Application of a magnetic extraction technique to assess radionuclide-mineral association in Cumbrian shoreline sediments.

Abstract An assessment has been made of the association of 239+240 Pu, 241 Am and 210 Po (in secular equilibrium with 210 Pb) with iron minerals using a magnetic extraction technique. Grab samples of beach sand from the Cumbrian (UK) coastline were subjected to successive extractions with a ~0.1 T ferrite magnet and a ~0.3 T rare-earth magnet procedure to separate magnetic iron oxide minerals. Radionuclide concentrations in the magnetic extracts were enhanced (by ~4–6-fold) relative to the residue. Those in the ~0.1 T magnet extracts were broadly similar to those in the antiferromagnetic material extracted by the ~0.3 T magnet, despite the very large differences in magnetic property values between the two fractions (one to two orders of magnitude). The percentage of magnetic material in terms of mass was small and therefore, the majority of these radionuclides (on average 88%) were associated with the residue. Removal of stable Fe was incomplete. Given that the radionuclides may also bind to paramagnetic (nonmagnetic) Fe minerals, the data were extrapolated by normalising the results to quantitative Fe removal. This yielded average values of 37%, 45% and 46% for 239+240 Pu, 241 Am and 210 Po( 210 Pb), respectively, as upper limits for the fraction associated with magnetic+nonmagnetic Fe minerals. There are significant uncertainties inherent in quantifying data from this extraction technique. Nevertheless, it seems reasonable to conclude that radionuclide association with Fe minerals is unlikely to have a significant impact upon the physical dispersion of sediment contaminated by Sellafield discharges in the Irish Sea. However, it may be an important factor in governing Pu redox and redissolution behaviour.