大規模原子力事故時の遠隔地における放射線被ばくへの対処

―長崎に飛来した福島第一原子力発電所の放射性物質からの推測―

抄録

Objective: To investigate human health effects of radiation exposure due to possible future nuclear accidents in distant places and other various findings of analysis of the radioactive materials contaminating the atmosphere of Nagasaki due to the Fukushima Daiichi Nuclear Power Plant accident. Methods: The concentrations of radioactive materials in aerosols in the atmosphere of Nagasaki were measured using a germanium semiconductor detector from March 2011 to March 2013. Internal exposure dose was calculated in accordance with ICRP Publ. 72. Air trajectories were analyzed using NOAA and METEX web-based systems. Results: ¹³⁴Cs and ¹³⁷Cs were repeatedly detected. The air trajectory analysis showed that ¹³⁴Cs and ¹³⁷Cs flew directly from the Fukushima Daiichi Nuclear Power Plant from March to April 2011. However, the direct air trajectories were rarely detected after this period even when ¹³⁴Cs and ¹³⁷Cs were detected after this period. The activity ratios (¹³⁴Cs/¹³⁷Cs) of almost all the samples converted to those in March 2011 were about unity. This strongly suggests that the ¹³⁴Cs and ¹³⁷Cs detected mainly originated from the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Although the ¹³⁴Cs and ¹³⁷Cs concentrations per air volume were very low and the human health effects of internal exposure via inhalation is expected to be negligible, the specific activities (concentrations per aerosol mass) were relatively high. Conclusion: It was found that possible future nuclear accidents may cause severe radioactive contaminations, which may require radiation exposure control of farm goods to more than 1000 km from places of nuclear accidents.