Modelling the environmental transfers of radioactivity following the Fukushima accident
Student thesis: Doctoral Thesis
After the massive tsunami on 11 March 2011, the explosions at 4 reactors of the Fukushima Daiichi(I) Nuclear Power Stations (Fukushima I NPSs) in Japan led to a large radioactive cloud being ejected into the atmosphere. Radionuclides released to the atmosphere were washed out in rain causing an area of remarkably high deposition in the area to the northwest of the Fukushima INPSs. Activity remaining in the atmosphere was dispersed further to other areas of Japan. At various times during the accident, plumes of contamination were transported from Japan to the Pacific Ocean, the North American continent, crossing the Atlantic Ocean to Europe, and eventually to Asia. Due to the lack of measurements in three major ecosystem compartments i.e.air, soil and surface water, information on the highest impact early-phase after fallout and the interpretation of long-term impacts is limited and unclear. The transfer of contaminated radionuclides to soil and air around the Fukushima I NPSs was studied using ratios of various radioisotopes to the long-lived nuclide 137Cs. It was found thatratios of three radionuclides i.e. 134,136Cs and 132Te were consistent with direction and distance in air and soil while the ratio 131I/137Cs was not. It was shown that, deposited 131I/137Cs had an inverse correlation with 137Cs activity in soil. These nuclides were key in forming the high-gamma dose rates in the early phase, particularly the high gamma energy of 132I from the 132Te/132I decay. The derived ratios of these key radionuclides in soil were used together with available measured gamma dose rates in the early phase (<30 days) to develop a model to reconstruct and predict external gamma dose rate. Model“blind” tests showed that more than 95% of predictions were within a factor of two of measurements from 15 sites to the north, northwest and west of the power station. It is demonstrated that generic isotope ratios provide a sound basis for reconstruction of early-phase external dose rates in these most contaminated areas. For contamination in surface runoff water, lake water and fish, a previous model developed following the Chernobyl accident (AQUASCOPE) was applied to the Fukushima situation. It was shown that by adjusting for the stronger absorption of radiocaesium in soils in Japan (compared with European countries) the model could be used successfully to predict long-term contamination in aquatic systems affected by Fukushima fall out. The results of the model showed good agreement with measured data, in particular in the long-term period (around 0.5-2 years)after the accident.
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