High levels of radioactive iodine-131 were detected in soil samples collected in areas extending northwest of the Fukushima No. 1 nuclear power plant.
High levels of radioactive iodine-131 were detected in soil samples collected in areas extending northwest of the Fukushima No. 1 nuclear power plant.
The finding emerged from maps released by the science ministry on Sept. 21.
The northwesterly pattern mirrors that of high levels of radioactive cesium-137 concentrations indicated on maps released Aug. 29.
The survey was conducted from June 6 to July 8. Soil was sampled at about 2,200 locations within a radius of 100 kilometers of the Fukushima No. 1 plant, and was analyzed by the Japan Chemical Analysis Center, the University of Tokyo and other institutions.
Iodine-131 has a short half-life of eight days, meaning it decreases to only about one-2,000th of the initial amount in three months. Iodine-131 was detected at about 400 locations.
The highest concentration of 55,000 becquerels per square meter was detected at one location in Tomioka town, within 20 km of the stricken nuclear plant. Locations of high concentrations spread to the northwest, well beyond a 30-km radius.
Relatively high iodine-131 concentrations were also found south of the nuclear plant. The ratios of iodine-131 to cesium-137 in the soil were different in the south than in the northwest. This possibly indicates that the radioactive substances in the northwest and in the south were discharged from the nuclear plant at different times, ministry officials said.