JAEA's experience in the source term estimation for the Fukushima Daiichi Nuclear Power Plant accident using WSPEEDI

Terada, Hiroaki   

To evaluate the environmental impacts and radiological doses to the public due to the Fukushima Daiichi Nuclear Power Plant accident in 2011, the source term of radioactive materials discharged into the atmosphere has been estimated and updated by Japan Atomic Energy Agency. The source term was reversely estimated by environmental monitoring data and atmospheric dispersion simulations mainly using the Worldwide version of System for Prediction of Environmental Emergency Dose Information, WSPEEDI. Just after the accident, the temporal variation of the release rates of I-131 and Cs-137 were estimated by using highly limited environmental monitoring data. Subsequently, the source term has been re-estimated by the usage of additionally opened monitoring data and the improvements of the atmospheric transport and deposition model (ATDM). In the latest study (Terada et al. 2020), we refined the source term and improved the dispersion simulation with an optimization method based on Bayesian inference. The estimated source term and spatiotemporal distribution of radionuclides reconstructed by the ATDM were used for evaluation of the doses to the public in the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2020 report.