Abstract
Geological media are omnipresent in nature. Lab-scale tests are frequently employed in radon exhalation measurements for these media. Thus, it is critical to find the thickness of the medium at an experimental scale that is equivalent to the medium thickness in a real geological system. Based on the diffusion-advection transport of radon, theoretical models of the surface radon exhalation rate for homogeneous semi-infinite and finite-thickness systems were derived (denoted as Jse and Jfi, respectively). Analysis of the equivalency of Jse and Jfi was subsequently carried out by introducing several dimensionless parameters, including the ratio of the exhalation rates for the semi-infinite and finite-thickness models, ε, and the number of diffusion lengths required to achieve a desired ε value, n. The results showed that when radon transport in geological media is dominantly driven by diffusion effect, if n > 3.6626, then ε > 95%; if n > 5.9790, then ε > 99.5%. When radon migration is dominantly driven by advection effect, if n > 2.5002, then ε > 95%; if n > 4.0152, then ε > 99.5%. Therefore, if the thickness of the geological media (x0) is greater than a certain n times the radon diffusion length of the media (L), the media can be modeled as semi-infinite. To validate the model, a pure radon diffusion experiment (no advection) was developed using uranium mill tailings, laterite, and radium-bearing rocklike material with different thicknesses (x0). The theoretical model was demonstrated to be reliable and valid. This study provides a basis for determining the appropriate thickness of geological media in lab-scale radon exhalation measurement experiments with open bottom.
Highlights
Rn is a major contributor to ionizing radiation, corresponding to about half of the total radiation exposure received by the human population
The most widely studied subjects are the radon profiles in various media for geophysical purposes, indoor radon concentrations, radon exhalation from uranium ore stockpiles and uranium mill tailings (UMTs) piles, and the radioactive impacts of radon the on surrounding environment
Most previous studies of radon exhalation from geological media considered cases in which the radon migration was dominantly driven by diffusion
Summary
Rn (hereafter referred to as radon) is a major contributor to ionizing radiation, corresponding to about half of the total radiation exposure received by the human population. Intrinsically present in geological media, such as the ground ( e.g., soil, sand, or bedrock), uranium ore stockpiles, and mill tailings piles, in various amounts (Girault et al 2010). Graaf et al (1992, 1994) and van der Spoel et al (1997, 1998a, 1998b, 1999) studied the diffusive and advective radon transport in soil and sand using laboratory experiments with a stainless-steel vessel (height and diameter of 2 m). Catalano et al (2015a) presented a numerical model to examine the radon transport through porous media and compared the model with experimental results. Analysis of the equivalent thickness of geological media for the lab-scale study of radon transport is presented in this paper.
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