Isotopic determination of U, Pu and Cs in environmental waters following the Fukushima Daiichi Nuclear Power Plant accident

Concentrations of the radionuclides, U, Pu, and Cs were measured in water samples (10-20 L) to study analyte dispersion and migration following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. A total of 8 water samples including oceanic water and paddy-field water were collected in the vicinity of the plant. Determinations of U, Pu and Cs isotopes were performed by accelerator mass spectrometry (AMS), inductively coupled plasma mass spectrometry (ICP-MS), and gamma-ray spectrometry. The U-236/(238) atom ratio was in the range 1.83-8.20 x 10(-9) for fresh water and around 0.57 x 10(-9) for seawater while the concentration of U-236 was about 10(4)-10(5) and 10(6) atoms/kg, respectively. Plutonium (Pu-239,Pu-240) was detected in one riverine sample and the marine samples at very low levels and with large uncertainty. The concentrations of Cs-132 in fresh riverine samples were 0.02-0.46 Bq/kg which are more than three orders of magnitude larger than the global fallout level. As for seawater samples within 80 km offshore of the FDNPP, the concentrations of Cs-137 were 10-20 times higher than that of the Japan Sea water. Also Cs-134 and Cs-137 were of similar concentrations in all samples. The results show that volatile and refractory nuclides such as Cs, U and Pu exist in the dissolved phase, which can be readily assimilated by plants/humans. However the environmental impact of Pu and U in the vicinity of the FDNPP is considered to be low in comparison to that of the volatile radionuclide Cs.