以下、読売新聞が最新版『Nature』の記事を報じました。
＊＊＊＊＊読売新聞（２０１１／１０・２７）＊＊＊＊＊
http://www.yomiuri.co.jp/science/news/20111027-OYT1T00072.htm
放射性物質放出量、政府推計の２倍か
東京電力福島第一原発事故の初期に放出された放射性物質セシウム１３７は約３万５０００テラ・ベクレルに上り、日本政府の推計の２倍を超える可能性があるとの試算を、北欧の研究者らがまとめた。
英科学誌「ネイチャー」が２５日の電子版で伝えた。世界の核実験監視網で観測した放射性物質のデータなどから放出量を逆算。太平洋上空に流れた量を多く見積もっている。
（2011年10月27日03時11分 読売新聞）
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『Nature』のもとの記事に当たっておきます。
重要と思われる部分は、下線を引きましたが、もっとも重要なことは、
「The latest analysis also presents evidence that xenon-133 began to vent from Fukushima Daiichi immediately after the quake, and before the tsunami swamped the area. This implies that even without the devastating flood, the earthquake alone was sufficient to cause damage at the plant. 」
「地震の直後に放射性キセノンが漏れていた証拠があり、津波が来るまでもなく、地震自体がプラントにダメージを来たし放射性漏れを起こしていた」ことを示しているということです。
上記の読売新聞記事では、この最も大事な部分が残念ながらネット版では報じられていません。（紙面では報じていたかもしれません。ネット版では、あえて誰かが消したのでしょうか？東京新聞では、紙面では報じていました。）
多くの人が誤解していると思います。
誤解：福島原発は地震には耐えたが、津波により、電源喪失が起こり、事故に至ったと。
真実は、福島原発は、津波の前に、すでに地震で壊れていました。
まずは、この真実を真摯に受け止め、きちんとした事故分析をすべきであると考えます。
＊＊＊＊＊＊＊＊＊＊＊Nature(25 October 2011)＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
http://www.nature.com/news/2011/111025/full/478435a.html
Published online 25 October 2011 | Nature 478, 435-436 (2011) | doi:10.1038/478435a
Fallout forensics hike radiation toll
Global data on Fukushima challenge Japanese estimates.
Geoff Brumfiel
The disaster at the Fukushima Daiichi nuclear plant in March released far more radiation than the Japanese government has claimed. So concludes a study1 that combines radioactivity data from across the globe to estimate the scale and fate of emissions from the shattered plant.
The study also suggests that, contrary to government claims, pools used to store spent nuclear fuel played a significant part in the release of the long-lived environmental contaminant caesium-137, which could have been prevented by prompt action. The analysis has been posted online for open peer review by the journal Atmospheric Chemistry and Physics.
Andreas Stohl, an atmospheric scientist with the Norwegian Institute for Air Research in Kjeller, who led the research, believes that the analysis is the most comprehensive effort yet to understand how much radiation was released from Fukushima Daiichi. "It's a very valuable contribution," says Lars-Erik De Geer, an atmospheric modeller with the Swedish Defense Research Agency in Stockholm, who was not involved with the study.
The reconstruction relies on data from dozens of radiation monitoring stations in Japan and around the world. Many are part of a global network to watch for tests of nuclear weapons that is run by the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna. The scientists added data from independent stations in Canada, Japan and Europe, and then combined those with large European and American caches of global meteorological data.
Stohl cautions that the resulting model is far from perfect. Measurements were scarce in the immediate aftermath of the Fukushima accident, and some monitoring posts were too contaminated by radioactivity to provide reliable data. More importantly, exactly what happened inside the reactors ― a crucial part of understanding what they emitted ― remains a mystery that may never be solved. "If you look at the estimates for Chernobyl, you still have a large uncertainty 25 years later," says Stohl.
Nevertheless, the study provides a sweeping view of the accident. "They really took a global view and used all the data available," says De Geer.
Challenging numbers
Japanese investigators had already developed a detailed timeline of events following the 11 March earthquake that precipitated the disaster. Hours after the quake rocked the six reactors at Fukushima Daiichi, the tsunami arrived, knocking out crucial diesel back-up generators designed to cool the reactors in an emergency. Within days, the three reactors operating at the time of the accident overheated and released hydrogen gas, leading to massive explosions. Radioactive fuel recently removed from a fourth reactor was being held in a storage pool at the time of the quake, and on 14 March the pool overheated, possibly sparking fires in the building over the next few days.
But accounting for the radiation that came from the plants has proved much harder than reconstructing this chain of events. The latest report from the Japanese government, published in June, says that the plant released 1.5 × 1016 bequerels of caesium-137, an isotope with a 30-year half-life that is responsible for most of the long-term contamination from the plant2. A far larger amount of xenon-133, 1.1 × 1019 Bq, was released, according to official government estimates.
The new study challenges those numbers. On the basis of its reconstructions, the team claims that the accident released around 1.7 × 1019 Bq of xenon-133, greater than the estimated total radioactive release of 1.4 × 1019 Bq from Chernobyl. The fact that three reactors exploded in the Fukushima accident accounts for the huge xenon tally, says De Geer.
Xenon-133 does not pose serious health risks because it is not absorbed by the body or the environment. Caesium-137 fallout, however, is a much greater concern because it will linger in the environment for decades. The new model shows that Fukushima released 3.5 × 1016 Bq caesium-137, roughly twice the official government figure, and half the release from Chernobyl. The higher number is obviously worrying, says De Geer, although ongoing ground surveys are the only way to truly establish the public-health risk.
Stohl believes that the discrepancy between the team's results and those of the Japanese government can be partly explained by the larger data set used. Japanese estimates rely primarily on data from monitoring posts inside Japan3, which never recorded the large quantities of radioactivity that blew out over the Pacific Ocean, and eventually reached North America and Europe. "Taking account of the radiation that has drifted out to the Pacific is essential for getting a real picture of the size and character of the accident," says Tomoya Yamauchi, a radiation physicist at Kobe University who has been measuring radioisotope contamination in soil around Fukushima.
Stohl adds that he is sympathetic to the Japanese teams responsible for the official estimate. "They wanted to get something out quickly," he says. The differences between the two studies may seem large, notes Yukio Hayakawa, a volcanologist at Gunma University who has also modelled the accident, but uncertainties in the models mean that the estimates are actually quite similar.
The new analysis also claims that the spent fuel being stored in the unit 4 pool emitted copious quantities of caesium-137. Japanese officials have maintained that virtually no radioactivity leaked from the pool. Yet Stohl's model clearly shows that dousing the pool with water caused the plant's caesium-137 emissions to drop markedly (see 'Radiation crisis'). The finding implies that much of the fallout could have been prevented by flooding the pool earlier.
The Japanese authorities continue to maintain that the spent fuel was not a significant source of contamination, because the pool itself did not seem to suffer major damage. "I think the release from unit 4 is not important," says Masamichi Chino, a scientist with the Japanese Atomic Energy Authority in Ibaraki, who helped to develop the Japanese official estimate. But De Geer says the new analysis implicating the fuel pool "looks convincing".
The latest analysis also presents evidence that xenon-133 began to vent from Fukushima Daiichi immediately after the quake, and before the tsunami swamped the area. This implies that even without the devastating flood, the earthquake alone was sufficient to cause damage at the plant.
The Japanese government's report has already acknowledged that the shaking at Fukushima Daiichi exceeded the plant's design specifications. Anti-nuclear activists have long been concerned that the government has failed to adequately address geological hazards when licensing nuclear plants (see Nature 448, 392–393; 2007), and the whiff of xenon could prompt a major rethink of reactor safety assessments, says Yamauchi.
The model also shows that the accident could easily have had a much more devastating impact on the people of Tokyo. In the first days after the accident the wind was blowing out to sea, but on the afternoon of 14 March it turned back towards shore, bringing clouds of radioactive caesium-137 over a huge swathe of the country (see 'Radioisotope reconstruction'). Where precipitation fell, along the country's central mountain ranges and to the northwest of the plant, higher levels of radioactivity were later recorded in the soil; thankfully, the capital and other densely populated areas had dry weather. "There was a period when quite a high concentration went over Tokyo, but it didn't rain," says Stohl. "It could have been much worse."
Additional reporting by David Cyranoski and Rina Nozawa.
References
１． Stohl, A. et al. Atmos. Chem. Phys. Discuss. 11, 28319-28394 (2011). | Article |
２. www.kantei.go.jp/foreign/kan/topics/201106/iaeahoukokushoe.html
3. Chino, M. et al. J. Nucl. Sci. Technol. 48, 1129-1134 (2011). | Article | ChemPort |
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