Distribution, geochemistry and geochronology of Sellafield waste in contaminated Solway Firth floodplain deposits
Allan, Robert Lindsay
PublisherUniversity of Glasgow
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This thesis describes a study of the distribution and geochemical behaviour of 137Cs, 241Am and 239+24PU within the floodplain deposits of south west Scotland. These sediments have been contaminated with low level effluent discharged from BNFL's reprocessing plant at Sellafield on the Cumbrian coast. The study establishes that the dominant supply mechanism of anthropogenic radionuclides to the floodplain is via on-shore transfer of contaminated particulate material which has been mathematically modelled. A series of lateral transects across the floodplain has confirmed previous observations of highest concentrations furthest inland, illustrating the relationship between particle size and radionuclide concentration. The inventories observed for 137Cs were of the order of - 106 to 10' Bq m-2 and were somewhat higher than previously published data. The vertical distribution of radionuclides was investigated at Southwick merse and indicated a declining depth of occurrence of maximum radionuclide concentrations with distance inland. The observations from a series of transects, using a variety of sample collection methods permitted the construction of a schematic model of the merse. This model identified three distinct zones within the floodplain and enabled estimation of the total inventory for the coastline of south west Scotland. By these estimates less than 1% of the total environmental inventory of 137Cs discharged from Sellafield has been returned on-shore. Speciation studies on the availability of 13C7 s and 239+240Pu indicate that 137Cs is strongly retained within the residual phase of the sediment and that 239+240Pu exhibits slightly enhanced availability, being associated with the organics and secondary Fe/Mn mineral phases. Evidence suggests that despite the increased environmental availability of 239+240Pu both these radionuclides and 131Csa re not generally in a form which is available for plant uptake. Flow desorption studies have indicated that137Cs is tightly bound to the sediment particles producing a Kd of -105 Ikg-1.