Estimation of time trend on atmospheric releases of I-131 and Cs-137 due to the Fukushima Daiichi nuclear power plant accident

The release rates and total amounts of ¹³¹ I and ¹³⁷Cs discharged into the atmosphere from March 12 to April 5, 2011 were estimated. The applied method is a reverse estimation by coupling environmental monitoring data with atmospheric dispersion simulations under the assumption of unit release rate (1 Bq/h). When air concentration data were not available, the release rates were estimated by comparing measured air dose rates due to radionuclides on the ground surface with calculated ones derived from simulations with unit release rate, assuming the composition of tellurium, iodine and cesium. The estimated temporal variation of releases indicates that the significant release, over 10¹⁵ Bq/h of ¹³¹I, occurred on March 15, following to relatively small releases, 10¹³, but the release rates from March 16 are estimated to be rather constant on the order 10¹⁴ Bq/h until March 24. The release rates have decreased with small day-to-day variations to the order of 10¹¹-10¹² Bq/h of ¹³¹I on the beginning of April. The estimated source term were examined by the comparison with other results. Furthermore, the ground depositions of ¹³⁷Cs calculated by various atmospheric dispersion models with estimated source term were compared with observed one. These examinations showed that the estimated source term was reasonably accurate at least during the period when the plume flowed over land in Japan.