Pu distribution in the marine sediments within the 30 km zone around the Fukushima Dai-ichi Nuclear Power Plant site

Atmospheric deposition of Pu isotopes from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has been observed in the terrestrial environment around the FDNPP site; however, their deposition in the marine environment has not been studied. To fully understand this possible contamination of Pu isotopes from the FDNPP accident to the marine environment, we collected marine sediment core samples within the 30 km zone around the FDNPP site in the western North Pacific about two years after the accident. Pu (²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu) and radiocesium (¹³⁴Cs, ¹³⁷Cs) isotopes were determined. The high activities of radiocesium and the ¹³⁴Cs/¹³⁷Cs activity ratios with values around 1 (decay corrected to 15 March 2011) revealed that these samples were heavily contaminated by the Fukushima-released radionuclides. However, the activities of ^239+240Pu and ²⁴¹Pu were comparable with the background level before the FDNPP accident. The Pu atom ratios (²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu) suggested that global fallout and the Pacific Proving Ground (PPG) close-in fallout are the main sources for Pu contamination in the marine sediments. As Pu isotopes are particle-reactive and they can be easily incorporated with the marine sediments, we concluded that the release of Pu isotopes from the FDNPP accident to the marine environment was negligible. (author)