《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 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, emerging research fronts include “renewable synthetic fuel,” “on-site conversion and efficient utilization of petroleum resources,” and “digital nuclear reactor and power plant intelligent simulation systems.” “Advanced nuclear spent fuel reprocessing and nuclear fuel recycling,” “fracture diagnosis and evaluation methods,” “study of mantle plume-related ore deposits of critical metals,” and “theory and technology for safe direct combustion of low-concentration gas” represent further developments in existing research fields. The research front of “fluidized mining of deep solid mineral resources and its process control mechanism” is revolutionizing the mining industry. The fronts of interdisciplinary integration include “basic theory and method for intelligent drilling,” “solar photovoltaic thermal (PV/T) coupling system based on nano phase change materials (nano-PCMs),” “security of intelligent grid cyber-physical systems,” and “Z-pinch driven inertial fusion mechanism based on pulse power technology.”

The number of core papers published annually from 2014 to 2019 for each of the top 12 engineering research fronts is listed in Table 1.1.2.

(1)  Renewable synthetic fuel

Increasing concerns regarding energy security and CO2 emission require a widespread use and development of renewable energy, which is also the key to address issues of energy storage, climate change and to achieve sustainable development worldwide. Although the installed renewable power capacity, from sources such as solar and wind power,

《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 top12 engineering research fronts in energy and mining engineering

is increasing dramatically, their seasonal and intermittent features limit the direct integration of renewable energy into power grids. One strategy to overcome this is to develop renewable synthetic fuels, which converts CO2 into stable, storable, and high-energy-density liquid fuels such as alcohols, ethers, and hydrocarbons using renewable energy sources. Renewable synthetic fuels can be utilized in power plants, internal combustion engines, aero engines, gas turbines, etc. The CO2 released by using these fuels can be captured from the environment and reduced into renewable synthetic fuels again, thus forming a carbon-neutral energy cycle. In the field of renewable synthetic fuels, major technological approaches include electrocatalytic, photocatalytic, photoelectrocatalytic, and chemical reductions of CO2 to fuel. This research front is focused on determining methods to improve product selectivity, conversion efficiency, and production rate.

(2)  Advanced nuclear spent fuel reprocessing and nuclear fuel recycling

The world is facing a major challenge in the coordinated development of energy and environmental protection, and the situation regarding energy conservation and emission reduction is serious. As a clean, efficient, and large-scale means of energy production, nuclear energy has received considerable attention from international communities. The development and utilization of nuclear energy must be supported by nuclear fuel. The sustainable development of nuclear energy requires the continuous recycling of nuclear fuel through reprocessing, so that unburned fuel can be fully utilized, and the newly generated nuclear fuel can be used effectively. Furthermore, this recycling ensures that other valuable nuclides can be used. The nuclear fuel cycle represents the entire process that involves the preparation of nuclear fuel before it is injected into the reactor, combustion of the fuel in the reactor, and post-processing of the nuclear fuel. The various processing procedures after nuclear fuel is discharged from the reactor comprise the post-nuclear fuel cycle. These procedures include intermediate storage of spent fuel, nuclear fuel reprocessing, preparation and recycling of recovered fuel, radioactive waste treatment, and final disposal. Among these, nuclear fuel reprocessing is the most critical step. The main task of nuclear fuel reprocessing is to separate the fission products from spent fuel using chemical treatment methods; recover and purify valuable fissionable substances, such as 235U and 239Pu, and turn them into fuel elements; and return these elements to nuclear power plants (thermal reactors or fast reactors) for use, which can increase the utilization rate of nuclear fuel and save a considerable amount of uranium resources. This process can also help extract transuranium elements and fission products to advance the application of isotopes in the medical and aerospace industries. The reprocessing of spent fuel significantly reduces the toxicity and volume of radioactive waste that requires final geological disposal and promotes safe disposal. Therefore, reprocessing is of great significance to the sustainable development of nuclear energy. At the same time, nuclear fuel reprocessing technology is a typical dual-use sensitive technology that is strictly restricted by the international Nuclear Non-Proliferation Treaty. Development of the reprocessing technology is an important indicator of the overall national strength and international status of a country.

(3)  On-site conversion and efficient utilization of petroleum resources

Research on on-site conversion and efficient utilization of petroleum resources comprises the integration of thermophysics, thermochemistry, materials science, and other multi-disciplinary areas; the transformation of low- grade resources into high-grade energy underground; and advanced research on the upgrade of high-viscosity crude oil and oil shale using in-situ conversion technology, which is expected to greatly increase the recovery rate or energy conversion rate and provide cutting-edge technology to deal with future structural changes in petroleum resources. The technology for in-situ upgrading of heavy oil has advanced from steam stimulation to steam flooding, steam assisted gravity drainage (SAGD), and fire flooding. In recent years, multi-media steam flooding technology has been developed. This technology is being further developed to include the application of highly active and universally applicable catalyst systems that allow the modification and viscosity reduction reactions of heavy oil to occur at a lower temperature, while simultaneously achieving ideal modification and viscosity reduction effects for different heavy oils. The shale oil underground in-situ conversion technology involves a physical and chemical process that uses horizontal well electric heating lightening technology to continuously heat the organic-rich shale interval to lighten multiple types of organic matter. Such a breakthrough in shale oil underground in-situ conversion technology would aid the development and utilization of shale oil resources and ensure long-term stability and an even higher production of crude oil. Promoting the in-situ conversion and efficient utilization of petroleum resources and realizing breakthroughs in commercialization are of great significance in the future development of the world petroleum industry.

(4)  Basic theory and method for intelligent drilling

Drilling is a key aspect of the discovery, exploration, and exploitation of oil and gas resources. However, existing drilling technology cannot meet the development requirements of complex oil and gas resources in terms of economy, safety, efficiency, and environmental protection. The application of smart technology in the field of oil and gas exploration and development has become the development trend of the petroleum industry. Major international oil companies are actively implementing various intelligent oil and gas exploration and development strategies. Therefore, a new generation of drilling technology must be developed. Intelligent drilling is a transformative technology that integrates big data, artificial intelligence, information engineering, downhole control engineering, and other theories and technologies. Advanced drilling, closed-loop control, precise guidance, and intelligent decision-making are realized through the application of surface automatic drilling rigs, downhole intelligent actuators, intelligent monitoring, decision-making technologies, etc. This is expected to greatly improve the drilling efficiency and reservoir encountering rate. In addition, this will help reduce drilling costs and significantly increase the single-well production and recovery efficiency of complex oil and gas reservoirs.

The basic theories and methods of intelligent drilling mainly involve intelligent characterization of the drilling environment, intelligent perception of drilling conditions, intelligent decision-making of drilling schemes, and intelligent control of drilling parameters. The intelligent characterization of the drilling environment can realize fine characterization of the drilling environment and the physical properties of the reservoir, which is the basis for the realization of intelligent drilling monitoring, diagnosis, decision-making, and regulation. Intelligent perception of drilling conditions uses intelligent monitoring and data transmission technologies to realize real-time analysis and acquisition of geological and engineering parameters and provide dynamic data support for intelligent drilling engineering. The intelligent decision- making of the drilling plan is based on the integration and processing of geological engineering data, revealing the cooperative response mechanism between the geological engineering parameters and objective functions such as drilling rate, cost, and risk. In addition, it helps optimize and determine engineering parameters and construction plans. Intelligent control of drilling parameters is based on an integrated coordination mechanism between drilling parameters. It utilizes the intelligent control theory to achieve intelligent control of drilling parameters such as well trajectory, wellbore pressure, and fluid performance.

(5)  Solar PV/T coupling system based on nano-PCMs

Based on the temperature characteristics of solar PV cells, the solar PV/T coupling system can consider both solar PV power generation and thermal energy conversion. While increasing electrical power generation, it can produce a certain amount of thermal energy. Currently, it is one of the most efficient methods for solar energy conversion and utilization. The use of nano-PCMs in PV/T systems can effectively control the operating temperature of the PV cells, with a high energy storage density and a fast heat storage and release speed, which is important for the efficient large-scale application of solar energy.

The main research topics include the micro-mechanisms of electricity and heat cogeneration for solar cells and optimization of the energy conversion process, new solar PV/T modules and their temperature characteristics, PV/T packaging with nano-PCMs, novel heat transfer mechanisms in nano-PCM energy storage and heat dissipation, the thermal characteristics of the PV/T coupling system using nano-PCMs, and the characteristics of electricity and heat cogeneration.

The combination of nano-PCMs and PV/T systems is important for realizing systematic solar thermoelectric conversion and improving energy efficiency. Research on and development of high-efficiency solar PV/T components and systems that integrate electricity and heat generation to meet the electricity, heating, and cooling loads of buildings and neighborhoods are receiving attention in the field of high- efficiency solar energy conversion and applications. Solar PV/T coupling systems are also an important energy source for future low-carbon cities, green buildings, and zero-energy buildings. Combined with nano-PCMs, PV/T technology can realize effective regulation and a stable energy supply for building energy systems.

(6)  Security of intelligent grid cyber–physical systems

A smart grid cyber–physical system includes a power network, an information network, and a power grid cyber–physical network. It facilitates the real-time perception, dynamic control, and information service of smart grids based on distributed intelligent sensors and communication networks. The security of smart grid cyber–physical systems focuses on the security of the information network and physical network as well as the risk of coupling the two networks. Power grids can be attacked by injecting false information through sensing devices. Therefore, the security problem of cyber– physical systems can limit the power grid security. Various cyber–physical system attack strategies and corresponding risk and vulnerability evaluation methods have been investigated previously. In addition, a test platform has been established for security assessment. Considering the dynamic propagation of the risk in cyber–physical systems, it is vital to conduct in-depth research on the risk propagation and evolution mechanism. Moreover, advanced network security technologies should be developed to strengthen the defenses of the information network, reduce risk, and resist cyber-attacks. Furthermore, future research should focus on the establishment of cyber-security situational awareness platforms, which will be used to identify risk events and simulate risk propagation processes. Big data and situational prediction techniques can be combined to predict cyber- physical system security and guarantee the safe and reliable operation of cyber-physical systems in an actual environment.

(7)   Digital nuclear reactor and power plant intelligent simulation systems

The development of a numerical reactor system refers to the entire process of providing a high-confidence expert reference database for the modeling of reactor operating conditions, the development of high-confidence numerical simulation software systems, and the establishment of virtual reactors for reactor engineering design, performance optimization, operating economy, safety, and reliability, based on the rapid development of high-performance computer systems. Using high-fidelity numerical methods and high-precision models, direct multi-physics simulations of the core design can be achieved, and more accurate calculation results can be obtained, improving the design margins from professional models and interprofessional interfaces, improving the safety and economy of design schemes and promoting reactor operation flexibility. In addition, the test project and scale can be optimized, thereby shortening the research and development cycle and reducing research and development costs. If high-fidelity numerical methods and high-precision models were used for the optimization of reactor operation, the competitiveness of nuclear power in the energy supply system can be enhanced.

Digitization/intelligence refers to the development of a digital design system integrating model research and development, engineering design, and comprehensive verification. Based on design data and new-generation intelligent technologies such as the Internet of Things, big data, and virtual reality, it realizes automated engineering design, intelligent computational analysis, and visualization of results and projects. In addition, it supports a multi-specialty, multi- project, fully collaborative, standardized digital construction system. Focusing on business scenarios such as nuclear power plant operation, maintenance, overhaul, decommissioning, and training, research is being conducted on key applications, such as production management, configuration management, intelligent monitoring and diagnosis, and intelligent robots in nuclear power plant operations. Other avenues of research include building digital twin power plants based on 3D simulation and virtual reality technologies, realizing the one- to-one mapping between the panoramic space of nuclear power plants and the real environment, and forming a digital twin with perception, analysis, decision-making, and execution capabilities. A database regarding the full life cycle of nuclear power plants should also be constructed. The integration of smart nuclear energy systems is being studied to realize nuclear power generation, nuclear energy supply at the site, and intelligent scheduling and management of various aspects of nuclear energy utilization, such as heat, nuclear energy, sea water desalination, and nuclear energy hydrogen production.

(8)  Z-pinch driven inertial fusion mechanism based on pulse power technology

Controllable fusion energy is the ideal clean energy of the future. The main techniques to realize this energy are magnetic and inertial confinement. For the inertial confinement, the fusion energy is obtained by supplying the heating energy directly or indirectly onto the fusion target, compressing the target, and then realizing thermonuclear ignition and combustion under the inertial constraint of internal explosive motion. Key technologies related to this front include driving sources with high heating power, repetition rate, and stability; the structural design of the implosion chamber and the fusion target; optimization of the design of the ignition method; high-efficiency energy conversion; and Tritium cycle and extraction. The fast Z-pinch technology is based on pulse power technology. Therefore, it can realize high-efficiency energy conversion from the electric energy storage of the driver to kinetic energy or X-ray radiation energy of the Z-pinch load. It is expected to help realize the repetitive operation of the driver, which will provide a powerful energy source for driving inertial confinement fusion and inertial fusion energy. The inertial confinement driven by the Z-pinch is a complex multi-scale and multi-physical process. Current experimental techniques are not capable of directly achieving fusion ignition. Numerical simulation is an important means to study the physical problems of Z-pinch driven inertial confinement fusion. In the future, numerical simulation tools will be further developed and improved to conduct research on the physics of the whole process and associated key problems.

(9)  Fracture diagnosis and evaluation methods

Fractures are the main spaces for the storage and circulation of oil and gas. Hydraulic fracturing occupies a high proportion of the production stimulation measures of low-permeability oil and gas reservoirs, and it is the main method for improving oil and gas recovery. Therefore, it is necessary to conduct research on fracturing diagnosis and evaluation methods to clarify the actual shape and parameters of fractures under current reservoir conditions. According to test results, and to compensate for the differences between theory and practical applications, the principles of hydraulic fracturing design are adjusted to ensure that the fracture parameters calculated by the fracturing model are in good agreement with the actual ones, which will further improve the applicability of the fracturing design plan. At the same time, this reduces fracturing investment and improves the effect of fracturing reconstruction. Although fracturing pressure fitting, tracers, production logging tools, micro- seismic fracture monitoring, micro-deformation fracture monitoring, and other methods have been used to diagnose and evaluate fractures for decades, little is known about them today and there are knowledge gaps pertaining to the complex shape, length, width, height, and distribution of diversion capacity. Owing to the characteristics of storage and hydraulic fracturing parameters, it is particularly important to accurately understand the fracture length and fracture orientation to optimize the overall development of low-permeability oilfields. In the future, research on fracture diagnosis and evaluation technology should be strengthened, as it is an important basis for deepening our understanding of fractures, correcting fracture prediction models, optimizing fracturing design and well pattern deployment, and ultimately helping oilfield developers optimize oilfield and single-well development plans to enhance the economic benefits.

(10)   Study of mantle plume-related ore deposits of critical metals

A mantle plume links the mass and energy exchange between the core, mantle, and crust, thus providing a long- term and steady material base for various critical metal mineralizations. Plume-related magmatic sequences are found to host some specific types of mineralization, such as mafic-ultramafic layered intrusions and Fe-Ti-V oxide (Co and Sc), Ni-Cu-PGE (platinum group element) sulfides and Cr- Ti-Fe oxides deposits, komatiite and related Cu-Ni sulfides deposits, peralkaline complex-carbonatites and Nb-Ta-Zr-REE deposits. In contrast, metals in the crust are transported and further concentrated by hydrothermal convection, which is activated by thermal anomalies induced by a mantle plume. This leads to the formation of hydrothermal ore deposits that are indirectly related to mantle plumes, such as Mn-Co ores, Carlin-style gold deposits, and tellurium ore deposits associated with the Emeishan large igneous provinces (LIPs). Previous research has identified the following key controlling factors in plume-related critical metal mineralization: structure of the plume, magma source, crystal fractionation, crustal contamination and liquid immiscibility, magma replenishment, mingling/mixing, and intrusion processes. Although the key metal mineralization related to mantle plumes is gaining an increasing amount of attention, its theoretical framework has not been established yet, and the understanding of the key controlling factors still needs to be improved. Notably, there are two Permian LIPs in China, which host various types of critical metal deposits and thus provide rare opportunities for us to reveal the genetic links between mantle plumes and critical metal mineralization and establish a systematic theory of critical metal metallogeny. Therefore, in addition to previous case studies on individual deposits, emphasis should be placed on research on the metallogenic sequence of key metals in mantle plumes, which will contribute to building an integrated theoretical system for critical metal mineralization associated with mantle plumes.

(11)  Theory and technology for safe direct combustion of low- concentration gas

Methane is the second largest greenhouse gas after CO₂, accounting for approximately 20% of global greenhouse gas emissions. The concentration of methane in the atmosphere is much lower than that of CO₂, but the global warming potential (GWP) of methane is 25 times that of the same amount of CO₂. According to the GWP, the methane emissions of the coal and oil energy industries are equivalent to more than 10 billion tons of CO₂, and the reduction of methane emissions in the energy industry is of great significance to environmental protection. As of 2018, the utilization rate of low-concentration coal mine gas with a concentration between 6% and 30% is only approximately 28%, and the utilization rate of ultra-low-concentration gas (including ventilation gas) with a concentration less than 6% is only 2%. Exploring the utilization technology of low-concentration gas, especially ultra-low-concentration gas, is the key to solving the problem of low gas utilization rate. For the combustion (oxidation) utilization of ultra-low-concentration gas and ventilation gas, research groups from various countries have proposed different thermal storage oxidation technologies, such as the thermal flow reversal reactor technology in the United States, the catalytic flow-reversal reactor technology in Canada, the catalytic gas turbine technology in Australia, the thermal regeneration oxidation technology in Germany, and the monolithic honeycomb catalytic oxidation technology in Germany. Some progress has been made, but the proportion of ultra-low-concentration gas in coal mines is large (more than 70% of the total gas emissions), and the gas concentration is low (< 6%). Furthermore, the concentration fluctuates greatly and crosses the explosion boundary range, and the content of impurities is high (dust, water vapor, and impurity gas). Therefore, the realization of safe transportation of explosive gas, efficient removal of impurity components, and efficient direct combustion (oxidation) of ultra-low- concentration gas is crucial for industrial application.

(12)   Fluidized mining of deep solid mineral resources and its process control mechanism

The exploitation of deep solid resources faces high stress, high well temperature, and high well depth. Fluidized mining of deep solid mineral resources involves injecting a leaching solution into a deep ore deposit via injection wells to allow the leaching solution and targeted minerals to react. Then, the leaching solution containing soluble metallic ions is lifted to the surface. Finally, metallic products are obtained via solvent extraction electrowinning. Studies in this research area aim to develop an innovative green mining method and a regulation mechanism of deep solid mineral resources, promote mining reform, and eventually provide a reference for safe and efficient mining of deep solid resources.

There are several research topics in this area such as exploring the inlay interaction between the in-situ pore structure and valuable minerals of deep solid resources to realize a clear representation of deep mining environments and studying the law of multi-scale seepage of solution under high- temperature and high-pressure conditions deep underground. Other research avenues include detecting the leaching reaction mechanism of metallic minerals under multi-physics effects and building a model to describe the response of in- situ mineral dissolution and precipitation to pore structure evolution. Furthermore, a mathematical model of leachable evaluation and prediction of deep solid mineral resources has been developed, and an ameliorated technology system for deep-ground solution seepage and leaching reaction has been built.

The future development of this research front will include non-disturbed detection of multi-scale pore structures during the deep fluidized leaching process, coupling of deep pore structure, solution seepage, and leaching reaction, well-net setups and seepage intelligent controls of deep fluidized leaching, and other future research interests, to eventually achieve a breakthrough in the theory of fluidized mining of deep solid resources and the equipment for this technology.

《1.2 Interpretations for four key engineering research fronts》

1.2 Interpretations for four key engineering research fronts

1.2.1 Renewable synthetic fuel

There is an urgent need to develop zero-carbon and carbon- neutral energy technologies to sustainably develop the world’s infrastructure. George Andrew Olah, the Nobel Prize winner in Chemistry, discussed the concept and drawbacks of hydrogen economy in his book Beyond Oil and Gas: The Methanol Economy in 2006, and further proposed a promising idea, namely to create a renewable energy resource by converting CO₂ from both nature and industry into carbon- neutral alcohol and ether fuels. In 2018, four academicians of the Chinese Academy of Sciences, published a perspective in Joule (Joule, 2018, 2, 1925–1949), where they pointed out that the key to utilizing solar energy is to convert it into stable, storable, and high-energy-density chemical fuels, which is also the concept of renewable synthetic fuel.

Owing to the potential to address the energy and environmental challenges, renewable synthetic fuels have attracted increasing attention in recent years. Major economies, including China, the United States, Europe, Japan, and South Korea are all pursuing researches in this field. The Department of Energy of the United States established 46 Energy Frontier Research Centers, which support the study of solar power generation and solar-to-fuel conversion technology. In 2010, it established the Joint Center for Artificial Photosynthesis (JCAP) with Caltech, University of California campuses in Irvine and San Diego, the Lawrence Berkeley National Laboratory, and the SLAC National Accelerator Laboratory, which is the largest research program dedicated to the advancement of solar fuel generation science and technology. In 2020, the JCAP started to investigate the solar-fuel reactor system in order to convert CO₂ into liquid fuel driven by solar energy at a large scale. The European Union and South Korea also established the Energy-X and KCAP programs, aiming to achieve the cyclic utilization of carbon energy on a CO₂ basis. According to a report by BOSCH, renewable synthetic fuels will be widely used in Europe by 2050, which could reduce CO₂ emissions by 2.8 billion tons (approximately three times the annual CO₂ emissions in Germany).

In June 2019, the China Association for Science and Technology announced a list of the top 20 scientific and engineering technical problems of 2019 which play a guiding role in scientific and technological innovation, of which the development of renewable synthetic fuel is one of the foremost scientific problems. Domestic scientific research institutions represented by Tsinghua University, the University of Science and Technology of China, the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences, and Shanghai Jiao Tong University have also been committed to this field for a long time. Currently, the major technological paths include electrocatalytic, photocatalytic, photoelectrocatalytic, and chemical reductions of CO₂ to fuel. However, each of these remains at the experimental exploration stage, and a number of technical bottlenecks need to be overcome before industrial application is possible. Significant further work on the reaction mechanism, catalytic materials, and reactor systems is required to greatly improve the product selectivity, conversion efficiency, and production rate in order to promote the practical application of this technology.

The top three countries with the greatest output of core papers on “renewable synthetic fuel” are the United States (37), China (31), and Canada (10), and the corresponding citations per paper reach 185, 222, and 123, respectively, as listed in Table 1.2.1. Among the major countries with publications on this research front, there is a large amount of cooperation among the United States, China, and Canada, as shown in Figure 1.2.1. The top two institutions with the greatest output of core papers are Stanford University (8) and the Chinese Academy of Sciences (6), as given in Table 1.2.2. Out of the top 10 institutions by publication volume, there is a large amount of cooperation among Stanford University, SLAC National Accelerator Laboratory, the University of Toronto, the University of Science and Technology of China, and Chinese Academy of Sciences, as demonstrated in Figure 1.2.2. The top three countries with the greatest output of citing papers are China (4 818), the United States (1 580), and Japan (523), as presented in Table 1.2.3. The top three institutions with the greatest output of citing papers are the Chinese Academy of Sciences (860), the University of Chinese Academy of Sciences (351), and the University of Science and Technology of China (239), as shown in Table 1.2.4.

《Table 1.2.1》

Table 1.2.1 Countries with the greatest output of core papers on “renewable synthetic fuel”

《Figure 1.2.1》

Figure 1.2.1 Collaboration network among major countries in the engineering research front of “renewable synthetic fuel”

《Table 1.2.2》

Table 1.2.2 Institutions with the greatest output of core papers on “renewable synthetic fuel”

《Figure 1.2.2》

Figure 1.2.2 Collaboration network among major institutions in the engineering research front of “renewable synthetic fuel”

《Table 1.2.3》

Table 1.2.3 Countries with the greatest output of citing papers on “renewable synthetic fuel”

《Table 1.2.4》

Table 1.2.4 Institutions with the greatest output of citing papers on “renewable synthetic fuel”

1.2.2 Advanced nuclear spent fuel reprocessing and nuclear fuel recycling

Nuclear fuel reprocessing is divided into two categories depending on whether it is conducted in an aqueous medium: water reprocessing and dry reprocessing. Water reprocessing comprises the chemical separation and purification processes in aqueous solutions such as precipitation, solvent extraction, and ion exchange, while dry reprocessing consists of processes, such as the fluorination volatilization process, high-temperature metallurgical treatment, high-temperature chemical treatment, the liquid metal process, the molten salt electrolysis process, and other chemical separation methods in an anhydrous state. Dry reprocessing has advantages in terms of processing high burnup spent fuel, especially fast reactor spent fuel, but is relatively difficult to implement and is still an important research direction at present.

Traditional spent fuel water reprocessing, e.g., the Plutonium uranium recovery by extraction (Purex) process, is currently used for the reprocessing of spent fuel in nuclear power plants. However, owing to its deep burnup, higher radioactivity, and high fission product content, spent fuel reprocessing in nuclear power plants is more focused on separating key elements such as neptunium and technetium more reliably and efficiently, and, through the separation of high radioactive nuclei and transmutation, to shorten the volume and storage duration for better environmental compatibility. In order to adapt to future requirements, thermal reactor spent fuel reprocessing plants will be reliable, safe, and economical. For this reason, research on and development of reprocessing technology, equipment, control, etc. are still under way. Further research on the post-treatment process includes improvements to the Purex process, such as simplifying the process flow, reducing investment costs, and using salt-free reagents to reduce waste generation.

In France, Areva developed the COEX process. In accordance with so-called non-proliferation considerations, the process does not produce pure Pu products, but produces U-Pu mixed products. It is said that this product is beneficial to the manufacture of mixed oxide fuel, but this is a controversial topic. The United States developed the UREX+ process, which separates U, Tc, and I, and the remaining products (including Pu) in the high-level radioactive waste liquid undergo further separation. Separation–transmutation is a strategic measure to minimize nuclear waste. In this method, the minor actinides (MAs) and long-lived fission products (LLFPs) are first separated from the high-level radioactive waste generated by the reprocessing of spent fuel. Then, the MAs and LLFPs are made into targets and transmuted in a transmuter (fast reactor or accelerator driven sub-critical system) to transform the long-lived MAs and LLFPs into short-lived or stable nuclides, in order to reduce the volume and long-term toxicity of high- level radioactive waste that requires geological disposal. Russia has rebuilt RT-1 and continues to reprocess the spent fuel. Meanwhile, the RT-2 plant can also reprocess the spent fuel.

In terms of the research and development of advanced main processes, based on the newly-built experimental platform of Nuclear Fuel Reprocessing Radiochemical Experimental Facility, in 2015, the China Institute of Atomic Energy successfully completed a 100-hour thermal experiment of continuous operation of the advanced process, and comprehensively obtained the key process indicators such as process flow fragment purification, uranium-plutonium yield, and uranium-plutonium separation. Furthermore, it highlighted the trend in the data of key elements, such as neptunium and technetium in the process, and investigated the stability and reliability of the process.

Regarding the separation process of high-level radioactive waste liquid, Tsinghua University developed the trialky- lphosphine oxide process in the 1970s. After 160 hours of radioactive verification, the conditions for a pilot-scale test were prepared. The next step is to conduct research on the separation of high-level radioactive waste liquids in the rep- rocessing of power reactor fuel.

In recent years, several domestic research institutions and universities have successfully conducted basic and applied research on different separation technologies, such as the chemical processes of actinides, the separation and extraction processes of neptunium, the separation of new actinium and lanthanum, and the separation of strontium and cesium. These institutions have also aimed to develop thorium-based molten salt reactors and conducted preliminary research on dry post-processing technology. Furthermore, they have researched electrolytic refining and the separation of uranium and rare earths, ionic liquid separation of rare earths, and supercritical CO2 extraction and separation of uranium.

In addition to the water separation process, another direction of research abroad is the research and development of the dry separation process as a candidate for advanced reprocessing. As nuclear fuel burnup further increases, spent fuel will become more radioactive, which may make it difficult for organic solvent-based water reprocessing. As a result, dry post-processing, which was studied by various countries in the 1960s and the 1970s, has become an active research field after approximately 20 years. Countries that are currently actively developing dry reprocessing techniques include the United States, Russia, Japan, France, India, and South Korea.

Dry reprocessing is a high-temperature chemical process, and the most promising method is the molten salt electrolytic refining method for metal fuel and oxide fuel. Compared with water reprocessing, the advantages of dry reprocessing are as follows: The inorganic reagents used have good high- temperature resistance and radiation resistance. In addition, the process flow is simple, and the equipment structure is compact. Furthermore, the equipment has good economic efficiency, and the recycling of reagents reduces waste generation. Moreover, Pu and MAs are recycled together, which helps prevent nuclear proliferation. However, dry reprocessing technology is difficult to implement. The strong irradiation of the components requires the entire process to be operated remotely, and the atmosphere needs to be strictly controlled to prevent hydrolysis and precipitation. Furthermore, the structural materials must have good high- temperature resistance, radiation resistance, and corrosion resistance.

As shown in Table 1.2.5, 30 core papers have been published in this direction, and the countries with the largest number of core papers are the United States, the United Kingdom, France, Germany, and China. It can be seen from Table 1.2.6 that the organizations that have published the largest number of core papers on this research topic are the French Alternative Energies and Atomic Energy Commission, the Joint Research Center of the European Commission, and the Korea Atomic Energy Research Institute. From the cooperation network among the top 10 core paper publishing countries (Figure 1.2.3), it can be seen that France, Germany, and the United Kingdom have frequent cooperation, and China and the United States have some cooperation. As demonstrated in Figure 1.2.4, out of the top 10 institutions by publication volume, there is a large amount of cooperation among French Alternative Energies and Atomic Energy Commission, Joint Research Center of the European Commission, Central Laboratory, Forschungszentrum Juelich, and Karlsruhe Institute of Technology; Korea Atomic Energy Research Institute, South Korea University of Science and Technology, and Ulsan National Institute of Science and Technology also have close cooperation on this front. The top five countries with the greatest output of citing papers are the United States, China, India, France, and Germany, as presented in Table 1.2.7. The top institutions with the greatest output of citing papers are the Chinese Academy of Sciences, French Alternative Energies and Atomic Energy Commission, University of Chinese Academy of Sciences, University of Tokyo, and Idaho National Laboratory, as shown in Table 1.2.8.

《Table 1.2.5》

Table 1.2.5 Countries with the greatest output of core papers on “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

《Table 1.2.6》

Table 1.2.6 Institutions with the greatest output of core papers on “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network among major countries in the engineering research front of “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network among major institutions in the engineering research front of “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

《Table 1.2.7》

Table 1.2.7 Countries with the greatest output of citing papers on “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

《Table 1.2.8》

Table 1.2.8 Institutions with the greatest output of citing papers on “advanced nuclear spent fuel reprocessing and nuclear fuel recycling”

1.2.3 On-site conversion and efficient utilization of petroleum resources

As the degree of exploration and development increases, the quality of petroleum resources is deteriorating. The remaining oil in old oilfields is extremely dispersed, and the development of crude oil is approaching the limit of profitability. A large amount of shale oil is difficult to flow and produce and faces common problems of efficient resource utilization. Flow and extraction are common challenges in the efficient use of these resources. Research on on-site conversion and efficient utilization of petroleum resources refers to the integration of thermophysics, thermochemistry, materials science, and other multi-disciplines to transform low-grade resources into high-grade energy underground. It also refers to advanced research on in-situ upgrading of high-viscosity crude oil and oil shale in-situ conversion technology, which is expected to greatly increase the recovery rate or energy conversion rate and provide cutting-edge technology to deal with future structural changes in petroleum resources.

The technology for in-situ upgrading of heavy oil has advanced from steam stimulation to steam flooding, SAGD, and fire flooding. In recent years, multi-media steam flooding technology has been developed, and this technology is being furthered by developing highly active and universally applicable catalysts. The system enables the modification and viscosity reduction reactions of heavy oil to occur at a lower temperature and, at the same time, achieve the ideal modification and viscosity reduction effects for different heavy oils.

Shale oil underground in-situ conversion involves a physical and chemical process that uses horizontal well electric heating lightening technology to continuously heat the organic-rich shale interval to lighten multiple types of organic matter. This advancement in shale oil underground in-situ conversion technology would help in the development and utilization of shale oil resources and ensure the long- term stability and even higher production of domestic crude oil. Promoting the in-situ conversion and efficient utilization of petroleum resources and realizing commercial breakthroughs are of great significance to improving China’s crude oil self-sufficiency and the future development of the world’s petroleum industry.

Relevant papers in this area are published primarily by China, the United States, India, and other countries, as given in Table 1.2.9. The foremost institutions include the Chinese Academy of Sciences, the University of Chinese Academy of Sciences, and Anna University, as listed in Table 1.2.10. In terms of international cooperation, Malaysia has cooperated with India, Canada with Turkey, and China with the United States (Figure 1.2.5). Close collaberation exists between Ann University and the University of Malaya; Gachon University, the Korea Advanced Institute of Science and Teahnology, and the Korea Institute of Energy Research; the Chinese Academy of Sciences and the University of Chinese Academy of Sciences (Figure 1.2.6). Moreover, the papers are cited primarily by countries such as China, the United States, and India, and by institutions such as the Chinese Academy of Sciences, the University of Chinese Academy of Sciences, and the University of Toronto (Tables 1.2.11 and 1.2.12).

《Table 1.2.9》

Table 1.2.9 Countries with the greatest output of core papers on “on-site conversion and efficient utilization of petroleum resources”

《Table 1.2.10》

Table 1.2.10 Institutions with the greatest output of core papers on “on-site conversion and efficient utilization of petroleum resources”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network among major countries in the engineering research front of “on-site conversion and efficient utilization of petroleum resources”

《Figure 1.2.6》

Figure 1.2.6 Collaboration network among major institutions in the engineering research front of “on-site conversion and efficient utilization of petroleum resources”

《Table 1.2.11》

Table 1.2.11 Countries with the greatest output of citing papers on “on-site conversion and efficient utilization of petroleum resources”

《Table 1.2.12》

Table 1.2.12 Institutions with the greatest output of citing papers on “on-site conversion and efficient utilization of petroleum resources”

1.2.4 Basic theory and method for intelligent drilling

As the core driving force of today’s technological revolution and industrial innovation, intelligent technology has become crucial. The application of smart technology in the field of oil and gas exploration and development has become an important trend in the development of the petroleum industry. Major international oil companies such as Schlumberger and Halliburton are actively implementing the strategic layout of intelligent oil and gas exploration and development. Drilling is a key part of the discovery, exploration, and exploitation of oil and gas resources. Intelligent drilling technology is a transformative technology that integrates big data, artificial intelligence, information engineering, downhole control engineering, and other theories and technologies.

This is expected to greatly improve the drilling efficiency and reservoir drilling encountering rate. Furthermore, this technology can reduce drilling costs and significantly increase the single-well production and recovery efficiency of complex oil and gas reservoirs.

The intelligent characterization of the drilling environment can provide a precise description of the drilling environment working conditions and the physical properties of the reservoir, which is the basis for the realization of intelligent drilling monitoring, diagnosis, decision-making, and regulation. The eDrilling Company has established a virtual wellbore to provide early warnings of gas invasion and well kick. Sinopec realized the direct characterization of rock physical properties using the amplitude variation with offset inversion method. The focus of research in this area is 3D geological modeling, drilling process visualization, and digital twin simulation.

Intelligent perception of drilling conditions is based on intelligent monitoring and data transmission technologies. It combines artificial intelligence algorithms, realizes real- time analysis and collection of geological and engineering data, and provides dynamic data support for the realization of intelligent drilling engineering. At present, King Fahd University of Petroleum and Minerals in Saudi Arabia has achieved a high-precision prediction of the rate of penetration (ROP) using neural network technology. Northeast Petroleum University in China used the improved back-propagation algorithm to predict the friction factor of the drill string to overcome individual differences in the friction factor. The main research trends in this area are the response mechanism of downhole monitoring devices, automatic diagnosis of risks in wells, and early warning theory.

The intelligent decision-making of the drilling plan is based on the integration and processing of geological engineering data, revealing the cooperative response mechanism between the geological engineering parameters and objective functions such as drilling rate, cost, and risk. Moreover, it helps optimize and determine engineering parameters and construction schemes. Drilling efficiency, construction cost, and drilling risk are the most important objective functions of intelligent decision-making. At present, Saudi Aramco analyzes the response of drilling parameters to changes in lithology in real time and obtains optimal control parameters through algorithms such as gradient search. The China Petroleum Engineering Institute proposed a bit selection database, which realized bit selection based on formation information. Research on multi-source data fusion, geological engineering model reconstruction, and intelligent coordinated control strategies for all drilling conditions will effectively promote the development of intelligent decision-making technology for drilling. The main research trend of intelligent decision- making is the two-way efficient real-time transmission of massive data, and the closed-loop optimization theory of downhole control parameters.

Intelligent control of drilling parameters is based on an integrated coordination mechanism between drilling parameters. It utilizes the intelligent control theory to achieve intelligent control of drilling parameters such as well trajectory, wellbore pressure, and fluid performance. The Halliburton fracturing parameter intelligent control system uses real-time measurement and an adaptive rate control algorithm to intelligently adjust the pump speed to achieve uniform fracturing. Intelligent control of drilling engineering parameters is a key component to realize intelligent drilling. The closed-loop control of ROP, intelligent steering drilling, and wellbore stability will become the focus of future research.

According to Table 1.2.13, the countries with the largest number of core papers on this research topic are China, Iran, and Australia. The percentage of core papers on this topic in each country other than China, Iran, and Australia is less than 10%. The percentage of Chinese core papers is 38.64%. The each country with the most citations of core papers in this direction is Iran, and the one with the most citations per core paper is Malaysia. Table 1.2.14 shows that Amirkabir University of Technology in Iran and Southwest Petroleum University in China have the largest number of core papers on this research topic, each with a fraction of more than 10%, while each of the other institutions has published less than 10% of the total papers. The core papers from the Amirkabir University of Technology are cited most frequently, and the core papers University of Technology Malaysia have the most citations per paper.

From Figure 1.2.7, it can be seen that China, Iran, the United States, and Australia cooperate more with other countries in this field. According to Figure 1.2.8, collaborative research among institutions is mainly carried out by Amirkabir University of Technology, University of Tehran, Iran Petroleum University of Technology, Islamic Azad University, and Sharif University of Technology, each of which has cooperative relations with three other institutions.

According to Table 1.2.15, the countries with the largest number of citing papers on this research topic are China, Iran, and Vietnam. The percentage of citing papers in China exceeds 30%. It can be seen from Table 1.2.16 that the institutions with the largest number of papers on this research topic are Duy Tan University, Amirkabir University of Technology, University of Technology Malaysia, and Ton Duc Thang University, with the percentage of citing papers written by each university exceeding 10%.

《Table 1.2.13》

Table 1.2.13 Countries with the greatest output of core papers on “basic theory and method for intelligent drilling”