Estimated levels of iodine-131 at 800 locations show how radiation likely spread as a result of the disaster that occurred at the Fukushima No. 1 nuclear power plant just over two years ago, according to a map released by researchers July 1.
Because the half-life--the time needed for the amount of radioactivity to be reduced in half--of iodine-131 is as short as eight days, it becomes difficult, once several months have elapsed following a nuclear accident, to determine how iodine-131 spread in the early stages of the disaster.
Instead, the researchers, led by Yasuyuki Muramatsu, an analytical chemistry professor at Gakushuin University, and Hiroyuki Matsuzaki, an applied engineering associate professor at the University of Tokyo, have estimated the amount of iodine-131 deposited in soil based on the soil concentration of iodine-129.
An advanced method was used to calculate the amount of deposited iodine-129, which has a half-life of 15.7 million years.
Though iodine-131 levels in soil do not directly show how much radiation a person has been exposed to since the disaster, Muramatsu said their investigation is rewarding.
“The map can be used as basic data to assess radioactive doses,” Muramatsu said.
Iodine-129 is believed to have been released from the crippled plant following the nuclear meltdowns, along with iodine-131, which is said to increase the risk of thyroid cancer.
After measuring levels of iodine-129 in the soil at 400 locations within 80 kilometers of the plant to estimate levels of iodine-131, the research group made a map that includes data obtained from its latest investigation, as well as information released to the public previously.
The map’s data is as of June 14, 2011, just over three months after the Great East Japan Earthquake and tsunami on March 11, 2011, triggered the accident at the Fukushima No. 1 nuclear plant.
It shows iodine-131 levels were relatively high in areas to the northwest of the plant, as well as sites to the south of the facility.
Even within 20 km of the plant, relatively low iodine-131 levels were detected in areas north of the crippled facility.
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