Abstract
The spatio-temporal distribution of atmospheric radioiodine immediately after the Fukushima Daiichi Nuclear Power Plant (FD1NPP) accident has not yet been clarified due to very limited observed data, compared with atmospheric radiocaesium data. Here, we first revealed that the ratios of 131I (decay-corrected to March 11, 2011) to 137Cs in radioactive plumes were divided into three groups (A, B, and C) by analysing all published data on atmospheric 131I concentrations independently measured immediately after the accident in eastern Japan. Groups A and C were found regardless of whether the measurement sites were located in eastern Fukushima or Kantou areas, while group B was observed only in the eastern Kantou area. The ratios in group A were approximately equal to 10 for the plumes on March 15, March 20, and on the morning of March 21, and those in group B were approximately 75 on March 16. Their possible sources were Unit 2 and/or Unit 3. In contrast, the ratios in group C were approximately equal to 360, much higher than those of groups A and B, and were observed from the afternoon of March 21 to March 25. These high 131I concentrations could be released after water supply to FD1NPP.
Highlights
The atmospheric distributions of radioiodine in the early period after the Fukushima Daiichi Nuclear Power Plant (FD1NPP) accident have not been clarified yet, because data on atmospheric 131I concentrations measured in the early period after the accident are very limited
The polluted air masses with high 131I(a + g) concentrations measured in the eastern Fukushima prefecture during March 20–25 were guessed to be directly transported from the FD1NPP by those wind systems, though detailed model simulations should be studied to confirm this point. These results strongly suggest that the release rates of 131I and 137Cs from the FD1NPP drastically changed between the morning and afternoon of March 21, 2011, because the ratios were nearly equal to 10 in plume P9 measured at Nuclear Science Research Institute (NSRI) in the Kantou area on the morning of March 21, as previously described
The ratios of 131I(a + g)*/137Cs in the plumes arrived in the Kantou area are expected to be almost equal to the ratios at NSRI if the environmental conditions did not change significantly
Summary
The atmospheric distributions of radioiodine in the early period after the Fukushima Daiichi Nuclear Power Plant (FD1NPP) accident have not been clarified yet, because data on atmospheric 131I concentrations measured in the early period after the accident are very limited. The spatio-temporal distributions of hourly atmospheric 137Cs concentrations have been retrieved by using filter-tapes installed in suspended particulate matter (SPM) monitors operated at the air quality monitoring stations in eastern and central Japan[5,6,7,8]. By analyzing these measurement data, nine major radioactive plumes (P1-P9) with high 137Cs concentrations were identified[5] (Table S1 and Fig. S1).
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