ABSTRACT

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 γ-ray spectrometry. The ²³⁶U/²³⁸U atom ratio was in the range 1.83-8.20 × 10^-9 for fresh water and around 0.57 × 10^-9 for seawater while the concentration of ²³⁶U was about 10⁴-10⁵ and 10⁶ atoms/kg, respectively. Plutonium (^239,240Pu) was detected in one riverine sample and the marine samples at very low levels and with large uncertainty. The concentrations of ¹³⁷Cs 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 ¹³⁷Cs were 10-20 times higher than that of the Japan Sea water. Also ¹³⁴Cs and ¹³⁷Cs 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.