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

The Irish sea contains elevated levels of Pu239 + 240 and Am-241 discharged , over several decades, from the British Nuclear Fuels reprocessing plant a t Sellafield, Cumbria. Data on both the temporal and spatial variations of these radionuclides are provided from surveys carried out over the period 1 973 - 1996. Seawater concentrations have varied from year to year, concomit ant with reductions in discharges, differences in transport behaviour (depe ndent upon the prevailing hydrographic conditions) and remobilisation. Over the period considered, the most noticeable change from the end of the 1970 s to the most recent time is a decrease in dissolved Pu239 + 240 levels (ap proximate to 4-fold). Concentrations of Am-241 have remained relatively unc hanged since 1976. Reductions in discharges have been significantly greater (e.g. approximate to 40-fold for Pu239 + 240) and remobilisation from sedi ments contaminated by large discharges in the 1970s is presently the predom inant source term to the water column. Estimates of the inventory in the di ssolved phase are small (approximate to 0.3 and 0.03% for Pu239 + 240 and A m-241, respectively) in comparison with that residing on the sub-tidal sedi ments. The dissolution half-time of Pu239 + 240 and Am-241 from the surface sediment layer (top 5 cm) is estimated to be in the order of 58 and 1000 y ears, respectively. Crown Copyright (C) 1999 Published by Elsevier Science Ltd. All rights reserved.