Radon transport carried by geogas: prediction model.

This paper provides an overview and information on radon migration in the crust. In the past several decades, numerous studies on radon migration have been published. However, there is no there is no comprehensive review of large-scale radon transport in the earth crust. A literature review was conducted to present the research on the mechanism of radon migration, geogas theory, investigation of multiphase flow, and modeling method of fractures. Molecular diffusion was long considered the primary mechanism for radon migration in the crust. However, a molecular diffusion mechanism cannot explain the understanding of anomalous radon concentrations. In contrast with early views, the process of radon migration and redistribution within the Earth may be determined by geogas (mainly CO2 and CH4). Microbubbles rising in fractured rocks may be a rapid and efficient way of radon migration, as reported by recent studies. All these hypotheses on the mechanisms of geogas migration are summarized into a theoretical framework, defined as "geogas theory." According to geogas theory, fractures are the principal channel of gas migration. The development of the discrete fracture network (DFN) method is expected to supply a new tool for fracture modeling. It is hoped that this paper will contribute to a deeper understanding of radon migration and fracture modeling.

A two-dimensional model for the analysis of radon migration in fractured porous media.

This paper develops a new two-dimensional model to estimate the radon exhalation rate of fractured porous media. The fractal discrete fracture network is used to characterize the fracture structure in the model. The finite element method solves the governing equations of radon migrations in fractures and porous matrix. Well-equipped laboratory tests validate the model with reasonable accuracy. The comparison of the model with the traditional radon migration model indicates that the model can simulate radon migration in fractured porous media more effectively than the traditional model. The effects of fracture intensity (P21), seepage velocity, and fracture connectivity on radon migration in fractured porous media are analyzed using the model. The radon exhalation rate increases with the fracture intensity and seepage velocity. There is an exponential relationship between fracture connectivity and radon concentration. The model provides a reliable method to analyze radon migration in fractured porous media and is helpful for radon pollution prevention and control.

Influence of Neutralization Accelerated by the Soak Method on the Radon Exhalation of Cement-based Materials.

ABSTRACT: Radon is considered a significant contaminant that affects indoor air quality. Neutralization is one of the most important environmental factors in changing pore structure of cement-based materials that may release radon. Therefore, it is necessary to study the radon exhalation of cement-based materials impacted by neutralization, which is extremely lacking. Some concrete blocks were accelerated to neutralization by soaking in sodium bicarbonate solution, and the radon activity concentrations over the blocks were measured to investigate the effect of neutralization on radon exhalation. Controlled experiments were conducted with blocks in the initial dry state and soaked in pure water. The results show that the radon activity concentration exhaled from block is promoted by neutralization, and the promoting role of water is quite limited. The radon surface exhalation rate increases by increasing the sodium bicarbonate solution concentration from 0.5% to saturation.

Novel method for measuring temperature-dependent diffusion coefficient of radon in porous media.

The temperature-dependent diffusion coefficient of radon is one of the most important parameters for predicting radon migration in porous media. In order to measure this parameter more effectively and accurately, theoretical formulas were derived by the steady-state one-dimensional equation of radon migration in porous media for designing the corresponding experimental device, which was used to measure the diffusion coefficients of radon in uranium mill tailings. The results show that the diffusion coefficient of radon in porous media increases with increasing the temperature, which is in agreement with existing researches, verifying the method effectiveness. The changes of the diffusion coefficient of radon with the absolute temperature follow a power law.

A fractal analysis of radon migration in discrete fracture network model.

A novel model is proposed to simulate radon migration by combining the fractal theory and the discrete fracture network (DFN) model. In the model, a power-law distribution based on fractal theory is applied to fracture length and aperture and the fracture locations and orientations are modeled with the Poisson distribution and von Mises-Fisher distribution, respectively. The model was applied to produce a computer code that can calculate the radon concentration, flux, and diffusivity of the fractured media. The key issues related to the model were analyzed and the results reveal that: (1) the threshold value of the ratio of the minimum fracture length to the maximum decreases as the fractal dimension of the fracture lengths and the relation between them follows an exponential law; (2) As the fractal dimension of the fracture lengths increases, more connected fractures are generated, resulting in a linear increase of the mean efficient radon diffusivity. (3) The dip angle is the parameter that has the greatest influence on radon migration in determining fracture orientations. (4) The radon exhalation rate increases exponentially with increasing advection velocity. (5) Models with larger fractal dimension for fracture lengths have larger representative elementary volume (REV) size and follow an exponential law.

Heat-air-moisture coupled model for radon migration in a porous media.

Radon is one of the main causes of environmental pollution and lung cancer. The precipitation of radon from porous media is affected by the coupling of heat and moisture, which has not been considered in the existing knowledge. We present a model for predicting radon migration in porous media. This model combines the heat-air-moisture (HAM) coupling model of porous media with a radon migration model to establish three-dimensional partial differential equations for steady-state radon migration under HAM coupling conditions. The finite element method (FEM) was used to obtain a numerical solution. Experimental verification showed that the model had high calculation accuracy; the calculated maximum relative error did not exceed 15%. The results of the model were compared with the results of a conventional model that does consider the coupling of heat and humidity; the results showed significant differences in the radon concentrations and radon flux distribution curves for the two models. The newly developed model revealed that there is a significant coupling effect between migration and the distribution of the temperature field, the humidity field, and radon flux in unsaturated porous media. The radon exhalation rate on the surface of porous media increases linearly with the increase of permeability. The exhalation rate decreased exponentially with the increase in relative humidity. When the trend of the temperature gradient was consistent with the concentration gradient, the radon exhalation rate decreased linearly with the increase in temperature gradient. We establish a new model to study the radon migration in porous media under the coupling of heat and moisture. The model provides a theoretical basis for an effective and accurate analysis of the impact of radon exhalation on the environment.

Experimental study on radon exhalation behavior of heap leaching uranium ore column with dilute sulfuric acid.

In order to study the radon release behavior when heap leaching uranium ores with dilute sulfuric acid, unleached uranium ores from a uranium mine in southern China were selected as test samples. Adopting parameters from leaching processes commonly used in uranium mines, a laboratory experiment was carried out for 21 days with a one-dimensional acid heap leaching experimental column. The surface radon exhalation rate of uranium ore column was determined by static accumulation method while spraying with deionized water and dilute sulfuric acid. The uranium leaching rate and ore column height for all 21 days of the experiment were also measured. The results show that (1) when sprayed with a leaching agent, the surface radon exhalation rate of uranium ore column initially increased with time sharply. After a maximum value was reached, the rate gradually decreased and stabilized. When the spraying stopped, the surface radon exhalation rate of uranium ore column initially decreased, before increasing until it tended to stabilize. (2) During the entirety of the 21-day leaching experiment, the cumulative leaching rate of uranium increased gradually with time. On the other hand, the surface radon exhalation rate of uranium ore column fluctuated, but the leaching of uranium from uranium ores had almost no effect on the radon exhalation rate. (3) There was no linear correlation between the surface radon exhalation rate and the residual height of ore column during leaching, but the collapsing event of ore column was the direct inducing factor of the fluctuation of surface radon exhalation rate.