Radioactive substances spewed out by the quake-stricken Fukushima No. 1 nuclear power plant have likely spread to western Japan and Hokkaido, according to a team of Japanese, U.S. and European scientists.
Their findings were published in the Nov. 14 online edition of the Proceedings of the U.S. National Academy of Sciences.
Japan's science ministry said radioactive contamination spread only as far west as the areas near the border between Nagano and Gunma prefectures. However, it conceded there was a possibility, albeit small, of contamination west of that line.
A team of scientists, led by Teppei Yasunari, research associate for the Universities Space Research Association in Maryland, simulated the dispersion of airborne cesium-137 on a 20-kilometer grid, taking account of the weather and rainfall conditions after the March 11 disaster.
The team inputted the science ministry's observation data on cesium-137 deposition into its numerical output to obtain quantitative estimates for cesium deposits between March 20 and April 19.
Overall, the simulated distribution of contamination was in line with measurements by the science ministry.
However, the study indicated possible contamination of radioactive fallout in mountainous areas in Gifu Prefecture, the Chugoku and Shikoku regions. The simulation also showed contamination in Hokkaido.
"These deposition levels do not have immediate impact on human health and do not require decontamination," said Tetsuzo Yasunari, a professor at Nagoya University who is a co-author of the study.
Cesium concentrations in soil were far below the safety standard for banning rice cultivation in most regions.
The analysis did not include the period through March 19 because of the unavailability of data, although most of the radioactive substances are thought to have been released during that period.
"The actual deposition is likely to be larger than our latest estimate," one scientist said.
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