Radon Migration Process and Its Influence Factors; Review

Radon (222Rn) generated within the grains of rocks, soils, building materials and other materials by the radioactive decay of radium (226Ra) can migrate to the atmosphere. This paper reviews the emanation coefficient, diffusion coefficient, and exhalation rate, and the factors that control the rate at which radon can enter atmosphere. The emanation coefficient which is the fraction of radon generated within the grains of materials and escaped to the pore space, varied from 2.1 to 32% for rocks, from 0.14 to 80% for soil, and from 0.10 to 58% in case of building materials. In addition, measurement methods used to evaluate emanation coefficient and its influence factors are also reviewed. The diffusion of radon is a process determined by radon concentration gradient across the radon sources and the surrounding air. The effective diffusion coefficient of some materials is summarized. Moreover, the radon exhalation rate process and the main influencing factors on the variation of exhalation rate data are reviewed. The exhalation rate varied in the range of 0.11 to 80 mBq m-2 s-1, 2.0×10-3 to 5.0×104 mBq m-2 s-1 and 4.0×10-3 to 5.0×101 mBq m-2 s-1 for rocks, soils and building materials, respectively.