Resulting from the Fukushima Daiichi Nuclear Power Plant accident

The release rates and total amounts of ¹³¹I and ¹³⁷Cs discharged into the atmosphere as a result of the nuclear accident at the Fukushima Daiichi Nuclear Power Plant were estimated for March 12 to April 5, 2011. The applied method was a reverse estimation by coupling environmental monitoring data with atmospheric dispersion simulations under the assumption of the unit release rate (1 Bq h^-1). The estimated temporal variation in the releases indicated that a significant release of greater than 10¹⁵ Bq h^-1 of ¹³¹I occurred on March 15, followed by relatively small releases on the order of 10¹³ Bq h^-1, while the release rates from March 16 until March 24 were estimated to be rather constant on the order of 10¹⁴ Bq h^-1. The release rates decreased with small day-to-day variations to approximately 10¹¹ - 10¹² Bq h^-1 of ¹³¹I at the beginning of April. The estimated source term was then examined on the basis of a comparison to other results. In addition, the ground depositions of ¹³⁷Cs calculated using various atmospheric dispersion models with the estimated source term in this study were compared with observed values. These examinations indicated that the estimated source term was reasonably accurate at least during the period when the plume flowed over the land in Japan. (author)