在福島核電一廠30公里半徑範圍外發現高輻射情況

在福島核電一廠30公里半徑範圍外發現高輻射情況

2011年4月9日 朝日新聞

原文位置：http://www.asahi.com/english/TKY201104080169.html

福島核電一廠發生爆炸後，大量放射性物質隨著爆炸而進到大氣之中。根據京都大學和廣島大學的專家研究團最新做的一項土壤測試之研究顯示，在福島核災發生後，其30公里半徑範圍外的一個社區的輻射水平竟比正常水平超出400倍。

根據這項研究的顯示，在福島縣飯館村（位於福島核電一廠30公里半徑範圍外的一個社區）土壤裡，接下來三個月內的輻射水平累積含量，將會遠超過中央政府規定，每年最高20毫希弗之撤離居民的標準。

目前，政府已撤離在福島核電一廠20公里半徑範圍內的居民，而在20 至30公里半徑範圍內的居民，政府則要求他們盡可能留在室內。

政府的各項研究還證明了30公里半徑範圍外的地方之輻射污染問題。他們的結果表示，核電廠所洩漏的輻射並非以同心圓的方式擴散。

研究是透過分析在泥土裡發現到的八種不同放射性物質之含量，以及在考慮到每種物質的半衰退期性後，所計算出地面所含輻射劑量之預期變化。

這項研究結果被認為比文部科學省所做的更為精確，因為文部科學省只公開兩種放射性物質的研究資料。

3月底時，京都大學核反應爐研究所的核能工程學系副教授Tetsuji Imanaka，及廣島大學的輻射物理學系副教授Satoru Endo，曾與一眾專家到飯館村進行考察。他們在村內五個地點收集5厘米深的土壤樣本。而所有採樣地點均是位於30公里半徑範圍外及連接各個村落的道路上。

他們從研究裡發現，銫137的水平大約每立方米59萬至219萬貝可（becquerels）之間。

1986年，車諾比核電廠事故發生後，前蘇聯政府撤離了所有居住在銫137含量超過55．5萬可區域內的居民。而根據國際原子能總署的一份報告顯示，當時約一萬平方公里內（大約是一個岐阜縣的面積），總數約27萬的居民被迫撤離。

但是，現時在飯館村找到的銫137之水平卻是車諾比時的四倍。如果有更多放射性物質在這殘破不堪的福島核電廠繼續洩漏出來的話，銫137的水平可能會再進一步上升。

他們也計算了，從3月15日福島核電廠爆炸後，連續三個月距離被污染地面一米的空氣之輻射水平變化。

研究發現，即使在核災發生的三個月後，這些放射性物質將會以每小時7至21微希弗的速度，從被輻射污染的泥土裡釋放到空氣之中。一個人若整整三個月都留在室外的話，他將會到曝曬在30至95毫希弗的輻射劑量之中。假設銫仍然留在土壤裡的話，一年後土地的累積輻射劑量大約是在70至220毫希弗。

但是，中央政府卻是正在考慮使用一年累積輻射總劑量為20毫希弗，來作為在政府應否發出疏散指令的其中一個指標。

而隨雨水和風，銫137有可能被移動或沖走。所以，實際的輻射累積含量有機會低於這個研究所做的預測。

另外，文部科學省所做的研究發現，被輻射污染的土壤並不是受到同心圓式的輻射污染，而是受到不規則的風向及其他因素所影響的。

文部科學省所做的土壤測試結果顯示，其他在30公里半徑範圍外的土壤，如大熊町（Okuma）和浪江町（Namie），每公斤泥土裡所含的銫137水平，都高於京都大學和廣島大學的專家團隊所得出的研究結果。

polly譯。

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原文如下：

High radiation levels found beyond 30-km radius

2011/04/09 asahi.com

A study of soil samples has revealed that as much as 400 times the normal levels of radiation could remain in communities beyond a 30-kilometer radius from the Fukushima No. 1 nuclear power plant, where explosions spewed radioactive materials into the atmosphere.

The study was conducted by a team of experts from Kyoto University and Hiroshima University.

According to the study, the accumulated amount of radiation in the soil at Iitate, Fukushima Prefecture--which is located outside of the 30-km radius--calculated over a three-month period would exceed the annual accumulated amount of 20 millisieverts that the central government is considering as a guideline for evacuating residents.

The government has asked residents living within a 20-km radius of the Fukushima No. 1 plant to evacuate and those living between a 20- to 30-km radius to remain indoors as much as possible.

Government studies have also found evidence of radiation contamination beyond the 30-km radius. The results show that radiation has not been spreading from the nuclear power plant in a concentric manner.

The predicted changes in the level of radiation at the ground surface were calculated after analyzing the amounts of eight kinds of radioactive materials found in the soil and taking into consideration the half-lives of each material.

The study results are considered more accurate than the study conducted by the science ministry, which only released information concerning two types of radioactive material.

Tetsuji Imanaka, an associate professor of nuclear engineering at the Kyoto University Research Reactor Institute, Satoru Endo, an associate professor of radiation physics at Hiroshima University, and other experts visited Iitate in late March.

They collected soil samples from five locations in the village at depths of five centimeters. All the locations were outside the 30-km radius and were by roadways in various hamlets.

The study found cesium-137 at levels between about 590,000 and 2.19 million becquerels per cubic meter.

After the Chernobyl nuclear accident in the former Soviet Union in 1986, residents who lived in areas where cesium-137 levels exceeded 555,000 becquerels were forced to move elsewhere.

According to a report by the International Atomic Energy Agency, the total land area from where residents were forced to move was about 10,000 square kilometers, about the land area of Gifu Prefecture. A total of about 270,000 residents were subject to the forced move.

The amounts of cesium-137 found in Iitate were at most four times the figure from Chernobyl.

If more radioactive materials are emitted from the crippled Fukushima plant, the level of cesium-137 could rise even further.

Calculations were also made of changes in the radiation level in the air one meter above contaminated ground over a three-month period from March 15, when an explosion occurred at the Fukushima plant.

The study found that even after three months there would be between 7 and 21 microsieverts per hour being emitted from the contaminated soil into the air.

If an individual remained outdoors for the entire three-month period, the person would be exposed to between 30 and 95 millisieverts over the period.

Assuming the cesium remains in the soil, the accumulated level of radiation after one year could be between about 70 and 220 millisieverts.

The central government is considering using an accumulated radiation exposure figure of 20 millisieverts over the course of a year as one indicator of whether an evacuation instruction should be issued.

The cesium-137 could move or be washed away by rain and wind, so there is the possibility that the actual accumulated radiation exposure figure could fall below the study's estimates.

Studies by the science ministry have found that contamination of the soil by radiation does not spread out in concentric circles, but is more irregular due to wind direction and other factors.

Soil-sample studies by the science ministry have found levels of cesium-137 per kilogram of soil in other areas beyond the 30-km radius, such as Okuma and Namie, that were higher than those in the study conducted by the experts at Kyoto and Hiroshima universities.