Radioactive cesium from the crippled Fukushima No. 1 nuclear plant may have reached as far as Hokkaido, Shikoku and the Chugoku region in the west, according to a recent simulation by an international research team.
Large areas of eastern and northeastern Japan were likely contaminated by the plant, with concentrations of cesium-137 exceeding 1,000 becquerels per kilogram of soil in some places, says the study, which was posted Monday on the website of the National Academy of Sciences.
Researchers for the U.S.-based organization said the study, which was based on partial data readings, is the first to estimate potential cesium contamination across the country. But they also played down the incident's impact on the three distant regions.
"The levels are not something that should raise concerns over agricultural production or human health," Ryugo Hayano, chairman of the physics department at the University of Tokyo, said in an email interview with The Japan Times.
The simulation indicated that eastern Hokkaido may have been contaminated with up to 250 becquerels per kilogram of cesium-137, while Shikoku and Chugoku were likely tainted with up to 25 becquerels .
The government's contamination limit for rice is 5,000 becquerels per kilogram for cesium-137, which has a 30-year half-life, and cesium-134, whose half-life is two years.
Although the study does not cover cesium-134, the results indicate contamination levels were under the limit for most parts of Japan. It's believed the two cesium isotopes were ejected in roughly equal amounts.
Hayano said that even before the Fukushima disaster, soil throughout Japan contained up to around 100 becquerels of cesium-137 per kilogram due to weapons tests in the Pacific and the 1986 Chernobyl accident.
But the study also confirmed that cesium contamination in eastern Fukushima Prefecture will result in extreme limitations on food production.
Food output in parts of Iwate, Miyagi, Yamagata, Niigata, Ibaraki, Tochigi and Chiba meanwhile face partial limitations because soil contamination exceeds 250 becquerels per kilogram, the simulation said.
Emphasizing that their study is not based on actual soil tests, the researchers called on the government to conduct measurements nationwide. So far, such surveys have been limited to eastern Japan, centered on Fukushima.
"The science ministry has been monitoring Fukushima and neighboring prefectures, but there are not enough data beyond such regions. We barely had contamination data covering all of Japan. That's why we relied on calculations," Tetsuzo Yasunari, a Nagoya University professor and climate system specialist, told The Japan Times Wednesday.
"The simulation's degree of accuracy is not that high. The gist of the paper is to recommend (that the government) actually survey the soil" not only in Fukushima and neighboring prefectures but across the nation, Hayano said.
The estimates were calculated by a computer simulation developed by a Norwegian research group based on ministry data compiled from daily fallout readings from March 20 to April 19 in each prefecture.
The study doesn't include fallout data on cesium-137 from before March 19."Because the science ministry didn't have daily deposition data between March 12 and 19 (a period in which the nuclear plant was racked by hydrogen explosions), the actual figure could be higher than our estimation," Yasunari said.
The science ministry doesn't have data of that crucial period because it took few days to prepare a monitoring device in each prefecture, a ministry official said. As for Fukushima and Miyagi prefectures, monitoring devices were damaged by the quake and tsunami, the official said.
There is no data for Fukushima up until March 26, and no data is available for Miyagi as of Thurday.
"But as for the western part of Japan and Hokkaido, I don't think the given figure would increase" even if data from the March 12-19 period — when the hydrogen explosions likely spread a great deal of fallout — were to be added, Yasunari said.
Yasunari also pointed to the need for a more detailed radiation survey in mountainous areas, as fallout from the Fukushima plant is more likely to have accumulated in ranges than in flatland.
The science ministry said the study is useful as a reference, and it will consider expanding its aerial monitoring to wider areas after completing the current monitoring in 22 prefectures from Aomori to Aichi.