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Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991-2005
Authors:Oikawa Shinji  Watabe Teruhisa  Inatomi Naohiko  Isoyama Naohiko  Misonoo Jun  Suzuki Chiyoshi  Nakahara Motokazu  Nakamura Ryoichi  Morizono Shigemitsu  Fujii Seiji  Hara Takeya  Kido Katsutoshi
Affiliation:a Marine Ecology Research Institute, Head Office, Research and Survey Group, Towa-Edogawabashi Bldg. 7F., 347 Yamabuki-cho, Shinjuku-ku, Tokyo 162-0801, Japan
b Marine Ecology Research Institute, Central Laboratory, 300 Iwawada, Onjuku-machi, Isumi-gun, Chiba 299-5105, Japan
Abstract:A radioactivity survey was launched in 1991 to determine the background levels of 239+240Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the 240Pu/239Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of 239+240Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the 239+240Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of 239+240Pu observed in the bottom water sample at sampling points having greater depth. The 240Pu/239Pu atom ratio in the seawater and sediment samples was higher than that of global fallout Pu, and comparable with the data in the other sea area around Japan which has likely been affected by close-in fallout Pu originating from the Pacific Proving Grounds. The 240Pu/239Pu atom ratio in bottom sediment samples decreased with sea depth. The land-originated Pu is not considered as the reason of the increasing 239+240Pu concentration and also decreasing the 240Pu/239Pu atom ratio with sea depth, and further study is required to clarify it.
Keywords:239+240Pu   240Pu/239Pu atom ratio   Seawater   Sediment   Reprocessing plant   Radioactivity monitoring
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