ABSTRACT
A radioactivity survey was launched in 1991 to determine the background levels of ²³9+²4°Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the ²4°Pu/²³9Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of ²³9+²4°Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the ²³9+²4°Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of ²³9+²4°Pu observed in the bottom water sample at sampling points having greater depth. The ²4°Pu/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 ²4°Pu/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+²4°Pu concentration and also decreasing the ²4°Pu/²³9Pu atom ratio with sea depth, and further study is required to clarify it.
