Departmental Bulletin Paper

In the afternoon on March 11,2011,a huge earthquake and tsunami which occurred in the Pacific Ocean off the coast of East Japan crippled the Fukushima I(Daiichi)Nuclear Power Plant. In the following two weeks,a large amount of radioactive materials leaked from the plant. Areas as far away as Iitate,40 kilometers northwest of the plant,got severely contaminated. Nakadori(middle third)of Fukushima Prefecture,and the northern Gunma and Tochigi were also badly contaminated. Eastern part of the Tokyo Metropolitan region and southern Iwate were also contaminated. In the evening of April 8,2011,the Nuclear Team of the Disaster Response Group of Fukushima Prefecture announced the dose rates at 1,648 schools and kindergartens in Fukushima. I found the data on the Internet,and picked one or two high numbers in each municipality,plotted them on the Google Map,and made the map public that day with the three isopleths of 8,2,and 0.5μSv/h. That was the first of the radiation contour maps. Ms.Sachiko Hagiwara beautifully stylized the map,and it was published on the Internet on April 21,2011 as the first version map. I continued to collect information,and published the revised version on June 18,2011. This version was the first to depict the contamination in northern Tochigi and Gunma,eastern Tokyo Metropolitan region,and southern Iwate. I revised the map every 3 months,and the current map is the 8th edition. Wind determined the distribution of radioactive contamination. Volcanic ashes from the volcanic eruption are transported by the wind at a high altitude of several kilometers and higher. However,radioactive materials leaked from the Fukushima I Nuclear Power Plant were transported by the wind near the ground surface. Looking at the meteorological data of that time period,wind directions at an altitude of 1 kilomete and higher cannot explain the distribution. I believe radioactive materials were carried by the surface wind whose altitude was dozens of meters off the ground at most. That is the reason why the isopleths of radiation seem to follow the ups and downs of geography,such as basins and mountain sides. The radioactive fog that passed over Minami Soma in Fukushima Prefecture at 9PM on March 12 continued over Sendai Bay and reached Onagawa in Miyagi Prefecture at 2AM on March 13. The contamination on March 15 was the worst. The radioactive fog that passed over Iwaki City in Fukushima Prefecture at 4AM reached Kanto Plain at 6AM. Since it was not raining,the fog then headed west and north until it hit the mountainous regions in Kanto and northern part of Gunma and Tochigi Prefecture, where, for the first time,it met the rain,and radioactive materials came fell to the ground with the rain. The contamination of Fukushima Nakadori also happened on that day. Radioactive materials that crossed the Abukuma Mountains came down with the snow. In the afternoon,a particularly dense radioactive fog was spewed from the nuclear plant. Carried by the wind,it went straight in the northwesterly direction,and contaminated the areas all the way to Iitate and Fukushima City,which the fog hit at 6PM. In the evening of March 20,areas near the Miyagi-Yamagata border and southern Iwate were contaminated. After that,the wind turned south,sending the fog past Mito City in Ibaraki Prefecture at 6AM on March 21,and reaching Shinjuku,Tokyo at 9AM. For 3 days from March 21 to 23,the Kanto region had intermittent rains. Moderate contamination seen in the eastern part of the Tokyo Metropolitan region is from this time period. The dates and times of contamination that I have just explained above do not coincide with the dates and times of explosions that took place at Fukushima I Nuclear Power Plant. Unit 1 exploded at 3:36PM on March 12,and Unit 3 exploded at 11:01AM on March 14. It was not at these moments that the large amount of radioactive materials leaked from the plant;the release of radioactive materials into the atmosphere coincide with sharp fluctuations in the reactor pressure.

2011年3月11日に起こった大きな地震と津波のあとしばらくして、大量の放射性物質が福島第一原発から大気中に出た。私はその汚染分布を迅速に把握して地図に表現し、インターネットで即座に公表した。放射性物質は原発から連続的定常的に放出されたのではなかった。3月12日、3月15日、それから3月20-21日に大きな放出イベントがあった。原子炉格納容器の圧力が低下あるいは上昇したときに対応する。順に1号機、2号機、3号機からの放出だった。大気中に出たセシウムは1京1000兆ベクレル（11PBq）。1986年のチェルノブイリ原発事故で出たセシウムの1/12である。しかし人口密度を考慮すると、被害は両者ほぼ同じかフクシマが3倍深刻である。芝生や森の林床だけでなく、アスファルトの上でも2011年3月に降り積もったセシウムはほとんど移動していない。そういった場所の放射線量率の自然減衰はセシウム134と137の半減期でよく説明できる。この事故で放出されたセシウムに起因するがん死の増加は5300人と見積もられる。今後50年間の福島中通りにおける放射能リスクは交通事故リスクの1/2である。