Effect of Different Cover Materials on Exhalation of Radon from the Radioactive Wastes

Abstract

Uranium tailings represent a huge radioactive waste contaminant, where radon emanation is considered a major health hazard. Many trials have been conducted to minimize radon exhalation rate by using different covering materials. In the present work, three covers as available materials (kaolin, white sand and bentonite) have been used with different thicknesses (1 cm, 1.5 cm, and 2 cm). 238U, 232Th, 40K and the radon exhalation rate were measured by using gamma spectrometry with a Hyper Pure Germanium (HPGe) detector and solid state nuclear track detectors (CR-39). Radon exhalation rates, calculated before and after covering, ranged from 2.80 ± 0.14 to 4.20 ± 0.21 Bq.m-2.h-1, and from 0.30 ± 0.01 to 4.00 ± 0.20 Bq.m-2.h 1, respectively. Also, the attenuation coefficients of different covering materials were calculated. The obtained results demonstrate that applying of kaolin, white sand and bentonite to uranium tailings has potentially minimized both the radon exhalation rate and the corresponding internal doses. Movement of radon gas out of the tailings is strongly influenced by the physicochemical characteristics of these tailings especially their radium content and the grain size. So, the tailing samples were size-fractionated into four sizes (> 250, 250-125, 125-74 and < 74 µm). The activity concentrations of different radionuclides in size-fractionated tailing samples have been shown to be strongly dependent on the size of the particles. In the range of > 250 : < 74 μm, the activity concentrations of 230Th, 226Ra, 214Pb, 214Bi, 210Pb, 232Th and 40K increased throughout with decreasing particle size, while that of 238U, 234U and 235U an opposite effect were observed. The results revealed an inverse relationship between the radon exhalation rate and size fractionation. Also, the results showed a good correlation between radium activity concentration and radon mass exhalation rate.