Dissolved plutonium and americium in surface waters of the Irish Sea, 1973–1996

The Irish Sea contains elevated levels of 239+240Pu and 241Am discharged, over several decades, from the British Nuclear Fuels reprocessing plant at Sellafield, Cumbria. Data on both the temporal and spatial variations of these radionuclides are provided from surveys carried out over the period 1973–1996. Seawater concentrations have varied from year to year, concomitant with reductions in discharges, differences in transport behaviour (dependent upon the prevailing hydrographic conditions) and remobilisation. Over the period considered, the most noticeable change from the end of the 1970s to the most recent time is a decrease in dissolved 239+240Pu levels (≈4-fold). Concentrations of 241Am have remained relatively unchanged since 1976. Reductions in discharges have been significantly greater (e.g. ≈40-fold for 239+240Pu) and remobilisation from sediments contaminated by large discharges in the 1970s is presently the predominant source term to the water column. Estimates of the inventory in the dissolved phase are small (≈0.3 and 0.03% for 239+240Pu and 241Am, respectively) in comparison with that residing on the sub-tidal sediments. The dissolution half-time of 239+240Pu and 241Am from the surface sediment layer (top 5 cm) is estimated to be in the order of 58 and 1000 years, respectively.