Anthropogenic radionuclides in the atmosphere observed at Tsukuba: characteristics of the radionuclides derived from Fukushima

During a serious accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), a huge quantity of radionuclides was released into the atmosphere and ocean. We measured anthropogenic radionuclides in surface air at Tsukuba, about 170 km from the FDNPP. On March 15, 2011, we detected the radioactivity released from the Fukushima accident in air samples at Tsukuba. The major radionuclides that we observed were radioiodine (131I, 132I, 133I) and radiocesium (134Cs, 136Cs, 137Cs). This radioiodine consisted of gaseous and particulate forms; the percentage of particulate 131I in the total 131I ranged from 0 to 86%. The percentage of the particulate 131I to the total 131I increased on the arrival of the plumes from major emissions of the FDNPP. After activities of the radionuclides attained the maximum on March 15, 2011, the FDNPP-derived radionuclides decreased rapidly in surface air. The activity median aerodynamic diameter of 131I-bearing particles was 0.7 μm, while those of 134Cs- and 137Cs-bearing particles were larger than 1 μm. Large variations of ratios of 131I/137Cs, 132Te/137Cs, and 99Mo (99mTc)/137Cs (all involving different elements) suggest that the behaviors of these radionuclides in the atmosphere, including the processes of their emission, differed each other.

► Surface air concentrations of Fukushima-derived radionuclides at Tsukuba were measured.
► Short-lived radionuclides (133I, 131I, 132mTe and others) as well as long-lived radionuclides were detected.
► The portion of particulate 131I to sum of gaseous and particulate 131I increased at arrival of radioactive plume.
► The 137Cs bearing particles were a kind of hot particles with particle size of >1 μm
► Decay-corrected activity ratios between isotopes such as 133I/131I were constant.