Radioactive strontium-90 from the crippled Fukushima No. 1 nuclear power plant has been detected for the first time in 10 prefectures outside Miyagi and Fukushima, the science ministry said July 24.
The highest reading was in Ibaraki Prefecture and nearly matched the maximum level of strontium-90 recorded in Japan following the Chernobyl nuclear disaster. The nine other prefectures are Iwate, Akita, Yamagata, Tochigi, Gunma, Saitama, Chiba, Tokyo and Kanagawa.
But experts say the current levels of strontium-90 will have little impact on health.
"The health impact of the strontium will be much smaller than that of cesium," said Takumaro Momose, deputy head of the Radiation Protection Department at the Japan Atomic Energy Agency. "There is no need to worry as long as we keep track of radiation doses derived from cesium. There is no problem about food, either, as long as the cesium content remains within the safety standard."
The latest measurements were part of the science ministry's release of data on the concentrations of strontium-90 in monthly dust deposits collected in outdoor receptacles at measurement stations in all 47 prefectures between April 2010 and December 2011.
Data was unavailable for Miyagi Prefecture because the tsunami that triggered the nuclear accident on March 11 last year also destroyed the measurement station. In Fukushima Prefecture, the ministry’s analysis was obstructed because the measurement station is located within the no-entry zone around the Fukushima No. 1 nuclear plant.
The science ministry said soil samples taken in June last year already detected strontium attributable to the Fukushima nuclear disaster in the two prefectures.
The ministry said the strontium found in the 10 other prefectures came from the Fukushima plant because the measurements exceeded the pre-disaster maximum of 0.30 becquerel per square meter and rose in March or April last year following the onset of the accident.
According to the ministry’s measurements, a maximum concentration of 6.0 becquerels of strontium-90 per square meter was recorded in Hitachinaka, Ibaraki Prefecture, in March 2011. That is 20 times the 0.30 becquerel per square meter observed in Hokkaido in February 2006, the highest measurement taken in Japan between April 2000 and February 2011.
Strontium-90 has a long half-life of about 50 years and tends to accumulate in human bones. It can easily be absorbed by food items, readily dissolves in water and can reach depths of 50-60 centimeters in the soil.
Strontium-90 readings in Japan peaked in the 1960s, a time when many nuclear tests were taking place around the world and releasing radioactive substances into the atmosphere. A record high 358 becquerels per square meter was recorded in Sendai in 1963.
The strontium levels later dropped as the number of nuclear tests declined, but they rose temporarily to 6.1 becquerels per square meter in Akita Prefecture following the Chernobyl disaster in 1986.
The latest strontium-90 fallout was only one-625th of the combined fallout of cesium-137 and cesium-134 in Akita Prefecture, where the strontium-to-cesium ratio was the largest. That ratio was less than one-10,000th in Chiba, Tokyo and Kanagawa prefectures, the science ministry said.
Thousands of becquerels of strontium-90 per square meter were deposited across Japan during the atmospheric nuclear tests of the 1950s and 1960s, according to Shunichi Hisamatsu, director of the Department of Radioecology at the Institute for Environmental Sciences.
"Adding 6 becquerels of strontium a month on top of that is not likely to have a major impact," Hisamatsu said.
(This article was compiled from reports by Hiroshi Ishizuka, Yuri Oiwa and Akiko Okazaki.)
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