Fa-Cheng Yi

This article presents a simulation of a long-term retardation performance Mock-up test of the multi-field coupling of buffer materials, with the aim to study the thermo-hydro-mechanical (THM) processes occurring in the engineered barrier system of a high-level waste (HLW) repository. In view of the theory of mixtures and mechanics of continuous media, the coupled THM mathematical model of unsaturated buffer materials is established, considering heat transport and multiphase fluid flow. Using the buffer material Mock-up test of multi-field coupling as a model, the partial differential equation (PDE) module in the general finite element software COMSOL Multiphysics was developed by a second development stage. The dynamic response process of buffer material under the condition of THM coupling was numerically simulated, and the spatial distribution and variation law of suction, porosity, horizontal displacement, temperature and swelling pressure in the engineered barrier were investigated. The porosity of the buffer material under THM coupling was influenced by the swelling pressure and the suction. The welling pressure evolution of the buffer material may be influenced by the thermal expansion induced by high temperature and the swelling pressure generated by buffer material saturation. The evolution of the horizontal displacement of the heater used to simulate a container with radioactive waste was validated. This paper provides technical reference for the design and safety evaluation of underground laboratory barrier engineering in China.

With the development of nuclear energy in China, the geological disposal of high-level radioactive waste (HLW) is increasingly receiving national attention. Among them, the study of nuclide migration is an important and complex technical system, which requires continuous in-depth research. Under the decay heat, radiation, and groundwater effects of HLW, buffer materials generate complex coupled thermo-hydro-mechanical-chemical (THMC) processes. The migration and diffusion of nuclides in buffer materials are controlled by the coupling effect of THMC. It is of great significance for the long-term safety of a HLW repository to study the long-term retarding effect of buffer material on nuclide strontium under the coupling effect of multiple factors. This study leverages the solving advantages of COMSOL Multiphysics 5.6, using a combination of the self-developed Mock-up experimental device as a geometric model and numerical simulations to study the multi-field coupling performance and radionuclide migration evolution characteristics of THMC buffer materials, which overcomes the difficulties due to the limitations of the experimental time and spatial scale. The simulation results can predict the migration range and distance variation of strontium in buffer materials at different time scales. In the initial stage, the migration and diffusion of nuclide in buffer materials are relatively slow, and the migration distance increases by about 0.03 m with time. In the mid-to-late stage, the migration distance increases by about 0.05 m over time; to ensure that, in 1000 years, core strontium does not penetrate the buffer material and migrate into the surrounding rock groundwater of the disposal facility, a buffer material thickness of 0.3 m needs to be set. The construction of THMC control equations for the migration and diffusion of nuclides in buffer materials under multi-field coupling conditions has been revealed, providing an important reference for a deeper understanding of the risk analysis of radionuclide contamination in disposal environments.

Compacted bentonite–sand mixture (CBM) is a kind of candidate buffer material of high-level radioactive waste (HLW) disposal in many countries. It is believed that the permeability of CBM is greatly related to its microstructure. The aim of this study was to search the effect of various aqueous mediums on the microstructure and pore characteristics of this buffer material. Permeation experiments and X-ray computerized tomography (X-ray CT) were used to explain the correlation between microstructure and permeability. Representative samples of CBM at a dry density of 1.7 g/cm³ were used. X-ray CT was used to study the CBM under the de-ionized water (DI) and three other aqueous medium conditions. After reconstruction with Dragonfly software, the pore characteristics and permeability of different solution-saturated samples were analyzed using AVIZO software, including pore size distribution (PSD), porosity, and connectivity. The results showed that the permeability coefficient of samples was NaOH > NaCl-Na₂SO₄ > Simulated Beishan groundwater > DI water, and the maximum swelling pressure of samples was NaOH < NaCl-Na₂SO₄ < Simulated Beishan groundwater < DI water. The permeability coefficient decreased with the increasing of maximum swelling pressure. Quantitative analysis indicated that the volume of interconnected pores increased owing to the infiltration of NaCl-Na₂SO₄ and NaOH.

The main purpose of this study is to analyze whether Cd²⁺ affects the absorption of Ca²⁺ and Fe²⁺ by the roots of five wetland plants and the toxic mechanism of cadmium on the subcellular structure. Five wetland plant samples were collected from the constructed wetland in the upper reaches of the Yangtze River. Based on the experiment and density function theory (DFT), we measured the Cd²⁺ content in the root, stem, and leaf, the morphological dimensions of plants, and in the subcellular structure the electronic activity of Cd compound was calculated to describe the stability and activity of the products. In general, Zephyranthes candida,Cynodon dactylon, Arundo donax, and Pontederia cordata have distinct cadmium uptake characteristics, while Phragmites communis does not. The results indicated tolerance to cadmium in all but Phragmites communis, which was due to cadmium distribution through the process of transpiration and a mechanical interception. The simulation results showed that Cd²⁺ imposed no obvious inhibition on the absorption of Ca²⁺ and Fe²⁺ in plants, as the energy barrier of the process is about 1–3 eV. Cd²⁺ could improve the amount of pyruvate and glucose by 30% via spd orbital hybridization, making them more chemically reactive. At the same time, Cd²⁺ could replace Mg²⁺ in chlorophyll through a copper substitution reaction, making the electron energy of chlorophyll more concentrated. As a result, the valence-band electron at −40 eV was vacant. In conclusion, we determined that Cd²⁺ has no obvious inhibitory effect on Ca²⁺ and Fe²⁺ in root absorption and that Cd²⁺ could affect the properties of compounds of the subcellular structure and thus produce physiological toxicity.

Here, we used a radioactive distribution approach for water samples from the Liu Shao Yan constructed wetland to investigate the horizontal advection of cadmium (Cd) in this urban constructed wetland. The objective of this study was to assess the effectiveness of Cd removal in constructed wetlands. Additionally, this study examined the factors affecting the horizontal distribution of Cd. Sediment samples were collected from an enclosed wet area. A predictive advection model was executed using a combination of observed Cd concentrations and predicted Cd concentrations from a genetic algorithm–backpropagation artificial neural network (GA–BPANN). A coefficient of variation was used to assess differences in Cd distribution due to flow rate, precipitation, and water plants. Scanning electronic microscopy–energy dispersive spectrometry (SEM–EDS) results suggested that the plant species Pontederia cordata could absorb Cd, but the influence was negligible. All plants investigated in our experiment were unsuitable for Cd removal. However, predictions from the GA–BPANN algorithm indicated that 13–25% of Cd loading was efficiently removed by constructed wetland, which mainly resulted from sediment sorption, bacterial uptake, and the dilution caused by water advection. Consequently, we conclude that the constructed wetlands are an environmentally friendly and cost-effective technology that can remove Cd to a certain extent.

In China, Gaomiaozi (GMZ) bentonite is recognized as a barrier material for isolating nuclear waste. Different chemical solutions may change the hydraulic conductivity and swelling capacity of bentonite. Consequently, a series of swelling pressure and permeability experiments was carried out on bentonite-sand mixtures with various dry densities and infiltrating solutions. X-ray diffraction (XRD) and the field emission scanning electron microscope (FESEM) were carried out on the samples experiencing the tests to identify the influence of chemistry pore solutions upon the mineralogical and microstructure changes. The results show that the swelling pressure experienced rapid swelling, slow expansion, and the stable expansion stage for the specimens of infiltrating solutions except for NaOH. For the specimens infiltrated with NaOH solutions, the swelling pressure experienced rapid increases, slow decreases, and a stable development stage. With hyper-alkaline and hyper-salinity infiltration, the swelling pressure decreased, and the permeability increased. In addition, swelling pressure attained stability more quickly on contact with hyper-alkaline and hyper-salinity solutions. Comparing the test results, the results indicate that the influence of NaOH on the expansion and permeability was higher than NaCl-Na₂SO₄ at the same concentration.

This work aimed to assess the water quality of the Tuojiang River Basin in recent years to provide a better understanding of its current pollution situation, and the potential pollution risks and causes. Water quality parameters such as dissolved oxygen (DO), ammonia–nitrogen (NH₃-N), total phosphorus (TP), the permanganate index (CODMn), five-day biochemical oxygen demand (BOD5), pH, and concentrations of various heavy metals were measured in the Tuojiang River, according to the national standards of the People’s Republic of China. Samples were collected between 2012 to 2018 at 11 national monitoring sites in the Tuojiang River Basin. The overall water pollution situation was evaluated with back propagation artificial neural network (BP-ANN) analysis. The pollution causes were analyzed considering both industrial wastewater discharge in the upper reaches and the current pollution situation. We found potential risks of excessive NH₃-N, TP, Cd, Hg, and Pb concentrations in the Tuojiang River Basin. Moreover, corresponding water pollution control suggestions were given.

A facile route has been developed to synthesise mesoporous SBA-15 using mild H₃PO₄ (replacing the most commonly used strong acid HC1) as the medium and the effects of H₃PO₄ concentration and synthesis temperature on the mesostructure and morphology of the resultant materials investigated. The results demonstrate that a more ordered structure can be obtained at low H₃PO₄ concentration (1–2 M), while a higher H₃PO₄ concentration (3–4 M) results in a less ordered structure. Furthermore, the morphology of SBA-15 particles transforms from rod-like to sphere-like with increasing H₃PO₄ concentration.