Temporal distribution of plutonium isotopes in marine sediment cores off Fukushima and Ibaraki for the last three years after the Fukushima Dai-ichi Nuclear Power Plant accident

Large amounts of radionuclides were released into the atmosphere as well as discharged into the sea as a consequence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident caused by the earthquake and subsequent tsunami on March 11, 2011. The radionuclide contamination in the marine environment due to the FDNPP accident is of great public and scientific concern. Radiocesium (137Cs and 134Cs) were detected in the sediment traps in the deep sea in Pacific Ocean one month after the FDNPP accident, indicating the quick incorporating of radiocesium in seawater with the sediments. In the coastal area of Fukushima and Ibaraki, spatial and temporal distributions of radiocesium in the marine sediments from June 2011 to February 2012 were determined. However, information about the temporal distribution of plutonium isotopes in the sediments off Fukushima and Ibaraki after the FDNPP accident is limited.Plutonium isotopes derived from the FDNPP accident have been detected in the soil and liter samples in the 20-30 km zone around the FDNPP, revealing the release of Pu from the accident. Although no detectable plutonium isotopes from the FNDPP accident in the marine environment 30 km outside the FDNPP till 2012 was reported, Pu characterization in the marine environment inside the 30 km zone remains unknown. The possible Pu contamination from the inside 30 km zone could be transported by ocean currents and thus extend the contamination area by time.Several sediment core samples were collected from the coastal areas off Fukushima and Ibaraki from July 2011 to January 2013. Vertical distribution of Pu activities and Pu atom ratios (240Pu/239Pu and 241Pu/239Pu) in the sediment cores were determined. Pu inventories in the sediment cores and temporal variation of Pu activities and isotopic composition are discussed to understand the source and transport of Pu in the sediments in the coastal areas off Fukushima and Ibaraki after the FDNPP accident.For the analysis of Pu isotopes in the sediment sample, ca. 2.0 g dried sample was weighted out and spiked with 1 pg 242Pu as yield monitor. The extraction of Pu was performed in a Teflon tube with 20 mL concentrated HNO3 on a hot plate at 180-200ºC for at least 4 h. A two-stage anion-exchange chromatographic method was employed for the separation of Pu and U and for the further purification of Pu. A double focusing SF-ICP-MS (Elemental Scientific Inc., Omaha, NE, USA) with membrane dosolvation unit (ACM) and a conical concentric nebulizer was used for the analysis of Pu concentration and Pu isotopic ratios.５th Asia-Pacific Symposium on Radiochemistry (APSORC 13