Transfer of Sellafield-derived 237Np to and within the terrestrial environment

The vertical distributions of 237Np in soil profiles collected from west Cumbria, UK show exponential decreases with depth, similar to those of the other transuranic and fission product nuclides. The maximum 237Np activity concentration in any of the soil samples was 2.87 ± 0.68 Bq kg-1 (dry) for the 0-5 cm section of a gley soil, compared to 3660 ± 460 Bq kg-1 (dry) for 239,240Pu in the same sample. A significant proportion of the total 237Np inventory is observed at depth in most sites, implying a greater mobility of neptunium than other transuranium nuclides within the profiles studied. Consideration of nuclide ratios, using BNFL Sellafield liquid discharges as the primary source term, indicates that 237Np is less bound to particles than 239,240Pu and 241Am. There appears to be a greater tendency for 237Np to behave more conservatively than the other transuranics on release to the Irish Sea. At one sampling site, transfer to the terrestrial environment is dominated by direct incursion of seawater. During this transition to land, 237Np is 'lost', relative to Pu, Am and Cs from the solid phase by remobilization. Further study by sequential leaching of soil and sediment samples confirms that a much greater proportion of the 237Np is associated with the more readily available fractions than is observed for Pu, Am or Cs. The organic phase nevertheless remains significant in Np retention and binding. (author)