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By means of a thermal diffusion column the range of the radiocarbon dating method can be extended by about 20 000 years. In order to calculate suitable dimensions and operation conditions, a theory of nonlinear enrichment is derived. On the basis of a formula which describes the dependence of the enrichment factor on time and the characteristic values of the column, a con-centric column of 11m length was constructed. The enrichment factors obtained for 13 CH4 as a function of time (up to 400 h) and pressure (150 -1500 Torr) and for

27Uranium / Tailings / Leaching / Laboratory columns / 28 Outdoor columns 29 Summary. The leaching of natural radionuclides from the 30 material of a uranium mill tailing at Crossen/Saxony was in-31 vestigated using eight columns (20 cm diameter, 50 cm 32 height). Six columns were irrigated in the laboratory by syn-33 thetic rainwater of pH 6, 3 and 2, two columns were exposed 34 outdoor to natural meteorological conditions. In the effluent 35 of the columns, the concentrations of the radionuclides were 36 measured periodically. With exception of the initial phase after 37 installation of the columns (ca. 0.4 y), the leaching rates of 38 radionuclides decreased slowly within the observation period 39 of 4.5 years. Due to the buffering capacity of the tailings 40 material, acidifying the irrigation water of the laboratory col-41 umns did not effect the effluent concentrations of the radio-42 nuclides. However, significant differences were observed be-43 tween laboratory and outdoor columns : outdoor, the concen-44 tration of 235 U in the effluents was for the first two years sub-45 stantially smaller than in the laboratory. As a consequence, 46 the accumulated leached activity of 235 U during 4.5 years was 47 19.0 Ϯ 0.9 % of the inventory for the laboratory columns, but 48 only 10.9 Ϯ 0.6 % for the outdoor columns. While uranium thus 49 proved to be rather soluble, the concentration of 226 Ra in the 50 effluents was quite small (Ͻ1 Bq/l). Thorium was retained 51 completely in the columns.52

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Quantitative Isotope Enrichment in Thermal Diffusion Separation ColumnIn applying the theory of the thermal diffusion separation column with containers at both ends to quantitative isotope enrichment, the volume of the column has not yet been exactly accounted for. A formula is derived which describes the effective volume as a function of the characteristic values of the enrichment apparatus. An iteration method for calculation of the effective volume is presented. The theoretical values are compared with experimental results in the case of time dependent enrichment of two isotopes ( 13 CH4, 14 CH4) and three different ways of enrichment. Agreement within the experimental limits of error of ± 6% was found.