RADIOACTIVE caesium and iodine has been deposited in Japan far from the Fukushima Daiichi nuclear plant, at levels that were considered highly contaminated after Chernobyl.
Readings taken by the Japanese science ministry, MEXT, reveal high levels of caesium-137 and iodine-131 outside the 30-kilometre evacuation zone, mostly to the north-north-west. Iodine-131, with a half-life of eight days, should disappear in a matter of weeks. The bigger worry concerns caesium-137, which has a half-life of 30 years and could pose a health threat for far longer. How serious that will be depends on where it lands, and whether remediation measures are possible.
The US Department of Energy has been surveying the area with an airborne gamma radiation detector. It reports that most of the 鈥渆levated readings鈥� are within 40 kilometres of the plant, but that 鈥渁n area of greater radiation extending north-west鈥� may be of interest to public safety officials鈥�.
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An analysis of MEXT鈥檚 data by New Scientist shows just how elevated the levels are. After the 1986 Chernobyl accident, the most highly contaminated areas were defined as those with over 1490 kilobecquerels (kBq) of caesium per square metre. Produce from soil with 550 kBq/m2 or more was destroyed.
People living within 30 kilometres of the plant have been evacuated or advised to stay indoors. Since 18 March, MEXT has repeatedly found caesium levels above 550 kBq/m2 in an area some 45 kilometres wide lying 30 to 50 kilometres north-west of the plant. The highest was 6400 kBq/m2, about 35 kilometres away, while levels reached 1816 kBq/m2 in Nihonmatsu City and 1752 kBq/m2 in the town of Kawamata, where iodine-131 levels of up to 12,560 kBq/m2 have also been recorded. 鈥淪ome of the numbers are really high,鈥� says Gerhard Proehl of the International Atomic Energy Agency.
Whether people鈥檚 health is at risk is not clear, however. Exposure depends on the local soil type, Proehl says. Sandy soil readily releases caesium, but clay binds it tightly, so contaminated clay can simply be buried. Otherwise, it depends on whether the fallout lands on farms and gardens, or forests and mountains.
With thousands of people in northern Japan made homeless by the tsunami, further evacuation of areas affected by the uncertain risk of fallout seems unlikely.