Study On Radon Longitudinal Transmission Rate In Overburden Of Coal Spontaneous Combustion Goaf

Goafs account for more than 60%of all spontaneous combustion disasters in coal mines as a result of their high concentration of coal.The proper setting of the spontaneous ignition site is the key to preventing and treating coal spontaneous combustion accidents.The isotope radon detection method has been used as an efficient technical means to determine the location of natural fire in mines,and has been applied in many mining areas with serious natural fire in China and Australia.The long-distance transport method of radon gas from deep beneath to near-surface has not been precisely defined,which severely limits the detection accuracy and practical application effect of isotope radon detection technology.It is vital to perform in-depth research on the critical parameter of radon transport rate in the overlying strata in order to systematically examine the long-distance transport mechanism of radon and create a thorough long-distance transport model of radon in the coal spontaneous combustion zone.However,there is minimal discussion of the rate of radon transmission in overlying strata in current radon migration theory research,and there is no known rate calculation method.To compensate for the lack of research on the transmission rate of radon in the overlying strata under the geological background of coal spontaneous combustion,this paper begins with the inherent emission characteristics of radon gas,calculates the formula for the residence time of nuclides based on the decay balance equation of radon and its daughters,and proposes a method for calculating the transmission rate of radon in the overlying strata.The surface drilling core sampling experiment was carried out with the spontaneous combustion goaf of the coal mine in Jinyuan District,Taiyuan City,Shanxi Province as the experimental sampling region to further test the feasibility and practical application effect of the rate calculation method.Following radionuclide determination experiments,the radioactive specific activities of ²¹⁰Po and ²¹⁰Pb in the core samples were determined,and the²¹⁰Po/²¹⁰Pb ratio data needed to calculate the rate was obtained.The transmission rate of radon in the overlying strata in the coal spontaneous combustion zone is eventually calculated using the difference in depth of the overlying strata and the difference in residence time.Based on the results of the oxidation heating experiment and the radon transmission rate calculation,a coal spontaneous combustion radon gas transmission model was built to simulate the invisible process of radon migration in the underground overlaying strata.The influence and effect of changes in radon source initial concentration,overburden diffusion coefficient,and transmission rate on radon migration process and concentration distribution in overburden are studied using a numerical simulation method in order to better understand the radon migration mechanism in overburden in the context of coal spontaneous combustion.The findings reveal that,the radon transmission rate can be determined using the radiation imbalance degree of radon progeny ²¹⁰Po and ²¹⁰Pb in overburden rock samples under the geological context of coal spontaneous combustion goaf.The nuclide detection results of coal spontaneous combustion goaf core samples show that the total amount of radionuclides ²¹⁰Po and ²¹⁰Pb adsorbed by the overlying strata in the spontaneous combustion zones is significantly higher than that in the non-spontaneous combustion.The specific activity of ²¹⁰Po in core samples from the spontaneous combustion area is around 3.71 times that of the non-spontaneous combustion area,whereas ²¹⁰Pb’s specific activity is 1.79 times that of the non-spontaneous combustion area.The radon gas transmission rate in the overlying strata was calculated,and the results reveal that spontaneous coal burning in the goaf accelerates the radon transmission process in the overlying strata.In the spontaneous combustion zone,the transmission rate of radon in the underlying strata ranges from2.4600×10^-4 to 6.2486×10^-4 m/s,with an average velocity of 4.2926×10^-4 m/s,which is about6.49 times that of the non-spontaneous combustion zone.The numerical simulation findings reveal that as the migration distance increases,the radon concentration in the underlying strata decreases exponentially.The migration process of radon in the strata is influenced by the initial concentration of radon gas source,diffusion coefficient,and transmission rate.The overall amount of radon gas engaged in the migration process is determined by the concentration of radon gas source.If all other factors remain constant,the concentration of radon in the overburden at the same depth rises in tandem with the concentration of the radon gas source.The diffusion coefficient is a crucial factor in radon transport.However,while increasing the diffusion coefficient alone will not improve radon transport in the longitudinal direction,it will improve radon transport in the lateral direction,allowing radon gas to migrate and spread horizontally and vertically from the radon gas source to the surrounding.The longitudinal transport of radon in the overburden is determined by the transmission rate.The longitudinal migration capacity of radon in the overlaying strata of spontaneous combustion goaf is strengthened when the radon transmission rate increases.More coal spontaneous combustion radon gas can travel to the surface under the same layer of overburden,generating abnormal surface radon concentrations.Increased transmission rate intensifies radon gas transfer between different depths of overburden layers,improves radon transmission efficiency in the overburden layers,and allows radon gas to migrate longitudinally over a longer distance in the overburden layers.