Measurement and spatial distribution pattern of natural radioactivity in a uranium tailings pond in Northwest China

ABSTRACT Numbers of radioactive tailings were produced in the process of uranium mining and milling.In this study,radiation environment monitoring was carried out on a decommissioned uranium mining and milling tailings pond in northwest China, and the governance effect and operation status of the tailings pond was investigated. The soil radionuclide activity concentration and ambient dose equivalent rate (H*(10)) were measured, and soil surface radon exhalation rate was calculated. The results shown that in the tailings pond and its surrounding area, the average soil radionuclide activity concentration was respectively 238U: 42.64±11.17 Bq·kg-1, 226Ra: 45.47±13.56 Bq·kg-1, 232Th: 33.30±7.33 Bq·kg-1, 40K: 631.33±72.05 Bq·kg-1, the average H*(10) was 63.32±6.46 nSv·h-1, and the average radon exhalation rate was 40.46±11.29 mBq·m-2·s-1.Radionuclide activity concentration, H*(10) and radon exhalation rate were within the range of world natural background. Spatial distribution of radioactivity was analyzed by OK interpolation based on the investigated data. According to the measured results and spatial interpolation analysis,the decommissioning governance effect of the uranium tailings reservoir is good, the operation is normal, and there is no migration or diffusion situation of radionuclides in the pond has been found.


Introduction
The continuous development and extensive utilization of nuclear energy have led to the increasing demand for uranium smelting products. However, a large number of radioactive contaminations will inevitably be produced in the process of uranium mining and milling (Craft et al., 2004;Jagetiya & Purohit, 2006;Portnov et al., 2017;Qin et al., 2020). The fine mineral slag produced in the process of uranium mining and milling is called uranium tailings which contain natural long-life radionuclides such as 238 U, 226 Ra, 232 Th, 40 K and some heavy metals associated with uranium mines (Dinis de & Fiúza, 2013;Landa, 2004;Laxman Singh et al., 2015;Momčilović et al., 2013;Zong et al., 2017). It is often hard to completely separate the long-life radionuclides and heavy metals in uranium tailings account for technical reasons and economic factors. Thus, most of the slag will be centrally managed through storage in the tailings pond, and the effect of the uranium tailings on the surrounding ecological environment safety and public health will be reduced through decommissioning governance of the tailings pond Ouyang et al., 2020;Portnov et al., 2017). However, with the increase of decommissioning years, the tailings pond may be disturbed by natural or anthropogenic factors, resulting in the migration and diffusion of radionuclides in the uranium tailings pond, affecting the stability of the tailings pond and its surrounding soil quality, and having a persistent potential effect on the regional ecological environment and public health Lou et al., 2018;Zong et al., 2017).
Due to the particularity, uranium tailings pond has been widely concerned and investigated all over the world (Carvalho et al., 2007;Momčilović et al., 2010). In this study, the investigation of the natural radioactivity levels in vertical profiles of the uranium tailings pond was conducted firstly, and then the spatial distribution pattern of the radionuclide, the ambient dose equivalent rate H*(10) and the soil radon exhalation rate were analyzed. So, this work is of eminent importance to identify the decommissioning governance effect and operation status of the uranium tailings pond, and prevent environmental radioactive contamination.

Overview of the study area
A uranium mining and milling tailings pond which is located in the northwest of China was taken as the object of investigation in this research. The tailings pond is a storage pond of solid uranium tailings, the decommissioning governance work has been completed through surface soil covering, slope protection, and other projects more than a decade ago, as shown in Figure 1. The area where the tailings pond is located belongs to arid and semi-arid climate, with less surface soil and vegetation coverage, abundant sunshine, and less rain all year round, and far away from the surface water source. The average altitude of the tailings pond is 926 m, the terrain in the north is higher than in the south, and the terrain in the west is higher than in the east. The concrete slope protection is located in the northeast of the tailings pond, and the hills are in the southeast.

Sampling and sample pretreatment
Soil samples were collected at different depths of 0-5 cm, 5-15 cm, and 15-30 cm. Three equal amounts of soil were randomly collected near each sampling point and mixed evenly as soil samples at this point and put into a sampling bag marked in advance. The soil sampling points were calibrated by coordinate information, including 4 sampling points in the tailings pond (points 01-04) and 4 sampling points around the tailings pond (points 05-08), with a total of 24 (8 × 3) soil samples. The ambient dose equivalent rate H*(10) was measured in the field, according to the grid of 36 m × 56 m, a total of 24 measurement points (points 01-24) were set (points 01,03, 04 to 08, 17, 21, 22, 24 are located in the surrounding area of the tailings pond, and points 02, 09 to 16, 18 to 20, 23 are located in the tailings pond). Soil sampling points and the ambient dose equivalent rate measurement points were shown in Figure 2.
The sample pretreatment was carried out according to the specifications. The soil samples after weed and gravel removal were placed in the blast drying oven, dried to constant weight at 105°C, then ground and sieved until the particle size was less than 80 mesh. After weighing with an electronic analysis balance, the pretreated samples were put into the cylindrical polyethylene sample container and sealed for 4 weeks to  ensure the radioactive nuclides uranium, radium, and their progenies reached the decay equilibrium (Ndontchueng et al., 2014).

Measurement of radionuclide activity concentration
In this research, the N-type high purity germanium (HPGe) γ spectrometer produced by ORTEC Company (model GMX40P4) was used to measure the activity concentration of natural radionuclides in samples. The crystal size of the detector is 42.4 mm × 73.2 mm, and the energy resolution at the peak of 1.33 MeV ( 60 Co reference source) is 1.92 keV. Before the sample measurement, the soil mixing reference volume source ( 226 Ra-60 Co-238 U-232 Th-40 K) produced by Shanghai Institute of Measurement and Testing Technology was used to calibrate the energy and efficiency of the spectrometer. The measurement time of each soil sample is no less than 21600s, and the quality control is carried out by repeated measurement. Take average value of measurement was taken as the final measurement result.
The energy of the characteristic peaks selected by the radionuclides are respectively, 238 U (63.29 keV and 92.6 keV), 226 Ra (351.92 keV, 609.31 keV, and 1120.29 keV), 232 Th (583.19 keV and 911.21 keV), and 40 K (1460.22 keV), and the calculation formula of radionuclide activity concentration mentioned above was quoted from reference (Turhan & Varinlioǧlu, 2012). The formula is as follows: Where A is the activity concentration of the radionuclide; N is the net count of the characteristic energy peak with energy E γ ; ε E γ À � is the detection efficiency of the characteristic energy peak with energy E γ ; P γ is the probability of the selected characteristic γ rays emitted by each decay of the radionuclide; t is the measurement time; M is the net weight of the sample.

Field measurement of ambient dose equivalent rate H*(10)
The ambient dose equivalent rate H*(10) with the unit nSv·h −1 was measured using the FH40G+FHZ672E-10 gamma dose rate detector, and the detection limit is 1 nSv·h −1 (UNSCEAR, 2013). The detector was preheated for 10 min before measurement, then it was moved to the measuring point and placed the sensitive area center of the detector probe at 1 m above ground. And it was read once every 10 s after the measured data stabilized, and the average value as the measurement result at the measuring point (Jónás et al., 2017).
The ambient dose equivalent rate H*(10) (nSv·h −1 ) was reported by Sanada et al. to be equivalent to the absorbed dose rate in air (nGy·h −1 ) (Sanada et al., 2020). In this research, H*(10) after deducting the contribution of cosmic rays is considered as the absorbed dose rate in air caused by terrestrial components, while the contribution of other gamma radiation sources in the air is considered negligible (Jónás et al., 2018;UNSCEAR, 2000). The theoretical formula for calculating the absorbed dose rate in air caused by the component of the cosmic rays is as follows: Where D C 0 ð Þ is the absorbed dose rate in air caused by cosmic rays' ionization component at sea level; h is the altitude of the measuring point with the unit of km; λ m is the geomagnetic latitude of the measuring point with the unit of N; λ and φ are the latitude and longitude of the measuring point respectively (UNSCEAR, 2000).

Calculation of radon exhalation rate
The radon exhalation rate of the tailings pond is calculated by a theoretical model, and its formula is as follows: ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi Where F is the radon exhalation rate in Bq·m −2 ·s −1 ; R is the activity of 226 Ra in soil, the unit is Bq·kg −1 , ρ is the bulk density of soil, the unit is kg·m −3 ε is the emanation coefficient of radon in the soil; T is soil temperature in K; λ is the decay constant of radon, with the value of 2.6 × 10 −6 s -1 ; D 0 is the diffusion coefficient of radon in free air, with the value of 1.1 × 10 −5 m 2 ·s −1 ; p is the soil porosity; S is the water saturation of soil (IAEA, 2013;Zhuo et al., 2006).
Where ε 0 is the emanation coefficient of radon at 298 K, a, b, and c are constants. For clay, ε 0 , a, b, and c are 0.10, 1.85, 18.8, and 0.012 respectively (Zhuo et al., 2008). The boxplot of soil radionuclide activity concentration inside and around the uranium tailings pond is shown in Figure 3. Although there are differences in the concentrations of radionuclide 238 U, 226 Ra, 232 Th, and 40 K inside and surrounding areas of the tailings pond, the results of one-way ANOVA show that the differences are not significant (p < 0.05).

Soil radionuclide activity concentration
The correlation between different radionuclides is present in Figure 4(a), Among the four radionuclides, only 238 U and 226 Ra show a strong positive correlation. It conforms to the theory that 226 Ra is the decay progeny of 238 U and is in the same decay series. Statistical tests prove that there is no significant difference between the activity concentration of 238 U and 226 Ra in the soil of the study area (p < 0.05). The results indicate that uranium and radium in the tailings pond and its surrounding area are in a state of decay equilibrium. Table 2 shows soil radionuclide activity concentrations from some countries or regions in the references. In contrast, the radionuclide activity of uranium tailings pond is in the natural soil radionuclide activity concentration in these countries and regions ( 238 U: 0.58-88.1, 226 Ra:6.81-265.58 Bq·kg −1 , 232 Th: 7-404 Bq·kg −1 , 40 K:

Ambient dose equivalent rate H*(10)
The average values of H*(10) in the study area after deducting the absorbed dose rate in air caused by the component of cosmic rays are presented in Table 3. The maximum and minimum values appear at point 07 and point 24 respectively, and the range of H*(10) is from 53.34 to 81.40 nSv·h −1 , the average value is 64.12 ± 7.45 nSv·h −1 , slightly lower than the average absorbed dose rate in air caused by outdoor terrestrial radiation in China (69.9 nGy·h −1 ), and within the range of the absorbed dose rate in air caused by outdoor terrestrial radiation (12.7-1300 nGy·h −1 ) (UNSCEAR, 2008). The average H*(10) inside the uranium tailings pond is 63.32 ± 6.46 nSv·h −1 , and the average H*(10) around the uranium tailings pond is 65.07 ± 8.46 nSv·h −1 , without significant difference (p < 0.05). Table 4 summarizes H*(10) or absorbed dose rate in air of some countries or regions reported in the Figure 4. (a) is hierarchical Cluster heat map based on Spearman correlation coefficient between radionuclide activity concentrations, *0.01 ≤ P < 0.05, **0.001 ≤ P < 0.01, *** P < 0.001.(b) is the activity concentration of the radionuclides 238 U and 226 Ra. The same lowercase letter above the box indicates that there is no significant difference between groups (p < 0.05) between groups (one-way ANOVA).  references. After deducting the contribution of cosmic rays, the field measurements of H*(10) are within the absorbed dose rate in air range of other countries and regions (10.7-750 nGy·h −1 ) (Aladeniyi et al., 2019;Antović et al., 2012;Durusoy & Yildirim, 2017;Gabdo et al., 2015;Qu et al., 2008;Shilfa et al., 2020).

Soil radon exhalation rate
The calculated radon exhalation rate on the soil surface at each point in the study area is summarized in Table 5. The average radon exhalation rate is 40.46 ± 11.29 mBq·m −2 ·s −1 , the highest and lowest radon exhalation rates are 65.02 and 28.41 mBq·m −2 ·s −1 , corresponding to point 07 and point 03, respectively. The calculated soil radon exhalation rates are within the world average range of soil radon exhalation rate as reported in the UNSCEAR (1982) report, 0.2-70 mBq·m −2 ·s −1 (UNSCEAR, 1982).
In Table 6, the estimated soil radon exhalation rate of uranium tailings pond is compared with the measured results of some countries or regions. The range of radon exhalation rates in these countries or regions is from 3.2 to 2100 mBq·m −2 ·s −1 , and the soil radon exudation rate in tailings pond and its surrounding areas is within this range (Jasaitis & Girgždys, 2007;Shweikani & Hushari, 2005;Vasidov, 2014;Zhang et al., 2015).

Spatial distribution of radiation in the study area
To further analyze the radiation distribution pattern of the environment and soil in the tailings pond and its surrounding area, the spatial distribution of radionuclides, H*(10), and radon exhalation rate in the uranium tailings pond and its surrounding area were predicted by the Ordinary Kriging (OK) spatial interpolation method in the study.
The distribution characteristics of soil radionuclides in the vertical profile of the study area are shown in Figure 5, the spatial distribution pattern between 238 U and 226 Ra is roughly similar, other radionuclides do not show relatively similar distribution pattern. 238 U has the highest activity concentration in the northwest of the study area, 226 Ra has higher activity concentration in the west and east of the study area, 232 Th and 40 K has higher activity concentrations in the northeast and southeast of the study area, respectively. In the vertical profile within 0 to 30 cm, the activity concentration of radionuclides 238 U, 226 Ra, and 40 K decreased with the increase of soil depth, in contrast, the activity concentration of 232 Th in the northeast direction of the study area increased with the increase of soil depth. Figure 6 shows the spatial distribution of H*(10) (the contribution of cosmic rays has been deducted) and soil radon exhalation rate in the study area. The ambient equivalent dose rate is higher in the northeast and south of the study area, and the soil radon exhalation rate is higher in the east of the study area but lower in the tailings pond. a H*(10) without deducting the contribution of cosmic rays. b The absorbed dose rate in air reported in the references is all calculated by theoretical formula, and the value is equal to the absorbed dose rate in air caused by outdoor terrestrial radiation. c H*(10) after deducting the contribution of cosmic rays. Radionuclide activity concentration and ambient dose equivalent rate are within the range of background range of China, in which the range of radionuclide activity concentration is 238 U: 2-690 Bq·kg −1, 226 Ra: 2-440 Bq·kg −1, 232 Th: 1-360 Bq·kg −1, 40 K: 9-1800 Bq·kg −1 , the range of absorbed dose rate in air caused by outdoor terrestrial radiation is 12.7 to 1300 nGy·h −1 . The calculated soil radon exhalation rates are within the world average range of soil radon exhalation rate as reported in the UNSCEAR (1982) report, 0.2 to 70 mBq·m −2 ·s −1 . Among the four radionuclides, the spatial distribution patterns of 238 U and 226 Ra are similar, while the spatial distribution patterns of 232 Th and 40 K are no obvious relationship in the study area. In the vertical profile within 0 to 30 cm, the activity concentrations of  radionuclides 238 U, 226 Ra, and 40 K decreased with the increase of soil depth, while the activity concentration of 232 Th in the northeast direction of the study area increased with the increase of soil depth.
These investigation results revealed that the decommissioning governance of the uranium tailings pond is a good effect, the tailing pond operates normally, and no migration or diffusion of radionuclides in the pond has been found.