Longterm leaching of natural radionuclides from uranium mill tailings material: comparison of indoor and outdoor column experiments

The leaching of natural radionuclides from the material of a uranium mill t ailing at Crossen/Saxony was investigated using eight columns (20 cm diamet er, 50 cm height). Six columns were irrigated in the laboratory by syntheti c rainwater of pH 6, 3 and 2, two columns were exposed outdoor to natural m eteorological conditions. In the effluent of the columns, the concentration s of the radionuclides were measured periodically. With exception of the in itial, phase after installation of the columns (ca. 0.4 y), the leaching ra tes of radionuclides decreased slowly within the observation period of 4.5 years. Due to the buffering capacity of the tailings material, acidifying t he irrigation water of the laboratory columns did not effect the effluent c oncentrations of the radionuclides. However, significant differences were o bserved between laboratory and outdoor columns: outdoor, the concentration of U-235 in the effluents was for the first two years substantially smaller than in the laboratory. As a consequence, the accumulated leached activity of 235U during 4.5 years was 19.0 +/- 0.9% of the inventory for the labora tory columns, but only 10.9 +/- 0.6% for the outdoor columns. While uranium thus proved to be rather soluble, the concentration of Ra-226 in the efflu ents was quite small (<1 Bq/1). Thorium was retained completely in the colu mns.