Abstract
Based on the radionuclide distributions in sedimentary coal-bearing strata, this study analyzed the microrelease mechanisms of radon in coal-bearing strata. It was found that the microrelease process includes three stages: emanation, migration, and exhalation. Based on this, an experimental apparatus was independently designed for monitoring radon during compression of coal/rock samples from coal-bearing strata, whose major components include an electrohydraulic servocontrolled rock mechanics testing system, an airtight container, coal/rock samples, radon output device, and a continuous emanometer. The developed apparatus was preliminarily utilized for uniaxial compression tests on mudstone samples taken from the #21105 coalface of the Fourth Coal Mine in Yili Coalfield, China. The test results show that before sample failure under the uniaxial compressive load (UCL), the radon concentration is negatively correlated with the applied UCL and the magnitude of imposed elastic deformation. Increasing the applied load shortens the period of stable deformation, gradually decreasing the porosity of the rock, and as a result of declining the concentration of radon emanation from the rock. Finally, suggestions for future research are proposed, including mathematical equations to express the correlations between different experimental parameters and fractal characteristics of radon release from porous media.
1. Introduction
Radon (Rn) was first discovered from minerals containing radium in 1990. Since then, this radioactive element and its decay products have received growing attention. A great deal of research has been conducted to investigate the movement mechanisms of radon from deep underground to the ground surface, e.g., diffusion and convection action [1], pore fluid action [2], transportation by microbubbles [3], relay transmission [4], stress squeezing [5], deep-penetrating geochemistry [6], composite cluster theory [7], and radon’s inherent tendency to move upward [8]. Despite different explanations, it is an indisputable fact that radon can migrate from underground to the surface.
In the decay series of uranium, uranium decays to radium and then radium decays to radon. Uranium is widely found in the natural environment, including the atmosphere, surface soil, subsurface coal-bearing strata, and groundwater, leading to its ubiquity in the natural world [9, 10]. The continuous development of modern science and technology has enabled scientists to use radon as a tracer to solve various engineering problems, e.g., exploration of subsurface mineral deposits [11], prediction of geological hazards [12], detection of concealed goaves in coal mines [13].
Mining-induced coal-rock failure is the major cause of serious accidents during underground coal mining. Therefore, understanding the failure mechanism of coal-rock mass and obtaining critical information about the failure has become an important direction of research in the field of rock mechanics. Conventional methods include microseismic monitoring [14], acoustic emission [15], electromagnetic radiation [16], infrared radiation [17], transient electromagnetics [18], electrical prospecting [19], and ultrasound detection [20]. In recent years, radiation measurement based on radon’s properties has been utilized in underground coal mining in China [21]. This led to initial attempts to detect radon released by mining-induced overburden deformation and failure, with an aim to develop an innovative radon detection method for advanced forecasting of accidents [22–24]. To ensure the reliability and accuracy of the current radon detection method in solving engineering problems related to coal mines, some fundamental issues need to be further studied, such as the relationship between radon concentration and coal/rock deformation.
For this reason, this study analyzed the microrelease mechanisms of radon in coal-bearing strata. On this basis, an experimental apparatus for monitoring radon during compression of coal/rock samples was independently developed. Then, the experimental apparatus was preliminarily utilized to conduct uniaxial compression tests on rock samples (mudstone). Furthermore, suggestions for future research are provided in this paper related to mathematical equations to show the correlations between the experimental parameters and fractal characteristics of radon release from porous media.
2. Microrelease Mechanism of Radon
Coal-bearing strata, a lithostratigraphic term, refer to a succession of sedimentary strata containing coal streaks or seams from a certain period of time in geological history, in which coal is often accompanied by other minerals [25]. The content of radionuclides in an underground coal seam depends on the elementary composition of the source material from which the coal has been produced and the geophysical and geochemical processes it has undergone during and after coal formation. Zhu et al. have found that the absorption of radioactive substances by humic acids and humus, two organic compounds of coal, during the initial stage of coal formation, is a key contributor to the enrichment of radioactive elements in coal [26]. Uranium has a great affinity for organic matters and is widely found in low-grade metamorphic coals, especially brown coals. Commercially recoverable coal seams that are rich in uranium were mostly deposited in continental environments. As we know, the uranium-absorbing capacity of coal gradually decreases with greater intensity of metamorphism, while the diffusion and convection can promote the transference and accumulation of the uranium released from a coal seam to the roof strata. As the gangue surrounding coal seams in continental facies is mostly composed of mudstone, it is likely to have a high level of radioactivity. Additionally, the radionuclides present in the overlying soil are normally sourced from bedrock, which has a bearing on the composition and structure of coal-bearing strata [27]. Therefore, the continental depositional environment provides a sound geological basis for radon release.
Coal seams, broken rocks, and mine water are the major sources of radon in underground coal mines [28]. Radon atoms originated in the decay of radium within the solid components of the strata may not be released into the pore space due to their very low diffusion coefficients. However, when radon atoms escape into the space between the particles, they may be released to the surface atmosphere [29]. Figure 1 illustrates the whole microrelease process of radon atoms from underground strata to the surface atmosphere. Strata or soil represent the U- and Ra-bearing materials that can release radon, which consists of fine particles with different shapes, sizes, and pores at a microscale. The microrelease process of radon includes the following three stages: (a) Emanation, the process by which radon atoms move from the grains of source material to the intergranular space; (b) Migration, the process of diffusion and convection that causes the movement of the emanated radon atoms through the material to the ground surface; (c) Exhalation, the process by which radon atoms move from the ground surface into the surface atmosphere.
3. Experimental Apparatus Design
The experimental apparatus for monitoring radon during compression of coal/rock samples mainly consists of an electrohydraulic servocontrolled rock mechanics testing system (MTS815, MTS Systems Corporation, Eden Prairie, Minnesota, America), an airtight container, coal/rock samples, radon output device, and a continuous emanometer (KJD-2000R, Xstar Company, Sichuan, China), as shown in Figure 2. The experimental apparatus is easy to assemble, disassemble, and operate, while the required raw materials are easily machinable. Moreover, it allows direct observation of the coal/rock sample’s morphological change during the test process. In particular, the background concentration of radon can be elevated by adjusting the radon output device to reduce test errors.
3.1. Design of the Airtight Container
The airtight container is made of 1 cm thick toughened glass. It is cylindrical in shape, with a height of 30 cm and a diameter of 25 cm, and consists of a bottom, a wall, and a removable cover, as shown in Figure 3. There is a pair of symmetric lifting eyes on the removable cover for manually lifting the removable cover. The airtight container wall has an air inlet and an air outlet in its lower-middle part, which are connected to the radon output device and the continuous emanometer, respectively. A rubber gasket is attached to the edge of the removable cover in order to prevent leakage between the cover and the wall. Moreover, to prevent the removable cover from sliding down during testing, the rubber gasket is required to offer a frictional force that can balance the force of gravity acting on the removable cover.
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3.2. Design of the Radon Output Device
The radon output device consists of a pedestal and a steel shell mounted on it, as shown in Figure 4. It is filled with about 1 kg of tailings from a uraninite residue, used as the radon source. The shell is lead-coated to shield the experimenters against the radioactive material radiation. One end of the shell is connected to a gas pipe with an adjustable valve to control the radon flow into the airtight container and thereby regulating the background concentration of radon. The pipe mouth is connected to the air inlet of the airtight container via the inlet rubber hose.
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3.3. Selection of the Continuous Emanometer
The traditional types of instantaneous emanometers, e.g., FD-3017 emanometer, can be used to accurately measure radon concentrations, but they are incapable of providing continuous measurements. Therefore, KJD-2000R emanometer, a continuous emanometer, was selected as the measuring instrument, as shown in Figure 5. KJD-2000R continuous emanometer has multiple advantages, including being portable, flexible, highly sensitive, and is able to provide continuous measurements over a wide concentration range. The KJD-2000R emanometer is linked by an outlet rubber hose to the air outlet in the airtight container wall.
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3.4. Specific Implementation Method
The utilized MTS815 test system has a base plate at its bottom and an indenter at its top, with the airtight container placed in between. The following steps were undertaken for conducting the tests: (a) Each coal/rock sample was appropriately put into the airtight container as required; (b) The valve was opened to let the radon output device input radon into the airtight container; (c) The MTS815 test system applied a uniaxial compressive load to the coal/rock sample and the KJD-2000R continuous emanometer measured the radon concentrations during compression of the sample. To enhance the airtightness of the whole experimental apparatus, the pipe mouth, air inlet, and outlet and the connection between the continuous emanometer and the outlet rubber hose were sealed with butter. After the completion of tests, the obtained data were processed to analyze the pattern of variation in radon concentration during compression of the coal/rock samples.
4. Preliminary Application
The experimental apparatus described above was preliminarily utilized for uniaxial compressive tests on rock samples to test its reliability. The measurements from the experimental apparatus were then analyzed to investigate the pattern of variation in radon concentration during the compression of the samples. The tests were conducted at the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou City, Jiangsu Province, China. For safety reasons, we have measured the radon concentration of the five places in the experimental hall (as shown in Figure 6). The results show that the average value of radon concentration is 8.5 Bq/m3, which is satisfied with the limiting value (400 Bq/m3) of second-class civil construction engineering in “State Standard of the People’s Republic of China: Code for Indoor Environmental Pollution Control of Civil Building Engineering (GB50325-2010).” Therefore, the experimental environment is safe enough for technicians [30].
4.1. Rock Samples Preparation and Test Procedure
The utilized mudstone samples were obtained through a combination of ground drilling and underground sampling from the #21105 coalface of the Fourth Coal Mine in Yili Coalfield, Xinjiang Autonomous Region, China. The sampled mudstone consisted of quartz, feldspar, and uranium gel, with particle sizes ranging from 0.3 to 0.65 mm. During sampling, cylindrical rock samples (#1 to #3) with a diameter of 50 mm and a height of 100 mm were obtained by drilling, following the relevant standard (GB/T 23561.7-2009: Methods for determining the physical and mechanical properties of coal and rock–Part 7: Methods for determining the uniaxial compressive strength and counting softening coefficient). Thereafter, the samples were polished so that the difference in the parallelism of the two end surfaces was less than 0.02 mm, the perpendicularity of the two end surfaces to the sample axis was less than 3.5 seconds, and the surface roughness of the samples was less than 0.3 mm. Finally, polished samples were dried for 24 hours, and they were sealed before being used for the uniaxial compression tests. The uniaxial compression tests were conducted using the MTS815 test system with a maximum axial pressure of 4600 kN and a loading rate in the range of 0.1 to 0.5 kN/s. Loading and strains were measured using a dynamic strainmeter. Figure 7 shows examples of the prepared rock samples and the test procedure.
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4.2. Analysis of Experiment Results
The results of uniaxial compression tests show that after the uniaxial compressive load exceeded 16.75 MPa (the corresponding maximum axial deformation was 2.4 mm) (as shown in Figure 8, specific data are presented in Tables 1 and 2), cracks formed in mudstone sample and it failed eventually, which indicates that the limit strength of this mudstone sample is about 16.75 MPa. Therefore, the loading strength for the test on samples (#1– #3) was set at 5 MPa, 10 MPa, and 15 MPa, respectively. The radon concentration in the radon output device delivered to the airtight container was set at 200 Bq/m3. The decay constant of radon is 2.097 × 10−6/s. It usually takes about 30 days to reach radioactive equilibrium for radium decay becoming radon, as well as the emanometer just measures the 222Rn itself. For this reason, we could take no account of the effect of radon decay, which would not affect the experiment precision. Radon concentration measurement was conducted when a constant strength of 2 MPa was applied for 5 min on three samples. Table 1 is the measured radon concentration of samples during the experiment. Figure 9 shows the axial deformations and radon concentrations of samples against loading time under different UCL. Uniaxial deformations of samples increased gradually and eventually stabilized before the sample’s failure. This process can be divided into three stages, including transient loading strain, attenuated strain, and allostasis. Although there were no microphotographs of rock microstructures, acoustic emissions, or porosity data, based on the variation features of radon concentrations, an inference can be made as follows: Initially, uniaxial deformations and strain rates of the samples were relatively high, indicating compression of the internal open structures and microcracks. The rate of axial deformation decreased with the increase in the loading time. The results also reveal that uniaxial deformations were positively related to UCL and the loading time for deformation to reach a stable state. These also reveal that the internal porosity of samples decreased as the UCL was increased [31].
5. Suggestions
Over the past century, new methods for measuring radiation have been developed and new applications have been discovered. It should be pointed out that various engineering applications of radiation measurements are still in the initial stages of development. In particular, the serious lack of fundamental knowledge poses an obstacle to the improvements in such technologies. The experimental results reported in this paper are based on limited rock samples with a specific condition. Thereby, more studies are needed to address the corresponding issues. For example, on the one hand, from the perspective of refinement, explore the correlations between different experimental parameters, especially how to use the mathematical equations to express the above correlations, which will be an important aspect of future research. On the other hand, the release of radon from coal/rock is a complex process that is affected by multiple external and internal factors [32], such as lithology, the content of radium isotopes, pore size, porosity, particle size, permeability, emanation coefficient, air pressure, temperature, humidity, and wind speed. In addition, considering the complex interior structures of porous media, future research can use the fractal theory to study the radon release from porous media and to apply it in the management of the radioactive environment. A recommended approach to radon prevention and mitigation is to reduce the radon release rate by changing the fractal structure of porous media.
6. Conclusion
In this study, the microrelease mechanisms of radon in coal-bearing strata were analyzed, an experimental apparatus was independently designed for monitoring radon during compression of coal/rock samples from coal-bearing strata, and the apparatus was preliminarily utilized for uniaxial compression tests on mudstone samples taken from #21105 coalface of the Fourth Coal Mine in Yili Coalfield, China. The following conclusions were obtained:(1)All coal-bearing strata are more or less radioactive. The continental depositional environment of coal provides a sound geological basis for the radon radon release. The microrelease process of radon atoms includes the following three stages: (a) Emanation, the process of radon atoms moving from the particles of source material to the intergranular space; (b) Migration, the process of diffusion and convection driving the movement of the emanated radon atoms through the material to the ground surface; (c) Exhalation, the process of radon atoms moving from the ground surface into the surface atmosphere.(2)The experimental apparatus during compression of coal/rock samples mainly includes a MTS815 test system, an airtight container, coal-rock samples, a radon output device, and a KJD-2000R continuous emanometer. This apparatus is easy to assemble, disassemble, and operate, while the required raw materials are easily machinable. Moreover, it allows direct observation of the tested sample’s morphological change during testing. In particular, the background concentration of radon can be elevated by adjusting the radon output device to reduce test errors.(3)The test results indicate that before the failure of rock samples, the elastic deformations and radon concentrations were correlated with the UCL, i.e., positively and negatively, respectively. Furthermore, the time needed for deformations to reach the stable state and internal porosity of samples decreased as the UCL was increased, resulting in reduced radon concentrations. The above results also further verified the reliability of the experimental apparatus for monitoring radon.
Data Availability
The article data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 51874278 and 51974291), the Foundation Research Project of Jiangsu Province (No. BK20181357), the “Six Talent Peaks” Project of Jiangsu Province (No. JNHB-087), the Talent Support Project of Jiangsu Association for Science and Technology (No. 2019-134), and the Independent Research Project of State Key Laboratory of Coal Resources and Safe Mining (No. SKLCRSM2020X04). The authors wish to thank the Fourth Coal Mine in Yili Coalfield, China, for supporting field help to conduct this important study. Special thanks are due to Mapletrans Company in Wuhan, China, for its professional English editing service.