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 ((131)I, (132)I, (133)I) and radiocesium ((134)Cs, (136)Cs, (137)Cs). This radioiodine consisted of gaseous and particulate forms; the percentage of particulate (131)I in the total (131)I ranged from 0 to 86%. The percentage of the particulate (131)I to the total (131)I 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 (131)I-bearing particles was 0.7 µm, while those of (134)Cs- and (137)Cs-bearing particles were larger than 1 µm. Large variations of ratios of (131)I/(137)Cs, (132)Te/(137)Cs, and (99)Mo ((99m)Tc)/(137)Cs (all involving different elements) suggest that the behaviors of these radionuclides in the atmosphere, including the processes of their emission, differed each other.