The effect of grain size on the radon emanation rate

Abstract

Tailings produced during mining and milling of uranium ores represent potentially large volumes of low level radioactive materials. A typical environmental problem associated with mill tailings is radon emanation. Covering tailings is widely applied to reduce radon exhalation rate. In this paper, the fractal theories and field covering tests are used to study the fractal characters of size distribution of six types of covering materials, including waste rock, sand, laterite, kaolin, mixture of sand and laterite, and mixture of waste rock and laterite, and their influences on radon exhalation. The size distributions of uranium tailings and the six aforementioned covering materials all exhibit a good fractal structure. The contents of fine grain increase with the increasing value of fractal dimension. The results of field radon measurement show that the radon emanation rate of tailings without covers is 14.7–18.6 Bq/m2 s. Covering tests were carried out of the six abovementioned covering materials with thickness of 0.4 m, 0.8 m, 1.2 m, 1.6 m and 2.0 m. The results indicate that the application of these materials for cover layers can decrease the radon exhalation rate markedly. The effectiveness of a cover layer in reducing radon exhalation is related to its fractal texture of size distribution. Under the same thickness conditions, the attenuation coefficient of radon exhalation rate increases with the increasing fractal dimension of size distribution of covers. The empirical expressions of the attenuation coefficients in relation to fractal dimension D of size distribution and thickness x of covers is obtained for evaluating the effectiveness of final covers for uranium tailings impoundments.