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Tuesday, Oct. 25, 2011
Kashiwa's hot spot just one of many to come, expert says
By MIZUHO AOKI
The hot spot discovered in Kashiwa, Chiba Prefecture, has local residents alarmed now that the science ministry has confirmed the source of the radiation is probably fallout from the Fukushima No. 1 power plant.
But another radiation expert warns that there are more hot spots to come.
Masahiro Fukushi warned citizens Monday that more hot spots can be found where rainwater accumulates, like near the ditch in Kashiwa, and urged them to go out and take readings of such places in their neighborhoods on their own, instead of waiting around for the government's plodding surveys.
Contamination in much of Kashiwa is far higher than other parts of the Tokyo metropolitan area, so the mini hot spot really wasn't much of a surprise, said Fukushi, a professor of radiation science at Tokyo Metropolitan University.
"If the (cesium) detected was 100 times higher than the amount measured by the science ministry, then it'd be strange. But in this case, it's just four or five times, so you should not be surprised," Fukushi said.
According to the science ministry, it is highly likely that cesium in rainwater condensed in the soil after leaking with it from the broken ditch.
The soil at the hot spot had a high 276,000 becquerels of cesium per kilogram, the Kashiwa Municipal Government said. This is four to five times higher than the level surrounding the hot spot and many other places in Kashiwa, he said. The condensation process will allow this level to be attained in any place where rainwater accumulates in a limited area, Fukushi said.
Typical examples are side ditches, openings near downspouts and soil under evergreen trees, Fukushi warned.
"As we now have the knowledge of where we can find hot spots, such as areas under downspouts, we should work together to monitor such places," he said. "I think this is where citizen volunteer efforts must come into play."
Fukushi also said that Kashiwa's residents should not worry too much about the hot spot. The highest radiation reading at the site was 15 microsieverts per hour, which is unlikely to harm anyone because most people will be unlikely to stand around the site for extended periods, Fukushi said.
"Even if a person walked through the site on every day (since March 11), the total exposure dose should not be a cause for fear," said Fukushi, who visited the hot spot last week.
According to Fukushi, residents in Tokyo, west of Kashiwa, should be less worried about hot spots because the contamination levels in Tokyo are much lower.
For example, one hot spot was found at the end of a downspout at an elementary school in Adachi Ward, but the reading was only 3.99 microsieverts per hour.
According to aerial monitoring surveys conducted by the science ministry since September, some areas in Kashiwa contain 60,000 to 100,000 becquerels per kilogram of radioactive cesium.
The highest exposure level recorded in the aerial survey was between 0.2 to 0.5 microsieverts per hour.