Results and Discussion

Preliminary Source Term Estimation

The release rates and total amounts of 131I and 137Cs discharged into the atmosphere were estimated for the period from 12 March to 5 April 2011 in the preliminary source term estimation [1]. Then, total release amounts were estimated by the time integration of release rates. Figure 15.3 shows the temporal variation in release rates of 131I and 137Cs. The results of source term estimation are as follows. Here, the first release rate estimated from the dust sampling data on the morning of 15 March at JCAC, Chiba-city, is assumed to continue from the occurrence of accidental release from Unit 1, 10:00 JST on 12 March to 23:00 JST on 14 March. The significant release, 1016 Bq h−1 of 131I, occurred on 15 March. Concerning the duration for the release rate of 1016 Bq h−1 of 131I in the daytime of March 15, which is critical to the total release amounts, several trial simulations of atmospheric dispersion were carried out with various release duration. The results indicated that the plume released during the period from about 12:00 to 15:00 JST March 15 flowed northwestward and resulted in high dose rates from wet deposition in the nighttime of that day. From 16 March, the release rates of 131I were estimated to be rather constant, on the order of 1014 Bq h−1, until 24 March. The release rates had decreased with small day-to-day variations to the order of 1011 to 1012 Bq h−1 of 131I in the beginning of April. The release rate of 137Cs was derived based on the radioactivity ratios of 131I to 137Cs from dust sampling data and had a similar trend with fluctuations of the 131I/137Cs ratio in the range from 1 to 100. The total amounts of 131I and 137Cs discharged into the atmosphere from 12 March to 6 April were estimated to be approximately 1.5 × 1017 and 1.3 × 1016 Bq, respectively. On 12 April 2011, the NSC announced that the total amounts of 131I and 137Cs discharged into the atmosphere in the same period were 1.5 × 1017 Bq and 1.2 × 1016 Bq as preliminary values [ 39], based on our research. The slight difference of the total amount of 137Cs from this chapter results from the revision of the activity ratio of 131I and 137Cs based on additional environmental data.

Fig. 15.3 Temporal variation in the preliminary estimated release rates of 131I and 137Cs. Solid and dashed lines show release rates of 131I and 137Cs, respectively. Japanese standard time (JST) is used

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