Estimating by inverse modeling the release of radioactive substances ( 133 Xe, 131 I, and 137 Cs) into the atmosphere from Fukushima Daiichi nuclear disaster

The estimate of the release of radioactive substances ( 133 Xe, 131 I, and 137 Cs) into the atmosphere from the Fukushima Daiichi nuclear disaster is presented. It was obtained using the FLEXPART Lagrangian dispersion model and the data of local ground-based measurements of radiation dose rate. The computation period covers the active phase of the nuclear disaster that lasted 20 days after the tsunami. To get the quantitative characteristics of emissions of radioactive substances, the inverse modeling based on the Bayesian approach is used. The emissions were estimated for three altitudes. The total emissions are equal to 2.1 + 0.4 kg (14 000 + 2700 PBq) for 133 Xe, (3.8 + 0.4) x 10 -2 kg (174 + + 18 PBq) for 131 I, and 5.7 + 1.2 kg (18 + 4 PBq) for 137 Cs that is consistent with the results of other studies. Retrieved emissions were used to provide the forward modeling for mapping the areas of radionuclide deposition. The developed method of retrieving the emission of radioactive substances makes a useful instrument that operationally estimates and localizes the areas of potential pollution in case of nuclear accidents and could be used for making decisions on the population evacuation.