Environmental Benefit Analysis of a Uranium Mine after Decommissioning

Uranium resources are an important strategic resource and the material basis for the development of new energy sources. After uranium mines have been decommissioned for decades, the environmental quality after decommissioning has been a concern of many scholars. The environmental remediation of decommissioned uranium mines should be based on environmental remediation planning, design, management, decision-making and pollution levels, environmental quality requirements, etc., combined with national financial and technical levels, and formulate their environmental remediation plans, striving for the most cost-effective, and the results obtained. optimal.

I. Overview of decommissioned uranium mines

(1) Decommissioned uranium mines and decommissioning standards

Decommissioning of uranium mines refers to the after-treatment of uranium mining and metallurgical facilities that are permanently suspended to ensure that workers and the public are protected from residual radioactivity and other possible hazards (according to GB14586-93). The remediation of decommissioned uranium mines is a very complicated and huge comprehensive project requiring wells and mines, subsidence pits, waste rock piles, tailings ponds, storage yards, hydrometallurgical plants, contaminated equipment and equipment. And contaminated farmland, water bodies, etc., to carry out thorough decontamination, treatment and disposal, so that the environmental quality of the mining area has reached the decommissioning standard.

In 1988, China promulgated the National Standard for Radiation Protection Regulations, which clearly states: “The selection, design, operation and decommissioning of all practices and facilities with radiation exposure must comply with these regulations and must be in accordance with the practice. The principles of sexual and radiation protection are optimized and the individual's exposure is determined to be below the corresponding dose limit." The three principles of radiation protection are the legitimacy of practice, the optimization of radiation protection and the limitation of individual doses. The standard also limits the radiation protection of individual dose limits, gamma radiation doses, alpha surface contamination levels, and sputum precipitation rates. After a series of treatment projects, the uranium mine can be decommissioned after the water environment, soil environment and atmospheric environment have reached the decommissioning standard.

(II) Overview of uranium mines

A uranium mine is located in southern China with abundant rain. The mining area is mainly composed of eastern and western mining areas. There are two open pit mine 5, a total of approximately 1.04 million mined ore T (metal 995t), waste rock 2,900,000 t, the size of the surface remaining waste dump 12, dumps the exposed area of about 173,000 m ² . The uranium and radium content in the pit sewage is relatively high. Before the treatment, the annual discharge of waste rock stack gas is 6.51×10 ¹² Bq. The mine belongs to the typical shallow “three down” (under the village, under the river, under the road) mining bed. It not only seriously threatens the safety of bridges, rivers and buildings, but also affects local land development and utilization. Decommissioning includes a secondary closed pit (well) port 26; waste rock management, industrial site 15 000 m ^2; demolish buildings and structures (processing) 9800m ^2; disposal of subsidence area 6, about 24 000 m ^2; grouting 4380m ³ ; backfilling and filling 3237m ³ ; slag wall 1644m, 6200m ³ ; drainage ditch 3025m, 3660m ³ ; covering soil 69,900m ³ ; planting trees 58900, vegetation 49500m ² ; waste stone export 87,800m ³ ;

(III) Analysis of environmental benefits after the treatment of decommissioned uranium mines

Through the comprehensive management of decommissioned uranium mines, all environmental indicators have reached the standards set by the state, and achieved good environmental benefits. In order to be representative, the sputum precipitation rate, collective effective dose, soil recovery improvement degree, gamma radiation dose rate, public personal radiation dose, water pollution degree, air pollution degree, vegetation coverage rate, land restoration utilization rate and mining area biomass are selected. As an indicator to measure the environmental benefits of decommissioned uranium mines.

Radiation environment. Taking a mine as an example, through the decommissioning of the system, the precipitation rate of surface ²²² Rn has dropped from the average of 1.73 Bq/m ² s before the treatment to the management limit (0.74 Bq/m ² s), and the total average value is 0.29. Bq/m ² s, up to 0.48 Bq/m ² s. The maximum individual dose due to gaseous effluent is reduced from 0.132 mSv/a before treatment to 0.0124 mSv/a after treatment in the E-direction 0 to 1 km sub-area of ​​the evaluation center, which is much less than the management limit of 0.25 mSv/a. Below, the reduction is 91.6%; in the range of 0-80km, the collective radiation dose is reduced from 0.59 people·Sv/a before treatment to 0.179 person·Sv/a after treatment, which is reduced by 69.6%; the dose rate of γ radiation is controlled by The former 76.3×10 ^-8 Gy/h dropped to 20.68×10 ^-8 Gy/h, a decrease of 73%; the concentration of ²²² Rn in the mining area decreased from an average of 36.4 Bq/m ³ before treatment to an average of 16.18 after treatment. Bq/m ³ , a 56% reduction.

Water Environment. The surface water system in the mining area is a river. After the implementation of the pit and water blocking project and the surface treatment project, the environmental water quality of the whole mining area is obviously improved compared with that before the treatment, and the river water quality meets the drinking water standard. The specific data is shown in Table 1.

Table 1 Environmental water quality index value of a decommissioned uranium mining area

The U and ²²⁶ Ra indicators in the river water near the uranium mine decreased from 0.146 Bq/L and 0.076 Bq/L before treatment to 0.037 Bq/L and 0.01 Bq/L after treatment. The pH value rose to 7.6, and the water quality reached the drinking water. standard. The pH value of the tailings water and waste rock seepage water decreased from 3.6 to 4.8 before treatment to 6.5 to 7.5. The concentration of BOD, SS and cadmium in water treated the metal ions reached the discharge standard.

Soil environment. After the soil of the polluted farmland in the mining area was replanted and changed to water, the soil environmental quality was significantly improved compared with that before the treatment. The content of U and ²²⁶ Ra in the soil decreased significantly compared with that before treatment, which were 11.1 mg/kg and 151 Bq/kg, respectively, which was far below the management limit. The content of U in vegetables was 0.16 mg/kg, which was lower than the management limit (1.5 mg/kg); the content of U in farmland rice was 1.3 mg/kg, which was lower than the management limit (1.9 mg/kg). The specific soil environmental quality indicators are shown in Table 2.

Table 2 Soil environmental quality indicators

The uranium mine collapse pits, waste rock piles, tailings ponds, storage yards, water and metallurgical plants have been rectified, and the heavy metal content after the soil cover can reach the standard. The soil in the mining area is low in pH due to acid leaching of uranium. After decommissioning, it can basically reach about 6.5.

Other environmental indicators. The deforestation project in the mining area planted 58900 trees with a vegetation of 49,500 m ² , the vegetation coverage rate reached 84%, and the atmospheric environment of the mining area also became better. For example, the concentrations of TSP and SO ₂ are basically reduced to 0.2 mg/m ³ and 0.15 mg/m ³ or less; after decommissioning, the previously acquired land (including farmland, barren hillsides and public welfare forest land) is replanted and resumed, and the land is restored. The usage rate is 90%. Since the mining area is in the mountainous area and the environment is the same as that in the surrounding area, the retired mining area continues to multiply due to the migration of surrounding organisms, so that the biomass is basically restored to the level before mining.

2. Calculation of comprehensive environmental benefits for decommissioned uranium mines

(1) Using AHP to determine the weight of various indicators of comprehensive environmental benefits

The comprehensive environmental benefit calculation starts from the natural environment and the ecological environment, and analyzes and evaluates the effects of regional treatment of decommissioned uranium mines. The uranium mine management benefit calculation system is a system of interrelated, restrictive and multi-factors. The Analytic Hierarchy Process (AHP) is a multi-objective decision-making method combining qualitative and quantitative analysis. It is often used to solve the multi-objective weights of a total target, or to solve the other factors associated with multiple factors. the weight of. The method steps are as follows:

1. Establish a benefit calculation index system. Reasonable selection of the evaluation index system and the amount of control indicators is the key to establishing an environmental quality assessment system for decommissioned uranium mines. After systematic analysis and research on the relationship between various factors of uranium mines, 10 representative index factors are selected to establish an environmental benefit calculation index system for decommissioned uranium mines, as shown in Figure 1.

Figure 1 Environmental benefit analysis framework of decommissioned uranium mine

2. Construct a judgment matrix. The judgment matrix P is a comparison indicating the relative importance of all factors of the layer to a certain factor of the upper layer. The element a _{ij of the} decision matrix is ​​given by Santy's 1-9 scale method.

3. The eigenvector corresponding to the maximum eigenvalue λ _max of the judgment matrix is ​​normalized (the sum of the elements in the vector is equal to 1), and the element is the same as the relative importance of the same level factor to the relative importance of a factor at the previous level. Value, this process is called hierarchical single sorting. The calculation method can be summarized as the eigenvalue and eigenvector of the judgment matrix by the average approximation method. Consistency test of the results:

Where CI is the consistency index, the larger the value, the worse the consistency of the judgment matrix; λ _max is the largest eigenvalue of the judgment matrix; n is the order of the judgment matrix; CR is the random consistency ratio; RI is the average random For consistency indicators, see Table 3. When CR<0.1, the degree of inconsistency of the judgment matrix is ​​within the allowable range, and the normalized feature vector can be used as the weight. Otherwise, the judgment matrix should be reconstructed and the judgment matrix adjusted.

Table 3 Comparison table of average random consistency indicators

4. Hierarchical total ordering and consistency check. Calculating the weight of all factors at a certain level for the relative importance of the highest level (total target) is called the total order of the levels. This process is carried out in order from the highest level to the lowest level.

(II) Normalized treatment of various indicators of comprehensive benefits

The dimensions of the various indicators of the decommissioned uranium mine environment are different, and there are significant differences in the order of magnitude between some indicators, which brings great difficulties to the analysis of environmental governance benefits. In order to reduce the order of magnitude and uniform dimension between indicators, this paper uses the extreme value method to process each indicator and normalize it. According to the order of magnitude and dimension of the selected 10 indicators, they are divided into 2 categories:

1. The forward indicator, that is, the processing of the maximum index value, the normalization formula is:

2. The negative indicator, that is, the processing of the extremely small index value, the normalization formula is:

Where u _i is the normalized index value of the i-th index; x _i is the actual index value of the i-th index; x _min and x _max are the actual minimum and maximum values ​​of the same type of indicators, respectively. After decommissioning, the environmental indicators of uranium mines have reached the standard of decommissioning. Most of the indicators in this paper use the waste treatment rate as the benefit value of uranium mine decommissioning.

(III) Results and analysis

According to the expert advice, input the judgment matrix value, calculate the maximum eigenvalue using the mathematical software MATLAB, and calculate the weight of each element. This paper regards the weight of the natural environment and the ecological environment as equally important, that is, the weight value is 0.5. This paper selects environmental benefits as the overall goal and gives a numerical indicator for the treatment effect of uranium mines. Taking a decommissioned mine as an example, the radon precipitation rate x ₁ , the collective effective dose x ₂ , the gamma radiation dose rate x ₃ , the public personal radiation dose x ₄ , the soil recovery improvement degree x ₅ , the water pollution degree x ₆ , the air pollution degree are selected. x ₇ , Vegetation Coverage x ₈ , Land Recovery Utilization Rate x ₉ and Mining Area Biomass x ₁₀ as a measure of the environmental benefits of decommissioning of the mine. The judgment matrix is ​​constructed by comparing the two pairs, and the maximum eigenvalue is obtained by using the mathematical software MATLAB. RI=1. 49 Therefore, the consistency test of the comprehensive weight is CR=0.0019<0.1, and the credibility is good. The weights of each benefit index are: w=(0.1639, 0.1437, 0.0714, 0.1294, 0.0714, 0. 0714, 0.0714, 0.1346, 0.0714, 0.0714). It can be seen that the weight of the raid rate x ₁ , the collective radiation dose x ₂ , the public personal radiation dose x ₅ , and the vegetation coverage x ₈ is more important, which is in line with the actual requirements.

(4) Weight determination and normalization of indicators

Through calculation and normalization, the weights of each indicator are obtained.

Table 4 Weights and normalized indicator values ​​of environmental benefit indicators

Note: λ _max = 8.0192, CI = 0.0027, CR = 0.0019 < 0.1, and the reliability is good.

(V) Calculation of comprehensive benefits of decommissioned uranium mine management

The comprehensive index of environmental governance of decommissioned uranium mines is equal to the weighted sum of the scores of the various indicators, namely:

Where, F is the comprehensive index of the environmental management benefits of the decommissioned uranium mine; u _j is the value of the normalized index j; W _j is the weight value of the index j, that is, the value obtained by hierarchical single ordering. The resulting environmental comprehensive benefit index can be used to assess the environmental benefits from the index comparison table (Table 5).

Table 5 Environmental Benefits Comprehensive Index Comparison Table

Therefore, the comprehensive index of environmental benefits is:

That is, the benefit of the decommissioned uranium mine is 0. 7985, indicating that the overall effect of this uranium mine decommissioning treatment is better. However, the collective effective dose, the degree of soil improvement and the management effect of the biomass in the mining area are not obvious, and further treatment is needed. In future work, a post-assessment of the uranium mine is required.

Third, the conclusion

The environmental benefit analysis of decommissioned uranium mines is to evaluate the effect of decommissioning of uranium mines from the perspective of natural environment and ecological environment after comprehensive decommissioning of uranium mines, and to give an evaluation grade from the perspective of mathematics. Taking a uranium mine as an example, this paper applies the analytic hierarchy process to calculate the environmental benefits of decommissioning. It is considered that the uranium mine has good governance effects, but it also has shortcomings. Since the selected indicators are not comprehensive enough and there is no comprehensive analysis of economic and social benefits, in-depth investigations should be conducted in subsequent studies to obtain reliable data for analysis. In the analysis process, it is only a benefit analysis of a uranium mine. In the vertical and horizontal comparison, it needs to be further explored in the follow-up study.

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