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

Abstract

The leaching of natural radionuclides from the material of a uranium mill tailing at Crossen/Saxony was investigated using eight columns (20 cm diameter, 50 cm height). Six columns were irrigated in the laboratory by synthetic rainwater of pH 6, 3 and 2, two columns were exposed outdoor to natural meteorological conditions. In the effluent of the columns, the concentrations of the radionuclides were measured periodically. With exception of the initial phase after installation of the columns (ca. 0.4 y), the leaching rates of radionuclides decreased slowly within the observation period of 4.5 years. Due to the buffering capacity of the tailings material, acidifying the irrigation water of the laboratory columns did not effect the effluent concentrations of the radionuclides. However, significant differences were observed between laboratory and outdoor columns: outdoor, the concentration of235U in the effluents was for the first two years substantially smaller than in the laboratory. As a consequence, the accumulated leached activity of235Uduring 4.5 years was 19.0±0.9% of the inventory for the laboratory columns, but only 10.9±0.6% for the outdoor columns. While uranium thus proved to be rather soluble, the concentration of226Ra in the effluents was quite small (<1 Bq/l). Thorium was retained completely in the columns.