Pu distribution in seawater in the near coastal area off Fukushima after the FDNPP accident

On March 11, 2011, a magnitude 9.0 earthquake occurred in the western North Pacific about 130 km off the northeast coast of Japan followed by a tremendous tsunami which arrived at Fukushima Daiichi Nuclear Power Plant (FDNPP) about 45 min later. The tsunami severely damaged the nuclear reactor cooling system, leading to hydrogen explosions in the reactor buildings. As a result, massive radionuclides were released into the environment. Intensive studies about the distribution of the FDNPP derived volatile radionuclides, such as 90Sr, 131I, 134Cs and 137Cs in the environment after the accident have been conducted. Compared with these plentiful studies on the volatile radionuclides, studies on the non-volatile radionuclides, especially for actinides, are much less. As one of the most important actinides, Pu isotopes attracted great public attention after the FDNPP accident because they present a high risk for internal radiation exposure via ingestion of contaminated agricultural crops and seafood. Although a large amount of radionuclides were released into the environment through air pathways, over 70% of them finally deposited in the western North Pacific as a result of the prevailing of westerly wind during spring in the accident area. Moreover, after the accident, contaminated water was leaked or directly discharged into the offshore of the FDNPP site, which is another possible pathway for Pu to enter the marine environment. Currently, the total amount of these high radioactive liquid wastes still remains unknown, whether the entrance of Pu derived from the FDNPP accident has a remarkable influence on the background Pu distribution in the western North Pacific needs to be studied. To better understand the Pu contamination in the marine environment after the accident, for the first time, we determined Pu isotopes 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 western North Pacific and its marginal seas before the FDNPP accident. 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.13th International Conference on the Biogeochemistry of Trace Element