Numerical simulation of atmospheric dispersion and high radiological dose zone due to accidental releases from Fukushima Power Plant

Atmospheric dispersion calculations are made for the accidental releases of radionuclides from Fukushima Dai-ichi Nuclear Power Plant (FDNPP) using a weather model coupled Lagrangian particle dispersion model FLEXPART-WRF in a 30 km range to understand the formation of high radiation impact zone on the land region near to the FDNPP. The period during 11-17 March 2011 is considered for the study with the available source term estimates for 137Cs, 134Cs and 113I released during the accident. Simulated high resolution (1km) flow field with nested WRF model was largely westerly from 11 to 14 March. It has been found that the flow changed to northeasterly, easterly and southeasterly on 15 March under the influence of a low pressure and precipitation occurred on 15 March. Dispersion simulations showed the plume moved in a clockwise direction from Pacific Ocean to the land region to the northwest sector of Fukushima resulting in large activity depositions in that sector. The simulations show depositions of 106 to 108 Bq/m2 for 131I and 134Cs; and air dose rates of 10 μSv/h to 100 μSv/h in a distance range of 20 km in the northwest sector of Fukushima prefecture. The model could simulate the observed spike in the instantaneous air dose and its time variation at off-site monitoring stations around the reactor in reasonable agreement with highest doses at Fukushima, Litate located in the northwest sector followed by other sectors. (author)