Studies on the Speciation of Plutonium and Americium in the Western Irish Sea

Abstract

The distributions of plutonium and americium between filtrate (<0.22 μm) and suspended solids in sea water sampled throughout the western Irish Sea in September 1988 and September 1989 are examined. It is shown that typically 2–15% of the plutonium present in the surface water and 30–60% of the americium, is associated with the particulate fraction. Furthermore, measurements of the oxidation state distribution of plutonium in filtered water taken in December 1989 at six stations throughout the eastern and western Irish Sea showed little variation with some 85 ± 6% of the plutonium in the filtrate in an oxidized (Pu V,VI) form. Little or no distinction was observed between near-surface and near-bottom waters, reflecting the shallow nature of much of the Irish Sea and the importance of turbulent mixing in addition to advective transport. Differences in the 241Am/239,240Pu ratio in sea water, suspended particulate matter and surface sediments have been interpreted and evidence presented to show that the dominant source term for the western Irish Sea has become remobilised plutonium and americium from the fine sediments close to Sellafield. The 241Pu/239,240Pu ratio in filtered water and sediment has been examined and an estimate of 15 years deduced for the effective ‘hold-up’ time of plutonium in the sediments of the eastern Irish Sea. Kd coefficients for plutonium and americium in the western Irish Sea have also been determined and differences between these values and those observed in the eastern Irish Sea are discussed.