Study on the effect of surface cracks in overburden on radon exhalation in uranium tailings ponds under freeze-thaw cycles

Freeze-thaw (F-T) climate causes cracking in the soil cover of uranium tailing ponds, and the exhalation of the carcinogenic gas radon needs to be monitored under the phenomenon. In laboratory experiment, cracking tests under 5 F-T cycles were carried out on 3 soil uranium tailing pond models, including Gansu loess (GS-L), Hengyang laterite (HY-L), and Hebei loess (HB-L). Each cycle consisted of −15 °C, 12 h of freezing and 15 °C, 12 h of thawing. The area, length, mean width and fractal dimension of the cracks on the soil surface were quantified using digital imaging techniques, the correlation of parameters with the radon exhalation was compared. Finally, the qualitative explanation was provided from the perspectives of visual and scanning electron microscopy (SEM). The results showed that: Fine, short and irregular cracks developed to penetrating wide cracks after the 3rd cycle. The radon barrier capacity of the soil was negatively correlated with the number of F-T cycles. After 5th cycles, the radon exhalation rates of GS-L, HY-L, and HB-L were 0.675 $Bq/\left(m^2·s\right)$, 0.555 $Bq/\left(m^2·s\right)$, and 0.462 $Bq/\left(m^2·s\right)$, which increased by 63.8 %, 43.1 %, and 39.6 %, respectively. The mean width of cracks showed the highest correlation with radon exhalation rate: HY-L (0.91), GS-L (0.89), and HB-L (0.87). SEM results showed fewer soil internal pores mean harder radon migration. The findings are relevant to the monitoring of radioactive gas in uranium tailing ponds.