Laboratory studies on the remobilisation of actinides from ravenglass estuary sediment

Abstract

Actinides from the BNFL Sellafield nuclear reprocessing plant are discharged into the Irish Sea and have accumulated in the Ravenglass Estuary 10 km to the south-east. As part of a programme to study the remobilisation of actinides in this estuary, laboratory experiments were carried out with actinide-bearing intertidal sediments to investigate the effect of different conditions on the distribution coefficients ( K d) for plutonium and americium. Using inactive seawater from the North Sea, K d values of 1.1 × 10 6 and 2.0 × 10 6 were obtained for 239+240Pu and 241Am, respectively. Further experiments, carried out to investigate the sorption—desorption equilibrium reactions, indicated that for plutonium an oxidation—reduction process operates, with the sediment itself able to reduce Pu(V/VI) to Pu(III/IV). With americium, on the other hand, the formation of a soluble carbonate-complex is thought to occur. With low values of salinity (e.g. at low tide and in the upper estuary), K d values were found to decrease by about two orders of magnitude. This was due, in part, to the associated drop in pH. A mechanism for the transport of remobilised actinides on suspended particulate material is postulated in order to explain the recent observations that, currently, there is a net loss of actinides from the Ravenglass Estuary to the Irish Sea.