The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released large amount of radionuclides into the marine environment. Compared with the fission products, data on the distributions of Pu in the marine environment of the western North Pacific after the accident is limited. To better understand the Pu contamination in the marine environment after the accident, for the first time, we determined Pu isotope ratio (²⁴⁰Pu/²³⁹Pu) in addition to ^239+240Pu activity in seawater collected in the near coastal area (mostly within the 30 km zone) off the FDNPP site. The ^239+240Pu activities were 4.16-5.52 mBq/m³ and the ²⁴⁰Pu/²³⁹Pu atom ratios varied from 0.221 to 0.295. These values were compared with the baseline data for Pu distribution in the near coast seawaters before the FDNPP accident (2008-2010). The results suggested that there is no significant Pu contamination in seawater in the near coastal area off the FDNPP site from the accident two years after the accident.

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In March 2014, a new sector-field (SF)-ICP-MS (Element XR) equipped with the Jet Interface was installed in the Research Center for Radiation Protection at the National Institute of Radiological Sciences (NIRS). This is the first Element XR installed for actinides measurements in Japan. In this paper, a detail evaluation is made of the Element XR on its analytical potential for ultra-trace determination of Pu isotopes. In order to enhance the sensitivity for Pu isotope analysis, high efficiency sample introduction systems (APEX-Q and Aridus II) were combined with the Element XR. A certified Pu isotope standard solution (NBS 947) and a multi element standard solution (Merck Multi Element Standard), which contained depleted U, were used to investigate four items: the formation rate of ²³⁸UH⁺, which could interfere with an ultra-trace level ²³⁹Pu determination; the detection limit for Pu; the precision and accuracy for ²⁴⁰Pu/²³⁹Pu atom ratio measurement; and the long-term (8- month) stability for Pu isotope composition analysis. Due to the exceptionally high sensitivity of the new SF-ICP-MS, a detection limit at the attogram (10^-18 g) level was achieved. This is comparable to or even surpasses the detection limit of accelerator mass spectrometry, which has long been regarded as the most sensitive technique for Pu isotope measurement.

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