Radon Release Features of Different Uranium Mines and the Relative Public Effective Doses

Abstract

This chapter attempts to introduce the development of a radon sampling device for the investigation of the return air shaft of mines. Also, different radon measurement methods were compared, which were based on the investigation of the radon release and diffusion pattern in the return air shaft, alkaline (CO₂ + O₂) in situ leaching uranium and sulfuric acid in situ leaching uranium, as well as analysis on continuous distribution characteristics of environmental radon activity concentration. Additionally, comparative studies on the normalized radon release of various mining processes and aerosol radionuclide distribution of various uranium mining facilities were implemented. Finally, the radiation dose prediction evaluation and cumulative radiation dose evaluation based on effluent monitoring and environmental monitoring, respectively, together with normalized public dose evaluation are also presented.

1. Introduction

Radon is known as a natural carcinogen. The release of radon during uranium mining has always been considered as a potential radioactive health hazard, and it is one of the most concerned radionuclides in uranium mining, which needs to be monitored and controlled.

El-Fawal established a joint solution model of radon, radon daughter, air volume, and wind pressure in mine ventilation network, and realized the prediction of radon activity concentration and radon daughter concentration in mine tunnel and working surface [1]. Sahu et al. reviewed various sources of radon in underground uranium mines (such as orebodies, filling tailings, crushed ore, and mine water) and factors affecting the radon release process on the basis of investigation on the effects of internal factors such as ore grade, ²²⁶Ra content, water content, material porosity, and surface area, and external factors such as atmospheric pressure, temperature and ventilation on radon activity concentration in uranium mines [2]. Long Huijia et al. carried out studies on the distribution of radon activity concentration and wind speed at the outlet of a uranium mine ventilation shaft by adopting the single-point sampling method combined with the interpolation and integration program [3]. Mudd implemented research on the radon release from uranium mining and milling in Australia in depth, and the cumulative radon release was analyzed and compared with the UNSCEAR method [4].

Most studies on the migration and diffusion of radionuclides in the atmosphere focus on the impact of leakage of nuclear power plants or nuclear installations on the environment and human beings and the evaluation of the consequences of nuclear accidents. The study on the impact of radionuclides in the exhaust gas of uranium mine on human beings and the environment is mainly based on field measurement, combining with numerical simulation. Xie Dong et al. conducted a numerical simulation study on the relationship between radon activity concentration and diffusion distance in the exhaust gas of uranium mine [5]. Wang Wenxian et al. conducted MATLAB simulation of atmospheric stability and wind speed on radon diffusion from exhaust releases of underground uranium mines [6]. Ye Yongjun et al. numerically simulated the distribution of surface radon activity concentration within 1 km around the return air shaft of uranium mines by the Gaussian plume model and analyzed the effects of initial radon activity concentration and air volume from the wellhead of return air shaft [7].

Li Xiaojun investigated and monitored the radon activity concentration and radon release rate into the ambient air of the in situ leaching mine, and grasped the characteristics of radon release with an in situ leaching uranium mine in Xinjiang. UAIR-FINE software was used to simulate the distribution of radon activity concentration, individual dose, and collective dose in each sub-region within 20 km around the leaching pool and evaporation pond [8]. In terms of the current problem of lack of measurement methods for radon release from recovery wells of in situ leaching field, Zhang Hui et al. calculated the characteristics of radon activity concentration release by using a theoretical model and verified it with field measured data, and found the normalized radon release of in situ leaching was only about a quarter of that of underground mining [9].

In China, uranium mining and milling industry was founded in 1958. Since the establishment of China’s uranium mining and milling industry, China has established underground mines, open-pit mines, and in situ leaching mines, and developed natural uranium extraction processes such as stirred leaching, pile leaching, in situ blasting leaching, and in situ leaching, among which the in situ leaching process includes sulfuric acid leaching and CO₂ + O₂ alkaline leaching, which is the only country in the world that covers various types of uranium mining and milling processes. China’s uranium mining and milling facilities have gone through three stages of development: operation, shutdown, and long-term stewardship of post decommissioning. In the process of production and operation of these uranium mining and milling facilities, to different extent the surrounding environment has been affected. In 1986, the first radiation environmental quality assessment of China’s nuclear industry over the past 30 years was organized. Due to the absence of shutdown and decommissioning of uranium mines, a comprehensive investigation and evaluation of the current radiation environment of uranium mines and milling plants in operation was only carried out. The evaluation results show that the collective dose resulting from uranium mining and milling to the public accounts for 91.5% of the entire nuclear fuel cycle system, and gaseous pathway dose in uranium mining and milling facilities accounts for 76% of total dose of gas and liquid pathway, in which ²²²Rn and short-lived daughters account for 92% of the gaseous pathway [10]. It can be inferred that ²²²Rn is the main cause for radiation environmental impact.

Since 1990s, uranium mines have experienced shutdown, decommissioning and newly built in China. In order to compare the radiation environmental effects of uranium mining and milling facilities with different uranium mining/milling processes and different stages including in operation and shut-down stages, a comprehensive and systematic investigation and evaluation were carried out.

2. Research content and methods

Underground mine return air shaft is the largest radon release source. The concentration is usually tens to hundreds of thousands Becquerel per cubic meter. The return air shaft wind speed is usually more than 10 m/s, humidity up to 98%, and temperature over 40°C. Therefore, the conventional radon monitoring device is not workable in the underground mine environment, and its release characteristics and its migration and diffusion characteristics in this environment have not yet been studied. For in situ leaching, there is no liquid pathway of public exposure by adoption of evaporation pool, and radon migration is the main public exposure pathway. Radon mainly comes from in situ leaching solution. A large amount of radon will be released during the changes of pressure, temperature, and disturbance. However, for the systematic studies of radon release and diffusion, key facility source terms have not been carried out. Due to the rapid change of environmental radon activity concentration with spatial and temporal variation, the research on the migration and diffusion of radon in return air shafts and in situ leaching uranium mining facilities need to carry out large-scale simultaneous distribution monitoring. Therefore, the monitoring technology of radon migration and diffusion of return air shaft is the primary problem. The challenges are to study and reveal the characteristics of release, migration, and diffusion of gas-borne pollutants.

In addition, the distribution of main radionuclides in aerosols is not clear according to previous studies. The relative distribution of the radionuclides ²³⁸U, ²²⁶Ra, ²¹⁰Pb, and ²¹⁰Po in aerosols is of common concern and needs to be revealed.

2.1 Radon monitoring method

On the basis of the available measurement technology for radon, the short-time, long-time, and continuous radon measurement technologies were combined by the authors. With help of a multi-directional simultaneous monitoring, the spatial and temporal distribution of radon and its daughter nuclides could be studied around uranium mines. Monitoring technology of radon release and migration of non in situ leaching uranium mines is represented by the maximum source term of return air shaft of underground uranium mine, and refers to the distribution characteristics of non-point source boundary of waste rock heap and tailings pond. The radon diffusion and migration characteristics in an in situ leaching uranium mine is revealed by the measurement of the radon in the liquid transport pipe, the uranium-rich lixiviate pool, the milling plant, the evaporation pool, and the surrounding environment.

The main pollutants in uranium mines are radon and aerosols, mainly ²³⁸U, ²²⁶Ra, ²²²Rn, ²¹⁰Pb, ²¹⁰Po, etc., including return air shaft, uranium-rich lixiviate pool, and nonpoint sources such as tailings pond and waste rock debris. Radon activity concentration monitoring is affected by a variety of factors, including measurement methods and devices, measurement point and time period, facilities and surrounding environmental conditions, etc.

Cumulative radon measurement refers to the way in which the radon activity concentration is characterized by cumulative mean radioactivity response over a period of time. The cumulative measurements include long-time and short-time radon measurements, represented by solid track etching and electret methods, respectively. The short-time measurement method refers to a cumulative detection time of more than 4 hours but no more than a half-life of radon. It monitors the fluctuations of the radon activity concentration. On the other side, long-time monitoring shows the integral activity over the whole measuring period. Short-time and long-time cumulative radon measuring techniques are complemented by a continuous instantaneous radon monitoring. The radon monitoring in this study applied these radon monitoring methods.

Passive radon measurement is mainly adopted in the selection of method to reduce the environmental disturbance caused by the sampling process, in which environmental disturbance could lead to the wrong judgment of radon activity concentration levels at different points. The electret radon measurement method was used as the short-time cumulative radon measurement method, and the solid-track radon detection method was used as the long-time cumulative radon measurement method.

Table 1 shows the radon monitoring methods of different facilities according to investigation and comparison with the characteristics of various types of facilities.

Monitoring object	Monitoring method	Monitoring device	Monitoring characteristics
Return air shaft	Electrets method	The E-perm electret	Short-time cumulative measurement
	Solid-track etching method	KF606B radon and γ personal dosimeters	Long-time cumulative measurements
Lixiviate preparation pool vent of in situ leaching	Solid-track etching method	KF606B radon and γ personal dosimeters	Long-time cumulative measurements
	Electrets method	The E-perm electret	Short-time cumulative measurement
	Electrostatic collection method	RAD7,PQ2000,SARAD1688	Instantaneous radon measurements
Milling plant (except lixiviate preparation pool) vent of in situ leaching	Solid-track etching method	KF606B radon and γ personal dosimeters	Long-time cumulative measurements
Uranium-rich lixiviate	The flicker chamber method	FD 125, Radon thorium analyzer	Instantaneous radon measurements
	Electrostatic collection method	RAD7,PQ2000,SARAD1688	Instantaneous radon measurements
Environmental radon	Electrets method	The E-perm electret	Short-time cumulative measurement
	Solid-track etching method	KF606B radon and γ personal dosimeters	Long-time cumulative measurements
	Electrostatic collection method	RAD7,PQ2000,SARAD1688	Instantaneous radon measurement, continuous radon measurement
Water surface radon flux	Solid-track etching method	KF606B radon and γ personal dosimeters	Long-time cumulative measurements
Ground radon flux	Electrostatic collection method	RAD7,PQ2000,SARAD1688	Instantaneous radon measurement, continuous radon measurement

Table 1.

Methods and device for radon monitoring.

2.2 Monitoring technology of radon release, migration, and diffusion in the return air shaft

2.2.1 Monitoring technology and device of radon release in the return air shaft

The monitoring technology of the characteristics of radon release and surrounding migration and diffusion distribution of underground uranium mining mainly applies to the return air shaft facilities. The return air shaft radon measuring device was developed considering the flow field and flow velocity characteristics of the return air shaft facilities. The cumulative radon activity concentration distribution was monitored simultaneously at different flow field locations in the return air shaft. In the return air shaft around a 1000-m range, we used the short-time and long-time cumulative radon monitoring to investigate the radon migration and diffusion distribution characteristics at the same time.

At the stable site of the 3.6-m diameter return air shaft, the parameters of wind speed, wind pressure, humidity, and stability are measured at the radial 0.11, 0.34, 0.62, 1.02, 1.75, 2.49, 2.88, 3.16, 3.40 m to determine the characteristics of radon discharge field of the return air shaft. Computational formula:

$$\mathrm{I}=0.16\times \left(Re\right)-0.125$$E1

$$Re=\mathrm{vd\rho }/\mathrm{\eta }$$E2

where is: I: turbulence strength; Re: Reynolds number; v: average velocity; d: hydraulic diameter; ρ: medium density; η: medium dynamic viscosity coefficient. Turbulent intensities of less than 1% were classified as “low turbulent intensity.” Turbulent intensities higher than 10% were considered as “high turbulent intensities.”

Radon activity concentration detectors were placed at different depths and at different points in the radial direction to monitor the radon activity concentration in the return air shaft and at the exhaust outlet. At the same time, the radon activity concentration of the flow field in the return shaft, near the exit and the outlet, is monitored. Radon monitoring device were handled with a bracket structure and lifting system (see Figures 1 and 2).

Figure 1.

Radon measuring device sketch in return air shaft.

Figure 2.

Schematic diagram of device operation.

A bracket system supports the monitoring devices and equipment. A lifting system is used to ensure the effective operation of the bracket system. The designed working positions are 0, −0.5, and − 1 m, measured from the shaft head. Two mutually vertical trusses fix the detectors on the upper two layers. Two monitoring trusses are set on the cross −1 m layer, and an aerosol sampler bracket is placed in the center to collect aerosol samples. The monitoring truss is arranged at the shafthead depth of 0, −0.5, and −1 m. Nine monitoring points are arranged on the truss with distance from the center of 9, 20, 40, 60, 80, and 100%. About 19 simultaneous monitoring data points are arranged in the same batch. The monitoring devices includes solid track detectors and electret detectors. Field operation is shown in Figure 3.

Figure 3.

Arrangement of cumulative radon detector.

The distribution of the radon activity concentration and flow field in the shaft is found out by measuring the radon activity concentration and wind speed in the shafthead at many points. A multi-point-interpolation-integral estimation method for investigation of the radon release in the return air shaft is established. The radon release concentration is calculated by integrating the change of the flow field and concentration of each monitoring point. Each radon activity concentration monitoring point controls the radon activity concentration flux of certain fan or in the field of fan column range, and calculates the radon activity concentration flow in the return air shaft. The following formulas are given:

$$P=\sum _{i=1}^n{v}_i{C}_i{S}_i$$E3

$$v_i=\frac{1}{x_{i+1}-x_i}{\int }_i^{i+1}v{d}_x$$E4

$$C_i=\frac{1}{x_{i+1}-x_i}{\int }_i^{i+1}C{d}_x$$E5

$$S_i=\frac{\pi \left(r_i^2-r_{i-1}^2\right)}{2}$$E6

where V_i is the mean wind speed in the control range of the concentration monitoring point i (m/s). C_i is the radon activity concentration at the control point i (Bq/m³). S_i is the interface control area for the control point i (m²). P is the release rate (Bq/s). According to the release rate and the release time t (s), it can be calculated as annual release Q (Bq/a).

$$Q=P·t$$E7

Comprehensive formula (3) ∼ formula (7), available as follows:

$$Q=\sum _{i=1}^n\frac{1}{x_{i+1}-x_i}{\int }_i^{i+1}v{d}_x·\frac{1}{x_{i+1}-x_i}{\int }_i^{i+1}C{d}_x·\frac{\pi \left(r_i^2-r_{i-1}^2\right)}{2}·t$$E8

2.2.2 Monitoring technology of the distribution characteristics of radon migration and diffusion around the return air shaft

The study on the distribution of radon around the return air shaft mainly adopts the combination of long-time and short-time cumulative radon monitoring, supplemented by the continuous monitoring of instantaneous radon-monitoring. The short-time radon measuring points are placed in 16 directions around the return air shaft. The main measuring points of long-time cumulative radon are placed at the dominant wind direction ENE and downwind direction WSW, the minimum wind frequency direction SE and downwind direction NW, and the six directions, in distances of 30, 50, 100, 200, 300, 500, 800, and 1000 m. About 10% parallel sample points were set to verify the quality of the monitoring data. Short-time radon monitoring period was between 8 and 48 h: The long-time radon monitoring was set to 90 days per batch (1 quarter/time, totaling 4 times). Instantaneous radon continuous monitoring was set for 72 hours. Both the operation and the shutdown phase were monitored. Temperature, the relative humidity, air pressure, wind direction, and wind speed were recorded during the monitoring process.

2.3 Monitoring technology of radon release, migration, and diffusion of in situ leaching uranium

The monitoring of the release, migration, and diffusion of radon at the in situ leaching uranium site includes the monitoring of the uranium-rich lixiviate pool, the uranium milling plant, the liquid transport pipe, the drilling, the evaporation pool, etc.

2.3.1 Sampling and monitoring of radon in liquid transport pipes

In the process of in situ leaching, the uranium-rich lixiviate of the ore-bearing layer is extracted to the surface from dozens of meters to several meters underground. More radon is dissolved under the underground pressure. As the groundwater is pumped to the surface, the pressure decreases, and a large amount of radon is released. It is also lost when the water flows during the passage through the transport pipe. In addition to its migration with water and airflow, radon can also be transferred in two phases.

Radon gas in liquid transport pipes is released into the air after contact with the atmosphere after entering the uranium-rich lixiviate pool. Radon gas that is not released in the pool will continue to flow into the plant and return to the underground. This process is a dynamic process, and such high concentrations cannot easily be monitored. Therefore, two invention patents, namely, the free gas radon-collecting device and the free gas radon-monitoring device, were adopted (see Figures 4 and 5). Radon-containing gas in the extraction bottle was introduced into the scintillation chamber and radon activity concentration was measured and calculated.

Figure 4.

Schematic diagram of infusion tube gas gathering device.

Figure 5.

Gas and radon sampling site of infusion tub device.

2.3.2 Calculation of radon activity concentrations in the recovery well

In this study, a one-dimensional vertical radon diffusion model was used to theoretically simulate the radon diffusion concentration. The process of radon migration and diffusion in the recovery well depends on the radon-containing extraction solution in the gas-phase interface, and the radon migration and diffusion into the upper air, connecting with the air through the gap of the top hole of the recovery well. The vertical constant temperature diffusion experiment of radon is designed to determine the effective diffusion coefficient of radon release in the recovery well environment and estimate the radon activity concentration at the recovery well outlet at different depths and different water radon activity concentrations. An experimental bench device for one-dimensional vertical diffusion of radon is built.

Figure 6 shows a schematic diagram of the device structure. It includes radon source, diffusion pipe, thermostatic casing, radon measurement device, and end radon absorption device. The diffusion behavior of radon gas in the extraction pipe belongs to the diffusion migration distribution of infinite vertical space, which behaves conforming to Fick’s theorem and is similar to the distribution of release gas in uniform non-radioactive media of layered ore body. The diffusion distribution can be described by the following equation:

Figure 6.

Schematic representation of the one-dimensional diffusion experimental setup.

$$N=N_0{e}^{-\sqrt{\raisebox{1ex}{$\lambda $}\!\left/ \!\raisebox{-1ex}{$D_e$}\right.·}x}$$E9

where N is the radon gas concentration from the space to the radon source x, x is the distance of a certain point in the pipe from the radon source, and λ is the decay constant of ²²²Rn, D_e is the effective diffusion coefficient of radon gas in the experimental environment, and N₀ is the radon gas concentration close to the location of the radon source point.

2.3.3 Monitoring and calculation of radon flux in the evaporation pool

A device for measuring water surface Radon flux is developed as shown in Figure 7. Passive diffusion electret is used to measure radon. The electret at the bottom of the air-collecting hood is installed and places the air-collecting hood on the surface of the measured medium to ensure good edge air tightness. At 4 to 10 h of accumulation, an electret is replaced and the measurement is completed after 20 h. Another electret was used to measure the radon activity concentration in the surface air. About 3 ∼ 5 radon activity concentration data in different time periods were obtained by the effective decay constant method.

Figure 7.

Schematic diagram of measuring device (left) and structure (right).

The calculation method of measured Radon flux includes slope method and effective decay constant method. Where the slope method is an approximation method, which assumes λt < < 1, C₀ is 0 (λ is the radon decay constant, t is the measurement time, and C₀ is the initial radon activity concentration) approximately replaces the Radon flux curve. The inherent relative error between the measured value and the true value of the slope method is (1-e^−λt)/(λt) −1. For this purpose, when the accumulation time exceeds 10 h its inherent system error is at least 5% using this method. Because the device adopts the radon measurement method and the short-time electret, the accumulation time should be at least 4 h, so the device adopts the effective decay constant method for the calculation of the radon flux.

The calculation method is performed as follows:

$$E_e=\frac{V{\mathrm{\lambda }}_e\left(C-C_0{e}^{-{\lambda }_{e}T}\right)}{S\left(1-e^{-{\lambda }_{e}T}\right)}$$E10

where Ee is the effective Radon flux; V is the volume of cumulative radon cover; S is the area of the cumulative radon cover; λe is the effective decay constant, which is related to the air leakage coefficient designed by the device, and the properties of the measured uniform medium; T is the measurement period; C is the measured radon activity concentration; C₀ is the initial radon activity concentration, usually is 0.

According to formula (10), the Radon flux can be calculated as long as λe is known. Let xi be the n radon activity concentration data with equal time period T and yi be the i radon activity concentration data with equal time period T, where xi is not equal to yi. Then:

$$\mathit{\lambda e}=\frac{1}{T}\mathit{ln}Y\frac{1}{b}Y$$E11

$$\mathrm{b}=\frac{\sum x_i{y}_i-\frac{1}{n}\left(\sum x_i\sum y_i\right)}{\sum x_i^2-\frac{1}{n}{\left(\sum x_i\right)}^2}=e^{-{\lambda }_{e}T}$$E12

$$\mathrm{a}=\frac{\sum y_i}{n}-\mathrm{b}\frac{\sum x_i}{n}=\frac{E_{e}S}{{\mathrm{\lambda }}_{e}V}\left(1-e^{-{\mathrm{\lambda }}_{e}T}\right)$$E13

$$E_e=\frac{V{\mathrm{\lambda }}_{e}a}{S\left(1-b\right)}$$E14

2.3.4 Monitoring technology of the distribution characteristics of radon migration and diffusion around the uranium-rich lixiviate pool and milling plant

The radon activity concentration around the uranium-rich lixiviate pool of in situ leaching mine was investigated by short-time cumulative radon-detectors. In the maximum wind frequency (up and down direction), minimum wind frequency (up and down direction), middle wind frequency (up and down direction), 11 distances of 2, 5, 10, 30, 30, 50, 100, 200, 300, 500, 800, and 1000 m were chosen for a 24-h monitoring.

At the larger and lower wind frequencies (up and down direction) of 50, 100, 300, and 500 m around the plant, namely four directions and four distances, the solid nuclear track detectors were exposed for the same time period.

3. Study results

3.1 Rules of radon release, migration, and diffusion in the return air shaft

3.1.1 Distribution law of radon activity concentration in the return air shaft

Solid nuclear track cumulative radon detectors were used to monitor the distribution of radon activity concentration in the return air shaft. The analysis of the data showed that the radon activity concentration at the center of radius was lower than at the edge and lower than at the center at both depths of −1 and −0.5 m in the return air shaft. However, the radon activity concentration at the outlet of the return air shaft showed high radon activity concentrations at the shaft’s center of radius (see Figures 8–10).

Figure 8.

Distribution of radon activity concentration in X and Y at a depth of −1 m.

Figure 9.

Distribution of radon activity concentration at a depth of −0.5 m.

Figure 10.

Distribution of radon activity concentration at the return air shaft exit (0 m).

The radon activity concentration in the return air shaft, measured by KF-606B solid track cumulative radon detectors, showed a radon activity concentration range of 21.0–63.7 kBq/m³ and a mean value of 37.5 kBq/m³. The radon monitored by electrets showed a radon activity concentration range and a mean value of 18.4–127 kBq/m³ and 47.6 kBq/m³, respectively. The two methods were comparable.

It is recommended to use the sampling points used in this study to achieve the optimal sampling effect and representativeness. Under appropriate conditions, at least one sampling point can be set at the center point of −1 m to obtain trusted radon activity concentration level data. It can be simplified as the return air shaft outlet of −1 m and the monitoring of radon activity concentration release level at the radial 1/3. The monitoring method could be a passive radon monitoring, while solid track detectors are recommended.

3.1.2 Distribution law of radon migration and diffusion in the environment around the return air shaft

From June 2014 to May 2015, a long-time radon monitoring study on the distribution law of radon activity concentration around the return air shaft was conducted in four batches for different directions and distances: two batches under normal production conditions from June to November 2014 and two batches at shutdown mode of the mine, from December 2014 to May 2015. Three blank samples were retained in each batch for background monitoring. The three blank background was 31, 27, and 26 Bq/m³.

For the short-time cumulative radon, the data were processed by the synergistic Kriging method (Co-Kriging), and a contour map of the radon activity concentration distribution was drawn. According to the meteorological data in 2014 of the nearest weather station within 50 km of the return air shaft, the distribution of radon activity concentration within 1000 m around the shaft was estimated by mode estimation. Figures 11 and 12 show the contour plots of the radon activity concentration within 1000 m around the shaft under operation condition and shutdown condition, respectively. At Figure 11, the mine was under the operation conditions, and the mean radon activity concentration of the return air shaft was 37 kBq/m³. The radon activity concentration outside a distance of 400 m from the shaft was about 75 Bq/m³. Furthermore, the radon contribution outside a distance of 500 m did not exceed 30 Bq/m³. The radon contribution outside 800 m did not exceed 20 Bq/m³, which is within the local background level. The diffusion of radon around the 15# return air shaft of J mine was mainly affected by the dominant wind direction, and a “sink” was formed at about 100 m southwest of the return air shaft, with the maximum concentration of 1732 Bq/m³. The radon activity concentration reduced to less than 100 Bq/m³ at about 400 m around the return air shaft, and the radon activity concentration outside the 500 m range was basically lower than 50 Bq/m³.

Figure 11.

Mine in operation mode. Radon-contour map.

Figure 12.

Mine at shutdown mode. Radon-contour map.

Figure 12 shows the radon distribution at operation and shutdown mode of the mine. The average radon activity concentration in the N, ENE, SE, and NW directions decreased by 70–97 Bq/m³, and the average radon activity concentration in the S, WSW directions was lower than at operation mode (330–373 Bq/m³). Compared with the shutdown status, the radon activity concentration outside the 500 m perimeter changed little. The boundary range of their influence on the radon in the surrounding environment is about 500 m to 800 m under operation and shutdown mode.

The distribution of the radon activity concentration in the WSW and ENE directions of the return air shaft had obvious distance distribution characteristics. Figures 13 and 14 show the overall behavior of the radon activity concentration decreased with distance. The maximum radon activity concentration in this direction occurs 100 m away from the return air shaft, while the radon activity concentration in the return air shaft at 500 m decreased to the local background level.

Figure 13.

Radon activity concentrations in the ENE and WSW directions at different distances and monitoring times.

Figure 14.

Comparison of the long-term and short-term monitoring data in the ENE and WSW directions.

Under the shutdown status of the mine, the radon activity concentration decreased, and the average at the outlet was 850 Bq/m³, generally lower than 100 Bq/m³ outside the air shaft. Among them, the radon activity concentrations outside 500 m were generally lower than 30 Bq/m³. A high concentration area appeared 400 m east-northeast of the return air shaft, as there was a uranium mine safety measure shaft at the bottom of the normal production state, and the unorganized air flow from the roadway in the shutdown state due to the change of the wind pressure at the bottom, resulting in an increase of radon. This phenomenon proved the effectiveness of the distribution scheme of radon activity concentration distribution in this study.

3.2 Rules of radon release, migration, and diffusion of the in situ leaching uranium mine

3.2.1 Investigation of the radon release from the uranium-rich lixiviate pool

Short-time cumulative radon monitoring was used for the survey of the radon activity concentration at the uranium-rich lixiviate pool (passive sampling integral measurement).

The monitoring locations are shown in Figure 15. The results are shown in Table 2. The average concentration of the radon release in the uranium-rich lixiviate pool was 65.9 kBq/m³. The radon activity concentration decreased rapidly at 2 m distance from the opening of the uranium-rich lixiviate pool, with an average concentration of about 2400 Bq/m³. It is recommended to seal the uranium-rich lixiviate pool.

Figure 15.

Electret monitoring position at the opening of uranium-rich lixiviate pool.

Parameter	Uranium-rich lixiviate pool	Milling plant							Wellfield drilling	Evaporation pool
		Lixiviate preparation pool	Leaching filter unit	Qualified lixiviate pool	Reverse osmosis area	Settling zone	Rejection zone	Chemical analysis unit
Mean radon activity concentration, Bq/m^3	65,582	52,690	1423	1012	1438	2117	1711	497	257	45
Radon release amount, GBq/a	6074	3859	70.34	16.47	23.40	32.92	27.84	0.86	30.83	19.01
Percentage	59.8	37.97	0.69	0.19	0.23	0.33	0.27	0.01	0.30	0.20

Table 2.

Radon activity concentration and annual radon release in uranium-rich lixiviate pool, milling plant, wellfield drilling and evaporation pool of CO₂ + O₂in situ leaching T mine.

For the convenience of calculation, assuming that at the opening of the open uranium-rich lixiviate pool, the amount of radon release in the uranium-rich lixiviate pool can be estimated by the following equation:

$$Q=0.25\mathit{\text{CRnυAT}}\sum _{x=1}^{x}P\left(x\right)\mathit{ABS}\left(\mathit{sin}x\right)$$E15

Q is the annual radon release (Bq); C_Rn is the calm wind mean radon activity concentration (Bq/m³, set 47.7 kBq/m³); V is the average annual long-time wind speed (m/s, set 3.6 m/s in this survey); A is the opening area (m², set 8m²); T is the time of 1 year; ABS is absolute; P is the wind frequency; x is the wind direction angle. Then, the annual release of radon in the uranium-rich lixiviate pool was 6.47 TBq/a.

The first phase uranium-rich lixiviate pool of T Mine was sealed in November 2016 and in September 2018. The radon activity concentration at the sealed uranium-rich lixiviate pool and its surrounding areas was measured with RAD 7-detectors, which were distributed along the downwind direction of the main wind direction. Results are shown in Table 3. As can be seen, the radon activity concentration close to the pool wall of the sealed uranium-rich lixiviate pool was 4.2 kBq/m³, which was more than one order of magnitude lower of radon activity concentration of the unsealed uranium-rich lixiviate pool. The sealing of the uranium-rich lixiviate pool had a significant effect on the radon release. The radon release of the trap was reduced by 95%, and the whole process release of the in situ leaching was reduced by 1–2 orders of magnitude.

State of the uranium-rich lixiviate pool	Radon activity concentration at the wall of the sump and downwind, Bq/m^3
	Pool wall	2 m	5 m	100 m	500 m	800 m	1000 m
Before sealing	46,000	6179	2712	315	85	49	39
After sealing	4230	376	257	101	52.7	26.7	13.5

Table 3.

Comparison of radon activity concentration around uranium-rich lixiviate pool of in situ leaching.

3.2.2 Investigation of radon release in the exhaust outlet of alkaline (CO₂ + O₂) in situ leaching uranium milling plant

The alkaline (CO₂ + O₂) in situ leaching refers to lixiviate formed by adding gaseous carbon dioxide and oxygen to native groundwater. The milling plant includes the main plant and a supporting plant. The main plant is separated from the supporting plant by walls. The main workshop includes an ion exchange area and a precipitation area, and the supporting workshop includes a storage pool area (qualified liquid pool, shower pool, lean liquid pool), an uranium-rich lixiviate filtration area, a reverse osmosis area, a recoil area, a liquid distribution pool, etc. The storage area and the distribution pool are sealed spaces, with only an exhaust outlet. There are 13 exhaust vents for the active ventilation facilities in the milling plant (see Figure 16). The cumulative radon activity concentration of each exhaust outlet was monitored and the annual radon release was calculated as shown in Table 2.

Figure 16.

Distribution of each exhaust outlet of milling plant of T uranium mine. 1. Filter area outlet. 2. Liquid pool outlet. 3. Reverse osmosis area outlet. 4. Laboratory outlet. 5. Recoil area outlet. 6. Settling zone outlet. 7. Storage pool area outlet. 8. Main building outlet. 9. Main building outlet.

3.2.3 Investigation of radon release in evaporation pool and recovery well and liquid transport pipe

3.2.3.1 Investigation of radon release in the evaporation pool

The electret radon flux-measuring device was used to float at the edge of the 0.5 m of the evaporation pool, and the release rate of radon was measured 25 m away from the inlet pipe of the evaporation pool. After freezing in winter, the radon flux of 5 points was measured in the center of the evaporation pool and in the diagonal position of the evaporation pool. The radon release rate is shown in Figure 17. The monitoring result of radon flux was that the radon flux was monitored for each water surface point and 5 ice surface points, and the monitoring data of radon flux of 7 water surface and 25 ice surface points were obtained. The mean value of water surface radon flux was 0.06 Bq/m²·s, and the mean radon flux on the ice surface was 0.035 Bq/m²·s. According to the area of the evaporation pool of 12160m² calculated, the icing period of no. 1 evaporation pool is 5 months, 12,160 m² × 8760 h × 3600 s (0.060 Bq/m²·s × 7/12 + 0.035 Bq/m²·s × 5/12) = 19.01 GBq.

Figure 17.

Schematic diagram of radon release rate on ice surface.

3.2.3.2 Investigation of radon release from recovery wells

The effective diffusion coefficient was calculated according to the fitting of the experimental results of the recovery well, and the radon activity concentration at the outlet of the recovery well could be calculated according to the formula. The fit is shown in Figure 18. The results using the least squares method can calculate the radon effective diffusion coefficient De. It was 0.0578 cm²·s⁻¹ at 16°C in vertical space. The radon activity concentration at the outlet of the recovery well was compared with the monitoring of the radon activity concentration. It is known that the radon activity concentration in the mining area is 2 × 10⁵ Bq/m³. At 16°C, the equilibrium Oswald coefficient of radon in the gas-liquid two phases is 0.3, and the air radon activity concentration at the in situ leaching liquid level is 60 kBq/m³. The distance between the borehole level of the stope is 8–10 m, and the mean value is 9.06 m. The radon activity concentration at the outlet of the extraction borehole was calculated to be 254.4 Bq/m³.

Figure 18.

Fitting results of radon activity concentration and diffusion distance.

3.2.3.3 Investigation of radon release in the liquid transport pipe

According to the measurement of water radon and gas radon in the seven centralized control rooms of T uranium mine, the mean activity concentration of gas radon in the total valve of the central control chamber was 75.5 and 2076 Bq/L, respectively, totaling 2152 Bq/L. The radon activity concentration of leaching solution lifted from the ground was reduced by about 70%, that was, by 1472 Bq/L in surface liquid distribution pool after the recovery process, resulting in the water radon activity concentration 680 Bq/L.

Gaseous radon and dissolved radon in liquid transport pipes are typical radon sources in situ leaching uranium mines. Gaseous radon contributed significantly to the increased activity concentration and release of radon in the non-closed liquid trap.

3.2.4 Investigation on the migration and diffusion law of radon in the surrounding environment uranium-rich lixiviate pool of in situ leaching facilities

The migration and diffusion of radon around the uranium-rich lixiviate pool is mainly affected by the dominant wind direction, as shown in Table 4. As the wind direction was SW on the day, the maximum was 6179 Bq/m³ at a distance of 2 m in the NNE direction. The radon activity concentration around the uranium-rich lixiviate pool in T mine was basically lower than 50 Bq/m³ outside the 800 m range. The radon activity concentration of the X1 mine by acid method was lower than 40 Bq/m³ outside the 500 m range and the outdoor background value of 9.76–41.48 Bq/m³ basically consistent, indicating that the environmental impact of radon release in the open uranium-rich lixiviate pool generally did not exceed 500 m.

In situ leaching	Radon activity concentration distribution around the uranium-rich lixiviate pool											Distribution of radon activity concentration around milling plant
	Distance/m	2	5	10	30	50	100	300	500	800	1000	Factory boundary	50	100	300	500
CO2 + O2in situ leaching uranium T mine	NNE	6179	2712	1232	1675	772	315	174	85	49	39	75	59	51	54	39
	SSW	264	251	87	85	85	83	60	43	42	40	646	548	145	79	56
	WNW	561	203	153	112	94	110	87	60	43	43	300	180	54	23	37
	ESE	3115	1898	1124	426	238	209	91	87	40	43	71	62	52	68	41
	N	143	97	102	101	99	87	53	42	42	36	—	—	—	—	—
	S	158	143	118	103	83	91	59	43	43	32	—	—	—	—	—
	W	2154	4318	285	131	94	84	43	43	43	31	—	—	—	—	—
	E	4187	2183	1246	2281	1238	310	184	86	60	49	—	—	—	—	—
Acid in situ leaching uranium X1 mine	E	381	—	—	—	35	34	41	24	—	—	691	575	122	81	27
	W	258	—	—	—	32	68	45	26	—	—	327	189	54	22	28
	N	151	—	—	—	46	49	38	31	—	—	—	—	—	—	—
	S	115	—	—	—	33	40	42	28	—	—	—	—	—	—	—
	SE	—	—	—	—	16	45	41	40	—	—	75	65	52	71.4	23
	NW	—	—	—	—	48	24	25	35	—	—	87	61	53	56	31

Table 4.

Radon activity concentration distribution in uranium-rich lixiviate pools, milling plant (Bq/m³).

3.2.5 Investigation on the migration and diffusion law of radon in the surrounding environment of the in situ leaching uranium milling plant

As can be seen in Table 4, radon activity concentration around milling plant was lower than around the uranium-rich lixiviate pool. The radon activity concentration decreased with the increasing distance from the milling plant. The environmental impact of radon release generally did not exceed 500 m.

3.3 Analysis of normalized radon release in various types of uranium mines

At present, only J uranium mine is conventional mixing leaching in underground mining mines in China, while the others are heap leaching. The release of radon is small. The normalized radon release amount in each mine is shown in Table 5. The release of normalized radon from underground mining is 0.327 ∼ 4.372 TBq/tU, while the normalized release of radon from in situ leaching uranium is only 0.0852 TBq/tU. L mine has the largest normalized radon release with 4.372 TBq/tU, followed by DB uranium mine with 3.912 TBq/tU, both deposits using in situ blasting leaching process. In 1978, the Pacific Northwest Research Institute measured and estimated the normalized radon release of 18 return air shafts, and its value was (0.205 ∼ 12.69) TBq/tU. The normalized radon release of return air shafts in China was lower than that of return air shafts in the United States. After removing the in situ blasting leaching, the normalized radon release of underground mines in China is 0.63 ∼ 2.83TBq/tU, which is generally higher than that reported in UNSCEAR 2000 by 0.018 ∼ 2.36 TBq/tU (1 ∼ 2000G/tU₃O₈) [11]. Mainly, China’s uranium resource endowment is poor, and the underground mining scale is small and scattered. The radon of underground milling mainly comes from the return air shaft, which accounts for 97.29% of the radon release amount of the whole underground mine. The amount of normalized radon released on in situ leaching uranium is significantly lower than that of underground uranium, which is about 4.27% of the amount of underground radon released.

Installation	Conventional underground mining								In situ blasting and leaching of underground mining				In situ leaching mining
	B mine	Q mine	C mine	J mine	M mine	DC mine	Weighted mean	Mean ratio,%	L mine	DB mine	Weighted mean	Mean ratio,%	CO2 + O2in situ leaching T mine	Acid in situ leaching X1 mine	Acid in situ leaching X2 mine
Return air shaft	0.63	0.22	1.15	0.63	1.42	2.83	1.08	96.26	4.34	3.81	4.15	98.67
Tailings pond	0.041	0.066	0.016	0.014	0.044	0.015	0.029	2.58	0.013	0.068	0.032	0.76
Heap leaching site	0.006	0.041	0.018	0.005	0.018	0.008	0.013	1.16	0.019	0.034	0.024	0.57
In situ leaching site													0.085	0.029	0.040
Total	0.67	0.327	1.184	0.649	1.482	2.853	1.122	100	4.372	3.912	4.206	100	0.085	0.029	0.040

Table 5.

Normalized radon release (TBq/tU) in each mine (shaft).

3.4 Distribution pattern of aerosol radionuclides

The aerosol radioactivity levels in the source terms and nearby atmospheric environment of various uranium mining and milling facilities are shown in Table 6.

Facilities type		^238U		^226Ra		^210Pb		^210Po
		Scope	Mean	Scope	Mean	Scope	Mean	Scope	mean
Source term	Underground mine in operation	0.0198 ∼ 33.6	29.3	0.0099 ∼ 0.143	0.041	0.78 ∼ 294	53.6	0.14 ∼ 37.0	2.62
	In situ leaching	0.038 ∼ 0.7	0.174	0.0056 ∼ 0.05	0.021	0.29 ∼1.87	0.61	0.042 ∼ 3.45	0.92
	Mine shutdown	0.0077 ∼ 6.5	0.66	0.0039 ∼ 0.034	0.013	0.33 ∼3.57	1.36	0.15 ∼ 1.83	0.66
Environment	Underground mine in operation	0.011 ∼ 1.4	0.748	0.0062 ∼ 0.1	0.021	0.59 ∼ 10.43	4.32	0.089 ∼ 2.82	1.04
	In situ leaching mine	0.014 ∼ 0.041	0.027	0.0063 ∼ 0.03	0.016	0.83 ∼ 4.38	2.03	0.142 ∼ 1.37	0.49
	Mine shutdown	0.0046 ∼ 6.5	0.488	0.003 ∼ 0.053	0.017	0.09 ∼ 5.44	1.66	0.13 ∼ 1.90	0.96
	Domestic	0.0059 ∼ 0.044	0.023	0.0047 ∼ 0.12	0.021	0.11 ∼ 9.4	1.70	0.03 ∼ 1.84	0.82

Table 6.

Aerosol radioactivity levels in different mine sources and environments (mBq/m³).

As can be seen in Table 6, ²³⁸U concentration in the aerosol from the operational underground uranium mining is higher, significantly higher than the shutdown and in situ leaching facilities. The high point of ²³⁸U concentration is the shafthead, which leads to higher ²³⁸U concentration resulting from the diffusion of uranium dust. The concentration of radionuclide in the in situ leaching mine is low. In most of the aerosol samples in the surrounding environment of various uranium mines, the radioactivity of ²¹⁰Pb and ²¹⁰Po was slightly higher than the UNSCEAR 2000 [11] recommended reference value (0.5 and 0.05 mBq/m³).

In aerosols, the activity concentrations of ²¹⁰Pb and ²¹⁰Po are mostly higher than that of ²³⁸U and ²²⁶Ra and show the following row: ²¹⁰Pb > ²¹⁰Po > ²³⁸U > ²²⁶Ra. In a few cases, the ²³⁸U activity is higher than the ²¹⁰Po-activity. The radionuclide (except radon) activity concentrations of aerosol from gaseous source term and environment are 6–10 and 4–6 orders of magnitude lower than the radon activity concentration, respectively. In the gas phase, the activity concentrations of ²¹⁰Pb and ²¹⁰Po in aerosols from other release sources (such as tailings ponds, waste rock yards, hydrometallurgy plants) except the shaft head are comparable with nearby residential sites. The average activity concentrations of ²¹⁰Pb and ²¹⁰Po in the shaft aerosol are 1 to 2 orders of magnitude higher than that of nearby settlements. ²¹⁰Pb and ²¹⁰Po in the uranium shafthead aerosol should be paid attention to.

3.5 Predictive evaluation of effective doses for the public based on the effluent

3.5.1 Expansion of the atmospheric evaluation model

AERMOD is selected as the application model to simulate and predict atmospheric migration and diffusion of radionuclides from uranium mines. A dose evaluation is carried out in combination with China’s Y30AIR radiation dose evaluation software, thus overcoming the shortcomings of AERMOD’s [12] non-dose evaluation function. Y30AIR’s non-point source integration, terrain and complex meteorological processing function, and synthesizing are the advantages of both models.

3.5.2 Prediction and evaluation results

From the radiation environmental impact assessment results of different types of facilities, the yearly emissions of different mine types are shown in Table 7. Public doses of critical resident groups (all contributed by gaseous pathway) from various mines are presented in Table 8. Table 9 shows the individual effective doses distribution of critical groups of various facilities. The collective doses from the gas phase and the liquid phase are shown in Table 10. Table 11 presents the comparison of the impact of radiation environment of various facilities. Finally, the collective effective doses of the public are given in Table 12.

Mine type	Pathway of irradiation	Release amount (Bq/a)
		Rn-222	U-238	U-234	Ra-226	Th-230	Po-210	Pb-210	total
Conventional mining in operation	Gas	4.47 × 10^14	8.94 × 10^10	4.60 × 10^8	4.38 × 10^8	4.60 × 10^8	4.38 × 10^8	4.38 × 10^8	4.47 × 10^14
	Liquid	—	3.59 × 10^9	3.59 × 10^9	1.33 × 10^9	7.10 × 10^8	1.19 × 10^9	1.92 × 10^9	1.23 × 10^10
	Total	4.47 × 10^14	9.30 × 10^10	4.05 × 10^9	1.76 × 10^9	1.17 × 10^9	1.63 × 10^9	2.36 × 10^9	4.47 × 10^14
In situ leaching mining	Gas	3.36 × 10^13	—	—	—	—	—	—	3.36 × 10^13
	Liquid	—	—	—	—	—	—	—	—
	Total	3.36 × 10^13	—	—	—	—	—	—	3.36 × 10^13
Shutdown mining	Gas	2.61 × 10^14	—	—	—	—	—	—	2.61 × 10^14
	Liquid	—	3.30 × 10^9	3.04 × 10^9	1.56 × 10^8	4.37 × 10^8	1.47 × 10^7	2.26 × 10^7	6.97 × 10^9
	Total	2.61 × 10^14	3.30 × 10^9	3.04 × 10^9	1.56 × 10^8	4.37 × 10^8	1.47 × 10^7	2.26 × 10^7	2.61 × 10^14
Decommissioning mining operation	Gas	4.59 × 10^11	—	—	—	—	—	—	4.59 × 10^11
	Liquid	—	3.92 × 10^6	3.92 × 10^6	3.13 × 10^6	2.31 × 10^6	9.03 × 10^5	2.18 × 10^6	—
	Total	4.59 × 10^11	3.92 × 10^6	3.92 × 10^6	3.13 × 10^6	2.31 × 10^6	9.03 × 10^5	2.18 × 10^6	4.59 × 10^11
Total	gas	7.42 × 10^14	8.94 × 10^10	4.60 × 10^8	4.38 × 10^8	4.60 × 10^8	4.38 × 10^8	4.38 × 10^8	7.42 × 10^14
	Liquid	—	6.89 × 10^9	6.63 × 10^9	1.49 × 10^9	1.15 × 10^9	1.21 × 10^9	1.95 × 10^9	1.93 × 10^10
	Total	7.42 × 10^14	9.63 × 10^10	7.09 × 10^9	1.92 × 10^9	1.61 × 10^9	1.65 × 10^9	2.39 × 10^9	7.42 × 10^14

Table 7.

Releases of gaseous and liquid radioactive effluent.

Critical resident groups and the main contributing sources		Conventional mining in in operation				In situ leaching mining			Shutdown mining							Decommissioned
		M Mine	J Mine	L Mine	Q Mine	X1 Mine	X2 mine	T mine	B Mine	H Plant	DC Mine	JH Mine	JR Mine		TC Mine	LC Mine
													JR-1	JR-2
Critical residents group	Position	S	NE	SE	W	NW	ENE	SW	WNW	ENE	W	ESE	WNW	N	SES	W
	Distance (km)	0.5	5.39	0.85	0.51	1.44	5.46	1.50	2.00	0.9	0.63	1.88	1.6	1.2	1.12	0.37
	Dose value (mSv/a)	0.33	0.48	0.94	0.002	0.016	0.017	0.004	0.006	0.39	0.014	0.024	0.027	0.028	0.076	0.008
Main contribution source terms		Cotton pit south return air shaft	Shannan waste rock debris	102 Return air shaft	Return air shaft	Wellfield, milling plant	Tower A uranium-rich lixiviate pool	Lixiviate preparation pool	Tailings pond	Tailings pond	Tailings pond	708 tailings pond	Tailings pond	Tailings pond e	Well field	Tailings pond

Table 8.

Critical groups of uranium mines and their effective dose based on effluents (key pathways are gaseous and key radionuclide is ²²²Rn).

Type	The ratio of public maximum individual dose distribution (%)
	<0.1	≥0.1, < 0.25	≥0.25, < 0.5	≥0.5, < 1	≥1
Conventional mining in operation	25	0	50	25	0
In situ leaching mining	100	0	0	0	0
Shutdown mining	75	12.5	12.5	0	0
Decommissioning mining	100	0	0	0	0
Total	64.3	7.1	21.4	7.1	0

Table 9.

Individual dose distribution in critical resident groups of various facilities.

Type	Pathway of irradiation	Collective Dose (person Sv/a)
		Rn-222	U-238	U-234	Ra-226	Th-230	Po-210	Pb-210	total
Conventional mining in in operation	Gas	3.67 × 10^1	4.09 × 10^−2	4.79 × 10^−2	4.79 × 10^−2	2.17 × 10^−1	2.93 × 10^−3	1.49 × 10^−2	3.70 × 10^1
	Liquid	—	2.49 × 10^−3	7.19 × 10^−4	3.83 × 10^−3	1.74 × 10^−4	4.46 × 10^−3	6.31 × 10^−2	7.48 × 10^−2
	Total	3.67 × 10^1	4.34 × 10^−2	4.86 × 10^−2	5.17 × 10^−2	2.17 × 10^−1	7.39 × 10^−3	7.80 × 10^−2	3.71 × 10^1
In situ leaching mining	Gas	6.45 × 10^−1	—	—	—	—	—	—	6.45 × 10^−1
	Liquid	—	—	—	—	—	—	—	—
	Total	6.45 × 10^−1	—	—	—	—	—	—	6.45 × 10^−1
Shutdown mining	Gas	1.94 × 10^1	—	—	—	—	—	—	1.94 × 10^1
	Liquid	—	3.87 × 10^−4	4.15 × 10^−4	1.17 × 10^−2	4.03 × 10^−4	2.48 × 10^−3	3.94 × 10^−2	5.48 × 10^−2
	Total	1.94 × 10^1	3.87 × 10^−4	4.15 × 10^−4	1.17 × 10^−2	4.03 × 10^−4	2.48 × 10^−3	3.94 × 10^−2	1.94 × 10^1
Decommissioning mining	Gas	2.00 × 10^−2	—	—	—	—	—	—	2.00 × 10^−2
	Liquid	—	2.04 × 10^−7	1.95 × 10^−7	3.49 × 10^−8	4.19 × 10^−8	4.02 × 10^−14	7.46 × 10^−11	4.76 × 10^−7
	Total	2.00 × 10^−2	2.04 × 10^−7	1.95 × 10^−7	3.49 × 10^−8	4.19 × 10^−8	4.02 × 10^−14	7.46 × 10^−11	2.00 × 10^−2
Total	Gas	5.67 × 10^1	4.09 × 10^−2	4.79 × 10^−2	4.79 × 10^−2	2.17 × 10^−1	2.93 × 10^−3	1.49 × 10^−2	5.71 × 10^1
	Liquid	—	2.88 × 10^−3	1.13 × 10^−3	1.56 × 10^−2	5.77 × 10^−4	6.94 × 10^−3	1.03 × 10^−1	1.30 × 10^−1
	Total	5.67 × 10^1	4.38 × 10^−2	4.90 × 10^−2	6.35 × 10^−2	2.18 × 10^−1	9.87 × 10^−3	1.17 × 10^−1	5.72 × 10^1

Table 10.

Collective dose distribution of air-liquid effluent.

Mine type	Total source term (Bq/a)	Maximum contribution of the public individual dose (mSv/a)		Public collective dose (person·Sv/a)
		Based on the effluents	Based on the environmental monitoring data
Conventional mining in operation	4.47 × 10^14	0.94	0.72	3.71 × 10^1
In situ leaching mining	3.36 × 10^13	0.017	0.28	6.45 × 10^−1
Shutdown mining	2.61 × 10^14	0.39	0.74	1.94 × 10^1
Decommissioning mining	4.59 × 10^11	0.008	—	2.00 × 10^−2

Table 11.

Summary of radiation environmental impact of different types of facilities.

Mine type	Pathway of irradiation	Collective Dose (person Sv/a)
		Rn-222	U-238	U-234	Ra-226	Th-230	Po-210	Pb-210	total
Conventional mining in operation	Gas	3.67 × 10^1	4.09 × 10^−2	4.79 × 10^−2	4.79 × 10^−2	2.17 × 10^−1	2.93 × 10^−3	1.49 × 10^−2	3.70 × 10^1
	Liquid	—	2.49 × 10^−3	7.19 × 10^−4	3.83 × 10^−3	1.74 × 10^−4	4.46 × 10^−3	6.31 × 10^−2	7.48 × 10^−2
	Total	3.67 × 10^1	4.34 × 10^−2	4.86 × 10^−2	5.17 × 10^−2	2.17 × 10^−1	7.39 × 10^−3	7.80 × 10^−2	3.71 × 10^1
In situ leaching mining	Gas	6.45 × 10^−1	—	—	—	—	—	—	6.45 × 10^−1
	Liquid	—	—	—	—	—	—	—	—
	Total	6.45 × 10^−1	—	—	—	—	—	—	6.45 × 10^−1
Shut down mining	Gas	1.94 × 10^1	—	—	—	—	—	—	1.94 × 10^1
	Liquid	—	3.87 × 10^−4	4.15 × 10^−4	1.17 × 10^−2	4.03 × 10^−4	2.48 × 10^−3	3.94 × 10^−2	5.48 × 10^−2
	Total	1.94 × 10^1	3.87 × 10^−4	4.15 × 10^−4	1.17 × 10^−2	4.03 × 10^−4	2.48 × 10^−3	3.94 × 10^−2	1.94 × 10^1
Decommissioning mining	Gas	2.00 × 10^−2	—	—	—	—	—	—	2.00 × 10^−2
	Liquid	—	2.04 × 10^−7	1.95 × 10^−7	3.49 × 10^−8	4.19 × 10^−8	4.02 × 10^−14	7.46 × 10^−11	4.76 × 10^−7
	Total	2.00 × 10^−2	2.04 × 10^−7	1.95 × 10^−7	3.49 × 10^−8	4.19 × 10^−8	4.02 × 10^−14	7.46 × 10^−11	2.00 × 10^−2
Total	Gas	5.67 × 10^1	4.09 × 10^−2	4.79 × 10^−2	4.79 × 10^−2	2.17 × 10^−1	2.93 × 10^−3	1.49 × 10^−2	5.71 × 10^1
	Liquid	—	2.88 × 10^−3	1.13 × 10^−3	1.56 × 10^−2	5.77 × 10^−4	6.94 × 10^−3	1.03 × 10^−1	1.30 × 10^−1
	Total	5.67 × 10^1	4.38 × 10^−2	4.90 × 10^−2	6.35 × 10^−2	2.18 × 10^−1	9.87 × 10^−3	1.17 × 10^−1	5.72 × 10^1

Table 12.

Collective dose distribution for the residents within 5 km.

Conclusions from Tables 8–12 are follows:

1. From the perspective of the total amount of source release, the percentage of releases of conventional operational mines, in situ leaching mines, closure and decommissioned mines are 60.2, 4.5, 35.2, and 0.1% of the total released amounts in uranium mining and milling facilities, respectively. The contribution of conventional mines in operation mode is the largest (Table 7).

2. Among all types of mines, the maximum individual effective dose for the public caused by gaseous effluent is mainly the conventional in-operation mine. The in situ blasting leaching mine (L Mine) gives the largest dose (0.94 mSv/a). The decommissioning LC Mine gives the smallest dose (0.008 mSv/a). The key radionuclide of the gaseous effluent of all mines is ²²²Rn. The maximum individual effective dose for the public due to liquid effluent is mainly in conventional in-operation mines, with a dose value of 1.84 × 10⁻² mSv/a. Here, the key radionuclide is ²¹⁰Pb. The critical residential groups caused by each mine are the adjacent mine residential points, with the key radionuclide ²²²Rn. The main pathway is by inhalation. During normal operation of some mine, the radiation effect on the surrounding living public is close to or even beyond the dose constraint value (0.5 mSv/a). Due to the large amount of source term release from nearby waste rock debris with numerous facilities in the region, the dose of critical resident group is close to the constraint value in J Mine (Table 8).

3. From the maximum value of public individual dose based on effluent evaluation, the main contribution is less than 0.1 mSv/a; the public dose caused by in situ leaching and decommissioned mines is less than 0.1 mSv/a, showing a small radiation impact on the public (Table 9).

4. Routine in-operation mines are the facility type with the greatest impact on the surrounding environment and the public. L Mine have relatively large influence, mainly related to the high grade of the deposit, the large source intensity, and the proximity of the settlement and facilities; among the closed facilities, the main contribution facility is 272 plant tailings pond, which is related to its large scale. It has not been decommissioned yet. Although the source term of in situ leaching mine is relatively large, its dose contribution to the public is relatively small due to its low emission and sparse surrounding population; the radiation impact of the decommissioning mines is minimal, and its source term and the public dose are lower than those of other types of mines.

5. The total public collective dose caused by the whole uranium mining and milling system is 57.2 person·Sv/a, ²²²Rn is the main contributor to the collective dose (98.9%); main pathway is by air (99.8%). Underground mines in operation and closure modes largely contribute to the collective dose (49.5 and 49.6%, respectively). This is related to the large release of the two kinds of mines, with their contributions of 60.2 and 35.2%, respectively.

3.6 Estimation of effective doses for the public based on environmental monitoring data

The estimation is based on the actual situation of public exposure caused by the cumulative discharge of radionuclides in various facilities after the operation of uranium mining and milling facilities for many years. The environmental monitoring data represent the cumulative results of the operation of the facilities over the years. Measuring the public exposure dose is difficult. Therefore, this project uses environmental monitoring data to calculate the cumulative maximum individual effective dose for the public.

3.6.1 Evaluation procedure

The cumulative evaluation involves two key data, namely, environmental monitoring and environmental background. They are important for the determination of the critical resident groups based on an effluent predictive evaluation. The evaluation procedure is as follows: source term determination → effluent monitoring → evaluation based on effluent data → identification of critical resident group → determining the environmental monitoring object and the pathway of irradiation → environmental monitoring → environmental background value determination and deducting → evaluation based on environmental monitoring data.

3.6.2 Radiation environment background determination technology

Environmental background determination is a key technology of the dose evaluation based on monitoring data. It mainly solves whether the monitoring data is at the environmental background level and is the key technology to judge the subtracted background value.

Grubbs method is used to test the background environmental data before operation or the level of comparison points during operation and to eliminate suspicious data. The distribution test of background data is carried out by Shapiro Wilke (S-W method) and skewness and kurtosis method.

If the natural background data X = {X₁,X₂,…X_n} obey the normal distribution, the arithmetic average value is usually used to characterize the environmental background value. When the test object does not meet the normal distribution, it is converted logarithmically to check, whether it is conform to a logarithmic or a normal distribution. If it conforms to the normal distribution after the test, the geometric mean value can be used to characterize the environmental background value. After testing, when natural background data cannot be converted to normal distribution by scale, or when the scale transformation is very complicated, natural background data is a skewed distribution, and then, the natural background data is usually characterized by the median and average difference of the sample data. The median represents the location of the median value of the sample data, which is not affected by the minimum and maximum of the sample data. It is used when there is a lack of sensitivity. The median is representative for all persons and recommended, when the sample size is small (n < 50).

3.6.3 Cumulative doses

Calculated effective doses based on environmental monitoring data are generally higher than those based on effluent data, with large differences (see Table 13).

Mine type	Conventional mining in operation			In situ leaching mining			Shutdown mining							Decommissioning mining
Mine name	M Mine	J Mine	L Mine	T Mine	X1 Mine	X2 mine	B Mine	Q Mine	DC Mine	JH Mine	JR-1 Mine	JR-2 Mine	TC Mine	LC Mine
Maximum possible public individual effective dose (mSv/a)	0.40	0.72	0.59	0.12	0.05	0.03	0.44	0.34	0.51	0.03	0.36	0.45	0.29	0.00
The inhalation pathway contributes to the (%)	100	25.8	100	100	98.4	—	100	100	100	100	100	100	100	—
The feeding pathway contributes (%)	—	74.2	—	—	1.6	100	—	—	—	—	—	—	—	—
The concentration contribution of radon (Bq/m^3)	18.8	15.4	66.4	12.9	2.3	—	20.6	18.4	23.9	1.5	28.0	21.0	32.6

Table 13.

Results of computational evaluated doses based on environmental monitoring data.

As Table 13 shows, the maximum individual effective dose for the public, based on environmental monitoring data, is calculated mostly between 0.25 and 0.5 mSv/a. The maximum individual effective dose for the public, based on environmental monitoring data, is dominated by the ²²²Rn inhalation contribution. J Mine Yangjia infant group is the largest individual dose for the public. The main contribution pathway is by ingestion (74.2%), dominated by ²¹⁰Po in rice (56.9% of the total dose).

3.7 Evaluation of normalized doses

According to the 30-year radiation environmental quality assessment of China’s nuclear industry and the environmental status of uranium mining and milling, before 1986, the collective dose of radioactive effluent produced by uranium mining and milling in China was 19.3 person Sv/a, among which the dose of gaseous and liquid effluent was 14.7 and 4.59 Sv/a, respectively. The key radionuclides that cause the largest environmental public doses are radon and its daughters. The key pathway is by inhalation. The radon and its daughters contribution of the uranium mine accounts for about 90.9% of the total gas-liquid dose, and that of mills 91.8% (Pan et al., 1989) [13].

Due to the development of the wastewater treatment technology, the contribution of the liquid pathway to the public collective dose has decreased further in the last 10 years. The contribution of the radon release caused by underground mining is as high as 97.29%, and other pathways are negligible. Therefore, the public effective dose caused by underground mining can be replaced by the public effective dose caused by radon in the return air shaft of mines. Calculated collective doses for normalized natural uranium caused by each return air shaft are shown in Table 14. The weighted average of conventional underground mining is 0.0121 person·Sv/tU, and the weighted average of in situ blasting underground mining is 0.373 person·Sv/tU. In addition, the normalized collective effective dose of the public caused by in situ leaching mine was 2.97 × 10⁻⁴ person·Sv/tU. According to the proportion of conventional underground mining, in situ blasting leaching underground mining, and in situ leaching mining each year, the annual average normalized collective effective dose is estimated (see Table 15).

Conventional underground mining							In situ blasting leaching mining
B mine	Q mine	C mine	J mine	M mine	DC mine	Weighted mean	L mine	DB mine	Weighted mean
4.73	2.56	21.3	8.88	10.0	30.7	12.1	554	38.1	373

Table 14.

Normalized natural uranium public collective dose person·mSv/tU.

Year	Normalized radon release, TBq/GWa	Average normalized collective effective dose, person·Sv/GWa	In situ leaching percentage,%
1986 ∼ 1995	284	8.09	1.0
1996 ∼ 2005	258	7.29	10.0
2006 ∼ 2010	224	6.15	20.8
2011 ∼ 2014	181	4.04	37.0
2015 ∼ 2019	115	3.51	50.0

Table 15.

Normalized radon release and the public collective effective dose from the uranium mines.

The normalized collective effective dose of the public caused by the Chinese uranium mining and milling production was 21 ∼ 33 times higher than the 5-year average value reported in UNSCEAR 2008 [11]. Due to the popularization and utilization of the in situ leaching process, the normalized collective effective dose of uranium production in China has shown a decreasing trend (Table 15).

In the nuclear power chain, the collective effective dose caused by mining and milling accounts for more than 97%, as shown in Table 16. The collective effective dose of the public caused by uranium mining and milling has increased from 80% after the establishment of the nuclear industry to 97%. It is mainly because uranium mining and milling is an open environment and the uranium deposits in China are small and scattered, and a large number of uranium mines have been developed in the past 30 years.

Year	Uranium mining and milling	Uranium purification	Uranium conversion	Uranium isotope separation	Nuclear fuel element manufacturing	Nuclear power plant operation	total	Uranium mining and milling ratio, %
2006 ∼ 2010	6.15	1.61 × 10^−2	7.62 × 10^−5	5.61 × 10^−5	4.60 × 10^−3	1.61 × 10^−1	6.312	97.44
2011 ∼ 2014	4.04	1.34 × 10^−2	1.33 × 10^−4	1.23 × 10^−5	2.44 × 10^−3	6.44 × 10^−2	4.120	98.05

Table 16.

Normalized public collective doses of nuclear power chains (person Sv/GWa).

Compared with the evaluation of the first 30 years of China nuclear industry, there are great differences in facilities or mine objects, production scale, monitoring and evaluation methods, and data statistics types. Therefore, a comparative analysis was performed only from the overall conclusions. The comparison of source release is shown in Table 17, and the comparison of individual effective dose of critical resident groups is shown in Table 18. The comparison of collective doses caused by each nuclide is shown in Table 19.

Evaluation period	Source term type	Release amount (Bq/a)								Ratio (%)
		Rn-222	U-238	U-234	Ra-226	Th-230	Po-210	Pb-210	total
Current stage evaluation	Gas	7.42 × 10^14	8.94 × 10^10	4.60 × 10^8	4.38 × 10^8	4.60 × 10^8	4.38 × 10^8	4.38 × 10^8	7.42 × 10^14	99.9
	Liquid	—	6.89 × 10^9	6.63 × 10^9	1.49 × 10^9	1.15 × 10^9	1.21 × 10^9	1.95 × 10^9	1.93 × 10^10	0.1
	Total	7.42 × 10^14	9.63 × 10^10	7.09 × 10^9	1.92 × 10^9	1.61 × 10^9	1.65 × 10^9	2.39 × 10^9	7.42 × 10^14	100
	Ratio (%)	100	1.30 × 10^−4	9.56 × 10^−4	2.59 × 10^−4	2.17 × 10^−4	2.22 × 10^−4	3.22 × 10^−4	100	—
Previous 30 years of evaluation of the nuclear industry	Gas	2.76 × 10^14	1.79 × 10^9	1.79 × 10^9	3.94 × 10^8	3.68 × 10^8	3.72 × 10^8	3.68 × 10^8	2.76 × 10^14	99.9
	Liquid	—	9.55 × 10^10	9.55 × 10^10	7.09 × 10^10	1.22 × 10^10	1.01 × 10^10	1.85 × 10^10	3.03 × 10^14	0.1
	Total	2.76 × 10^14	9.73 × 10^10	9.73 × 10^10	7.13 × 10^10	1.26 × 10^10	1.05 × 10^10	1.89 × 10^10	2.76 × 10^14	100
	Ratio (%)	99.9	3.5 × 10^−2	3.5 × 10^−2	2.6 × 10^−2	4.5 × 10^−3	3.8 × 10^−3	6.8 × 10^−3	100	—

Table 17.

Comparison of gaseous and liquid radioactive effluent releases from uranium mines.

Evaluation period	Maximum individual effective dose in critical resident groups (mSv/a) *
	<0.1	≥0.1, < 0.25	≥0.25, < 0.5	≥0.5, < 1	≥1
Current stage evaluation	64.3	7.1	21.4	7.1	0
Previous 30 years of evaluation of the nuclear industry	37.3	22.0	33.8		6.9

Table 18.

Proportion of individual effective dose distribution from uranium mines (%).

*

Results based on effluent calculation.

Evaluation period	Pathway of Irradiation	Collective Dose (person Sv/a)								Ratio (%)
		Rn-222	U-238	U-234	Ra-226	Th-230	Po-210	Pb-210	total
Current stage Evaluation	Gas	56.7	0.041	0.048	0.048	0.217	0.003	0.015	57.1	99.8
	Liquid	—	0.003	0.001	0.016	0.001	0.007	0.103	0.130	0.2
	Total	56.7	0.044	0.049	0.064	0.218	0.010	0.118	57.2	100
	Ratio (%)	99.1	0.077	0.086	0.111	0.381	0.017	0.205	100	—
Previous 30 years of evaluation of the nuclear industry	Gas	12.70	0.885	0.055	0.026	—	0.013	0.115	13.80	76.7
	Liquid	—	0.233	0.014	0.948	—	0.055	2.94	4.19	23.3
	total	12.70	1.12	0.069	0.974	0.00	0.068	3.06	18.0	100
	Ratio (%)	70.6	6.2	0.4	5.4	0	0.4	17.0	100	—

Table 19.

Comparison of collective dose caused by each nuclide of effluent from uranium mines.

Compared with the previous 30-year evaluation period of the nuclear industry, the total release of gaseous and liquid radioactive effluent from the uranium mining and milling facilities has increased by a factor of about 2.7. The contribution of gaseous effluent and ²²²Rn remained basically unchanged, above 99.9%, due to the growing development of Chinese nuclear industry and increasing trend of the production scale. Due to the increased proportion of in situ leaching facilities and shutdown stage of underground mining facilities, the total amount of radionuclide release from the uranium mining and milling facilities increases, public maximum individual effective dose of the surrounding critical residential group is reduced, and in particular, the group part with less than 0.1 mSv/a has increased. The public collective dose of uranium mining and milling facilities has increased by about two times compared with the previous 30-year evaluation period of the nuclear industry. This is related to increased emissions of source terms (see Table 17) and increased population around the facilities. In terms of the exposure pathway, the contribution of the liquid exposure pathway decreased by 23.2% and the gaseous exposure pathway increase by 28.5% due to the closure of facilities and the application of in situ leaching process and increasing trend of production scale.

4. Conclusions

This study develops the monitoring device for the determination of the radon release in the return air shaft of mines and the monitoring technology, which reveals release, migration, and diffusion behavior of radon and aerosols inside and outside the return air shaft. The method to monitor the radon release of liquid transport pipes and evaporation pools of in situ leaching mines, the radon release, and the monitoring method of radon in each vent of a uranium milling plant of in situ leaching are also established. The law of release, migration, and diffusion of radon and the distribution characteristics of gas-borne pollutants of in situ leaching mines are analyzed and determined.

²²²Rn is still the dominant radionuclide, which is released from uranium mining and milling sources (more than 99.9% of the total gas-and-liquid pathway). The normalized radon release amount of various uranium mining and milling facilities types in decreasing order is operational in situ blasting leaching mine, operational underground mine, and operational in situ leaching mine, of which in situ leaching appears obvious environmental advantages.

The radon activity concentration at the outlet of the return air shaft of a mine shows an inverse parabolic function of its activity profile. Below the return air shaft head (−0.5 m, −1.0 m) a wavy line pattern of the activity profile was measured. The maximum radon activity concentration around the return air shaft was in the area of 100- to 300-m distance downwind. The maximum radon activity concentration around in situ leaching facilities occurs within a radius of 100 m. The radon activity concentration outside a distance of 500 m drops to the background level. In the gaseous effluent, the activity concentration of ²¹⁰Pb is several times higher than that of ²³⁸U and 1–2 orders of magnitude higher than ²²⁶Ra. The source term data are 1–2 orders of magnitude higher than for nearby residents. The activities of radionuclides in aerosols from source terms and in aerosols from the environment are ranged generally in: ²¹⁰Pb > ²¹⁰Po > ²³⁸U > ²²⁶Ra. In a few cases, ²³⁸U > ²¹⁰Po. Radionuclide concentrations in aerosols from mines are 6 to 10 orders of magnitude lower than the corresponding radon activity concentration. Radionuclide concentrations in environmental aerosols are 4–6 orders of magnitude lower than the corresponding radon activity concentration.

The public effective doses caused by uranium mining/milling facilities are in descending order: in situ leaching operational mines > non-in situ leaching closed mines > in situ leaching operational mines > decommissioning mines. Over the past 30 years, the normalized public collective effective dose was reduced yearly, from 8.09 Sv/Gwa (during 1986–1995) to 3.51 Sv/Gwa (2015–2019). The normalized public collective effective dose is only 0.063 person Sv/GWa for residents around in in situ leaching facilities. The contribution of gaseous effluent and ²²²Rn to the collective effective dose of the public increased from 70% to more than 99%. And the contribution of the liquid pathway to the collective dose was reduced from 23.3 to 0.2% due to the application of the in situ leaching uranium technique, closure and decommissioning of some facilities, and an effective management of the wastewater from mines. In situ leaching uranium mines show environmental advantages.

In short, this study provides methods and technical support for the daily operation at China’s uranium mines and mills, such as the management of effluents, the environmental monitoring, pollution prevention and control, decommissioning, and environmental governance. It improves the radiation management of uranium mining and milling. It also provides important guidance for a pollution prevention and control and the supervision of the development of associated radioactive mineral resources. It has promoted the technological development of related industries and fields.