Source term estimation of the radioactive materials released into the atmosphere during the Fukushima Daiichi Nuclear Power Station accident

Terada, Hiroaki   ; Nagai, Haruyasu  ; Tsuzuki, Katsunori  ; Kadowaki, Masanao   

To evaluate the environmental impacts and radiological doses to the public due to the Fukushima Daiichi nuclear power station accident, 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. In the latest estimation in Terada et al. (2020), we refined the source term and improved the dispersion simulation with an optimization method based on Bayesian inference. This optimization improved not only the source term but also the wind field in meteorological calculation by feeding back comparison results between the dispersion calculations and measurements of radionuclides. By expanding this optimization method for more wide range of spatial scale, we have enabled to estimate the source term by comprehensively comparing multi-scale dispersion calculations for local to hemispheric regions and various environmental monitoring data acquired over a wide area from the vicinity of a site to the hemisphere.