Introduction

Since the twentieth century, with the expansion of energy structures and the demand for sustainable development, the advantages of nuclear energy have become increasingly important. The stable development of nuclear energy relies on the exploitation of uranium mines, which generate tailings, waste residue, liquid effluents, and gaseous effluents, all of which are radioactive. Their long-term natural deposition negatively affects the surrounding ecological environment, causing the surface γ-ray dose rate and concentration of radon in the surrounding residential areas to substantially exceed normal background values. Consequently, the treatment of radioactive waste produced by uranium mining has become an urgent issue in the field of environmental protection [1,2,3,4,5].

The decommissioning management of uranium mines has a long cycle and requires a large amount of capital investment. Many countries have been conducting research on the environmental monitoring of uranium mines since the 1950s and have developed many effective decommissioning plans, which have achieved significant results in the environmental and ecological restoration of uranium mines [6,7,8,9,10]. In China, due to historical reasons and lack of experience, a "pollute first, manage later" approach has historically been taken in regard to the environmental management of uranium mines, which has led to a noticeable gap between China and international standards in the field of decommissioning and remediation. Since the 1990s, China has successfully implemented decommissioning plans and environmental management measures for a number of resource-exhausted uranium mines. Over time, decommissioned uranium mines may be damaged by human or natural disasters, and the long-term effectiveness of the technologies applied in the decommissioning process remains to be verified [11, 12]. Therefore, understanding the effectiveness of decommissioning management for uranium mines is essential for identifying potential future issues and safety risks. This paper focuses on a decommissioned uranium mine in southern China and thoroughly reviews the domestic and international standards for the decommissioning of uranium mines. We measured the activity concentration of radionuclides in the soil, water, and air surrounding the mine, evaluated the long-term effectiveness of the decommissioning project, and proposed rectification measures for sources that fail to meet the standards. This research aims to provide insights for long-term environmental radioactivity monitoring of decommissioned uranium mines.

Materials and methods

Production process and pollution sources

Located in Hunan Province, China, this mine consists of an eastern and a western mining area. The decommissioned workplaces include opencast mining ruins, an ore dressing plant, a waste rock site, and a wastewater treatment plant (see Fig. 1). Uranium ore is rich in natural radionuclides such as 238U, 226Ra and 232Th, that generate a significant amount of radioactive waste during the mining, processing, and decommissioning stages. Radioactive materials spread into the environment through diffusion, deposition, percolation, and leaching in the forms of gases, liquids, and solids [13,14,15]. As shown in Fig. 2, the production process of this mine involves the preliminary screening of uranium ore extracted from opencast mining sites and mine pits, and the separation of low-grade ore with no heap leaching value from high-grade ore, which is then transported to the hydrometallurgical mill. The waste rock and residues produced during mining accumulate in the open air within the mine area. Due to current technological and process limitations, completely separating and extracting these radionuclides remains challenging. The stacking of these solid wastes causes radionuclides to migrate continuously through the soil pores.

Fig. 1
figure 1

Arrangement of the monitoring points of the uranium mine

Full size image
Fig. 2
figure 2

Production process of the uranium mine

Full size image

Decommissioning governance schemes and regulatory limits

The mine was closed in 1995, and its decommissioning work was completed in 2002. The decommissioning efforts mainly included: (1) backfilling opencast mining ruins with waste rock and scattered ore from the surrounding environment to reduce the accumulation of waste rock on the surface, (2) all production equipment was washed with high-pressure water and repurposed for other mining operations, (3) metal parts contaminated with radionuclides were cut off and filled into the opencast mining ruins, (4) after decontamination and ensuring surface radioactivity levels were below 0.08 Bq cm–2, the remaining decommissioned equipment was sent to a smelting plant in Hunan Province, and (5) after all the decommissioned areas were cleaned, leveled and stabilized in situ, low-level radioactive areas nearby were selected as soil excavation sites, and sticky loess was excavated and spread over the decommissioned areas to a thickness of 60–80 cm. This mixture was then mechanically compacted to reduce the surface γ-ray dose rate and suppress the exhalation of radon and its progeny. Finally, grass was planted to restore vegetation.

According to China's "Regulations for Radiation Protection and Radiation Environment Protection in Uranium Mining and Milling" and relevant regulations from other countries on uranium mine decommissioning [16,17,18,19,20], a dose limit of no more than 1 mSv a–1 is widely adopted worldwide as the main recommended guidance limit for evaluating uranium mine decommissioning projects. For specific measurement items, the decommissioning regulatory limits for uranium mines we selected are shown in Table 1.

Table 1 Radionuclide concentration/dose regulatory limits in soil, water and air
Full size table

Survey methods and instruments

From May 2021 to April 2023, in accordance with China's "Regulations for Radiation Environmental Monitoring in Uranium Mine and Mill" [21], measurements were conducted in the eastern and western mining areas of the mine and the surrounding regions. These measurements included the surface γ-ray dose rate, the concentration of radon and its progeny in air, the radon exhalation rate, and the concentration of natural radionuclides in the soil and surface water. To comprehensively and accurately reflect the environmental radioactivity status within the surveyed area, the frequency of monitoring for each measurement target was set at once per month. Multiple measurements were continually performed at each monitoring point, and their average values were taken. Additionally, each monitoring point was measured repeatedly every year during the survey period. To obtain contrast point data, measurements were taken in areas 80 km away, that were unaffected by the production of the mine.

To ensure the accuracy of the measurement results, comprehensive quality assurance measures were adopted for this survey. The collection, processing, and analysis of samples were conducted based on China's national standards. All measuring instruments employed were verified by the China Institute of Metrology. The personnel conducting the measurement analysis received professional training. The instruments and parameters introduced for each monitoring item are listed in Table 2.

Table 2 Monitoring instruments and their parameters
Full size table

Results and discussion

Radioactivity measurement in air

Surface γ-ray dose rate

The instrument used for measuring the surface γ-ray dose rate is an HA1310 high-pressure ionization chamber (see Fig. 3), which adopts an integrated structure design of the host and detector and has the characteristics of high detection sensitivity, stable performance, long service life, and good energy response. The measurement results of each monitoring point in the mining area are listed in Table 3. During the measurement, the locations of monitoring points were organized into a grid: a 20 m × 20 m layout was used within the uranium mine area, and a 50 m × 50 m layout was used for other areas. If abnormal values were detected, the number of monitoring points was increased. The measuring instruments were placed 1 m above the ground at the centre of each grid. The monitoring results from Table 3 indicate that the average γ-ray dose rates at the decommissioned areas, including opencast mining ruins, waste rock sites, mine pits, ore dressing plants, and wastewater treatment plants, ranged from 90.2 ~ 286.6 nGy h–1. This range aligns with the decommissioning management goal of ensuring the environmental surface γ-ray dose rate does not exceed 450 nGy h–1. The comparison with the final decommissioning stage revealed a slight reduction in the overall surface γ-ray dose rate. In nearby residential areas, the surface γ-ray dose rates averaged between 83.2 and 102.2 nGy h–1, which is within the range of the measurement values for the control point.

Fig. 3
figure 3

Instruments used for measuring the surface γ-ray dose rate and concentration of radon and its progeny in the air

Full size image
Table 3 Surface γ-ray dose rates at the monitoring points
Full size table

The annual effective dose for residents and staff caused by γ-ray was calculated through the dose calculation model recommended in the 1982 report by the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) [22]:

$$H_{e1} = k\sum\limits_{i} {X_{i} \cdot T_{i} }$$
(1)

where He1 is the annual effective dose caused by γ ray, k is the ratio of the effective dose to the surface γ-ray dose rate, with UNSCEAR recommending a value of 0.7, Xi is the γ-ray dose rate at the i-th monitoring point, and Ti is the actual time spent by staff or residents at the i-th monitoring point. According to UNSCEAR, the occupancy factors for residents indoors and outdoors are 0.8 and 0.2, respectively, which translates to 7008 h spent indoors and 1752 h spent outdoors. For staff who currently reside in the residential area and work in the monitoring area for 2000 h, their time spent indoors and outdoors in the residential area amounts to 5408 h and 1352 h, respectively. The calculations demonstrate that the maximum value of the average annual effective dose caused by γ-ray in the mining area was 0.76 mSv for residents and 1.06 mSv for staff.

Concentration of radon and its progeny in the air

The layout for monitoring the concentration of radon and its progeny in the air was consistent with that for monitoring the surface γ-ray dose rate. The instrument used for measurement is a DHZM-II radon monitor (see Fig. 3), which uses the double-filter method and can measure the concentration of radon and its progeny in real time. The measurement results are presented in Table 4. According to these results, the concentration of radon and its progeny in the mining area remained at the same level as at the time of decommissioning. The western mining area, which is in a low-lying and relatively enclosed space with poor air circulation, had initially exhibited moderately high concentrations of radon and its progeny. However, when monitoring measurements were compared with the contrast point data, no significant difference was found, which indicates that the potential impact of radon on the public and the environment post-remediation is acceptable.

Table 4 Concentrations of radon and its progeny in the air
Full size table

Based on the dose calculation model proposed in the UNSCEAR 2000 report [23], the annual effective dose caused by the inhalation of 222Rn and its progeny is calculated as follows:

$$H_{e2} = G_{R} \sum\limits_{i} {0.180 \cdot D_{i} \cdot T_{i} }$$
(2)

where Ti is the actual time spent by staff or residents at the i-th monitoring point, with the specific time distribution consistent with the effective dose calculation for the surface γ-ray dose rate; Di is the concentration of radon progeny (nJ m–3); and 0.180 is the conversion factor from nJ m–3 to Bq m–3. GR is the dose conversion coefficient; according to the UNSCEAR 2000 report, the effective dose conversion coefficient for inhaling the unit equilibrium equivalent radon concentration per unit time is 9 nSv (Bq m–3 h)–1. The calculation results indicate that the maximum annual effective dose caused by the inhalation of radon and its progeny in the mining area was 1.25 mSv for residents and 1.42 mSv for staff.

Radioactivity measurement in soil

Radon exhalation rate

The layout for measuring the radon exhalation rate was consistent with that for the surface γ-ray dose rate. The measurement device is shown in Fig. 4. The RaD-7 radon monitor includes a host, filter, drying tube, and desiccant. It is widely used because of its simple sampling and good stability. To minimize the impact of air humidity fluctuations on measurement accuracy, the measurements were typically conducted between 10 a.m. and 4 p.m. Additionally, field measurements were carried out on clear days and at least 24 h after the last rainfall. Table 5 presents the radon exhalation rates measured in the mining area. The results from Table 5 show that after decommissioning and remediation, the radon exhalation rates measured in the opencast mining ruins ranged from 123.2 to 336.6 mBq m–2 s–1, and those in the waste rock sites ranged from 44.5 to 292.7 mBq m–2 s–1. The average radon exhalation rates across various monitoring points in other workplaces ranged from 39.5 to 138.6 mBq m–2 s–1, which meets the regulatory limits set for decommissioning. Compared with the measurements taken at the final state of decommissioning, there was a significant decrease which indicates that the remediation efforts have been effectively maintained.

Fig. 4
figure 4

Measurement device used for determining the radon exhalation rate

Full size image
Table 5 Radon exhalation rates from the soil surface
Full size table

Natural radionuclides in soil

Soil samples for this survey were collected from the buried cover layers of the mining area and nearby farmlands. The soil sampling followed a 10 m × 10 m grid pattern and utilized a plum distribution method to ensure no fewer than 10 sampling points per grid. After removing all tubers and grass layers, samples were mixed on-site, stored in cloth bags labelled with GPS tags, and transported to the laboratory, where they were spread out on enamel trays to dry. The dry samples were then sifted through a 40–60 mesh screen, weighed, sealed in sample boxes, and stored for 5 weeks prior to measurement. The energy resolution of the GEM HPGe γ spectrometer used for measurement (see Fig. 5) is 1.92 keV @ 1332.5 keV, and the relative detection efficiency is 28.3%. Before measuring the samples, the energy and detection efficiency of the γ spectrometer were calibrated using a standard soil source. In the measured energy spectrum data, according to the decay diagrams of 238U, 226Ra and 232Th, the activity concentration of 238U was determined by γ photon with an energy of 92.6 keV emitted by 234Th, the activity concentration of 226Ra was determined by γ photon with an energy of 351.9 keV emitted by 214Pb and γ photon with an energy of 609.3 keV emitted by 214Bi, and the activity concentration of 232Th was determined by γ photon with an energy of 238.6 keV emitted by 212Pb and γ photon with an energy of 911.2 keV emitted by 228Bi. Figure 6 shows the energy spectrum of a soil sample. The measurement results of the soil samples are shown in Table 6. As shown in Table 6, the measurement results imply that the activities of 238U and 226Ra in the soil of the remediated areas, such as the opencast mining ruins and waste rock sites, were slightly greater than those in the farmlands and control points. This likely results from the leaching of 238U and 226Ra from buried waste rock into the surface soil. The average activities of 226Ra in the soil of both the remediated areas and surrounding farmlands were below the levels at the time of decommissioning, which suggests a gradual improvement in the amount of radioactive pollution affecting the farmlands surrounding the mine post-remediation.

Fig. 5
figure 5

GEM HPGe γ spectrometer

Full size image
Fig. 6
figure 6

Energy spectrum of a soil sample

Full size image
Table 6 Activity concentration of radionuclides in soil (Bq kg–1)
Full size table

Radioactivity measurement in surface water

Water samples were primarily collected from mountain springs, rivers, and residential water sources surrounding the mining area, with focus on the middle of these water bodies for sampling. Polyethylene plastic barrels were chosen as sample containers. Before collection, these barrels were thoroughly cleaned and then rinsed three times with water from the sampling site. Field sampling was conducted on clear days, and the time interval between the last rainfall was at least 48 h. Upon returning to the laboratory, nitric acid was introduced to adjust the pH value of the water samples from each monitoring point to 2. If the water contained suspended particles, it was first filtered through a 0.45 μm membrane filter. The water samples were then saved in sample boxes for measurements. The activity concentrations of 226Ra and 232Th in the water samples were measured using a GEM HPGe γ spectrometer. The measurement of total uranium was carried out using the FYWY uranium analyser (see Fig. 7). The FYWY uranium analyser uses a new digital laser fluorescence measurement technology, which can easily adjust the measurement parameters according to the actual situation and has a wide range of adaptations. An Fj-2604 α/β radioactivity monitor was used for gross α and gross β measurements. The Fj-2604 α/β radioactivity monitor uses a passive implemented planar silicon (PIPS) detector, which is mainly used for radioactivity measurement of low-level environmental samples. The measurement results of the water samples are shown in Table 7. The results reveal that after remediation, the concentrations of nuclides in surface water at various monitoring points were generally lower than those at the time of decommissioning and did not exceed the established regulatory limits, implying the effectiveness of the decommissioning efforts. The activity concentrations of uranium and radium in the surface water of the residential area in the western mining area are higher than those in the mining area. It can be inferred that uranium and radium in the mining area have migrated. There was no significant difference in the gross α or gross β values of the drinking water in the residential area compared to those at the control point, which was similar to the results reported by many scientific researchers [24,25,26].

Fig. 7
figure 7

Instruments used for measuring total uranium, gross α and gross β

Full size image
Table 7 Concentrations of radionuclides in water
Full size table

Conclusions

Over the two years, the investigation collected comprehensive environmental radioactivity measurements and assessments of a decommissioned uranium mine and its surrounding area, including soil, surface water, and air, in southern China. The findings indicate that after years of decommissioning efforts, the surface γ-ray dose rate and concentrations of radon and its progeny in the air are all below the regulatory limits. There is a significant reduction in the radon exhalation rate, and the concentrations of radionuclides, such as uranium, thorium, and radium, in the soil and surface water are low which demonstrates that management has been effective. The maximum annual effective dose due to internal and external radiation for the staff was 2.48 mSv, which was lower than the management limit of China (5 mSv). For residents living around the mine, after subtracting the background dose (data from the contrast point), the maximum annual effective dose due to internal and external radiation was 0.18 mSv, which was also below the management limit of 1 mSv.

The comparison of the environmental radioactivity measurements from the current survey to those at the time of the final decommissioning phase illustrates the positive correlation between the radon exhalation rate of the soil and the activity concentrations of uranium and radium. Following remediation efforts, there has been a significant improvement in the environment surrounding the mine. Radon exhalation rates and surface γ-ray dose rates have fallen below the regulatory limits, thereby significantly reducing the impact of the mine on nearby communities. However, data analyses suggest that after decommissioning, the main route of radioactive influence on surrounding areas is attributed to the migration of radionuclides facilitated by the permeation of groundwater and surface water, which emphasizes the necessity for intensified monitoring of these aspects in the future.