Tritium and iodine-129 concentrations in precipitation at Tsukuba, Japan, after the Fukushima Daiichi Nuclear Power Plant accident

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 (H-3) and iodine-129 (I-129) in rainwater samples collected at Tsukuba, 170 km southwest of the plant, during the year following the accident. High H-3 concentrations were observed in the rainwater samples collected within one month after the FNPP1 accident. H-3 concentrations decreased steadily over time and returned to the levels before the accident. Concentrations of I-129 also decreased over time. However, pulses of high I-129 concentrations were observed at several other times following the accident. The I-129 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 I-129 and also I-131, 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.