《1 Engineering research fronts》

1 Engineering research fronts

《1.1 Trends in Top 12 engineering research fronts》

1.1 Trends in Top 12 engineering research fronts

The Top 12 engineering research fronts as assessed by the Energy and Mining Engineering Group are shown in Table 1.1.1. These fronts involve the fields of energy and electrical science, technology, and engineering; nuclear science, technology, and engineering; geology resources science, technology, and engineering; and mining science, technology, and engineering. Among these 12 research fronts, “core materials of high safety and high energy-density batteries”, “flexibility improvement of renewable energy power generation and power grid support theory”, and “synthesis of ammonia by electrochemical nitrogen reduction in organic system” represent energy and electrical science, technology, and engineering research fronts; “research on spent fuel reprocessing and high-level radioactive material separation process”, “research on nuclear waste glass properties”, and “interaction mechanism between plasma and reactor materials” represent nuclear science, technology, and engineering research fronts; “3D fracture propagation model for hydraulic fracturing”, “hydrogen-carbon cycle process in deep earth and its relation to oil and gas resources”, and “intelligent prospecting and comprehensive quantitative research for continental potash and lithium resources” represent geology resources science, technology, and engineering research fronts; “induced mechanism and early warning method of rockburst in deep mining”, “mechanism of carbon dioxide flooding recovery-capture and storage of crude oil”, and “intelligent identification method for rock properties” represent research fronts of mining science, technology, and engineering.

The annual publication status of the core papers related to each front from 2016 to 2021 is shown in Table 1.1.2.

(1)   Core materials of high safety and high energy-density batteries

The emergence of battery technology has changed our lives.

《Table 1.1.1》

Table 1.1.1 Top 12 engineering research fronts in energy and mining engineering

《Table 1.1.2》

Table 1.1.2 Annual number of core papers published for the Top 12 engineering research fronts in energy and mining engineering

It is a core component of electronic devices, electric vehicles, and smart grids. The energy density of existing lithium-ion batteries (LIBs) is close to its theoretical value, which is not adapted to the electric vehicle market. Safety issues have also become a significant obstacle hindering the development of battery technology in large-scale applications. Batteries are mainly composed of cathodes, anodes, and electrolytes. The energy density of batteries depends on the electrodes, whereas electrolytes determine their safety. The energy density of an electrode depends on voltage and capacity. To realize high energy- density cathodes, high-voltage cathode materials and high-capacity sulfur materials are being explored; concerning anodes, lithium metal anodes and silicon materials are being researched; finally, regarding electrolytes, solid-state electrolytes are being investigated. Solid-state electrolytes fundamentally solve safety problem of batteries, and they enable the use of high energy-density electrodes. Solid-state electrolytes can be divided into solid polymer electrolytes, solid inorganic electrolytes, and solid composite electrolytes. Solid composite electrolytes, which are actively researched for next-generation batteries, combine the advantages of good processability of polymer electrolytes and high ionic conductivity of inorganic electrolytes.

(2)   Research on spent fuel reprocessing and high-level radioactive material separation process

Nuclear fuel reprocessing aims to separate fission products in the spent fuel with chemical treatment methods, recover and purify valuable fissionable substances such as 235U and 239Pu, and remake them into fuel elements for the further use in the nuclear power plant (thermal reactor or fast reactor). This can jointly improve the utilization ratio of nuclear fuel and greatly save uranium resources; it can also extract transuranic elements and fission products required to develop isotopes for application in medical treatment, aerospace, etc. Nuclear fuel reprocessing can be categorized into aqueous reprocessing and non-aqueous reprocessing, based on whether it is carried out in an aqueous medium or not. Aqueous reprocessing refers to the chemical separation and purification processes in aqueous solutions, such as precipitation, solvent extraction, or ion exchange; non- aqueous reprocessing refers to the chemical separation and purification methods in an anhydrous state, such as fluorination volatilization, high-temperature metallurgical treatment, high-temperature chemical treatment, liquid metal process, electrolysis of fused salts. Non-aqueous reprocessing has advantages in the treatment of high burnup spent fuel, especially in the reprocessing of fast reactor spent fuel. Research on “separation/transmutation” and “advanced nuclear fuel recycling” has been internationally conducted. In this case, the separation and recovery of minor actinides (MA) and long-lived fission products (LLFPs) from nuclear waste is a key step. Compared with existing spent fuel reprocessing and disposal methods, the integrated process of spent fuel reprocessing and high-level waste liquid separation is simpler and more economical, produces less secondary waste, and can better meet the separation requirements of advanced nuclear fuel recycling.

(3)  3D fracture propagation model for hydraulic fracturing

Hydraulic fracturing is a reservoir reconstruction technology that communicates natural fractures by pumping high displacement fluid into a formation to generate artificial fractures in the reservoir. Owing to the introduction of idealized assumptions concerning fracture geometry in the fracture height direction, the traditional two-dimensional and quasi-three-dimensional hydraulic-fracturing numerical models cannot describe the propagation process of the actual hydraulic fracturing. In a three-dimensional hydraulic fracturing model, the geometric constraints of the fracture are removed. Consequently, a more practical fracture design scheme can be obtained. At present, three-dimensional hydraulic-fracturing models can be divided into plane three- dimensional models and non-plane three-dimensional models. In a plane three-dimensional model, it is assumed that the hydraulic fracturing can only expand in the original fracture plane, while a non-plane three-dimensional model relaxes this condition and can simulate the non-plane deflection of the hydraulic fracturing. In a three-dimensional hydraulic-fracturing model, the crack propagation mode is more complex, and a mixed propagation mode is often coupled with three basic modes. Therefore, new crack propagation criteria need to be adopted. Hence, the fracture morphology of three-dimensional models is more complex than that of two-dimensional models and requires a new algorithm to accurately capture the crack front and calculate the stress field at the crack tip. In addition, problems addressed by two-dimensional models, such as the influence of in-situ stress, filtration effect, reservoir heterogeneity, and fracturing mode, on the expansion of hydraulic fractures, migration of proppant still need to be further developed in three-dimensional models.

(4)    Induced mechanism and early warning method of rockburst in deep mining

Rockburst is a phenomenon in which the elastic deformation potential energy accumulated in rock mass is suddenly and violently released under certain conditions, resulting in rockburst and ejection. It is a serious dynamic disaster. With the increase of mining depth, rockburst in coal mines is becoming increasingly dangerous. Research on the underlying mechanism of rockburst disasters, as well as on monitoring, early warning, and prevention and control technology, is increasingly important for safe coal mining. The main research topics in this field include dynamic response and catastrophe mechanisms of deep coal mine rockburst, intelligent monitoring of early warning theory and technology for the deep coal mine rockburst, and mechanisms and methods for prevention and control of deep coal mine rockburst. Regarding the underlying mechanism of rockburst, a new approach is based on the analysis of the sudden change mechanism of strain energy release of surrounding rock. Concerning rockburst early warning technology, future research lines will be based on integration of artificial intelligence algorithms, such as data mining and big data processing and analysis, to realize intelligent monitoring and early warning of rockburst disasters, and multi-field multi-dimensional perception.

(5)   Flexibility improvement of renewable energy power generation and power grid support theory

The development of renewable energy generation, such as wind and solar power, is a promising approach for China to facilitate a clean and low-carbon transition of the energy and power industry and achieve the “30·60” carbon neutrality goal. Different from conventional electricity generators with continuous and controllable power output, the features of renewable generation are strong volatility and weak controllability. This fundamentally changes the physical form and operation mode of power systems. With the rapid development of centralized and distributed renewable generation, as well as the gradual decommissioning of thermal power units, the uncertainty continues to increase in both the generation and consumption sides of power systems, and flexible regulation resources are becoming increasingly scarce. The power supply-demand balance and secure economic operation of power systems are facing unprecedented challenges. It is of utmost importance to investigate the theory and key technologies for flexibility improvement of renewable generation and power grid support, and for improvement of wide-area and efficient integration of a high proportion of renewable generation. Related research topics include mechanisms, operation strategies, and market mechanisms of flexibility balance in power systems; techno-economic analysis of the flexible transformation of thermal generation units and their conversion to regulating units; transient voltage support technology for large-scale wind and solar power generation systems; stability control technology of large-scale wind and solar power generation system; coordinated planning and operation of multi-period and multi-type energy storage technologies; and hierarchical aggregation, coordinated control, and distributed transaction of massive heterogeneous demand-side resources.

(6)   Synthesis of ammonia by electrochemical nitrogen reduction in organic system

Ammonia is the second most produced commodity in the world. Motivated by the global shift toward sustainable energy, ammonia synthesis by electrochemical nitrogen reduction reaction (NRR) has attracted wide attention and is regarded as a promising alternative to traditional Haber-Bosch process. The electrochemical NRR can be performed in aqueous and organic systems and the later usually have higher Faradaic efficiency. Among the investigated modes of NRR, a lithium- mediated process in organic electrolytes distinguishes itself by outstanding ammonia yield rate and Faradaic efficiency. The lithium-mediated process takes place in three steps. Electrodeposition of metallic lithium first occurs, and then lithium-nitrogen based compound is produced by spontaneous reaction with reactive lithium. Based on this, the ammonia is formed by the protonation reaction. The performance of organic lithium-mediated systems is directly correlated with characteristics of lithium depositions. The study of lithium-mediated NRR is mainly focused on optimization of electrolytes, including the selection of proton donor, lithium salt, and additive. The highest reported current density and Faradaic efficiency are approximately 1 A·cm−2 and 100%, respectively, which are even better than the targets set by the REFUEL program of the US Department of Energy (current density is 0.3 A·cm−2; faradaic efficiency is 90%; energy efficiency is 60%). Before practical use, higher energy efficiency and better durability must be accomplished.

(7) Research on nuclear waste glass properties

The safe treatment and disposal of high-level radioactive waste liquid is a common challenge faced by all countries in the world, and it is also one of the key factors restricting the sustainable development of the nuclear industry. Glass solidification is currently the only industrially applied high- level waste liquid treatment method in the world. Glass has a short-range-ordered and long-range-disordered network structure, and most of the nuclides in the periodic table can enter the network of the glass. Therefore, Glass solidification used for treatment of high-level waste liquid have been extensively studied by scholars domestically and abroad, involving aluminosilicate glass, phosphate glass, silicate glass, and borosilicate. Phosphate glass has a relatively low melting temperature and are easy to prepare; however, they are seriously corrosive to other equipment. To date, only Russia has adopted phosphate glass solidification. Silicate glass contains more SiO2, although it has good anti-leaching performance. However, its solidification process is relatively complicated, and the solidification of high-level waste liquid with high cesium content is poor. Borosilicate glass has the advantages of good stability, simple preparation process, high mechanical strength, and large waste packaging capacity. It has become a solidified material widely used in the treatment of high-level waste liquid. Borosilicate glass is the substrate of choice. The first vitrification plant in China has now entered the operation stage, and research on the solidification behavior and anti-leaching performance of glass solidified bodies is of great significance for the safe production, temporary storage, and disposal of such bodies.

(8)   Interaction mechanism between plasma and reactor materials

Controlled fusion energy is the ideal clean energy source of the future. Currently, the most likely method to achieve controlled thermonuclear fusion is magnetic confinement fusion. Solving the material problem of tokamak devices and future reactors is a key issue to realize magnetic-confinement fusion energy. The solution strongly depends on the deep understanding of the processes and mechanisms of plasma-wall interactions (PWIs), which occur mainly in the boundary plasma beyond the outermost closed magnetic surface of the tokamak field, also known as the scrapped-off layer (SOL), and in the region of the plasma-facing material in direct contact with the SOL. PWIs directly determine the operation safety of fusion devices, development process of wall material components, and future wall lifetime. The understanding of the various physical processes and underlying mechanisms of PWIs and their effective control is one of the most important aspects in the realization of future fusion energy. The International Tokamak Physics Activity (ITPA) Working Group on scraped- off layer/divertor (SOL/Div) is dedicated to identifying the PWI issues and coordinating joint efforts at international scale. The results will have important implications for the design, manufacture, and operation of future fusion demonstration plants and commercial reactors. Currently, the main research directions include selection of plasma-facing materials, basic physical processes of boundary plasma, surface damage and structural effects under plasma irradiation, impurities and dust, and wall conditioning. Although a lot of research work on PWI process has been conducted domestically and abroad, there are still many issues to be solved; for example, the data on atomic-molecular processes in boundary plasma are not perfect, and the behavior of particle transport and redeposition is not completely clear. In view of the urgent demand for PWI-related data for national fusion projects and ITER, it is necessary to conduct comprehensive and in-depth basic research on PWIs both domestically and abroad as soon as possible.

(9)   Hydrogen-carbon cycle process in deep earth and its relation to oil and gas resources

The long-term hydrogen-carbon cycle in deep earth is closely related to the short-term hydrogen-carbon cycle on the earth’s surface. As the “processing plant” of the earth material cycle, the plate subduction zone is a window for understanding the deep earth. However, its internal complexity also restricts the understanding of the “generation, transportation, and accumulation” process of the earth’s deep material. The exchange of material and energy between the ocean- sphere, lithosphere, and atmosphere is strengthened in the subduction “plant”. Volatile components such as H2O and CO2 are brought into the mantle along with crustal sediments in the subduction zone. The products of deep- seated action are recycled into and out of the crust through the magmatic system of island arc volcanoes and rift basins, deep sea, and hydrothermal systems. By clarifying the link and correlation between plate subduction processes, deep geological processes, and inorganic origin oil and gas resources, the dynamics and material energy exchange mechanism across the lithosphere, and the possibility of independent accumulation of inorganic origin oil and gas can be revealed. In the future, the work in this research field will focus on various sedimentary basins in the east of China that are under the influence of the subduction zone. By analyzing the direct or indirect relationships between these basins and deep geological processes, the interaction and dynamic mechanisms between the ocean-sphere, lithosphere, and atmosphere under the framework of the subduction “plant” will be revealed, and the resource effect of material-energy exchange between the lithosphere and asthenosphere will be clarified. Meanwhile, the hydrocarbon cycle process under organic-inorganic interaction between the ocean-sphere, lithosphere, and atmosphere, as well as the enrichment mechanism of inorganic origin oil and gas, and geothermal and other resources under plate subduction background can be clarified. Thus, the evaluation standard of inorganic-origin oil and gas resources and selection method of enrichment area can be established.

(10)  Intelligent prospecting and comprehensive quantitative research for continental potash and lithium resources

Terrestrial potassium-lithium salts are formed in a terrestrial sedimentary environment. At present, China’s potassium salt resource industry is based on modern terrestrial salt lakes mainly located in the Qaidam basin. Recently, Paleocene- Neogene deep-brine-type terrestrial potassium-lithium salts in western Qaidam have been discovered. The future trend of this type of resource-industry development is to carry out intelligent mineral search and comprehensive quantitative research.

Big data, artificial intelligence, blockchain, and other new technologies will be used to conduct efficient and intelligent collaborative research on key technologies for deep-brine potassium-lithium resources. In particular, new technologies will be applied to achieve deep data mining and multi- dimensional intelligent evaluation of deep-brine potassium- lithium resources. Therefore, it is of great significance to support intelligent mineral search and comprehensive quantitative research on deep-brine potassium-lithium resources and safeguard the national strategic resource security.

The next steps are as follows. First, using potassium and lithium exploration combined with seismic and logging data for oil and gas exploration will allow mapping the distribution of the reservoir brine layer. Second, well investigation and water release tests will be conducted to obtain hydrogeological parameters of the aquifer section, determine the water richness, brine composition, aquifer permeability coefficient (K), specific yield (m), storage coefficient (S), coefficient of recharge from precipitation (a), irrigation infiltration coefficient (b), evaporation limit depth of diving (D), and other key parameters for resource evaluation. Third, core calibration or nuclear magnetic logging will be used to identify original and movable water saturations. Fourth, tectonic and lithium-bearing reservoir phase models will be created to obtain comprehensive well data, seismic data, and various types of results. Fourth, a phase-control and trend-control constraint modeling method, reservoir property model, and the use of intelligent means will be combined to achieve batch-computing quantitative evaluation of potassium and lithium resources.

(11)  Mechanism of carbon dioxide flooding recovery-capture and storage of crude oil

Carbon dioxide-enhanced oil recovery with carbon capture and storage technology refers to the injection of captured carbon dioxide into low-permeability reservoirs to achieve carbon dioxide burial and storage while enhancing the recovery of oil. This technology is a win-win solution to help reduce carbon emissions and enhance the production from oil and gas wells, which is of great significance for China to enhance “carbon peaking and carbon neutrality goals”. Carbon dioxide flooding technology was first developed in the USA in the early 1950s. By now, the carbon dioxide injection capacity amounts to 68 million tons/year and has become the main method of enhanced recovery in low-permeability and very-low-permeability blocks in the USA. There are abundant low-permeability oil reserves in China that approximately represent half of the total resources. However, the reservoir conditions of the onshore low-permeability reservoirs are complex and feature special crude oil properties. Likewise, existing carbon dioxide flooding technology is complicated. Carbon dioxide capture and transportation incur high cost, and carbon dioxide is difficult to bury, which limits the scale of carbon dioxide flooding storage. Therefore, it is urgent to explore mechanisms of carbon dioxide flooding and carbon capture and storage for the low-permeability reservoirs in China. The main research areas include efficient carbon dioxide capture technology, secure carbon dioxide transport technology, carbon dioxide enhanced oil recovery technology, and efficient carbon dioxide storage mechanism.

Understanding of the mechanism of carbon dioxide flooding recovery-capture and storage of crude oil, and accelerating the widespread promotion of carbon dioxide flooding and carbon capture and storage technology in China’s onshore low- permeability reservoirs will lead to important contributions to the optimization of energy structures and realization of “carbon peaking and carbon neutrality goals”.

(12) Intelligent identification method for rock properties

Rock lithology classification identification is the primary task of underground engineering. This identification is the basis of quantitative analysis of rock engineering geology and an important aspect in geological safety evaluation. The main research areas in lithology identification mainly includes physical methods, such as ultrasonic, rebound index, density, and other physical test methods; mathematical and statistical methods, such as statistics, analysis, and comparison of the main elements of stratum rocks; and lithology intelligent identification methods. These development trends show that intelligent recognition methods are more dependent on sample data, and the generalization ability of models is easily affected when the sample data are insufficient; moreover, the classification error of rock lithology is larger when only a single feature type of the rock is considered, such as image optical features. Therefore, it is also necessary to analyze other physical features or properties of the rock to improve the classification accuracy. Coupling rock image recognition models and rock audio intensity regression models achieves rapid recognition of rock lithology and effectively improves the generalization ability and accuracy of models. Combined with the recognition results of comparative expert methods, it assists engineers to make the correct classification of rock properties.

《1.2 Interpretations for four key engineering research fronts》

1.2 Interpretations for four key engineering research fronts

1.2.1 Core materials of high safety and high energy- density batteries

Safety and energy density are two obstacles hindering the development of the battery industry. Batteries are mainly composed of cathodes, anodes, and electrolytes. The energy density of batteries depends on the electrodes, and electrolytes determine the safety of batteries. LIBs, the most extensively employed battery system, are essentially a rocking-chair battery in which lithium ions are intercalated back and forth between the positive and negative electrodes. As early as 1990, Sony Company commercialized the first LIBs. The development of batteries has shaped our lives. It is a core component of electronic devices, electric vehicles, and smart grids. The most widely used anodes of LIBs are graphite and lithium titanate. Carbonate electrolytes are typically employed, and the positive electrodes are oxides such as lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt manganese oxide.

The energy density of electrodes is determined by voltage and capacity. The main research areas in high energy- density cathodes are high-voltage cathode materials and high-capacity sulfur materials; in anodes, the main research areas are lithium metal anodes and silicon materials. High- voltage cathode materials include lithium-rich manganese- based cathodes, lithium nickel manganese oxides, and high- voltage lithium cobalt oxides; high-capacity cathode materials are mainly sulfur cathodes. The cost of cathode materials accounts for more than 40% of the total cost of batteries. Ideal cathode materials featuring both high capacity and high voltage still need to be explored. Lithium-metal and silicon anodes have been extensively studied both in academia and industry in recent years. However, dendrite problems for lithium-metal anodes and long-cycling problems for silicon anodes at large scale hinder their commercialization. A possible tradeoff involves the use of Si/C composite anodes. Besides, the overall capacity of Si/C composite anodes can be improved by adding a certain proportion of silicon material.

The main research area in electrolytes is the development of solid-state electrolytes. These electrolytes can fundamentally solve the safety problem of batteries. In addition, they enable the use of high energy-density electrodes. Solid- state electrolytes can be categorized into solid polymer electrolytes, solid inorganic electrolytes, and solid composite electrolytes. On the one hand, solid polymer electrolytes have good processing ability, wide electrochemical window, and excellent stability. On the other hand, they are severely impeded by their limited ionic conductivity and transference number at room temperature. The ionic conductivities of inorganic solid electrolytes have reached values close to those of liquid electrolytes, but their poor stability and huge interfacial impedance with electrode materials seriously hinder their application. Composite solid electrolytes combine the advantages of polymer electrolytes and inorganic electrolytes. They are expected to achieve a breakthrough in the technology of all-solid-state battery industry if the solid interface between electrolytes and electrodes and the active loading of electrodes can be improved.

In the engineering research front of “core materials of high safety and high energy-density batteries”, the top three countries in terms of the number of core papers published (Table 1.2.1) are China, the USA, and Germany, all of which have been cited more than 60 times. Among the Top 10 countries with the most published papers, the

《Table 1.2.1》

Table 1.2.1 Countries with the greatest output of core papers on “core materials of high safety and high energy-density batteries”

USA and China have more cooperation, followed by China and Australia (Figure 1.2.1). The institutions that produce a large number of core papers include Chinese Academy of Sciences, Central South University, and Huazhong University of Science and Technology (Table 1.2.2). Among these 10 institutions, Chinese Academy of Sciences has more cooperation with the Tsinghua University (Figure 1.2.2). In the amount of citing core papers, the top three countries are China, the USA, and South Korea (Table 1.2.3). The main output institutions of citing core papers are Chinese Academy of Sciences, Zhengzhou University, and Central South University (Table 1.2.4).

Material innovation is indispensable for new developments in the battery industry. Concerning lithium batteries, cathode materials will evolve from high-voltage cathode materials to low-cost high-capacity sulfur cathodes, and anode materials will evolve from Si/C composite anodes to pure silicon anodes and finally to lithium metal anodes. Electrolytes will evolve from solid composite electrolytes to all-solid-state inorganic electrolytes. Terminal all-solid-state high-voltage lithium metal batteries or lithium-sulfur batteries are expected to be used in electronic devices, electric vehicles, and smart grids. Figure 1.2.3 shows the roadmap of the engineering research front of “core materials of high safety and high energy-density batteries”.

《Figure 1.2.1》

Figure 1.2.1 Collaboration network among major countries in the engineering research front of “core materials of high-safety and high energy-density batteries”

《Table 1.2.2》

Table 1.2.2 Institutions with the greatest output of core papers on “core materials of high safety and high energy-density batteries”

《Figure 1.2.2》

Figure 1.2.2 Collaboration network among major institutions in the engineering research front of “core materials of high safety and high energy-density batteries”

《Table 1.2.3》

Table 1.2.3 Countries with the greatest output of citing papers on “core materials of high safety and high energy-density batteries”

《Table 1.2.4》

Table 1.2.4 Institutions with the greatest output of citing papers on “core materials of high safety and high energy-density batteries”

《Figure 1.2.3》

Figure 1.2.3 Roadmap of the engineering research front of “core materials of high safety and high energy-density batteries”

1.2.2 Research on spent fuel reprocessing and high-level radioactive material separation process

(1) Spent nuclear fuel reprocessing research

World-wide industrial-scale spent fuel reprocessing has a history of more than 40 years. A total of 17 countries have developed spent fuel reprocessing and built 32 reprocessing plants, including intermediate and experimental devices. The total reprocessing capacity is approximately 4 800 tons/year. Britain and France are the world leaders in terms of large- scale commercial spent fuel reprocessing.

The development of spent fuel reprocessing in China has gone through two stages: production reactor reprocessing and power reactor reprocessing. This development has been basically determined by the three-stage idea of “pilot scale–demonstration scale–large commercial scale”. The successful thermal commissioning and operation of the pilot plant shows that China has mastered the core technology of power-reactor spent fuel reprocessing, which is a milestone. The reprocessing industrial demonstration plant is expected to be completed in 2023, when China will have industrial- scale spent fuel reprocessing capability. China will complete the technology integration during the “14th Five-Year Plan” period. As a whole, it will have the ability to independently design and build large-scale post-processing plants.

In terms of technology development trends, spent fuel reprocessing methods can be categorized into two types: aqueous reprocessing and non-aqueous reprocessing, according to the state of the spent fuel being processed in the main stage. Aqueous reprocessing is mostly based on the PUREX process, whereas non-aqueous reprocessing uses electrolytic melting of halogen salts to separate fissionable material and fission products. Considering the differences in the spent fuel reprocessing objects and improvement in fuel consumption, the mainstream development directions of aqueous reprocessing are as follows. First, improving the separation efficiency of U and Pu, strengthening the separation of Np and Tc, and improving and strengthening the control of 14C and 129I radioactive gas emissions. Second, to reduce the toxicity and volume of the final radioactive waste, MA and LLFP require “separation/transmutation”. In addition to the improvement of the conventional spent fuel reprocessing—PUREX process, the concept of “advanced spent fuel reprocessing” has been proposed internationally in terms of the “separation/transmutation” of MA and LLFP. It includes separation-based and transmutation-based spent fuel reprocessing technologies. Concerning fast reactor and accelerator technology, separation can be realized through two schemes, namely “total separation” and “spent fuel reprocessing—high-level waste liquid separation”. In addition, the design concept, key equipment, instrument control, waste management, nuclear safety, and other aspects of reprocessing plants are constantly improving and developing.

Non-aqueous reprocessing mainly includes halogenated volatilization, pyrometallurgy, and electrolytic refining. Electrodeposition and electrolytic refining are generally considered to be the most promising, feasible, economical, and reliable non-aqueous reprocessing technologies. As such, they have been widely studied internationally. At present, most countries are still in the laboratory research stage in terms of non- aqueous reprocessing. Only the USA has completed laboratory- scale and engineering-scale simulation experiments, and is preparing for pilot-scale thermal experiments.

(2) Research on separation of high-level waste liquid

The USA, France, Russia, Japan, and China have proposed their own high-level waste liquid separation processes. In the 1990s, Russia built a pilot-scale facility (UE-35) for separation of high-level radioactive waste using CCD-PEG (chlorinated cobalt dicarbollide and polyethylene glycol) in Mayak to recover Sr and Cs from high-salt waste liquid. At present, only the USA has completed research on high-level waste liquid separation technology from laboratory to engineering scale. The high-level waste liquid separation plant was built at the Savannah River National Laboratory, and over 4 000 cubic meters of high-level waste liquid have been treated there by June 2016.

Since the 1980s, Tsinghua University in China has conducted research on the separation technology of high-level radioactive waste liquid, and proposed a separation process with independent intellectual property rights. At present, experimental research on the separation of high-level waste liquid of power reactor is being carried out, setting a solid foundation for thermal verification and application of high- level waste liquid separation technology, and meeting the conditions for engineering applications.

Concerning technology development trends, the separation process of transuranic elements (TRU) includes American TRUEX-TALSPEAK (Transuranic Extraction Process-Trivalent Actinide Lanthanide Separation by Phosphorous reagent Extraction from Aqueous Komplexe) and its improved process, American ALSEP (Actinide Lanthanide Separation Process) process, French DIAMEX-SANEX (DIAMide Extraction-Selective Actinide Extraction) process, European GANEX (Group Actinide Extraction) process, Japanese ARTIST (Amide-based Radio- Resources Treatment with Interim Storage of Transuranics) process, and Russian dibutyl zirconium phosphate extraction process. The Sr and Cs separation process includes the Russian CCD-PEG process, French CSSEX process, American FPEX (Fission Product Extraction) process, and Japanese extraction chromatography process.

In China, the trialkylphosphine oxide (TRPO) process for separating actinides, crown ether process for separating strontium, and calixarene crown ether process for separating cesium have been proposed. At present, China has basically completed research on the separation process and equipment of high-level waste liquid, and is investigating on thermal experimental verification of the separation process of high- level waste liquid in power reactors. China aims to master the engineering technology by 2030 and then realize industrial applications of high-level waste liquid separation.

As shown in Table 1.2.5, the top four countries in terms of the number of core papers published on this research topic are China, the USA, the UK, and Germany. Among them, China occupies the first place, with more than 30%. The USA, the UK, and Germany exceed 10%. It can be seen from Table 1.2.6 that the top six institutions in terms of the number of core papers on this research topic are the Chinese Academy of Sciences, The University of Manchester, Sichuan University, Forschungszentrum Julich, National Nuclear Energy Laboratory, and Eidgenössische Technische Hochschule Zürich.

As shown in Figure 1.2.4, China, the USA, Japan, Germany, the UK, France, and Spain pay more attention to cooperation among countries in this field. The number of published papers in the USA is relatively large, mainly in cooperation with China, France, Germany, the UK, and Japan. As shown in Figure 1.2.5,

《Table 1.2.5》

Table 1.2.5 Countries with the greatest output of core papers on “research on spent fuel reprocessing and high-level radioactive material separation process”

《Table 1.2.6》

Table 1.2.6 Institutions with the greatest output of core papers on “research on spent fuel reprocessing and high-level radioactive material separation process”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network among major countries in the engineering research front of “research on spent fuel reprocessing and high-level radioactive material separation process”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network among major institutions in the engineering research front of “research on spent fuel reprocessing and high-level radioactive material separation process”

The University of Manchester, Forschungszentrum Julich, and National Nuclear Energy Laboratory also cooperate.

Table 1.2.7 shows that China, which is the main output country, accounts for 32.28%, whereas the USA accounts for 20.47%. As shown in Table 1.2.8, the institutions with the main output of citing papers are Chinese Academy of Sciences, Chongqing University, Qingdao University of Science and Technology, Tsinghua University, University of New South Wales, University of Texas at Austin, and Shandong University of Science and Technology. Among them, Chinese Academy of Sciences, Chongqing University, and Qingdao University of Science and Technology all account for more than 10% on average.

From the above data analysis, it can be concluded that China and the USA are at the forefront of the world in the number of core papers on “research on spent fuel reprocessing and high-level radioactive material separation process”, and the number of the output of citing core papers by institutions in China is relatively large.

With different reprocessing objects and the improvement of fuel burnup, the mainstream development direction of aqueous reprocessing is to strengthen the separation of Np and Tc on the basis of improving the separation efficiency of U and Pu. To reduce the toxicity and volume of the final radioactive waste, it is necessary to perform “separation/ transformation” on MA and LLFP. Separation technology can be realized through two schemes, namely “total separation” and “spent fuel reprocessing—high-level waste

《Table 1.2.7》

Table 1.2.7 Countries with the greatest output of citing papers on “research on spent fuel reprocessing and high-level radioactive material separation process”

《Table 1.2.8》

Table 1.2.8 Institutions with the greatest output of citing papers on “research on spent fuel reprocessing and high-level radioactive material separation process”

liquid separation”. Most countries are still in the laboratory research stage in terms of non-aqueous reprocessing. Only the USA has completed laboratory-scale and engineering- scale simulation experiments, and is preparing for pilot-scale thermal experiments. By 2025, the spent fuel reprocessing industrial demonstration plant will be completed and put into stable operation, conduct special work on reprocessing scientific research, and have the ability to independently build large reprocessing plants. By 2030, the objectives are to carry out further pilot-scale thermal verification of the high-level waste liquid separation process and master the engineering technology. By 2035, the objectives are to realize special scientific research technology of spent fuel reprocessing and master the core technology of large-scale spent fuel reprocessing plants. China will independently construct a large-scale reprocessing plant. By 2050, research on advanced aqueous reprocessing technology will be conducted, verification of non-aqueous reprocessing technology and demonstration of non-aqueous reprocessing projects will be realized, and advanced and complete reprocessing process and radioactive waste treatment along with disposal technologies and industries will be established. Figure 1.2.6 shows the roadmap of the engineering research front of “research on spent fuel reprocessing and high-level radioactive material separation process”.

1.2.3 3D fracture propagation model for hydraulic fracturing

Since the 1950s, research on fracture propagation models for hydraulic fracturing has been conducted both analytically and numerically. PKN (Perkins-Kern-Nordgren), KGD (Khristianovic-Geertsma-de Klerk), and Penny are three classical hydraulic fracturing models that were developed in early stages. In the 1980s, with the progress of computational solid mechanics technology, fracture propagation models that consider the phase change of fracturing fluid and reservoir heat exchange were gradually developed. In 1983, Cleary established a 3D fracture propagation model for hydraulic fracturing that included singular integral equations to describe the deformation of rock, and finite element methods to simulate the fluid flow in the fracture. This model is used to analyze the effect of stress difference and bedding on fracture propagation and can be directly used in fracture design. Several researchers, including Settari, Hongren Gu, and Donsov, have developed various 3D fracture propagation models for hydraulic fracturing that account for proppant, heat transfer, and fracturing fluid compressibility using finite differences, a finite element mesh, and implicit level set algorithms. The research objective is to closely combine the characteristics and actual conditions of hydraulic fracturing, achieve a comprehensive description of hydraulic fracture geometry, and master the law of hydraulic fracturing. Many countries around the world have conducted extensive research on 3D fracture propagation model for hydraulic fracturing. In particular, China and the USA have made the most remarkable efforts and published most of the core papers. Statistical data (Table 1.2.9 and Figure 1.2.7) show that China and the USA are the countries that pay most attention to the research on 3D fracture propagation model of hydraulic fracturing and produce most relevant core papers. Chinese core papers amount to 143, accounting for 59.83% of the total number of papers, whereas American papers amount to 90, accounting for 37.66%. Together, both countries account for 97.49% of the global production. The main institutions in terms of core papers related to 3D fracture propagation model for hydraulic fracturing also comprise mostly universities and research centers in China and the USA. The data (Table 1.2.10 and Figure 1.2.8) show that there are 6 Chinese universities and 3 American universities in the Top 10. Among them, China

《Figure 1.2.6》

Figure 1.2.6 Roadmap of the engineering research front of “research on spent fuel reprocessing and high-level radioactive material separation process”

《Table 1.2.9》

Table 1.2.9 Countries with the greatest output of core papers on “3D fracture propagation model for hydraulic fracturing”

《Figure 1.2.7》

Figure 1.2.7 Collaboration network among major countries in the engineering research front of “3D fracture propagation model for hydraulic fracturing”

《Table 1.2.10》

Table 1.2.10 Institutions with the greatest output of core papers on “3D fracture propagation model for hydraulic fracturing”

University of Petroleum (Beijing) has 32 papers, accounting for 13.39% of the total number of papers, and Texas A&M University has 22 papers, accounting for 9.21% of the total number of papers. These are the institutions that present the largest production of core papers in China and the USA. Likewise, China and the USA are also the countries producing most citing papers on 3D fracture propagation model for hydraulic fracturing (Tables 1.2.11 and 1.2.12). Chinese citing papers amount to 1 676, accounting for 55.77%, while the citing papers from USA are 617, accounting for 20.53%. Together, both countries account for 76.3% of the global production. Among the Top 10 universities, 8 are from China and 2 from the USA. The above data clearly demonstrate that China and the USA pay attention to 3D hydrofracture propagation model as a cutting-edge engineering research topic.

Domestic research on 3D fracture propagation model for hydraulic fracturing started earlier. To date, different studies have revealed factors that influence models. A large amount of simulations has been realized using tools such as the ABAQUS large calculation software. However, the development speed is notably slow, mainly because 3D fracture propagation model for hydraulic fracturing involves coupling several complex problems. Strong knowledge of mathematics and mechanics related to coupling is required. In the next 5 to 10 years, 3D fracture propagation model research will still be focused on the mechanisms underlying influence factors such as proppant, heat transfer, fluid compressibility, three- phase flow, and hydraulic and natural fractures. Verification of 3D fracture propagation model by a large number of physical simulation experiments will be required. After ensuring the accuracy of model prediction and achieving reliable high-

《Figure 1.2.8》

Figure 1.2.8 Collaboration network among major institutions in the engineering research front of “3D fracture propagation model for hydraulic fracturing”

《Table 1.2.11》

Table 1.2.11 Countries with the greatest output of citing papers on “3D fracture propagation model for hydraulic fracturing”

performance computer and software, multi-factor coupled model software will be developed to carry out high-precision 3D hydraulic pressure fracture prediction, setting a theoretical basis for hydraulic fracturing technology transformation. Figure 1.2.9 shows the roadmap of the engineering research front of “3D fracture propagation model for hydraulic fracturing”.

1.2.4 Induced mechanism and early warning method of rockburst in deep mining

Rockburst is a phenomenon in which the elastic deformation potential energy accumulated in rock mass is suddenly and violently released under certain conditions, resulting in rockburst and ejection. It is a serious dynamic disaster. With the increase of mining depth, rockburst in coal mines is becoming increasingly dangerous. Research on the mechanism of rockburst disaster, monitoring, early warning and prevention, and control technology is increasingly important for safe coal mining.

At present, among the main coal-producing countries in the world, China faces the most deep mining conditions, and the most serious potential harm from rockburst. To improve the monitoring, early warning, and control level of rockburst, China has conducted extensive scientific research on the induced mechanism and early warning methods of rockburst in deep mining. The country with the highest output of core papers in this field is China, which has produced 78.5% of the core papers, followed by Canada and Australia (Table 1.2.13). Portugal has the highest citation frequency with 35.67 times (Table 1.2.13). Institutions with high output of core papers are mainly Chinese universities, among which China University of Mining and Technology, Shandong University of Science and Technology and University of Science and Technology Beijing occupy the top three in the output of core papers in this field (Table 1.2.14). Among the countries with greatest paper output, China and Australia have more cooperation, followed by China and Canada, China and the USA (Figure 1.2.10). Among the institutions, China University of Mining and Technology and Shandong University of Science and Technology, University of Science and Technology Beijing and North China Institute of Science and Technology have the closest cooperation (Figure 1.2.11). The top two countries with greatest output of cited core papers are China and

《Table 1.2.12》

Table 1.2.12 Institutions with the greatest output of citing papers on “3D fracture propagation model for hydraulic fracturing”

Figure 1.2.9 Roadmap of the engineering research front of “3D fracture propagation model for hydraulic fracturing”

《Table 1.2.13》

Table 1.2.13 Countries with the greatest output of core papers on “induced mechanism and early warning method of rockburst in deep mining”