Dynamics of atmospheric (131)I in radioactive plumes in eastern Japan immediately after the Fukushima accident by analysing published data

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 (131)I (decay-corrected to March 11, 2011) to (137)Cs in radioactive plumes were divided into three groups (A, B, and C) by analysing all published data on atmospheric (131)I 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 (131)I concentrations could be released after water supply to FD1NPP.