Distribution coefficient Kd of transuranics in soils: Experimental determination and consequences for dose assessment

The distribution coefficient (or Kd) of 241Am between soil and soil solution was estimated on the basis of experiments carried out under controlled conditions for soils displaying pH and organic matter content (which are the relevant parameters for transuranic nuclides sorption ability) found in cultivated soils. In these experiments, designed with five experimental units (batches) per studied contact time, 10 g of dry soil are contaminated with 50 mL of osmosed water containing proper concentrations of the radionuclide. The tracer, initially in nitric form, is neutralized prior to addition with the water, to ensure a constant ionic strength and avoid disturbance of the soil chemistry. After shaking and sedimentation, for each batch, the supernatant is sampled and analyzed by either liquid scintillation or alpha spectrometry. The sorption-desorption kinetic time necessary to reach apparent equilibrium is evaluated (> 10 days). Fixation curves, e.g. activity of the soil vs activity of the solution are established, after measurements, for a single delay representative of the equilibrium, and for a range of several orders of magnitude for radionuclide concentrations. On the basis of several assumptions, Kd values are evaluated by simple linear adjustment. A numerical application to retention half lives in soil is made. Finally, the weight of using site-specific Kd values instead of default parameters, on the doses due to ingestion of terrestrial foodstuffs is discussed for different scenarios