The source term of the atmospheric release of 131 I and 137 Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident estimated by previous studies was validated and refined by coupling atmospheric and oceanic dispersion simulations with observed 134 Cs in seawater collected from the Pacific Ocean. By assuming the same release rate for 134 Cs and 137 Cs, the sea surface concentration of 134 Cs was calculated using the previously estimated source term and was compared with measurement data. The release rate of 137 Cs was refined to reduce underestimation of measurements, which resulted in a larger value than that previously estimated. In addition, the release rate of 131 I was refined to follow the radioactivity ratio of 137 Cs. As a result, the total amounts of 131 I and 137 Cs discharged into the atmosphere from 5 JST on March 12 to 0 JST on March 20 were estimated to be approximately 2.0 × 10 17 and 1.3 × 10 16 Bq, respectively.
Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived 129I, the 129I concentrations in seawater before and after the accident were compared.
Before the accident (2008–2009), the 129I concentrations in the western margin of the North Pacific between 32° N and 44° N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average 129I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of 129I in surface seawater, the accident-derived 129I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively.
As of October 2011, depth profiles of 129I revealed that 129I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived 129I were estimated to be (1.6–9.6) × 1012 atoms m−2 in this study area.
On the basis of the 129I data in the seawater near Fukushima, the effective dose of 129I from seafood ingestion was much smaller than the annual dose limit
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