Association of 210Po 210Pb/ , 239q240Pu and 241Am with different mineral fractions of a beach sand at Seascale, Cumbria, UK

The degree of association of 210Po in secular equilibrium with 210Pb., 239q240Pu and 241Am with iron oxide minerals in beach sand has been assessed using samples collected by panning and selective magnetic extraction techniques. The samples were obtained from Seascale beach Cumbria., close to the BNF reprocessing plant at Sellafield UK. and physically and chemically characterised in terms of their grain-size distribution, radionuclide content and magnetic properties. Low frequency 0.46 kHz. susceptibility xLF. and saturation isothermal remanent magnetisation SIRM. measurements were used to determine the magnetic properties of the bulk samples and the grain size fractions. Samples were also subjected to magnetic extraction, to concentrate the iron oxide minerals, and thence determine their degree of association with the radionuclides of interest. The efficiency of the magnetic extraction technique was estimated from rock magnetic measurements, before and after the extraction procedure. The placer deposits, enriched in iron oxide grains by environmental sorting processes, were both finer grained and more magnetic than the bulk sand. The mean particle diameter of the bulk sand was largely ;75%. greater than 200 mm, whereas for the placer material, )88% of particles have mean diameters of -200 mm. Whilst xLF and SIRM values of the 100]200 mm placer material were approximately 40]50-fold higher than those of the bulk sand of equivalent grain size, the radionuclide concentrations were approximately 4]13-fold higher. In terms of mass, the percentage of magnetic material extracted from samples of bulk sand using a powerful, ;0.3 T, rare earth magnet., was small approx. 1]3%.. Nevertheless, )70% of the magnetic signal was associated with these extracts. Concentrations of all three radionuclides in the magnetic extracts were enhanced relative to the weakly magnetic residue. The proportions of 210Po 210Pb., 239q240Pu and 241Am associated with the magnetic extracts were ;18%, ;12% and ;11%, respectively. Therefore, although the strongly magnetic iron oxide fraction contains measurably enhanced levels of these radionuclides, the majority of the a-radioactivity appears to be associated with the weakly magnetic residue. Crown Copyright