Experimental Study on the Effect of Wetting-Drying Cycles on Crack Development and Radon Retardation of the Covering Soil in Uranium Mill Tailing Impoundments.

The majority of uranium mill tailing impoundments in the southern part of China are located in humid subtropical regions where persistent rainfall and rapid evaporation of water after rain often occur. Under the prolonged influence of alternating wet and dry conditions, the covering soil layer of uranium mill tailing impoundments develops cracks, leading to the issue of degradation or even failure of the radon retardation effect. A beach surface of uranium mill tailing impoundments in the southern part of China is selected as the research object. Through use of a self-made simulation test device, a degradation experiment of uranium mill tailing covering soil models under wetting-drying cycles was conducted indoors. The experimental results indicate that with a constant amplitude of wetting-drying cycles, microcracks characterized by a narrow width and high abundance were mainly generated in the early-to-mid-stage of wetting-drying cycles. The main cracks, characterized by their wide width and less abundance, were mainly formed in the mid-to-late stage of wetting-drying cycles. After seven wetting-drying cycles, the total length of cracks showed a "stair-step" increase and the surface crack ratio exhibited a trend of moving from rapid growth to stable growth and then to a slight decline. The cumulative damage degree showed a rapid increase to stable growth with an increase in the number of wetting-drying cycles. Grey relational analysis revealed that, compared to other surface crack indicators, radon exhalation rate was the most closely correlated with the surface crack ratio. With a constant amplitude of wetting-drying cycles, the radon exhalation rate underwent four stages as the number of wetting-drying cycles increased: stage I witnessed a rapid increase, stage II witnessed a rapid decrease, stage III witnessed a gradual increase, and stage IV witnessed a stable or even slight decrease. With a constant number of wetting-drying cycles, the radon exhalation rate correspondingly increased with the amplitude of wetting-drying cycles, particularly noticeable when the alternation amplitude was 30 ± 20%. From the early mid-stage to the late stage of wetting-drying cycles, the curves of the radon exhalation rate, surface crack ratio, and cumulative damage degree tended to be consistent, showing a gradual increase. The research provided in this study offers valuable insights into radon control measures and environmental assessments on the beach surface of uranium mill tailing impoundments.

Research on the influence of surface crack development on radon exhalation in uranium tailing ponds under wet and dry cycles.

Compacted soil layers effectively prevent the migration of radon gas from uranium tailings impoundments to the nearby environment. However, surface damage caused by wet and dry cycles (WDCs) weakens this phenomenon.In order to study the effect of crack network on radon exhalation under WDCs, a homemade uranium tailing pond model was developed to carry out radon exhalation tests under five WDCs. Based on image processing and morphological methods, the area, length, mean width and fractal dimension of the drying cracks were quantitatively analyzed, and multiple linear regression was used to establish the relationship between the geometric characteristics of the cracks and the radon exhalation rate under multiple WDCs. The results suggested that the radon release rate and crack network of the uranium tailings pond gradually stabilized as the water content decreased, following rapid development in a single WDC process. The radon release rate increased continuously after each cycle, with a cumulative increase of 25.9% over 5 cycles. The radon release rate and average crack width remained consistent in size, and a binary linear regression considering width and fractal dimension could explain the changes in radon release rate after multiple WDCs.

Influence of temperature on the radon concentration distribution in ramp under low-speed wind field: A numerical simulation study.

By introducing the parameters of radon exhalation rate and radon diffusion coefficient, the distribution of radon concentration field on ramp under the condition of superposition of temperature field and flow field is simulated. The simulation results show that the distribution of radon concentration in the ramp under the condition of low-speed ventilation is greatly affected by the temperature field and flow field, and the change of radon exhalation caused by temperature is the main factor leading to the change of radon concentration in the ramp. The change of temperature will cause the overall increase of radon concentration in the ramp. Under the condition of constant flow field, the radon concentration in the chamber is more than two times higher than the average radon concentration in the ramp. Some areas severely exceeded the limit.

A comprehensive review of radioactive pollution treatment of uranium mill tailings.

Natural uranium is a crucial resource for clean nuclear energy, which has brought significant economic and social benefits to humanity. However, the development and utilization of uranium resources have also resulted in the accumulation of vast amounts of uranium mill tailings (UMTs), which pose a potential threat to human health and the ecological environment. This paper reviews the research progress on UMTs treatment technologies, including cover disposal, solidification disposal, backfilling disposal, and bioremediation methods. It is found that cover disposal is a versatile method for the long-term management of UMTs, the engineering performance and durability of the cover system can be improved by choosing suitable stabilizers for the cover layer. Solidification disposal can convert UMTs into solid waste for permanent disposal, but it produces a large amount of waste and requires high operating costs; it is necessary to explore the effectiveness and efficiency of solidification disposal for UMTs, while minimizing the bad environmental impact. Backfilling disposal realizes the resource utilization of solid waste, but the high radon exhalation rate caused by the UMTs backfilling also needs to be considered. Bioremediation methods have low investment costs and are less likely to cause secondary pollution, but the remediation efficiency is low, it can be combined with other treatment technologies to remedy the defects of a single remediation method. The article concludes with key issues and corresponding suggestions for the current UMTs treatment methods, which can provide theoretical guidance and reference for further development and application of radioactive pollution treatment of UMTs.

Radon attenuation characteristics of compacted soil layer for uranium mill tailings pond subjected to drying-wetting cycles.

Compacted soil layer (CSL) is generally designed for uranium mill tailings (UMTs) pond to form the radon seals, whereas it is usually affected by drying-wetting environmental conditions. To summarize the radon attenuation degradation performance of CSL subjected to drying-wetting cycles, an experiment with the application of meteorological data was developed. This paper focuses on the evolution of the soil apparent porosity, soil integrity and radon attenuation characteristics of CSL during continuous drying-wetting cycles. Image processing and a nonmetal acoustic wave detector were applied to analyze variations in the soil surface and internal defects, and the radon concentration was measured to reveal the radon attenuation performance of the CSL. The results reveal that with successive drying-wetting cycles, the soil apparent porosity increased, and the soil pores were enlarged. The soil integrity underwent dynamic recombination or reorganization and eventually reached a steady state. Moreover, it was observed that the saturated state of the uppermost soil led to a sharp increase in radon concentration (capping effect), and the decrease in accumulated radon concentration during the initial period resulted from the coupling effect of soil moisture, temperature and ambient pressure. The observations confirm that the drying-wetting environmental conditions markedly affect the radon migration channels and environment in the CSL, which provides a theoretical foundation for UMTs pond governance and radiation safety management.

Effect of simulated earthquake loading on radon exhalation from uranium tailings dam.

Uranium tailings sand will continuously release radon-222. When the external condition changes, the exhalation of radon will also change. Thus, radon is being recommended as a tracer for dam damage assessment. When an earthquake is simulated on the uranium tailings dam with a shaking table test and the change in radon concentration is measured, it is observed that the earthquake causes micro-fissures in the uranium tailings dam, which aggregate to form fractures. During the process, the radon concentration will climb dramatically, as will the radon exhalation rate. To verify that the radon monitoring date is accurate, the acceleration response, surface displacement, and interior displacement are all monitored. The results show that radon can be utilized as a tracer to evaluate uranium tailings dam damage.

Identification of sources with abnormal radon exhalation rates based on radon concentrations in underground environments.

When there is poor ventilation or an irregular radon exhalation rate in an underground environment, it is necessary to judge whether the radon concentration is abnormal. To protect personal safety and health from radon gas, it is necessary to track the location of an abnormal radon source and measure its release rate to formulate emergency control and eradication measures. However, in an underground environment, it is impossible to fully monitor the radon concentration at every location, and as a result, blind spots are present, making it difficult to obtain timely early warnings in the event of an abnormal radon exhalation rate. Based on the distribution of radon concentration in an underground environment, this research establishes a theoretical mathematical model of an underground ventilation network containing radon. We combined particle swarm optimization with the long short-term memory (PSO-LSTM) method, which uses part of a time series signal of monitored radon concentrations to track the location of an abnormal radon source and determine an abnormal radon exhalation rate. Performing experiments of theoretical examples and actual underground ventilation environment examples, we prove the necessity of optimizing the monitoring position of the angle-connected ventilation network. The results show that the PSO-LSTM model based on radon concentration monitoring can process time series signals. Its accuracy and decision coefficient greater that is than 0.9 indicate the reliability of the model and method.

Preparation and related properties of geopolymer solidified uranium tailings bodies with various fibers and fiber content.

Uranium tailing ponds are a potential major source of radioactive pollution. Solidification treatment can control the diffusion and migration of radioactive elements in uranium tailings to safeguard the surrounding ecological environment. A literature review and field investigation were conducted in this study prior to fabricating 11 solidified uranium tailing samples with different proportions of PVA fiber, basalt fiber, metakaolin, and fly ash, and the weight percentage of uranium tailings in the solidified body is 61.11%. The pore structure, volume resistivity, compressive strength, radon exhalation rate variations, and U(VI) leaching performance of the samples were analyzed. The pore size of the solidified samples is mainly between 1 and 50 nm, the pore volume is between 2.461 and 5.852 × 10-2 cm3/g, the volume resistivity is between 1020.00 and 1937.33 Ω·m, and the compressive strength is between 20.61 and 36.91 MPa. The radon exhalation rate is between 0.0397 and 0.0853 Bq·m-2·s-1. The cumulative leaching fraction of U(VI) is between 2.095 and 2.869 × 10-2 cm, and the uranium immobilization rate is between 83.46 and 85.97%. Based on a comprehensive analysis of the physical and mechanical properties, radon exhalation rates, and U(VI) leaching performance of the solidified samples, the basalt fiber is found to outperform PVA fiber overall. The solidification effect is optimal when 0.6% basalt fiber is added.

Analysis of equivalent thickness of geological media for lab-scale study of radon exhalation.

Geological media are omnipresent in nature. Lab-scale tests are frequently employed in radon exhalation measurements for these media. Thus, it is critical to find the thickness of the medium at an experimental scale that is equivalent to the medium thickness in a real geological system. Based on the diffusion-advection transport of radon, theoretical models of the surface radon exhalation rate for homogeneous semi-infinite and finite-thickness systems were derived (denoted as Jse and Jfi, respectively). Analysis of the equivalency of Jse and Jfi was subsequently carried out by introducing several dimensionless parameters, including the ratio of the exhalation rates for the semi-infinite and finite-thickness models, ε, and the number of diffusion lengths required to achieve a desired ε value, n. The results showed that when radon transport in geological media is dominantly driven by diffusion effect, if n > 3.6626, then ε > 95%; if n > 5.9790, then ε > 99.5%. When radon migration is dominantly driven by advection effect, if n > 2.5002, then ε > 95%; if n > 4.0152, then ε > 99.5%. Therefore, if the thickness of the geological media (x0) is greater than a certain n times the radon diffusion length of the media (L), the media can be modeled as semi-infinite. To validate the model, a pure radon diffusion experiment (no advection) was developed using uranium mill tailings, laterite, and radium-bearing rocklike material with different thicknesses (x0). The theoretical model was demonstrated to be reliable and valid. This study provides a basis for determining the appropriate thickness of geological media in lab-scale radon exhalation measurement experiments with open bottom.

STUDY ON THE INFLUENCE OF DRY DENSITY AND PARTICLE SIZE FRACTAL DISTRIBUTION ON THE RADON CONTROL PERFORMANCE OF OVERLYING SOIL IN URANIUM TAILINGS POND.

In order to explore the influence of dry density and particle size fractal distribution on the radon control performance of overlying soil in uranium tailings pond, overlying soil samples with different particle size fractal dimension and dry density were prepared for radon exhalation experiments. According to the principle of radon generation and diffusion, a set of radon measuring device was designed independently. In addition, the radon concentration on the surface of different overlying soil layers was measured by local static method, and the radon exhalation rate was further calculated. The relationship between radon exhalation law, dry density and fractal dimension of overlying soil was studied by correlation analysis method. Furthermore, a piecewise linear function between radon exhalation rate, dry density and fractal dimension was constructed. Then, the average value of the experimental data was compared with the calculated results. Results show that: at the same dry density and different fractal dimension, the radon exhalation rate decreased with the increase of fractal dimension. When the fractal dimension increased to a certain value, the radon exhalation rate tended to be stable. At the same fractal dimension and different dry density, the radon exhalation rate decreased with the increase of dry density. The calculated results of piecewise linear function were relatively close to the experimental data, which verified the practicability of the formula.

Experimental study on permeability characteristics and radon exhalation law of overburden soil in uranium tailings pond.

The permeability characteristics of overburden soil have a significant effect on radon exhalation in uranium tailings pond. To understand this change, the effects of dry density, particle size range, and moisture content on the gas permeability and radon exhalation were studied by correlation analysis method. According to the mechanism of radon generation, the permeability characteristics of overburden, and the weather conditions, a theoretical model of radon exhalation was constructed and an experimental setup was designed. A series of tests on different properties of overburden soil were conducted by uranium tailings permeameter, and the radon exhalation rate of corresponding overburden surface was measured by local static method. Results show that the permeability of overburden decreased, which led to the decrease of radon exhalation: (1) At the same moisture content, with the increase of dry density, both the gas permeability and radon exhalation rate decrease. (2) At the same moisture content, with the increase of the particle size range, both the gas permeability and radon exhalation rate increase. (3) At the same dry density, with the increase of moisture content, the gas permeability decreases, while the radon exhalation rate increases slightly at first and then decreases.

Predictive analysis of shaft station radon concentrations in underground uranium mine: A case study.

This paper presented a method for predicting shaft station radon concentrations in a uranium mine of China through theoretical analysis, mathematical derivation and Monte-Carlo simulation. Based upon the queuing model for tramcars, the average waiting time of tramcars and average number of waiting tramcars were determined, which were further used in developing the predictive model for calculating shaft station radon concentrations. The results exhibit that the extent of variation of shaft station radon concentration in the case study mine is not significantly affected by the queuing process of tramcars, and is always within the allowable limit of 200 Bq m(-3). Thus, the empirical limit of 100,000 T annual ore-hoisting yields has no value in ensuring radiation safety for this mine. Moreover, the developed model has been validated and proved useful in assessing shaft station radon levels for any uranium mine with similar situations.