The availability of plutonium and americium in Irish Sea sediments for re-dissolution

The availability of plutonium and americium, for re-dissolution from offsho re sediments into Irish Sea water, has been examined. Sediments collected f rom the mud-patch near the Cumbrian coast were characterized in terms of sp atial location, particle size, partitioning of radionuclides with respect t o physico-chemical bonds and availability of actinides for release into sea water. Sequential extraction investigations revealed that plutonium was pre dominantly associated with strongly bound sesquioxide and organic complex f ractions. Americium was associated mainly with the organic complex fraction , but a significant fraction was in carbonate form. Sediment/water re-disso lution experiments with and without stirring were compared to simulate the effect of disturbing bed sediment. After 1 week, neither set of re-dissolut ion data provided significant trends between dissolved activity and time. S tirred systems appeared to release 2.5 times more plutonium and americium i nto seawater than unstirred systems. Measured Pu-239,Pu-240 and Am-241 dist ribution coefficients (K-d values) were both typically approximately 10(5) 1 kg(-1). Am-241 K-d values are an order of magnitude lower than previously reported for the north-eastern Irish Sea, but similar to western Irish Sea values. Overall, the fractions of plutonium and americium available for re -dissolution from bed sediment are very low at < 0.1%, with proportionally more plutonium being released than americium. These findings lend further s upport for the extrapolation of laboratory-derived information to environme ntal conditions. (C) 2001 Elsevier Science B.V. All rights reserved.