Studies of Np and Pu in the marine environment of Swedish–Danish waters and the North Atlantic Ocean

The long-lived anthropogenic radionuclides 237Np, 239Pu and 240Pu were determined in marine environmental samples (seaweed and seawater) collected from Swedish–Danish waters and the North Atlantic Ocean at various locations on different occasions during the period 1991–2001. The measurements were performed with sector field Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and conventional alpha spectrometry. The 237Np activity concentrations in Fucus vesiculosus and surface seawater from the Swedish west coast and Danish waters ranged from 0.16 ± 0.02 to 1.02 ± 0.09 mBq kg−1 (dry weight) and 0.65 ± 0.02 to 1.69 ± 0.02 mBq m−3, respectively, depending on the location and sampling year. Most of the 237Np in these waters is believed to originate from the Sellafield nuclear reprocessing plant, with some contribution from global fallout. The 240Pu/239Pu atomic ratios in F. vesiculosus samples are reported in this study with an overall average of 0.17 ± 0.03. The 237Np and 239Pu activity concentrations observed in surface seawater collected in North Atlantic waters ranged from 0.16 ± 0.01 to 0.62 ± 0.08 mBq m−3 and from 0.64 ± 0.05 to 4.27 ± 0.08 mBq m−3, respectively, and the 237Np/239Pu atomic ratios were a good indicator of conservative behaviour of Np in marine waters.