Plutonium Isotopes (239-241Pu) 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-240 to Pu-239 and Pu-241 to Pu-239 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-241 is a well-suited indicator for a recent entry of Pu because Pu-241 from fallout of nuclear weapon testings has already significantly decayed. A total of 10 ocean water samples were prepared at the Radiochemie Munchen of the TUM and analyzed at the Vienna Environmental Research Laboratory (VERA). Several samples showed a slightly elevated Pu-240/Pu-239 ratio of up to 0.22 +/- 0.02 compared to global fallout (Pu-240/Pu-239 = 0.180 +/- 0.007), whereas all measured Pu-241-to-Pu-239 ratios were consistent with nuclear weapon fallout (Pu-241/Pu-239 < 2.4 X 10(-3)), which means that no impact from the Fukushima accident was detected. From the average Pu-241-to-Pu-239 ratio of 8(-2)(+3) X10(-4) at a sampling station located at a distance of 39.6 km to FDNPP, the 1-sigma upper limit for the FDNPP contribution to the Pu-239 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.