Concentrating of natural origin radionuclides using nuclear filters

Full text: Presently, territories of uranium mines have accumulated plenty of dumps which processing is unprofitable because of low concentration of uranium. However, mining rocks in these dumps are subject to natural influence (atmospheric precipitations, wind) as a result of which there is an environmental contamination. The purpose of this work is to study the element contents of water from these dumps with the help of nuclear filters. The factor of radionuclides filtration by nuclear filters was defined by means of water in which some weeks earlier rocks from uranium mine dumps were crushed. Acidity of the solution was pH =5. The analysis was performed on the gamma-spectrometer with the Ge-detector with relative efficiency of 20%. Gamma-spectra have shown presence in this solution of five radionuclides: 214Bi, 214Pb, 234Th, 234mPa and 235U, activities of which were measured daily. Initial activity of 214Bi and 214 Pb was 6 * 103 Bq/l, but quickly reduced, and in 10 days reached 6*10 Bq/l. On the contrary, activities of 234Th and 234Pa grew, and from approximately 100 Bq/l in 20 days increased up to 800 Bq/l and 1700 Bq/l, respectively. Activity of 235U radionuclide was constant and made (115 ± 2) Bq/l. In 20 days, the behavior of radionuclides in a studied solution has been determined. The solution has been passed consequently through nuclear filters with diameter of holes 0.2 and 0.03 microns, and then the activities of this solution and a deposit were measured by filters. It was revealed, that 234Th and 234mPa activities have decreased 8 - 9 times, and then increased with the same rate, as that for initial solution. Activity of 235U after filtration has not changed. In 27 days after filtration when activities of 234Th and 234mPa have considerably increased (up to ∼1* 103 Bq/l and ∼ 2 * 103Bq/l, respectively), the solution has been passed again through the 0.2 micron nuclear filter. The subsequent analysis has shown similar result of 234Th and 234mPa activities change. Activity 235U radionuclide has remained at initial level. Thus, we were twice convinced that after filtration, the activities of 234Th and 234mPa decreased 8 - 9 times, and activity of 235U remained constant. Then similar measurements have been conducted with the solution at pH = 7. After filtration through the 0.2-micron nuclear filter, it was again observed that the activities of 234Th and 234mPa reduced 8 - 9 times. At the same time, 235U activity fell 1.9 times. Further alkalinity of the solution has been increased up to pH =11 and the filtration through the 0.2-micron filter has been conducted. Factor of 234Th and 234mPa filtration is KF 5.6, and activity of uranium has decreased 1.9 times. By increasing to pH=14, we have considerably increased quantity of a deposit. Actually, all elements that were contained in a solution have precipitated in a deposit. We shall notice that the 226Ra nuclide was not found in the solution. Energy of its gamma-line is equal to 186.2 keV and it is very close to energy of 235U gamma-line (185.7 keV). These lines cannot be resolved by spectrometer, but interactive adjustment of the form of peak (program Genie -2000) resolve them, when amplification factor of the spectrometer increases up to such degree that all interesting gamma-lines of other nuclides were still on a scale of the analyzer. Interactive adjustment of peak has shown that 226Ra was absent in water (Ra lixiviation at pH = 5 did not occur). Absence of Ra in water is also indicated by sharp reduction of 214Pb and 214Bi radionuclides activity - quickly breaking up elements of 238U radioactive chain, in which 226Ra is the part of this chain. The following filtration experiments were conducted with water, in which Pb, Bi, U, and Ra were lixiviated from rocks at pH =3. These samples water contained 210Pb, 214Pb, 214Bi nuclides, for which activity behavior could be observed at filtration. The filtration at pH =3 has not affected concentration radionuclides. The element contents in deposits on nuclear filters were also measured by X-ray fluorescence and activation analyses. Results of analyses of structure of water from refuse dumps of uranium mines with the help of nuclear filters show that lixiviation of heavy elements can occur in the natural way. (authors)