Simulating air dose rate of radioactivity in selected area of Fukushima by PHITS

It has already passed nearly three years since the accident of a Fukushima Dai-ichi Nuclear Power Plant (FDNPP). Now, the main nuclide which is contributing to radiation dose rate is radioactive caesium (¹³⁴Cs and ¹³⁷Cs, hereafter, radioactive Cs). Predicting the distribution of radioactive Cs deposited in the soil surface of Fukushima Prefecture after the accident and understanding the adsorption/desorption mechanism between soil and radioactive Cs in the environment are indispensable for evaluating the impact of radiological exposure dose. It is familiar that Cs tends to adsorb with clay rich soils, and therefore its primary transportation mechanism is considered to be in the form of soil erosion on the land surface and sediment-sorbed contaminants transport in the water system. In our project entitled 'Long-term Assessment of Transport of Radioactive Contaminant in the Environment of Fukushima' (F-TRACE project), we have been simulating radioactive Cs transport by watershed models. Based on these predictions, we evaluate a 1-cm dose equivalent rate with PHITS (Particle and Heavy Ion Transport code System) in a selected environment and present the results attained thus far. (author)