The thickness and compaction degree of the overburden soil on the beach of the uranium tailings reservoir has an important influence on the radon reduction rate. A theoretical model of radon exhalation is established and an experimental device is designed. The main results are as follows. (1) The radon reduction rate increases with the increase of thickness. When the soil compaction degree is 85.5%, 90.2%, and 94.8%, the radon reduction efficiency increases significantly when the thickness increases from 5 cm to 10 cm, and when the soil thickness is over 10 cm, the increase of radon reduction efficiency tends to be stable. When the compaction degree is 80.9%, the radon reduction rate always increases obviously with the increase of the thickness of the overburden soil, but the increase rate shows a downward trend. (2) The radon reduction rate increases gradually with the increase of compaction degree, and the increasing trend becomes less obvious when the compaction degree is more than 85.5%. Besides, the effect of the change of soil compaction on radon reduction rate decreases with the increase of soil thickness. The calculation formulas about the effect of thickness and compaction degree on radon reduction rate can guide the design and construction of radiation protection of uranium tailings reservoir.

In order to find out radon reduction performance of the overburden layer on uranium mill tailings (UMTs) pond beach surface after rainfall, the rainfall simulation experiment of the overburden layer was carried out with the self-developed equipment. Based on the radon migration model of the overburden layer on the UMTs pond beach surface, the change rule of radon exhalation in four types of compactness of the overburden layer within 120 hours after rainfall was studied, and the corresponding moisture content was also analyzed. The results show that the radon concentration in the overburden layer of UMTs increases nonlinearly; the dynamic change in moisture content of the overburden layer on the beach surface leads to the unsteady radon exhalation. The variation of radon exhalation shows three stages: increase, linear decrease, and stability tendency. After rainfall, radon exhalation rate increases due to water vapor and there is free radon seepage in pores. With the decrease of free radon production rate, radon exhalation rate gradually decreases until it reaches stability again. When the thickness of the overburden layer reduces, the porosity decreases with the increase in compactness of the overburden layer. While the decrease in radon reduction is more obvious, the less time it takes for radon exhalation to vary from unstable to stable overburden after rainfall.

The majority of uranium mill tailing impoundments in the southern part of China are located in humid subtropical regions where persistent rainfall and rapid evaporation of water after rain often occur. Under the prolonged influence of alternating wet and dry conditions, the covering soil layer of uranium mill tailing impoundments develops cracks, leading to the issue of degradation or even failure of the radon retardation effect. A beach surface of uranium mill tailing impoundments in the southern part of China is selected as the research object. Through use of a self-made simulation test device, a degradation experiment of uranium mill tailing covering soil models under wetting-drying cycles was conducted indoors. The experimental results indicate that with a constant amplitude of wetting-drying cycles, microcracks characterized by a narrow width and high abundance were mainly generated in the early-to-mid-stage of wetting-drying cycles. The main cracks, characterized by their wide width and less abundance, were mainly formed in the mid-to-late stage of wetting-drying cycles. After seven wetting-drying cycles, the total length of cracks showed a "stair-step" increase and the surface crack ratio exhibited a trend of moving from rapid growth to stable growth and then to a slight decline. The cumulative damage degree showed a rapid increase to stable growth with an increase in the number of wetting-drying cycles. Grey relational analysis revealed that, compared to other surface crack indicators, radon exhalation rate was the most closely correlated with the surface crack ratio. With a constant amplitude of wetting-drying cycles, the radon exhalation rate underwent four stages as the number of wetting-drying cycles increased: stage I witnessed a rapid increase, stage II witnessed a rapid decrease, stage III witnessed a gradual increase, and stage IV witnessed a stable or even slight decrease. With a constant number of wetting-drying cycles, the radon exhalation rate correspondingly increased with the amplitude of wetting-drying cycles, particularly noticeable when the alternation amplitude was 30 ± 20%. From the early mid-stage to the late stage of wetting-drying cycles, the curves of the radon exhalation rate, surface crack ratio, and cumulative damage degree tended to be consistent, showing a gradual increase. The research provided in this study offers valuable insights into radon control measures and environmental assessments on the beach surface of uranium mill tailing impoundments.