Investigation of Removal Factors of Various Materials inside Houses after Nuclear Power Station Accident

Large quantities of radionuclides were released as a result of Fukushima Daiichi Nuclear Power Station accident. It is known that these radionuclides contaminated inside houses as well as outdoor environment. Considering the radiation protection of residents after a nuclear power station accident, it is important to know the influence of radionuclides inside houses to radiation dose to residents. In this study, we investigated removal factors and fractions of fixed contamination of various materials inside houses in Okuma Town, Futaba Town, and Namie Town to assess the contamination level inside house appropriately. Nine kinds of materials: fibers, woods (smooth), woods (rough), glasses, concretes (smooth), concretes (rough), plastics, PVCs and metals, were used in examinations. The lowest and the highest removal factors were 23% ± 16% of woods (rough) and 79% ± 7.7% of glasses, respectively. Removal factors of all materials were higher than 10% which is recommended by Japanese Industrial Standard. The negative correlation was found between removal factors and fractions of fixed contamination. Using this correlation, the decontamination efficiency, which means the ratio of the activity removed from the surface by one smear sample to the activity of the total surface activity, was proposed. The air dose rate from the contamination inside house was calculated using obtained decontamination efficiencies and removal factor of 10%. In the case using the removal factor of 10%, the air dose rate derived by indoor contamination was approximately 2 times higher than the case using obtained decontamination efficiencies. We found that the air dose rate derived by indoor contamination was much lower than the air dose rate outside house, and the influence of indoor contamination on the external exposure was small.