Fukushima radiation spread to residential areas hours before venting (original article in Japanese below)

(Source) http://mainichi.jp/english/english/newsselect/news/20130222p2a00m0na009000c.html

Radioactive material from the damaged Fukushima No. 1 Nuclear Power Plant spread to residential areas hours before workers vented the containment vessel of the plant’s No. 1 reactor on March 12, 2011, to release pressure, it has emerged.
In one area, the level of radiation had surged to more than 700 times the normal level, indicating that many local residents were exposed to high levels of radiation before they evacuated.
The Fukushima Prefectural Government operated 25 monitoring posts around the nuclear power plant before it was crippled by the March 11, 2011 Great East Japan Earthquake and tsunami. Five monitoring posts were swept away by the tsunami, and 20 couldn’t send data because the quake caused power cuts. Accordingly, officials were unable to put the data to use when evacuating residents.
Over the period up until September last year, the prefectural government collected and analyzed data from the 20 monitoring posts that survived the disaster. Results of its analysis were published on the prefectural government’s website and the prefecture notified local bodies. However, it was not revealed that radiation had spread before the plant’s operator, Tokyo Electric Power Co. (TEPCO), commenced venting operations — and neither the Diet nor the government’s nuclear accident investigation committees were aware of the fact.
Workers are believed to have first tried opening a vent at the plant at 10:17 a.m. on March 12, 2011. TEPCO reported success after a fourth venting operation at about 2:30 p.m. the same day.
However, data at four monitoring posts in the Koriyama, Yamada, Kamihatori and Shinzan districts in the Fukushima Prefecture town of Futaba indicated that radiation levels had risen hours before TEPCO starting opening the vents.
Radiation dosages in the four areas before the disaster ranged between 0.04 and 0.05 microsieverts per hour, but as of 5 a.m. the level in the Koriyama district, located about 2.5 kilometers north of the plant, had swelled to 0.48 microsieverts per hour and at 6 a.m. it stood at 2.94 microsieverts per hour. By 9 a.m., roughly one hour before officials started opening the vent, the hourly radiation level had surged to 7.8 microsieverts. In the Yamada district 5.5 kilometers west of the power plant, the radiation level at 10 a.m. had increased to 32.47 microsieverts per hour — roughly 720 times the normal figure.
The average radiation dosage permitted by the government under normal conditions works out at 0.23 microsieverts per hour. The data obtained from the monitoring posts shows that radiation levels shot up rapidly over a short period of time. Officials believe that the radiation levels were affected by changes in the wind direction.
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The Fukushima Prefectural Government ordered residents within a two-kilometer radius of the plant to evacuate at 8:50 p.m. on March 11. At 9:23 p.m. the central government expanded the scope of the evacuation zone to three kilometers around the plant. The following morning at 5:44 a.m. the central government ordered residents within a 10 kilometer radius to evacuate on the presumption that reactor vents would be opened. However, it was not until 8 a.m. on March 12 that many of the roughly 50,000 residents within the zone started evacuating. It is believed that radiation spread over a wide area after the fourth venting operation, as well as after a hydrogen explosion at 3:36 p.m. Thirty minutes after the fourth venting operation, the radiation level in the Kamihatori district stood at 1,591 microsieverts.
[ .....]
The prefectural government did not finish analyzing data from the monitoring posts until after the government and Diet compiled their final reports on the Fukushima disaster, and the data is not reflected in heath checks currently performed on Fukushima Prefectural residents.
“If the prefectural government was thinking firstly about the health of its residents, then it would have considered the data vital information that needed to be analyzed quickly,” said Reiko Hachisuka, who represented Fukushima Prefecture at the Diet’s Fukushima accident investigation committee. “As a prefectural resident, I find the Fukushima Prefectural Government’s response shameful.”
February 22, 2013(Mainichi Japan)

毎日新聞　2013年02月22日　02時30分（最終更新　02月22日　02時35分）

２０１１年３月１２日の福島第１原発周辺で観測された空間放射線量推移


東日本大震災による東京電力福島第１原発事故で、１１年３月１２日に１号機格納容器の水蒸気を外部に放出する「ベント」を始める約５時間前から、放 射性物質が約１０キロ圏に拡散していたことがわかった。福島県の放射線モニタリングポストに蓄積されていた観測データの解析で判明した。放射線量が通常の ７００倍超に達していた地点もあり、避難前の住民が高線量にさらされていた実態が初めて裏づけられた。
県が原発周辺に設置していたモニタリングポストは２５基。５基が津波で流され、２０基は地震による電源喪失でデータ送信できず、事故当時、住民の避 難に活用することはできなかった。県は昨年９月下旬までに２０基の蓄積データを回収し解析。県のホームページに解析結果を掲載し、関係自治体に連絡した。 しかし、ベント前に放射性物質が拡散していたことは周知されておらず、国会と政府の原発事故調査委員会も把握していなかった。
最初のベントは３月１２日午前１０時１７分に試みられ、４回目の同日午後２時半ごろに「成功した」とされる。しかし、観測データによると、主に双葉 町の▽郡山地区▽山田地区▽上羽鳥地区▽新山地区−−の４地点でベント前に放射線量が上昇していた。震災前の線量は毎時０．０４〜０．０５マイクロシーベ ルトだったが、原発の北２．５キロの郡山地区では３月１２日午前５時に０．４８マイクロシーベルト、同６時に２．９４マイクロシーベルトと上昇。さらにベ ント開始約１時間前の同９時には７．８マイクロシーベルトになった。西５．５キロの山田地区ではベント直前の同１０時に３２．４７マイクロシーベルトと通 常の約７２０倍を記録した。
国の平時の被ばく許容線量は毎時に換算すると０．２３マイクロシーベルトで、各地で瞬間的に上回ったことになる。数値の変動は風向きの変化によると みられる。国会事故調の最終報告書などによると、１号機では１１日夜から１２日未明にかけて、全電源喪失を原因として炉心溶融（メルトダウン）が発生。圧 力容器などが損傷し、放射性物質が外部に漏出したと推定されている。http://mainichi.jp/select/news/20130222k0000m040136000c.html