Artificial Radionuclides in the Environment
2015
環境における人工放射能の研究 2015

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Koenji, Tokyo
Fukushima Accident

2Radioactivity deposition (mBq/m /month)

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Chernobyl Accident
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Nuclear tests byformer USSR, USA, etc.
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Nuclear tests by China
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Atmospheric Environment and Applied Meteorology Research Department&
Forecast Research Department, Meteorological Research InstituteJAPAN
August 2016

気象研究所

環境・応用気象研究部
予報研究部

2020

最終ページに図の詳しい説明があります。
See figure caption at the end page of this publication.

「環境における人工放射能の研究 2015」について

気象研究所では、1954 年以来、大気及び海洋の環境放射能の研究を実施してきました。
特に人工放射性核種の降下量を 60 年余りの長期にわたり、東京・つくばで精密測定してきました。この観測は、世界でも最長の定点観測です。このモニタリング観測の期間では、1950年代から 1960 年代にかけては、旧ソ連や米国等による核実験が行われ、また、その後も 1986
年には旧ソ連のチェルノブイリ原子力発電所の事故が発生するなどがありましたが、この観測によって、それらの影響を含めたバックグラウンドの人工放射性核種の長期的変動を明らかにし、国民の安全・安心に寄与してきました。2011 年には不幸にして東日本大震災に伴う福島第一原子力発電所事故が発生し、それによって放出された放射性核種の推移を調査するという課題も担うこととなりました。今後も長期的視点で、人工放射性核種の変動を監視する必要があります。
この研究によるこれまでの観測や解析から、核実験や事故などによる一次放出に加え、長期的変動に対しては、一旦地面などに沈着した人工放射性核種が、風送、燃焼、植物などの働きにより空気中へ再飛散する過程が重要であることが明らかになりつつあります。このような変動メカニズムを正しく理解するためには、長期的観測に加えて、数値計算モデルを用いた解析が必要です。気象研究所では、気候変動や降水予測の研究のため、大気中の微粒子の動きを表現する数値モデルを開発してきましたが、そのような数値モデルを利用して、バックグラウンド放射能の変動メカニズムを解析する研究にも着手しています。
本論文集「環境における人工放射能の研究 2015」 は、その研究成果を､関係省庁の担当者の方々及び大学や試験研究機関の研究者の方々に広く周知するために、福島原発事故後二年から現在までの期間に出版された論文（主に英語論文）を、過去から現在までの成果と最近のトピックスに関するテーマ毎に分類し、各テーマの冒頭に簡単な日本語の解説を加えて､一冊にまとめたものです。
なお、この研究は原子力規制庁放射能調査研究費により実施されています。

平成 28 年 8 月
気象研究所 研究総務官 齊藤 和雄

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気象研究所（現在は環境・応用気象研究部および予報研究部）では、1954 年以来、環境放射能の観測・測定法の開発、放射能汚染の実態の把握、大気や海洋における物質輸送解明のト
レーサーとしての利用を目的として、60 年余にわたって環境放射能の研究を実施してきた。
1957 年以降、各省庁がそれぞれの所掌で実施してきた環境放射能調査研究関連業務は、まず旧科学技術庁、その後文部科学省が所管してきた。さらに原子力規制委員会と原子力規制庁の発足に伴って 2013 年からは、原子力規制庁が所管することとなった。このような長期にわたり多数の研究者が係わり、観測・研究を継続してきた結果、環境放射能について世界的にも他に類を見ない貴重な時系列データが内外に提供され、また様々な気象学・海洋学的発見をもたらしてきた。この間の研究成果は 200 編以上の原著論文や解説資料として内外の雑誌や成書で公表されている。
1954 年 3 月 1 日に米国によりビキニ環礁で行われた水爆実験により、危険水域外で操業していた第五福竜丸乗組員が放射性物質を含む降灰（いわゆる死の灰）による被ばくを受けた事件を契機にして、日本における環境放射能研究が本格的に始まった。当時の気象研究所地球化学研究室は環境の放射能の分析・研究において日本で有数の研究室であり、海洋及び大気中の放射能汚染の調査・研究に精力的に取り組んだ。その結果、当時予想されていなかった海洋の
放射能汚染、さらに大気を経由した日本への影響など放射能汚染の拡大の実態を明らかにすることができた。1958 年には、放射能調査研究費による特定研究課題の一つである「放射化学分析（落下塵・降水・海水中の放射性物質の研究）」を開始し、以降、地点の変動はあるが、札幌、仙台、東京、大阪、福岡の五つの管区気象台、秋田、稚内、釧路、石垣島の 4 地方気象
台、輪島、米子の 2 測候所の全国 11 気象官署及び観測船で採取した海水中の人工放射性核種（90Sr、137Cs、3H 及びプルトニウム）の分析を実施してきた。
東京・つくばにおける大気中の人工放射性核種の降下量は 1961 年から 1962 年に行われた大規模な大気圏核実験の翌年である 1963 年に最大値を観測した。その後、「部分的核実験禁止条約」の締結により米ソの大気圏核実験が中止された結果、降下量はおよそ１年の半減時間で減少した。この放射性核種の降下量の減少率は成層圏にまきあげられた物質の滞留時間を反映している。その後も、中国及びフランスにより大気圏核実験は続けられ、人工放射性核種の降下量は増減を繰り返した。1980 年の中国による最後の大気圏核実験の後、放射性フォールアウト
は再度 1 年の半減期間で減少し、1985 年には 1957 年の観測開始以降最も低いレベルになった。
しかし、1986 年の旧ソ連のチェルノブイリ原子力発電所事故により、大気中の人工放射性核種濃度（特に揮発性の高い 131I、134Cs、137Cs など）は日本でも 1963 年に近いレベルに達するほど著しく増加した。大部分の放射性核種は対流圏の滞留時間（25 日）で減少したが一部 137Cs は成層圏にも輸送されていることが分かった。1988 年以降は低いレベルで推移しているが、明瞭な減少の傾向は見られなかった。この原因は一度地上に降下した放射性核種の再浮遊に由来すると考えている。さらに、再浮遊がどこで起るかについて研究を進め、東アジアの砂漠域で発生する黄砂が有力な候補であることを明らかにした。黄砂の発生は大陸域の環境変化と関連しており、降下物中の人工放射性核種は、砂漠化や関連する植生変化など大陸域の環境変化の指標となりうることが分かった。
大気フォールアウトの研究と共に、海洋における放射性核種の挙動も長期に調査研究を実施し-3-てきた。日本周辺海域ばかりでなく、太平洋の広域に亘って海水試料の採取を実施し、放射能汚染の実態を明らかにした。1960 年代後半から 1970 年代の調査で、海洋表面水中の放射能が北半球の中緯度で高いことを明らかにし、フォールアウトの緯度分布を反映していることが分かった。その後、海洋表面水中の放射性核種は海洋の物質循環に支配されていることを明らかにした。さらに、海水中の人工放射性核種の分析法の高度化を実現し少量の試料で分析可能にし、海洋の 137Cs 濃度の精密鉛直断面を描き、核実験由来の 137Cs の主な部分は北太平洋の亜熱帯中層に存在していることを明らかにした。フォールアウトによる人工放射性核種の海洋への主な降下以来数 10 年以上経過し、その広がりは北太平洋から、インド洋など他の海盆に及んでいる。これに関連して 2003－4 年に実施された BEAGLE2003 の航海で採取された海水について人工放射性核種の分析を行って、その広がりについて知見が得られた。1993 年旧ソ連/ロシアによる放射性廃棄物の日本海等への海洋投棄の実態が明らかにされ、それに伴う日本海の放射能調査の実施に参加した。放射性廃棄物による影響は検出されなかったが、調査の結果を踏まえ、日本海における固有水の生成過程及び生成場所（ウラジオストック沖）についての知見を得ることができた。これらは、海洋放射能のデータベースとして公開されている。
1990 年以降の環境放射能汚染として、1997 年の旧動力炉核燃料開発事業団「アスファルト
固化処理施設」の火災爆発事故や 1999 年の JCO ウラン燃料工場の臨界事故があるが、いずれも環境中に放出された放射能は極めて低いレベルで、その影響は殆どなかった。しかしながら、1372011 年 3 月に起きた東日本大震災にともなう東京電力福島第一原子力発電所事故により、Csの総量だけで 10 PBq（ペタベクレル）以上という過去日本では経験したことのない大量の人
工放射性核種が環境中に放出され、その 2 割程度が日本の国土に降下し、残りの 8 割は北太平洋に降下したと考えられている。これらの影響評価やその後の環境中での拡散状況の把握のためにも、環境放射能調査・研究は今後とも重要である。
2006 年 4 月より 2011 年 3 月まで、気象研究所では放射能調査研究費による特定研究課題
として「放射性降下物の長期変動と再浮遊に関する研究」及び「海洋環境における放射性核種の長期挙動に関する研究」の二課題で環境放射能研究に取り組んできた。また、2011 年 8月からは、「大気を通じた人工放射性核種の陸圏・水圏への沈着およびその後の移行過程の解明研究」の課題のもと、過去の知見を十分に活用しながら、同年 3 月 11 日に発生した東日本大震災にともなう福島第一原子力発電所の事故以降の新たな事態に対応し、放出された
大量の人工放射性核種の挙動を解明する研究に着手した。目の前の大気に加えて、北太平洋
におけるこれらの核種による汚染実態の把握やその動態の解明に取り組んだわけである。
過去 60 年余にわたり実施されてきたこうした研究成果を踏まえ、2015 年 4 月からは「人工放射性核種のバックグラウンド大気監視と数値解析に関する研究」を新たに開始した。本報告書では、この研究課題で得られた成果を含め、最新の成果やトピックスをテーマ毎に記述した。関係各位の今後の研究や業務に役立つ資料とすべく、編集作業を行った。我が国における環境放射能研究や大気科学研究に多少なりとも寄与できたとすれば、著者一同の望外の喜びである。

2016 年 8 月
気象研究所 環境・応用気象研究部
予報研究部

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Contents
1. Temporal trends of radioactivity concentrations in the atmosphere
2. Physical and chemical characteristics of the radioactive materials

3. Evaluations of radioactive materials in the atmosphere using numerical models
4. The results of analysis on the samples which were collected before the accident of Fukushima
Daiichi Nuclear Power Plant but their data were not reported.
5. Publications

2013-2015

6. Publication list

1954-2015

7. Publication list (in Japanese)

1954-2015

8. History of the studies at the Meteorological Research Institute
9. A caption of cover art

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目次
1. 大気中の放射性物質の濃度の推移
2. 大気中の放射性物質の物理・化学性状
3. 大気中の放射性物質のモデルによる評価
4. 福島第一原発事故前に採取されデータ未報告だった試料の分析・測定結果について
5. 出版論文

2013-2015

6. 論文リスト（英文誌）1954-2015
7. 論文リスト（和文誌）1954-2015
8. 表：研究の歴史
9. 表紙説明-7-

1. 大気中放射能濃度の推移
1）はじめに気象研究所では、大気圏内核実験が盛んに実施された 1950 年代後期から 60 年近くの期間にわたり、大気圏での人工放射性核種の濃度水準の実態とその変動要因を明らかにすべく、
環境影響の大きい重要な核種について観測を継続してきた。特に、90Sr（半減期 28.8 年）および 137Cs（半減期 30.2 年）の月間降下量（大気降下物）の長期観測結果は、2016 年 4 月
で満 59 年を迎える。また、2011 年 3 月の東京電力福島第一原子力発電所事故（福島第一原
発事故）前後では、大気エアロゾル試料の採取及び放射能分析を継続し、その結果をいち早く学会で報告するとともに、研究所のホームページにも掲載してきた。ここでは前報に続き蓄積したデータ、
すなわち Igarashi et al. (2015) に掲載した茨城県つくば市での約三年間にわたる福島第一原発事故による大気環境影響に関する観測結果（時系列データ）の概略を紹介
し、事故直後における放射性物質の再浮遊（二次飛散）などに関する考察について解説する。

2）研究方法月間大気降下物の捕集は、茨城県つくば市長峰にある気象研究所の露場観測実験棟屋上に設置した大気降下物捕集用のプラスチック製水盤（面積 4m2）で 1980 年代以降実施してきた。福島第一原発事故以降（2011 年 4 月以降）は、放射能水準の上昇を考慮し、1 m2 水盤
２基での捕集を実施している。得られた試料はポリ製保管容器で保存しつつ、ロータリーエ
バポレーターや蒸発皿等を使用して全量を蒸発濃縮・固化し、まず Ge 半導体検出器によりγ線放出核種（放射性 Cs 等）を測定した。次いで試料の一部を分取して濃硝酸、過酸化水素を添加し加熱酸分解操作によって溶液化した。その後放射化学分離により 90Sr を精製し、最終的に炭酸 Sr として固定した。数週間放置して 90Sr と 90Y とが放射平衡に達した後に、
低バックグラウンド 2πガスフロー検出器でβ放射能を測定した。福島第一原発事故後の試料については、事故により放出された 89Sr の影響があるため、生成固定した炭酸 Sr 線源のβ放射能を繰り返し測定することで、90Y の放射平衡達成と 89Sr 放射能の減衰の様子とを確認しつつ評価し、必要な場合は計算で 89Sr の影響を除去した。

3）結果・議論
3.1) 90Sr および 137Cs 大気降下量の変動と推移
Fig.1 に気象研究所における月間降下量の変動を 1950 年代後半より福島第一原発事故以降、2014 年末までのデータを含めて描画した。単位は毎月当たりの降下量（mBq/m2）とした。
福島第一原発事故が発生した 2011 年 3 月の 137Cs 月間降下量は 23±0.9 kBq/m2 であり、震災前の水準よりも 6～7 桁大きかった。ここで示した福島第一原発事故後のデータの絶対値については、発生源が近いためその空間代表性は小さくなっていることに注意いただきたい。
しかし、2012 年以降は（二次的な放出）が主な過程となりつつあることから、時系列変動の傾向は関東地方全体でほぼ同様となり、それなりの空間代表性を持つと考えている。2011
年全体では、気象研究所における 137Cs 降下量は 25.5 kBq/m2/年であった。
Fig. 1 に示すように 1957 年から福島原発事故以前についての単純な 137Cs 降下量積算は、
およそ 7 kBq/m2 である。福島第一原発事故は単一事象としてこの数倍をもたらした。また、
137
Cs の放射壊変を考慮した現存量（およそ 2 kBq/m2）と比較したときには 10 倍強の量をもたらした。これに加え 134Cs（半減期 2.1 年）がほぼ同量降下し、両核種併せての地表面汚染はおよそ 50 kBq/m2 を超えた。この値は、文部科学省が実施した航空機マッピングによる
つくば市周辺の値とほぼ整合する。その後の降下量は急速に低下してきたが、2005－2010 年
における 137Cs 降下量は 1.2－97 mBq/m2/月の範囲で、2013 年における降下量は 2－39 Bq/m2/
月の範囲であり、依然として 3～4 桁の差がある。2011 年 3 月の 90Sr 降下量は 4.4±0.1 Bq/m2-8-であり、同月の 137Cs 降下量の約 1/5,000 だった。この降下量水準は、震災前の水準からすると 2～3 桁大きい。2011 年全体では 90Sr 降下量は 10.6 Bq/m2/年であり、137Cs 降下量の約1/2,500 であった。これに対し、1957 年からの 90Sr の放射壊変を考慮した蓄積量はおよそ 0.9
kBq/m2 である。したがって、90Sr による関東地方の汚染はかなり軽微であった。また、事故前の 2005－2011 年における 90Sr 月間降下量は、0.5－19 mBq/m2/月であったのに対し、2013
年での降下量は 1－33 mBq/m2/月となった。すなわち、90Sr については福島第一原発事故以前の降下量水準に戻りつつあるが、137Cs については 1970 年代～1980 年代前半の水準に依然として匹敵する。降下量水準の低下は事故直後とは異なり、全体に 2012 年以降緩やかなものとなっている。

Fig. 1 Monthly deposition is expressed in mBq/m2 on a logarithmic scale. Sr-90 and 137Cs analyses
from deposition samples taken 6 and 8 months before the accident, respectively, are ongoing to avoid
possible sample contamination at the MRI because of the accident. Thus, these data are missing not
only in Figure 1. The measurement uncertainty (1σ) is shown only for the data obtained after the
Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, and is reasonably small compared tothe analytical data. For comparison, uncertainty for the monthly data in 2010 is also given. The
effects of atmospheric nuclear bomb tests have been recorded since 1957. Until the Partial Test Ban
Treaty (PTBT) became effective in 1963, the USA, Soviet Union, and UK conducted atmospherictests. France and China continued atmospheric testing until 1974 and 1980, respectively. Since 1981,
all the nuclear bomb tests have shifted underground, so additional radioSr and Cs contamination
should be negligible. However, the Chernobyl accident in 1986 also affected the time series. The
simple summation of the deposition from 1957 to the time before the FDNPP accident (mid-2010)
and decay-corrected summations for 90Sr and 137Cs can be compared to the FDNPP-derived
deposition. From Igarashi et al. (2015)-9-Fig. 2 Activity concentration is expressed in Bq/m3 on a logarithmic scale. The measurement
uncertainty (1σ) is shown. The maximum concentration of 38 Bq/m3 of 137Cs was observed during
March 20–21, 2011. After that, the radioCs concentrations rapidly decreased until fall 2011, when
the decrease slowed. The levels before (approximately 1 μBq/m3) and three years after the FDNPP
accident (12 μBq/m3 from March to August 2014) are also compared. A difference of at least oneorder of magnitude is observed between the concentration level from March to August 2014 and thelevel before the FDNPP accident. From Igarashi et al. (2015)3.2) 137Cs 大気中濃度の変動と推移
大気中 137Cs 濃度は、2011 年 3 月 20〜21 日の期間、最高値として 38 Bq/m3 を記録した。
その後、濃度は 2011 年秋までは速やかに減少し、それ以降は減少が鈍くなった。福島第一原発事故以前の 137Cs 濃度水準は高くとも約 1 μBq/m3 であったが、事故の 3 年後、2014 年での 137Cs の平均濃度水準は約 12 μBq/m3 で、事故前の水準には戻っていない。この原因は大気中に放射性 Cs を供給する過程として、汚染した環境からの再浮遊（二次的な放出）があるためと考えられる。時折、濃度上昇が観測され、グラフではスパイク状に見える。これら
は事故サイトからの空気の流れがあった際に発生していた。
事故以来数年間の大気データは、放射性 Cs 濃度がどのような減少傾向を示したか、その時間変動を表し、再浮遊過程について検討・考察するための重要な根拠となる。
これらに加えて、Igarashi et al. (2015)では、福島第一原発事故発生直後からのモニタリン
グ即応活動の結果（90Sr の大気中濃度を他のγ線放出核種と併せて示した）や、大気放射能汚染に対するモデリング、モデルを利用したデータ解析アプローチなど、関連した情報が付属資料で提供されている。

3.3)

事故直後の 137Cs 大気中濃度の変動とエアロゾルモデルによる解析－事故直後の再浮遊Fig. 3 に a) 放射性核種の福島第一原発事故直後の大気中濃度の時間変動、b) そのエアロゾルモデルによる再現計算の結果の比較を示した。b)では実測値とモデル計算値は全体によく一致するが、モデル値ではプルームが通過すると濃度値はほぼゼロにまで低下し、たとえば 3 月 16－20 日の期間は実測値とまったく一致しない。この不一致の原因はモデルでは再浮遊をまったく計算していないことにあると考えた。そこで、観測値とモデル計算値を組み合わせて、放射性物質が沈着した直後にどの程度の再浮遊が生じているのか、求めてみた（Igarashi et al., 2015）。
放射性物質、137Cs の大気中濃度の時間変動（𝜕𝜕𝐶𝐶/𝜕𝜕𝑡𝑡）は、連続の式を考えたとき、
𝜕𝜕𝐶𝐶/𝜕𝜕𝑡𝑡 = 𝛻𝛻 (𝑲𝑲𝑑𝑑𝑖𝑖𝑓𝑓𝛻𝛻𝐶𝐶) – 𝛻𝛻 (𝑼𝑼𝐶𝐶) – 𝜆𝜆𝐶𝐶 + Φ と表される。
ここで、Kdif は 3 次元の拡散係数、U は風の場、𝜆𝜆は壊変定数、そして Φ が放射性核種の再浮遊となる。他方 ボックスモデルにおいて 137Cs の濃度変動を考えると、以下のように表される。
Δ𝐶𝐶/Δ𝑡𝑡 = Φ = 𝑘𝑘𝑖𝑖 ⋅ 𝐷𝐷𝑖𝑖 ⋅ (Δ𝑥𝑥Δ𝑦𝑦/Δ𝑥𝑥Δ𝑦𝑦Δ𝑧𝑧)- 10 -

ここで ki と Di はそれぞれ、核種 i の再浮遊係数（毎秒 s-1）、および地表面汚染（Bq/m2）である。つまり、地表面汚染 Di に比例して、比例定数 ki で、常時大気へ 137Cs が供給されていると考える。Δx、Δy および Δz はモデルのボックスの縦・横および高さである。放射性壊変、水平拡散を無視し、積雲による急速な対流がなかったとして、流入と流出の収支をとったとき、最終的に次式が得られる。
(𝑘𝑘𝑖𝑖⋅𝐷𝐷𝑖𝑖)/(△𝑧𝑧) = (Δ𝐾𝐾𝑧𝑧/Δ𝑧𝑧) ⋅(Δ𝐶𝐶𝑖𝑖/Δ𝑧𝑧) + (Δ𝑢𝑢/Δ𝑥𝑥 + Δ𝑣𝑣/Δ𝑦𝑦) ⋅ 𝐶𝐶𝑖𝑖
ここで i は核種を、Di はモデル計算 (ガス+エアロゾル) で得られた 3 月 17 日 9 時までの累積沈着量 (Bq/m2)を、ki は地表面汚染がどの程度の割合で再浮遊するかを表す比率（再浮遊係数；s-1）であり、u、v と Kz はそれぞれモデルによる平均水平風速(m/s) 、鉛直の乱流拡
散係数 (m2/s)である。Ci は 137Cs の平均実測濃度で（9.75×10-4 Bq/m3）、Δx、Δy、を 3 km、9 km または 15km とし、Δz を 100、200 または 400 m と変化させた。上式で流入濃度がゼロ
とおいて ki を求めると、最小で 1.6×10-6~1.5×10-5 毎秒 (平均 6.1×10-6 毎秒) となり、放射性Cs が沈着した直後には 1 時間当たり沈着量の 2%程度が大気へ再浮遊していたと考えられた。
この値は過去のチェルノブイリ事故によるデータとも整合していた。事故直後の再浮遊はかなり大きな値を取るが、時間経過とともに急速に小さくなっていくらしい。

a)Fig. 3 Atmospheric activity concentrations of radionuclides from the FDNPP accident in March2011 a) Observed data from filter samples collected at the MRI, Tsukuba, Japan; b) comparison ofobserved (black) and simulated results (red) for 137Cs The abscissa is expressed in dates in March 2011
and is labeled at the start of the day in a) and the middle of the day in b). Contamination of the filter
samples cannot be totally ruled out for the period before March 14 in a), which is depicted by theleft-right pointing double arrow. From Igarashi et al. (2015)- 11 -

4）まとめと今後に向けて
福島第一原発事故後、茨城県つくば市の気象研究所における 90Sr および 137Cs の月間降下
量（大気降下物）観測の結果について報告した。2014 年末の時点では、福島第一原発事故直後と比較すると降下量は数桁も低下し、甚大な汚染というべき水準ではない。90Sr については、事故以前の降下量とほぼ同じ水準に戻りつつある。しかし、137Cs については依然として 3～4 桁より大きな値となっている。水準低下にはまだ時間がかかるであろう。ただし、福島第一原発事故サイトからの放出はあっても、現状の大気中の濃度水準を説明できるほどの量にはないと考えられる（Igarashi et al., 2015）。
一次放出に替わり、
大気へのこれらの核種の供給を続けている過程は、
いわゆる再浮遊（二次放出）である。沈着直後の二次放出はかなり大きな値を取ることがわかった。二次放出は周辺の環境から由来すると考えられるから、周辺の汚染度がつくばに比し数桁高い地域においては、降下量もそれに比例し数桁高い。従って、二次放出過程の解明は必須な科学的課題である。二次放出源には、従来、汚染した地表面からの表土ダスト、汚染植生からの再浮遊、または汚染したゴミの燃焼、野焼き等による放射性物質の大気への揮散などが想定されてきた。最近、夏季における発生源として森林生態系からのバイオエアロゾル発生が寄与するのではないかとの仮説が急浮上してきている。今後は、その解明に努めねばならない。
上記に加えて、こうした長期の観測においては分析やデータの品質管理が必ず問題となる。
このような点についても常に留意しながら、観測の継続を図っていくことが肝要である。
（木
村ほか、2015）- 12 -

参考文献
Igarashi, Y., Kajino, M., Zaizen, Y., Adachi, K., and Mikami, M., Atmospheric radioactivity overTsukuba, Japan: a summary of three years of observations after the FDNPP accident, Progress inEarth and Planetary Science 2:44, 2015. doi: 10.1186/s40645-015-0066-1
木村徹, 五十嵐康人, 「大気降下物試料の放射能分析の品質管理」Proceedings of the 16th
Workshop Environmental Radioactivity. KEK Proceedings 2015-4, 23-27, 2015

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- 14 -

2. 大気中の放射性物質の物理・化学性状
1）はじめに
大気中で観測される放射性物質は、ガスや粒子（エアロゾル）として大気中に漂い、やがて地上や海水面に降下する。その大気拡散や環境動態、また発生メカニズムを理解するには、
放射性物質がどのような物理化学性状を持ち、またどのようなエアロゾルを担体としているかを解明する必要がある。本章では、一例として東日本大震災で発生した福島第一原子力発
電所事故によって直接放出された放射性物質の物理化学特性の理解を目指した研究成果を報告する。
2011 年 3 月に発生した事故によって様々な放射性物質が大気・海洋に放出されたが、放出された放射性物質は重量としては非常に微量であるので、放射能測定以外の分析手法で環境試料から検出することは困難であった。しかしながら、放射性物質の環境動態は、それ自体の性質だけで決まるものではなく、その運び手となる粒子の化学組成、イオン状態、粒径、表面状態といった物理化学特性に支配されるため、担体を含めた物理特性や化学特性の分析は重要である。微量化学物質の有効な分析手法として、電子顕微鏡や放射光分析などの微小領域化学分析がある。本研究においては、それらの手法を用いて 2011 年 3 月 14-15 日につくば市で採取されたフィルター試料中の放射性粒子の特定を行った（Adachi et al., 2013）。
またそれらの粒子の組成、化学状態、結晶状態をマイクロビーム放射光によって分析し（Abeet al., 2014）、さらに放射性粒子の断面を透過型電子顕微鏡により詳細に解析した（Yamaguchi
et al., 2016）。なお、Adachi et al. (2013)と Abe et al., (2014)は総説として日本語の論文にまとめた（足立,2015）。

2）主な研究成果
一連の研究の中で、最初の研究（Adachi et al., 2013）では、走査型電子顕微鏡（SEM-EDS）
を用いて 2011 年 3 月 14－15 日にかけて関東に飛来した放射性粒子を分析し、セシウム、鉄、亜鉛や多くの核燃料由来と考えられる元素を含み、水に溶けにくい２マイクロメートル前後の球状ガラス様粒子であることを示した。そして、これらの詳細な物理特性は、事故時における粒子の拡散モデルの精度向上に重要であることを示唆した。
続いて行った研究（Abe et al., 2014）では、大型放射光施設 SPring8 において、感度の高い極微量元素分析を行い、SEM-EDS 分析で得られた元素（鉄、亜鉛、セシウム）に加え、ルビジウム、ジルコニウム、モリブデン、スズ、アンチモン、テルル、バリウムが全ての放射性粒子から検出されることを示した。加えて、マンガン、クロム、銀、ウラン、鉛などの元素も 1 個もしくは 2 個の粒子から検出された。また、化学状態や結晶状態を X 線吸収端
近傍構造分析（XANES）や X 線回折法（XRD）を用いて分析し、大気中に放出された放射性粒子が非晶質のガラス状であることを示した。これらの研究成果は、炉内での粒子発生メ
カニズムの解明へ貢献することが期待される。
さらに、Yamaguchi et al. (2016) は、事故によって放出された放射性粒子を、収束イオンビーム装置を用いて薄片化し、透過型電子顕微鏡（TEM）による分析に供した。その結果、表面分析ではわからなかった内部の構造、すなわちセシウムが粒子表面に濃集していることを示した。また、銀やテルルなどのナノ粒子が放射性粒子に内在していることも明らかにした。これらの発見は、放射性粒子の長期的環境動態の予測や粒子生成メカニズムの解明において重要な知見である。

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3）まとめ
このように、気象研究所で行った研究により、福島第一原子力発電所事故において放出された放射性物質の大気中の物理化学特性の一部が明らかとなった。これらの研究はケースス
タディであるが、今後様々な発生源から放出される放射性物質の発生源メカニズムの解明や環境動態の予測への貢献が期待される。

- 16 -

参考文献
Abe, Y., Iizawa, Y., Terada, Y., Adachi, K., Igarashi, Y., and Nakai, I., Detection of uranium andchemical state analysis of individual radioactive microparticles emitted from the Fukushima nuclear
accident using multiple synchrotron radiation X-ray analyses, Analytical Chemistry, 86, 8521-8525,
2014.
Adachi, K., Kajino, M., Zaizen, Y., and Igarashi, Y., Emission of spherical cesium-bearing particles
from an early stage of the Fukushima nuclear accident, Scientific Reports, 3, 2554, 2013.
Yamaguchi, N., Mitome, M., Kotone, A. H., Asano, M., Adachi, K., and Kogure, T., Internal structure ofcesium-bearing radioactive microparticles released from Fukushima nuclear power plant. Scientific
Reports, 6, 20548, 2016.
足立光司, 電子顕微鏡がとらえた放射性粒子, 地球化学, 49, 185‒193, 2015.

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- 18 -

3. 大気中の放射性物質のモデルによる評価
1）放射性物質移流拡散沈着モデル
本研究を進めるにあたり、Kajino et al.（2012）によって開発された領域大気質モデル（Regional Air Quality Model 2; RAQM2）を放射性物質移流拡散モデルとして使用できるよ
う改変した（Adachi et al., 2013）。このモデル（RAQM2）はエアロゾルの粒径分布を対数
正規分布関数で仮定して、核生成・凝結・凝集・乾性沈着・グリッドスケールの雲凝結／氷晶核活性化といったエアロゾル力学過程をトリプルモーメント法で記述する独自のエアロ
ゾルモジュールを実装している。また、エアロゾル力学過程とそれに続く雲微物理過程（雲内部での降水除去過程）と雲底下での降水除去過程に関する方程式系がすべてモデル（RAQM2）内で記述されている。
RAQM2 はオフラインモデルであり、気象場（気象物理量の格子点解析値）は外部から与えられる。標準的なシミュレーション計算では、気象庁非静力学気象モデル（JMA-NHM;
Saito et al., 2007）を水平解像度 3km（東日本を 215×259 グリッドに分割）で実行することによって気象場を作成し、同じ解像度とドメインで RAQM2 の計算も行った。この際、JMA-NHM の鉛直解像度 50 層（最上層は 50hPa＝高度 20km 程度）を RAQM2 では 20 層（最上層は高度 10km）に変換した。JMA-NHM の初期値および境界値（スペクトルナッジング）には気象庁メソ客観解析値（水平解像度 5km；時間解像度 3 時間）を用いた。また、異なる気象場の計算法として、JMA-NHM を局所アンサンブル変換カルマンフィルタ（LETKF）データ同化システムに組み込み、気象観測値（アメダス、ラジオゾンデ、海上風等）をデータ
同化することによって独自に格子点解析値を作成することも試みた（Sekiyama et al., 2015）。
福島第一原子力発電所事故による大気汚染シミュレーションをモデル計算のテストケー（2011）
スとして実施する場合には、原子力発電所から放出された 137Cs の量として Katata et al.
の時系列インベントリーを利用した。137Cs は全量が硫酸エアロゾルと有機炭素エアロゾルの内部混合粒子に含まれていると仮定し、その 137Cs 含有粒子は数等価幾何平均の乾燥直径を 0.5μm、標準偏差を 1.6、粒子密度を 1.83 g/cm3、吸湿性を 0.4 としてモデル内の方程式系を計算した。137Cs 含有粒子の放出高度は Katata et al.（2011）の推定シナリオに合わせて高度 20m から 150m の間で時間変化させた。

2）エアロゾルの物理・化学特性に関するモデル感度実験
Kajino（2015）では、1）節で解説したモデルを用いて、2011 年 3 月における原発事故由来の 137Cs の放出、輸送、沈着過程の再現実験を行った。そして、2 章で解説した放射性セシウムを含むエアロゾルの物理・化学特性の違いや、複数の異なる気象モデルの計算結果を用いた比較・感度実験を実施した。放射性セシウムを運ぶエアロゾルは 2 種類考慮した：1）
節で述べた水溶性のサブミクロン粒子、および 2 章で述べた Adachi et al.（2013）で発見された非水溶性の粗大粒子である。
同じエアロゾル特性や沈着モジュールを用いた計算であっても、用いた気象モデルの違い
により 137Cs の沈着量は 3 倍程度変化した。またエアロゾル特性の違いによる沈着量の変化
は、気象モデルによる違いよりは小さいが、最大で 2.1 倍程度の変化をもたらした。したがって、気象モデルの違い（すなわち力学コアや物理スキームの違い）やエアロゾル微物理特性の違いもまた放射性セシウムの動態モデリングにとって重要なパラメータであることが判明した。

- 19 -

参考文献
Adachi, K., Kajino, M., Zaizen, Y., and Igarashi, Y., Emission of spherical cesium-bearing particles
from an early stage of the Fukushima nuclear accident, Sci. Rep., 3, 2554, 2013.
Kajino, M., Inomata, Y., Sato, K., Ueda, H., Han, Z., An, J., Katata, G., Deushi, M., Maki, T., Oshima,N., Kurokawa, J., Ohara, T., Tamami, A., and Hatakeyama, S., Development of the RAQM2
aerosol chemical transport model and prediction of the Northeast Asian aerosol mass, size,
chemistry, and mixing type, Atmos, Chem. Phys., 12, 11833–11856, 2012.
Kajino, M ., G-3. NHM-Chem: Sensitivity of Cs deposition to the size and hygroscopicity of Cs-bearing
aerosols, Technical Reports of the Meteorological Research Institute, 76, 132–142, 2015.
Katata, G., Ota, M., Terada, H., Chino, M., and Nagai, H., Atmospheric discharge and dispersion ofradionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident. Part I: Source term
estimation and local-scale atmospheric dispersion in early phase of the accident, J. Environ.
Radioactivity, 109, 103–113, 2011.
Saito, K., Ishida, J., Aranami, K., Hara, T., Segawa, T., Narita, M., and Honda, Y., Nonhydrostatic
atmospheric models and operational development at JMA, J. Meteorol. Soc. Jpn., 85B, 271–304,
2007.
Sekiyama, T. T., Kunii, M., Kajino, M., and Shinbori, T., Horizontal Resolution Dependence ofAtmospheric Simulations of the Fukushima Nuclear Accident Using 15-km, 3-km, and 500-m Grid
Models, J. Meteorol. Soc. Jpn., 93B, 49–64, 2015.

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4．福島第一原発事故前に採取されデータ未報告だった試料の分析・測定結果について
1）はじめに
福島第一原発事故前に採取された試料で、事故当時分析や前処理の途上にあった試料はつ
くば市の研究所の実験や測定環境が著しく汚染されたために、その汚染を極力避け、濃縮操
作・測定自体を汚染度の低い関西の大阪大学や㈱環境総合テクノス及び㈱アトックスにおいて実施した。そのデータの分析結果をここでは報告する。

2）主な研究成果
2.1) 137Cs のデータ
Table 1 に福島第一原発事故前につくばと榛名山で観測された 137Cs の月間降下量を示す。
Cs の降下量は、福島第一原発事故によって 134Cs がほぼ 1:1 の放射能比で放出されたということに基づき、試料に含まれる 137Cs 放射能はグローバルフォールアウトと原発事故の両者から由来するが、134Cs は原発事故からのみ由来したと考え、差分放射能を求めた。この差分放射能を減衰補正することで、グローバルフォールアウト由来の 137Cs 放射能を得る。
しかし、差分放射能の値はほとんどの試料において僅かな数値であり、不確実なデータとなったことから ND（検出限界値以下）と判断せざるを得なかった。一部、差分放射能値が有為となった試料がある（榛名山 1010、同 1011、同 1012）。しかしながら、榛名山 1010 の値は 1 Bq/m2/月を超えており、事故以前の水準を超えている。試料の保管、輸送、その後の操作では細心の注意を払って、厳重に汚染管理に努めたものの、汚染を受けている可能性も否定できない。榛名山 1011、同 1012 については、汚染を受けなかったと判断される。
1372.1) 90Sr のデータ
つくば市での 90Sr の月間降下量については、Table 2 に掲げるとおりである。90Sr の汚染水
準は放射性 Cs に比べ何桁も小さいことから、細心の注意を払った汚染管理によって充分に事故の影響を避けることができた。しかし、残念ながら、2011 年 1 月の降下量は ND となって定量化できていない。引き続き値を出せないか、再測定を行っていく。

3）まとめ以上のように、大変残念ではあるが、福島第一原発事故による実験・測定環境の汚染により、気象研究所の月間降下量時系列データに欠測が発生したことを報告する。
試料の前処理・化学分析・放射能測定に協力いただいた大阪大学、(株)環境総合テクノス、(株)アトックスと関係者各位に感謝する。

- 21 -

Table 1 Monthly 137Cs deposition at the MRI, Tsukuba and Mt. Haruna

放射能(2011/3/11に減衰補正)
試料名

分析番 採取地
採取月（期間）
号
点

差分放射能
137
（ Cs-134Cs）

差分放射能（137Cs-134Cs）(採取日に減衰補正）

誤差
誤差
誤差
誤差
137
137
137
Cs
Cs
Cs
Cs
(mBq/
(mBq/
(mBq/
(mBq/
(mBq/㎡)(mBq/㎡)(mBq/㎡)(mBq/㎡)
㎡)
㎡)
㎡)
㎡)
134

MRI 1009 4㎡
MRI 1010
4㎡
MRI 1011
4㎡
MRI 1012
4㎡
MRI 1101
4㎡
MRI 11024㎡

榛名山 1008
4㎡

榛名山 1009
4㎡

榛名山 1010
4㎡

-

MRI

2010年9月

42.2

3.6

41.5

2.6

-0.7

4.4

ND

-

1

MRI

2010年10月

1452

73.0

1492

74.8

41

105

ND

-

-

MRI

2010年11月

36.2

3.3

37.3

2.3

1.1

4.0

ND

-

2

MRI

2010年12月

44.6

3.6

57.8

3.2

13.2

4.8

ND

-

3

MRI

2011年1月

0.4

2.0

5.1

1.1

4.6

2.3

ND

-

4

MRI

2011年2月

51.0

3.9

48.4

2.8

-2.5

4.8

ND

-

144

117

ND

-

5

6

7

榛名山

榛名山

2010年8月

2010年9月

榛名山 2010年10月

32.6

6.9

60.3

3.9

27.7

7.9

注1

1521

81.3

1637

83

117

116

注2

57.4

8.4

85.2

4.9

27.8

9.7

ND

-

1387

172

1398

173

26.0

15.4

1310

67.0

1283

69

注1

2139

110

2242

113

103

158

注2

榛名山 1011
8
榛名山 2010年11月
11.9
2.8
54.9
3.2
43.0
4.2
4㎡
榛名山 1012
9
榛名山 2010年12月
71.7
5.6
32.3
8.7
6.6
104.0
4㎡
榛名山 1101
10
榛名山 2011年1月
5.7
3.6
14.1
1.5
8.4
3.9
4㎡
榛名山 110211
榛名山 2011年2月
33.3
38.8
2.6
5.5
6.1
5.5
4㎡
*134Csの標準が無いため、標準に同形の土壌試料を使用した*差分放射能については、137Csがマイナス（統計的誤差）となっているものもあるがそのまま表記した*減衰補正後の差分放射能については、誤差の3倍を超えないものについては、NDとして放射能を算出した
注1 保管容器、ロンテナに収納した試料のみを濃縮乾燥し測定した値
注2 濃縮途中試料のみを濃縮乾燥し測定した値- 22 -

43.3

4.2

32.4

8.7

ND

-

ND

-

Table 2 Monthly deposition of 90Sr and 137Cs at the MRI, Tsukuba during the years of 2010 and 2011
Regarding some of the samples, we could not obtain the correct data due to the heavy contaminations of
the sample itself, experimental environment, measurement environment as well as detectors themselves.
年

90

月

137

Sr
2

2

mBq/m

2010

合計
2011

合計

１月
２月
３月
４月
５月
６月
７月
８月
９月
１０月
１１月
１２月
１月
２月
３月
４月
５月
６月
７月
８月
９月
１０月
１１月
１２月

2.55
3.36
5.98
3.56
3.67
0.97
1.19
0.60
1.44
1.09
0.97
4.31
29.7
ND
1.84
5160
4660
376
152
46.0
76.8
25.7
31.3
15.2
31.0
10580

137

Cs

mBq/m

±
±
±
±
±
±
±
±
±
±
±
±

0.89
0.72
0.80
0.67
0.35
0.19
0.73
0.05
0.62
0.49
0.14
0.77

±
±
±
±
±
±
±
±
±
±
±
±

0.86
46.6
40.7
14.9
10.9
7.13
6.18
6.65
4.98
1.38
4.84

6.76
17.1
22.5
12.3
8.69
1.87
4.23
ND
ND
ND
ND
ND
73.4
ND
ND
23100000
1780000
330000104000
82000
32000
45900
25800
5850
20300
25530000

±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±

Activity unit: mBq/m2/month
ND: We could not obtain the significant data.

- 23 -

1.27
3.05
3.40
2.14
1.69
0.35
0.62

924000
1300
273
142
125
99.1
88.8
103
47.4
83.8

Cs/90Sr

降水量

残渣重量

放射能比

mm

g/m2

2.7
5.1
3.8
3.4
2.4
1.9
3.6
2.5
4480
382
878
683
1780
417
1780
824
385
654
2410

9.5
96.5
101.0
169.0
132.0
95.0
69.5
5.0
379.5
157.5
81.5
102.0
1398.0
0.0
104.5
74.0
74.5
210.0
138.5
184.0
142.5
186.0
160.5
79.0
41.0
1394.5

1.26
2.84
2.92
2.48
1.72
0.94
1.39
0.81
1.34
0.70
0.30
1.11
17.81
0.74
1.03
4.65
5.97
3.90
2.09
2.53
1.17
2.76
1.29
0.73
1.79
28.65

- 24 -

5. Publications 2013-2015
出版論文 2013-2015- 25 -

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44
DOI 10.1186/s40645-015-0066-1

RESEARCH ARTICLE

Open Access

Atmospheric radioactivity over Tsukuba,
Japan: a summary of three years ofobservations after the FDNPP accident
Yasuhito Igarashi1*, Mizuo Kajino1, Yuji Zaizen1, Kouji Adachi1 and Masao Mikami1,2

Abstract
A severe accident occurred in March 2011 at the Fukushima Dai-ichi nuclear power plant (FDNPP) operated by the
Tokyo Electric Power Company (TEPCO), causing serious environmental pollution over a wide range covering easternJapan and the northwestern Pacific. This accident created a large mark in the atmospheric radionuclide chronologicalrecord at the Meteorological Research Institute (MRI). This paper reports the impacts from the FDNPP accident overapproximately 3 years in Tsukuba, Ibaraki (approximately 170 km southwest from the accident site), as a typicalexample of the atmospheric pollution from the accident. The monthly atmospheric 90Sr and 137Cs depositional
fluxes in March 2011 reached approximately 5 Bq/m2/month and 23 kBq/m2/month, respectively. They are 3–4
and 6–7 orders of magnitude higher, respectively, than before the accident. Sr-90 pollution was relatively insignificant
compared to that of 137Cs. The 137Cs atmospheric concentration reached a maximum of 38 Bq/m3 during
March 20–21, 2011. After that, the concentrations quickly decreased until fall 2011 when the decrease slowed.
The pre-FDNPP accident 137Cs concentration levels were, at most, approximately 1 μBq/m3. The average level3 years after the accident was approximately 12 μBq/m3 during 2014. The atmospheric data for the 3 years
since the accident form a basis for considering temporal changes in the decreasing trends and re-suspension(secondary emission), supporting our understanding of radioCs’ atmospheric concentration and deposition.
Information regarding our immediate monitoring, modeling, and data analysis approaches for pollution fromthe FDNPP accident is provided in the Appendices.
Keywords: Temporal change, 90Sr, 137Cs, Atmospheric deposition, Atmospheric concentration, FDNPP accident

BackgroundWe have conducted observational research on radionuclides in the environment for almost 60 years at
the Meteorological Research Institute (MRI) in Japan,
ever since the 1950s when the USA, Soviet Union,and others performed vigorous nuclear tests in the atmosphere. The atmosphere is the major medium into
which radioactive materials were directly injected by
the nuclear tests and accidents, and within it, transport, diffusion, and wet and dry removal of these materials occur. During the nuclear testing era, themajor purpose of our research was to clarify theradioactive pollution situation and its major controlling* Correspondence: yigarash@mri-jma.go.jp
1
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052,
Japan
Full list of author information is available at the end of the article

factors in the atmosphere (Hirose et al. 1986; Katsuragi
1983; Miyake 1954; Miyake et al. 1963, 1975) andhydrosphere (Miyake et al. 1955, 1962, 1988). Afterthe Chernobyl accident, the purpose of the research
gradually shifted to obtaining more data about various
processes in the atmosphere (Aoyama 1988; Aoyama
et al. 1986, 1987, 1991, 2006; Hirose et al. 1993,
2001; Igarashi et al. 1996, 2003, 2009) and hydrosphere (Aoyama 1995, Aoyama and Hirose 2004; Hirose
et al. 1999, Hirose and Aoyama 2003; Miyao et al. 2000).
Of particular interest in this study, observation of monthly
radionuclide deposition (atmospheric total deposition/
radioactive fallout) for 90Sr (half-life, 28.8 years) and 137Cs(half-life, 30.2 years) had continued for 57 years as of April
2014, although the location of the observations moved
from Koenji, Tokyo, to Tsukuba in 1980 when the sciencecity was built (Katsuragi 1983). Both radionuclides are© 2015 Igarashi et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

scientifically important because of their health andenvironmental impacts (e.g., see U.S. Department of
Health and Human Services, Public Health Service,
Agency for Toxic Substances and Disease Registry
ATSDR2004Cs 2004; U.S. Department of Health andHuman Services, Public Health Service, Agency for
Toxic Substances and Disease Registry ATSDR2004Sr
2004). We continued collecting and analyzing atmospheric samples after the accident at Tokyo ElectricPower Company’s (TEPCO) Fukushima Dai-ichi Nuclear
Power Plant (FDNPP) in Ohkuma-machi and Futabamachi, Fukushima prefecture (37.42 °N, 140.97 °E) in
March 2011.
Many authors have attempted to determine the environmental impacts of the FDNPP accident, which have
gradually come to light (e.g., Aoyama et al. 2012, 2013;
Hirose 2012; Kusakabe et al. 2013; Masson et al.
2011; Masumoto et al. 2012; MEXT 2011a ; MEXT
and USDOE 2011; Povinec et al. 2013a, b; Tsumune
et al. 2013; Yamamoto et al. 2012; Yoshida and Kanda
2012; Yoshida and Takahashi 2012). We still need to
study the following issues from an atmospheric science point of view (Igarashi 2009): (1) primary source
terms including emissions inventory and temporalchanges (e.g., Chino et al. 2011; Katata et al. 2012, b,
2014; Maki et al. 2013; Stohl et al. 2012; Terada et al.
2012; Winiarek et al. 2012), (2) transport and diffusion (e.g., Masson et al. 2011; Morino et al. 2011;
Sekiyama et al. 2015; Stohl et al. 2012; Takemura etal. 2011; Tanaka 2013; Terada et al. 2012), and (3)dry and wet removal (e.g., Adachi et al. 2013; Hirose
et al. 1993; Kristiansen et al. 2012), which governedradioactive surface contamination during the earlyphase of the accident. In addition, the physical andchemical properties of the radioactive materials (e.g.,
Adachi et al. 2013; Kaneyasu et al. 2012) are important factors that influence the second and third subjects to be investigated. Here, we summarize theobservations, present a time series of the atmospheric
impacts of the TEPCO FDNPP accident over approximately 3 years in Tsukuba, Ibaraki, Japan, and compare the levels to the situation before the accident asvery basic scientific information (Igarashi, 2009). In
addition, secondary emissions from contaminated surfaces to the atmosphere (re-suspension; Igarashi 2009)
have become important during the later phases. Resuspension comes from contaminated surfaces, terrestrial
ecosystems, and open-field burning. These sources haveundoubtedly supported atmospheric radionuclides but are
not yet well understood and are thus considered briefly.
Other information about the accident, related to our immediate monitoring and modeling endeavors and data
analysis approaches to short-lived γ-emitters and 89Sr, issummarized in the Appendices.

Page 2 of 19

Methods
Atmospheric deposition samples

The monthly atmospheric total deposition/atmosphericfallout has been sampled using a weathering-resistant plastic tray (area = 4 m2) installed on a cottage roof in an open
field of the MRI in Tsukuba, Ibaraki (36.1 °N, 140.1 °E; approximately 170 km southwest of the FDNPP) since the1980s. After April 2011, the sample size was reduced to
two trays, each 1 m2, which we considered sufficient for
the levels present after the FDNPP accident. The collected
samples were evaporated and concentrated into a grossquantity with a rotary evaporator (Eyela NE-12) or anevaporating dish, and the samples were saved in apolyethylene safekeeping container. Each evaporated
sample, packed in a cylindrical plastic container, was measured for γ-ray emitting radionuclides (134Cs and 137Cs)
using a Ge semiconductor detector (coaxial-type from
ORTEC EG&G or Eurisys) coupled with a computed
spectrometric analyzer (Oxford-Tennelec Multiport or
Seiko EG&G 92x). The precision, accuracy, and quality control of the measurements are described elsewhere (Otsuji-Hatori et al. 1996).
Part of the sample was then stored for future reanalysis. The remaining sample was added to concentrated nitric acid along with H2O2 and digested in a heating
operation. Sr-90 was radiochemically recovered from theobtained sample solution, purified and finally fixed as Srcarbonate precipitate, an activity measurement source.
After the source was left for several weeks to achieve90
Sr and 90Y radioequilibrium, its β-activity was measured using a low-background 2π gas-flow detector(Tennelec LB5100) with P10 gas (Otsuji-Hatori et al.
1996). Within several months after the FDNPP accident,
89
Sr (half-life, 50.5 days) from the accident coexistedwith 90Sr and affected the β-activity measurement. To
remove the 89Sr influence, we occasionally repeated theSr source measurement and evaluated the radioequilibrium between 90Sr and 90Y, as well as the decrease in89
Sr activity (see Appendix 2). When required, the influence of the 89Sr activity was subtracted from the βactivity counts to obtain the 90Sr activity. The activity
was always decay-corrected mid-sampling. The detection
limit for 90Sr was approximately 7.0 mBq/sample, approximately 3.5 mBq/m2 using a total of 30,000 s ofmeasurement. For 137Cs, the limit was approximately
16.0 mBq/sample, approximately 8.0 mBq/m2 for an
average of 120,000 s of measurement.
Atmospheric radioactive aerosols

Aerosol samples were collected weekly using a highvolume air sampler (HV; Sibata Scientific Technology
Ltd., HV-1000 F) on a quartz fiber filter (Advantech
QR100; 203 mm × 254 mm) (Igarashi et al. 1999a). During March 2011, the sampling frequency was intensified.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

The flow rate was set at 0.7 m3/min, and the daily
sucked air volume was approximately 1000 m3. After
collection, the filters were compressed into pellets
using a hydraulic press device. They then underwent
conventional γ-ray spectrometry with Ge detectors as
described above. Current detection limits for 134Cs and137
Cs are approximately 9.0 mBq/sample (1.3 μBq/m3)
and 10 mBq/sample (1.5 μBq/m3) for approximately
1,000,000 s measurements, respectively.
The filter samples collected before the radioactive
plume arrived at Tsukuba were measured at the Kyoto
University Research Reactor Institute to achieve lower
detection limits and avoid contamination from the
FDNPP accident. This was necessary because the Ge detectors, measurement environment, and experimental
materials at the MRI were somehow contaminated by
the radioactive plume’s passage on March 14–15 and
20–23, 2011 (see Appendix 1). To date, radioSr analysis
has been performed on only a limited number of aerosol
samples collected during March 2011. The results arepresented in Appendix 2.

Results and discussion
Figures 1 and 2 depict the results of the atmospheric90
Sr and 137Cs deposition observations at the MRI for

Page 3 of 19

different durations. The temporal changes in monthly
radionuclide depositions shown in Fig. 1 include thosefrom the late 1950s to more recently available data, i.e.,
after the FDNPP accident. Figure 2 compares theamounts of atmospheric deposition after the FDNPP accident and from the late 2000s. Analyses of 90Sr and137
Cs deposition samples taken 6 and 8 months before
the accident are ongoing to control for possible sample
contamination at the MRI caused by the accident. Thus,
these data are missing in Figs. 1 and 2.
Figure 3 depicts the temporal change in atmosphericactivity concentrations of radioCs since March 2011. Before the FDNPP accident, it was difficult to detect 137Cs
below about 1 μBq/m3 in the air (the global fallout background level).
Although there were small-scale Japanese nuclear accidents in the 1990s (Igarashi et al. 1999a, 2000; Komura
et al. 2000), they did not cause significant marks in thepresent time series of monthly 90Sr and 137Cs depositions. The effects of the Chernobyl accident that occurred in 1986 were more evident for 137Cs than 90Sr(e.g., Aoyama et al. 1991) as illustrated in Fig. 1. However, the previous maximum 137Cs deposition was twoorders of magnitude lower than those caused by the
FDNPP accident. Thus, the impact of the FDNPP

Fig. 1 Sr-90 and 137Cs monthly deposition observed at the Meteorological Research Institute (MRI) from 1957 to 2014. Monthly deposition isexpressed in millibecquerel per square meter on a logarithmic scale. Sr-90 and 137Cs analyses from deposition samples taken 6 and 8 months before
the accident, respectively, are ongoing to avoid possible sample contamination at the MRI because of the accident. Thus, these data are missing not
only in Fig. 1 but also in Fig. 2. The measurement uncertainty (1σ) is shown only for the data obtained after the FDNPP accident and is reasonably small
compared to the analytical data. For comparison, uncertainty for the monthly data in 2010 is also given. The effects of atmospheric nuclear bomb tests
have been recorded since 1957. Until the Partial Test Ban Treaty (PTBT) became effective in 1963, the USA, Soviet Union, and UK conducted atmospheric tests. France and China continued atmospheric testing until 1974 and 1980, respectively. Since 1981, all the nuclear bomb tests have shifted
underground, so additional radioSr and Cs contamination should be negligible. However, the Chernobyl accident in 1986 also affected the time series.
The simple summation of the deposition from 1957 to the time before the FDNPP accident (mid-2010) and decay-corrected summations for 90Sr and137
Cs can be compared to the FDNPP-derived deposition

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Page 4 of 19

Fig. 2 Monthly 90Sr and 137Cs deposition levels in pre- and post-accident periods. Partial enlargement of Fig. 1. The monthly deposition is expressed inmillibecquerel per square meter on a logarithmic scale. The atmospheric depositions of 90Sr and 137Cs in 2013 observed at the MRI were a few ordersof magnitude higher than those from 2005 to 2011 before the FDNPP accident. For 90Sr and 137Cs, monthly depositions during 2005 to 2010 were0.5–19 mBq/m2/month and 1.2–97 mBq/m2/month, whereas they were 1–33 mBq/m2/month and 2–39 Bq/m2/month in 2013, respectively

accident was more remarkable than any previous incident in our time series.
Temporal changes in monthly 137Cs atmosphericdeposition

The monthly 137Cs deposition in March 2011, when the
FDNPP accident occurred, was 23 ± 0.9 kBq/m2/month,which is six to seven orders of magnitude higher thanthe level before the Fukushima disaster (Figs. 1 and 2).
Because the pollution source of the FDNPP accident iscloser to the observation site (170 km) than it is to theweapons testing sites and Chernobyl (several thousand
kilometers), the spatial representativeness of the MRI
data (as an absolute value) is lower.
The cumulative 137Cs deposition at the MRI was
25.5 kBq/m2/year for the year 2011. The sum of theFig. 3 Temporal change in atmospheric radioCs concentrations at the MRI before and after the FDNPP accident (“Mar.-Aug. 2014”). Activity
concentration is expressed in milli becquerel per cubic meter on a logarithmic scale. The measurement uncertainty (1σ) is shown. The maximum
concentration of 38 Bq/m3 of 137Cs was observed during March 20–21, 2011. After that, the radioCs concentrations rapidly decreased until fall2011 when the decrease slowed. The levels before (approximately 1 μBq/m3) and 3 years after the FDNPP accident (12 μBq/m3 from March toAugust 2014) are also compared. A difference of at least one order of magnitude is observed between the concentration level from March toAugust 2014 and the level before the FDNPP accident

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

simple monthly 137Cs depositions from 1957 to mid2010, the time before the Fukushima disaster, is approximately 7.0 kBq/m2 (this figure is thought to containsome error since the pre-1970s data did show individual
undefined errors), as shown in Fig. 1. Considering theradioactive decay of the individual monthly 137Cs depositions, this past total contribution represents 2.3 kBq/m2.
The FDNPP accident’s influence was over ten times larger than that of any past event. Almost the same amountof 134Cs (half-life, 2.1 years) was simultaneously deposited with the 137Cs; thus, the total cesium depositioncame to more than 50 kBq/m2. This value agrees
quite well with figures for the area around Tsukuba
in observation mapping provided by the Ministry of
Education, Culture, Sports, Science and Technology(MEXT 2011a).
Later, the deposition decreased rapidly, but the monthly
137
Cs deposition in 2012 and 2013 ranged from 8–36
and 2–39 Bq/m2/month, respectively, where deposition
during 2005–2010 had been in the range of 1.2–
97 mBq/m2/month, i.e., three to four orders of magnitude higher. The deposition level at the end of 2013
was still as high as values registered when atmosphericnuclear tests were conducted by China in the 1970s to
the early 1980s. The deposition rate slowly decreased in
the following years.
Atmospheric concentrations of radioCs

Figure 3 displays the temporal change in the atmospheric radioCs activity concentrations at the MRI in
Tsukuba since the FDNPP accident. The temporal trend
shows an abrupt increase (peak) of several orders ofmagnitude, followed by a rather rapid concentration decrease over a short period (3 to 4 months after the
FDNPP accident), with a smaller decreasing rate after.
The highest 137Cs atmospheric concentrations (38 Bq/m3
in a 12 h sampling period) were registered on March
20–21, 2011, which slightly exceeded the limit stipulated by Japanese regulations and ordinances (30 Bq/m3).
Although the pre-accident activity concentration level was
not measured, it had been observed for a short period,
from February to April 1997, which includes the time
when the Power Reactor and Nuclear Fuel Development
Corporation Tokai accident occurred (Igarashi et al.
1999a). The background level was approximately 1 μBq/m3
and did not decrease far below half that value (approximately 0.5 μBq/m3) until 2011. The decrease inmonthly 137Cs deposition was small during the same
period (Igarashi et al. 2003, 2009). Thus, the 137Cs activity concentration level registered during summer
2014 appears at least 10 times higher than that before
the accident. During 2011 and 2012, small spikes wererecorded from time to time (Fig. 4). In these cases, daily
forward trajectory analysis suggested that the polluted

Page 5 of 19

air masses were transported from the accident site during the corresponding observation period as shown in
the figure. In addition, relatively high concentrations
were registered in the winter (Fig. 3). This phenomenon
was noted at other places in northern and eastern Japan(Hirose 2013), so there is most likely a common explanation, as described in the literature.
Temporal change in monthly 90Sr atmospheric deposition

In contrast to 137Cs, the monthly 90Sr deposition in
March 2011 was 5.2 ± 0.1 Bq/m2/month. This was approximately 1/5000 the amount of 137Cs deposited in
the same month. This deposition was 2–3 orders ofmagnitude larger than the level before the FDNPP disaster. The annual 90Sr deposition was 10.6 Bq/m2/year
during 2011, approximately 1/2500 of the quantity of137
Cs deposited. The simple sum of the monthly 90Sr depositions from 1957 to mid-2010, before the Fukushimadisaster, was approximately 2.7 kBq/m2, as shown in
Fig. 1. Taking the radioactive decay of the individual
monthly 90Sr depositions into account, the sum represents approximately 0.9 kBq/m2. The FDNPP accident’s
impact on 90Sr was very small. The most extreme
monthly 90Sr deposition, recorded during the global fallout era of May 1963 in Tokyo, was 170 Bq/m2/month.
The FDNPP accident’s impact on the monthly 90Sr deposition was less than one-thirtieth of this maximum. Therefore, it is probable that 90Sr pollution over the Kanto Plain
from the accident was relatively insignificant; the environmental and health impacts of 90Sr are relatively minor.
In addition, the 137Cs/90Sr activity ratio fluctuated between approximately 400 and 5000 (Fig. 5), except forsome abnormal cases described below. This confirmsthat the degree of radioSr pollution is relatively insignificant compared to that of radioCs. However, it is still unknown why the 137Cs/90Sr activity ratio varied so widely
despite the radionuclides having a common accident
emission source, namely, the FDNPP accident. More discussion on the 137Cs/90Sr activity ratio is given in Appendix 2. The reason for the variability is worth studyingin the future. The monthly 90Sr deposition recorded in2012 was 10–31 mBq/m2/month, whereas during 2005–
2010, it was 0.5–19 mBq/m2/month, a difference of up
to two orders of magnitude.
A 90Sr deposition anomaly in October 2012

In October 2012, the monthly 90Sr deposition showed apeak of 145 ± 2 mBq/m2/month (see the arrow in Figs. 5
and 6), which is 1–2 orders of magnitude higher than
any monthly 90Sr deposition registered that year, and its
influence lasted a few months (Fig. 6). This small 90Sr
event remains puzzling. By applying forward trajectory
analysis and closely examining the precipitation overTsukuba, we believe that the 90Sr may have come fromAtmospheric activity concn. (mBq/m3)

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

1x10Page 6 of 19

0

Fukushima Accident
8x10

-1

6x10

-1

4x10

-1

2x10

-1

0

ND level

11/3/2

11/4/30 11/6/30 11/8/30 11/10/30 11/12/30 12/3/1

12/4/30 1

134

Cs

137

Cs

/30 13/3/2

Date (yy/m/d)Fig. 4 Atmospheric concentration increases observed during 2011 and 2012 and their air mass trajectories. Note that the activity concentration
scale is linear. The forward air mass trajectory calculated by the NOAA’s HYSPLIT model is depicted for the radioCs activity concentration peaks,
suggesting that the plume from the FDNPP site passed over the Tsukuba region. The shown trajectory cases are December 1, 2011 and April 5,2012. The increases seem to be attributable to the transport of primary radioCs from the accident site

the FDNPP and encountered precipitation on October 7
and 18–19, 2012. However, this increase was not accompanied by a radioCs deposition peak, and the major
radionuclide emitted by the FDNPP accident is radioCs,
which is inconsistent with FDNPP accident being thesource of the October anomaly.
The Japanese Radioactivity Survey data on the Internet
were checked, but no consistent data were evident forthe corresponding period. In addition, no such anomaly

was reported in Europe (Masson 2014, personal communication). Based on the timescale of this contamination,
however, the source should be neither very local nor
very small. This episode shows some similarities to thecase in fall 1995 in Tsukuba (Igarashi et al. 1999b).
We also assume unidentified, unreported incidents ofburning and/or melting of industrial 90Sr sources in
the Far East region as a possible explanation, such as theAlgeciras (Spain) incident in 1998 with its 137Cs source ofFig. 5 Activity ratio of 137Cs/90Sr in monthly depositions since March 2011 at the MRI. The temporal changes do not show a clear decreasing or
increasing trend. The arrow shows the month during which an anomalous deposition of 90Sr was observed. Except for the anomaly, the 137Cs/
90
Sr activity ratio fluctuated from approximately 150 to 6700

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Page 7 of 19

Fig. 6 Exponential fitting of the decreasing monthly 137Cs deposition trend since March 2011 at the MRI. The curve is composed of three exponentialfunctions. These are attributable to the decreasing intensity of primary emission, tropospheric aerosol residence and re-suspension. The arrow shows
the month during which an anomalous 90Sr deposition was observed. Possible causes are mentioned in the text

3.7 TBq (Estevan 2003). Sr-90 is widely used in industrial applications, such as in thickness gauges, andits activity size ranges from 740 MBq to 3.7 GBq in
Japan. Because 90Sr is a pure β-emitter, it is more difficult to determine the sources of its environmental
pollution than it is for 137Cs.
Decrease in monthly 137Cs deposition after the FDNPP
accident

Although researchers do not agree precisely on the
FDNPP radioactivity emission inventory (Chino et al.
2011; Katata et al. 2012, 2012b, 2014; Maki et al. 2013;
Stohl et al. 2012; Terada et al. 2012; Winiarek et al.
2012), if the 137Cs emission in March 2011 is assumed
to be 10 PBq/month, the deposition/emission ratio (themonthly deposition at the MRI divided by the monthly
emissions from TEPCO (2012)) would be approximately
10−12. If the MRI is included in the so-called “hot spot”
area, the deposition could be approximately 100 kBq/m2(five times larger). This would give a deposition/emission
ratio of approximately 10−11. After March 2011, the ratio
is calculated to be in the range of 10−10 to 10−9, whichappears to be large, if the emission-deposition relation
above is correct. We can presume that this excess deposition at the MRI, Tsukuba came from secondary
emissions. Thus, Tsukuba can be regarded as representative of a typical suburban area in the Kanto Plain, andthe relative trend of temporal changes there can be considered comparable to surface contamination levels for
similar geographical domains. The temporal trends(holding time constant) may also be spatially representative, although this potential is limited.
To study the decreasing trend in monthly 137Cs atmospheric deposition caused by the FDNPP accident
and to make future projections, a curve was fitted on
the temporal trends using multiple components. A
drawing software was employed, and the fitting operation was put through 100 iterations, each time changing the initial value so that the calculation results
would converge, as shown in Fig. 6. A trinomial exponential function of the form a × (e−k×t) was applied to fit
the data (where a is a constant and k is an inverted timescale; Ln2/T1/2), and the individual half-times (T1, T2,
and T3 in Fig. 6) were approximately 5.9 (±11 %) days,
16 (±18 %) days, and 1.1 (±32 %) years, respectively. The
relative uncertainty is shown in parentheses. These appear to correspond to the time scale of (1) the reductionin the original FDNPP accident surge (primary emission
source), (2) the tropospheric transportation and diffusionof the radioactive plume (equivalent to the removal ofradioactive aerosols from the atmosphere), and (3) theemission intensity of re-suspension (secondary emission
sources). We posit that some primary radiological release to the atmosphere continues because the FDNPP is
not isolated from the neighboring environment (Hirose
2013; TEPCO 2012). The results, then, cannot be assumed to be completely free of primary release. However, the first and second terms can be reasonableestimates corresponding to the primary emission andtropospheric aerosol residence, respectively.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

The second term is almost identical to figures obtained by other recent studies (e.g., Hirose 2012, 2013;
Kristiansen et al. 2012). Hirose (2013) analyzed radioCs
deposition data obtained during 2011–2012 from several
places over the Kanto Plain and Fukushima prefecture,
Japan. According to his report, “The apparent half-lives at
Ichihara, Tokyo, Utsunomiya, Hitachinaka and Maebashi
were 11.9, 10.6, 13.5, 11.5 and 12 d, respectively.” Hirose(2012) states that “the residence times of aerosols in the
troposphere, which are in the range of 5–30 d, have been
determined by natural and anthropogenic radionuclides,
which depend on particle size and altitude (Ehhalt, 1973).”
Hirose (2012) also argues “the temporal change of the
Fukushima-derived 137Cs revealed that the apparent atmospheric residence time of the Fukushima-derived 137Cs
in sites within 300 km from the Fukushima Dai-ichi
NPP is about 10 d.” This long residence time mightreflect the Fukushima radioactive plume’s circulation
over the Northern Hemisphere, which takes about
20 days (Hernández-Ceballos et al. 2012). As shown in
Fig. 8a in the Appendix 1, the third Fukushima plume’s
arrival over the Kanto Plain was observed from March
28–31, 2011. It was well reconstructed by the aerosol
transport model. Other observations over the Kanto Plain
also revealed this transport event (e.g., Amano et al. 2012;
Haba et al. 2012). However, we cannot clearly determine
whether this concentration peak is due to delayed primaryemission (e.g., Terada et al. 2012), hemispheric circulation,
or a combination of both. This is because the current
model simulation uses the emission inventory, which isalso based on atmospheric monitoring results (e.g., Terada
et al. 2012). Regarding this connection, Kristiansen et al.
(2012) investigated the 131I and 137Cs removal times
from the atmosphere using global-scale monitoringdata. Their estimated 137Cs removal times were in therange of 10.0–13.9 days, which is closer to our present
result. They also noted the difference from the typicalvalues of 3–7 days obtained by aerosol model simulations, suggesting that the aerosol transportation models
need improvement. We would like to add that the deposition results should be interpreted to reflect not
only the surface air but also the air column up to atleast the mixed layer. Therefore, the deposition may beaffected by large-scale transportation, in contrast to indications obtained from the surface concentration only.
For further reference, based on the monthly emissionof radioCs until the end of 2011 estimated by TEPCO
2012, the primary emission decrease can be fitted using
two exponential laws with half-time constants of
2.3 days (±2 %) and 48 days (±23 %).
The third term’s half-time of 1.1 years for the MRI
data, despite its relatively large associated uncertainty,
appears to reveal the total re-suspension of radioCs fromcontaminated surfaces. This value is too large toPage 8 of 19

correspond to any primary releases from the FDNPP in
the early phases. In addition, it agrees with the value forthe re-suspension “descending trend” due to the Chernobyl accident reported by Garger et al. (2012), whichwas 300 days. It was possible to fit a two-term exponential curve to the present 137Cs data by fixing the 1.1-year
half-time, obtaining a value of 7.8 days for the first term.
When compared with the triple exponential (three-term)
model, the fitting distance (defined by the ratio of thecalculation to the observation) for the double exponential (two-term) model was larger for elapsed times of 2–
12 months, although there were exceptions. The meanand standard deviation for the two- and three-term fit
distances are 2.50 ± 2.02 and 1.54 ± 1.14, respectively.
The medians are 1.82 and 1.09, respectively, suggestingthat the three-term model fits better. Although we do
not provide an illustration here, we found that fittingwith three-term functions for the decrease in monthly
90
Sr deposition after the disaster was also possible.
Therefore, we preferred fitting with a trinomial exponential function to reproduce the deposition flux of radionuclides from the FDNPP accident. Again, the primaryemissions of radioCs to the atmosphere are anticipated
to continue at a non-negligible level (less than 7.2 GBq/
month is assumed in TEPCO’s latest press release (inJapanese) at http://www.tepco.co.jp/life/custom/faq/images/d150129-j.pdf ) because the FDNPP is not isolated
from the surrounding environment (Hirose 2013). These
delayed primary emissions of approximately 7 GBq are
6–7 orders of magnitude lower than the emissions inMarch 2011 (e.g., 15 PBq for 137Cs; NISA 2011). If the
primary emission deposits were delayed in a fashion
similar to those from March 2011, recent MRI records
after the FDNPP accident would correspondingly be 6–7
orders of magnitude lower than the peak value causedby the accident (see Fig. 2). Therefore, we consider thatthe present decrease in the third term reflects secondary
emission (re-suspension) trends over the Kanto Plain
moderately well. In future, we plan to confirm this byapplying different evaluation methods such as transport
simulations or others.
Consideration of re-suspension and its persistence

Currently, there may be interest and concern about how
long it will take for the atmospheric radionuclide deposition fluxes to return to pre-FDNPP accident levels (cf.
Garger et al. 2012; Hatano and Hatano 2003). Although
it seems slightly arbitrary, the monthly 137Cs depositions
can be estimated if the fitted curve described above is
extrapolated. The result of this extrapolation is illustrated in Fig. 7. This simple estimation shows that
more than a decade will likely be required for theactivity levels to return to pre-accident levels. Thus,
re-suspension (secondary emission to the atmosphere;

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Page 9 of 19

Fig. 7 Future projection for monthly 137Cs deposition level using a trinomial exponential function. The present simple estimation shows that
more than a decade would be necessary for the 137Cs atmospheric deposition level to return to pre-accident levels

e.g., Igarashi 2009) must be scrutinized with long-termmonitoring. Because it seems natural that radionuclide
emission flux would be proportional to surface pollution
density, there could be radioCs fluxes several orders ofmagnitude higher than those measured in Tsukuba inareas nearer the FDNPP site whose Cs surface pollution isseveral orders of magnitude higher than in Tsukuba.
Therefore, elucidating the secondary emission processesof the FDNPP radionuclides remains an imminent scientific challenge, especially for heavily polluted areas. Secondary sources can include soil dust suspension from
polluted earth surfaces, emissions from polluted vegetation and forests, and volatilization and release from combustion of polluted garbage and open field burning (e.g.,
Igarashi 2009). Although the main emission sources are
not yet well understood, this elucidation must be performed as soon as possible.

radioCs. During 2013, the Fukushima fallout decreasedby 3–4 orders from its magnitude at the time of the accident, yet some becquerel per square meter of monthly
deposition continues. This corresponds to the level in the
1970s and early 1980s when China performed atmospheric nuclear tests. During 2013, the 137Cs concentration
remained at a level of tens of micro becquerel per cubic
meter. Because re-suspension (secondary emission) willcontinue over a long time, it is necessary to monitor its
future trends and variability. An apparent decrease inatmospheric radioCs deposition was fitted by trinomial
exponentials, giving information regarding the reducing
trend of airborne radionuclide persistence through resuspension into the atmosphere. Extrapolation of thedecreasing rate suggests that it would take at least a
decade for the activity to return to pre-disaster period
levels. Further monitoring efforts are essential.

Conclusions
The authors conducted atmospheric monitoring of airborne radioSr and Cs and their deposition at the MRI in
Tsukuba, Japan. The monitoring period encompasses the
FDNPP accident and the subsequent few years. The
monthly 137Cs deposition at the MRI was (23 ± 0.9) ×
103 Bq/m2/month in March 2011, which is 6–7 ordersof magnitude higher than pre-accident levels. Almost
equal amounts of 134Cs and 137Cs were deposited,causing surface pollution of more than 50 kBq/m2 in
Tsukuba in 2011, in close agreement with the Ministry of
Education, Culture, Sports, Science and Technology,
Japan (MEXT)’s airborne mapping. Deposition of 90Sr
was 5.2 ± 0.1 Bq/m2/month in March 2011, which isless than 0.02 % of the total 137Cs deposition in thatmonth. The level of 90Sr deposition was 3–4 orders ofmagnitude higher than pre-accident levels and did notreach the level registered during the 1960s after nuclear
tests; the effects from 90Sr will not be as large as from

Appendix

Appendix 1 Temporal changes in radioactive
aerosol concentrations and plume transport fromthe FDNPP accident over Tsukuba in March 2011
Introduction

The heat and blast at the FDNPP accident resulted in
the leakage of a huge amount of anthropogenic radionuclides, near the levels of the Chernobyl accident in 1986,
into the environment (IAEA 2006; Janžekovič andKrižman 2011; NISA 2011), as seen on both the domestic
and Northern Hemispheric scale (Hernández-Ceballos etal. 2012; Masson et al. 2011; Takemura et al. 2011;
Tanaka 2013). The transport of the radioactive plumeand its deposition over the Pacific Ocean (Aoyama etal. 2013; Honda et al. 2012), North America (e.g.,
Schwantes et al. 2012; Zhang et al. 2011), and Europe(e.g., Masson et al. 2011) as well as within the JapaneseIgarashi et al. Progress in Earth and Planetary Science (2015) 2:44

territories (Hirose 2012; Kinoshita et al. 2011; Morino
et al. 2011; Terada et al. 2012; Tsuruta et al. 2014) has
been well depicted by many researchers. The pattern ofdomestic pollution of the land by local fallout was madefairly clear by the creation of a contamination map basedon many university investigations (Kinoshita et al. 2011;
Tanihata 2013) and airborne surveys by Japan’s MEXT and
the USA’s NASA/DOE (MEXT and USDOE 2011; Sanada
et al. 2014; Torii et al. 2013; USDOE 2013). The transportof the radioactive plume and its subsequent deposition
over the capital area (the Kanto Plain; Amano et al. 2012;
Haba et al. 2012; Tsuruta et al. 2014) has been reported
and monitored in Tsukuba (Doi et al. 2013; Kanai 2012).
The MRI in Tsukuba suffered almost no electricity outage
soon after the earthquake. Thus, aerosol sampling at theobservation field continued from before the FDNPP accident through its aftermath. Here, we add our independent
observations of the temporal changes in atmosphericradionuclide concentrations over Tsukuba covering all ofMarch 2011, with our specific transport model simulation
for reference.
Experiment
Intensified aerosol sampling

Aerosol samples were collected onto quartz fiber filters
using a high-volume sampler, as described in the body ofthe paper; the only change was the duration of sampling,from 1 day to 6 h—which was altered as soon as the accident was made public. The total sucked air volume
was thus between 250 and 1000 m3.
Activity measurement

After collection, the filters were treated in the samemanner as usual and measured with Ge detectors, as
described previously. The filter samples collected before the radioactive plume’s arrival at Tsukuba were
measured at the Kyoto University Research Reactor
Institute (KURRI) to lower the detection limits. This
was necessary because the Ge detector and the laboratory environment at the MRI building were contaminated by the radioactive plume on March 14–15
and 20–22, increasing the background levels. Before
the compression procedure, portions of the filter
were punched out (33 mmφ × 4 pieces), of which one
piece was selected for radioSr analysis, as noted inAppendix 2.
Transport modeling

The Eulerian chemical transport model RAQM2 (Kajino
et al. 2012; Adachi et al. 2013; Sekiyama et al. 2015) wasused to simulate radioactive plume transport from the
FDNPP accident over the Kanto Plain. The JMA/MRI
non-hydrostatic meteorological model (NHM; Saito
et al. 2007) was used to simulate the meteorologicalPage 10 of 19

field to calculate the transport and deposition processes of radionuclides using RAQM2. The horizontal
domain and its grid resolution (3 km) were common
to both NHM and RAQM2, with 50 vertical layers
from the surface up to 22 km for NHM and 20 layers
to 10 km for RAQM2. The JMA’s Meso-Regional Objective Analysis (MANAL), which has a horizontal
resolution of 5 km, was used to define the boundary
conditions for NHM. The calculated domains coversouthern Tohoku and the central part of Honshu. Details of the transport (advection, diffusion, and convective transport) and deposition schemes (dry andwet (in cloud and below cloud, grid-scale and subgrid-scale)) are described in Kajino et al. (2012) andSekiyama et al. (2015).
We simulated five species of particulate radionuclides (volatile and reactive 131I (I2), volatile and nonreactive 131I (CH3I), non-volatile 131I, 134Cs, and
137
Cs). We conducted dispersion and deposition simulation of radioCs in two very different forms—hygroscopic submicrons vs. hydrophobic supermicrons—ina previous study (Adachi et al. 2013) and showed thatthe deposition regions were significantly different.
However, because the proportions of hygroscopic andhydrophobic radioCs in emissions have never been estimated, we assumed the hygroscopic submicron aerosols to be the carriers of radionuclides and useddimensions equivalent to the geometric mean of thedry diameter Dg,n,dry = 102 nm, geometric standard deviation σg = 1.6, particle density ρp = 1.83 g/cm3, andhygroscopicity κ = 0.4 (Petters and Kreidenweis 2007;
Adachi et al. 2013). The emission inventories of 131I
and 137Cs were taken from Katata et al. (2014).
RAQM2 incorporates aerosol dynamic processes, suchas nucleation, condensation/volatilization, and coagulation, within and among different aerosol categories,
but the size distribution of the aerosols was assumed
to remain unchanged in this simulation.
Results and discussion
Particulate fission products and radioCs

The detected γ-emitting radionuclides were 99Mo-99mTc(half-life, 65.9–6 hours), 129mTe (33.6 days), 131I (8.02 days),132
Te-132I (3.20 days–2.3 hours), 133I (20.8 hours), 134Cs(2.07 years), 136Cs (13.2 days), and 137Cs (30.0 years) as
shown in Fig. 8a in the Appendix 1. Note that gaseous iodine was not captured by the present sampling. The 90Sr results are also plotted in the figure(for analytical details, please refer to Appendix 2).
There were two significant transport events thatbrought the radioactive plume toward the Kanto Plainin March 2011. One was during March 14–15 andthe other occurred during March 20–22. Plume transport is determined by temporal changes in emission

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

intensity and the wind field near the ground surface,which have been addressed by many authors (e.g., Katata
et al. 2012, 2014; Morino et al. 2011; Terada et al.
2012). The releasing sources are attributed to a venting operation at an individual reactor vessel, reactor
core damage, buildings damaged by a hydrogen explosion, and continuous release through a reactor building (see, e.g., TEPCO 2012; Katata et al. 2014). The
activity concentrations of these radionuclides wereconsistent with those described in previous reports
regarding Tsukuba (e.g., Doi et al. 2013; Kanai 2012).
The March 7–12, 12–13, 13, and 13–14 samples exhibited detectable levels of radioCs and 131I, for which
we cannot totally rule out the possibility of sample
contamination despite their measurement at KURRI.
The two events exhibited different radionuclide compositions, reflecting different source at the accident
site. Although the 134Cs/137Cs ratio was unity forboth transport events, the activity ratios were 131I/
137
Cs ≈ 5 and 132Te/137Cs ≈ 8 during the first event
and 131I/137Cs ≈ 2.5 and 132Te/137Cs ≈ 1 during thesecond event. Te-132 was significant during the firsttransport event. Because the melting point of metallic
Te is 450 °C, whereas that of Cs is only 28 °C, thefinding may suggest a higher temperature for thesource in the earlier phase. For comparison, 90Sr data
are included in Fig. 8a in the Appendix 1; the detailsof the measurements are given in Appendix 2.
After the FDNPP accident, unlike in Chernobyl, noradioRu was found (Aoyama et al. 1986, 1987). This may
be because of the different accident scenarios; the melting temperature of metallic Ru is very high (approximately 2500 °C).
Another notable point is the magnitude of the concentration drop between the first and second plume events.
RadioCs and 132Te concentrations were 4–5 orders ofmagnitude lower for the second plume than the concentration peaks, and those for 131I were 2–3 ordersof magnitude lower. This difference appeared to becaused by either the re-suspension of radioI or thecontamination of our materials and instruments. The
latter seems unlikely, however, because the filter samples were treated identically and the maximum contamination levels would be those found for the March
7–14 samples (measured at the KURRI). We gave sufficient attention to reducing contamination during
sampling and sample handling. Nevertheless, the entire environment was contaminated, and therefore, it
was difficult to avoid entirely. In any case, the volatilenature of iodine (the boiling point of CH3I is 42 °C,
while the melting point of I2 is 113 °C) is likely partof the cause. Therefore, immediate re-suspension ofradioI should be given more attention. This is briefly
addressed below.

Page 11 of 19

Transport model simulation

The aerosol simulation model captures the events thattransported the radioactive plume to the Kanto Plainvery well (see Fig. 8b and 9 in the Appendix 1). The
transport of the plume from the southern Tohoku district is not considered very exceptional (the MRI is approximately 170 km southwest from the accident site).
Aoyama et al. (1999) and Igarashi et al. (1999a) analyzed
the radioactive plume over the Kanto Plain from theearlier PNC accident in Tokai, Ibaraki, in 1997. Igarashi
et al. (2000a,b) conducted continuous observations at
the MRI of 85Kr, of which the local source was the Tokai
nuclear fuel reprocessing plant approximately 60 km
northeast of Tsukuba. They noted the incidence ofplume transport from a point source in northern Ibaraki
over the Kanto Plain with a northeasterly wind, a prevalent weekly wind pattern occurring during the spring inJapan. Similar meteorological situations appeared tooccur on March 14–15 and March 20–22, 2011 over
the Kanto Plain. Notably, the drop in activity concentration between the plume advections is evident in
the simulation results ( Fig. 8b and c in the Appendix 1)
despite only primary emissions coming from the FDNPP
accident. The reality of the observations differed from thesimulations (Fig. 8a in the Appendix 1). As describedabove, contamination in the observation procedures
cannot be totally ruled out, but by coupling the model andobservations, it is possible to evaluate the immediate resuspension of the atmospheric Fukushima radionuclides(see section below).
Finally, we argue that aerosol transport modeling is anindispensable tool for the assessment of accident effects.
However, many uncertainties remain, especially concerning the emission inventory, wet and dry deposition, andcloud processes. Data and information are collected to
improve the transport model schemes, and comparisonof different models has been performed to contribute toan accurate evaluation of the source term and transportand deposition processes (SCJ 2014).
Estimation of immediate re-suspension factor

The quantity of the deposited radionuclides that could
return again to the air (re-suspension) is notable. Maximum re-suspension is known to occur just after radioactive plume passage (hereafter, we call this immediate
re-suspension). Thus, as a primary approach, immediate
re-suspension factors were roughly estimated with modeled amounts deposited in the Kanto Plain by the firstplume and the observed minimum activity concentration
between the two plume events, i.e., March 17 09JST to
March 20 09JST. We assumed mass closure between resuspension from the contaminated surface and outflow
by horizontal advection and turbulence vertical mixing
as below.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Page 12 of 19

Fig. 8 Atmospheric activity concentrations of radionuclides from the FDNPP accident in March 2011. a Observed data from filter samples collected at
the MRI, Tsukuba, Japan; b comparison of observed (black) and simulated results (red) for 137Cs; and c similar to b but for particulate 131I. The abscissa isexpressed in dates in March 2011 and is labeled at the start of the day in a and the middle of the day in b and c. Contamination of the filter samples
cannot be totally ruled out for the period before March 14 in a, which is depicted by the left-right pointing double arrow

The continuity equation is expressed as∂C=∂t ¼ ∇ðK dif ∇C Þ−∇ðUC Þ−λC þ Φ;
in which C is concentration, Kdif indicates threedimensional diffusion terms, U denotes the wind field, λ isthe decay constant, and Φ is a re-suspension term for individual radionuclides. On the other hand, the concentration increase in one unit of time from re-suspension is expressed as

ΔC=Δt ¼ Φ ¼ k i  Di  ðΔxΔy=ΔxΔyΔzÞ;Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

aPage 13 of 19bFig. 9 Radioactive plume transport from the FDNPP accident in the Kanto Plain in March 2011. a On March 15 07JST and b on March 20 12JST.
The figures show the simulated surface 137Cs concentration in shaded colors with the model topography in grayscalein which ki and Di are a re-suspension factor (/s) andsurface contamination (Bq/m2) for individual radionuclides, respectively. Also, Δx, Δy, and Δz are the horizontal and vertical lengths of the space where the massclosure is obtained.
We can disregard radioactive decay, horizontal diffusion, and convective wind. Balancing the mass between
inflow and outflow, we finally obtain the following
relationship:ðk i ⋅Di Þ=ðΔzÞ ¼ ðΔK z =ΔzÞ  ðΔC=ΔzÞ þ ðΔu=Δxþ Δv=ΔyÞ  C i ;in which i indicates the radionuclides, namely, 137Cs
and 131I; Di indicates the modeled total (gas + aerosol)
cumulative deposition (Bq/m2) by March 17 09JST; ki is
the re-suspension factor (s−1); U and Kz are the modeled
space- and time-averaged horizontal wind speed (m/s)
and vertical turbulent diffusivity (m2/s), respectively; Ci
indicates the time-averaged observed concentrations of
the radionuclides (9.75 × 10−4 and 3.14 × 10−1 Bq/m3 for137
Cs and 131I, respectively); and Δx, Δy, and Δz are thehorizontal and vertical distances in space over which theabove mass closure is obtained. To obtain the horizontal
and vertical gradient terms on the right-hand side of theequation, the concentrations outside the space are assumed to be zero (no inflow into the space).
The re-suspension factors for 137Cs and 131I are 7.0 ×
10−6 /s and 5.3 × 10−4 /s, respectively, for the smallest
volume of the RAQM2 model grid (Δx = 3 km, Δy =3 km, and Δz = 100 m). Those for 137Cs and 131I varied
from 1.6 × 10−6 /s to 1.5 × 10−5 /s (6.1 × 10−6 /s on average) and from 5.3 × 10−4 /s to 1.3 × 10−3 /s (4.6 × 10−4 /son average), respectively, for the various horizontal
spaces plus neighboring zero, one, or two RAQM2 grids
from the grid where the MRI is located (i.e., Δx, Δy = 3,

9, or 15 km) and vertical spaces plus zero, one, or two
RAQM2 grids from the bottom (Δz = 100, 200, or 400 m).
In summary, the immediate re-suspension factors ki of137
Cs and 131I are estimated to be on the order of 10−6–−5
10 /s and 10−4–10−3 /s, respectively, and that of 131I isapproximately two orders of magnitude larger than thatof 137Cs. These values are converted correspondingly,
often quoting the concentration ratio over the contaminated surface as follows: 5.8 × 10−6 − 1.7 × 10−5and 4.4 ×
10−4 − 1.3 × 10−3 /m) for 137Cs and 131I, respectively. The
present data do not display the large deviation hitherto
reported (e.g., 10−6–10−4 /m; Maxwell and Anspaugh2011). Because those values are based on rough assumptions, further studies based on surface flux measurements need to be conducted to more accurately estimate
the re-suspension factors.

Appendix 2 RadioSr in the aerosol samples
collected during March 2011
Introduction

There are several reports containing estimates of theradioactive contamination from the FDNPP accident,
presented in the form of mapped images produced fromthe results of investigations of radionuclides in the soil
(e.g., MEXT 2011a ; Sanada et al. 2014; Torii et al. 2013)
and in the form of air dose rate figures produced fromaircraft observations. Among the radionuclides, radioSr
is an important indicator of contamination. The former
Nuclear and Industrial Safety Agency (NISA) in Japan
reported the following emission estimates within the atmosphere: 89Sr (half-life, 50.5 days) as 2.0 × 1015 Bq and90
Sr (half-life, 28.8 years) as 1.4 × 1014 Bq (NISA 2011).
Nevertheless, there have been no reports on 89Sr and90
Sr in air samples because of analytical difficulty. The
detection of nine different γ-emitting radionuclides, including 99Mo, is described in Appendix 1. However, 89Sr

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

and 90Sr emit no γ-rays with their radioactive decay,
making it impossible to determine their presence by γspectrometry. To evaluate their radioactive pollution
levels, the aerosol components were radiochemically extracted from the HV filter sample to analyze the radioSr
and assess the emission ratios of 137Cs, 89Sr, and 90Sr.
Experiment
Sub-HV filter sample for Sr analysis

HV filter samples from the γ-spectrometry measurements noted earlier were used for the radioSr analysis.
Approximately 2 % of the filter area was punched out(as circles) and provided for this analysis, which was performed on sub-filter samples collected during March2011 (Table 1 in the Appendix 2).
Analysis of radioSr

To dissolve the aerosols on the filter, 100–200 ml of concentrated nitric acid was added and heated on a 200 °C
hotplate, then 1–5 ml of hydrogen peroxide solution wasadded to accelerate the decomposition of any organic matter. This was followed by further thermolysis for morethan an hour. The obtained solution was subjected toseparation, which was conducted through radiochemical
analysis comprising several precipitation separations, suchas oxalate, fuming nitric acid, hydroxide, carbonate, andbarium chromate precipitations. The last separation was
repeated twice, which allowed the Sr fraction to be freed
from radioBa and Ra isotopes. The final strontium carbonate deposit was β-counted with the low-background
2π gas-flow counter described earlier (Tennelec LB5100).
Estimating the activity ratio of 89Sr and 90Sr

The atmospheric aerosol sample contained 89Sr and 90Sr,
indicating that the total β-activity must be deconvoluted.
The measurement sensitivity of the gas-flow counter
was confirmed for possible energy independence; therefore, the temporal change in the β-counting rate of apurified 90Sr (maximum β-ray energy 0.546 MeV) source
and 90Y (maximum β-ray energy 2.24 MeV) growth fromthe parent nuclide was observed in five specimens of the
MRI reference fallout samples (Otsuji-Hatori et al. 1996)
that contained no 89Sr. The following equation was thenapplied to find the counting efficiency of 90Sr and 90Y:
N total ¼ ASr−90  m1 þ AY −90  ð1−e−λt Þ  m2 :Ntotal is the total counting rate (cpm); A stands foreach nuclide’s β-activity (dpm); λ is the decay constantof 90Y; t is the elapsed time; and m1 and m2 are thecounting efficiencies of 90Sr and 90Y, respectively. The βray energy emitted by 90Y is approximately 4 times thatof 90Sr, and the average values of m1 and m2 from the fivespecimens were 27.3 ± 1.8 % and 24.8 ± 3.7 %, respectively.

Page 14 of 19

Table 1 Temporal variation of 90Sr activity concentration in the
air over Tsukuba
Sampling start date
and time (JST)End date andtime (JST)90

March 12 21 pm

March 13 9 amndMarch 13 21 pm

March 14 9 amndMarch 14 9 am

March 14 21 pm

ndMarch 14 21 pm

March 15 9 am

1.50 ± 0.13

March 15 9 am

March 15 15 pm

1.04 ± 0.095

March 15 15 pm

March 15 21 pm

ndMarch 15 21 pm

March 16 9 amndMarch 16 9 am

March 17 8 amndMarch 17 9 am

March 18 8 amndMarch 18 8 am

March 19 9 amndMarch 19 9 am

March 20 8 amndMarch 20 9 am

March 20 21 pm

ndMarch 20 21 pm

March 21 9 am

1.32 ± 0.13

March 21 9 am

March 22 9 amndMarch 22 9 am

March 23 8 amndMarch 23 9 am

March 24 9 amndMarch 24 9 am

March 25 9 amndMarch 25 9 am

March 28 9 amndMarch 28 9 am

March 29 9 amndMarch 29 9 am

March 30 9 amnd

Sr activity concentration(mBq/m3)

Although the “nd” measurements change, depending mainly on the sample
volume, the average level was approximately 0.2 mBq/m3
nd not detected

There were no statistically significant differences. Thus,
the β-activities of radioSr were interpreted to have thesame counting efficiency regardless of the β-energy. The
activity ratio of 89Sr and 90Sr was elucidated from thevalue traced back to the date of sample collection as wellas the fixed date when the strontium carbonate precipitated. The activity was always decay corrected in the middle of the sampling time. The current detection limit forradioSr in air at that time was approximately 230 μBq/m3.
Results and discussion
Estimation of 90Sr in the aerosol sample

We will now quantify and describe the radioSr found in the
air over Tsukuba. The radioactivity in Tsukuba indicated a
two-fold concentration increase in March 2011, as shownin Fig. 8 in the Appendix 1. The amount of radioSr in thesample was smaller than what was anticipated based on
past experience (e.g., Aoyama et al. 1991). 90Sr was unable
to be detected except when plume transport occurred.
From March 14 9 pm (JST) to March 15 9 am, from March
15 9 am to 3 pm, and March 20 9 pm to March 21 9 am,
the results were 1.5 ± 0.13, 1.0 ± 0.10, and 1.3 ± 0.13 mBq/
m3, respectively. For the other samples, the radioSr was

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

lower than the detection limits (Table 1 in the Appendix 2).
The 90Sr activity results shown here were calculated basedon β-counts made long enough after the events that thecontribution of 89Sr could be negligible (less than 5 % of90
Sr activity). For example, we waited at least 200 days
after chemical separation (separation was performed after
December 2011). The accompanying uncertainty was estimated from the average of the relative β-count uncertainties in the five latest individual measurements.
The activity ratio of 137Cs/90Sr in the aerosol samples,
which was in the range of 4700–23,000, is very large compared with the activity ratio of radioactive fallout, which
was 1.63 during the 1960–1970s; this indicates a clear difference in the data before and after the FDNPP accident.
Furthermore, the MRI’s estimated 137Cs/90Sr ratio for the
Chernobyl radionuclides in May 1986 in Japan was 96(Aoyama et al. 1991), which indicates that the Fukushimaradionuclide composition was dominated by radioCs. In
the activity peak on March 14–15, the ratio was 4700–
6000, and the peak on March 20–21 was 23,000 times
higher with 137Cs, which also shows that the compositionof the radioactive plume differed between the earlier andlater dates during the course of the FDNPP accident.
The measured 137Cs/90Sr activity ratio in Tsukuba wasmore than 40 times higher than the emission assessment
by NISA 2011 for the FDNPP accident (137Cs: 90Sr = 15:0.14). The IAEA (2006) had estimated that the amount of90
Sr emitted (approximately 10 PBq) for the Chernobyl
accident was only 12 % that of 137Cs (approximately
85 PBq), yet in reality, the atmosphere/precipitation observations in Japan showed approximately the amount of 90Sr
to be only 1/100 that of 137Cs (Aoyama et al. 1991), indicating that less than 1/10 of the emitted 90Sr was transported. Thus, the 8000 km long-range transportationfrom Chernobyl produced the radionuclide separation.
With that in mind, it could be possible that fractionation

Page 15 of 19

Table 2 Curve fitting results with assumed 89Sr over 90Sr activity
ratio
March 14–15, 2011

March 20–21, 2011

89

σ

89

σ

9:0.14

0.3356

8:0.140.455010:0.140.33409:0.14

0.4165

11:0.140.3493

10:0.140.5432

12:0.14.

0.3795

11:0.140.5869

Sr:90Sr

Sr:90Sr

The results for two air filter samples collected in March 2011. The minimum
standard deviation σ suggests the best estimate

caused by particle size deviation (Hirose et al. 1993) occurred in the FDNPP plume. The plume was transported
less than a few hundred kilometers in the present case,but fractionation could be very effective.
89

Sr/90Sr activity ratio

The emissions estimated by NISA 2011 showed that the89
Sr proportion was 14 times higher than that of 90Sr after
the nuclear accident, which indicated that the radioactivity
estimate would be 1/3 that of 90Sr after a year. The resultsfrom the aerosol sample observations suggest the presenceof 89Sr; therefore, the temporal change in the β-counts
was fitted based on emission estimates by the former
NISA (89Sr:90Sr = 2:0.14). Figure 10 in the Appendix 2
shows the fitted results of the aerosol sample measurements for March 14–15. As shown in the figure, the sample counting values exhibited a large decay after 40 daysof fixation as strontium carbonate, which indicates thatthe amount of coexisting 89Sr was relatively large. Therefore, appropriately different ratios were examined insteadof the 2:0.14 ratio, which could not be fitted. Therefore,
the emitted ratio for the sample collected on March 14–
15 was 10:0.14 for 89Sr:90Sr. The peak data for March 20–
21 indicated that a ratio of 9:0.14 fit perfectly. Table 2 inFig. 10 Deconvolution of the 89Sr, 90Sr and 90Y activities. It is possible to deconvolute radionuclides by measuring the temporal change in the
total β-activity (cpm) of the purified radioSr source (March 14–15, 2011 sample). Elapsed days means the time after the radiochemical separation.
An initial activity ratio of 89Sr/90Sr was assumed and applied to the curve fit as 2/0.14 and 10/0.14.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Page 16 of 19

Table 3 Efficiency of 137Cs extracted from air filter samples by heated concentrated nitric acid
Sampling date and time (JST)Before ext. 137Cs (Bq)

After ext. 137Cs (Bq)

Extraction efficiency (%)

March 14 21 pm–March15 9 am

243 ± 0.772 ± 0.4

70.4

March 15 9 am–March 15 15 pm

41 ± 0.3

7.4 ± 0.12

82.0

March 15 15 pm–March 15 21 pm

20 ± 0.2

6.1 ± 0.11

69.5

March 15 21 pm–March 16 9 am

19 ± 0.2

0.51 ± 0.04

97.3

March 16 9 am–March 17 9 am

9.2 ± 0.19

0.52 ± 0.04

94.3

March 19 9 am–March 20 21 pm

94.2 ± 0.6

0.20 ± 0.03

99.8

March 20 21 pm–March 21 9 am

423 ± 0.9

1.2 ± 0.05

99.7

March 21 9 am–March 22 9 pm

30.8 ± 0.24

0.15 ± 0.03

99.5

Air filter samples were subjected to heated conc. nitric acid extraction for Sr analysis. Cs-137 was measured to confirm the extraction efficiency. Samples shown as“nd” before extraction were excluded from the table. Some samples exhibited significantly lower extraction efficiencies of 70–80 %. Insoluble and refractory radioactive particles must have been incorporated in these samples

the Appendix 2 shows these fitting results. Therefore, theemission ratio of 89Sr/90Sr for both March 14–15 and
20–21 was approximately 70 (10:0.14), which was fivetimes bigger than what NISA 2011 had estimated.
The MEXT has reported 89,90Sr in approximately 50
soil samples within 80 km of the FDNPP (MEXT
2011b). The decay data are corrected as of June 2011,
and the activity ratio was reported to be in the range of
1.9–6.5 (average: 4). Another decay correction as ofMarch 11, 2011 gives 89Sr/90Sr ratios of 7–24 with anaverage of 15. The ratio is not consistent with our
results, and the fluctuation was large. The cause of
the discrepancy and fluctuation is still unknown. The
most likely explanation is that stable Sr, alreadypresent in reactor materials or seawater components,
absorbed neutrons and formed 89Sr. The extent of andfluctuation in mixing (inhomogeneity) might produce
the discrepancy.
Efficiency of acid extraction of 137Cs from filter specimens

The rates at which 137Cs could be extracted from thefilter and aerosol samples using acid are shown in
Table 3 in the Appendix 2. The samples collected on
March 14–15 and 20–21 have different extraction
rates, indicating that the 137Cs in the sample from
the March 14–15 was refractory to some extent (20–30 %), even in a heated solution of nitric acid. This ispossibly because of the difference in the physical andchemical nature of the radioactive aerosol. Thus, it ispossible that the current radioSr concentration has beenslightly underestimated (20–30 %) because of the low
water dissolution rate of the radioactive material, especially for the March 14–15 sample.
As shown here, observations of the radioactive plumeover Tsukuba at different times demonstrated that the89
Sr/90Sr ratio was almost constant, but the 137Cs/90Sr
ratio and the extraction efficiency of 137Cs with nitric aciddiffered. Moreover, it was shown earlier that the activity
ratios among other γ-emitters differed (see Appendix 1).

These findings confirm that the characteristics of theaerosol particles that carried major radionuclides fromthe first plume differed from later advected radioactive
plumes. Adachi et al. (2013) addressed this sort of contrast in the characteristics of the two plumes’ radioactive aerosols in detail, and Abe et al. (2014) added
more information. They documented the discovery of
insoluble, glassy spherules containing radioCs and assumed that the major fraction came from the firstevent. Indeed, no such particles were detected in thelater event. This should also affect the ratio of 137Cs/90Srin the air, and evidence regarding this will be obtainedin future work. In conclusion, the present results support the previous findings of less 90Sr contamination
than radioCs contamination from the FDNPP accident
and indicate the necessity of further investigations ofradioSr in the atmospheric environment.
Abbreviations
ATSDR: US Department of Health and Human Services, Public Health Service,
Agency for Toxic Substances and Disease Registry; DOE: US Department ofEnergy; FDNPP: Fukushima Dai-ichi nuclear power plant; HYSPLIT: Hybrid
Single Particle Lagrangian Integrated Trajectory Model; IAEA: International
Atomic Energy Agency; JMA: Japan Meteorological Agency; KURRI: Kyoto
University Research Reactor Institute; MANAL: Meso-regional objective
analysis; MEXT: Ministry of Education, Culture, Sports, Science andTechnology, Japan; MRI: Meteorological Research Institute, Japan;
NASA: National Aeronautics and Space Administration, USA; NHM: The JMA/MRI
non-hydrostatic meteorological model; NISA: The former Nuclear and Industrial
Safety Agency, Japan Carriage Return; RAQM2: Regional Air Quality Model 2;
SCJ: Science Council of Japan Carriage Return; TEPCO: Tokyo Electric Power
Company.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YI designed and supervised the study and summarized the manuscript. MK
conducted the transport simulation and wrote that part of the manuscript.
Both YI and MK analyzed the data and helped in their interpretation. YZ
helped conduct the sampling. KA and MM provided important suggestions
for summarizing the work. They collaborated with the corresponding author
in the preparation of the manuscript. All the authors read and approved thefinal manuscript.

Igarashi et al. Progress in Earth and Planetary Science (2015) 2:44

Authors’ information
YI received his PhD degree in chemistry from the University of Tsukuba in
1987. From 1987 to 1991, he was at the National Institute of Radiological
Sciences and studied radiochemical analysis and radioecology. He moved to
the MRI in 1991 because of his scientific ambition to be involved in moreglobal issues. His current interests are atmospheric aerosols and theirprecursors, including Asian dust and PM2.5, and their possible influences on
climate, general environmental change, and other phenomena. He is
working enthusiastically and is a member of several organizations, including
the Japan Association of Aerosol Science and Technology, the Geochemical
Society of Japan, the Japan Society of Nuclear and Radiochemical Sciences,
the Meteorological Society of Japan, the Japan Radioisotope Association, theJapan Society of Analytical Chemistry, and the Japan Geoscience Union. He
considers collaboration between observational researchers and modelers as
a basic requisite in pursuing the geo- and environmental sciences.
MK received his PhD from the Graduate School of Science at Kyoto
University in 2005. Since then, he has been engaged in the development ofchemical transport models coupled with regional-scale meteorologicalmodels at the Disaster Prevention Research Institute, Kyoto University, Research Center for Advanced Science and Technology, University of Tokyo,
and currently, the MRI. His main research interest is elemental processes ofchemistry and microphysics of airborne particles and its impacts on air quality, ecosystem, and climate.
YZ graduated Meteorological College in 1984 and worked at the Nagasaki
Marine Observatory, Japan Meteorological Agency from 1984 to 1989. He
has been studying atmospheric aerosols at the MRI since 1989 and received
his PhD from Nagoya University in 2005.
KA received his PhD from Kobe University in 2005, worked at Arizona State
University between 2005 and 2011 as a postdoctoral/faculty research
associate, and is currently studying atmospheric aerosols at the MRI.
MM received his PhD from Tohoku University in 1996, has worked at the MRI
from 1985 to 2015, and finally holds the position of Senior Researcher for
Research Affairs at the institute. Currently, he is working at Japan
Meteorological Business Support Center.
Acknowledgements
The authors are deeply indebted to the following part-time and temporary
staff members at the MRI: Chitsuko Takeda, Tokyo Nuclear Service Co. Ltd.,and Kazue Inukai and Keiko Kamioka, currently at ATOX Co. Ltd. for thedifficult work of analyzing samples and preparing samples measurements
under an unusually severe accident situation; Hiroshi Sakou, Toru Kimura,and Sakae Mayama, ATOX Co. Ltd. for sampling, sample preparation, andgeneral analysis; Wakari Iwai and Kazuma Nabeshima for radiochemical
analysis of Sr isotopes and sample preparation, respectively; and Yuriko
Kamiya, Kayo Yanagida, and Rina Mori for sampling, logistical support,
figure preparation, and manuscript editing. Takeshi Ito helped with the
trajectory analysis in the revised manuscript. The authors are also gratefulto the following MRI academic colleagues: Hiroaki Naoe, Michio Aoyama(currently at Fukushima University), and Hiroshi Takahashi (currently at
the Japan Meteorological Agency) for their help with sampling.
Additionally, they acknowledge the assistance of Naoyuki Osada(currently at Okayama University) and Yuichi Oki (Kyoto University
Research Reactor Institute) with low-background γ-ray measurements.
The authors benefitted from discussions with Kazuyuki Kita (Ibaraki
University) and Yuko Hatano (University of Tsukuba) regarding there-suspension issue. Olivier Masson, IRSN, France, kindly gave criticalcomments on an early version of the manuscript, for which the authors
are grateful. This study was financially supported by the former MEXT andcurrent Nuclear Regulation Authority “Japanese Radioactivity Survey”
fund and partially by the MEXT “Kakenhi” (a Grant-in-Aid for Scientific
Research on Innovative Areas under the A01-1 and A01-2 research teamsin the “Interdisciplinary Study on Environmental Transfer of Radionuclides
from the Fukushima Daiichi NPP Accident (ISET-R; leader: Professor Yuichi
Onda, University of Tsukuba)”; grant nos. 24110002 and 24110003) andthe JSPS “Kakenhi” (leader Dr. Tsuyoshi T Sekiyama, MRI; grant no.
24340115). We gratefully acknowledge NOAA’s ARL for providing the
HYSPLIT transport and dispersion model and the READY website used inthis study. The present paper was written and organized based on
previous proceedings (Iwai et al. 2012; Igarashi et al. 2013a) and presentations
at the ICAS, AMS, EGU, domestic meetings, and other settings (Igarashi et al.
2011, 2013b , 2013c) and the MRI home page (MRI 2011).

Page 17 of 19

Author details
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052,
Japan. 2Present address: Japan Meteorological Business Support Center,
Chiyoda-ku, Tokyo 101-0054, Japan.
1

Received: 16 March 2015 Accepted: 22 October 2015

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KEK Proceedings 2015-4
November 2015 R
Proceedings of the 16th Workshop on Environmental Radioactivity
KEK, Tsukuba, Japan March 9-11, 2015
High Energy Accelerator Research Organization

Quality control of radioactivity analysis
in atomospheric fallout sample

Toru Kimura1,Yasuhito Igarashi2
1ATOX Co.,Ltd.,2MeteorologicalResearch InstituteIn the Meteorological Research Institute, observations of 90Sr and 137Cs in
atmospheric deposition have been carried out for more than 50 years from the late
1950s. And a part of this observation, especially of radioactivity analysis, wasundertaken by ATOX Co. from a few years ago. In this report analytical results (90Sr,
137Cs) that have been obtained in the past are compared with our recent values andreference values, indicating the current status of quality control have been made with agood accuracy. 137Cs analysis was made by measuring the reference fallout samples byusing a Ge detector. For 90Sr analysis, after decomposition of organic substances, Sr
fraction was radiochemically-separated and finally purified strontium carbonate wasmeasured with a low background 2π gas flow counter. Comparing the reference value
with the analytical ones, 90Sr and 137Cs analysis results were generally within thefavorable range. The average values were also in good agreement with the past
analytical values. However, distribution normality for the 90Sr analytical values was
not confirmed clearly due to small total sample analysis. Also, the difference between
the average analytical values for 90Sr and 137Cs and the reference values was subjected
to t-test at the 5% significance level, significant difference was not observed. Therefore,
it is concluded that quality control of the whole analytical procedures has been done
properly.

大気降下物試料の放射能分析の品質管理
木村徹＊ １ ，五十嵐康人＊ ２＊１㈱アトックス、＊ ２ 気象研究所 環境・応用気象研究部
１. はじめに気象研究所では、1950 年代後半から 50 年以上の期間にわたり大気降下物中の 90Sr と137Cs について観測を行っている。アトックスでは数年前からこの観測の一部を請負い、放射能分析を実施してきた。その中で定期的に技術水準を確認することは、適正な品質を確保する観点から重要な事である。今回はこれまでの結果（90Sr，137 Cs）をまとめ、過去に実施された分析結果や参照値と比較し、良好な精度で品質管理がなされている現状を報告する。
２. 分析試料技術水準の確認に用いた試料は、気象研究所から供された標準試料(Fig.1)である。この標準試料は 1963 年～1979 年にかけて日本全国 14 地点で採取された降下物試料を、気象研究所において混合・調製したものである。なおこの試料は、調整後に他の分析機関においても分析が行われ、参照値が与えられている[1,2]。

Fig.1 Reference fallout materials

Fig.2 A Ge semiconductor detector
with a sample auto-changer

３. 分析方法
137Cs については、標準試料約 4g～60g をそれぞれ外径 6cm のプラスチック容器（丸型

V 式容器 V-1 又は V-7)に詰め、高さを測定後、Ge 半導体検出器(Fig.2)で測定した。90Sr
については、約 4g の標準試料をテフロン容器内に入れ、濃硝酸と過酸化水素を加えて 200℃に加熱し有機物を分解後、不溶残渣をろ過で除去する。担体として Sr 50mg 相当を加えたろ液に水酸化ナトリウムを加えアルカリにして、炭酸塩沈殿(Fig.3a)、つづいてシュウ酸塩
沈殿(Fig.3b)、発煙硝酸法によるカルシウム除去(Fig.3c)、重クロム酸バリウム共沈によるバリウム・ラジウム除去、鉄共沈による Y 除去により化学分離し、最終的に炭酸ストロン
チウムとしてろ紙ばさみに固定した。二週間以上放置し、90Sr と 90Ｙとが放射平衡に達した後に、低バックグラウンド 2π ガスフロー検出器(Tennelec LB-5100)(Fig.4)で測定した[2]。
Sr の回収率については、以下の計算式による。
𝑊𝑊2
Y=
∗ 100
𝑊𝑊１
Y : Sr yield(%)、W1 ： The quantity of added strontium as a carrier (mg)、
W2 ： The quantity of recovered strontium (mg)Fig.3a Radiochemical separation (carbonate)Fig.3b Radiochemical separation (oxalate)

Fig.3c Radiochemical separation (calcium removal)

Fig.4 A 2π gas-flow detector

Fig.5 The ratio between the standard deviation
deviation and the 137Cs analysis value

Fig.6 The ratio between the standard
and the 90Sr analysis value

Table-1 Comparison of 90Sr and 137Cs between analysis value and reference value (Bq/g)

137Cs
90Sr

Past analysis value
307±9（n=699）208±8(n=95)QC analysis value
315±10（n=135）206±10(n=15)

Reference value
312±16211±33

Table-2 t-test for 137Cs data
Average
Variance
Observation number
Degrees of freedomt

QC analysis value

Reference value

QC analysis value

315
109
135

312
246
9

315
109
135

Past analysis value Past analysis value

307
88
699

Reference value

307
88
699

312
246
9

P(T<=t) one-sided test

142
0.92
0.18

832
9.22
1.2E-19

706
-1.53
0.06

P(T<=t) two-sided test

0.36

2.4E-19

0.13

Table-3 t-test for 90Sr data
Average
Variance
Observation number
Degrees of freedomt

QC analysis value

Reference value

QC analysis value

206
109
15

211
1081
3

206
109
15

Past analysis value Past analysis value

208
63
95

Reference value

208
63
95

211
1081
3

P(T<=t) one-sided test

16
-0.46
0.33

108
-0.91
0.18

96
-0.43
0.33

P(T<=t) two-sided test

0.65

0.37

0.67

４. データ解析
これまでにアトックスが実施した標準試料の分析結果を QC 分析値（137Cs については、9 試料×Ge 5 台/半年の直近 135 試料、90Sr については 5 試料/年、2012～2014 年の 15 試料の分析値）とし、核種毎にその値から標準偏差を求め、標準偏差との差を 1σ 毎基準化した度数分布か
らヒストグラムを作成し、正規分布と比較した（Fig.5 および Fig.6）。
また、過去の分析値（弊社以前に分析業務を請負っていた他社の標準試料分析値。137Cs につ
いては 1996～2007 年、90Sr については 1992～2007 年の分析値）・QC 分析値・参照値[2]の平均値を比較(Table-1）し、平均値の差が統計的に有意かを確かめるために、有意水準 5％で両側検定の t 検定を行った(Table-2 および Table-3)。
５. 結果と考察
90Sr および 137Cs の分析結果は概ね良好なばらつきの範囲内であったが、90Sr については分

析試料総数が少ないため正規性の確認は確実ではない。しかし、平均値は参照値とよく一致している。また、t 検定結果から 137Cs については過去の分析値と QC 分析値の間で平均値は統計的に有意であったものの、その平均値の差は 2.5%であり、環境放射能分析においては許容範囲内であると判断できる。その他の 90Sr 分析値や 137Cs 分析値の t 検定結果は、P（両側検定）>0.05 であることから平均値の差に有意差は見られない。従って、分析の品質管理が適正に行われていると判断している。結果として、アトックスが分析を請負って以降も分析の質において変化はなく、気象研究所の分析データの信頼性について、大きく変わることは無かった。
今後もこのような分析データの品質管理を継続し、安全・安心の確保に貢献していきたい。
[参考文献][1] 五十嵐康人,広瀬勝己,「放射性降下物標準試料の調整を通じた 90Sr、137Cs、プルトニウムなどの人工放射性核種分析の相互比較と参照値の決定」、放射化分析研究会,17,39-42(2004)[2] M. Otsuji-Hatori, Y. Igarashi & K. Hirose, Preparation of Reference Fallout Material
for Activity Measurements, J. Environ. Radioactivity, 31, 143-155 (1996)

Letterpubs.acs.org/ac

Detection of Uranium and Chemical State Analysis of IndividualRadioactive Microparticles Emitted from the Fukushima Nuclear
Accident Using Multiple Synchrotron Radiation X‑ray Analyses
Yoshinari Abe,*,† Yushin Iizawa,† Yasuko Terada,‡ Kouji Adachi,§ Yasuhito Igarashi,§ and Izumi Nakai*,††

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601,
Japan‡
Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan§
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, JapanS Supporting Information*

ABSTRACT: Synchrotron radiation (SR) X-ray microbeam analyses
revealed the detailed chemical nature of radioactive aerosol microparticles
emitted during the Fukushima Daiichi Nuclear Power Plant (FDNPP)
accident, resulting in better understanding of what occurred in the plant
during the early stages of the accident. Three spherical microparticles (∼2
μm, diameter) containing radioactive Cs were found in aerosol samples
collected on March 14th and 15th, 2011, in Tsukuba, 172 km southwestof the FDNPP. SR-μ-X-ray ﬂuorescence analysis detected the following
10 heavy elements in all three particles: Fe, Zn, Rb, Zr, Mo, Sn, Sb, Te,
Cs, and Ba. In addition, U was found for the ﬁrst time in two of theparticles, further conﬁrmed by U L−edge X-ray absorption near-edge
structure (XANES) spectra, implying that U fuel and its ﬁssion products were contained in these particles along with radioactive
Cs. These results strongly suggest that the FDNPP was damaged suﬃciently to emit U fuel and ﬁssion products outside thecontainment vessel as aerosol particles. SR-μ-XANES spectra of Fe, Zn, Mo, and Sn K−edges for the individual particles revealed
that they were present at high oxidation states, i.e., Fe3+, Zn2+, Mo6+, and Sn4+ in the glass matrix, conﬁrmed by SR-μ-X-ray
diﬀraction analysis. These radioactive materials in a glassy state may remain in the environment longer than those emitted aswater-soluble radioactive Cs aerosol particles.

T

In this study, we conducted a more detailed study of thenature of the Cs-bearing radioactive aerosol microparticles bymeans of advanced analytical techniques using a synchrotron
radiation (SR)-X-ray microbeam. In the previous study,5 ascanning electron microscope (SEM) with an energy dispersive
X-ray spectrometer (EDS) was used for chemical characterization of the particles. In this study, X-ray ﬂuorescence (XRF)
analysis using a high-energy SR-X-ray microbeam, which ismuch more sensitive to heavy elements than SEM-EDS
analysis, was introduced to carry out nondestructive identiﬁcation and qualitative detection of trace amounts of heavy
elements in individual microparticles. Although chemical
analyses such as a laser ablation-inductively coupled plasma
mass spectrometry (LA-ICPMS) may have a better sensitivity
than SR-XRF, it is diﬃcult to analyze single microparticle
sample. Moreover, chemical state and crystal structure
information could not be obtained with LA-ICPMS.
To evaluate the conditions under which these particles wereformed, chemical state analysis of the transition elements in thehe Fukushima Daiichi Nuclear Power Plant (FDNPP)
accident is the largest nuclear incident since the 1986
Chernobyl disaster and has been rated at the maximum level of
7 on the International Nuclear Event Scale.1 Large amounts ofradioactive materials were released into the environment during
the accident.2−4 Although more than 3 years have passed since
the accident, the radioactive materials emitted from the FDNPP
have been detectable in the environment. However, little is
known about the physical and chemical natures of radioactive
materials released during the early stages of the accident.5−7
Adachi et al.5 found spherical microparticles containing
radioactive Cs in aerosol samples collected on March 14th and15th, 2011, in Tsukuba, 172 km southwest of the FDNPP and
about 60 km northeast of central Tokyo. They revealed thatthese microparticles consisted of Fe, Zn, and Cs and wereinsoluble in water. Additionally, they calculated deposition areaof these particles based on the size and hygroscopicity of theparticles and concluded that these particles mainly fell to theground through dry deposition. Such knowledge of theradioactive materials from the accident is important tounderstand potential environmental and human health impacts,
an assessment of the accident sequence, and methods fordecontamination of the radioactive pollution.
© 2014 American Chemical Society

Received: May 17, 2014
Accepted: August 1, 2014
Published: August 1, 20148521

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Analytical Chemistry

Letterparticles was carried out by applying X-ray absorption nearedge structure (XANES) analysis. X-ray diﬀraction (XRD)
analysis was also conducted to reveal the crystal structures ofthe particles. Brilliant X-rays from an advanced SR light source
at SPring-8 enabled us to use a combination of these three
analytical techniques. The SR-X-ray microbeam was particularly
suitable to obtain detailed information from individual
microparticles.8,9 This study aims to apply these analyticaltechniques to the Cs-bearing microparticles from the FDNPP
accident and to reveal their nature to further understand theaccident as well as their eﬀects on the environment and humanhealth.

■EXPERIMENTAL SECTION
Sample Collection. From March 14th at 21:10 to March
15th at 9:10 (JST), aerosol particles containing radioactive
materials were collected at the Meteorological ResearchInstitute (Tsukuba, Japan; 36.05° N, 140.13° E) using a highvolume aerosol sampler (HV-1000F, 1000 m3/24 h; Sibata
Scientiﬁc Technology Ltd.) on a quartz ﬁber ﬁlter (QR100;
Advantec). The detailed sampling procedures were described
elsewhere.5 An imaging plate (IP; GE Measurement andControl, CR×25P computed radiography scanner) and micromanipulator (AP-xy-01; Micro Support Corp.) were used todetect and separate the radioactive particles from the ﬁlter.
Approximately 100 small dots, each of which suggests thepresence of radioactive material, appeared on the IP image of
the ﬁlter (Figure 1a). In the previous study,5 the particle
number concentration was estimated to be around 10
radioactive particles/m3. Three radioactive particles, designatedparticles A, B, and C, were sampled from the ﬁlter and placedon glass substrates. Particles A and C in this study are the sameas the particle nos. 3 and 2, respectively, in the previous study.5
They were subjected to the SEM-EDS analysis and gamma-ray
spectrometry (see details of the measurements in the
Supporting Information). After these analyses, the radioactive
particles on the carbon tape fragment were removed and placedon a ﬂat Kapton tape with a plastic holder for the SR X-ray
analyses.
Synchrotron Radiation X-ray Analyses. The SR experiments were carried out at the BL37XU,8,9 a hard X-ray
undulator beamline at SPring-8, at Japan Synchrotron Radiation
Research Institute (JASRI). We used two beamtimes: beamtime
A for measurements with a high-energy X-ray beam (17.1−37.5keV) and beamtime B for those with a low-energy X-ray beam
(7.0−15.0 keV). The sample was placed on an automatic XY
stage. Monochromatic X-rays were obtained with a Si(111)
double crystal monochromator, and the X-ray microbeam wasproduced by focusing Kirkpatrick−Baez mirrors. The area ofthe X-ray microbeam in beamtime A was 1.0 μm (V) × 1.2 μm(H), while that of beamtime B was 0.6 μm (V) × 0.8 μm (H).
Using these X-ray microbeams, we applied three X-ray
analytical techniques, SR-μ-XRF, SR-μ-XANES, and SR-μXRD. The intensity of the incident X-ray (I0 intensity) was
continuously monitored using an ionization chamber locatedbefore the focusing mirror. The SR-μ-XRF analysis, including
two-dimensional imaging analysis, was carried out using 37.5
keV X-rays and a Si (Li) detector in beamtime A. The SR-μXRF spectrum was measured for 1 000 s in live time persample. The intensity of each spectrum was normalized to thatof the Thomson scattering peak. To visualize the distributionsof the elements in each particle, SR-μ-XRF imaging analysis of
the particle was conducted with a step size of 0.5 μm (V) × 0.5

Figure 1. Characterization of radioactive aerosol particles prior to SR
experiments. (a) IP autoradiography of the aerosol ﬁlter collected in
Tsukuba after the FDNPP accident.5 Black dots indicate the presenceof radioactive materials. (b−d) SEM images of (b) particle A (2.0 μm
diameter), (c) particle B (2.8 μm diameter), and (d) particle C (1.4
μm diameter). (e) Comparison of the EDS spectra of the threeparticles. The intensity of each spectrum is displayed on a logarithmic
scale and shifted in a longitudinal direction. A rodlike extraneous
fouling over particle C (d) is a fragment of quartz ﬁber ﬁlter attachedto the carbon tape.

μm (H) with an integration time of 4.0 s/point. The XRF
intensities for each measured point were normalized to the I0
intensity.
The SR-μ-XANES spectra of the particles and the reference
samples were measured in ﬂuorescence mode for the following
absorption edges: the Fe−K edge (7 111 eV), Zn−K edge (9661 eV), U−L3 edge (17 171 eV), Mo−K edge (20 000 eV),and Sn−K edge (29 200 eV). The absorption edge energies
used were based on experimental values in Deslattes et al.10
In the SR-μ-XRD analysis, the X-ray diﬀraction patterns of
the samples were measured with a Debye−Scherrer opticalsystem using a two-dimensional detector (CMOS ﬂat panel)placed 200 mm behind the sample in beamtime B. Si powder(NIST SRM640c) was also measured as a reference material.
The energy of the incident X-ray was set to 15.0 keV with an
exposure time of 440 ms and an integration of 100 times/
sample. Details of the SR measurements are given in the
Supporting Information.

■

RESULTS AND DISCUSSION
Scanning Electron Microscope and Gamma-Ray
Spectra Analyses of Radioactive Aerosol Microparticles.
Figure 1b−d shows SEM images of the three microparticles
analyzed in the SR experiments. They are spherical withdiameters of ∼2 μm. EDS spectra of the three particles areshown in Figure 1e. There were no apparent diﬀerences among8522

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Analytical Chemistry

LetterFigure 2. Results of SR-μ-XRF analyses. (a) Comparison of the SR-μ-XRF spectra obtained for particles A, B, and C and the carbon tapebackground. The intensity of each spectrum was displayed on a logarithmic scale and shifted in a longitudinal direction. (b−d) Distributions ofrepresentative elements extracted from the SR-μ-XRF images of (b) particle A, (c) particle B, and (d) particle C with enlarged SEM image
corresponding to the imaging area (scale bar: 2 μm).

the three spectra, consistent with the previous results5
indicating that their major components were Fe, Zn, and Cs.
Some of the peaks for light elements (e.g., Si and Ca) may haveoriginated from both the glass substrate and the particle itself.
Gamma-ray spectra of the three particles detected both 134Cs
and 137Cs in each particle with activity ratios of ∼1 (decay
corrected as of March 2011). The decay-corrected activities for134
Cs and 137Cs were 1.20 (±0.05) Bq and 1.29 (±0.02) Bq forparticle A, 1.49 (±0.06) Bq and 1.49 (±0.03) Bq for particle B,
and 1.07 (±0.05) Bq and 1.10 (±0.02) Bq for particle C,
respectively. In the previous study,5 it is pointed out that theactivity ratios between 134Cs and 137Cs of the radioactive
materials released by the FDNPP accident were ∼1. It is thus
conﬁrmed that these three particles are radioactive ones derived
from the FDNPP accident.5
Detailed Chemical Composition Analysis. The SR-μXRF spectra of the three particles and the carbon tapebackground are shown in Figure 2a. In addition to Fe, Zn, andCs, all of which were previously reported,5 the following eightheavy elements were detected in all three particles: Rb, Zr, Mo,
Sn, Sb, Te, Ba, and Pb. Several unique elements were also
detected from speciﬁc particles, i.e., Mn and Cr in particle A
and Ag in particle B.
In addition to these elements, U−L lines in the SR-μ-XRF
spectra appeared in particles A and B. To address potential
interferences from elemental contamination such as W, which
could be due to contamination from the micromanipulator
needles during the particle separation procedure, we used SR-μXRF imaging analysis and visualized the elemental distributions
within each particle. Figure 2b−d shows the distributions of
selected elements from the SR-μ-XRF imaging of the threeparticles with enlarged SEM images corresponding to theimaging area (additional SR-μ-XRF images are shown in Figure
S1 in Supporting Information). In particles A and B, the twodimensional distributions of characteristic elements, including
U corresponded well to the particle shapes in the SEM images
and the Cs distributions identiﬁed by the SEM-EDS analysis.

In these images, we found homogeneous distributions ofmost elements in the particles except that of Pb in particle C(Figure 2d). Although strong peaks for Pb−L lines weredetected in the spectrum of particle C, the Pb distribution was
distinctly diﬀerent from those of the other elements and theSEM image of the particle, indicating that the Pb did not
originate from the particle components.
Veriﬁcation of the Presence of Uranium. In order toobtain additional evidence for the presence of U in the
microparticles, we conducted U−L3 edge SR-μ-XANES analysis(Figure 3a). While no absorption edge for U was observed forparticle C, clear edge jumps were observed for both particles A
and B at the energy of the U−L3 edge, conﬁrming the presenceof U within the aerosol microparticles in the environment. This
result implies that elements other than radioactive Cs were
emitted along with Cs from the reactor into the atmosphere.
Chemical State and Crystal Structure. SR-μ-XANES
spectra of the Fe, Mo, and Sn K−edges for these three particles
are shown in Figure 3b−d (see Figure S2 in Supporting
Information for Zn K−edge). Peak positions and the shapes ofthe pre-edges between the particles and the reference materials
agreed well, indicating that these elements occurred as Fe3+,
Mo6+, Sn4+, and Zn2+. In addition, features of the SR-μ-XANES
spectra of the three particles corresponded to those of the glass
references.
SR-μ-XRD patterns of the three particles and Si powder as areference material (see Figure S3 in Supporting Information)
showed that the particles had no diﬀraction peak while the Si
powder showed clear Debye−Scherrer rings. This result
suggests that the particles are amorphous, glassy materials.
These observations together with their spherule shapes impliedthat they experienced melting at a high temperature and rapid
cooling as aerosol under oxidative conditions.
Relevant Element Sources around the Reactors. We
explored the possible sources of the 14 elements (Cr, Mn, Fe,
Zn, Rb, Zr, Mo, Ag, Sn, Sb, Te, Cs, Ba, and U) found within the
microparticles by the SR-μ-XRF analysis. The reactors of the8523

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Analytical Chemistry

Lettertimes higher than that presently associated with the radioactive
Cs. In addition to the previous report that these particles are
insoluble in water,5 our study revealed that they are glassy
materials with highly oxidized states. These characteristics
suggest that they could have a relatively long-term impact on
the environment, i.e., continued release of soluble radioactive
Cs into the environment as these insoluble glassy particles
degrade. Similar radioactive particles have been detected insoils, plants, and mushrooms collected from the areasurrounding the FDNPP as shown by IP autoradiography.7
Although there is no chemical and size information for theparticles reported in other studies, it is probable that someradioactive particles found in these previous studies are thesame as the microparticles characterized in our study.

■

CONCLUSIONSThe present study has provided better understanding theaccident based on chemical information recorded in individual
2-μm radioactive Cs-bearing particles emitted from the FDNPP
accident using an SR-X-ray microbeam. The SR-μ-XRF analyses
directly identiﬁed U and heavy elements, that may originate
from the fuel, FPs and materials used in the FDNPP, containedin the aerosol particles together with radioactive Cs, althoughisotope ratios should be identiﬁed to conclude their exact
sources. The SR-μ-XANES and XRD analyses showed thatthese particles were highly oxidized glassy materials. Clarifying
the nature of these microparticles assists in understanding what
occurred in the reactors during the early stages of the accident.
Simulation of distribution and deposition of the radioactive
materials depends on physical and chemical natures of materials
of interest, and our results could improve models simulating
how radioactive materials were formed and were distributed
from the reactors into the environment during the accident.
Further quantitative investigations of the chemical nature of theradioactive particles including quantiﬁcation and chemical state
analysis of U and FPs in the particles will be important tounderstand further mechanisms of particle formation andemissions, as well as their potential human health andenvironmental impacts.

Figure 3. Results of SR-μ-XANES analyses. (a) Comparison of theU−L3 edge SR-μ-XANES spectra of the three radioactive particles
demonstrating the presence of U in particles A and B. (b−d)
Comparisons of the (b) Fe−K edge, (c) Mo−K edge, and (d) Sn−K
edge SR-μ-XANES spectra of the three particles and the referencematerials.

FDNPP (see Figure S4 in Supporting Information) wereboiling-water reactors (BWR),11 and the ﬁssion fuels composedof U (only no. 3 reactor used mixed oxide fuel11). As a result of
the nuclear ﬁssion reaction of U, the ﬁssion products (FPs)
could yield 9 elements (Rb, Zr, Mo, Ag, Sn, Sb, Te, Cs, andBa)11−13 found in the particles. Zr−Sn alloy was used for fuelcladding within the reactors.14 Stainless steel, which commonly
consists of Fe, Cr, and Mn, was used in the structure of thevessel. Zn had been added to the primary cooling water in the
FDNPP15 for corrosion control to reduce 60Co. On the other
hand, given the possibility of a molten core as a result of thenuclear meltdown may react with a concrete base as suggestedby the presence of Si in the particles, it should be noted that apercentage of some elements (e.g., Rb and Zn) may beoriginated from components of the concrete. Because of thelack of the access to the damaged reactors, we do not havedirect evidence to identify the source of these elements.
However, we conclude that U fuel, FPs, and components of thereactors are very likely the sources of the elements identiﬁed
within the three radioactive microparticles, although further
investigation will be needed to conﬁrm their sources. We
assume that, because these elements could have originated frommultiple sources, they were melted together during the accident
and eventually formed spherical microparticles.
Environmental Impacts of the Microparticles. If ourhypothesis that some heavy elements in the particles were
produced by nuclear ﬁssion reactions is correct, these particles
likely contained additional short-lived radionuclides when they
were released during the accident.11,12 Thus, the speciﬁc activityof these particles at the time of release may have been several■

ASSOCIATED CONTENT

S Supporting Information*

Additional material as described in the text. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors*E-mail: y.abe@rs.tus.ac.jp.
*E-mail: inakai@rs.kagu.tus.ac.jp.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by MEXT/JSPS KAKENHI Grants(Grant-in-Aid for Scientiﬁc Research on Innovative Areas underthe A01-02 research teams and publicly oﬀered research on theInterdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident; Grant
Numbers 24110003 and 25110510, respectively). The
synchrotron radiation experiments were performed with theapproval of the SPring-8 Program Advisory Committee(Proposal Numbers 2013A1392 and 2013B1309).
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Letter

REFERENCES(1) INES. The International Nuclear and Radiological Event Scale
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2014.
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Hayakawa, S. AIP Conf. Proc. 2004, 705, 376−379.
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(10) Deslattes, R. D.; Kessler, E. G., Jr.; Indelicato, P.; de Billy, L.;
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(14) Zinkle, S. J.; Was, G. S. Acta Mater. 2013, 61 (3), 735−758.
(15) Hori, S.; Suzuki, A. TEPCO’s Challenges for Occupational
Exposure Reductioninstallation of Additional CF in Fukushima
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OPEN
SUBJECT AREAS:
ENVIRONMENTALSCIENCESENVIRONMENTAL MONITORING
ATMOSPHERIC CHEMISTRY

Emission of spherical cesium-bearing
particles from an early stage of the
Fukushima nuclear accident
Kouji Adachi, Mizuo Kajino, Yuji Zaizen & Yasuhito Igarashi

GEOCHEMISTRY
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki, Japan 305-0052.

Received
12 June 2013
Accepted
15 August 2013
Published
30 August 2013

Correspondence andrequests for materials
should be addressed toK.A. (adachik@mrijma.go.jp)

The Fukushima nuclear accident released radioactive materials into the environment over the entire
Northern Hemisphere in March 2011, and the Japanese government is spending large amounts of money toclean up the contaminated residential areas and agricultural fields. However, we still do not know the exact
physical and chemical properties of the radioactive materials. This study directly observed sphericalCs-bearing particles emitted during a relatively early stage (March 14–15) of the accident. In contrast to theCs-bearing radioactive materials that are currently assumed, these particles are larger, contain Fe, Zn, andCs, and are water insoluble. Our simulation indicates that the spherical Cs-bearing particles mainly fell onto
the ground by dry deposition. The finding of the spherical Cs particles will be a key to understand theprocesses of the accident and to accurately evaluate the health impacts and the residence time in theenvironment.

An earthquake with a magnitude of 9.0 occurred in Japan on March 11, 2011. In addition to the earthquake
itself, a tsunami struck along the east coast of Honshu and damaged the Fukushima Daiichi nuclear power
plant (FNPP). The FNPP eventually lost its ability to cool the nuclear fuel, which caused hydrogen
explosions and released radioactive materials into the environment1,2. Because of the local contamination, many
people still cannot return home. Currently, the Japanese government is spending considerable amounts of money
to clean up the contaminated residential environment and agricultural fields.
The radioactive materials released to the atmosphere traveled over the entire Northern Hemisphere3–7. In
Europe, for example, Masson et al.3 measured increases in radioactive cesium and iodine in the air from March 19,
2011 and showed that the maximum levels occurred between March 28 and 30. Although the accident has global
impacts, we still do not know exactly what happened in the reactors during the accident, and the estimates of theradioactive Cs releases vary largely from 9 to 36 PBq6,8–10.
The chemical and physical properties (i.e., chemical forms, particle sizes, shape, phases (gas or aerosol), watersolubility, and residence time) of the radioactive materials released into the environment are not well known11.
Such knowledge is necessary to improve the numerical models to estimate the geographical distributions andevaluate the human exposures during and after the accident. Because the mass of released radioactive material was
small, i.e., the release of 137Cs from the accident was ,20 kg, and the material spread globally, it is extremely
difficult to chemically detect it other than by radioactivity detectors. In this study, we chemically detected Cs
within single particles for the first time by using electron microscopy and report the shape, composition, watersolubility, and sizes of the particles to evaluate the implications of their formation process, occurrences in theenvironment, and potential health effects. This knowledge is still important for preventing further accidents,
finding effective ways to remove the radioactive materials from the environment, and preventing further resuspention of the materials.

Results
Two plume events. The major radioactive material release events occurred during March 12 and 23, 2011 at the
FNPP5. We collected aerosol samples on quartz fiber filters (25 cm 3 20 cm) at the Meteorological ResearchInstitute, Tsukuba, Japan, which is located 170 km southwest of FNPP (see the Method section). In these samples,
we found two significant peaks in the radioactivity concentrations between March 14 and 15 (Plume 1) andbetween March 20 and 22 (Plume 2) (Fig. 1). These significant plumes in the air were also reported in easternJapan and polluted the water and soil2,12–15. Meteorological conditions, such as rain and the wind direction, andthe releases of radioactive materials were the main causes of the high surface deposition events14. We used the
SCIENTIFIC REPORTS | 3 : 2554 | DOI: 10.1038/srep02554

1

www.nature.com/scientificreports

Figure 1 | The radioactivity of the aerosol particles after the Fukushima Daiichi nuclear power plant accident in Tsukuba, Japan. Red dots indicate the
midpoints of each sampling period.

filters that had the maximum radioactivity levels from each plume(from March 14, 21:10 (local time) to March 15, 09:10 and fromMarch 20, 21:30 to March 21, 09:13) and analyzed portions ofthese filters (,10 cm2 per filter) using an imaging plate (IP) and ascanning electron microscope (SEM) to directly observe theradioactive materials. In addition, we measured the aerosol particle
size distributions within the plumes (Figs. S1 and S2).
First plume (March 14–15). We measured the radioactive materials
that were collected in the filter at ground level on March 14–15 using
the IP (Fig. 2). The radioactive materials were distributed spotty,suggesting that the number of radioactive particles was relatively
small but that their activity levels were relatively strong. Within
this filter sample, we counted approximately 100 spots caused byradioactive materials, suggesting a concentration of approximately
10 radioactive particles per m3. For reference, the average particle
number concentration was 4.1 3 107 per m3 for particles larger than
0.5 mm from March 15 (Fig. S1). The spotty distribution in the IP

image was also observed in the rooftop filter sample from March 15(Fig. S3).
To detect radioactive particles using SEM, it is necessary to reduce
the number of non-radioactive particles on the filter. Therefore, we
cut the filter into many small parts to include the radioactive spots
features (Fig. S4). We then measured the radioactivity of each segment using IP and/or a Ge detector to chase the radioactive particle.
After reducing the particle numbers in the segments from the three
radioactive spots, we used the SEM and found three radioactive Csbearing particles.
In Figure 3, we show a particle containing Cs (Cs Particle 1).
The particle is spherical with a diameter of 2.6 mm. The energy
dispersive X-ray spectrometer (EDS) spectrum shows Cs peaks.
The Cs distribution in the elemental mapping image indicates thatthe spherical particles consist of Cs along with substantial
amounts of Fe and Zn and minor amounts of Cl, Mn, and O.
The decay-corrected activity (as of March 2011) of Cs Particle 1 is
3.27 6 0.04 and 3.31 6 0.06 Bq for 137Cs and 134Cs, respectively

Figure 2 | The distribution of radioactive materials on the filter samples measured with the IP. Black dots indicate the presence of radioactive materials.
The outer rims (dotted line) of the filters were added artificially. This study focused on the filter samples from March 14, 21:10 to March 15, 09:10 (upper
left), and from March 20, 21:30 to March 21, 09:13 (bottom center).
SCIENTIFIC REPORTS | 3 : 2554 | DOI: 10.1038/srep02554

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Figure 3 | SEM and EDS mapping images of a radioactive Cs-bearing particle from the sample collected during March 14, 21:10 and March 15, 09:10.
(a) A Cs-bearing particle partially embedded within a carbon paste. (b) The same Cs-bearing particle as a) but measured the next day. The particle shows aspherical shape. (c) An elemental mapping (Cs) of the particle (a). (d) The EDS spectrum of the particle a) (black line). The red line shows the
spectrum from the glass substrate. The Cs in the particle shows multiple peaks. (e) An elemental mapping of the other elements within the area. O, Si, Cl,
Mn, Fe, and Zn are possibly coexistent with Cs within the particle.

(Fig. S5). Assuming a particle density of 2.0 g/cm3, the Cs masspercentage within the particle is estimated from its activity (Bq) to
be 5.5. Another Cs-bearing particles (Cs particles 2 and 3) fromthe same filter but different spots are similar to Cs Particle 1,
although they have weaker activity (Fig. S6). The particles consistof Fe, Zn, and Cs and are approximately 2.0 mm in diameter. The
radioactivity for Cs particle 2 is 0.66 6 0.02 and 0.78 6 0.04 Bq
for 137Cs and 134Cs, respectively. The Cs mass percentage within
Particle 2 is estimated from its activity to be 2.5. Assuming thatthe entire the radioactivity in the Plume 1 was from the Cs-bearing spherical particles gives an average of 1.4 Bq per particle,
which is comparable to that of Particle 2.
We analyzed the water solubility of Cs Particle 1 by comparing theparticle’s shape before and after exposure to water (Fig. S7). The
results show that there was no change in shape, suggesting thatSCIENTIFIC REPORTS | 3 : 2554 | DOI: 10.1038/srep02554

the particle was insoluble to water at least during atmospheric transportation.
Second plume (March 20–21). The IP image of the filter collected on
March 20–21 indicates that the radioactive materials are evenly
distributed within the filter with approximately 10 diffused spots(Fig. 2). We cut the filter including a diffused spot, formed several
layers, and captured the IP image (Fig. S8). The radioactivity is
distributed along the filter segments, a result which differs fromthat of Plume 1. We interpret that small amounts of theradioactive Cs attach to other dominant aerosol particles and thatthe occurrence is consistent with the results of Kaneyasu et al.16 about
the role of sulfate aerosols as carriers of 137Cs. An SEM analysis with
EDS elemental mapping shows numerous sulfate and mineral dust,
as is commonly found in aerosol samples (Fig. S8). The aerosol filter3www.nature.com/scientificreports

Figure 4 | The model simulation of the total deposition of the 137Cs
released from the FNPP in Plume 1 (between March 14, 17:00 and March
15, 02:00 (JST)). We assumed that all Cs consists of relatively large, waterinsoluble particles. The Regional Air Quality Model 2 (RAQM217) was used
for the model calculation. We used the Generic Mapping Tools (GMT)
developed at University of Hawaii to draw the figure. The model elevationin the figures was generated based on a 1 km resolution Global 30 ArcSecond Elevation (GTOPO30) of U.S. Geological Survey (USGS).

sample from the rooftop site also indicates a similar distribution ofradioactive materials (Fig. S3).
Model. We simulated the depositions of the radioactive particles from
Plumes 1 and 2 using a tagged chemical transport model. Unlike othersimulation models (e.g.4,6,9,12), we considered the aerosol dynamical
processes explicitly and used the measured values and assumptionsof the particle physical and chemical properties from our observations,
i.e., on March 14–15, radioactive Cs consisted of 2.3 mm hydrophobic
particles, whereas on March 20–21, the Cs was carried by hydrophilic
submicron particles (e.g., sulfate).
The tagged simulation result indicates that the aerosol particles on
the filter sample from Plume 1 were mainly emitted during March 14,
17:00 to March 15, 02:00 (JST) from the FNPP. On the other hand,the aerosol particles on the filter sample from Plume 2 were mainly
emitted during March 19, 20:00 and March 20, 07:00.
In Figure 4, we show simulation of the total (dry 1 wet) 137Cs
deposition. Within Plume 1, 17% and 5.1 3 1023% of the 137Cs
released from the FNPP fell onto the ground by dry and wet deposition processes, respectively, and the rest (83%) was deposited into
the ocean or was transported out of the model domain. In contrast,
the deposition ratios onto the ground for the particles within Plume 2were 1.9% and 3.8% by dry and wet deposition, respectively.

Discussion
This study reports for the first time the presence of spherical radioactive Cs-bearing particles emitted from the FNPP during a relatively
early stage (March 14–15) of the accident. The particles coexist withFe, Zn, and possibly other elements, and their diameters are approximately 2 mm. Because these elements were evenly distributed within
the particle, we conclude that they are internally mixed and form analloy. This result differs from that reported by Kaneyasu et al.16, whoshowed that the Cs measured on samples collected during April andMay 2011 was carried by sulfate aerosol particles approximately
0.5 mm in size. Due to its spherical shape and composition, theparticle is likely solid and is largely insoluble in water. Sphericalaerosol particles, such as fly ash, commonly form from liquidized
SCIENTIFIC REPORTS | 3 : 2554 | DOI: 10.1038/srep02554

materials or during the condensation of vaporized materials depending on their sizes17.
The spherical Cs-bearing particles were larger and less water soluble than sulfate particles, resulting in more dry deposition and lessdeposition in the region northwest of the FNPP (Figs. 4 and S9). If weassume that all of the Cs had been carried by the sulfate aerosol
particles in Plume 1, 5.6% and 9.3% of the released 137Cs should havebeen deposited through dry and wet deposition processes, respectively. Accordingly, the geographical distribution of Cs depositiondiffers depending on the physical and chemical properties (Fig.
S9), although the quantitative radioactivity levels for the total deposition in the model depend on assumptions such as cloud microphysics and the total amount of emissions from the FNPP, which is still
under debate. Our model results suggest that because the dry and wet
deposition processes are sensitive to the chemical form and sizes of
the Cs carriers, multiple numerical simulation models based on theaccurate chemical and physical properties of Cs-bearing particles will
be needed to reevaluate how the Cs from the early stages of theaccident was deposited.
It is probable that the emission processes had changed between theemissions of Plumes 1 and 2 as the accident and water injection
progressed; however, further studies will be needed to reveal theemission process during the accident.
This study aims to show the presence of spherical radioactive Csbearing particles to stimulate and facilitate further studies acrossmultidisciplinary fields that will enable the proper understanding
and evaluation of particle effects. We believe the finding of the Csbearing particles will have implications to the following studies.
1)

2)

3)

The composition and the spherical shape of the Cs-bearing
particles emitted by the FNPP accident will be a key to understand what happened in the nuclear reactors during theaccident.
The spherical Cs-bearing particles likely have longer retention
times on the land surface than those of the water-soluble Csparticles. The retention time of the particles in the soil or otherenvironments needs to be reconsidered.
The health effects of the particles should be evaluated based on
the particle sizes and insolubility in water.

Methods
Sampling. The samples were collected at the Meteorological Research Institute(Tsukuba, 36.05N, 140.13E) using both a high-volume aerosol sampler (Sibata
Scientific Technology ltd., HV-1000F; 1000 m3/24 h) placed on the ground and aPM2.5 aerosol sampler (24 m3/24 h) placed on the roof of the 6-floor building(approximately 25 m from ground level). Quartz fiber filters were used in both
samplers. The sampling times were 6, 12, or 24 h for the high volume air sampler and
24 h for the PM2.5 aerosol sampler. The particle size distributions were monitored
using an aerosol particle sizer (APS; TSI-3321) and a scanning mobility particle sizer(SMPS; TSI-3080 and TSI-3775) on the rooftop site for particle sizes .0.5 mm andbetween 7 and 289 nm in diameter, respectively, with a 2.5 mm cutoff size.
Analyses. An imaging plate (IP; GE CRx25P) was used to detect the radioactivity on
the filters with a pixel spatial resolution of 50 mm. An intrinsic Ge detector (SEIKO
EG&G) coupled with a multi-channel analyzer was used to obtain the gamma spectraof each Cs-bearing particle and filters. A scanning electron microscope (SEM; Hitachi
high-Technologies SU 3500) and an energy dispersive X-ray spectrometer (EDS;
Horiba ltd. X-max 50 mm) were used to observe and analyze the shapes andcompositions of the particles. The particles attached to the filter fibers were mounted
within a carbon tape (Fig. S4). A manipulator (Micro Support Corp., AP-xy-01) wasused to cut the carbon tape into as small segments as possible (,0.1 mm).
Model. We used the Regional Air Quality Model 2 (RAQM218), which implements a
triple-moment modal aerosol dynamics module assuming a log-normal size
distribution of the aerosol populations. This model describes the nature of the aerosol
dynamical processes, such as nucleation, condensation, coagulation, dry deposition,
grid-scale cloud condensation and ice nuclei activation, and the subsequent cloud
microphysical processes (rainout) and the washout processes. A non-hydrostatic
meteorological model (NHM)19 was used to produce the meteorological field. There
were 215 3 259 grids with a 3 km horizontal grid resolution in both the NHM andRAQM2. There were 50 vertical layers to 50 hPa in the NHM, and 20 layers to 10 km
in the RAQM2. The Japan Meteorological Agency (JMA) Meso-Regional Objective

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www.nature.com/scientificreports
Analysis data sets (3 h, 5 km 3 5 km) were used for the initial and boundary
conditions for the NHM and for the spectral nudging method. The 137Cs released fromthe FNPP was tagged with a temporal resolution of 1 hour. We assumed 0.43 PBq(Plume 1) and 0.39 PBq (Plume 2) for the total amounts of 137Cs activity released fromthe FNPP by using the inventory of Katata et al.20. We used a number equivalent
geometric mean dry diameter Dg,n,dry 5 2.3 mm (an averaged value of the Cs-bearing
particles 1 and 2), geometric standard deviation sg 5 1.3, particle density rp 5 2.0 g/
cm3, and hygroscopicity k 5 0 for Plume 1. For Plume 2, we used Dg,n,dry 5 102 nm(measured by SMPS), sg 5 1.6, rp 5 1.83 g/cm3, and k 5 0.4. The calculated dry
deposition velocities of the particles in Plume 1 are approximately 4–5 times greaterthan those of the particles in Plume 2. For the particles in Plume 2, both the rainout
and washout processes were considered, whereas for particles in Plume 1, only
washout was considered because no cloud condensation nuclei activity is assumed(i.e., k 5 0).
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Science 335, 1184–1188 (2012).
12. Morino, Y., Ohara, T. & Nishizawa, M. Atmospheric behavior, deposition, andbudget of radioactive materials from the Fukushima Daiichi nuclear power plantin March 2011. Geophys. Res. Lett. 38, GL048689 (2011).
13. Yasunari, T. J. et al. Cesium-137 deposition and contamination of Japanesesoils due to the Fukushima nuclear accident. Proc. Natl. Acad. Sci. USA 108,
19447–19448 (2011).

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14. Kinoshita, N. et al. Assessment of individual radionuclide distributions from the
Fukushima nuclear accident covering central-east Japan. Proc. Natl. Acad. Sci.
USA 108, 19526–19529 (2011).
15. Doi, T. et al. Anthropogenic radionuclides in the atmosphere observed at
Tsukuba: characteristics of the radionuclides derived from Fukushima. J. Environ
Radioactivity 122, 55–62 (2013).
16. Kaneyasu, N., Ohashi, H., Suzuki, F., Okuda, T. & Ikemori, F. Sulfate aerosol as apotential transport medium of radiocesium from the Fukushima nuclear accident.
Environ. Sci. Technol. 46, 5720–5726 (2012).
17. Damle, A. S., Ensor, D. S. & Ranade, M. B. Coal combustion aerosol formation
mechanisms: A review. Aerosol Sci. Technol. 1, 119–133(1981).
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type. Atmos. Chem. Phys. 12, 11833–11856 (2012).
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at JMA. J. Meteorol. Soc. Jpn. 85B, 271–304 (2007).
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Acknowledgements
This study was supported by MEXT KAKENHI (a Grant-in-Aid for Scientific Research on
Innovative Areas under the A01-01 and A01-02 research teams in the ‘‘Interdisciplinary
Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP
Accident’’; grant numbers 24110002 and 24110003, respectively) and the MEXT JapaneseRadioactivity Survey. We acknowledge Mr. H. Sako and Mr. T. Kimura (Atox Co. ltd.) for
their help with the IP and gamma spectrometry, Ms. C. Takeda (Tokyo Nuclear Services co.
ltd.), Ms. K. Inukai, and Ms. K. Kamioka for their help with the HV sampling, and Mr. Y.
Iizawa (Tokyo University of Science) for his help with finding the Cs-bearing particle 3. The
PM2.5 filters sampled by Mr. K. Kuchiki and Dr. T. Aoki (MRI) were courteously provided
for the present analyses.

Author contributions
K.A. found the Cs-bearing particles, performed the IP and SEM analyses, and was theprimary author of the manuscript; M.K. performed numerical model calculations; Y.Z.
performed the SMPS and APS analyses; Y.I. performed the filter sampling and supervised
the study; all authors contributed to the manuscript.

Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Adachi, K., Kajino, M., Zaizen, Y. & Igarashi, Y. Emission of
spherical cesium-bearing particles from an early stage of the Fukushima nuclear accident.
Sci. Rep. 3, 2554; DOI:10.1038/srep02554 (2013).
This work is licensed under a Creative Commons AttributionNonCommercial-NoDerivs 3.0 Unported license. To view a copy of this license,
visit http://creativecommons.org/licenses/by-nc-nd/3.0

5SUPPORTING INFORMATION FOR
Emission of spherical cesium-bearing particles from anearly stage of the Fukushima nuclear accident
Kouji Adachi1*, Mizuo Kajino1, Yuji Zaizen1, and Yasuhito Igarashi1
1: Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki, Japan 305-0052*Corresponding author: Kouji Adachi, Meteorological Research Institute, 1-1 Nagamine, Tsukuba,
Ibaraki, Japan 305-0052, tel: +81-29-853-8570, adachik@mri-jma.go.jp

1

SI 1. Particle number concentrations for particles of 0.5-1.0, 1.0-2.0, and >2.0 µm. These data were
measured using an aerosol particle sizer (APS). The data around March 15 were missing because ofa regional unstable power supply caused by the earthquake. The APS was located on the rooftop
site.

2

SI 2. The size distribution of aerosol particles between 7 and 289 nm using a scanning mobility
particle sizer (SMPS). The averaged particle number concentration was 1.9×109 per m3 during thesecond plume (March 20-21). The data prior to March 15, 17:18 (local time) were missing becauseof the emergency stop caused by the earthquake. The SMPS was located on the rooftop site.

3

SI 3. The IP images of the PM2.5 filter samples collected on March 15 (left) and March 21 (right)
on the rooftop site. The left filter includes many spots whereas the right filter appears dark all over.
The features are similar to those in the Figure 2, which were collected at the ground level of the site.
A part of each filter was cut for carbonaceous measurements and replaced (the trace is apparent in
the right filter). A two hour exposure time was used for the IP analysis.

4

SI 4. The selection of radioactive particles for the SEM analysis. a) The IP image of the filter was
captured, and a part of the filter that includes radioactive material was cut off (dotted line). b) The
segment of the filter was separated into approximately 10 layers and put onto a carbon tape attached
to a glass substrate. c) The IP image was taken to identify the position of the radioactive material. d)
The carbon tape including the radioactive material was cut into small pieces. e) The radioactive
material was detected using a Ge detector from the filter segments. f) The processes d) and e) wererepeated to make the filter and the number of particles as small as possible. g) The radioactive
particle embedded within carbon paste was analyzed using the SEM.

5

SI 5. The gamma spectrum of radioactive Cs particle 1. A 50,000 sec detection time was used.
Peaks marked with * were from the background or the glass substrate. The sample was analyzed
two years after the accident, and short-lived radionuclides would not have been detected even if
they had been present when emitted.

6

7

SI 6. The SEM analyses of radioactive Cs particle 2 from the March 14-15 sample. 1) An SEM
image of the Cs-bearing particle. The particle was embedded within a carbon paste and waspartially covered with a quartz fiber. b) The elemental mapping of Cs in the particle. c) The
elemental mapping for Zn, Fe, and O in the particle. Unlike in particle 1, oxygen is not apparent. d)
The line profile of the selected elements over the particle. The analyzed line is shown in a). e) The
EDS spectra of the particle (black line) and glass substrate (red line).

8

SI 7. SEM images of the Cs particle 1 before and after exposure to water. There is no change
between the images, suggesting that the particle is largely insoluble to water. Scale bars: 1 µm.

9

SI 8. The SEM analyses of the filter sample from March 20-21. a) An SEM image of a filter
segment containing a diffused spot (Fig. 2). The filter was divided into several layers (SI 4-b), andthis image corresponds to one of those layers. b) An IP image of the filter. Radioactive materials are
distributed all over the filter. c) Elemental mapping images for Al and S, which represent
aluminosilicate minerals and sulfate particles, respectively. These particles are distributed all overthe filter.

10

SI 9. The total (dry + wet) deposition of 137Cs released from the FNPP between (a) March 14, 17:00
and March 15, 02:00 (JST) and (b) March 19, 20:00 and March 20, 07:00. The Cs carriers are
assumed to be water-soluble submicron particles, such as sulfates, in both plume. The Regional Air
Quality Model 2 (RAQM2) was used for the model calculation. We used the Generic Mapping
Tools (GMT) developed at University of Hawaii to draw the figure. The model elevation in the
figures was generated based on a 1km resolution Global 30 Arc-Second Elevation (GTOPO30) of
U.S. Geological Survey (USGS).

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OPEN

received: 03 June 2015
accepted: 06 January 2016
Published: 03 February 2016

Internal structure of cesiumbearing radioactive microparticles
released from Fukushima nuclear
power plant
Noriko Yamaguchi1,*, Masanori Mitome2, Akiyama-Hasegawa Kotone2, Maki Asano1,3,
Kouji Adachi4 & Toshihiro Kogure5,*
Microparticles containing substantial amounts of radiocesium collected from the ground in Fukushima
were investigated mainly by transmission electron microscopy (TEM) and X-ray microanalysis withscanning TEM (STEM). Particles of around 2 μm in diameter are basically silicate glass containing Fe
and Zn as transition metals, Cs, Rb and K as alkali ions, and Sn as substantial elements. These elements
are homogeneously distributed in the glass except Cs which has a concentration gradient, increasingfrom center to surface. Nano-sized crystallites such as copper- zinc- and molybdenum sulfide, andsilver telluride were found inside the microparticles, which probably resulted from the segregationof the silicate and sulfide (telluride) during molten-stage. An alkali-depleted layer of ca. 0.2 μm thickexists at the outer side of the particle collected from cedar leaves 8 months after the nuclear accident,
suggesting gradual leaching of radiocesium from the microparticles in the natural environment.
Although almost five years have passed since the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP),
radioactive contamination in the surrounding area is still a serious problem in Japan. The largest radionuclide
deposition event occurred on March 15–16 and the second largest on 21–23, 2011. Wet deposition was a major
source of radiocesium contamination of terrestrial environment1, while contribution of dry deposition was larger
near the FDNPP2. In order to understand and predict the fate of radioactive materials contaminating the terrestrial environment, it is important to clarify the physicochemical properties of the deposited materials. Fromprevious cases of radionuclide release, it is known that the chemical species of released radiocesium is monovalent cation (Cs+) which is soluble3. Deposition of radiocesium as insoluble particles has also been pointed out.
Autoradiography analyses using imaging plate (IP) showed spots of particulate materials on plant tissues collectedfrom Fukushima4–6. On the aerosol filter collected from March 14–15, 2011 in Tsukuba, 170 km south-southwestof FDNPP, Adachi et al.7 discovered spherical particulate radiocesium of 2.0–2.6 μ m in diameter, with particles
insoluble in water having a glass-like structure8. These microparticles contain several fission products of U-235
other than radiocesium, and Fe and Zn which are also used in nuclear reactors8. Hence, they were considered tobe released directly from nuclear reactors.
Kaneyasu et al.9 suggested that vaporized radiocesium was transported with sulfate aerosol in the air, dissolved
to cloud droplets and fell as rain. On the aerosol filter collected on March 20–21, 2011, rainy days in Tsukuba,
the majority of radiocesium was in water-soluble form7. Such water-soluble radiocesium that reached the groundsurface as a solute was fixed to soils, especially to clay minerals10. In the terrestrial environment, the majority ofradiocesium is present in solid form regardless of the initial form of deposition. However, compared to clay minerals originally contaminated by soluble radiocesium in soil, the solid radiocesium, which was initially depositedas radioactive microparticles, had stronger radioactivity. Although the contribution or percentage of such radioactive microparticles in the contamination level of Fukushima has not been evaluated, its influence on human1

National Institute for Agro-Environmental Sciences, Kannondai, Tsukuba 305-0864, Japan. 2National Institutefor Materials Science, Namiki, Tsukuba 305-0044, Japan. 3Graduate School of Life and Environmental Sciences,
University of Tsukuba, Tennodai, Tsukuba, 305-8572, Japan. 4Meteorological Research Institute, Nagamine, Tsukuba,
305-0052, Japan. 5Graduate School of Sciences, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan. *Theseauthors contributed equally to this work. Correspondence and requests for materials should be addressed to N.Y.
(email: nyamag@affrc.go.jp) or T.K. (email: kogure@eps.s.u-tokyo.ac.jp)
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

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Figure 1. (a) Bright-field (BF) image of NWC-1. The opaque material outside the microparticle is tungsten(W) and copper (Cu) deposited in the FIB process. (b) Electron diffraction (ED) pattern from the particle.
(c,d) EDS spectrum acquired from almost the whole area of the particle for the energy range of (c) 0–11 keV
and (d) 10–30 keV. (e) BF image of CB-8. The opaque material outside the sphere is W and the thin transparent
material is Kapton tape. (f) ED pattern from the particle.

health may be serious in terms of its intense radioactivity. Moreover, the structural detail of the microparticles
may give insights into the state of the broken reactor and fuel debris.
In the present study, we investigated radioactive microparticles, similar to those reported by Adachi et al.7, butcollected from the ground, by observing their internal structure with transmission electron microscopic (TEM)
techniques.

Results

Structure and composition of Cesium-bearing radioactive microparticles.

Cesium-bearing radioactive microparticles that had been deposited on non-woven fabric cloth (NWC-1) and on a needle of Japanesecedar (Cryptomeria japonica) (CB-8) were investigated. They were in the field for five and eight months, respectively, until sampling. Scanning electron microscope (SEM) images of NWC-1 of the whole microparticle before
preparing thin sections for TEM analyses; and elemental composition of the whole particle determined by synchrotron radiation microbeam X-ray fluorescence (SXRF) are shown in Supplementary Figs S1 and S2 online,
respectively. The activities of 137Cs for the NWC-1 and CB-8 were 5.04 ±  0.472 and 3.14 ±  0.178 Bq, respectively.
Bright-field (BF) images and selected-area electron diffraction (SAED) patterns recorded in TEM from thewhole area of NWC-1 and CB-8 are shown in Fig. 1, along with the energy-dispersive X-ray spectrum (EDS)
acquired in TEM from NWC-1. Preparation of thin specimens from NWC-1 microparticle by focused-ion-beam(FIB) process was successful only for the upper half of the sphere. The BF contrast of NWC-1 (Fig. 1a) was almost
uniform except for two dark nanoparticles inside the sphere. These nanoparticles will be described in a later subsection. The SAED pattern from the whole particle consists of only a halo (Fig. 1b) and the EDS from the whole

Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

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particle (Fig. 1c,d) mainly consist of peaks of Si and O (Cu is from the supporting mesh), indicating that the particle was basically silicate glass. Additionally, Cl, K, Cs, Fe, Zn, Rb, and Sn were definitely identified. The presence ofNa was not confirmed since its peak overlapped with the L-peak of Zn. These elements were previously reported7,8
except for K. EDS Semi-quantitative analysis for almost the entire area of the TEM specimen, without considering
the absorption effect estimated the glass composition (wt.%) to be SiO2; 69.3, K2O; 1.9, Fe2O3; 8.6, ZnO; 11.0,
Rb2O; 1.3, SnO2; 1.4, Cs2O; 3.4 with a small amount of Cl (1.4 wt.%). The valence states of cations were assumed to
be in line with results in an X-ray absorption spectroscopic study8. Rubidium, Cs and Sn are the fission productsof U-235. Tin is also used for fuel cladding11. Iron is used as steel for reactor pressure vessels, and Si and O aremain components of concrete at the bottom of the containment vessel, where the melt-down fuel or core debris
is thought to exist. Zinc had been added to the primary cooling water12. Potassium and Cl may originate fromconcrete and/or seawater, which was used to cool down the reactor.
The BF TEM image of CB-8 showed a two-layer structure having an inner core with darker contrast, an outercrust with a lighter one, and a small dark nanoparticle near the center (Fig. 1e). The thickness of the crust wasaround 0.2 μ m. SAED pattern indicated that both are amorphous (Fig. 1f). Locally, a bubble-like structure wasobserved at a certain radius in the crust, or the outside part was peeled off at the radius (Fig. 1e). The chemicalcomposition of the entire area of the specimen was SiO2; 73.3, K2O; 1.4, Fe2O3; 7.2, ZnO; 11.4, Rb2O; 1.2, SnO2;
1.5, Cs2O; 3.3 and Cl; 0.7, which is similar to that of NWC-1.

Distributions of elements in microparticles. Elemental maps from NWC-1 using STEM-EDS are
shown in Fig. 2a. Brighter color indicates higher concentration of the elements. All elements were almost uniformly distributed in the particle with Cs the only exception. Cesium concentration was lower near the center
than the outside of the particle. The ratio of the amounts between the center and outside is around two. It is
possible that vapor-phase cesium in the reactor was absorbed to the molten silicate microparticles, from theirsurface. The low diffusion velocity of cesium in the glass may have left the concentration gradient. On the contrary, elemental mapping for CB-8 (Fig. 2b) indicates that K, Rb and Cs were distinctively depleted in the crust
but concentrated in the vicinity of the surface. Although the reason for the high concentration at the surface is
not certain, it might be an artifact by diffusion of alkali cations caused by electron-beam radiation in the analysis.
Chlorine was concentrated in the crust.
Crystalline nanoparticles in the glass. Two nanoparticles with dark contrast were observed in the TEM
image of NWC-1 (Fig. 1a). Although EDS spectrum only from the particles cannot be obtained since they arestill buried in the glass in spite of the thinning by FIB, S was distinctively identified from the particles (Fig. 3a).
Elemental mapping by STEM-EDS indicated enrichment of Cu, Zn and Mo at the nanoparticles along with S,
indicating that they are sulfide. The possibility of sulfate was excluded because oxygen is deficient in the nanoparticles in the oxygen map (Fig. 3a). Moreover, the locations of Cu, Zn and Mo do not overlap within thenanoparticles, suggesting that these metal elements form different sulfide phases in the nanoparticle (Fig. 3a).
SAED from the left particle showed a single-crystal diffraction pattern (Fig. 3b), which can be explained by the
crystallographic parameters for a high-temperature polymorph of digenite (Cu2−xS). The origin of Cu and S may
be minor elements in the concrete. On the other hand, Ag and Te were distinctively detected by EDS in TEM
from the nanoparticle in CB-8 (Supplementary Fig. S3 online), indicating the particle to be silver telluride. Bothelements can be fission products of U. Diffraction spots were observed in the SAED pattern from the particle,
indicating that it is crystalline but the phase could not be identified. These sulfide and telluride were probably
segregated and crystallized in the microparticles at the molten state.

Discussion

Our most significant finding is that the matrix of the Cs-bearing microparticles is silicate glass, based on the
TEM-EDS analysis with FIB sample preparation. Previous studies suggested that Fe, Mo, Sn and Zn in the
Cs-bearing microparticles had a similar X-ray absorption near-edge structure to those composed of glass8, however the presence of Si in the microparticles has not been verified7,8. It is probable that the high-temperature
melt-down fuel from the reactor came into contact with and melted the concrete, and then splashed microparticles of silicate melt, which were solidified by cooling to form silicate glass in the atmosphere. However, there areseveral questions with respect to the selection of the constituting elements in the glass from various ones in thereactor. For instance, Ca which is one of the major elements in concrete, was almost absent in the microparticles
of NWC-1. Since TEM observed only a small portion of the microparticles, by making them thin using FIB, there
may have been other elements in the microparticles, for instance, as a form of chalcogenide nanoparticle.
The next important finding is the alkali-depleted crust in CB-8 microparticle. This is probably the result of
elution of alkali ions by contact with acidic solution in the field, commonly observed in silicate glass13. On theother hand, such alkali-depleted crust was not observed in NWC-1. This may be attributed to the different environments of the two microparticles after release from the nuclear plant. NWC-1 was on non-woven cloth andCB-8 on a cedar leaf before it was collected. It is well-known that silicate glass elutes alkali components fromtheir surface by ion-exchange with proton or hydronium ions to form an alkali-leaching layer on the surface if
pH of reacting solution is low, whereas the silicate framework of the glass itself is dissolved with high-pH solution13,14. Coniferous forest canopy induces acidic condition due to ammonia uptake, nitrification and leachingof plant-derived acid15. It was likely that CB-8 deposited on the cedar leaf had been in acidic conditions, which
derives alkali depletion in the crust, for eight months. The finding of the alkali-depleting crust on the surface of
the Cs-bearing radioactive microparticle indicates that radiocesium in the particles can be released by “weathering” of the glass in natural environments, and considering its small size, duration for the total release of theradioactive cesium from the particles is probably not long, from several years to a few decades, though it willstrongly depends on the environment.
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

3www.nature.com/scientificreports/

Figure 2. (a) Element maps of NWC-1 acquired by STEM-EDS with a STEM Dark-field (DF) image at thesame area, and (top) TEM Bright-field image in which the rectangle indicates the area analyzed by STEM-EDS.
The thin area around the top of the particle which was not observed in Fig. 1a was formed by a further thinning
process by FIB. (b) Element maps and images of CB-8 with the same framing as in (a).

In order to investigate the dissolution rate and detailed Cs-leaching properties of the Cs-bearing radioactive
microparticles, a leaching experiment should be conducted as a function of temperature and pH. However, collecting and isolating the Cs-bearing microparticles is time-consuming and it is difficult to obtain a large enoughnumber of Cs-bearing microparticles to investigate dissolution properties. Alternatively, synthesized silicate glasswith the same composition as the microparticles presented in this study may help to obtain information on thefate of Cs-bearing radioactive microparticles. However, in our preliminary experiment, we were unable to makeuniform glass with the same composition at present. This is probably due to the liquid immiscibility at the composition. A solution to this problem is now being considered.
There are mainly two types of solid-phase radiocesium in the terrestrial environment affected by FDNPP accident; that fixed to clay minerals in the soil via wet deposition and that contained in the microparticles of silicate
glass flown directly from the nuclear reactors. The radioactivity of the former is rather in low-density but distributed widely, therefore it is a major source of external radiation from ground. Part of the radiocesium adsorbed on
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

4www.nature.com/scientificreports/

Figure 3. (a) Element maps and STEM bright-field (BF) image of the two nanoparticles inside NWC-1.
The circular contrasts outside the nanoparticles in the BF image are radiation damage in glass formed by theelectron probe for chemical analyses. (b) (Left) ED pattern from the left nanoparticle. (Right) Drawing of ED
pattern for the high-temperature polymorph of digenite (Cu2−xS) along < 121> .

the soil clay is desorbed and transferred to crops thereby causing internal radiation. On the other hand, contribution of the microparticles to the air radiation is most likely not significant, but their radiation density is very high,which is particularly problematic for organisms including humans if the microparticles are inhaled or ingested.
The plant availability of radiocesium in the microparticles should depend on its solubility. Consequently, further
research on this material should be carried out as soon as possible.

Methods

Samples. Radioactive microparticles attached on non-woven fabric cloth and needles of Japanese cedar, collected from Fukushima, were identified by autoradiography followed by point-by-point analyses with scanningelectron microscope (SEM; Hitachi high-Technologies SU3500) equipped with an energy dispersed X-ray spectrometer (EDS; Horiba X-max 50 mm). The identified particles were further determined by SXRF with excitation
X-ray energy of 37.5 keV to ensure the detection of Cs-K line from the particle. The activities of 137Cs was determined by a Ge detector (GCW2523S Canberra, USA). Detailed methods to identify the Cs-bearing microparticles are described in the supplementary information.
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

5www.nature.com/scientificreports/
TEM Analyses.

Cross-sectional thin TEM specimens were prepared from radioactive microparticles using
a focused ion beam (FIB) instrument with micro-sampling system (Hitachi FB-2100) as described in the supplementary information. Then specimens were initially examined using a TEM (JEOL JEM-2010UHR) operated at
200 kV with an EDS analyzer system (JEOL JED-2200). Elemental mapping in the microparticles and quantitative
analyses were performed using a JEOL JEM-3100FEF operated at 300 kV in the STEM mode, with an EDS analyzer system (Thermo Fisher Scientific NORAN System SIX). Finally, elemental maps for nanoparticulates inside
the microparticles were acquired using a JEOL JEM-2800 operated at 200 kV with double wide-area (0.95 sr.)silicon drift detectors (SDD) for EDS analyses.

References

1. Yoshida, N. & Takahashi, Y. Land-surface contamination by radionuclides from the Fukushima Daiichi Nuclear Power Plant
accident. Elements 8, 201–206 (2012).
2. Katata, G., Ota, M., Terada, H., Chino, M. & Nagai, H. Atmospheric discharge and dispersion of radionuclides during the Fukushima
Dai-ichi Nuclear Power Plant accident. Part I: Source term estimation and local-scale atmospheric dispersion in early phase of theaccident. J. Environ. Radioactiv. 109, 103–113 (2012).
3. Santschi, P. H. et al. Chernobyl radionuclides in the environment- tracers for the tight coupling of atmospheric, terrestrial, andaquatic geochemical processes. Environ. Sci. Technol. 22, 510–516 (1988).
4. Nakanishi, T. M., Kobayashi, N. I. & Tanoi, K. Radioactive cesium deposition on rice, wheat, peach tree and soil after nuclear
accident in Fukushima. J. Radioanal. Nuclear Chem. 296, 985–989 (2013).
5. Nakanishi, H. et al. Radioactive cesium distribution in bamboo Phyllostachys reticulata (Rupr) K. Koch shoots after the TEPCO
Fukushima Daiichi Nuclear Power Plant disaster. Soil Sci. Plant Nutr. 60, 801–808 (2014).
6. Itoh, S., Eguchi, T., Kato, N. & Takahashi, S. Radioactive particles in soil, plant, and dust samples after the Fukushima nuclear
accident. Soil Sci. Plant Nutr. 60, 540–550 (2014).
7. Adachi, K., Kajino, M., Zaizen, Y. & Igarashi, Y. Emission of spherical cesium-bearing particles from an early stage of the Fukushima
nuclear accident. Sci. Rep. 3, No. 2554 (2013).
8. Abe, Y. et al. Detection of uranium and chemical state analysis of individual radioactive microparticles emitted from the Fukushima
nuclear accident using multiple synchrotron radiation X-ray analyses. Anal. Chem. 86, 8521–8525 (2014).
9. Kaneyasu, N., Ohashi, H., Suzuki, F., Okuda, T. & Ikemori, F. Sulfate aerosol as a potential transport medium of radiocesium fromthe Fukushima Nuclear accident. Environ. Sci. Technol. 46, 5720–5726 (2012).
10. Mukai, H. et al. Speciation of radioactive soil particles in the Fukushima contaminated area by IP autoradiography andmicroanalyses. Environ. Sci. Technol. 48, 13053–13059 (2014).
11. Zinkle, S. J. & Was, G. S. Materials challenges in nuclear energy. Acta Mater. 61, 735–758 (2013).
12. Hori, S. & Suzuki, A. TEPCO’s challenges for occupational exposure reduction - installation of additional CF in Fukushima Daiichi
NPP., < www.nsra.or.jp/isoe/english/…/pdf/atc2009-3-1ga.pdf> (2009). (Accessed 18th May 2015).
13. Bunker, B. C. Molecular mechanisms for corrosion of silica and silicate-glasses. J. Non-Cryst. Solids 179, 300–308 (1994).
14. Gin, S. et al. Origin and consequences of silicate glass passivation by surface layers. Nat. Commun. 6, No. 6360 (2015).
15. Cronan, C. S. & Reiners, W. A. Canopy processing of acidic precipitation by coniferous and hardwood forests in New England.
Oecologia 59, 216–223 (1983).

Acknowledgements

Part of this work was supported by a Grant-in-Aid for science research (15H04222 and 24110003) by JSPS, Japan.
This work was also supported by 2014 JAEA contract research for Fukushima environment recovery, “Studyon Cs adsorption and desorption process on clay minerals” and by MAFF contract research “Development of
Decontamination technologies for radioactive substances in agricultural land”. The synchrotron radiation
experiment was performed under approval of the SPring-8 Program Advisory Committee (Proposal Number:
2014B1282). The authors wish to thank Dr. M. Kobayashi for providing the Japanese cedar samples, Ms. E. Fujii
for preparing TEM specimens, Dr. T. Hara for providing the opportunity to use the STEM instrument, Prof. Clive
Langham for English edition, and Dr. Y. Igarashi for comments on the manuscript.

Author Contributions

N.Y. and T.K. designed the research and wrote the paper; N.Y. performed the IP, SEM and SXRF analyses; M.A.
performed SEM and SXRF analyses; M.M. and K.A.-H. performed TEM analyses; K.A. performed SEM analyses;
T.K. performed SEM and TEM analyses and supervised the study; all authors contributed to the manuscript. N.Y
and T.K. contributed equally to this work.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Yamaguchi, N. et al. Internal structure of cesium-bearing radioactive microparticles
released from Fukushima nuclear power plant. Sci. Rep. 6, 20548; doi: 10.1038/srep20548 (2016).
This work is licensed under a Creative Commons Attribution 4.0 International License. The images
or other third party material in this article are included in the article’s Creative Commons license,
unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license,
users will need to obtain permission from the license holder to reproduce the material. To view a copy of thislicense, visit http://creativecommons.org/licenses/by/4.0/

Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

6地 球 化 学 49，185‒193（2015）‒193（2015）
Chikyukagaku（Geochemistry）49，185
電子顕微鏡がとらえた放射性粒子
doi:10.14934/chikyukagaku.49.185

総

185

説

電子顕微鏡がとらえた放射性粒子：
福島第一原子力発電所事故初期に大気中に放出された
放射性粒子の物理化学的性質
足

立

光

司*

（2015 年 8 月 13 日受付，2015 年 10 月 9 日受理）

Radioactive particles revealed by electron microscopy:
Chemical and physical properties of radioactive particles inaerosol samples emitted during the early stage ofFukushima Dai-ichi Nuclear Power Plant accident
Kouji Adachi*
Meteorological Research Institute1‒1 Nagamine, Tsukuba, Ibaraki 305‒0052, Japan* E-mail: adachik@mri-jma.go.jp
Water-insoluble radioactive materials emitted during an early stage of the Fukushima
Dai-ichi Nuclear Power Plant accident in 2011 were identiﬁed, and their chemical and physicalproperties were characterized as particulate matters. In this report, studies on radioactive particles collected from ﬁlter samples in Tsukuba on March 14‒15, 2011 are summarized. Their
compositions, chemical states, sizes, shapes, crystallinity, and hygroscopicity were analyzed using microscopic analyses such as electron microscopy and synchrotron with a micro-beam. The
results indicate that they include Cs, Fe, and Zn as well as elements from ﬁssion products andare water insoluble, spherical-glassy particles with ca. 2 micrometer in size. Understanding of
their detailed properties is signiﬁcant to improve the numerical models during the accident
and to understand their occurrences in soil as well as the accident itself. In addition to the water-insoluble radioactive materials, water-soluble radioactive materials, which were likely
emitted in different events during the accident, should be investigated to have comprehensive
understanding of the accident and its environmental effects. More samples from various environments such as soil will be needed, and more detailed chemical and physical analyses will
help to understand their formation process, inﬂuences on human health, and long term decrements in ambient conditions.
Key words: Radioactive particles, Fukushima Dai-ichi Nuclear Power Plant Accident, Electron microscopy, Synchrotron Radiation, Cesium

1. は じ め に沈着し環境汚染を引き起こしている（Masson et al.,2011; Buesseler et al., 2012; Kaneyasu et al., 2012;

東日本大震災時に発生した福島第一原子力発電所事

鶴 田・ 中 島，2012; Yoshida and Kanda, 2012）。 そ

故によって，さまざまな放射性物質が大気・海洋に放の沈着プロセスは，放射性物質の運び手となるエアロ

出 さ れ た（Chino et al., 2011; Tsuruta et al., 2014;

ゾルの大きさ・化学組成・水溶性といった物理化学

Katata et al., 2015）。大気中にエアロゾルとして放特性（Adachi et al., 2013）や，降水・降雪・霧など出された放射性物質は，地面，森林，また海面などにの 気 象 の 影 響 を 大 き く 受 け る（Hososhima andKaneyasu, 2015）。これらの放射性物質は，植物（梅気象研究所〒305‒0052

村ほか，2015），土壌（Kinoshita et al., 2011; Fujii
茨城県つくば市長峰 1‒1

et al., 2014），大気中（Igarashi et al., in press）か

186

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司

ら検出されており，それらがどのように環境で移行すら大気中放射性物質のモニタリングを行っている（Ig-

るかといった長期的な環境動態に対する理解は，汚染

arashi et al., in press）。原発事故時においても敷地を受けた地域の将来にとって重要である。

内で空気中に含まれる放射性物質の採取を行ってお事故によって放出された放射性物質は重量換算にすり，ゲルマニウム半導体検出器を使った放射能濃度のると非常に微量であり，放射能測定以外の化学分析手

時系列変化が測定されている（Fig. 1）。その結果，

法で環境試料から放射性物質を検出することは困難で近隣のモニタリング結果と同様に（Doi et al., 2013）,ある。一方，放射性物質の環境動態は，放射性物質自3 月 14‒15 日，3 月 20‒21 日に採取された試料が高い体の性質だけで決まるものではなく，その運び手とな

放射能強度を示す結果が得られている。フィルターかる粒子の化学組成，イオン状態，粒径，表面状態とら検出された放射性元素のうち，短寿命核種を除き放いった物理化学特性に大きく支配されるため，その特射能強度が高かったものはセシウム 134・137 であ定および化学分析は重要である。本稿では，2013 年

り，最大で大気 1 m3 中に 40 Bq 程度であった。

に筆者らが報告した水に溶けにくい性質をもった放射フィルター上に捕集された放射性物質の分布を可視性粒子を電子顕微鏡で直接分析した研究（Adachi et

化して分析する，イメージングプレート（IP）と呼ば

al., 2013）および，その研究を発展させた放射光 X 線

れる方法がある（箕輪，2013; 田中ほか，2013; Itoh分析を用いた粒子の詳細な化学状態・組成分析の結果

et al., 2014）。この IP を使って放射能強度の高い 3 月（Abe et al., 2014）を中心にまとめる。それらの結果

14‒15 日と 20‒21 日のフィルターを分析すると，Fig.

を踏まえ，事故初期に放出された放射性粒子の物理化

2 のような結果が得られた。3 月 14‒15 日に捕集され学的特徴およびその研究がもたらす応用について，いたフィルターからは，比較的放射能強度の強いスポッ

くつかの提案を行う。

ト状の放射性物質が見つかり，後者の 3 月 20‒21 日に2.

関東に飛来した放射性物質採取されたフィルターからは，ほぼ全面に均一な強度の放射性物質が検出された。これらの結果は，原発事

原発事故によって大気中に放出された放射性物質

故によって大気中にエアロゾルとして放出された放射は，大気の流れによって原発周辺をはじめ東日本，関性物質が，原発から直接放出された比較的高濃度に放東，さらには北半球全域に拡散した（Masson et al.,

射性物質を濃縮した粒子と，ガスや水溶性の放射性物

2011; Morino et al., 2011; Christoudias and Le-

質として放出された後大気での反応や水に溶けて他の

lieveld, 2013; de Vismes Ott et al., 2013）。関東周辺

物質と混ざり合ったものの二種類があることを示唆しでは，3 月 15 日，3 月 21 日前後に高濃度汚染空気塊がていた。3 月 14‒15 日に捕集された放射性物質は，そ到達したことが報告されている（例えば Doi et al.,の分布状態から事故によって直接放出された粒子であ2013）。

ることが考えられ，次章に示す方法でその単離を試み

茨城県つくば市にある気象研究所では，1957 年かFig. 1た。

(Color online) The radioactivity of the aerosol particles after the Fukushima Dai-ichi nuclear
power plant accident in Tsukuba, Japan. Dots indicate the midpoints of each sampling period.
Reprinted from Fig. 1 of Adachi et al. (2013)．

電子顕微鏡がとらえた放射性粒子

3.

放射性粒子の単離

187

3 月 14 日の空気中には約 4×107 個 m−3 の濃度で粒子が存在した（Adachi et al., 2013）。そのため，電子顕エアロゾル粒子をフィルター上に捕集した試料に微鏡等でそのままフィルターを分析しても放射性物質

は，放射性粒子に加え他の発生源から生じた放射性でを特定することはほぼ不可能であり，フィルター上のはない粒子が無数にある。例えばつくば市の 2011 年

放射性物質を単離して顕微鏡分析に供する必要があFig. 2The distribution of radioactive materials on the ﬁlter samples measured with the IP. Black dots
indicate the presence of radioactive materials. The outer rims (dotted line) show peripheries of
the ﬁlters. This study focused on the ﬁlter samples from March 14, 21 : 10 to March 15, 09 : 10
(upper left), and from March 20, 21 : 30 to March 21, 09 : 13 (bottom center). Reprinted from
Fig. 2 of Adachi et al. (2013)．

Fig. 3The selection method of radioactive particles for the SEM analysis. a) The IP image of the ﬁlter
was captured, and a part of the ﬁlter that includes radioactive material was cut off (dotted line).
b) The segment of the ﬁlter was separated into approximately 10 layers and put onto a carbon
tape attached to a glass substrate. c) The IP image was taken to identify the position of the radioactive material. d) The carbon tape including the radioactive material was cut into small
pieces. e) The radioactive material was detected using a Ge detector from the ﬁlter segments. f)
The processes d) and e) were repeated to make the ﬁlter and the number of particles as small as
possible. g) The radioactive particle embedded within carbon paste was analyzed using theSEM. Reprinted from SI 4 of Adachi et al. (2013)．

188

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立

る。著者らが 2013 年に報告した論文では，1）フィル光司e），4）3）の作業を繰り返して放射性物質を単離した

ターを IP にかけ，黒点で示された放射性物質をフィ状態に近くして（Fig. 3f），かつ走査型電子顕微鏡

ルター繊維ごと切り取る（Fig. 3a），2）切り取った（SEM）で分析可能なように粒子を表面に露出させる

フィルター繊維をガラスプレート上に貼られたカーボ（ガラスプレート上で放射性物質を含んだカーボン

ンテープ等に移す（Fig. 3b），3）マイクロマニュピ

テープを薄く延ばす）（Fig. 3 g），という放射性物質

レータ等を使ってカーボンテープごとできるだけ小さの単離方法を提案した。そして，最後に SEM で試料

く分割し，IP やゲルマニウム半導体検出器を使っての観察を行い，セシウムを含んだ球状の粒子を発見し放 射 性 物 質 が 含 ま れ た 分 画 を 特 定 す る（Fig. 3c‒

た。

Fig. 4(Color online) SEM and EDS mapping images of a radioactive particle from the sample collectedduring March 14, 21 : 10 and March 15, 09 : 10. a) A radioactive particle partially embedded
within a carbon paste. b) The same radioactive particle as a) but measured the next day. The
particle shows a spherical shape. c) An elemental mapping (Cs) of the particle a). d) The EDS
spectrum of the particle a) (black line). The red line shows the spectrum from the glass substrate. The Cs in the particle shows multiple peaks. e) An elemental mapping of the other elements within the area. O, Si, Cl, Mn, Fe, and Zn are possibly coexistent with Cs within the particle. Reprinted from Fig. 3 of Adachi et al. (2013)．

電子顕微鏡がとらえた放射性粒子

4.

放射性粒子の走査型電子顕微鏡分析

189

能強度から換算した放射性セシウムの質量パーセントは，この粒子の密度を鉱物に近い 2.0 g/cm3 と仮定す

気象研で 3 月 14‒15 日に採取されたフィルター中かると 5.5％となる。また，EDS 分析では，この粒子は

ら最初に見つかった放射性粒子は，直径 2.6 マイクロセシウムに加え，酸素，ケイ素，鉄，亜鉛を主体としメートルの球状粒子で，SEM に付属したエネルギーて，微量の塩素，マンガンなどを約 0.1％以上含んで分散型 X 線分析装置（EDS）を使った分析でセシウいることがわかった。同じフィルターの別の黒点からムを含むことが確認された（Fig. 4）。ゲルマニウム

分離された放射性粒子も，放射能強度は低いながら定半導体検出器を用いたガンマ線スペクトル分析では，

性的には同様の組成を有していた。また，この放射性事故から 2 年後の分析において放射性セシウム以外の粒子の水溶性を簡易的に調べるため，放射性粒子に水

ガンマ線放出核種は確認されなかった（Fig. 5）。こ

滴を垂らして約 1 時間自然乾燥させた前後の形態観察の粒子の試料採取時における（2011 年 3 月 14 日）放を行った（Fig. 6）。その結果，前後において形態に射能強度を減衰補正して計算すると，セシウム 134 が変化は見られなかったことから，この粒子は大気中を3.31±0.06 Bq， セ シ ウ ム 137 が 3.27±0.04 Bq で あ運ばれる数日以内の短時間では水に溶けない性質（非り，セシウム 137, 134 比はほぼ 1 : 1 であった。放射水溶性）であることが示された。

Fig. 5

The gamma spectrum of the radioactive particle. A 50,000 sec detection time was used. Peaks
marked with* were from the background or the glass substrate. The sample was analyzed two
years after the accident, and short-lived radionuclides would not have been detected even if
they had been present when emitted. The particle was measured on March 28, 2013. Reprinted
from SI 5 1 of Adachi et al. (2013)．

Fig. 6

SEM images of the radioactive particle 1 before and after exposure to water. There is no change
between the images, suggesting that the particle is largely insoluble to water. Scale bars: 1 μm.
Reprinted from SI 7 of Adachi et al. (2013)．

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放射性粒子の放射光分析

光司の背後にある粒子（フィルター繊維など）からの寄与が考えられる。XANES による化学状態分析の結果

2011 年 3 月 14‒15 日 に 気 象 研 究 所 で 採 取 さ れ たは，鉄，モリブデン，スズが各粒子中に高い酸化数をフィルターから単離された放射性粒子 3 個に対して，持ったガラス状態で存在することを示している（Fig.

大型放射光施設である SPring8 でさらに詳細な組成，

8b‒d）。また各粒子について行われた XRD では回折化 学 状 態， 結 晶 性 の 分 析 が 行 わ れ た（Abe et al.,ピークが検出されず，これらの粒子が非晶質であるこ2014）。SPring8 で行われた分析では，BL37XU におとを示していた。

ける縦横 1 マイクロメートル前後に絞られた高輝度のSPring8 放射光分析と SEM-EDS の分析結果から，

X 線ビームを用いて，1）放射光 X 線分析による組成

放射性粒子の生成に関し以下のように考えられ分析および粒子内元素分布解析，2）X 線吸収端近傍

る。1）つくばで得られた放射性粒子は，核燃料物質

構造分析（XANES）による化学状態分析，3）X 線回からの分裂生成物である元素や，核燃料以外の起源

折法（XRD）による結晶性の分析，を行った。

（例えば構造物）から発生した物質（ケイ素，鉄，亜SPring8 での蛍光 X 線分析では SEM-EDS 分析で得鉛など）を含んだ複合的な組成をしている。2）球形られた元素（鉄，亜鉛，セシウム）に加え，ルビジウの形態や，ガラス状の構造をしていることなどから，ム，ジルコニウム，モリブデン，スズ，アンチモン，

いったん溶けた溶融物が自由落下条件で急冷して粒子

テ ル ル， バ リ ウ ム が す べ て の 粒 子 か ら 検 出された状になった。3）なんらかの原因（水素爆発等）によ（Fig. 7a）。加えて，マンガン，クロム，銀，ウラン，り原子炉内部からこのような粒子が環境中に放出さ鉛などの元素も 1 もしくは 2 個の粒子から検出されれ，高濃度の放射性物質を含むプルームとして輸送さた。特にウランは L3 吸収端の XANES 分析でも微量れた。

ながらその存在が確認された（Fig. 8a）。また，元素の粒子内での分布を示す図（Fig. 7b‒d）では，鉛を除き球状粒子の厚さに応じた強度勾配を示しており，

6.

水に溶けない放射性粒子， 溶ける放射性粒子

それぞれの元素が粒子全体に分布していることを示唆

本稿で紹介したセシウムを含む比較的放射能強度のしていた。図 7d に見られる鉛の分布は，放射性粒子

高い放射性粒子は，事故によって直接放出されたもの

Fig. 7(Color online) Results of Synchrotron Radiation (SR)-μ-X-Ray Fluorescence (XRF) analyses. (a)Comparison of the SR-μ-XRF spectra obtained for articles A, B, and C and the carbon tape back-ground. The intensity of each spectrum was displayed on a logarithmic scale and shifted in alongitudinal direction. (b‒d) Distributions of representative elements extracted from the SRμ-XRF images of (b) particle A, (c) particle B, and (d) particle C with enlarged SEM image corresponding to the imaging area (scale bar: 2 μm). Reprinted with permission from Fig. 2 of Abe etal. (2014). Copyright 2014 American Chemical Society.

電子顕微鏡がとらえた放射性粒子

Fig. 8191(Color online) Results of SR-μ-XANES analyses. (a) Comparison of the U‒L3 edge SR-μ-XANES
spectra of the three radioactive particles demonstrating the presence of U in particles A and B.
(b‒d) Comparisons of the (b) Fe‒K edge, (c) Mo‒K edge, and (d) Sn‒K edge SR-μ-XANES spectraof the three particles and the reference materials. Reprinted with permission from Fig. 3 of Abeet al. (2014). Copyright 2014 American Chemical Society.

がそのままの形態で環境中に存在すると考えられる。

ような水に溶ける放射性粒子，溶けない放射性粒子の一方で，大気中で硫酸塩などのエアロゾルと混合した両方が混在して試料中の放射能強度や濃度を決定してと考えられる水溶性セシウムも事故により放出されいると考えられる（例えば Ohse et al., 2015）。また，（Kaneyasu et al., 2012），主に雨などによって地上に例えばつくば市に飛来した放射性粒子のうち，硝酸溶沈着したと考えられる。これらの水に溶ける放射性物

液に溶ける放射性セシウムの割合は，3 月 14‒15 日が質は，植物根からの吸収（梅村ほか，2015），土壌中70％程度であるのに対し，3 月 20‒21 日に飛来した試の鉱物への吸着（Mukai et al., 2014），河川や海洋へ料はほぼ 100％近く溶出することが知られており（Ig-

流出（Yamashiki et al., 2014），など不溶性放射性粒

arashi et al., in press），炉からの放出タイミングの子とは異なった環境動態を示すことが推測される。こ違いで異なった物理化学組成をもったと考えられる。

れらの水溶性放射性物質は，極めて低い濃度で鉱物等

そのため，事故時の放射性物質放出プロセスの理解に吸着されているため，上記に紹介した走査型電子顕と，どの日時に放出された放射性物質によって汚染さ微鏡での直接観察や，通常の化学分析では検出が非常れた環境試料なのかを知ることが，その試料中に含まに困難である。土壌や植物などの環境試料では，このれる水溶性と非水溶性の放射性物質の割合を推定する

192

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ために必要である。

7.

放射性粒子の物理化学特性を応用した研究の可能性

放射性粒子の物理化学特性の理解が進むことで，今光

司件 に 応 じ て 変 化 す る と 考 え ら れ る（Haste et al.,2013）。よって，環境中に放出された放射性粒子の詳
細な物理化学特性から逆に生成プロセスを推定することで，事故時に炉内がどのような状況であったかを推測する研究の進展も期待される。

後，放射性物質拡散の数値モデル，放射性物質による8.

健康影響評価，また事故時の炉内事象推定に対する研究への応用の進展が期待される。

今後の研究課題

福島第一原子力発電所事故はその事故の詳細や放出

放射性物質の大気への飛散推定や，沈着後の環境中された放射性物質の環境動態についていまだに十分理での中長期的に放射能が減少する時間スケールの見積

解されていない。そのため，水溶性放射物質の理解に

もり等には，数値計算を使ったモデル予測が有効であ

加えて，本稿で紹介した不溶性の放射性粒子の詳細なる。一方，事故当時の放射性物質の大気を通じた拡散分析を通じて，環境中に放出された放射性物質の拡散過程にはいまだに十分理解されていない現象も多くあプロセス，長期的な環境からの移動速度予測などに対り，異なったシナリオをもとにした複数の数値計算結する理解が進むことが期待される。特に，大気エアロ

果を比較した結果にはばらつきがある（例えば Kata-

ゾルとして放出された不溶性放射性粒子がどのように

ta et al., 2015）。そのため，観測とモデル結果を一致

環境中に分布し，また挙動をしているかを理解するたさせることも困難である。数値モデルを使った拡散予

めにも，環境試料として簡易に採取可能な土壌試料中測では，当時の気象条件や放射性物質の放出タイミンからの放射性粒子の検出，分析が求められる。今後，グなどに加え，放射性物質の物理化学性状の理解がモ単離された放射性粒子に対して，地球化学分野で利用

デル精度向上に有効である（Adachi et al., 2013）。例

されるさまざまな機器分析を適用することで，汚染実

えば，放射性粒子の吸湿性は，それらがエアロゾルと態の理解を深めるとともに，炉内事象の理解に向けた

して大気中を輸送されるとき雨によって大気から除去

地球化学，環境科学と原子炉工学などの複数の分野にされるかどうかを決める重要な要素として放射性物質

またがる共同研究が望まれる。また，放射性粒子の物の大気寿命および沈着分布を決定する。また，粒子の理化学的理解を通じて，再飛散プロセスの理解，人体

大きさは重力沈降速度や輸送距離を決定する。

に取り込まれた際の健康影響に関する調査研究，効果

放射性物質が土壌等に沈着した後の数十年間に及ぶ的な除染方法の検討，除染作業で発生する汚染物質の中長期の挙動も，数値計算による予測が可能である。

量的軽減化等，学際的な研究課題の進展が期待される。

特に，土壌中での移行は，主に土壌鉱物との吸着や降水による溶脱，動植物による輸送などが考えられ，その推定には放射性物質の組成や水溶性などの情報が必
要となる。

謝

辞
本稿は東京理科大学，阿部善也博士，気象研究所，

五十嵐康人博士と議論し，貴重な意見をいただいた。

大気中に浮遊する微粒子の体内への取り込み効率やまた，本研究の一部は，研費新学術領域「福島原発事沈着部位の推定は，その粒子の大きさによって異なる故により放出された放射性核種の環境動態に関する学（梶野ほか，2014）。例えば，10 マイクロメートル前際的研究」A01‒02 班（科研費番号 24110003）の活動

後の比較的大きな粒子は鼻やのどの粘膜に付着しやすの一環として行った。ここに記して感謝いたします。

く，それよりも小さな数十ナノメートルの粒子は肺の内部まで侵入しやすい。さらに小さな 10 ナノメート

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Journal
Februaryof2015
the Meteorological Society of Japan, Vol.
T. T.93,
SEKIYAMA
No. 1, pp. 49−64,
et al. 2015
DOI:10.2151/jmsj.2015-002

49

Horizontal Resolution Dependence of Atmospheric Simulations of
the Fukushima Nuclear Accident Using 15-km, 3-km,
and 500-m Grid Models
Tsuyoshi T. SEKIYAMA, Masaru KUNII, Mizuo KAJINO, and Toshiki SHIMBORI
Meteorological Research Institute, Tsukuba, Japan(Manuscript received 16 June 2014, in final form 1 October 2014)

Abstract
We investigated the horizontal resolution dependence of atmospheric radionuclide (Cs-137) simulations of the
Fukushima nuclear accident on March 15, 2011. We used Eulerian and Lagrangian transport models with low(15-km), medium- (3-km), and high- (500-m) resolutions; both models were driven by the same meteorologicalanalysis that was prepared by our data assimilation system (NHM-LETKF) for each horizontal resolution. Thispreparation was necessary for the resolution-dependent investigation, excluding any interpolation or averaging
of meteorological fields. In the results, the 15-km grid analysis could not reproduce Fukushima’s mountainous
topography in detail, and consequently failed to depict a complex wind structure over mountains and valleys. In
reality, the Cs-137 plume emitted from the Fukushima Daiichi Nuclear Power Plant (FDNPP) was mostly blockedby Mt. Azuma and other mountains along the Naka-dori valley after crossing over Abukuma Mountains on March
15, 2011. However, the 15-km grid simulations could not represent the blockage of the Cs-137 plume, which
unnaturally spread through the Naka-dori valley. In contrast, the 3-km and 500-m grid simulations produced very
similar Cs-137 concentrations and depositions, and successfully produced the plume blockage and deposition along
the Naka-dori valley. In conclusion, low-resolution (15-km grid or greater) atmospheric models should be avoided
for assessing the Fukushima nuclear accident when a regional analysis is needed. Meanwhile, it is reasonable touse 3-km grid models instead of 500-m grid models due to their similarities and the high computational burdenof 500-m grid model simulations.
Keywords

1.

model simulation; resolution dependence; data assimilation; Fukushima nuclear accident; cesium-137

Introduction

1.1 	
Atmospheric simulations of the Fukushima
nuclear accident
The 2011 Tohoku earthquake occurred off the
Pacific coast of Japan on March 11, 2011 and triggered tsunami waves, which caused severe disaster
Corresponding author: Tsuyoshi Thomas Sekiyama,
Atmospheric Environment and Applied Meteorology
Research Department, Meteorological Research Institute,
1-1 Nagamine, Tsukuba, Ibaraki 305-0052 Japan
E-mail: tsekiyam@mri-jma.go.jp©2015, Meteorological Society of Japan

at the Fukushima Daiichi Nuclear Power Plant(FDNPP). The accident became the largest nuclear
disaster since Chernobyl, and resulted in the dispersion and deposition of a large amount of radionuclides in the environment of eastern Japan. Since thenmany numerical simulations have been performed
to predict or assess the effects of the nuclear accident using atmospheric chemistry transport models(e.g., Chino et al. 2011; Morino et al. 2011, 2013;
Yasunari et al. 2011; Takemura et al. 2011; Schöppner et al. 2011; Sugiyama et al. 2012; Mathieu
et al. 2012; Stohl et al. 2012; Katata et al. 2012a, b;
Terada et al. 2012; Christoudias and Lelieveld 2013;

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Table 1. Spatial resolution of Fukushima radionuclide atmospheric simulation models with a global
domain.
model

spatial resolution

Schöppner et al. (2011)
Takemura et al. (2011)
Yasunari et al. (2011)
Stohl et al. (2012)
Mathieu et al. (2012)
Christoudias and Lelieveld (2013)
Arnold et al. (2015)

1° × 1° (≈ 90 km × 110 km)
0.56° × 0.56° (≈ 50 km × 60 km)
1° × 1° (≈ 90 km × 110 km) [global domain]
0.18° × 0.18° (≈ 16 km × 20 km) [regional domain]
0.5° × 0.5° (≈ 45 km × 55 km)
0.5° × 0.5° (≈ 45 km × 55 km) [global domain]
0.125° × 0.125° (≈ 11 km × 14 km) [regional domain]
0.5° × 0.5° (≈ 45 km × 55 km)
0.2° × 0.2° (≈ 18 km × 22 km)
0.5° × 0.5° (≈ 45 km × 55 km)

Table 2. Spatial resolution of Fukushima radionuclide atmospheric simulation
models with a regional domain.
model

spatial resolution

Chino et al. (2011)
Morino et al. (2011)
Katata et al. (2012a, 2012b) and Terada et al. (2012)
Sugiyama et al. (2012)

Adachi et al. (2013)
Morino et al. (2013)
Hu et al. (2014)
Saito et al. (2015)

Adachi et al. 2013; Saito et al. 2015; Hu et al. 2014;
Arnold et al. 2015). Some models were used to estimate the amount of radionuclides released from the
FDNPP, coupled with a reverse analysis. Others wereused to estimate the health damage to local residents or to investigate the meteorological causes ofinhomogeneous radioactive contamination. Thosemodels can be categorized into two types according
to their domains: global and regional models. Global
models have a large domain covering not only Japanbut also America and Europe; however, their resolutions are not very fine compared with regionalmodels. The horizontal resolutions of global models
are approximately 50 km varying from 11 km to
110 km (Table 1). In contrast, regional models havea relatively finer resolution, but their domains often
only cover eastern Japan. The horizontal resolutionsof regional models are often 3 km, varying from 1

2 km × 2 km
3 km × 3 km
6 km × 6 km
9 km × 9 km
3 km × 3 km
1 km × 1 km
27 km × 27 km
9 km × 9 km
3 km × 3 km
1 km × 1 km
3 km × 3 km
3 km × 3 km
3 km × 3 km
5 km × 5 km

km to 9 km (Table 2). Incidentally, the atmosphericsimulation models of the Chernobyl nuclear accident have often had lower resolutions, even when
they were used with regional domains (Table 3). The
lower model resolutions are most likely attributed
to the topographical features of Chernobyl, which islocated on very flat terrain. The nearest mountains are
more than 500 km away from Chernobyl. It would be
unnecessary to make the model resolution very highfor continental-scale or very flat area simulations.
However, it is unclear what resolution is good enoughto regionally represent the advection and depositionof radionuclides in the case of Fukushima because
Japan has a complex topography. More than 70% of
the Japanese territory is mountainous. Small plains
and basins are scattered throughout the remaining area
where the population is concentrated. Fukushima is
not an exception to the topographical complexity.

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Table 3. Spatial resolution of Chernobyl radionuclide atmospheric simulation models.
model
Bradt et al. (2002)
Davoine et al. (2007)
Evangeliou et al. (2013)

spatial resolution
25 km × 25 km [outer domain]
5 km × 5 km [inner domain]
1.125° × 1.125° (≈ 110 km × 140 km)
2.5° × 1.27° (≈ 230 km × 140 km) [global domain]
0.66° × 0.51° (≈ 60 km × 55 km) [Europe domain]

Fig. 1. Fukushima’s topography depicted by the (a) 15-km, (b) 3-km, and (c) 500-m grid scales. The cross indicates the location of FDNPP. The Abukuma Mountains have peaks of approximately 1000 m. Fukushima City is
located in a narrow basin 70 m above sea level (asl). Mt. Azuma has several peaks of which the highest is 2035
m asl.

1.2 Fukushima’s topography
Fukushima is a mountainous region with few
plains. FDNPP is located on the coastline of the
Pacific Ocean, but the flat area around the power
plant is very narrow (Fig. 1c). The Abukuma Mountains, which are as high as 1000 m, are located justbehind the power plant; the distance between the
FDNPP premises and Abukuma Mountain foothills
is only a few kilometers. Fukushima City, the capitalof Fukushima Prefecture, is situated beyond themountain range; it is located in a long narrow basinthat is approximately 70 km from FDNPP. Furthermore, 2000-m mountain peaks (Mt. Azuma) are seen
just behind the city. These mountains and the basin
are well depicted in Fig. 1c with a 500-m horizontal
resolution grid. In contrast, it is difficult to recognize
those features in Fig. 1a with a 15-km horizontal resolution grid. Fine structures, such as small valleys, are

not depicted in Fig. 1b (3-km grid) compared with
Fig. 1c (500-m grid), but the rough shapes of themountains and basins are apparent in the 3-km and
500-m grid maps.
As mentioned above, the 3-km grid is the mostpopular horizontal resolution in the regional simulation models of the Fukushima nuclear accident (Table2). However, it is not evident that the 3-km grid resolution can properly reproduce the advection and deposition of radionuclides associated with the FDNPP
accident. On the other hand, the global simulation
models of the FDNPP accident (Table 1) commonlyuse much lower resolutions than the 15-km grid scalethat cannot depict Fukushima’s complex topography
in detail (Fig. 1a). Nevertheless, some researchers
validated their global simulation results with domestic
observations in Japan (e.g., Stohl et al. 2012) andinvestigated the health damage to local residents (e.g.,

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Christoudias and Lelieveld 2013). If their simulations
are accurate, then there is no need to use higher resolution models, because the computational burdens
exponentially increase with finer model resolutions.
However, the validity of using 15-km or lower resolutions for the regional assessment of the FDNPP accident has not been proven.

trary-resolution analysis independent from other resolution models and analyses. The details of our data
assimilation are described in Section 2.1. Then, thechemistry transport models are described in Sections
2.2 and 2.3. We present the results and discussion in
Section 3, and the conclusion in Section 4.

1.3 Objective
This study investigated whether the 3-km grid
(representative of the regional model resolution)and 15-km grid (representative of the global model
resolution) models adequately simulate the radioactive pollution of the FDNPP accident compared witha very high-resolution model (500-m grid). Thesesimulations focused on Cs-137 radionuclides only
because the available observations were spatiallydispersed throughout Fukushima and its neighboringareas. To double-check the resolution dependence, wemanaged two types of chemistry transport models:the Eulerian and Lagrangian models. Both models
were driven by the same meteorological analysis with
15-km, 3-km, and 500-m horizontal resolution grids.
Eulerian models indirectly describe air motion byfocusing on specific locations through which the airflows over time. Lagrangian models directly describe
air motion by following an individual air parcel as it
moves. Both models have advantages and disadvantages. For example, Eulerian models explicitly forecast radionuclide concentrations, and can implement
chemical and physical processes in detail, but are notgood at handling point source emissions. In contrast,
Lagrangian models are good at handling point source
emissions, but are susceptible to sampling noise when
there are few Lagrangian particles.
In addition, we generally encounter difficulties in
obtaining meteorological analyses with arbitrary horizontal resolutions. It is impossible for Lagrangianmodels to arbitrarily obtain a high- (low-) resolution
analysis just by using simple interpolation (averaging)of the meteorological fields because the amount of
information is not adequately increased (decreased
enough) in the meteorological field by such simple
calculation. Meanwhile, many Eulerian chemistrytransport models are driven by the meteorologicalanalysis calculated by their own dynamical module,
wherein the meteorological field is nudged by anotheranalysis with a different resolution. Consequently, thecalculated meteorological analysis may be a mixtureof the dynamical module simulation and anotheranalysis with a different resolution. Therefore, we
conducted the data assimilation to make the arbi-

2.1 Preparation of meteorological analyses
Prior to calculating the radionuclide transport, we
prepared meteorological analyses, which had threedifferent horizontal resolutions, using a flow-dependent data assimilation system assembled and validated by Kunii (2013). This data assimilation systemcomprises the Japan Meteorological Agency’s nonhydrostatic model (JMA-NHM) and the local ensemble
transform Kalman filter (LETKF), which are together
called NHM-LETKF. The system calculated all thenecessary meteorological variables, which were stored
every 10 min, to subsequently drive the radionuclidetransport models. In this study, the horizontal resolutions were set to 15 km, 3 km, and 500 m. Here, aone-way nested data assimilation scheme was implemented, wherein the first guess of a lower resolution
model is used as a boundary condition for finer-resolution model integration (Kunii 2013). The nested
inner model runs independently from the outer coarse
model, except for the boundary condition. We defined
the 15-km resolution as the typical grid scale ofhigh-resolution global simulations (cf. Table 1). The
3-km resolution was defined as the typical grid scaleof regional simulations implemented for the FDNPP
accident (cf. Table 2). The 500-m resolution wasconsidered higher than that of any other Fukushima
simulation models and was expected to depict
Fukushima’s complex topography.
The meteorological model JMA-NHM was developed by JMA (Saito et al. 2006, 2007) and has beenused for government-operated weather forecasts.
Operationally, JMA-NHM is initialized by the JMA
non-hydrostatic-model
four-dimensionalvariational data assimilation system (JNoVA, Honda
et al. 2005). Most of the regional simulation models
for the FDNPP accident use the JNoVA grid-pointvalue (GPV) data as the initial/boundary conditions
or pseudo-observations (Chino et al. 2011; Morino
et al. 2011, 2013; Katata et al. 2012a, b; Terada et al.
2012; Adachi et al. 2013; Draxler et al. 2015; Saito
et al. 2015). In contrast, we calculated the meteorological analyses by running our own data assimilation system (NHM-LETKF) instead of using the
JNoVA GPV. The data assimilation scheme LETKF

2.

Methodology

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T. T. SEKIYAMA et al.

53

Fig. 2. NHM-LETKF model domains of the (a) 15-km, (b) 3-km, and (c) 500-m grid spacing analyses. The crossin panel (c) indicates the location of FDNPP.

is an ensemble Kalman filter (EnKF) implementation developed by Hunt et al. (2007). The application of EnKFs enables an evaluation of uncertainties of the analysis fields and leads to a probabilistic
prediction through ensemble forecasting. LETKF has
been applied to various simulations, such as weather
forecast modeling (e.g., Miyoshi and Aranami 2006;
Miyoshi and Yamane 2007; Miyoshi and Kunii
2012; Kunii and Miyoshi 2012; Kunii et al. 2012;
Kunii 2013) and chemistry transport modeling (e.g.,
Sekiyama et al. 2010, 2011a, b; Kang et al. 2011;
Miyazaki et al. 2012; Nakamura et al. 2013).
In this study, the 15-km grid spacing analysis
was calculated by the outer NHM-LETKF, whose
domain covered East Asia (Fig. 2a), and was initiated at 6:00 UTC March 9, 2011 with 20 ensemble
members. This domain comprises 241 × 193 horizontal grid points on the Lambert conformal projection and 50 vertical levels up to approximately 22
km in the terrain-following hybrid vertical coordinates, which include 11 levels below 1 km but above
ground level (agl). The horizontal coverage of thisconfiguration is similar to that of the JNoVA systemroutinely operated by JMA. The initial and boundary
conditions of the NHM-LETKF cycle were obtained
from the JMA operational global prediction system.
We added initial and lateral boundary perturbations
derived from the JMA operational one-week ensemble
prediction system to each ensemble member inaccordance with Saito et al. (2012). The 15-km grid
spacing JMA-NHM implemented a modified Kain–Fritsch convective parameterization scheme (Kain andFritsch 1993) along with the 3-ice-bulk cloud micro-

physics (Lin et al. 1983). For the turbulence scheme,
the improved Mellor–Yamada level 3 closure model(Nakanishi and Niino 2004, 2006) was implemented.
This LETKF analyzed all the prognostic variables of
JMA-NHM, i.e., wind components (u, v, w), temperature (T), pressure (p), water vapor mixing ratio (qv),
and water/ice microphysics variables. Here, LETKF
employed an adaptive inflation scheme, which adaptively estimates the multiplicative inflation factors at
each grid point (Miyoshi 2011; Miyoshi and Kunii
2012). The covariance localization parameters were
set to 150 km in the horizontal, 0.2 ln p coordinate
in the vertical, and 3 h in the time dimension. These
localization parameters correspond to the length atwhich the Gaussian localization function becomes
e−0.5. The NHM-LETKF cycle was succeeded with a
6-h time interval, wherein the observation data weregrouped into 1-h time slots and were four-dimensionally assimilated (Hunt et al. 2004). To assimilate the observation data, we used JMA’s operational
dataset, which was integrated and quality-controlled
for the JNoVA mesoscale weather prediction like
Kunii (2013). The JNoVA dataset contains observations acquired by radiosondes, weather observatories, pilot balloons, wind profilers, aircrafts, ships,
buoys, and satellites. However, satellite radiancesand radar precipitation analyses were not assimilated in this study because observation operators
for these observations have not yet been installedin the NHM-LETKF system. Instead, we additionally assimilated surface wind observations acquiredby the automated meteorological data acquisition system (AMeDAS) and Tokyo Electric Power

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Company (TEPCO) monitoring posts. AMeDAS is asurface observation network managed by JMA. The
network currently comprises approximately 1,300
stations throughout the country, which are laid out at
average intervals of 17 km. The TEPCO monitoring
posts are placed at FDNPP, and the Fukushima Dai-ni
Nuclear Power Plant that is located to 12 km south of
FDNPP. The monitoring post datasets are freely available to the public at the TEPCO website (http:// www.
tepco.co.jp/nu/fukushima-np/index-j.html).
The 3-km grid spacing analysis was calculated by
the nested NHM-LETKF, whose domain covered
eastern Japan (Fig. 2b), and was initiated at 6:00 UTC
March 10, 2011 with 20 ensemble members. Thisdomain comprises 215 × 259 horizontal grid points
in the Lambert conformal projection and 60 verticallevels up to approximately 22 km in the terrain-following hybrid vertical coordinate. The lateralboundary conditions were supplied from the outputsof the outer (15-km grid) NHM-LETKF cycle. Here,nearly the same configurations were implemented inboth the 3-km grid spacing JMA-NHM and the 15-km
simulation, but the convective parameterization
scheme was not activated for the former. The covariance localization parameters were set to 50 km in thehorizontal, 0.1 ln p coordinate in the vertical, and 3
h in the time dimension. The inner NHM-LETKF
cycle was succeeded with a 3-h time interval, andits data assimilation was performed four-dimensionally. We used the same observation data (i.e., JNoVA,
AMeDAS, and TEPCO datasets) that were used in
the 15-km data assimilation. Furthermore, the 500-m
grid spacing analysis was calculated by the second
nested NHM-LETKF whose domain mostly covered
Fukushima Prefecture (Fig. 2c) with 281 × 301 horizontal grid points in the Lambert conformal projection
and 60 vertical levels up to approximately 22 km in
the terrain-following hybrid vertical coordinate. In
Fig. 2c, the location of the FDNPP is indicated witha cross along the coastline. The lateral boundary
conditions were supplied from the outputs of the
3-km NHM-LETKF cycle. The covariance localization parameters were set to 25 km in the horizontal,
0.1 ln p coordinate in the vertical, and 3 h in the time
dimension. Basically, the same configurations were
implemented for the 500-m grid spacing JMA-NHM
and the 3-km simulation. Moreover, the same observation data were assimilated during the second nested
NHM-LETKF cycle. The triple-nested data assimilation experiments were conducted with horizontal resolutions of 15 km, 3 km, and 500 m.
To generate the terrain features in the computa-Vol. 93, No. 1

tional domains of these 15-km, 3-km, and 500-m
grid JMA-NHMs, we used the global digital elevation data with a horizontal grid spacing of 30 arc s(GTOPO30), which is approximately 900 m in theeast–west direction and 700 m in the south–northdirection, provided from the U.S. Geological Survey(https://lta.cr.usgs.gov/GTOPO30). For each domain,
the GTOPO30 data was smoothed in the approximately 1.5 times coarser resolution of each model(i.e., 800 m for the 500-m grid model, 5 km for the
3-km grid model, and 25 km for the 15-km gridmodel). In general, this smoothing procedure isindispensable for meteorological model simulation
to avoid the computational instability and error-amplification caused by precipitous terrains (= coordinate distortions). Figure 1 shows the terrains after thesmoothening.
2.2 Eulerian transport model
We conducted Eulerian simulations using theRegional Air Quality Model 2 (RAQM2) that was
originally developed to simulate Asian air quality
and aerosol properties (Kajino et al. 2012). RAQM2
implements a Eulerian transport scheme and a
triple-moment modal aerosol dynamics module. The
aerosol dynamics, such as dry deposition, grid-scalerainout (cloud condensation nuclei and ice nuclei
activations, and subsequent cloud microphysicalprocesses), and washout (coagulation between aerosols and settling hydrometeors) processes, as well as
advection, diffusion, sub-grid-scale convection/scavenging, and gravitational settling are calculated offlineby coupling with meteorological models. In this study,
RAQM2 was driven by the meteorological analyses
with 15-km, 3-km, or 500-m resolutions prepared by
NHM-LETKF. The analyses were taken into RAQM2
every 10 min and linearly interpolated within each
10-min interval. RAQM2 and NHM-LETKF shared
the same model domain and horizontal grid resolution, but their vertical resolutions were converted
from NHM-LETKF’s 50 or 60 layers (expandedfrom the surface to approximately 22 km asl) to
RAQM2’s 20 layers (expanded from the surface toapproximately 10 km asl). The radionuclide transportversion of RAQM2 was developed for the Fukushima
nuclear accident simulation (Adachi et al. 2013). Thisversion of RAQM2 implements simplified aerosol
dynamics compared with those of Kajino et al. (2012)by assuming aerosol hygroscopicity and perpetualparticle size distribution; nucleation, condensation,and coagulation were not considered here. Still, we
described the nature of the aerosol dynamics, such as

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dry deposition and grid-scale rainout/washout (i.e.,
wet deposition) processes, based on the prescribedsize distribution. Details of the dry and wet deposition
processes are described in Sections 2.2.7 and 2.2.8 ofKajino et al. (2012), respectively. In this study, all theradioactive Cs-137 was contained in sulfate aerosol
particles (mixed with organic compounds) when
it was transported in the atmosphere. The particle
size distribution of sulfate aerosols was assumed to
be log-normal with a number equivalent geometricmean dry diameter = 0.5 µm, a geometric standard
deviation = 1.6, a particle density = 1.83 g cm−3, anda hygroscopicity = 0.4, assuming a sulfate–organic
mixture. We used the emission scenario of the Cs-137
release from the FDNPP that was estimated by theJapan Atomic Energy Agency (JAEA) (cf. Chinoet al. 2011; Katata et al. 2012a; Terada et al. 2012).
Following the time series of the emission scenario,
Cs-containing sulfate aerosols were injected into agrid cell above the FDNPP at every time step. The
grid cell was assigned by the Cs-137 release heightscenario estimated by JAEA that varied temporally
from 20 m to 150 m agl.
2.3 Lagrangian transport model
We conducted Lagrangian simulations using the
JMA operational regional atmospheric transportmodel (JMA-RATM) that was originally developed for volcanic ash fall forecasts (Shimbori et al.
2009, 2010). JMA-RATM uses a Lagrangian scheme(Iwasaki et al. 1998; Seino et al. 2004) and implements advection, horizontal and vertical diffusions,gravitational settling, dry deposition, and wet scavenging processes offline. The radionuclide transportversion of JMA-RATM was developed by Saito et al.
(2015) for the World Meteorological Organization(WMO) at the request of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, Draxler et al. 2015). In this study,
JMA-RATM was driven by the NHM-LETKF meteorological analyses with 15-km, 3-km, or 500-m
resolutions. The analyses were taken into JMA-RATM
every hour, and the vertical advection was calculated using a spatially averaged (9 grid cells) verticalvelocity and was assumed to be terrain-following at
the lowermost model layer. All the radioactive Cs-137
was assumed to be contained in sulfate aerosol particles when it was transported in the atmosphere.
For the wet deposition of Cs-containing aerosol
particles, only a washout process (i.e., below-cloudscavenging) was considered, wherein the scavenging
rate was set as 2.98 × 10−5 × P0.75 s−1 for liquid rain55

and 2.98 × 10−5 × P0.30 s−1 for solid snow; P is thesurface precipitation intensity [mm h−1]. The wet
scavenging was applied under the height of approximately 1500 m asl. The dry deposition was simply
computed by the deposition rate of 10−5 s−1 according
to Sportisse (2007) and Draxler and Rolph (2012). For
the gravitational settling, the particle size distributionof sulfate aerosols was assumed to be log-normal witha mean diameter of 1.0 µm, a standard deviation of
1.0, and a uniform particle density of 1.0 g cm−3. The
number of Lagrangian particles was set to a constant
100,000 per hour. The particles were uniformly
emitted at the location of FDNPP from the groundsurface to 100 m agl. After the model calculation, thehourly particle concentration and deposition outputs
were multiplied by the hourly Cs-137 emission rateof JAEA (cf. Chino et al. 2011; Katata et al. 2012a;
Terada et al. 2012) at the time of particles emission.
Then, the multiplied values were summed at each grid
point.
3.

Results and discussion

3.1 Meteorology
In this study, we focused the simulation on March
15, 2011 (UTC) because we needed to investigate
the radioactive plumes that outflowed landward.
According to Morino et al. (2013), their standard
experiment with the JAEA emission scenario indicatesthat most of the Cs-137 deposition over land (mainly
in Fukushima Prefecture) occurred during the period
from March 15 to 16 (Japanese Standard Time; JST)and accounts for 72 % of the total deposition overland between March 10, 2011 and April 20, 2011.
The comparison of the meteorological analyses of
NHM-LETKF is shown in Figs. 3 and 4. It was eitherraining or snowing over eastern Japan on March 15,
2011, as depicted by the JMA Radar/rain gauge-Analyzed Precipitation (RAP) data in Fig. 3a. JMA operationally produces the RAP data by calibrating radar
echo observations with 1-h accumulated rain gauge
precipitation data (AMeDAS) throughout Japan.
Figure 3a shows a 24-h accumulation of RAP interpolated onto 1-km grids on March 15, 2011 UTC.
In general, the quantitative simulation of precipitation is difficult for models, compared with dynamical variables, such as wind, pressure, and temperature because precipitation involves a chemical phasetransition. Nonetheless, the NHM-LETKF analyses(Figs. 3b, c, d) demonstrated good agreement with
RAP in regard to synoptic-scale precipitation. Notethat the 24-h accumulated precipitation of the 15-km,
3-km, and 500-m grid spacing analyses were very

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Fig. 3. Precipitation intensity [mm/day] accumulated from 00:00 UTC March 15 to 00:00 UTC March 16, 2011derived from (a) the JMA RAP radar observations, (b) the outputs of the 15-km grid model JMA-NHM, (c) the
outputs of the 3-km grid model JMA-NHM, and (d) the outputs of the 500-m grid model JMA-NHM.

similar. The difference among them was smaller
than that between RAP and the simulated precipitation. This similarity suggests that there would not
be a large difference in the wet deposition due to theprecipitation distribution of each model resolution.
In contrast, there was a large difference among
the analyses of the horizontal winds in the planetary
boundary layer (PBL), as shown in Fig. 4. The winddirection and speed (10-min mean) in the lowermost
layer (from the surface to 40 m agl) at 15:00 UTC
March 15, 2011 is depicted. The three scenes of Fig.
4 project the same area, including the northern partof the Abukuma Mountains, FDNPP, and Fukushima
City. At the two AMeDAS points that were located inSouma City near the coastline and Fukushima City in
the Naka-dori valley, we compared the wind direc-

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57

tions and speeds with the corresponding AMeDAS
observations (Table 4). The 15-km analysis (Fig.
4a) did not adequately simulate the northerly winds
around Fukushima City along the Naka-dori valley
because the model could not represent the Abukuma
Mountains or the Naka-dori valley. As shown in Table
4, the wind direction of the 15-km grid analysis at
Fukushima City was completely different from thecorresponding observations that were consistent withthe 3-km and 500-m grid analyses. For transport
simulations, it was important that the wind direction was southerly or northerly in Fukushima City,
because the Naka-dori valley is oriented in the north–south direction. Between the 3-km (Fig. 4b) and
500-m (Fig. 4c) grid analyses, the wind fields wereroughly similar, although only the 500-m grid analysis depicted a fine wind structure over the mountains
and valleys. When the wind blows in the east–west
direction at the Naka-dori valley, the 15-km grid analysis performs reasonably in some situations. Indeed,
the predominant wind direction was often westerly
in the period between March 14 and 16, 2011 except
the afternoon of March 15, in Fukushima City. Duringthat period the 15-km grid analysis was sometimes
consistent with the 3-km and 500-m grid analyses;
however, the land contamination did not occur when
the wind direction was westerly. Furthermore, while
there was a large dependence of the PBL wind fieldon the spatial resolution, there was little difference inthe free-tropospheric winds among the 15-km, 3-km,
and 500-m grid analyses (not shown). These features
are consistent with the previous study (Takemi 2013)that compared local-scale wind fields over Eastern
Fukushima in March 2011 between 2-km and 400-m
grid models. In the case of the FDNPP accident, theradionuclide release height was estimated to be low(20–150 m agl; Chino et al. 2011; Katata et al. 2012a;
Terada et al. 2012); also winter is responsible for the
Fig. 4. Surface wind direction and speed (10-minmean) in the northern area of the Abukuma
Mountains, Fukushima, at 15:00 UTC March 15,
2011 as simulated by the (a) 15-km grid model
JMA-NHM, (b) 3-km grid model JMA-NHM,
and (c) 500-m grid model JMA-NHM. Theorange lines depict the contours of altitude.
The cross indicates the location of FDNPP. The
diamond symbol indicates the location of theAMeDAS Souma observatory, and the squaresymbol indicates the location of the AMeDAS
Fukushima City observatory (cf. Table 4). Notethat the wind vectors of the 500-m grid modelare drawn for every other grid point.

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Table 4. Surface wind direction and speed (10-min mean) of AMeDAS and model simulations at
Souma observatory and Fukushima City observatory on 15:00 UTC March 15, 2011.

Souma
Fukushima City

AMeDAS15-km model

3-km model

500-m modelSW
1.0 m s–1
NNE
0.6 m s–1

ESE
3.6 m s–1ESE
1.4 m s–1SE
1.6 m s–1NE
1.6 m s–1SE
1.2 m s–1N
1.6 m s–1

relatively stable PBL. Considering these factors, theresolution dependence of the PBL wind field is likely
to cause a large difference in the advection of theradioactive plumes between the 15-km simulation andthe other simulations.
3.2 Surface concentration of Cs-137
According to the TEPCO monitoring post data, thenortherly wind in the vicinity of FDNPP graduallyshifted clockwise to a southeasterly wind between
6:00 JST (21:00 UTC the day before) and 12:00 JST(3:00 UTC) on March 15, 2011. Then, the southeasterly wind continued for more than 10 h and flowedfrom FDNPP on the coastline to an inland area.
During that time, the radioactive plume is supposed to
sweep across Fukushima and the neighboring prefectures. The simulation results of this dispersal is shownin Fig. 5, which illustrates the 24-h averaged Cs-137
concentrations in the lowermost layer (from thesurface to 40 m agl) between 00:00 UTC on March
15 and 00:00 UTC on March 16.
Looking at the 3-km (Fig. 5b) and 500-m (Fig.
5c) grid Eulerian simulations, the Cs-137 distributions agreed well. The Cs-137 plume crossed over
the Abukuma Mountains, and was mostly blockedby Mt. Azuma and other mountains along the Nakadori valley. However, the 15-km grid Eulerian simulation (Fig. 5a) did not represent the blockage of theCs-137 plume, which broadly spread out through the
Naka-dori valley as far as Yamagata Prefecture. As
expected by the PBL wind errors, the 15-km gridtransport simulation behaved unnaturally. Similar
characteristics were also observed in the Lagrangiansimulation results. The 15-km grid Lagrangian simulation (Fig. 5d) behaved completely differently
from the 3-km (Fig. 5e) and 500-m (Fig. 5f) grid
Lagrangian simulations. Similar to the Eulerian simulations, the 3-km and 500-m grid Lagrangian simulations agreed well; the blockage of the Cs-137 plume
was successfully reproduced along the Naka-dorivalley. In addition, the 15-km grid Lagrangian simulation (Fig. 5d) had a very similar Cs-137 distribution

to that of the Eulerian simulation with the same grid
resolution (Fig. 5a); however, their concentrations
were quantitatively different. This similarity was notexpected because Eulerian models are generally notgood at handling point-source-emissions comparedwith Lagrangian models.
An analogous diagnosis can be obtained by thecalculation of the absolute spatial correlation coefficients among the 15-km, 3-km, and 500-m gridCs-137 distributions (Table 5). All the correlations
were calculated within the area of the 500-m gridmodel domain using the concentration values illustrated in Fig. 5. The values were used without logarithmic transformation. Both Eulerian and Lagrangiansimulations presented high correlations between
the 3-km and 500-m grid distributions (0.90–0.85).
In contrast, there was a relatively low correlation
between the 15-km and 500-m grid simulations (0.64–0.65), and between the 15-km and 3-km grid simulations (0.75–0.78).
3.3 Deposition of Cs-137
The Japanese government has conducted aerial
surveys since the nuclear accident to measure cesium
deposition amounts on the ground over the entire areaof eastern Japan with JAEA (Torii et al. 2012), as
shown in Fig. 6. Note that this deposition represents
all of the cesium emissions from the FDNPP accident
until the autumn of 2011. However, as mentioned
above, most of the Cs-137 deposition over land (especially in Fukushima) occurred only on March 15,
2011. Therefore, it is possible to validate the simulation results of March 15, 2011, whether they are plausible or not, by comparing them with the aerial observations of the accumulated deposition. In fact, Fig. 6
clearly illustrates the northwestward-flowing Cs-137
pollution emitted from FDNPP and its blockage by
the mountains along the Naka-dori valley. The model
results of the one-day accumulated deposition of
Cs-137 on March 15, 2011 UTC are shown in Fig. 7.
In both the 15-km grid Eulerian (Fig. 7a) and
Lagrangian (Fig. 7d) model results, a highly polluted

February 2015

59

T. T. SEKIYAMA et al.

Fig. 5. Surface Cs-137 concentration averaged from 00:00 UTC March 15 to 00:00 UTC March 16, 2011 as simulated by the Eulerian model RAQM2 with the (a) 15-km grid spacing meteorological analysis, (b) 3-km grid
spacing meteorological analysis, and (c) 500-m grid spacing analysis. The same as simulated by the RAQM2,
but simulated by the Lagrangian model JMA-RATM with the (d) 15-km grid spacing meteorological analysis, (e)3-km grid spacing meteorological analysis, and (f) 500-m grid spacing analysis.

Table 5. Horizontal distribution correlations of the surface Cs-137 concentrations as simulated by the models.

Eulerian models
Lagrangian models

15-km grid ↔ 3-km grid

3-km grid ↔ 500-m grid

500-m grid ↔ 15-km grid

0.75
0.78

0.9
0.85

0.64
0.65

area broadly extended beyond Mt. Azuma and theother mountains through the Naka-dori valley as far
as Yamagata and Niigata Prefectures. This distribution is similar to that of the surface concentrations shown in Figs. 5a and 5d. In addition, the mostpolluted area was not located near FDNPP but in thevicinity of the inland border between Fukushima andYamagata Prefectures. This unrealistic pollution wascaused by the wet deposition of the Cs-137 plumethat passed over the mountains beyond the Naka-dorivalley and intercepted a heavy precipitation area (cf.,Fig. 3b). This pollution hotspot does not exist in theaerial observations (Fig. 6), but Arnold et al. (2015)
also reported similar inland pollution with theirsimulation using a 0.2° (≈ 20 km) model resolution.
This pollution might be a common characteristic of
low-resolution models. In contrast, both the 3-km
(Figs. 7b, e) and 500-m (Figs. 7c, f) grid models

60

Journal of the Meteorological Society of Japan

4.

Fig. 6. Measured Cs-137 deposition based on
aerial sampling, which was conducted by theJapanese government (Torii et al. 2012). Notethat this deposition represents all of the Cs-137
emissions from the FDNPP until the autumn of
2011.

limited the heavily polluted area to mostly withineastern Fukushima near FDNPP and did not produce
an unrealistic deposition in comparison with the aerial
observations. However, there was a large discrepancy in the distribution and strength of Cs-137 deposition between the Eulerian and Lagrangian simulations. This discrepancy was caused by the differenceof dynamical and physical processes in the aerosol
models, such as the treatment of rainout and washout,which should be examined in a future study. The
correlation coefficients among the simulated Cs-137
deposition distributions presented a good agreement
between the 3-km and 500-m grid simulations, and avery poor agreement between the 15-km and 500-m
grid simulations (Table 6). The correlations were
calculated in the same way as in Table 5. The coefficient of the 3-km and 500-m Eulerian (Lagrangian)model simulations was 0.84 (0.63), while the coefficient was 0.42 (0.27) for the 15-km and 500-m simulations and 0.54 (0.54) for the 15-km and 3-km simulations. This result indicated that a 3-km grid model
could be alternated with a 500-m grid model, but it
was unacceptable to use a 15-km grid model insteadof a 500-m grid model.

Vol. 93, No. 1

Conclusion

We investigated the horizontal resolution dependence of atmospheric radionuclide simulations of the
Fukushima nuclear accident on March 15, 2011. We
used Eulerian and Lagrangian transport models withlow- (15-km), medium- (3-km), and high- (500-m)
resolutions. Both Eulerian and Lagrangian models
were driven by the same meteorological analysis thatwas independently prepared by an EnKF data assimilation system using the JMA operational weather
forecast model. This independent preparation of eachresolution analysis was definitely necessary for the
resolution-dependence investigation, excluding any
interpolation or averaging of meteorological fields.
Regarding the meteorological analyses, there wasa large difference in the PBL wind field between the
15-km grid resolution and the other grid resolutions.
The 15-km grid analysis could not reproduce Fukushima’s mountainous topography in detail. Consequently,
it failed to depict a complex wind structure overmountains and valleys. This error in the wind field
caused a large difference in the radionuclide transport and deposition simulations. In reality, the Cs-137
plume emitted from FDNPP was mostly blocked by
Mt. Azuma and other mountains along the Naka-dorivalley after crossing over the Abukuma Mountains.
However, the 15-km grid simulations did not represent the blockage of the Cs-137 plume, which unnaturally spread through the Naka-dori valley. In contrast,
the 3-km and 500-m grid simulations successfully
replicated the Cs-137 plume blockage along the Nakadori valley. The 3-km and 500-m grid simulations
produced very similar Cs-137 surface concentrations
and deposition distributions. Notably, the Eulerian and
Lagrangian models qualitatively behaved in the samemanner, but quantitatively yielded different results,
although they were driven by the same meteorological analysis. This was most likely because these twomodels had completely different aerosol physics (cf.
sensitivity studies conducted by Draxler et al. 2015;
Morino et al. 2013; Hu et al. 2014) and vertical advection handling at the lowermost model layer.
In conclusion, we suggest that it is illogical to use
low-resolution (15-km or more grid) atmosphericmodels to assess the Fukushima nuclear accident
when a regional analysis is needed due to the unrealistic performance of the 15-km grid simulations in
the local Fukushima area. Meanwhile, it is reasonable to use 3-km grid models instead of 500-m gridmodels due to the similarities between the 3-km and
500-m grid simulations and the high computational

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T. T. SEKIYAMA et al.

Fig. 7. Total deposition of Cs-137 accumulated from 00:00 UTC March 15 to 00:00 UTC March 16, 2011 assimulated by the Eulerian model RAQM2 with the (a) 15-km grid spacing meteorological analysis, (b) 3-km grid
spacing meteorological analysis, and (c) 500-m grid spacing analysis. The same as simulated by the RAQM2,
but simulated by the Lagrangian model JMA-RATM with the (d) 15-km grid spacing meteorological analysis, (e)3-km grid spacing meteorological analysis, and (f) 500-m grid spacing analysis.

Table 6. Horizontal distribution correlations of the Cs-137 depositions as simulated by the models.

Eulerian models
Lagrangian models

15-km grid ↔ 3-km grid

3-km grid ↔ 500-m grid

500-m grid ↔ 15-km grid

0.54
0.54

0.84
0.63

0.42
0.27

burden of 500-m grid simulations. In general, a
500-m grid simulation requires more than 200 times
as many CPU resources as a 3-km grid simulation.
Therefore, after ensuring the computational resources
to perform a 3-km grid Fukushima simulation, otherresources should be allotted for performing ensemble
analysis or high-quality data assimilation. However, it
is noted that the Fukushima nuclear accident occurred
during winter, and a different resolution-dependence

would have been found if the accident had occurred inanother season.
Acknowledgments
We are grateful to Dr. K. Saito (MRI) and Prof.
T. Iwasaki (Tohoku University) for their help andencouragement. We thank Dr. M. Chino (JAEA)
for providing the emission inventory data and Dr. T.
Torii (JAEA) for providing the aerial survey data.

62

Journal of the Meteorological Society of Japan

This study was supported by JSPS KAKENHI Grant
Number 24340115 and MEXT KAKENHI Grant
Number 24110003.
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ONLINE ISSN 2189-8871

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No. 76

Contribution of JMA to the WMO Technical Task Teamon Meteorological Analyses for Fukushima Daiichi
Nuclear Power Plant Accident and Relevant AtmosphericTransport Modeling at MRI
BY

K. Saito, T. Shimbori, R. Draxler, T. Hara, E. Toyoda, Y. Honda,
K. Nagata, T. Fujita, M. Sakamoto, T. Kato, M. Kajino, T.T. Sekiyama,
T.Y. Tanaka, T. Maki, H. Terada, M. Chino, T. Iwasaki, M.C. Hort,
S.J. Leadbetter, G. Wotawa, D. Arnold, C. Maurer, A. Malo, R. Servranckx
and P. Chen

気象研究所技術報告
第 76 号

ＷＭＯ福
福島第一原発事故に関する気象解析技術タスク
チーム活
活動と気象研究所の大気拡散モデリング斉藤和雄・・新堀敏基・
・R. Draxler
原旅人・豊田英司・・本田有機・
・永田和彦・
・藤田司・
・坂本雅巳
巳
加藤輝之・
・梶野瑞王・関山剛・田中泰宙・眞木貴史
寺田宏
宏明・
・茅野政
政道・
・岩崎俊樹
M.C. Hort, S.J. Leadbetter, G. Wotawa, D. Arnold, C. Maurer,
A. Malo, R. Servranckx, P. Chen

気 象 研 究 所
METEOROLOGICAL RESEARCH INSTITUTE, JAPANOCTOBER 2015

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気象研究所技術報告 第76号 平成27年10月
TECHNICAL REPORTS OF THE METEOROLOGICALRESEARCH INSTITUTE No.76, October 2015
doi:10.11483/mritechrepo.76

Contribution of JMA to the WMO Technical Task Teamon Meteorological Analyses for Fukushima Daiichi
Nuclear Power Plant Accident and Relevant AtmosphericTransport Modeling at MRI
byKazuo Saito and Toshiki Shimbori
Meteorological Research Institute, Japan Meteorological Agency

Roland Draxler
Air Resource Laboratory, National Oceanic and Atmospheric Administration, USA

Tabito Hara, Eizi Toyoda, Yuki Honda, Kazuhiko Nagata, Tsukasa Fujita
and Masami Sakamoto
Japan Meteorological Agency

Teruyuki Kato, Mizuo Kajino, Tsuyoshi T. Sekiyama, Taichu Y. Tanaka
and Takashi Maki
Meteorological Research Institute, Japan Meteorological Agency

Hiroaki Terada and Masamichi Chino
Japan Atomic Energy Agency

Toshiki Iwasaki
Tohoku University

Matthew C. Hort and Susan J. Leadbetter
Met Office, United Kingdom

Gerhard Wotawa, Delia Arnold and Christian Maurer
Central Institute for Meteorology and Geodynamics, Austria

Alain Malo and Rene Servranckx
Canadian Meteorological CentrePeter Chen
World Meteorological Organization

Contents
A

Preface

1

B

Overview

3

B-1

Overview of the WMO Task Team3

B-2

Task Team meetings

5

B-3

Overview of JMA's contribution to the WMO Task Team

9

B-4

Offer of data

13

C

JMA data and meteorological analyses

14

C-１

Observation data of JMA

14

C-2

NWP system at JMA

18

C-3

Data configurations of JMA mesoscale analysis

22

C-4
C-5

Quantitative Precipitation Estimation (QPE) and Quantitative
Precipitation Forecasting by JMA
GRIB2 templates for JMA Radar/Rain gauge-Analyzed Precipitation
data

24
39

C-6

Radar / Rain gauge-Analyzed Precipitation dataset by JMA

51

C-7

File converter tool

56

C-8

JMA Meso-scale 4D-VAR analysis

65

C-9

Meteorological field

68

D

ATDM experiments

73

D-1

Design of the Task Team experiment

73

D-2

Reverse estimation of amounts of 131I and 137Cs discharged into theatmosphere

77

D-3

Verification methods

81

D-4

The NOAA ARL website

84

D-5

Task team final report and follow-up

88

E

JMA-RATM

89

E-1

Original and preliminary RATM

89

E-2

Revision of RATM

95

E-3

Experiments with RATM

97

F

ATDM simulations by TT members

108

F-1

The NOAA ATDM experiments

108

i

F-2

The Met Office ATDM experiments

110

F-3

Impact of different meteorological input on ATM with FLEXPART

113

F-4

The CMC ATDM experiments

119

F-5

Results of ATDM simulations

122

G

Relevant modeling at MRI and JMA

126

G-1

Numerical Atmospheric Transport and Dispersion Models

126

G-2
G-3

WMO emergency response activities and the operational
atmospheric transport modelling at JMA
NHM-Chem: Sensitivity of Cs deposition to the size andhygroscopicity of Cs-bearing aerosols

127
133

G-4

NHM-Chem-LETKF

143

G-5

Emission source estimation by an inverse model

150

G-6

Science Council of Japan atmospheric transport modelintercomparison

154

H

References

159

I

Appendix172

I-1

Final report of the first meeting of WMO Task Team

174

I-2

Final report of the second meeting of WMO Task Team

201

I-3

Final report of the third meeting of WMO Task Team

215

total

225

ii

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

G-3. NHM-Chem: Sensitivity of Cs deposition to the size and hygroscopicity of
Cs-bearing aerosols 1
G-3-1. Abstract
The emission, transport, and deposition of 137Cs released by the Fukushima Daiichi Nuclear Power
Plant (FDNPP) accident were simulated with consideration of the microphysical properties (i.e. sizeand hygroscopicity) of the Cs-bearing aerosols (carrier aerosols of radioactive Cs). The sensitivity ofthe simulated deposition of 137Cs to the size and hygroscopicity of the carrier aerosols was assessedand compared with the sensitivity to meteorological fields simulated using different dynamics andphysics modules. Two types of Cs-bearing aerosols were considered in the simulation, supermicronwater-insoluble and submicron water-soluble particles, in accordance with previously publishedobservational evidence (Adachi et al., 2013 and Kaneyasu et al., 2012). Even though the sametransport model was used, the simulated depositions were very different when meteorological models
with different dynamics and physics modules were used. The sensitivity of 137Cs deposition to thecarrier aerosol size and hygroscopicity, in which the proportion of water-insoluble aerosol emission
ranged from 10% to 90%, during the early stage ranged from March. 11-12 to Mar. 12-20, was foundto be lower but still as important as the sensitivity to meteorological fields simulated using different
dynamics and physics modules. To better understand the environmental behavior of the radioactive Csdischarged from the FDNPP, knowledge of the carrier aerosol microphysical properties is as importantas the accuracy of the meteorological simulation and the emission scenario.

G-3-2. Introduction
Three months after the FDNPP accident, Chino et al. (2011) estimated the emission amounts ofradioactive 137Cs and 131I associated with the accident by using a reverse estimation method in which
both the environment monitoring data and an atmospheric dispersion simulation were used (seesection D-2). Since then, many modeling studies have been conducted to assess the emission,
dispersion, and deposition amounts of radionuclides associated with the accident (Morino et al. 2011;
Yasunari et al., 2011; Schöppner et al. 2011; Takemura et al., 2011; Sugiyama et al., 2012; Stohl et al.,2012; Terada et al., 2012; Katata et al., 2012a, 2012b; Morino et al., 2013; Adachi et al., 2013; Hu etal., 2014; Katata et al., 2015; Sekiyama et al., 2015).
Becausenumerical

modelsuse

uncertain

parameters

and

rough

assumptions,model

inter-comparison and intra-comparison (or sensitivity) studies are essential to assess the uncertainties
of numerical simulations. In fact, previous model inter-comparison studies have shown that simulation
results vary substantially among models (Draxler et al., 2013a; Katata et al., 2015; SCJ, 2014).
Although model inter-comparison studies can show how the simulation results of models using
different dynamics, physics, and chemistry modules and emission scenarios differ overall, the reasonsfor the differences cannot be easily identified. In contrast, model intra-comparison (or sensitivity)

1

M. Kajino

133

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

studies can identify the modules or parameters that are responsible for different results, but underlimited conditions that the simulations are performed only by a single model.
Morino et al. (2013) investigated the sensitivity of radioactive Cs dispersion and deposition to thewet-scavenging modules and emission scenarios. Like most previous studies, for the meteorologicalfield they used the output of only one meteorology model. In this study, we used several different
meteorology models and simulation techniques to evaluate sensitivity of the transport model results todifferent meteorological simulations as well.
Another important aspect of this study is that we examined the sensitivity of the simulated
deposition to the microphysical properties of Cs-carrying aerosols for the first time. Adachi et al.
(2013) reported that in the early stage of the accident, the carrier aerosols of radioactive Cs were
spherical, water-insoluble particles (hereafter, Cs-balls), and they predicted that the atmosphericbehavior of these aerosols would be different from that of the submicron water-soluble particles
described by Kaneyasu et al. (2012). Washout (or below-cloud scavenging) of aerosol particles (i.e., ofboth types described in this paragraph) is not usually efficient because of their small inertia and slow
Brownian motion. In contrast, the submicron water-soluble particles are efficiently scavenged via
rainout (or in-cloud scavenging) because the Kelvin (curvature) effect is enough small. Washout isprobably the dominant scavenging process of water-insoluble aerosols, because very highsupersaturation conditions are needed for rainout of water-insoluble aerosols to occur.
The purpose of this study was to assess the sensitivities described above in order to evaluate theuncertainties of the simulated deposition of 137Cs caused by aerosol microphysical properties (i.e.,
aerosol size and hygroscopicity) and to compare it to the uncertainty caused by the use of different
meteorological simulations.

G-3-3. NHM-Chem
NHM-Chem is a chemical transport model, offline- or online-coupled with Japan Meteorological
Agency’s non-hydrostatic model (JMA-NHM; Saito et al., 2007). NHM is a numerical weather
prediction model of JMA. An Eulerian regional chemical transport model, Regional Air Quality
Model 2 (RAQM2) (Kajino et al., 2012a), is used to simulate emission, transport, and deposition of
trace gases and aerosols. RAQM2 implements a triple-moment modal aerosol microphysics modulethat assumes a log-normal size distribution of aerosol populations. This model describes the nature ofaerosol dynamical processes, such as nucleation, condensation, coagulation, hygroscopic growth, dry
deposition, grid-scale rainout (cloud condensation and ice nuclei activation and subsequent
mixed-phase cloud microphysical processes) and washout (coagulation between aerosols and settlinghydrometeors) processes, and sub-grid-scale convection and scavenging processes. In the study, theoffline-coupled NHM-Chem was used in order to use different meteorological models alternatively,
such as the Weather Research and Forecasting (WRF) model (Skamarock et al., 2008) to drive
RAQM2.

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TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

G-3-4. Simulation settings
In this study, meteorology simulations were performed with NHM and WRF with two different
cloud microphysics modules, Morrison et al. (2009) and Lim and Hong (2010), referred to as
WRF-MORR and WRF-WDM6, respectively. The two WRF simulations were used so that thesensitivity to just the cloud microphysical process (grid-scale) could be assessed, because wet
deposition of 137Cs over Japan dominated over dry deposition in this study.
NHM, WRF, and RAQM2 shared the same domain, which consisted of 215 × 259 grid cells with a
3 km horizontal resolution; this model domain is slightly larger than the area shown in Fig. G-3-1.
There were 50 vertical layers up to 50 hPa in NHM, 28 layers up to 100 hPa in WRF, and 20 layers up
to 10 km in RAQM2. JMA Meso-Regional Objective Analysis data sets (3 hourly, 5 km × 5 km) wereused for the initial and boundary conditions of NHM and WRF. The same analysis data sets were used
for the spectral nudging in NHM and for the grid nudging in WRF.
The radionuclide transport version of NHM-Chem was developed for simulations of nuclear power
plant accidents such as the FDNPP accident (Adachi et al., 2013; Sekiyama et al., 2015). This versionof NHM-Chem uses an aerosol dynamics module that is simplified from that described by Kajino et al.
(2012a) because aerosol hygroscopicity and the particle size distribution are assumed to be constantduring transport. The nature of the aerosol dynamics such as dry deposition and grid-scalerainout/washout processes are thus described on the basis of the prescribed size distribution andhygroscopicity. Details of the dry and wet deposition processes are described by Kajino et al. (2012a;
their sections 2.2.7 and 2.2.8). Even aerosols that are completely water-insoluble (i.e., hygroscopicity
κ = 0) can act as cloud condensation nuclei under highly supersaturated conditions. Althoughwater-insoluble aerosols can coagulate with cloud droplets within a cloud (this is also rainout process),for simplicity, in this study we did not consider rainout of Cs-balls and only washout in their wet
deposition modeling. Sub-grid scale convection and scavenging processes were not considered. The
fog deposition process of Katata et al., (2015) was considered.
We used the emission scenario for 137Cs discharged from the FDNPP from Terada et al. (2012), andconsidered Cs-bearing aerosols to be of two types. Supermicron water-insoluble particles (Cs-balls)
had a lognormal size distribution, number equivalent geometric mean dry diameter D g,n,dry = 2.3 μm,
geometric standard deviation σ g = 1.3, particle density ρ p = 2.0 g/cm3, and κ = 0 (Adachi et al., 2013),
and submicron water-soluble particles (Kaneyasu et al., 2012) had a lognormal size distribution,
D g,n,dry = 0.1 μm, σ g = 1.6, ρ p = 1.83 g/cm3, and κ = 0.4.
For the sensitivity studies, taking into consideration the findings of Adachi et al. (2013), we allowed
the proportion of early-stage emissions consisting of Cs-balls during the early stage to range from 10%
to 90%, and the ending date of the early stage to range from 12 to 20 March 2011.
The analysis period was from 00:00 UTC on 11 March to 00:00 UTC on 1 April, with a spin-up
period of 3 days. Thus, the entire simulation period was from 8 March to 1 April 2011.

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TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

Fig. G-3-1. Cumulative precipitation (mm) from 11 March to 1 April: (a) Radar/rain gauge-analyzed
precipitation (RAP) data and precipitation simulated by (b) NHM, (c) WRF-MORR, and (d) WRF-WDM6.

Fig. G-3-2. Scattergrams of simulated and observed (RAP) cumulative precipitation: (a–c) from 11 March to 1
April and (d–f) in the afternoon of 15 March. Simulations were by (a, d) NHM, (b, e) WRF-MORR, and (c,f) WRF-WDM6. The plotted data are for all grids for which both observation and simulation data wereavailable. Although the data are plotted on a log-log scale, the statistics R, mean bias (MB), and theobservation average (Obs. Ave.) were calculated on a linear-linear basis.

136

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

G-3-5. Results and discussion
We compared observed and simulated cumulative precipitation from 11 March to 1 April among thethree meteorology models (Figs. G-3-1 and G-3-2). For observed data, we used JMA’s radar/raingauge-analyzed precipitation (RAP) data, which were interpolated to the 3 km resolution grid. We also
compared observed and simulated cumulative precipitation on the afternoon of 15 March (Fig. G-3-2,
lower panels), when substantial deposition occurred on land (e.g., Morino et al., 2013). All threesimulations overestimated precipitation over the ocean by a factor of more than 10 (data points above
the 10:1 simulation:observation line in Fig. G-3-2), and the two WRF simulations also underestimated
precipitation in the southwestern part of the domain by a factor of more than 10 (data points below the
1:10 simulation:observation line in Fig G-3-2). Our focus was on land regions where the 137Cs
deposition was large (>10 kBq/m2) (see Fig. G-3-3), and we did not expect the large discrepancies
between the simulated and observed precipitation over the ocean to substantially affect the modelingof 137Cs deposition in land areas.
The differences due to the different cloud microphysics modules were notable. The simulated
precipitation spatial distribution patterns of the two WRFs were similar and different from the NHM
pattern, whereas the precipitation amounts in WRF-MORR were fairly close to those in NHM, andthose in WRF-WDM6 were much larger than the amounts in the other two simulations (Fig. G-3-2).
WRF-WDM6 overestimated precipitation substantially over high-altitude regions (corresponding tolocations where the simulated precipitation was >600 mm; Fig. G-3-1d). In the afternoon of 15 March,
the overestimation of WRF-WDM6 was substantial; the mean bias (MB) was 4.1 mm and theobservation average was 2.36 mm. Judging from the values of the correlation coefficient (R), theperformance of NHM was best among the three meteorological simulations (R = 0.86, MB = 0.35 mm).
The MB of WRF-MORR was smallest (MB = 0.24 mm), but owing to the square shape of the plotted
data, R was 0.67.
Comparison of cumulative 137Cs deposition amounts between aircraft observations (Torii et al.,2012) and simulations by NHM, WRF-MORR, and WRF-WDM6 (Fig. G-3-3), performed under theassumption that 100% of 137Cs was carried by water soluble particles, showed that NHM simulated too
much deposition in northern areas (Yamagata, Miyagi, and Iwate prefectures). This deposition wascaused by rainout of ice phase precipitation (snow and graupel). For accurate simulation of rainoutof 137Cs, the vertical distribution of the 137Cs and the hydrometeor mixing ratio must be accurately
predicted. However, because no observations of the vertical profiles of 137Cs are available for the time
period of this study, the reason for this overestimation is impossible to identify.
WRF-MORR also simulated too much deposition in Yamagata and Miyagi prefectures, but
WRF-WDM6 simulated less deposition in this area. The two WRF simulations reasonably reproduced
depositions in the highest deposition areas (>1000 kBq/m2), but depositions in those areas were
underestimated by NHM. The two WRF simulations also reasonably reproduced the higher
depositions in the mountainous regions of Tochigi and Gunma prefectures, but they overestimateddepositions in the southern area (Tokyo, Kanagawa, Shizuoka, and Chiba prefectures). The NHM
137

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

Fig. G-3-3. Cumulative 137Cs deposition amounts (kBq/m2) in (a) aircraft observations and (b) NHM, (c)WRF-MORR, and (d) WRF-WDM6 simulations. Simulated depositions are shown only for land areas tofacilitate visual comparison with the observed deposition.

simulation underestimated deposition in all of these areas (i.e., in Tochigi and Gunma prefectures aswell as in Tokyo, Kanagawa, Ibaraki, and Chiba prefectures). It is notable that, even though thetransport model was the same, the simulated depositions varied substantially among the different
meteorological simulations.
Figure G-3-4 shows depositions of water-soluble and water-insoluble particles simulated by using
the three meteorological fields on both land and ocean areas. We compared the simulated depositions
with aircraft observation data (Fig. G-3-3a) interpolated to the 3 km resolution grids of the models in
Fig. G-3-5. Note that following Morino et al. (2013) and Katata et al. (2015), R and MB were
calculated only when the observed values were larger than 10 kBq/m2.

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TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

Fig. G-3-4. Cumulative depositions (kBq/m2) simulated using the meteorological fields of (a, d) NHM, (b, e)
WRF-MORR, and (c, f) WRF-WDM6 under the assumption that 100% of Cs was carried by (a–c) watersoluble or (d–f) water insoluble particles.

Fig. G-3-5. Scattergrams of cumulative deposition between the (a, d) NHM, (b, e) WRF-MORR, and (c, f)
WRF-WDM6 simulations and aircraft observations. The simulations were performed under the assumptionthat 100% of Cs was carried by (a–c) water soluble or (d–f) water insoluble particles. Although the data areplotted on a log-log scale, the statistics R and mean bias (MB) were calculated on a linear-linear basis.

139

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

The simulated results were substantially different between Cs-bearing particles assumed to be 100%water soluble or water insoluble, because rainout of the Cs-balls was not considered to occur (compare
upper and lower panels in Figs. G-3-4 and G-3-5), whereas the dry deposition velocity of Cs-balls wasapproximately four times that of the water-soluble submicron aerosols. It is interesting that although
the simulation of precipitation by NHM/WRF-WDM6 was good/poor, the simulation of deposition by
NHM/WRF-WDM6 was poor/good.
Adachi et al. (2013) detected Cs-balls only in samples collected during the early stage of theaccident (14–15 March), but they reported that radioactive Cs was carried by water-soluble aerosols
later (20–22 March). Kaneyasu et al. (2012), who analyzed samples collected from 28 April to May 12
and during 12–26 May (i.e., after the later sampling period of Adachi et al., 2013), also reported theradioactive Cs to be carried by water-soluble aerosols. Therefore, the assumption of 100%water-insoluble or water-soluble particles (Figs. G-3-4 and G-3-5) was not realistic; rather, reality
must lie somewhere in between. Therefore, under the assumption that Cs-balls, as indicated by Adachi
et al. (2013), were emitted only in the early stage of the accident, we used the following settings to test
the sensitivity to aerosol microphysical properties:
1. We set the proportion of Cs-ball emissions to values from 10% to 90% during the early stage.
2. We started the early stage on 11 March but varied its ending date between 12 and 20 March2011 (i.e., before the later sampling of Adachi et al., 2013).
We next compared cumulative depositions simulated using the meteorological fields calculated by
NHM and the two WRF simulations between two extreme cases: 10% Cs-ball emission until 12 Marchand 90% Cs-ball emission until March 20 (Fig. G-3-6). The statistics (MB, root mean square error(RMSE) and R) of these comparisons are presented in Table G-3-1, together with the statistics for thethree meteorological simulations when emissions were assumed to consist of 100% water-soluble
submicron particles.
It is notable that even when the transport model settings and aerosol properties were the same, thefractional bias (MB divided by the observation average) ranged from 0.25 to 0.74, differing by
approximately threefold, among the three different meteorological simulations. This difference ismarked, because it means, for example, that the emission amount estimated by an inverse model fromthe deposition amount could vary threefold, depending on the meteorological model used. The rangesof MB, RMSE and R in the sensitivity to aerosol properties test results were smaller than their ranges
in the sensitivity to meteorology test results (Table G-3-1), but the differences were similarly marked.
The fractional bias range differed by approximately twofold between WRF-MORR and WRF-WDM6(0.35–0.74 and 0.38–0.66, respectively). Therefore, the sensitivity of 137Cs deposition to aerosol
microphysical properties was as important as its sensitivity to the meteorological simulation used.

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TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

Fig. G-3-6. Simulated cumulative depositions (kBq/m2) using the meteorological fields calculated by (a, b)
NHM, (c, d) WRF-MORR, and (e, f) WRF-WDM6 between the two extreme aerosol microphysicalassumptions: (a, c, e) 10% Cs-ball emission from 11 to 12 March and (b, d, f) 90% Cs-ball emission from 11
to 20 March.
Table G-3-1. Ranges of the statistics between observed and simulated cumulative depositions between the two
extreme sensitivity tests (10% or 90% Cs-balls and 12 or 20 March as the ending date of the early stage) witheach meteorological simulation (first to third row). The bottom row shows the same statistics among themeteorological simulations when no Cs-balls were assumed.
Sensitivity to

MB 1(min:max)

RMSE1(min:max)

R1(min:max)

Obs.
Ave.

(kBq/m2)(kBq/m2)(-)(kBq/m2)

274.3 : 295.0

0.39 : 0.55

88.2

251.1 : 272.5

0.53 : 0.59

225.9 : 244.2

0.65 : 0.70

Aerosol-68.5 : -65.5
properties
Aerosol-57.2 : -22.8
properties
Aerosol-54.6 : -30.2
properties
Meteorology-66.0 : -23.2
simulations
1
linear-linear statistics

233.0 : 274.9

0.54 : 0.66

Simulation settings
Cs-ball fraction

Ending date ofearly stage

NHM

10-90%

Mar.12 - 20

88.2

WRF-MORR

10-90%

Mar.12 - 20

88.2

WRF-WDM6

10-90%

Mar.12 - 20

88.2

NHM, WRF-MORR,
WDM6

0%

-

141

Meteorologicalsimulation

TECHNICAL REPORTS OF THE METEOROLOGICAL RESEARCH INSTITUTE No.76 2015

G-3-6. Summary
We simulated the emission, transport, and deposition of 137Cs released due to the FDNPP accident.
The sensitivity of the simulated depositions of radioactive Cs to the size and hygroscopicity of thecarrier aerosols was assessed and compared with the sensitivity to the meteorological simulation.
Two types of Cs-bearing aerosols, water-insoluble supermicron particles (Cs-balls) andwater-soluble submicron particles were considered in the simulation. The simulated depositions of thetwo aerosols were significantly different because rainout was not considered to occur with Cs-balls,
and the dry deposition velocities of Cs-balls were approximately four times those of the water-soluble
particles.
Even when the transport model was used with exactly the same settings, the simulated depositions
were very different among the different meteorological simulations: The fractional bias (MB divided
by observation average) ranged from 0.25 to 0.74, an approximately threefold difference. The
sensitivity of 137Cs deposition to particle size and hygroscopicity (determined by adjusting the
proportion of water-insoluble Cs emission between 10% and 90% and the ending date of the earlystage between 12 and 20 March 2011) was smaller but just as important as the sensitivity to themeteorological simulation (in which 100% of Cs was assumed to be water soluble, as in previous
studies). To better understand the environmental behavior of radioactive Cs discharged from the
FDNPP, knowledge of the aerosol microphysical properties is as important as the accuracy of themeteorological simulations and emission scenarios.
In future work, several new wet deposition modules and emission scenarios, together with new
meteorological simulations (for example, NHM-LETKF as in Sekiyama et al., 2015), will be added tothe current sensitivity analysis study to provide a more robust uncertainty estimation of the numericalsimulation techniques. It would also be interesting to estimate the sensitivity to the modeling approach(Lagrangian or Eulerian), because the both approaches have been used in FDNPP accident simulation
studies.

142

1 9 4 6（昭和2 1）年

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中 野 英 之藤 貞 夫
小 野 景 子気 象 研 究 所 技 術 報 告 は ， 1978（ 昭 和 53） 年 の 初 刊 以 来 ,気 象 研 究 所 が 必 要 の 都 度 発 行 す る 刊 行 物 で あ り ,気 象 研 究 所 の 研 究 計 画 に 基づ き 実 施 し た 研 究 に 関 す る 手 法 、 デ ー タ 、 結 果 等 に つい て の ま と め、 ま た は 、す で に 公 表 し た 研 究 論 文 類 を と り ま と め 総 合的報告とした ものを 掲載する 。

本 紙 に 掲 載 され た 報 告 の 著 作 権 は 気 象 研究 所に帰 属する 。本紙に掲載された報告を 引用 する場合 は,出所を 明 示 す れ ば 気 象 研 究 所 の 許 諾 を 必 要 と し な い 。 本 紙に 掲 載 さ れ た報 告 の 全 部 又は 一 部 を 複 製， 転 載 ， 翻
訳 ， あ る い は そ の 他 に 利 用 す る 場 合 は 気 象 研 究 所 の 許諾 を 得 な け れば な ら な い 。個 人 が 研 究 ，学 習 ， 教 育に 使 用 する場 合 は,出 所 を 明 示 す れ ば 気 象 研 究所の許諾を 必要と しない。

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〒305-0052 茨城県つくば市長峰１－１
TEL(029)853-8535

6. Publication list 1954-2015
論文リスト（英文誌）1954-2015- 26 -

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2015
Igarashi, Y., Kajino, M., Zaizen, Y., Adachi, K., and Mikami, M., Atmospheric radioactivity
over Tsukuba, Japan: a summary of three years of observations after the FDNPP accident,
Progress in Earth and Planetary Science 2:44, 2015. doi: 10.1186/s40645-015-0066-1.
Kajino, M., G-3. NHM-Chem: Sensitivity of Cs deposition to the size and hygroscopicity of
Cs-bearing aerosols, Technical Reports of the Meteorological Research Institute, 76, 132–142,
2015.
Sekiyama, T. T., Kunii, M., Kajino, M., and Shinbori, T., Horizontal Resolution Dependenceof Atmospheric Simulations of the Fukushima Nuclear Accident Using 15-km, 3-km, and
500-m Grid Models, J. Meteorol. Soc. Jpn., 93B, 49–64, 2015.
2014
Abe, Y., Iizawa, Y., Terada, Y., Adachi, K., Igarashi, Y., and Nakai, I., Detection of uranium
and chemical state analysis of individual radioactive microparticles emitted from the
Fukushima nuclear accident using multiple synchrotron radiation X-ray analyses, Analytical
Chemistry, 86, 8521-8525, 2014.
2013
Aoyama, M., Uematsu, M., Tsumune, D., and Hamajima, Y., Surface pathway of radioactive
plume of TEPCO Fukushima NPP1 released 134Cs and 137Cs, Biogeosciences, 10,3067-3078, 2013. doi: 10.5194/bg-10-3067-2013.
Tsumune, D., Tsubono, T., Aoyama, M., Uematsu, M., Misumi, K., Maeda, Y., Yoshida, Y.,
and Hayami, H., One-year, regional-scale simulation of 137Cs radioactivity in the ocean
following the Fukushima Daiichi Nuclear Power Plant accident, Biogeosciences, 10,5601-5617, 2013.
Povinec, P.P., Aoyama, M., Biddulph, D., Breier, R., Buesseler, K., Chang, C., Golser, R.,
Hou, X., Ješkovský, M., and Jull, A., Cesium, iodine and tritium in NW Pacific waters-acomparison of the Fukushima impact with global fallout. Biogeosciences, 10, 5481-5496.
2013.
Kumamoto, Y., Murata, A., Kawano, T., and Aoyama, M., Fukushima-derived radiocesium inthe northwestern Pacific Ocean in February 2012, Applied Radiation and Isotopes, 81,
335-339, 2013. doi: 10.1016/j.apradiso.2013.03.085.
Toyama, C., Muramatsu, Y., Igarashi, Y., and Aoyama, M., Atmospheric fallout of 129I inJapan before the Fukushima accident: Regional and global contributions, 1963-2005,
Environmental Science & Technology, 47(15), 8383-8390, 2013. doi: 10.1021/es401596z.
Adachi, K., Kajino, M., Zaizen, Y., and Igarashi, Y., Emission of spherical cesium-bearing
particles from an early stage of the Fukushima nuclear accident. Scientific Reports, 3, Article
number 2554, 2013. doi:10.1038/srep02554.
2012
Toyama, C., Muramatsu, Y., Uchida, Y., Igarashi, Y., Aoyama, M., and Matsuzaki, H.,
Variations of 129I in the atmospheric fallout of Tokyo, Japan 1963-2003, Journal ofEnvironmental Radioactivity, 113, 116-122, 2012. doi: 10.1016/j.jenvrad.2012.04.014.
Inomata, Y., Aoyama, M., Tsumune, D., Motoi, T., and Nakano, H., Optimum interpolation
analysis of basin-scale 137Cs transport in surface seawater in the North Pacific Ocean, Journalof Environmental Monitoring, 14, 3146-3155, 2012. doi: 10.1039/C2EM30707C.
Aoyama, M., Tsumune, D., Uematsu, M., Kondo, F., and Hamajima, Y., Temporal variationof 134Cs and 137Cs activities in surface water at stations along the coastline near the Fukushima
Dai-ichi Nuclear Power Plant accident site, Japan. Geochem. J, 46, 321-325, 2012.
Aoyama, M., Tsumune, D., and Hamajima, Y., Distribution of 137Cs and 134Cs in the North
Pacific Ocean: impacts of the TEPCO Fukushima-Daiichi NPP accident, Journal ofRadioanalytical and Nuclear Chemistry, 1-5, 2012. doi: 10.1007/s10967-012-2033-2.

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Honda, M. C., Aono, T., Aoyama, M., Hamajima, Y., Kawakami, H., Kitamura, M.,
Masumoto, Y., Miyazawa, Y., Takigawa, M., and Saino, T., Dispersion of artificial
caesium-134 and-137 in the western North Pacific one month after the Fukushima accident,
Geochem. J, 46, e1-e9, 2012.
Povinec, P. P., Hirose, K., and Aoyama, M., Radiostrontium in the Western North Pacific:
characteristics, behavior, and the Fukushima impact, Environmental Science & Technology,
46, 10356-10363, 2012. doi: 10.1021/es301997c.
Tsumune, D., Tsubono, T., Aoyama, M., and Hirose, K., Distribution of oceanic 137Cs from the
Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model,
Journal of Environmental Radioactivity, 111, 100-108, 2012.

2011
Aoyama, M., Fukasawa, M., Hirose, K., Hamajima, Y., Kawano, T., Povinec, P.P., andSanchez-Cabeza, J. A., Cross Equator transport of 137Cs from North Pacific Ocean to South
Pacific Ocean, BEAGLE2003 cruises, Progress in Oceanography, 89, 7-16, 2011.
doi:10.1016/j.pocean.2010.12.003.
Buesseler, K., Aoyama, M., and Fukasawa, M., Impacts of the Fukushima nuclear power
plants on marine radioactivity. Environmental Science & Technology, 45(23), 9931-9935,
2011. doi: 10.1021/es202816c.
Povinec, P. P., Aoyama, M., Fukasawa, M., Hirose, K., Komura, K., Sanchez-Cabeza, J.A.,
Gastaud, J., Jeskovsky, M., Levy, I., and Sykora, I., 137Cs water profiles in the South Indian
Ocean - an evidence for accumulation of pollutants in the subtropical gyre. Progress in
Oceanography, 89 , 17-30, 2011. doi:10.1016/j.pocean.2010.12.004.
Gastaud, J., Povinec, P.P., Aoyama, M., Hirose, K., Sanchez-Cabeza, J.A., Levy, I., Roos, P.,
Eriksson, M., Bosc, E. and Rezzoug, S., Transport and scavenging of Pu in surface waters of
the Southern Hemisphere Oceans, Progress in Oceanography, 89, 92-100, 2011.
doi:10.1016/j.pocean.2010.12.009.
Hirose, K., Kim, C.S., Yim, S.A., Aoyama, M., Fukasawa, M., Komura, K., Povinec, P.P., andSanchez-Cabeza, J.A., Vertical profiles of plutonium in the central South Pacific, Progress in
Oceanography, 89, 101-107, 2011. doi:10.1016/j.pocean.2010.12.010.
Tsumune, D., Aoyama, M., Hirose, K., Bryan, F., Lindsay, K. and Danabasoglu, G., Transportof 137Cs to the Southern Hemisphere in an Ocean General Circulation Model, Progress in
Oceanography, 89, 38-48, 2011. doi:10.1016/j.pocean.2010.12.006.
Sanchez-Cabeza, J.A., Levy, I., Gastaud, J., Eriksson, M., Osvath, I., Aoyama, M., Povinec,
P.P. and Komura, K., Transport of North Pacific 137Cs labeled waters to the south-eastern
Atlantic
Ocean.
Progressin
Oceanography,
89,
31-37,
2011.
doi:10.1016/j.pocean.2010.12.005.
Levy, I., Povinec, P.P., Aoyama, M., Hirose, K., Sanchez-Cabeza, J.A., Comanducci, J-F.,
Gastaud, J., Eriksson, M., Hamajima, Y., Kim, C.S., Komura, K., Osvath, I., Roos, P. and Yim,
S.A., Marine anthropogenic radiotracers in the Southern Hemisphere: new sampling andanalyticalstrategies,
Progressin
Oceanography,
89,
120-133,
2011.
doi:10.1016/j.pocean.2010.12.012.
2010
Inomata,Y., Igarashi, Y., Yoshioka, K., Tanaka, T. Y., and M, Chiba., Temporal variation of222
Rn at the summit of Mt. Fuji associated with the Asian continental outflow, Atmospheric
Environment, 44,3856-3865, 2010.
Nakano, H., Motoi, T., Hirose, K., and Aoyama, M., Analysis of 137Cs concentration in the
Pacific using a Lagrangian approach. Journal of Geophysical Research, 115, C06015, 2010.
doi:10.1029/2009JC005640.
Hirose, K., Igarashi, Y., Aoyama, M., and Inomata, Y., Depositional behaviors ofplutonium and thorium isotopes at Tsukuba and Mt. Haruna in Japan indicate the sourcesof atmospheric dust. Journal of Environmental Radioactivity, 101,106-112, 2010.

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Zhang, Y., Zeng, J., Yamada, M., Wu, F., Igarashi, Y., and Hirose, K., Characterization of
Pu concentration and its isotopic composition in a reference fallout material. Science of the
Total Environment, 408(5), 1139-1144, 2010.

2009
Aoyama, M., Hamajima, Y., Fukasawa, M., Kawano, T., and Watanabe, S., Ultra low level
deep water 137Cs activity in the South Pacific Ocean, Journal of Radioanalytical and Nuclear
Chemistry, 2009. doi:10.1007/s10967-009-0253-x.
Hirose, K., Aoyama, M., and Povinec, P.P., 239, 240Pu /137Cs ratios in the water column of theNorth Pacific: a proxy of biogeochemical processes, Journal of Environmental Radioactivity,
100, 258-262, 2009.
Igarashi, Y., Inomata, Y., Aoyama, M., Hirose, K., Takahashi, H., Shinoda, Y., Sugimoto, N.,
Shimizu, A., and Chiba, M., Possible change in Asian dust source suggested by atmosphericanthropogenic radionuclides during the 2000s, Atmospheric Environment, 43, 2971-2980,
2009.
Inomata, Y., Aoyama, M., and Hirose, K., Analysis of 50-y record of surface 137Csconcentrations in the global ocean using the HAM-global database, Journal of Environmental
Monitoring, 11(1), 116-125, 2009. doi: 10.1039/b811421h.
Hirose, K., Igarashi, Y., Aoyama, M., and Inomata, Y., Depositional behaviors of plutonium
and thorium isotopes at Tsukuba and Mt.Haruna in Japan indicate the sources of atmospheric106-112,
2009.
dust,
JournalofEnvironmental
Radioactivity,
101(2),
doi:10.1016/j.jenvrad.2009.09.003.
Igarashi Y., Kikawada, Y., Oda, K., Yamauchi, R., Nomura, M., Honda, T., Oi, T., and Hirose,K., Anomalous Uranium Isotope Ratio in Atmospheric Deposits in Japan, Journal of Nuclear
Science and Technology, 46(12), 1094-1098. 2009.
2008
Aoyama, M., Hirose, K., Nemoto, K., Takatsuki, Y., and Tsumune, D., Water masses labeledwith global fallout 137Cs formed by subduction in the North Pacific, Geophysical Research
Letters, 35, L01604, 2008. doi:10.1029/2007GL031964.
Hirose, K., Igarashi, Y., and Aoyama, M., Analysis of the 50-year records of the atmosphericdeposition of long-loved radionuclides in Japan, Applied Radiation and Isotopes, 66,
1675-1678, 2008.
Hirose, K., Aoyama, M., Igarashi, Y., and Komura, K., Improvement of 137Cs analysis in small
volume seawater samples using the Ogoya underground facility, Journal of Radioanalyticaland Nuclear Chemistry, 276,3,795-798, 2008.
Aoyama, M., Hirose, K., Nemoto, K., Takatsuki, Y., and Tsumune, D., Water masses labeledwith global fallout 137Cs formed by subduction in the North Pacific, Geophysical Research
Letters, 35, L01604, 2008. doi:10.1029/2007GL031964.
2007
Hirose, K., Aoyama, M., and Kim, C.S., Plutonium in Seawater of the Pacific Ocean, Journalof Radioanalytical and Nuclear Chemistry, 274 No. 3, 635-638, 2007.
Hirose, K., Igarashi, Y., and Aoyama, M., 50 years records of atmospheric deposition of
long-lived anthropogenic radionuclides in Japan, International Conference on Environmental
Radioactivity: From Measurements and Assessments to Regulation, IAEA-CN-145, 95-96,
2007.
Hirose, K., Igarashi, Y., and Aoyama, M., Recent trends of plutonium fallout observed inJapan: Comparison with natural lithogenic radionuclides, thorium isotopes, Journal ofRadioanalytical and Nuclear Chemistry, 273 No.1, 115-118, 2007.
Hirose, K., Aoyama, M., Fukasawa, M., Kim, C. S., Komura, K., Povinec, P. P., andSanchez-Cabeza, J. A., Plutonium and 137Cs in surface water of the South Pacific Ocean,
Science of the Total Environment, 381, 243-255, 2007.

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2006
Yamamoto, M., Sakaguchi, A., Sasaki, K., Hirose, K., Igarashi, Y., and Kim, C.K., Seasonaland spatial variation of atmospheric 210Pb and 7Be deposition: features of the Japan Sea side ofJapan, Journal of Environmental Radioactivity, 86, 110-131, 2006.
Hirose, K., Aoyama, M., Kim, C.S., Kim, C.K., and Povinec, P.P., Plutonium isotopes inseawater of the North Pacific: effects of close-in fallout, INTERNATIONAL CONFERENCE
ON ISOTOPES AND ENVIRONMENTAL STUDIES. Eds. P. Povinec, J.A. Sanchez-Cabeza.
Vol. 8 of Radioactivity in the Environment, Elsevier, London, 2006.
Aoyama, M., Hirose, K., and Igarashi, Y., Re-construction and updating our understanding on
the global weapons tests 137Cs fallout, Journal of Environmental Monitoring, 8, 431-438, 2006.
Aoyama, M., Fukasawa, M., Hirose, K., Mantoura, R.F.C., Povinec, P.P., Kim, C.S., andKomura, K., Southern Hemisphere Ocean Tracer Study (SHOTS): An overview andpreliminary results, INTERNATIONAL CONFERENCE ON ISOTOPES AND
ENVIRONMENTAL STUDIES. Eds. P. Povinec, J.A. Sanchez-Cabeza, Vol. 8 ofRadioactivity in the Environment, Elsevier, London, 2006.
Inoue, H.Y., Matsueda, H., Igarashi, Y., Sawa, Y., Wada, A., Nemoto, K., Sartorius, H., andSchlosser, C., Seasonal and long-term variations in atmospheric CO2 and 85Kr in Tsukuba,
Central Japan, Journal of the Meteorological Society of Japan, 84/6, 959-968, 2006.
2005
Hirose, K., Aoyama, M., Igarashi, Y., and Komura, K., Extremely low backgroundmeasurements of 137Cs in seawater samples using an underground facility (Ogoya), Journal ofRadioanalytical and Nuclear Chemistry, 263, 349-353, 2005.
Povinec, P.P., Aarkrog, A., Buesseler, K.O., Delfanti, R., Hirose, K., Hong, G.H., Ito, T.,
Livingston, H.D., Nies, H., Noshkin, V.E., Shima, S., and Togawa, O., 90Sr, 137Cs and 239,
240
Pu concentration surface water time series in the Pacific and Indian Oceans- WOMARS
results, Journal of Environmental Radioactivity, 81, 63-87, 2005.
Igarashi, Y., Aoyama, M., Hirose, K., Povinec, P.P., and Yabuki, S., What anthropogenic
radionuclides (90Sr and 137Cs) in atmospheric deposition, surface soils and Aeoliandusts
suggest for dust transport over JAPAN, Water, Air, and Soil Pollution: Focus, 5, 51-69, 2005.
2004
Hirose, K., Kim, C.K., Kim, C.S., Chang, B.W., Igarashi, Y., and Aoyama, M., Wet and dry
depositions of plutonium in Daejeon, Korea, The Science of the Total Environment, 332,
243-252, 2004.
Povinec, P.P., Hirose, K., Honda, T., Ito, T., MarianScott, E., and Togawa, O., Spatial
distribution of 3H, 90Sr, 137Cs and 239, 240Pu in surface waters of the Pacific and Indian Oceans GLOMARD database, Journal of Environmental Radioactivity, 76, 113-137, 2004.
Aoyama, M., and Hirose, K., Artificial Radionuclides database in the Pacific Ocean: Ham
database, TheScientificWorldJOURNAL, 4, 200-215, 2004.
Hirota, M., Nemoto, K., Wada, A., Igarashi, Y., Aoyama, M., Matsueda, H., Hirose, K.,
Sartorius, H., Schlosser, C., Schmid, S., Weiss, W., and Fujii, K., Spatial and Temporal
Variations of Atmospheric 85Kr Observed During 1995-2001 in Japan: Estimation of
Atmospheric 85Kr inventory in the Northern Hemisphere, Journal of Radiation Research,
45, 405-413, 2004.
Hirose, K., Chemical Speciation of Thorium in Marine Biogenic Particulate matter, The
Scientific World JOURNAL 4, 67-76, 2004.
2003
Hirose, K., and Aoyama, M., Analysis of 137Cs and 239, 240Pu concentrations in surface watersof the Pacific Ocean, Deep-Sea Research II, 50, 2675-2700, 2003.
Hirose, K., Aoyama, and Povinec, P.P., Concentration of particulate plutonium in surface anddeep-water, samples collected during the IAEA’97 expedition, Deep-Sea Research II, 50,
2639-2647, 2003.

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Povinec, P.P., Livingston, H.D., Shima, S., Aoyama, M., Gastaud, J., Goroncy, I., Hirose, K.,
Huynh-Ngoc, L., Ikeuchi, Y., Ito, T., Rosa, J.L., Kwong, L.L.W., Lee, S.H., Moriya, H.,
Mulsow, S., Oregioni, B., Pettersson, H., and Togawa, O., IAEA’97 expedition to the NW
Pacific Ocean-Results of oceanographic and radionuclide investigations of the water column,
Deep-Sea Research II, 50,2607-2637, 2003.
4. Ito, T., Povinec, P.P., Togawa, O., and Hirose, K., Temporal and Spatial variation ofanthropogenic radionuclides in seawater of the Japan Sea, Deep-Sea Research II, 50,
2701-2711, 2003.
5. Tsumune, D., Aoyama, M., and Hirose, K., Behavior of 137Cs and concentrations in the North
Pacific in an ocean general circulation model, Journal of Geophysical Research, 108, C8, 3262,
2003.
6. Hirose, K., and Aoyama, M., Present Background Levels of Surface 137Cs and 239, 240Pu
Concentrations in the Pacific, Journal of Environmental Radioactivity, 69(1-2), 53-60, 2003.
7. Tsumune, D., M. Aoyama, and K. Hirose, Numerical simulation of 137Cs and 239, 240Pu
concentrations by an ocean general circulation model, Journal of Environmental Radioactivity,69(1-2), 61-84, 2003.
8. Aoyama, M., and Hirose, K., Temporal variation of 137Cs water column inventory in the North
Pacific since the 1960s, Journal of Environmental Radioactivity, 69(1-2),107-117, 2003.
9. Hirose, K., Igarashi, Y., Aoyama, M., Kim, C.K., Kim, C.S., Chang, B.W., Recent trends ofplutonium fallout observed in Japan: plutonium as a proxy for desertification, J. Environ.
Monit, 5, 302-307, 2003.
10. Igarashi, Y., Aoyama, M., Hirose, K., Miyao, T., Nemoto, K., Tomita, M., and Fujikawa, T.,
Resuspension: Decadal Monitoring Time Series of the Anthropogenic Radioactivity
Deposition in Japan, Journal of Radiation Research, 44, 319-328, 2003.
11. Kim, C.K., Kim, C.S., Chang, B.U., Choi, S.W., Chung, C.S., Hong, G.H., Hirose, K., andIgarashi, Y., Plutonium isotopes in seas around the Korean Peninsula, The Science of the Total
Environment, 318, 197-209, 2003.
12. Kim, C.K., Kim, C.S., Chang, B.U., Choi, S.W., Chung, C.S., Hong, G.H., Hirose, K., andPettersson, H.B.L., 240Pu /239Pu atom ratios in the bottom sediments of the NW Pacific
Ocean, Journal of Radioanalytical and Nuclear Chemistry, 258(2), 265-268, 2003.
13. Lee, H.N., Tanaka, T., Chiba, M., and Igarashi, Y., Long range transport of Asian dust fromdust storms and its impact on Japan, Water, Air, and Soil Pollution, 3, 231-243, 2003
14. Hirose, K., Implication of POC/234Th ratios in oceanic particulate matter: an approach toparticle aggregation, Papers of Meteorology and Geophysics, 53, 4, 109-118, 2003.
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2002
Hirose, K., Miyao, T., Aoyama, M., and Igarashi, Y., Plutonium isotopes in the Sea of Japan,
Journal of Radioanalytical and Nuclear Chemistry, 252, 293-299, 2002.
Hirose, K., and Aoyama, M., Chemical speciation of plutonium in seawater, Analytical andBioanalytical Chemistry, 372, 418-420, 2002.
Hong, G.H., Kim, Y.I., Lee, S.H., Cooper, L.W., Choe, S.M., Tkalin, A.V., Lee, T., Kim, S.H.,
Chung, C.S., and Hirose, K., 239+240Pu and 137Cs concentrations for zooplankton and nekton in
the Northwest Pacific and Antarctic Oceans 1993-1996, Marine Pollution Bulletin, 44,
660-665, 2002.
2001
Igarashi, Y., Aoyama, M., Nemoto, K., Hirose, K., Miyao, T., Fushimi, K., Suzuki, M., Yasui,
S., Asai, Y., Aoki, I., Fujii, K., Yamamoto, S., Sartorius, H., and Weiss, W., 85Kr measurement
system for continuous monitoring at Meteorological Research Institute, Japan, Journal ofEnvironmental Monitoring, 3, 688-696, 2001.
Igarashi, Y., Aoyama, M., Hirose, K., Miyao, T., and Yabuki, S., Is it Possible to use 90Sr and137
Cs as tracers for the aeolian transport?, Water. Air. And Soil Pollution, 130, 349-354, 2001
Hirose, K., Igarashi, Y., Aoyama, M., and Miyao, T., Long-term trends of plutonium fallout
observed in Japan, In Plutonium in the Environment, 251-266, 2001.

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Aoyama, M., Hirose, K., Miyao, T., Igarashi, Y., and Povinec, P.P., 137Cs activity in surface
water in the western North Pacific, J. Radioanal. Nucl. Chem., 248, 3, 789-793, 2001.
Aoyama, M., Hirose, K., Miyao, T., Igarashi, Y., and Povinec, P.P., Temporal variation of137
Cs inventory in the western North Pacific, J Radioanal Nucl Chem, 248, 3, 785-787, 2001
Hirose, K., Aoyama, M., Miyao, T., and Igarashi, Y., Plutonium in seawaters on the western
North Pacific, J Radioanal Nucl Chem, 248, 3, 771-776, 2001.
Tsumune. D., Aoyama, M., Hirose, K., Maruyama, K., and Nakashiki, N., Caluculation of
Artificial Radionuclides in the Ocean by an Ocean General Circulation Model, J Radioanal
Nucl Chem, 248, 3, 777-783, 2001.

2000
Shiraishi, K., Kimura, S., Yonehara, H., Takada, J., Ishikawa, M., Igarashi, Y., Aoyama, M.,
Komura, K., and Nakazima, T., Survey of external dose around the JCO facility using sugar
samples and ESR method, Adv. ESR Appl, 16, 9-14, 2000.
Komura, K, Yamamoto, M., Muroyama, T., Murata, Y., Nakanishi, T., Hoshi, M., Takada, J.,
Ishikawa, M., Kitagawa, K., Suga, S., Endo, A., Tozaki, N., Mitsugashira, T., Hara, M.,
Hashimoto, T., Takano, M., Yanagawa, Y., Tsuboi, T., Ichimasa, M., Ichimasa, Y., Imura, H.,
Sasajima, E., Seki, R., Saito, Y., Kondo, M., Kojima, S., Muramatsu, Y., Yoshida, S., Shibata,S., Yonehara, H., Watanabe, Y., Kimura, S., Shiraishi, K., Bannai, T., Sahoo, S.K., Igarashi,
Y., Aoyama, M., Hirose, K., Uehiro, M., Doi, T., and Matsuzawa, T., The JCO criticality
accident at Tokai-mura, Japan: an overview of the sampling campaign and preliminary results,
J. Environ. Radioactivity, 50, 3-14, 2000.
Igarashi, Y., Miyao, T., Aoyama, M., Hirose, K., Sartorius, H., and Weiss, W., Radioactive
noble gases in the surface air monitored at MRI, Tsukuba, before and after the JCO accident, J.
Environ. Radioactivity, 50, 107-118, 2000.
Miyao, T., Hirose, K., Aoyama, M., and Igarashi, Y., Trace of the recent deep water formationin the Japan Sea deduced from historical 137Cs data, Geophys. Res. Lett., 27, 22, 3731-3734,
2000.
Aoyama, M., Hirose, K., Miyao, T., and Igarashi, Y., Low level 137Cs measurements in deep
seawater samples, Appl. Radiat. Isot, 53, 159-162, 2000.
Igarashi, Y., Sarutorius, H., Miyao, T., Weiss, W., Fushimi, K., Aoyama, M., Hirose, K., andInoue, H.Y., 85Kr and 133Xe monitoring at MRI, Tsukuba and its importance, J. Environ.
Radioactivity, 48, 191-202, 2000.
1999
Hirose, K., Amano, H., Baxter, M.S., Chaykovskaya, E., Chumichev, V.B., Hong, G.H., Isogai,
K., Kim, C.K., Kim, S.H., Miyao, T., Morimoto. T, Nikitin, A., Oda, K., Pettersson, H.B.L.,
Povinec, P.P., Seto, Y., Tkalin, A., Togawa, O., and Veletova, N.K., Anthropogenic
radionuclides in seawater in the East Sea/Japan Sea: Results of the first-stage
Japanese-Korean-Russian expedition, J. Environ. Radioactivity, 43, 1-13, 1999.
Igarashi, Y., Aoyama, M., Miyao, T., Hirose, K., Komura, K., and Yamamoto, M., Airconcentration of radiocaesium in Tsukuba, Japan following the release from the Tokai waste
treatment plant: comparisons of observations with predictions, Applied Radiation and Isotopes,
50, 1063-1073, 1999.
Igarashi, Y., Aoyama, M., Miyao, T., Hirose, K., and Tomita, M., Anomalous 90Sr deposition
during the fall, 1995 at MRI, Tsukuba, Japan, J. Radioanal. Nucl. Chem, 239(3), 539-542,
1999.
Ikeuchi, Y., Amano, H., Aoyama, M., Berezhnov, V.I., Chaykovskaya, E., Chumichev, V.B.,
Chung, C.S., Gastaud, J., Hirose, K., Hong, G.H., Kim, C.K., Kim, S.H., Miyao, T., Morimoto,T., Nikitin, A., Oda, K., Pettersson, H.B.L., Povinec, P.P., Tkalin, A., Togawa, O., andVeletova, N.K., Anthropogenic radionuclides in seawater of the Far Eastern Seas, Sci. Total
Environ., 237/238, 203-212, 1999.

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1.

1.
2.
3.

1.

2.

Pettersson, H.B.L., Amano, H., Berezhnov, V.I., Chaykovskaya, E., Chumichev, V.B., Chung,C.S., Gastaud, J., Hirose, K., Hong, G.H., Kim, C. K., Lee, S.H., Morimoto, T., Nikitin, A.,
Oda, K., Povinec, P.P., Suzuki, E., Tkalin, A., Togawa, O., Veletova, N.K., Volkov, Y., andYoshida, K., Anthropogenic radionuclides in sediments in the NW Pacific Ocean and its
marginal seas:results of the 1994-1995 Japanese-Korean-Russian expeditions, Sci. Total
Environ, 237/238, 213-224, 1999.

1998
Igarashi, Y., Hirose, K., and Otsuji-Hatori, M., Beryllium-7 Deposition and Its Relation to
Sulfate Deposition, J. Atmos. Chem, 29, 217-231, 1998.
Miyao, T., Hirose, K., Aoyama, M., and Igarashi, Y., Temporal variation of 137Cs and 239, 240Pu
in the sea of Japan, J. Environ. Radioactivity, 40, 239-250, 1998.
1997
Hirose, K., Complexation-scavenging of plutonium in the ocean, Radioprotection - Colloq, 32,
C2-225 - C2-230, 1997.
1996
Igarashi, Y., Otsuji-Hattori, M., and Hirose, K., Recent deposition of 90Sr and 137Cs observedin Tsukuba, J. Environ. Radioactivity, 31, 157-169, 1996.
Otsuji-Hatori. M., Igarashi, Y., and Hirose, K., Preparation of a Reference Fall out Material for
Activity Measurements, J. Environ Radioactivity, 31, 2, 143-155, 1966.
1995
Aoyama, M., and Hirose, K., The temporal and spatial variation of 137Cs concentration in thewestern North Pacific and its marginal seas during the period from 1979 to 1988, J. Environ.
Radioactivity, 29, 57-74, 1995.
Hirose, K., Geochemical studies on the Chernobyl radioactivity in environmental samples, J.
Radioanal. Nucl. Chem., Articles, 197, 315-335, 1995.
1994
Hirose, K., Takatani, S., and Aoyama, M., Deposition of 90Sr and plutonium isotopes derived
from the Chernobyl accident in Japan, J. Radioanal. Nucl. Chem., Articles, 182, 349-358,
1994.
1993
Hirose, K., and Sugimura, Y., Chemical speciation of particulate 238U, 239, 240Pu and Th
isotopes in seawater, Sci. Total Environ., 130/131, 517-524, 1993.
Hirose, K., Takatani, S., and Aoyama, M., Wet deposition of radionuclides derived from the
Chernobyl accident, J. Atmos. Chem., 17, 61-71, 1993.
Shiraishi, K., Igarashi, Y., Yamamoto, M., and Nakajima, T., Concentrations of thorium anduranium in freshwater samples collected in the former USSR, J. Radioanal. Nucl. Chem.,
185,157-165, 1993.
1992
Aoyama, M., Hirose, K., and Takatani, S., Particle size dependent dry deposition velocity of
the Chernobyl radioactivity, In Precipitation Scavenging and Atmosphere - Surface
Exchange.volume 3, The 22e Summers Volume: Application and Appraisals. Coordinated by
S.E. Schwartz and W.G.N. Slinn. Hemisphere Publishing Corporation, Washington,
1581-1593, 1992.
Hirose, K., Sugimura, Y., and Aoyama, M., Plutonium and 137Cs in the western North Pacific:
estimation of residence time of plutonium in surface water, Appl. Radiat. Isot, 43, 349-359,
1992.

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2.

1.
2.
3.
4.

1991
Aoyama, M., Hirose, K., and Sugimura, Y., The temporal variation of stratospheric fallout
derived from the Chernobyl accident, J. Environ. Radioactivity, 13, 103-115, 1991.
Hirose, K., and Sugimura, Y., Chemical speciation of particulate uranium in seawater, J.
Radioanal. Nucl. Chem., Articles, 149, 83-96, 1991.
1990
Hirose, K., and Sugimura, Y., Plutonium isotopes in the surface air in Japan: effect ofChernobyl accident, J. Radioanal. Nucl. Chem., Articles, 138, 127-138, 1990.
Hirose, K., Aoyama, M., and Sugimura, Y., Plutonium and cesium isotopes in river waters inJapan, J. Radioanal. Nucl. Chem., Articles, 141, 191-202, 1990.
Hirose, K., Aoyama, M., and Sugimura, Y., Short and long term effects of Chernobyl
radioactivity on deposition and air concentrations in Japan, IAEA-SM 306/129, 141-149,
1990.
1988
Aoyama, M., Evidence of stratospheric fallout of caesium isotopes from Chernobyl accident,
Geophys. Res. Lett., 15, 327-330, 1988.
Miyake, Y., Saruhashi, K., Sugimura, Y., Kanazawa, T., and Hirose, K., Contents of 137Cs,
plutonium and americium isotopes in the southern ocean waters, Pap. Met. Geophys, 39,
95-113, 1988.
1987
Aoyama, M., Hirose, K., and Sugimura, Y., Deposition of gamma-emitting nuclides in Japanafter the reactor-IV accident at Chernobyl, J. Radioanal. Nucl. Chem., Articles, 116, 291-306,
1987.
Hirose, K., Aoyama, M., Katsuragi, Y., and Sugimura, Y., Annual deposition of Sr-90, Cs-137
and Pu-239, 240 from the 1961-1980 nuclear explosions: a simple model, J. Meteor. Soc.
Japan, 65, 259-277, 1987.
1986
Aoyama, M., Hirose, K., Suzuki, Y., and Sugimura, Y., High level radioactive nuclides inJapan in May, Nature, 321, 819-820, 1986.
Hirose, K., Sugimura, Y., and Katsuragi, Y., 90Sr and 239+240Pu in the surface air in Japan: theirconcentrations and size distributions, Pap. Met. Geophys, 37, 255-269, 1986.
Katsuragi, Y., and Aoyama, M., Seasonal variation of Sr-90 fallout in Japan through the end of
1983, Pap. Met. Geophys., 37, 15-36, 1986.
Suzuki, Y., Inoue, H., Katsuragi, Y., and Sugimura, Y., The distribution of 85Krin the air over
the North and South Pacific Ocean Mem., Natl. Inst. Polar Res., Spec. Issue, 40, 462-466,
1986.

1985
1. Hirose, K., and Sugimura, Y., A new method of plutonium speciation in seawater, J. Radioanal.
Nucl. Chem, Articles, 92, 363-369, 1985.
1.
2.

1.
2.

1984
Hirose, K., and Sugimura, Y., Plutonium in the surface air in Japan, Health Phys., 46,
1281-1285, 1984.
Inoue, H., Katsuragi, Y., and Shigehara, K., Tritiated water vapor in the surface air at Tokyo,
Pap. Met. Geophys, 35, 11-20, 1984.
1983
Katsuragi, Y., A study of 90Sr fallout in Japan, Pap. Met. Geophys., 33, 277-291, 1983.
Katsuragi, Y., Kawamura, K., and Inoue, H., Tritium fallout in Tokyo, Pap. Met. Geophys, 34,
21-30, 1983.
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1.
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3.
4.
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2.
3.

4.
5.

1982
Inoue, H., and Katsuragi, Y., A study of tritium fallout in Japan, Pap. Met. Geophys, 32, 21-28,
1982.
Katsuragi, Y., Hirose, K., and Sugimura, Y., A study of plutonium fallout in Japan, Pap. Met.
Geophys, 33, 85-93, 1982.
1981
Hirose, K., and Sugimura, Y., Concentration of Uranium and the activity ratio of 234U /238U insurface air-effect of atmospheric burn-up of cosmos-954, Pap. Met. Geophys., 32, 317-322,
1981.
Hirose, K., and Sugimura, Y., Plutonium content of river water in Japan, Pap. Met. Geophys,32, 301-305, 1981.
Sugimura, Y., and M. Mayeda, An improved method of analysis of uranium in sea water using
chelating resin, Pap. Met. Geophys, 32, 167-171, 1981.
1980
Sugimura, Y., The uranium content and the activity ratio 234U /238U in sea water in the Pacificocean, Isotope Marine Chemistry, 211-246, 1980.
1979
Sugiura, Y., Saruhashi, K., and Miyake, Y., Evaluation on the disposal of radioactive wastes
into the North Pacific. - the effect of steady flow and up - welling, Proc. The 3rd NEA
Seminar on Marine Radioecology, 1979.
1978
Miyake, Y., Nuclear weapons and radioactive pollution of the earth's environment, In, Proc.
Internal Symp. On the damage and after-effects of the atomic bombing of Hiroshima andNagasaki July 21-Aug., 9, 1977, Tokyo, 164-188, 1978.
1976
Miyake, Y., and Saruhashi, K., A critical study on the IAEA definition of high level
radioactive waste unsuitable for dumping at sea, Pap. Met. Geophys. 27, 75-80, 1976.
Miyake, Y., and Saruhashi, K., Disposal of radioactive waste into deep seas, J. Radiat. Res., 17,
42-43, 1976.
Miyake, Y., and Sugimura, Y., The plutonium content in the Pacific Ocean waters, Proc. of
Symp. on Transuranium Nuclides in the Environment, San Francisco, 17-21 Nov. 1975,
USERDA and IAEA, IAEA-SM-199/22, 91-105,1976.
Sugimura, Y., Symposium II plutonium in environment, II-1.Distribution and behavior ofplutonium in the global environments, J. Radiat. Res., 17, 4-5, 1976.
Sugiura, Y., Saruhashi, K., and Miyake, Y., Evaluation on the disposal of radioactive wastes
into the North Pacific, Pap. Met. Geophys, 27, 81-87, 1976.
1975
Miyake, Y., Shimada, T., Kawamura, K., Sugimura, Y., Shigehara, K., and Saruhashi, K.,
Distribution of tritium in the Pacific ocean, Rec. Oceanogr. Works Japan, 13, 17, 1975.
Miyake, Y., Katsuragi, Y., and Sugimura, Y., Plutonium fallout in Tokyo, Pap. Met. Geophys,
26, 1-8, 1975.
Miyake, Y., Sugimura, Y., and Hirao, Y., Uranium, thorium and potassium contents in granitic
and basaltic rocks in Japan, In, The Natural Radiation Environment II, II, Proc. 2nd Intnl.
Symp. On Natural Radiation Environment, Aug. 7-11, 1972, Houston, Texas, USA, ed. J. A. S.
Adams, W. M. Lowder, T. F. Gesell, 535-558,1975.
Saruhashi, K., I-1 The global radioactive contamination due to nuclear weapon testings, J.
Radiat. Res., 16, 47, 1975.
Saruhashi, K., Katsuragi, Y., Kanazawa, T., Sugimura, Y., and Miyake, Y., 90Sr and 137Cs in
the Pacific waters, Rec. Oceanogr. Works Japan, 13, 1-15, 1975.

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2.
3.
4.

1.

1973
Miyake, Y., and Saruhashi, K., The radio-carbon model of the ocean, Pap. Met. Geophys, 24,
263-271, 1973.
Miyake, Y., Saruhashi, K., and Sugimura, Y., The isotopic ratio 234U /238U in sea water and its
bearings on the isotopic ratio in river waters, Oceanogr. Works Japan, 12, 23-25, 1973.
Miyake, Y., Sugimura, Y., and Saruhashi, K., Content of plutonium in river water in Japan,
Pap. Met. Geophys, 24, 75-78, 1973.
Miyake, Y., Sugimura, Y., and Yasujima, T., Thorium isotope content in river water in Japan,
Pap. Met. Geophys, 24, 67-73, 1973.
1972
Miyake, Y., Sugimura, Y., and Uchida, T., A new method of spectrophotometric determinationof uranium in sea water and uranium content with 234U /238U ratio in the Pacific water, Rec.
Oceanogr. Works Japan, 11, 53-63, 1972.

2.

1970
Miyake, Y., Katsuragi, Y., and Sugimura, Y., A study on plutonium fallout, J. Geophys. Res.,75, 2329-2330, 1970.
Miyake, Y., Sugimura, Y., and Mayeda, M., The uranium content and the activity ratio 234U
/238U in marine organisms and sea water in the western North Pacific, J. Oceanogr. Soc. Japan,
26, 123-129, 1970.

1.

1969
Miyake, Y., Radioactive contamination of the ocean, Bull. Jpn. Soc. Fisheries Oceanogr., Nov.,
1969, 1969.

1.

1.
2.

1.
2.
3.
1.
2.

1.
2.
3.

1968
Miyake, Y., and Sugimura, Y., Plutonium content in the western North Pacific waters, Pap.
Met. Geophys, 19, 481-485, 1968.
Miyake, Y., Katsuragi, Y., and Sugimura, Y., Deposition of plutonium in Tokyo through theend of 1966, Pap. Met. Geophys, 19, 267-276, 1968.
1967
Miyake, Y., and Kanazawa, T., Atmospheric ozone and radioactive fallout, Pap. Met.
Geophys., 18, 311-326, 1967.
Miyake, Y., Radioactive contamination of the ocean, J. Radiat. Res., 8, 1, 1967.
Miyake, Y., Sea, radioactivity in, International Dictionary of Geophysics, 1-7, 1967.
1966
Miyake, Y., and Saruhashi, K., On the radio-carbon age of the ocean waters, Pap. Met.
Geophys., 17, 218-223, 1966.
Miyake, Y., and Sugimura, Y., Ratio 234U /238U and the uranium concentration in seawater inthe western North Pacific, J. Geophys. Res., 71, 3083-3087, 1966.
1965
Kuroda, P. K., Miyake, Y., and Nemoto, J., Strontium isotopes Global circulation after the
Chinese nuclear explosion of 14 May 1965, Science, 150, 1289-1290, 1965.
Miyake, Y., Saruhashi, K., Katsuragi K., and Kanazawa, T., Radioactivity of dust and rain the ratio of Cs-137 and Sr-90 in the radioactive fallout, Proc. Internl. Conf. On CloudPhysics,Tokyo, IAMAP, 395-399, 1965.
Park, K., George, M.J., Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T.,
Strontium-90 and caesium-137 in Columbia river plume, July 1964, Nature, 208, 1084-1085,
1965.

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6.
7.
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2.
3.
4.
5.
6.

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2.
3.
4.

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2.

1.
2.

1964
Miyake, Y., and Sugimura, Y., Uranium and radium in the western North Pacific waters,
Studies on Oceanography, Prof. K. Hidaka Sixtieth Birthday Commemorative Volume, Univ.
of Tokyo Press, Tokyo, 274-278, 1964.
Miyake, Y., and Ohtsuka, Y., Beryllium-7 in rain water, Pap. Met. Geophys, 15, 89-92, 1964.
Miyake, Y., A sequential procedure for the radiochemical analysis of marine material, Annex
to safety series 11."Method of surveying and monitoring marine radioactivity, IAEA, Vienna,
26, 1964.
Miyake, Y., Atomic weapons and the pollution of the sea, The East, 3, 64-68, 1964.
Miyake, Y., Saruhashi, K., Katsuragi, Y., Kanazawa, T., and Sugimura, Y., Uranium, radium,
thorium, Ionium, strontium 90 and cesium 137 in coastal waters of Japan, In, Recent
Researches in the Fields of Hydrosphere, Atmosphere and Nuclear Geochemistry, publishedby Editorial Committee of Sugawara Festival Volume, Maruzen Co. Ltd., 127-141, 1964.
Miyake, Y., Sugimura, Y., and Tsubota, H., Content of uranium, radium, and thorium in riverwaters in Japan, In, The Natural Radiation Environment, ed. By J. A. S. Adams and W. M.
Lowder, Univ. of Chicago Press, Chicago, Rice Univ. Semicentennial Series, 219-225, 1964.
Sugimura, Y., Torii, T., and Murata, S., Uranium distribution in drake passage waters, Nature,
204, 464-465, 1964.
1963
Folsom, T.R., and Saruhashi, K., A comparison of analytical techniques used for determinationof fallout cesium in sea water for oceanographic purpose, J. Radiat. Res., 4, 39-53, 1963.
Miyake, Y., and Tsubota, H., Estimation of the direct contribution of meteoric water to riverwaters by means of fall-out radio-cesium and radiostrontium, "Radioisotopes in Hydrology"
Proc. IAEA Symp., Tokyo. IAEA, Vienna, 425-431, 1963.
Miyake, Y., Artificial radioactivity in the Pacific ocean, IUGG Monograph, 20 Radioactive
traces in oceanography, 1963.
Miyake, Y., Saruhashi, K., Katsuragi, K., and Kanazawa, T., Deposition of Cs-137 and Sr-90
in Tokyo through the end of July 1963, Pap. Met. Geophys, 13, 180-181, 1963.
Miyake, Y., Saruhashi, K., Katsuragi, Y., Kanazawa, T., and Tsunogai, S., Deposition of 90Sr
and 137Cs in Tokyo through the end of July 1963, Pap. Met. Geophys, 14, 58-65, 1963.
Miyake, Y., Sugiura, Y., and Tsubota, H., II. Contents of uranium, radium, and thorium inriver waters in Japan, The Natural Radiation Environment, RICE Univ. semicentennial Pub,
1963.
1962
Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., Seasonal variation of radioactive
fallout, J. Geophys. Res., 67, 189-193, 1962.
Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., Penetration of 90Sr and 137Cs indeep layers of the Pacific and vertical diffusion rate of deep water, J. Radiat. Res., 3, 141-147,
1962.
Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., The peak in radioactive fallout inthe temperate zone of the northern hemisphere, J. Radiat. Res., 3,148-152, 1962.
Sugimura, Y., and Sugimura, T., Uranium in recent Japanese sediments, Nature, 194, 568-569,
1962.
1961
Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., Cesium 137 and strontium 90 insea water, J. Radiat. Res., 2 25-28, 1961.
Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., Cesium 137 and strontium 90 insea water, Pap. Met. Geophys., 12, 85-88, 1961.
1960
Miyake, Y., and Saruhashi, K., Vertical and horizontal mixing rates of radioactive material in
the ocean, Disposal of Radioactive wastes IAEA Vienna, 167-173, 1960.
Miyake, Y., and Katsuragi, Y., Strontium 90 in western North Pacific surface waters, Pap. Met.
Geophys., 11, 188-190, 1960.
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4.
5.
6.
7.

Miyake, Y., Saruhashi, K., Katsuragi, Y., and Kanazawa, T., Radioactive fallout in Japan andits bearings on meteorological conditions, Pap. Met. Geophys., 11, 151-158, 1960.

1959
Miyake, Y., Saruhashi, K., and Katsuragi, Y., The Sr-90 fallout and the air motion, Pap. Met.
Geophys., 9, 172-176, 1959.
Miyake, Y., Special committee on oceanic research, Working Group on Radioactivity in the
Ocean, 1959.
1958
Miyake, Y., and Saruhashi, K., Distribution of man-made radio-activity in the North Pacificthrough summer 1955, J. Mar. Res., 17, 383-389, 1958.
Miyake, Y., and Sugiura, Y., The method of measurement of radioactivity in sea water, Pap.
Met. Geophys., 9, 48-50, 1958.
Miyake, Y., Hazards to human health of radioactive dust, Radio Japan, 2, 3-5, 1958.
Miyake, Y., The distribution of artificial radioactivity in the equatorial region in the Pacific in
the summer of 1956, The Proc. 9th Pacific Science Congress, 16, 227, 1958.
1957
Miyake, Y., and Saruhashi, K., The world-wide strontium 90 deposition during the period from1951 to the fall of 1955, Pap. Met. Geophys., 8, 241-243, 1957.
Miyake, Y., The biological effects of nuclear tests - warning by Japanese scientists, RadioJapan, 1, 3-5, 1957.
Miyake, Y., XI. Methods for the measurement of radioactivity in sea water, Annuals ofInternational Geophysical Year, 5, 1957.
Miyake, Y., XII. Radioactivity as a tracer of air motions in the atmosphere, Symp. On
radioactivity, IGY, Utrecht, 360, 1957.
Miyake, Y., Sugiura, Y., Saruhashi, K., and Kanazawa, T., The estimation of the amount of
Sr-90 deposition and the external infinite gamma does in Japan due to man-made radioactivity,
Pap. Met. Geophys., 8, 222-231, 1957.
1956
Miyake, Y., On the distribution of radioactivity in the North Pacific ocean in 1954-1955,
Internl. Conf. For Peaceful Uses of Atomic Energy in Geneva, August 1955, 13, pp.381-384,
1956.
Miyake, Y., Radioactivity in rain water and air, Proc. Internl. Conf. For Peaceful Uses ofAtomic Energy in Geneva, August 1955, 13, 345-349, 1956.
Sugiura, Y., and Kanazawa, T., On the radioactive fallout collected in Tokyo on 26th
November, 1955, Pap. Met. Geophys., 7, 128-135, 1956.
1955
Miyake, Y., and Sugiura, Y., Radiochemical analysis of radio-nuclides in sea water collected
near BIKINI atoll, Pap. Met. Geophys., 6, 90-92, 1955.
Miyake, Y., Effects of atomic explosions on the atmosphere and sea, The research in theeffects and influences of the nuclear bomb text explosions, 1-6, 1955.
Miyake, Y., Kigoshi, K., and Saruhashi, K., Radiochemical analysis of fission products
contained in the soil collected at Tokyo, May, 1954, Pap. Met. Geophys., 6, 1, 93-94, 1955.
Miyake, Y., The artificial radioactivity in rain water observed in Japan, 1954-1955, Researchin the Effects and Influences of the Nuclear Bomb Test Explosions Science, 151-159, 1955.
Miyake, Y., The artificial radioactivity in rain water observed in Japan, from autumn 1954 tospring 1955, Pap. Met. Geophys., 6, 26-32, 1955.
Miyake, Y., Sugiura, Y., and Kameda, K., On the artificial radioactivity in the sea near Japan,
Pap. Met. Geophys., 6, 90-92, 1955.
Miyake, Y., Sugiura, Y., and Kameda, K., On the distribution of radioactivity in the sea around
Bikini atoll in June, 1954, Pap. Met. Geophys., 5, 253-262, 1955.
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Miyake, Y., Sugiura, Y., and Kameda, K., Research in the effects and influence of the nuclear
bomb test explosions, Soc. For Promotion of Science, 415-417, 1955.

1954
Miyake, Y., Kigoshi, K., Sugiura, Y., and Saruhashi, K., A study on the productivity in coastal
waters by means of the radio-carbon, Pap. Met. Geophys., 5, 89-94, 1954.
Miyake, Y., The artificial radioactivity in rain water observed in Japan from May to August,
1954, Pap. Met. Geophys, 5, 173-177, 1954.

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7. Publication list (in Japanese) 1954-2015

論文リスト（和文誌）1954-2015- 40 -

1.
2.

2015
足立光司, 電子顕微鏡がとらえた放射性粒子, 地球化学, 49,185‒193, 2015.
木村徹, 五十嵐康人,「大気降下物試料の放射能分析の品質管理」，Proceedings of the 16th
Workshop Environmental Radioactivity. KEK Proceedings 2015-4 , 23-27, 2015.

1.

2014
青山道夫, 東電福島原発事故後の海洋での放射能汚染の推移 （With English Abstract）.
YAKUGAKU ZASSHI, Journal of the Pharmaceutical Society of Japan. 2014.

1.

2013
青山道夫, 極低レベル放射能測定：少量表層海水および極深層海水試料への応用.放射線,39(1), 17-20, 2013.

1.
2.
3.
4.

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2.

1.
2.
3.

1.
2.

1.
2.

2012
青山道夫, 福島第一原子力発電所から放出された放射性セシウム同位体の北太平洋における総量と分布, 日本原子力学会誌, 54, 780-783, 2012.
青山道夫, 五十嵐康人, 1957 年から始まる気象研究所における環境放射能研究の歴史, 放射線, 38(3), 123-128, 2012.
青山道夫, 海洋に放出された放射性物質の長期地球規模での挙動, 検証！福島第一原発事故-放射性物質の実際と科学者たちの活動の記録（化学 4 月号別冊）. 4, 33-38, 2012.
青山道夫, 五十嵐康人, 広瀬勝己, 月間降下物測定 660 ヶ月が教えること-90Sr, 137Cs および Pu 降下量 1957 年 4 月～2012 年 3 月-. 科学, 82(4), 0442-0457, 2012.
2011
青山道夫, 福島原子力発電所事故による海洋汚染. 理大科学フォーラム, 11(通 巻 329 号),
42-45, 2011.
津旨大輔, 坪野考樹, 青山道夫, 廣瀬勝巳, 福島第一原子力発電所から漏洩した 137Cs の海
洋拡散シミュレーション(With English Abstract). 電力中央研究所報告書, V11002, PP.18,
2011.
2009
五十嵐康人, 放射性同位体分析, In 岩坂泰信, 西川雅高, 山田丸, 洪天祥（編）, 黄砂, 古今書院, pp81-87, 東京, 2009.
Igarashi, Y., Anthropogenic Radioactivity in Aerosol -A Review Focusing on Studies during the
2000s-, 保健物理, 44(3), 2009.
猪股弥生, 五十嵐康人, 千葉長, 青山道夫, 廣瀬勝己, 再現期間を用いた降水時における高ガンマ線線量率の異常値判定. Radioisotopes, 58, 37-42, 2009.
2007
三浦誓也, 貝森優希, 工藤英嗣, 野呂幸男, 時枝隆之, 廣瀬勝己, 青森県における大気中クリプトン-85 濃度調査, 青森県原子力センター所報, 1, 43-44, 2007.
廣瀬勝己, 青山道夫, Kim、S.A. Kim, 太平洋の海水中のプルトニウムの挙動について, 日本
放射化学会誌 別冊, 8 supplement, 103, 2007.
2006
青山道夫, 大気と海洋深層における核実験起源フォールアウトの超低レベル放射能測定,
ADIOISOTOPES, 55, 429-438, 2006.
小村和久, 青山道夫, 秋山正和, 氏平祐輔, 大田裕二, 大塚良仁, 大西勝基, 櫻井敬久, 篠原
伸夫, Ge 検出器の使用状況に関するアンケート報告, RADIOISOTOPES, 55, 699-707,2006.

- 41 -

1.
2.

1.

1.

1.
2.
3.

1.
2.

1.

2004
廣瀬勝己, 化学トレーサーを利用した海洋変動の研究, 測候時報第 71 巻特別号, 71,
S81-S86, 2004.
五十嵐康人, 廣瀬勝己, 放射性降下物標準試料の調製を通じた 90Sr, 137Cs,プルトニウムなどの人工放射性核種分析の相互比較と参照値の決定, 放射化分析, No. 17, 39-42, 2004.
90

五十嵐康人, Sr と
2002.

2002
Cs を用いたダスト輸送過程と再飛散, エアロゾル研究, 12, 17(4), 252,

137

1986
鈴木款, 井上久幸, 杉村行勇, 大気中の放射性クリプトンの分布, 地球化学, 20, 106-108,
1986.
1978
葛城幸雄, 人工放射性物質, 天気, 25, 第 11 号, 20-23, 1978.
猿橋勝子, 海洋中の放射性物質を指標とした物質の移動・拡散, 水汚染の機構と解析－環境
科学持論－, 日本地球化学編, 東京, 産業図書, 271-287, 1978.
三宅泰雄, 猿橋勝子, 杉村行勇, 葛城幸雄, 金沢照子, 核兵器と地球環境の放射能汚染, 被爆の実相と被爆者の実情 －1977 年 NGO 被爆問題シンポジウム報告書, ISDA－ JNPC 編集,
朝日イブニングニュース社発行, 182-212, 1978.
1976
葛城幸雄, 日本における放射性降下物, 天気, 23, No. 7, 333-345, 1976.
杉村行雄, 環境のなかのプルトニウム, 自然, 3 月号, 1976.
1975
三宅泰雄, 島田利夫, 川村清, 重原好次, 地上大気のトリチウム含量と気象との関係, 日本
地球化学会 “環境問題特別号”, 131-135, 1975.

1.

1974
三宅泰雄, 葛城幸雄, 最近の Sr 降下物の起源, 天気, 21, 639-644, 1974.

1.

1971
葛城幸雄, 人工放射能による大気汚染, 気象研究ノート, 107, 258-282, 1971

1.

1970
三宅泰雄, 葛城幸雄, 最近の日本における放射性塵の降下, 天気, 17, 593-598, 1970.

1.
2.

90

1965
葛城幸雄, 日本における Cs-137 および Sr-90 降下について(I), 天気, 12, 323-328, 1965.
葛城幸雄, 日本における Cs-137 および Sr-90 降下について(II) "成層圏における人工放射性物質の滞留時間の推定", 天気, 12, 377-384, 1965.
1964

1.

三宅泰雄, 葛城幸雄, 金沢照子, 放射性降下物の現状と将来, 科学, 34, 142-148, 1964.

- 42 -

1.

1963
三宅泰雄, フォールアウトの気象学の側面, 天気, 9, No. 1, 1962.

1962
1. 三宅泰雄, フォールアウトの気象学的側面, 天気, 9, 1-6, 1962.
2. 三宅泰雄, 猿橋勝子, 放射能による水の汚染, 水利科学, 23, 1-14, 1962.

1.

1.

1.
2.
3.
4.

1.
2.
3.

1958
三宅泰雄, 猿橋勝子, 放射化学と海洋, 科学, 28, 510-513, 1958.
1956
三宅泰雄, 杉浦吉雄, 葛城幸雄, 1955 年 4 月旭川地方に降った放射性の落下塵,気象集誌, 第2 号, 34, 226-230, 1956.
1955
三宅泰雄, 放射能雨の性格, 測候時報, 22, 196-202, 1955.
三宅泰雄, 日本に降った人工放射性雨・雪[II]9 月～12 月, 天文と気象, 21, 1-8, 1955.
三宅泰雄, 杉浦吉雄, 亀田和久, 日本近海の海水放射能について, 昭和 29 年度海洋資源開発
調査報告（黒潮班）, 81-82, 1955.
三宅泰雄, 放射能雨の性格, 測候時報, 22, No. 7, 196-202, 1955.
1954
三宅泰雄, 日本に降った人工放射性雨(1954 年 5-7 月), 天文と気象, 20, 1-8, 1954.
三宅泰雄, 杉浦吉雄,亀田和久, ビキニ海域における人工放射能の分布とその海洋学的考察"
ビキニ灰とそれによる傷害 第三", 科学, 24, 601-605, 1954.
三宅泰雄, 有住直介, 磯野謙治, 田島英三, ビキニの灰とそれによる傷害（つづき）日本に降った放射能雨, 科学, 24, No. 8, 405-409, 1954.

- 43 -

年代・雑誌名不明
1.
2.
3.
4.

三宅泰雄, 放射能とその作用, 遺伝,（1975？）
三宅泰雄, 第 II 編 環境の放射能汚染と化学分析,（1970？）
三宅泰雄, 猿橋勝子, 杉村行勇, 海洋における放射性核種, 海洋化学講座, 第 6 巻, 海洋無機
化学, 第 4 章, 109-170,（1969？）
三宅泰雄, 田島英三, 日米放射能会議, 8-18,（1954？）- 44 -

8. History of the studies at the Meteorological Research Institute

表：研究の歴史- 45 -

研究の歴史
放射能調査研究費以前および、放射能調査研究費での研究
西暦

年度

1954

昭和29年

1955

S30

北太平洋観測（日・米・加）

1956

S31

北太平洋赤道海域観測（日・米・仏）

1957

S32

国際地球観測年事業費

1958

S33

国際地球観測年事業費

1959

予算項目

予算項目
課題名

課題名

課題名

課題名

課題名

放射化学分析(落下
放射能調査研究費 塵・降水・海水中の放射性物質の研究)

深海水の循環に関する研究(以下深海水）S34

放射能調査研究費 放射化学分析

深海水

1960

S35

国際原子力機関委託研究 放射能調査研究費 放射化学分析

深海水

1961

S36

国際原子力機関委託研究 放射能調査研究費 放射化学分析

深海水

1962

S37

国際インド洋観測（日・米・放射能調査研究費 放射化学分析
ソ・英・仏）

深海水

1963

S38

国際インド洋観測（日・米・放射能調査研究費 放射化学分析
ソ・英・仏）

深海水

1964

S39

オレゴン州立大学との共同研究

放射能調査研究費 放射化学分析

深海水

1965

S40

放射能調査研究費 放射化学分析

海洋中の放射性廃棄物のモニタリングの測定法
深海水に関する研究（以下海洋廃棄物モニタリング）

海水中における放射廃棄物の化学的挙動の研究(以下廃棄物化学的挙動）1966

S41

放射能調査研究費 放射化学分析

海洋廃棄物モニタリング 深海水

廃棄物化学的挙動

1967

S42

放射能調査研究費 放射化学分析

海洋廃棄物モニタリング 深海水

廃棄物化学的挙動

1968

S43

放射能調査研究費 放射化学分析

海洋廃棄物モニタリング 深海水

廃棄物化学的挙動

1969

S44

放射能調査研究費 放射化学分析

深海水

廃棄物化学的挙動

1970

S45

放射能調査研究費 放射化学分析

深海水

廃棄物化学的挙動

1971

S46

放射能調査研究費 放射化学分析

深海水

廃棄物化学的挙動

課題名課題名

放射能調査研究費以前および、放射能調査研究費での研究
西暦

年度

予算項目

予算項目
課題名

課題名

課題名

課題名

課題名

1972

S47

放射能調査研究費 放射化学分析

放射性固体廃棄物の海洋処分に伴う鉛直拡散に関する研究(以下廃棄物鉛直
拡散）

1973

S48

放射能調査研究費 放射化学分析

廃棄物鉛直拡散課題名課題名

大気中におけるKr-85
およびH-3挙動と蓄積
廃棄物鉛直拡散に関する調査研究(以下Kr-85／H-3）
廃棄物鉛直拡散Kr-85／H-3
廃棄物鉛直拡散Kr-85／H-3数種の放射性核種 原子力施設に由来するの同時測定による深 放射性気体の広域分
海拡散の研究(以下 布に関する調査研究(以下放射性気体）
深海拡散）

1974

S49

放射能調査研究費 放射化学分析

1975
1976

S50
S51

放射能調査研究費 放射化学分析
放射能調査研究費 放射化学分析

1977

S52

放射能調査研究費 放射化学分析

1978

S53

放射能調査研究費 放射化学分析

深海拡散

放射性気体

環境における超ウラン元素の分布と挙動に関する研究（以下超ウラン元素）

1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991

S54
S55
S56
S57
S58
S59
S60
S61
S62
S63
H1
H2
H3

放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費
放射能調査研究費

深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散
深海拡散

放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体
放射性気体

超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素超ウラン元素1992

H4

放射能調査研究費 放射化学分析

海洋における放射性核種の挙動に関する調査研究（以下海洋放射性
核種）

放射性気体

超ウラン元素

1993
1994
1995

H5
H6
H7

放射能調査研究費 放射化学分析
放射能調査研究費 放射化学分析
放射能調査研究費 放射化学分析

海洋放射性核種
海洋放射性核種
海洋放射性核種

放射性気体
放射性気体
放射性気体

超ウラン元素超ウラン元素超ウラン元素放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析
放射化学分析

放射能調査研究費以前および、放射能調査研究費での研究
西暦

年度

予算項目

予算項目
課題名

課題名

課題名

課題名

課題名

課題名

海洋環境における放射性核種の挙動に関する研究（以下海洋環境放
射性核種）

1996

H8

大気圏の放射性核種の動態に関する
放射能調査研究費
研究（以下大気圏
放射性核種）

1997

H9

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

1998

H10

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

1999

H11

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

2000

H12

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

2001

H13

大気圏の放射性核 海洋環境における放射種の長期的動態に 性核種の長期挙動に関
放射能調査研究費
関する研究(以下大 する研究(以下海洋環境
気圏放射性核種） 放射性核種）

大気中の放射性気体の実態把握に関する研究(以下放射性
気体)

2002

H14

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

放射性気体

2003

H15

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

放射性気体

2004

H16

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

放射性気体

2005

H17

放射能調査研究費 大気圏放射性核種 海洋環境放射性核種

放射性気体

2006

H18

放射性降下物の長 海洋環境における放射期変動と再浮遊に 性核種の長期挙動に関
放射能調査研究費
関する研究（以下放 する研究(以下海洋環境
放射性核種）
射性降下物）

2007

H19

放射能調査研究費 放射性降下物

海洋環境放射性核種

2008

H20

放射能調査研究費 放射性降下物

海洋環境放射性核種

2009

H21

放射能調査研究費 放射性降下物

海洋環境放射性核種

2010

H22

放射能調査研究費 放射性降下物

海洋環境放射性核種課題名

放射能調査研究費以前および、放射能調査研究費での研究
西暦

年度

予算項目

予算項目
課題名

2011

H23(８月から）「大気を通じた人工
放射性核種の陸圏・水圏への沈着
放射能調査研究費 およびその後の移行過程の解明研究（以下陸圏水圏人工放射性核種）」

2012

H24

放射能調査研究費

2013

H25

2014

H26

2015

H27(4月から）

陸圏水圏人工放射性核種
陸圏水圏人工放射
放射能調査研究費
性核種
陸圏水圏人工放射
放射能調査研究費性核種「人工放射性核種のバックグラウンド
大気監視と数値解
放射能調査研究費
析に関する研究（以下ＢＧ放射性核
種）」

課題名

課題名

課題名

課題名

課題名

課題名

表紙の図説明2015 年 8 月までの 90Sr および 137Cs 月間降下量
福島第一原発事故後の大気の放射能汚染の実態把握を継続しています。2011 年 3 月の事故以降に確定できた90Sr および137Cs 月間降下量を対数軸で誤差と併せてプロットしています。
134

Cs は事故直後には 137Cs とほぼ等量降下していますから、放射性セシウム全体ではこのプロットのほぼ倍量となります。ただし、90Sr、137Cs はどちらもおよそ 30 年の半減期で減衰しますが、134Cs はおよそ 2 年で半減します。図示した誤差は計測の統計誤差で、１シグマです。測定誤差は本来全ての測定値につき表示すべきですが、グラフが見づらくなるため、従来はあえて表示してきませんでした。また、数十年前のデータについては誤差データが伝えられていません。
福島第一原発事故以前に採取され、分析や前処理途上だった試料については、事故の汚染によって実験室環境および測定室環境、測定機器のバックグラウンドなどが大幅に上昇したため、蒸発濃縮工程においても試料の汚染（コンタミネーションと言います）の問題が発生します。そのため、観測値を求めること自体が困難になっていましたが、試料を汚染の水準の低い関西にて分析しこの問題の回避に努めました。しかし、本文中にあるように、細心の注意と努力にもかかわらず、2010 年の後半および 2011 年初のデータは欠測となってしまいました。
徐々に実験環境の除染や測定機器の入れ替えなどを実施して事故以前のきれいな実験環境を追求し、データを求めるように努めています。また、IAEA が実施した分析相互比較に参画するなどし、その精度の維持管理に努めていますので、現状のデータの信頼性は担保できていると考えています。

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A caption of the cover art
Monthly depositions of 90Sr and 137Cs before August 2015.
We have been monitoring the deposition amounts for the purpose of understanding the actual conditionof radioactive pollution after the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP). The
monthly deposition amounts of both 90Sr and 137Cs, which were successfully fixed values after theaccident in March 2011, are shown in logarithmic axis with error widths. Since the deposition amounts of134

Cs were nearly equal to those of 137Cs, the total amounts of radioactive cesium were approximately twotimes of the values in this figure. We would like to note that the half-lives of 90Sr and 137Cs areapproximately 30 years, however, 134Cs decays to half in about 2 years. The error widths are one sigma instatistical error of measurements. Although the error widths of all measurement values were better to beshown, error values used to be not shown on purpose to avoid busy figure in the former versions. In
addition, error data before several decades are not handed down.
For the analysis of samples collected before the accident of FDNPP and not measured then, significant
increase of background values were severe problem, because environments including laboratories andinstruments were polluted, and it was also problem that samples could have been polluted in theoperations of concentration or other preprocesses (contamination problem). We overcame the difficulty bycarrying the analysis out in western Japan (Kansai area) where the pollution level was low, however, thedata during the latter part of 2010 and the former part of 2011 were lacked in spite of our efforts as
described in the main text.
We are making efforts to recover clean working environment as before the accident, by cleaning of theenvironments, exchanging instruments, etc. for the purpose of acquire the correct data. In addition, we
maintain the precision of the data, for example, by participating inter-comparison programs performed by
IAEA, so that, the credibility of the data are assured (please refer to the section: quality control ofradioactive analysis of atmospheric deposition samples ).

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注：本報告書は一般配布を目的として
2016

年 8 月 1 日に作成された。