ABSTRACT

The earthquake off the Pacific coast of Japan and the subsequent tsunami on March 11, 2011, triggered a series of accidents in the Fukushima Daiichi Nuclear Power Plant (FNPP1). The accidents caused the release of a mixture of radioactive substances into the environment. This study measured the concentration of tritium (³H) and iodine-129 (¹²⁹I) in rainwater samples collected at Tsukuba, 170 km southwest of the plant, during the year following the accident. High ³H concentrations were observed in the rainwater samples collected within one month after the FNPP1 accident. ³H concentrations decreased steadily over time and returned to the levels before the accident. Concentrations of ¹²⁹I also decreased over time. However, pulses of high ¹²⁹I concentrations were observed at several other times following the accident. The ¹²⁹I concentrations were found to be correlated with iron concentrations in rainwater. It is likely that iron oxide, which can absorb iodate ions (IO₃^–), was the carrier of radiogenic iodine. This study concludes that ¹²⁹I and also ¹³¹I, which is one of the most harmful radionuclides produced in nuclear reactors, can be redistributed to the atmosphere in the months following the deposition of radiogenic iodine on the ground.