Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991–2005

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.

Research highlights
► A radioactivity survey on Pu isotopes was carried out in the coastal sea adjacent the nuclear fuel reprocessing plant in Aomori, Japan during 1991-2005.
► The 239+240Pu concentrations were uniform in surface water from one sampling point to another and decreased with a half-life of 17.8 y on the average.
► The time dependent decrease of 239+240Pu concentration in bottom water became more gradual with the increase of the depth.
► The 239+240Pu concentrations were relatively low in sediments at the shallow sampling points, but higher at the deeper points, without time dependent changes.
► The higher concentration of 239+240Pu was found in the sediments at deeper sampling points, and vice versa for 240Pu/239Pu atom ratio.