Plutonium Isotopes (Pu) Dissolved in Pacific Ocean Waters Detected by Accelerator Mass Spectrometry: No Effects of the Fukushima Accident Observed.

The concentration of plutonium (Pu) and the isotopic ratios of Pu to Pu and Pu to Pu were determined by accelerator mass spectrometry (AMS) in Pacific Ocean water samples (20 L each) collected in late 2012. The isotopic Pu ratios are important indicators of different contamination sources and were used to identify a possible release of Pu into the ocean by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. In particular, Pu is a well-suited indicator for a recent entry of Pu because Pu from fallout of nuclear weapon testings has already significantly decayed. A total of 10 ocean water samples were prepared at the Radiochemie München of the TUM and analyzed at the Vienna Environmental Research Laboratory (VERA). Several samples showed a slightly elevated Pu/Pu ratio of up to 0.22 ± 0.02 compared to global fallout (Pu/Pu = 0.180 ± 0.007), whereas all measured Pu-to-Pu ratios were consistent with nuclear weapon fallout (Pu/Pu < 2.4 × 10), which means that no impact from the Fukushima accident was detected. From the average Pu-to-Pu ratio of 8 ×10 at a sampling station located at a distance of 39.6 km to FDNPP, the 1-σ upper limit for the FDNPP contribution to the Pu inventory in the water column was estimated to be 0.2%. Pu, with the signature of weapon-grade Pu was found in a single sample collected around 770 km off the west coast of the United States.