Experimental study on radon retardation effect of modular covering floats in radon-containing water☆
Graphical abstract
Introduction
Radon (222Rn) is a naturally occurring radioactive inert gas. It was classified as a Class I carcinogen for humans by the International Agency for Research on Cancer and as one of the 19 major human carcinogenic factors by the World Health Organization (WHO) (Tong, 2007; Gooding, 2018). In recent years, the radon concentration in uranium mines and their surrounding areas has increased significantly. The risk of radioactive pollution from radon gas released from radon-containing water bodies in heap leaching and in-situ leaching uranium mines, including dispensing pools, impounding basins and artificial reservoirs containing transformation waste water pools, is inevitable (Li et al., 2015; Wang et al., 2021). Therefore, it is necessary to develop methods and devices to retard radon exhalation from radon-containing water in uranium mining areas.
The migration and exhalation of radon in water are complex processes. The surface radon exhalation rate of water is affected by factors such as water level, diffusion coefficient (Ye et al., 2016; Ye et al., 2019), and wind field which changes the transfer velocity at the gas-liquid interface (Cockenpot et al., 2015; Ye et al., 2019). While these studies mainly focus on the migration and exhalation of radon in water, researches on reducing radon release in water, as well as preventing and controlling its pollution, are not well developed. In China's uranium mines, caps are typically placed on top of radon-containing pools, with an air layer between the cap and the water surface. However, the dissolved radon in water can enter such air layer very easily. This could create a workplace with high radon concentration for staff in the pool, or worse, release radon to the atmosphere outside due to changes in water level and poor tightness of the cap. A more efficient method to retard the release of radon from water is therefore warranted. In recent years, covering methods were shown as one of the most effective methods in retarding the exhalation of airborne radon in rock, soil, and housing building materials (Daoud and Renken, 2001; Singh et al., 2005; Qu, 2012; Deng et al., 2017), but few researches have been focused on retarding radon exhalation in water. In view of this, this paper proposes a radon exhalation retardation device (RERD) with flexible modular floats based upon theories of radon migration and exhalation in water. Experiments were also conducted to investigate the radon retardation effect of such modular surface-covering floats.
Compared with the aforementioned method of covering the water with a cap, RERD can retard radon exhalation more effectively. This is because the modular surface-covering floats allows radon to decay in water as much as possible, thus reducing radon exhalation from water. In addition, the floats float up and down with the water level, allowing contact with the gas-liquid interface to be maintained at all times. This provides new possibilities for studying of radon migration rules and protection against radon in uranium mines.
With the goal of effectively reducing the release of radon in water and satisfying low-carbon environmental protection requirements, experiments were conducted to study the effect of radon retardation of modular surface-covering floats using the RERD. Two experimental conditions were investigated: unperturbed water bodies, which represents static water such as those in lakes or tailings pond beach areas, as well as perturbed water bodies, which represents water with inflow and outflow, such as those in the underground mine pit pools or the leaching solution pools of in-situ leaching uranium mines. The results of the experiments shows that RERD has an excellent radon retardation effect, proving great scientific value in protecting the surrounding environment of mines and the health of miners.
Section snippets
Radon migration model in water
The migration of radon in water includes the migration process of radon in the sediment area, overlying water area, sediment-water interface and gas-liquid interface. According to Fick ‘s law (Webb and Pruess, 2003) and the radon transfer theory at the gas-liquid interface (Kawabata et al., 2003; Webb and Pruess, 2003; Cockenpot et al., 2015; Ongori et al., 2015), a one-dimensional mathematical model for radon diffusion and migration is established as shown in Fig. 1.
Principle
According to theories of radon migration and exhalation in water, when the free radon production rate in water is constant: 1) The surface radon exhalation rate of water increases with the difference in radon transfer velocity and radon concentration between water phase and gas phase at the gas-liquid interface. 2) The amount of radon exhaled from water surface increases with the exposed surface area of the water body and the radon exhalation rate.
Components of RERD
The design of RERD is shown in Fig. 2. It
Experimental setup
There are four parts to our experimental setup: a sample holder, an RERD that covers radon-containing water, a circulating water pump (The brand is Kodiak and the circulation flow rate is about 450 L/H,with a power of 6 W), and a radon concentration measurement system, as shown in Fig. 3. The sample holder (an acrylic container with inner length of 0.485 m, inner width of 0.335 m, and inner height of 0.24 m) has a scale on the wall for measuring the water level with a water sample collection
Accumulated radon concentration
The accumulated radon concentration of the radon collection system increased with time as shown in Fig. 6. As can be in Fig. 6, the accumulated radon concentration of the radon collection system increased linearly with time for different area coverage rates and different immersion depths from 0 to 24 h.
Radon exhalation rate, radon retardation rate and transfer velocity
Based on the monitoring data of accumulated radon concentration under different coverage conditions, the radon exhalation rate, amount radon exhaled and radon retardation rate under different
Conclusion
Based on theories of radon migration and exhalation in water bodies, an RERD with flexible modular surface-covering floats was designed and manufactured. The radon exhalation rate of the exposed water surface under different coverage conditions was measured by experiments in unperturbed and perturbed water bodies. The radon retardation effect of the modular floats was concluded as follows.
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In both unperturbed and perturbed water, the larger the area coverage, the higher the radon exhalation rate
Author statement
Yongjun Ye: Conceptualisation, Methodology, Formal analysis, Data curation, Funding acquisition, Writing – review & editing. Shuyuan Liu: Sample collection, experimental design and implementation; Data curation, Writing – original draft, Writing – review & editing, Visualization. Ming Xia: Sample collection, Visualization, experimental design and implementation. Ting Yu: experimental implementation. Shanwei Shang: Sample collection, experimental implementation.
Funding
This work was supported by the National Natural Science Foundation of China (Grant No. 11575080), the National Natural Science Foundation of Hunan Province, China(Grant No. 2022JJ30482).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 11575080), the National Natural Science Foundation of Hunan Province, China (Grant No. 2022JJ30482). The manuscript was improved for proper English language and grammar by Long Kiu Chung.
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This paper has been recommended for acceptance by Sarah Harmon.