details are provided in attachment A-3.

H. Comparisons with dose estimates in UNSCEAR 2013 Report
A141. To illustrate the differences between the doses estimated in this report with those estimated in
the UNSCEAR 2013 Report [U10], figures A-XIX and A-XX show scatter plots comparing estimates inthe two reports. Figure A-XIX compares estimates of the municipality-average effective dose in the firstyear and over a lifetime to an adult for each of the municipalities in Fukushima Prefecture. The diagonalline in each figure indicates where the doses would be equal; points below the line are where current
estimates of doses are lower than in the UNSCEAR 2013 Report, and points above the line are where
current estimates of doses are higher than in the UNSCEAR 2013 Report. The comparison for the184

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municipality-average effective dose in the first year to adults shows that the current estimates are
systematically lower than those in the UNSCEAR 2013 Report for all municipalities by an amount that
increases with decreasing estimated dose (from a few tens of per cent for municipalities with the higher
doses to a factor of several for municipalities with lower doses). Over a lifetime, however, the effective
doses to adults estimated in this report are similar to those estimated in the UNSCEAR 2013 Report inmany municipalities, but higher (by up to 30%) for municipalities with higher doses. The general
reduction in the current estimates of effective doses in the first year compared with those in the
UNSCEAR 2013 Report are largely due to the more realistic and lower estimates of doses from ingestion;in the UNSCEAR 2013 Report, the dose from ingestion dominated the total dose in municipalities where
the doses from other pathways were low. Other factors that have led to a decrease in the current estimatesof effective dose in the first year include the reduction in the dose from inhalation because account has
been taken of filtration inside buildings, and the dose coefficients used that are specific to the Japanese
population. Over the longer term, these decreases in the estimated effective doses are counterbalanced byan increase in the estimated dose from external exposure to deposited radionuclides (see section III.C.1(a)of this appendix).
A142. Figure A-XX compares estimates of the municipality-average absorbed doses to the thyroid in
the first year to infants from all pathways and from inhalation only, again for each of the municipalities
in Fukushima Prefecture. The comparison for the municipality-average absorbed dose to the thyroid fromall pathways shows very clearly that the current estimates are consistently, and considerably (by up to anorder of magnitude or more in some cases), lower than those in the UNSCEAR 2013 Report [U10]. Thislarge difference can be attributed to the dose from ingestion having been overestimated in that reportbecause of the unduly cautious assumptions made. Consideration of inhalation dose only shows that, formost municipalities, the dose estimates in this report are lower as a result of using Japanese specific dosecoefficients for intakes of radioiodine, and because account has been taken of filtering by buildings in
reducing the concentrations of radionuclides indoors. However, the different source term and ATDM
used in this report has also introduced additional scatter. The Committee considers that, as a result ofusing more realistic estimates of doses from ingestion and taking due account of filtering by buildings
and of the iodine-rich Japanese diet, the estimates made in this report are much more realistic.

Figure A-XIX. Scatter plot of the effective dose (a) in the first year and (b) over a lifetime to adults estimated in this report compared with that estimated in the
UNSCEAR 2013 Reporta

The straight line indicates the 1:1 relationship (i.e., where the dose estimates would be equal).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

a

185

186

Figure A-XX. Scatter plot of the absorbed dose to the thyroid in the first year to infants (a) from all pathways and (b) from inhalation only estimated in thisreport compared with that estimated in the UNSCEAR 2013 Reporta

UNSCEAR 2020/2021 REPORTa

The straight line indicates the 1:1 relationship (i.e., where the dose estimates would be equal).

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

V. ACKNOWLEDGEMENTS OF ORGANIZATIONS THATPROVIDED DATACountry
Belarus

Organization
Belarusian State Medical University
Hirosaki University
Minamisoma City

Japan

Naraha Town
National Institute of Radiological Sciences of the
National Institutes for Quantum and Radiological
Science and Technology

Russian Federation
UkraineFederal Medical Biological Agency
National Research Centre for Radiation Medicine
National Academy of Medical Sciences of Ukraine187

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

189

APPENDIX B. COMPARISON BETWEEN VARIOUS ATTRIBUTESAND CONSEQUENCES OF THE ACCIDENTS AT CHERNOBYLAND FUKUSHIMA DAIICHI NUCLEAR
POWER STATIONS
B1.
The main features or attributes of the accidents that occurred at Chernobyl Nuclear Power
Station (CNPS) and Fukushima Daiichi Nuclear Power Station (FDNPS) from peer reviewed literatureare summarized in the table below together with estimates of the resulting exposures of, and health effects
in, workers and the public. The consequences of the accident at FDNPS were much lower than those at
CNPS (e.g., for the FDNPS accident, estimated average effective doses to adult evacuees were less thanabout 6 mSv and average absorbed doses to the thyroid were less than about 15 mGy, compared witharound 30 mSv and 500 mGy for the CNPS accident). These lower consequences were due to thefollowing main reasons (see table B1):(a) A much larger fraction of the more significant radionuclides (isotopes of iodine and caesium) in
the fuel of the respective reactors (also referred as units) was released to the environment from CNPS
than FDNPS. The reactors at FDNPS had purpose-built containments within which most of theradionuclides released from the molten fuel were retained; by contrast, the reactor at CNPS did nothave a containment and the core was directly exposed to the atmosphere as a result of the explosionthat occurred at the beginning of the accident;(b) The explosion at CNPS resulted in a fire on the roof of the reactor and turbine buildings that had
to be extinguished to minimize the risk of the accident escalating and propagating to other reactor
units. In this process more than a hundred emergency workers were exposed to high levels ofradiation (exposure of the whole body to external radiation and of the skin to beta radiation) that
resulted in acute radiation syndrome (ARS);(c) The radionuclides released to the atmosphere from CNPS were dispersed and largely depositedover the land mass of Europe. By contrast, some 80% of the release to the atmosphere from FDNPS
was dispersed and largely deposited over the Pacific Ocean, where the consequences (in terms ofdoses to and effects on humans) were considerably less than for radionuclides dispersed anddeposited over land. Beyond the evacuation area, deposition densities were also much lower from
Fukushima compared with Chernobyl;(d) The accident at FDNPS occurred earlier in the agricultural growing season than that at CNPS
(11 March compared with 26 April) with the result that a much lower fraction of depositedradionuclides was transferred to foodstuffs;(e) The binding or fixation of radiocaesium in many soils in Japan and in the former Soviet Unionis broadly comparable, as is its bioavailability. However, much enhanced and sustained uptake ofradiocaesium occurred from some poor agricultural soils (sandy, organic) that occurred in affected
regions of Belarus, the Russian Federation and Ukraine around CNPS and in highly organic andupland soils in Western Europe. There is no evidence of such enhanced uptake from Japanese soils;(f) Agricultural practice and animal husbandry differ between Japan and the former Soviet Union (i.e.,
in Japan by far the majority of cattle are housed in barns and do not graze pasture) with the result thatthe transfer of radionuclides to milk and meat following the accident at FDNPS was much lower;

190

UNSCEAR 2020/2021 REPORT(g) Protective measures (in respect of people, e.g., precautionary evacuation, and foodstuffs) were
implemented more effectively following the accident at FDNPS than at CNPS, resulting in a larger
fraction of the potential doses being averted;(h) The failure to inform the population, in particular in more rural areas, of the need to restrict oravoid the consumption of potentially affected foodstuffs especially milk, following the accident at
CNPS resulted in very large doses to the thyroids of many people and an increased incidence,
attributable to radiation exposure, of thyroid cancer, in particular among those who were children at
the time;(i) The restrictions imposed in Japan on the concentrations of radionuclides in foodstuffs that weremarketed for consumption were much lower than in the former Soviet Union, resulting in lower
doses from ingestion.

Table B1. Main attributes and features of the accidents at Chernobyl and Fukushima Daiichi Nuclear Power Stations
Chernobyl Nuclear Power Station (Unit 4)

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

Reactor type

RBMKa

BWRb

Electric power

1 000 MWe

460, 784 and 784 MWe

Location

Inland site

Coastal site

Containment

No

Yes

Date of accident

26 April 1986

11 March 2011

Origin/s of the accident

Uncontrolled nuclear chain reaction occurred during a safety test toExtended loss of off-site and on-site power and heat removal capacity as result
demonstrate that cooling water circulation could be maintained following of the great east-Japan earthquake and the associated tsunami. Fuelan electric power outage pending back up power becoming available.
overheated/melted with hydrogen generated from water-fuel cladding (Zr)This resulted in: a rapid and large increase of energy that vaporized
reactions. Fission and activation products released from the fuel, the relative
superheated cooling water; a steam explosion; melting of the fuel andamounts depending on their volatility
other core components; reactor building extensively damaged; reactor
core exposed to the atmosphere and on fire for about 10 days (burning of High volatility (low vapour pressure) elements (e.g., noble gases, I, Te, Cs)
released almost totally from the fuel; percentage release from the fuel ofthe graphite moderator and other core materials); and fragments of themoderate volatility elements (e.g., Ba, Sb, Nb, Sr) were a few orders ofcore (fuel, graphite moderator, control rods and other core components)
magnitude lower than elements of high volatility; and percentage release ofejected from the building in the initial explosion
refractory elements (e.g., La, actinides) was even smaller
Little retention of fission and activation products released from the molten
fuel in the absence of a containment system and the core being exposed Most of the fission products (apart from noble gases) released from the fuelwere retained in the reactor pressure vessel and/or the primary containment
directly to the atmosphere. Release of less volatile elements (e.g., Sr,
as a result of condensation, plate out on various surfaces and scrubbing. A
actinides) larger than would have been expected on account of theirfraction was released to the atmosphere though breaches in the reactor
volatility alone – a result of the fragmentation of the fuel during thepressure vessel and primary containment and/or by deliberate venting,
explosion and physical generation of aerosols and particles containinghydrogen
explosions, etc. Releases occurred over an extended period, withless volatile elements. Releases occurred over an extended period withmost occurring in the period from 12 to the end of March 2011
most occurring within the first 10 days after the accident

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

REACTOR CHARACTERISTICS AND SOURCE TERM

191

133Xe6 500 (≈100%)

7 300 (≈61%)

131I≈1 760 (≈60%)

120 (≈2%)

134Cs≈47 (≈30%)

10 (≈1.3%)

137Cs≈85 (≈30%)

10 (≈1.3%)

90Sr≈10 (≈4%)<0.01 (<0.001%)d239Pu≈0.013 (≈1.5%)

Very smalle

Dispersed and largely deposited over the land mass of Europe≈80% of the release dispersed and largely deposited overthe Pacific Ocean

Pattern of release

Direct release (PBq) to
the seaf(% of core inventory)

3H

0.3–0.7

131I

11–18 (≈0.2%)

134Cs

Not applicableg

3.5–5.6 (≈0.6%)

137Cs

3.5–5.6 (≈0.6%)

90Sr

0.04–1 (not applicable)
Pacific Ocean

Indirect release (PBq) to the seah(deposited from the atmosphere
onto the sea surface)

Black Sea – ≈2.8 (137Cs)

131I: ≈ 57–100

Baltic Sea – ≈3.0 (137Cs)

134Cs: 5–11
137Cs: 5–11

UNSCEAR 2020/2021 REPORT(% of core inventory)

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

192

Release (PBq) to theatmospherec

Chernobyl Nuclear Power Station (Unit 4)

Chernobyl Nuclear Power Station (Unit 4)

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

EMERGENCY RESPONSEi
Precautionary evacuation of 78 000 from within 20 km zone
116 000 (in 1986)

Deliberate evacuation of 10 000 on the basis of measurements
Voluntary evacuation of 30 000 from 20–30 km evacuation prepared zone

Number of people relocated

220 000 (in 1989–1992) plus those evacuated in 1986
(i.e., ≈340 000 in total)

Those evacuated (i.e., 118 000 in total, as indicated above)

Iodine prophylaxis

Iodine tablets were issued but their distribution/issue was neither consistentnor uniform. They were not distributed to evacuees within the 20 km zone
Not implemented in a timely or effective manner (except for evacuees from despite this being the intent. They were distributed (on or after 14 March
Pripyat)2011) to everyone younger than 40 years living within the Fukushima
Prefecture between about 20 and 50 km of FDNPS (about 1 million tablets
were distributed)

Food restrictions

Various countermeasures applied (e.g., evacuation of animals from 30 km
zone, supply of clean feed, controlling levels of radionuclides in food,Countermeasures relating to food, drinking water and agriculture
converting milk to storable products, etc.). The focus was on collective
implemented generally in a timely and effective manner along withfarms. The population was not adequately informed of the need forarrangements for informing the public. “Provisional regulation values”
protective actions, in particular the private farming sector prevalent in(replaced in April 2012 by “standard limits”) established controlling the levels
rural areas. Countermeasures in some areas were not applied or wereof radionuclides in food and drinking water, and extensive monitoring
delayed with the result that contaminated milk and vegetables were
campaigns were implemented
consumed that should, otherwise, have been restricted; milk consumption
resulted in large (and avoidable) doses to the thyroids of many people

TRANSFER IN THE TERRESTRIAL ENVIRONMENT

Fixation and bioavailability ofradiocaesium in soil

Radiocaesium is strongly bound to many soils in the former Soviet Unionand most European countries, which limits its bioavailability. Fixation,
The fixation and bioavailability of radiocaesium in Japanese soils are broadly
however, in poor quality agricultural soils (sandy, organic) present in somecomparable with many soils in the former Soviet Union. No evidence,affected areas of Belarus, Russian Federation and Ukraine and highly
however, of enhanced and sustained uptake of radiocaesium as experienced
organic and upland soils in Western Europe is much lower (andin poor quality agricultural soils in areas affected by the CNPS accident
bioavailability much higher); this resulted in sustained and relatively highuptake of radiocaesium from such soils

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Number of people evacuated193

Larger fraction of cattle grazing pasture at the time of the accident in theformer Soviet Union – resulting in high transfer of depositedradionuclides to meat and milk (especially in rural areas)

Majority of cattle housed in barns with only a small fraction grazing pasture –resulting in much lower transfer of radionuclides to meat and milk

DOSES TO EMERGENCY/RECOVERY WORKERSi
Number of emergency and recovery
workers

Workers with acuteradiation syndrome

Numberdiagnosed
Doses

Emergency workers: ≈ 600
Recovery workers: ≈530 000

About 21 000 in the first year and varying within a range of ≈14 000 to 21 000
in each subsequent year

134

None

Bone marrow doses ranged from 0.8 to 16 Gy
Skin doses 10 to 30 times greater
Average annual dose: 13 mSv in first year (declining in subsequent years withina range from ≈6 to ≈2 mSv in the year ending 31 March 2020)

Effective dose

Average individual dose: ≈120 mSv

Other emergency andrecovery workers

168 and 6 workers, respectively with doses >100 mSv and 250 mSv in first year
Maximum absorbed dose in first year – 32 Gy
About 180 workers with thyroid equivalent dose >100 mSv

Thyroid doseCollective
effective dose
Iodine prophylaxis

Maximum annual dose: 680 mSv in first year (declining in subsequent years
within a range from ≈50 to ≈20 mSv in the year ending 31 March 2020)

61 000 man Svj
Stable iodine tablets administered to emergency workers and to somepersonnel operating other reactors at CNPS

≈860 man Svk up to March 2020
About 17 500 stable iodine tablets issued to about 2 000 workers during theaccident although records of their issue are incomplete

UNSCEAR 2020/2021 REPORT

Agricultural practice and animalhusbandry

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

194

Chernobyl Nuclear Power Station (Unit 4)

Chernobyl Nuclear Power Station (Unit 4)

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

DOSES TO PUBLICi

Average individual doses to residents
in different regions, municipalities orprefectures

Effective
dosem,n(mSv)

Group (adults)

Time period

≈500

≈50

EvacueesFirst year

≈100

≈13

Municipalities inFukushima
Prefecture

NumberThyroid dosem(mGy)(thousands)

Group

Time period

EvacueesFirst year

115

“Contaminatedareas”o inBelarus, Russian First year forthyroid
Federationp anddose
Ukraine

6 400

1986-2005
foreffectivedose

98 000≈20≈2500 000≈1≈0.4

Belarus, RussianFederationp andUkraineRest of Europe

Ranges of individual doses

1 200

400 000l

Number Thyroid dosem(mGy)(thousands)

Effective
dosem
(mSv)

118

≈0.8–15≈0.05–6

1 900≈0.5–10≈0.2–10

Municipalities in First ten years
for effective
neighbouring
dose
prefectures

17 000≈0.3–3≈0.3–3Prefectures inrest of Japan

110 000≈0.03–0.5≈0.009–1

First year forthyroid dose

Absorbed doses to the thyroids of evacuees ranged from <50 mGy to >5 Gy,
Absorbed doses to the thyroids of evacuees varied over a wide range (5th towith several hundred evacuees receiving doses in excess of 5 Gy
95th percentile) from less than about 1 mGy to about 15 mGy
Absorbed doses to the rest of the population of Belarus, Russian Federation
Absorbed doses to the thyroids of non-evacuees range up to about 15 mGy
and Ukraine (98 million) varied over a wide range, with most receiving(95th percentile) with about 1% >20 mGy
thyroid doses <50 mGy and about 1% doses >200 mGy

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Measurements of radioiodine inthyroid195

Evacuees (first year)

Number(thousands)

Thyroidr(thousandman Gy) in thefirst year

Effectiver,s(thousandman Sv)in the period
1986–2005

115

57

≈6

6 400

650

85

“Contaminated areas”oCollective dosesq

in Belarus, RussianFederationp andUkraineCollective doses to the whole of the Japanese population (128 million) in thefirst ten years
Effective: 32 thousand man Sv
Thyroid: 44 thousand man Gy (24 thousand man Gy in the first year)

Belarus, RussianFederationp andUkraine98 000

1 600

≈200

Rest of Europe

500 000

660≈160

HEALTH EFFECTS IN EMERGENCY AND RECOVERY WORKERSt
Early

ARS

134

None

Deaths attributable to
ARSu

28

None

ARS survivors

Skin injuries and radiation induced cataracts. Some increases in otherdiseases probably due to ageing and other factors not attributable toradiation

Not applicable

Other emergency andrecovery workers

Evidence of a dose related increase in the incidence of leukaemia andcataracts among those who received higher doses; no evidence of other
health effects that can be attributed to radiation exposure

None to date(Nuclear Emergency Workers Study ongoing)

Late

UNSCEAR 2020/2021 REPORTGroup

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

196

Chernobyl Nuclear Power Station (Unit 4)

Chernobyl Nuclear Power Station (Unit 4)

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

HEALTH EFFECTS IN, AND SCREENING OF, THE PUBLICv

Thyroid cancer

Systematic screening of thyroids of about 600 000 young people using
ultrasoundLower resolution (compared with Fukushima) ultrasound equipment used
for screening with lower sensitivity to detect thyroid abnormalities

High resolution ultrasound equipment used for screening with greater
sensitivity to detect thyroid abnormalities

Substantial fraction of the 19 000 thyroid cancers observed (up to 2016)
among people who were children or adolescents at the time of theaccident attributable to radiation exposure

Greater incidence of thyroid cancer and abnormalities observed in thosescreened than were expected based on national statistics – most likely the
result of using high resolution ultrasound in the screening. Increasing bodyof evidence that the observed thyroid cancers are not attributable toradiation exposure

Other effects (e.g., other cancers, birthdefects, fetal deaths, non-cancer
No persuasive evidence of any other health effect attributable to radiation exposure at CNPS or FDNPS
diseases, etc.)
Social/psychological impact

Increases observed in a wide range of health conditions following both accidents but no evidence that any are directly attributable to radiation exposure;rather, they are the result of changes in lifestyle (e.g., following evacuation/relocation), psychological stress, social stigma, etc., associated with theaccident. Also, in the case of the accident at FDNPS, the wider impacts of the earthquake and associated tsunami

DOSES AND EFFECTS ON NON-HUMAN BIOTAw
Maximum accumulated doses(first months post deposition)

Pine trees = >100 Gy (4 km2 zone near CNPS)

Pine trees = 0.5 Gy (first month)

Mammals = 110 Gy (first 5 months, gamma-radiation)

Mammals = 0.2 Gy (first month)Complete death of pine trees was observed over an area covering 4 km2
Effects in terrestrial plants(within the first years post-accident)

No confirmed instances of forest die off attributable to the accident atand sublethal effects in conifers including death of meristems occurred
over and area of 38 km2. The colour of dead pine stands, appearing during Fukushima have been published although some sublethal, morphological
the summer of 1986, resulted in the forest being referred to as the “redaberrations in conifers have been documented
forest”

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

Screening

Systematic screening of thyroids of a large number of young people(as of 1986) using ultrasound197

Fukushima Daiichi Nuclear Power Station (Units 1, 2 and 3)

Effects in invertebrates(within the first years post-accident)

Effects in other animals(within the first years post-accident)

Substantial impacts in terrestrial animals were observed. The number of
No population level impacts on animals in the impacted areas have beenrodents (autumn 1986) in highly contaminated areas had decreased by acorroborated. There have been observations of sublethal cytogenetic andfactor of 2–10 times. Chronic radiation effects were observed in autopsied
haematological impacts on various groups including chromosomal
larger animals including reduced body mass and the presence ofaberrations in mice and low blood cell counts in Macaque. The few studies
haematomas in organs. Cattle suffered severe impacts on their thyroids
where population impacts have been inferred, notably for birds, remain
connected with disruption of the hormonal balance and reproductive
unsubstantiatedfailure and in some cases deatha

RBMK – graphite moderated, water cooled reactor.
BWR – boiling water reactor.
cValues taken from [I2] and/or [U8] for CNPS and from [T28] or [U10] for FDNPS unless otherwise stated; inventory and percentages released from CNPS taken from [I1].
dThree to four orders of magnitude less than radiocaesium [I8].
e
No notable increase in levels present from global fallout from the testing of nuclear weapons in the atmosphere prior to the accident [I8].
fValues from this annex (see table 2).
g
CNPS sited inland so no direct release to the sea.
h
Values for CNPS from [U8]; those for FDNPS from this annex (see table 2).
iValues taken from [U8] for CNPS and from this annex for FDNPS.
j
For the period 1986 to 2005.
k
On-site workers at FDNPS; about 40% of the collective dose up to the end of March 2020 was received in the first year after the accident.
lDirect measurements of thyroids in Belarus, Russian Federation and Ukraine [U20].
m
Rounded values (one significant figure) generally quoted.
nIncluding the contribution to effective dose from internal exposure due to intake of 131I.
o“Contaminated” areas in Belarus, Russian Federation and Ukraine designated as those with 137Cs deposition in excess of 37 Bq/m2 (1 Ci/km).
p
Nineteen affected regions of the Russian Federation.
qNote that the doses reported for CNPS and FDNPS may not be directly comparable (i.e., different population groups and time periods over which doses integrated).
rRounded to one or two significant figures.
s
From [U8], vol. 2, table 2. Collective effective dose includes the contribution from internal exposure due to intake of 131I.
t
Values for Chernobyl from [U8] (effects up to 2006).
uDeaths within 4 months – other deaths occurred later for those diagnosed with ARS but in general the cause of death was not attributable to radiation exposure.
v
Values for Chernobyl from [U15] (effects up to 2016) and those for FDNPS from this annex.
w
From [U8] and [U10].
b

UNSCEAR 2020/2021 REPORT

Sublethal damage in invertebrates has been observed including genetic
damage in earthworms, and morphological aberrations in aphids andbutterflies. Impacts on populations linked to these sublethal effects havebeen inferred as opposed to directly observed and the few studies
suggesting significant populations impacts have not been corroborated

Population level effects in invertebrates were observed in the mostcontaminated areas. The number of invertebrates within 3–7 km was
reduced by a factor of 30 and reproduction was strongly impacted, i.e.,
larvae and nymphs were absent

198

Chernobyl Nuclear Power Station (Unit 4)

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

199

SUPPLEMENTARY INFORMATION
A number of attachments are cited in this annex that provide supplementary information on the detailed
analyses that have been made of doses to the public and their outcomes.
The attachments cited in this annex are electronically available for download fromhttp://www.unscear.org/unscear/en/publications/2020_2021_2_Attachments.html.
They comprise the following:
A-1

The estimation of external doses from deposited radionuclides following the accident at the
Fukushima Daiichi Nuclear Power Station and their validation

A-2

The estimation of thyroid doses resulting from the accident at the Fukushima Daiichi Nuclear
Power Station and their validation

A-3

The estimation of whole-body doses from the intake of radiocaesium following the accident at the
Fukushima Daiichi Nuclear Power Station

A-4

Japan-specific data

A-5

Data sets downloaded from the Japan Atomic Energy Agency Environment Monitoring Database

A-6

MEXT survey of ground deposition: results of the radionuclide analysis of soil sampling at
2,200 locations in Fukushima Prefecture and neighbouring prefectures

A-7

MEXT survey of ground deposition: radioactivity concentration analysis of iodine in the
distribution survey of radioactive substances

A-8

MEXT survey of ground deposition combined with demographic data

A-9

Atmospheric transport, dispersion and deposition modelling of air concentration over Japan

A-10 Methodology for estimating external doses from the plume and internal doses from inhalation ofradionuclides in the plumeA-11 Evacuation scenarios and number of evacuees
A-12 Approach for assessing uncertainties and variability in estimated doses
A-13 Estimates of effective dose to people in Japan for the first year after the accident at the Fukushima
Daiichi Nuclear Power Station
A-14 Estimates of absorbed dose to the thyroid of people in Japan for the first year after the accident at
the Fukushima Daiichi Nuclear Power Station
A-15 Estimates of absorbed dose to the red bone marrow of people in Japan for the first year after theaccident at the Fukushima Daiichi Nuclear Power Station

200

UNSCEAR 2020/2021 REPORT

A-16 Estimates of absorbed dose to the colon of people in Japan for the first year after the accident at
the Fukushima Daiichi Nuclear Power Station
A-17 Estimates of absorbed dose to the breast of people in Japan for the first year after the accident at
the Fukushima Daiichi Nuclear Power Station
A-18 Estimates of effective dose and absorbed dose to the thyroid of evacuees in Japan for the first year
after the accident at the Fukushima Daiichi Nuclear Power Station
A-19 Estimates of the effective dose to people in Japan over longer periods
A-20 Estimates of collective dose for the Japanese population resulting from the accident at the
Fukushima Daiichi Nuclear Power Station
A-21 Distributions of doses in municipalities and evacuation groups
A-22 Estimation of thyroid doses to evacuees from inhalation of radioiodine and comparison with thosederived from measurements of people and estimated by Ohba et al.
A-23 Power calculations for epidemiological detection of health effects from the accident at the
Fukushima Daiichi Nuclear Power Station

ANNEX B: LEVELS AND EFFECTS OF RADIATION EXPOSURE DUE TO THE ACCIDENT AT THE FUKUSHIMA ...

201

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A2

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A3

Adachi, K., M. Kajino, Y. Zaizen et al. Emission of spherical cesium-bearing particles from anearly stage of the Fukushima nuclear accident. Sci Rep 3: 2554 (2013).

A4

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A6

Akahane, K., S. Yonai, S. Fukuda et al. NIRS external dose estimation system for Fukushimaresidents after the Fukushima Dai-ichi NPP accident. Sci Rep 3: 1670 (2013).

A7

Akimoto, K. Annual and weekly cycles of radioactivity concentration observed in Fukushima
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A8

Akimoto, S. Morphological abnormalities in gall-forming aphids in a radiation-contaminatedarea near Fukushima Daiichi: selective impact of fallout? Ecol Evol 4(4): 355-369 (2014).

A9

Akleyev, A., I. Deltour, L. Krestinina et al. Incidence and mortality of solid cancers in people
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A10

Amano, H., M. Akiyama, B. Chunlei et al. Radiation measurements in the Chiba Metropolitan
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A11

Ambe, D., H. Kaeriyama, Y. Shigenobu et al. Five-minute resolved spatial distribution ofradiocesium in sea sediment derived from the Fukushima Dai-ichi Nuclear Power Plant.
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Andoh, M., S. Mikami, S. Tsuda et al. Decreasing trend of ambient dose equivalent rates over awide area in eastern Japan until 2016 evaluated by car-borne surveys using KURAMA systems.
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A13

Aoyama, M., Y. Hamajima, M. Hult et al. 134Cs and 137Cs in the North Pacific Ocean derived
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one: surface pathway and vertical distributions. J Oceanogr 72(1): 53-65 (2015).

A14

Aoyama, M., Y. Hamajima, Y. Inomata et al. Radiocaesium derived from the TEPCO
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A15

Arai, T. Radioactive cesium accumulation in freshwater fishes after the Fukushima nuclear
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202

UNSCEAR 2020/2021 REPORTA16Asano, T. and A. Ito. Experience and perspective on radiation dose registry in Japan. JPN J
Health Phys 54(2): 135-136 (2019).

A17

Aung, H.P., Y.S. Aye, A.D. Mensah et al. Relations of fine-root morphology on 137Cs uptake by
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GLOSSARY
Absorbed dose

Fundamental dosimetric quantity defined as the energy imparted
by ionizing radiation in unit mass of matter; measured in grays (Gy);
1 gray is equal to 1 joule per kilogram (J/kg).

Activity

The rate at which spontaneous transformations occur in a given
amount of radioactive material. (Strictly, the expectation value ofthe number of nuclear transformations occurring in a given quantityof material per unit time). It is measured in becquerels (Bq);
1 becquerel equals one transformation per second.

Acute exposure

Exposure received within a short time period (see also protracted
exposure).

Adjustment

The process of statistically accounting for effects of differences
between groups or populations under comparison in order tocontrol confounding. Adjustment is frequently performed whenestimating effect measures from epidemiological data, for example
by stratification or multivariate regression analysis.

Attained age

Age at observation or during the follow-up, usually refers to age astime-dependent characteristic in a cohort study.

Baseline risk

Baseline risk refers to the probability that an event of interest (e.g.,diagnosis of cancer) will occur in an individual among an unexposed
population over a given time period (e.g., lifetime following
exposure).

Bayesian inference

The method of inference that quantifies the degree of belief (or the
analyst’s state of knowledge) of a true value or sets of values of aquantity of interest by combining their prior knowledge about
these quantities with recent measurements, observations, or
estimates. The degree of belief of the quantity of interest is often
described by a subjective probability distribution representing
possibly true values for that quantity. The distribution resulting
from Bayesian inference is called a posterior distribution.

Becquerel (Bq)

The SI unit of activity, equal to one transformation per second. As
the unit is so small, multiples are frequently used such as
megabecquerels (MBq) which is 106 or a million becquerels(1 GBq is 109 Bq; 1 TBq is 1012 Bq; and 1 PBq is 1015 Bq).

Bias

A statistical estimation procedure is “biased” if the expected valueof the estimate of the quantity of interest is not equal to the true
value of the quantity. The “bias” of the procedure is the difference
between the expected and true values.

Biodosimetry

The use of biological (or chemical, physiological) markers of
exposure to reconstruct acute or protracted radiation doses.

Biokinetic model

A mathematical model for the behaviour of radionuclides in the
body which takes account of the movement, retention andexcretion of a radionuclide as a function of time.

Chronic exposure

Exposure persisting in time. (See also: “Acute exposure” or “Protracted
exposure”.)

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Cold shutdown

Defined in the context of this report by TEPCO and the Nuclear
Emergency Response Headquarters as the state where the coolant
water temperatures of Units 1–3 were less than 100ºC, the pressure
inside the reactor vessels was the same as the outside air pressure,and where any further releases would not result in dose rates
greater than 1 mSv per year at the site boundary.

Collective dose

The total radiation dose incurred by a population (that is the sum of
all the individual doses); measured in man–sievert (man Sv) or man–gray (man Gy).

Committed dose

The integral, over a defined period of time, of the dose rate in aparticular tissue or organ that will be received following the intakeof radioactive material into the body.

Confidence interval

In “frequentist inference”, a confidence interval is an interval
defined in terms of the sampling distribution of a statistic of interest(i.e., the distribution of estimates of the statistic that would arise
from repeated—generally hypothetical—realizations of data
generated from the same underlying distribution as the observeddata) such that, for example, the probability that a 95% confidence
interval for a given parameter contains the true value of thatparameter is 0.95. (Compare this with “Credible intervals” used in“Bayesian inference”.)

Confounding factor or confounder

A confounding factor is a variable that is correlated with both theexposure (e.g., radiation exposure or dose) and the outcome
variable (e.g., risk of lung cancer) and, if not controlled foranalytically, may distort the conclusions. For example, occupation
may be a confounding factor in a study of the relation between lungcancer incidence among non-smokers (dependent variable) andmedical radiation exposure (independent variable). For instance, aircrew are exposed to higher levels of radiation due to theiremployment (correlation with radiation exposure) while staff
working in certain recreation industries are often occupationally
exposed to cigarette smoke (correlation with outcome lung cancer).
This confounding might be controlled by introducing into theanalysis an indicator for the occupational group.

Decontamination

The complete or partial removal of contamination by a deliberate
physical, chemical or biological process.

Deterministic effectA health effect of radiation exposure for which a threshold level ofdose generally exists, above which the severity of the effect isgreater for a higher dose.

Dose

A measure of the energy deposited by radiation in a target. Dose canbe used as a shorthand for absorbed dose and effective dose when
the context is clear.

Dose coefficient/dose per unit intake

The committed effective dose or committed absorbed or equivalentdose in a tissue or organ resulting from an intake, by a specified
means (usually ingestion or inhalation), of unit activity of a specified
radionuclide in a specified chemical form.

Dose rate

Dose delivered or received per unit time. Measurements are usually
made in terms of the dosimetric quantity, ambient dose equivalentrate, H*(10), in units of μSv/h.

Effective dose

Fundamental dosimetric quantity for the purposes of radiologicalprotection defined as the tissue-weighted sum of the equivalentdoses in all specified tissues and organs of the body; measured insieverts (Sv).

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Equivalent dose

Fundamental dosimetric quantity for the purposes of radiologicalprotection defined as the product of the absorbed dose in the tissueor organ and the appropriate radiation weighting factor for the typeof radiation giving rise to the dose; measured in sieverts (Sv).

Error

The difference between an observed or estimated value and thetrue (but unknown) value. Since the true value is unknowable, error
cannot be quantified. Error can be contrasted with uncertainty,
which can be estimated by repeated measurements or Bayesianinference.

Evacuation

The rapid, temporary removal of people from an area to avoid or
reduce short-term radiation exposure in an emergency.

Excess risk/rate

A measure of the statistical relationship between a given risk factor
and a specific outcome. Depending on the context it can refer tosome characterization of the influence on rates such as the “relativerisk”, “excess relative risk”, or “excess (absolute) rate”, or, perhaps
most appropriately, to estimates of the risk over some period of time(lifetime risk) associated with an exposure of interest. The excess
rate is strictly a statistic calculated from observed frequencies/rates,
while the excess risk is a prospective estimate inferred from theobservations and reasoning.

Exposure

The act or condition of being subject to irradiation. External
exposure is exposure to radiation from a source outside of the body.
Internal exposure is exposure to radiation from a source within thebody.

External dose

Dose from an external radiation source; obtained from being withina radiation field.

Frequency (of occurrence of disease)

The number of new cases of the disease under study divided by the
number of people in a population over a defined time period.

Gene

The fundamental physical and functional unit of heredity. A gene isan ordered sequence of nucleotides located in a particular position
on a particular chromosome that encodes a specific functional
product.

In vivo measurement

A procedure used to determine the nature, activity, location or
retention of radionuclides in the body by direct measurement.

Inference

The process of drawing conclusions from scientific observations,evidence and reasoning in the presence of uncertainty. While thisreport is focussed on prospectively inferring risk, note that
estimating an assigned share (or probability of causation) is alsoinference, but retrospective.

Internal dose

Dose from radioactive material deposited in the body.

Latency (period)

The period between exposure and manifestation of a health effect.
This is also the period after which statistically significant increases infrequency of occurrence of the health effect in a population havebeen seen; theoretically, there might be an undetectable increased
frequency of occurrence of the health effect in an exposed
population during the presumed latency period, but this possibility
becomes vanishingly small in the period shortly after exposure
because there is a finite time required for damaged cells to replicate
in an uncontrolled manner and manifest as a cancerous growth.

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Lifetime risk

Probability that a disease occurs from a given point of time (e.g., atexposure) until the end of life. The lifetime baseline risk refers to theprobably of a disease occurring over a lifetime without exposure
additional to the background from natural and other sources ofradiation. Lifetime risk due to exposure is the additional probabilityof a disease occurring over a lifetime due to additional radiation
exposure.

Likelihood

Generally, the state or fact of being likely or probable. The term may
also be used to express a defined statistical concept. Specifically,
given a set of data and a statistical model that describes the
distribution of the data in terms of some parameters, the statisticalconcept of “likelihood” is a function of the model parameters that is
proportional to the probability density function for the data (given
the parameter values). For independent observations, the likelihoodof the data is the product of the likelihood values for eachobservation. Likelihood functions play a central role in both
“frequentist inference” and “Bayesian inference” (albeit withdifferent interpretations). Frequentist inference often proceeds byfinding parameters that maximize the likelihood given the data
(maximum likelihood estimation) and using (asymptotic) propertiesof the maximized (log-) likelihood as the basis for inference.

Monte Carlo analysis

Computation of a probability distribution of an output of a modelon the basis of repeated calculations using random sampling ofvalues of uncertain input variables specified as probability
distributions. The numerical sampling strategy used may be Simple
Random Sampling, or a form of stratified sampling such as Latin
Hypercube Sampling.

Pooled analysis

A combined analysis of original data from two or more data setsbearing on a common question of interest. The analysis may include
parameters that distinguish between the different data sets.
(Contrast with “meta-analysis”.)

Protracted exposure

Exposure persisting in time (see also acute exposure).

Radionuclide

A radioactive isotope of an element. Different isotopes of anelement have the same number of protons but different numbers ofneutrons and hence different atomic masses. If there are too manyor too few neutrons, the nuclei of the isotope tend to be unstable
and transform into the nuclei of another element and in the processemit radiation.

Relative risk

Ratio of the risk for two groups, estimated from the relative rates of
disease between, for example, an exposed group and an unexposedgroup.

Remediation

Any measures that may be carried out to reduce the radiation
exposure from existing contamination of land areas through actions
applied to the contamination itself (the source) or to the exposure
pathways to humans.

Shielding

The absorbing property of material between a radiation source anda receptor which results in reduced exposure.

Sievert (Sv)

Unit of equivalent dose and of effective dose, equal to 1 joule perkilogram (J/kg).

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Source term

Mathematical expression used to denote information about theactual or potential release of radiation or radioactive material froma given source. Here this term includes the release rate, radionuclide
composition, physico-chemical form and their changes over time of
the radionuclides released.

Statistical power

The expected probability of being able to detect an effect of a
specified magnitude, estimated before the start of a study. It
depends on study size, baseline risk and effect size. Statistical powerreflects only random error in outcome occurrence, while ignoringmeasurement error and typically assumes no bias in the study.

Stochastic effectA radiation-related health effect, the probability of occurrence of
which is greater for a higher radiation dose and the severity of which(if it occurs) is independent of dose.

True dose

The true but unknown value of dose for a specific individual. The
state of knowledge about this unknown true value can becharacterized by measurements and estimates that produce a
distribution representing state of knowledge about possibly true
values. In an epidemiological cohort, there will be a unique set of
unknown true doses.

Uncertainty

Expression of having doubt, or being unsure about study results,
hypotheses, model-based estimations or results of measurements,and specifically the true value of a quantity of interest. This may be
due to lack of complete knowledge about true values for anindividual or to a lack of complete knowledge of factors explaining
the inter-individual variability of true values in a defined subgroupor population. Unlike error, uncertainties can be quantified.
Estimates of uncertainty represent the amount or percentage bywhich an observed or calculated value might differ from its true
value. For a quantity of interest that has a true fixed value,
uncertainty is defined here as a degree of belief probability
distribution comprising many realizations of possibly true values.
For a quantity of interest that is a group or population of true values,
uncertainty can be characterized as many alternative realizations ofsets of true values.

Variability

Heterogeneity, diversity or a range which characterizes variation inestimated, measured, or true values of a quantity of interest. The
term variability is often used to describe differences in measured ortrue values among individuals in a population or cohort. Examples
include inter-individual differences in body weight and/or dose orinter-cohort differences in exposure–response due to differences insensitivity to a hazardous agent. Further study cannot reduce
variability, but may provide additional information to explain
reasons why some of this variability occurs. This additional
information can reduce the fraction of inter-individual variability
initially treated as stochastic.

unscear.org

This publication contains:
VOLUME II
Scientific annex with appendices
Annex B: Levels and effects of radiation exposure due to the accident at the Fukushima
Daiichi Nuclear Power Station: implications of information published since the
UNSCEAR 2013 Report

E VA L U AT I N G R A D I AT I O N S C I E N C E F O R I N F O R M E D D E C I S I O N - M A K I N G
In 1955 the United Nations General Assembly established the Scientific Committee on the Effectsof Atomic Radiation (UNSCEAR) in response to concerns about the effects of ionizing radiation
on human health and the environment. At that time fallout from atmospheric nuclear weapons
tests was reaching people through air, water and food. UNSCEAR was to collect and evaluate
information on the levels and effects of ionizing radiation. Its first reports laid the scientific grounds
on which the Partial Test Ban Treaty prohibiting atmospheric nuclear weapons testing was
negotiated in 1963.
Over the decades, UNSCEAR has evolved to become the world authority on the global levels
and effects of exposure to ionizing radiation. UNSCEAR’s independent and objective evaluationsof the science are to provide for—but not address—informed policymaking and decision-making
related to radiation risks and protection.

V. 21-09816

ISBN 978-92-1-139207-4