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内容記述 |
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has raised wide concerns on drinking water contamination by artificial radionuclides. Neptunium-237 (237Np) and plutonium isotopes (239+240Pu) have high radiotoxicity and long half-lives, whereas they are rarely determined in drinking or natural waters owing to their extremely low abundance. This study established a sequential separation method, including primary Fe(OH)2 co-precipitation, secondary CaF2/LaF3 co-precipitation, and anion exchange chromatographic separation, to determine 237Np and 239+240Pu in drinking and river/lake water by SF-ICP-MS. The method detection limits of 237Np, 239Pu, and 240Pu were 0.035 fg/L (0.0009 μBq/L), 0.012 fg/L (0.027 μBq/L), and 0.010 fg/L (0.085 μBq/L), respectively for 20 L of water. The method was validated by Milli-Q water spiked with the IAEA-443 reference material. Based on the method, 237Np concentrations in 27 tap water and 6 river/lake samples collected from 9 prefectures of Japan were determined, ranging from 0.004±0.002 to 0.116±0.010 μBq/L, with the 242Pu recovery of 70.2-93.8%. However, 239Pu was only determined in 27% of the samples, ranging from 0.080±0.035 to 0.111±0.064 μBq/L, and 240Pu was not detected in all samples. The 237Np activity concentrations in drinking waters were close to that in lake/river waters, indicating that the conventional drinking water treatment techniques cannot effectively remove 237Np from natural waters. Moreover, the significant relationship between the 237Np activity concentrations in drinking water and soil suggested that global fallout was the primary source of 237Np in drinking water. The determined 237Np and 239Pu concentrations in drinking water were far below the guidelines for drinking water quality (1 Bq/L) issued by the World Health Organization. The estimated annual effective dose of 237Np for adults ranged from 3.17×10-10 to 9.29×10-10 mSv, much lower than the safety threshold. The findings suggested that it is necessary to routinely monitor the 237Np levels in drinking tap water after nuclear accidents to protect the public from excessive internal exposure. |
