Numerical reconstruction of high dose rate zones due to the Fukushima Dai-ichi Nuclear Power Plant accident.

To understand how the high dose rate zones were created during the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident on March 2011, the atmospheric dispersion of radionuclides during the period from 15 to 17 March was reproduced by using a computer-based nuclear emergency response system, WSPEEDI-II. With use of limited environmental monitoring data, prediction accuracy of meteorological and radiological fields by the system was improved to obtain best estimates of release rates, radiation dose maps, and plume movements. A large part of current high dose rate zones in Fukushima was explained by simulated surface deposition of radionuclides due to major releases of radionuclides on 15 March. In the simulation, the highest dose rate zones to the northwest of FNPP1 were created by a significant deposition of radionuclides discharged from FNPP1 during the afternoon. The results indicate that two environmental factors, i.e., rainfall and topography, strongly affected the spatial patterns of surface deposition of radionuclides. The wet deposition due to rainfall particularly played an important role in the formation of wide and heterogeneous distributions of high dose rate zones. The simulation also demonstrated that the radioactive plume flowed along the valleys to its leeward, which can expand the areas of a large amount of surface deposition in complex topography.