Summary of atmospheric measurements of radioactive materials released by the Fukushima Dai-ichi Nuclear Power Plant accident and their transport pathways

Continual monitoring of radioactive nuclides in the atmosphere after the Fukushima Dai-Ichi Nuclear Power Plant (FD1NPP) accident is needed to evaluate internal exposure dose, to estimate the emission inventory of radionuclides released by the accident in the atmosphere, and to validate results of numerical simulation by atmospheric dispersion/transport models for transport and deposition of atmospheric radioactivity. In this report, by acquisition of those data in the Kanto region south of Fukushima prefecture, we summarize what we have found for dynamic behavior of atmospheric radioactivity in a regional scale. In the Fukushima area, short-time sampling (10-20 min.) of radioactive materials in the atmosphere by dust monitor once or twice a day was conducted at more than ten sites to detect direct effect of the accident near the FD1NPP by the Environmental Radioactivity Monitoring Center of Fukushima and the Ministry of Education, Culture, Sports, Science and Technology, and in the site of FD1NPP by the Tokyo Electric Power Company. Continuous monitoring was independently carried out at several stations in the Kanto area. A sampling duration among these stations was in a wide range from one hour to two days with most of sampling duration of one day. The sampling method was different between the stations, and a low-volume or high-volume air sampler was used to collect atmospheric aerosols with different types of filters. In spite of these differences, at most stations, high radioactive concentrations of I-131, Cs-134, and Cs-137 were measured in a period of 15-16 March, and of 20-23 March 2011. The concentration of I-131 was always higher than that of Cs-137, and the ratio of I-131 to Cs-137 was much lower in the high radioactivity periods than in the other periods, at all the stations. Day-to-day variations in the ratio were much higher than the spatial variations among the stations which could be attributed to the day-to-day difference in the release rate of radioactive nuclides from FD1NPP into the atmosphere. According to the data at two stations where I-131 was separately measured in particulate phase (I-131a) and gas phase (I-131g), time series of the ratio (R) of I-131a to the sum of I-131a and I-131g was similar between two stations, while the filters used in the sampling were different and these two stations are located at a large distance of 65km. The value of R at both stations was 0.5-0.8 and 0.2-0.4, in high and low radioactivity periods, respectively. The release rate of I-131 may be underestimated without any consideration of R, although further investigation is needed to consider what happened in the nuclear reactors of the FD1NPP, and gas to particle conversion in the atmosphere. Possible transport pathways of polluted air masses from the FD1NPP to the stations where high atmospheric radioactivity was observed were also studied. A numerical simulation was performed by an atmospheric transport model, and the high radioactivity periods showed good agreement at most stations between the model result and the observational data.