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Emanation fraction of radon atoms created in a granular material was estimated using Monte Carlo method. Two models based on different arrangements of grains were considered: one of them representing simple cubic structure and the other one based on face centered cubic structure. As it was expected, the structure that provided higher density of grain packing (FCCS) gave lower values of radon emanation fraction. Both models showed that the probability of radon embedding in the neighboring grains would approach zero for large values of grain radius. Assuming homogenous distribution of 226 Ra within the whole grain volume, radon emanation fraction decreased with increasing grain radius while it showed the opposite behavior in the case of surface distribution of 226 Ra. The order of magnitude of radon emanation fraction decreased from 10 −2 % (for grain radius of few micrometers) to 10 −4 % (for grain radius larger than few millimeters) when homogenous distribution of 226 Ra was considered. If the distribution of 226 Ra was assumed only within the surface layer of a grain, emanation increased from less than 1% (obtained for grain size of few micrometers) to almost 25% (for large grain radius). The effect of moisture on the emanation fraction was confirmed by examining the case of water filling the space among the grains. Radon emanation was also considered analytically and the results were compared with Monte Carlo calculations. [ABSTRACT FROM AUTHOR]