Association of 210Po(210Pb), 239+240Pu 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), 239+240Pu 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 (X(LF)) 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 (approximately 75%) greater than 200 microm, whereas for the placer material, > 88% of particles have mean diameters of < 200 microm. Whilst X(LF) and SIRM values of the 100-200 microm 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, approximately 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), 239+240Pu and 241Am associated with the magnetic extracts were approximately 18%, approximately 12% and approximately 11%, respectively. Therefore, although the strongly magnetic iron oxide fraction contains measurably enhanced levels of these radionuclides, the majority of the alpha-radioactivity appears to be associated with the weakly magnetic residue.
McCubbin
D
CEFAS Lowestoft Laboratory, Lowestoft, Suffolk, UK. d.mccubbin@cefas.co.uk
Leonard
KS
Maher
BA
Hamilton
EI